Relative number and distribution of murine hypothalamic proopiomelanocortin neurons innervating distinct target sites

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.

Proopiomelanocortin projections to the nucleus accumbens modulate acquisition and maintenance of operant palatable pellet administration in mice

Obesity is a crisis in the United States, producing many co-morbid diseases that can drastically decrease quality of life. While diet is a major focus for therapeutic intervention, the need to understand underlying appetitive neurocircuitry persists. Proopiomelanocortin (POMC) peptides are well-known for their anorexigenic activity, but also mediate reward and learning. The nucleus accumbens (NAcc) is best known for its role in reward-based learning, but the contribution of POMC projections to NAcc on feeding are controversial since the two major POMC-derived peptides (β-endorphin and α-MSH) have opposite effects on food intake. Our objective was to determine the effect of stimulating POMC projections in the NAcc on acquisition and maintenance of operant self-administration of a palatable food. Adult POMCCre mice were microinjected into the NAcc with a Cre-dependent retrograde adeno-associated viral vector expressing Gq Designer Receptors Exclusively Activated by Designer Drugs (DREADDs). Mice were trained to self-administer palatable 20-mg pellets in daily operant sessions. Acquisition of self-administration (fixed ratio 30) and baseline self-administration were measured in daily sessions, with mice receiving injections of either JHU37152 (DREADD agonist) or saline (i.p.) 15 min prior to the sessions. POMC neuron stimulation (JHU injection) before training sessions produced a significant increase in rate of acquisition and accuracy compared to the saline treated group, with no significant effect on rewards earned. Removal of POMC neuron stimulation before sessions initially reduced consumption with a gradual increase in responding for reinforcer over 3 days of saline injections. Reinstatement of POMC neuron stimulation (JHU) before the session resulted in a significant decrease in responding and rewards earned. These results suggest a complex role of POMC peptides within the NAcc that increase reward learning for a novel palatable food while decreasing consumption of the reinforcer following experience with it.

The life and times of endogenous opioid peptides: Updated understanding of synthesis, spatiotemporal dynamics, and the clinical impact in alcohol use disorder

The opioid G-protein coupled receptors (GPCRs) strongly modulate many of the central nervous system structures that contribute to neurological and psychiatric disorders including pain, major depressive disorder, and substance use disorders. To better treat these and related diseases, it is essential to understand the signaling of their endogenous ligands. In this review, we focus on what is known and unknown about the regulation of the over two dozen endogenous peptides with high affinity for one or more of the opioid receptors. We briefly describe which peptides are produced, with a particular focus on the recently proposed possible synthesis pathways for the endomorphins. Next, we describe examples of endogenous opioid peptide expression organization in several neural circuits and how they appear to be released from specific neural compartments that vary across brain regions. We discuss current knowledge regarding the strength of neural activity required to drive endogenous opioid peptide release, clues about how far peptides diffuse from release sites, and their extracellular lifetime after release. Finally, as a translational example, we discuss the mechanisms of action of naltrexone (NTX), which is used clinically to treat alcohol use disorder. NTX is a synthetic morphine analog that non-specifically antagonizes the action of most endogenous opioid peptides developed in the 1960s and FDA approved in the 1980s. We review recent studies clarifying the precise endogenous activity that NTX prevents. Together, the works described here highlight the challenges and opportunities the complex opioid system presents as a therapeutic target.

Magel2 knockdown in hypothalamic POMC neurons innervating the medial amygdala reduces susceptibility to diet-induced obesity

Hyperphagia and obesity profoundly affect the health of children with Prader-Willi syndrome (PWS). The Magel2 gene among the genes in the Prader-Willi syndrome deletion region is expressed in proopiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus (ARC). Knockout of the Magel2 gene disrupts POMC neuronal circuits and functions. Here, we report that loss of the Magel2 gene exclusively in ARC^POMC neurons innervating the medial amygdala (MeA) causes a reduction in body weight in both male and female mice fed with a high-fat diet. This anti-obesity effect is associated with an increased locomotor activity. There are no significant differences in glucose and insulin tolerance in mice without the Magel2 gene in ARC^POMC neurons innervating the MeA. Plasma estrogen levels are higher in female mutant mice than in controls. Blockade of the G protein-coupled estrogen receptor (GPER), but not estrogen receptor-α (ER-α), reduces locomotor activity in female mutant mice. Hence, our study provides evidence that knockdown of the Magel2 gene in ARC^POMC neurons innervating the MeA reduces susceptibility to diet-induced obesity with increased locomotor activity through activation of central GPER.

Dopamine systems and biological rhythms: Let's get a move on

How dopamine signaling regulates biological rhythms is an area of emerging interest. Here we review experiments focused on delineating dopamine signaling in the suprachiasmatic nucleus, nucleus accumbens, and dorsal striatum to mediate a range of biological rhythms including photoentrainment, activity cycles, rest phase eating of palatable food, diet-induced obesity, and food anticipatory activity. Enthusiasm for causal roles for dopamine in the regulation of circadian rhythms, particularly those associated with food and other rewarding events, is warranted. However, determining that there is rhythmic gene expression in dopamine neurons and target structures does not mean that they are bona fide circadian pacemakers. Given that dopamine has such a profound role in promoting voluntary movements, interpretation of circadian phenotypes associated with locomotor activity must be differentiated at the molecular and behavioral levels. Here we review our current understanding of dopamine signaling in relation to biological rhythms and suggest future experiments that are aimed at teasing apart the roles of dopamine subpopulations and dopamine receptor expressing neurons in causally mediating biological rhythms, particularly in relation to feeding, reward, and activity.

Actions of feeding-related peptides on the mesolimbic dopamine system in regulation of natural and drug rewards

The mesolimbic dopamine system is the primary neural circuit mediating motivation, reinforcement, and reward-related behavior. The activity of this system and multiple behaviors controlled by it are affected by changes in feeding and body weight, such as fasting, food restriction, or the development of obesity. Multiple different peptides and hormones that have been implicated in the control of feeding and body weight interact with the mesolimbic dopamine system to regulate many different dopamine-dependent, reward-related behaviors. In this review, we summarize the effects of a selected set of feeding-related peptides and hormones acting within the ventral tegmental area and nucleus accumbens to alter feeding, as well as food, drug, and social reward.

Development of "Hunger Neurons" and the Unanticipated Relationship Between Energy Metabolism and Mother-Infant Interactions

Over the course of a lifetime, the perinatal period plays an outsized role in the function of physiological systems. Here, we discuss how neurons that regulate energy metabolism contribute to the infant's relationship with the mother. We focus our discussion on Agrp neurons, which are located in the arcuate nucleus of the hypothalamus. These neurons heavily regulate energy metabolism. Because offspring transition from a period of dependence on the caregiver to independence, we discuss the importance of the caregiver-offspring relationship for the function of Agrp neurons. We present evidence that in the adult, Agrp neurons motivate the animal to eat, while in the neonate, they motivate the offspring to seek the proximity of the caregiver. We specifically highlight the peculiarities in the development of Agrp neurons and how they relate to the regulation of metabolism and behavior over the course of a lifetime. In sum, this review considers the unique insights that ontogenetic studies can offer toward our understanding of complex biological systems, such as the regulation of energy metabolism and mother-infant attachment.

