Role of melanocortinergic neurons in feeding and the agouti obesity syndrome

Melanin-concentrating hormone: a functional melanocortin antagonist in the hypothalamus

Melanin-concentrating hormone (MCH) and alpha-melanocyte-stimulating hormone (alpha-MSH) demonstrate opposite actions on skin coloration in teleost fish. Both peptides are present in the mammalian brain, although their specific physiological roles remain largely unknown. In this study, we examined the interactions between MCH and alpha-MSH after intracerebroventricular administration in rats. MCH increased food intake in a dose-dependent manner and lowered plasma glucocorticoid levels through a mechanism involving ACTH. In contrast, alpha-MSH decreased food intake and increased glucocorticoid levels. MCH, at a twofold molar excess, antagonized both actions of alpha-MSH. alpha-MSH, at a threefold molar excess, blocked the orexigenic properties of MCH. MCH did not block alpha-MSH binding or the ability of alpha-MSH to induce cAMP in cells expressing either the MC3 or MC4 receptor, the principal brain alpha-MSH receptor subtypes. These data suggest that MCH and alpha-MSH exert opposing and antagonistic influences on feeding behavior and the stress response and may function in a coordinate manner to regulate metabolism through a novel mechanism mediated in part by an MCH receptor.

The genetics of obesity

Five genes have been identified, each capable of causing obesity in mice and each with a human homologue. One of them codes for a signal expressed by adipose tissue, and another for the signal's brain receptor. The rest reveal brain pathways probably downstream from the receptor. Together, the genes offer glimpses of an intricate system that defends adipose stores--and in some persons maintains an unhealthful set-point.

Leptin

Leptin (from the Greek leptos=thin) was identified only 3 years ago. It has attracted huge attention both scientifically, with more than 600 publications, and in the media, where this protein has been portrayed as the way to a cure for obesity. Indeed, leptin was first described as an adipocyte-derived signalling factor, which, after interaction with its receptors, induced a complex response including control of bodyweight and energy expenditure. Leptin seems in addition to its role in metabolic control to have important roles in reproduction and neuroendocrine signalling. Human obesity is a complex disorder, with many factors playing a part; the pathophysiology of leptin is not as simple as it seems to be in rodent models of obesity.

Orexins and orexin receptors: a family of hypothalamic neuropeptides and G protein-coupled receptors that regulate feeding behavior

The hypothalamus plays a central role in the integrated control of feeding and energy homeostasis. We have identified two novel neuropeptides, both derived from the same precursor by proteolytic processing, that bind and activate two closely related (previously) orphan G protein-coupled receptors. These peptides, termed orexin-A and -B, have no significant structural similarities to known families of regulatory peptides. prepro-orexin mRNA and immunoreactive orexin-A are localized in neurons within and around the lateral and posterior hypothalamus in the adult rat brain. When administered centrally to rats, these peptides stimulate food consumption. prepro-orexin mRNA level is up-regulated upon fasting, suggesting a physiological role for the peptides as mediators in the central feedback mechanism that regulates feeding behavior.

Different developmental patterns of melanocortin MC3 and MC4 receptor mRNA: predominance of Mc4 in fetal rat nervous system

Melanocortins are thought to be involved in neuronal development and regeneration. Pro-opiomelanocortin (POMC), the precursor of alpha-melanocyte stimulating hormone (alpha-MSH), gamma-MSH, ACTH, and beta-endorphin, becomes detectable in rat hypothalamic neurons from gestational day (E) 12.5. We recently described stage- and region-specific ontogenetic patterns of binding sites for the alpha-MSH analogue [125I]-Nle4,D-Phe7-alpha-MSH ([125I]-NDP), with the first localizations in epithalamus and sympathetic chain at E13. [125I]-NDP binds to all known melanocortin receptors, including MC3-R and MC4-R, the predominant melanocortin receptors in nervous system. To identify the receptor type expressed during ontogeny, the developmental pattern of MC3-R and MC4-R mRNA was investigated by in situ hybridization in fetuses and offspring of time-pregnant Long Evans rats between E14 and postnatal day (P) 27. MC4-R mRNA was found to be the predominant species during the entire fetal period. It was localized in all fetal areas exhibiting distinct [125I]-NDP binding, starting with sympathetic ganglia and epithalamus (E14), and including sensory trigeminal nuclei (E16), dorsal motor nucleus of vagus (E16) and cranial nerve ganglia, inferior olive (E18) and cerebellum (E18), striatal regions (E16), and entorhinal cortex (E22). In contrast, MC3-R mRNA was detectable only in the postnatal period, with a fast increase in expression in the ventromedial and arcuate nuclei. The early presence of MC4-R mRNA in central and peripheral nervous system and transient regional peaks of mRNA expression, often concomitant with periods of neural network formation, suggest a role of this receptor type in early ontogeny. The MC3 receptor may be involved in analogous processes during postnatal development.

