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

Melanin-concentrating hormone regulation by estradiol and progesterone in the incerto-hypothalamic area

Melanin-concentrating hormone (MCH) is a peptide related to the reproductive function by interacting with the hypothalamus-pituitary-gonadal axis. In addition to the MCH central production, it is also found in the blood with a putative role as a neurohormone. Thereby, our focus is on steroid hormones' role in regulating centrally produced MCH in the incerto-hypothalamic area (IHy) and the peripheral MCH in the serum. For this, we investigated the effect of estradiol and/or progesterone injection on the number of MCH immunoreactive (MCH-ir) neurons at the IHy and serum levels. For further study of the role of progesterone, we analyzed the effect of blockade of progesterone receptors by its antagonist on MCH-ir neurons at the IHy and serum. To identify whether such regulation over MCH is established before sexual maturation, we assessed the effect of peripubertal removal of steroid hormones on MCH-ir neurons at the IHy and serum levels at adult age. Our results show that injecting estradiol in ovariectomized female rats reduces the number of MCH-ir neurons in the IHy, in addition to its serum levels. Blockade of progesterone receptors in intact females increases the number of MCH-ir neurons in the IHy and its serum concentration. The regulation of these hormones over the MCH peptidergic system is established before sexual maturation, once the peripubertal removal of the ovaries changes the serum levels of MCH and the number of MCH-ir neurons in the IHy of adult females. Such results support the inhibitory role of steroid hormones over the MCH system.

Melanin-concentrating hormone and food intake control: Sites of action, peptide interactions, and appetition

Given the increased prevalence of obesity and its associated comorbidities, understanding the mechanisms through which the brain regulates energy balance is of critical importance. The neuropeptide melanin-concentrating hormone (MCH) is produced in the lateral hypothalamic area and the adjacent incerto-hypothalamic area and promotes both food intake and energy conservation, overall contributing to body weight gain. Decades of research into this system has provided insight into the neural pathways and mechanisms (behavioral and neurobiological) through which MCH stimulates food intake. Recent technological advancements that allow for selective manipulation of MCH neuron activity have elucidated novel mechanisms of action for the hyperphagic effects of MCH, implicating neural "volume" transmission in the cerebrospinal fluid and sex-specific effects of MCH on food intake control as understudied areas for future investigation. Highlighted here are historical and recent findings that illuminate the neurobiological mechanisms through which MCH promotes food intake, including the identification of various specific neural signaling pathways and interactions with other peptide systems. We conclude with a framework that the hyperphagic effects of MCH signaling are predominantly mediated through enhancement of an "appetition" process in which early postoral prandial signals promote further caloric consumption.

Concentration-dependent activation of dopamine receptors differentially modulates GABA release onto orexin neurons

Dopamine (DA) and orexin neurons play important roles in reward and food intake. There are anatomical and functional connections between these two cell groups: orexin peptides stimulate DA neurons in the ventral tegmental area and DA inhibits orexin neurons in the hypothalamus. However, the cellular mechanisms underlying the action of DA on orexin neurons remain incompletely understood. Therefore, the effect of DA on inhibitory transmission to orexin neurons was investigated in rat brain slices using the whole-cell patch-clamp technique. We found that DA modulated the frequency of spontaneous and miniature IPSCs (mIPSCs) in a concentration-dependent bidirectional manner. Low (1 μM) and high (100 μM) concentrations of DA decreased and increased IPSC frequency, respectively. These effects did not accompany a change in mIPSC amplitude and persisted in the presence of G-protein signaling inhibitor GDPβS in the pipette, suggesting that DA acts presynaptically. The decrease in mIPSC frequency was mediated by D2 receptors whereas the increase required co-activation of D1 and D2 receptors and subsequent activation of phospholipase C. In summary, our results suggest that DA has complex effects on GABAergic transmission to orexin neurons, involving cooperation of multiple receptor subtypes. The direction of dopaminergic influence on orexin neurons is dependent on the level of DA in the hypothalamus. At low levels DA disinhibits orexin neurons whereas at high levels it facilitates GABA release, which may act as negative feedback to curb the excitatory orexinergic output to DA neurons. These mechanisms may have implications for consummatory and motivated behaviours.

