Characterization of melanin-concentrating hormone in chum salmon pituitaries

Increased susceptibility to LTP generation and changes in NMDA-NR1 and -NR2B subunits mRNA expression in rat hippocampus after MCH administration

The present study attempts to determine which mechanisms underlie the retrograde facilitation of memory induced by microinjection hippocampal melanin-concentrating hormone (MCH) on the inhibitory avoidance paradigm. Previous reports using this test on the hippocampus suggest that NMDA receptor-mediated mechanisms are involved in memory processing and are also necessary for the induction of long-term potentiation (LTP) of the hippocampal dentate gyrus. In addition, alterations in expression of synaptic NMDA subunits in the hippocampus have been associated with memory formation of an inhibitory avoidance task. We have studied the effects of the neuropeptide upon the electrophysiological parameters using hippocampal slices from rats injected with the peptide and tested in step-down tests as well as possible changes in the mRNA expression of NMDA receptor subunits. We postulate that the increased facility to induce LTP, and the overexpression of this N-methyl-D-aspartate mRNA receptor subunits induced by MCH, could be behind the retrograde facilitation observed after MCH hippocampal microinjection.

Chronic MCH-1 receptor modulation alters appetite, body weight and adiposity in rats

Central administration of the neuropeptide melanin-concentrating hormone (MCH) stimulates feeding in rodents. We studied the effects of intracerebroventricular (i.c.v.) administration of an MCH-1 receptor agonist (Compound A) and an MCH-1 receptor antagonist (Compound B) on feeding in satiated rats. Compound B (10 microg, i.c.v.) blocked the acute orexigenic effect of Compound A (5 microg, i.c.v.). In an experiment designed to either stimulate or inhibit MCH-1 receptor signaling over an extended period, rats received continuous i.c.v. infusions of vehicle (saline), Compound A (30 microg/day), Compound B (30 or 48 microg/day) or neuropeptide Y (24 microg/day, as positive control) via implantable infusion pumps. Continuous MCH-1 receptor activation recapitulated the obese phenotype of MCH-over-expressor mice, manifest as enhanced feeding (+23%, P<0.001), caloric efficiency and body weight gain (+38%, P<0.005) over the 14-day period relative to controls. Chronic MCH-1 receptor activation also elevated plasma insulin and leptin levels significantly. Conversely, continuous MCH-1 receptor antagonism led to sustained reductions in food intake (-16%, P<0.001), body weight gain (-35%, P<0.01), and body fat gain relative to controls, without an effect on lean mass. Antagonism of the MCH-1 receptor may be an effective approach for the treatment of obesity.

Melanin-concentrating hormone functions in the nervous system: food intake and stress

Melanin-concentrating hormone (MCH) is a cyclic neuropeptide, which centrally regulates food intake and stress. MCH induces food intake in rodents and, more generally, acts as an anabolic signal in energy regulation. In addition, MCH seems to be activatory on the stress axis. Two receptors for MCH in humans have very recently been characterised, namely, MCH-R1 and MCH-R2. MCH-R1 has received considerable attention, as potent and selective antagonists acting at that receptor display anxiolytic, antidepressant and/or anorectic properties. Feeding and affective disorders are both debilitating conditions that have become serious worldwide health threats. There are as yet no efficient and/or safe cures that could contain the near-pandemia phenomen of both diseases. Thus, the discovery of MCH-R1 antagonists may lead to the development of valuable drugs to treat obesity, anxiety and depressive syndromes. In addition, it opens wide avenues to probe additional functions of the peptide, both in the brain and in the peripheral nervous system.

Melanin-concentrating hormone receptor 1 activates extracellular signal-regulated kinase and synergizes with G(s)-coupled pathways

Melanin-concentrating hormone (MCH) is a hypothalamic neuropeptide that plays a key role in energy homeostasis. Like many neuropeptides, it signals through two G protein-coupled receptors. MCH receptor 1 (MCHR1) is the sole receptor expressed in rodents and couples to G(i) and G(q) proteins. Little is known about the intracellular pathways engaged by MCH and its receptor. Using HEK293 cells stably expressing MCHR1, we demonstrate that MCH, acting through MCHR1, antagonizes the action of forskolin, an adenylate cyclase activator that increases intracellular levels of cAMP. MCH also inhibits cAMP induction by the G(s)-coupled beta-adrenergic receptor. Activation of either the G(i)- or G(s)-dependent pathway typically results in ERK phosphorylation in HEK293 cells. In contrast to opposing actions on cAMP synthesis, simultaneous MCH and forskolin treatment results in synergistic activation of ERK. This synergy proceeds through pertussis toxin-independent pathways and requires several enzymatic activities such as protein kinase A, protein kinase C, phospholipase C, and Src kinase. Finally, we provide evidence that such positive interactions are not limited to cell lines but can also be observed in the brain.

Lateral hypothalamic neuropeptides in reward and drug addiction

The hypothalamus has been long considered important in feeding and other motivated behaviors. The identification of neuropeptides expressed in the hypothalamus has initiated efforts to better elucidate the underlying molecular mechanisms involved. The neuropeptides orexin and melanin-concentrating hormone (MCH) are expressed in the lateral hypothalamus (LH) and have been implicated in regulation of feeding behavior. Neurons expressing these neuropeptides have extensive projections to regions of the brain important for behavioral responses to drugs of abuse, raising the possibility that the pathways may also be important in addiction. Regulation of LH intracellular signaling pathways in response to drugs of abuse supports a role for the LH neuropeptides in addiction.

Evidence of melanin-concentrating hormone-containing neurons supplying both cortical and neuroendocrine projections

In the rat, melanin-concentrating hormone-containing projections are detected in the median eminence and in the neural lobe of the pituitary. After vascular injections of the retrograde tracers fluorogold or fastblue, melanin-concentrating hormone neurons are retrogradely labeled in the rostromedial zona incerta and adjacent perifornical region. These neurons may be the source of the melanin-concentrating hormone projections toward the median eminence and posterior pituitary, and may release their secretory products into the bloodstream. After fastblue injections in the cerebral cortex and vascular fluorogold injections, some melaninconcentrating hormone neurons contain both tracers, indicating that they send collaterals in the cerebral cortex and in the median eminence/posterior pituitary. No such collaterals have been described for the classical neuroendocrine systems. The melanin-concentrating hormone system is thought to play a role in arousal in correlation with specific goal oriented behaviors such as feeding or reproduction. Some MCH neurons may be involved in such functions by modulating directly cortical activity as well as being neuroendocrine.

Characterization of MCH-mediated obesity in mice

Melanin-concentrating hormone (MCH) is a cyclic orexigenic peptide expressed in the lateral hypothalamus. Recently, we demonstrated that chronic intracerebroventricular infusion of MCH induced obesity accompanied by sustained hyperphagia in mice. Here, we analyzed the mechanism of MCH-induced obesity by comparing animals fed ad libitum with pair-fed and control animals. Chronic infusion of MCH significantly increased food intake, body weight, white adipose tissue (WAT) mass, and liver mass in ad libitum-fed mice on a moderately high-fat diet. In addition, a significant increase in lipogenic activity was observed in the WAT of the ad libitum-fed group. Although body weight gain was marginal in the pair-fed group, MCH infusion clearly enhanced the lipogenic activity in liver and WAT. Plasma leptin levels were also increased in the pair-fed group. Furthermore, MCH infusion significantly reduced rectal temperatures in the pair-fed group. In support of these findings, mRNA expression of uncoupling protein-1, acyl-CoA oxidase, and carnitine palmitoyltransferase I, which are key molecules involved in thermogenesis and fatty acid oxidation, were reduced in the brown adipose tissue (BAT) of the pair-fed group, suggesting that MCH infusion might reduce BAT functions. We conclude that the activation of MCH neuronal pathways stimulated adiposity, in part resulting from increased lipogenesis in liver and WAT and reduced energy expenditure in BAT. These findings confirm that modulation of energy homeostasis by MCH may play a critical role in the development of obesity.

