Response of melanocortin-4 receptor-deficient mice to anorectic and orexigenic peptides

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

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

Gene expression in hypothalamus, liver, and adipose tissues and food intake response to melanocortin-4 receptor agonist in pigs expressing melanocortin-4 receptor mutations

Transcriptional profiling was used to identify genes and pathways that responded to intracerebroventricular injection of melanocortin-4 receptor (MC4R) agonist [Nle4, d-Phe7]-α-melanocyte stimulating hormone (NDP-MSH) in pigs homozygous for the missense mutation in the MC4R, D298 allele ( n = 12), N298 allele ( n = 12), or heterozygous ( n = 12). Food intake (FI) was measured at 12 and 24 h after treatment. All pigs were killed at 24 h after treatment, and hypothalamus, liver, and back-fat tissue was collected. NDP-MSH suppressed ( P < 0.004) FI at 12 and 24 h in all animals after treatment. In response to NDP-MSH, 278 genes in hypothalamus ( q ≤ 0.07, P ≤ 0.001), 249 genes in liver ( q ≤ 0.07, P ≤ 0.001), and 5,066 genes in fat ( q ≤ 0.07, P ≤ 0.015) were differentially expressed. Pathway analysis of NDP-MSH-induced differentially expressed genes indicated that genes involved in cell communication, nucleotide metabolism, and signal transduction were prominently downregulated in the hypothalamus. In both liver and adipose tissue, energy-intensive biosynthetic and catabolic processes were downregulated in response to NDP-MSH. This included genes encoding for biosynthetic pathways such as steroid and lipid biosynthesis, fatty acid synthesis, and amino acid synthesis. Genes involved in direct energy-generating processes, such as oxidative phosphorylation, electron transport, and ATP synthesis, were upregulated, whereas TCA-associated genes were prominently downregulated in NDP-MSH-treated pigs. Our data also indicate a metabolic switch toward energy conservation since genes involved in energy-intensive biosynthetic and catabolic processes were downregulated in NDP-MSH-treated pigs.

Food demand and meal size in mice with single or combined disruption of melanocortin type 3 and 4 receptors

Mice with homozygous genetic disruption of the melanocortin-4 receptor gene (MC4R-/-) are known to be hyperphagic and become obese, while those with disruption of the melanocortin-3 receptor gene (MC3R-/-) do not become markedly obese. The contribution of MC3R signaling in energy homeostasis remains little studied. In the present work, we compare MC3R-/- mice with wild-type (WT), MC4R-/-, and mice bearing disruption of both genes (double knockout, DKO) on select feeding and neuroanatomical dimensions. DKO mice were significantly more obese than MC4R-/-, whereas MC3R-/- weighed the same as WT. In a food demand protocol, DKO and MC4R-/- were hyperphagic at low unit costs for food, due primarily to increased meal size. However, at higher costs, their intake dropped below that of WT and MC3R-/-, indicating increased elasticity of food demand. To determine whether this higher elasticity was due to either the genotype or to the obese phenotype, the same food demand protocol was conducted in dietary obese C57BL6 mice. They showed similar elasticity to lean mice, suggesting that the effect is of genotypic origin. To assess whether the increased meal size in MC4R-/- and DKO might be due to reduced CCK signaling, we examined the acute anorectic effect of peripherally administered CCK and subsequently the induction of c-Fos immunoreactivity in select brain regions. The anorectic effect of CCK was comparable in MC4R-/-, DKO, and WT, but it was unexpectedly absent in MC3R-/-. CCK-induced c-Fos was lower in the paraventricular nucleus in MC3R-/- than the other genotypes. These data are discussed in terms of demand functions for food intake, MC receptors involved in feeding, and their relation to actions of gut hormones, such as CCK, and to obesity.

Corticotropin-releasing hormone receptor-1 in cerebral microvessels changes during development and influences urocortin transport across the blood-brain barrier

In this study we tested the hypothesis that receptor-mediated transport of urocortin across the blood-brain barrier (BBB) undergoes developmental changes. Urocortin is a peptide produced by both selective brain regions and peripheral organs, and it is involved in feeding, memory, mood, cardiovascular functions, and immune regulation. In BBB studies with multiple-time regression analysis, we found that neonatal mice had a significant influx of (125)I-urocortin. By contrast, adult mice did not transport urocortin across the BBB. Quantitative RT-PCR showed that corticotropin-releasing hormone receptor (CRHR)-1 was developmentally regulated in enriched cerebral microvessels as well as hypothalamus, being significantly higher in neonatal than adult mice. This change was less dramatic in agouti viable yellow mice, a strain that develops adult-onset obesity. The level of expression of CRHR1 mRNA was 33-fold higher in the microvessels than in hypothalamic homogenates. The mRNA for CRHR2 was less abundant in both regions and less prone to changes with development or the agouti viable yellow mutation. Supported by previous findings of receptor-mediated endocytosis of urocortin, these results suggest that permeation of urocortin across the BBB is dependent on the level of CRHR1 expression in cerebral microvessels. These novel findings of differential regulation of CRH receptor subtypes help elucidate developmental processes in the brain, particularly for the urocortin system.

Regulation of energy homeostasis by bombesin receptor subtype-3: selective receptor agonists for the treatment of obesity

Bombesin receptor subtype 3 (BRS-3) is a G protein coupled receptor whose natural ligand is unknown. We developed potent, selective agonist (Bag-1, Bag-2) and antagonist (Bantag-1) ligands to explore BRS-3 function. BRS-3-binding sites were identified in the hypothalamus, caudal brainstem, and several midbrain nuclei that harbor monoaminergic cell bodies. Antagonist administration increased food intake and body weight, whereas agonists increased metabolic rate and reduced food intake and body weight. Prolonged high levels of receptor occupancy increased weight loss, suggesting a lack of tachyphylaxis. BRS-3 agonist effectiveness was absent in Brs3(-/Y) (BRS-3 null) mice but was maintained in Npy(-/-)Agrp(-/-), Mc4r(-/-), Cnr1(-/-), and Lepr(db/db) mice. In addition, Brs3(-/Y) mice lost weight upon treatment with either a MC4R agonist or a CB1R inverse agonist. These results demonstrate that BRS-3 has a role in energy homeostasis that complements several well-known pathways and that BRS-3 agonists represent a potential approach to the treatment of obesity.

Ethanol-induced increase of agouti-related protein (AgRP) immunoreactivity in the arcuate nucleus of the hypothalamus of C57BL/6J, but not 129/SvJ, inbred mice

BACKGROUND

The melanocortin (MC) system is composed of peptides that are cleaved from the polypeptide precursor, pro-opiomelanocortin (POMC). Previous research has shown that MC receptor (MCR) agonists reduce, and MCR antagonists increase, ethanol consumption in rats and mice. Consistently, genetic deletion of the endogenous MCR antagonist, agouti-related protein (AgRP), causes reductions of ethanol-reinforced lever pressing and binge-like ethanol drinking in C57BL/6J mice. Ethanol also has direct effects on the central MC system, as chronic exposure to an ethanol-containing diet causes significant reductions of alpha-melanocyte stimulating hormone (alpha-MSH) immunoreactivity in specific brain regions of Sprague-Dawley rats. Together, these observations suggest that the central MC system modulates neurobiological responses to ethanol. To further characterize the role of the MC system in responses to ethanol, here we compared AgRP and alpha-MSH immunoreactivity in response to an acute injection of saline or ethanol between high ethanol drinking C57BL/6J mice and moderate ethanol drinking 129/SvJ mice.

