Endogenous activation of spinal 5-hydroxytryptamine (5-HT) receptors contributes to the thermoregulatory activation of brown adipose tissue

Brown Adipose Tissue and Novel Management Strategies for Polycystic Ovary Syndrome Therapy

Brown adipose tissue (BAT), a unique tissue, plays a key role in metabolism and energy expenditure through adaptive nonshivering thermogenesis. It has recently become a therapeutic target in the treatment of obesity and metabolic diseases. The thermogenic effect of BAT occurs through uncoupling protein-1 by uncoupling adenosine triphosphate (ATP) synthesis from energy substrate oxidation. The review discusses the recent developments and progress associated with the biology, function, and activation of BAT, with a focus on its therapeutic potential for the treatment of polycystic ovary syndrome (PCOS). The endocrine activity of brown adipocytes affects the energy balance and homeostasis of glucose and lipids, thereby affecting the association of BAT activity and the metabolic profile. PCOS is a complex reproductive and metabolic disorder of reproductive-age women. Functional abnormalities of adipose tissue (AT) have been reported in patients with PCOS. Numerous studies have shown that BAT could regulate the features of PCOS and that increases in BAT mass or activity were effective in the treatment of PCOS through approaches including cold stimulation, BAT transplantation and compound activation in various animal models. Therefore, BAT may be used as a novel management strategy for the patients with PCOS to improve women's health clinically. It is highly important to identify key brown adipokines for the discovery and development of novel candidates to establish an efficacious therapeutic strategy for patients with PCOS in the future.

5-HT Receptors and Temperature Homeostasis

The present review summarizes the data concerning the influence of serotonin (5-HT) receptors on body temperature in warm-blooded animals and on processes associated with its maintenance. This review includes the most important part of investigations from the first studies to the latest ones. The established results on the pharmacological activation of 5-HT1A, 5-HT3, 5-HT7 and 5-HT2 receptor types are discussed. Such activation of the first 3 type of receptors causes a decrease in body temperature, whereas the 5-HT2 activation causes its increase. Physiological mechanisms leading to changes in body temperature as a result of 5-HT receptors' activation are discussed. In case of 5-HT1A receptor, they include an inhibition of shivering and non-shivering thermogenesis, as well simultaneous increase of peripheral blood flow, i.e., the processes of heat production and heat loss. The physiological processes mediated by 5-HT2 receptor are opposite to those of the 5-HT1A receptor. Mechanisms of 5-HT3 and 5-HT7 receptor participation in these processes are yet to be studied in more detail. Some facts indicating that in natural conditions, without pharmacological impact, these 5-HT receptors are important links in the system of temperature homeostasis, are also discussed.

Serotonergic Regulation of Hepatic Energy Metabolism

The liver is a vital organ that regulates systemic energy metabolism and many physiological functions. Nonalcoholic fatty liver disease (NAFLD) is the commonest cause of chronic liver disease and end-stage liver failure. NAFLD is primarily caused by metabolic disruption of lipid and glucose homeostasis. Serotonin (5-hydroxytryptamine [5-HT]) is a biogenic amine with several functions in both the central and peripheral systems. 5-HT functions as a neurotransmitter in the brain and a hormone in peripheral tissues to regulate systemic energy homeostasis. Several recent studies have proposed various roles of 5-HT in hepatic metabolism and inflammation using tissue-specific knockout mice and 5-HT-receptor agonists/antagonists. This review compiles the most recent research on the relationship between 5-HT and hepatic metabolism, and the role of 5-HT signaling as a potential therapeutic target in NAFLD.

Systemic serotonin inhibits brown adipose tissue sympathetic nerve activity via a GABA input to the dorsomedial hypothalamus, not via 5HT1A receptor activation in raphe pallidus

AIM

Serotonin (5-hydroxytryptamine, 5-HT), an important neurotransmitter and hormone, modulates many physiological functions including body temperature. We investigated neural mechanisms involved in the inhibition of brown adipose tissue (BAT) sympathetic nerve activity (SNA) and BAT thermogenesis evoked by 5-HT.

METHODS

Electrophysiological recordings, intravenous (iv) injections and nanoinjections in the brains of anaesthetized rats.

RESULTS

Cooling-evoked increases in BAT SNA were inhibited by the intra-rostral raphé pallidus (rRPa) and the iv administration of the 5-HT_1A receptor agonist, 8-OH-DPAT or 5-HT. The intra-rRPa 5-HT, the intra-rRPa and the iv 8-OH-DPAT, but not the iv 5-HT-induced inhibition of BAT SNA were prevented by nanoinjection of a 5-HT_1A receptor antagonist in the rRPa. The increase in BAT SNA evoked by nanoinjection of NMDA in the rRPa was not inhibited by iv 5-HT, indicating that iv 5-HT does not inhibit BAT SNA by acting in the rRPa or in the sympathetic pathway distal to the rRPa. In contrast, under a warm condition, blockade of 5HT_1A receptors in the rRPa increased BAT SNA and BAT thermogenesis, suggesting that endogenous 5-HT in the rRPa contributes to the suppression of BAT SNA and BAT thermogenesis. The increases in BAT SNA and BAT thermogenesis evoked by nanoinjection of NMDA in the dorsomedial hypothalamus (DMH) were inhibited by iv 5-HT, but those following bicuculline nanoinjection in the DMH were not inhibited.

CONCLUSIONS

The systemic 5-HT-induced inhibition of BAT SNA requires a GABAergic inhibition of BAT sympathoexcitatory neurones in the DMH. In addition, during warming, 5-HT released endogenously in rRPa inhibits BAT SNA.

Genetic deletion of mast cell serotonin synthesis prevents the development of obesity and insulin resistance

Obesity is linked with insulin resistance and is characterized by excessive accumulation of adipose tissue due to chronic energy imbalance. Increasing thermogenic brown and beige adipose tissue futile cycling may be an important strategy to increase energy expenditure in obesity, however, brown adipose tissue metabolic activity is lower with obesity. Herein, we report that the exposure of mice to thermoneutrality promotes the infiltration of white adipose tissue with mast cells that are highly enriched with tryptophan hydroxylase 1 (Tph1), the rate limiting enzyme regulating peripheral serotonin synthesis. Engraftment of mast cell-deficient mice with Tph1^−/− mast cells or selective mast cell deletion of Tph1 enhances uncoupling protein 1 (Ucp1) expression in white adipose tissue and protects mice from developing obesity and insulin resistance. These data suggest that therapies aimed at inhibiting mast cell Tph1 may represent a therapeutic approach for the treatment of obesity and type 2 diabetes.

Serotonin inhibits adipose tissue thermogenesis. Here the authors show that obese mice housed in thermoneutrality have increased mast cell serotonin synthesis, and that inhibiting this pathway through deletion of mast cell Tph1 increases white adipose tissue browning and protects against diet-induced obesity, insulin resistance and liver steatosis.