Distribution of melanin-concentrating hormone neurons projecting to the medial mammillary nucleus

The melanin-concentrating hormone and neuropeptide glutamic acid-isoleucine are expressed in neurons located mainly in the hypothalamus that project widely throughout the CNS. One of the melanin-concentrating hormone main targets is the medial mammillary nucleus, but the exact origin of these fibers is unknown. We observed melanin-concentrating hormone and neuropeptide glutamic acid-isoleucine immunoreactive fibers coursing throughout the mammillary complex, showing higher density in the pars lateralis of the medial mammillary nucleus, while the lateral mammillary nucleus showed sparse melanin-concentrating hormone innervation. The origins of these afferents were determined by using implant of the retrograde tracer True Blue in the medial mammillary nucleus. Double-labeled neurons were observed in the lateral hypothalamic area, rostromedial zona incerta and dorsal tuberomammillary nucleus. A considerable population of retrogradely labeled melanin-concentrating hormone perikaryal profiles was also immunoreactive to neuropeptide glutamic acid-isoleucine (74+/-15% to 85+/-15%). The afferents from the lateral hypothalamic area, rostromedial zona incerta and dorsal tuberomammillary nucleus to the medial mammillary nucleus were confirmed using implant of the anterograde tracer Phaseolus vulgaris leucoagglutinin. In addition, using double-labeled immunohistochemistry, we found no co-localization between neurons expressing melanin-concentrating hormone and adenosine deaminase (histaminergic marker) in the dorsal tuberomammillary nucleus. We hypothesize that these melanin-concentrating hormone projections participate in spatial memory process mediated by the medial mammillary nucleus. These pathways would enable the animal to look for food during the initial moments of appetite stimulation.

Chronic intracerebroventricular infusion of MCH causes obesity in mice. Melanin-concentrating hormone

Melanin-concentrating hormone (MCH) is a cyclic amino acid neuropeptide localized in the lateral hypothalamus. Although MCH is thought to be an important regulator of feeding behavior, the involvement of this peptide in body weight control has been unclear. To examine the role of MCH in the development of obesity, we assessed the effect of chronic intracerebroventricular infusion of MCH in C57BL/6J mice that were fed with regular or moderately high-fat (MHF) diets. Intracerebroventricular infusion of MCH (10 microg/day for 14 days) caused a slight but significant increase in body weight in mice maintained on the regular diet. In the MHF diet-fed mice, MCH more clearly increased the body weight accompanied by a sustained hyperphagia and significant increase in fat and liver weights. Plasma glucose, insulin, and leptin levels were also increased in the MCH-treated mice fed the MHF diet. These results suggest that chronic stimulation of the brain MCH system causes obesity in mice and imply that MCH may have a major role in energy homeostasis.

The structure and evolution of the melanocortin and MCH receptors in fish and mammals

Zebrafish are an excellent genetic model system for studying developmental and physiological processes. Pigment patterns in zebrafish are affected by mutations in three types of chromatophores. The behavior of these cells is influenced by alpha-melanocyte-stimulating hormone (alphaMSH) and melanin-concentrating hormone (MCH). Mammals have five alphaMSH receptors (melanocortin receptors) and one or two MCH receptors. We have identified the full complement of melanocortin and MCH receptors in both zebrafish and the pufferfish, Fugu. Zebrafish have six melanocortin receptors, including two MC5R orthologues, while Fugu, lacking MC3R, has only four. We also demonstrate that Fugu and zebrafish have two and three MCHR genes, respectively. MC2R and MC5R are physically linked in all species examined. Unlike other species, we find the Fugu genes contain introns, one of which is in a conserved location and is probably ancestral. We also detail the differential expression of the zebrafish genes throughout development.

Different structural requirements for melanin-concentrating hormone (MCH) interacting with rat MCH-R1 (SLC-1) and mouse B16 cell MCH-R

Melanin-concentrating hormone (MCH) is a neuropeptide occurring in all vertebrates and some invertebrates and is now known to stimulate pigment aggregation in teleost melanophores and food-intake in mammals. Whereas the two MCH receptor subtypes hitherto cloned, MCH-R1 and MCH-R2, are thought to mediate mainly the central effects of MCH, the MCH-R on pigment cells has not yet been identified, although in some studies MCH-R1 was reported to be expressed by human melanocytes and melanoma cells. Here we present data of a structure-activity study in which 12 MCH peptides were tested on rat MCH-R1 and mouse B16 melanoma cell MCH-R, by comparing receptor binding affinities and biological activities. For receptor binding analysis with HEK-293 cells expressing rat MCH-R1 (SLC-1), the radioligand was [125I]-[Tyr13]-MCH with the natural sequence. For B16 cells (F1 and G4F sublines) expressing B16 MCH-R, the analog [125I]-[D-Phe13, Tyr19]-MCH served as radioligand. The bioassay used for MCH-R1 was intracellular Ca2+ mobilization quantified with the FLIPR instrument, whereas for B16 MCH-R the signal determined was MAP kinase activation. Our data show that some of the peptides displayed a similar relative increase or decrease of potency in both cell types tested. For example, linear MCH with Ser residues at positions 7 and 16 was almost inactive whereas a slight increase in side-chain hydrophilicity at residues 4 and 8, or truncation of MCH at the N-terminus by two residues hardly changed binding affinity or bioactivity. On the other hand, salmonic MCH which also lacks the first two residues of the mammalian sequence but in addition has different residues at positions 4, 5, 9, and 18 exhibited a 5- to 10-fold lower binding activity than MCH in both cell systems. A striking difference in ligand recognition between MCH-R1 and B16 MCH-R was however observed with modifications at position 13 of MCH: whereas L-Phe13 in [Phe13, Tyr19]-MCH was well tolerated by both MCH-R1 and B16 MCH-R, change of configuration to D-Phe13 in [D-Phe13, Tyr19]-MCH or [D-Phe13]-MCH led to a complete loss of biological activity and to a 5- to 10-fold lower binding activity with MCH-R1. By contrast, the D-Phe13 residue increased the affinity of [D-Phe13, Tyr19]-MCH to B16 MCH-R about 10-fold and elicited MAP kinase activation as observed with [Phe13, Tyr19]-MCH or MCH. These data demonstrate that ligand recognition by B16 MCH-R differs from that of MCH-R1 in several respects, indicating that the B16 MCH-R represents an MCH-R subtype different from MCH-R1.

