Brain stem melanocortinergic modulation of meal size and identification of hypothalamic POMC projections

Identification of AgRP cells in the murine hindbrain that drive feeding

OBJECTIVE

The central melanocortin system is essential for the regulation of food intake and body weight. Agouti-related protein (AgRP) is the sole orexigenic component of the central melanocortin system and is conserved across mammalian species. AgRP is currently known to be expressed exclusively in the mediobasal hypothalamus, and hypothalamic AgRP-expressing neurons are essential for feeding. Here we characterized a previously unknown population of AgRP cells in the mouse hindbrain.

METHODS

Expression of AgRP in the hindbrain was investigated using gene expression analysis, single-cell RNA sequencing, immunofluorescent analysis and multiple transgenic mice with reporter expressions. Activation of AgRP neurons was achieved by Designer Receptors Exclusively Activated by Designer Drugs (DREADD) and by transcranial focal photo-stimulation using a step-function opsin with ultra-high light sensitivity (SOUL).

RESULTS

AgRP expressing cells were present in the area postrema (AP) and the adjacent subpostrema area (SubP) and commissural nucleus of the solitary tract (cNTS) of the mouse hindbrain (termed AgRPHind herein). AgRPHind cells consisted of locally projecting neurons as well as tanycyte-like cells. Food deprivation stimulated hindbrain Agrp expression as well as neuronal activity of subsets of AgRPHind cells. In adult mice that lacked hypothalamic AgRP neurons, chemogenetic activation of AgRP neurons resulted in hyperphagia and weight gain. In addition, transcranial focal photo-stimulation of hindbrain AgRP cells increased food intake in adult mice with or without hypothalamic AgRP neurons.

CONCLUSIONS

Our study indicates that the central melanocortin system in the hindbrain possesses an orexigenic component, and that AgRPHind neurons stimulate feeding independently of hypothalamic AgRP neurons.

A human obesity-associated MC4R mutation with defective Gq/11α signaling leads to hyperphagia in mice

Melanocortin 4 receptor (MC4R) mutations are the most common cause of human monogenic obesity and are associated with hyperphagia and increased linear growth. While MC4R is known to activate Gsα/cAMP signaling, a substantial proportion of obesity-associated MC4R mutations do not affect MC4R/Gsα signaling. To further explore the role of specific MC4R signaling pathways in the regulation of energy balance, we examined the signaling properties of one such mutant, MC4R (F51L), as well as the metabolic consequences of MC4RF51L mutation in mice. The MC4RF51L mutation produced a specific defect in MC4R/Gq/11α signaling and led to obesity, hyperphagia, and increased linear growth in mice. The ability of a melanocortin agonist to acutely inhibit food intake when delivered to the paraventricular nucleus (PVN) was lost in MC4RF51L mice, as well as in WT mice in which a specific Gq/11α inhibitor was delivered to the PVN; this provided evidence that a Gsα-independent signaling pathway, namely Gq/11α, significantly contributes to the actions of MC4R on food intake and linear growth. These results suggest that a biased MC4R agonist that primarily activates Gq/11α may be a potential agent to treat obesity with limited untoward cardiovascular and other side effects.

Obesity: A Review of Pathophysiology and Classification

Obesity is a chronic, multifactorial, and morbid disease. In the United States, 69% of adults are overweight or have obesity, and the global prevalence of obesity is increasing. Obesity is influenced by genetic, neurologic, metabolic, enteric, and behavioral processes. It remains a key modifiable risk factor for many comorbid diseases, including cardiovascular disease, diabetes mellitus, and cancer. Whereas there are recent and significant advances in obesity therapy, including diets, lifestyle modifications, pharmacotherapies, endoscopic procedures, and bariatric surgeries, there is an immense need for a better understanding of the heterogeneity in the pathophysiologic process of obesity and outcomes. Here we review salient pathophysiologic mechanisms underlying the development and morbidity of obesity as well as pathophysiologically based classification systems that inform current obesity management and may inform improved and individualized management in the future.

Increased body weight in mice with fragile X messenger ribonucleoprotein 1 (Fmr1) gene mutation is associated with hypothalamic dysfunction

Mutations in the Fragile X Messenger Ribonucleoprotein 1 (FMR1) gene are linked to Fragile X Syndrome, the most common monogenic cause of intellectual disability and autism. People affected with mutations in FMR1 have higher incidence of obesity, but the mechanisms are largely unknown. In the current study, we determined that male Fmr1 knockout mice (KO, Fmr1-/y), but not female Fmr1-/-, exhibit increased weight when compared to wild-type controls, similarly to humans with FMR1 mutations. No differences in food or water intake were found between groups; however, male Fmr1-/y display lower locomotor activity, especially during their active phase. Moreover, Fmr1-/y have olfactory dysfunction determined by buried food test, although they exhibit increased compulsive behavior, determined by marble burying test. Since olfactory brain regions communicate with hypothalamic regions that regulate food intake, including POMC neurons that also regulate locomotion, we examined POMC neuron innervation and numbers in Fmr1-/y mice. POMC neurons express Fmrp, and POMC neurons in Fmr1-/y have higher inhibitory GABAergic synaptic inputs. Consistent with increased inhibitory innervation, POMC neurons in the Fmr1-/y mice exhibit lower activity, based on cFOS expression. Notably, Fmr1-/y mice have fewer POMC neurons than controls, specifically in the rostral arcuate nucleus, which could contribute to decreased locomotion and increased body weight. These results suggest a role for Fmr1 in the regulation of POMC neuron function and the etiology of Fmr1-linked obesity.

Functional anatomy of the vagus system: How does the polyvagal theory comply?

Due to its pivotal role in autonomic networks and interoception, the vagus attracts continued interest from both basic scientists and therapists of various clinical disciplines. In particular, the widespread use of heart rate variability as an index of autonomic cardiac control and a proposed central role of the vagus in biopsychological concepts, e.g., the polyvagal theory, provide a good opportunity to recall basic features of vagal anatomy. In addition to the "classical" vagal brainstem nuclei, i.e., dorsal motor nucleus, nucleus ambiguus and nucleus tractus solitarii, the spinal trigeminal and paratrigeminal nuclei come into play as targets of vagal afferents. On the other hand, the nucleus of the solitary tract receives and integrates not only visceral but also somatic afferents.

The ins and outs of the caudal nucleus of the solitary tract: An overview of cellular populations and anatomical connections

The body and brain are in constant two-way communication. Driving this communication is a region in the lower brainstem: the dorsal vagal complex. Within the dorsal vagal complex, the caudal nucleus of the solitary tract (cNTS) is a major first stop for incoming information from the body to the brain carried by the vagus nerve. The anatomy of this region makes it ideally positioned to respond to signals of change in both emotional and bodily states. In turn, the cNTS controls the activity of regions throughout the brain that are involved in the control of both behaviour and physiology. This review is intended to help anyone with an interest in the cNTS. First, I provide an overview of the architecture of the cNTS and outline the wide range of neurotransmitters expressed in subsets of neurons in the cNTS. Next, in detail, I discuss the known inputs and outputs of the cNTS and briefly highlight what is known regarding the neurochemical makeup and function of those connections. Then, I discuss one group of cNTS neurons: glucagon-like peptide-1 (GLP-1)-expressing neurons. GLP-1 neurons serve as a good example of a group of cNTS neurons, which receive input from varied sources and have the ability to modulate both behaviour and physiology. Finally, I consider what we might learn about other cNTS neurons from our study of GLP-1 neurons and why it is important to remember that the manipulation of molecularly defined subsets of cNTS neurons is likely to affect physiology and behaviours beyond those monitored in individual experiments.

