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Neuropeptide‐E‐1 Antagonizes the Action of Melanin‐Concentrating Hormone on Stress‐Induced Release of Adrenocorticotropin in the Rat

The physiological role of melanin‐concentrating hormone (MCH) in mammals is still very elusive, but this peptide might participate in the central control of the hypothalamopituitary adrenal (HPA) axis during adaptation to stress. Cloning and sequencing of the rat MCH (rMCH) cDNA revealed the existence of additional peptides encoded into the MCH precursor. Among these peptides, neuropeptide (N) glutamic acid (E) isoleucine (I) arnide (NEI) is co‐processed and secreted with MCH in rat hypothalamus. In the present work we examined: (1) The pattern of rMCH mRNA expression during the light and dark conditions in the rat hypothalamus and (2) The effect of intracerebroventricular (ICV) injections of rMCH and NEI in the control of basal or ether stress‐modified release of corticotropin (ACTH), prolactin (PRL) and growth hormone (GH) secretion in vivo in light‐on and light‐off conditions. Our data indicate that rMCH mRNA levels do not change during the light‐on period, but increase after the onset of darkness. Either alone or co‐administered, rMCH and NEI do not modify basal secretion of GH and PRL at any time tested nor do they alter ether stress‐induced changes in these two hormonal secretions. At the end of the light on period corresponding to the peak of the circadian rhythm in ACTH, administration of rMCH but not NEI leads to a decrease in ACTH levels while MCH is not effective during the light off period of the cycle (i.e. when basal ACTH levels are already low). Using a moderate ether induced stress, ACTH levels are only stimulated during the dark phase of the cycle. rMCH (63 or 210 nmoles) prevents the rise in ACTH release while NEI alone does not modify the stress response. Co‐administration of both peptides before stress results in an abolition of the rMCH induced inhibition of ACTH plasma levels. Taken together, these data indicate that rMCH may act as a central corticotropin inhibitory factor involved in the circadian rhythmicity of plasma ACTH levels and that NEI antagonizes its action.

Structure and Regulation of the Mouse Melanin-Concentrating Hormone mRNA and Gene

Melanin-concentrating hormone (MCH) and associated peptides, designated NEI and NGE, are predominantly expressed in hypothalamic neurons which project widely throughout the mammalian brain. These peptides might be involved as neuromodulators in the control of goal-directed behaviors, the integration of the stress response, and/or the regulation of arousal in general. In vivo studies of this peptidic system using a transgenic mouse model call for information about the structure and regulation of the mouse MCH (mMCH) gene. One complementary DNA (cDNA) for mouse prepro-MCH was isolated from a brain library by using a rat MCH cDNA as probe. This cDNA contains an open reading frame coding for a 165-amino acid precursor that displays about 90% sequence identity with rat and human prepro-MCHs. Most of the structural portion of the mMCH gene was cloned and characterized using the polymerase chain reaction (PCR). Strong conservation in exon-intron organization and primary sequences was found among the mouse, rat, and human genes, suggesting that coding and noncoding regions have important biological functions. Developmentally regulated expression of mMCH gene in mouse hypothalamus was examined by Northern blot hybridization. Up to 10-fold changes in the relative mMCH mRNA contents were observed during postnatal development, characterized by a peak at the weaning period. Moreover, striking variations in mMCH mRNA length, due to a poly(A) tail, were revealed during postnatal life. Tissue distribution of mMCH gene transcripts was investigated by means of the PCR and Northern blot procedures. Expression of the mMCH gene was revealed in heart, intestine, spleen, and testis and was found to be regulated in a developmentally programmed manner. Strikingly, short as well as long mMCH RNA species were identified at the periphery. Taken together, our data indicate that both transcriptional and post-transcriptional mechanisms regulate the expression of the MCH gene in mouse brain and at the periphery.

Development of Posterior Hypothalamic Neurons Enlightens a Switch in the Prosencephalic Basic Plan