Conformational features of human melanin-concentrating hormone: an NMR and computational analysis

The conformational features of human melanin-concentrating hormone (hMCH) [Asp1-Phe2-Asp3-Met4-Leu5-Arg6-cyclo(S[bond]S)(Cys7-Met8-Leu9-Gly10-Arg11-Val12-Tyr13-Arg14-Pro15-Cys16)-Trp17-Gln18-Val19], in water and in a CD(3)CN/H(2)O (1:1 v/v) mixture at 298 K, have been determined by NMR spectroscopy followed by simulated annealing and molecular dynamics analyses to identify conformer populations. Backbone clustering analysis of NMR-spectroscopy-derived structures in the 7-16 peptide region led to the identification of a single representative structure in each solvent. Both root mean square deviation clustering and secondary structure analysis of the final conformers in both solvents show substantial convergence of most conformers into a single fold in the 4-17 region, with a limited variability around Gly10 and Tyr13 on going from CD(3)CN/H(2)O to pure water. The main feature deduced from the analysis of secondary structures is the occurrence of an N-terminal alpha helix of variable length, which spans an overall residue range of 2-9. A comparative analysis in the two solvents highlights that these structures are substantially different from that reported in the literature for the cyclic MCH(5-14) subunit of salmon MCH, which was used to perform a molecular characterization of the MCH/receptor complex. Our conformational data call for a critical revision of the proposed MCH/receptor complex model.

Functional role of N-linked glycosylation on the rat melanin-concentrating hormone receptor 1

Melanin-concentrating hormone (MCH) is known to act through two G-protein-coupled receptors MCHR1 and MCHR2. MCHR1 has three potential sites (Asn13, Asn16 and Asn23) for N-linked glycosylation in its extracellular amino-terminus which may modulate its reactivity. Site-directed mutagenesis of the rat MCHR1 cDNA at single or multiple combinations of the three potential glycosylation sites was used to examine the role of the putative carbohydrate chains on receptor activity. It was found that all three potential N-linked glycosylation sites in MCHR1 were glycosylated, and that N-linked glycosylation of Asn23 was necessary for full activity. Furthermore, disruption of all three glycosylation sites impaired proper expression at the cell surface and receptor activity. These data outline the importance of the N-linked glycosylation of the MCHR1.

Endogenous receptor for melanin-concentrating hormone in human neuroblastoma Kelly cells

Melanin-concentrating hormone (MCH), a cyclic nonadecapeptide, is predominantly expressed in mammalian neurons located in the zona incerta and lateral hypothalamus. Current interest in MCH relates to its role in the control of feeding behaviour. Two receptors for MCH were recently found: MCH-R(1) and MCH-R(2). We show here by RT-PCR analysis and immunofluorescence studies that the human neuroblastoma cell line Kelly expresses MCH and MCH-R(1) but not MCH-R(2). In competition assays using 125I-labelled MCH an inhibitory concentration 50% (IC(50)) of 76nM was determined for MCH, indicating a high affinity of Kelly cells for MCH. MCH induces mitogen-activated protein kinase (MAPK) phosphorylation in Kelly cells but no increase in the intracellular free Ca(2+) concentration. This suggests that MCH signals via Galpha(i)/Galpha(0) in these cells. The presence and functionality of MCH-R(1) renders this neuronal cell a very useful model for future structure-activity studies in a physiological environment mimicking the human brain for the evaluation of potential appetite-regulating drugs.

Human immune cells express ppMCH mRNA and functional MCHR1 receptor

Melanin-concentrating hormone (MCH) is highly expressed in the brain and modulates feeding behavior. It is also expressed in some peripheral tissues where its role remains unknown. We have investigated MCH function in human and mouse immune cells. RT-PCR analysis revealed a low expression of prepro-MCH and MCH receptor 1 (MCHR1) but not of MCHR2 transcript in tissular and peripheral blood immune cells. FACS and in vitro assay studies demonstrated that MCHR1 receptor expression on most cell types can trigger, in the presence of MCH, cAMP synthesis and calcium mobilization in peripheral blood mononuclear cells (PBMCs). Moreover, MCH treatment decreases the CD3-stimulated PBMC proliferation in vitro. Accordingly, our data indicate for the first time that MCH and MCHR1 may exert immunomodulatory functions.

Melanin-concentrating hormone neuron system: the Wide Web that controls the feeding

Recent evidence indicates that the hypothalamic control system of food intake involves many feeding-related neuropeptides. Melanin-concentrating hormone (MCH), one of a group of potent orexigenic peptides, is exclusively produced in neurons of the lateral hypothalamic area that give off fibers to the widespread brain regions. The receptor of MCH was recently identified to be localized in cell bodies and dendritic processes of particular neurons throughout the brain, in close spatial relation to MCH fibers. Leptin, an anorectic hormone secreted from the adipose tissue, acts on the specific receptor present on its target neurons in the brain, and suppresses the expression of both MCH and its receptor. Leptin receptor and STAT3, a transcription factor mediating the leptin signaling, are distributed in the widespread brain regions including the cerebral neocortex, hippocampal formation and lower brainstem as well as the hypothalamus where MCH fibers and the MCH receptor are abundantly present. These findings suggest that MCH exerts the effect through its specific receptor distributed throughout the brain and that the function of MCH is influenced by the condition of peripheral energy balance via leptin, the leptin receptor and STAT3, not only in the hypothalamus, but also in other brain regions. In the brain, MCH might be involved in various feeding-related functions, such as appetite, food-searching behavior, eating muscle movement, and control of energy balance, depending upon the physiological role of each region.

Melanin-concentrating hormone and its receptor are expressed and functional in human skin

In this study, we have demonstrated the presence of melanin-concentrating hormone (MCH) and melanin-concentrating hormone receptor (MCHR1) transcripts in human skin. Sequence analysis confirmed that the transcripts of both genes were identical to those previously found in human brain. In culture, endothelial cells showed pro-MCH expression whereas no signal was found in keratinocytes, melanocytes, and fibroblasts. MCHR1 expression was restricted to melanocytes and melanoma cells. Stimulation of cultured human melanocytes with MCH reduced the alpha-MSH-induced increase in cAMP production. Furthermore, the melanogenic actions of alpha-MSH were inhibited by MCH. We propose that the MCH/MCHR1 signalling system is present in human skin and may have a role with the melanocortins in regulating the melanocyte.

Appetite-boosting property of pro-melanin-concentrating hormone(131-165) (neuropeptide-glutamic acid-isoleucine) is associated with proteolytic resistance

Melanin-concentrating hormone (MCH) is a cyclic neuropeptide, with a major role in stimulation of feeding behavior in mammals. MCH signals in the brain occur via two seven-transmembrane G protein-coupled receptors, namely MCH1 (SLC-1, MCH(1), MCH-R1, or MCH-1R) and MCH2 (SLT, MCH(2), MCH-R2, or MCH-2R). In this study, we demonstrate that the pro-MCH(131-165) peptide neuropeptide-glutamic acid-isoleucine (NEI)-MCH is more potent than MCH in stimulating feeding in the rat. Using rat MCH1-expressed human embryonic kidney 293 cells, we show that NEI-MCH exhibits 5-fold less affinity in a binding assay and 2-fold less potency in a cAMP assay than MCH. A similar 7- to 8-fold shift in potency was observed in a Ca(2+)(i) assay using rat MCH1 or human MCH2-transfected Chinese hamster ovary cell models. This demonstrates that NEI-MCH is not a better agonist than MCH at either of the MCH receptors. Then, we compared the proteolysis resistance of MCH and NEI-MCH to rat brain membrane homogenates and purified proteases. Kinetics of peptide degradation using brain extracts indicated a t(1/2) of 34.8 min for MCH and 78.5 min for NEI-MCH with a specific pattern of cleavage of MCH but not NEI-MCH by exo- and endo-proteases. Furthermore, MCH was found highly susceptible to degradation by aminopeptidase M and endopeptidase 24.11, whereas NEI-MCH was fully resistant to proteolysis by these enzymes. Therefore, our results strongly suggest that reduced susceptibility to proteases of NEI-MCH compared with MCH account for its enhanced activity in feeding behavior. NEI-MCH represents therefore the first MCH natural functional "superagonist" so far described.

