Energetic responses are triggered by caudal brainstem melanocortin receptor stimulation and mediated by local sympathetic effector circuits

Vagal oxytocin receptors are necessary for esophageal motility and function

Oxytocin plays a key role in reproductive physiology but has recently garnered interest for its involvement in modulating feeding behavior. The vagus nerve contributes to feeding behavior control, as well as other gastrointestinal functions. Oxytocin receptors (OTR) are expressed on the vagus, but their role is poorly understood. Herein, we evaluated the contribution of the vagal OTR to food intake and body weight control in male and female rats. Virogenetic knockdown of vagal OTR resulted in reduced body weight and food intake in male rats. Loss of OTR in the vagus also resulted in suppressed locomotor activity in males but hyperactivity in females. Importantly, rats with vagal OTR knockdown, but not controls, exhibited a significantly elevated mortality rate starting 4 weeks after knockdown, with males being disproportionately affected. Mortality followed large eating bouts and was accompanied by abnormal presence of food in the mouth and esophagus, suggesting death by aspiration or food in the airways and suggesting a crucial role of vagal OTR in upper gastrointestinal tract motility. Furthermore, in vivo experiments revealed impaired esophageal transit. Ex vivo findings indicated oxytocin's contribution to lower esophageal sphincter contraction. Our findings demonstrated a critical role for the oxytocin system: essential function of vagal OTR for esophageal transit and swallowing.

Hindbrain REV-ERB nuclear receptors regulate sensitivity to diet-induced obesity and brown adipose tissue pathophysiology

OBJECTIVE

The dorsal vagal complex (DVC) of the hindbrain is a major point of integration for central and peripheral signals that regulate a wide variety of metabolic functions to maintain energy balance. The REV-ERB nuclear receptors are important modulators of molecular metabolism, but their role in the DVC has yet to be established.

METHODS

Male REV-ERBα/β floxed mice received stereotaxic injections of a Cre expressing virus to the DVC to create the DVC REV-ERBα/β double knockout (DVC RDKO). Control littermates received stereotaxic injections to the DVC of a green fluorescent protein expressing virus. Animals were maintained on a normal chow diet or a 60% high-fat diet to observe the metabolic phenotype arising from DVC RDKO under healthy and metabolically stressed conditions.

RESULTS

DVC RDKO animals on high-fat diet exhibited increased weight gain compared to control animals maintained on the same diet. Increased weight gain in DVC RDKO animals was associated with decreased basal metabolic rate and dampened signature of brown adipose tissue activity. RDKO decreased gene expression of calcitonin receptor in the DVC and tyrosine hydroxylase in the brown adipose tissue.

CONCLUSIONS

These results suggest a previously unappreciated role of REV-ERB nuclear receptors in the DVC for maintaining energy balance and metabolic rate potentially through indirect sympathetic outflow to the brown adipose tissue.

Targeting the central melanocortin system for the treatment of metabolic disorders

A large body of preclinical and clinical data shows that the central melanocortin system is a promising therapeutic target for treating various metabolic disorders such as obesity and cachexia, as well as anorexia nervosa. Setmelanotide, which functions by engaging the central melanocortin circuitry, was approved by the FDA in 2020 for use in certain forms of syndromic obesity. Furthermore, the FDA approvals in 2019 of two peptide drugs targeting melanocortin receptors for the treatment of generalized hypoactive sexual desire disorder (bremelanotide) and erythropoietic protoporphyria-associated phototoxicity (afamelanotide) demonstrate the safety of this class of peptides. These approvals have also renewed excitement in the development of therapeutics targeting the melanocortin system. Here, we review the anatomy and function of the melanocortin system, discuss progress and challenges in developing melanocortin receptor-based therapeutics, and outline potential metabolic and behavioural disorders that could be addressed using pharmacological agents targeting these receptors.

Deciphering an AgRP-serotoninergic neural circuit in distinct control of energy metabolism from feeding

Contrasting to the established role of the hypothalamic agouti-related protein (AgRP) neurons in feeding regulation, the neural circuit and signaling mechanisms by which they control energy expenditure remains unclear. Here, we report that energy expenditure is regulated by a subgroup of AgRP neurons that send non-collateral projections to neurons within the dorsal lateral part of dorsal raphe nucleus (dlDRN) expressing the melanocortin 4 receptor (MC4R), which in turn innervate nearby serotonergic (5-HT) neurons. Genetic manipulations reveal a bi-directional control of energy expenditure by this circuit without affecting food intake. Fiber photometry and electrophysiological results indicate that the thermo-sensing MC4RdlDRN neurons integrate pre-synaptic AgRP signaling, thereby modulating the post-synaptic serotonergic pathway. Specifically, the MC4RdlDRN signaling elicits profound, bi-directional, regulation of body weight mainly through sympathetic outflow that reprograms mitochondrial bioenergetics within brown and beige fat while feeding remains intact. Together, we suggest that this AgRP neural circuit plays a unique role in persistent control of energy expenditure and body weight, hinting next-generation therapeutic approaches for obesity and metabolic disorders.

POMC: The Physiological Power of Hormone Processing

Pro-opiomelanocortin (POMC) is the archetypal polypeptide precursor of hormones and neuropeptides. In this review, we examine the variability in the individual peptides produced in different tissues and the impact of the simultaneous presence of their precursors or fragments. We also discuss the problems inherent in accurately measuring which of the precursors and their derived peptides are present in biological samples. We address how not being able to measure all the combinations of precursors and fragments quantitatively has affected our understanding of the pathophysiology associated with POMC processing. To understand how different ratios of peptides arise, we describe the role of the pro-hormone convertases (PCs) and their tissue specificities and consider the cellular processing pathways which enable regulated secretion of different peptides that play crucial roles in integrating a range of vital physiological functions. In the pituitary, correct processing of POMC peptides is essential to maintain the hypothalamic-pituitary-adrenal axis, and this processing can be disrupted in POMC-expressing tumors. In hypothalamic neurons expressing POMC, abnormalities in processing critically impact on the regulation of appetite, energy homeostasis, and body composition. More work is needed to understand whether expression of the POMC gene in a tissue equates to release of bioactive peptides. We suggest that this comprehensive view of POMC processing, with a focus on gaining a better understanding of the combination of peptides produced and their relative bioactivity, is a necessity for all involved in studying this fascinating physiological regulatory phenomenon.

The hindbrain is a site of energy balance action for prolactin-releasing peptide: feeding and thermic effects from GPR10 stimulation of the nucleus tractus solitarius/area postrema

PURPOSE

Prolactin-releasing peptide (PrRP) is a neuropeptide that suppresses food intake and increases body temperature when delivered to the forebrain ventricularly or parenchymally. However, PrRP's receptor GPR10 is widely distributed throughout the brain with particularly high levels found in the dorsomedial hindbrain. Thus, we hypothesized that hindbrain-directed PrRP administration would affect energy balance and motivated feeding behavior.

