17beta-estradiol regulation of melanin-concentrating hormone and neuropeptide-E-I contents in cynomolgus monkeys: a preliminary study

The roles of melanin-concentrating hormone in energy balance and reproductive function: Are they connected?

Melanin-concentrating hormone (MCH) is an anabolic neuropeptide with multiple and diverse physiological functions including a key role in energy homoeostasis. Rodent studies have shown that the ablation of functional MCH results in a lean phenotype, increased energy expenditure and resistance to diet-induced obesity. These findings have generated interest among pharmaceutical companies vigilant for potential anti-obesity agents. Nutritional status affects reproductive physiology and behaviours, thereby optimising reproductive success and the ability to meet energetic demands. This complex control system entails the integration of direct or indirect peripheral stimuli with central effector systems and involves numerous mediators. A role for MCH in the reproductive axis has emerged, giving rise to the premise that MCH may serve as an integratory mediator between those discrete systems that regulate energy balance and reproductive function. Hence, this review focuses on published evidence concerning i) the role of MCH in energy homoeostasis and ii) the regulatory role of MCH in the reproductive axis. The question as to whether the MCH system mediates the integration of energy homoeostasis with the neuroendocrine reproductive axis and, if so, by what means has received limited coverage in the literature; evidence to date and current theories are summarised herein.

Estradiol signaling in the regulation of reproduction and energy balance

Our knowledge of membrane estrogenic signaling mechanisms and their interactions that regulate physiology and behavior has grown rapidly over the past three decades. The discovery of novel membrane estrogen receptors and their signaling mechanisms has started to reveal the complex timing and interactions of these various signaling mechanisms with classical genomic steroid actions within the nervous system to regulate physiology and behavior. The activation of the various estrogenic signaling mechanisms is site specific and differs across the estrous cycle acting through both classical genomic mechanisms and rapid membrane-initiated signaling to coordinate reproductive behavior and physiology. This review focuses on our current understanding of estrogenic signaling mechanisms to promote: (1) sexual receptivity within the arcuate nucleus of the hypothalamus, (2) estrogen positive feedback that stimulates de novo neuroprogesterone synthesis to trigger the luteinizing hormone surge important for ovulation and estrous cyclicity, and (3) alterations in energy balance.

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.

Electrophysiological effects of MCH on neurons in the hypothalamus

Melanin concentrating hormone (MCH) has been implicated in many brain functions and behaviors essential to the survival of animals. The hypothalamus is one of the primary targets where MCH-containing nerve fibers and MCH receptors are extensively expressed and its actions in the brain are exerted. Since the identification of MCH receptors as orphan G protein coupled receptors, the cellular effects of MCH have been revealed in many non-neuronal expression systems (including Xenopus oocytes and cell lines), however, the mechanism by which MCH modulates the activity in the neuronal circuitry of the brain is still under investigation. This review summarizes our current knowledge of electrophysiological effects of MCH on neurons in the hypothalamus, particularly in the lateral hypothalamus. Generally, MCH exerts inhibitory effects on neurons in this structure and may serve as a homeostatic regulator in the lateral hypothalamic area. Given the contrast between the limited data on cellular functions of MCH in the hypothalamus versus a fast growing body of evidence on the vital role of MCH in animal behavior, further investigations of the former are warranted.

Neuropeptide glutamic-isoleucine (NEI) specifically stimulates the secretory activity of gonadotrophs in primary cultures of female rat pituitary cells

The neuropeptide EI (NEI) is derived from proMCH. It activates GnRH neurons, and has been shown to stimulate the LH release following intracerebroventricular administration in several experimental models. The aim of the present paper was to evaluate NEI actions on pituitary hormone secretion and cell morphology in vitro. Pituitary cells from female rats were treated with NEI for a wide range of concentrations (1-400x10(-8)M) and time periods (1-5h). The media were collected and LH, FSH, PRL, and GH measured by RIA. The interaction between NEI (1, 10 and 100x10(-8)M) and GnRH (0.1 and 1x10(-9)M) was also tested. Pituitary cells were harvested for electron microscopy, and the immunogold immunocytochemistry of LH was assayed after 2 and 4h of NEI incubation. NEI (100x10(-8)M) induced a significant LH secretion after 2h of stimulus, reaching a maximum response 4h later. A rapid and remarkable LH release was induced by NEI (400x10(-8)M) 1h after stimulus, attaining its highest level at 2h. However, PRL, GH and FSH were not affected. NEI provoked ultrastructural changes in the gonadotrophs, which showed accumulations of LH-immunoreactive granules near the plasma membrane and exocytotic images, while the other populations exhibited no changes. Although NEI (10x10(-8)M), caused no action when used alone, its co-incubation with GnRH (1x10(-9)M), promoted a slight but significant increase in LH. These results demonstrate that NEI acts at the pituitary level through a direct action on gonadotrophs, as well as through interaction with GnRH.