METHODS

Mice received an intraperitoneal (i.p.) injection of ethanol (1.5 g/kg or 3.5 g/kg; mixed in 0.9% saline) or an equivolume of 0.9% saline. Two hours after injection, animals were sacrificed and their brains were processed for AgRP and alpha-MSH immunoreactivity.

RESULTS

Results indicated that acute ethanol administration triggered a dose-dependent increase in AgRP immunoreactivity in the arcuate (ARC) of C57BL/6J mice, an effect that was not evident in the 129/SvJ strain. Although acute administration of ethanol did not influence alpha-MSH immunoreactivity, C57BL/6J mice had significantly greater overall alpha-MSH immunoreactivity in the ARC, dorsomedial, and lateral regions of the hypothalamus relative to the 129/SvJ strain. In contrast, C57BL/6J mice displayed significantly lower alpha-MSH immunoreactivity in the medial amygdala.

CONCLUSIONS

The results show that acute ethanol exposure has direct effects on endogenous AgRP activity in ethanol preferring C57BL/6J mice. It is suggested that ethanol-induced increases in AgRP may be part of a positive feedback system that stimulates excessive binge-like ethanol drinking in C57BL/6J mice. Inherent differences in alpha-MSH immunoreactivity may contribute to differences in neurobiological responses to ethanol that are characteristically observed between the C57BL/6J and 129/SvJ inbred strains of mice.

Distribution and function of melanocortin receptors within the brain

Biological responses to pro-opiomelanocortin (POMC)-derived peptides administered in the brain were documented in the 1950s but their molecular mechanisms of action only began to be resolved with the mapping of melanocortin receptor subtypes to specific brain regions in the 1990s. Out of the five melanocortin receptor subtypes, MC3R and MC4R are widely recognised as 'neural' melanocortin receptors. In situ hybridization anatomical mapping of these receptor subtypes to distinct hypothalamic nuclei first indicated their roles in energy homeostasis, roles that were later confirmed with the obese phenotypes exhibited by Mc3R and Mc4R knockout mice. It is perhaps less well known however, that all five melanocortin receptor subtypes have been detected in developing and/or adult brains of various species. This chapter provides a comprehensive summary of the detection and mapping of each melanocortin receptor subtype in mammalian, chicken and fish brains and relates the sites of expression to functions that are either known or proposed for each receptor subtype.

Recent advances in understanding leptin signaling and leptin resistance

The brain controls energy homeostasis and body weight by integrating various metabolic signals. Leptin, an adipose-derived hormone, conveys critical information about peripheral energy storage and availability to the brain. Leptin decreases body weight by both suppressing appetite and promoting energy expenditure. Leptin directly targets hypothalamic neurons, including AgRP and POMC neurons. These leptin-responsive neurons widely connect to other neurons in the brain, forming a sophisticated neurocircuitry that controls energy intake and expenditure. The anorexigenic actions of leptin are mediated by LEPRb, the long form of the leptin receptor, in the hypothalamus. LEPRb activates both JAK2-dependent and -independent pathways, including the STAT3, PI 3-kinase, MAPK, AMPK, and mTOR pathways. These pathways act coordinately to form a network that fully mediates leptin response. LEPRb signaling is regulated by both positive (e.g., SH2B1) and negative (e.g., SOCS3 and PTP1B) regulators and by endoplasmic reticulum stress. Leptin resistance, a primary risk factor for obesity, likely results from impairment in leptin transport, LEPRb signaling, and/or the neurocircuitry of energy balance.

The role of GNAS and other imprinted genes in the development of obesity

Genomic imprinting is an epigenetic phenomenon affecting a small number of genes, which leads to differential expression from the two parental alleles. Imprinted genes are known to regulate fetal growth and a 'kinship' or 'parental conflict' model predicts that paternally and maternally expressed imprinted genes promote and inhibit fetal growth, respectively. In this review we examine the role of imprinted genes in postnatal growth and metabolism, with an emphasis on the GNAS/Gnas locus. GNAS is a complex imprinted locus with multiple oppositely imprinted gene products, including the G-protein alpha-subunit G(s)alpha that is expressed primarily from the maternal allele in some tissues and the G(s)alpha isoform XLalphas that is expressed only from the paternal allele. Maternal, but not paternal, G(s)alpha mutations lead to obesity in Albright hereditary osteodystrophy. Mouse studies show that this phenomenon is due to G(s)alpha imprinting in the central nervous system leading to a specific defect in the ability of central melanocortins to stimulate sympathetic nervous system activity and energy expenditure. In contrast mutation of paternally expressed XLalphas leads to opposite metabolic effects in mice. Although these findings conform to the 'kinship' model, the effects of other imprinted genes on body weight regulation do not conform to this model.

Analysis of the therapeutic functions of novel melanocortin receptor agonists in MC3R- and MC4R-deficient C57BL/6J mice

Melanocortin receptor agonists act in the brain to regulate food intake and body weight and, independently of these actions, affect insulin sensitivity. These experiments investigated the function of novel non-selective melanocortin receptor agonists (BIM-22493, BIM-22511) that cross the blood-brain barrier when administered peripherally. Treatment of diet induced obese C57BL/6J (B6) mice with melanocortin agonists administered peripherally improved obesity, hyperinsulinemia (approximately 50%) and fatty liver disease. Specificity of function was determined using B6 melanocortin-3 and melanocortin-4 receptor knockout mice (MC3RKO, MC4RKO). Chow fed MC4RKO but not MC3RKO used for these tests exhibited obesity, hyperinsulinemia and severe hepatosteatosis associated with increased expression of insulin-stimulated genes involved in lipogenesis. Reduced food intake associated with acute BIM-22493 treatment, and weight loss associated with 14 days of treatment with BIM-22511, required functional MC4R but not MC3R. However, while 14 days of treatment with BIM-22511 did not affect body weight and even increased cumulative food intake in MC4RKO, a significant reduction (approximately 50%) in fasting insulin was still observed. Despite lowering insulin, chronic treatment with BIM-22511 did not improve hepatosteatosis in MC4RKO, and did not affect hepatic lipogenic gene expression. Together, these results demonstrate that peripherally administered melanocortin receptor agonists regulate body weight, liver metabolism and glucose homeostasis through independent pathways. MC4R are necessary for melanocortin agonist-induced weight loss and improvements in liver metabolism, but are not required for improvements in hyperinsulinemia. Agonists with activity at MC4R improve glucose homeostasis at least partially by causing weight loss, however other melanocortin receptors may have potential for treating aberrations in glucose homeostasis associated with obesity.