METHODS

To address this hypothesis, a range of behavioral and physiologic variables were measured in Sprague-Dawley rats that received PrRP delivered to the fourth ventricle (4V) or the nucleus of the solitary tract (NTS) at the level of the area postrema (AP).

RESULTS

4V PrRP delivery decreased chow intake and body weight, in part, through decreasing meal size in ad libitum maintained rats tested at dark onset. PrRP inhibited feeding when delivered to the nucleus tractus solitarius (NTS), but not to more ventral hindbrain structures. In addition, 4V as well as direct NTS administration of PrRP increased core temperature. By contrast, 4V PrRP did not reduce ad libitum intake of highly palatable food or the motivation to work for or seek palatable foods.

CONCLUSIONS

The dorsomedial hindbrain and NTS/AP, in particular, are sites of action in PrRP/GPR10-mediated control of chow intake, core temperature, and body weight.

Subdiaphragmatic Vagotomy With Pyloroplasty Ameliorates the Obesity Caused by Genetic Deletion of the Melanocortin 4 Receptor in the Mouse

Background/Objectives: We tested the hypothesis that abolishing vagal nerve activity will reverse the obesity phenotype of melanocortin 4 receptor knockout mice (Mc4r^−/−).

Subjects/Methods: In two separate studies, we examined the efficacy of bilateral subdiaphragmatic vagotomy (SDV) with pyloroplasty in the prevention and treatment of obesity in Mc4r^−/− mice.

Results: In the first study, SDV prevented >20% increase in body weight (BW) associated with this genotype. This was correlated with a transient reduction in overall food intake (FI) in the preventative arm of the study. Initially, SDV mice had reduced weekly FI; however, FI normalized to that of controls and baseline FI within the 8-week study period. In the second study, the severe obesity that is characteristic of the adult Mc4r^−/− genotype was significantly improved by SDV with a magnitude of 30% loss in excess BW over a 4-week period. Consistent with the first preventative study, within the treatment arm, SDV mice also demonstrated a transient reduction in FI relative to control and baseline levels that normalized over subsequent weeks. In addition to the accompanying loss in weight, mice subjected to SDV showed a decrease in respiratory exchange ratio (RER), and an increase in locomotor activity (LA). Analysis of the white fat-pad deposits of these mice showed that they were significantly less than the control groups.

Conclusions: Altogether, our data demonstrates that SDV both prevents gain in BW and causes weight loss in severely obese Mc4r^−/− mice. Moreover, it suggests that an important aspect of weight reduction for this type of monogenic obesity involves loss of signaling in vagal motor neurons.

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.

Effects of Endogenous Oxytocin Receptor Signaling in Nucleus Tractus Solitarius on Satiation-Mediated Feeding and Thermogenic Control in Male Rats

Central oxytocin receptor (OT-R) signaling reduces food intake and increases energy expenditure, but the central sites and mechanisms mediating these effects are unresolved. We showed previously that pharmacological activation of OT-R in hindbrain/nucleus tractus solitarius (NTS) amplifies the intake-inhibitory effects of gastrointestinal (GI) satiation signals. Unexplored were the energetic effects of hindbrain OT-R agonism and the physiological relevance of NTS OT-R signaling on food intake and energy expenditure control. Using a virally mediated OT-R knockdown (KD) strategy and a range of behavioral paradigms, this study examined the role of endogenous NTS OT-R signaling on satiation-mediated food intake inhibition and thermogenic control. Results showed that, compared with controls, NTS OT-R KD rats consumed larger meals, were less responsive to the intake-inhibitory effects of a self-ingested preload, and consumed more chow following a 24-hour fast. These data indicate that NTS OT-R signaling is necessary for normal satiation control. Whereas both control and NTS OT-R KD rats increased core temperature following high-fat diet maintenance (relative to chow maintenance), the percent increase in core temperature was greater in control compared with NTS OT-R KD rats during the light cycle. Hindbrain oxytocin agonist delivery increased core temperature in both control and NTS OT-R KD rats and the percent increase relative to vehicle treatment was not significantly different between groups. Together, data reveal a critical role for endogenous NTS OT-R signaling in mediating the intake-inhibitory effects of endogenous GI satiation signals and in diet-induced thermogenesis.

Physical Activity, Energy Expenditure, and Defense of Body Weight in Melanocortin 4 Receptor-Deficient Male Rats

Melanocortin 4 receptor (MC4R) variants contribute to human obesity, and rats lacking functional MC4R (Mc4r^K314X/K314X) are obese. We investigated the hypothesis that low energy expenditure (EE) and physical activity contribute to this obese phenotype in male rats, and determined whether lack of functional MC4R conferred protection from weight loss during 50% calorie restriction. Though Mc4r^K314X/K314X rats showed low brown adipose Ucp1 expression and were less physically active than rats heterozygous for the mutation (Mc4r^+/K314X) or wild-type (Mc4r^+/+) rats, we found no evidence of lowered EE in Mc4r^K314X/K314X rats once body weight was taken into account using covariance. Mc4r^K314X/K314X rats had a significantly higher respiratory exchange ratio. Compared to Mc4r^+/+ rats, Mc4r^K314X/K314X and Mc4r^+/K314X rats lost less lean mass during calorie restriction, and less body mass when baseline weight was accounted for. Limited regional overexpression of Mc3r was found in the hypothalamus. Although lower physical activity levels in rats with nonfunctional MC4R did not result in lower total EE during free-fed conditions, rats lacking one or two functional copies of Mc4r showed conservation of mass, particularly lean mass, during energy restriction. This suggests that variants affecting MC4R function may contribute to individual differences in the metabolic response to food restriction.

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.

The medial preoptic nucleus as a site of the thermogenic and metabolic actions of melanotan II in male rats

The present study was designed to investigate the role of the medial preoptic nucleus (MPO) as a site of the thermogenic and metabolic effects of the α-melanocyte-stimulating hormone analog melanotan II (MTII). We also assessed the involvement of the dorsomedial hypothalamic nucleus (DMH) by investigating the effects of the MPO infusion of MTII in rats with DMH lesions produced by kainic acid. Infusion of MTII in the MPO led to increases in interscapular brown adipose tissue (iBAT) temperature and iBAT uptake of 14C-bromopalmitate. Both increases were blocked by DMH lesions. iBAT temperature increase (area under curve) and 14C-bromopalmitate uptake emerged as two correlated variables ( r = 0.63, P < 0.001). DMH lesions also blocked MTII-induced expression of mRNAs coding for proteins involved in 1) thermogenesis [type II iodothyronine deiodinase ( Dio2) and peroxisome proliferator-activated receptor gamma coactivator 1-α ( Pgc1α)], 2) lipolysis [hormone-sensitive lipase ( Hsl)], and 3) lipogenesis [diacylglycerol-O-acyltransferase 2 ( Dgat2), fatty acid synthase ( Fas)], in iBAT of rats killed 1 h after MPO infusion of MTII. MTII also stimulated expression of genes in iWAT but only in rats with DMH lesions. These genes included glucose transporter member 4 ( Glut4), glycerol-3-phosphate acyltransferase 3 ( Gpat3), Dgat1, Dgat2, triglyceride lipase ( Atgl), Hsl, and carnitine palmitoyltransferase 1β ( Cpt1β). Altogether, the present results reveal the MPO as a site of the thermogenic and metabolic actions of MTII. They also contribute to establish the MPO-DMH duet as a significant target for melanocortins to modulate energy homeostasis.

