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

Fast neurotransmitter identity of MCH neurons: Do contents depend on context?

Hypothalamic melanin-concentrating hormone (MCH) neurons participate in many fundamental neuroendocrine processes. While some of their effects can be attributed to MCH itself, others appear to depend on co-released neurotransmitters. Historically, the subject of fast neurotransmitter co-release from MCH neurons has been contentious, with data to support MCH neurons releasing GABA, glutamate, both, and neither. Rather than assuming a position in that debate, this review considers the evidence for all sides and presents an alternative explanation: neurochemical identity, including classical neurotransmitter content, is subject to change. With an emphasis on the variability of experimental details, we posit that MCH neurons may release GABA and/or glutamate at different points according to environmental and contextual factors. Through the lens of the MCH system, we offer evidence that the field of neuroendocrinology would benefit from a more nuanced and dynamic interpretation of neurotransmitter identity.

SNAPshots of the MCHR1: a Comparison Between the PET-Tracers [18F]FE@SNAP and [11C]SNAP-7941

PURPOSE

The melanin-concentrating hormone receptor 1 (MCHR1) has become an important pharmacological target, since it may be involved in various diseases, such as diabetes, insulin resistance, and obesity. Hence, a suitable positron emission tomography radiotracer for the in vivo assessment of the MCHR1 pharmacology is imperative. The current paper contrasts the extensive in vitro, in vivo, and ex vivo assessments of the radiotracers [18F]FE@SNAP and [11C]SNAP-7941 and provides comprehensive information about their biological and physicochemical properties. Furthermore, it examines their suitability for first-in-man imaging studies.

PROCEDURES

Kinetic real-time cell-binding studies with [18F]FE@SNAP and [11C]SNAP-7941 were conducted on adherent Chines hamster ovary (CHO-K1) cells stably expressing the human MCHR1 and MCHR2. Small animal imaging studies on mice and rats were performed under displacement and baseline conditions, as well as after pretreatment with the P-glycoprotein/breast cancer resistant protein inhibitor tariquidar. After the imaging studies, detailed analyses of the ex vivo biodistribution were performed. Ex vivo metabolism was determined in rat blood and brain and analyzed at various time points using a quantitative radio-HPLC assay.

RESULTS

[11C]SNAP-7941 demonstrates high uptake on CHO-K1-hMCHR1 cells, whereas no uptake was detected for the CHO-K1-hMCHR2 cells. In contrast, [18F]FE@SNAP evinced binding to CHO-K1-hMCHR1 and CHO-K1-hMCHR2 cells. Imaging studies with [18F]FE@SNAP and [11C]SNAP-7941 showed an increased brain uptake after tariquidar pretreatment in mice, as well as in rats, and exhibited a significant difference between the time-activity curves of the baseline and blocking groups. Biodistribution of both tracers demonstrated a decreased uptake after displacement. [11C]SNAP-7941 revealed a high metabolic stability in rats, whereas [18F]FE@SNAP was rapidly metabolized.

CONCLUSIONS

Both radiotracers demonstrate appropriate imaging properties for the MCHR1. However, the pronounced metabolic stability as well as superior selectivity and affinity of [11C]SNAP-7941 underlines the decisive superiority over [18F]FE@SNAP.

Insulin-Sensitizer Effects of Fenugreek Seeds in Parallel with Changes in Plasma MCH Levels in Healthy Volunteers

In developed, developing and low-income countries alike, type 2 diabetes mellitus (T2DM) is one of the most common chronic diseases, the severity of which is substantially a consequence of multiple organ complications that occur due to long-term progression of the disease before diagnosis and treatment. Despite enormous investment into the characterization of the disease, its long-term management remains problematic, with those afflicted enduring significant degradation in quality-of-life. Current research efforts into the etiology and pathogenesis of T2DM, are focused on defining aberrations in cellular physiology that result in development of insulin resistance and strategies for increasing insulin sensitivity, along with downstream effects on T2DM pathogenesis. Ongoing use of plant-derived naturally occurring materials to delay the onset of the disease or alleviate symptoms is viewed by clinicians as particularly desirable due to well-established efficacy and minimal toxicity of such preparations, along with generally lower per-patient costs, in comparison to many modern pharmaceuticals. A particularly attractive candidate in this respect, is fenugreek, a plant that has been used as a flavouring in human diet through recorded history. The present study assessed the insulin-sensitizing effect of fenugreek seeds in a cohort of human volunteers, and tested a hypothesis that melanin-concentrating hormone (MCH) acts as a critical determinant of this effect. A test of the hypothesis was undertaken using a hyperinsulinemic euglycemic glucose clamp approach to assess insulin sensitivity in response to oral administration of a fenugreek seed preparation to healthy subjects. Outcomes of these evaluations demonstrated significant improvement in glucose tolerance, especially in patients with impaired glucose responses. Outcome data further suggested that fenugreek seed intake-mediated improvement in insulin sensitivity correlated with reduction in MCH levels.

In vivo evaluation of radiotracers targeting the melanin-concentrating hormone receptor 1: [11C]SNAP-7941 and [18F]FE@SNAP reveal specific uptake in the ventricular system

The MCHR1 is involved in the regulation of energy homeostasis and changes of the expression are linked to a variety of associated diseases, such as diabetes and adiposity. The study aimed at the in vitro and in vivo evaluation of [11C]SNAP-7941 and [18F]FE@SNAP as potential PET-tracers for the MCHR1. Competitive binding studies with non-radioactive derivatives and small-animal PET/CT and MRI brain studies were performed under baseline conditions and tracer displacement with the unlabelled MCHR1 antagonist (±)-SNAP-7941. Binding studies evinced high binding affinity of the non-radioactive derivatives. Small-animal imaging of [11C]SNAP-7941 and [18F]FE@SNAP evinced high tracer uptake in MCHR1-rich regions of the ventricular system. Quantitative analysis depicted a significant tracer reduction after displacement with (±)-SNAP-7941. Due to the high binding affinity of the non-labelled derivatives and the high specific tracer uptake of [11C]SNAP-7941 and [18F]FE@SNAP, there is strong evidence that both radiotracers may serve as highly suitable agents for specific MCHR1 imaging.