The physiological control of eating: signals, neurons, and networks

During the past 30 yr, investigating the physiology of eating behaviors has generated a truly vast literature. This is fueled in part by a dramatic increase in obesity and its comorbidities that has coincided with an ever increasing sophistication of genetically based manipulations. These techniques have produced results with a remarkable degree of cell specificity, particularly at the cell signaling level, and have played a lead role in advancing the field. However, putting these findings into a brain-wide context that connects physiological signals and neurons to behavior and somatic physiology requires a thorough consideration of neuronal connections: a field that has also seen an extraordinary technological revolution. Our goal is to present a comprehensive and balanced assessment of how physiological signals associated with energy homeostasis interact at many brain levels to control eating behaviors. A major theme is that these signals engage sets of interacting neural networks throughout the brain that are defined by specific neural connections. We begin by discussing some fundamental concepts, including ones that still engender vigorous debate, that provide the necessary frameworks for understanding how the brain controls meal initiation and termination. These include key word definitions, ATP availability as the pivotal regulated variable in energy homeostasis, neuropeptide signaling, homeostatic and hedonic eating, and meal structure. Within this context, we discuss network models of how key regions in the endbrain (or telencephalon), hypothalamus, hindbrain, medulla, vagus nerve, and spinal cord work together with the gastrointestinal tract to enable the complex motor events that permit animals to eat in diverse situations.

Individual arcuate nucleus proopiomelanocortin neurons project to select target sites

Proopiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus (ARH) are a diverse group of neurons that project widely to different brain regions. It is unknown how this small population of neurons organizes its efferent projections. In this study, we hypothesized that individual ARH POMC neurons exclusively innervate select target regions. To investigate this hypothesis, we first verified that only a fraction of ARH POMC neurons innervate the lateral hypothalamus (LH), the paraventricular nucleus of the hypothalamus (PVN), the periaqueductal gray (PAG), or the ventral tegmental area (VTA) using the retrograde tracer cholera toxin B (CTB). Next, two versions of CTB conjugated to distinct fluorophores were injected bilaterally into two of the regions such that PVN and VTA, PAG and VTA, or LH and PVN received tracers simultaneously. These pairs of target sites were chosen based on function and location. Few individual ARH POMC neurons projected to two brain regions at once, suggesting that there are ARH POMC neuron subpopulations organized by their efferent projections. We also investigated whether increasing the activity of POMC neurons could increase the number of ARH POMC neurons labeled with CTB, implying an increase in new synaptic connections to downstream regions. However, chemogenetic enhancement of POMC neuron activity did not increase retrograde tracing of CTB back to ARH POMC neurons from either the LH, PVN, or VTA. Overall, subpopulations of ARH POMC neurons with distinct efferent projections may serve as a way for the POMC population to organize its many functions.

Tanycytic networks mediate energy balance by feeding lactate to glucose-insensitive POMC neurons

Hypothalamic glucose sensing enables an organism to match energy expenditure and food intake to circulating levels of glucose, the main energy source of the brain. Here, we established that tanycytes of the hypothalamic arcuate nucleus, specialized glia that line the wall of the third ventricle, convert brain glucose supplies into lactate that they transmit through monocarboxylate transporters to arcuate proopiomelanocortin neurons, which integrate this signal to drive their activity and to adapt the metabolic response to meet physiological demands. Furthermore, this transmission required the formation of extensive Connexin-43 gap-junction-mediated metabolic networks by arcuate tanycytes. Selectively suppressing either tanycytic monocarboxylate transporters or gap junctions resulted in altered feeding behavior and energy metabolism. Tanycytic intercellular communication and lactate production are thus integral to the mechanism by which hypothalamic neurons that regulate energy and glucose homeostasis efficiently perceive alterations in systemic glucose levels as a function of the physiological state of the organism.

Loss of CTRP4 alters adiposity and food intake behaviors in obese mice

Central and peripheral mechanisms are both required for proper control of energy homeostasis. Among circulating plasma proteins, C1q/TNF-related proteins (CTRPs) have recently emerged as important regulators of sugar and fat metabolism. CTRP4, expressed in brain and adipose tissue, is unique among the family members in having two tandem globular C1q domains. We previously showed that central administration of recombinant CTRP4 suppresses food intake, suggesting a central nervous system role in regulating ingestive physiology. Whether this effect is pharmacological or physiological remains unclear. We used a loss-of-function knockout (KO) mouse model to clarify the physiological role of CTRP4. Under basal conditions, CTRP4 deficiency increased serum cholesterol levels and impaired glucose tolerance in male but not female mice fed a control low-fat diet. When challenged with a high-fat diet, male and female KO mice responded differently to weight gain and had different food intake patterns. On an obesogenic diet, male KO mice had similar weight gain as wild-type littermates. When fed ad libitum, KO male mice had greater meal number, shorter intermeal interval, and reduced satiety ratio. Female KO mice, in contrast, had lower body weight and adiposity. In the refeeding period following food deprivation, female KO mice had significantly higher food intake due to longer meal duration and reduced satiety ratio. Collectively, our data provide genetic evidence for a sex-dependent physiological role of CTRP4 in modulating food intake patterns and systemic energy metabolism.

Regulation of melanocortin-4-receptor (MC4R) expression by SNP rs17066842 is dependent on glucose concentration

Melanocortin-4-receptor (MC4R) gene codes for a G-protein-coupled receptor that is highly expressed in the hypothalamus and involved in the regulation of appetite. Single-nucleotide polymorphisms (SNPs) in the MC4R gene region have been associated with obesity, type 2-diabetes (T2D) and with antipsychotic-induced weight gain. Of these, rs17066842 (G>A) in the MC4R promoter region is the top variant associated with obesity and diabetes. In this study, we investigated the effect of rs17066842 on MC4R expression at various glucose concentrations using reporter gene expression in the SH-SY5Y cell line and regulation of MC4R expression in human cerebral organoids. We observed that higher glucose concentrations significantly reduced MC4R mRNA expression in SH-SY5Y cells. In addition, at high glucose concentrations, the luciferase reporter plasmid containing the MC4R promoter insert with the G-allele of rs170066842 showed significantly reduced activity compared to the A-allele carrying plasmid. The immediate early gene product, early growth-response 1 (EGR-1), was identified to bind to the sequence containing the G-allele at rs17066842 but not to the A-allele-containing sequence. Interestingly, in human induced pluripotent stem cell (hiPSC)-derived cerebral organoids, we observed increased MC4R expression in response to high glucose exposure. These opposite observations might suggest that glucose regulation is complex and may be cell-specific. This study provides evidence that rs17066842 regulates MC4R gene expression through binding of EGR-1 and that this process is influenced by glucose concentration.

Brain-Derived Neurotrophic Factor Affects mRNA and miRNA Expression of the Appetite Regulating Centre in the Sheep Arcuate Nucleus

The neuromodulatory effects of brain-derived neurotrophic factor (BDNF) on appetite regulation centre peptide gene activity in the sheep hypothalamus have not been examined yet. The aim of this study was to determine whether BDNF participates in modulation of neuropeptide Y (npy), agouti-related peptide (agrp), cocaine and amphetamine regulated transcript (cart), and proopiomelanocortin (pomc) mRNA expression and selected microRNAs in the sheep hypothalamic arcuate (ARC) nucleus. Animals (Polish Merino sheep, n=24) were divided into three groups. The control group received a central infusion of Ringer-Locke solution (480 µl/day) whereas the experimental groups were treated with BDNF in two doses: 10 or 60 μg/480 µl/day. All sheep received four intracerebroventricular infusions (performed from 08:40 a.m. to 01:30 p.m.; infusion scheme: 4 x 50 min infusions with 30 min intervals between them) on each of three consecutive days. Immediately after the last infusion, the sheep were slaughtered, and selected structures of the hypothalamus were frozen for further real-time qPCR analysis. Central infusion of BDNF evoked dose-dependent changes in npy, agrp, cart, pomc and peptidylglicine alpha-amidating monooxygenase (pam) mRNA expression in the sheep ARC nucleus. An increase in npy, agrp and pomc mRNA expression but also a decrease in cart mRNA expression in the ARC nucleus were detected. Moreover, a decrease in pam (gene encoding an enzyme that converts POMC into α-MSH) mRNA expression, was also noted. Furthermore, after central BDNF administration, changes in miRNA-33a-5p, miRNA-33b-5p, miRNA-377-3p, miRNA-214-3p, miRNA-485 and miRNA-488 expression were observed. Based on the presented results, it can be concluded that BDNF may affect the appetite regulating centre activity through modulation of npy, agrp, cart, pomc and pam mRNA expression in the ARC nucleus. It was also revealed that BDNF modulates miRNA expression in the sheep ARC nucleus.