Brown adipose tissue thermogenesis in the resistance to and reversal of obesity: A potential new mechanism contributing to the metabolic benefits of proglucagon-derived peptides

The capacity for increased thermogenesis through brown adipose tissue (BAT) activation is important for body weight homeostasis. Differences in BAT thermogenesis can underlie significant differences in body weight and body composition, as we demonstrate in a rat model of obesity. This mini-review focuses on our current understanding of physiological BAT regulation, with a view to how it may be exploited therapeutically. BAT activation is under central nervous system control, with the most potent activator of BAT being the sympathetic nervous system, although other humoral and hormonal factors also contribute to BAT regulation. The peptide products of the proglucagon gene are important in energy homeostasis, with well-described effects on feeding and body weight. We recently demonstrated that the peptides glucagon-like peptide 1, glucagon, and oxyntomodulin are also able to induce BAT thermogenesis by a central, sympathetic mechanism. Given the wide spread use of GLP-1 receptor based therapies for type 2 diabetes, drugs targeting this system may be useful in a wider energy balance context.

Integrated effects of leptin in the forebrain and hindbrain of male rats

Leptin receptors (ObRs) in the forebrain and hindbrain have been independently recognized as important mediators of leptin responses. It is unclear how leptin activity in these areas is integrated. We tested whether both forebrain and hindbrain ObRs have to be activated simultaneously to change energy balance and to maintain metabolic homeostasis. Previous studies used acute leptin injections in either the third ventricle (1-5 μg) or the fourth ventricle (3-10 μg); here we used 12-day infusions of low doses of leptin in one or both ventricles (0.1 μg/24 h in third, 0.6 μg/24 h in fourth). Male Sprague Dawley rats were fitted with third and fourth ventricle cannulas, and saline or leptin was infused from Alzet pumps for 6 or 12 days. Rats that received leptin into only the third or the fourth ventricle were not different from controls that received saline in both ventricles. By contrast, rats with low-dose leptin infusions into both the third and fourth ventricle showed a dramatic 60% reduction in food intake that was reversed on day 6, a 20% weight loss that stabilized on day 6, and a 50% decrease in body fat at day 12 despite the correction of food intake. They displayed normal activity and maintained energy expenditure despite weight loss, indicating inappropriately high thermogenesis that coincided with increased signal transducer and activator of transcription 3 (STAT3) phosphorylation in the brainstem. Altogether, these findings show that with low doses of leptin, chronic activation of both hypothalamic and brainstem ObRs is required to reduce body fat.

Hindbrain neurons as an essential hub in the neuroanatomically distributed control of energy balance

This Review highlights the processing and integration performed by hindbrain nuclei, focusing on the inputs received by nucleus tractus solitarius (NTS) neurons. These inputs include vagally mediated gastrointestinal satiation signals, blood-borne energy-related hormonal and nutrient signals, and descending neural signals from the forebrain. We propose that NTS (and hindbrain neurons, more broadly) integrate these multiple energy status signals and issue-output commands controlling the behavioral, autonomic, and endocrine responses that collectively govern energy balance. These hindbrain-mediated controls are neuroanatomically distributed; they involve endemic hindbrain neurons and circuits, hindbrain projections to peripheral circuits, and projections to and from midbrain and forebrain nuclei.

Food intake reductions and increases in energetic responses by hindbrain leptin and melanotan II are enhanced in mice with POMC-specific PTP1B deficiency

Leptin regulates energy balance through central circuits that control food intake and energy expenditure, including proopiomelanocortin (POMC) neurons. POMC neuron-specific deletion of protein tyrosine phosphatase 1B (PTP1B) (Ptpn1(loxP/loxP) POMC-Cre), a negative regulator of CNS leptin signaling, results in resistance to diet-induced obesity and improved peripheral leptin sensitivity in mice, thus establishing PTP1B as an important component of POMC neuron regulation of energy balance. POMC neurons are expressed in the pituitary, the arcuate nucleus of the hypothalamus (ARH), and the nucleus of the solitary tract (NTS) in the hindbrain, and it is unknown how each population might contribute to the phenotype of POMC-Ptp1b(-/-) mice. It is also unknown whether improved leptin sensitivity in POMC-Ptp1b(-/-) mice involves altered melanocortin receptor signaling. Therefore, we examined the effects of hindbrain administration (4th ventricle) of leptin (1.5, 3, and 6 μg) or the melanocortin 3/4R agonist melanotan II (0.1 and 0.2 nmol) in POMC-Ptp1b(-/-) (KO) and control PTP1B(fl/fl) (WT) mice on food intake, body weight, spontaneous physical activity (SPA), and core temperature (T(C)). The results show that KO mice were hypersensitive to hindbrain leptin- and MTII-induced food intake and body weight suppression and SPA compared with WT mice. Greater increases in leptin- but not MTII-induced T(C) were also observed in KO vs. WT animals. In addition, KO mice displayed elevated hindbrain and hypothalamic MC4R mRNA expression. These studies are the first to show that hindbrain administration of leptin or a melanocortin receptor agonist alters energy balance in mice likely via participation of hindbrain POMC neurons.

Volume transmission of beta-endorphin via the cerebrospinal fluid; a review

There is increasing evidence that non-synaptic communication by volume transmission in the flowing CSF plays an important role in neural mechanisms, especially for extending the duration of behavioral effects. In the present review, we explore the mechanisms involved in the behavioral and physiological effects of β-endorphin (β-END), especially those involving the cerebrospinal fluid (CSF), as a message transport system to reach distant brain areas. The major source of β-END are the pro-opio-melano-cortin (POMC) neurons, located in the arcuate hypothalamic nucleus (ARH), bordering the 3^rd ventricle. In addition, numerous varicose β-END-immunoreactive fibers are situated close to the ventricular surfaces. In the present paper we surveyed the evidence that volume transmission via the CSF can be considered as an option for messages to reach remote brain areas. Some of the points discussed in the present review are: release mechanisms of β-END, independence of peripheral versus central levels, central β-END migration over considerable distances, behavioral effects of β-END depend on location of ventricular administration, and abundance of mu and delta opioid receptors in the periventricular regions of the brain.