SVK14 cells express an MCH binding site different from the MCH1 or MCH2 receptor

Melanin-concentrating hormone (MCH) is a cyclic peptide, mainly involved in the regulation of skin pigmentation in teleosts and feeding behavior in mammals. The human keratinocyte SVK14 cell line has been previously shown to express binding sites for the MCH analog [125I]-[Phe13,3-iodo-Tyr19]MCH. We report here that: (1) this binding site similarly recognized [125I]-[3-iodo-Tyr13]MCH; (2) its pharmacological profile clearly differed from those observed at the two human MCH receptor subtypes, MCH1-R and MCH2-R; (3) MCH did not induce any effect on second messenger systems (including cAMP, calcium, and MAP kinase signaling pathways), and (4) no mRNAs corresponding to the MCH receptors were found. In conclusion, the binding site characterized in the SVK14 cell line is distinct from the MCH1 and MCH2 receptors and deserves therefore further investigation.

Melanin-concentrating hormone receptor subtypes 1 and 2: species-specific gene expression

To assess the contribution of potential central nervous system pathways implicated in the control of appetite regulation and energy metabolism, it is essential to first identify appropriate animal models. Melanin-concentrating hormone (MCH), a conserved cyclic neuropeptide implicated in the modulation of food intake, has been shown to bind and activate two G-protein-coupled receptors, called GPR24 and MCHR2, expressed in human brain and other tissues. Here we show that several non-human species (rat, mouse, hamster, guinea pig, and rabbit) do not have functional MCHR2 receptors, or encode a nonfunctional MCHR2 pseudogene while retaining GPR24 expression. We identified three species for further evaluation that express both MCH receptor subtypes. We cloned and functionally characterized dog, ferret, and rhesus GPR24 and MCHR2 in mammalian cells and studied their brain distribution patterns by in situ hybridization. The homology, expression profile, and functional similarity of the receptors in the dog, ferret, and rhesus to that of human support the potential use of these species as preclinical animal models in the development of therapeutic agents for obesity or other MCH-mediated disorders.

Melanin-concentrating hormone and its receptors: state of the art

Melanin-concentrating hormone (MCH) is a cyclic neuropeptide of nineteen amino acids in mammals. Its involvement in the feeding behaviour has been well established during the last few years. A first receptor subtype, now termed MCHIR, was discovered in 1999, following the desorphanisation of the SLCI orphan receptor, using either reverse pharmacology or systematic screening of agonist candidates. A second MCH receptor, MCH2R, has been discovered recently, by several groups working on data mining of genomic banks. The molecular pharmacology of these two receptors is only described on the basis of the action of peptides derived from MCH. The present review tentatively summarizes the knowledge on these two receptors and presents the first attempts to discover new classes of antagonists that might have major roles in the control of obesity and feeding behaviour.

Novel G-protein-coupled receptor genes expressed in the brain: continued discovery of important therapeutic targets

The rhodopsin family of G-protein-coupled receptors (GPCRs) is the largest known group of cell-surface mediators of signal transduction. The vast majority of these receptors were discovered by methods based upon shared sequence homologies found throughout this family. While such efforts identified a multitude of receptor subtypes for previously known ligands, numerous receptors have been discovered for which endogenous ligands were unknown. These receptors are commonly referred to as orphan receptors. One of the most important tasks of modern pharmacology lies in elucidating the functions of these receptors. Of particular interest are receptors with recognised expression in the central nervous system, given that many psychiatric and neurodegenerative disorders are mediated by unknown mechanisms. Hence, this collection of putative neurotransmitter and neuromodulator signal mediators represents a substantial and untapped resource for novel drug discovery. Recently, various methodologies have accelerated the discovery of novel ligands for these orphan receptors, identifying the basic components required for further physiological ligand/receptor system characterisation. Equipped with proven ligand identification strategies, the characterisation of all orphan GPCRs and the exploitation of their exciting potential as targets for the discovery of novel drugs is anticipated.

The melanin-concentrating hormone receptor 1, a novel target of autoantibody responses in vitiligo

Vitiligo is a common depigmenting disorder resulting from the loss of melanocytes in the skin. The pathogenesis of the disease remains obscure, although autoimmune mechanisms are thought to be involved. Indeed, autoantibodies and autoreactive T lymphocytes that target melanocytes have been reported in some vitiligo patients. The objective of this study was to identify pigment cell antigens that are recognized by autoantibodies in vitiligo. Using IgG from vitiligo patients to screen a melanocyte cDNA phage-display library, we identified the melanin-concentrating hormone receptor 1 (MCHR1) as a novel autoantigen related to this disorder. Immunoreactivity against the receptor was demonstrated in vitiligo patient sera by using radiobinding assays. Among sera from healthy controls and from patients with autoimmune disease, none exhibited immunoreactivity to MCHR1, indicating a high disease specificity for Ab's against the receptor. Inhibition of MCH binding to its receptor by IgG from vitiligo patients was also shown.

Characterization of orexins (hypocretins) and melanin-concentrating hormone in genetically obese mice

Orexins (hypocretins) and the melanin-concentrating hormone (MCH) are neuropeptides localized to the lateral hypothalamic area and are potential regulators of energy homeostasis. Using highly sensitive radioimmunoassay for orexins and MCH, we determined their contents in the lateral hypothalamus (LH) of genetically obese ob/ob and db/db mice and their controls, C57BL/6J and C57BL/KSJ. The orexin contents in the lateral hypothalamus significantly increased in the ob/ob mice, whereas the orexin contents significantly decreased in the db/db mice. Mature orexin-A and -B peptides were the major endogenous orexin molecules in the lateral hypothalamus. Conversely, the MCH contents in the lateral hypothalamus of both obese mice increased compared to the control mice. MCH contents in the lateral hypothalamus were two- to five-fold higher than that of orexin contents. These results suggest that the regulatory mechanism of orexin and MCH may be different in the genetically obese mice.

T-226296: a novel, orally active and selective melanin-concentrating hormone receptor antagonist

Through the screening of our in-house chemical compound library, we found a novel melanin-concentrating hormone (MCH) receptor antagonist, T-226296, a (-) enantiomer of N-[6-(dimethylamino)-methyl]-5,6,7,8-tetrahydro-2-naphthalenyl]-4'-fluoro[1,1'-biphenyl]-4-carboxamide. T-226296 exhibited high affinity for cloned human and rat MCH receptors (SLC-1) in receptor binding assays (IC50=5.5+/-0.12 nM for human SLC-1; 8.6+/-0.32 nM for rat SLC-1). T-226296 had high selectivity over other receptors, including the second subtype of the MCH receptor, SLT (MCH2), transporters and ion channels. In Chinese hamster ovary (CHO) cells expressing human SLC-1, T-226296 reversed the MCH-mediated inhibition of forskolin-stimulated cAMP accumulation, inhibited MCH-induced intracellular Ca2+ increase, and also inhibited MCH-stimulated arachidonic acid release. In rats, oral administration of T-226296 (30 mg/kg) almost completely suppressed the food intake induced by intracerebroventricular injection of MCH. These results clearly indicate that T-226296 is a novel, orally active and selective MCH receptor antagonist that will be promising for further exploring the physiology and pathophysiology of MCH-SLC-1 signaling.