Hindbrain melanocortin 3/4 receptors modulate the food intake and body weight suppressive effects of the GLP-1 receptor agonist, liraglutide

Simultaneously targeting multiple energy balance control systems is a promising direction for the development of obesity pharmacotherapies. Here, we explore the interaction between the GLP-1 and melanocortin system within the dorsal vagal complex (DVC) of the caudal brainstem. Using a pharmacological approach, we demonstrate that the full anorectic potential of liraglutide, an FDA-approved GLP-1 analog for the treatment of obesity, requires DVC melanocortin 3/4 receptor (MC3/4R) signaling. Specifically, the food intake and body weight suppressive effects of liraglutide were attenuated by DVC administration of the MC3/4R antagonist SHU9119. In contrast, the anorectic effects of liraglutide were enhanced by combined activation of DVC MC3/4Rs using the agonist MTII. Our findings highlight the modulation of liraglutide-induced anorexia by DVC MC3/4R signaling, thereby suggesting a site of action at which two important energy balance control systems interact.

Ciliary gene RPGRIP1L is required for hypothalamic arcuate neuron development

Intronic polymorphisms in the α-ketoglutarate-dependent dioxygenase gene (FTO) that are highly associated with increased body weight have been implicated in the transcriptional control of a nearby ciliary gene, retinitis pigmentosa GTPase regulator-interacting protein-1 like (RPGRIP1L). Previous studies have shown that congenital Rpgrip1l hypomorphism in murine proopiomelanocortin (Pomc) neurons causes obesity by increasing food intake. Here, we show by congenital and adult-onset Rpgrip1l deletion in Pomc-expressing neurons that the hyperphagia and obesity are likely due to neurodevelopmental effects that are characterized by a reduction in the Pomc/Neuropeptide Y (Npy) neuronal number ratio and marked increases in arcuate hypothalamic-paraventricular hypothalamic (ARH-PVH) axonal projections. Biallelic RPGRIP1L mutations result in fewer cilia-positive human induced pluripotent stem cell-derived (iPSC-derived) neurons and blunted responses to Sonic Hedgehog (SHH). Isogenic human ARH-like embryonic stem cell-derived (ESc-derived) neurons homozygous for the obesity-risk alleles at rs8050136 or rs1421085 have decreased RPGRIP1L expression and have lower numbers of POMC neurons. RPGRIP1L overexpression increases POMC cell number. These findings suggest that apparently functional intronic polymorphisms affect hypothalamic RPGRIP1L expression and impact development of POMC neurons and their derivatives, leading to hyperphagia and increased adiposity.

Alpha-melanocyte stimulating hormone in ghrelin-elicited feeding and gut motility

This review evaluates published studies regarding alpha-melanocyte stimulating hormone (α-MSH) in ghrelin-elicited feeding and gut motility. We have sought to integrate all available evidences to provide a complete review on the properties of melanocortin receptors (MCR) and the potential clinical treatment of α-MSH after ghrelin-elicited feeding and gut motility. The available studies were grouped into four categories: food intake, gastric emptying, small intestinal transit, and colonic transit. As we describe, the literature provides evidence of the ability of ghrelin to increase food intake, gastric emptying, small intestinal transit, and colonic transit. α-MSH, which displays high affinity for the MC3 and MC4 receptors, can competitively activate MCRs with agouti-related protein stimulated by ghrelin, and partly attenuates the effect of acyl ghrelin on food intake. Central ghrelin-induced acceleration of gastric emptying is not mediated by MCRs, but the acceleration of the small intestinal transit is at least partly mediated via MCRs in the brain. Similar to fecal pellets and total fecal weight, distal colonic motility and secretion are partly mediated by MCRs in the brain. The interplay between acyl ghrelin and MCRs may provide a new therapeutic avenue to ameliorate anorexia and constipation.

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.

TAp63 contributes to sexual dimorphism in POMC neuron functions and energy homeostasis

Sexual dimorphism exists in energy balance, but the underlying mechanisms remain unclear. Here we show that the female mice have more pro-opiomelanocortin (POMC) neurons in the arcuate nucleus of hypothalamus than males, and female POMC neurons display higher neural activities, compared to male counterparts. Strikingly, deletion of the transcription factor, TAp63, in POMC neurons confers "male-like" diet-induced obesity (DIO) in female mice associated with decreased POMC neural activities; but the same deletion does not affect male mice. Our results indicate that TAp63 in female POMC neurons contributes to the enhanced POMC neuron functions and resistance to obesity in females. Thus, TAp63 in POMC neurons is one key molecular driver for the sexual dimorphism in energy homeostasis.

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.

Integrated insights into the role of alpha-melanocyte stimulatory hormone in the control of food intake and glycaemia

Identifying peptide hormones with multipotent actions on both weight and glycaemia can have a significant impact on therapeutic options in the treatment of obesity and diabetes. This has been exemplified by recent advances involving pharmacological exploitation of glucagon-like peptide 1 biology. Herein, we summarise evidence supporting the potential candidacy in this light of alpha-melanocyte stimulatory hormone, an endogenous peptide hormone and a breakdown product of the neuropeptide pro-opiomelanocortin. We reference its well described central actions in the control of food intake and moreover highlight new data pointing to an important role for this peptide hormone in the periphery, in relation to glycaemic control.

α-MSH Influences the Excitability of Feeding-Related Neurons in the Hypothalamus and Dorsal Vagal Complex of Rats

Alpha-melanocyte-stimulating hormone (α-MSH) is processed from proopiomelanocortin (POMC) and acts on the melanocortin receptors, MC3 and MC4. α-MSH plays a key role in energy homeostasis. In the present study, to shed light on the mechanisms by which α-MSH exerts its anorectic effects, extracellular neuronal activity was recorded in the hypothalamus and the dorsal vagal complex (DVC) of anesthetized rats. We examined the impact of α-MSH on glucose-sensing neurons and gastric distension (GD) sensitive neurons. In the lateral hypothalamus (LHA), α-MSH inhibited 75.0% of the glucose-inhibited (GI) neurons. In the ventromedial nucleus (VMN), most glucose-sensitive neurons were glucose-excited (GE) neurons, which were mainly activated by α-MSH. In the paraventricular nucleus (PVN), α-MSH suppressed the majority of GI neurons and excited most GE neurons. In the DVC, among the 20 GI neurons examined for a response to α-MSH, 1 was activated, 16 were depressed, and 3 failed to respond. Nineteen of 24 GE neurons were activated by α-MSH administration. Additionally, among the 42 DVC neurons examined for responses to GD, 23 were excited (GD-EXC) and 19 were inhibited (GD-INH). Fifteen of 20 GD-EXC neurons were excited, whereas 11 out of 14 GD-INH neurons were suppressed by α-MSH. All these responses were abolished by pretreatment with the MC3/4R antagonist, SHU9119. In conclusion, the activity of glucose-sensitive neurons and GD-sensitive neurons in the hypothalamus and DVC can be modulated by α-MSH.