Galanin knockout mice show disturbances in ethanol consumption and expression of hypothalamic peptides that stimulate ethanol intake

BACKGROUND

There is growing evidence suggesting that hypothalamic galanin (GAL), which is known to stimulate intake of a fat-rich diet, has a role in promoting the consumption of ethanol. The present study further examined this possibility in GAL knockout (GALKO) mice.

METHODS

Two groups of female and male GALKO mice, compared to wild-type (WT) controls, were trained to voluntarily drink increasing concentrations of ethanol, while maintained on lab chow and water. They were examined in terms of their daily ethanol intake and preference, acute consumption of a high-fat diet, preference for flavored solutions, and expression of different peptides shown to stimulate ethanol intake.

RESULTS

In the GALKO mice compared to WT, the results revealed: (i) a 35 to 45% decrease in ethanol intake and preference, which was evident only at the highest (15%) ethanol concentration, was stronger in female than in male mice, and was seen with comparisons to littermate as well as nonlittermate WT mice; (ii) a 48% decrease in acute intake of a fat-rich diet, again stronger in female than male mice; (iii) no difference in consumption of sucrose or quinine solutions in preference tests; (iv) a total loss of GAL mRNA in the hypothalamic paraventricular nucleus (PVN) of female and male mice; and (v) a gender-specific change in mRNA levels of peptides in the perifornical lateral hypothalamus (PFLH), orexin and melanin-concentrating hormone, which are known to stimulate ethanol and food intake and were markedly decreased in females while increased in males.

CONCLUSIONS

These results provide strong support for a physiological role of PVN GAL in stimulating the consumption of ethanol, as well as a fat-rich diet. Ablation of the GAL gene produced a behavioral phenotype, particularly in females, which may reflect the functional relationship of galanin to ovarian steroids. It also altered the peptides in the PFLH, with their reduced expression contributing to the larger behavioral effects observed in females and their increased expression attenuating these effects in males.

Anatomy, function and regulation of neuropeptide EI (NEI)

This review is focused on the anatomy, role and behavior of neuropeptide-glutamic acid-isoleucine (NEI), providing a general report on the neuropeptide. In addition to hormone release, this peptide also takes part in the regulation of grooming behavior and locomotor activity. NEI is produced by cleavage of prepro-MCH that probably takes place at the Lys(129)-Arg(130) and Arg(145)-Arg(146) sites (the glycine residue on the C-terminus of NEI strongly suggests that this peptide is amidated). This same prohormone is also the precursor of MCH, widely studied in relation to food and water intake, and NGE, of which little is known. NEI and MCH are extensively colocalized throughout the central nervous system (CNS), and NEI is also present in peripheral tissues. The latter is also effective in stimulating luteinizing hormone (LH) release and, to a lesser extent, FSH from primary pituitary cell cultures. In addition to releasing LH from the medial eminence, NEI also acts directly on gonadotropes. Lastly, this neuropeptide also acts at the CNS level on gonadotropin-releasing hormone (GnRH) neurons.