Distribution of melanin-concentrating hormone-like immunoreactivity in the central nervous system of Rana esculenta

The distribution of salmon and rat melanin-concentrating hormone (MCH)-like and neuropeptide glutamate-isoleucine (NEI)-like immunoreactivity in the brain and spinal cord of the frog Rana esculenta was studied with immunohistochemistry. In the telencephalon, only fibers showed immunoreactivity in the olfactory bulb, lateral pallium, diagonal band, septum, and the amygdala. Immunoreactive fibers were abundant in all diencephalic structures, except the optic tract, the visual neuropils, and the habenula. Several cells in the central thalamic nucleus and a few in the suprachiasmatic nucleus were stained with the MCH antisera. Cells and their processes were intensely stained in the dorsal hypothalamus with the MCH and NEI antisera. Immunoreactive fibers were found in all tegmental nuclei and the white matter of the mesencephalon. They formed terminal plexuses in the deep layers of the optic tectum and the laminar nucleus of the torus semicircularis. Immunoreactive fibers were sparse in the rhombencephalon and the spinal cord.

Melanin concentrating hormone increase hippocampal synaptic transmission in the rat

A retrograde facilitation has been demonstrated in the one trial step-down inhibitory avoidance of melanin-concentrating hormone (MCH), when it was infused into rat hippocampal formation. Considering the high density of specific binding sites for the MCH peptide on the hippocampus and the participation of this structure on learning and memory processes we have studied the effects of MCH on the hippocampal synaptic transmission. For this purpose, slices of rat hippocampus were perfused with different concentration of MCH. The main result of the present study was a long-lasting potentiation on the hippocampal evoked response on dentate gyrus induced by MCH (4-11 microM) at 30, 60 and 120 min with a maximum effect at 120 min. Previous perfusion of DL - 2- amino - 5 phosphonovaleric acid (APV, 20 microM) was unable to impair the increased hippocampal evoked response induced by MCH 4 microM. On the other hand, the channel blocker Dizocilpine (MK-801, 10 microM) completely impaired the increased hippocampal synaptic plasticity induced by MCH perfusion. We postulate the increased hippocampal synaptic efficacy induced by MCH as one of the mechanisms underlying the retrograde facilitation on the inhibitory avoidance paradigm, observed after MCH hippocampal microinjection. We cannot rule out other MCH neurochemical mechanism and other areas of the brain involved in the MCH effects.

Endogenous melanin-concentrating hormone receptor SLC-1 in human melanoma SK-MEL-37 cells

Melanin-concentrating hormone (MCH) is a hypothalamic neuropeptide that regulates several physiological functions. The orphan G protein-coupled receptors SLC-1 and MCHR2 were recently found to bind MCH with high affinity. We show here that the human melanoma cell line SK-MEL-37 expresses SLC-1 mRNA but not MCHR2 by RT-PCR analysis and immunofluorescence studies. Using Chinese hamster ovary cells and 293 cells overexpressing SLC-1 by cDNA transfection, it was shown that SLC-1 coupled to both G alpha(i)/G alpha(o) and G alpha(q) proteins. In SK-MEL-37 cells, MCH inhibited forskolin-stimulated cyclic AMP accumulation and induced mitogen-activated protein kinase (MAPK) in a pertussis toxin-(PTX)-sensitive manner. The MAPK activity leads to the production of phosphorylated forms of p42/p44 MAPK. However, an increase in the intracellular free Ca(2+) concentration was not elicited by MCH in SK-MEL-37 cells. These results show that SLC-1 is coupled only to PTX-sensitive G alpha(i)/G alpha(o) in SK-MEL-37 cells. This study provides for the first time a skin-derived cellular model to analyze the molecular mechanism of the MCH signaling pathway.

Hypothalamic and vagal neuropeptide circuitries regulating food intake

It has been recognized for some time that a number of different neuropeptides exert powerful effects on food intake. During the last few years, the neurocircuitry within which these peptides operate has also begun to be elucidated. Peptidergic feeding-regulatory neurones are found both in the hypothalamus and the brainstem, where they act as input stations for hormonal and gastrointestinal information, respectively. These cell populations both project to several other brain regions and interconnect extensively. The present review summarizes the neuroanatomy and connectivity of some prominent peptides involved in food intake control, including neuropeptide Y, melanocortin peptides, agouti gene-related protein, cocaine- and amphetamine-regulated transcript, orexin/hypocretin, melanin-concentrating hormone and cholecystokinin. Disturbances in the hypothalamic neuropeptide systems have been implicated in the phenotype of a genetic model of fatal hypophagia, the mouse anorexia (anx) mutation, which is also discussed.

Identification and pharmacological characterization of a novel human melanin-concentrating hormone receptor, mch-r2

Melanin-concentrating hormone (MCH) is a neuropeptide highly expressed in the brain that regulates several physiological functions mediated by receptors in the G protein-coupled receptor family. Recently an orphan receptor, SLC-1, has been identified as an MCH receptor (MCH-R1). Herein we identify and characterize a novel receptor for human MCH (MCH-R2). The receptor is composed of 340 amino acids encoded by a 1023-base pair cDNA and is 35% homologous to SLC-1. (125)I-MCH specifically bound to Chinese hamster ovary cells stably expressing MCH-R2. MCH stimulated dose-dependent increases in intracellular free Ca(2+) and inositol phosphate production in these cells but did not affect cAMP production. The pharmacological profile for mammalian MCH, [Phe(13),Tyr(19)]MCH, and salmon MCH at MCH-R2 differed compared with MCH-R1 as assessed by intracellular signaling and radioligand binding assays. The EC(50) in signaling assays and the IC(50) in radioligand binding assays of salmon MCH was an order of magnitude higher than mammalian MCH at MCH-R2. By comparison, the EC(50) and IC(50) values of salmon MCH and mammalian MCH at MCH-R1 were relatively similar. Blot hybridization revealed exclusive expression of MCH-R2 mRNA in several distinct brain regions, particularly in the cortical area, suggesting the involvement of MCH-R2 in the central regulation of MCH-mediated functions.

[125I]-S36057: a new and highly potent radioligand for the melanin-concentrating hormone receptor

Shortened, more stable and weakly hydrophobic analogues of melanin-concentrating hormone (MCH) were searched as candidates for radioiodination. Starting from the dodecapeptide MCH(6 - 17), we found that: (1) substitution of Tyr(13) by a Phe residue; (2) addition of a 3-iodo-Tyr residue at the N-terminus; and (3) addition of a hydrophilic spacer 8-amino-3,6-dioxyoctanoyl between the 3-iodo-Tyr and MCH(6 - 17) (compound S36057), led to an agonist more potent than MCH itself in stimulating [35S]-GTPgammaS binding at membranes from HEK293 cells stably expressing the human MCH receptor. Specific binding of [125I]-S36057 was found in HEK293 and CHO cell lines stably expressing the human MCH receptor. This radioligand recognized a similar number of binding sites (ca. 800 fmol mg(-1)) than [125I]-[3-iodo Tyr(13)]-MCH. However, the K(D) for [125I]-S36057 obtained from saturation studies (0.037 nM) or from binding kinetics (0.046 nM) was at least 10 fold higher to that of [125I]-[3-iodo Tyr(13)]-MCH (0.46 nM). Affinities determined for a series of MCH analogues were similar with both radioligands, S36057 being the most potent compound tested (K(i)=0.053 nM). Finally, [125I]-S36057 also potently labelled the MCH receptor in membranes from whole rat brain (K(D) 0.044 nM, B(max)=11 fmol mg(-1)). In conclusion, [125I]-S36057 is a more potent and more stable radioligand than [125I]-[3-iodo Tyr(13)]-MCH that will represent a reliable tool for binding assays in the search of novel MCH ligands. It should also provide great help for autoradiographic studies of the MCH receptor distribution in the central nervous system.