Leptin, leptin receptors, and the control of body weight

The assimilation, storage, and disposition of nutrient energy constitute a complex homeostatic system central to the survival of both prokaryotic and eukaryotic organisms. In vertebrates, and especially among land dwelling mammalian species, the ability to store large quantities of energy-dense fuel in the form of adipose tissue triglyceride permits survival during prolonged periods of food deprivation. In order to maintain such fuel stores during times of dietary scarcity or surfeit, some balance between energy intake and expenditure must be achieved. Lesions of the hypothalamus alter body weight suggesting that this brain region regulates nutritional state. These and other studies led to the hypothesis that body weight was regulated by a feedback loop in which peripheral signals reported nutritional information to an integratory center in the brain. However, the identity of these nutrition signals proved elusive.

Interaction of Agouti protein with the melanocortin 1 receptor in vitro and in vivo

Agouti protein and Agouti-related protein (Agrp) are paracrine-signaling molecules that normally regulate pigmentation and body weight, respectively. These proteins antagonize the effects of alpha-melanocyte-stimulating hormone (alpha-MSH) and other melanocortins, and several alternatives have been proposed to explain their biochemical mechanisms of action. We have used a sensitive bioassay based on Xenopus melanophores to characterize pharmacologic properties of recombinant Agouti protein, and have directly measured its cell-surface binding to mammalian cells by use of an epitope-tagged form (HA-Agouti) that retains biologic activity. In melanophores, Agouti protein has no effect in the absence of alpha-MSH, but its action cannot be explained solely by inhibition of alpha-MSH binding. In 293T cells, expression of the Mc1r confers a specific, high-affinity binding site for HA-Agouti. Binding is inhibited by alpha-MSH, or by Agrp, which indicates that alpha-MSH and Agouti protein bind in a mutually exclusive way to the Mc1r, and that the similarity between Agouti protein and Agrp includes their binding sites. The effects of Agouti and the Mc1r in vivo have been examined in a sensitized background provided by the chinchilla (Tyrc-ch) mutation, which uncovers a phenotypic difference between overexpression of Agouti in lethal yellow (Ay/a) mice and loss of Mc1r function in recessive yellow (Mc1re/Mc1re) mice. Double and triple mutant studies indicate that a functional Mc1r is required for the pigmentary effects of Agouti, and suggest that Agouti protein can act as an agonist of the Mc1r in a way that differs from alpha-MSH stimulation. These results resolve questions regarding the biochemical mechanism of Agouti protein action, and provide evidence of a novel signaling mechanism whereby alpha-MSH and Agouti protein or Agrp function as independent ligands that inhibit each other's binding and transduce opposite signals through a single receptor.

Central infusion of melanocortin agonist MTII in rats: assessment of c-Fos expression and taste aversion

Like leptin (OB protein), central infusion of the nonspecific melanocortin agonist MTII reduces food intake for relatively long periods of time (i.e., 12 h; W. Fan, B. A. Boston, R. A. Kesterson, V. J. Hruby, and R. D. Cone, Nature; 385: 165-168, 1997). To test the hypothesis that MTII may influence ingestive behavior via mechanisms similar to those that mediate the effects of leptin, we infused a single dose of MTII into the third ventricle (i3vt) of Long-Evans rats and examined three dependent measures that have been studied following i3vt infusion of leptin: 1) effects on long-term food intake and body weight (48 h), 2) patterns of c-Fos expression in the brain, and 3) conditioned taste aversion learning. Similar to leptin, MTII reduced 48-h food intake (1.0 nmol dose), reduced body weight at 24 and 48 h (0.1 and 1.0 nmol doses, respectively), and induced c-Fos expression in the paraventricular nucleus of the hypothalamus and the central nucleus of the amygdala. In contrast to leptin, MTII was found to produce conditioned taste aversions. These results are consistent with the hypothesis that MTII may influence regulatory behavior via mechanisms similar to those that mediate the effects of leptin.

