Localisation of melanin-concentrating hormone receptor 1 in rat brain and evidence that sleep parameters are not altered despite high central receptor occupancy

Orexin and MCH neurons: regulators of sleep and metabolism

Sleep-wake and fasting-feeding are tightly coupled behavioral states that require coordination between several brain regions. The mammalian lateral hypothalamus (LH) is a functionally and anatomically complex brain region harboring heterogeneous cell populations that regulate sleep, feeding, and energy metabolism. Significant attempts were made to understand the cellular and circuit bases of LH actions. Rapid advancements in genetic and electrophysiological manipulation help to understand the role of discrete LH cell populations. The opposing action of LH orexin/hypocretin and melanin-concentrating hormone (MCH) neurons on metabolic sensing and sleep-wake regulation make them the candidate to explore in detail. This review surveys the molecular, genetic, and neuronal components of orexin and MCH signaling in the regulation of sleep and metabolism.

The melanin-concentrating hormone system as a target for the treatment of sleep disorders

Given the widespread prevalence of sleep disorders and their impacts on health, it is critical that researchers continue to identify and evaluate novel avenues of treatment. Recently the melanin-concentrating hormone (MCH) system has attracted commercial and scientific interest as a potential target of pharmacotherapy for sleep disorders. This interest emerges from basic scientific research demonstrating a role for MCH in regulating sleep, and particularly REM sleep. In addition to this role in sleep regulation, the MCH system and the MCH receptor 1 (MCHR1) have been implicated in a wide variety of other physiological functions and behaviors, including feeding/metabolism, reward, anxiety, depression, and learning. The basic research literature on sleep and the MCH system, and the history of MCH drug development, provide cause for both skepticism and cautious optimism about the prospects of MCH-targeting drugs in sleep disorders. Extensive efforts have focused on developing MCHR1 antagonists for use in obesity, however, few of these drugs have advanced to clinical trials, and none have gained regulatory approval. Additional basic research will be needed to fully characterize the MCH system’s role in sleep regulation, for example, to fully differentiate between MCH-neuron and peptide/receptor-mediated functions. Additionally, a number of issues relating to drug design will continue to pose a practical challenge for novel pharmacotherapies targeting the MCH system.

Centrally Projecting Edinger-Westphal Nucleus in the Control of Sympathetic Outflow and Energy Homeostasis

The centrally projecting Edinger-Westphal nucleus (EWcp) is a midbrain neuronal group, adjacent but segregated from the preganglionic Edinger-Westphal nucleus that projects to the ciliary ganglion. The EWcp plays a crucial role in stress responses and in maintaining energy homeostasis under conditions that require an adjustment of energy expenditure, by virtue of modulating heart rate and blood pressure, thermogenesis, food intake, and fat and glucose metabolism. This modulation is ultimately mediated by changes in the sympathetic outflow to several effector organs, including the adrenal gland, heart, kidneys, brown and white adipose tissues and pancreas, in response to environmental conditions and the animal's energy state, providing for appropriate energy utilization. Classic neuroanatomical studies have shown that the EWcp receives inputs from forebrain regions involved in these functions and projects to presympathetic neuronal populations in the brainstem. Transneuronal tracing with pseudorabies virus has demonstrated that the EWcp is connected polysynaptically with central circuits that provide sympathetic innervation to all these effector organs that are critical for stress responses and energy homeostasis. We propose that EWcp integrates multimodal signals (stress, thermal, metabolic, endocrine, etc.) and modulates the sympathetic output simultaneously to multiple effector organs to maintain energy homeostasis under different conditions that require adjustments of energy demands.

Sleep-Wake Control by Melanin-Concentrating Hormone (MCH) Neurons: a Review of Recent Findings

PURPOSE OF THE REVIEW

Melanin-concentrating hormone (MCH)-expressing neurons located in the lateral hypothalamus are considered as an integral component of sleep-wake circuitry. However, the precise role of MCH neurons in sleep-wake regulation has remained unclear, despite several years of research employing a wide range of techniques. We review recent data on this aspect, which are mostly inconsistent, and propose a novel role for MCH neurons in sleep regulation.

RECENT FINDINGS

While almost all studies using "gain-of-function" approaches show an increase in rapid eye movement sleep (or paradoxical sleep; PS), loss-of-function approaches have not shown reductions in PS. Similarly, the reported changes in wakefulness or non-rapid eye movement sleep (slow-wave sleep; SWS) with manipulation of the MCH system using conditional genetic methods are inconsistent. Currently available data do not support a role for MCH neurons in spontaneous sleep-wake but imply a crucial role for them in orchestrating sleep-wake responses to changes in external and internal environments.

Melanin-concentrating hormone neurons contribute to dysregulation of rapid eye movement sleep in narcolepsy

The lateral hypothalamus contains neurons producing orexins that promote wakefulness and suppress REM sleep as well as neurons producing melanin-concentrating hormone (MCH) that likely promote REM sleep. Narcolepsy with cataplexy is caused by selective loss of the orexin neurons, and the MCH neurons appear unaffected. As the orexin and MCH systems exert opposing effects on REM sleep, we hypothesized that imbalance in this REM sleep-regulating system due to activity in the MCH neurons may contribute to the striking REM sleep dysfunction characteristic of narcolepsy. To test this hypothesis, we chemogenetically activated the MCH neurons and pharmacologically blocked MCH signaling in a murine model of narcolepsy and studied the effects on sleep-wake behavior and cataplexy. To chemoactivate MCH neurons, we injected an adeno-associated viral vector containing the hM3Dq stimulatory DREADD into the lateral hypothalamus of orexin null mice that also express Cre recombinase in the MCH neurons (MCH-Cre::OX-KO mice) and into control MCH-Cre mice with normal orexin expression. In both lines of mice, activation of MCH neurons by clozapine-N-oxide (CNO) increased rapid eye movement (REM) sleep without altering other states. In mice lacking orexins, activation of the MCH neurons also increased abnormal intrusions of REM sleep manifest as cataplexy and short latency transitions into REM sleep (SLREM). Conversely, a MCH receptor 1 antagonist, SNAP 94847, almost completely eliminated SLREM and cataplexy in OX-KO mice. These findings affirm that MCH neurons promote REM sleep under normal circumstances, and their activity in mice lacking orexins likely triggers abnormal intrusions of REM sleep into non-REM sleep and wake, resulting in the SLREM and cataplexy characteristic of narcolepsy.