[(18)F]FE@SNAP-a specific PET tracer for melanin-concentrating hormone receptor 1 imaging?

BACKGROUND

The melanin-concentrating hormone receptor 1 (MCHR1), which is highly expressed in the lateral hypothalamus, plays a key role in energy homeostasis, obesity and other endocrine diseases. Hence, there is a major interest in in vivo imaging of this receptor. A PET tracer would allow non-invasive in vivo visualization and quantification of the MCHR1. The aim of the study was the ex vivo evaluation of the MCHR1 ligand [(18)F]FE@SNAP as a potential PET tracer for the MCHR1.

METHODS

[(18)F]FE@SNAP was injected directly into the jugular vein of awake naïve rats for ex vivo brain autoradiography, biodistribution and additional blood metabolite analysis. Blocking experiments were conducted using the unlabeled MCHR1 ligand SNAP-7941.

RESULTS

A high uptake of [(18)F]FE@SNAP was observed in the lateral hypothalamus and the ventricular system. Both regions were significantly blocked by SNAP-7941. Biodistribution evinced the highest uptake in the kidneys, adrenals, lung and duodenum. Specific blocking with SNAP-7941 led to a significant tracer reduction in the heart and adrenals. In plasma samples, 47.73 ± 6.1 % of a hydrophilic radioactive metabolite was found 45 min after tracer injection.

CONCLUSIONS

Since [(18)F]FE@SNAP uptake was significantly blocked in the lateral hypothalamus, there is strong evidence that [(18)F]FE@SNAP is a highly suitable agent for specific MCHR1 imaging in the central nervous system. Additionally, this finding is supported by the specific blocking in the ventricular system, where the MCHR1 is expressed in the ependymal cells. These findings suggest that [(18)F]FE@SNAP could serve as a useful imaging and therapy monitoring tool for MCHR1-related pathologies.

Comparative autoradiographic in vitro investigation of melanin concentrating hormone receptor 1 ligands in the central nervous system

The MCHR1 is an interesting pharmacological and pharmaceutical target, due to its involvement in pathologies as diabetes, gut inflammation and adiposity. in vivo PET-studies of the MCHR1 in energy homeostasis and diabetes could be of great value for deeper understanding of endocrinological hormone status and consequential pharmacological interactions. Furthermore, PET-tracers would facilitate compound dose selection of MCHR1 antagonists for treatment. Therefore, we developed two potential PET-tracers, [(11)C]SNAP-7941 and [(18)F]FE@SNAP, for the in vivo visualization of this receptor. Aim of this study was a preclinical in vitro evaluation of both unlabeled ligands. Therefore, a comparative autoradiographic investigation on CNS (coronal rat brain and 4 different human brain regions) and peripheral tissues (rat tongue as target and rat testes as non-target region) was conducted. Competition experiments, using the two radioligands [(125)I]-MCH and [(125)I]-S36057, were performed with selective and specific MCHR1 ligands as PMC-3886, a MCHR1 agonist, SNAP-7941 and FE@SNAP, two MCHR1 antagonists. Additionally, immunohistochemical staining with a specific MCHR1 antibody was performed. Specific binding was found in all tissues known to express the MCHR1 as human and rat CNS and peripheral rat tongue tissue. No specific binding was found in the non-target region of rat testes. MCHR1 antibody staining complemented the outcome of the autoradiographic experiments. The compounds SNAP-7941 and FE@SNAP were generally comparable with PMC-3886. Hence, the in vitro autoradiographic study of the unlabeled compounds SNAP-7941 and FE@SNAP further qualifies the potential of the PET-tracers [(11)C]SNAP-7941 and [(18)F]FE@SNAP as useful MCHR1 PET-tracers.

Olanzapine-induced changes in glucose metabolism are independent of the melanin-concentrating hormone system

Atypical antipsychotic drugs such as Olanzapine (Ola) induce weight gain and metabolic changes associated with the development of type 2 diabetes. The mechanisms underlying these undesired side-effects are currently unknown. Chagnon et al. showed that the common allele rs7973796 of the prepro-melanin-concentrating hormone (PMCH) gene is associated with a greater body mass index in Ola-treated schizophrenic patients. As PMCH encodes for the orexigenic neuropeptide melanin-concentrating hormone (MCH), it was hypothesized that MCH is involved in Ola-induced metabolic changes. We have recently reported that the intragastric infusion of Ola results in hyperglycaemia and insulin resistance in male rats. In order to test in vivo the possible involvement of the PMCH gene in the pathogenesis of Ola side-effects, we administered Ola intragastrically in wild-type (WT) and PMCH knock-out (KO) rats. Our results show that glucose and corticosterone levels, as well as endogenous glucose production, are elevated by the infusion of Ola in both WT and KO animals. Thus, the lack of MCH does not seem to affect the acute effects of Ola on glucose metabolism. On the other hand, these effects might be obliterated by compensatory changes in other hypothalamic systems. In addition, possible modulatory effects of the MCH KO on the long term effects of Ola, i.e. increased adiposity, body weight gain, have not been investigated yet.