Cardiovascular consequences of obesity: role of leptin

1. Several mechanisms have been implicated in the association between obesity and hypertension, including salt-sensitivity, insulin resistance and sympathetic activation. Obese animals and humans exhibit exaggerated blood pressure responses to increases in salt intake. 2. Although insulin resistance is common in obesity, it is clear that abnormal insulin action is not the sole or sufficient cause of hypertension in obesity. Obesity is associated with increased activity of the sympathetic nervous system. Sympathetic blockade has been reported to attenuate sodium retention and hypertension in experimental models of obesity. 3. The mediators responsible for salt sensitivity, insulin resistance and sympathetic activation in obesity remain unclear. 4. The novel protein hormone leptin is produced almost exclusively by adipose tissue and acts in the central nervous system through a specific receptor and multiple neuropeptide pathways to decrease appetite and increase energy expenditure. 5. Increasing evidence suggests that leptin may have wider actions influencing autonomic, cardiovascular, renal and endocrine function. We have shown that leptin increases sympathetic nerve activity to kidney, hindlimb and adrenal gland, in addition to brown adipose tissue. 6. Despite this sympathoexcitatory action, acute systemic administration of leptin does not acutely increase arterial pressure or heart rate in anaesthetized animals. This may reflect opposing antihypertensive actions of leptin. For example, leptin increases renal sodium and water excretion, apparently through a direct tubular action. In addition, leptin increases systemic insulin sensitivity, even in the absence of weight loss. 7. In conclusion, leptin may act as a mediator linking body adiposity with changes in insulin action, sympathetic neural outflow and renal sodium excretion. Alterations in leptin generation or action may, in part, underlie the sympathetic, endocrine and renal consequences of obesity.

Exocrine gland dysfunction in MC5-R-deficient mice: evidence for coordinated regulation of exocrine gland function by melanocortin peptides

The effects of pituitary-derived melanocortin peptides are primarily attributed to ACTH-mediated adrenocortical glucocorticoid production. Identification of a widely distributed receptor for ACTH/MSH peptides, the melanocortin-5 receptor (MC5-R), suggested non-steroidally mediated systemic effects of these peptides. Targeted disruption of the MC5-R produced mice with a severe defect in water repulsion and thermoregulation due to decreased production of sebaceous lipids. High levels of MC5-R was found in multiple exocrine tissues, including Harderian, preputial, lacrimal, and sebaceous glands, and was also shown to be required for production and stress-regulated synthesis of porphyrins by the Harderian gland and ACTH/MSH-regulated protein secretion by the lacrimal gland. These data show a requirement for the MC5-R in multiple exocrine glands for the production of numerous products, indicative of a coordinated system for regulation of exocrine gland function by melanocortin peptides.

Characterization of the binding of MSH-B, HB-228, GHRP-6 and 153N-6 to the human melanocortin receptor subtypes

We determined the binding affinities of the MSH analogues MSH-B, HP-228 and 153N-6 and of the enkephalin analogue GHRP-6 on a single eukaryotic cell line transiently expressing the human MC1, MC3, MC4 and MC5 receptors. Moreover, we tested the binding and cAMP response of MSH-B in comparison with alpha-MSH on murine B16 melanoma cells. Our results indicate that MSH-B has a potency similar to that of alpha-MSH and that these two peptides induce similar cAMP responses in murine B16 melanoma cells. HP-228 has its highest affinity for the MC1 receptor. For the other receptors, it has slightly higher affinity for the MC5 receptor than for the MC3 and MC4 receptors. 153N-6 was found to be selective for the MC1 receptor. GHRP-6 was found to bind to the MC1 and the MC5 receptors despite its low structural homology with alpha-MSH. [D-Lys3]GHRP-6 bound to all the four MC receptors with similar affinities. The structurally related Met-enkephalin and the functionally related GHRH, as well as LHRH and somatostatin-14 did not bind to these MC receptors. The low affinity of the GH-releasing/enkephalin peptides may indicate that they do not interact with the MC receptors at pharmacologically relevant concentrations.

Phage display selection on whole cells yields a peptide specific for melanocortin receptor 1

A phage display system for the selection of peptides binding to heterologously expressed human melanocortin receptor 1 on the surface of insect cells has been established. It could be shown that phage particles displaying the natural ligand alpha-melanocyte-stimulating hormone bind selectively to cells expressing this receptor and that these phages exhibit biological activity on mouse B16F1 melanoma cells. Insect cells were superior to other cell lines tested and have been used to select binders from a small library, in which critical determinants (Phe7-Arg8-Trp9) were kept, whereas the flanking regions where allowed to variate freely. One peptide displaying little similarity with native hormone was found that binds to the receptor also in its free form with an affinity of 7 nM. It showed a remarkable selectivity for this receptor, because it binds to the other melanocortin receptor subtypes with a maximum affinity of 21 microM. This is the first time phage display has been used successfully with G-protein-coupled receptors lacking an extracellular binding domain.