Regulation of lean mass, bone mass, and exercise tolerance by the central melanocortin system

Signaling via the type 4-melanocortin receptor (MC4R) is an important determinant of body weight in mice and humans, where loss of function mutations lead to significant obesity. Humans with mutations in the MC4R experience an increase in lean mass. However, the simultaneous accrual of fat mass in such individuals may contribute to this effect via mechanical loading. We therefore examined the relationship of fat mass and lean mass in mice lacking the type-4 melanocortin receptor (MC4RKO). We demonstrate that MC4RKO mice display increased lean body mass. Further, this is not dependent on changes in adipose mass, as MC4RKO mice possess more lean body mass than diet-induced obese (DIO) wild type mice with equivalent fat mass. To examine potential sources of the increased lean mass in MC4RKO mice, bone mass and strength were examined in MC4RKO mice. Both parameters increase with age in MC4RKO mice, which likely contributes to increases in lean body mass. We functionally characterized the increased lean mass in MC4RKO mice by examining their capacity for treadmill running. MC4R deficiency results in a decrease in exercise performance. No changes in the ratio of oxidative to glycolytic fibers were seen, however MC4RKO mice demonstrate a significantly reduced heart rate, which may underlie their impaired exercise performance. The reduced exercise capacity we report in the MC4RKO mouse has potential clinical ramifications, as efforts to control body weight in humans with melanocortin deficiency may be ineffective due to poor tolerance for physical activity.

TrkB receptor signaling in the nucleus tractus solitarius mediates the food intake-suppressive effects of hindbrain BDNF and leptin

Brain-derived neurotrophic factor (BDNF) and TrkB receptor signaling contribute to the central nervous system (CNS) control of energy balance. The role of hindbrain BDNF/TrkB receptor signaling in energy balance regulation is examined here. Hindbrain ventricular BDNF suppressed body weight through reductions in overall food intake and meal size and by increasing core temperature. To localize the neurons mediating the energy balance effects of hindbrain ventricle-delivered BDNF, ventricle subthreshold doses were delivered directly to medial nucleus tractus solitarius (mNTS). mNTS BDNF administration reduced food intake significantly, and this effect was blocked by preadministration of a highly selective TrkB receptor antagonist {[N2-2-2-Oxoazepan-3-yl amino]carbonyl phenyl benzo (b)thiophene-2-carboxamide (ANA-12)}, suggesting that TrkB receptor activation mediates hindbrain BDNF's effect on food intake. Because both BDNF and leptin interact with melanocortin signaling to reduce food intake, we also examined whether the intake inhibitory effects of hindbrain leptin involve hindbrain-specific BDNF/TrkB activation. BDNF protein content within the dorsal vagal complex of the hindbrain was increased significantly by hindbrain leptin delivery. To assess if BDNF/TrkB receptor signaling acts downstream of leptin signaling in the control of energy balance, leptin and ANA-12 were coadministered into the mNTS. Administration of the TrkB receptor antagonist attenuated the intake-suppressive effects of leptin, suggesting that mNTS TrkB receptor activation contributes to the mediation of the anorexigenic effects of hindbrain leptin. Collectively, these results indicate that TrkB-mediated signaling in the mNTS negatively regulates food intake and, in part, the intake inhibitory effects of leptin administered into the NTS.

Melanocortin control of energy balance: evidence from rodent models

Regulation of energy balance is extremely complex, and involves multiple systems of hormones, neurotransmitters, receptors, and intracellular signals. As data have accumulated over the last two decades, the CNS melanocortin system is now identified as a prominent integrative network of energy balance controls in the mammalian brain. Here, we will review findings from rat and mouse models, which have provided an important framework in which to study melanocortin function. Perhaps most importantly, this review attempts for the first time to summarize recent advances in our understanding of the intracellular signaling pathways thought to mediate the action of melanocortin neurons and peptides in control of longterm energy balance. Special attention will be paid to the roles of MC4R/MC3R, as well as downstream neurotransmitters within forebrain and hindbrain structures that illustrate the distributed control of melanocortin signaling in energy balance. In addition, distinctions and controversy between rodent species will be discussed.

Characterization of a novel melanocortin receptor-containing node in the SNS outflow circuitry to brown adipose tissue involved in thermogenesis

The melanocortins (MC) can affect interscapular brown adipose tissue (IBAT) thermogenesis via its sympathetic nervous system (SNS) innervation. We chose a site of high MC4-receptor (MC4-R) mRNA co-localization with SNS outflow neurons to IBAT, the subzona incerta (subZI) to test whether IBAT thermogenesis could be increased or decreased. We first performed immunohistochemical characterization of the subZI and found neurons and/or fibers in this area positive for melanin concentrating hormone, oxytocin, arginine vasopressin, agouti-related protein and alpha-melanocyte stimulating hormone. Functional characterization of the subZI was tested via site-specific microinjections. The MC3/4-R agonist, melanotan II [MTII (0.025, 0.05 and 0.075nmol)], and specific MC4-R agonist (cyclo [ß-Ala-His-D-Phe-Arg-Trp-Glu]-NH2; 0.024nmol) both significantly increased IBAT temperature (T(IBAT)) and pretreatment with the MC4R antagonist, HS024 (0.072nmol) blocked the MC4-R agonist-induced increased T(IBAT) in conscious, freely-moving Siberian hamsters. Injection of the MC4-R antagonist alone significantly decreased T(IBAT) up to 3h post injection. Collectively, these results highlight the identification of a brain area that possesses high concentrations of MC4-R mRNA and SNS outflow neurons to IBAT that has not been previously reported to be involved in the control of T(IBAT). These results add to previously identified neural nodes that are components of the central circuits controlling thermogenesis.

Neural controls of prostaglandin 2 pyrogenic, tachycardic, and anorexic actions are anatomically distributed

Fever and anorexia are induced by immune system challenges. Because these responses are adaptive when short lasting but deleterious when prolonged, an understanding of the mediating neural circuitry is important. Prostaglandins (PGE) are a critical signaling element for these immune responses. Despite the widespread distribution of PGE receptors throughout the brain, research focuses on the hypothalamic preoptic area as the mediating site of PGE action. Paraventricular nucleus of the hypothalamus (PVH), parabrachial nucleus (PBN), and nucleus tractus solitarius (NTS) neurons also express PGE receptors and are activated during systemic pathogen infection. A role for these neurons in PGE-induced fever, tachycardia, and anorexia is unexplored and is the subject of this report. A range of PGE₂ doses was microinjected into third or fourth ventricles (v), or directly into the dorsal PVH, lateral PBN, and medial NTS, and core and brown adipose tissue temperature, heart rate, locomotor activity, and food intake were measured in awake, behaving rats. PGE₂ delivery to multiple brain sites (third or fourth v, PVH, or PBN) induced a short- latency (< 10 min) fever and tachycardia. By contrast, an anorexic effect was observed only in response to third v and PVH stimulation. NTS PGE₂ stimulation was without effect; locomotor activity was not affected for any of the sites. The data are consistent with a view of PGE₂-induced effects as mediated by anatomically distributed sites rather than a single center. The data also underscore a potential anatomical dissociation of the neural pathways mediating pyrogenic and anorexic effects of PGE₂.