Preparation and First Preclinical Evaluation of [(18)F]FE@SNAP: A Potential PET Tracer for the Melanin-Concentrating Hormone Receptor-1 (MCHR1)

The melanin-concentrating hormone (MCH) system is a new target for the treatment of human disorders. Since the knowledge of the MCH system’s involvement in a variety of pathologies (obesity, diabetes, and deregulation of metabolic feedback mechanism) is based on in vitro or preclinical studies, a suitable positron emission tomography (PET) tracer needs to be developed. We herein present the preparation and first preclinical evaluation of [¹⁸F]FE@SNAP – a new PET tracer for MCH receptor-1 (MCHR1). The synthesis was performed using a microfluidic device. Preclinical evaluation included binding affinity, plasma stability, plasma free fraction, stability against the cytochrome P-450 (CYP450) system using liver microsomes, stability against carboxyl-esterase, and methods to assess the penetration of the blood-brain barrier (BBB) such as logD analysis and immobilized artificial membrane (IAM) chromatography. Levels at 374 ± 202 MBq [¹⁸F]FE@SNAP were obtained after purification. The obtained K_d value of [¹⁸F]FE@SNAP was 2.9 nM. [¹⁸F]FE@SNAP evinced high stability against carboxylesterase, CYP450 enzymes, and in human plasma. LogD (3.83) and IAM chromatography results (P_m=0.51) were in the same range as for known BBB-penetrating compounds. The synthesis of [¹⁸F]FE@SNAP was reliable and successful. Due to high binding affinity and stability, [¹⁸F]FE@SNAP is a promising tracer for MCHR1.

[¹⁸F]FE@SNAP-A new PET tracer for the melanin concentrating hormone receptor 1 (MCHR1): microfluidic and vessel-based approaches

Changes in the expression of the melanin concentrating hormone receptor 1 (MCHR1) are involved in a variety of pathologies, especially obesity and anxiety disorders. To monitor these pathologies in-vivo positron emission tomography (PET) is a suitable method. After the successful radiosynthesis of [(11)C]SNAP-7941-the first PET-Tracer for the MCHR1, we aimed to synthesize its [(18)F]fluoroethylated analogue: [(18)F]FE@SNAP. Therefore, microfluidic and vessel-based approaches were tested. [(18)F]fluoroethylation was conducted via various [(18)F]fluoroalkylated synthons and direct [(18)F]fluorination. Only the direct [(18)F]fluorination of a tosylated precursor using a flow-through microreactor was successful, affording [(18)F]FE@SNAP in 44.3 ± 2.6%.

Radiosynthesis of [11C]SNAP-7941--the first PET-tracer for the melanin concentrating hormone receptor 1 (MCHR1)

The melanin concentrating hormone (MCH) system is a new target to treat human disorders. Our aim was the preparation of the first PET-tracer for the MCHR1. [(11)C]SNAP-7941 is a carbon-11 labeled analog of the published MCHR1 antagonist SNAP-7941. The optimum reaction conditions were 2 min reaction time, ≤25°C reaction temperature, and 2 mg/mL precursor (SNAP-acid) in acetonitrile, using [(11)C]CH(3)OTf as methylation agent. [(11)C]SNAP-7941 was prepared in a reliable and feasible manner with high radiochemical yields (2.9±1.6 GBq; 11.5±6.4% EOB, n=15).

Antiobesity effects of melanin-concentrating hormone receptor 1 (MCH-R1) antagonists

Despite remarkable progress in the elucidation of energy balance and regulation, the development of new antiobesity drugs is still at the stage of infancy. This review describes the MCH and MCH receptor system with regard to its involvement in energy homeostasis and summarizes the pharmacological profiles of selected small molecule MCH-R1 antagonists that are relevant for their development as antiobesity drugs. Although their clinical value still has to be demonstrated, and challenges with regard to unwanted side effects remain to be resolved, MCH-R1 antagonists may provide an effective pharmacotherapy for the treatment of obesity in the near future.

Peptides and their potential role in the treatment of diabetes and obesity

It is estimated that 347 million people worldwide have diabetes and that over 1.5 billion adults worldwide are overweight. Predictions suggest these rates are increasing. Diabetes is a common complication in overweight and obese subjects, and in 2004, an estimated 3.4 million people died from consequences of high blood sugar. Thus, there is great interest in revealing the physiological systems that regulate body weight and blood sugar. Several peptidergic systems within the central nervous system and the periphery regulate energy homeostasis. A number of these systems have been investigated as potential treatments for obesity and the metabolic syndrome. However, manipulation of peptidergic systems poses many problems. This review discusses the peptidergic systems currently attracting research interest for their clinical potential to treat obesity. We consider first neuropeptides in the brain, including the orexigenic neuropeptide Y and melanin-concentrating hormone, and anorectic factors such as the melanocortins, ciliary neurotrophic factor, and neuromedin U. We subsequently discuss the utility of targeting peripheral gut peptides, including pancreatic polypeptide, peptide YY, amylin, and the gastric hormone ghrelin. Also, we analyze the evidence that these factors or drugs based on them may be therapeutically useful, while considering the disadvantages of using such peptides in a clinical context.