Gut peptide regulation of food intake - evidence for the modulation of hedonic feeding

The number of people living with obesity has tripled worldwide since 1975 with serious implications for public health, as obesity is linked to a significantly higher chance of early death from associated comorbidities (metabolic syndrome, type 2 diabetes, cardiovascular disease and cancer). As obesity is a consequence of food intake exceeding the demands of energy expenditure, efforts are being made to better understand the homeostatic and hedonic mechanisms governing food intake. Gastrointestinal peptides are secreted from enteroendocrine cells in response to nutrient and energy intake, and modulate food intake either via afferent nerves, including the vagus nerve, or directly within the central nervous system, predominantly gaining access at circumventricular organs. Enteroendocrine hormones modulate homeostatic control centres at hypothalamic nuclei and the dorso-vagal complex. Additional roles of these peptides in modulating hedonic food intake and/or preference via the neural systems of reward are starting to be elucidated, with both peripheral and central peptide sources potentially contributing to central receptor activation. Pharmacological interventions and gastric bypass surgery for the treatment of type 2 diabetes and obesity elevate enteroendocrine hormone levels and also alter food preference. Hence, understanding of the hedonic mechanisms mediated by gut peptide action could advance development of potential therapeutic strategies for the treatment of obesity and its comorbidities.

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.

Perinatal exposure to high fat diet alters expression of MeCP2 in the hypothalamus

Obesity is a complex disease that is the result of a number of different factors including genetic, environmental, and endocrine abnormalities. Given that monogenic forms of obesity are rare, it is important to identify other mechanisms that contribute to its etiology. Methyl-Cp-G binding protein 2 (MeCP2) is a neuroepigenetic factor that binds to methylated regions of DNA to influence transcription. Past studies demonstrate that disruption in MeCP2 function produces obesity in mice. Using a diet-induced obesity mouse model, we show that perinatal exposure to high fat diet significantly decreases MeCP2 protein expression in the hypothalamus of female mice, effects not seen when high fat diet is given to mice during adulthood. Moreover, these effects are seen specifically in a subregion of the hypothalamus known as the arcuate nucleus with females having decreased MeCP2 expression in rostral areas and males having decreased MeCP2 expression in intermediate regions of the arcuate nucleus. Interestingly, mice gain more weight when exposed to high fat diet during adulthood relative to mice exposed to high fat diet perinatally, suggesting that perhaps high fat diet exposure during adulthood may be affecting mechanisms independent of MeCP2 function. Collectively, our data demonstrate that there are developmentally sensitive periods in which MeCP2 expression is influenced by high fat diet exposure and this occurs in a sexually dimorphic manner.

VTA MC3R neurons control feeding in an activity- and sex-dependent manner in mice

Increasing evidence indicates that the melanocortin and mesolimbic dopamine (DA) systems interact to regulate feeding and body weight. Because melanocortin-3 receptors (MC3R) are highly expressed in the ventral tegmental area (VTA), we tested whether VTA neurons expressing these receptors (VTA MC3R neurons) control feeding and body weight in vivo. We also tested whether there were sex differences in the ability of VTA MC3R neurons to control feeding, as MC3R -/- mice show sex-dependent alterations in reward feeding and DA levels, and there are clear sex differences in multiple DA-dependent behaviors and disorders. Designer receptors exclusively activated by designer drugs (DREADD) were used to acutely activate and inhibit VTA MC3R neurons and changes in food intake and body weight were measured. Acutely altering the activity of VTA MC3R neurons decreased feeding in an activity- and sex-dependent manner, with acute activation decreasing feeding, but only in females, and acute inhibition decreasing feeding, but only in males. These differences did not appear to be due to sex differences in the number of VTA MC3R neurons, the ability of hM3Dq to activate VTA MC3R neurons, or the proportion of VTA MC3R neurons expressing tyrosine hydroxylase (TH). These studies demonstrate an important role for VTA MC3R neurons in the control of feeding and reveal important sex differences in behavior, whereby opposing changes in neuronal activity in male and female mice cause similar changes in behavior.

Hypothalamic Expression of Neuropeptide Y (NPY) and Pro-OpioMelanoCortin (POMC) in Adult Male Mice Is Affected by Chronic Exposure to Endocrine Disruptors

In the arcuate nucleus, neuropeptide Y (NPY) neurons, increase food intake and decrease energy expenditure, and control the activity of pro-opiomelanocortin (POMC) neurons, that decrease food intake and increase energy expenditure. Both systems project to other hypothalamic nuclei such as the paraventricular and dorsomedial hypothalamic nuclei. Endocrine disrupting chemicals (EDCs) are environmental contaminants that alter the endocrine system causing adverse health effects in an intact organism or its progeny. We investigated the effects of long-term exposure to some EDCs on the hypothalamic NPY and POMC systems of adult male mice that had been previously demonstrated to be a target of some of these EDCs after short-term exposure. Animals were chronically fed for four months with a phytoestrogen-free diet containing two different concentrations of bisphenol A, diethylstilbestrol, tributyltin, or E₂. At the end, brains were processed for NPY and POMC immunohistochemistry and quantitatively analyzed. In the arcuate and dorsomedial nuclei, both NPY and POMC immunoreactivity showed a statistically significant decrease. In the paraventricular nucleus, only the NPY system was affected, while the POMC system was not affected. Finally, in the VMH the NPY system was affected whereas no POMC immunoreactive material was observed. These results indicate that adult exposure to different EDCs may alter the hypothalamic circuits that control food intake and energy metabolism.

The Melanocortin System behind the Dysfunctional Eating Behaviors

The dysfunction of melanocortin signaling has been associated with obesity, given the important role in the regulation of energy homeostasis, food intake, satiety and body weight. In the hypothalamus, the melanocortin-3 receptor (MC3R) and melanocortin-4 receptor (MC4R) contribute to the stability of these processes, but MC3R and MC4R are also localized in the mesolimbic dopamine system, the region that responds to the reinforcing properties of highly palatable food (HPF) and where these two receptors seem to affect food reward and motivation. Loss of function of the MC4R, resulting from genetic mutations, leads to overeating in humans, but to date, a clear understanding of the underlying mechanisms and behaviors that promote overconsumption of caloric foods remains unknown. Moreover, the MC4R demonstrated to be a crucial modulator of the stress response, factor that is known to be strictly related to binge eating behavior. In this review, we will explore the preclinical and clinical studies, and the controversies regarding the involvement of melanocortin system in altered eating patterns, especially binge eating behavior, food reward and motivation.

Disruption of GABA or glutamate release from POMC neurons in the adult mouse does not affect metabolic end points

Proopiomelanocortin (POMC) neurons contribute to the regulation of many physiological processes; the majority of which have been attributed to the release of peptides produced from the POMC prohormone such as α-MSH, which plays key roles in food intake and metabolism. However, it is now clear that POMC neurons also release amino acid transmitters that likely contribute to the overall function of POMC cells. Recent work indicates that constitutive deletion of these transmitters can affect metabolic phenotypes, but also that the expression of GABAergic or glutamatergic markers changes throughout development. The goal of the present study was to determine whether the release of glutamate or GABA from POMC neurons in the adult mouse contributes notably to energy balance regulation. Disturbed release of glutamate or GABA specifically from POMC neurons in adult mice was achieved using a tamoxifen-inducible Cre construct (Pomc-CreER^T2) expressed in mice also carrying floxed versions of Slc17a6 (vGlut2) or Gad1 and Gad2, encoding the vesicular glutamate transporter type 2 and GAD67 and GAD65 proteins, respectively. All mice in the experiments received tamoxifen injections, but control mice lacked the tamoxifen-inducible Cre sequence. Body weight was unchanged in Gad1- and Gad2- or vGlut2-deleted female and male mice. Additionally, no significant differences in glucose tolerance or refeeding after an overnight fast were observed. These data collectively suggest that the release of GABA or glutamate from POMC neurons in adult mice does not significantly contribute to the metabolic parameters tested here. In light of prior work, the data also suggest that amino acid transmitter release from POMC cells may contribute to separate functions in the adult versus the developing mouse.

