Distribution of melanin-concentrating hormone (MCH)-like immunoreactivity in neurons of the diencephalon of sheep

The Melanin-Concentrating Hormone (MCH) System: A Tale of Two Peptides

The melanin-concentrating hormone (MCH) system is a robust integrator of exogenous and endogenous information, modulating arousal and energy balance in mammals. Its predominant function in teleosts, however, is to concentrate melanin in the scales, contributing to the adaptive color change observed in several teleost species. These contrasting functions resulted from a gene duplication that occurred after the teleost divergence, which resulted in the generation of two MCH-coding genes in this clade, which acquired distinctive sequences, distribution, and functions, examined in detail here. We also describe the distribution of MCH immunoreactivity and gene expression in a large number of species, in an attempt to identify its core elements. While initially originated as a periventricular peptide, with an intimate relationship with the third ventricle, multiple events of lateral migration occurred during evolution, making the ventrolateral and dorsolateral hypothalamus the predominant sites of MCH in teleosts and mammals, respectively. Substantial differences between species can be identified, likely reflecting differences in habitat and behavior. This observation aligns well with the idea that MCH is a major integrator of internal and external information, ensuring an appropriate response to ensure the organism’s homeostasis. New studies on the MCH system in species that have not yet been investigated will help us understand more precisely how these habitat changes are connected to the hypothalamic neurochemical circuits, paving the way to new intervention strategies that may be used with pharmacological purposes.

Melanin-concentrating hormone peptidergic system: Comparative morphology between muroid species

Melanin-concentrating hormone (MCH) is a conserved neuropeptide, predominantly located in the diencephalon of vertebrates, and associated with a wide range of functions. While functional studies have focused on the use of the traditional mouse laboratory model, critical gaps exist in our understanding of the morphology of the MCH system in this species. Even less is known about the nontraditional animal model Neotomodon alstoni (Mexican volcano mouse). A comparative morphological study among these rodents may, therefore, contribute to a better understanding of the evolution of the MCH peptidergic system. To this end, we employed diverse immunohistochemical protocols to identify key aspects of the MCH system, including its spatial relationship to another neurochemical population of the tuberal hypothalamus, the orexins. Three-dimensional (3D) reconstructions were also employed to convey a better sense of spatial distribution to these neurons. Our results show that the distribution of MCH neurons in all rodents studied follows a basic plan, but individual characteristics are found for each species, such as the preeminence of a periventricular group only in the rat, the lack of posterior groups in the mouse, and the extensive presence of MCH neurons in the anterior hypothalamic area of Neotomodon. Taken together, these data suggest a strong anatomical substrate for previously described functions of the MCH system, and that particular neurochemical and morphological features may have been determinant to species-specific phenotypes in rodent evolution.

The Melanin-Concentrating Hormone as an Integrative Peptide Driving Motivated Behaviors

The melanin-concentrating hormone (MCH) is an important peptide implicated in the control of motivated behaviors. History, however, made this peptide first known for its participation in the control of skin pigmentation, from which its name derives. In addition to this peripheral role, MCH is strongly implicated in motivated behaviors, such as feeding, drinking, mating and, more recently, maternal behavior. It is suggested that MCH acts as an integrative peptide, converging sensory information and contributing to a general arousal of the organism. In this review, we will discuss the various aspects of energy homeostasis to which MCH has been associated to, focusing on the different inputs that feed the MCH peptidergic system with information regarding the homeostatic status of the organism and the exogenous sensory information that drives this system, as well as the outputs that allow MCH to act over a wide range of homeostatic and behavioral controls, highlighting the available morphological and hodological aspects that underlie these integrative actions. Besides the well-described role of MCH in feeding behavior, a prime example of hypothalamic-mediated integration, we will also examine those functions in which the participation of MCH has not yet been extensively characterized, including sexual, maternal, and defensive behaviors. We also evaluated the available data on the distribution of MCH and its function in the context of animals in their natural environment. Finally, we briefly comment on the evidence for MCH acting as a coordinator between different modalities of motivated behaviors, highlighting the most pressing open questions that are open for investigations and that could provide us with important insights about hypothalamic-dependent homeostatic integration.