Cellular models for the study of the pharmacology and signaling of melanin-concentrating hormone receptors

Cellular models for the study of the neuropeptide melanin-concentrating hormone (MCH) have become indispensable tools for pharmacological profiling and signaling analysis of MCH and its synthetic analogues. Although expression of MCH receptors is most abundant in the brain, MCH-R(1) is also found in different peripheral tissues. Therefore, not only cell lines derived from nervous tissue but also from peripheral tissues that naturally express MCH receptors have been used to study receptor signaling and regulation. For screening of novel compounds, however, heterologous expression of MCH-R(1) or MCH-R(2) genes in HEK293, Chinese hamster ovary, COS-7, or 3T3-L1 cells, or amplified MCH-R(1) expression/signaling in IRM23 cells transfected with the G(q) protein gene are the preferred tools because of more distinct pharmacological effects induced by MCH, which include inhibition of cAMP formation, stimulation of inositol triphosphate production, increase in intracellular free Ca(2+) and/or activation of mitogen-activated protein kinases. Most of the published data originate from this type of model system, whereas data based on studies with cell lines endogenously expressing MCH receptors are more limited. This review presents an update on the different cellular models currently used for the analysis of MCH receptor interaction and signaling.

Recent updates on the melanin-concentrating hormone (MCH) and its receptor system: lessons from MCH1R antagonists

Melanin-concentrating hormone (MCH) is a 19-amino-acid cyclic peptide which was originally found to lighten skin color in fish that is highly conserved among many species. MCH interacts with two G-protein-coupled receptors, MCH1R and MCH2R, but only MCH1R is expressed in rodents. MCH is mainly synthesized in the lateral hypothalamus and zona incerta, while MCH1R is widely expressed throughout the brain. Thus, MCH signaling is implicated in the regulation of many physiological functions. The identification of MCH1R has led to the development of small-molecule MCH1R antagonists that can block MCH signaling. MCH1R antagonists are useful not only for their potential therapeutic value, but also for understanding the physiological functions of the endogenous MCH system. Here, we review the physiological functions of the MCH system which have been investigated using MCH1R antagonists such as food intake, anxiety, depression, reward, and sleep. This will help us understand the physiological functions of the MCH system and suggest some of the potential applications of MCH1R antagonists in human disorders.

Pmch expression during early development is critical for normal energy homeostasis

Postnatal development and puberty are times of strong physical maturation and require large quantities of energy. The hypothalamic neuropeptide melanin-concentrating hormone (MCH) regulates nutrient intake and energy homeostasis, but the underlying mechanisms are not completely understood. Here we use a novel rat knockout model in which the MCH precursor Pmch has been inactivated to study the effects of loss of MCH on energy regulation in more detail. Pmch(-/-) rats are lean, hypophagic, osteoporotic, and although endocrine parameters were changed in pmch(-/-) rats, endocrine dynamics were normal, indicating an adaptation to new homeostatic levels rather than disturbed metabolic mechanisms. Detailed body weight growth and feeding behavior analysis revealed that Pmch expression is particularly important during early rat development and puberty, i.e., the first 8 postnatal weeks. Loss of Pmch resulted in a 20% lower set point for body weight that was determined solely during this period and remained unchanged during adulthood. Although the final body weight is diet dependent, the Pmch-deficiency effect was similar for all diets tested in this study. Loss of Pmch affected energy expenditure in both young and adult rats, although these effects seem secondary to the observed hypophagia. Our findings show an important role for Pmch in energy homeostasis determination during early development and indicate that the MCH receptor 1 system is a plausible target for childhood obesity treatment, currently a major health issue in first world countries.

Melanin-concentrating hormone receptor 2 affects 3T3-L1 preadipocyte differentiation

To investigate the influence of melanin-concentrating hormone receptor 2 (MCHR2) on preadipocyte proliferation and differentiation, a 3T3-L1-MCHR2 cell line stably expressing human MCHR2 was established. The expression level and biological activity of MCHR2 were evaluated by immunofluorescence and radioligand binding assays. A 3-(4,5-dimethylthiazol-2yl)-2,5-diphenyltetrazolium bromide (MTT) assay and cell cycle analysis showed that MCH did not affect 3T3-L1-MCHR2 and 3T3-L1-mock cell proliferation. MCH accelerated preadipocyte differentiation of the two cell lines when they were administered various doses of standard adipogenic inducers. The rate and degree of differentiation of the 3T3-L1-MCHR2 cells were higher than those of the 3T3-L1-mock cells. The expression levels of adipocyte-specific marker genes, including peroxisome proliferator-activated receptorgamma2 (PPARgamma2), CCAAT enhancer-binding proteinalpha (C/EBPalpha), adipocyte-specific fatty acid-binding protein (aP(2)) and leptin were significantly up-regulated, especially in 3T3-L1-MCHR2 cells. These findings suggest that MCHR2 positively mediates the regulation of MCH during preadipocyte differentiation and is involved in energy balance regulation without affecting preadipocyte proliferation.

MCH receptors/gene structure-in vivo expression

Melanin-concentrating hormone (MCH) is a cyclic peptide which was originally discovered in fish to lighten skin color by affecting melanosomes aggregation. This peptide is highly conserved and also found in rodents whose gene is overexpressed upon fasting. However, the site of MCH action remained obscure until its receptor was discovered in 1999 as a G protein-coupled receptor. After this receptor structure was identified, the functional domains important for MCH-MCHR interaction were revealed. Moreover, the cloning of the MCH receptor led us to identify the in vivo sites of MCH action which suggested potential physiological functions of the MCH system. Furthermore, the MCH receptor identification allow for designing surrogate molecules which can block MCH activity. Studies using these molecules revealed various physiological functions of the MCH system not only in feeding but also in other physiological responses such as stress and emotion. This review will discuss how the MCH receptor was discovered and its impact on many studies investigating the MCH receptor's structure, signaling pathways, and expression pattern.