Melanin-concentrating hormone is expressed in the laterodorsal tegmental nucleus only in female rats

Melanin-concentrating hormone (MCH) is a neuropeptide originating from prepro-MCH. In male rats, neurons expressing MCH are found in the lateral hypothalamic area and medial zona incerta, as well as, sparsely, in the olfactory tubercle and pontine reticular formation. The wide distribution of MCH fibers suggests the involvement of this neuropeptide in a variety of functions, including arousal, neuroendocrine control and energy homeostasis. In lactating females, MCH is expressed in the preoptic area, indicating sexual dimorphism in MCH gene activation according to the female reproductive state. We hypothesized that MCH is also expressed differentially in the brainstem of female rats. Adult male rats and female rats (in the afternoon of diestrus and proestrus days; ovariectomized; or on lactation days 5, 12 and 19) were perfused between 2 and 4 p.m., and the brainstems were processed for in situ hybridization using a 35S-labeled prepro-MCH riboprobe. As described in males, prepro-MCH was expressed in the pontine reticular formation of females. We also observed consistent prepro-MCH expression in the caudal laterodorsal tegmental nucleus (LDT) of females but no differential expression comparing the various female reproductive states. Using dual-label immunohistochemistry or dual-label in situ hybridization, we found that brainstem MCH neurons coexpress glutamic acid decarboxylase mRNA, the gamma aminobutyric acid (GABA) processing enzyme, but do not colocalize choline acetyl transferase (acetylcholine processing enzyme). Since changes in LDT GABAergic cell activity are associated with rapid eye movement (REM) sleep, our findings suggest that MCH interacts with LDT GABAergic neurons and plays a role in REM sleep regulation.

Neuropeptide glutamic acid-isoleucine may induce luteinizing hormone secretion via multiple pathways

Neuropeptide glutamic acid-isoleucine (NEI) is a 14-amino acid peptide processed from prepro-melanin-concentrating hormone (ppMCH). In males, the localization of NEI is almost identical to that of MCH, the cell bodies of both being located primarily in the lateral hypothalamic area and zona incerta, projecting fibers throughout the brain. Although MCH has been widely studied, the role that NEI plays in brain circuitry has been poorly investigated. Recently, we showed that intracerebroventricular injection of NEI increases serum luteinizing hormone (LH) levels. In order to identify the anatomical substrate underlying this effect, we used combined immunohistochemistry methods to analyze the forebrains of females on the diestrus and proestrus days, as well as those of ovariectomized females treated with estradiol benzoate, with estradiol benzoate plus progesterone or with sesame oil (control animals). We found that ovariectomized females with no steroid treatment showed an increased number of NEI-immunoreactive neurons in the medial zona incerta. In addition, we observed dense to moderate NEI innervation of areas related to reproduction, including the organum vasculosum of the lamina terminalis, the anteroventral periventricular nucleus (AVPV) and the median eminence. The NEI fibers were in close apposition with the AVPV and gonadotropin-releasing hormone (GnRH) neurons expressing Fos in the afternoon of the proestrus day or following administration of estradiol benzoate plus progesterone. In the median eminence, NEI varicosities and terminal-like structures were in close proximity to blood vessels and GnRH fibers. Our results suggest that NEI might induce LH secretion in one of the following ways: by direct release into the median eminence, by modulation of GnRH neurons located in the preoptic area, by modulation of the GnRH terminals located in the median eminence or by an additive effect involving other neurotransmitters or neurohormones. Release of NEI might also induce LH secretion indirectly by modulating AVPV neurons.

Regulation of the hypothalamic melanin-concentrating hormone neurons by sex steroids in the goldfish: possible role in the modulation of luteinizing hormone secretion

In teleost fish, melanin-concentrating hormone (MCH) is a cyclic heptadecapeptide released from the pituitary during white background adaptation. In the periphery MCH concentrates melanin granules in melanophores thus lightening the body color of fish. Evidence from mammalian studies has demonstrated the involvement of MCH in the control of energy balance and the reproductive axis. Information about the hormonal regulation of MCH neurons in non-mammalian systems is scarce and nothing is known about its role in the regulation of the reproductive axis. We here report the molecular characterization of two MCH precursors in the goldfish. Both precursors are peripherally expressed and the expression in the central nervous system is restricted to the mediobasal hypothalamus. Hypothalamic MCH-mRNA production is upregulated during white background adaptation. Both testosterone and estradiol stimulate MCH mRNA expression in the hypothalamus in a sex-dependent manner, with females showing the greatest responsiveness. In addition, in vitro experiments demonstrated that graded doses of salmon MCH stimulate LH, but not GH, secretion from dispersed pituitary cells. Results suggest that hypothalamic MCH may participate in the steroid positive feedback loop on pituitary LH secretion.