The MCH neuron population as a model for the development and evolution of the lateral and dorsal hypothalamus

The LHA contains neurons producing melanin-concentrating hormone (MCH) or hypocretin (Hcrt) that have emerged as being more conspicuous and representative of the posterior LHA. In this review, we focus on MCH neurons and show that they have unique qualities. Their distribution is conserved in the posterior hypothalamus of all vertebrates and their ontogenetic differentiation is very precocious in the rodent embryo. In mammals, interspecific differences in their medio-lateral distribution suggest that the LHA differentiation may follow species specific strategies. These characteristics make a very valuable tool of MCH neurons to study the development as well as the phylogenetical origin and differentiation of the LHA.

Lateral hypothalamic orexin and melanin-concentrating hormone neurons provide direct input to gonadotropin-releasing hormone neurons in the human

Hypophysiotropic projections of gonadotropin-releasing hormone (GnRH)-synthesizing neurons form the final common output way of the hypothalamus in the neuroendocrine control of reproduction. Several peptidergic neuronal systems of the medial hypothalamus innervate human GnRH cells and mediate crucially important hormonal and metabolic signals to the reproductive axis, whereas much less is known about the contribution of the lateral hypothalamic area to the afferent control of human GnRH neurons. Orexin (ORX)- and melanin-concentrating hormone (MCH)-synthesizing neurons of this region have been implicated in diverse behavioral and autonomic processes, including sleep and wakefulness, feeding and other functions. In the present immunohistochemical study, we addressed the anatomical connectivity of these neurons to human GnRH cells in post-mortem hypothalamic samples obtained from autopsies. We found that 38.9 ± 10.3% and 17.7 ± 3.3% of GnRH-immunoreactive (IR) perikarya in the infundibular nucleus of human male subjects received ORX-IR and MCH-IR contacts, respectively. On average, each 1 mm segment of GnRH dendrites received 7.3 ± 1.1 ORX-IR and 3.7 ± 0.5 MCH-IR axo-dendritic appositions. Overall, the axo-dendritic contacts dominated over the axo-somatic contacts and represented 80.5 ± 6.4% of ORX-IR and 76.7 ± 4.6% of MCH-IR inputs to GnRH cells. Based on functional evidence from studies of laboratory animals, the direct axo-somatic and axo-dendritic input from ORX and MCH neurons to the human GnRH neuronal system may convey critical metabolic and other homeostatic signals to the reproducive axis. In this study, we also report the generation and characterization of new antibodies for immunohistochemical detection of GnRH neurons in histological sections.

Different distributions of preproMCH and hypocretin/orexin in the forebrain of the pig (Sus scrofa domesticus)

Neurons producing melanin-concentrating hormone (MCH) or hypocretin/orexin (Hcrt) have been implicated in the sleep/wake cycle and feeding behavior. Sleep and feeding habits vary greatly among mammalian species, depending in part of the prey/predatory status of animals. However, the distribution of both peptides has been described in only a limited number of species. In this work, we describe the distribution of MCH neurons in the brain of the domestic pig. Using in situ hybridization and immunohistochemistry, their cell bodies are shown to be located in the posterior lateral hypothalamic area (LHA), as expected. They form a dense cluster ventro-lateral to the fornix while only scattered cells are present dorsal to this tract. By comparison, Hcrt cell bodies are located mainly dorsal to the fornix. Therefore, the two populations of neurons display complementary distributions in the posterior LHA. MCH projections are, as indicated by MCH-positive axons, very abundant in all cortical fields ventral to the rhinal sulcus, as well as in the lateral, basolateral and basomedial amygdala. In contrast, most of the isocortex is sparsely innervated. To conclude, the distribution of MCH cell bodies and projections shows some very specific features in the pig brain, that are clearly different of that described in the rat, mouse or human. In contrast, the Hcrt pattern seems more similar to that in these species, i.e. more conserved. These results suggest that the LHA anatomic organization shows some very significant interspecies differences, which may be related to the different behavioral repertoires of animals with regard to feeding and sleep/wake cycles.