Animals models of MCH function and what they can tell us about its role in energy balance

Melanin-concentrating hormone (MCH) has attracted considerable attention because of its effects on food intake and body weight and the MCH receptor (MCHR1) remains one of the viable targets for obesity therapy. This review summarizes the literature examining the effects of MCH on body weight, food intake and energy expenditure in rodent models, and the central sites where MCH acts in regulating energy homeostasis. Emphasis is given on the discrepancies between the genetic and pharmacologic models of MCHR1 inactivation. We propose some solutions to resolve these discrepancies and discuss some future directions in MCH research.

The pharmacological properties of a novel MCH1 receptor antagonist isolated from combinatorial libraries

Melanin-concentrating hormone (MCH) is a neuropeptide that exhibits potent orexigenic activity. In rodents, it exerts its actions by interacting with one receptor, MCH(1) receptor which is expressed in many parts of the central nervous system (CNS). To study the physiological implications of the MCH system, we need to be able to block it locally and acutely. This necessitates the use of MCH(1) receptor antagonists. While MCH(1) receptor antagonists have been previously reported, they are mainly not accessible to academic research. We apply here a strategy that leads to the isolation of a high affinity and selective MCH(1) receptor antagonist amenable to in vivo analyses without further chemical modifications. This antagonist, TPI 1361-17, was identified through the screening of multiple non-peptide positional scanning synthetic combinatorial libraries (PS-SCL) totaling more than eight hundred thousand compounds in conditions that allow for the identification of only high-affinity compounds. TPI 1361-17 exhibited an IC(50) value of 6.1 nM for inhibition of 1 nM MCH-induced Ca(2+) mobilization and completely displaced the binding of [(125)I] MCH to rat MCH(1) receptor. TPI 1361-17 was found specific, having no affinity for a variety of other G-protein coupled receptors and channels. TPI 1361-17 was found active in vivo since it blocked MCH-induced food intake by 75%. Our results indicate that TPI 1361-17 is a novel and selective MCH(1) receptor antagonist and is an effective tool to study the physiological functions of the MCH system. These results also illustrate the successful application of combinatorial library screening to identify specific surrogate antagonists in an academic setting.

Melanin-concentrating hormone as a mediator of intestinal inflammation

Melanin-concentrating hormone (MCH) is expressed primarily in the hypothalamus and has a positive impact on feeding behavior and energy balance. Although MCH is expressed in the gastrointestinal tract, its role in this system remains elusive. We demonstrate that, compared to wild type, mice genetically deficient in MCH had substantially reduced local inflammatory responses in a mouse model of experimental colitis induced by intracolonic administration of 2,4,6 trinitrobenzene sulfonic acid (TNBS). Likewise, mice receiving treatments with an anti-MCH antibody, either prophylactically or after the establishment of colitis, developed attenuated TNBS-associated colonic inflammation and survived longer. Consistent with a potential role of MCH in intestinal pathology, we detected increased colonic expression of MCH and its receptor in patients with inflammatory bowel disease. Moreover, we found that human colonic epithelial cells express functional MCH receptors, the activation of which induces IL-8 expression. Taken together, these results clearly implicate MCH in inflammatory processes in the intestine and perhaps elsewhere.

Orphan GPCR research

Orphan G protein-coupled receptors (GPCRs) are receptors lacking endogenous ligands. Found by molecular biological analyses, they became the roots of reverse pharmacology, in which receptors are attempted to be matched to potential transmitters. Later, when high-throughput screening technology was applied to reverse pharmacology, dozens of orphan GPCRs became deorphanized. Furthermore, novel neuropeptides were discovered. This review retraces the history of the orphan GPCRs and of the discoveries of their endogenous ligands, it also discusses the difficulties that the search for new ligands is presently encountering.

Effect of ppMCH derived peptides on PBMC proliferation and cytokine expression

The mRNA encoding prepro-Melanin concentrating hormone (ppMCH) is mainly expressed in the central nervous system but has also been detected at lower amount in many peripheral tissues including spleen and thymus. At the peptide level however, several forms of the precursor can be detected in these tissues and are sometimes expressed at similar levels compared to brain. In the present work, we have studied the in vitro action of a wide range of concentration (1 nM to 1 microM) of the different peptides encoded by ppMCH i.e. neuropeptide glycine-glutamic acid (NGE), neuropeptide glutamic acid-isoleucine (NEI), Melanin concentrating hormone (MCH) and the dipeptide NEI-MCH on peripheral blood mononuclear cells (PBMC) proliferation and cytokine production following anti-CD3 stimulation. Among them only MCH decreased PBMC proliferation with a maximal effect of 35% at 100 nM. Moreover as demonstrated by using ELISA, MCH significantly decreases IL-2 production by 25% but not IL-4, INF-gamma or TNF-alpha expression. Interestingly, exogenous IL-2 decreases significantly MCH-mediated inhibition, suggesting that it is an important downstream mediator of MCH action. Finally, we showed that after 7 to 9 days of incubation, MCH also inhibits proliferation of non-stimulated PBMC. Altogether, these data demonstrate that fully mature MCH modulates proliferation of anti-CD3 stimulated PBMC partially through regulation of IL-2 production.

Melanin concentrating hormone is a novel regulator of islet function and growth

Melanin concentrating hormone (MCH) is a hypothalamic neuropeptide known to play a critical role in energy balance. We have previously reported that overexpression of MCH is associated with mild obesity. In addition, mice have substantial hyperinsulinemia and islet hyperplasia that is out of proportion with their degree of obesity. In this study, we further explored the role of MCH in the endocrine pancreas. Both MCH and MCHR1 are expressed in mouse and human islets and in clonal beta-cell lines as assessed using quantitative real-time PCR and immunohistochemistry. Mice lacking MCH (MCH-KO) on either a C57Bl/6 or 129Sv genetic background showed a significant reduction in beta-cell mass and complemented our earlier observation of increased beta-cell mass in MCH-overexpressing mice. Furthermore, the compensatory islet hyperplasia secondary to a high-fat diet, which was evident in wild-type controls, was attenuated in MCH-KO. Interestingly, MCH enhanced insulin secretion in human and mouse islets and rodent beta-cell lines in a dose-dependent manner. Real-time PCR analyses of islet RNA derived from MCH-KO revealed altered expression of islet-enriched genes such as glucagon, forkhead homeobox A2, hepatocyte nuclear factor (HNF)4alpha, and HNF1alpha. Together, these data provide novel evidence for an autocrine role for MCH in the regulation of beta-cell mass dynamics and in islet secretory function and suggest that MCH is part of a hypothalamic-islet (pancreatic) axis.