Leptin amplifies the action of thyrotropin-releasing hormone in the solitary nucleus: an in vitro calcium imaging study

Leptin exerts a powerful permissive influence on neurogenic thermogenesis. During starvation and an absence of leptin, animals cannot produce thermogenic reactions to cold stress. However, thermogenesis is rescued by restoring leptin. We have previously observed a highly cooperative interaction between leptin and thyrotropin-releasing hormone [TRH] to activate hindbrain-generated thermogenic responses (Hermann et al., 2006). In vivo physiological studies (Rogers et al., 2009) suggested that the thermogenic impact of TRH in the hindbrain is amplified by the action of leptin through a leptin receptor-mediated production of phosphoinositol-trisphosphate [PIP3]. In turn, PIP3 can activate a tyrosine kinase whose target is the Src-SH2 regulatory site on the phospholipase C [PLC] complex. The TRH receptor signals through the PLC complex. Our immunohistochemical studies (Barnes et al., 2010) suggest that this transduction interaction between leptin and TRH occurs within neurons of the solitary nucleus [NST], though this interaction had not been verified. The present in vitro live cell calcium imaging study shows that while medial NST neurons are rarely activated by leptin alone, leptin pre-treatment significantly augments NST neurons' responsiveness to TRH. This leptin-mediated priming of NST neurons was uncoupled by pre-treatment with the phosphoinositide 3-kinase [PI3K] inhibitor [wortmannin], the phospholipase C inhibitor [U73122] and the Src-SH2 antagonist [PP2]. TTX did not eliminate the synergistic response of the agonists, thus the sensitization cannot be attributed to pre-synaptic mechanisms. It seems likely that NST neurons are involved in the leptin-mediated increase in BAT temperature by sensitizing the TRH-PLC-IP3-calcium release mechanism.

Pro-opiomelanocortin gene transfer to the nucleus of the solitary track but not arcuate nucleus ameliorates chronic diet-induced obesity

Short-term pharmacological melanocortin activation deters diet-induced obesity (DIO) effectively in rodents. However, whether central pro-opiomelanocortin (POMC) gene transfer targeted to the hypothalamus or hindbrain nucleus of the solitary track (NTS) can combat chronic dietary obesity has not been investigated. Four-weeks-old Sprague-Dawley rats were fed a high fat diet for 5 months, and then injected with either the POMC or control vector into the hypothalamus or NTS, and body weight and food intake recorded for 68 days. Insulin sensitivity, glucose metabolism and adrenal indicators of central sympathetic activation were measured, and voluntary wheel running (WR) assessed. Whereas the NTS POMC-treatment decreased cumulative food consumption and caused a sustained weight reduction over 68 days, the hypothalamic POMC-treatment did not alter cumulative food intake and produced weight loss only in the first 25 days. At death, only the NTS-POMC rats had a significant decrease in fat mass. They also displayed enhanced glucose tolerance, lowered fasting insulin and increased QUICK value, and elevated adrenal indicators of central sympathetic activation. Moreover, the NTS-POMC animals exhibited a near 20% increase in distance ran relative to the respective controls, but the ARC-POMC rats did not. In conclusion, POMC gene transfer to the NTS caused modest anorexia, persistent weight loss, improved insulin sensitivity, and increased propensity for WR in DIO rats. These metabolic improvements may involve stimulation of energy expenditure via centrally regulated sympathetic outflow. The similar POMC treatment in the hypothalamus had minimal long-term physiological or metabolic impact. Thus, melanocortin activation in the brainstem NTS region effectively ameliorates chronic dietary obesity whilst that in the hypothalamus fails to do so.

The endogenous actions of hypothalamic peptides on brown adipose tissue thermogenesis in the rat

Although the neuronal pathways within the hypothalamus critical in controlling feeding and energy expenditure and projecting to brown adipose tissue (BAT) have been identified and their peptidergic content characterized, endogenous action of such peptides in the control of BAT activity has not been elucidated. Here male Sprague Dawley rats received infusions of either melanin-concentrating hormone antagonist (SNAP-7941) (1 microg/microl x h), orexin A receptor antagonist (SB-334867-A; 1 microg/microl x h), combined SB-334867-A (1 microg/microl x h), and SNAP-7941 (1 microg/microl x h), or melanocortin-3/4 receptor antagonist (SHU9119) (1 microg/microl x h) via an indwelling cannula in the lateral ventricle attached to s.c. implanted osmotic minipump. BAT temperature, physical activity, body weight, food intake, and changes in uncoupling protein (UCP)-1 were measured. SB-334867-A and SNAP-7941 significantly increased BAT temperature and UCP1 expression and reduced food intake and body weight. Combined infusion of SB-334867-A and SNAP-7941 produced a pronounced response that was greater than the addition of the individual effects in all parameters measured. SHU9119 significantly decreased BAT temperature and UCP1 expression and increased feeding and body weight. In a second series of experiments, the effect of SB-334867-A and SNAP-7941 alone or combination on the expression of the Fos protein was determined. SB-334867-A and SNAP-7941 increased Fos expression in key hypothalamic and brainstem feeding-related regions. In combination, these antagonists produced a greater than additive elevation of Fos expression in most of the regions evaluated. These findings support a role for endogenous orexigenic and anorexigenic hypothalamic peptides acting in concert to create a thermogenic tone via BAT activity.

The melanocortin-4 receptor: physiology, pharmacology, and pathophysiology

The melanocortin-4 receptor (MC4R) was cloned in 1993 by degenerate PCR; however, its function was unknown. Subsequent studies suggest that the MC4R might be involved in regulating energy homeostasis. This hypothesis was confirmed in 1997 by a series of seminal studies in mice. In 1998, human genetic studies demonstrated that mutations in the MC4R gene can cause monogenic obesity. We now know that mutations in the MC4R are the most common monogenic form of obesity, with more than 150 distinct mutations reported thus far. This review will summarize the studies on the MC4R, from its cloning and tissue distribution to its physiological roles in regulating energy homeostasis, cachexia, cardiovascular function, glucose and lipid homeostasis, reproduction and sexual function, drug abuse, pain perception, brain inflammation, and anxiety. I will then review the studies on the pharmacology of the receptor, including ligand binding and receptor activation, signaling pathways, as well as its regulation. Finally, the pathophysiology of the MC4R in obesity pathogenesis will be reviewed. Functional studies of the mutant MC4Rs and the therapeutic implications, including small molecules in correcting binding and signaling defect, and their potential as pharmacological chaperones in rescuing intracellularly retained mutants, will be highlighted.