Emerging Roles for Serotonin in Regulating Metabolism: New Implications for an Ancient Molecule

Serotonin is a phylogenetically ancient biogenic amine that has played an integral role in maintaining energy homeostasis for billions of years. In mammals, serotonin produced within the central nervous system regulates behavior, suppresses appetite, and promotes energy expenditure by increasing sympathetic drive to brown adipose tissue. In addition to these central circuits, emerging evidence also suggests an important role for peripheral serotonin as a factor that enhances nutrient absorption and storage. Specifically, glucose and fatty acids stimulate the release of serotonin from the duodenum, promoting gut peristalsis and nutrient absorption. Serotonin also enters the bloodstream and interacts with multiple organs, priming the body for energy storage by promoting insulin secretion and de novo lipogenesis in the liver and white adipose tissue, while reducing lipolysis and the metabolic activity of brown and beige adipose tissue. Collectively, peripheral serotonin acts as an endocrine factor to promote the efficient storage of energy by upregulating lipid anabolism. Pharmacological inhibition of serotonin synthesis or signaling in key metabolic tissues are potential drug targets for obesity, type 2 diabetes, and nonalcoholic fatty liver disease (NAFLD).

Central nervous system circuits that control body temperature

Maintenance of mammalian core body temperature within a narrow range is a fundamental homeostatic process to optimize cellular and tissue function, and to improve survival in adverse thermal environments. Body temperature is maintained during a broad range of environmental and physiological challenges by central nervous system circuits that process thermal afferent inputs from the skin and the body core to control the activity of thermoeffectors. These include thermoregulatory behaviors, cutaneous vasomotion (vasoconstriction and, in humans, active vasodilation), thermogenesis (shivering and brown adipose tissue), evaporative heat loss (salivary spreading in rodents, and human sweating). This review provides an overview of the central nervous system circuits for thermoregulatory reflex regulation of thermoeffectors.

Central Mechanisms for Thermoregulation

Maintenance of a homeostatic body core temperature is a critical brain function accomplished by a central neural network. This orchestrates a complex behavioral and autonomic repertoire in response to environmental temperature challenges or declining energy homeostasis and in support of immune responses and many behavioral states. This review summarizes the anatomical, neurotransmitter, and functional relationships within the central neural network that controls the principal thermoeffectors: cutaneous vasoconstriction regulating heat loss and shivering and brown adipose tissue for heat production. The core thermoregulatory network regulating these thermoeffectors consists of parallel but distinct central efferent pathways that share a common peripheral thermal sensory input. Delineating the neural circuit mechanism underlying central thermoregulation provides a useful platform for exploring its functional organization, elucidating the molecular underpinnings of its neuronal interactions, and discovering novel therapeutic approaches to modulating body temperature and energy homeostasis.

Stress-induced hyperthermia and hypothermia

Stress affects core body temperature (T_c). Many kinds of stress induce transient, monophasic hyperthermia, which diminishes gradually if the stressor is terminated. Stronger stressors produce a longer-lasting effect. Repeated/chronic stress induces anticipatory hyperthermia, reduces diurnal changes in T_c, or slightly increases T_c throughout the day. Animals that are exposed to chronic stress or a cold environment exhibit an enhanced hyperthermic response to a novel stress. These changes persist for several days after cessation of stress exposure. In contrast, long-lasting inescapable stress sometimes induces hypothermia. In healthy humans, psychologic stress induces slight increases in T_c, which are within the normal range of T_c or just above it. Some individuals, however, develop extremely high T_c (up to 41°C) when they are exposed to emotional events or show persistent low-grade high T_c (37-38°C) during or after chronic stress situations. In addition to the nature of the stressor itself, such stress-induced thermal responses are modulated by sex, age, ambient temperature, cage mates, past stressful experiences and cold exposure, and coping. Stress-induced hyperthermia is driven by mechanisms distinct from infectious fever, which requires inflammatory mediators. However, both stress and infection activate the dorsomedial hypothalamus-rostral medullary raphe region-sympathetic nerve axis to increase T_c.

Efferent neural pathways for the control of brown adipose tissue thermogenesis and shivering

The fundamental central neural circuits for thermoregulation orchestrate behavioral and autonomic repertoires that maintain body core temperature during thermal challenges that arise from either the ambient or the internal environment. This review summarizes our understanding of the neural pathways within the fundamental thermoregulatory reflex circuitry that comprise the efferent (i.e., beyond thermosensory) control of brown adipose tissue (BAT) and shivering thermogenesis: the motor neuron systems consisting of the BAT sympathetic preganglionic neurons and BAT sympathetic ganglion cells, and the alpha- and gamma-motoneurons; the premotor neurons in the region of the rostral raphe pallidus, and the thermogenesis-promoting neurons in the dorsomedial hypothalamus/dorsal hypothalamic area. Also included are inputs to, and neurochemical modulators of, these efferent neuronal populations that could influence their activity during thermoregulatory responses. Signals of metabolic status can be particularly significant for the energy-hungry thermoeffectors for heat production.

Tonic inhibition of brown adipose tissue sympathetic nerve activity via muscarinic acetylcholine receptors in the rostral raphe pallidus

KEY POINTS

A tonically active, muscarinic cholinergic inhibition of rostral raphe pallidus (rRPa) neurons influences thermogenesis of brown adipose tissue (BAT) independent of ambient temperature conditions. The tonically active cholinergic input to rRPa originates caudal to the hypothalamus. Muscarinic acetylcholine receptor (mAChR) activation in rRPa contributes to the inhibition of BAT sympathetic nerve activity (SNA) evoked by activation of neurons in the rostral ventrolateral medulla (RVLM). The RVLM is not the sole source of the muscarinic cholinergic input to rRPa. Activation of GABA receptors in rRPa does not mediate the cholinergic inhibition of BAT SNA.

ABSTRACT

We sought to determine if body temperature and energy expenditure are influenced by a cholinergic input to neurons in the rostral raphe pallidus (rRPa), the site of sympathetic premotor neurons controlling thermogenesis of brown adipose tissue (BAT). Nanoinjections of the muscarinic acetylcholine receptor (mAChR) agonist, oxotremorine, or the cholinesterase inhibitor, neostigmine (NEOS), in the rRPa of anaesthetized rats decreased cold-evoked BAT sympathetic nerve activity (SNA, nadirs: -72 and -95%), BAT temperature (Tbat, -0.5 and -0.6°C), expired CO₂ (Exp. CO₂ , -0.3 and -0.5%) and heart rate (HR, -22 and -41 bpm). NEOS into rRPa reversed the increase in BAT SNA evoked by blockade of GABA receptors in rRPa. Nanoinjections of the mAChR antagonist, scopolamine (SCOP), in the rRPa of warm rats increased BAT SNA (peak: +1087%), Tbat (+1.8°C), Exp. CO₂ (+0.7%), core temperature (Tcore, +0.5°C) and HR (+54 bpm). SCOP nanoinjections in rRPa produced similar activations of BAT during cold exposure, following a brain transection caudal to the hypothalamus, and during the blockade of glutamate receptors in rRPa. We conclude that a tonically active cholinergic input to the rRPa inhibits BAT SNA via activation of local mAChR. The inhibition of BAT SNA mediated by mAChR in rRPa does not depend on activation of GABA receptors in rRPa. The increase in BAT SNA following mAChR blockade in rRPa does not depend on the activity of neurons in the hypothalamus or on glutamate receptor activation in rRPa.