Changes in mRNA expression of arcuate nucleus appetite-regulating peptides during lactation in rats

The contribution of hypothalamic appetite-regulating peptides to further hyperphagia accompanying the course of lactation in rats was investigated by using PCR array and real-time PCR. Furthermore, changes in the mRNA expression for appetite-regulating peptides in the hypothalamic arcuate nucleus (ARC) were analyzed at all stages of pregnancy and lactation, and also after weaning. Food intake was significantly higher during pregnancy, lactation, and after weaning than during non-lactation periods. During lactation, ARC expression of mRNAs for agouti-related protein (AgRP) and peptide YY was increased, whereas that of mRNAs for proopiomelanocortin (POMC) and cholecystokinin (CCK) was decreased, in comparison with non-lactation periods. The increase in AgRP mRNA expression during lactation was especially marked. The plasma level of leptin was significantly decreased during the course of lactation, whereas that of acyl-ghrelin was unchanged. In addition, food intake was negatively correlated with the plasma leptin level during lactation. This study has clarified synchronous changes in the expression of many appetite-regulating peptides in ARC of rats during lactation. Our results suggest that hyperphagia during lactation in rats is caused by decreases in POMC and CCK expression and increases in AgRP expression in ARC, the latter being most notable. Together with the decrease in the blood leptin level, such changes in mRNA expression may explain the further hyperphagia accompanying the course of lactation.

MCHergic projections to the nucleus pontis oralis participate in the control of active (REM) sleep

Neurons that utilize melanin-concentrating hormone (MCH) as a neuromodulator are located in the lateral hypothalamus and incerto-hypothalamic area and project diffusely throughout the central nervous system, including areas that participate in the generation and maintenance of sleep and wakefulness. Recent studies have shown that hypothalamic MCHergic neurons are active during active sleep (AS), and that intraventricular microinjections of MCH induce AS sleep; however, there are no data available regarding the manner in which MCHergic neurons participate in the control of this behavioral state. Utilizing immunohistochemical and retrograde tracing techniques, we examined, in the cat, projections from MCHergic neurons to the nucleus pontis oralis (NPO), which is considered to be the executive area that is responsible for the generation and maintenance of AS. In addition, we explored the effects on sleep and waking states produced by the microinjection of MCH into the NPO. We first determined that MCHergic fibers and terminals are present in the NPO. We also found that when a retrograde tracer (cholera toxin subunit B) was placed in the NPO MCHergic neurons of the hypothalamus were labeled. When MCH was microinjected into the NPO, there was a significant increase in the amount of AS (19.8+/-1.4% versus 11.9+/-0.2%, P<0.05) and a significant decrease in the latency to AS (10.4+/-4.2 versus 26.6+/-2.3 min, P<0.05). The preceding anatomical and functional data support our hypothesis that the MCHergic system participates in the regulation of AS by modulating neuronal activity in the NPO.

Infectobesity: Obesity of Infectious Origin

The rapid increase in obesity and the associated health care costs have prompted a search for better approaches for its prevention and management. Such efforts may be facilitated by better understanding the etiology of obesity. Of the several etiological factors, infection, an unusual causative factor, has recently started receiving greater attention. In the last two decades, 10 adipogenic pathogens were reported, including human and nonhuman viruses, scrapie agents, bacteria, and gut microflora. Some of these pathogens are associated with human obesity, but their causative role in human obesity has not been established. This chapter presents information about the natural hosts, signs and symptoms, and pathogenesis of the adipogenic microorganisms. If relevant to humans, "Infectobesity" would be a relatively novel, yet extremely significant concept. A new perspective about the infectious etiology of obesity may stimulate additional research to assess the contribution of hitherto unknown pathogens to human obesity and possibly to prevent or treat obesity of infectious origins.

Innervation of the paraventricular nucleus of the thalamus from cocaine- and amphetamine-regulated transcript (CART) containing neurons of the hypothalamus

The paraventricular nucleus of the thalamus (PVT) is a midline thalamic nucleus with heavy projections to the nucleus accumbens and other limbic regions. Previous studies have shown that the PVT contains fibers immunoreactive for cocaine- and amphetamine-related transcript (CART). The purpose of the present study was to determine the location of CART neurons innervating the PVT of the rat by using retrograde tracing with cholera toxin B (CTb) combined with immunofluorescence for CTb and CART (amino acid sequence 55-102). Immunohistochemical analysis of CART in the dorsal thalamus showed that the PVT is densely innervated by CART fibers whereas adjacent midline and intralaminar thalamic nuclei are unlabeled. Injections of CTb in the dorsal midline thalamus retrogradely labeled neurons in several areas of the hypothalamus and brainstem which also contained CART neurons. The largest number of double-labeled neurons (CTb/CART) was found in the arcuate nucleus of the hypothalamus. CTb/CART neurons were also found in the lateral hypothalamus, zona incerta, and periventricular hypothalamus. These results indicate that the arcuate nucleus is a major source of CART fibers in the PVT. CART neurons in the arcuate nucleus monitor circulating hormonal signals and may regulate food intake and hypothalamic-pituitary-adrenal (HPA) activity. Consequently, CART neurons in the arcuate nucleus may transmit signals to the PVT which in turn may influence limbic regions involved in regulating food intake and the HPA.