Changes of melanin-concentrating hormone related to LHRH release in the median eminence of rats

The present research was carried out to study the distribution of melanin-concentrating hormone (MCH) fibers in the median eminence of rats and to evaluate if changes in the MCH content of the median eminence could be related to the release of LHRH. Immunocytochemical studies in the median eminence of males and estrous females showed the presence of MCH fibers, mainly in its internal layer. Diestrous and proestrous animals displayed MCH immunoreactivity in both the internal and external layers of the median eminence. Longitudinal sections of the median eminence in proestrous animals showed that MCH-immunoreactive (ir) density is higher at 12 than at 9 h in both layers of the median eminence. MCH was assayed by radioimmunoassay in median eminences of males and in females in all stages of the estrous cycle at 10 h. It was observed that the content of MCH at diestrus-1 and -2 was higher than in estrus and in male rats. In the day of proestrus, MCH and LHRH were assayed at 10, 12, 13, 14, 15 and 17 h. At 12 h, the content of MCH and LHRH showed the maximal values. At 13 h, MCH content showed a decline, while LHRH was still high. At 14 h, the LHRH content started to decrease. The present results suggest that MCH is involved in the regulation of LHRH release in the female rat.

Intracerebroventricular injection of neuropeptide EI increases serum LH in male and female rats

Melanin concentrating hormone (MCH) precursor-derived neuropeptide EI (NEI) has not yet been extensively studied. The aim of this study was to determine the effect of neuropeptide EI on serum levels of LH in normal male rats and chronically ovariectomized (CHR-OVX) female rats treated with estrogen benzoate (EB) and with a low dose of progesterone. The peptide, administered intracerebroventricularly in male and chronically ovariectomized female rats, increased LH serum levels compared to the controls injected with artificial cerebrospinal fluid. It is important to note that there is some relation between neuropeptide EI-melanin concentrating hormone and alpha-melanocyte stimulating hormone (alpha-MSH) indicating that all three peptides are associated in a complex inter-relationship. Therefore, the question that arises is if neuropeptide EI could also be related with the receptors for melanin concentrating hormone or alpha-melanocyte stimulating hormone.

Diencephalic neurons producing melanin-concentrating hormone are influenced by local and multiple extra-hypothalamic tachykininergic projections through the neurokinin 3 receptor

As melanin-concentrating hormone (MCH) neurons express the neurokinin 3 receptor (NK3) in the rat diencephalon, their innervation by tachykininergic fibers, the origin of this innervation and the effect of a NK3 agonist on MCH mRNA expression were researched. The obtained results show that the tachykininergic system develops complex relationships with MCH neurons. Overall, MCH cell bodies appeared targeted by both NKB- and SP-inputs. These afferents have multiple hypothalamic and extra-hypothalamic origins, but a local (intra-lateral hypothalamic area) origin from small interneurons was suspected as well. MCH cell bodies do not express NK1, but around 2.7% of the MCH neurons contained SP after colchicine injection. Senktide, a NK3 agonist, produced an increase of the MCH mRNA expression in cultured hypothalamic slices. This effect was reversed by two NK3 antagonists. Tachykinins enhance MCH mRNA expression, and, thus, may modulate the effect of MCH in functions such as feeding and reproductive behaviors in which this peptide has been experimentally involved.

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.

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.

Melanin-concentrating hormone depresses L-, N-, and P/Q-type voltage-dependent calcium channels in rat lateral hypothalamic neurons