Serotonergic Control of Metabolic Homeostasis

New treatments are urgently needed to address the current epidemic of obesity and diabetes. Recent studies have highlighted multiple pathways whereby serotonin (5-HT) modulates energy homeostasis, leading to a renewed interest in the identification of 5-HT-based therapies for metabolic disease. This review aims to synthesize pharmacological and genetic studies that have found diverse functions of both central and peripheral 5-HT in the control of food intake, thermogenesis, and glucose and lipid metabolism. We also discuss the potential benefits of targeting the 5-HT system to combat metabolic disease.

Sinoaortic denervation attenuates vasopressin-induced hypothermia and reduction of sympathetic nerve activity innervating brown adipose tissue in rats

It is well known that sympathetic nerve activity innervating brown adipose tissue (BAT sympathetic nerve activity) plays an important role in BAT thermogenesis. We have found that peripheral administration of arginine vasopressin (AVP) induced hypothermia by reduced thermogenesis in BAT. However, little is known about AVP-induced hypothermic response and its relationship with BAT sympathetic nerve activity. Because increases in baroreceptor inputs inhibit peripheral sympathetic nervous activity, we hypothesized that AVP-induced hypothermia is related to baroreceptor reflex suppression of BAT sympathetic nerve activity. To test this hypothesis, Male Sprague-Dawley rats were subjected to sinoaortic denervation or sham denervation, and implanted with radiotelemetry transmitters to assess the effects of peripheral administration of AVP on BAT sympathetic nerve activity, core and BAT temperatures. In sham-operated rats, an intraperitoneal (i.p.) injection of 10µg/kg AVP led to a significant decrease in core and BAT temperatures. However, sinoaortic denervation significantly reduced the fall of core and BAT temperatures induced by AVP, compared with levels in sham-operated rats. AVP (10µg/kg i.p.) rapidly decreased BAT sympathetic nerve activity in control and sham-operated rats, with the greatest levels of suppression occurring at 35min and these lowest levels attained were with 30.6% and 29.24%, respectively. Furthermore, we found that sinoaortic denervation attenuated the suppressive effects of AVP (10µg/kg i.p.) on BAT sympathetic nerve activity. The greatest level of suppression was only 20.8% occurring at 35min after AVP. Therefore, these results indicate that the hypothermic effects of peripheral administration of AVP are partially mediated by the arterial baroreceptor reflex suppression of BAT sympathetic nerve activity and BAT thermogenesis.

Glycinergic inhibition of BAT sympathetic premotor neurons in rostral raphe pallidus

The rostral raphe pallidus (rRPa) contains sympathetic premotor neurons controlling thermogenesis in brown adipose tissue (BAT). We sought to determine whether a tonic activation of glycine_A receptors (Gly_AR) in the rRPa contributes to the inhibitory regulation of BAT sympathetic nerve activity (SNA) and of cardiovascular parameters in anesthetized rats. Nanoinjection of the Gly_AR antagonist, strychnine (STR), into the rRPa of intact rats increased BAT SNA (peak: +495%), BAT temperature (T_BAT, +1.1°C), expired CO₂, (+0.4%), core body temperature (T_CORE, +0.2°C), mean arterial pressure (MAP, +4 mmHg), and heart rate (HR, +57 beats/min). STR into rRPa in rats with a postdorsomedial hypothalamus transection produced similar increases in BAT thermogenic and cardiovascular parameters. Glycine nanoinjection into the rRPa evoked a potent inhibition of the cooling-evoked increases in BAT SNA (nadir: -74%), T_BAT (-0.2°C), T_CORE (-0.2°C), expired CO₂ (-0.2%), MAP (-8 mmHg), and HR (-22 beats/min) but had no effect on the increases in these variables evoked by STR nanoinjection into rRPa. Nanoinjection of GABA into the rRPa inhibited the STR-evoked BAT SNA (nadir: -86%) and reduced the expired CO₂ (-0.4%). Blockade of glutamate receptors in rRPa reduced the STR-evoked increases in BAT SNA (nadir: -61%), T_BAT (-0.5°C), expired CO₂ (-0.3%), MAP (-9 mmHg), and HR (-33 beats/min). We conclude that a tonically active glycinergic input to the rRPa contributes to the inhibitory regulation of the discharge of BAT sympathetic premotor neurons and of BAT thermogenesis and energy expenditure.

Central monoaminergic systems are a site of convergence of signals conveying the experience of exercise to brain circuits involved in cognition and emotional behavior

Physical activity can enhance cognitive function and increase resistance against deleterious effects of stress on mental health. Enhanced cognitive function and stress resistance produced by exercise are conserved among vertebrates, suggesting that ubiquitous mechanisms may underlie beneficial effects of exercise. In the current review, we summarize the beneficial effects of exercise on cognitive function and stress resistance and discuss central and peripheral signaling factors that may be critical for conferring the effects of physical activity to brain circuits involved in cognitive function and stress. Additionally, it is suggested that norepinephrine and serotonin, highly conserved monoamines that are sensitive to exercise and able to modulate behavior in multiple species, could represent a convergence between peripheral and central exercise signals that mediate the beneficial effects of exercise. Finally, we offer the novel hypothesis that thermoregulation during exercise could contribute to the emotional effects of exercise by activating a subset of temperature-sensitive serotonergic neurons in the dorsal raphe nucleus that convey anxiolytic and stress-protective signals to forebrain regions. Throughout the review, we discuss limitations to current approaches and offer strategies for future research in exercise neuroscience.

Central neural control of thermoregulation and brown adipose tissue

Central neural circuits orchestrate the homeostatic repertoire that maintains body temperature during environmental temperature challenges and alters body temperature during the inflammatory response. This review summarizes the experimental underpinnings of our current model of the CNS pathways controlling the principal thermoeffectors for body temperature regulation: cutaneous vasoconstriction controlling heat loss, and shivering and brown adipose tissue for thermogenesis. The activation of these effectors is regulated by parallel but distinct, effector-specific, core efferent pathways within the CNS that share a common peripheral thermal sensory input. Via the lateral parabrachial nucleus, skin thermal afferent input reaches the hypothalamic preoptic area to inhibit warm-sensitive, inhibitory output neurons which control heat production by inhibiting thermogenesis-promoting neurons in the dorsomedial hypothalamus that project to thermogenesis-controlling premotor neurons in the rostral ventromedial medulla, including the raphe pallidus, that descend to provide the excitation of spinal circuits necessary to drive thermogenic thermal effectors. A distinct population of warm-sensitive preoptic neurons controls heat loss through an inhibitory input to raphe pallidus sympathetic premotor neurons controlling cutaneous vasoconstriction. The model proposed for central thermoregulatory control provides a useful platform for further understanding of the functional organization of central thermoregulation and elucidating the hypothalamic circuitry and neurotransmitters involved in body temperature regulation.