The vertebrate diencephalic MCH system: a versatile neuronal population in an evolving brain

Neurons synthesizing melanin-concentrating hormone (MCH) are described in the posterior hypothalamus of all vertebrates investigated so far. However, their anatomy is very different according to species: they are small and periventricular in lampreys, cartilaginous fishes or anurans, large and neuroendocrine in bony fishes, or distributed over large regions of the lateral hypothalamus in many mammals. An analysis of their comparative anatomy alongside recent data about the development of the forebrain, suggests that although very different, MCH neurons of the caudal hypothalamus are homologous. We further hypothesize that their divergent anatomy is linked to divergence in the forebrain - in particular telencephalic evolution.

Selected hormonal and neurotransmitter mechanisms regulating feed intake in sheep

Appetite control is a major issue in normal growth and in suboptimal growth performance settings. A number of hormones, in particular leptin, activate or inhibit orexigenic or anorexigenic neurotransmitters within the arcuate nucleus of the hypothalamus, where feed intake regulation is integrated. Examples of appetite regulatory neurotransmitters are the stimulatory neurotransmitters neuropeptide Y (NPY), agouti-related protein (AgRP), orexin and melanin-concentrating hormone and the inhibitory neurotransmitter, melanocyte-stimulating hormone (MSH). Examination of messenger RNA (using in situ hybridization and real-time PCR) and proteins (using immunohistochemistry) for these neurotransmitters in ruminants has indicated that physiological regulation occurs in response to fasting for several of these critical genes and proteins, especially AgRP and NPY. Moreover, intracerebroventricular injection of each of the four stimulatory neurotransmitters can increase feed intake in sheep and may also regulate either growth hormone, luteinizing hormone, cortisol or other hormones. In contrast, both leptin and MSH are inhibitory to feed intake in ruminants. Interestingly, the natural melanocortin-4 receptor (MC4R) antagonist, AgRP, as well as NPY can prevent the inhibition of feed intake after injection of endotoxin (to model disease suppression of appetite). Thus, knowledge of the mechanisms regulating feed intake in the hypothalamus may lead to mechanisms to increase feed intake in normal growing animals and prevent the wasting effects of severe disease in animals.

Melanin concentrating hormone (MCH) in the cerebrospinal fluid of ewes during spontaneous oestrous cycles and ram effect induced follicular phases

The melanin-concentrating hormone (MCH) is a neuropeptide synthesized by neurons of the lateral hypothalamus and incerto-hypothalamic area that project throughout the central nervous system. The aims of the present report were: (1) to determine if MCH levels in cerebrospinal fluid (CSF) of ewes vary between the mid-luteal and the oestrous phase of spontaneous oestrous cycles; and (2) to study if MCH levels in CSF of ewes vary acutely during the follicular phase induced with the ram effect in anoestrous ewes. In the first experiment, CSF was collected from 8 adult ewes during spontaneous oestrous and during the mid-luteal phase (8-10 days after natural oestrus). In the second experiment, performed during the mid non-breeding season, a follicular phase was induced with the ram effect. After isolating a group of 16 ewes from rams, CSF was obtained from 5 of such ewes (control group). Three rams were joined with the ewes, and samples were obtained 12h (n=6) and 24h (n=5) later. In Experiment 1, there were no differences in MCH concentrations in CSF measured during the mid-luteal phase and spontaneous oestrus (0.14 ± 0.04 vs. 0.16 ± 0.05 ng/mL respectively). In Experiment 2, MCH concentrations tended to increase 12h after rams introduction (0.15 ± 0.08 vs. 0.35 ± 0.21 ng/mL, P=0.08), and increased significantly 24h after rams introduction (0.37 ± 0.15 ng/mL, P=0.02). We concluded that MCH concentration measured in the CSF from ewes increased markedly during the response to the ram effect but not during the natural oestrous cycle of ewes.

