Plants traditionally used in age-related brain disorders (dementia): an ethanopharmacological survey

Abstract

Context: Epidemiological studies have shown that despite mortality due to communicable diseases, poverty and human conflicts, the incidence of dementia increases in the developing world in tandem with the ageing population. Although some FDA approved drugs are available for the treatment of dementia, the outcomes are often unsatisfactory. In traditional practices of medicine, numerous plants have been used to treat cognitive disorders, including neurodegenerative diseases such as Alzheimer’s disease (AD) and other memory-related disorders. In western medicine most of the drugs used for the treatment of neurodegenerative disorders are derived from plant sources.

Objective: This article reviews plants and their active constituents that have been used for their reputed cognitive-enhancing and antidementia effects.

Methods: A literature survey in Science Direct, Pubmed, and Google Scholar was performed to gather information regarding drug discovery from plants sources for the treatment of congnitive disorders and dementia.

Results: More than forty herbal remedies were identified with cholinesterase inhibitory, anti-inflammatory, or antioxidant activities. Bioactive compounds include alkaloids, flavonoids, steroids, saponins, terpenoids, and essential oils. About eleven herbal plants with multipotent activity against AD are discussed.

Conclusion: Literature surveys show that most of the research has been conducted on herbal remedies effect on cholinesterase inhibitory and antioxidant activities. Studies regarding the effect of herbal drugs on β-secretase inhibitory activity and antiaggregation property are lacking. This review provides leads for identifying potential new drugs from plant sources for the treatment of neurodegenerative disorders.

Keywords::

• β-Secretase
• acetylcholinesterase
• butyrylcholinesterase
• phytoinhibitors
• antioxidant

Introduction

Neurodegenerative disorder (Greek neuro – “nerval” and Latin degenerare, “to decline” or “to worsen”) is a heterogeneous group of degenerative conditions affecting specific areas of the central and peripheral nervous system, leading to gradual and progressive cognitive or movement impairments, depending on the type of neuronal cells undergoing selective degeneration with this disease. It is a condition in which cells of the brain and spinal cord are lost. The brain and spinal cord are composed of neurons with different functions such as controlling movements, processing sensory information, and making decisions. Since these cells are not readily regenerated, excessive damage can be devastating. Some sources limit the term “degenerative” to conditions primarily affecting grey matter that are not associated with an obvious inciting event. This disorder, characterized by progressive loss of motor, sensory neurons and the ability of the mind to refer sensory information to an external object, is affected in different kinds of neurological disorders (Coppede et al., 2006). Neurodegenerative disorders are crudely divided into two groups according to phenotypic effects: 1) conditions causing problems with movements, such as ataxia, and 2) conditions affecting memory and related to dementia. The majority of neurodegenerative pathologies are age-related dementia, thus becoming an increasing health and socioeconomical problem in industrialized countries, where the frequency has increased in the last decades, together with an increased life expectancy of the individuals (Mayeux, 2003). Neurodegenerative disorders may be inherited, sporadic or transmitted. Environmental factors, including metals, pesticides, food, head injuries and infections, have been extensively studied as potential risk factors for sporadic forms of neurodegeneration (Brown et al., 2005).

Dementia is the most common form of neurodegenerative disorder affecting several million people world wide. Dementia is not a disease itself, but rather a group of symptoms that might accompany certain diseases or conditions. It is characterized by chronic progressive mental disorder, which adversely affects memory, thinking, comprehension, calculation and language. Dementia is alarmingly prevalent in the elderly population, affecting 5% of people over the age of 65 and up to 50% of people over the age of 85 (Evans et al., 1989). Current data from developing countries suggest that age-adjusted dementia prevalence estimates in 65 year olds are high (≥5%) in certain Asian and Latin American countries, but consistently low (1–3%) in India and sub-Saharan Africa; Alzheimer’s disease accounts for 60% whereas vascular dementia accounts for 20% of the prevalence (Figure 1) (Kalaria et al., 2008). An estimated 24 million people worldwide have dementia – two-third of them in developing countries. Unless we find a cure, this figure could increase to more than 80 million people by 2040 (Melzer et al., 1997). Some of the commonest forms of dementia causing degeneration of neurons are Alzheimer’s disease, dementia with Lewy bodies, Parkinson’s, Huntington’s disease, and Myasthenia gravis (Holden & Kelly, 2002).

Figure 1.  Prevalence of different forms of dementia.

Alzheimer’s disease

Alzheimer’s disease (AD) one of the fourth leading disorders in the world accounts for ~60% of dementia in elderly persons. AD is characterized clinically by global cognitive dysfunction, especially memory loss, behavior and personality changes, and impairments in the performance of activities of daily living that leaves end-stage patients bedridden, incontinent and dependent on custodial care. Patient death occurs, on average, 9 years after diagnosis. The risk of AD dramatically increases with aging, affecting 7–10% of individuals over age 65, and about 40% of persons over 80 years of age, and it is predicted that the incidence of AD will increase threefold within the next 50 years if no therapy intervenes (Sisodia, 1999). In developed societies where life expectancy has been considerably extended, this devastating disease actually represents a major public health concern, being estimated that 22 million people worldwide will develop this progressive neurodegenerative disorder by 2025 (Sleegers & van Duijn, 2001). Despite the strong progresses made in AD research in the last decades, no treatment with a strong disease-modifying effect is currently available.

Neuropathological hallmarks

The neuropathological hallmarks of AD are neuritic (senile) plaques, which are extracellular deposits predominantly composed of fibrillar β-amyloid (Aβ) peptide, usually surrounded by reactive astrocytes, activated microglia and dystrophic neurites (altered axons and dendrites), and intracellular neurofibrillary tangles (NFT) composed of filamentous aggregates called paired helical filaments (PHF) of hyperphosphorylated protein tau, frequently conjugated to ubiquitin (Selkoe, 2001). Plaques and tangles are present mainly in brain regions involved in learning and memory and emotional behaviors such as the entorhinal cortex, hippocampus, basal forebrain and amygdala. Brain regions with plaques typically exhibit reduced numbers of synapses, and neurites associated with the plaques are often damaged, which suggests that Aβ damages synapses and neurites. Neurons that use glutamate or acetylcholine as neurotransmitters appear to be particularly affected, but neurons that produce serotonin and norepinephrine are also damaged. Neurons that degenerate in AD exhibit increased oxidative damage, impaired energy metabolism, and perturbed cellular calcium homeostasis; Aβ appears to be an important instigator of these abnormalities (Mattson, 2004).

Etiology

Early onset dementia, termed as familial type AD, is inherited in autosomal dominant fashion. Mutation of the genes shown in Table 1 augments the amyloidogenic pathway of β APP processing in cells in a way that favors production of the Aβ 42 variant. Aβ 42, which is selectively deposited in affected brain regions in humans, is more prone to oligomerization and fibril formation than the slightly shorter and less hydrophobic Aβ 40 form. In addition persons suffering from Downs’s syndrome (trisomy 21) showed over expression of structurally normal APP almost invariably leading to the premature occurrence of classical AD neuropathology (neuritic plaques and neurofibrillary tangles) during middle adult years. Other than genetic abnormalities, environmental factors also cause AD which is termed as late onset or sporadic type AD. Epidemiological findings supported by in vivo studies suggest that a low education level, history of head trauma, consumption of high-caloric/high-fat diets and a sedentary life may each increase the risk of occurrence of AD (Mattson, 2003). Insulin resistance may also be a risk factor for the development of AD (Watson & Craft, 2003) as insulin appears important for learning and memory, and it was shown to inhibit the phosphorylation of tau (Hong & Lee, 1997), to stimulate the secretion of APPsα, and also to reduce the intracellular pool of Aβ (Tanzi & Bertram, 2001). The causes of altered βAPP metabolism and Aβ deposition in sporadic cases of AD are not understood, but may include age-related increases in oxidative stress, impaired energy metabolism, and perturbed cellular ion homeostasis. Altered metabolism βAPP by β- and γ- secretase increases the level of lipophilic Aβ (1–42) fragment, which in the presence of Fe²⁺ and Cu⁺ is highly susceptible to aggregation forming β-amyloid plague in the synaptic junction of cholinergic neurons. When Aβ aggregation occurs at the cell membrane, membrane associated oxidative stress occurs leading to 4-hydroxynonenal formation a neurotoxic aldehyde that covalently modifies membrane transporters (ion motive ATPases, glucose transporter, glutamate transporter), G protein, NMDA receptors. Oxidative modification of tau protein leads to neurofibrillary tangle formation. Aβ can also induce Ca²⁺ dysregulation, mitochondrial impairment, and oxidative stress in endoplamic reticulum, apoptosis and finally death of neurons (Miranda et al., 2000; Chauhan & Chauhan, 2006). Until now, the only treatment for AD is based on drugs that act as acetylcholinesterase inhibitors (AChE), enhancing the level of acetylcholine in the brain. Recently, several new strategies have been proposed to ameliorate the neurodegenerative developments associated with AD. As production of neurotoxic forms of Aβ from APP appears to be pivotal event in AD pathogenesis there is intense increase in developing drugs that block βand γ secretase inhibitors (Dewachter & Van Leuven, 2002). Another approach to reducing amyloid accumulation in the brain is agents that chelate copper and iron, such chelators would also be expected to reduce oxidative stress in neurons so they are termed as indirect antioxidants (Ritchie et al., 2003). The emerging link between cholesterol levels and AD has led to trials of cholesterol lowering statins in AD patients. Other therapeutic approaches being tested include anti-inflammatory agents such as COX-2 inhibitors and steroids that decline during normal ageing such as estrogen and testosterone (Hoozemans et al., 2003), activation of proteases that degrades β-amyloid protein, activation of neuronal growth factors and insulin like receptors and glutamate selective drugs, etc.

Table

Chromosome	Genetic defect	Phenotype
21	β APP mutation	Production of all Aβ peptides or Aβ40 peptides
19	ApoE4 polymorphism	Density of Aβ plaques and vascular deposits
14	Presenilin 1 mutation	Production of Aβ42 peptides
1	Presenilin 2 mutation	Production of Aβ42 peptide

Vascular dementia

Multi-infarct dementia, also known as vascular dementia is the second most common type of dementia in elderly persons (Brown, 1993) where the brain has been damaged by repeated small strokes. Causatives of vascular dementia are high blood pressure (hypertension), irregular heart rhythms (arrhythmias) and diseases which cause damage to the arteries in the brain. The prevalence of the illness is 1.5% in western countries and approximately 2.2% in Japan. It accounts for 50% of all dementias in Japan, 20–40% in Europe and 15% in Latin America. The incidence of dementia is 9 times higher in patients who had a stroke than in controls. The prevalence rate is higher in men than in women and it increases with age (Hagnell et al., 1992). Executive dysfunction is often seen in patients with vascular dementia, but memory dysfunction may be minimal or nonexistent in patients with a mild form of the disease (Roman, 2003). Microvascular pathology, including foci of pallor, neuronal loss and gliosis, has been found to play a causal role in dementia. Dementia was also found to be better correlated with the amount of hippocampal and cortical atrophy than with the volume of the lacunae (Fein et al., 2000). Cholinergic deficits are observed in patients in later stage of vascular dementia. Cholinesterase inhibitors have been used successfully in patients diagnosed with vascular dementia (Roman et al., 2003).

Parkinson’s disease (PD)

Parkinson’s disease is another most common neurodegenerative disorder, after AD that often impairs motor skills, speech and other functions. Epidemological studies shows that the prevalence rate of PD is approximately 0.5–1% among persons 65–69 years of age, rising to 1–3% among persons 80 years of age and older (Tanner & Goldman, 1996). PD is thought to affect more than 1 million people in the United States alone, 1 of every 100 individuals above the age of 55 (Zigmond & Burke, 2002). It is characterized clinically by parkinsonism (resting tremor, bradykinesia, rigidity, and postural instability) (Jankovic, 2008) and pathologically by the loss of neurons in the nucleus basalis of Meynert as well as the septal forebrain areas (Whitehouse et al., 1983) in association with the presence of ubiquinated protein deposits in the cytoplasm of neurons (Lewy bodies) (Pollanen et al., 1993) and thread-like proteinaceous inclusions within neurites (Lewy neurites). Significant reductions of choline acetyltransferase (ChAT), a marker of cortical cholinergic activity, were found in all 4 cortical lobes in PD, and the degree of reduction of ChAT in the temporal lobe correlated with the degree of mental impairment in PD (Perry et al., 1985). The loss of cholinergic neurons in the basal forebrain is probably the principal pathological cause of impaired cortical cholinergic activity in PD. Moreover oxidative stress in the substantia nigra leads to significant loss of neurons which produce Dopamine (DA) resulting in the characteristic deficiency of DA in the substantia nigra (Kidd, 2000). The treatment of PD also relies on elevation of DA levels by use of monoamine oxidase inhibitors, l-hydroxyphenylalanine (l-DOPA), its precursor, and by the administration of dopaminergic agonists, especially the ergot alkaloid derivatives. In PD, enhancement of cholinergic activity is by the use of cholinesterase inhibitors.

Dementia with Lewy bodies

The third most common type of dementia in elderly person is dementia with Lewy bodies (DLB) which constitutes 15% of all dementia cases (Zaccai et al., 2005). Within DLB, the loss of cholinergic (ACh-producing) neurons is thought to account for the degradation of cognitive functioning, as in AD, while the loss of dopaminergic (dopamine-producing) neurons is thought to account for the degradation of motor control, as in PD (Heidebrink, 2002). It is characterized anatomically by the presence of Lewy bodies, clumps of α-synuclein and ubiquitin protein in neurons, detectable in postmortem brain biopsies (Spillantini et al., 1997). Pathophysiology of DLB involves global cognitive impairment, neuropsychiatric disturbance with visual hallucinations, and Parkinsonism (Tiraboschi et al., 2000). Cholinesterase inhibitors are more effective in patients who have dementia with Lewy bodies than in those with Alzheimer’s disease. The symptoms of all types of dementia are presumed to be related to impaired neurotransmission and degeneration of neuronal circuits in the brain areas affected. These observations suggest that impairment of cholinergic function contributes to the symptoms of all three forms of dementia and that all patients with dementia could potentially benefit from cholinergic replacement therapy (Poirier, 2002).

Huntington disease

Hutington disease (HD) is a progressive neurodegenerative disorder with an established genetic origin and symptoms that are preferable to specific regions of brain disease. HD can be described as a triad of motor, cognitive, and emotional disturbances (Folstein, 1989). Symptoms usually begin between the ages of 35 and 50 years, although the onset may occur at any time from childhood to old age. Death occurs an average of 15–20 years after symptoms first appears. Cognitive difficulties usually begin about the same time and proceed at the same rate as the abnormal movements (Brandt & Butters, 1986), although some patients may have considerable motor impairment with very little dementia, or the reverse. In contrast to AD, patients with HD seem to have trouble with retrieval rather than storage of memories. This distinction has led to the classification of HD as a subcortical dementia (Brandt et al., 1988). In later stage of disease cognitive losses accumulate progressively leading to deficits in memory, visuospatial abilities, and judgment develop. Patients suffer from irritability and aggression. Rarely, patients develop schizophrenia-like syndrome, with prominent delusions, hallucinations, or thought disorder in the absence of an abnormal mood.

Myasthenia gravis

Chronic autoimmune disorder characterized by production of antibodies against ACh nicotinic receptors thereby inhibiting the stimulative effect of ACh at the post synaptic neuromuscular junction (Conti-Fine et al., 2006). Over time, the motor end plate is destroyed leading to muscle weakness and fatigue. Majority of dementia and other neurodegenerative disorders are related to abnormalities in central cholinergic system, which shows a decline in ACh level. Decline in neurotransmitter ACh leads to impairment in cognitive functions. So enhancement of cholinergic neurotransmission has been put forward as the most promising strategy to improve the cognitive function.

Traditional medicine in the treatment of dementia

Nature is a rich source of biological and chemical diversity. The unique and complex structures of natural products cannot be obtained easily by chemical synthesis. Traditional medicinal practice based on the use of plants and plant extracts are termed as herbal medicine. In traditional practices numerous plants have been used to treat cognitive disorders, including neurodegenerative diseases, and different neuropharmacological disorders. An ethnopharmacological approach has provided leads to identify plants and potential new drugs that are relevant for the treatment of cognitive disorders, including AD, and may aid the discovery of a more varied and efficacious selection of drugs for AD treatment. Nowadays, herbal medicine has received much attention and is recommended as a natural alternative to maintain one’s health. This review focuses on the recently reported medicinal plants with pharmacological activities relevant to the treatment of dementia including anticholinesterase (anti-ChE), anti-inflammatory, antioxidant, secretase inhibitory and metal chelating properties.