Central nervous system regulation of brown adipose tissue

Thermogenesis, the production of heat energy, in brown adipose tissue is a significant component of the homeostatic repertoire to maintain body temperature during the challenge of low environmental temperature in many species from mouse to man and plays a key role in elevating body temperature during the febrile response to infection. The sympathetic neural outflow determining brown adipose tissue (BAT) thermogenesis is regulated by neural networks in the CNS which increase BAT sympathetic nerve activity in response to cutaneous and deep body thermoreceptor signals. Many behavioral states, including wakefulness, immunologic responses, and stress, are characterized by elevations in core body temperature to which central command-driven BAT activation makes a significant contribution. Since energy consumption during BAT thermogenesis involves oxidation of lipid and glucose fuel molecules, the CNS network driving cold-defensive and behavioral state-related BAT activation is strongly influenced by signals reflecting the short- and long-term availability of the fuel molecules essential for BAT metabolism and, in turn, the regulation of BAT thermogenesis in response to metabolic signals can contribute to energy balance, regulation of body adipose stores and glucose utilization. This review summarizes our understanding of the functional organization and neurochemical influences within the CNS networks that modulate the level of BAT sympathetic nerve activity to produce the thermoregulatory and metabolic alterations in BAT thermogenesis and BAT energy expenditure that contribute to overall energy homeostasis and the autonomic support of behavior.

Central serotonergic neurons activate and recruit thermogenic brown and beige fat and regulate glucose and lipid homeostasis

Thermogenic brown and beige adipocytes convert chemical energy to heat by metabolizing glucose and lipids. Serotonin (5-HT) neurons in the CNS are essential for thermoregulation and accordingly may control metabolic activity of thermogenic fat. To test this, we generated mice in which the human diphtheria toxin receptor (DTR) was selectively expressed in central 5-HT neurons. Treatment with diphtheria toxin (DT) eliminated 5-HT neurons and caused loss of thermoregulation, brown adipose tissue (BAT) steatosis, and a >50% decrease in uncoupling protein 1 (Ucp1) expression in BAT and inguinal white adipose tissue (WAT). In parallel, blood glucose increased 3.5-fold, free fatty acids 13.4-fold, and triglycerides 6.5-fold. Similar BAT and beige fat defects occurred in Lmx1b(f/f)ePet1(Cre) mice in which 5-HT neurons fail to develop in utero. We conclude 5-HT neurons play a major role in regulating glucose and lipid homeostasis, in part through recruitment and metabolic activation of brown and beige adipocytes.

Endogenous ways to stimulate brown adipose tissue in humans

Obesity is the result of disequilibrium between energy intake and energy expenditure (EE). Successful long-term weight loss is difficult to achieve with current strategies for the correction of this caloric imbalance. Non-shivering thermogenesis (NST) in brown adipose tissue (BAT) is a possible therapeutic target for the prevention and treatment of obesity and associated metabolic diseases. In recent years, more knowledge about the function and stimulation of bat has been obtained. The sympathetic nervous system (SNS) is currently seen as the main effector for brown fat function. Also, interplay between the thyroid axis and SNS plays an important role in BAT thermogenesis. Almost daily new pathways for the induction of BAT thermogenesis and 'browning' of white adipose tissue (WAT) are identified. Especially the activation of BAT via endogenous pathways has received strong scientific attention. Here we will discuss the relevance of several pathways in activating BAT and their implications for the treatment of obesity. In this review we will focus on the discussion of the most promising endocrine and paracrine pathways to stimulate BAT, by factors and pathways that naturally occur in the human body.

Role of the Oxytocin Receptor Expressed in the Rostral Medullary Raphe in Thermoregulation During Cold Conditions

Recent papers have reported that oxytocin (Oxt) and the oxytocin receptor (Oxtr) may be involved in the regulation of food intake in mammals. We therefore suspected the Oxt/Oxtr system to be involved in energy homeostasis. In previous studies, we found a tendency toward obesity in Oxtr-deficient (Oxtr (-/-)) mice, as well as impaired thermoregulation when these mice were exposed to cold conditions. In the present study, we observed the expression of Oxtr in the rostral medullary raphe (RMR), the brain region known to control thermogenesis in brown adipose tissue (BAT). Through immunohistochemistry, we detected neurons expressing Oxtr and c-Fos in the RMR of mice exposed to cold conditions. Up to 40% of Oxtr-positive neurons in RMR were classified as glutamatergic neurons, as shown by immunostaining using anti-VGLUT3 antibody. In addition, mice with exclusive expression of Oxtr in the RMR were generated by injecting an AAV-Oxtr vector into the RMR region of Oxtr (-/-) mice. We confirmed the recovery of thermoregulatory ability in the manipulated mice during exposure to cold conditions. Moreover, mice with RMR-specific expression of Oxtr lost the typical morphological change in BAT observed in Oxtr (-/-) mice. Additionally, increased expression of the β3-adrenergic receptor gene, Adrb3, was observed in BAT. These results are the first to show the critical role of RMR Oxtr expression in thermoregulation during cold conditions.

Diphtheria toxin treatment of Pet-1-Cre floxed diphtheria toxin receptor mice disrupts thermoregulation without affecting respiratory chemoreception

In genetically-modified Lmx1b(f/f/p) mice, selective deletion of LMX1B in Pet-1 expressing cells leads to failure of embryonic development of serotonin (5-HT) neurons. As adults, these mice have a decreased hypercapnic ventilatory response and abnormal thermoregulation. This mouse model has been valuable in defining the normal role of 5-HT neurons, but it is possible that developmental compensation reduces the severity of observed deficits. Here we studied mice genetically modified to express diphtheria toxin receptors (DTR) on Pet-1 expressing neurons (Pet-1-Cre/floxed DTR or Pet1/DTR mice). These mice developed with a normal complement of 5-HT neurons. As adults, systemic treatment with 2-35μg of diphtheria toxin (DT) reduced the number of tryptophan hydroxylase-immunoreactive (TpOH-ir) neurons in the raphe nuclei and ventrolateral medulla by 80%. There were no effects of DT on minute ventilation (VE) or the ventilatory response to hypercapnia or hypoxia. At an ambient temperature (TA) of 24°C, all Pet1/DTR mice dropped their body temperature (TB) below 35°C after DT treatment, but the latency was shorter in males than females (3.0±0.37 vs. 4.57±0.29days, respectively; p<0.001). One week after DT treatment, mice were challenged by dropping TA from 37°C to 24°C, which caused TB to decrease more in males than in females (29.7±0.31°C vs. 33.0±1.3°C, p<0.01). We conclude that the 20% of 5-HT neurons that remain after DT treatment in Pet1/DTR mice are sufficient to maintain normal baseline breathing and a normal response to CO2, while those affected include some essential for thermoregulation, in males more than females. In comparison to models with deficient embryonic development of 5-HT neurons, acute deletion of 5-HT neurons in adults leads to a greater defect in thermoregulation, suggesting that significant developmental compensation can occur.