CART neurons in the arcuate nucleus and lateral hypothalamic area exert differential controls on energy homeostasis

OBJECTIVE

The cocaine- and amphetamine-regulated transcript (CART) codes for a pivotal neuropeptide important in the control of appetite and energy homeostasis. However, limited understanding exists for the defined effector sites underlying CART function, as discrepant effects of central CART administration have been reported.

METHODS

By combining Cart-cre knock-in mice with a Cart adeno-associated viral vector designed using the flip-excision switch (AAV-FLEX) technology, specific reintroduction or overexpression of CART selectively in CART neurons in the arcuate nucleus (Arc) and lateral hypothalamic area (LHA), respectively, was achieved. The effects on energy homeostasis control were investigated.

RESULTS

Here we show that CART neuron-specific reintroduction of CART into the Arc and LHA leads to distinct effects on energy homeostasis control. Specifically, CART reintroduction into the Arc of otherwise CART-deficient Cart^cre/cre mice markedly decreased fat mass and body weight, whereas CART reintroduction into the LHA caused significant fat mass gain and lean mass loss, but overall unaltered body weight. The reduced adiposity in Arc^CART;Cart^cre/cre mice was associated with an increase in both energy expenditure and physical activity, along with significantly decreased Npy mRNA levels in the Arc but with no change in food consumption. Distinctively, the elevated fat mass in LHA^CART;Cart^cre/cre mice was accompanied by diminished insulin responsiveness and glucose tolerance, greater spontaneous food intake, and reduced energy expenditure, which is consistent with the observed decrease of brown adipose tissue temperature. This is also in line with significantly reduced tyrosine hydroxylase (Th) and notably increased corticotropin-releasing hormone (Crh) mRNA expressions in the paraventricular nucleus (PVN).

CONCLUSIONS

Taken together, these results identify catabolic and anabolic effects of CART in the Arc and LHA, respectively, demonstrating for the first time the distinct and region-specific functions of CART in controlling feeding and energy homeostasis.

Inhibition of central de novo ceramide synthesis restores insulin signaling in hypothalamus and enhances β-cell function of obese Zucker rats

Objectives

Hypothalamic lipotoxicity has been shown to induce central insulin resistance and dysregulation of glucose homeostasis; nevertheless, elucidation of the regulatory mechanisms remains incomplete. Here, we aimed to determine the role of de novo ceramide synthesis in hypothalamus on the onset of central insulin resistance and the dysregulation of glucose homeostasis induced by obesity.

Methods

Hypothalamic GT1-7 neuronal cells were treated with palmitate. De novo ceramide synthesis was inhibited either by pharmacological (myriocin) or molecular (si-Serine Palmitoyl Transferase 2, siSPT2) approaches. Obese Zucker rats (OZR) were intracerebroventricularly infused with myriocin to inhibit de novo ceramide synthesis. Insulin resistance was determined by quantification of Akt phosphorylation. Ceramide levels were quantified either by a radioactive kinase assay or by mass spectrometry analysis. Glucose homeostasis were evaluated in myriocin-treated OZR. Basal and glucose-stimulated parasympathetic tonus was recorded in OZR. Insulin secretion from islets and β-cell mass was also determined.

Results

We show that palmitate impaired insulin signaling and increased ceramide levels in hypothalamic neuronal GT1-7 cells. In addition, the use of deuterated palmitic acid demonstrated that palmitate activated several enzymes of the de novo ceramide synthesis pathway in hypothalamic cells. Importantly, myriocin and siSPT2 treatment restored insulin signaling in palmitate-treated GT1-7 cells. Protein kinase C (PKC) inhibitor or a dominant-negative PKCζ also counteracted palmitate-induced insulin resistance. Interestingly, attenuating the increase in levels of hypothalamic ceramides with intracerebroventricular infusion of myriocin in OZR improved their hypothalamic insulin-sensitivity. Importantly, central myriocin treatment partially restored glucose tolerance in OZR. This latter effect is related to the restoration of glucose-stimulated insulin secretion and an increase in β-cell mass of OZR. Electrophysiological recordings also showed an improvement of glucose-stimulated parasympathetic nerve activity in OZR centrally treated with myriocin.

Conclusion

Our results highlight a key role of hypothalamic de novo ceramide synthesis in central insulin resistance installation and glucose homeostasis dysregulation associated with obesity.

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de novo ceramide synthesis induces hypothalamic insulin resistance through PKCζ.

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Hypothalamic ceramides induce glucose homeostasis dysregulation seen with obesity.

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Hypothalamic ceramides mediate inhibition of insulin secretion induced by obesity.

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Hypothalamic ceramides decreases β cell mass in obese rats.

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Hypothalamic ceramides decreases parasympathetic tonus.

α-melanocyte stimulating hormone modulates the central acyl ghrelin-induced stimulation of feeding, gastrointestinal motility, and colonic secretion

Background

Acyl ghrelin-induced intake depends on hypothalamic neuropeptide Y and agouti-related protein (AgRP) neurotransmitters. Intracerebroventricular (ICV) injection of AgRP increases feeding through competitive antagonism at melanocortin receptors. ICV administration of α-melanocyte stimulating hormone (α-MSH), a natural antagonist of AgRP, may modulate the acyl ghrelin-induced orexigenic effect.

Objective

This study aimed to investigate the modulating effect of α-MSH on the central acyl ghrelin-induced food intake, gastrointestinal motility, and colonic secretion in rats.

Methods and procedures

We examined the effects of α-MSH and acyl ghrelin on food intake, gastric emptying, small intestinal transit, colonic motility, and secretion in conscious rats with a chronic implant of ICV catheters.

Results

ICV injection of O-n-octanoylated ghrelin (0.1 nmol/rat) significantly increased the cumulative food intake up to 8 h (P<0.01), enhanced non-nutrient semi-liquid gastric emptying (P<0.001), increased the geometric center and running percentage of small intestinal transit (P<0.001), accelerated colonic transit time (P<0.05), and increased fecal pellet output (P<0.01) and total fecal weight (P<0.01). Pretreatment with ICV injection of α-MSH (1.0 and 2.0 nmol/rat) attenuated the acyl ghrelin-induced hyperphagic effect, fecal pellet output, and total fecal weight, while higher dose of α-MSH (2.0 nmol/rat) attenuated the increase in the geometric center of small intestinal transit (P<0.01). However, neither dose of α-MSH altered acyl ghrelin-stimulated gastroprokinetic effect, increase in the running percentage of small intestinal transit, nor accelerated colonic transit time.

Conclusion

α-MSH is involved in central acyl ghrelin-elicited feeding, small intestinal transit, fecal pellet output, and fecal weight. α-MSH does not affect central acyl ghrelin-induced acceleration of gastric emptying and colonic transit time in rats.

Adiponectin potentiates the acute effects of leptin in arcuate Pomc neurons

Objective

Adiponectin receptors (AdipoRs) are located on neurons of the hypothalamus involved in metabolic regulation – including arcuate proopiomelanocortin (Pomc) and Neuropeptide Y/Agouti-related peptide (NPY/AgRP) neurons. AdipoRs play a critical role in regulating glucose and fatty acid metabolism by initiating several signaling cascades overlapping with Leptin receptors (LepRs). However, the mechanism by which adiponectin regulates cellular activity in the brain remains undefined.

Methods

In order to resolve this issue, we utilized neuron-specific transgenic mouse models to identify Pomc and NPY/AgRP neurons which express LepRs for patch-clamp electrophysiology experiments.

Results

We found that leptin and adiponectin synergistically activated melanocortin neurons in the arcuate nucleus. Conversely, NPY/AgRP neurons were inhibited in response to adiponectin. The adiponectin-induced depolarization of arcuate Pomc neurons occurred via activation of Phosphoinositide-3-kinase (PI3K) signaling, independent of 5′ AMP-activated protein kinase (AMPK) activity. Adiponectin also activated melanocortin neurons at various physiological glucose levels.