Central and peripheral dysregulation of melanin-concentrating hormone in obese Zucker rats

Melanin concentrating hormone (MCH) is a peptide synthesized in the lateral hypothalamus which stimulates food ingestion and leptin secretion in rodents. In this experiment, we measured the expressions of MCH as well as of its receptor (SLC-1) in the hypothalamus of obese hyperphagic and lean Zucker rats by quantitative real time RT-PCR. MCH mRNA expression in the obese rats was significantly increased by a factor of five (P<0.01) whereas expression of SLC-1 was decreased by more than 50% (P<0.05). Circulating levels of leptin and MCH were increased in the plasma of obese Zucker rats when compared to lean rats (38-fold and 1.7-fold, respectively, P<0.001 and P<0.01). However, individual MCH levels were not directly correlated to leptin levels in the lean (functional leptin receptor) or in the obese (non-functional leptin receptor) Zucker rats. These results indicate that the absence of leptin signaling in rats is associated with an increased hypothalamic expression and circulating release of MCH, contributing to their obesity syndrome.

Melanin concentrating hormone depresses synaptic activity of glutamate and GABA neurons from rat lateral hypothalamus

The neuropeptide melanin concentrating hormone (MCH) is synthesised only by neurons of the lateral hypothalamic (LH) area in the CNS. MCH cells project widely throughout the brain. Despite the growing interest in this peptide, in part related to its role in feeding, little has been done to characterise its physiological effects in neurons. Using whole-cell recording with current and voltage clamp, we examined the cellular actions in neurons from the LH. MCH induced a consistent decrease in the frequency of action potentials and reduced synaptic activity. Most fast synaptic activity in the hypothalamus is mediated by GABA or glutamate. MCH inhibited the synaptic activity of both glutamatergic and GABAergic LH neurons, each tested independently. MCH reduced the amplitude of glutamate-evoked currents and reduced the amplitude of miniature excitatory currents, indicating an inhibitory modulation of postsynaptic glutamate receptors. In the presence of tetrodotoxin to block action potentials, MCH caused a depression in the frequency of miniature glutamate-mediated postsynaptic currents, suggesting a presynaptic site of receptor expression. In voltage clamp experiments, MCH depressed the amplitude of calcium currents, suggesting that a mechanism of inhibition may involve a reduced calcium-dependent release of amino acid transmitter. Previous reports have suggested that MCH activated potassium channels in non-neuronal cells transfected with the MCH receptor gene. We found no effect of MCH on voltage-dependent potassium channels in LH neurons. Baclofen, a GABAB receptor agonist, activated G-protein gated inwardly rectifying potassium (GIRK)-type channels; in the same neurons, MCH had no effect on GIRK channels. MCH showed no modulation of sodium currents. Blockade of the Gi/Go protein with pertussis toxin eliminated the actions of MCH. The inhibitory actions of MCH on both excitatory and inhibitory synaptic events, coupled with opposing excitatory actions of hypocretin, another LH peptide that projects to many of the same loci, suggest a substantial level of complexity in neuropeptide modulation of LH actions.

Expression of the melanin-concentrating hormone (MCH) receptor mRNA in the rat brain

The melanin-concentrating hormone (MCH) system is thought to be an important regulator of food intake. Recently the orphan G protein-coupled receptor SLC-1 was identified as the MCH receptor (MCHR). Preliminary analyses of MCHR mRNA distribution have supported a role for the MCH system in nutritional homeostasis. We report here a complete anatomical distribution of the MCHR mRNA. We have found high levels of expression of MCHR mRNA in most anatomical areas implicated in control of olfaction, with the exception of the main olfactory bulb. Dense labeling was also detected in the hippocampal formation, subiculum, and basolateral amygdala, all of which are important in learning and memory, and in the shell of the nucleus accumbens, a substrate for motivated behavior and feeding. Within the hypothalamus, MCHR mRNA was moderately expressed in the ventromedial nucleus, arcuate nucleus, and zona incerta, all of which serve key roles in the neuronal circuitry of feeding. In the brainstem, strong expression was observed in the locus coeruleus, which is implicated in arousal, as well as in nuclei that contribute to orofacial function and mastication, including the facial, hypoglossal, motor trigeminal, and dorsal motor vagus nuclei. In most regions there was a good correspondence between MCHR mRNA distribution and that of MCH-immunoreactive fibers. Taken together, these data suggest that MCH may act at various levels of the brain to integrate various aspects of feeding behavior. However, the extensive MCHR distribution throughout the brain suggests that this receptor may play a role in other functions, most notably reinforcement, arousal, sensorimotor integration, and autonomic control.

Cloning of a novel G protein-coupled receptor, SLT, a subtype of the melanin-concentrating hormone receptor

A DNA fragment encoding an amino acid sequence possessing common features to the G protein-coupled receptor (GPCR) superfamily was found in the human genomic sequence, and from this information, the full-length cDNA of a novel GPCR, designated SLT, was cloned from the human hippocampus cDNA library. SLT showed the highest homology to the melanin-concentrating hormone (MCH) receptor, SLC-1 (31.5% identity), and to a lesser extent, to the somatostatin (SST) receptor subtypes. MCH exhibited agonistic behavior when applied to the SLT-expressing CHO cells at subnanomolar doses whereas more than 200 known peptides, including SST and cortistatin, did not. These results indicated that MCH is the cognate ligand of the SLT receptor and that this newly cloned GPCR is the second subtype of the MCH receptor. Quantitative polymerase chain reaction analysis of the SLT gene expression in human tissues showed that the SLT receptor is expressed mainly in brain areas including the cerebral cortex, amygdala, hippocampus, and corpus callosum, as well as in a limited number of peripheral tissues. The distribution of the SLT nearly overlapped that of SLC-1, suggesting that some of the neural functions of MCH may be mediated by both of these receptor subtypes.

Identification and characterization of a melanin-concentrating hormone receptor

Melanin-concentrating hormone (MCH), a neuropeptide expressed in central and peripheral nervous systems, plays an important role in the control of feeding behaviors and energy metabolism. An orphan G protein-coupled receptor (SLC-1/GPR24) has recently been identified as a receptor for MCH (MCHR1). We report here the identification and characterization of a G protein-coupled receptor as the MCH receptor subtype 2 (MCHR2). MCHR2 has higher protein sequence homology to MCHR1 than any other G protein-coupled receptor. The expression of MCHR2 has been detected in many regions of the brain. In contrast to MCHR1, which is intronless in the coding region and is located at the chromosomal locus 22q13.3, the MCHR2 gene has multiple exons and is mapped to locus 6q21. MCHR2 is specifically activated by nanomolar concentrations of MCH, binds to MCH with high affinity, and signals through Gq protein. This discovery is important for a full understanding of MCH biology and the development of potential therapeutics for diseases involving MCH, including obesity.