A novel and selective melanin-concentrating hormone receptor 1 antagonist ameliorates obesity and hepatic steatosis in diet-induced obese rodent models

Melanin-concentrating hormone (MCH), a cyclic neuropeptide expressed predominantly in the lateral hypothalamus, plays an important role in the control of feeding behavior and energy homeostasis. Mice lacking MCH or MCH₁ receptor are resistant to diet-induced obesity (DIO) and MCH₁ receptor antagonists show potent anti-obesity effects in preclinical studies, indicating that MCH₁ receptor is a promising target for anti-obesity drugs. Moreover, recent studies have suggested the potential of MCH₁ receptor antagonists for treatment of non-alcoholic fatty liver disease (NAFLD). In the present study, we show the anti-obesity and anti-hepatosteatosis effect of our novel MCH₁ receptor antagonist, Compound A. Repeated oral administration of Compound A resulted in dose-dependent body weight reduction and had an anorectic effect in DIO mice. The body weight lowering effect of Compound A was more potent than that of pair-feeding. Compound A also reduced lipid content and the expression level of lipogenesis-, inflammation-, and fibrosis-related genes in the liver of DIO mice. Conversely, intracerebroventricular infusion of MCH caused induction of hepatic steatosis as well as increase in body weight in high-fat diet-fed wild type mice, but not MCH₁ receptor knockout mice. The pair-feeding study revealed the MCH-MCH₁ receptor system affects hepatic steatosis through a mechanism that is independent of body weight change. Metabolome analysis demonstrated that Compound A upregulated lipid metabolism-related molecules, such as acylcarnitines and cardiolipins, in the liver. These findings suggest that our novel MCH₁ receptor antagonist, Compound A, exerts its beneficial therapeutic effect on NAFLD and obesity through a central MCH-MCH₁ receptor pathway.

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.

The role of melanin-concentrating hormone and its receptors in energy homeostasis

Extensive studies in rodents with melanin-concentrating hormone (MCH) have demonstrated that the neuropeptide hormone is a potent orexigen. Acutely, MCH causes an increase in food intake, while chronically it leads to increased weight gain, primarily as an increase in fat mass. Multiple knockout mice models have confirmed the importance of MCH in modulating energy homeostasis. Animals lacking MCH, MCH-containing neurons, or the MCH receptor all are resistant to diet-induced obesity. These genetic and pharmacologic studies have prompted a large effort to identify potent and selective MCH receptor antagonists, initially as tool compounds to probe pharmacology in models of obesity, with an ultimate goal to identify novel anti-obesity drugs. In animal models, MCH antagonists have consistently shown efficacy in reducing food intake acutely and inhibiting body-weight gain when given chronically. Five compounds have proceeded into clinical testing. Although they were reported as well-tolerated, none has advanced to long-term efficacy and safety studies.

Association between the activation of MCH and orexin immunorective neurons and REM sleep architecture during REM rebound after a three day long REM deprivation

Rapid eye movement (REM) sleep rebound following REM deprivation using the platform-on-water method is characterized by increased time spent in REM sleep and activation of melanin-concentrating hormone (MCH) expressing neurons. Orexinergic neurons discharge reciprocally to MCH-ergic neurons across the sleep-wake cycle. However, the relation between REM architecture and the aforementioned neuropeptides remained unclear. MCH-ergic neurons can be divided into two subpopulations regarding their cocaine- and amphetamine-regulated transcript (CART) immunoreactivity, and among them the activation of CART-immunoreactive subpopulation is higher during the REM rebound. However, the possible role of stress in this association has not been elucidated. Our aims were to analyze the relationship between the architecture of REM rebound and the activation of hypothalamic MCH-ergic and orexinergic neurons. We also intended to separate the effect of stress and REM deprivation on the subsequent activation of subpopulations of MCH-ergic neurons. In order to detect neuronal activity, we performed MCH/cFos and orexin/cFos double immunohistochemistry on home cage, sleep deprived and sleep-rebound rats using the platform-on-water method with small and large (stress control) platforms. Furthermore, REM architecture was analyzed and a triple MCH/CART/cFos immunohistochemistry was also performed on the rebound groups in the same animals. We found that the activity of MCH- and orexin-immunoreactive neurons during REM rebound was positively and negatively correlated with the number of REM bouts, respectively. A negative reciprocal correlation was also found between the activation of MCH- and orexin-immunoreactive neurons during REM rebound. Furthermore, difference between the activation of CART-immunoreactive (CART-IR) and non-CART-immunoreactive MCH-ergic neuron subpopulations was found only after selective REM deprivation, it was absent in the large platform (stress control) rebound group. These results support the role of CART-IR subpopulation of MCH-ergic neurons and the inverse relationship of MCH and orexin in the regulation of REM sleep after REM sleep deprivation.

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.