Therapeutic strategies for dementia

Dementia represents one of the most life threatening diseases to the elderly population with steady increase of occurrence in recent years; a trend that is set to continue in the future. Unfortunately, no definitive therapy for the prevention and resolution of dementia exists. Despite urgent need for effective therapeutic treatment, progress towards this goal has been painstakingly slow. Although dementia is most common neurodegenerative disorder, multiple etiological factors makes it difficult to identify appropriate targets that would promise fast, effective strategies to combat the disease onset and progression. Some of the most relevant therapies for the treatment of dementia are:

• Cholinesterase inhibitors: Inhibit acetylcholinesterase and increase the level of acetylcholine in the synaptic junction thereby improving cholinergic neurotransmission

• β- and γ-Secretase inhibitors: Prevent formation of Aβ 1–42 fragment from APP which is responsible for amyloid plaque formation

• Antinflammatory drugs: Nonsteroidal anti-inflammatory drugs inhibit proinflammatory mediators such as cyclooxygenase which is considered to be neurotoxic

• Antioxidants as drugs for dementia: Scavenge ROS and RNS, the major causative agent for oxidative injury in neurons.

• Amyloid antiaggregant therapies: Prevent aggregation of Aβ 1–42 fragment the precursor in β amyloid plague formation

• Metal chelators (indirect antioxidants): Chelate Cu²⁺ and Zn²⁺ which play major roles in Aβ 1–42 fragment aggregation resulting in the formation β amyloid plaque.

• Monoamine oxidase inhibitors: Monoamine oxidase (MAO) is a major enzyme responsible for the fast breakdown of DA and related compounds at the synapse. Inhibitors of this enzyme cause a net increase in DA levels and, although their major therapeutic use has been as antidepressants, they have potential use in PD.

Cholinesterase inhibitors

A deficit in central cholinergic transmission caused by the degeneration of the basal forebrain nuclei is an important pathological and neurochemical feature of AD and other dementia. As a result of these pathological changes, there are decreases in biochemical indices of cholinergic function in the neocortex and hippocampus that correlate with dementia severity. The observation of a deficiency in cholinergic neurotransmission in AD led to the development of cholinesterase (ChE) inhibitors as the first approved treatment for dementia symptoms to enhance cholinergic activity. Acetylcholinesterase (AChE) inhibitors have therapeutic applications in AD, senile dementia, ataxia, myasthenia gravis and PD (Enz et al., 1993; Siddiqui & Levey, 1999). Several ChE inhibitors are being investigated for the treatment of AD. However, only tacrine, donezepil, rivastigmine and galanthamine have been approved by the Food and Drug Administration (FDA) in the United States (Zarotsky et al., 2003) for the symptomatic treatment of AD and other dementia. These drugs improve cognitive and neuropsychiatric symptoms, and stabilize functioning over at least 6 months during clinical trials in patients with mild to moderate AD (Farlow, 2002). Although of the same drug class, ChEIs are structurally diverse (Brufani, & Filocamo, 2000). Donepezil and galantamine possess relative selectivity for AChE, whereas tacrine and rivastigmine coinhibit both AChE and BuChE. However, these drugs are known to have limitations for clinical use due to their short half-lives and unfavorable side effects such as hepatotoxicity and gastrointestinal disorders. So there is still a great interest in finding better cholinesterase inhibitors from natural sources (Sung et al., 2002).

A variety of plants and their phytoconstitutents has been reported to show AChE and BuChE inhibitory activity and so may be relevant to the treatment of AD and other related dementia. Plants belonging to families Acanthaceae, Apocynaceae, Amaryllidaceae, Angelicae, Araceae, Asclepiadaceae, Berberidaceae, Buxaceae, Combretaceae, Compositae, Coniferae, Cyperaceae, Ebenaceae, Ericaceae, Euphorbiaceae, Fumariaceae, Gentianaceae, Guttiferae, Lamiaceae, Leguminosae, Lilliaceae, Lycopodiaceae, Malvaceae, Magnoliaceae, Menispermaceae, Molluginaceae, Moraceae, Musaceae, Nelumbonaceae, Papaveraceae, Piperaceae, Rubiaceae, Rutaceae, Sapotaceae, Solanaceae and Tamaricaceae have been reported to have AChE inhibitory potential. Thus the search for plant-derived inhibitors of AChE has accelerated in view of the benefits of these drugs not only in the treatment of AD but in other forms of dementia, such as dementia with Lewy bodies (Perry et al., 1994), vascular dementia (Erkinjuntti et al., 2002), and Down’s syndrome (Kishnani et al., 1999).

Cholinesterase inhibitors from plants

Bacopa monniera L. (Scrophulariaceae) and Ginkgo biloba L. (Ginkgoaceae) are well-known cognitive enhancers in Indian and Chinese traditional medicine systems. Standardized extracts of B. monniera and G. biloba both showed a dose-dependent inhibitory effect on AChE activity (Das et al., 2002). Methanol extract of Myricaria elegans Royle (Tamaricaceae) was found to have significant AChE inhibitory activity (Ahmad et al., 2003). Methanol extracts of seven herbs Acorus calamus L. (Acoraceae), Acorus gramineus Sol. (Acoraceae), Bupleurm facaltum L. (Apiaceae), Dioscorea batatas L. (Dioscorea), Epimedium koreanum L. (Berberidaceae), Poria cocos F. (Polyporaceae) and Zizyphi jujube var. (Rhamnaceae), used in traditional Korean medicine for improvement of memory and cognition in old age have been tested for cholinesterase inhibitory properties and significant inhibition of the enzyme was shown by extracts from A. calamus and E. koreanum (Oh et al., 2004). Ingkaninan et al. (2000, 2003) screened the methanol extracts of 32 plants used in Thai traditional rejuvenating and neurotonic remedies, for inhibitory activity on AChE and found that the extracts from roots of Stephania suberosa Lour. (Menispermaceae) and Tabernaemontana divaricata L. (Menispermaceae) showed significant inhibitory activity. The chloroform:methanol (1:1) extracts of a number of the plant species namely Corydalis solida L. (Papaveraceae), Glaucium corniculatum L. (Papaveraceae), Rhododendron ponticum L. (Ericaceae), Rhododendron luteum Sweet (Ericaceae), Buxus sempervirens L. (Buxaceae), Vicia faba L. (Fabaceae), Robinia pseudoacacia L. (Caeselpiniaceae), Tribulus terrestris L. (Zygophyllaceae), Zygophyllum fabago.L. (Zygophyllaceae), Lycopodium clavatum L. (Lycopodiaceae), Fumaria vaillantii L., Fumaria capreolata L., Fumaria kralikii L., Fumaria asepala Boiss, Fumariadensiflora DC., Fumaria flabellate L., Fumaria petteri, Fumaria macrocarpa Parl., Fumaria cilicica Hauskkn., Fumaria parviflora Lam. and Fumaria judaica Boiss. (Fumariaceae) were screened for their anticholinesterase activity (Orhan et al., 2004). The extracts of R. ponticum, R. luteum, C. solida, G. corniculatum and B. sempervirens showed remarkable inhibitory activity above 50% inhibition rate at 1 mg/ml.

Among plants that investigated for dementia therapy, Salvia is one of the most numerous genera within the family Lamiaceae and grows in many parts of the world. It causes inhibition of AChE as well as nicotinic activity (Perry et al., 2000; 2001).

Urosolic acid (1) the active component of Origanum majorana L. (Labiatae) exhibits AChE inhibitory activity with IC₅₀ value of 7.5 nM. Widespread occurrence of ursolic acid accounts for the traditional use of several plant species for memory improvement and AD related conditions.

Lecuas urticifolia Vahl. (Lamiaceae) is an annual herb commonly found in Karachi and other parts of Sind province of Pakistan. Ethanol extract of whole plant exhibited potent BuChE inhibitory activity due to presence of leufolin A and leufolin B (2 and 3). These compounds exhibited BuChE inhibitory activity with IC ₅₀ values of 1.6 ± 0.98 and 3.6 ± 1.7 µM, respectively (Atia-tun-Noor et al., 2007).

The root and stem bark of Magnolia officinalis Rehder et Wils. (Magnoliaceae) inhibits AChE activity in vitro and increased hippocampal ACh release in vivo and the activity is due to presence of the biphenolic lignans, honokiol and magnolol (4 and 5).

Hydroalcoholic extract of Areca catechu L. (Piperaceae) inhibited AChE and BuChE in a dose-dependent manner (Gilani et al., 2002).

Vitis amurenis Rupr. (Vitaceae), wild-growing grape, in Japan, China and Korea, has been widely used in traditional medicine for the treatment of cancer and various pains. Root extracts possess anti-inflammatory and antitumor activity. Vitisin A and heyneanol A (6 and 7), two polymers of reseveratol isolated from the butanol root extract, inhibited both AChE and BuChE in a dose-dependent manner.

The hexane extract of the fruit of Schizandra chinensis Turcz. (Schisndraceae) showed significant inhibition of the activity of AChE due to the presence of lignans (Ingkaninan et al., 2003).

Zerumbone (8) (ZER), a sesquiterpene from the edible plant Zingiber zerumbet L. Roscoe ex S. (Zingiberacea), is known to possess enzymolytic effect towards AChE. It could be suggested that ZER might be a potential candidate for the development of anti-AChE for AD treatment (Bustamam et al., 2008).

Green and black tea extract from Camellia sinensis L. (Theaceae) inhibited human AChE

with IC₅₀ values of 0.03 and 0.06 mg/ml, respectively, and human BuChE with IC₅₀ 0.05 mg/ml, making it an effective drug for AD (Okello et al., 2004).

Orhan et al. (2011) reported that hexane and acetone extract of leaves of Ficus carica L.

(Moraceae) showed potent cholinesterase inhibitory activity against AChE (62.9 ± 0.9% and 50.8 ± 2.1%, respectively) and BuChE (76.9 ± 2.2% and 45.6 ± 1.3%, respectively).

(+)-(S)-psi-Ribalinine, (R)-(+)-ribalinine and methyl isoplatydesmine isolated from the aerial parts of Skimmia laureola Sieb. (Rutaceae) showed potent AChE and BuChE inhibitory activity with IC₅₀ values of 62.46 ± 1.58, 153.31 ± 1.9, 74.5 ± 1.05 µM, respectively (Sultana & Khalid, 2008).

The ethanol fraction of the stem of Esenbeckia leiocarpa Engl. (Rutaceae) showed potent AChE inhibitory activity of 91.1 ± 0.2% at 200 µg/ml. Bioactivity-guided fractionation of the ethanol extract showed the presence of six alkaloids such as leiokinine A, leptomerine, kokusaginine, skimmianine, maculine and flindersiamine. All the compounds showed AChE inhibitory activity with leptomerine showed highest activity (IC₅₀ = 2.5 μM), when compared to reference compound galanthamine (IC₅₀ = 1.7 μM) (Cardoso-Lopes et al., 2010).

Coumarin and phenol derivatives such as umbelliprenin, coladonin, coladin, epilaserine, and epielmanticine isolated from the dichloromethane extract of roots of Ferulago campestris Besser, showed potent AChE inhibitory activity with IC₅₀ values ranging from 1.2–0.1 mM (Dall’Acqua et al., 2010).

Stigma sterol isolated from the fruit of Rhazya stricta Decne. (Apocynaceae) showed AChE inhibitory activity with an IC₅₀ of 644.0 ± 11.75 µM (Sultana & Khalid, 2010).

Bisbenzylisoquinoline alkaloids isolated from Cocculus pendulus Diels. (Atta-ur-Rahman et al., 2009) showed inhibitory activities against acetyl- and butyrylcholinesterases.

Trigonella foenum graecum L. (Fabaceae) ethyl acetate fraction of the alcohol extract (IC₅₀ 53.00 ± 17.33 µg/ml), and total alkaloid fraction (IC₅₀ 9.23 ± 6.08 µg/ml), showed potential AChE inhibition. Trigonelline showed IC₅₀ 233 ± 0.12 µM. Galanthamine was used as standard and it showed inhibition of acetyl cholinesterase with an IC₅₀ of 1.27 ± 0.21 µM (Kumar et al., 2012).

The essential oil and methanol extract of Zatraia multiflora Saatar. (Lamiaceae) was evaluated for anticholinesterase activity using modified Ellman method. Both the essential oil and methanol extract of the plant exhibited high anticholinesterase activity (95.3 ± 3.4 and 87.9 ± 2.2% inhibition, respectively) which was similar to eserine (96.2 ± 1.7% inhibition). The IC₅₀ value of essential oil was determined as 0.97 ± 0.12 µg/mL in comparison to eserine (0.13 ± 0.02 µg/ml) (Sharififar et al., 2012).

Five steroidal alkaloids isolated from Holarrhena antidysenterica Linn. (Apocynaceae), conessine, isoconessimine, conessimin, conarrhimin, and conimin, showed potent AChE inhibiting activity with IC₅₀ ranging from 4 to 28 μM with conessimin a potent AChE inhibitor. The mode of inhibition was observed to reversible noncompetitive type (Yang et al., 2012).

Hydrodistilled leaf essential oil from Pulicaria stephanocarpoil Balf.f (Asteraceae) revealed an AChE inhibitory activity of 47% at a concentration of 200 µg/mL (Ali et al 2012).

Belladine, an alkaloid extracted from bulbs of Nerine bowdenii Watson (Amaryllidaceae), showed promising cholinesterase inhibitory activities against human AChE and human plasma BuChE in a dose-dependent manner with IC₅₀ values of 781 ± 12.5 µM and 284.8 ± 4.2 µM, respectively (Cahlíková et al., 2011).

Fruits of Semecarpus anacardium L.F. (Anacardiaceae) used in Ayurvedic medicine for the treatment of dementia showed the presence of 1′-2′-dihydroxy-3′-pentadec-8-enylbenzene and 1′-2′-dihydroxy-3′-pentadeca-8,11-dienylbenzene which exhibited potent AChE inhibition with IC₅₀ values of 12 and 34 μg/ml (Adhami et al., 2012).

Leaf and stem petroleum ether, dichloromethane and 50% aqueous methanol extracts of

Leucosidea sericea Eckl. & Zeyh. (Rosaceae) were screened for AChE inhibitory activity. Leaf sample showed potent inhibitory activity with IC₅₀ 0.16 ± 0.020, 0.14 ± 0.010 and 0.24 ± 0.010 mg/ml when compared with stem extracts (Aremu et al., 2011).

Plant extracts of Armeria rouyana Daveau. and Thymus capitellatus L. clearly demonstrated effective AChE inhibitory activity (480 ± 98 and 490 ± 46 μg/ml, respectively) that could be associated with polyphenols (Tavares et al., 2012).

Recently, Okello et al. (2004) found that water extracts of green tea, black tea, and coffee had AChE inhibitory activity with IC₅₀ values of 0.03 ± 0.004, 0.06 ± 0.005, and 0.41 ± 0.004 mg/ml, respectively.

Mukherjee et al. (2007a) confirmed that the hydroalcohol extracts from Centella asiatica L. (Umbelliferae), Nardostachys jatamansi DC. (Valerianaceae), Myristica fragrans (Myristicaceae) Gronov., Evalvulus alsinoides (Convolvulaceae) L., used for the treatment of ADin Indian systems of medicine, could inhibit 50% of AChE activity at concentrations of 100–150 µg/ml.

Kaufmann and Dogra (2011) also found that 1,8-cineole, carvacrol, myrtenal and verbenone could apparently inhibit AChE, and myrtenal showed the highest inhibitory activity (IC₅₀ 0.17 mg).

The methanol extract of Angelica gigas Nakai. (Umbelliferae) showed potent AChE inhibitory activity due to presence of decursinol, a coumarin (IC₅₀ 0.28 µM).

The hexane extract from bark of Mesua elegans (King) Kosterm. showed the presence of 4-phenyl coumarins with mesuagenin B, the potent AChE inhibitor with IC₅₀ of 0.71 µM.

Furanocoumarins isolated from the hexane extract of Citrus hystrix fruits contained (R)-(+)-6-hydroxy-7-methoxybergamottin and (R)-(+)-6-ihydroxybergamottin showing IC₅₀ values of 11.2 ± 0.1 and 15.4 ± 0.3 µM, respectively (Anand & Singh, 2012).