Conclusions

Our results demonstrate a requirement for PI3K signaling in the acute adiponectin-induced effects on the cellular activity of arcuate melanocortin neurons. Moreover, these data provide evidence for PI3K as a substrate for both leptin and adiponectin to regulate energy balance and glucose metabolism via melanocortin activity.

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Adiponectin activates arcuate Pomc neurons.

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Adiponectin-induced activation of Pomc neurons requires PI3K (independent of AMPK).

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Adiponectin inhibits adjacent NPY/AgRP neurons (disinhibiting arcuate Pomc neurons).

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Leptin potentiates the effects of adiponectin arcuate Pomc neurons.

Melanocortin 3 Receptor Signaling in Midbrain Dopamine Neurons Increases the Motivation for Food Reward

The central melanocortin (MC) system mediates its effects on food intake via MC3 (MC3R) and MC4 receptors (MC4R). Although the role of MC4R in meal size determination, satiation, food preference, and motivation is well established, the involvement of MC3R in the modulation of food intake has been less explored. Here, we investigated the role of MC3R on the incentive motivation for food, which is a crucial component of feeding behavior. Dopaminergic neurons within the ventral tegmental area (VTA) have a crucial role in the motivation for food. We here report that MC3Rs are expressed on VTA dopaminergic neurons and that pro-opiomelanocortinergic (POMC) neurons in the arcuate nucleus of the hypothalamus (Arc) innervate these VTA dopaminergic neurons. Our findings show that intracerebroventricular or intra-VTA infusion of the selective MC3R agonist γMSH increases responding for sucrose under a progressive ratio schedule of reinforcement, but not free sucrose consumption in rats. Furthermore, ex vivo electrophysiological recordings show increased VTA dopaminergic neuronal activity upon γMSH application. Consistent with a dopamine-mediated effect of γMSH, the increased motivation for sucrose after intra-VTA infusion of γMSH was blocked by pretreatment with the dopamine receptor antagonist α-flupenthixol. Taken together, we demonstrate an Arc POMC projection onto VTA dopaminergic neurons that modulates motivation for palatable food via activation of MC3R signaling.

PCSK1 Mutations and Human Endocrinopathies: From Obesity to Gastrointestinal Disorders

Prohormone convertase 1/3, encoded by the PCSK1 gene, is a serine endoprotease that is involved in the processing of a variety of proneuropeptides and prohormones. Humans who are homozygous or compound heterozygous for loss-of-function mutations in PCSK1 exhibit a variable and pleiotropic syndrome consisting of some or all of the following: obesity, malabsorptive diarrhea, hypogonadotropic hypogonadism, altered thyroid and adrenal function, and impaired regulation of plasma glucose levels in association with elevated circulating proinsulin-to-insulin ratio. Recently, more common variants in the PCSK1 gene have been found to be associated with alterations in body mass index, increased circulating proinsulin levels, and defects in glucose homeostasis. This review provides an overview of the endocrinopathies and other disorders observed in prohormone convertase 1/3-deficient patients, discusses the possible biochemical basis for these manifestations of the disease, and proposes a model whereby certain missense mutations in PCSK1 may result in proteins with a dominant negative action.

The role of GluN2A and GluN2B NMDA receptor subunits in AgRP and POMC neurons on body weight and glucose homeostasis

OBJECTIVE

Hypothalamic agouti-related peptide (AgRP) and pro-opiomelanocortin (POMC) expressing neurons play critical roles in control of energy balance. Glutamatergic input via n-methyl-d-aspartate receptors (NMDARs) is pivotal for regulation of neuronal activity and is required in AgRP neurons for normal body weight homeostasis. NMDARs typically consist of the obligatory GluN1 subunit and different GluN2 subunits, the latter exerting crucial differential effects on channel activity and neuronal function. Currently, the role of specific GluN2 subunits in AgRP and POMC neurons on whole body energy and glucose balance is unknown.

METHODS

We used the cre-lox system to genetically delete GluN2A or GluN2B only from AgRP or POMC neurons in mice. Mice were then subjected to metabolic analyses and assessment of AgRP and POMC neuronal function through morphological studies.

RESULTS

We show that loss of GluN2B from AgRP neurons reduces body weight, fat mass, and food intake, whereas GluN2B in POMC neurons is not required for normal energy balance control. GluN2A subunits in either AgRP or POMC neurons are not required for regulation of body weight. Deletion of GluN2B reduces the number of AgRP neurons and decreases their dendritic length. In addition, loss of GluN2B in AgRP neurons of the morbidly obese and severely diabetic leptin-deficient Lep (ob/ob) mice does not affect body weight and food intake but, remarkably, leads to full correction of hyperglycemia. Lep (ob/ob) mice lacking GluN2B in AgRP neurons are also more sensitive to leptin's anti-obesity actions.

CONCLUSIONS

GluN2B-containing NMDA receptors in AgRP neurons play a critical role in central control of body weight homeostasis and blood glucose balance via mechanisms that likely involve regulation of AgRP neuronal survival and structure, and modulation of hypothalamic leptin action.

Pituitary Adenylate Cyclase-Activating Peptide in the Central Amygdala Causes Anorexia and Body Weight Loss via the Melanocortin and the TrkB Systems

Growing evidence suggests that the pituitary adenylate cyclase-activating polypeptide (PACAP)/PAC1 receptor system represents one of the main regulators of the behavioral, endocrine, and autonomic responses to stress. Although induction of anorexia is a well-documented effect of PACAP, the central sites underlying this phenomenon are poorly understood. The present studies addressed this question by examining the neuroanatomical, behavioral, and pharmacological mechanisms mediating the anorexia produced by PACAP in the central nucleus of the amygdala (CeA), a limbic structure implicated in the emotional components of ingestive behavior. Male rats were microinfused with PACAP (0-1 μg per rat) into the CeA and home-cage food intake, body weight change, microstructural analysis of food intake, and locomotor activity were assessed. Intra-CeA (but not intra-basolateral amygdala) PACAP dose-dependently induced anorexia and body weight loss without affecting locomotor activity. PACAP-treated rats ate smaller meals of normal duration, revealing that PACAP slowed feeding within meals by decreasing the regularity and maintenance of feeding from pellet-to-pellet; postprandial satiety was unaffected. Intra-CeA PACAP-induced anorexia was blocked by coinfusion of either the melanocortin receptor 3/4 antagonist SHU 9119 or the tyrosine kinase B (TrKB) inhibitor k-252a, but not the CRF receptor antagonist D-Phe-CRF(12-41). These results indicate that the CeA is one of the brain areas through which the PACAP system promotes anorexia and that PACAP preferentially lessens the maintenance of feeding in rats, effects opposite to those of palatable food. We also demonstrate that PACAP in the CeA exerts its anorectic effects via local melanocortin and the TrKB systems, and independently from CRF.

Conditional expression of Pomc in the Lepr-positive subpopulation of POMC neurons is sufficient for normal energy homeostasis and metabolism

Peptides derived from the proopiomelanocortin (POMC) precursor are critical for the normal regulation of many physiological parameters, and POMC deficiency results in severe obesity and metabolic dysfunction. Conversely, augmentation of central nervous system melanocortin function is a promising therapeutic avenue for obesity and diabetes but is confounded by detrimental cardiovascular effects including hypertension. Because the hypothalamic population of POMC-expressing neurons is neurochemically and neuroanatomically heterogeneous, there is interest in the possible dissociation of functionally distinct POMC neuron subpopulations. We used a Cre recombinase-dependent and hypothalamus-specific reactivatable PomcNEO allele to restrict Pomc expression to hypothalamic neurons expressing leptin receptor (Lepr) in mice. In contrast to mice with total hypothalamic Pomc deficiency, which are severely obese, mice with Lepr-restricted Pomc expression displayed fully normal body weight, food consumption, glucose homeostasis, and locomotor activity. Thus, Lepr+ POMC neurons, which constitute approximately two-thirds of the total POMC neuron population, are sufficient for normal regulation of these parameters. This functional dissociation approach represents a promising avenue for isolating therapeutically relevant POMC neuron subpopulations.