Central neural regulation of brown adipose tissue thermogenesis and energy expenditure

Thermogenesis, the production of heat energy, is the specific, neurally regulated, metabolic function of brown adipose tissue (BAT) and contributes to the maintenance of body temperature during cold exposure and to the elevated core temperature during several behavioral states, including wakefulness, the acute phase response (fever), and stress. BAT energy expenditure requires metabolic fuel availability and contributes to energy balance. This review summarizes the functional organization and neurochemical influences within the CNS networks governing the level of BAT sympathetic nerve activity to produce the thermoregulatory and metabolically driven alterations in BAT thermogenesis and energy expenditure that contribute to overall energy homeostasis.

Autonomic regulation of brown adipose tissue thermogenesis in health and disease: potential clinical applications for altering BAT thermogenesis

From mouse to man, brown adipose tissue (BAT) is a significant source of thermogenesis contributing to the maintenance of the body temperature homeostasis during the challenge of low environmental temperature. In rodents, BAT thermogenesis also contributes to the febrile increase in core temperature during the immune response. BAT sympathetic nerve activity controlling BAT thermogenesis is regulated by CNS neural networks which respond reflexively to thermal afferent signals from cutaneous and body core thermoreceptors, as well as to alterations in the discharge of central neurons with intrinsic thermosensitivity. Superimposed on the core thermoregulatory circuit for the activation of BAT thermogenesis, is the permissive, modulatory influence of central neural networks controlling metabolic aspects of energy homeostasis. The recent confirmation of the presence of BAT in human and its function as an energy consuming organ have stimulated interest in the potential for the pharmacological activation of BAT to reduce adiposity in the obese. In contrast, the inhibition of BAT thermogenesis could facilitate the induction of therapeutic hypothermia for fever reduction or to improve outcomes in stroke or cardiac ischemia by reducing infarct size through a lowering of metabolic oxygen demand. This review summarizes the central circuits for the autonomic control of BAT thermogenesis and highlights the potential clinical relevance of the pharmacological inhibition or activation of BAT thermogenesis.

Efferent projections of neuropeptide Y-expressing neurons of the dorsomedial hypothalamus in chronic hyperphagic models

The dorsomedial hypothalamus (DMH) has long been implicated in feeding behavior and thermogenesis. The DMH contains orexigenic neuropeptide Y (NPY) neurons, but the role of these neurons in the control of energy homeostasis is not well understood. NPY expression in the DMH is low under normal conditions in adult rodents but is significantly increased during chronic hyperphagic conditions such as lactation and diet-induced obesity (DIO). To understand better the role of DMH-NPY neurons, we characterized the efferent projections of DMH-NPY neurons using the anterograde tracer biotinylated dextran amine (BDA) in lactating rats and DIO mice. In both models, BDA- and NPY-colabeled fibers were limited mainly to the hypothalamus, including the paraventricular nucleus of the hypothalamus (PVH), lateral hypothalamus/perifornical area (LH/PFA), and anteroventral periventricular nucleus (AVPV). Specifically in lactating rats, BDA-and NPY-colabeled axonal swellings were in close apposition to cocaine- and amphetamine-regulated transcript (CART)-expressing neurons in the PVH and AVPV. Although the DMH neurons project to the rostral raphe pallidus (rRPa), these projections did not contain NPY immunoreactivity in either the lactating rat or the DIO mouse. Instead, the majority of BDA-labeled fibers in the rRPa were orexin positive. Furthermore, DMH-NPY projections were not observed within the nucleus of the solitary tract (NTS), another brainstem site critical for the regulation of sympathetic outflow. The present data suggest that NPY expression in the DMH during chronic hyperphagic conditions plays important roles in feeding behavior and thermogenesis by modulating neuronal functions within the hypothalamus, but not in the brainstem.

Pharmacological strategies for targeting BAT thermogenesis

Biopsies following positron emission tomography coupled to computer tomography (PET-CT) imaging have confirmed the presence of thermogenically active brown adipose tissue (BAT) in adult humans, leading to suggestions that it could be stimulated to treat obesity and its associated morbidities. The mechanisms regulating thermogenesis in BAT are better understood than ever before, and many new hypotheses for increasing the amount of brown fat or its activity are currently being explored. The challenge now is to identify safe ways to manipulate specific aspects of the physiological regulation of thermogenesis, in a manner that will be bioenergetically effective. This review outlines the nature of these regulatory mechanisms both in terms of their cellular specificity and probable effectiveness given the physiological paradigms in which thermogenesis is activated. Similarly, their potential for being targeted by new or existing drugs is discussed, drawing on the known mechanisms of action of various pharmacological agents and some probable limitations that should be considered.

The inhibition of neurons in the central nervous pathways for thermoregulatory cold defense induces a suspended animation state in the rat

The possibility of inducing a suspended animation state similar to natural torpor would be greatly beneficial in medical science, since it would avoid the adverse consequence of the powerful autonomic activation evoked by external cooling. Previous attempts to systemically inhibit metabolism were successful in mice, but practically ineffective in nonhibernators. Here we show that the selective pharmacological inhibition of key neurons in the central pathways for thermoregulatory cold defense is sufficient to induce a suspended animation state, resembling natural torpor, in a nonhibernator. In rats kept at an ambient temperature of 15°C and under continuous darkness, the prolonged inhibition (6 h) of the rostral ventromedial medulla, a key area of the central nervous pathways for thermoregulatory cold defense, by means of repeated microinjections (100 nl) of the GABA(A) agonist muscimol (1 mm), induced the following: (1) a massive cutaneous vasodilation; (2) drastic drops in deep brain temperature (reaching a nadir of 22.44 ± 0.74°C), heart rate (from 440 ± 13 to 207 ± 12 bpm), and electroencephalography (EEG) power; (3) a modest decrease in mean arterial pressure; and (4) a progressive shift of the EEG power spectrum toward slow frequencies. After the hypothermic bout, all animals showed a massive increase in NREM sleep Delta power, similarly to that occurring in natural torpor. No behavioral abnormalities were observed in the days following the treatment. Our results strengthen the potential role of the CNS in the induction of hibernation/torpor, since CNS-driven changes in organ physiology have been shown to be sufficient to induce and maintain a suspended animation state.