Narciprimine, a phenanthridone alkaloid isolated from ethanol bulb extract of Cyrtanthus contractus Aiton. (Amaryllidaceae), exhibited AChE inhibitory activity (IC₅₀ 78.9 µM) (Nair et al., 2011).

Five alkyl phenyl and salicylic acid derivatives isolated from the hexane and dichloromethane stem bark of Knema laurina Warb. showed AChE inhibitory activity with 2-hydroxy-6-[10 (Z)-heptadecenyl]benzoic acid as a potent inhibitor whose IC₅₀ value was 0.573 ± 0.0260 µM (Akhtar et al., 2011).

Kermadecins D and J, and isokermadecin D, isolated from the ethyl acetate bark extract of Kermadecia rotundifolia Brongn. & Gris (Proteaceae), exhibited significant AChE inhibitory activity (IC₅₀ 3.6 ± 0.6, 3.4 ± 0.3 and 3.4 ± 0.8 mM, respectively). AChE inhibitory activity is due to presence of ether linkage (Beniddir et al., 2010).

Random screening for AChE inhibitory activity in about 100 Korean medicinal plants showed that the methanol fruit extract (5 mg/ml) of Terminalia chebula Retz. (Combretaceae) showed 95 ± 1 and 85 ± 1% inhibition against AChE and BuChE. Activity-guided fractionation of the methanol extract showed the presence of gallotanin-1,2,3,4,6-penta-O-galloyl-β-d-glucose which exhibited dose-dependent inhibitory activities against AChE and BChE with IC₅₀ values 29.9 ± 0.3 μM and 27.6 ± 0.2 μM, respectively (Sancheti et al., 2010).

Broussonetia papyrifera (L.) Vent., (Moraceae) is known as paper mulberry. Two prenylated flavonols isolated from the ethanol root extracts of paper mulberry showed potent inhibitory activity against AChE (IC₅₀ 0.8 and 3.1 μM) and BuChE (IC₅₀ 0.5 and 24.7 μM) in a dose-dependant manner. Inhibition was observed to be mixed type inhibition (Ryu et al., 2012).

Four stilbenes isolated from the methanol extract of Ficus foveolata Wall. (Moraceae) vines showed potent BuChE inhibitory activity of which Gnetol had the most potent inhibitory activity with an IC₅₀ value of 1.3 μM. BuChE inhibitory activity is due to the presence of OH group in the stilbene ring and the type of inhibition was observed to be competitive. In the case of AChE, the four stilbenes showed much less activity (Sermboonpaisarn & Sawasdee, 2012).

Illicium verum Hook.f. (Illiciaceae), a well-known spice in traditional Indian system for its therapeutic potential, was assessed for its AChE and BuChE inhibitory activity. Oil obtained by hydrodistillation of aqueous extract of I. verum showed potent AChE and BuChE inhibitory activity with IC₅₀ values of 36.00 ± 0.44 and 70.65 ± 0.96 μg/ml which might be due to presence of anethole (Bhadra et al., 2011).

Guo et al. (2010) screened AChE inhibitors from flavonoids used in traditional Chinese medicine. Among the 21 flavonoids of different subclasses, galangin, isolated from rhizome of Alpiniae Officinarum Hance (Zingiberaceae), showed the highest inhibitory activity of 55% with an IC₅₀ 120 µM and an Ki value of 74 µM.

Isoquinolinic alkaloid montanine isolated from the ethanol extract of the fresh bulb of Hippeastrum psittacinum Atibaia. (Amaryllidaceae) showed AChE inihibitory activity in a dose-dependant manner with an IC₅₀ value of 1 mM (Pagliosa et al., 2010).

The aqueous extract (infusion and decotion) from the leaves of Peumus boldus Molina. (Monimiaceae) showed potent AChE inhibitory activity with IC₅₀ values of 1.24 ± 0.03 0.93 ± 0.02 mg/ml. The activity is due to the presence of flavonoids (Falé et al., 2012).

The aqueous and ethanol extracts from the root of Salvia miltiorrhiza Bunge. (Lamiaceae) were assessed for AChE inhibitory activity using rat brain homogenate as enzyme source in vitro. The ethanol extract showed potent AChE inhibitory activity in a dose-dependant manner with highest inhibition rate of 73% at 2 mg/ml. Activity is due to the presence of tanshinones (Zhou et al., 2011).

The aqueous extract of the rhizome of red and white ginger [Zingiber officinale Roscoe. (Zingiberaceae)] was assessed for AChE inhibitory activity. Results showed that white ginger had the highest inhibitory activity with an IC₅₀ value of 2.8 mg/ml when compared red ginger (IC₅₀ value 3.03 mg/ml). Both extracts when given together showed highest inhibitory activity due to a synergistic effect (Oboh et al., 2011).

Aerial parts of 55 Salvia species from Turkish origin were screened for AChE inhibitory activity in dichloromethane, ethyl acetate and methanol extracts. The dichloromethane extract of S. fruticosa Mill. showed 51.08% inhibition at 100 µg/ml, followed by ethyl acetate extracts of S. pomifera subsp. (36.39%) and S. fruticosa (34.27%) at the same concentration. The methanol extracts showed no inhibitory activity (Senol et al., 2010).

Phytoconstituents having cholinesterase inhibitory activity

Work on new bioactive compounds from medicinal plants has led to the isolation and structure elucidation of a number of exciting new pharmacophores (Figure 2). Phytochemicals having significant AChE inhibitory activity are listed in Table 2.

Table

Class of the compound	Name of the compound	Source	Plant family	Activity	References
Steroidal alkaloid	Galantamine	Galanthus nivalis	Amaryllidaceae	50% inhibition at 1.077 ± 0.18 µM	Rhee et al., (2001); Lopez et al., (2002)
Steroidal alkaloid	Assoanine	Narcissus assoanus	Amaryllidaceae	50% inhibition at 3.877 ± 0.24 µM	Lopez et al., (2002)
Steroidal alkaloid	Buxamine B	Bucus hyrcana,Bucus papillosa	Buxaceae	50% inhibition at 7.567 ± 0.008 µM	Rahman and Choudhary (2001)
Steroidal alkaloid	N, N-dimethyl buxapapine	Bucus papillosa	Buxaceae	50% inhibition at 7.287 ± 0.06 µM	Rahman and Choudhary (2001)
Steroidal alkaloid	Epinorgalantamine	Narcissus confuses, N. perezchiscanoi, Narcissus leonensis, N. legionensis, Narcissus poeticus	Amaryllidaceae	50% inhibition at 9.607 ± 0.65 µM	Lopez et al., (2002)
Steroidal alkaloid	11-Hydroxygalantamine	Narcissus poeticus	Amaryllidaceae	50% inhibition at 1.617 ± 0.21 µM	Lopez et al., (2002)
Steroidal alkaloid	Oxoassoanine	Narcissus assoanus	Amaryllidaceae	50% inhibition at 47.217 ± 1.13 µM	Lopez et al., (2002)
Steroidal alkaloid	Galantamine	Galanthus nivalis Narcissus confuses Lycorus radiate	Amaryllidaceae	50% inhibition at 1.077 ± 0.18 µM	Rhee et al.,. (2001); Lopez et al., (2002)
Steroidal alkaloid	Sanguinine	Eucharis grandiflora	Amaryllidaceae	50% inhibition at 0.107 ± 0.01 µM	Lopez et al.,, (2002)
Steroidal alkaloid	Sarsalignone	Sarcococca saligna	Buxaceae	50% inhibition at 7.0287 ± 0.007 µM	Rahman and Choudhary (2001)
Steroidal alkaloid	Vaganine	Sarcococca saligna	Buxaceae	50% inhibition at 8.597 ± 0.155 µM	Rahman and Choudhary (2001)
Quinolizidine alkaloid	(-)-Huperzine A	Huperzia serrata	Lycopodiaceae	50% inhibition at 10^−4 µM	Orhan et al. (2004)
Indole alkaloid	Physostigmine	Physostigma venenosum	Leguminosae	50% inhibition 6 × 10^−4 µM	Karczmar (1998)
Indole alkaloid	Coronaridine	Tabernaemontana australis	Apocynaceae	Minimum concentration of 25 µM to produce detectable spot in TLC	Andrade et al., (2005)
Indole alkaloid	Rupicoline	Tabernaemontana australis	Apocynaceae	Minimum concentration of 25 µM to produce detectable spot in TLC	Andrade et al., (2005)
Indole alkaloid	Voacangine	Tabernaemontana australis	Apocynaceae	Minimum concentration of 25 µM to produce detectable spot in TLC	Andrade et al., (2005)
Cynatroside A	Pregnane glycoside	Cynanchum atratum	Asclepiadaceae	50% inhibition at 6.4 µM)	Lee et al., (2003)
Isoquinoline alkaloid	Corynoline	Corydalis incisa	Papaveraceae	50% inhibition at 30.6 µM	Kim (2002)
Isoquinoline alkaloid	Palmatine	Corydalis speciosa	Papaveraceae	50% inhibition at 5.8 µM	Kim et al., (2004)
Isoquinoline alkaloid	Protopine	Corydalis speciosa	Papaveraceae	50% inhibition at 16.1 µM	Kim et al., (2004)
Glycoalkaloid	α-Solanine	Solanum tuberosum	Solanaceae	44.3% inhibition at 10 µM	Roddick (1989)
Pregnane glycoside	Cynatroside A	Cynanchum atratum	Asclepiadaceae	50% inhibition at 6.4 µM	Lee et al., (2003)
Pregnane glycoside	Cynatroside B	Cynanchum atratum	Asclepiadaceae	50% inhibition at 3.6 µM	Lee et al., (2003)
Bellidin 8-O-β-glucopyranoside	Norswertianolin	Gentiana cambpestris	Coniferae	Minimum concentration of 1.20 nM to produce detectable spot in TLC	Urbain et al., (2004)
Bellidifolin 8-O- β-glucopyranoside	Swertianolin	Gentiana cambpestris	Coniferae	Minimum concentration of 0.18 nM to produce detectable spot in TLC	Urbain et al., (2004)
Stilbene oligomer	(+)-α-Viniferin	Caragana chamlague	Leguminosae	50% inhibition at 2.0 µM	Sung et al., (2002)
Xanthone	Bellidin	Gentiana cambpestris	Coniferae	Minimum concentration of 0.03 nM to produce detectable spot in TLC bioassay	Urbain et al.,(2004)
Xanthone	Bellidifolin	Gentiana cambpestris	Coniferae	Minimum concentration of 0.15 nM to produce detectable spot in TLC	Urbain et al., (2004)
Hydroxy-heptamethyl-icosahydropicene carboxylic acid	Ursolic acid	Origanum majorana	Lamiaceae	50% inhibition at 7.5 nM	Chung et al., (2001)

Figure 2.  Phytoconstituents exhibiting cholinesterase inhibitory activity.

Alkaloids

Physostigmine (Indole Alkaloid)

Physostigmine (9) is a reversible AChE inhibitor originally isolated from the seed of calabar bean [Physostigma venenosum L. (Fabaceae)]. It has been widely used for different purposes, ranging from an historical role in rituals and primitive medicine, to its present-day used for the treatment neurological disorders such as AD and Myasthenia gravis. Physostigmine has been approved by the FDA as an anticholinergic drug for the treatment of mild to moderate AD. It enhances short-term memory in dementia patients (Coelho et al., 2008). Limitation factors are shorter plasma half-life (~30 min) and high incidence of adverse effects such as nausea, vomiting and diarrhea. Despite limitations, it is currently used in the formulation physostigmine salicylate (Synapton®).

Rivastigmine

Rivastigmine (10), an AChE inhibitor, is licensed for use in UK for the symptomatic treatment of mild-to-moderately severe AD. The chemical structure of physostigmine has provided a template for the development of rivastigmine (Foye et al., 1995). Rivastigmine is reported to inhibit AChE in the cortex and hippocampus, brain areas involved in cognition. Thus, it is apparent that plant-derived alkaloid AChE inhibitors may be important for the development of more appropriate drug candidates for the treatment of AD (Foye et al., 1995).

Huperzine

Huperzine A (11) is a lycopodium alkaloid isolated from the clubmoss Lycopodium serratum Thunb. (Lycopodiaceae), which has been used in TCM for its memory-enhancing properties for centuries. Over 100 alkaloids (huperzin A-R) have been isolated from the genus Lycopodium. Of them, only huperzine A possessed remarkable AChE inhibitory activity. The activity of huperzine A has been found to be as high as or even greater than physostigmine, galanthamine, donepezil and tacrine, the commercial drugs already used against AD. In various in vivo and ex vivo experiments, huperazine A has been shown to inhibit AChE reversibly and also prevents oxidative cell damage induced by β-amyloid plaques (Tang 1996; Tang & Han 1999). α-Onocerin (12), a triterpene-type compound from L. clavatum, showed ca. 50% activity. Huperzine A is also a NMDA receptor antagonist that protects the brain against glutamate-induced damage, and it increases nerve growth factor levels. Side effects may include breathing problems, tightness in the throat or chest, chest pain, skin hives, rash, itchy or swollen skin, upset stomach, diarrhea, vomiting, hyperactivity, and insomnia (Orhan et al., 2003).

Galanthamine

Galanthamine (13), an alkaloid isolated from Galanthus nivalis L. (Amaryllidaceae), has been recently used in the treatment of AD. It acts as a reversible competitive AChE inhibitor rather than BuChE and modulates the nicotinic ACh receptors. Initially derived from extracts of snowdrop and daffodil bulbs, this phenanthrene alkaloid is now synthetically produced. It provides complete oral bioavailability. The half-life of galanthamine is 6 h. Galanthamine (Nivalin®) is approved as HBr salt in Austria and licensed as Reminyl® in the USA and some European countries in the treatment of AD. Extracts from Narcissus and Galanthus species of Turkey were screened for AChE inhibitory activity, and suggests that the alkaloids having galanthamine and lycorine skeletons such as assoanine, epinorgalantamine, oxoassoanine, sanguinine, 11-hydroxygalantamine have been reported to possess AChE activity (Lopez et al., 2002).

Protopine

In the course of screening Korean natural products for cholinesterase inhibitory activity, the crude methanol extract prepared from tubers of Corydalis ternate (Papaveraceae) exhibited potent AChE inhibitory activity. Bioactivity-directed fractionation of these extracts afforded protopine (14), an alkaloid-type compound which exhibited reversible competitive-type inhibition. This result was supported by a passive avoidance test, which is used to measure antiamnesic activity, in male mice. Anti-AChE activity and antiamnestic activity of protopine increases the therapeutic value in the treatment of dementia (Davis et al., 1999).

Corynoline

Corydalis incisa Thunb. (Papaveraceae), which is widely distributed in Korea, is used as a folk medicine in China and Japan for the treatment of inflammation, skin diseases, and headache. It is also used for the treatment of stomach, liver, and abdominal pains, as well as a detoxifying remedy. The methanol extract of aerial parts of C. incise exhibited AChE activity. Corynoline (15), a isoquinonline alkaloid, exhibited reversible noncompetitive inhibition which can be used for the treatment of AD (Ma et al., 1999).

Dehydroevodiamine

Twenty-nine medicinal plants from South Korea were screened for anti-ChE activity. Results showed that the dichloromethane extract of Evodia rutaecarpa Juss. (Rutaceae) exhibited a maximum inhibition of 84.3%. The dichloromethane extract of E. rutaecarpa also exhibited antiamnestic activity in the passive avoidance test in rats (Sprague Dawley) with scopolamine-induced memory loss. The bioactive compound responsible for the activity is dehydroevodiamine HCl (16) (Park et al., 1996; Park et al., 2000).

Steroid alkaloids from Buxus species

Buxus species, a widespread plant in Turkey, have long been known as rich sources of new and biologically active triterpenoidal alkaloids and the ethyl acetate extract of aerial parts of Buxus species are reported to be useful in various disorders such as malaria, rheumatism and skin infections. Screening for anti-ChE activity in B. hyrcana L. (Buxaceae) showed that steroidal alkaloids homomoenjodaramine and moenjodaramine (17) are the bioactive compounds responsible for the AChE inhibitory activity with IC₅₀ values of 19.2 and 50.8 mM, respectively. B. sempervirens L. showed 50% inhibition to both AChE and BuChE at the concentration 1 mg/ml (Rahman A-ur et al., 1998). Triterpenoids from B. papillosa also exhibited inhibitory activity to both AChE and BuChE.

