Preoptic bombesin-like receptor-3 neurons heat it up

Comment on: Piñol RA, Mogul AS, Hadley CK, et al. Preoptic BRS3 neurons increase body temperature and heart rate via multiple pathways. Cell Metab. 2021;33(7):1389–1403.e6.

The preoptic area and BRS3

A defining characteristic of endotherms, including mammals, is a warm, highly regulated, and stable core body temperature (Tb). Identifying the network of neurons controlling Tb is essential for understanding this fundamental physiology. The preoptic area (POA) is a brain region that receives afferent and local Tb sensory information and harbors efferent neurons of autonomic and behavioral thermoregulatory pathways [1]. These pathways contribute to thermoregulatory behavior, shivering and non-shivering thermogenesis, cutaneous vasomotion and cardiovascular responses. Researchers have identified POA neuronal populations in mice that reduce Tb when activated, regulating heat defense, torpor, and thermoregulation during sleep. A dozen such populations are marked by the expression of genes encoding enzymes, neuropeptides, and/or receptors and are predominantly glutamatergic. Future studies will need to better characterize these heterogenous neuronal populations, uncovering overlaps and defining subpopulations with more precise roles. A POA population that increases Tb when activated has been proposed and likely uses glutamatergic projections to the dorsomedial hypothalamus (DMH) [2]. We have now identified POA neurons expressing bombesin-like receptor-3 (POA^BRS3) as the first defined, specific population whose activation increases Tb [3]. This is driven by non-shivering thermogenesis through brown adipose tissue (BAT) activation, with concomitant increases in heart rate and blood pressure.

BRS3 is an orphan G protein-coupled receptor in mammals, although both gastrin-releasing peptide and neuromedin B are endogenous ligands in some other vertebrates. Brs3 is expressed in limited hypothalamic and other brain regions and in some peripheral cells. Mice with germline ablation of Brs3 become obese and have a lower light phase resting Tb and resting heart rate. Ablation of Brs3 in glutamatergic neurons caused a similar obesity and Tb phenotype as the global Brs3 null mice, and re-expression of Brs3 only in glutamatergic neurons reversed the null phenotype. Thus, BRS3 function, during and/or after glutamatergic neuronal development, has a role in regulating Tb and other aspects of energy homeostasis.

More recently, we used expression of BRS3 as a marker for identification of and genetic access to specific neuronal populations. Ablation or silencing of DMH^BRS3 or POA^BRS3 neurons did not produce obesity or affect mean light phase Tb [3,4], indicating that BRS3 functions may be redundant or dispersed among multiple neuronal populations. While POA^BRS3 neurons may not affect mean light phase Tb, we did find impairments (described below) in adaptions to changing ambient temperatures and larger Tb swings in mice with this population silenced.

A cold responsive POA^BRS3 population

The POA is a heterogenous region containing at least 70 classes of neurons [5], including one glutamatergic and five GABAergic POA^BRS3 clusters. However, two of the GABAergic POA^BRS3 clusters express markers suggesting that they are from anatomically adjacent areas (bed nucleus of the stria terminalis and periventricular nucleus). Only ~3% of the POA neurons express BRS3 and glutamatergic POA^BRS3 neurons are only ~0.2% of the total [3,5]. For perspective, about 4/5^th of the POA neuron population is GABAergic and only 1/5^th is glutamatergic. The POA^BRS3 neurons project widely, including to the paraventricular nucleus of the thalamus, paraventricular nucleus of the hypothalamus, DMH, periaqueductal gray, and raphe pallidus, and some of these projections are involved in the regulation of Tb, heart rate, and blood pressure. Future investigations, examining patterns of gene expression, afferent inputs, axonal projections, and/or functional assays will determine how these POA^BRS3 neuron clusters can be subdivided further.

We speculate that the glutamatergic, not GABAergic, neurons mediate the thermogenic POA^BRS3 effects for two reasons. First, BRS3 in glutamatergic, but not in GABAergic, neurons is necessary and sufficient for BRS3ʹs metabolic effects. Second, one of the pathways that POA^BRS3 neurons use to increase Tb is to the DMH. Glutamate is the likely neurotransmitter in a POA→DMH pathway mediating cold-evoked BAT activation [2].

The POA thermoregulation system receives sensory information (both of warm and cold) notably via the brain stem lateral parabrachial nucleus (see Figure 1) and is involved in behavioral and autonomic thermoeffector responses. Adaptations to cold exposure are behavioral (eg., nesting, huddling) and physiologic (eg., cutaneous vasoconstriction, piloerection, thermogenesis through BAT activation or shivering). Despite the POA’s role in thermoregulation, its ablation in rats or loss of glutamatergic or GABAergic neuron function in mice does not affect mean Tb at 22°C, which is a cold environment for small mammals [6]. Further acute exposure to 4°C produced differing results, depending on the research group, possibly due to the extent of the lesion. Similarly, silencing POA^BRS3 neurons did not cause changes in mean light phase Tb at 22°C or upon acute switch to colder temperatures for 4 hours. However, POA^BRS3 neurons are cold responsive with ~40% of median preoptic area BRS3 neurons expressing FOS (a marker for neuronal activation) after a 4 hour exposure to cold. In addition, acute activation of POA^BRS3 neurons increases Tb. POA^BRS3 neurons contribute to Tb regulation at a longer time scale. In mice housed at 30°C for ≥3 days, silencing of POA^BRS3 neurons impaired acute adaptation to a colder environment (22°C) (leading to a lower Tb and energy expenditure) for a duration of ~2 days, after which they maintained Tb well. Therefore, one contribution of POA^BRS3 neurons is acute response to cold after long-term adaptation to warm ambient temperatures.