Whole-brain mapping of the direct inputs and axonal projections of POMC and AgRP neurons

Pro-opiomelanocortin (POMC) neurons in the arcuate nucleus (ARC) of the hypothalamus and nucleus tractus solitarius (NTS) of the brainstem play important roles in suppressing food intake and maintaining energy homeostasis. Previous tract-tracing studies have revealed the axonal connection patterns of these two brain areas, but the intermingling of POMC neurons with other neuron types has made it challenging to precisely identify the inputs and outputs of POMC neurons. In this study, we used the modified rabies virus to map the brain areas that provide direct inputs to the POMC neurons in the ARC and NTS as well as the inputs to the ARC AgRP neurons for comparison. ARC POMC neurons receive inputs from dozens of discrete structures throughout the forebrain and brainstem. The brain areas containing the presynaptic partners of ARC POMC neurons largely overlap with those of ARC AgRP neurons, although POMC neurons receive relatively broader, denser inputs. Furthermore, POMC neurons in the NTS receive direct inputs predominantly from the brainstem and show very different innervation patterns for POMC neurons in the ARC. By selectively expressing fluorescent markers in the ARC and NTS POMC neurons, we found that almost all of their major presynaptic partners are innervated by POMC neurons in the two areas, suggesting that there are strong reciprocal projections among the major POMC neural pathways. By comprehensively chartering the whole-brain connections of the central melanocortin system in a cell-type-specific manner, this study lays the foundation for dissecting the roles and underlying circuit mechanisms of specific neural pathways in regulating energy homeostasis.

Glycemic state regulates melanocortin, but not nesfatin-1, responsiveness of glucose-sensing neurons in the nucleus of the solitary tract

The nucleus of the solitary tract (NTS) is a medullary integrative center with critical roles in the coordinated control of energy homeostasis. Here, we used whole cell current-clamp recordings on rat NTS neurons in slice preparation to identify the presence of physiologically relevant glucose-sensing neurons. The majority of NTS neurons (n = 81) were found to be glucose-responsive, with 35% exhibiting a glucose-excited (GE) phenotype (mean absolute change in membrane potential: 9.5 ± 1.1 mV), and 21% exhibiting a glucose-inhibited (GI) response (mean: 6.3 ± 0.7 mV). Furthermore, we found glucose-responsive cells are preferentially influenced by the anorexigenic peptide α-melanocyte-stimulating hormone (α-MSH), but not nesfatin-1. Accordingly, alterations in glycemic state have profound effects on the responsiveness of NTS neurons to α-MSH, but not to nesfatin-1. Indeed, NTS neurons showed increasing responsiveness to α-MSH as extracellular glucose concentrations were decreased, and in hypoglycemic conditions, all NTS neurons were depolarized by α-MSH (mean 10.6 ± 3.2 mV; n = 8). Finally, decreasing levels of extracellular glucose correlated with a significant hyperpolarization of the baseline membrane potential of NTS neurons, highlighting the modulatory effect of glucose on the baseline excitability of cells in this region. Our findings reveal individual NTS cells are capable of integrating multiple sources of metabolically relevant inputs, highlight the rapid capacity for plasticity in medullary melanocortin circuits, and emphasize the critical importance of physiological recording conditions for electrophysiological studies pertaining to the central control of energy homeostasis.

NMDA-type glutamate receptors participate in reduction of food intake following hindbrain melanocortin receptor activation

Hindbrain injection of a melanocortin-3/4 receptor agonist, MTII, reduces food intake primarily by reducing meal size. Our previously reported results indicate that N-methyl-D-aspartate-type glutamate receptors (NMDAR) in the nucleus of the solitary tract (NTS) play an important role in the control of meal size and food intake. Therefore, we hypothesized that activation of NTS NMDARs contribute to reduction of food intake in response to fourth ventricle or NTS injection of MTII. We found that coinjection of a competitive NMDAR antagonist (d-CPP-ene) with MTII into the fourth ventricle or directly into the NTS of adult male rats attenuated MTII-induced reduction of food intake. Hindbrain NMDAR antagonism also attenuated MTII-induced ERK1/2 phosphorylation in NTS neurons and prevented synapsin I phosphorylation in central vagal afferent endings, both of which are cellular mechanisms previously shown to participate in hindbrain melanocortinergic reduction of food intake. Together, our results indicate that NMDAR activation significantly contributes to reduction of food intake following hindbrain melanocortin receptor activation, and it participates in melanocortinergic signaling in NTS neural circuits that mediate reduction of food intake.

Central vagal afferent endings mediate reduction of food intake by melanocortin-3/4 receptor agonist

Injection of the melanocortin-3/4 receptor agonist melanotan-II (MTII) into the nucleus of the solitary tract (NTS) produces rapid and sustained reduction of food intake. Melanocortin-4 receptors (MC4Rs) are expressed by vagal afferent endings in the NTS, but it is not known whether these endings participate in MTII-induced reduction of food intake. In experiments described here, we evaluated the contribution of central vagal afferent endings in MTII-induced reduction of food intake. Examination of rat hindbrain sections revealed that neuronal processes expressing immunoreactivity for the endogenous MC4R agonist α-melanoctyte-stimulating hormone course parallel and wrap around anterogradely labeled vagal afferent endings in the NTS and thus are aptly positioned to activate vagal afferent MC4Rs. Furthermore, MTII and endogenous MC4R agonists increased protein kinase A (PKA)-catalyzed phosphorylation of synapsin I in vagal afferent endings, an effect known to increase synaptic strength by enhancing neurotransmitter release in other neural systems. Hindbrain injection of a PKA inhibitor, KT5720, significantly attenuated MTII-induced reduction of food intake and the increase in synapsin I phosphorylation. Finally, unilateral nodose ganglion removal, resulting in degeneration of vagal afferent endings in the ipsilateral NTS, abolished MTII-induced synapsin I phosphorylation ipsilateral to nodose ganglion removal. Moreover, reduction of food intake following MTII injection into the NTS ipsilateral to nodose ganglion removal was significantly attenuated, whereas the response to MTII was not diminished when injected into the contralateral NTS. Altogether, our results suggest that reduction of food intake following hindbrain MC4R activation is mediated by central vagal afferent endings.

CART in the regulation of appetite and energy homeostasis

The cocaine- and amphetamine-regulated transcript (CART) has been the subject of significant interest for over a decade. Work to decipher the detailed mechanism of CART function has been hampered by the lack of specific pharmacological tools like antagonists and the absence of a specific CART receptor(s). However, extensive research has been devoted to elucidate the role of the CART peptide and it is now evident that CART is a key neurotransmitter and hormone involved in the regulation of diverse biological processes, including food intake, maintenance of body weight, reward and addiction, stress response, psychostimulant effects and endocrine functions (Rogge et al., 2008; Subhedar et al., 2014). In this review, we focus on knowledge gained on CART's role in controlling appetite and energy homeostasis, and also address certain species differences between rodents and humans.