Whole-brain efferent and afferent connectivity of mouse ventral tegmental area melanocortin-3 receptor neurons

The mesolimbic dopamine (DA) system is involved in the regulation of multiple behaviors, including feeding, and evidence demonstrates that the melanocortin system can act on the mesolimbic DA system to control feeding and other behaviors. The melanocortin-3 receptor (MC3R) is an important component of the melanocortin system, but its overall role is poorly understood. Because MC3Rs are highly expressed in the ventral tegmental area (VTA) and are likely to be the key interaction point between the melanocortin and mesolimbic DA systems, we set out to identify both the efferent projection patterns of VTA MC3R neurons and the location of the neurons providing afferent input to them. VTA MC3R neurons were broadly connected to neurons across the brain but were strongly connected to a discrete set of brain regions involved in the regulation of feeding, reward, and aversion. Surprisingly, experiments using monosynaptic rabies virus showed that proopiomelanocortin (POMC) and agouti-related protein (AgRP) neurons in the arcuate nucleus made few direct synapses onto VTA MC3R neurons or any of the other major neuronal subtypes in the VTA, despite being extensively labeled by general retrograde tracers injected into the VTA. These results greatly contribute to our understanding of the anatomical interactions between the melanocortin and mesolimbic systems and provide a foundation for future studies of VTA MC3R neurons and the circuits containing them in the control of feeding and other behaviors.

Nicotinic Acetylcholine Receptor Signaling in the Hypothalamus: Mechanisms Related to Nicotine's Effects on Food Intake

Despite health risks associated with smoking, up to 20% of the US population persist in this behavior; many smoke to control body weight or appetite, and fear of post-cessation weight gain can motivate continued smoking. Nicotine and tobacco use is associated with lower body weight, and cessation yields an average weight gain of about 4 kg, which is thought to reflect a return to the body weight of a typical nonsmoker. Nicotine replacement therapies can delay this weight gain but do not prevent it altogether, and the underlying mechanism for how nicotine is able to reduce weight is not fully understood. In rodent models, nicotine reduces weight gain, reduces food consumption, and alters energy expenditure, but these effects vary with duration and route of nicotine administration. Nicotine, acting through nicotinic acetylcholine receptors (nAChRs), increases the firing rate of both orexigenic agouti-related peptide and anorexigenic proopiomelanocortin neurons in the arcuate nucleus of the hypothalamus (ARC). Manipulation of nAChR subunit expression within the ARC can block the ability of nicotine and the nicotinic agonist cytisine from decreasing food intake; however, it is unknown exactly how this reduces food intake. This review summarizes the clinical and preclinical work on nicotine, food intake, and weight gain, then explores the feeding circuitry of the ARC and how it is regulated by nicotine. Finally, we propose a novel hypothesis for how nicotine acts on this hypothalamic circuit to reduce food intake. Implications: This review provides a comprehensive and updated summary of the clinical and preclinical work examining nicotine and food intake, as well as a summary of recent work examining feeding circuits of the hypothalamus. Synthesis of these two topics has led to new understanding of how nAChR signaling regulates food intake circuits in the hypothalamus.

Deletion of liver kinase B1 in POMC neurons predisposes to diet-induced obesity

AIMS

Liver kinase B1 (LKB1) is a serine/threonine kinase. Although many biological functions of LKB1 have been identified, the role of hypothalamic LKB1 in the regulation of central energy metabolism and susceptibility to obesity is unknown. Therefore, we constructed POMC neuron-specific LKB1 knockout mice (PomcLkb1 KO) and studied it at the physiological, morphological, and molecular biology levels.

MAIN METHODS

Eight-week-old male PomcLkb1 KO mice and their littermates were fed a standard chow fat diet (CFD) or a high-fat diet (HFD) for 3 months. Body weight and food intake were monitored. Dual-energy X-ray absorptiometry was used to measure the fat mass and lean mass. Glucose and insulin tolerance tests and serum biochemical markers were evaluated in the experimental mice. In addition, the levels of peripheral lipogenesis genes and central energy metabolism were measured.

KEY FINDINGS

PomcLkb1 KO mice did not exhibit impairments under normal physiological conditions. After HFD intervention, the metabolic phenotype of the PomcLkb1 KO mice changed, manifesting as increased food intake and an enhanced obesity phenotype. More seriously, PomcLkb1 KO mice showed increased leptin resistance, worsened hypothalamic inflammation and reduced POMC neuronal expression.

SIGNIFICANCE

We provide evidence that LKB1 in POMC neurons plays a significant role in regulating energy homeostasis. LKB1 in POMC neurons emerges as a target for therapeutic intervention against HFD-induced obesity and metabolic diseases.

Cocaine Self-administration Regulates Transcription of Opioid Peptide Precursors and Opioid Receptors in Rat Caudate Putamen and Prefrontal Cortex

The brain opioid system plays an important role in cocaine reward. Altered signaling in the opioid system by chronic cocaine exposure contributes to cocaine-seeking and taking behavior. The current study investigated concurrent changes in the gene expression of multiple components in rat brain opioid system following cocaine self-administration. Animals were limited to 40 infusions (1.5 mg/kg/infusion) within 6 h per day for five consecutive days. We then examined the mRNA levels of opioid receptors including mu (Oprm), delta (Oprd), and kappa (Oprk), and their endogenous opioid peptide precursors including proopiomelanocortin (Pomc), proenkephalin (Penk), prodynorphin (Pdyn) in the dorsal striatum (CPu) and the prefrontal cortex (PFC) 18 h after the last cocaine infusion. We found that cocaine self-administration significantly increased the mRNA levels of Oprm and Oprd in both the CPu and PFC, but had no effect on Oprk mRNA levels in either brain region. Moreover, cocaine had a greater influence on the mRNA levels of opioid peptide precursors in rat CPu than in the PFC. In the CPu, cocaine self-administration significantly increased the mRNA levels of Penk and Pdyn and abolished the mRNA levels of Pomc. In the PFC, cocaine self-administration only increased Pdyn mRNA levels without changing the mRNA levels of Pomc and Penk. These data suggest that cocaine self-administration influences the expression of multiple genes in the brain opioid system, and the concurrent changes in these targets may underlie cocaine-induced reward and habitual drug-seeking behavior.

Hypothalamic Pomc expression restricted to GABAergic neurons suppresses Npy overexpression and restores food intake in obese mice

Objective

Hypothalamic arcuate proopiomelanocortin (Arc-POMC) neurons are involved in different physiological processes such as the regulation of energy balance, glucose homeostasis, and stress-induced analgesia. Since these neurons heterogeneously express different biological markers and project to many hypothalamic and extrahypothalamic areas, it is proposed that Arc-POMC neurons could be classified into different subpopulations having diverse physiological roles. The aim of the present study was to characterize the contribution of the subpopulation of Arc-POMC neurons cosecreting gamma-aminobutyric acid (GABA) neurotransmitter in the control of energy balance.

Methods

Arc-Pomc expression restricted to GABAergic-POMC neurons was achieved by crossing a reversible Pomc-deficient mouse line (arcPomc⁻) with a tamoxifen-inducible Gad2-CreER transgenic line. Pomc expression was rescued in the compound arcPomc^−/−:Gad2-CreER female and male mice by tamoxifen treatment at postnatal days 25 (P25) or 60 (P60), and body weight, daily food intake, fasting glycemia, and fasting-induced hyperphagia were measured. POMC recovery was quantified by immunohistochemistry and semiquantitative RT-PCR. Neuropeptide Y (NPY) and GABAergic neurons were identified by in situ hybridization. Arc-POMC neurons projecting to the dorsomedial hypothalamic nucleus (DMH) were studied by stereotactic intracerebral injection of fluorescent retrobeads into the DMH.