Hypothalamic melanocortin signaling and leptin resistance--perspective of therapeutic application for obesity-diabetes syndrome

The adipocyte-derived hormone, leptin controls feeding behavior, augments fatty acid beta-oxidation in the skeletal muscle, attenuates insulin secretion but enhances whole body insulin sensitivity and glucose disposal, thereby serving as a promising therapeutic candidate for the treatment of insulin resistance and dyslipidemia. Along with other researchers, we demonstrated the clinical efficacy and safety of leptin in the treatment of diabetes and dyslipidemia for patients with generalized lipodystrophy. However, the clinical application of leptin has been hampered by the notion that leptin does not fully exert its metabolic effects in human obesity and diet-induced obese rodents. We found that the activity of skeletal muscle AMP-activated protein kinase (AMPK) parallels hypothalamic leptin sensitivity and metabolic phenotype in transgenic mice overexpressing leptin. Our data indicate that the activation of skeletal muscle AMPK is mediated by the hypothalamic melanocortin pathway. In fact, intracerebroventricular administration of melanocortin agonist, MT-II in mice robustly overcomes high-fat diet-induced leptin resistance and ameliorates fuel dyshomeostasis and hyperphagia, with a concomitant recovery of AMPK activity in skeletal muscle. Conversely, AMPK/ACC phosphorylation by leptin was abrogated by the co-administration of melanocortin antagonist, SHU9119 and in the KKA(y) mice, which centrally express endogenous melanocortin antagonist. Importantly, high-fat diet-induced attenuation of AMPK/ACC phosphorylation in leptin-overexpressing transgenic mice was not reversed by central leptin per se, but was markedly recovered by MT-II. Our data provide evidence for the critical role of the central melanocortin system in leptin-skeletal muscle AMPK axis, and highlight the system as a therapeutic target for leptin insuffciency in obese humans.

Central nervous system imprinting of the G protein G(s)alpha and its role in metabolic regulation

In Albright hereditary osteodystrophy, a monogenic obesity disorder linked to heterozygous mutations of G(s)alpha, the G protein that mediates receptor-stimulated cAMP generation, obesity develops only when the mutation is on the maternal allele. Likewise, mice with maternal (but not paternal) germline G(s)alpha mutation develop obesity, insulin resistance, and diabetes. These parent-of-origin effects are due to G(s)alpha imprinting, with preferential expression from the maternal allele in some tissues. As G(s)alpha is ubiquitously expressed, the tissue involved in this metabolic imprinting effect is unknown. Using brain-specific G(s)alpha knockout mice, we show that G(s)alpha imprinting within the central nervous system underlies these effects and that G(s)alpha is imprinted in the paraventricular nucleus of the hypothalamus. Maternal G(s)alpha mutation impaired melanocortin stimulation of energy expenditure but did not affect melanocortin's effect on food intake, suggesting that melanocortins may regulate energy balance in the central nervous system through both G(s)alpha-dependent and -independent pathways.

Loss of hypothalamic response to leptin during pregnancy associated with development of melanocortin resistance

Hypothalamic leptin resistance during pregnancy is an important adaptation that facilitates the state of positive energy balance required for fat deposition in preparation for lactation. Within the arcuate nucleus, pro-opiomelanocortin (POMC) neurones and neuropeptide Y (NPY)/agouti-related gene protein (AgRP) neurones are first-order leptin responsive neurones involved in the regulation of energy balance. The present study aimed to investigate whether the regulation of these neuropeptides is disrupted during pregnancy in association with the development of leptin resistance. As measured by quantitative in situ hybridisation, POMC and AgRP mRNA levels were not significantly different during pregnancy, whereas NPY mRNA levels increased such that, by day 21 of pregnancy, levels were significantly higher than in nonpregnant, animals. These data suggest that these neurones were not responding normally to the elevated leptin found during pregnancy. To further characterise the melanocortin system during pregnancy, double-label immunohistochemistry was used to quantify leptin-induced phosphorylation of signal transducer and activator of transcription 3 (pSTAT3) in POMC neurones, using α-melanocyte-stimulating hormone (MSH) as a marker. The percentage of α-MSH neurones containing leptin-induced pSTAT3 did not significantly differ from nonpregnant animals, indicating that there was no change in the number of POMC neurones that respond to leptin during pregnancy. Treatment with α-MSH significantly reduced food intake in nonpregnant rats, but not in pregnant rats, indicating resistance to the satiety actions of α-MSH during pregnancy. The data suggest that multiple mechanisms contribute to leptin resistance during pregnancy. As well as a loss of responses in first-order leptin-responsive neurones in the arcuate nucleus, there is also a downstream disruption in the melanocortin system.

The role of mPer2 clock gene in glucocorticoid and feeding rhythms

The circadian clock synchronizes the activity level of an organism to the light-dark cycle of the environment. Energy intake, as well as energy metabolism, also has a diurnal rhythm. Although the role of the clock genes in the sleep-wake cycle is well characterized, their role in the generation of the metabolic rhythms is poorly understood. Here, we use mice deficient in the clock protein mPer2 to study how the circadian clock regulates two critical metabolic rhythms: glucocorticoid and food intake rhythms. Our findings indicate that mPer2-/- mice do not have a glucocorticoid rhythm even though the corticosterone response to hypoglycemia, ACTH, and restraint stress is intact. In addition, the diurnal feeding rhythm is absent in mPer2-/- mice. On high-fat diet, they eat as much during the light period as they do during the dark period and develop significant obesity. The diurnal rhythm of neuroendocrine peptide alphaMSH, a major effector of appetite control, is disrupted in the hypothalamus of mPer2-/- mice even though the diurnal rhythm of ACTH, the alphaMSH precursor, is intact. Peripheral injection of alphaMSH, which has been shown to enter the brain, restored the feeding rhythm and induced weight loss in mPer2-/- mice. These findings emphasize the requirement of mPer2 in appetite control during the inactive period and the potential role of peripherally administered alphaMSH in restoring night-day eating pattern in individuals with circadian eating disorders such as night-eating syndrome, which is also associated with obesity.

High ambient temperature reverses hypothalamic MC4 receptor overexpression in an animal model of anorexia nervosa

The potential involvement of the melanocortin system in the beneficial effects of heat application in rats submitted to activity-based anorexia (ABA), an analogous model of anorexia nervosa (AN), was studied. Once ABA rats had lost 20% of body weight, half of the animals were exposed to a high ambient temperature (HAT) of 32 degrees C, whereas the rest were maintained at 21 degrees C. Control sedentary rats yoked to ABA animals received the same treatment. ABA rats (21 degrees C) showed increased Melanocortin 4 (MC4) receptor and Agouti gene Related Peptide (AgRP) expression, and decreased pro-opiomelanocortin (POMC) mRNA levels (Real Time PCR), with respect to controls. Heat application increased weight gain and food intake, and reduced running rate in ABA rats, when compared with ABA rats at 21 degrees C. However, no changes in body weight and food intake were observed in sedentary rats exposed to heat. Moreover, heat application reduced MC4 receptor, AgRP and POMC expression in ABA rats, but no changes were observed in control rats. These results indicate that hypothalamic MC4 receptor overexpression could occur on the basis of the characteristic hyperactivity, weight loss, and self-starvation of ABA rats, and suggest the involvement of hypothalamic melanocortin neural circuits in behavioural changes shown by AN patients. Changes in AgRP and POMC expression could represent an adaptative response to equilibrate energy balance. Moreover, the fact that HAT reversed hypothalamic MC4 receptor overexpression in ABA rats indicates the involvement of brain melanocortin system in the reported beneficial effects of heat application in AN. A combination of MC4 receptor antagonists and heat application could improve the clinical management of AN.

