The biosynthesis of melanin-concentrating hormone in a fish

The melanocortins and melanin-concentrating hormone in the central regulation of feeding behavior and energy homeostasis

A number of different neuropeptides exert powerful concerted controls on feeding behavior and energy balance, most of them being produced in hypothalamic neuronal networks under stimulation by anabolic and catabolic peripheral hormones such as ghrelin and leptin, respectively. These peptide-expressing neurons interconnect extensively to integrate the multiple opposing signals that mediate changes in energy expenditure. In the present review I have summarized our current knowledge about two key peptidic systems involved in regulating appetite and energy homeostasis, the melanocortin system (alpha-MSH, agouti and Agouti-related peptides, MC receptors and mahogany protein) and the melanin-concentrating hormone system (proMCH-derived peptides and MCH receptors) that contribute to satiety and feeding-initiation, respectively, with concurrent effects on energy expenditure. I have focused particularly on recent data concerning transgenic mice and the ongoing development of MC/MCH receptor antagonists/agonists that may represent promising drugs to treat human eating disorders on both sides of the energy balance (anorexia, obesity).

Comparative analysis of melanin-concentrating hormone structure and activity in fishes and mammals

A comparative analysis of the structure of the melanin-concentrating hormone (MCH) precursor reveals that this sequence has been subjected to a higher selection pressure in mammals than in teleosts, suggesting that the structural constraints have not been the same throughout the vertebrate lineage. In contrast, the MCH peptide sequence has been very well conserved in all species. A sensitive and reproducible eel skin assay was developed and allowed us to define the structural features needed for a full MCH bioactivity. It was shown that the minimal structure carrying the critical residues was the same in fishes and in mammals. A pharmacological approach confirmed that MCH receptor activation decreased the cAMP levels in the fish skin, but this effect appeared to be independent from a Galphai protein. We propose that one of the intracellular signaling pathways of the MCH receptor in fish skin is the activation of one or several cellular phosphodiesterases.

Corticotropin-releasing hormone (CRH) in the teleost fish Oreochromis mossambicus (tilapia): in vitro release and brain distribution determined by a novel radioimmunoassay

The quantitative distribution of corticotropin-releasing hormone (CRH) in the brain and pituitary of the fish Oreochromis mossambicus (tilapia) was studied following the validation of a radioimmunoassay. Compared to the pituitary content, the brain contained 20 times more CRH. Eighty percent of the total brain content was located outside the hypothalamus, particularly in the telencephalon. Substantial amounts of CRH were also present in other regions devoid of hypophysiotropic neurons, such as the vagal lobe and optic tectum. Telencephalic and pituitary CRH co-eluted with the tilapia CRH(1-41)standard on reverse phase HPLC. In vitro CRH release by the telencephalon amounted to 5% of its content per hour, whereas release from the pituitary was negligible. We conclude that CRH in the brain of tilapia regulates pituitary and non-pituitary related functions, probably as a neurotransmitter or neuromodulator.

Cloning and sequence analysis of hypothalamus cDNA encoding tilapia melanin-concentrating hormone

Melanin-concentrating hormone (MCH) is a neuroendocrine peptide involved in the regulation of skin pigmentation in teleosts. We isolated and sequenced a 543 bp hypothalamic cDNA encoding the MCH-preprohormone of tilapia (Oreochromis mossambicus). Initially, polymerase chain reaction (PCR) experiments were performed on hypothalamic RNA with a synthetic oligonucleotide primer corresponding to a conserved region of salmon and mammalian MCH peptide and an oligo dT primer. A 0.2 kb PCR fragment was obtained and found to have low but significant nucleotide sequence similarity with the 3' ends of known MCH-mRNAs. Subsequently, the PCR fragment was used to screen λZAP cDNA libraries constructed from tilapia hypothalamic poly(A(+)) RNA. The cloned tilapia MCH preprohormone cDNA encodes a 133-amino acid protein of which 17 amino acids belong to the signal peptide. The MCH peptide sequence is located at the carboxy terminus of the preprohormone structure and is preceded by a pair of arginine residues which can serve as a proteolytic cleavage site. 23 to 25 amino acids further upstream in the prohormone structure three consecutive basic residues are present. Cleavage at this site would yield a 22-amino acid MCH gene-related peptide (Mgrp), which is much larger than (12- to 13-amino acid) salmon and mammalian Mgrp. A comparative structural analysis between tilapia preproMCH and its salmon and mammalian counterparts revealed that the MCH peptide sequence is very well conserved (100% identity with salmon and 75% identity with both rat and human MCH). In contrast, the remaining parts of the preproMCH structures have diverged considerably. Northern blot analysis revealed the presence of tilapia preproMCH mRNA in the hypothalamus and not in other brain regions nor in several peripheral tissues.

The Biosynthesis of Melanin-Concentrating Hormone in Trout Kept Under Different Conditions of Background Colour and Stress, as Determined by an in vitro Method

The aim of this study was to develop a method to monitor the synthetic activity of neurons which secrete the neurohypophysial melanin-concentrating hormone (MCH), a hormone implicated in two separate physiological roles in fish-pigmentary regulation and the response to stress. Trout hypothalamic fragments, containing the MCH neuronal cell bodies, were incubated in vitro in a medium including [(35) S]methionine. Labelled MCH-related products were separated by immunoprecipitation. Gel electrophoresis showed that radioactive methionine was incorporated into MCH precursors and into mature MCH, much as in vivo. Thus, de novo hormone synthesis continues in vitro. Trout reared at a fish-farm and adapted to black or white tanks for 39 days displayed nearly a 2-fold difference in their rate of methionine incorporation. Transferring fish from a black to a white background also doubled the rate of incorporation within 7 days and this rate increased only very slightly during the following 3 weeks. The rate of methionine incorporation by tissue from trout reared in black tanks was very depressed, and 4-fold lower than that of fish reared in white tanks, suggesting that very long-term adaptation to one or other background has increasingly marked effects on the activity and perhaps the number of synthetically active neurons. Stress also influenced the rate of methionine incorporation: a mild daily stress was stimulatory but more frequent stress had an inhibitory effect in many cases. The effects of daily dexamethasone administration were inconclusive. It is suggested that these differences in methionine incorporation reflect the relative rates of MCH synthesis in vivo, and that the method is useful to investigate conditions which modulate the biosynthesis of MCH in the trout.