Results

Tamoxifen treatment of arcPomc^−/−:Gad2-CreER mice at P60 resulted in Pomc expression in ∼23–25% of Arc-POMC neurons and ∼15–23% of Pomc mRNA levels, compared to Gad2-CreER control mice. Pomc rescue in GABAergic-POMC neurons at P60 normalized food intake, glycemia, and fasting-induced hyperphagia, while significantly reducing body weight. Energy balance was also improved in arcPomc^−/−:Gad2-CreER mice treated with tamoxifen at P25. Distribution analysis of rescued POMC immunoreactive fibers revealed that the DMH is a major target site of GABAergic-POMC neurons. Further, the expression of the orexigenic neuropeptide Y (NPY) in the DMH was increased in arcPomc^−/− obese mice but was completely restored after Pomc rescue in arcPomc^−/−:Gad2-CreER mice. Finally, we found that ∼75% of Arc-POMC neurons projecting to the DMH are GABAergic.

Conclusions

In the present study, we show that the expression of Pomc in the subpopulation of Arc-GABAergic-POMC neurons is sufficient to maintain normal food intake. In addition, we found that DMH-NPY expression is negatively correlated with Pomc expression in GABAergic-POMC neurons, suggesting that food intake may be regulated by an Arc-GABAergic-POMC → DMH-NPY pathway.

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The subpopulation of arcuate GABAergic-POMC neurons is sufficient to maintain normal food intake.

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Overweight induced by Pomc deficiency is reduced by arcuate Pomc expression restricted to GABAergic-POMC neurons.

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DMH-Npy overexpression in POMC-deficient mice is restored by Pomc rescue restricted to GABAergic-POMC neurons.

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Arcuate POMC neurons projecting to the DMH are mainly GABAergic.

Diet-Induced Growth Is Regulated via Acquired Leptin Resistance and Engages a Pomc-Somatostatin-Growth Hormone Circuit

Anorexigenic pro-opiomelanocortin (Pomc)/alpha-melanocyte stimulating hormone (αMSH) neurons of the hypothalamic melanocortin system function as key regulators of energy homeostasis, also controlling somatic growth across different species. However, the mechanisms of melanocortin-dependent growth control still remain ill-defined. Here, we reveal a thus-far-unrecognized structural and functional connection between Pomc neurons and the somatotropic hypothalamo-pituitary axis. Excessive feeding of larval zebrafish causes leptin resistance and reduced levels of the hypothalamic satiety mediator pomca. In turn, this leads to reduced activation of hypophysiotropic somatostatin (Sst)-neurons that express the melanocortin receptor Mc4r, elevated growth hormone (GH) expression in the pituitary, and enhanced somatic growth. Mc4r expression and αMSH responsiveness are conserved in Sst-expressing hypothalamic neurons of mice. Thus, acquired leptin resistance and attenuation of pomca transcription in response to excessive caloric intake may represent an ancient mechanism to promote somatic growth when food resources are plentiful.

Alpha-melanocyte stimulating hormone increases the activity of melanocortin-3 receptor-expressing neurons in the ventral tegmental area

KEY POINTS

Alpha-melanocyte stimulating hormone (α-MSH) is an anorexigenic peptide. Injection of the α-MSH analog MTII into the ventral tegmental area (VTA) decreases food and sucrose intake and food reward. Melanocortin-3 receptors (MC3R) are highly expressed in the VTA, suggesting that the effects of intra-VTA α-MSH may be mediated by α-MSH changing the activity of MC3R-expressing VTA neurons. α-MSH increased the firing rate of MC3R VTA neurons in acute brain slices from mice, although it did not affect the firing rate of non-MC3R VTA neurons. The α-MSH induced increase in MC3R neuron firing rate is probably activity-dependent, and was independent of fast synaptic transmission and intracellular Ca²⁺ levels. These results help us to better understand how α-MSH acts in the VTA to affect feeding and other dopamine-dependent behaviours.

ABSTRACT

The mesocorticolimbic dopamine system, the brain's reward system, regulates multiple behaviours, including food intake and food reward. There is substantial evidence that the melanocortin system of the hypothalamus, an important neural circuit controlling feeding and body weight, interacts with the mesocorticolimbic dopamine system to affect feeding, food reward and body weight. For example, melanocortin-3 receptors (MC3Rs) are expressed in the ventral tegmental area (VTA) and our laboratory previously showed that intra-VTA injection of the MC3R agonist, MTII, decreases home-cage food intake and operant responding for sucrose pellets. However, the cellular mechanisms underlying the effects of intra-VTA alpha-melanocyte stimulating hormone (α-MSH) on feeding and food reward are unknown. To determine how α-MSH acts in the VTA to affect feeding, we performed electrophysiological recordings in acute brain slices from mice expressing enhanced yellow fluorescent protein in MC3R neurons to test how α-MSH affects the activity of VTA MC3R neurons. α-MSH significantly increased the firing rate of VTA MC3R neurons without altering the activity of non-MC3R expressing VTA neurons. In addition, the α-MSH-induced increase in MC3R neuron activity was independent of fast synaptic transmission and intracellular Ca²⁺ levels. Finally, we show that the effect of α-MSH on MC3R neuron firing rate is probably activity-dependent. Overall, these studies provide an important advancement in the understanding of how α-MSH acts in the VTA to affect feeding and food reward.

Hypothalamic mechanism of corticotropin-releasing factor's anorexigenic effect in Japanese quail (Coturnix japonica)

Central administration of corticotropin-releasing factor (CRF), a 41-amino acid peptide, is associated with anorexigenic effects across various species, with particularly potent reductions in food intake in rodents and chickens (Gallus gallus domesticus), a species for which the most is known. The purpose of the current study was to determine the hypothalamic mechanism of CRF-induced anorexigenic effects in 7 day-old Japanese quail (Coturnix japonica), a less-intensely-selected gallinaceous relative to the chicken that can provide more evolutionary perspective. After intracerebroventricular (ICV) injection of 2, 22, or 222 pmol of CRF, a dose-dependent decrease in food intake was observed that lasted for 3 and 24 h for the 22 and 222 pmol doses, respectively. The 2 pmol dose had no effect on food or water intake. The numbers of c-Fos immunoreactive cells were increased in the paraventricular nucleus (PVN) and lateral hypothalamic area (LHA) at 1 h post-injection in quail injected with 22 pmol of CRF. The hypothalamic mRNA abundance of proopiomelanocortin, melanocortin receptor subtype 4, CRF, and CRF receptor sub-type 2 was increased at 1 h in quail treated with 22 pmol of CRF. Behavior analyses demonstrated that CRF injection reduced feeding pecks and jumps and increased the time spent standing. In conclusion, results demonstrate that the anorexigenic effects of CRF in Japanese quail are likely influenced by the interaction between CRF and melanocortin systems and that injection of CRF results in species-specific behavioral changes.