Isoquinoline alkaloids from Fumaria species (Fumarioideae, Papaveraceae)

Fumaria, a widespread species in Turkey, is the richest source of isoqunioline alkaloids with remarkable biological activities. Nineteen species were screened for cholinesterase inhibitory activity and results showed that all Fumaria spp. exhibited, significantly higher activity (ranging from 84.9 to 96.8%) when compared to standard galanthamine. Of the 19 species, F. vaillantii L. exhibited a maximum inhibition of 94.2%. Bioassay-guided fractionation of F. vaillantii afforded many isoquinoline alkaloids responsible for activity. Among them, ophiocarpine, β-allocryptopine, berberine and protopine (18, 19, 20, 14), exhibited major AChE activity and the activity of the methanol extract may be due to the synergistic interaction between these alkaloids, which may be of therapeutic value in the treatment of AD (Orhan, 2003).

Steroidal alkaloids from Caragana chamlague

The total methanol extract of the underground parts of C. chamlague Fabr. (Leguminosae) showed significant inhibition towards AChE. Inhibitory activity is due to the presence of stilbene oligomers, (+)-α-viniferin (21) and kobophenol A (22). Both compounds inhibited AChE activity in a dose-dependent manner, and their IC₅₀ values are 2.0 and 115.8 mM, respectively. The type of inhibition is specific reversible noncompetitive. Structure-activity relationships suggest that the nitrogen substituent at C-3 and/or C-20 of the steroidal skeleton and the hydrophobic properties of the pregnane skeleton are the key structural features contributing to the inhibitory potency of pregnane-type steroidal alkaloids against AChE.

Other alkaloids

Three quaternary protoberberine alkaloids (stepharanine, cyclanoline and N-methyl stepholidine) isolated from the aqueous extract of Stephania venosa Spreng. (Menispermaceae) tubers were assessed for AChE inhibitory activity. All three alkaloids showed potent AChE inhibitory activity with IC₅₀ values of 14.10 ± 0.81, 9.23 ± 3.47, and 31.30 ± 3.67 µM, respectively. The ethanol extract of the aerial portion of Chelidonium majus L. (Papapervacea) inhibited AChE activity without a significant inhibition of BuChE. Bioactive-guided fractionation showed that 8-hydroxydihydrochelerythrine, 8-hydroxydihydrosanguinarine and berberine exhibited potent inhibitory activity against AChE (IC₅₀ 0.61–1.85 mM) (Cho et al., 2006).

Terpenoids

Terpenoids, a very large group of natural products, comprise two or more branched 5 carbon units, formed from a common precursor named mevalonic acid. Skeletons consisting of multiplets of 2, 3, 4 or 6 of these linked together in many different ways are found in a variety of mostly cyclic compounds named monoterpenes (10 carbons in the skeleton), sesquiterpenes (15 carbons), diterpenes (20 carbons) and triterpenoids (30 carbons), respectively. These compounds tend to be lipophilic, so they are able to cross the blood-brain barrier; the monoterpenes, and some of the sesquiterpenes, are volatile, so effects could occur through inhalation. Monoterpenes consist of a hydrocarbon skeleton that contributes to their anti-ChE activity. These compounds are responsible for the strong odors and flavors of many herbs, spices, and traditional medicines. An effect on CNS activity by the volatile substances in perfumes and other odoriferous materials has attracted interest in recent years and one of the first findings that monoterpenes had AChE inhibitory effects was made only in the mid-1990s in studies investigating historical records that monoterpene-containing plants were ‘good for the memory’ (Perry et al., 1996).

Terpenoids from Lamiaceae family

Amongst plants investigated for dementia therapy, Salvia is one of the most numerous genera within the family Lamiaceae and grows in many parts of the world. Salvia is used as memory enhancer in European folk medicine. An ethanol extract and the oil of S. officinalis L. and S. lavandulaefolia Vahl. were investigated for anti-ChE activity and it was found that all inhibited of AChE at quite low concentrations (Perry et al., 1996). The cholinesterase inhibition by S. lavandulaefolia oil was shown to be partly due to the cyclic monoterpenes 1-8-cineole and α-pinene (23 and 24), which are known to inhibit AChE in vitro, with some contribution from other constituents, perhaps by acting synergistically (Perry et al., 2000a). Since the effects of the oil were better than those of individual monoterpenes, further in vivo and clinical studies (described below) were carried out on the essential oils, which consist of a mixture of monoterpenes, rather than isolated compounds. Oral administration of S. lavandulaefolia essential oil to rats decreased striatal AChE activity in both the striatum and the hippocampus compared to control rats. Thus, it appeared that constituents of the S. lavandulaefolia oil, or their metabolites, reach the brain and inhibit AChE in select brain areas, consistent with evidence of inhibition of the brain enzyme in vivo (Perry et al., 2002). Clinical studies on human volunteers and even patients with AD have been reported in recent years. A small trial with 11 patients showing mild to moderate symptoms of AD showed that oral administration of the essential oil of S. lavandulaefolia significantly improved cognitive function (Perry et al., 2003). Anti-BuChE activity of S. lavandulaefolia, S. fructicosa Mill. and S. officinalis show that essential oil of S. officinalis and S. fructicosa exhibited activity in a time-dependent manner due to the presence of β-pinene, 3-carene, sabinene and camphor. Thus, oils of S. officinalis and S. fructicosa which possess dual cholinergic activity may be used to treat severe AD, while S. lavandulaefolia can be used to treat mild AD (Savelev et al., 2004). Another sage species, S. miltiorrhiza, Bunge. used in TCM to heart problems and calm nerves was investigated for AChE inhibitory effect. The root extract showed an inhibitory effect due to the presence of diterpenes tanshinones. Dihydrotanshinone (25) was shown to be the most active (IC₅₀1.0 µM) with cryptotanshinone (26) (IC₅₀7.0 µM) also showing activity (Ren et al., 2004). A feature that appears to be necessary for activity is the saturated bond in the furan ring of the molecules.

Rosmarinus officinalis L. and Melissa officinalis L. extracts were investigated for cholinesterase inhibitory activity. The methanol extract of R. officinalis showed the highest in vitro inhibitory activity. Bioactive-guided fractionation showed that the activity is due to the presence of essential oil; 1,-8-cineol exhibited 44.42% and α-pinene showed 12.57% inhibition. Rosmarinic acid, a phenolic compound in rosemary, exhibited the highest inhibitory activity of 85.8% towards AChE (Orhan et al., 2007).

Steroids

Withania somnifera (L.) Dunal (Solanaceae)

The root of this plant is one of the most highly regarded herbs in Ayurvedic medicine where it is known as ‘ashwagandha’ and has a history of use for almost 4,000 years. It is classified among the rejuvenative tonics known as ‘Rasayanas’. Root extract was administered orally to mice and the effect of neurotransmitter system in brain was observed. The extract showed enhanced AChE inhibitory activity in the lateral septum and globus pallidus areas of the brain and enhanced muscarinic M1 receptor binding in cortical regions. The active compounds responsible for the activity were sitoindosides and withaferin A. The extract containing the active compounds also reversed the reduction in cholinergic markers (e.g., ACh, choline acetyltransferase; ChAT) in rats (Bhattacharya et al., 1995). These activities could explain the reputed cognition enhancing effects of W. somnifera root because of preferential action on cholinergic neurotransmission in the cortical and basal forebrain, brain areas involved in cognitive function. Based on this information, it could be speculated that the sitoindosides and withaferin A (27 and 28) could have potential in AD therapy.

Xanthones

Bellidifolin

The methanol leaf extract of Gentiana campestris (L.) Boerner. (Coniferae) exhibited significant inhibition of AChE activity. Four xanthones, bellidin, bellidifolin (29), bellidin 8-O-β-glucopyranoside (norswertianolin), and bellidifolin 8-O-β-glucopyranoside (swertianolin), were found to be responsible for the anti-AChE activity effects (Urbain et al., 2004). Bellidifolin showed similar activity to galanthamine in this enzyme assay.

Glycosides

Cynatroside B

The roots of Cynanchum atratum Bunge. (Asclepiadaceae) were investigated for AChE inhibitory activity and the activity is due to the presence of pregnane glycosides. Of all the glycosides, cynatroside B exhibited potent inhibitory activity with IC₅₀ value of 3.6 pM. The mode of AChE inhibition by cynatroside B was reversible and noncompetitive in nature (Lee et al., 2003). Cynatroside B (1.0 mg/kg body weight i.p.) significantly ameliorated memory impairments induced in mice by scopolamine (1.0 mg/kg body weight s. c.) as measured in the passive avoidance and the Morris water maze tests. The presence of both anti-AChE and antiamnesic activities makes it significant in therapeutics in alleviating certain memory impairments observed in Alzheimer’s disease.

Flavonoids

The ethyl acetate extract of Agrimonia pilosa Ledeb. (Rosaceae) whole plants was assessed for AChE inhibitory activity. The activity is due to presence of 4 flavones [tiliroside, 3-methoxy quercetin, quercitrin and quercetin (30)] (Jung & Park, 2007). Quercetin showed twice the activity of dehydroevodiamine (DHED).

Gingko bilbo

Gingko bilbo extract was assessed for cholinesterase inhibitory. The ginkobilbo EGb 761 fraction exhibited cholinesterase inhibitory activity equivalent to standard drug tacrine, donepezil (Wettstein, 2000). EGb 761 is a standardized extract that contains approximately 24% flavone glycosides (primarily quercetin, kaempferol, and isorhamnetin) and 6% terpene lactones (2.8–3.4% ginkgolides A, B and C, and 2.6–3.2% bilobalide). Ginkgolide B and bilobalide account for about 0.8 and 3% of the total extract, respectively.

β- and γ-Secretase inhibitors

Blocking the production of Aβ by specific inhibition of the key proteases (β & γ secretase) required for Aβ generation is another major focus research in AD therapy. Pathologically AD is characterized by intracellular neurofibrallary tangles and extracellular senile plaques in synaptic terminals. The main components of these plaques are the amyloid β peptides (Aβ) in pleated sheet conformation, which is formed by the action of β- and γ-secretase on amyloid precursor protein (APP). β-Secretase, also termed as memapsin (membrane aspartyl protease of the statin family) or β-site APP-cleaving enzyme (BACE) encoded by chromosome 11, initiates the first step of Aβ peptide formation and is the rate limiting enzyme in Aβ production. So β-secretase is considered as a potential therapeutic drug for the treatment of AD (Vassar, 2004).

Currently β-secretase (BACE1) inhibitors are known synthetic chemicals and of limited therapeutic potential because of high molecular weights likely to restrict transfer across the blood-brain barrier. Some of the BACE1 inhibitors from natural sources are listed below: luteolin and rosmarinic acid from the methanol leaf extract of Perilla frutescens (L.) Britton (Lamiaceae) showed potent BACE1 inhibitory activity in a noncompetitive manner with IC₅₀ values of 5.0 × 10⁻⁷ and 2.1 × 10⁻⁵ M, respectively. Ki values of luteolin and rosmarinic acid were observed to be 6.2 × 10⁻⁵ and 3.9 × 10⁻⁵ M, respectively. These compounds showed less inhibitory activity against other enzymes such as α-secretase (TACE), acetylcholine esterase (AchE), chymotrypsin, and elastase, indicating that they were relatively specific inhibitors of BACE1 (Choi et al., 2008). Ellagic acid and punicalagin isolated from the husk of Punica granatum L. (Lythraceae) showed potent BACE1 inhibitory activity with IC₅₀ values of 3.9 × l0⁻⁶ and 4.1 × 10⁻⁷ M and Ki values of 2.4 and 5.9 × 10⁻⁷ M, respectively. Ellagic acid and punicalagin exhibited highly specific noncompetitive type inhibition with less specificity towards other enzymes (Kwak et al., 2005). BACE1 inhibitory activity of ellagic acid might be due to the presence of extensive peripheral substitution of OH groups in the aromatic ring. In the course for screening BACE1 inhibitors from natural resources, the ethyl acetate fraction of Sanguisorbae Radix L. (Rosaceae) showed potent BACE1 inhibitory activity. 1,2,3-Trigalloyl-4,6-hexahydroxydiphenoyl-13-d-glucopyranoside (Tellimagrandin II) and 1,2,3,4,6-pentagalloyl-13-d-glucopyranoside showed potent noncompetitive type inhibition against BACE1 with IC₅₀ values of 3.10 × 10⁶ and 3.76 × 10⁶ M, respectively. The Ki values were 6.84 × 10⁶ and 5.13 × 10⁶ M (Lee et al., 2005). Clove extract exhibited dose-dependent inhibition of BACE1 with an EC₅₀ of 64 µg/ml; however, the major essential oil of clove, eugenol, did not account for BACE1 inhibitory activity (Ayoola et al., 2008). Green tea infusion at a concentration of 0.03 mg/ml inhibited BACE 1 activity by 38% (Okello et al., 2004). The ethyl acetate fraction of green tea, rich in catechin, showed potent inhibitory activity. Bioactive-guided fractionation of the ethyl acetate fraction showed the presence of constituents such as (-)-epigallocatechin gallate (EGCG),(-)-epicatechin gallate (ECG), and (-)-gallocatechin gallate (GCG) with BACE1 inhibitory with IC₅₀ values of 1.6 × 10⁻⁶, 4.5 × 10⁻⁶ and 1.8 × 10⁻⁶ M. Inhibition was observed to be of the noncompetitive type. BACE1 inhibitory activity might be due to presence of pyrogallol moiety on C-2 and/or C-3 of catechin skeleton, while the stereochemistry of C-2 and C-3 did not have an effect on the inhibitory activity (Jeon et al., 2003). In vivo studies showed that oral administration of EGCG in mice treated with lipopolysaccharides dose-dependently (1.5–3 mg/kg) reduced LPS induced β- and γ-secretase activation both in the cortex and hippocampus region of the brain (Lee et al., 2009). The ethyl acetate fraction of dried rhizomes of Smilax china L. (Liliaceae) showed potent inhibitory activity against BACE1. The active compounds were identified as a trans/cis-resveratrol mixture, oxyresveratrol, veraphenol, and cis-scirpusin A which showed noncompetitive type of inhibition with IC₅₀ values of 1.5 × 10⁻⁵, 7.6 × 10⁻⁶, 4.2 × 10⁻⁶ and 1.0 × 10⁻⁵ M, respectively. Ki values were observed to be 5.4 × 10⁻⁶, 5.4 × 10⁻⁶, 3.4 × 10⁻⁶, and 5.4 × 10⁻⁶ M, respectively (Jeon et al., 2007). Epiberberine and groenlandicine isolated from the rhizome of Coptis chinensis Franch. (Ranunculaceae) showed potent BACE1 inhibitory activity with IC₅₀ values of 8.55 and 19.68 µM, respectively. The mode of inhibition is noncompetitive type with Ki values of 10.0 and 21.2, respectively (Jung et al., 2009). In the course for screening antidementia agents from natural products, five BACE1 inhibitors were isolated from the chloroform root extract of Angelica dahurica Fisch.ex Hoffm. (Apiaceae). Bioactive constituents were identified as furanocoumarins (isoimperatorin, imperatorin, (+)-oxypeucedanin, (+)-byakangelicol and (+)-byakangelicin). Among the compounds, isoimperatonin and (+)-oxypeucedanin showed potent BACE1 inhibitory activity with IC₅₀ values of 91.8 ± 7.5 and 104.9 ± 2.4 µM, respectively (Marumoto & Miyazawa, 2010). Owing to their low molecular weight, these compounds can easily cross the blood-brain barrier (BBB) and reach the brain. A new isoflavone, neocorylin was isolated from the extract of Psoralea corylifolia L. (Fabaceae) seeds together with eight known constituents {(bakuchiol, psoralen bavachromene, isobavachromene, bavachalcone, isobavachalcone, 7,8-dihydro-8-(4-hydrophenyl)-2,2-dimethyl-2H,6H-[1,2-b:5,4-b′]dipyran-6-one, and bavachinin)} showed significant inhibitory effect on baculovirus-expressed BACE1 under in vitro conditions (Choi et al., 2008). Prenylated flavones isolated from methanol extracts of Morus lhou L. stem bark (Moraceae) exhibited significant BACE1 inhibitory activity with an IC₅₀ of 78.4 µg/mL. Activity-guided fractionation of M. lhou stem bark showed the presence of norartocarpetin, kuwanon C, morusin, kuwanon A, cyclomorusin, morusinol, neocyclomorusin and mormin. The IC₅₀ values of compounds for BACE1 inhibition were determined to range between 3.4 and 146.1 µM. Kuwanon C (IC₅₀ = 3.4 µM) exhibited activity 20-times more than its parent compound noratocarpetin (IC₅₀ = 60.6 µM). The stronger activity was related with a resorcinol moiety on the B-ring and isoprenyl functionality at C-3. Kinetic analysis revealed the type of inhibition is noncompetitive (Cho et al., 2011). Sophora flavescens Aiton (Fabaceae), one of the most ubiquitous traditional herbal medicines in East Asia, with an array of biological activities such as anticancer, anti-inflammatory, and tyrosinase inhibitory properties, was screened for BACE1 inhibitory activity. Bioactive-guided fractionation of chloroform root extract showed the presence of five lavandulyl flavanones with potent BACE1 inhibitory activities whose IC₅₀ value ranged from 5.2, 3.3, 8.4, 2.6, and 6.7µM, respectively. The type of inhibition was observed to be noncompetitive (Hwang et al., 2008). Resveratrol dimer (+)-vitisinol E isolated from the stem bark extract of Vitis vinifera L. (Vitaceae) together with four known resveratrol oligomers, (+)-epsilon-viniferin, (+)-ampelopsin A, (+)-vitisin A and (-)-vitisin B, showed potent BACE1 inhibitory activity under in vitro conditions in dose-dependent manner (Choi et al., 2009). Similarly, resveratrol oligomer isolated from the seed of Paeonia lactiflora (Paeoniaceae) showed potent inhibitory activity against BACE1 (Choi et al., 2011). 2,2′-4′-Trihydroxychalcone (TDC) from Glycyrrhiza glabras L. (Fabaceae) a kind of flavonoid belonging to the chalcone family, has been found to exhibit antioxidant, anti-inflammatory, and antitumor properties. Screening for an antidementia effect showed that TDC exhibited noncompetitive type of inhibition with an IC₅₀ value of 2.45 µM under in vitro conditions in human embryo kidney cells. The Ki value was 3.08 (Zhu et al., 2010). Fourteen diterpenoids from the n-hexane extract of Aralia cordata Thunb. (Araliaceae) roots were screened for β-secretase inhibitiory activity. About three ent-kaurane compounds (16α-hydroxy-17-isovaleroyloxy-ent-kauran-19-oic acid, 17-hydroxy-ent-kaur-15-en-19-oic acid, 15α,16α-epoxy-17-hydroxy-ent-kauran-19-oic acid) showed potent BACE1 inhibitory activity with IC₅₀ values of 18.58 ± 1.77, 23.40 ± 1.69, 46.09 ± 4.11µM, respectively (Jung et al., 2009). Panax notoginseng Wall. (Araliaceae), a Chinese herb widely used in Chinese traditional medicine to improve learning and memory, was screened for BACE1 inhibitory activity in vitro. Ginsenoside Rg (steroid glycoside), one of the major active compounds of root of Panax notoginseng, showed potent inhibitory activity against BACE1 in PC12 cells (Wang & Du, 2009). In the course of screening plant extracts for BACE1 inhibitory activity, Aloe vera L. (Xanthorrhoeaceae) Burm.f., a traditional Chinese medicine, was found to potently inhibit BACE1 at 10^–5 g/mL. Among the eight known and four unknown chromosome glycosides isolated from the ethanol extract of Aloe vera and Aloe nobilis, aloersin D and C-2′-decoumaroyl-aloeresin G showed significant inhibitory activity against BACE1 with IC₅₀ values of 39.0 and 20.5 × 10⁻⁶ M (Lv et al., 2008). Root extract of Polygala tenuifolia Wild. (Polygalaceae), commonly used as a traditional Chinese medicine to treat memory loss, was assessed for its BACE1 inhibitory activity. Tenuigenin the active constituents decreased the β-secretase activity thereby reducing Aβ production (Jia et al., 2004).

