Circadian Rhythms in Neurotransmitter Receptors in Discrete Rat Brain Regions

Abstract

Circadian rhythms were measured in α₁ -, α₂ and β-adrenergic, acetylcholine muscarinic (ACh), and benzodiazepine (BDZ) receptor binding in small regions of rat brain. Rhythms α₁-receptor binding were measured in olfactory bulb, frontal, cingulate, piriform, parietal, temporal and occipital cortex, hypothalamus, hippocampus, pons-medulla, caudate-putamen and thalamus-septum. No rhythm was found in cerebellum. Rhythms in α₂-receptor binding were measured in frontal, parietal and temporal cortex, and pons-modulla. No rhythm was found in cingulate, piriform or occipital cortex, or hypothalamus. Rhythms in binding to β-receptors were measured in olfactory bulb, piriform, insular, parietal and temporal cortex, hypothalamus and cerebellum. No rhythms were found in frontal, entorhinal, cingulate, or occipital cortex, hippocampus, caudate-putamen, or pons-medulla. Rhythms in ACh receptor binding were measured in olfactory bulb, parietal cortex and caudate-putamen. No rhythms were found in frontal or occipital cortex, nucleus accumbens, hippocampus, thalamus-septum, pons-medulla or cerebellum. Rhythms in BDZ receptor binding were measured in olfactory bulb, olfactory and occipital cortex, olfactory tubercle, nucleus accumbens, amygdala, caudate-putamen, hippocampus and cerebellum. No rhythms were found in parietal cortex, pons-medulla or thalamus-septum. The 24-hr mean binding to receptors varied between 3- and 10-fold, the highest in cortex and the lowest, usually, in cerebellum. The piriform cortex was particularly high in α₁- and α₂-adrenergic receptors; the nucleus accumbens and caudate, in ACh receptors; and the amygdala, in BDZ receptors. Most adrenergic and ACh receptor rhythms peaked in subjective night (the period when lights were off under L: D conditions), whereas most BDZ receptor rhythms peaked in subjective day (the time lights were on in L: D). Perhaps in the rat, a nocturnal animal, the adrenergic and ACh receptors mediate activity and the functions that accompany it, and the BDZ receptors mediate rest, and with it, sleep.

Key Words:

• Circadian rhythm
• adrenergic
• cholinergic
• benzodia zepine receptors