Molecular and cellular characterization of nicotinic acetylcholine receptor subtypes in the arcuate nucleus of the mouse hypothalamus

Nicotine, acting through nicotinic acetylcholine receptors (nAChRs), increases the firing rate of both orexigenic agouti-related peptide (AgRP) and anorexigenic pro-opiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus (ARC), yet nicotine and other nAChR agonists decrease food intake in mice. Viral-mediated knockdown of the β4 nAChR subunit in all neuronal cell types in the ARC prevents the nicotinic agonist cytisine from decreasing food intake, but it is not known whether the β4 subunit is selectively expressed in anorexigenic neurons or how other nAChR subtypes are distributed in this nucleus. Using translating ribosome affinity purification (TRAP) on ARC tissue from mice with ribosomes tagged in either AgRP or POMC cells, we examined nAChR subunit mRNA levels using real-time PCR. Both AgRP and POMC cells express a comparable panel of nAChR subunits with differences in α7 mRNA levels and a trend for difference in α4 levels, but no differences in β4 expression. Immunoprecipitation of assembled nAChRs revealed that the β4 subunit forms assembled channels with α3, β2 and α4, but not other subunits found in the ARC. Finally, using cell type-selective, virally delivered small hairpin RNAs targeting either the β4 or α7 subunit, we examined the contribution of each subunit in either AgRP or POMC cells to the behavioural response to nicotine, refining the understanding of nicotinic regulation of this feeding circuit. These experiments identify a more complex set of nAChRs expressed in ARC than in other hypothalamic regions. Thus, the ARC appears to be a particular target of nicotinic modulation.

POMC neurons in heat: A link between warm temperatures and appetite suppression

When core body temperature increases, appetite and food consumption decline. A higher core body temperature can occur during exercise, during exposure to warm environmental temperatures, or during a fever, yet the mechanisms that link relatively warm temperatures to appetite suppression are unknown. A recent study in PLOS Biology demonstrates that neurons in the mouse hypothalamus that express pro-opiomelanocortin (POMC), a neural population well known to suppress food intake, also express a temperature-sensitive ion channel, transient receptor potential vanilloid 1 (TRPV1). Slight increases in body temperature cause a TRPV1-dependent increase in activity in POMC neurons, which suppresses feeding in mice. Taken together, this study suggests a novel mechanism linking body temperature and food-seeking behavior.

Activation of temperature-sensitive TRPV1-like receptors in ARC POMC neurons reduces food intake

Proopiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus (ARC) respond to numerous hormonal and neural signals, resulting in changes in food intake. Here, we demonstrate that ARC POMC neurons express capsaicin-sensitive transient receptor potential vanilloid 1 receptor (TRPV1)-like receptors. To show expression of TRPV1-like receptors in ARC POMC neurons, we use single-cell reverse transcription-polymerase chain reaction (RT-PCR), immunohistochemistry, electrophysiology, TRPV1 knock-out (KO), and TRPV1-Cre knock-in mice. A small elevation of temperature in the physiological range is enough to depolarize ARC POMC neurons. This depolarization is blocked by the TRPV1 receptor antagonist and by Trpv1 gene knockdown. Capsaicin-induced activation reduces food intake that is abolished by a melanocortin receptor antagonist. To selectively stimulate TRPV1-like receptor-expressing ARC POMC neurons in the ARC, we generate an adeno-associated virus serotype 5 (AAV5) carrying a Cre-dependent channelrhodopsin-2 (ChR2)–enhanced yellow fluorescent protein (eYFP) expression cassette under the control of the two neuronal POMC enhancers (nPEs). Optogenetic stimulation of TRPV1-like receptor-expressing POMC neurons decreases food intake. Hypothalamic temperature is rapidly elevated and reaches to approximately 39 °C during treadmill running. This elevation is associated with a reduction in food intake. Knockdown of the Trpv1 gene exclusively in ARC POMC neurons blocks the feeding inhibition produced by increased hypothalamic temperature. Taken together, our findings identify a melanocortinergic circuit that links acute elevations in hypothalamic temperature with acute reductions in food intake.

Intense exercise acutely decreases appetite and subsequent food intake. As exercise is accompanied by increased body temperature, we hypothesized that a rise in body temperature during exercise plays a role in reducing food intake. The hypothalamic neurons are major components of the neural circuits that control feeding in response to hormones and neural signals. Among hypothalamic neurons, those that express proopiomelanocortin (POMC) in the arcuate nucleus of the hypothalamus are important in controlling food intake. In this study, we found that these POMC-expressing neurons express TRPV1-like thermoreceptors that are activated by an increase in temperature within the physiological range in mice. We also showed that an increase in body temperature during exercise is directly sensed by these POMC-expressing neurons through activation of the TRPV1-like receptors. Hence, this study provides a novel perspective on the cellular mechanisms underlying energy balance: body temperature reduces food intake via TRPV1-like receptors in POMC-expressing neurons in the arcuate nucleus of the hypothalamus.

Effect of long-term treatment with classical neuroleptics on NPQ/spexin, kisspeptin and POMC mRNA expression in the male rat amygdala

Neuroleptics modulate the expression level of some regulatory neuropeptides in the brain. However, if these therapeutics influence the peptidergic circuits in the amygdala remains unclear. This study specifies the impact profile of the classical antipsychotic drugs on mRNA expression of the spexin/NPQ, kisspeptin-1 and POMC in the rat amygdala. Animals were treated with haloperidol and chlorpromazine for 28 days prior to transcript quantification via qPCR. Haloperidol and chlorpromazine induced a change in the expression of all neuropeptides analyzed. Both drugs led to the decrease of Kiss-1 expression, whereas in POMC and spexin/NPQ their up-regulation in the amygdala was detected. These modulating effects on may represent alternative, so far unknown mechanisms, of classical antipsychotic drugs triggering pharmacological responses.

GTRAP3-18 regulates food intake and body weight by interacting with pro-opiomelanocortin

Pro-opiomelanocortin (POMC)-expressing neurons provide α-melanocyte-stimulating hormone (α-MSH), which stimulates melanocortin 4 receptor to induce hypophagia by AMPK inhibition in the hypothalamus. α-MSH is produced by POMC cleavage in secretory granules and released. However, it is not known yet whether any posttranscriptional regulatory mechanism of POMC signaling exists upstream of the secretory granules in neurons. Here we show that glutamate transporter-associated protein 3-18 (GTRAP3-18), an anchor protein that retains interacting proteins in the endoplasmic reticulum, is a critical regulator of food intake and body weight by interacting with POMC. GTRAP3-18-deficient mice showed hypophagia, lean bodies, and lower blood glucose, insulin, and leptin levels with increased serum and brain α-MSH levels, leading to AMPK inhibition. Intraperitoneal glucose tolerance tests revealed significantly decreased blood glucose levels and areas under the curve in GTRAP3-18-deficient mice compared to wild-type mice. An intracerebroventricular infusion of a selective melanocortin 4 receptor antagonist to GTRAP3-18-deficient mice significantly increased their food intake and body weight. A fluorescence resonance energy transfer study showed an interaction between GTRAP3-18 and POMC in vitro These findings suggest that activation of the melanocortin pathway by modulating GTRAP3-18/POMC interaction could be an alternative strategy for obesity and/or type 2 diabetes.-Aoyama, K., Bhadhprasit, W., Watabe, M., Wang, F., Matsumura, N., Nakaki, T. GTRAP3-18 regulates food intake and body weight by interacting with pro-opiomelanocortin.

Altered sucrose self-administration following injection of melanocortin receptor agonists and antagonists into the ventral tegmental area

RATIONALE AND OBJECTIVES

Alpha-melanocyte stimulating hormone (αMSH) and agouti-related protein (AgRP) are antagonistic neuropeptides that play an important role in the control of feeding and body weight through their central actions on the melanocortin-3 and melanocortin-4 receptors. Increasing evidence indicates that αMSH and AgRP can interact with the mesolimbic dopamine system to regulate feeding as well as other behaviors. For example, we have shown previously that injection of melanocortin receptor agonists and antagonists into the ventral tegmental area (VTA) alters both normal home-cage feeding and the intake of sucrose solutions, but it remains unknown whether αMSH and AgRP can also act in the VTA to affect reward-related feeding.