Melanin-Concentrating Hormone Receptor-1 Antagonists as Antiobesity Therapeutics

There is compelling genetic and pharmacologic evidence to indicate that melanin-concentrating hormone receptor-1 (MCHR1) signaling is involved in the regulation of food intake and energy expenditure. The medical need for novel therapies to treat obesity and related metabolic disorders has led to a great deal of interest by pharmaceutical companies in the discovery of MCHR1 antagonists. Recent publications describing preclinical studies have demonstrated that small-molecule MCHR1 antagonists decrease food intake, bodyweight, and adiposity in rodent models of obesity. Results from ongoing early-stage clinical trials with MCHR1 antagonists are eagerly awaited, as is the movement of other MCHR1 antagonists into the clinic.

Potent, Selective, and Orally Efficacious Antagonists of Melanin-Concentrating Hormone Receptor 1

The high expression of MCH in the hypothalamus with the lean hypophagic phenotype coupled with increased resting metabolic rate and resistance to high fat diet-induced obesity of MCH KO mice has spurred considerable efforts to develop small molecule MCHR1 antagonists. Starting from a lead thienopyrimidinone series, structure-activity studies at the 3- and 6-positions of the thienopyrimidinone core afforded potent and selective MCHR1 antagonists with representative examples having suitable pharmacokinetic properties. Based on structure-activity relationships, a structural model for MCHR1 was constructed to explain the binding mode of these antagonists. In general, a good correlation was observed between pKas and activity in the right-hand side of the template, with Asp123 playing an important role in the enhancement of binding affinity. A representative example when evaluated chronically in diet-induced obese mice resulted in good weight loss effects. These antagonists provide a viable lead series in the discovery of new therapies for the treatment of obesity.

6-(4-Chlorophenyl)-3-substituted-thieno[3,2-d]pyrimidin-4(3H)-one-Based Melanin-Concentrating Hormone Receptor 1 Antagonist

Genetic manipulation studies in mice at both the MCH receptor 1 (MCHR1) as well as the MCH peptide levels have implicated MCHR1 as a key player in energy homeostasis. The phenotype exhibited by these studies, that is, increased metabolic rate, resistance to high fat diet, and subsequent weight loss, has spurred considerable efforts to develop antagonists of MCHR1. In continuation of efforts directed toward this goal, the present work capitalizes on the putative binding mode of an MCH antagonist, resulting in the identification of several novel chemotypes that are potent and selective MCHR1 antagonists. In addition, the favorable pharmacokinetics of representative examples has allowed for the evaluation of an MCHR1 antagonist in a high fat diet-induced obese rodent model of obesity. The tolerability of the right-hand side of the template for diverse chemotypes accompanied by favorable effects on weight loss enhances the attractiveness of this template in the pursuit toward development of effective anti-obesity agents.

Expanding the scales: The multiple roles of MCH in regulating energy balance and other biological functions

Melanin-concentrating hormone (MCH) is a cyclic peptide originally identified as a 17-amino-acid circulating hormone in teleost fish, where it is secreted by the pituitary in response to stress and environmental stimuli. In fish, MCH lightens skin color by stimulating aggregation of melanosomes, pigment-containing granules in melanophores, cells of neuroectodermal origin found in fish scales. Although the peptide structure between fish and mammals is highly conserved, in mammals, MCH has no demonstrable effects on pigmentation; instead, based on a series of pharmacological and genetic experiments, MCH has emerged as a critical hypothalamic regulator of energy homeostasis, having effects on both feeding behavior and energy expenditure.

Further insights into the neurobiology of melanin-concentrating hormone in energy and mood balances

Melanin-concentrating hormone (MCH) is a critical hypothalamic anabolic neuropeptide, with key central and peripheral actions on energy balance regulation. The actions of MCH are, so far, known to be transduced through two seven-transmembrane-like receptor paralogues, named MCH1R and MCH2R. MCH2R is not functional in rodents. MCH1R is an important receptor involved in mediating feeding behaviour modulation by MCH in rodents. Pharmacological antagonism at MCH1R in rodents diminishes food intake and results in significant and sustained weight loss in fat tissues, particularly in obese animals. Additionally, MCH1R antagonists have been shown to have anxiolytic and antidepressant properties. The purpose of this review is to highlight the recent numerous pieces of evidence showing that pharmacological blockade at MCH1R could be a potential treatment for obesity and its related metabolic syndrome, as well as for various psychiatric disorders.