Melanin-concentrating hormone (MCH), a cyclic 19-amino-acid peptide, is synthesized exclusively by neurons in the lateral hypothalamic (LH) area. It is involved in a number of brain functions and recently has raised interest because of its role in energy homeostasis. MCH axons and receptors are found throughout the brain. Previous reports set the foundation for understanding the cellular actions of MCH by using non-neuronal cells transfected with the MCH receptor gene; these cells exhibited an increase in cytoplasmic calcium in response to MCH, suggesting an excitatory action for the peptide. In the study presented here, we have used whole-cell recording in 117 neurons from LH cultures and brain slices to examine the actions of MCH. MCH decreased the amplitude of voltage-dependent calcium currents in almost all tested neurons. The inhibition desensitized rapidly (18 s to half maximum at 100 nM concentration) and was dose-dependent (IC(50) = 7.8 nM) when activated with a pulse from -80 mV to 0 mV. A priori activation of G-proteins with GTPgammaS completely eliminated the MCH-induced effect at low MCH concentrations and reduced the MCH-induced effect at high MCH concentrations. Inhibition of G-proteins with pertussis toxin (PTX) blocked the MCH-induced inhibitory effect at high MCH concentrations. Pre-pulse depolarization resulted in an attenuation of the MCH-induced inhibition of calcium currents in most neurons. These data suggest that MCH exerts an inhibitory effect on calcium currents via PTX-sensitive G-protein pathways, probably the G(i)/G(o) pathway, in LH neurons. L-, N- and P/Q-type calcium channels were identified in LH neurons, with L- and N-type channels accounting for most of the voltage-activated current (about 40 % each); MCH attenuated each of the three types (mean 50 % depression), with the greatest inhibition found for N-type currents. In contrast to previous data on non-neuronal cells showing an MHC-evoked increase in calcium, our data suggest that the reverse occurs in LH neurons. The attenuation of calcium currents is consistent with an inhibitory action for the peptide in neurons.

Melanin concentrating hormone depresses synaptic activity of glutamate and GABA neurons from rat lateral hypothalamus

The neuropeptide melanin concentrating hormone (MCH) is synthesised only by neurons of the lateral hypothalamic (LH) area in the CNS. MCH cells project widely throughout the brain. Despite the growing interest in this peptide, in part related to its role in feeding, little has been done to characterise its physiological effects in neurons. Using whole-cell recording with current and voltage clamp, we examined the cellular actions in neurons from the LH. MCH induced a consistent decrease in the frequency of action potentials and reduced synaptic activity. Most fast synaptic activity in the hypothalamus is mediated by GABA or glutamate. MCH inhibited the synaptic activity of both glutamatergic and GABAergic LH neurons, each tested independently. MCH reduced the amplitude of glutamate-evoked currents and reduced the amplitude of miniature excitatory currents, indicating an inhibitory modulation of postsynaptic glutamate receptors. In the presence of tetrodotoxin to block action potentials, MCH caused a depression in the frequency of miniature glutamate-mediated postsynaptic currents, suggesting a presynaptic site of receptor expression. In voltage clamp experiments, MCH depressed the amplitude of calcium currents, suggesting that a mechanism of inhibition may involve a reduced calcium-dependent release of amino acid transmitter. Previous reports have suggested that MCH activated potassium channels in non-neuronal cells transfected with the MCH receptor gene. We found no effect of MCH on voltage-dependent potassium channels in LH neurons. Baclofen, a GABAB receptor agonist, activated G-protein gated inwardly rectifying potassium (GIRK)-type channels; in the same neurons, MCH had no effect on GIRK channels. MCH showed no modulation of sodium currents. Blockade of the Gi/Go protein with pertussis toxin eliminated the actions of MCH. The inhibitory actions of MCH on both excitatory and inhibitory synaptic events, coupled with opposing excitatory actions of hypocretin, another LH peptide that projects to many of the same loci, suggest a substantial level of complexity in neuropeptide modulation of LH actions.

Melanin-concentrating hormone stimulates the release of luteinizing hormone-releasing hormone and gonadotropins in the female rat acting at both median eminence and pituitary levels

The purpose of this study was to investigate whether melanin-concentrating hormone (MCH) acts directly on the median eminence and on the anterior pituitary of female rats regulating LHRH and gonadotropin release. In addition, immunohistochemistry was used to examine the density and distribution of MCH-immunoreactive fibers in the median eminence of proestrous rats. MCH-immunoreactive fibers were found in both the internal and external layers of the median eminence and in close association with hypophysial portal vessels. In the first series of in vitro experiments, median eminences and anterior pituitaries were incubated in Krebs-Ringer bicarbonate buffer containing two MCH concentrations (10(-10) and 10(-8) M). The lowest MCH concentration (10(-10) M) increased (P < 0.01) LHRH release only from proestrous median eminences. Anterior pituitaries incubated with both MCH concentrations also showed that 10(-10) M MCH increased gonadotropin release only from proestrous pituitaries. In the second series of experiments, median eminences and pituitaries from proestrous rats were incubated with graded concentrations of MCH. MCH (10(-10) and 10(-9) M) increased (P < 0.01) LHRH release from the median eminence, and only 10(-10) M MCH increased (P < 0.01) LH and FSH release from the anterior pituitary. The effect of MCH on the stimulation of both gonadotropins from proestrous pituitaries was similar to the effect produced by LHRH. Simultaneous incubation of pituitaries with MCH and LHRH did not modify LH but increased the FSH release induced by LHRH. The present results suggest that MCH could be involved in the regulation of preovulatory gonadotropin secretion.