METHODS

We tested whether injection of the melanocortin receptor agonist, MTII, or the melanocortin receptor antagonist, SHU9119, directly into the VTA affected operant responding maintained by sucrose pellets in self-administration assays.

RESULTS

Injection of MTII into the VTA decreased operant responding maintained by sucrose pellets on both fixed ratio and progressive ratio schedules of reinforcement, whereas SHU9119 increased operant responding under fixed ratio, but not progressive ratio schedules. MTII also increased and SHU9119 decreased 24-h home-cage food intake.

CONCLUSIONS

This study demonstrates that αMSH and AgRP act in the VTA to affect sucrose self-administration. Thus, it adds critical information to the growing literature showing that in addition to their well-characterized role in controlling "need-based" feeding, αMSH and AgRP can also act on the mesolimbic dopamine system to control reward-related behavior.

Using Kalirin conditional knockout mice to distinguish its role in dopamine receptor mediated behaviors

Background

Mice lacking Kalirin-7 (Kal7^KO), a Rho GDP/GTP exchange factor, self-administer cocaine at a higher rate than wildtype mice, and show an exaggerated locomotor response to experimenter-administered cocaine. Kal7, which localizes to post-synaptic densities at glutamatergic synapses, interacts directly with the GluN2B subunit of the N-methyl-d-aspartate (NMDA; GluN) receptor. Consistent with these observations, Kal7 plays an essential role in NMDA receptor dependent long term potentiation and depression, and glutamatergic transmission plays a key role in the response to chronic cocaine. A number of genetic studies have implicated altered Kalirin expression in schizophrenia and other disorders such as Alzheimer’s Disease.

Results

A comparison of the effects of experimenter-administered cocaine on mice lacking all Kalirin isoforms to its effects on mice lacking only Kalirin-7 identified Kal7 as the key isoform whose deletion produces exaggerated locomotor responses to cocaine. Pretreatment of Kal7^KO mice with a low dose of ifenprodil, a selective GluN2B antagonist, eliminated their enhanced locomotor response to cocaine, revealing an important role for GluN2B in this behavior. Selective knockout of Kalirin in dopamine transporter expressing neurons produced a transient enhancement of cocaine-induced locomotion, while knockout of Kalirin in Drd1a- or Drd2-dopamine receptor expressing neurons was without effect. As observed in Kalirin global knockout mice, eliminating Kalirin expression in Drd2-expressing neurons increased exploratory behavior in the elevated zero maze, an effect eliminated by pretreatment with ifenprodil.

Conclusions

The cocaine-sensitive neuronal pathways which are most sensitive to altered Kalirin function may be the pathways most dependent on GluN2B and Drd2.

Electronic supplementary material

The online version of this article (doi:10.1186/s12868-017-0363-2) contains supplementary material, which is available to authorized users.

The Role of the Melanocortin System in Metabolic Disease: New Developments and Advances

Obesity is increasing in prevalence across all sectors of society, and with it a constellation of associated ailments including hypertension, type 2 diabetes, and eating disorders. The melanocortin system is a critical neural system underlying the control of body weight and other functions. Deficits in the melanocortin system may promote or exacerbate the comorbidities of obesity. This system has therefore generated great interest as a potential target for treatment of obesity. However, drugs targeting melanocortin receptors are plagued by problematic side effects, including undesirable increases in sympathetic nervous system activity, heart rate, and blood pressure. Circumnavigating this roadblock will require a clearer picture of the precise neural circuits that mediate the functions of melanocortins. Recent, novel experimental approaches have significantly advanced our understanding of these pathways. We here review the latest advances in our understanding of the role of melanocortins in food intake, reward pathways, blood pressure, glucose control, and energy expenditure. The evidence suggests that downstream melanocortin-responsive circuits responsible for different physiological actions do diverge. Ultimately, a more complete understanding of melanocortin pathways and their myriad roles should allow treatments tailored to the mix of metabolic disorders in the individual patient.

Mechanisms and significance of brain glucose signaling in energy balance, glucose homeostasis, and food-induced reward

The concept that hypothalamic glucose signaling plays an important role in regulating energy balance, e.g., as instantiated in the so-called "glucostat" hypothesis, is one of the oldest in the field of metabolism. However the mechanisms by which neurons in the hypothalamus sense glucose, and the function of glucose signaling in the brain, has been difficult to establish. Nevertheless recent studies probing mechanisms of glucose signaling have also strongly supported a role for glucose signaling in regulating energy balance, glucose homeostasis, and food-induced reward.

Homeostasis Meets Motivation in the Battle to Control Food Intake

Signals of energy homeostasis interact closely with neural circuits of motivation to control food intake. An emerging hypothesis is that the transition to maladaptive feeding behavior seen in eating disorders or obesity may arise from dysregulation of these interactions. Focusing on key brain regions involved in the control of food intake (ventral tegmental area, striatum, hypothalamus, and thalamus), we describe how activity of specific cell types embedded within these regions can influence distinct components of motivated feeding behavior. We review how signals of energy homeostasis interact with these regions to influence motivated behavioral output and present evidence that experience-dependent neural adaptations in key feeding circuits may represent cellular correlates of impaired food intake control. Future research into mechanisms that restore the balance of control between signals of homeostasis and motivated feeding behavior may inspire new treatment options for eating disorders and obesity.

Estradiol Membrane-Initiated Signaling and Female Reproduction

The discoveries of rapid, membrane-initiated steroid actions and central nervous system steroidogenesis have changed our understanding of the neuroendocrinology of reproduction. Classical nuclear actions of estradiol and progesterone steroids affecting transcription are essential. However, with the discoveries of membrane-associated steroid receptors, it is becoming clear that estradiol and progesterone have neurotransmitter-like actions activating intracellular events. Ultimately, membrane-initiated actions can influence transcription. Estradiol membrane-initiated signaling (EMS) modulates female sexual receptivity and estrogen feedback regulating the luteinizing hormone (LH) surge. In the arcuate nucleus, EMS activates a lordosis-regulating circuit that extends to the medial preoptic nucleus and subsequently to the ventromedial nucleus (VMH)--the output from the limbic and hypothalamic regions. Here, we discuss how EMS leads to an active inhibition of lordosis behavior. To stimulate ovulation, EMS facilitates astrocyte synthesis of progesterone (neuroP) in the hypothalamus. Regulation of GnRH release driving the LH surge is dependent on estradiol-sensitive kisspeptin (Kiss1) expression in the rostral periventricular nucleus of the third ventricle (RP3V). NeuroP activation of the LH surge depends on Kiss1, but the specifics of signaling have not been well elucidated. RP3V Kiss1 neurons appear to integrate estradiol and progesterone information which feeds back onto GnRH neurons to stimulate the LH surge. In a second population of Kiss1 neurons, estradiol suppresses the surge but maintains tonic LH release, another critical component of the estrous cycle. Together, evidence suggests that regulation of reproduction involves membrane action of steroids, some of which are synthesized in the brain.