Phosphodiesterase inhibitor-dependent inverse agonism of agouti-related protein on melanocortin 4 receptor in sea bass (Dicentrarchus labrax)

The melanocortin 4 receptor (MC4R) is a G protein-coupled receptor mainly expressed in the central nervous system of vertebrates. Activation of the MC4R leads to a decrease in food intake, whereas inactivating mutations are a genetic cause of obesity. The binding of agouti-related protein (AGRP) reduces not only agonist-stimulated cAMP production (competitive antagonist) but also the basal activity of the receptor, as an inverse agonist. Transgenic zebrafish overexpressing AGRP display increased food intake and linear growth, indicative of a physiological role for the melanocortin system in the control of the energy balance in fish. We report on the cloning, pharmacological characterization, tissue distribution, and detailed brain mapping of a sea bass (Dicentrarchus labrax) MC4R ortholog. Sea bass MC4R is profusely expressed within food intake-controlling pathways of the fish brain. However, the activity of the melanocortin system during progressive fasting does not depend on the hypothalamic/pituitary proopiomelanocortin (POMC) and MC4R expression, which suggests that sea bass MC4R is constitutively activated and regulated by AGRP binding. We demonstrate that AGRP acts as competitive antagonist and reduces MTII-induced cAMP production. AGRP also decreases the basal activity of the receptor as an inverse agonist. This observation suggests that MC4R is constitutively active and supports the evolutionary conservation of the AGRP/MC4R interactions. The inverse agonism, but not the competitive antagonism, depends on the presence of a phosphodiesterase inhibitor (IBMX). This suggests that inverse agonism and competitive antagonism operate through different intracellular signaling pathways, a view that opens up new targets for the treatment of melanocortin-induced metabolic syndrome.

Melanocortin peptides affect the motivation to feed in rainbow trout (Oncorhynchus mykiss)

In this study, we investigated the effects of one melanocortin receptor (MCR) agonist and two antagonists on food intake in juvenile rainbow trout. Baseline food intake was established prior to 1 microl intracerebroventricular injection (ICV) of the non-specific agonist MTII, the MC4R antagonist HS024 and the MC3/4R antagonist SHU9119 at concentrations of 0.3, 1 or 3 nM. Saline-injected fish and untreated fish served as controls. Changes in food intake were observed 1h after the ICV injections. Our results showed that treatment with MTII significantly decreased food intake at 3 nM compared to control, HS024 significantly increased food intake at 3 nM compared to control and saline-treated fish, and SHU9119 significantly increased food intake at 3 nM compared to saline-treated fish. In conclusion, our study provides further evidence, and hence strengthens the hypothesis, that MC4R participates in the control of energy balance in fish in the same manner as in mammals. Our findings that HS024 is more potent than SHU9119 in increasing food intake suggest that the effects of melanocortin on energy balance in rainbow trout are mainly regulated by activation of MC4R. Hence, HS024 seems an excellent tool as a MC4R antagonist in rainbow trout.

Hypothalamic lipids and the regulation of energy homeostasis

The hypothalamus is a specialised area in the brain that integrates the control of energy homeostasis, regulating both food intake and energy expenditure. The classical theory for hypothalamic feeding control is mainly based on the relationship between peripheral signals and neurotransmitters/neuromodulators in the central nervous system. Thus, hypothalamic neurons respond to peripheral signals, such as hormones and nutrients, by modifying the synthesis of neuropeptides. Despite the well-established role of these hypothalamic networks, increasing evidence indicates that the modulation of lipid metabolism in the hypothalamus plays a critical role in feeding control. In fact, the pharmacologic and genetic targeting of key enzymes from these pathways, such as AMP-activated protein kinase, acetyl-CoA carboxylase, carnitine palmitoyltransferase 1, fatty acid synthase, and malonyl-CoA decarboxylase, has a profound effect on food intake and body weight. Here, we review what is currently known about the relationship between hypothalamic lipid metabolism and whole body energy homeostasis. Defining these novel mechanisms may offer new therapeutic targets for the treatment of obesity and its associated pathologies.

Six new loci associated with body mass index highlight a neuronal influence on body weight regulation

Common variants at only two loci, FTO and MC4R, have been reproducibly associated with body mass index (BMI) in humans. To identify additional loci, we conducted meta-analysis of 15 genome-wide association studies for BMI (n > 32,000) and followed up top signals in 14 additional cohorts (n > 59,000). We strongly confirm FTO and MC4R and identify six additional loci (P < 5 x 10(-8)): TMEM18, KCTD15, GNPDA2, SH2B1, MTCH2 and NEGR1 (where a 45-kb deletion polymorphism is a candidate causal variant). Several of the likely causal genes are highly expressed or known to act in the central nervous system (CNS), emphasizing, as in rare monogenic forms of obesity, the role of the CNS in predisposition to obesity.

Regulation of feeding and anxiety by alpha-MSH reactive autoantibodies

alpha-Melanocyte-stimulating hormone (alpha-MSH) is a stress-related neuropeptide involved in the regulation of motivated behavior, appetite and emotion including stimulation of satiety and anxiety. Although autoantibodies (autoAbs) reactive with alpha-MSH have been identified in human subjects and in rats, it remained unknown if these autoAbs are involved in the regulation of feeding and anxiety and if their production is related to stress. Here we show that repeated exposure of rats to anxiolytic mild stress by handling increases the levels and affinity of alpha-MSH reactive IgG autoAbs and that these changes are associated with adaptive feeding and anxiety responses during exposure of rats to a strong stress by food restriction. Importantly, an increase in affinity of alpha-MSH reactive autoAbs was associated with changes of their functional roles from stimulation to inhibition of alpha-MSH-mediated behavioural responses, suggesting that these autoAbs can be a carrier or a neutralizing molecule of alpha-MSH peptide, respectively. Using a model of passive transfer into the brain, we show that alpha-MSH autoAbs affinity purified from blood of rats exposed to repeated mild stress, but not from control rats, are able to increase acutely food intake, suppress anxiety and modify gene expression of hypothalamic neuropeptides in naïve rats. These data provide the first evidence that autoAbs reactive with alpha-MSH are involved in the physiological regulation of feeding and mood, supporting a further role of the immune system in the control of motivated behavior and adaptation to stress.