Interaction of melanin-concentrating hormone (MCH), neuropeptide E-I (NEI), neuropeptide G-E (NGE), and alpha-MSH with melanocortin and MCH receptors on mouse B16 melanoma cells

Melanin-concentrating hormone (MCH) and alpha-melanocyte-stimulating hormone (alpha-MSH) are known to exhibit mostly functionally antagonistic, but in some cases agonistic activities, e.g., in pigment cells and in the brain. Neuropeptide E-I (NEI) displays functional MCH-antagonist and MSH-agonist activity in different behavioral paradigms; the role of neuropeptide G-E (NGE) is not known. This study addressed the question of possible molecular interactions between alpha-MSH, MCH and the MCH-precursor-derived peptides NEI and NGE at the level of the pigment cell MCH receptor subtype (MCH-Rpc) and the different melanocortin (MC) receptors. Radioreceptor assays using [125I]MCH, [125l]alpha-MSH and [125I]NEI as radioligands and bioassays were performed with MCI-R-positive and MC1-R-negative mouse B16 melanoma cells and with COS cells expressing the different MC receptors. The IC50s of alpha-MSH and NEI or NGE for [125I]MCH displacement from mouse MCH-Rpc were 80-fold and, respectively, >300-fold higher than that of MCH, and the IC50s for MCH and NEI or NGE for [125I]alpha-MSH displacement from mouse MC1-R were 50,000-fold and >200,000-fold higher than that of alpha-MSH. No high-affinity binding sites for NEI were detected on B16 melanoma cells and there was no significant displacement of [1251]alpha-MSH by MCH, NEI or NGE with MC3-R, MC4-R and MC5-R expressed in COS cells. At concentrations of 100 nM to 10 microM, however, MCH, NEI and NGE induced cAMP formation and melanin synthesis which could be blocked by agouti protein or inhibitors of adenylate cyclase or protein kinase A. This shows that mammalian MCH-precursor-derived peptides may mimic MSH signalling via MC1-R activation at relatively high, but physiologically still relevant concentrations, as e.g. found in autocrine/paracrine signalling mechanisms.

Gonadal steroids and energy homeostasis in the leptin era

Gonadal steroids influence food intake and body weight. Although the specific mechanisms underlying these effects are not clear, a consideration of their effects in the context of current models of energy homeostasis may ultimately lead to the identification of these mechanisms. When compared with leptin, the prototypical humoral signal of energy balance, sex steroids share many common properties related to food intake and body weight. Specifically, gonadal steroids circulate in proportion to fat mass and current energy balance, and administration of these compounds influences food intake, energy expenditure, body weight, and body composition. Moreover, both estrogens and androgens modulate central nervous system effectors of energy homeostasis that are targets for the action of leptin, including pathways that contain neuropeptide Y, pro-opiomelanocortin, or melanin-concentrating hormone. Sex steroids and leptin also regulate one another's production. Although gonadal steroids, unlike leptin, are clearly not critical to the maintenance of normal energy homeostasis, they do appear to function as physiologic modulators of this process. Identifying the specific central mediators of their effects will contribute to our understanding of their role in energy homeostasis.

Melanin-concentrating hormone receptor: an orphan receptor fits the key

The melanin-concentrating hormone (MCH), a hypothalamic peptide, was identified initially in teleost fish as a regulator of pigmentary changes in background adaptation, and was later also found, in mammals, to be a regulator of feeding and energy homeostasis. Its specific receptor remained an enigma until very recently when it was identified as the orphan G-protein-coupled receptor SLC-1. This review focuses on the identification, structure and signaling of the MCH receptor and discusses some of the implications of its discovery.