Role of the Paraventricular Nucleus of the Hypothalamus in the Sympathoexcitatory Effects of Leptin

Leptin binds to receptors in multiple hypothalamic nuclei to increase sympathetic nerve activity; however, the neurocircuitry is unclear. Here, using anesthetized male Sprague-Dawley rats, we investigated the role of the paraventricular nucleus of the hypothalamus. Intracerebroventricular injection of leptin slowly increased lumbar sympathetic nerve activity (LSNA), heart rate, mean arterial pressure, and baroreflex control of LSNA and heart rate. Inhibition of the paraventricular nucleus with muscimol completely reversed leptin's effects. Blockade of paraventricular melanocortin 3/4 receptors with SHU9119 or ionotropic glutamate receptors with kynurenate, alone or together, each partially reversed the effects of leptin, implicating increased activation of glutamate and melanocortin 3/4 receptors. Conversely, although blockade of neuropeptide Y Y1 receptors in the paraventricular nucleus increased LSNA, mean arterial pressure, and heart rate, these responses were prevented by intracerebroventricular or arcuate nucleus injections of leptin, suggesting that, at least in part, leptin also increases sympathetic nerve activity by suppression of tonic neuropeptide Y inhibitory inputs from the arcuate nucleus. Injection of the melanocortin 3/4 receptor agonist melanotan-II into the paraventricular nucleus increased LSNA, mean arterial pressure, and heart rate only after blockade of neuropeptide Y Y1 receptors. Therefore, we conclude that leptin increases LSNA in part via increased glutamatergic and α-melanocyte-stimulating hormone drive of paraventricular sympathoexcitatory neurons, the latter of which requires simultaneous withdrawal of tonic neuropeptide Y inhibition.

Cognitive and autonomic determinants of energy homeostasis in obesity

Obesity ensues from an imbalance between energy intake and expenditure that results from gene-environment interactions, which favour a positive energy balance. A society that promotes unhealthy food and encourages sedentary lifestyle (that is, an obesogenic environment) has become a major contributory factor in excess fat deposition in individuals predisposed to obesity. Energy homeostasis relies upon control of energy intake as well as expenditure, which is in part determined by the themogenesis of brown adipose tissue and mediated by the sympathetic nervous system. Several areas of the brain that constitute cognitive and autonomic brain systems, which in turn form networks involved in the control of appetite and thermogenesis, also contribute to energy homeostasis. These networks include the dopamine mesolimbic circuit, as well as the opioid, endocannabinoid and melanocortin systems. The activity of these networks is modulated by peripheral factors such as hormones derived from adipose tissue and the gut, which access the brain via the circulation and neuronal signalling pathways to inform the central nervous system about energy balance and nutritional status. In this Review, I focus on the determinants of energy homeostasis that have emerged as prominent factors relevant to obesity.

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.

Central melanocortins regulate the motivation for sucrose reward

The role of the melanocortin (MC) system in feeding behavior is well established. Food intake is potently suppressed by central infusion of the MC 3/4 receptor agonist α-melanocyte stimulating hormone (α-MSH), whereas the MC 3/4 receptor inverse-agonist Agouti Related Peptide (AGRP) has the opposite effect. MC receptors are widely expressed in both hypothalamic and extra-hypothalamic brain regions, including nuclei involved in food reward and motivation, such as the nucleus accumbens (NAc) and the ventral tegmental area. This suggests that MCs modulate motivational aspects of food intake. To test this hypothesis, rats were injected intracerebroventricularly with α-MSH or AGRP and their motivation for sucrose was tested under a progressive ratio schedule of reinforcement. Food motivated behavior was dose-dependently decreased by α-MSH. Conversely, AGRP increased responding for sucrose, an effect that was blocked by pretreatment with the dopamine receptor antagonist α-flupenthixol. In contrast to progressive ratio responding, free intake of sucrose remained unaltered upon α-MSH or AGRP infusion. In addition, we investigated whether the effects of α-MSH and AGRP on food motivation were mediated by the NAc shell. In situ hybridization of MC3 and MC4 receptor expression confirmed that the MC4 receptor was expressed throughout the NAc, and injection of α-MSH and AGRP into the NAc shell caused a decrease and an increase in motivation for sucrose, respectively. These data show that the motivation for palatable food is modulated by MC4 receptors in the NAc shell, and demonstrate cross-talk between the MC and dopamine system in the modulation of food motivation.

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.

Elimination of Kalrn expression in POMC cells reduces anxiety-like behavior and contextual fear learning

Kalirin, a Rho GDP/GTP exchange factor for Rac1 and RhoG, is known to play an essential role in the formation and maintenance of excitatory synapses and in the secretion of neuropeptides. Mice unable to express any of the isoforms of Kalrn in cells that produce POMC at any time during development (POMC cells) exhibited reduced anxiety-like behavior and reduced acquisition of passive avoidance behavior, along with sex-specific alteration in the corticosterone response to restraint stress. Strikingly, lack of Kalrn expression in POMC cells closely mimicked the effects of global Kalrn knockout on anxiety-like behavior and passive avoidance conditioning without causing the other deficits noted in Kalrn knockout mice. Our data suggest that deficits in excitatory inputs onto POMC neurons are responsible for the behavioral phenotypes observed.

Gender-specific roles for the melanocortin-3 receptor in the regulation of the mesolimbic dopamine system in mice

The melanocortin-3 receptor (MC3R) and MC4R are known to play critical roles in energy homeostasis. However, the physiological functions of the MC3R remain poorly understood. Earlier reports indicated that the ventral tegmental area (VTA) is one of the highest sites of MC3R expression, and we sought to determine the function of the receptor in this brain region. A MC3R-green-fluorescent protein transgenic mouse and a MC3R knockout mouse strain were used to characterize the neurochemical identity of the MC3R neurons in the VTA and to determine the effects of global MC3R deletion on VTA dopamine (DA) homeostasis. We demonstrate that the MC3R, but not MC4R, is expressed in up to a third of dopaminergic neurons of the VTA. Global deletion of the MC3R increases total dopamine by 42% in the VTA and decreases sucrose intake and preference in female but not male mice. Ovariectomy restores dopamine levels to normal, but aberrant decreased VTA dopamine levels are also observed in prepubertal female mice. Because arcuate Agouti-related peptide/neuropeptide Y neurons are known to innervate and regulate VTA signaling, the MC3R in dopaminergic neurons provides a specific input for communication of nutritional state within the mesolimbic dopamine system. Data provided here suggest that this input may be highly sexually dimorphic, functioning as a specific circuit regulating effects of estrogen on VTA dopamine levels and on sucrose preference. Overall, this data support a sexually dimorphic function of MC3R in regulation of the mesolimbic dopaminergic system and reward.

Temporal changes in nutritional state affect hypothalamic POMC peptide levels independently of leptin in adult male mice

Hypothalamic proopiomelanocortin (POMC) neurons constitute a critical anorexigenic node in the central nervous system (CNS) for maintaining energy balance. These neurons directly affect energy expenditure and feeding behavior by releasing bioactive neuropeptides but are also subject to signals directly related to nutritional state such as the adipokine leptin. To further investigate the interaction of diet and leptin on hypothalamic POMC peptide levels, we exposed 8- to 10-wk-old male POMC-Discosoma red fluorescent protein (DsRed) transgenic reporter mice to either 24-48 h (acute) or 2 wk (chronic) food restriction, high-fat diet (HFD), or leptin treatment. Using semiquantitative immunofluorescence and radioimmunoassays, we discovered that acute fasting and chronic food restriction decreased the levels of adrenocorticotropic hormone (ACTH), α-melanocyte-stimulating hormone (α-MSH), and β-endorphin in the hypothalamus, together with decreased DsRed fluorescence, compared with control ad libitum-fed mice. Furthermore, acute but not chronic HFD or leptin administration selectively increased α-MSH levels in POMC fibers and increased DsRed fluorescence in POMC cell bodies. HFD and leptin treatments comparably increased circulating leptin levels at both time points, suggesting that transcription of Pomc and synthesis of POMC peptide products are not modified in direct relation to the concentration of plasma leptin. Our findings indicate that negative energy balance persistently downregulated POMC peptide levels, and this phenomenon may be partially explained by decreased leptin levels, since these changes were blocked in fasted mice treated with leptin. In contrast, sustained elevation of plasma leptin by HFD or hormone supplementation did not significantly alter POMC peptide levels, indicating that enhanced leptin signaling does not chronically increase Pomc transcription and peptide synthesis.