Forebrain melanocortin signaling enhances the hindbrain satiety response to CCK-8

Melanocortin 4 receptors (MC4R) are hypothesized to mediate the central nervous system actions of leptin to enhance the satiety effects of cholecystokinin (CCK). To further elucidate this mechanism, we confirmed that peripheral administration of CCK-8 is less effective in producing this effect in MC4R-deficient mice (MC4R(-/-)). Whereas intraperitoneal (ip) CCK-8 at 0.75 nmol/kg lean body mass (lbm) suppressed food intake in wild-type mice, CCK-8 doses of 7.5 nmol/kg lbm were required to attenuate food intake in MC4R(-/-) mice. To determine whether melanocortin signaling in the hypothalamic paraventricular nucleus (PVN) participates in regulating this CCK satiety response, we administered the MC3/MC4R antagonist, SHU9119, into the PVN of rats before ip CCK-8 administration. PVN administration of SHU9119 attenuated the ability of CCK-8 to reduce 30-min food intake by 20%. To determine whether MC4R are expressed by PVN neurons that project directly to hindbrain nuclei involved in the satiety response to ip CCK-8, the retrograde tracer fluorescent cholera toxin subunit B was injected into the nucleus tractus solitarius (NTS) of the hindbrain. After 4 days, labeled PVN neurons were collected by laser capture microdissection and found to express MC4R mRNA by quantitative RT-PCR analysis. These data provide evidence for a neuroanatomical link between hypothalamic melanocortin signaling in the PVN and NTS neurons that regulate food intake. These findings highlight the contribution of melanocortin signaling in the PVN toward regulating the satiety effects of CCK-8 while acknowledging that melanocortin-dependent pathways in other brain regions and/or melanocortin-independent mechanisms are also important in this mechanism.

Corticotropin-releasing factor (CRF) is involved in the acute anorexic effect of alpha-melanocyte-stimulating hormone: a study using CRF-deficient mice

Alpha-melanocyte-stimulating hormone (alpha-MSH) and its receptors are critical and indispensable for maintaining appropriate feeding behavior and energy homeostasis in both mice and humans. Corticotropin-releasing factor (CRF) is a candidate for mediating the anorexic effect of alpha-MSH. In the present study, we examined whether CRF and its receptors are involved in the anorexic effect of alpha-MSH, using CRF-deficient (CRFKO) mice and a CRF receptor antagonist. Intracerebroventricular administration of NDP-MSH, a synthetic alpha-MSH analogue, suppressed food intake in wild-type (WT) mice. This effect was abolished by pretreatment with a non-selective CRF receptor antagonist, astressin, suggesting that the effect of alpha-MSH-induced anorexia was mediated by a CRF receptor. In CRFKO mice, administration with NDP-MSH did not affect food intake at an early phase (0-4h). In addition, CRF mRNA levels in the hypothalamus were significantly increased in NDP-MSH-treated mice. Therefore, our findings, using CRFKO, strongly support evidence that CRF is involved in the acute anorexic effect of alpha-MSH. On the other hand, NDP-MSH administered to CRFKO mice led to suppressed food intake at the late phase (4-12h), similar to the effect in WT mice. Further, NDP-MSH similarly reduced food intake during the late phase in all types of mice, including WT, CRFKO, and CRFKO with corticosterone replacement. The results would suggest that alpha-MSH-induced suppression of food intake at late phase was independent of glucocorticoids and CRF.

The melanocortin-3 receptor is required for entrainment to meal intake

Entrainment of anticipatory activity and wakefulness to nutrient availability is a poorly understood component of energy homeostasis. Restricted feeding (RF) paradigms with a periodicity of 24 h rapidly induce entrainment of rhythms anticipating food presentation that are independent of master clocks in the suprachiasmatic nucleus (SCN) but do require other hypothalamic structures. Here, we report that the melanocortin system, which resides in hypothalamic structures required for food entrainment, is required for expression of food entrainable rhythms. Food anticipatory activity was assessed in wild-type (WT) and melanocortin-3 receptor-deficient (Mc3r-/-) C57BL/J mice by wheel running, spontaneous locomotory movement, and measurement of wakefulness. WT mice housed in wheel cages subject to RF exhibited increased wheel activity during the 2 h preceding meal presentation, which corresponded with an increase in wakefulness around meal time and reduced wakefulness during the dark. WT mice also exhibited increased x- and z-movements centered around food initiation. The activity-based responses to RF were significantly impaired in mice lacking Mc3r. RF also failed to increase wakefulness in the 2 h before food presentation in Mc3r-/- mice. Food entrainment requires expression of Neuronal PAS domain 2 (Npas2) and Period2 (Per2) genes, components of the transcriptional machinery maintaining a clock rhythm. Analysis of cortical gene expression revealed severe abnormalities in rhythmic expression of clock genes (Bmal1, Npas2, Per2) under ad libitum and RF conditions. In summary, Mc3r are required for expression of anticipatory patterns of activity and wakefulness during periods of limited nutrient availability and for normal regulation of cortical clock function.

Voluntary exercise prevents the obese and diabetic metabolic syndrome of the melanocortin-4 receptor knockout mouse

Exercise is a mechanism for maintenance of body weight in humans. Morbidly obese human patients have been shown to possess single nucleotide polymorphisms in the melanocortin-4 receptor (MC4R). MC4R knockout mice have been well characterized as a genetic model that possesses phenotypic metabolic disorders, including obesity, hyperphagia, hyperinsulinemia, and hyperleptinemia, similar to those observed in humans possessing dysfunctional hMC4Rs. Using this model, we examined the effect of voluntary exercise of MC4R knockout mice that were allowed access to a running wheel for a duration of 8 wk. Physiological parameters that were measured included body weight, body composition of fat and lean mass, food consumption, body length, and blood levels of cholesterol and nonfasted glucose, insulin, and leptin. At the termination of the experiment, hypothalamic mRNA expression levels of neuropeptide Y (NPY), agouti-related protein (AGRP), proopiomelanocortin (POMC), cocaine- and amphetamine-regulated transcript (CART), orexin, brain-derived neurotropic factor (BDNF), phosphatase with tensin homology (Pten), melanocortin-3 receptor (MC3R), and NPY-Y1R were determined. In addition, islet cell distribution and function in the pancreas were examined. In the exercising MC4R knockout mice, the pancreatic islet cell morphology and other physiological parameters resembled those observed in the wild-type littermate controls. Gene expression profiles identified exercise as having a significant effect on hypothalamic POMC, orexin, and MC3R levels. Genotype had a significant effect on AGRP, POMC, CART, and NPY-Y1R, with an exercise and genotype interaction effect on NPY gene expression. These data support the hypothesis that voluntary exercise can prevent the genetic predisposition of melanocortin-4 receptor-associated obesity and diabetes.