Figure 1. Preoptic BRS3 neurons in central thermoregulatory pathways. Selected Tb regulation pathways [1] are shown in green and functional projections of POA^BRS3 and DMH^BRS3 neurons [3,4] in purple. BRS3 and non-BRS3 pathways may be excitatory, inhibitory, or both, and may include parallel circuits carrying discrete information. For visual clarity, non-BRS3 pathways parallel to BRS3 pathways are not depicted.

Abbreviation: BAT: brown adipose tissue; DH: dorsal horn; DMH: dorsomedial hypothalamus, IML: intermediolateral column of the spinal cord; LPB: lateral parabrachial nucleus; PAG: periaqueductal grey; POA: preoptic area; PVH: paraventricular nucleus of the hypothalamus; RPa: raphe pallidus; SG: sympathetic ganglion; Tb: core body temperature

Further POA^BRS3 neuron fine tuning of Tb and possible redundant pathways

In addition to a role in cold defense, POA^BRS3 neurons contribute to fine-tuning Tb regulation. Specifically, when POA^BRS3 neurons were constitutively silenced, the mice had a larger Tb range: a lower Tb with interventions that lowered Tb (fasting-induced hypothermia and cold ambient temperature) and a higher Tb with interventions that increased Tb (hot ambient temperature, and non-significantly in dark phase, handling, lipopolysaccharide treatment, and BRS3 agonist treatment). Multiple models could explain this observation, including loss or reduction of sensory input or of integration of sensory input. It is not known if the Tb overshoot/undershoot is regulated by the same POA^BRS3 neuron classes that activate BAT, increase heart rate, raise and/or regulate Tb on an extended time scale. Another possibility is that BRS3 is expressed in some fraction of the populations that regulate the warm response. Irrespective of the explanation for our observations, the most likely way for POA^BRS3 neurons to contribute to the fine-tuning is through regulation of BAT thermogenesis, as activating POA^BRS3 neurons does not increase tail temperature.

In addition to the POA, many other nuclei form part of the thermoregulatory network. We found that cold defense dorsal DMH^BRS3→RPa neurons receive robust input from neurons in the LPB [4]. Others have reported that an LPB→DMH pathway could be involved in Tb regulation in the absence of rostral input (e.g., from POA) to DMH in certain situations (e.g., torpor). This suggests that redundant pathways, which may bypass the POA, contribute to Tb regulation. Acute chemogenetic inhibition of POA^BRS3 neurons decreases Tb by just 1.5°C, which strengthens our and others’ observation [6] that maintaining Tb in a cool environment does not readily rely on POA populations. This raises the possibility that redundant pathways, bypassing POA, are important for maintaining Tb in a cool environment.

Fitting with cold response pathways bypassing POA is the speculation that the contribution of POA populations to thermoregulation is skewed toward heat defense and torpor promotion rather than to cold defense. To date, the thermoeffector functions of most identified POA neuron populations focus on dissipating heat and inhibiting BAT. We have confirmed this by studying the effect of global POA activation. This may explain why a cold defense thermogenic population in the POA has not been identified earlier, versus the multiple identified populations that decrease Tb when activated. The overall contribution of POA neuronal populations of thermoeffectors in review diagrams is often equally represented between responses to cold sensory and warm sensory input. However, it could be speculated that on balance more preoptic thermoeffectors weigh toward cooling off. In contrast, BRS3 neurons can increase Tb. It remains to be discovered what other populations can also increase Tb as a role in cold defense.

The POA^BRS3 neurons are involved in the thermoregulatory functions of BAT and the cardiovascular system. Since the discovery of functional BAT in humans, there is increased interest in central pathways that regulate BAT. While infants rely on BAT for thermogenesis, later in life there is little BAT thermogenesis in most people. BAT activation is potentially beneficial in obese patients, since it can improve glucose regulation and cardiovascular health, in addition to increasing energy expenditure. Therefore, BAT activation is a conceivable therapy in patients with obesity or diabetes. Hence, detailed knowledge of central circuits of BAT and Tb regulation in mice contributes to better understanding our own physiology in health and disease. The newly identified functions of the POA^BRS3 neurons provide a glimpse into the complexity of neural circuitry network controlling Tb and BAT. It is exciting to apply modern tools of neurobiology to study this ancient, conserved physiology. These insights will contribute to the understanding of homeothermy, a defining characteristic of mammalian biology.