Triennial Growth Symposium: neural regulation of feed intake: modification by hormones, fasting, and disease

Appetite is a complex process that results from the integration of multiple signals at the hypothalamus. The hypothalamus receives neural signals; hormonal signals such as leptin, cholecystokinin, and ghrelin; and nutrient signals such as glucose, FFA, AA, and VFA. This effect is processed by a specific sequence of neurotransmitters beginning with the arcuate nucleus and orexigenic cells containing neuropeptide Y or agouti-related protein and anorexigenic cells containing proopiomelanocortin (yielding the neurotransmitter α-melanocyte-stimulating hormone) or cells expressing cocaine amphetamine-related transcript. These so-called first-order neurons act on second-order orexigenic neurons (containing either melanin-concentrating hormone or orexin) or act on anorexigenic neurons (e.g., expressing corticotropin-releasing hormone) to alter feed intake. In addition, satiety signals from the liver and gastrointestinal tract signal through the vagus nerve to the nucleus tractus solitarius to cause meal termination, and in combination with the hypothalamus, integrate the various signals to determine the feeding response. The activities of these neuronal pathways are also influenced by numerous factors such as nutrients, fasting, and disease to modify appetite and hence affect growth and reproduction. This review will begin with the central nervous system pathways and then discuss the ways in which hormones and metabolites may alter the process to affect feed intake with emphasis on farm animals.

Intracerebroventricular melanin-concentrating hormone stimulates food intake in sheep

Melanin-concentrating hormone (MCH) stimulates feeding when injected intracerebroventricularly (ICV) in rats. At present it is not clear whether the function of MCH is similar in ruminants, which are species with a continuous delivery of nutrients. Therefore the current investigation sought to determine the role of MCH in sheep. In the first experiment, six, castrate male sheep were satiated and received one of four treatments [saline, 0.1, or 1.0 nmol/kg MCH, and NPY (0.1 nmol/kg)] injected ICV over 30s, then infused ICV for 6 h ( approximately 500 microl/h). Food intake was measured for 2 h before and at 2, 4, 6, 8, 12 and 24 h. In this experiment, feed intake was increased (P<or=0.05) in NPY treated sheep only. In the second experiment, the same sheep were fed to satiety and then randomized to receive one of six treatments [saline and either 0.1, 1.0 or 5.0 nmol/kg MCH, 0.1 nmol/kg NPY, or MCH+NPY (0.1 nmol/kg)] injected ICV over 30 s. Food intake was measured for 2 h before and at 2, 4, 6, 8, 12, and 24 h after ICV injection. All doses of MCH as well as NPY resulted in greater (P<or=0.05) food intake than saline. In order to determine whether MCH expression was regulated by fasting, brains from fed and 3-day fasted sheep were fixed in situ, sectioned in the coronal plane, and subjected to dual-label immunohistochemistry using Fos as a marker for neuronal activity. Nutritional state (fed or fasted) did not alter Fos expression in MCH neurons. Finally, using real time PCR, MCH mRNA was unchanged by fasting. In this study we found bolus ICV MCH to be a potent stimulus to food intake in sheep, but MCH was not regulated by fasting.