Antagonism of central melanocortin receptors in vitro and in vivo by agouti-related protein

Expression of Agouti protein is normally limited to the skin where it affects pigmentation, but ubiquitous expression causes obesity. An expressed sequence tag was identified that encodes Agouti-related protein, whose RNA is normally expressed in the hypothalamus and whose levels were increased eightfold in ob/ob mice. Recombinant Agouti-related protein was a potent, selective antagonist of Mc3r and Mc4r, melanocortin receptor subtypes implicated in weight regulation. Ubiquitous expression of human AGRP complementary DNA in transgenic mice caused obesity without altering pigmentation. Thus, Agouti-related protein is a neuropeptide implicated in the normal control of body weight downstream of leptin signaling.

Molecular basis for the interaction of [Nle4,D-Phe7]melanocyte stimulating hormone with the human melanocortin-1 receptor

The melanocortin-1 receptor (MC1R) is a seven-transmembrane (TM) G-protein-coupled receptor whose natural ligands are the melanocortin peptides, adrenocorticotropic hormone, and alpha-, beta-, and gamma- melanocyte stimulating hormone (MSH). To test a previously constructed three-dimensional model of the molecular interaction between the long-acting, superpotent alpha-MSH analog [Nle4,D-Phe7]MSH (NDP-MSH) and the human MC1R we examined the effects of site-directed receptor mutagenesis on the binding affinity and potency of NDP-MSH. In addition, we also examined the effects of these same mutations on the binding affinity and potency of the structurally related agonists alpha-MSH, gamma-MSH, and Ac-Nle4-cyclic-[Asp5,His6,D-Phe7,Arg8,Trp9,Lys10]NH2 (MT-II). Mutagenesis of acidic receptor residues Glu94 in TM2 and Asp117 or Asp121 in TM3 significantly altered the binding affinity and potency of all four agonists suggesting that these receptor residues are important to the ligand-receptor interactions of all. A disproportionate change in agonist potency versus affinity observed with simultaneous mutation of these acidic residues (mutant constructs D117A/D121A or E94A/D117A/D121A) or introduction of a single positive charge (mutant construct D121K) also implicates these residues in receptor activation. In addition, results from the individual mutation of aromatic receptor residues Phe175, Phe196, and Phe257, and simultaneous mutation of multiple TM4, -5, and -6 tyrosine and phenylalanine residues suggests that aromatic-aromatic ligand-receptor interactions also participate in binding these melanocortins to the MC1R. These experiments appear to have identified some of the critical receptor residues involved in the ligand-receptor interactions between these melanocortins and the hMC1R.

Sympathetic and cardiorenal actions of leptin

Body weight is tightly regulated physiologically. The recent discovery of the peptide hormone leptin has permitted more detailed evaluation of the mechanisms responsible for control of body fat. Leptin is almost exclusively produced by adipose tissue and acts in the CNS through a specific receptor and multiple neuropeptide pathways to decrease appetite and increase energy expenditure. Leptin thus functions as the afferent component of a negative feedback mechanism to control adipose tissue mass. Increasing evidence suggests that leptin may have wider actions influencing autonomic, cardiovascular, and endocrine function. Intravenous leptin increases norepinephrine turnover and sympathetic nerve activity to thermogenic brown adipose tissue. Studies from our laboratory suggest that leptin also increases sympathetic nerve activity to kidney, hindlimb, and adrenal gland. However, systemic administration of leptin does not acutely increase arterial pressure or heart rate in anesthetized animals. Thus, longer-term exposure to hyperleptinemia may be necessary for full expression of the expected pressor effect of renal sympathoexcitation. Alternatively, leptin may have additional cardiovascular actions to oppose sympathetically mediated vasoconstriction. Leptin in high doses increases renal sodium and water excretion, apparently through a direct tubular action. In addition, leptin appears to increase systemic insulin sensitivity, even in the absence of weight loss. Although we are at an early stage of understanding, we speculate that abnormalities in the actions of leptin may have implications for the sympathetic, cardiovascular, and renal changes associated with obesity.