Arterial chemoreceptor activation reduces the activity of parapyramidal serotonergic neurons in rats

The parapyramidal (ppy) region targets primarily the intermediolateral cell column and is probably involved in breathing and thermoregulation. In the present study, we tested whether ppy serotonergic neurons respond to activation of central and peripheral chemoreceptors. Bulbospinal ppy neurons (n=30) were recorded extracellularly along with the phrenic nerve activity in urethane/α-chloralose-anesthetized, paralyzed, intact (n=7) or carotid body denervated (n=6) male Wistar rats. In intact animals, most of the ppy neurons were inhibited by hypoxia (n=14 of 19) (8% O2, 30s) (1.5 ± 0.03 vs. control: 2.4 ± 0.2 Hz) or hypercapnia (n=15 of 19) (10% CO2) (1.7 ± 0.1 vs. control: 2.2 ± 0.2 Hz), although some neurons were insensitive to hypoxia (n=3 of 19) or hypercapnia (n=4 of 19). Very few neurons (n=2 of 19) were activated after hypoxia, but not after hypercapnia. In carotid body denervated rats, all the 5HT-ppy neurons (n=11) were insensitive to hypercapnia (2.1 ± 0.1 vs. control: 2.3 ± 0.09 Hz). Biotinamide-labeled cells that were recovered after histochemistry were located in the ppy region. Most labeled cells (90%) showed strong tryptophan hydroxylase immunocytochemical reactivity, indicating that they were serotonergic. The present data reveal that peripheral chemoreceptors reduce the activity of the serotonergic premotor neurons located in the ppy region. It is plausible that the serotonergic neurons of the ppy region could conceivably regulate breathing automaticity and be involved in autonomic regulation.

In a model of Batten disease, palmitoyl protein thioesterase-1 deficiency is associated with brown adipose tissue and thermoregulation abnormalities

Infantile neuronal ceroid lipofuscinosis (INCL) is a fatal neurodegenerative disorder caused by a deficiency of palmitoyl-protein thioesterase-1 (PPT1). We have previously shown that children with INCL have increased risk of hypothermia during anesthesia and that PPT1-deficiency in mice is associated with disruption of adaptive energy metabolism, downregulation of peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α), and mitochondrial dysfunction. Here we hypothesized that Ppt1-knockout mice, a well-studied model of INCL that shows many of the neurologic manifestations of the disease, would recapitulate the thermoregulation impairment observed in children with INCL. We also hypothesized that when exposed to cold, Ppt1-knockout mice would be unable to maintain body temperature as in mice thermogenesis requires upregulation of Pgc-1α and uncoupling protein 1 (Ucp-1) in brown adipose tissue. We found that the Ppt1-KO mice had lower basal body temperature as they aged and developed hypothermia during cold exposure. Surprisingly, this inability to maintain body temperature during cold exposure in Ppt1-KO mice was associated with an adequate upregulation of Pgc-1α and Ucp-1 but with lower levels of sympathetic neurotransmitters in brown adipose tissue. In addition, during baseline conditions, brown adipose tissue of Ppt1-KO mice had less vacuolization (lipid droplets) compared to wild-type animals. After cold stress, wild-type animals had significant decreases whereas Ppt1-KO had insignificant changes in lipid droplets compared with baseline measurements, thus suggesting that Ppt1-KO had less lipolysis in response to cold stress. These results uncover a previously unknown phenotype associated with PPT1 deficiency, that of altered thermoregulation, which is associated with impaired lipolysis and neurotransmitter release to brown adipose tissue during cold exposure. These findings suggest that INCL should be added to the list of neurodegenerative diseases that are linked to alterations in peripheral metabolic processes. In addition, extrapolating these findings clinically, impaired thermoregulation and hypothermia are potential risks in patients with INCL.

Orexin modulates brown adipose tissue thermogenesis

Non-shivering thermogenesis in brown adipose tissue (BAT) plays an important role in thermoregulation. In addition, activations of BAT have important implications for energy homeostasis due to the metabolic consumption of energy reserves entailed in the production of heat in this tissue. In this conceptual overview we describe the role of orexins/hypocretins within the central nervous system in the modulation of thermogenesis in BAT under several physiological conditions. Within this framework, we consider potential neural mechanisms underlying the pathological conditions associated with the absence of the central orexinergic modulation of BAT thermogenesis and energy expenditure. Overall, the experimental basis for our understanding of the role of central orexin in regulating body temperature and energy homeostasis provides an illustrative example that highlights several general principles and caveats that should help to guide future investigations of the neurochemical regulation of thermogenesis and metabolism.

Central control of brown adipose tissue thermogenesis

Thermogenesis, the production of heat energy, is an essential component of the homeostatic repertoire to maintain body temperature during the challenge of low environmental temperature and plays a key role in elevating body temperature during the febrile response to infection. Mitochondrial oxidation in brown adipose tissue (BAT) is a significant source of neurally regulated metabolic heat production in many species from mouse to man. BAT thermogenesis is regulated by neural networks in the central nervous system which responds to feedforward afferent signals from cutaneous and core body thermoreceptors and to feedback signals from brain thermosensitive neurons to activate BAT sympathetic nerve activity. This review summarizes the research leading to a model of the feedforward reflex pathway through which environmental cold stimulates BAT thermogenesis and includes the influence on this thermoregulatory network of the pyrogenic mediator, prostaglandin E(2), to increase body temperature during fever. The cold thermal afferent circuit from cutaneous thermal receptors, through second-order thermosensory neurons in the dorsal horn of the spinal cord ascends to activate neurons in the lateral parabrachial nucleus which drive GABAergic interneurons in the preoptic area (POA) to inhibit warm-sensitive, inhibitory output neurons of the POA. The resulting disinhibition of BAT thermogenesis-promoting neurons in the dorsomedial hypothalamus activates BAT sympathetic premotor neurons in the rostral ventromedial medulla, including the rostral raphe pallidus, which provide excitatory, and possibly disinhibitory, inputs to spinal sympathetic circuits to drive BAT thermogenesis. Other recently recognized central sites influencing BAT thermogenesis and energy expenditure are also described.

Brown adipose tissue: Recent insights into development, metabolic function and therapeutic potential

Obesity is currently a global pandemic, and is associated with increased mortality and co-morbidities including many metabolic diseases. Obesity is characterized by an increase in adipose mass due to increased energy intake, decreased energy expenditure, or both. While white adipose tissue is specialized for energy storage, brown adipose tissue has a high concentration of mitochondria and uniquely expresses uncoupling protein 1, enabling it to be specialized for energy expenditure and thermogenesis. Although brown fat was once considered only necessary in babies, recent morphological and imaging studies have provided evidence that, contrary to prior belief, this tissue is present and active in adult humans. In recent years, the topic of brown adipose tissue has been reinvigorated with many new studies regarding brown adipose tissue differentiation, function and therapeutic promise. This review summarizes the recent advances, discusses the emerging questions and offers perspective on the potential therapeutic applications targeting this tissue.