Sensitivity of galanin- and melanin-concentrating hormone-containing neurones to nutritional status: an immunohistochemical study in the ovariectomized ewe

The sensitivities of galanin and melanin-concentrating hormone (MCH) neuronal systems to nutrition are poorly understood in sheep compared to rodents. The aim of this study was to describe the changes in the numbers of galanin and MCH neurones in ovariectomized ewes submitted to different nutritional levels. In the first experiment, ewes were fed ad libitum or food deprived for 24 h. In the second experiment, two groups of ewes were fed at maintenance level (group 100) or undernourished (group 40) for 167 days, after which one-half of each group was killed or refed ad libitum (group 100R and 40R) for 4 days. The MCH neuronal population located in the lateral hypothalamic area was not affected by these nutritional changes. Long-term undernutrition enhanced the number of galanin neurones located in the infundibular nucleus and the dorsal hypothalamic area (DHA), refeeding resulted in an increase of neurones in the DHA and preoptic area, but short-term starvation had no effect on any galanin subpopulations. Our data suggest that the sensitivity of MCH neuronal populations to nutrition in sheep differs from that of rodents. Various populations of galanin-containing neurones differ in sensitivity in ewes subjected to long undernutrition and refeeding but not to short starvation.

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.

Immunohistochemical characterization of localization of long-form leptin receptor (OB-Rb) in neurochemically defined cells in the ovine hypothalamus

Leptin, a hormone secreted from the adipose tissue, is involved in the regulation of food intake and neuroendocrine function, by modulation of the expression and/or function of various neuropeptides in the hypothalamus. The long isoform (OB-Rb) is the major signaling form of the leptin receptor in the hypothalamus. We have used double-labeling immunohistochemistry to examine the extent of OB-Rb expression in neurochemically defined cell types in the ovine hypothalamus. OB-Rb-like immunoreactivity was widespread within cells localized to the periventricular, paraventricular, supraoptic, dorsomedial hypothalamic, ventromedial hypothalamic and arcuate nuclei, as well as the median eminence, perifornical, anterior hypothalamic and lateral hypothalamic areas and the zona incerta. Double-labeling showed expression of OB-Rb in 59.6+/-6.0% neuropeptide Y-containing cells, 60.8+/-4.7% galanin-containing cells, 89.8+/-2.65% pro-opiomelanocortin-containing cells, 73.4+/-3.5% tyrosine hydroxylase-containing cells and 31.8+/-2.8% corticotropin-releasing factor-containing cells. Interestingly 100% of melanin-concentrating hormone and orexin positive cells were also OB-Rb immunoreactive. These data provide semi-quantitative information on the extent to which various cell types express OB-Rb in the hypothalamus. Expression of OB-Rb within specific neuropeptidergic neurons provides evidence for the direct action of leptin upon the various neurochemical systems that regulate food intake, neuroendocrine and autonomic function in the brain.

Differential expression of cocaine- and amphetamine-regulated transcript and agouti related-protein in chronically food-restricted sheep

Recently, much attention has focused on the role of the melanocortin system in the regulation of energy homeostasis, especially the satiety effects of the pro-opiomelanocortin (POMC)-derived peptide alpha-melanocyte stimulating hormone (alpha-MSH). We have found that POMC mRNA levels are similar in fat and thin sheep and the current study sought to further characterize the effects of nutritional status on the melanocortin system. To this end, we studied the expression of agouti-related peptide (AGRP) (an endogenous antagonist of alpha-MSH) and cocaine- and amphetamine-regulated transcript (CART), which is co-localized within POMC cells of the arcuate nucleus (ARC) in rodents. Twelve ovariectomized ewes were randomly divided into two groups and fed a maintenance (n=6) or restricted diet (n=6). At the time of experimentation, the animals had significantly (P<0.0001) different bodyweights (53.4+/-2.2 kg, ad libitum vs. 30.4+/-1.2 kg, food-restricted), which was largely due to altered body fat deposits. In situ hybridization was used to study the expression of POMC, AGRP and CART. The expression of POMC in the ARC was similar in ad libitum and food-restricted animals but the expression of AGRP was profoundly increased in the food-restricted group. The expression of CART was abundant throughout the hypothalamus but was not found in the ARC. In food-restricted animals, the expression of CART was lower in the retrochiasmatic nucleus (P<0.01), paraventricular nucleus (P<0.001), the dorsomedial nucleus and the lateral hypothalamic area (P<0.05), but was higher (P<0.01) in the posterior hypothalamic area. Thus, long-term changes in nutritional status have profound effects on the expression of AGRP and CART in the hypothalamus.