Neuropeptide Y and alpha-melanocyte-stimulating hormone: interaction in obesity and possible role in the development of hypertension

AIM

Obesity and hypertension frequently coexist and both represent important risk factors for cardiovascular disease. The mechanisms implicated in the regulation of food intake have not been completely elucidated. Recent data suggests that peripheral and central neuropeptides play an important role in the maintenance of energy balance. More specifically, leptin, neuropeptide Y (NPY) and alpha-melanocyte-stimulating hormone (a-MSH) appear to be implicated in the pathogenesis of obesity and also contribute to the development of hypertension in obesity.

METHODS

Analysis of the pertinent bibliography published in PubMed database.

RESULTS

Leptin is produced in the adipose tissue directly correlated with fat tissue mass. Leptin acts on two distinct neural populations in the hypothalamus: the first expresses the orexigenic peptides NPY and agouti-related protein (AgRP), the second pro-opiomelanocortin (POMC). The activation of POMC neurons increases the production of the anorexigenic hormone a-MSH and inhibits the release of NPY and AgRP. In addition, the hypothalamus integrates the neuroendocrine systems with the autonomic nervous system and controls the activity of the latter. Stimulation of hypothalamic nuclei elicits sympathetic responses including blood pressure elevation. Both NPY and a-MSH appears to be implicated in the hypothalamic regulation of sympathetic nervous system (SNS) activity.

CONCLUSION

Alterations in leptin, NPY and a-MSH are frequently observed in obesity and might stimulate SNS activity, contributing to the development of hypertension in obese patients. These neuropeptides might provide a pathophysiologic link between excess weight and hypertension. However, more research is needed before the pharmacologic manipulation of these complex neuroendocrine systems can be applied in the treatment of obesity and hypertension.

Molecular and neural mediators of leptin action

The adipose tissue-derived hormone, leptin, acts via its receptor (LepRb) in the brain to regulate energy balance and neuroendocrine function. Parsing the biology of leptin requires understanding LepRb signaling and the roles for specific signaling pathways in neural and physiological leptin action. Since the leptin acts via a broadly distributed network of LepRb-expressing neurons, understanding the function of each of these LepRb neural populations will also be crucial. Here, we review the status of knowledge regarding the molecular mediators of leptin action and the neural substrate via which leptin acts to regulate physiologic processes.

Intracellular retention of human melanocortin-4 receptor: a molecular mechanism underlying early-onset obesity in F261S pedigree of Chinese

OBJECTIVE

To investigate how F261S mutation identified from Chinese obese patients affects the function of melanocortin 4 receptor (MC4R) and to analyze the obesity-related phenotypes in subjects carrying the F261S mutation.

METHODS

F261S mutant of MC4R was generated by site-directed mutagenesis. Plasmids encoding wild-type or F261S mutant of MC4R were transfected into HEK293 and COS-7 cells to examine their functional characteristics. Signaling properties of F261S MC4R were assessed by measuring intracellular cAMP levels in response to alpha-MSH stimulation. Cell surface expression of F261S MC4R was compared with that of wild-type MC4R. Clinical examinations were performed in subjects carrying F261S mutation and in non-mutated controls.

RESULTS

The alpha-MSH-stimulated reporter gene activity was significantly reduced in cells expressing F261S MC4R, with a maximal response equal to 57% of wild-type MC4R. The F261S mutation also led to a significant change in the Es50 value compared with the wild-type receptor (P<0.01). Immunofluorescent assay revealed a marked reduction in plasma membrane localization of the MC4R in cells expressing the F261S mutant receptor. The resting metabolic rate and fat composition of the mutant carriers were not significantly different from those of the non-mutated obese controls.

CONCLUSIONS

The decreased response to alpha-MSH due to the intracellular retention of MC4R may cause early-onset obesity in the F261S pedigree of Chinese.

Effects of fluoxetine administration on hypothalamic melanocortin system in obese Zucker rats

The aim of the present work was to study the potential involvement of melanocortin system in the anorectic mechanism of fluoxetine, a selective serotonin reuptake inhibitors, in obese Zucker rats. Male obese Zucker (fa/fa) rats were administered fluoxetine (10 mg/kg; i.p.) daily for two weeks. The control group was given 0.9% NaCl solution. RT-PCR for pro-opiomelanocortin (POMC), Agouti gene related peptide (AgRP) and melanocortin receptor 4 (MC4-R) in the hypothalamus, as well as regional immunostaining for alpha-melanocyte stimulating hormone (alpha-MSH) and MC4-R were carried out. Fluoxetine administration increased POMC expression and reduced MC4-R expression in the hypothalamus, without changes in AgRP mRNA levels. Moreover, an increase in the numbers of alpha-MSH positively immunostained neural cells in the hypothalamic arcuate nucleus (ARC), as well as a significant decrease in the numbers of neural cells positively immunostained for MC4-R in the paraventricular nucleus (PVN), without changes in lateral hypothalamic area (LHA), were observed. These results suggest the involvement of alpha-MSH in central fluoxetine anorectic action.

Adeno-associated virus-mediated knockdown of melanocortin-4 receptor in the paraventricular nucleus of the hypothalamus promotes high-fat diet-induced hyperphagia and obesity

Pharmacological and genetic studies have suggested that melanocortin-4 receptor (MC4R) signaling in the paraventricular nucleus of hypothalamus (PVN) regulates appetite and energy balance. However, the specific role of MC4R signaling in PVN neurons in these processes remains to be further elucidated in normally developed animals. In the present study, we employed RNA interference to determine whether MC4R knockdown in the PVN modulates food intake and body weight in adult rats. Adeno-associated viral (AAV) vectors encoding short hairpin RNAs targeting MC4R (AAV-shRNA-MC4R) were generated to induce MC4R knockdown in the PVN. By in situ hybridization, we detected a high-level expression of Dicer, a key enzyme required for shRNA-mediated gene silencing, along the entire rostrocaudal extent of the PVN. Bilateral injection of AAV-shRNA-MC4R vectors into the PVN of the adult rat resulted in significant and specific reduction of MC4R mRNA expression. Animals with MC4R knockdown exhibited an increase in food intake and excessive body weight gain when exposed to a high-fat diet. Our results provide evidence that AAV-mediated silencing of MC4R on PVN neurons promotes hyperphagia and obesity in response to the dietary challenge in the adult animal.

Mechanisms of leptin action and leptin resistance

The adipose tissue-derived hormone leptin acts via its receptor (LRb) in the brain to regulate energy balance and neuroendocrine function. LRb signaling via STAT3 and a number of other pathways is required for the totality of leptin action. The failure of elevated leptin levels to suppress feeding and mediate weight loss in common forms of obesity defines a state of so-called leptin resistance. A number of mechanisms, including the leptin-stimulated phosphorylation of Tyr(985) on LRb and the suppressor of cytokine signaling 3, attenuate leptin signaling and promote a cellular leptin resistance in obesity. Several unique features of the arcuate nucleus of the hypothalamus may contribute to the severity of cellular leptin resistance in this region. Other mechanisms that govern feeding behavior and food reward may also underlie the inception of obesity.