The CNS glucagon-like peptide-2 receptor in the control of energy balance and glucose homeostasis

The gut-brain axis plays a key role in the control of energy balance and glucose homeostasis. In response to luminal stimulation of macronutrients and microbiota-derived metabolites (secondary bile acids and short chain fatty acids), glucagon-like peptides (GLP-1 and -2) are cosecreted from endocrine L cells in the gut and coreleased from preproglucagonergic neurons in the brain stem. Glucagon-like peptides are proposed as key mediators for bariatric surgery-improved glycemic control and energy balance. Little is known about the GLP-2 receptor (Glp2r)-mediated physiological roles in the control of food intake and glucose homeostasis, yet Glp1r has been studied extensively. This review will highlight the physiological relevance of the central nervous system (CNS) Glp2r in the control of energy balance and glucose homeostasis and focuses on cellular mechanisms underlying the CNS Glp2r-mediated neural circuitry and intracellular PI3K signaling pathway. New evidence (obtained from Glp2r tissue-specific KO mice) indicates that the Glp2r in POMC neurons is essential for suppressing feeding behavior, gastrointestinal motility, and hepatic glucose production. Mice with Glp2r deletion selectively in POMC neurons exhibit hyperphagic behavior, accelerated gastric emptying, glucose intolerance, and hepatic insulin resistance. GLP-2 differentially modulates postsynaptic membrane excitability of hypothalamic POMC neurons in Glp2r- and PI3K-dependent manners. GLP-2 activates the PI3K-Akt-FoxO1 signaling pathway in POMC neurons by Glp2r-p85α interaction. Intracerebroventricular GLP-2 augments glucose tolerance, suppresses glucose production, and enhances insulin sensitivity, which require PI3K (p110α) activation in POMC neurons. Thus, the CNS Glp2r plays a physiological role in the control of food intake and glucose homeostasis. This review will also discuss key questions for future studies.

α-MSH overexpression in the nucleus tractus solitarius decreases fat mass and elevates heart rate

The POMC pathway is involved in the regulation of energy and cardiovascular homeostasis in the hypothalamus and the brain stem. Although the acute effects of POMC-derived peptides in different brain locations have been elucidated, the chronic site-specific effects of distinct peptides remain to be studied. To this end, we used a lentiviral gene delivery vector to study the long-term effects of α-MSH in the nucleus tractus solitarius (NTS) of the brain stem. The α-MSH vector (LVi-α-MSH-EGFP) based on the N-terminal POMC sequence and a control vector (LVi-EGFP) were delivered into the NTS of C57BL/6N male mice fed on a western diet. Effects on body weight and composition, feeding, glucose metabolism, and hemodynamics by telemetric analyses were studied during the 12-week follow-up. The LVi-α-MSH-EGFP-treated mice had a significantly smaller gain in the fat mass compared with LVi-EGFP-injected mice. There was a small initial decrease in food intake and no differences in the physical activity. Glucose metabolism was not changed compared with the control. LVi-α-MSH-EGFP increased the heart rate (HR), which was attenuated by adrenergic blockade suggesting an increased sympathetic activity. Reduced response to muscarinic blockade suggested a decreased parasympathetic activity. Fitting with sympathetic activation, LVi-α-MSH-EGFP treatment reduced urine secretion. Thus, the results demonstrate that long-term α-MSH overexpression in the NTS attenuates diet-induced obesity. Modulation of autonomic nervous system tone increased the HR and most probably contributed to an anti-obesity effect. The results underline the key role of NTS in the α-MSH-induced long-term effects on adiposity and in regulation of sympathetic and parasympathetic activities.

High-fat diet induces site-specific unresponsiveness to LPS-stimulated STAT3 activation in the hypothalamus

Hypophagia induced by inflammation is associated with Janus kinase (JAK)-2/signal transducer and activator of transcription (STAT) 3 signaling pathway, and leptin-mediated hypophagia is also mediated by JAK2-STAT3 pathway. We have previously reported that lipopolysaccharide (LPS) did not reduce food intake in leptin-resistant high-fat diet (HFD) rats but maintained body weight loss. We investigated whether changes in p-STAT3 expression in the hypothalamus and brain stem could account for the desensitization of hypophagia in HFD animals after a low LPS dose (100 μg/kg). Wistar rats fed standard diet (3.95 kcal/g) or HFD (6.3 kcal/g) for 8 wk were assigned into control diet-saline, control diet-LPS, HFD-saline, and HFD-LPS groups. LPS reduced feeding in the control diet but not HFD. This group showed no p-STAT3 expression in the paraventricular nucleus (PVN) and ventromedial hypothalamic nucleus (VMH), but sustained, though lower than control, p-STAT3 in the nucleus of the solitary tract (NTS) and raphe pallidus (RPa). LPS decreased body weight in HFD rats and increased Fos expression in the NTS. LPS increased body temperature, oxygen consumption, and energy expenditure in both control diet and HFD rats, and this response was more pronounced in HFD-LPS group. Brown adipose tissue (BAT) thermogenesis and increased energy expenditure seem to contribute to body weight loss in HFD-LPS. This response might be related with increased brain stem activation. In conclusion, LPS activates STAT3-mediated pathway in the hypothalamus and brain stem, leading to hypophagia, however, LPS effects on food intake, but not body weight loss, are abolished by leptin resistance induced by HFD. The preserved STAT3 phosphorylation in the brain stem suggests that unresponsiveness to LPS on STAT3 activation under HFD might be selective to the hypothalamus.

α-MSH exerts direct postsynaptic excitatory effects on NTS neurons and enhances GABAergic signaling in the NTS

The central melanocortin system plays an essential role in the regulation of energy balance. While anorexigenic effects of α-melanocyte-stimulating hormone (α-MSH) acting in the nucleus of the solitary tract (NTS), a critical medullary autonomic control center, have been established, the cellular events underlying these effects are less well characterized. In this study, we used whole-cell patch-clamp electrophysiology to examine firstly whether α-MSH exerts direct postsynaptic effects on the membrane potential of rat NTS neurons in slice preparation, and secondly whether α-MSH influences GABAergic signaling in the NTS. In normal artificial cerebrospinal fluid, perfusion of α-MSH (500 nM) resulted in a depolarization in 39% of cells (n=16, mean 6.14±0.54 mV), and a hyperpolarization in 22% of cells (n=9, -6.79±1.02 mV). Studies using tetrodotoxin to block neuronal communication revealed α-MSH exerts direct depolarizing effects on some NTS neurons, and indirect inhibitory effects on others. A third subset of neurons is simultaneously directly depolarized and indirectly hyperpolarized by α-MSH, resulting in a net lack of effect on membrane potential. The inhibitory inputs influenced by α-MSH were identified as GABAergic, as α-MSH increased the frequency, but not amplitude, of inhibitory postsynaptic currents (IPSCs) in 50% of NTS neurons. α-MSH had no effect on the frequency or amplitude of miniature IPSCs. Furthermore, pharmacological blockade of GABAA and GABAB receptors, and physical removal of all synaptic inputs via cellular dissociation, abolished hyperpolarizations induced by α-MSH. We conclude α-MSH exerts direct, postsynaptic excitatory effects on a subset of NTS neurons. By exciting GABAergic NTS neurons and presynaptically enhancing GABAergic signaling, α-MSH also indirectly inhibits other NTS cells. These findings provide critical insight into the cellular events underlying medullary melanocortin anorexigenic effects, and expand the understanding of the circuitries involved in central melanocortin signaling.

Role of hypothalamic autophagy in the control of whole body energy balance

Autophagy is a catabolic process involving the rearrangement of subcellular membranes to sequester cytoplasm and organelles for delivery to lysosomes, where the sequestered material is degraded and recycled. Autophagy is important for maintenance of intracellular energy homeostasis and the quality control of organelles such as the endoplasmic reticulum (ER) and mitochondria, which suggests that dysregulated autophagy might play a role in the pathogenesis of metabolic disorders and diabetes. In an attempt to elucidate the role of autophagy in metabolic disorders, diverse in vivo and in vitro models have been employed. Site-specific autophagy knockout models that are autophagy-deficient specifically in pancreatic β-cells, skeletal muscle, adipose tissues or liver have been produced. These models have generated valuable information regarding the role of autophagy in body metabolism. The role of autophagy in the hypothalamus, which controls whole body energy balance, appetite and energy expenditure, has also been investigated. Thus, mice with autophagy deficiency in the hypothalamus have shown diverse phenotypes (lean vs. obese) depending on the site of autophagy deficiency or the method of autophagy abrogation.