The role of the agouti gene in the yellow obese syndrome

The yellow obese syndrome in mice encompasses many pleiotropic effects including yellow fur, maturity-onset obesity, hyperinsulinemia, insulin resistance, hyperglycemia, increased skeletal length and lean body mass, and increased susceptibility to neoplasia. The molecular basis of this syndrome is beginning to be unraveled and may have implications for human obesity and diabetes. Normally, the agouti gene is expressed during the hair-growth cycle in the neonatal skin where it functions as a paracrine regulator of pigmentation. The secreted agouti protein antagonizes the binding of the alpha-melanocyte-stimulating hormone to its receptor (melanocortin 1 receptor) on the surface of hair bulb melanocytes, causing alterations in intracellular cAMP levels. Widespread, ectopic expression of the mouse agouti gene is central to the yellow obese phenotype, as demonstrated by the molecular cloning of several dominant agouti mutations and the ubiquitous expression of the wild-type agouti gene in transgenic mice. Recent experiments have revealed that the hypothalamus and adipose tissue are biologically active target sites for agouti in the yellow obese mutant lines.

Physiological response to long-term peripheral and central leptin infusion in lean and obese mice

Recent data have identified leptin as an afferent signal in a negative-feedback loop regulating the mass of the adipose tissue. High leptin levels are observed in obese humans and rodents, suggesting that, in some cases, obesity is the result of leptin insensitivity. This hypothesis was tested by comparing the response to peripherally and centrally administered leptin among lean and three obese strains of mice: diet-induced obese AKR/J, New Zealand Obese (NZO), and Ay. Subcutaneous leptin infusion to lean mice resulted in a dose-dependent loss of body weight at physiologic plasma levels. Chronic infusions of leptin intracerebroventricularly (i.c.v.) at doses of 3 ng/hr or greater resulted in complete depletion of visible adipose tissue, which was maintained throughout 30 days of continuous i.c.v. infusion. Direct measurement of energy balance indicated that leptin treatment did not increase total energy expenditure but prevented the decrease that follows reduced food intake. Diet-induced obese mice lost weight in response to peripheral leptin but were less sensitive than lean mice. NZO mice were unresponsive to peripheral leptin but were responsive to i.c.v. leptin. Ay mice did not respond to subcutaneous leptin and were 1/100 as sensitive to i.c.v. leptin. The decreased response to leptin in diet-induced obese, NZO, and Ay mice suggests that obesity in these strains is the result of leptin resistance. In NZO mice, leptin resistance may be the result of decreased transport of leptin into the cerebrospinal fluid, whereas in Ay mice, leptin resistance probably results from defects downstream of the leptin receptor in the hypothalamus.

Obesity and impaired prohormone processing associated with mutations in the human prohormone convertase 1 gene

Human obesity has an inherited component, but in contrast to rodent obesity, precise genetic defects have yet to be defined. A mutation of carboxypeptidase E (CPE), an enzyme active in the processing and sorting of prohormones, causes obesity in the fat/fat mouse. We have previously described a women with extreme childhood obesity (Fig. 1), abnormal glucose homeostasis, hypogonadotrophic hypogonadism, hypocortisolism and elevated plasma proinsulin and pro-opiomelanocortin (POMC) concentrations but a very low insulin level, suggestive of a defective prohormone processing by the endopeptidase, prohormone convertase 1 (PC1; ref. 4). We now report this proband to be a compound heterozygote for mutations in PC1. Gly-->Arg483 prevents processing of proPC1 and leads to its retention in the endoplasmic reticulum (ER). A-->C+4 of the intro-5 donor splice site causes skipping of exon 5 leading to loss of 26 residues, a frameshift and creation of a premature stop codon within the catalytic domain. PC1 acts proximally to CPE in the pathway of post-translational processing of prohormones and neuropeptides. In view of the similarity between the proband and the fat/fat mouse phenotype, we infer that molecular defects in prohormone conversion may represent a generic mechanism for obesity, common to humans and rodents.

Anatomic localization of alternatively spliced leptin receptors (Ob-R) in mouse brain and other tissues

Leptin's effects are mediated by interactions with a receptor that is alternatively spliced, resulting in at least five different murine forms: Ob-Ra, Ob-Rb, Ob-Rc, Ob-Rd, and Ob-Re. A mutation in one splice form, Ob-Rb, results in obesity in mice. Northern blots, RNase protection assays, and PCR indicate that Ob-Rb is expressed at a relatively high level in hypothalamus and low level in several other tissues. Ob-Ra is expressed ubiquitously, whereas Ob-Rc, -Rd, and -Re RNAs are only detectable using PCR. In hypothalamus, Ob-Rb is present in the arcuate, ventromedial, dorsomedial, and lateral hypothalamic nuclei but is not detectable in other brain regions. These nuclei are known to regulate food intake and body weight. The level of Ob-Rb in hypothalamus is reduced in mice rendered obese by gold thioglucose (GTG), which causes hypothalamic lesions. The obesity in GTG-treated mice is likely to be caused by ablation of Ob-Rb-expressing neurons, which results in leptin resistance.