G protein-coupled receptors disrupted in human genetic disease

Genetic variation in G protein-coupled receptors (GPCRs) results in the disruption of GPCR function in a wide variety of human genetic diseases. In vitro strategies have been used to elucidate the molecular pathologies that underlie naturally occurring GPCR mutations. Various degrees of inactive, overactive, or constitutively active receptors have been identified. These mutations often alter ligand binding, G protein coupling, receptor desensitization, and receptor recycling. The role of inactivating and activating calcium-sensing receptor (CASR) mutations is discussed with respect to familial hypocalciuric hypercalemia (FHH) and autosomal dominant hypocalemia (ADH). Among ADH mutations, those associated with tonic-clonic seizures are discussed. Other receptors discussed include rhodopsin, thyrotropin, parathyroid hormone, melanocortin, follicle-stimulating hormone, luteinizing hormone, gonadotropin-releasing hormone (GnRHR), adrenocorticotropic hormone, vasopressin, endothelin-beta, purinergic, and the G protein associated with asthma (GPRA). Diseases caused by mutations that disrupt GPCR function are significant because they might be selectively targeted by drugs that rescue altered receptors. Examples of drug development based on targeting GPCRs mutated in disease include the calcimimetics used to compensate for some CASR mutations, obesity therapeutics targeting melanocortin receptors, interventions that alter GnRHR loss from the cell surface in idiopathic hypogonadotropic hypogonadism and novel drugs that might rescue the P2RY12 receptor in a rare bleeding disorder. The discovery of GPRA suggests that drug screens against variant GPCRs may identify novel drugs. This review of the variety of GPCRs that are disrupted in monogenic disease provides the basis for examining the significance of common pharmacogenetic variants.

Development of anti-obesity agents: drugs that target neuropeptide and neurotransmitter systems

BACKGROUND

Obesity results from an imbalance of energy intake and energy expenditure. While the prevalence of obesity is increasing and the negative impact on public health is being recognized, highly effective and safe therapy for obesity is not yet available.

OBJECTIVE

This review summarizes the current state of the art in the late-stage development area of anti-obesity drugs, provides a framework for understanding the potential disparity of animal data and human disease modification for some agents, and highlights novel agents that may provide significant clinical benefits in the future.

METHODS

Published literature and meeting proceedings were surveyed to review the late-stage development of new anti-obesity agents.

RESULTS/CONCLUSIONS

Most of the current clinical candidates for obesity treatment are targeting neurotransmitter receptors or peptide receptors. While new agents have been discovered, a significant failure rate also emerged. Rigorous scientific investigations, developing target engagement tools, and understanding any potential rodent versus human species difference will be necessary to ascertain mechanism-based efficacy and increase the probability of success.

Effects of selective modulation of the central melanocortin-3-receptor on food intake and hypothalamic POMC expression

Hypothalamic POMC neurons regulate energy balance via interactions with brain melanocortin receptors (MC-Rs). POMC neurons express the MC3-R which can function as an inhibitory autoreceptor in vitro. We now demonstrate that central activation of MC3-R with ICV infusion of the specific MC3-R agonist, [D-Trp(8)]-gamma-MSH, transiently suppresses hypothalamic Pomc expression and stimulates food intake in rats. Conversely, we also show that ICV infusion of a low dose of a selective MC3-R antagonist causes a transient decrease in feeding and weight gain. These data support a functional inhibitory role for the MC3-R on POMC neurons that leads to changes in food intake.

Urocortin and the brain

Urocortin is a member of the corticotropin-releasing hormone (CRH) family of peptides. In the brain, its potent suppression of food intake is mediated by CRH receptors (CRHR). Urocortin also participates in the regulation of anxiety, learning, memory, and body temperature, and it shows neuroprotection. This review will summarize the location of urocortin-producing neurons and their projections, the pharmacological evidence of its actions in the CNS, and information acquired from knockout mice. Urocortin interacts with leptin, neuropeptide Y, orexin, and corticotropin in the brain. Also produced by the GI tract, heart, and immune cells, urocortin has blood concentrations ranging from 13 to 152 pg/ml. Blood-borne urocortin stimulates the cerebral endothelial cells composing the blood-brain barrier and crosses the blood-brain barrier by a unique transport system. Overall, urocortin acts on a broad neuronal substrate as a neuromodulator important for basic survival.

Disruption of the RIIbeta subunit of PKA reverses the obesity syndrome of Agouti lethal yellow mice

Agouti lethal yellow (A(y)) mice express agouti ectopically because of a genetic rearrangement at the agouti locus. The agouti peptide is a potent antagonist of the melanocortin 4 receptor (MC4R) expressed in neurons, and this leads to hyperphagia, hypoactivity, and increased fat mass. The MC4R signals through Gs and is thought to stimulate the production of cAMP and activation of downstream cAMP effector molecules such as PKA. Disruption of the RIIbeta regulatory subunit gene of PKA results in release of the active catalytic subunit and an increase in basal PKA activity in cells where RIIbeta is highly expressed. Because RIIbeta is expressed in neurons including those in the hypothalamic nuclei where MC4R is prominent we tested the possibility that the RIIbeta knockout might rescue the body weight phenotypes of the A(y) mice. Disruption of the RIIbeta PKA regulatory subunit gene in mice leads to a 50% reduction in white adipose tissue and resistance to diet-induced obesity and hyperglycemia. The RIIbeta mutation rescued the elevated body weight, hyperphagia, and obesity of A(y) mice. Partial rescue of the A(y) phenotypes was even observed on an RIIbeta heterozygote background. These results suggest that the RIIbeta gene mutation alters adiposity and locomotor activity by modifying PKA signaling pathways downstream of the agouti antagonism of MC4R in the hypothalamus.

Counterintuitive effects of double-heterozygous null melanocortin-4 receptor and leptin genes on diet-induced obesity and insulin resistance in C57BL/6J mice

Circulating levels of leptin correlate with food intake and adiposity. A decline in serum leptin associated with calorie restriction instigates behavioral and metabolic adaptation, increasing appetite and conserving energy. Brain melanocortin-4 receptors (Mc4rs) are important mediators of leptin's effects on appetite and energy expenditure. Because subtle changes in function associated with heterozygous null mutations for either the Leptin (Lep-HET) or Mc4r genes (Mc4r-HET) increase adiposity, we tested the hypothesis that combined heterozygous mutations (Dbl-HET) would severely exacerbate diet-induced obesity (DIO) and insulin resistance in C57BL/6J mice. Serum leptin levels were lower as a function of adiposity in heterozygous Leptin mutants (Lep-HET, Dbl-HET) matched with mice homozygous for the wild-type (WT) Lep gene (Mc4r-HET). Evidence for an additive interaction on adiposity in Dbl-HET mice maintained on a low-fat diet was observed at 10 wk of age. Male but not female mice developed DIO and insulin resistance on a high-fat diet. Compared with WT mice, DIO was more severe in Mc4r-HET but not Lep-HET mice, regardless of sex. However, the response of male and female Dbl-HET mice was different, with males being less and females being more responsive relative to Mc4r-HET. Glucose tolerance of Dbl-HET mice was not significantly different from WT mice in either sex. These results show a complex interaction between the Leptin and Mc4r genes that is influenced by age, gender, and diet. Remarkably, while heterozygous Lep mutations initially exacerbate obesity, in situations of severe obesity, reduced leptin levels may act oppositely and have beneficial effects on energy homeostasis.