Functions for pro-opiomelanocortin-derived peptides in obesity and diabetes

Melanocortin peptides, derived from POMC (pro-opiomelanocortin) are produced in the ARH (arcuate nucleus of the hypothalamus) neurons and the neurons in the commissural NTS (nucleus of the solitary tract) of the brainstem, in anterior and intermediate lobes of the pituitary, skin and a wide range of peripheral tissues, including reproductive organs. A hypothetical model for functional roles of melanocortin receptors in maintaining energy balance was proposed in 1997. Since this time, there has been an extraordinary amount of knowledge gained about POMC-derived peptides in relation to energy homoeostasis. Development of a Pomc-null mouse provided definitive proof that POMC-derived peptides are critical for the regulation of energy homoeostasis. The melanocortin system consists of endogenous agonists and antagonists, five melanocortin receptor subtypes and receptor accessory proteins. The melanocortin system, as is now known, is far more complex than most of us could have imagined in 1997, and, similarly, the importance of this system for regulating energy homoeostasis in the general human population is much greater than we would have predicted. Of the known factors that can cause human obesity, or protect against it, the melanocortin system is by far the most significant. The present review is a discussion of the current understanding of the roles and mechanism of action of POMC, melanocortin receptors and AgRP (agouti-related peptide) in obesity and Type 2 diabetes and how the central and/or peripheral melanocortin systems mediate nutrient, leptin, insulin, gut hormone and cytokine regulation of energy homoeostasis.

Thyroid hormone exerts negative feedback on hypothalamic type 4 melanocortin receptor expression

The type 4 melanocortin receptor MC4R, a key relay in leptin signaling, links central energy control to peripheral reserve status. MC4R activation in different brain areas reduces food intake and increases energy expenditure. Mice lacking Mc4r are obese. Mc4r is expressed by hypothalamic paraventricular Thyrotropin-releasing hormone (TRH) neurons and increases energy usage through activation of Trh and production of the thyroid hormone tri-iodothyronine (T(3)). These facts led us to test the hypothesis that energy homeostasis should require negative feedback by T(3) on Mc4r expression. Quantitative PCR and in situ hybridization showed hyperthyroidism reduces Mc4r mRNA levels in the paraventricular nucleus. Comparative in silico analysis of Mc4r regulatory regions revealed two evolutionarily conserved potential negative thyroid hormone-response elements (nTREs). In vivo ChIP assays on mouse hypothalamus demonstrated association of thyroid hormone receptors (TRs) with a region spanning one nTRE. Further, in vivo gene reporter assays revealed dose-dependent T(3) repression of transcription from the Mc4r promoter in mouse hypothalamus, in parallel with T(3)-dependent Trh repression. Mutagenesis of the nTREs in the Mc4r promoter demonstrated direct regulation by T(3), consolidating the ChIP results. In vivo shRNA knockdown, TR over-expression approaches and use of mutant mice lacking specific TRs showed that both TRalpha and TRbeta contribute to Mc4r regulation. T(3) repression of Mc4r transcription ensures that the energy-saving effects of T(3) feedback on Trh are not overridden by MC4R activation of Trh. Thus parallel repression by T(3) on hypothalamic Mc4r and Trh contributes to energy homeostasis.

Leptin and the systems neuroscience of meal size control

The development of effective pharmacotherapy for obesity will benefit from a more complete understanding of the neural pathways and the neurochemical signals whose actions result in the reduction of the size of meals. This review examines the neural control of meal size and the integration of two principal sources of that control--satiation signals arising from the gastrointestinal tract and CNS leptin signaling. Four types of integrations that are central to the control of meal size are described and each involves the neurons of the nucleus tractus solitarius (NTS) in the dorsal hindbrain. Data discussed show that NTS neurons integrate information arising from: (1) ascending GI-derived vagal afferent projections, (2) descending neuropeptidergic projections from leptin-activated arcuate and paraventricular nucleus neurons, (3) leptin signaling in NTS neurons themselves and (4) melanocortinergic projections from NTS and hypothalamic POMC neurons to NTS neurons and melanocortinergic modulation of vagal afferent nerve terminals that are presynaptic to NTS neurons.

Hypothalamic nutrient sensing in the control of energy homeostasis

The hypothalamus is a center of convergence and integration of multiple nutrient-related signals. It can sense changes in circulating adiposity hormones, gastric hormones and nutrients, and receives neuroanatomical projections from other nutrient sensors, mainly within the brainstem. The hypothalamus also integrates these signals with various cognitive forebrain-descending information and reward/motivation-related signals coming from the midbrain-dopamine system, to coordinate neuroendocrine, behavioral and metabolic effectors of energy balance. Some of the key nutrient-sensing hypothalamic neurons have been identified in the arcuate, the ventro-medial and the lateral nuclei of the hypothalamus, and the molecular mechanisms underlying intracellular integration of nutrient-related signals in these neurons are currently under intensive investigation. However, little is known about the neural pathways downstream from hypothalamic nutrient sensors, and how they drive effectors of energy homeostasis under physiological conditions. This manuscript will review recent progress from molecular, genetic and neurophysiological studies that identify and characterize the critical intracellular signalling pathways and neurocircuits involved in determining hypothalamic nutrient detection, and link these circuits to behavioral and metabolic effectors of energy balance. We will provide a critical analysis of current data to identify ongoing challenges for future research in this field.

Hypothalamic and hindbrain melanocortin receptors contribute to the feeding, thermogenic, and cardiovascular action of melanocortins

Forebrain ventricular delivery of melanocortin receptor (MC3/4R) agonist increases energy expenditure and decreases food intake (FI). Because forebrain ventricular delivery provides ligand to various anatomically distributed MC3/4R-bearing nuclei, it is unclear which of the receptor subpopulations contributes to the feeding suppression and the sympathetic-thermogenic effects observed. The literature indicates that reexpression of MC4R in the paraventricular nucleus (PVH) affects the feeding but not the energetic phenotype of the MC4R knockout, suggesting that divergent MC4R populations mediate energy expenditure (hindbrain) and FI (hypothalamus) effects of stimulation. Not consistent with this view are data indicating that PVH sympathetic projection neurons express MC4Rs and that feeding effects are induced from hindbrain MC4R sites. Therefore, we hypothesize an opposing perspective: that stimulation of anatomically diverse MC3/4R-bearing nuclei triggers energetic as well as feeding effects. To test this hypothesis, ventricle subthreshold doses of MC3/4R agonist (5 and 10 pmol) were applied in separate experiments to six hindbrain and hypothalamic sites; core temperature (Tc), heart rate (HR), spontaneous activity (SPA), and FI were measured in behaving rats. Nucleus tractus solitarius and PVH stimulation increased Tc, HR, and SPA and decreased FI. Rostral ventrolateral medulla, parabrachial nucleus, and retrochiasmatic area stimulation increased Tc, HR, but not SPA, and decreased FI. The response profile differed to some extent for each nucleus tested, suggesting differential output circuitries for the measured parameters. Data are consistent with the view that energetic and feeding responses are not controlled by regionally divergent MC3/4Rs and can be elicited from multiple, anatomically distributed MC3/4R populations.