In rats and mice, ascending and descending axons from neurons producing melanin-concentrating hormone (MCH) reach the cerebral cortex and spinal cord. However, these ascending and descending projections originate from distinct sub-populations expressing or not “Cocaine-and-Amphetamine-Regulated-Transcript” (CART) peptide. Using a BrdU approach, MCH cell bodies are among the very first generated in the hypothalamus, within a longitudinal cell cord made of earliest delaminating neuroblasts in the diencephalon and extending from the chiasmatic region to the ventral midbrain. This region also specifically expresses the regulatory genes Sonic hedgehog (Shh) and Nkx2.2. First MCH axons run through the tractus postopticus (tpoc) which gathers pioneer axons from the cell cord and courses parallel to the Shh/Nkx2.2 expression domain. Subsequently generated MCH neurons and ascending MCH axons differentiate while neurogenesis and mantle layer differentiation are generalized in the prosencephalon, including telencephalon. Ascending MCH axons follow dopaminergic axons of the mesotelencephalic tract, both being an initial component of the medial forebrain bundle (mfb). Netrin1 and Slit2 proteins that are involved in the establishment of the tpoc and mfb, respectively attract or repulse MCH axons. We conclude that first generated MCH neurons develop in a diencephalic segment of a longitudinal Shh/Nkx2.2 domain. This region can be seen as a prosencephalic segment of a medial neurogenic column extending from the chiasmatic region through the ventral neural tube. However, as the telencephalon expends, it exerts a trophic action and the mfb expands, inducing a switch in the longitudinal axial organization of the prosencephalon.

Peripheral injections of melanin-concentrating hormone receptor 1 antagonist S38151 decrease food intake and body weight in rodent obesity models

The compound S38151 is a nanomolar antagonist that acts at the melanin-concentrating hormone receptor 1 (MCH1). S38151 is more stable than its purely peptide counterpart, essentially because of the blockade of its N-terminus. Therefore, its action on various models of obesity was studied. Acute intra-cerebroventricular (i.c.v.) administration of S38151 in wild-type rats counteracted the effect of the stable precursor of melanin-concentrating hormone (MCH), NEI-MCH, in a dose-dependent manner (from 0.5 to 50 nmol/kg). In genetically obese Zucker fa/fa rats, daily i.c.v. administration of S38151 induced dose-dependent (5, 10, and 20 nmol/kg) inhibition of food intake, water intake, and body weight gain, as well as increased motility (maximal effect observed at 20 nmol/kg). In Zucker fa/fa rats, intraperitoneal injection of S38151 (30 mg/kg) induced complete inhibition of food consumption within 1 h. Daily intraperitoneal injection of S38151 (10 and 30 mg/kg) into genetically obese ob/ob mice or diet-induced obese mice is able to limit body weight gain. Furthermore, S38151 administration (10 and 30 mg/kg) does not affect food intake, water intake, or body weight gain in MCHR1-deleted mice, demonstrating that its effects are linked to its interaction with MCH1. These results validate MCH1 as a target of interest in obesity. S38151 cannot progress to the clinical phase because it is still too poorly stable in vivo.

The weaning period promotes alterations in the orexin neuronal population of rats in a suckling-dependent manner

The orexin-immunoreactive neurons are part of an important arousal-promoting hypothalamic population. Several groups have investigated these neurons during the lactation period, when numerous physiological alterations occur in the dam’s body to cope with the newly acquired metabolic needs of the litter. Although those studies have probed this population during the early and intermediate stages of lactation, few works have examined its response to weaning, including the cessation of the tactile suckling stimulus as the litter stops nursing. Using double immunohistochemistry for orexin and FOS combined with three-dimensional reconstruction techniques, we investigated orexin-synthesizing neurons and their activation at different times during weaning, in addition to the role played by the suckling stimulus. We report here that weaning promoted a decline in the anterior population of orexin-immunoreactive neurons and decreased the number of double orexin–FOS neurons labeled in the central dorsomedial hypothalamus, in addition to reducing the overall number of FOS-immunoreactive cells in the whole tuberal hypothalamus. Disruption of the suckling stimulus from the pups impaired the decrease in the number of anteriorly located orexin-immunoreactive neurons, attenuated the activation of orexin-synthesizing cells in the dorsomedial hypothalamus and reduced the number of FOS-immunoreactive neurons across the tuberal hypothalamus. When taken together, our data suggest that the weaning period is necessary to restore neurochemical pathways altered during the lactation period and that the suckling stimulus plays a significant role in this process.

Chapter II The melanin-concentrating hormone

Publisher Summary This chapter presents the most recent findings on structural analysis and neurobiology of melanin-concentrating hormone (MCH), with special emphasis on the cloning, functional characterization and neuronal distribution of at least two MCH receptors. It was a long-standing and important achievement. MCH is classified as a melanotropin—that is, a factor implicated in regulating color tegument. However, unlike the MSH and adrenocorticotropic hormone (ACTH) peptide family, all derived from the prepro-opio-melanocortin (POMC) precursor, MCH is not classified as a melanocortin because it lacks the consensus melanocortin sequence -His-Phe-Arg-Trp-. Consistent with that, MCH does not signal through the specific set of not less than six G-protein-coupled receptor (GPCR) proteins binding MSHs and ACTH, and termed from MC-1 to MC-6 binding. Intriguingly, it may be possible that MCH and MSH are evolutionary related.