Emerging drugs for obesity: linking novel biological mechanisms to pharmaceutical pipelines

Obesity is associated with hypertension, diabetes, dyslipidaemias and metabolic syndrome, and causes substantial morbidity and mortality from cardiovascular and other diseases. The cost to treat obesity and its complications in the US has increased steeply and is currently estimated to be USD 100 billion. Current therapy for obesity is mainly based on changes in lifestyle that often fail. Existing pharmacological treatment is marginally efficient and poorly tolerated. The discovery of leptin and related neural mechanisms of energy metabolism regulation has opened the doors to potential targets for new antiobesity drugs. In this review, new pharmacological targets are discussed and an update on the development of emerging antiobesity drugs is provided. Despite intense investigation, the pipelines for antiobesity drugs in late stages of development are relatively empty. Breakthrough treatments for obesity may take some years to emerge. Clinical trials will be necessary to clarify the impact of new antiobesity drugs on hard cardiovascular and metabolic end points.

Characterization of a neuronal cell line expressing native human melanin-concentrating hormone receptor 1 (MCHR1)

Melanin-concentrating hormone (MCH), an orexigenic neuropeptide in mammals, activates a G-protein coupled receptor, MCHR1. It is expected that antagonists of MCHR1 function will prove therapeutically useful as anti-obesity agents. Intracellular signaling by MCHR1 has been investigated primarily using non-neural cell lines expressing the recombinant receptor, in which MCHR1 has been shown to couple to G alpha(i/o) and G alpha(q) G-proteins. While these cell lines have been widely utilized to discover and optimize small molecule antagonists, it is unknown whether the intracellular signaling pathways in these cells accurately reflect those in neurons. Thus, we sought to develop a neurally derived cell line endogenously expressing MCHR1. IMR32, a human neuroblastoma cell line, has been shown to express MCHR1 mRNA; however, we were unable to detect either MCH-binding or MCH-stimulated Ca++-mobilization in these cells. Following transfection of IMR32 cells with a plasmid encoding human G alpha(16) G-protein, we isolated a cell line, I3.4.2, which responded to MCH in Ca++-mobilization assays. We found that the expression level of MCHR1 mRNA in I3.4.2 cells was 2000-fold higher than in the parent cell line. Using [125I]MCH saturation-binding to I3.4.2 cell membranes, we estimated the Bmax as 0.72 pmol/mg protein and the Kd as 0.35 nM. We report that Ca++-mobilization in I3.4.2 cells was insensitive to pertussis toxin (Ptx) treatment, indicating that signaling was via G alpha(q) G-proteins. Furthermore, negative results in cAMP accumulation assays confirmed the lack of signaling via the G alpha(i/o) G-proteins. Our results suggest that the I3.4.2 cell line may be useful for characterization of MCHR1 activity in a neural-derived cell line.

6-Acylamino-2-amino-4-methylquinolines as potent melanin-concentrating hormone 1 receptor antagonists: Structure–activity exploration of eastern and western parts

SAR explorations of the eastern and western parts of recently disclosed 2-aminoquinoline MCH1R-antagonists are reported. Eastern part investigations confirmed a high degree of structural freedom, and a number of additional single digit nanomolar antagonists were identified. Investigations of the western part also confirmed the initial SAR analysis, requiring a para-substituted phenyl ring spaced from the 6-amide by two connecting atoms. The exploration led to the discovery of a novel sub-series with a 4-biphenylcarboxamide western part, also exhibiting single digit nanomolar affinity.

Origins of lateral hypothalamic afferents associated with N-methyl-d-aspartic acid-elicited eating studied using reverse microdialysis of NMDA and Fluorogold

Afferent projections to the tuberal lateral hypothalamus (tLH), where excitatory amino acid application is most effective in eliciting feeding, and to the anterior, posterior and medial regions of the hypothalamus were studied using reverse microdialysis of N-methyl-D-aspartic acid (NMDA) and Fluorogold (FG). NMDA at 660 microM delivered for 10 min was effective in stimulating food intake only when administered into the tLH, causing a mean intake of 9.3 g compared to less than 1 g in any other site. Subsequent administration of FG through the dialysis probe retrogradely in labeled neurons in brain structures associated with the feeding response including the frontal cortex, amygdala, nucleus accumbens (NA), preoptic areas, substantia nigra, ventral tegmental area (VTA), parabrachial nucleus, and the nucleus of the solitary tract (NST). Labeling after anterior and posterior LH infusion of FG was similar to that seen after tLH delivery with some apparent differences, whereas FG administration into the medial hypothalamus produced a distinctly different pattern of labeling compared to the other groups. Some of the observed labeling appeared to be almost exclusively associated with the tLH where NMDA elicits feeding. In particular, amygdala, preoptic area and shell of the accumbens labeling was noticeably denser in tLH eaters than in all other groups. These findings are consistent with the role of LH glutamate and NMDA receptors in the regulation of food intake and identify afferents to the region which possibly mediate endogenous LH glutamate's effects on feeding.