γ-Secretase determines the ratio of Aβ 1–40 to Aβ 1–42 and has several unusual properties including its ability to cut substrates in the middle of the transmembrane domain in a water-free environment. γ-Secretase is a multiprotein complex and presenilin proteins are the catalytic site of this complex. Presenilins are involved in the cleavage of the notch receptor and blocking of this pathway in the embryo is lethal whereas presenilin-1-knockout adult mice have no overt pathological phenotype. Recently, a γ secretase inhibitor was developed that is able to reduce Aβ production without affecting Notch signalling, which raises the possibility that targeted γ-secretase inhibitor therapy will enter into clinical trials (Petit et al., 2001). In vivo studies showed that γ-secretase inhibitors reduce amyloid burden in animal models and several compounds have already entered phase I clinical trials (Dovey et al., 2001). Tarenflubril (flurizan) a nonosteriodal anti-inflammatory drug (NSAID) and potent γ-secretase inhibitor under both in vitro and in vivo conditions, showed no significant cognitive improvement in clinical trials (Jamie, 2010). Semagacestat (LY450139), a dipeptide, showed significant reduction in γ-secretase activity in neuronal cells. In Phase I clinical trials, single dosing of Semagacestat (LY450139) demonstrated a biphasic response of Aβ (reduction followed by elevation) and hence no significant changes in CSF Aβ was observed. In addition, patients treated with LY450139 suffered from severe side effects particularly in the gastrointestinal tract (Siemers et al., 2007). Thus for, γ-secretase inhibitors were not successful under clinical trials due to their severe side effects.

Antinflammatory drugs

Local plaque-associated inflammation with activated microglia, reactive astrocytes, cytokines, and complement components is a characteristic pathological feature of AD (Akiyama et al., 2000). Proinflammatory cytokines and neurotoxic factors might contribute to neurodegeneration and nonsteroidal anti-inflammatory drugs could reduce the inflammatory responses in the plaques through cyclooxygenase inhibition and direct effects on amyloid processing (Ho et al., 2001). Cyclooxygenase expression is increased in the brains of patients with mild AD and recent findings show that cyclooxygenase 2 promotes amyloidosis in transgenic animal models of AD. Epidemiological studies found patients with rheumatoid arthritis, who often use NSAIDs, have a lower incidence of AD (Breitner, 1996; Jenkinson et al., 1989; McGeer et al., 1996). Weggen and colleagues (2001) found that ibuprofen, indomethacin, and sulindac, but not other NSAIDs, decrease the release of Aβ 1–42 from different types of cultured cells overexpressing APP. In particular, the lowering of Aβ 1–42 concentrations is associated with an increase of Aβ1-38 concentrations without any effect on Notch signalling. These effects seem to be independent of a direct anti-inflammatory action and suggest that some nonsteroidal anti-inflammatory drugs are able to change the processing of APP by γ-secretase to produce the nonamyloidogenic Aβ1-38. Therefore, the use of anti-inflammatory drugs may also have potential in the treatment of AD. Over the past decade, particularly in the past 5 years, several placebo-controlled, randomised trials have investigated the efficacy of various anti-inflammatory approaches for the treatment of AD. Indomethacin was effective but owing to intolerance it was discontinued. Nimesulide, naproxen, COX inhibitors such as celecoxib and rofecoxib had no beneficial effect. So there is need to identify potential NSAIDS from alternate source for treatment of AD.

Numerous plants and plant constituents have demonstrated anti-inflammatory properties (Bingol & Sener, 1995), thus, there is potential for novel anti-inflammatory agents to be identified from plant sources for the treatment of AD, with fewer adverse effects than drugs currently available. Numerous flavonoid compounds (e.g., gossypin, quercetin, gnaphalin) have been associated with anti-inflammatory activity (de la Puerta et al., 1999; Harbone & Baxter, 1993) and may have potential in the management of dementia disorders. The presence of a pyrocatechol group in least one of the flavonoid rings is responsible for cyclooxygenase (COX) inhibition; however, flavones without such substituents can also inhibit COX (Alcaraz & Ferrandiz, 1987; Welton et al., 1986). Other compounds with potential for use in inflammatory disorders include ferulic acid, which is an antioxidant and anti-inflammatory compound derived from plants. Ferulic acid ameliorated the β-amyloid-induced reduction in ACh levels in the cortex, and the β-amyloid-induced inflammatory responses in the hippocampus in mice, and improved cognitive function (Yan et al., 2001). Dietary polyphenols such as epigallocatechin, epigallocatechin gallate, curcumin and myricetin attenuates the expression of COX-2, inhibits prostaglandin E₂ production thereby inhibiting inflammation in neurons. Therefore, dietary polyphenols are termed as neuroprotector (Kim et al., 2007). Resveratol inhibited proinflammatory gene expression via inhibition of inhibitory κB (IκB), thus inhibiting NF-κB transactivation, as well as restoring transrepressive pathways through the activation of histone deacetylase in RAW 264.7 cells (Tsai et al., 1999). Several plant-derived compounds exhibit direct regulation of PPARγ and NF-κB that plays a major role inflammation. Root extract of Withania somnifera exhibits anti-inflammatory effect in vivo and the extract has been shown to reduce levels of the proinflammatory interleukins IL-1 and TNF-α, which are considered involved in senile plaque formation and neurodegeneration (Dhuley, 1997; Begum & Sadique, 1988). Bilignans such as honokiol and magnolol isolated from the root and stem extracts of Magnolia officinalis have been widely used in TCM for the treatment of nerve disturbances and also mediate anti-inflammatory effects (Wang & Mineshita, 1996). 4′-Geranyloxyferulic acid, obtained from Acronychia baueri f. baueri Schott (Rutaceae), increases PARγ activity significantly (Genovese et al., 2010). Incensole acetate and its nonacetylated form, incensole (IN), isolated from Boswellia serrata Triana & Planch. (Burseraceae), a major anti-inflammatory agent in herbal medical tradition, inhibits NF-κB activation (Moussaieff et al., 2007). Parthenolide from the medicinal herb feverfew [Tanacetum parthenium (L.) Sch. Bip. (Asteraceae)] directly binds and inhibits I-κB kinase (Kwok et al., 2001). Hesperetin, a flavanone derived from Citrus fruits, suppresses NF-κB activation in both young and old rats through multiple signal transduction pathways (Kim et al., 2006). The methanol extract of Gastrodia elata Blume. (Orchidaceae) inhibits NO production and expression of iNOS and COX-2 upon stimulation by LPS in RAW264.7 macrophages (Ahn et al., 2007).

These exciting studies indicate that nuclear transcription factors may serve as the direct target of natural anti-inflammatory compounds. More attention should be paid to the role of these transcription factors in the neuroprotective effects of natural anti-inflammatory compounds. Gingerol, inflexin and icariin, the active constituents of Zingiber officinale, Isodon excises (Lamiaceae), and Epimedium brevicornum L. (Berberidaceae), decreased the synthesis of proinflammatory markers such as prostaglandins and leukotrienes via inhibition of COX-2 and 5-lipoxygenase (5-LOX) enzymes and proinflammatory cytokines, including IL-1β, IL-2, IL12, TNF-α, and interferon (IFN)-γ, which are the targets for numerous anti-inflammatory pharmaceuticals. They also attenuated the mitogen-activated protein kinases (MAPKs), extracellular signal-regulated kinases 1 and 2 (ERK1/2), the major pathways of neurodegeneration, which suggest these compounds are suitable for the treatment of neuroinflammation (Jung et al., 2009; Ko et al., 2010, Zeng et al., 2010). Obovatol, the main component of leaves of Magnolia obovata Thunb. (Magnoliaceae), inhibited microglia mediated neuroinflammation both under in vitro and in vivo conditions (Choi et al., 2007). Neolignans isolated from Piper kadsura Trel. & Yunck. (Piperaceae) showed reduction in the level of NO production in LPS-activated BV-2 microglial cells (Kim et al., 2009). Ganoderma lucidum (Curtis) P. Karst (Ganodermataceae) extract significantly prevents the production of microglia-derived proinflammatory and cytotoxic factors (nitric oxide, TNF-α, and IL-1β), and down-regulate the TNF-α and IL-1β gene expression at the mRNA level indicating it may be a promising agent for the treatment of neuroinflammatory diseases. Blueberry significantly enhances microglial clearance of Aβ1-42, inhibits aggregation of Aβ1-42, and suppresses microglial activation, all via suppression of the p44/42 MAPK pathway (Zhu et al., 2008). Berberine, isolated from Rhizoma coptidis, suppresses neuroinflammatory responses through AMP-activated protein kinase activation in BV-2 microglial cells (Lu et al., 2010). Isodojaponin D, derived from Isodon japonicas var. (Lamiaceae), significantly decreased LPS-induced production of COX-2 and iNOS and proinflammatory cytokines, including IL-1β, IL-6, TNF-α, through NF-κB and MAPK signaling pathways (Lim et al., 2010). Tetrandrine and fangchinoline, found in Stephania tetrandra S. Moore (Menispermaceae), have been shown to decrease IL-1β, IL-6, IL-8 and TNF-α by microglial cells, which damage nerve cells (Xue et al., 2008). Urtica dioica L. (Urticaceae), also known as “stinging nettle,” reduces IL-1β, IL-2, IFN-α, and TNF-α. The ethanol root extract of Mahonia oiwakensis Hayata. (Berberidaceae) attenuated the expression of COX-2 and of iNOS and neutrophil infiltration in Chao et al. (2009). Thai red curry paste significantly suppressed the nitric oxide production and expression of iNOS, COX-2, TNFα dose-dependently in RAW264.7 cells treated cells without a cytotoxic effect (Tuntipopipat et al., 2011). Jolkinolides the diterpenoids isolated from the root of Euphorbia fischeriana inhibited inflammatory mediators such as prostaglandin E₂ (PGE₂), nitric oxide (NO), and proinflammatory cytokines [interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α)] at the protein level and mRNA expression level. In addition, it causes suppression of MAPK phosphorylation and NF-κB activation (Uto et al. 2012). Ginsenosides from Panax ginseng L. (Araliacea) exhibited potent anti-inflammatory effects by suppressing the activity of NF-κB and MAP kinase followed by a decrease in inducible nitric oxide synthase (iNOS) and proinflammatory cytokine (Park & Cho, 2009). Curcumin reduces the release of proinflammatory cytokines, such as IL-1β, IL-6, and TNF-α (Jin et al., 2007), attenuates NO production and decreases the expression of COX-2. The anti-inflammatory properties of curcumin are exerted mainly by suppression of the NF-κB signaling pathway (Kang et al., 2007). Wogonin, isolated from the root of Scutellaria baicalensis Georgi. (Lamiaceae), inhibited the production of NO and the expression of iNOS in cultured rat astrocytes (Kim et al., 2000). Baicalein (5,6,7-trihydroxyflavone), derived from the root of S. baicalensis, exerts anti-inflammatory effects by decreasing microglial activation accompanied by down regulation of TNFα, NO, super oxide and free radical formation (Suk et al., 2003). Triptolide, isolated from Tripterygium wilfordii Hook.f. (Celastraceae), inhibited the inflammation-mediated damage involved in the signaling of the p38-NF-κB-COX-2-PGE2 and JNK-PGE2 pathways (Gong et al., 2008).

Antioxidants as drugs for dementia

Oxidative stress resulting from excessive generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS) has been suggested as a causal factor in various neurodegenerative disorders including Parkinson’s disease (Emerit et al., 2004) and Alzheimer’s disease. These free radicals are released during inflammatory reactions, whereas others are formed during normal oxidative metabolism and auto-oxidation of certain neurotransmitters and by β-amyloid. Neurons are most susceptible to direct oxidative injury by ROS and RNS. ROS and RNS can also indirectly contribute to tissue damage by activating a number of cellular pathways resulting in the expression of stress-sensitive genes and proteins to cause oxidative injury. Moreover, oxidative stress also activates mechanisms that result in a glia-mediated inflammation that also causes secondary neuronal damage. Associated with neuronal injuries caused by many CNS insults is an activation of glial cells (particularly astrocytes and microglia) at the sites of injury. Activated glial cells are thus histopathological hallmarks of neurodegenerative diseases. Even though direct contact of activated glia with neurons per se may not necessarily be toxic, the immune mediators (e.g., nitric oxide and reactive oxygen species, proinflammatory cytokines and chemokines) released by activated glial cells are currently considered to be candidate neurotoxins. Therefore, study of the protective role of antioxidant compounds on inhibition of the inflammatory response and correcting the fundamental oxidant/antioxidant imbalance in patients suffering from neurodegenerative diseases are important vistas for further research. The use of antioxidants may slow AD progression and minimize neuronal degeneration. Single use of antioxidants may not be adequate for the treatment of these diseases; use of cocktail of drugs (antioxidants + anti-inflammatory + neuroprotectants) may be beneficial for the treatment of neurodegenerative disorder (Wang et al., 2006). Clinical studies revealed that high doses of antioxidants such as vitamin E and NADH (Sano et al., 1997; Birkmayer, 1996) had beneficial effect in the treatment of AD.