Identification and characterization of a second melanin-concentrating hormone receptor, MCH-2R

Melanin-concentrating hormone (MCH) is a 19-aa cyclic neuropeptide originally isolated from chum salmon pituitaries. Besides its effects on the aggregation of melanophores in fish several lines of evidence suggest that in mammals MCH functions as a regulator of energy homeostasis. Recently, several groups reported the identification of an orphan G protein-coupled receptor as a receptor for MCH (MCH-1R). We hereby report the identification of a second human MCH receptor termed MCH-2R, which shares about 38% amino acid identity with MCH-1R. MCH-2R displayed high-affinity MCH binding, resulting in inositol phosphate turnover and release of intracellular calcium in mammalian cells. In contrast to MCH-1R, MCH-2R signaling is not sensitive to pertussis toxin and MCH-2R cannot reduce forskolin-stimulated cAMP production, suggesting an exclusive G(alpha)q coupling of the MCH-2R in cell-based systems. Northern blot and in situ hybridization analysis of human and monkey tissue shows that expression of MCH-2R mRNA is restricted to several regions of the brain, including the arcuate nucleus and the ventral medial hypothalamus, areas implicated in regulation of body weight. In addition, the human MCH-2R gene was mapped to the long arm of chromosome 6 at band 6q16.2-16.3, a region reported to be associated with cytogenetic abnormalities of obese patients. The characterization of a second mammalian G protein-coupled receptor for MCH potentially indicates that the control of energy homeostasis in mammals by the MCH neuropeptide system may be more complex than initially anticipated.

Molecular cloning and functional characterization of MCH2, a novel human MCH receptor

Melanin-concentrating hormone (MCH) is involved in the regulation of feeding and energy homeostasis. Recently, a 353-amino acid splice variant form of the human orphan receptor SLC-1 () (hereafter referred to as MCH(1)) was identified as an MCH receptor. This report describes the cloning and functional characterization of a novel second human MCH receptor, which we designate MCH(2), initially identified in a genomic survey sequence as being homologous to MCH(1) receptors. Using this sequence, a full-length cDNA was generated with an open reading frame of 1023 base pairs, encoding a polypeptide of 340 amino acids, with 38% identity to MCH(1) and with many of the structural features conserved in G protein-coupled receptors. This newly discovered receptor belongs to class 1 (rhodopsin-like) of the G protein-coupled receptor superfamily. HEK293 cells transfected with MCH(2) receptors responded to nanomolar concentrations of MCH with an increase in intracellular Ca(2+) levels and increased cellular extrusion of protons. In addition, fluorescently labeled MCH bound with nanomolar affinity to these cells. The tissue localization of MCH(2) receptor mRNA, as determined by quantitative reverse transcription-polymerase chain reaction, was similar to that of MCH(1) in that both receptors are expressed predominantly in the brain. The discovery of a novel MCH receptor represents a new potential drug target and will allow the further elucidation of MCH-mediated responses.

Comparative analyses of the pigment-aggregating and -dispersing actions of MCH on fish chromatophores

In melanophores of the peppered catfish and the Nile tilapia, melanin-concentrating hormone (MCH) at low doses (<1 microM) induced pigment aggregation, and the aggregated state was maintained in the presence of MCH. However, at higher MCH concentrations (such as 1 and 10 microM), pigment aggregation was immediately followed by some re-dispersion, even in the continued presence of MCH, which led to an apparent decrease in aggregation. This pigment-dispersing activity at higher concentrations of MCH required extracellular Ca(2+) ions. By contrast, medaka melanophores responded to MCH only by pigment aggregation, even at the highest concentration employed (10 microM). Since it is known that medaka melanophores possess specific receptors for alpha-melanophore-stimulating hormone (alpha-MSH), the possibility that interaction between MSH receptors and MCH at high doses in the presence of Ca(2+) might cause pigment dispersion is ruled out. Cyclic MCH analogs, MCH (1-14) and MCH (5-17), failed to induce pigment dispersion, whereas they induced aggregation of melanin granules. These results suggest that another type of MCH receptor that mediates pigment dispersion is present in catfish and tilapia melanophores, and that intact MCH may be the only molecule that can bind to these receptors. Determinations of cAMP content in melanophores, which were isolated from the skin of three fish species and treated with 10 nM or 10 microM MCH, indicate that MCH receptors mediating aggregation may be coupled with Gi protein, whereas MCH receptors that mediate dispersion may be linked to Gs. The response of erythrophores, xanthophores and leucophores to MCH at various concentrations was also examined, and the results suggest that the distribution patterns of the two types of MCH receptors may differ among fish species and among types of chromatophore in the same fish.

Melanin-concentrating hormone stimulates the release of luteinizing hormone-releasing hormone and gonadotropins in the female rat acting at both median eminence and pituitary levels

The purpose of this study was to investigate whether melanin-concentrating hormone (MCH) acts directly on the median eminence and on the anterior pituitary of female rats regulating LHRH and gonadotropin release. In addition, immunohistochemistry was used to examine the density and distribution of MCH-immunoreactive fibers in the median eminence of proestrous rats. MCH-immunoreactive fibers were found in both the internal and external layers of the median eminence and in close association with hypophysial portal vessels. In the first series of in vitro experiments, median eminences and anterior pituitaries were incubated in Krebs-Ringer bicarbonate buffer containing two MCH concentrations (10(-10) and 10(-8) M). The lowest MCH concentration (10(-10) M) increased (P < 0.01) LHRH release only from proestrous median eminences. Anterior pituitaries incubated with both MCH concentrations also showed that 10(-10) M MCH increased gonadotropin release only from proestrous pituitaries. In the second series of experiments, median eminences and pituitaries from proestrous rats were incubated with graded concentrations of MCH. MCH (10(-10) and 10(-9) M) increased (P < 0.01) LHRH release from the median eminence, and only 10(-10) M MCH increased (P < 0.01) LH and FSH release from the anterior pituitary. The effect of MCH on the stimulation of both gonadotropins from proestrous pituitaries was similar to the effect produced by LHRH. Simultaneous incubation of pituitaries with MCH and LHRH did not modify LH but increased the FSH release induced by LHRH. The present results suggest that MCH could be involved in the regulation of preovulatory gonadotropin secretion.

Structure-activity relationship studies of melanin-concentrating hormone (MCH)-related peptide ligands at SLC-1, the human MCH receptor

Melanin-concentrating hormone (MCH) is a cyclic nonadecapeptide involved in the regulation of feeding behavior, which acts through a G protein-coupled receptor (SLC-1) inhibiting adenylcyclase activity. In this study, 57 analogues of MCH were investigated on the recently cloned human MCH receptor stably expressed in HEK293 cells, on both the inhibition of forskolin-stimulated cAMP production and guanosine-5'-O-(3-[(35)S]thiotriphosphate ([(35)S]- GTPgammaS) binding. The dodecapeptide MCH-(6-17) (MCH ring between Cys(7) and Cys(16), with a single extra amino acid at the N terminus (Arg(6)) and at the C terminus (Trp(17))) was found to be the minimal sequence required for a full and potent agonistic response on cAMP formation and [(35)S]- GTPgammaS binding. We Ala-scanned this dodecapeptide and found that only 3 of 8 amino acids of the ring, namely Met(8), Arg(11), and Tyr(13), were essential to elicit full and potent responses in both tests. Deletions inside the ring led either to inactivity or to poor antagonists with potencies in the micromolar range. Cys(7) and Cys(16) were substituted by Asp and Lys or one of their analogues, in an attempt to replace the disulfide bridge by an amide bond. However, those modifications were deleterious for agonistic activity. In [(35)S]- GTPgammaS binding, these compounds behaved as weak antagonists (K(B) 1-4 microm). Finally, substitution in MCH-(6-17) of 6 out of 12 amino acids by non-natural residues and concomitant replacement of the disulfide bond by an amide bond led to three compounds with potent antagonistic properties (K(B) = 0.1-0.2 microm). Exploitation of these structure-activity relationships should open the way to the design of short and stable MCH peptide antagonists.