?Blood-contacting neurons? in the brain of the Japanese eelAnguilla japonica

To discriminate "blood-contacting neurons" within the brain of the eel, Evans blue (EB) was injected intraperitoneally. After five days, six brain areas were externally stained blue with the dye; the saccus dorsalis (SD), the epiphysis (E), the area postrema (AP), the posterior part of the magnocellular preoptic nucleus (PM), the pituitary (Pit), and the saccus vasculosus (SV). Among the EB-positive area, some cells in the PM, the anterior tuberal nucleus (NAT) and the AP were discriminated as the "blood-contacting neurons" histologically, whereas EB-positive neurons were not detected in the SD, the E, the Pit and the SV regions. In the PM, most EB-positive neurons (90 %) were immunoreactive to vasotocin (AVT) antibody, indicating that these neurons are vasotocinergic. The remaining EB-positive neurons (10 %) were not immunoreactive to ANG II and tyrosine hydroxylase (TH) antibodies. Although some neurons in the PM were immunoreactive to ANG II antibody, they were EB-negative. In contrast, almost all EB-positive neurons in the AP showed TH-like immunoreactivity (-lir), indicating that these neurons utilize catecholamine(s) as a neurotransmitter. The EB-positive neurons in the NAT were not immunoreactive to AVT, ANG II and TH antibodies, whereas some neurons without EB-staining showed ANG II-lir. Possible roles of these neurons in regulating drinking behavior in eels are discussed.

4-Acylamino-and 4-ureidobenzamides as melanin-concentrating hormone (MCH) receptor 1 antagonists

Synthesis, in vitro biological evaluation and structure-activity relationships of 4-acylamino-and 4-ureidobenzamides as novel hMCH1R-antagonists are disclosed. The nature of the amine side chains could be varied considerably in contrast to the central benzamide scaffold and aromatic substituents.

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.

Melanin-concentrating hormone signaling systems in fish

Melanin-concentrating hormone (MCH) is a cyclic neuropeptide synthesized as a preprohormone in the hypothalamus of all vertebrates. This neuropeptide binds to G-protein-coupled seven transmembrane receptor(s) to mediate its function. MCH was named after its function in teleosts, in which it causes aggregation or concentration of melanin granules in melanophores, thus regulating body color. The function of central MCH that has attracted most attention is its involvement in regulating food intake and energy homeostasis in mammals, a role confirmed through a series of experiments, including central administration of MCH or MCH receptor blockers, and genetic manipulation of MCH and its receptors. The aim of this article is to review the recent data on MCH and MCH receptor signaling systems in fish.

Hypothalamic control of energy balance: different peptides, different functions

Energy balance is maintained via a homeostatic system involving both the brain and the periphery. A key component of this system is the hypothalamus. Over the past two decades, major advances have been made in identifying an increasing number of peptides within the hypothalamus that contribute to the process of energy homeostasis. Under stable conditions, equilibrium exists between anabolic peptides that stimulate feeding behavior, as well as decrease energy expenditure and lipid utilization in favor of fat storage, and catabolic peptides that attenuate food intake, while stimulating sympathetic nervous system (SNS) activity and restricting fat deposition by increasing lipid metabolism. The equilibrium between these neuropeptides is dynamic in nature. It shifts across the day-night cycle and from day to day and also in response to dietary challenges as well as peripheral energy stores. These shifts occur in close relation to circulating levels of the hormones, leptin, insulin, ghrelin and corticosterone, and also the nutrients, glucose and lipids. These circulating factors together with neural processes are primary signals relaying information regarding the availability of fuels needed for current cellular demand, in addition to the level of stored fuels needed for long-term use. Together, these signals have profound impact on the expression and production of neuropeptides that, in turn, initiate the appropriate anabolic or catabolic responses for restoring equilibrium. In this review, we summarize the evidence obtained on nine peptides in the hypothalamus that have emerged as key players in this process. Data from behavioral, physiological, pharmacological and genetic studies are described and consolidated in an attempt to formulate a clear statement on the underlying function of each of these peptides and also on how they work together to create and maintain energy homeostasis.

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.

Perspectives: molecular genetic research in human obesity

Within the past decade the molecular basis of single forms of monogenic obesity has been elucidated. With the exception of functionally relevant mutations in the melanocortin-4 receptor gene, which occur in approximately 2-4% of extremely obese individuals, all other currently known monogenic forms are rare and additionally associated with distinct endocrinological abnormalities. A large number of association studies have been performed in 'normal' obesity. Whereas many associations have been reported, it is largely unclear which of these represent true positive findings. Over 20 genome scans pertaining to obesity and related phenotypes have been performed; specific chromosomal peak regions have been identified in different scans. We review the current status and discuss relevant issues related to phenotyping, association and linkage studies. We recommend that the procedure via which a consensus is reached as to what constitutes a true positive association finding requires formalization.