Brain regulation of energy balance and body weight

Body weight is determined by a balance between food intake and energy expenditure. Multiple neural circuits in the brain have evolved to process information about food, food-related cues and food consumption to control feeding behavior. Numerous gastrointestinal endocrine cells produce and secrete satiety hormones in response to food consumption and digestion. These hormones suppress hunger and promote satiation and satiety mainly through hindbrain circuits, thus governing meal-by-meal eating behavior. In contrast, the hypothalamus integrates adiposity signals to regulate long-term energy balance and body weight. Distinct hypothalamic areas and various orexigenic and anorexigenic neurons have been identified to homeostatically regulate food intake. The hypothalamic circuits regulate food intake in part by modulating the sensitivity of the hindbrain to short-term satiety hormones. The hedonic and incentive properties of foods and food-related cues are processed by the corticolimbic reward circuits. The mesolimbic dopamine system encodes subjective "liking" and "wanting" of palatable foods, which is subjected to modulation by the hindbrain and the hypothalamic homeostatic circuits and by satiety and adiposity hormones. Satiety and adiposity hormones also promote energy expenditure by stimulating brown adipose tissue (BAT) activity. They stimulate BAT thermogenesis mainly by increasing the sympathetic outflow to BAT. Many defects in satiety and/or adiposity hormone signaling and in the hindbrain and the hypothalamic circuits have been described and are believed to contribute to the pathogenesis of energy imbalance and obesity.

Melanocortin signaling in the brainstem influences vagal outflow to the stomach

Activation of melanocortin 4 receptors (MC4-Rs) in brain nuclei associated with food intake profoundly influences consummatory behavior. Of these nuclei, the dorsal motor vagal nucleus (DMV), which has a dense concentration of MC4-Rs, is an important regulator of gastric tone and motility. Hence, the present study sought to examine the role of MC4-Rs in this nucleus on these activities. Using an in vivo approach, MC4-R agonists, melanotan-II (MT-II) or α-melanocyte stimulating hormone (α-MSH), were unilaterally microinjected into the DMV of rats, and their effects were noted on gastric activity. MT-II decreased phasic contractions, whereas α-MSH increased their amplitude. Both effects were blocked by the MC4-R antagonist SHU9119 or by ipsilateral vagotomy. Microinjection of the agonists (MT-II and α-MSH) into the overlying nucleus of the solitary tract (NTS), an important component of "vago-vagal" gastric circuitry, decreased phasic contractions. In addition, α-MSH reduced gastric tone and mean arterial blood pressure. To study the underlying mechanisms of the effect of MC4-R stimulation on gastric activity, electrophysiological recordings were made from labeled DMV antrum neurons in rat pups and MC4-R(-/-) mice. Bath application of MT-II or α-MSH significantly reduced spontaneous action potentials (but not in MC4-R(-/-) mice). However, in low-calcium ACSF, MT-II decreased neuronal firing, whereas α-MSH increased it. These effects mirror those of our in vivo DMV studies. Altogether, our novel findings show that activation of MC4-Rs in the brainstem, particularly in the medial NTS by the endogenous peptide α-MSH, modulates gastric activity, which may have physiological relevance for food intake and gastric function.

Melanocortin MC(4) receptor-mediated feeding and grooming in rodents

Decades ago it was recognized that the pharmacological profile of melanocortin ligands that stimulated grooming behavior in rats was strikingly similar to that of Xenopus laevis melanophore pigment dispersion. After cloning of the melanocortin MC1 receptor, expressed in melanocytes, and the melanocortin MC4 receptor, expressed mainly in brain, the pharmacological profiles of these receptors appeared to be very similar and it was demonstrated that these receptors mediate melanocortin-induced pigmentation and grooming respectively. Grooming is a low priority behavior that is concerned with care of body surface. Activation of central melanocortin MC4 receptors is also associated with meal termination, and continued postprandial stimulation of melanocortin MC4 receptors may stimulate natural postprandial grooming behavior as part of the behavioral satiety sequence. Indeed, melanocortins fail to suppress food intake or induce grooming behavior in melanocortin MC4 receptor-deficient rats. This review will focus on how melanocortins affect grooming behavior through the melanocortin MC4 receptor, and how melanocortin MC4 receptors mediate feeding behavior. This review also illustrates how melanocortins were the most likely candidates to mediate grooming and feeding based on the natural behaviors they induced.

Acute and long-term suppression of feeding behavior by POMC neurons in the brainstem and hypothalamus, respectively

POMC-derived melanocortins inhibit food intake. In the adult rodent brain, POMC-expressing neurons are located in the arcuate nucleus (ARC) and the nucleus tractus solitarius (NTS), but it remains unclear how POMC neurons in these two brain nuclei regulate feeding behavior and metabolism differentially. Using pharmacogenetic methods to activate or deplete neuron groups in separate brain areas, in the present study, we show that POMC neurons in the ARC and NTS suppress feeding behavior at different time scales. Neurons were activated using the DREADD (designer receptors exclusively activated by designer drugs) method. The evolved human M3-muscarinic receptor was expressed in a selective population of POMC neurons by stereotaxic infusion of Cre-recombinase-dependent, adeno-associated virus vectors into the ARC or NTS of POMC-Cre mice. After injection of the human M3-muscarinic receptor ligand clozapine-N-oxide (1 mg/kg, i.p.), acute activation of NTS POMC neurons produced an immediate inhibition of feeding behavior. In contrast, chronic stimulation was required for ARC POMC neurons to suppress food intake. Using adeno-associated virus delivery of the diphtheria toxin receptor gene, we found that diphtheria toxin-induced ablation of POMC neurons in the ARC but not the NTS, increased food intake, reduced energy expenditure, and ultimately resulted in obesity and metabolic and endocrine disorders. Our results reveal different behavioral functions of POMC neurons in the ARC and NTS, suggesting that POMC neurons regulate feeding and energy homeostasis by integrating long-term adiposity signals from the hypothalamus and short-term satiety signals from the brainstem.

Altered feeding and body weight following melanocortin administration to the ventral tegmental area in adult rats

RATIONALE

The melanocortin system is an important component of the brain circuitry controlling feeding and body weight, and most of the effects of melanocortins are attributed to their actions in hypothalamic and brainstem nuclei. The mesolimbic dopamine system is another component of the central circuitry controlling feeding, and there is evidence that melanocortins can act on mesolimbic dopamine pathways. It is unknown, however, whether melanocortins can act on the mesolimbic dopamine system to regulate feeding.

OBJECTIVE

These studies tested whether injection of melanocortin receptor agonists and antagonists directly into the ventral tegmental area (VTA) of adult rats affects feeding and body weight.

METHODS

Varying doses of the melanocortin receptor agonist, MTII, or the melanocortin receptor antagonist, SHU9119, were injected directly into the VTA, and food intake was measured at specific intervals. In addition, melanocortin receptors in the VTA were chronically blocked through repeated daily injections of SHU9119 into the VTA, and the resulting effects on food intake and body weight were determined.

RESULTS

Injection of MTII into the VTA dose-dependently inhibited feeding for up to 24 h, while injection of SHU9119 into the VTA dose-dependently stimulated feeding for up to 24 h. In addition, chronic blockade of melanocortin receptors in the VTA increased feeding, body weight, and caloric efficiency.

CONCLUSIONS

These studies demonstrate that melanocortins can control feeding and body weight by acting in the VTA and suggest that endogenous melanocortins control feeding in part through actions on the mesolimbic dopamine system in vivo.