Distinct effects of leptin and a melanocortin receptor agonist injected into medial hypothalamic nuclei on glucose uptake in peripheral tissues

OBJECTIVE

The medial hypothalamus mediates leptin-induced glucose uptake in peripheral tissues, and brain melanocortin receptors (MCRs) mediate certain central effects of leptin. However, the contributions of the leptin receptor and MCRs in individual medial hypothalamic nuclei to regulation of peripheral glucose uptake have remained unclear. We examined the effects of an injection of leptin and the MCR agonist MT-II into medial hypothalamic nuclei on glucose uptake in peripheral tissues.

RESEARCH DESIGN AND METHODS

Leptin or MT-II was injected into the ventromedial (VMH), dorsomedial (DMH), arcuate nucleus (ARC), or paraventricular (PVH) hypothalamus or the lateral ventricle (intracerebroventricularly) in freely moving mice. The MCR antagonist SHU9119 was injected intracerebroventricularly. Glucose uptake was measured by the 2-[(3)H]deoxy-d-glucose method.

RESULTS

Leptin injection into the VMH increased glucose uptake in skeletal muscle, brown adipose tissue (BAT), and heart, whereas that into the ARC increased glucose uptake in BAT, and that into the DMH or PVH had no effect. SHU9119 abolished these effects of leptin injected into the VMH. Injection of MT-II either into the VMH or intracerebroventricularly increased glucose uptake in skeletal muscle, BAT, and heart, whereas that into the PVH increased glucose uptake in BAT, and that into the DMH or ARC had no effect.

CONCLUSIONS

The VMH mediates leptin- and MT-II-induced glucose uptake in skeletal muscle, BAT, and heart. These effects of leptin are dependent on MCR activation. The leptin receptor in the ARC and MCR in the PVH regulate glucose uptake in BAT. Medial hypothalamic nuclei thus play distinct roles in leptin- and MT-II-induced glucose uptake in peripheral tissues.

Role of central melanocortin pathways in energy homeostasis

The rise in the global prevalence of human obesity has emphasized the need for a greater understanding of the physiological mechanisms that underlie energy homeostasis. Numerous circulating nutritional cues and central neuromodulatory signals are integrated within the brain to regulate both short- and long-term nutritional state. The central melanocortin system represents a crucial point of convergence for these signals and, thus, has a fundamental role in regulating body weight. The melanocortin ligands, synthesized in discrete neuronal populations within the hypothalamus and brainstem, modulate downstream homeostatic signalling via their action at central melanocortin-3 and -4 receptors. Intimately involved in both ingestive behaviour and energy expenditure, the melanocortin system has garnered much interest as a potential therapeutic target for human obesity.

Endogenous hindbrain glucagon-like peptide-1 receptor activation contributes to the control of food intake by mediating gastric satiation signaling

Exogenous activation of central nervous system glucagon-like peptide-1 (GLP-1) receptors (GLP-1Rs) reduces food intake. Experiments addressed whether endogenous central GLP-1R activity is involved in the control of normal feeding and examined which gastrointestinal satiation signals contribute to this control. Given that nucleus tractus solitarius (NTS) neurons are the source of central GLP-1, that caudal brainstem circuits mediate the intake suppression triggered by exogenous hindbrain GLP-1R activation, and that these neurons process gastrointestinal vagal signals, the role of endogenous hindbrain GLP-1R activation to intake control was the focus of the analysis. Food intake increased with GLP-1R antagonist [Exendin-(9-39) (Ex-9)] [10 microg, fourth intracerebroventricular (icv)] delivery to overnight food-deprived rats after ingestion of 9 ml Ensure diet. Direct medial NTS injection of a ventricle subthreshold dose (1.0 microg) of Ex-9 increased food intake and established the contribution of this GLP-1R population to the effect observed with ventricular administration. To determine whether satiation signals of gastric vs. intestinal origin drive the GLP-1R-mediated NTS effect on food intake, two experiments were performed in overnight-fasted rats. In one, Ensure was infused intraduodenally (0.4 ml/min for 20 min); in another, the stomach was distended (9 ml SILASTIC brand balloon) for 15 min before fourth icv Ex-9. The intake suppression by duodenal nutrient infusion was not affected by GLP-1R blockade, but the feeding suppression after gastric distension was significantly attenuated by fourth icv Ex-9. We conclude that endogenous NTS GLP-1R activation driven by gastric satiation signals contributes to the control of normal feeding.

Nesfatin-1 exerts cardiovascular actions in brain: possible interaction with the central melanocortin system

Nesfatin-1 is a recently discovered hypothalamic peptide that was shown to suppress food intake through a melanocortin-3/4 receptor-dependent mechanism. Since nesfatin-1 mRNA is detected in the paraventricular nucleus of the hypothalamus, and because many peptides that alter food intake also influence cardiovascular function, we tested the ability of centrally administered nesfatin-1 to affect mean arterial pressure (MAP) in conscious, freely moving rats. Significant increases in MAP were observed following intracerebroventricular administration of nesfatin-1. Pretreatment with either the melanocortin-3/4 receptor antagonist, SHU9119 (intracerebroventricular), or the alpha-adrenergic antagonist, phentolamine (intra-arterial), abrogated the rise in MAP induced by nesfatin-1, indicating that nesfatin-1 may interact with the central melanocortin system to increase sympathetic nerve activity and lead to an increase in MAP. Thus we have identified a novel action of nesfatin-1, in addition to its anorexigenic effects, to stimulate autonomic nervous system activity.

Hindbrain cocaine- and amphetamine-regulated transcript induces hypothermia mediated by GLP-1 receptors

Cocaine- and amphetamine-regulated transcript (CART) peptides are widely distributed throughout the neuraxis, including regions associated with energy balance. CART's classification as a catabolic neuropeptide is based on its inhibitory effects on feeding, coexpression with arcuate nucleus proopiomelanocortin neurons, and on limited analysis of its energy expenditure effects. Here, we investigate whether (1) caudal brainstem delivery of CART produces energetic, cardiovascular, and glycemic effects, (2) forebrain-caudal brainstem neural communication is required for those effects, and (3) glucagon-like peptide-1 receptors (GLP-1Rs) contribute to the mediation of CART-induced effects. Core temperature (Tc), heart rate (HR), activity, and blood glucose were measured in rats injected fourth intracerebroventricularly with CART (0.1, 1.0, and 2.0 microg). Food was withheld during physiologic recording and returned for overnight measurement of intake and body weight. CART induced a long-lasting (>6 h) hypothermia: a 1.5 degrees C and 1.6 degrees C drop in Tc for the 1.0 and 2.0 microg doses. Hindbrain CART application reduced food intake and body weight and increased blood glucose levels; no change in HR or activity was observed. Supracollicular decerebration eliminated the hypothermic response observed in intact rats to hindbrain ventricular CART, suggesting that forebrain processing is required for hypothermia. Pretreatment with the GLP-1R antagonist (exendin-9-39) in control rats attenuated CART hypothermia and hypophagia, indicating that GLP-1R activation contributes to hypothermic and hypophagic effects of hindbrain CART, whereas CART-induced hyperglycemia was not altered by GLP-1R blockade. Data reveal a novel function of CART in temperature regulation and open possibilities for future studies on the clinical potential of the hypothermic effect.