Injury of plant cells, as well as mammalian cells, is associated with the activation of lipoxygenases, which catalyze the formation of hydroperoxides of polyunsaturated fatty acids (PUFAs); a hydroperoxide radical may react with fatty acids to produce dioxoenes, which are regarded as plant defense compounds (Spiteller, 1993). The occurrence of oxidative mechanisms in plants may explain why an abundance of antioxidant compounds have been identified in plant tissue. Their antioxidant effects in plants may therefore have relevance in mammals, particularly in disorders involving oxidative stress such as AD. Plants such as Aeseclus hippocastanum L. (Hippocastanaceae), Allium nutans L. (Alliaceae), Artemisia spp. (Asteraceae), Guiera senegalensis J.F. Gmel. (Combretaceae), Hamamelis virginiana L. (Hamamelidaceae), Rosmarinus officinalis, Salvia officinalis, Taraxacum officinale F.H. Wigg (Asteraceae) and Thymus vulgaris L. (Labiatae) and their phytochemicals including cinnamic acids, coumarins, diterpenes, flavonoids, lignans, monoterpenes, phenylpropanoids, tannins and triterpenes exhibit excellent antioxidant properties (Aruoma et al., 1996; Bouchet et al., 1998; Cuvelier et al., 1996; Deans et al., 1993; Masaki et al., 1995; Stajner et al., 1999; Youdim & Deans, 1999). Some plants and plant compounds with antioxidant activity have shown favorable effects in the CNS, so they may be appropriate for use in AD treatment. Curcumin from Curcuma longa L. (Zingiberaceae) reduced lipid peroxidation in rat brain following oral administration to rats with ethanol-induced brain injury (Rajakrishnan et al., 1999), and Bacopa monniera, which is reported to have cognition enhancing effects, induced a dose related increase in superoxide dismutase, catalase and glutathione peroxidase activities in the rat frontal cortex, striatum and hippocampus (Bhattacharya et al., 2000b). In another study, Thymus vulgaris essential oil maintained higher PUFA levels in various tissues, including the brain in rats, indicating protective antioxidant effects (Youdim & Deans, 1999). Natural polyphenolic compounds exhibit their antioxidant effect by quenching free radical species and/or promoting endogenous antioxidant capacity. In addition, they stimulate synthesis of endogenous antioxidant molecules in cells via activating the Nrf/ARE pathway. Green tea polyphenols such as epicatechin, epicatechin-3-gallate, epigallocatechin-3-gallate acts as efficient ROS scavengers (Rice-Evans et al., 1996). Besides green tea polyphenols other flavonoids such as quercetin, gossypetin, myricetin, quercitrin, isoquercitrin, and rutin are known for antioxidant properties are currently used for AD therapy (Taniguchi et al., 2005). Several species of Salvia such as S. officinalis, S. lavandeulafolia, S. miltiorrhiza exhibited excellent antioxidant activity The antioxidant effect is due to the presence of essential oils, caffeic acid, carnosic acid, carnosol, rosmarinic acid and dihydrotanshinones (Wang et al., 2000; Perry et al., 2001; Du et al., 2000). The crude extract of Withaneia somnifera exhibited excellent antioxidant and anti-inflammatory activities that may be relevant in AD therapy. Compounds responsible for antioxidant activity include the withanolides, glycowithanolides that reduced lipid peroxidation in brain of rodents. Moreover, glycowithanolides and sitoindosides (VII-X) enhanced catalase and glutathione peroxidase activities in rat frontal cortex and striatum (Bhattacharya et al., 2001). Gardenia jasminodes J.Ellis (Rubiaceae) is traditionally used as an anti-inflammatory agent. Its extract reduces the cytotoxicity of Aβ in PC12 cells by reducing oxidative stress (Choi et al., 2007). Bacopa monnieri (Scrophulariaceae), commonly known as Brahmi, is a traditional Ayurvedic medicinal plant and is used as a nerve tonic. Aqueous extract of aerial parts of brahmi lowers the level of reactive oxygen species lowering the oxidative stress induced damage in neurons (Limpeanchob et al., 2008). Honokiol and magnolol, isolated from bark extract of Magnolia officinalis, protected neurons against toxicity induced by hydrogen peroxide, glutamate and N-methyl-d-aspartic acid (NMDA) through suppression of ROS production, caspase 3 activity, and intracellular calcium elevation. Honokiol pretreatment restores the level of glutathione, glutathione peroxidase activity and induces the activation of the ERK pathway and suppresses NF-κB activation thereby mitigating neuroinflammation (Ock et al., 2010). Polyphenols present in Vaccinium cyanococcus Rydb. (Ericaceae) play a major role as antioxidants by attenuating ROS production and the stress-signaling pathway (Joseph et al., 2010). EGb761, the active component of Ginko biloba, scavenges free radicals including NO, hydroxyl radical, superoxide anion and peroxyl radical thereby leading to neuroprotection of primary hippocampal neurons and PC12 cells against neurotoxicity of amyloid-β peptide (Bastianetto et al., 2000). Ginsenosides, isolated from Panax spp., inhibited the overproduction of NO and malondialdehyde, and the influx of calcium. Vinpocetine, an alkaloid derived from Vinca spp. (Apocynaceae), acts as a free radical scavenger and protects PC12 cells from β-amyloid toxicity (Grundman et al., 2002). Ferulic acid, found in many fruits and vegetables such as tomato, showed antioxidant activity against peroxynitrite formation, protein carbonyl formation and lipid peroxidation in synaptosomal membranes. Strong antioxidant capacity is due to the presence of resonance-stabilizing structural motifs on the benzene ring that could stabilize a phenoxyl radical. The strong antioxidative property of ferulic acid makes it a promising candidate for therapeutic intervention in AD (Srinivasan et al., 2007). Resveratrol in red wine plays a major role in protecting against oxidative insult both under in vivo and in vitro conditions by scavenging ROS and nitric oxide related toxicity that is due to the presence of the para-hydroxyl group of trans-resveratrol responsible for efficient scavenging activity (Stojanovic et al., 2001). Celesterol, a triterpenoid derived from Tripterygium wilfordii Hook.f. (Celastraceae), showed potent inhibition of lipid peroxidation and NO production thereby preventing oxidant damage in neurons (Allison et al., 2001). Aqueous seed extract of Celastrus paniculatus Wall. (Celastreacea), showed an antioxidant effect in rat brain that may be responsible for congnitive improvement (Gupta, Kumar, 2002). The aqueous extract of Coptis chinesis Franch. (Ranunculaceae) also showed in vitro antioxidant activity (Liu & Ng, 2000). The hydroalcholic extract of dried stigmase of Crocus sativus L. (Iridaceae) exhibited antioxidant activity in vitro (Papandreou et al., 2006). Hypericin and hyperforin, isolated form Hypericum perforatum L. (Hypericaceae), showed potent in vitro antioxidant activity (Hunt e al., 2001). The volatile oil of Melissa officinalis showed potent antioxidant activities (Mimica-Dukic et al., 2004) which might be due to presence of oxygenated monterpens and sesquiterpene hydrocarbons. The methanol extract from the rhizomes of Rheum spp. (Polygonaceae) exhibited in vitro antioxidant properties (Matsuda et al., 2001). In addition, the aqueous extract of dried fruits of Terminalia chebual Retz. (Combretaceae) showed in vitro antioxidant activity (Naik et al., 2004).

Amyloid antiaggregant therapies

The “amyloid hypothesis” suggests that normal soluble Aβ undergoes a conformational change that causes it to aggregate as a fibril rich β-pleated sheet structure. Soto (2001) have designed short synthetic peptides homologous to the central hydrophobic region of Aβ that disrupts β-sheet stabilization. Recently, the same group has shown a reduction in aggregated amyloid in transgenic mice overproducing APP treated with a five-residue β-sheet breaker peptide (iAb5p). The peptide did not induce antibody production and could cross the blood-brain barrier. Clinical trials of antiaggregants as potential therapies for AD are now possible. Curcumin disrupts existing plaques and restores distorted neuritis in vivo. Many dietary polyphenolic compounds are reported to have an inhibitory effect on amyloid fibril formation. Red wine derived polyphenols not only exhibit antioxidant activity but also prevents deposition of β amyloid deposition (Ono et al., 2003). Curcumin and rosmarinic acid inhibits polymerization of Aβ peptide by specific binding with free β-amyloid (Ono et al., 2004a,2004b). Resveratol from red wine and Gingko biloba also act as deaggregaters of β-amyloid (Conte et al., 2003). Green tea polyphenol such as epigallocatechin gallate inhibits Aβ fibrillation. Quercitin, gossypetin and myricetin specifically bind with Aβ preventing aggregation (Kocisko et al., 2003; Taniguchi et al., 2005). 1,2,3,4,6-Penta-O-galloyl-β-d-glucopyranose isolated from Paeonia suffruticosa Andrews inhibits the formation of Aβ aggregates (Fujiwara et al., 2009). Tannic acid was effective in inhibiting the formation of Aβ aggregates and destabilizing preformed Aβ in a dose-dependent manner, even though the precise mechanisms of tannic acid on the inhibition of Aβ oligomer formation were not fully elucidated (Ono et al., 2004a). Uncaria rhynchophylla (Miq.) Jacks. (Rubiaceae) reduced the formation of Aβ aggregates in a concentration-dependent manner and readily destabilizes Aβ aggregates. Bioactive constituents such as oxindole and indole alkaloids such as corynoxeine, hirsutine and hirsuteine are responsible for the anti-Aβ aggregating properties (Fujiwara et al., 2006). Aged aqueous extract of garlic also exhibited inhibitory activity against Aβ oligomerization (Gupta et al., 2009). Salvianolic acid B (Sal B), an active ingredient of Salvia miltiorrhiza, inhibits fibril aggregation (IC₅₀ 1.54- 5.37 mM) as well as destabilizes preformed Aβ fibril (IC₅₀ 5.00-5.19 mM) in a dose- and time-dependent manner (Durairajan et al., 2008). Rutin inhibits the formation of Aβ aggregates in a dose-dependant manner (Wang et al., 2012). Ginkgo biloba active fraction EGb71 inhibits fibrillation of amyloid-β via iron chelation at the concentration of 20-100 µg/mL (Luo et al., 2002). Resveratrol and ampelopsin the stilbenes isolated from Vitis vinifera exhibited 63 and 46% against Aβ aggregation (Zga et al., 2009). Aqueous extract of Caesalpinia crista L. (100 µg/ml) not only inhibited the Aβ (42) aggregation from monomers and oligomers but also disaggregate the preformed fibrils that might be due to the presence of high polyphenolic content (Ramesh et al., 2010). Walnut extract and Withania sominfera aqueous extract inhibited the fibrillization of synthetic Aβ and also solubilized mature Aβ fibrils (Chauhan et al., 2004; Kumar et al., 2012).

Metal chelators (indirect antioxidants)

Metal ions may have a role in the pathogenesis of AD. Aβ seems to act as a metalloprotein with high affinity for Cu²⁺ and Zn²⁺, which might mediate its aggregation and toxicity. In particular, metal binding to Aβ might cause the production of reactive oxygen species, which are involved in neurodegeneration. Cu²⁺ and Zn²⁺ concentrations are high in the neocortex in patients with AD. This evidence supports an approach of targeting the interaction between Aβ and metal ions (Atwood et al., 1998). In 1999, Cherny and coworkers showed that Cu/Zn chelators solubilise Aβ plaques from brains of patients with AD after death. Clioquinol is a hydrophobic Cu/Zn chelator that freely crosses the blood-brain barrier and was used as an oral antiamoebic treatment for many years before being withdrawn from the market in the 1970s because of its association with subacute myelo-optic neuropathy. This effect is now believed to be preventable with B₁₂ supplementation (Yassin et al., 2000). Curcumin is good metal chelator which readily chelates Cu²⁺ and curcumin-Cu²⁺ is more active than the parent compound in scavenging ROS by catalyzing the dismutation of superoxide anion radical or by donating the electron (Craig et al., 2005). Green tea polyphenols such as epicatechin, EGC, EGCG, flavaonoids such as quercetin, gossypetin, myricetin, quercitrin, isoquercitrin, rutin, and xanthones from natural sources act as very good metal chelators (Rice-Evans et al., 1996; Taniguchi et al., 2005). TCM phytochemicals such as bellidifolin, isogntisin, swerchirin, glycyrrhisoflavone and morin act as effective metal chelators.

Monoamine oxidase inhibitors

Monoamine oxidase (MAO) is a major enzyme responsible for the fast breakdown of dopamine (DA) and related compounds at the synapse. Inhibitors of this enzyme, known as MAOIs, cause a net increase in DA levels and, although their major therapeutic use has been as antidepressants, they have potential use in PD. This has not generally been realized because of the side effects associated with elevation of peripheral DA levels. The β-carboline alkaloids harmane and harmaline are found in several traditional medicine plant species, including Banisteriopsis caapi Spruce ex Griseb. (Malpighiaceae), prevents metabolism of amines thereby acting as MAOI. Following reports of the successful use of B. caapi root extracts for treating PD patients in Ecuador, it was shown that, in addition to the MAOI properties, the alkaloids harmane and harmaline stimulated the release of DA from striatal cells (Schwarz et al., 2003). These compounds would therefore have a double effect in helping improve DA levels, and this may underlie the reputed improvements in PD patients when the extract is taken. Ethyl acetate and petroleum ether leaf extract of Ruta graveolens exhibited MAO inhibitory activity with an IC₅₀ value of 5 ± 1 & 3 ± 1 μg/ml) and specific MAO-B inhibition with an IC₅₀ value of 7.4 ± 6; 3 ± 1 μg/ml, respectively (Stafford et al., 2007).

An enthnobotanical survey for MAO-B inhibitors from traditional Chinese medicine showed that 50% aqueous methanol extracts of Arisaema amurense Maxim. (Araceae), Lilium brownii var. colchesteri (Liliaceae), Lycium chinense Mill. (Solanaceae), and Uncaria rhynchophylla Miq. Jacks (Rubiaceae) exhibited the best activity and selectivity towards MAO-B with IC₅₀ values of 0.44, 0.29, 0.40, and 0.03 mg/ml, respectively. Kinetic studies showed that A. amurense and L. brownii var. colchesteri exhibited mixed type inhibition with Ki values of 0.59 and 0.58 mg/ml while L. chinense and U. rhynchophylla showed noncompetitive type inhibition with Ki values of 5.01 and 0.02 mg/ml, respectively (Lin et al., 2003). The Methanol extract of dried bark of Gentiana lutea L. (Gentianaceae) showed the presence four compounds of which 3-3-(2-hydroxy-4-O-isoprenylchalcone)-(2-hydroxy-4-O-isoprenyldihydrochalcone) exhibited potent MAO-B inhibitory activity with an IC₅₀ value of 48.7 µM and the mode of inhibition was observed to be the competitive type (Haraguchi et al., 2004). (+)-Catechin and (−)-epicatechin, isolated from the hook of Uncaria rhynchophylla (Rubiaceae), showed potent MAO-B inhibitory activity (IC₅₀ 88.6 and 58.9 µM) in a dose-dependent manner. The mode of inhibition is mixed type with Ki values of 74 and 21µM, respectively (Hou et al., 2005). Narigenin isolated from the methanol extract of Mentha aquatic L. (Lamiaceae) showed potent MAO-B inhibitory activity with an IC₅₀ value of 288 ± 18 μM (Olsen et al., 2008). Diermen et al. (2009) reported that methanol and water extracts of Rhodiola rosea L. (Crassulaceae) roots showed potent MAO inhibition of 81.8 and 88.9% on MAO B, at a concentration of 100 µg/ml. The most active compound (rosiridin) presented an inhibition over 80% on MAO-B at a concentration of 10⁻⁵ M (pIC₅₀ = 5.38 ± 0.05). The role of flavonoids as important antioxidant contributors in the diet has received much attention and the neuroprotective properties of this effect have been demonstrated by several workers. Kaempferol from the leaves of Ginkgo biloba and quercetin have been shown to inhibit MAO-B (Sloley et al., 2000) and reverse the effects of induced catalepsy, which mimics the bradykinesia often seen in PD (Singh et al., 2003). Tangeretin has been shown to inhibit MAO-B and to cross the blood brain barrier in a rat model and consequently reduce DA depletion, so it may have therapeutic potential (Datla et al., 2002). A compound related to the flavonoids, the polyphenol (–)-epigallocatechin-3-gallate, found in green tea, has a similar polyvalent activity which may be sufficient to have a protective effect in PD and other conditions (Mandel et al., 2004). Since most of these flavonoids occur in reasonably large amounts in common fruits and vegetables, the question is raised as to whether the apparent increase in incidence of neurodegenerative disease is related to some extent with the decline in consumption of such foods in the diet in some sectors of the industrialized world.