A derivative of the melanocortin receptor antagonist SHU9119 (PG932) increases food intake when administered peripherally

Melanocortin receptors are considered promising candidates for the treatment of behavioral and metabolic disorders ranging from obesity to anorexia and cachexia. These experiments examined the response of mice to peripheral injections of two compounds. PG932 is a derivative of SHU9119 which is non-selective antagonist of melanocortin-3 and melanocortin-4 receptors (Mc3r and Mc4r). PG946 is a derivative of a hybrid of alpha- and beta-MSH, and is a moderately selective Mc3r antagonist. SHU9119 increases food intake when administered intracerebroventricularly but is without effect when injected into the periphery. In contrast, PG932 was found to be highly effective at stimulating food intake when administered peripherally by intraperitoneal injection. The orexigenic effect of PG932 required functional Mc4r, suggesting that inhibition of this receptor is involved in the stimulation of food intake. PG946 did not significantly affect on feeding behavior. PG932 is thus a useful new compound for studies examining the regulation of appetite and energy balance, and may also prove useful for the treatment of cachectic conditions.

LRb signals act within a distributed network of leptin-responsive neurones to mediate leptin action

The adipose tissue-derived hormone, leptin, acts via its receptor (LRb) in the brain to regulate energy balance and neuroendocrine function. In order to understand leptin action we have explored the physiological function of LRb signalling pathways, defining important roles for signal transducer and activator of transcription-3 (STAT3) in positive signalling and for LRbTyr(985)-mediated feedback inhibition in leptin signal attenuation. As the cells on which leptin acts are not homogeneous, but rather represent a broadly distributed network of neurones with divergent projections and functions, it is also crucial to consider how each of these populations responds to LRb signals to contribute to leptin action. While well-known LRb-expressing neurones within the arcuate nucleus of the hypothalamus mediate crucial effects on satiety and energy expenditure, other populations of LRb-expressing neurones in the ventral tegmental area and elsewhere likely control the mesolimbic dopamine system. Additional populations of LRb-expressing neurones likely contribute to other aspects of neuroendocrine regulation. It will be important to define the molecular mechanisms by which leptin acts to regulate neurophysiology in each of these LRb-expressing neural populations in order to understand the totality of leptin action.

New agents in development for the management of obesity

Obesity is fast becoming the major cause of premature death in the developed world. The rising prevalence of obesity and obesity-related comorbidities also elevates healthcare costs, and reduced quality of life. The National Institute of Clinical Excellence in the UK recommends pharmacotherapy, in conjunction with lifestyle modification, for obese individuals [i.e. body mass index (BMI) of 30 kg/m(2)] and for overweight persons with a BMI greater than 27 kg/m(2), accompanied by at least one comorbidity. However, the current pharmaceutical treatment available to combat this epidemic remains limited. We review the efficacy and pharmacology of the anti-obesity agents currently used in clinical practice as well as some of the potential agents in phase II and III trials.

The melanocortinergic pathway is rapidly recruited by emotional stress and contributes to stress-induced anorexia and anxiety-like behavior

Neurons producing melanocortin receptor agonist, alpha-MSH derived from proopiomelanocortin, and antagonist, agouti-related protein, are known to be sensitive to metabolic stress such as food deprivation and glucoprivation. However, how these neurons respond to emotional/psychological stress remained to be elucidated. We report here that acute emotional stressors, i.e. restraint and forced swim, evoked mRNA expression of c-fos, a neuronal activation marker, in a high percentage of proopiomelanocortin neurons (up to 53% for restraint stress and 62% for forced swim), with marked variations along the rostro-caudal axis of the arcuate nucleus. In contrast, only a small population of agouti-related protein neurons in this brain region was activated. These neuronal activation patterns were correlated with behavioral reactions. Both stressors suppressed feeding and induced anxiety-like behavior in the elevated plus-maze test, as reflected by a reduction in the percentage of entries and time spent in the open arms. Central pretreatment with SHU9119, a melanocortin receptor antagonist, dose dependently attenuated the anorectic and anxiogenic effects elicited by acute restraint or forced swim. These results indicate that the melancortinergic pathway can be rapidly recruited by acute emotional stress, and that activation of melanocortin signaling is involved in mediating stress-induced anorexia and anxiety.

Dmbx1 is essential in agouti-related protein action

Dmbx1 is a paired-class homeodomain transcription factor. We show here that mice deficient in Dmbx1 exhibit severe leanness associated with hypophagia and hyperactivity and that isolation of a Dmbx1(-/-) mouse from its cohabitants induces self-starvation, sometimes leading to death, features similar to those of anorexia nervosa in humans. Interestingly, overexpression of agouti in Dmbx1(-/-) mice failed to induce aspects of the A(y)/a phenotype, including hyperphagia, obesity, and diabetes mellitus. In Dmbx1(-/-) mice, administration of agouti-related protein increased cumulative food intake for the initial 6 h but significantly decreased it over 24- and 48-h periods. In addition, Dmbx1 was shown to be expressed at embryonic day 15.5 in the lateral parabrachial nucleus, the rostral nucleus of the tractus solitarius, the dorsal motor nucleus of the vagus, and the reticular nucleus in the brainstem, all of which receive melanocortin signaling, indicating involvement of Dmbx1 in the development of the neural network for the signaling. Thus, Dmbx1 is essential for various actions of agouti-related protein and plays a role in normal regulation of energy homeostasis and behavior.

The role of proopiomelanocortin (POMC) neurones in feeding behaviour

The precursor protein, proopiomelanocortin (POMC), produces many biologically active peptides via a series of enzymatic steps in a tissue-specific manner, yielding the melanocyte-stimulating hormones (MSHs), corticotrophin (ACTH) and β-endorphin. The MSHs and ACTH bind to the extracellular G-protein coupled melanocortin receptors (MCRs) of which there are five subtypes. The MC3R and MC4R show widespread expression in the central nervous system (CNS), whilst there is low level expression of MC1R and MC5R. In the CNS, cell bodies for POMC are mainly located in the arcuate nucleus of the hypothalamus and the nucleus tractus solitarius of the brainstem. Both of these areas have well defined functions relating to appetite and food intake. Mouse knockouts (ko) for pomc, mc4r and mc3r all show an obese phenotype, as do humans expressing mutations of POMC and MC4R. Recently, human subjects with specific mutations in β-MSH have been found to be obese too, as have mice with engineered β-endorphin deficiency. The CNS POMC system has other functions, including regulation of sexual behaviour, lactation, the reproductive cycle and possibly central cardiovascular control. However, this review will focus on feeding behaviour and link it in with the neuroanatomy of the POMC neurones in the hypothalamus and brainstem.