Neuronal projections to the lateral retrochiasmatic area of sheep with special reference to catecholaminergic afferents: immunohistochemical and retrograde tract-tracing studies

The retrochiasmatic area contains the A15 catecholaminergic group and numerous monoaminergic afferents whose discrete cell origins are unknown in sheep. Using tract-tracing methods with a specific retrograde fluorescent tracer, fluorogold, we examined the cells of origin of afferents to the retrochiasmatic area in sheep. The retrogradely labeled cells were seen by observation of the tracer by direct fluorescence or by immunohistochemistry with specific antibodies raised in rabbits or horses. Among the retrogradely labeled neurons, double immunohistochemistry for tyrosine hydroxylase, dopamine-beta-hydroxylase, and serotonin were used to characterize catecholamine and serotonin FG labeled neurons. The retrochiasmatic area, which included the A15 dopaminergic group and the accessory supraoptic nucleus (SON), received major inputs from the lateral septum (LS), the bed nucleus of the stria terminalis (BNST), the thalamic paraventricular nucleus, hypothalamic paraventricular and supraoptic nuclei, the perimamillary area, the amygdala, the ventral part of the hippocampus and the parabrachial nucleus (PBN). Further, numerous scattered retrogradely labeled neurons were observed in the preoptic area, the ventromedial part of the hypothalamus. the periventricular area, the periaqueductal central gray (CG), the ventrolateral medulla and the dorsal vagal complex. Most of the noradrenergic afferents came from the ventro-lateral medulla (Al group), and only a few from the locus coeruleus complex (A6/A7 groups). A few dopaminergic neurons retrogradely labeled with flurogold were observed in the periventricular area of the hypothalamus. Rare serotoninergic fluorogold labeled neurons belonged to the dorsal raphe nucleus. Most of these afferents came from both sides of the brain, except for hypothalamic supraoptic and paraventricular nuclei. In the light of these anatomical data, we compared our results with data obtained from rats, and we discussed the putative role of these afferents in sheep in the regulation of several specific functions in which the retrochiasmatic area may be involved, such as reproduction.

Melanin-concentrating hormone-producing neurons in birds

The peptidergic melanin-concentrating hormone (MCH) system was investigated by immunocytochemistry in several birds. MCH perikarya were found in the periventricular hypothalamic nucleus near the paraventricular organ and in the lateral hypothalamic areas. Immunoreactive fibers were very abundant in the ventral pallidum, in the nucleus of the stria terminalis, and in the septum/diagonal band complex, where immunoreactive pericellular nets were prominent. Many fibers innervated the whole preoptic area, the lateral hypothalamic area, and the infundibular region. Some fibers also reached the dorsal thalamus and the epithalamus. The median eminence contained only sparse projections, and the posterior pituitary was not labeled. Thus, in birds, a neurohormonal role for MCH is not likely. Immunoreactive fibers were observed in other regions, such as the intercollicular nucleus, stratum griseum periventriculare (mesencephalic tectum), central gray, nigral complex (especially the ventral tegmental area), reticular areas, and raphe nuclei. Although no physiological investigation concerning the role of MCH has been performed in birds, the distribution patterns of the immunoreactive perikarya and fibers observed suggest that MCH may be involved in functions similar to those described in rats. In particular, the projections to parts of the limbic system (ventropallidal ganglia, septal complex, hypothalamus, dorsal thalamus, and epithalamus) and to structures concerned with visceral and other sensory information integration suggest that MCH acts as a neuromodulator involved in a wide variety of physiological and behavioral adaptations (arousal) with regard to feeding, drinking, and reproduction.