Neonatal insulin action impairs hypothalamic neurocircuit formation in response to maternal high-fat feeding

Maternal metabolic homeostasis exerts long-term effects on the offspring's health outcomes. Here, we demonstrate that maternal high-fat diet (HFD) feeding during lactation predisposes the offspring for obesity and impaired glucose homeostasis in mice, which is associated with an impairment of the hypothalamic melanocortin circuitry. Whereas the number and neuropeptide expression of anorexigenic proopiomelanocortin (POMC) and orexigenic agouti-related peptide (AgRP) neurons, electrophysiological properties of POMC neurons, and posttranslational processing of POMC remain unaffected in response to maternal HFD feeding during lactation, the formation of POMC and AgRP projections to hypothalamic target sites is severely impaired. Abrogating insulin action in POMC neurons of the offspring prevents altered POMC projections to the preautonomic paraventricular nucleus of the hypothalamus (PVH), pancreatic parasympathetic innervation, and impaired glucose-stimulated insulin secretion in response to maternal overnutrition. These experiments reveal a critical timing, when altered maternal metabolism disrupts metabolic homeostasis in the offspring via impairing neuronal projections, and show that abnormal insulin signaling contributes to this effect.

Feed your head: neurodevelopmental control of feeding and metabolism

During critical periods of development early in life, excessive or scarce nutritional environments can disrupt the development of central feeding and metabolic neural circuitry, leading to obesity and metabolic disorders in adulthood. A better understanding of the genetic networks that control the development of feeding and metabolic neural circuits, along with knowledge of how and where dietary signals disrupt this process, can serve as the basis for future therapies aimed at reversing the public health crisis that is now building as a result of the global obesity epidemic. This review of animal and human studies highlights recent insights into the molecular mechanisms that regulate the development of central feeding circuitries, the mechanisms by which gestational and early postnatal nutritional status affects this process, and approaches aimed at counteracting the deleterious effects of early over- and underfeeding.

Distinct glutamatergic and GABAergic subsets of hypothalamic pro-opiomelanocortin neurons revealed by in situ hybridization in male rats and mice

Pro-opiomelanocortin (POMC) and agouti-related protein (AGRP) neurons in the hypothalamus regulate various aspects of energy homeostasis and metabolism. POMC and AGRP neurons, respectively, agonize and antagonize melanocortin receptors on their common downstream neurons. However, it is unknown whether they also reciprocally stimulate and inhibit the same neurons by amino acid transmitters. Whereas AGRP neurons are mostly GABAergic, surprisingly, only a small population of POMC neurons has been found to be glutamatergic, and a significantly larger subpopulation to be GABAergic. To further examine amino acid phenotypes of POMC neurons, we studied mRNA expression for the glutamatergic marker, type 2 vesicular glutamate transporter (VGLUT2), and the GABA synthetic enzyme, glutamic acid decarboxylase 67 (GAD67), in POMC neurons of both rats and mice by using in situ hybridization techniques. In rats, approximately 58% of POMC neurons were labeled for VGLUT2 and 37% for GAD67 mRNA. In mice, approximately 43% of POMC neurons contained VGLUT2, and 54% contained GAD67 mRNA. In both species, a prominent mediolateral distribution pattern was observed at rostral and mid levels of the POMC cell group with VGLUT2-POMC neurons dominating in lateral portions and GAD67-POMC neurons in medial portions. These data demonstrate that both glutamatergic and GABAergic cells are present in comparably significant numbers among POMC neurons. Their glutamatergic or GABAergic phenotype may represent a major functional division within the POMC cell group.

Addiction and the adrenal cortex

Substantial evidence shows that the hypophyseal–pituitary–adrenal (HPA) axis and corticosteroids are involved in the process of addiction to a variety of agents, and the adrenal cortex has a key role. In general, plasma concentrations of cortisol (or corticosterone in rats or mice) increase on drug withdrawal in a manner that suggests correlation with the behavioural and symptomatic sequelae both in man and in experimental animals. Corticosteroid levels fall back to normal values in resumption of drug intake. The possible interactions between brain corticotrophin releasing hormone (CRH) and proopiomelanocortin (POMC) products and the systemic HPA, and additionally with the local CRH–POMC system in the adrenal gland itself, are complex. Nevertheless, the evidence increasingly suggests that all may be interlinked and that CRH in the brain and brain POMC products interact with the blood-borne HPA directly or indirectly. Corticosteroids themselves are known to affect mood profoundly and may themselves be addictive. Additionally, there is a heightened susceptibility for addicted subjects to relapse in conditions that are associated with change in HPA activity, such as in stress, or at different times of the day. Recent studies give compelling evidence that a significant part of the array of addictive symptoms is directly attributable to the secretory activity of the adrenal cortex and the actions of corticosteroids. Additionally, sex differences in addiction may also be attributable to adrenocortical function: in humans, males may be protected through higher secretion of DHEA (and DHEAS), and in rats, females may be more susceptible because of higher corticosterone secretion.

The role of melanocortins and Neuropeptide Y in food reward

The Neuropeptide Y and the melanocortin peptides are two well-described hypothalamic feeding peptides regulating energy balance. Predominantly expressed within the arcuate nucleus, these neurons project to different brain areas and modulate various aspects of feeding. Hedonic feeding, where one overindulges in palatable food consumption beyond one's nutritional necessities, is one such aspect regulated by NPY/melanocortin signaling. Research suggests that NPY/melanocortin regulate hedonic aspects of feeding through its projections to the brain reward circuitry (ventral tegmental area, lateral hypothalamus, nucleus accumbens etc.), however, exact target areas have not yet been identified. The current work explores literature to provide a mechanistic explanation for the effects of these peptides on food reward.

Expression of GABAergic and glutamatergic phenotypic markers in hypothalamic proopiomelanocortin neurons

Hypothalamic proopiomelanocortin (POMC) neurons have traditionally been defined by their peptide transmitters, which are important regulators of energy balance and reward. Recent work shows that POMC neurons can also release the amino acid transmitters γ-aminobutyric acid (GABA) and glutamate, although studying GABAergic and glutamatergic populations of POMC neurons has been hindered by the difficulty in reliably identifying amino acid (AA) transmitter phenotypes. In the present study, fluorescent in situ hybridization and immunohistochemistry were used to identify POMC neurons and to detect the presence of mRNA for the transporters responsible for packaging either GABA (vesicular GABA transporter [vGAT]) or glutamate (vesicular glutamate transporter [vGLUT]) into vesicles, as well as the enzymes responsible for GABA synthesis, glutamic acid decarboxylase (GAD)65 and GAD67. Approximately 7% of POMC neurons expressed vGlut2 and the highest percentage of vGlut2-positive POMC cells were located in the rostral arcuate nucleus. Despite the reports of GABA release from POMC neurons, vGat was not detected in POMC neurons, although Gad65 and Gad67 were present in ~40% of POMC neurons. Approximately half of the vGlut2-expressing POMC cells also expressed Gad65. Markers of neurotransmitter phenotype were better detected by using in situ hybridization techniques rather than transgenic expression of fluorophores under the control of the vGat or Gad67 promoters. It is now clear that the expression of markers of AA phenotype provides a useful means to identify distinct subpopulations of POMC neurons. Additionally, the method described will be useful to explore the possibility that plasticity of AA phenotype is an important aspect of POMC neuron function.