Interaction of melanin-concentrating hormone (MCH), neuropeptide E-I (NEI), neuropeptide G-E (NGE), and alpha-MSH with melanocortin and MCH receptors on mouse B16 melanoma cells

Melanin-concentrating hormone (MCH) and alpha-melanocyte-stimulating hormone (alpha-MSH) are known to exhibit mostly functionally antagonistic, but in some cases agonistic activities, e.g., in pigment cells and in the brain. Neuropeptide E-I (NEI) displays functional MCH-antagonist and MSH-agonist activity in different behavioral paradigms; the role of neuropeptide G-E (NGE) is not known. This study addressed the question of possible molecular interactions between alpha-MSH, MCH and the MCH-precursor-derived peptides NEI and NGE at the level of the pigment cell MCH receptor subtype (MCH-Rpc) and the different melanocortin (MC) receptors. Radioreceptor assays using [125I]MCH, [125l]alpha-MSH and [125I]NEI as radioligands and bioassays were performed with MCI-R-positive and MC1-R-negative mouse B16 melanoma cells and with COS cells expressing the different MC receptors. The IC50s of alpha-MSH and NEI or NGE for [125I]MCH displacement from mouse MCH-Rpc were 80-fold and, respectively, >300-fold higher than that of MCH, and the IC50s for MCH and NEI or NGE for [125I]alpha-MSH displacement from mouse MC1-R were 50,000-fold and >200,000-fold higher than that of alpha-MSH. No high-affinity binding sites for NEI were detected on B16 melanoma cells and there was no significant displacement of [1251]alpha-MSH by MCH, NEI or NGE with MC3-R, MC4-R and MC5-R expressed in COS cells. At concentrations of 100 nM to 10 microM, however, MCH, NEI and NGE induced cAMP formation and melanin synthesis which could be blocked by agouti protein or inhibitors of adenylate cyclase or protein kinase A. This shows that mammalian MCH-precursor-derived peptides may mimic MSH signalling via MC1-R activation at relatively high, but physiologically still relevant concentrations, as e.g. found in autocrine/paracrine signalling mechanisms.

Expression of melanin-concentrating hormone receptor messenger ribonucleic acid in tumor tissues of pheochromocytoma, ganglioneuroblastoma, and neuroblastoma

Expression of melanin-concentrating hormone (MCH) receptor messenger ribonucleic acid (mRNA) was studied by RT-PCR and Northern blot analysis in human brain; pituitary; adrenal glands; tumor tissues of adrenal tumors, ganglioneuroblastomas, and neuroblastomas; and various cultured tumor cell lines. RT-PCR analysis showed that MCH receptor mRNA was widely expressed in brain tissues, pituitary, normal portions of adrenal glands (cortex and medulla), tumor tissues of adrenocortical tumors (12 of 13 cases), pheochromocytoma (all 7 cases), ganglioneuroblastoma (1 case), neuroblastoma (all 5 cases), and various cultured tumor cell lines (6 of 7 cell lines), including 2 neuroblastoma cell lines. Northern blot analysis showed the expression of MCH receptor mRNA ( approximately 2.4 kb) only in the tumor tissues of 5 pheochromocytomas, 1 ganglioneuroblastoma, and 4 neuroblastomas, indicating that the expression levels of MCH receptor mRNA are much higher in these tumors than in the other tissues. These findings raised the possibility that MCH or MCH-like peptides may be related to the pathophysiology of these neural crest-derived tumors.

The AROM gene, spliced mRNAs encoding new DNA/RNA-binding proteins are transcribed from the opposite strand of the melanin-concentrating hormone gene in mammals

Melanin-concentrating hormone (MCH) mRNA expression is induced by nerve growth factor and lithium in PC12 cells, whereas three large MCH RNA species are found in untreated cells. In this study, we investigated the structures, regulations of expression, and putative functions of these transcripts. Northern blot, rapid amplification of cDNA ends-polymerase chain reaction, reverse transcriptase-polymerase chain reaction, and sequencing experiments demonstrated that they are antisense RNAs complementary to the MCH gene. Two classes of antisense RNAs could be discriminated as follows: 1) non-coding unspliced RNAs that overlap mainly the coding part of the MCH gene; 2) spliced variant mRNAs complementary to the 3'-flanking end of the MCH gene and that encode putative proteins containing DNA/RNA binding domains. We named this new transcriptional unit AROM for antisense-RNA-overlapping-MCH gene. Spliced variant AROM mRNAs are expressed in a broad range of rat organs. Western blot and immunohistochemistry experiments revealed several proteins with cytoplasmic but also nuclear localization in PC12 cells. Time course studies during nerve growth factor and lithium treatment of PC12 cells indicated a reciprocal regulation of the MCH and AROM gene transcripts, reflected also at the level of AROM proteins. The major translational product is a 64-kDa protein (AROM-p64). Recombinant AROM-p64 displayed high binding to single-stranded DNA and poly(A) homopolymers suggesting that this protein could play a role in mRNA maturation/metabolism.

Melanin-concentrating hormone receptor: an orphan receptor fits the key

The melanin-concentrating hormone (MCH), a hypothalamic peptide, was identified initially in teleost fish as a regulator of pigmentary changes in background adaptation, and was later also found, in mammals, to be a regulator of feeding and energy homeostasis. Its specific receptor remained an enigma until very recently when it was identified as the orphan G-protein-coupled receptor SLC-1. This review focuses on the identification, structure and signaling of the MCH receptor and discusses some of the implications of its discovery.

Expression of melanin-concentrating hormone receptors in insulin-producing cells: MCH stimulates insulin release in RINm5F and CRI-G1 cell-lines

Melanin-concentrating hormone (MCH) is a hypothalamic orexigenic peptide. Recently, an orphan G-protein-coupled receptor (SLC-1) was identified that binds MCH with high affinity. Here, we demonstrate the mRNA expression of this receptor in insulin-producing cells including CRI-G1 and RINm5F cells, and in rat islets of Langerhans. Immunofluorescence studies in CRI-G1 and RINm5F cell-lines demonstrated cell-surface expression of the receptor. Rat MCH significantly stimulated insulin secretion in both cell-lines. The potency and the efficacy of MCH were significantly increased in the simultaneous presence of forskolin, suggesting that MCH may amplify the insulinotropic effect of cyclic AMP elevating stimuli. Salmon MCH, which differs from rat/human MCH by six amino acids, was less efficacious than rat/human MCH in stimulating insulin release. The data provide evidence for the expression of MCH receptors in insulin producing cells. The insulinotropic effect of MCH may contribute to the regulation of metabolism and energy balance by this peptide.

Brain regulation of feeding behavior and food intake in fish

In mammals, the orexigenic and anorexigenic neuronal systems are morphologically and functionally connected, forming an interconnected network in the hypothalamus to govern food intake and body weight. However, there are relatively few studies on the brain control of feeding behavior in fish. Recent studies using mammalian neuropeptides or fish homologs of mammalian neuropeptides indicate that brain orexigenic signal molecules include neuropeptide Y, orexins, galanin and beta-endorphin, whereas brain anorexigenic signal molecules include cholecystokinin, bombesin, corticotropin-releasing factor, cocaine- and amphetamine-regulated transcript, and serotonin. Tachykinins may also have an anorectic action in fish. The brain hypothalamic area is associated with regulation of food intake, while sites outside the hypothalamus are also involved in this function. There is correlation between short-term changes in serum growth hormone levels and feeding behavior, although possible mechanisms integrating these functions remain to be defined.