Melanin-concentrating hormone receptor 1 deficiency increases insulin sensitivity in obese leptin-deficient mice without affecting body weight

The hypothalamic peptide melanin-concentrating hormone (MCH) plays important roles in energy homeostasis. Animals overexpressing MCH develop hyperphagia, obesity, and insulin resistance. In this study, mice lacking both the MCH receptor-1 (MCHr1 knockout) and leptin (ob/ob) double-null mice (MCHr1 knockout ob/ob) were generated to investigate whether the obesity and/or the insulin resistance linked to the obese phenotype of ob/ob mice was attenuated by ablation of the MCHr1 gene. In MCHr1 knockout ob/ob mice an oral glucose load resulted in a lower blood glucose response and markedly lower insulin levels compared with the ob/ob mice despite no differences in body weight, food intake, or energy expenditure. In addition, MCHr1 knockout ob/ob mice had higher locomotor activity and lean body mass, lower body fat mass, and altered body temperature regulation compared with ob/ob mice. In conclusion, MCHr1 is important for insulin sensitivity and/or secretion via a mechanism not dependent on decreased body weight.

Increases in melanin-concentrating hormone and MCH receptor levels in the hypothalamus of dietary-obese rats

Melanin-concentrating hormone (MCH) is a hypothalamic neuropeptide that stimulates feeding and increases body weight in rodents. We studied the role of the system in energy homeostasis and its regulation by the satiety signals, leptin and insulin. We used real-time PCR to measure the hypothalamic expression of MCH and its receptor (MCHR1) in two contrasting models of altered nutritional status, namely, obesity induced by 8 weeks' voluntary overeating and food restriction for 10 days. Diet-fed rats were stratified according to final total fat-pad mass into a 'high fat gain' group (HG) and 'low fat gain' group (LG). MCH mRNA levels were increased by 31% (p>0.05) and 49% (p<0.05) in the LG and HG, respectively, compared with controls. MCHR1 mRNA levels rose by 118% in the LG (p<0.01) and 85% in the HG (p<0.01). There were significant positive correlations (p<0.05) between plasma leptin concentration and both MCH and MCHR1 mRNA levels, and between plasma insulin and MCHR1 expression. A positive correlation was also observed between MCH and MCHR1 mRNA levels (p<0.05). Food-restricted rats showed no significant alterations in the levels of either MCH mRNA or MCHR1 mRNA. In a second experiment, we measured MCH peptide levels in five discrete hypothalamic areas of dietary-obese rats. MCH concentrations were significantly increased in the arcuate nuclei of the HG (p<0.05) and the paraventricular nuclei of both the LG (p<0.05) and HG (p<0.05), compared with their lean counterparts. These results suggest that the MCH system becomes more active in dietary obesity and could be involved in enhancing appetite for palatable food. The possibility that MCH and MCHR1 expression are positively regulated by leptin and insulin, which normally inhibit feeding, is a putative explanation for how appetite for palatable food is able to override mechanisms that prevent the development of obesity.

Expression and characterization of melanin-concentrating hormone receptors on mammalian cell lines

The neuropeptide melanin-concentrating hormone (MCH) is expressed in central and peripheral tissues where it participates in the complex network regulating energy homeostasis as well as in other physiologically important functions. Two MCH receptor subtypes, MCH-R1 and MCH-R2, have been cloned which signal through activation of Gi/o/q proteins and hence regulate different intracellular signals, such as inhibition of cAMP formation, stimulation of IP3 production, increase in intracellular free Ca2+ and/or activation of MAP kinases. Most of the data were obtained with cell systems heterologously expressing either of the MCH receptors. Fewer reports exist on studies with cell lines which endogenously express MCH receptors. Here, we describe human and other mammalian cell lines with which MCH receptor activation can be studied under "natural" conditions and we summarize the characteristics and signaling pathways of the MCH receptors in the different cell systems.

Comparative analysis of melanin-concentrating hormone structure and activity in fishes and mammals

A comparative analysis of the structure of the melanin-concentrating hormone (MCH) precursor reveals that this sequence has been subjected to a higher selection pressure in mammals than in teleosts, suggesting that the structural constraints have not been the same throughout the vertebrate lineage. In contrast, the MCH peptide sequence has been very well conserved in all species. A sensitive and reproducible eel skin assay was developed and allowed us to define the structural features needed for a full MCH bioactivity. It was shown that the minimal structure carrying the critical residues was the same in fishes and in mammals. A pharmacological approach confirmed that MCH receptor activation decreased the cAMP levels in the fish skin, but this effect appeared to be independent from a Galphai protein. We propose that one of the intracellular signaling pathways of the MCH receptor in fish skin is the activation of one or several cellular phosphodiesterases.

Beyond skin color: emerging roles of melanin-concentrating hormone in energy homeostasis and other physiological functions

Melanin-concentrating hormone (MCH) is a cyclic peptide that mediates its effects by the activation of two G-protein-coupled seven transmembrane receptors (MCHR1 and MCHR2) in humans. In contrast to its primary role in regulating skin color in fish, MCH has evolved in mammals to regulate dynamic physiological functions, from food intake and energy expenditure to behavior and emotion. Chronic infusion or transgenic expression of MCH stimulates feeding and increases adipocity, whereas targeted deletion of MCH or its receptor (MCHR1) leads to resistance to diet-induced obesity with increased energy expenditure and thermogenesis. The involvement of MCH in energy homeostasis and in brain activity has also been validated in mice treated with non-peptide antagonists, suggesting that blockade of MCHR1 could provide a viable approach for treatment of obesity and certain neurological disorders. This review focuses on emerging roles of MCH in regulating central and peripheral mechanisms.

Promoter characterization of the mouse melanin-concentrating hormone receptor 1

The gene encoding the mouse melanin-concentrating hormone receptor 1 was isolated and its structural organization and flanking regions were characterized. The 3' flanking region is marked by the presence of two polyadenylation signals but used with different frequencies. RNase protection and 5' rapid amplification of cDNA ends (RACE) identified multiple transcription initiation sites between -150 and -203 bp upstream of the ATG initiation codon. Functional analysis of deletion mutants reveals a cell independent transcriptional activity localized between nucleotide -305 and -589. The proximal 1.5 kb region does not possess consensus TATA or CAAT boxes but has several consensus sequences for regulatory elements including USF, GATA, AP1, AP4, MyoD, GKLF and Ikaros that could explain the broad expression of the receptor.