Testosterone replacement does not normalize carcass composition in chronically decerebrate male rats

Chronically decerebrate (CD) rats, in which the forebrain and its descending projections are completely neurally isolated from hindbrain and rostral projections, gain substantial amounts of body fat, lose lean tissue, and have low circulating testosterone concentrations. We tested whether testosterone replacement would normalize body composition of male CD rats. Five groups of rats were used: CD placebo, CD testosterone, control placebo, castrate placebo, and castrate testosterone. Testosterone replacement was initiated at the first stage of CD surgery in both CDs and castrate controls. The second stage of CD surgery occurred 8 days later, and the study ended 15 days later. Testosterone implants produced 10-fold normal circulating concentrations. Food intake was fixed for all rats by tube feeding. CD rats had substantially more body fat and less lean tissue than neurally intact rats. Testosterone replacement did not affect adiposity of CD rats but did increase carcass water content. Energy expenditure of CD rats was significantly lower than that of control placebo and castrated rats. Testosterone lowered respiratory equivalency ratio and ameliorated a fall in energy expenditure late in the intermeal interval in CD rats. Castration increased, and testosterone decreased luteinizing hormone (LH) and follicle stimulating hormone (FSH) in neurally intact controls. LH was undetectable, and FSH was equivalent to neurally intact controls in CD rats, and neither was affected by testosterone. Collectively, low testosterone did not explain obesity or decreased lean body mass of CD rats, although CD rats exhibited abnormal levels of circulating reproductive hormones and disrupted testosterone negative feedback.

Hindbrain leptin stimulation induces anorexia and hyperthermia mediated by hindbrain melanocortin receptors

Of the central nervous system receptors that could mediate the energy balance effects of leptin, those of the hypothalamic arcuate nucleus receive the greatest attention. Melanocortin receptors (MC-Rs) contribute to the feeding and energetic effects of hypothalamically delivered leptin. Energy balance effects of leptin are also mediated by extrahypothalamic neurons including the hindbrain nucleus tractus solitarius. Hindbrain leptin receptors play a role in leptin's anorectic effects, but their contribution to its energetic effects and their functional interaction with melanocortin systems within the hindbrain remains unexplored. Here rats implanted with telemetric devices for recording energetic/cardiovascular responses were examined to determine whether: 1) hindbrain (fourth ventricular) leptin receptor stimulation triggers energetic and cardiovascular effects, 2) these effects are altered by a 6-wk high-fat diet maintenance, and 3) hindbrain MC-Rs mediate the thermogenic, cardiovascular, and anorexic effects of hindbrain leptin delivery. Results show that hindbrain leptin receptor stimulation produced long-lasting (>6 h) increases in core temperature and heart rate and also decreased food intake and body weight. These responses were not altered by high-fat maintenance, in contrast to what has been reported for forebrain leptin delivery. Fourth ventricular pretreatment with MC-R antagonist SHU 9119 completely abolished the hyperthermia, anorexia, and body weight loss seen with hindbrain-directed leptin but had no effects of its own. These data highlight a role for hindbrain leptin receptors in the initiation of energetic and anorexic responses and show that MCRs are part of the downstream mediation of hindbrain leptin-induced energy balance effects, paralleling effects observed for hypothalamic leptin receptors.

Energetic responses to cold temperatures in rats lacking forebrain-caudal brain stem connections

Hypothalamic neurons are regarded as essential for integrating thermal afferent information from skin and core and issuing commands to autonomic and behavioral effectors that maintain core temperature (T(c)) during cold exposure and for the control of energy expenditure more generally. Caudal brain stem neurons are necessary elements of the hypothalamic effector pathway and also are directly driven by skin and brain cooling. To assess whether caudal brain stem processing of thermal afferent signals is sufficient to drive endemic effectors for thermogenesis, heart rate (HR), T(c), and activity responses of chronic decerebrate (CD) and control rats adapted to 23 degrees C were compared during cold exposure (4, 8, or 12 degrees C) for 6 h. Other CDs and controls were exposed to 4 or 23 degrees C for 2 h, and tissues were processed for norepinephrine turnover (NETO), a neurochemical measure of sympathetic drive. Controls maintained T(c) for all temperatures. CDs maintained T(c) for the 8 and 12 degrees C exposures, but T(c) declined 2 degrees C during the 4 degrees C exposure. Cold exposure elevated HR in CDs and controls alike. Tachycardia magnitude correlated with decreases in environmental temperature for controls, but not CDs. Cold increased NETO in brown adipose tissue, heart, and some white adipose tissue pads in CDs and controls compared with their respective room temperature controls. These data demonstrate that, in neural isolation from the hypothalamus, cold exposure drives caudal brain stem neuronal activity and engages local effectors that trigger sympathetic energetic and cardiac responses that are comparable in many, but not in all, respects to those seen in neurologically intact rats.

Melanocortin-4 receptor mRNA expressed in sympathetic outflow neurons to brown adipose tissue: neuroanatomical and functional evidence

A precise understanding of neural circuits controlling lipid mobilization and thermogenesis remains to be determined. We have been studying the sympathetic nervous system (SNS) contributions to white adipose tissue (WAT) lipolysis largely in Siberian hamsters. Central melanocortins are implicated in the control of the sympathetic outflow to WAT, and, moreover, the melanocortin 4 receptors (MC4-R) appear to be principally involved. We previously found that acute third ventricular melanotan II (MTII; an MC3/4-R agonist) injections increase sympathetic drive (norepinephrine turnover) to interscapular brown adipose tissue (IBAT) and IBAT temperature. Here we tested whether MC4-R mRNA is expressed in IBAT SNS outflow neurons using in situ hybridization for the former and injections of the transneuronal viral retrograde tract tracer, pseudorabies virus (PRV) into IBAT, for the latter. Significant numbers of double-labeled cells for PRV and MC4-R mRNA were found across the neuroaxis (mean of all brain sites approximately 60%), including the hypothalamic paraventricular nucleus (PVH; approximately 80%). Acute parenchymal MTII microinjections into the PVH of awake, freely-moving hamsters, using doses below those able to increase IBAT temperature when injected into the third ventricle, increased IBAT temperature for as long as 4 h, as measured by temperature transponders implanted below the tissue. Collectively, these data add significant support to the view that central melanocortins are important in controlling IBAT thermogenesis via the SNS innervation of this tissue, likely through the MC4-Rs.