Impaired ventilatory and thermoregulatory responses to hypoxic stress in newborn phox2b heterozygous knock-out mice

The Phox2b genesis necessary for the development of the autonomic nervous system, and especially, of respiratory neuronal circuits. In the present study, we examined the role of Phox2b in ventilatory and thermoregulatory responses to hypoxic stress, which are closely related in the postnatal period. Hypoxic stress was generated by strong thermal stimulus, combined or not with reduced inspired O(2). To this end, we exposed 6-day-old Phox2b(+/-) pups and their wild-type littermates (Phox2b(+/+)) to hypoxia (10% O(2)) or hypercapnia (8% CO(2)) under thermoneutral (33°C) or cold (26°C) conditions. We found that Phox2b(+/-) pups showed less normoxic ventilation (V(E)) in the cold than Phox2b(+/+) pups. Phox2b(+/-) pups also showed lower oxygen consumption (VO(2)) in the cold, reflecting reduced thermogenesis and a lower body temperature. Furthermore, while the cold depressed ventilatory responses to hypoxia and hypercapnia in both genotype groups, this effect was less pronounced in Phox2b(+/-) pups. Finally, because serotonin (5-HT) neurons are pivotal to respiratory and thermoregulatory circuits and depend on Phox2b for their differentiation, we studied 5-HT metabolism using high pressure liquid chromatography, and found that it was altered in Phox2b(+/-) pups. We conclude that Phox2b haploinsufficiency alters the ability of newborns to cope with metabolic challenges, possibly due to 5-HT signaling impairments.

Central circuitries for body temperature regulation and fever

Body temperature regulation is a fundamental homeostatic function that is governed by the central nervous system in homeothermic animals, including humans. The central thermoregulatory system also functions for host defense from invading pathogens by elevating body core temperature, a response known as fever. Thermoregulation and fever involve a variety of involuntary effector responses, and this review summarizes the current understandings of the central circuitry mechanisms that underlie nonshivering thermogenesis in brown adipose tissue, shivering thermogenesis in skeletal muscles, thermoregulatory cardiac regulation, heat-loss regulation through cutaneous vasomotion, and ACTH release. To defend thermal homeostasis from environmental thermal challenges, feedforward thermosensory information on environmental temperature sensed by skin thermoreceptors ascends through the spinal cord and lateral parabrachial nucleus to the preoptic area (POA). The POA also receives feedback signals from local thermosensitive neurons, as well as pyrogenic signals of prostaglandin E(2) produced in response to infection. These afferent signals are integrated and affect the activity of GABAergic inhibitory projection neurons descending from the POA to the dorsomedial hypothalamus (DMH) or to the rostral medullary raphe region (rMR). Attenuation of the descending inhibition by cooling or pyrogenic signals leads to disinhibition of thermogenic neurons in the DMH and sympathetic and somatic premotor neurons in the rMR, which then drive spinal motor output mechanisms to elicit thermogenesis, tachycardia, and cutaneous vasoconstriction. Warming signals enhance the descending inhibition from the POA to inhibit the motor outputs, resulting in cutaneous vasodilation and inhibited thermogenesis. This central thermoregulatory mechanism also functions for metabolic regulation and stress-induced hyperthermia.

Raphe pallidus modulates Bötzinger complex-induced inhibition of the phrenic nerve activity in rats

The raphe pallidus (RPa) and Bötzinger complex (BötC) represent two important nuclei which project to spinal phrenic motor neurons. Stimulation of the RPa produces facilitative effects on respiratory activity, whereas stimulation of the BötC induces inhibitory effects on respiratory activity. In the present study, we examined the modulatory effects of serotonergic (5-hydroxytryptamine, 5-HT) RPa neurons on the inhibitory response of the phrenic nerve activity elicited from the BötC in rats. Experiments were performed on spontaneously breathing, urethane-anesthetized adult rats. Either high-frequency stimulation or glutamatergic chemical activation of the RPa region significantly attenuated the BötC-induced inhibition of the phrenic nerve. This attenuation showed a post-stimulation time and intensity dependency. Pharmacological experiments showed that intravenous injection of methysergide, a broad-spectrum antagonist of 5-HT receptors, markedly reduced the respiratory facilitation induced by electrical stimulation of the RPa. Furthermore, microinjections of methysergide into the cerebrospinal fluid around the phrenic motor nucleus (PMN) region at spinal cord segments C4 and C5 significantly decreased the RPa-related attenuation effects on BötC-evoked inhibition of phrenic nerve discharge. These results suggest that RPa serotonergic neurons could modulate the inhibition of phrenic nerve activity induced by BötC. Moreover, as the relevant 5-HT receptors for RPa's modulatory effects are located in the cervical spinal cord, 5-HT may, in part, function as a modulator to suppress the BötC neuronal activity via direct RPa-PMN and BötC-PMN convergent projection pathways to phrenic motoneurons.

Brainstem serotonin deficiency in the neonatal period: autonomic dysregulation during mild cold stress

Based on previous studies in adult animals, devoid of 5-HT neurones, showing altered thermoregulation in cold stress (4°C) and a reduced ventilatory response to CO₂, we hypothesized that neonatal mice lacking 60-70% of their 5-HT neurones (Pet-1(-/-)) would have: (1) a reduced thermogenic response to a mild drop in ambient temperature (TA), (2) reduced V(E) and heart rate (HR) responses to mild cooling that reflect this reduced thermogenic response, and (3) a reduced ventilatory response to CO₂ after postnatal day 12 (P12), when 5-HT neurones become chemosensitive in vitro. We first determined that a 60-70% loss of 5-HT-positive neurones results in a ~90% loss of 5-HT from the brainstems of Pet-1(-/-) animals. We then subjected Pet-1(-/-) and wild-type (WT) mice (N = 5) to mild environmental cooling (T(A) = 29°C) at ~P12. T(A) was initially held at 34°C for ~20 min, reduced to 29°C over 15 min and held for an additional 10 min at steady state, and then returned to 34°C. From 34°C to 29°C, there was a robust increase in V(O₂) in P12WT, but not Pet-1(-/-) animals (68±19.9% versus -16±8%, respectively; P = 0.002). On average, body temperature (T(B)) dropped 1.1°C more in Pet-1(-/-) compared to WT animals (P = 0.03). HR remained unchanged in WT but dropped 22±2.3% in Pet-1(-/-) animals (P = 0.01). Genotype had no effect on tail temperature (T(T)), either at 34°C or 29°C. After cooling, values for V(O₂) and HR of Pet-1(-/-) animals were no different to values predicted by Q₁₀ effects alone, while values of WT animals were greater than predicted. V(E) increased in WT with cooling, while it decreased in Pet-1(-/-) animals (P = 0.002). Still, Pet-1(-/-) animals hyperventilated relative to WT (increased V(E)/V(O₂)) irrespective of T(A) (P = 0.002). As tested in a separate group of pups, there was no difference in the ventilatory response to CO₂ between WT and Pet-1(-/-) animals, either at P5 or P15. We conclude that during neonatal life in mouse pups: (1) brainstem 5-HT is critical for the thermogenic response to a mild drop in environmental temperature probably via a sympathetically-mediated increase in brown fat metabolism; (2) reduced thermogenesis probably contributes to the reduced HR and V(O₂) observed with 5-HT deficiency; and (3) the presence of some brainstem 5-HT is sufficient for an appropriate ventilatory response to hypercapnia up until P15. Infants with reduced brainstem 5-HT could be prone to cardiovascular and respiratory abnormalities resulting from compromised thermogenesis.