Agouti signaling protein inhibits melanogenesis and the response of human melanocytes to alpha-melanotropin

In mouse follicular melanocytes, the switch between eumelanin and pheomelanin synthesis is regulated by the extension locus, which encodes the melanocortin-1 receptor (MC1R) and the agouti locus, which encodes a novel paracrine-signaling molecule that inhibits binding of melanocortins to the MC1R. Human melanocytes express the MC1R and respond to melanotropins with increased proliferation and eumelanogenesis, but a potential role for the human homolog of agouti-signaling protein, ASIP, in human pigmentation has not been investigated. Here we report that ASIP blocked the binding of alpha-melanocyte-stimulating hormone (alpha-MSH) to the MC1R and inhibited the effects of alpha-MSH on human melanocytes. Treatment of human melanocytes with 1 nM-10 nM recombinant mouse or human ASIP blocked the stimulatory effects of alpha-MSH on cAMP accumulation, tyrosinase activity, and cell proliferation. In the absence of exogenous alpha-MSH, ASIP inhibited basal levels of tyrosinase activity and cell proliferation and reduced the level of immunoreactive tyrosinase-related protein-1 (TRP-1) without significantly altering the level of immunoreactive tyrosinase. In addition, ASIP blocked the stimulatory effects of forskolin or dibutyryl cAMP, agents that act downstream from the MC1R, on tyrosinase activity and cell proliferation. These results demonstrate that the functional relationship between the agouti and MC1R gene products is similar in mice and humans and suggest a potential physiologic role for ASIP in regulation of human pigmentation.

The genetics of obesity

Despite the influence of obesity in predisposing to many diseases, and evidence for high heritability, efforts to identify human genes with major effects on bodyweight have not yet been successful. In contrast, remarkable progress has been made in the identification and characterization of the genes mutated in five monogenic mouse models of obesity. These genes have led to new insights into the etiology of obesity and provide promising targets for therapeutic intervention.

Hypothalamic expression of ART, a novel gene related to agouti, is up-regulated in obese and diabetic mutant mice

We have isolated cDNA clones that encode a novel human gene related to agouti. Sequence analysis of this gene, named ART, for agouti-related transcript, predicts a 132-amino-acid protein that is 25% identical to human agouti. The highest degree of identity is within the carboxyl terminus of both proteins. Like agouti, ART contains a putative signal sequence and a cysteine rich carboxyl terminus, but lacks the region of basic residues and polyproline residues found in the middle of the agouti protein. Both agouti and ART contain 11 cysteines, and 9 of these are conserved spatially. ART is expressed primarily in the adrenal gland, subthalamic nucleus, and hypothalamus, with a lower level of expression occurring in testis, lung, and kidney. The murine homolog of ART was also isolated and is predicted to encode a 131-amino-acid protein that shares 81% amino acid identity to humans. The mouse was found to have the same expression pattern as human when assessed by RT-PCR. Examination by in situ hybridization using mouse tissues showed localized expression in the arcuate nucleus of the hypothalamus, the median eminence, and the adrenal medulla. In addition, the hypothalamic expression of ART was elevated approximately 10-fold in ob/ob and db/db mice. ART was mapped to human chromosome 16q22 and to mouse chromosome 8D1-D2. The expression pattern and transcriptional regulation of ART, coupled with the known actions of agouti, suggests a role for ART in the regulation of melanocortin receptors within the hypothalamus and adrenal gland, and implicates this novel gene in the central control of feeding.

Neuropeptides responding to leptin

Leptin, the circulating protein that inhibits food intake and energy expenditure, was thought to function through inhibition of the hypothalamic neuropeptide Y (NPY), a stimulator of food intake. However, mouse mutants lacking NPY are normal, suggesting that alternative neuromodulators of food intake must exist. Recently, melanocortin, a neuropeptide acting on the hypothalamic receptor melanocortin4-R, was discovered in mice, controlling energy regulation. This receptor is antagonized by the "agouti" protein in the mutant obese agouti mouse.