Multipotent target for the treatment of dementia

With the accelerated aging of human society, dementia is becoming one of the biggest threats to human health (Mattson, 2004). Although the etiology of dementia is not very clear, multiple pathogenic factors have been identified as causative for this disorder, which include amyloid-β (Aβ) peptide and/or τ protein aggregation, excessive metal ions (e.g., Cu²⁺, Zn²⁺, Fe³⁺), oxidative stress and reduced ACh level, DA level, etc. (Forman et al., 2004). Besides, genetic factors and lifestyles, such as diet, exercise and cognitive stimulation, are also associated with AD development. Despite the diverse pathogenic factors involved in AD, the current anti-AD strategy depends largely on single-targeted drugs, especially acetylcholinesterase (AChE) inhibitors. As these drug’s effects are quite limited (Maggini et al., 2006), attention is given to finding multiple-targeted agents to hit more than one target implicated in AD (Zhang, 2005). Although the new anti-AD strategy may be fulfilled by combining different anti-AD drugs in one pill (cocktail therapeutics) (Youdim & Buccafusco, 2005), an alternative approach that aims at multiple AD-targets with a single structure (termed multipotent agent) is also attractive, because of its advantages in reducing risks of drug-drug interactions and controlling pharmacokinetic behaviors. Despite the preliminary successes of synthetic hybrid agents, the latent risks in safety and bioavailability is a big concern in their further development. Thus, finding multipotent natural agents to combat AD is attracting more and more attention.

Multipotent antidementia agents derived from foods

Epidemiological investigations revealed that high consumption of foods such as fruits, vegetable juices, green tea, wine, Mediterranean diet, curry spice turmeric and even cigarettes, all of which rich in antioxidants (polyphenols) were inversely associated with AD incidence (Dai et al., 2006). As it is well known that polyphenols are excellent antioxidants both as ROS scavengers and transition metal chelators, the anti-AD effects of these foods were naturally linked to their antioxidant potential (Zhang, 2005). Antioxidants derived from foods such as flavonoids like quercitin, (-)-epigallocatechin gallate (EGCG), resveratrol, olive oil phenols and curcumin, etc., possess multiple pharmacological effects such as inhibition of MAO A and B, cholinesterase inhibition, blocking of Aβ- or τ-aggregation, anti-inflammatory activity, many of which are beneficial to combat AD (Lee et al., 2001a,b).

Plants: A source of multipotent antidementia agents

Preliminary clinical trials revealed that in addition to foods, some herbal plants, such as G biloba, H. serrata, S. officinalis, M. officinalis, also hold anti-AD potential. Herbal plants used in traditional medicine with multipotent pharmacological activity that might have beneficial effects in dementia patients are discussed below.

Centella asiatica (Umbelliferae)

Centella asiatica leaf, an ancient Ayurvedic remedy, is used as revitalizing herb to strengthen nervous function and memory, restore youth, and longevity (Kapoor, 1990). The herb is taken as a tonic for poor digestion and rheumatism; the latter suggesting it may have anti-inflammatory effects. C. asiatica is also used in TCM for combating physical and mental exhaustion (Brinkhaus et al., 2000). The monoterpenes present in the essential oil from C. asiatica such as bornyl acetate, α-pinene, β-pinene, and γ-terpinene exhibited AChE inhibitory activity (Miyazawa et al., 1997). An alcohol extract of the leaves is reported to be tranquillizing in rats, an activity that was attributed to a triterpene, brahmoside (Kapoor, 1990; Sakina & Dandiya, 1990). In mice, an extract of C. asiatica leaf was used as a sedative, antidepressant, and showed cholinomimetic activity, which was blocked by atropine (Sakina & Dandiya, 1990). These results indicate that multipotent activity of C. asiatica may be appropriate to treat symptoms of depression and anxiety in AD, and may enhance cholinergic activity and, thus, cognitive function. An aqueous extract of C. asiatica leaf improved learning and memory processes in rats, and modulated dopamine, 5-hydroxytryptamine (5-HT) and noradrenaline systems in rat brain in vivo (Nalini et al., 1992). These results suggest that the more polar compounds (perhaps triterpene saponins) present in C. asiatica leaf may enhance cognitive function by influencing neurotransmitter systems in the CNS. The triterpene asiatic acid and its derivatives have been shown to protect cortical neurons from glutamate-induced excitotoxicity in vitro (Lee et al., 2000). Further studies are necessary to confirm this to identify any potential relevance in dementia treatment.

Ginkgo biloba L. (Coniferae)

Ginkgo biloba has been used in TCM for respiratory disorders, and its use in Western medicine for circulatory disorders dates back to the 1960s. G. biloba has been used traditionally in Iran to improve memory loss associated with blood circulation abnormalities (Ross, 2001). This herb has been subjected to numerous investigations regarding its potential in cognitive disorders. The G. biloba extract EGb 761 has shown favorable effects on cerebral circulation and neuronal cell metabolism (Loffler et al., 2001), on the muscarinic cholinergic system (Kristofikova et al., 1992), and showed antioxidant activity (Marcocci et al., 1994; Topic et al., 2002). EGb761 was also neuroprotective against β-amyloid and NO-induced toxicity in vitro (Bastianetto et al., 2000a,2000b), and could reduce apoptosis both in vitro and in vivo (Yao et al., 2001). G. biloba extracts were also evaluated for their effect on cognitive function. Treatment with G. biloba extracts attenuated scopolamine-induced amnesia in rats (Chopin & Briley, 1992), enhanced memory retention in young and old rats (Petkov et al., 1993) and improved short-term memory in mice (Stoll et al., 1996). The clinical efficacy of G. biloba extracts, including EGb 761, was observed (modest improvements in cognitive function) following administration to AD and non-AD patients in various studies, including randomized, double-blind, placebo-controlled, multicentre trials (Le Bars et al., 2000; Oken et al., 1998; Rigney et al., 1999). The compounds responsible for these observations require further investigation, but activity is perhaps due to vasodilatory flavonoids, although other mechanisms of action may also be responsible for the favorable effects observed. For example, ginkgolide B from G. biloba is a platelet-activating factor (PAF) antagonist (Barquet et al., 1994), which indicates activity against inflammatory processes. It is apparent that G. biloba can be useful in the treatment of AD symptoms, but further research is necessary to identify appropriate dosing regimens, the potential effects of long-term use, interactions with other medicines, and standardization of extracts must also be a consideration.

Melissa officinalis (Labiatae)

Melissa officinalis leaf has been used as a medicinal plant for more than 2000 years in traditional European medicine. M. officinalis was used as a calming and strengthening remedy and to treat migraines, melancholia, neuroses and hysteria, and the plant has been acclaimed for promoting long life and for restoring memory (Bisset, 1994; Kenner & Requena, 1996; Yarnell, 1998). In 1751, the herbalist John Hill stated that M. officinalis was ‘Good for disorders of the head and stomach’ (Crellin & Philpott, 1990), indicating potential benefits for the use of M. officinalis for CNS disorders. In Arabic medicine it was used to treat depression (McVicar, 1994), and in Greek medicine it is used to treat hysteria (Malamas & Marselos, 1992). The Commission E Monograph in Germany approves the use of M. officinalis for nervous insomnia. In modern alternative medicine, M. officinalis essential oil is used in aromatherapy to alleviate depression and insomnia (McVicar, 1994). The primary monoterpenes identified in the essential oil of M. officinalis include citral (geranial and neral) (Bisset, 1994; Guenther, 1949; Sarer & Kodkdil, 1991) which is a weak inhibitor of AChE (Ryan & Byrne, 1988). M. officinalis has been the subject of research regarding its potential as a sedative and anxiolytic, activities that may be appropriate to provide symptomatic relief for behavioral problems such as agitation in AD. M. officinalis leaf is reported to alleviate mild anxiety and nervousness in a double-blind study, and, in combination with Valeriana officinalis root, was reported to be as effective as triazolam (Yarnell, 1998). A hydroalcohol (30% ethanol) extract of M. officinalis leaf was sedative in mice and potentiated barbiturate induced sleep, but the M. officinalis essential oil did not demonstrate these sedative effects (Soulimani et al., 1991). Other activities of M. officinalis leaf extracts that may be useful for AD therapy include antioxidant effects (Hohmann et al., 1999; Tagashira & Ohtake, 1998) and binding to muscarinic and nicotinic receptors in vitro (Perry et al., 1996; Wake et al., 2000), which suggests that favorable effects on cholinergic function may occur in AD patients.

Polygala tenuifolia

Polygala tenuifolia (Polygalaceae) root is used in TCM as a cardiotonic and cerebrotonic, as a sedative and tranquillizer, and for amnesia, forgetfulness, neuritis, nightmares and insomnia (Chang & But, 1987). According to the Chinese Materia Medica, the root is supposed to have a special effect upon the will and mental powers, giving strength of character, improving understanding, strengthening the memory, and increasing physical powers. There have been numerous studies regarding the reputed memory-enhancing potential of P. tenuifolia root. For example, the traditional Chinese prescription DX-9386, composed of four herbs (Panax ginseng, Polygala tenuifolia, Acorus gramineus and Poria cocos) has shown favorable effects in relation to AD symptoms in several animal models. DX-9386 improved motor activity, reduced lipid peroxidation, ameliorated memory impairment and prolonged the lifespan of senescence accelerated mice and, ameliorated ethanol- and scopolamine-induced memory impairment in mice (Nishiyama et al., 1994a,b). Further investigations are required to clarify the contribution of each of the four herbs in DX-9386 to the observed pharmacological activities.

Kami-utan-to (KUT), a prescription containing 13 herbs including P. tenuifolia root, is used in traditional Japanese medicine to treat psychoneurological diseases. KUT dose-dependently upregulated choline acetyltransferase (ChAT) activity and increased nerve growth factor (NGF) secretion in vitro, and improved passive avoidance behavior and induced ChAT activity in the cerebral cortex of aged rats, and in scopolamine-induced memory impaired rats in vivo (Yabe et al., 1997; Yamada & Yabe, 1997). The effects on ChAT activity and NGF secretion in vitro were not as pronounced when treated with KUT in the absence of P. tenuifolia root, but P. tenuifolia root extract alone did upregulate ChAT activity and increase NGF secretion in vitro (Yabe et al., 1997). The cinnamic acid derivative sinapinic acid, from P. tenuifolia root, increased ChAT activity in the frontal cortex in brain-lesioned rats (Yabe et al., 1997). These results suggest that P. tenuifolia root, particularly the cinnamic acid derivatives, significantly contribute to the pharmacological activities of KUT, and may explain the reputed beneficial effects of KUT in Japanese medicine. KUT treatment in AD patients is also reported to improve memory-related behavior (Yamada & Yabe, 1997). P. tenuifolia also inhibited AChE activity in vitro (Park et al., 1996). An aqueous extract of P. tenuifolia root, inhibited interleukin-1 (IL-1) mediated tumor necrosis factor (TNF) secretion by astrocytes in vitro (Kim et al., 1998) and also dose-dependently inhibited ethanol-induced IL- 1 secretion in vitro (Koo et al., 2000). This suggests that the reputed favorable effects of the herb in CNS disorders may also involve anti-inflammatory activity, but this requires confirmation in vivo. The aqueous extract of P. tenuifolia root is reported to prolong hexobarbital sleeping time in mice (due to onjisaponin F) (Chang & But, 1987; Tang & Eisenbrand, 1992). Thus, it may have potential as a tranquillizer.

Panx ginseng

The root of Panx ginseng, commonly termed as ginseng, is widely in used TCM as a general tonic and adaptogen to help the body to resist the adverse influences of a wide range of physical, chemical, and biological factors and to restore homeostasis (Nocerino et al., 2000). Jiang et al. (2000) and Lee et al. (2001) reported that ginseng and its components increased cell survival, extension of neurite growth, and rescuing of neurons from death in consequence of different insults either in vivo or in vitro. Ginseng, in particular ginsenoside Rg3, inhibits both N-methyl-d-aspartate (NMDA) and non-NMDA glutamate receptors (Kim et al., 2002), and thereby reduces increased influx of Ca²⁺ into neurons and protects the neuronal cells from degenerative processes. Ginseng increases choline acetyltransferase levels in rodent brains suggesting that these compounds may improve central cholinergic function in humans and may be used to treat memory deficit (Rudakewich et al., 2001).

Itoh et al. (1989) reported that ginsenosides increased dopamine and norepinephrine in the cerebral cortex, which may explain the favorable effects of ginseng extract upon attention, cognitive processing, integrated sensory motor function, and auditory reaction time in healthy subjects. Cognitive effects of ginseng has become increasingly popular during recent years and some studies have shown its enhancing effects on learning and memory either in aged and/or brain damaged rodents (Yamaguchi et al., 1996). In humans, Terasawa et al. (1997) have shown that ginseng or ginseng extract had significant effects on neurological and psychiatric symptoms in aged humans and psychomotor functions in healthy subjects, respectively. This positive effect of ginseng on cognition performance is due to the direct action of ginseng on the hippocampus.

Shen and Zhang (2003) suggested that the influence of ginsenoside Rg1 on the proliferating ability of neuronal progenitor cells may serve as an important mechanism underlying its nootropic and anti-aging effects particularly on learning and memory. Moreover, ginsenosides alleviated oxidative stress by scavenging of free radicals, inhibiting of NO production which usually accompanies glutamate excitotoxicity, inducing superoxide dismutase (SOD1) and catalase genes and reducing lipid peroxidation (Chu & Chen, 1990). Anti-inflammatory activities of ginseng are due to its inhibitory activity on cytokine production such as IL-1β, IL-6, and TNF-α and it abrogates cyclooxygenase-2 gene expression. Multipotent pharmacological activity of this ginseng indicates that this herb may have multiple beneficial effects in AD patients.

Salvia lavandulaefolia and Salvia officinalis (Labiatae)

Research into historical literature has revealed that some activities of sage, particularly its reputation as being good for the memory, may be relevant to AD treatment (Perry et al., 1998). Thus, in the late 16th century, English herbalist, Gerard writes about sage ‘It is singularly good for the head and brain and quicken the nerves and memory’. Half a century later, Nicholas Culpeper claimed that the ‘herb heals the the memory, warming and quickening the senses’ whilst Hill in 1756 poignantly encapsulates the tragic effects associated with ageing by stating ‘Sage will retard that rapid progress of decay that treads upon our heels so fast in later years of life, will preserve faculty and memory more valuable to the rational mind than life itself’ (Perry et al., 2001). The ancient reports, together with current usage, indicates that sage might possess anti-inflammatory properties and also alleviate conditions associated with estrogen imbalance. Newall et al. (1996) suggest that sage might be of possible use in AD. Numerous in vitro and in vivo tests have been employed to monitor the effects of sage extracts on some of the factors associated with AD, or with a reduction in its incidence.

Cholinesterase inhibition in vitro

An ethanol extract, the steam-distilled oil of S. officinalis and the oil of S. lavandulaefolia were investigated for anti-ChE activity. It was found that all three samples gave inhibition of AChE at quite low concentrations (Perry et al., 1996). This activity was of considerable interest since the cholinesterase inhibition shown by the S. lavandulaefolia oil was likely to be due to the cyclic monoterpenes 1,8-cineole and α-pinene, which were shown to inhibit AChE in vitro, with some contribution from other constituents perhaps by acting synergistically (Perry et al., 2001).