2010 Carl Ludwig Distinguished Lectureship of the APS Neural Control and Autonomic Regulation Section: Central neural pathways for thermoregulatory cold defense

Central neural circuits orchestrate the homeostatic repertoire to maintain body temperature during environmental temperature challenges and to alter body temperature during the inflammatory response. This review summarizes the research leading to a model representing our current understanding of the neural pathways through which cutaneous thermal receptors alter thermoregulatory effectors: the cutaneous circulation for control of heat loss, and brown adipose tissue, skeletal muscle, and the heart for thermogenesis. The activation of these effectors is regulated by parallel but distinct, effector-specific core efferent pathways within the central nervous system (CNS) that share a common peripheral thermal sensory input. The thermal afferent circuit from cutaneous thermal receptors includes neurons in the spinal dorsal horn projecting to lateral parabrachial nucleus neurons that project to the medial aspect of the preoptic area. Within the preoptic area, warm-sensitive, inhibitory output neurons control heat production by reducing the discharge of thermogenesis-promoting neurons in the dorsomedial hypothalamus. The rostral ventromedial medulla, including the raphe pallidus, receives projections form the dorsomedial hypothalamus and contains spinally projecting premotor neurons that provide the excitatory drive to spinal circuits controlling the activity of thermogenic effectors. A distinct population of warm-sensitive preoptic neurons controls heat loss through an inhibitory input to raphe pallidus sympathetic premotor neurons controlling cutaneous vasoconstriction. The model proposed for central thermoregulatory control provides a platform for further understanding of the functional organization of central thermoregulation.

Ccl22/MDC, is a prostaglandin dependent pyrogen, acting in the anterior hypothalamus to induce hyperthermia via activation of brown adipose tissue

CC Chemokine ligand 22 (Ccl22) is a selective, high affinity ligand at the CC chemokine receptor 4 (Ccr4). We have identified cDNAs encoding both ligand and receptor of the Ccl22-Ccr4 pair in cDNA libraries of the anterior hypothalamus/pre-optic area (AH/POA) by PCR. The AH/POA is the key brain region where endogenous pyrogens have been shown to act on warm sensitive neurons to affect thermogenesis in brown adipose tissue (BAT) and other thermogenically responsive tissues. We show that functional Ccr4 receptors are present in the AH/POA neurons as injection of Ccl22 into the POA but not to other hypothalamic nuclei induces an increase in core body temperature as measured by radiotelemetry. Indomethacin (5 mg/kg s.c) pre-treatment markedly reduced the hyperthermia evoked by POA injection of Ccl22 (10 ng/0.5 ul) and thus suggests that this hyperthermia is mediated through cyclooxygenase activation and thus likely through the formation and action of the pyrogen prostaglandin E2. The temperature elevation involves a decrease in the respiratory exchange ratio and increased activation of the brown adipose tissue as demonstrated by ¹⁸F-FDG-PET imaging. We describe a novel role to the ligand Ccl22 and its receptor Ccr4 in the anterior hypothalamus in temperature regulation that depends on the synthesis of the endogenous pyrogen, prostaglandin E2.

Co-localization of TRHR1 and LepRb receptors on neurons in the hindbrain of the rat

We have reported a highly cooperative interaction between leptin and thyrotropin releasing hormone (TRH) in the hindbrain to generate thermogenic responses (Hermann et al., 2006) (Rogers et al., 2009). Identifying the locus in the hindbrain where leptin and TRH act synergistically to increase thermogenesis will be necessary before we can determine the mechanism(s) by which this interaction occurs. Here, we performed heat-induced epitope recovery techniques and in situ hybridization to determine if neurons or afferent fibers in the hindbrain possess both TRH type 1 receptor and long-form leptin receptor [TRHR1; LepRb, respectively]. LepRb receptors were highly expressed in the solitary nucleus [NST], dorsal motor nucleus of the vagus [DMN] and catecholaminergic neurons of the ventrolateral medulla [VLM]. All neurons that contained LepRb also contained TRHR1. Fibers in the NST and the raphe pallidus [RP] and obscurrus [RO] that possess LepRb receptors were phenotypically identified as glutamatergic type 2 fibers (vglut2). Fibers in the NST and RP that possess TRHR1 receptors were phenotypically identified as serotonergic [i.e., immunopositive for the serotonin transporter; SERT]. Co-localization of LepRb and TRHR1 was not observed on individual fibers in the hindbrain but these two fiber types co-mingle in these nuclei. These anatomical arrangements may provide a basis for the synergy between leptin and TRH to increase thermogenesis.

Inhibition of brown adipose tissue thermogenesis by neurons in the ventrolateral medulla and in the nucleus tractus solitarius

Neurons in the ventrolateral medulla (VLM) and in the nucleus tractus solitarius (NTS) play important roles in the regulation of cardiovascular and other autonomic functions. In the present study, we demonstrate an inhibition of brown adipose tissue (BAT) thermogenesis evoked by activation of neurons in the VLM, as well as by neurons in the intermediate NTS, of chloralose/urethane-anesthetized, artificially ventilated rats. Activation of neurons in either rostral VLM or caudal VLM with N-methyl-d-aspartate (12 nmol) reversed the cold-evoked increase in BAT sympathetic nerve activity (SNA), BAT temperature, and end-expired CO(2). Disinhibition of neurons in either VLM or NTS with the GABA(A) receptor antagonist, bicuculline (30 pmol), reversed the increases in BAT SNA, BAT temperature, and end-expired CO(2) that were elicited 1) by cold defense; 2) during the febrile model of nanoinjection of prostaglandin E(2) into the medial preoptic area; 3) by activation of neurons in the dorsomedial hypothalamus or in the rostral raphe pallidus (rRPa); or 4) by the micro-opioid receptor agonist fentanyl. Combined, but not separate, inhibitions of neurons in the VLM and in the NTS, with the GABA(A) receptor agonist, muscimol (120 pmol/site), produced increases in BAT SNA, BAT temperature, and expired CO(2), which were reversed by nanoinjection of glycine (30 nmol) into the rRPa. These findings suggest that VLM and NTS contain neurons whose activation inhibits BAT thermogenesis, that these neurons receive GABAergic inputs that are active under these experimental conditions, and that neurons in both sites contribute to the tonic inhibition of sympathetic premotor neuronal activity in the rRPa that maintains a low level of BAT thermogenesis in normothermic conditions.