Effects of ghrelin and other neuropeptides (CART, MCH, orexin A and B, and GLP-1) on the release of insulin from isolated rat islets

BACKGROUND AND AIMS

Ghrelin, a neuropeptide containing 28 amino acids, shows a reciprocal diurnal plasma fluctuation to that of plasma insulin. The aim of this study is to clarify the dose and glucose-dependency of ghrelin on the insulin secretion and to compare its effect with that of other neuropeptides-GLP-1, CART (55-102), CART (55-76), CART (62-76), MCH, orexin A, and B.

MATERIALS AND METHODS

Rat islets were incubated with 1 pmol/l-1 micromol/l of ghrelin, CART fragments, MCH, orexin A or B, or GLP-1 (n = 16-32) in the presence of 16.7 mmol/l glucose. Ghrelin (10 nmol/l) was added to islets at glucose concentrations of 3.3, 6.6, 16.7 and 25 mmol/l, respectively (n = 28-32). Also, INS-1E cells were incubated with ghrelin (1 nmol/l) in the presence of glucose (3.3, 6.6, 16.7, and 25 mmol/l). In addition, we measured the mRNA expression of the ghrelin receptor using RT-PCR.

RESULTS

Ghrelin inhibited insulin secretion from islets and INS-1E cells in a dose- and glucose-dependent manner. Neither 10 pmol/l-1 micromol/l of CART fragments, MCH, orexin A, nor orexin B changed the insulin secretion at 16.7 mmol/l glucose, while GLP-1, as expected, stimulated the insulin release from rat islets. Interestingly, ghrelin receptors were expressed both in islets, INS-1E, MIN 6 and alpha cell Tca-9 lines.

CONCLUSIONS

Ghrelin inhibits the insulin secretion in vitro in a dose- and glucose-dependent manner. Beta cells contain ghrelin receptors. CART fragments did not affect the insulin secretion. Ghrelin may play a physiological role for the regulation of insulin secretion.

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.

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.

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 activates signaling pathways in 3T3-L1 adipocytes

Energy homeostasis is regulated by peripheral signals, such as leptin, and by several orexigenic and anorectic neuropeptides. Recently, we reported that the orexigenic neuropeptide melanin-concentrating hormone (MCH) stimulates leptin production by rat adipocytes and that the MCH receptor (MCH-R1) is present on these cells. Here, we show that MCH-R1 is present on murine 3T3-L1 adipocytes. Treatment of 3T3-L1 adipocytes with 1 micromolar MCH for up to 2 h acutely downregulated MCH-R1, indicating a mechanism of ligand-induced receptor downregulation. Potential signaling pathways mediating MCH-R1 action in adipocytes were investigated. Treatment of 3T3-L1 adipocytes with 1 micromolar MCH rapidly induced a threefold and a fivefold increase in p44/42 MAPK and pp70 S6 kinase activities, respectively. In addition, 3T3-L1 adipocytes transiently transfected with a murine leptin-luciferase promoter construct showed a fourfold and a sixfold increase in leptin promoter-reporter gene expression at 1 h and 4 h, respectively, in response to MCH. Activity decreased to basal levels at 8 h. Furthermore, MCH-stimulated leptin promoter-driven luciferase activity was diminished in the presence of the MAP/ERK kinase inhibitor PD-98059 and in the presence of rapamycin, an inhibitor of pp70 S6 kinase activation. These results provide further evidence for a functional MCH signaling pathway in adipocytes.

Cell and molecular cell biology of melanin-concentrating hormone

Recent advances in the study of melanin-concentrating hormone (MCH) have depended largely on molecular biological techniques. In mammals, which have attracted the most attention, novel findings concern (i) the MCH gene, which can yield several peptides by either posttranslational cleavage or alternative splicing, as well as bidirectional transcription; (ii) the identification of two G protein-coupled MCH receptors in the brain and peripheral tissues; and (iii) the evidence for subpopulations of MCH neurons in the central nervous system, characterized by their chemical phenotypes, connections, and individual physiological responses to different physiological paradigms. The involvement of central MCH in various functions, including feeding, reproduction, stress, and behavior patterns, is reviewed. The stage during evolution at which MCH may have acquired hypophysiotrophic and hormonal functions in lower vertebrates is considered in light of morphological data. Evidence that MCH also has peripheral paracrine/autocrine effects in mammals is provided.

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.

Expression of receptors for melanin-concentrating hormone (MCH) in different tissues and cell lines

Melanin-concentrating hormone (MCH) is a potent orexigenic neuropeptide and a physiological antagonist of alpha-melanocyte-stimulating hormone (alpha-MSH) in the brain as well as at peripheral sites, including the pigmentary systems of specific vertebrates. Two receptor subtypes for MCH, MCH-R1 and MCH-R2, have been cloned, but other receptor subtypes are likely to exist. Based on our own data and the current literature, we have compared the expression of different receptors for MCH in various mammalian cell lines and tissues. Summarizing all data currently available, we conclude that the two cloned MCH receptors, MCH-R1 and MCH-R2, exhibit differences in their expression pattern, although MCH-R1 is generally colocalized in all tissues where MCH-R2 expression is found. It appears that MCH-R1 is more abundant and has a wider distribution pattern than MCH-R2. Other hypothetical MCH-R subtypes may be expressed in specific tissues, e.g., in the pigment cell system.

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.

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.

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.