Antioxidant activity in vitro

An aqueous methanol extract of S. officinalis dose-dependently inhibited lipid peroxidation (Hohmann et al., 1999). Several antioxidant compounds have been identified in S. officinalis, including caffeic acid, carnosic acid, carnosol, rosmarinic acid (Cuvelier et al., 1996; Wang et al., 2000), salvianolic acids I, K and L and various other phenolic compounds (Lu et al., 2010; Wang et al., 1999). Weak antioxidant effects were also shown to be present in an ethanol extract of S. lavandulaefolia (when compared with the standard antioxidant 10 µM propyl gallate), and both the water and chloroform-soluble fractions of this extract had similar activity (Perry et al., 2010). Some of the individual components of the volatile oil from S. lavandulaefolia have been investigated and antioxidant effects were observed with 1-8-cineole, α- and β-pinene, but a prooxidant effect was given by camphor, a relatively major component of the oil (Perry et al., 2001). Given that the monoterpenoids with antioxidant activity in this study were present in the essential oil at a slightly higher relative percentage (collectively over 30%) than camphor (27% of essential oil), the prooxidant activity of camphor may not have its effect in the whole essential oil. Further investigations are necessary, in particular in vivo, before characterizing any of the extracts or oil constituents as antioxidant or prooxidant.

Effect of extract on eicosanoid synthesis in vitro

An ethanol extract of S. lavandulaefolia showed only weak inhibition of eicosanoid synthesis, giving only about 10% inhibition of thromboxane-B₂ (TXB₂) synthesis but more inhibition (60%) of leukotriene B₄ (LTB₄) synthesis (Perry et al., 2001). The essential oil constituents of S. lavandulaefolia were evaluated for anti-inflammatory effects. S. lavandulaefolia oil constituent α-pinene (comprising 5% of the essential oil) exhibited significant activity (52% inhibition at 200 µM) and showed weak selectivity for inhibition of LTB₄ generation. LTB₄ is produced via the enzyme 5-lipoxygenase (5-LOX), the gene of which is up regulated during neurodegeneration and although the role of this inflammatory mediator in AD is not entirely apparent, selective inhibition over cyclooxygenase (COX) may be relevant therapeutically (Sugaya et al., 2000).

Estrogenic activity in vitro

Dose-dependent estrogenic activity was present in the ethanol extract (which appeared to be concentrated in the water-soluble fraction) of S. lavandulaefolia (Perry et al., 2001). Geraniol (0.1–2 mM; 1% of essential oil), a monoterpenoid constituent of S. lavandulaefolia essential oil, exhibited estrogenic activity (Perry et al., 2001). The potential estrogenic activity of sage extracts and essential oil constituents require further investigation.

In vivo and clinical studies

Oral administration of S. lavandulaefolia essential oil (in a standard dose of sunflower oil) once daily for 5 days to rats, decreased striatal AChE activity with doses of 20 µL; at doses of 50 µL, a decrease in AChE activity was observed in both the striatum and the hippocampus compared with the control treated rats (administered sunflower oil alone (Perry et al., 2001). Thus, it is apparent that following oral administration, one or more constituents of the S. lavandulaefolia oil, or their metabolites, reach the brain (crossing the gastrointestinal and blood–brain barriers) and inhibit AChE in select brain areas, consistent with evidence of inhibition of the brain enzyme in vitro (Perry et al., 1996). The effect of sage in healthy volunteers was studied. Results represent the first systematic evidence that Salvia is capable of acute modulation of mood and cognition in healthy young adults (Tildesley et al., 2005).

Salvia miltiorrhiza

Dried root of S. miltiorrhiza was used in folk medicine for the management of blood disorders and in TCM to stabilize the heart and calm nerves (Huang, 1993). Traditionally, root extract was used for the treatment of blood circulation disorders, insomnia, neurasthenia and alleviation of inflammation (Tang & Eisenbrand, 1992). S. miltiorrhiza, root has been implicated in attenuating dysfunction of vasoactive intestinal peptide (VIP), a neuropeptide distributed within the gastrointestinal tract and CNS, which may participate in the changes that occur in cerebral ischaemia (Kuang et al., 1989). Distribution abnormalities of the neuropeptide substance P have also been associated with some CNS disorders, including AD. Decreased levels of substance P have been suggested as a consequence of neuronal damage following cerebral ischaemia; S. miltiorrhiza root has been implicated in protecting neurons from ischaemia (Kuang et al., 1991), and so may actively protect against cerebral ischaemia and perhaps other CNS disorders via this mechanism.

Other beneficial effects of S. miltiorrhiza root against cerebral ischaemia have also been

explored. S. miltiorrhiza root may inhibit neuronal cell death by inhibition of presynaptic glutamate release (Kuang & Xiang, 1994), and it has been suggested that inhibition of nitric oxide (NO) formation may also explain CNS protective effects observed with S. miltiorrhiza root (Kuang et al., 1996). Further investigations indicate S. miltiorrhiza root may modify ischemic cell changes by modulating somatostatin, a CNS neuropeptide that has been implicated in learning and memory (Kuang et al., 1993). S. miltiorrhiza root may offer an additional therapeutic approach to management of stroke and ischemia. Reperfusion to aid recovery of ischemia can cause further brain damage. During reperfusion, metabolism of free fatty acids from the breakdown of lipid membranes during ischemia has been proposed to generate oxygen free radicals leading to further brain injury. S. miltiorrhiza root has been shown to offer protection against this process by reducing lipid peroxidation (Zhao et al., 1996). The antioxidant effects of S. miltiorrhiza root have been studied and several compounds have been identified with significant antioxidant activity useful in AD therapy.

‘Salvia compositus’, an herbal mixture of the Chinese herbs S. miltiorrhiza and Delbergia odorifera, has been used traditionally for management of coronary heart disease (Fan et al., 1979). Investigations suggest this herbal remedy has a potential role in the antioxidation of lipids (Zhang et al., 1994a), and in amelioration of cerebral edema (Kuang et al., 1995), providing further evidence for therapeutic advantage in cerebral disorders. S. compositus has shown effects on electrical activities of the cerebral cortex, showing a CNS depressant action (Fan et al., 1979). There are also reports of S. miltiorrhiza root being analgesic and sedative (Huang, 1993; Chang & But, 1987).

Rosmariquinone and perhaps other quinones from S. miltiorrhiza may explain the tranquillizing effects observed, and could be developed as anxiolytic agents for managing the behavioral disturbances often observed in AD patients. Tanshinones, isolated from S. miltiorrhiza root, have demonstrated anti-inflammatory activity in mice and were active against 5-LOX in porcine leukocytes, but were not as active as the crude extracts (Chang & But, 1986; Paulus & Bauer, 2000). The tanshinones are also reported to show weak estrogenic activity (Chang & But, 1986). These activities require further investigation for confirmation, but could be relevant in AD therapy. Other studies have reported Salvia species to have pharmacological activities, or reputed activities in traditional medicine that may be relevant in the treatment of various CNS disorders. For example, S. guaranitica was used by Amazonian Indians for its sedative action; this use may be explained by the constituents cirsiliol and caffeic acid ethyl ester, which are benzodiazepine receptor ligands (Marder et al., 1996).

Withania somnifera L. (Solanaceae)

Withania somnifera root (ashwagandha) is one of the most highly regarded herbs in Ayurvedic medicine and its use date back almost 4000 years. It is classified among the rejuvenative tonics (‘Rasayanas’), which was emphasized by the Ayurvedic scholar Charaka (10 BC) who wrote about W. somnifera, ‘One obtains longevity, regains youth, gets a sharp memory and intellect and freedom from diseases, gets a lustrous complexion, and strength of a horse’ (Upton, 2000).

The herb is also traditionally used to treat inflammatory conditions, such as arthritis. Nicotine is reported to be present in W. somnifera root (Kapoor, 1990), which is considered to be associated with cognitive enhancement and protection against AD development (Newhouse & Kelton, 2000). W. somnifera root extract containing the sitoindosides VII-X and withaferin A was administered to mice and effects on the neurotransmitter systems in the brain were observed. The results showed that the extract enhanced AChE activity in the lateral septum and globus pallidus and decreased AChE activity in the vertical diagonal band, enhanced muscarinic M1 receptor binding in the lateral and medial septum and in the frontal cortices, and increased muscarinic M2 receptor binding sites in cortical regions (Schliebs et al., 1997).

The extract containing the sitoindosides VII-X and withaferin A also reversed the ibotenic acid-induced cognitive deficit and reversed the reduction in cholinergic markers (e.g., ACh, ChAT) in rats (Bhattacharya & Kumar, 1995). The reputed cognition enhancing effects of W. somnifera root may be explained by a preferential action on cholinergic neurotransmission in the cortical and basal forebrain, brain areas involved in cognitive function. These observations indicate that the sitoindosides VII-X and withaferin A could have potential in AD therapy. The root extract of W. somnifera also reversed scopolamine-induced disruption of acquisition and attention, and attenuated amnesia following electroconvulsive shock in mice, effects that are possibly attributed to the root extract having nootropic activity (Dhuley, 2001). A methanol extract of W. somnifera root dose-dependently promoted dendrite formation in human neuroblastoma cells in vitro (Tohda et al., 2000). If this effect occurred in the CNS, treatment of AD patients with the root extract may promote synaptic formation, which involves neurite outgrowth; thus, cholinergic function may be enhanced.

A root preparation of W. somnifera has been shown to significantly reduce the number of degenerating cells in the hippocampal brain region of stressed rats, which indicates the root extract may have potential neuroprotective effects in neurodegenerative disorders (Jain et al., 2001). The glycowithanolides showed anxiolytic and antidepressant activities in rats (Bhattacharya et al., 2000c), which may be applicable in the symptomatic treatment of AD. An alkaloid extract also had tranquillizing effects in vivo and potentiated barbiturate-, ethanol- and urethane-induced hypnosis in mice (Malhotra et al., 1965). The pharmacological basis for these observations is unknown, but could reflect the reported GABA-mimetic effect of W. somnifera extract (Mehta et al., 1991).

W. somnifera root and some constituents are also reported to have antioxidant and anti-

inflammatory activities, which may also be relevant in AD therapy. The root extract and the glycowithanolides (consisting of equimolar concentrations of sitoindosides VII-X and withaferin A) are hepatoprotective in rats and mice, an effect attributed to the antioxidant activity against hepatic lipid peroxidation (Bhattacharya et al., 2000a; Chaurasia et al., 2000). The glycowithanolides decreased lipid peroxidation in various tissues including the brain in rodents, and both the glycowithanolides and the sitoindosides (VII-X) enhanced catalase and glutathione peroxidase activities in rat frontal cortex and striatum (Bhattacharya et al., 1997, 2001; Chaurasia et al., 2000). W. somnifera administration to mice treated with a carcinogen reduced IL-1 and TNF-α levels (Dhuley, 1997), which may also be relevant in AD treatment, considering the possible involvement of these inflammatory mediators in senile plaque formation and neurodegeneration. W. somnifera leaves are also reported to have anti-inflammatory activity (Sudhir et al., 1986). The numerous pharmacological activities of W. somnifera root indicate that this herb may have multiple beneficial effects in AD patients.

Angelica archangelica L. (Apiaceae)

A. archangelica has been used in TCM for cerebral diseases (Hoss et al., 2001). The crude alcohol extract of Angelica archangelica displaced nicotine binding to nicotine receptors in a concentration-dependent manner (Perry et al., 1996) and inhibited AChE activity in vitro. It also enhances cerebral blood flow (Ross, 2001).

Biota orientalis L. Franco. (Cupressaceae)

It is used in TCM for insomnia and amnesia. An herbal prescription composed of Biota orientalis, Panx ginseng and Schisandra chinensis preferentially improved memory registration and consolidation in mice (Nishiyama et al., 1995b). B. orientalis seed extract ameliorated the memory acquisition disorders induced by amygdale and also basal forebrain lesions in mice (Nishiyama et al., 1992, 1995b).

Codonopsis pilulosa var. (Campanulaceae)

Codonopsis pilulosa root is used in TCM for the treatment of various disorders including amnesia and promotes blood circulation. An extract reduced the impairment of memory acquisition in vivo (Wang et al., 1998) and has shown nootropic effects (Zhu & Liu, 1990).

Crocus sativus (saffron)

Saffron (Iridaceae) is the world’s most expensive spice and apart from its traditional value as a food additive, recent studies indicate its potential as an anticancer agent and memory enhancer. The value of saffron is determined by the existence of three main secondary metabolites: crocin and its derivatives, which are responsible for color; picrocrocin, responsible for taste; and safranal, responsible for odor. Saffron is used for depression in Persian traditional medicine. Crocin improved ethanol-induced impairment of learning behavior in mice (Abe & Saito, 2000). Crocin suppressed TNF-α induced apoptosis of neuronally differentiated PC12 cells in vitro (Soeda et al., 2001).

Magnolia officinalis

The bark and stem of Magnolia officinalis has been used in TCM to treat anxiety and nervous disturbances; the anxiolytic effects of honokiol (Kuribara et al., 1999, 2000) and mangnolol have been attributed to their ability to potentiate GABAergic. Both honokiol and magnolol increased ChAT activity and inhibited AChE activity in vitro and increased hippocampal ACh release in vivo (Hou et al., 2001). Both the compounds are reported to have antioxidant activity. Magnolol protected neurons against chemical hypoxic damage or necrotic cell death in cortical neuron-astrocyte cultures in vivo. Magnolol also showed antinflammatory activity in vitro and in vivo via inhibition of COX and 5-LOX (Wang et al., 1995).

Allium sativum L. (Liliaceae)

Allium sativum, commonly known as garlic, ranks highly among foods that help to prevent disease, largely due to its high content of organosulfur compounds and antioxidant activity. The highly standardized AGE is produced by extraction with water and ageing of organic fresh garlic, at room temperature (37°C), for 20 months. The process increases antioxidant levels well above those of the fresh bulb and converts harsh unstable compounds, such as allicin to stable health-promoting substances. AGE contains mostly stable water-soluble organosulfur compounds S-allyl mercaptocysteine, some oil soluble organosulfur compounds, flavonoids, a phenolic compound (allixin) and other beneficial nutrients, including selenium (Borek, 2001). AGE has potential to protect the brain against neurodegenerative conditions (Numagami et al., 1996), by preventing brain injury following ischemia, protecting neuronal cells against apoptosis, by inhibiting caspase and preventing Aβ induced oxidative death (Griffin, 1998). Preclinical studies in models that are genetically prone to early aging show that AGE has additional anti-aging effects (Moriguchi, et al., 1997). Treatment with AGE or S-allyl cysteine prevented the degeneration of the brain’s frontal lobe, improved learning and memory retention and extended lifespan. Isolated neurons from the hippocampus area, grown in the presence of AGE or S-allyl cysteine showed an unusual ability to grow and branch, which may be linked to the findings that AGE increases learning and cognition (Moriguchi et al., 1996).

Conclusions

It is apparent that a variety of plants show, or have the potential to show, numerous activities that may be relevant to the treatment of neurodegenerative disorders such as AD. The majority of studies have focused on ChE inhibitors. This is perhaps a reflection of the relative success of the use of AChE inhibitors in AD patients, senile dementia, ataxia, myasthenia gravis and Parkinson’s disease. Although some plants, such as Ginkgo biloba and Withania somnifera, have shown beneficial effects on cognitive function, further studies regarding the compounds responsible for activity are necessary, to identify which compounds are responsible for the pharmacological activities observed, or if compounds act synergistically to enhance activity. For many of the plants and compounds that have demonstrated activities relevant to AD therapy, clinical data are very limited. Clinical efficacy and potential toxicity of active plants and compounds in larger trials requires further assessment before recommendations concerning their routine use can be identified. Future trends could involve the use of a polyvalent ‘cocktail’ of drugs which act in different ways by mechanisms such as antioxidant and anti-inflammatory activity and the inhibition of the formation of fibrillary tangles and β amyloid plaques. Although the introduction of l-DOPA and other dopaminergic compounds over the last three decades has improved the condition of many suffering of PD, the side effects and their unpredictability of occurrence highlight the fact that more work is needed. In this context also, the exploitation of compounds derived from plants with other mechanisms, such as monoamine oxidase inhibition, may provide a better treatment experience in due course. The ethnopharmacological approach for selecting plants with multipotent activities to investigate for the treatment of neurodegenerative disorders is a relatively successful method for the identification of plants and compounds that may be exploited for use therapeutically in neurodegenerative and other cognitive disorders.

Declaration of interest

KPD wishes to thanks UGC, India for financial assistance and NS wishes to thanks CSIR, India for providing a Senior Research Fellowship. The authors gratefully acknowledge the computational and bioinformatics facility provided by the Alagappa University Bioinformatics Infrastructure Facility (funded by Department of Biotechnology, Government of India; Grant No. BT/BI/25/001/2006). The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.