A sensitive bioassay for melanotropic hormones using isolated medaka melanophores

Comparative analyses of the pigment-aggregating and -dispersing actions of MCH on fish chromatophores

In melanophores of the peppered catfish and the Nile tilapia, melanin-concentrating hormone (MCH) at low doses (<1 microM) induced pigment aggregation, and the aggregated state was maintained in the presence of MCH. However, at higher MCH concentrations (such as 1 and 10 microM), pigment aggregation was immediately followed by some re-dispersion, even in the continued presence of MCH, which led to an apparent decrease in aggregation. This pigment-dispersing activity at higher concentrations of MCH required extracellular Ca(2+) ions. By contrast, medaka melanophores responded to MCH only by pigment aggregation, even at the highest concentration employed (10 microM). Since it is known that medaka melanophores possess specific receptors for alpha-melanophore-stimulating hormone (alpha-MSH), the possibility that interaction between MSH receptors and MCH at high doses in the presence of Ca(2+) might cause pigment dispersion is ruled out. Cyclic MCH analogs, MCH (1-14) and MCH (5-17), failed to induce pigment dispersion, whereas they induced aggregation of melanin granules. These results suggest that another type of MCH receptor that mediates pigment dispersion is present in catfish and tilapia melanophores, and that intact MCH may be the only molecule that can bind to these receptors. Determinations of cAMP content in melanophores, which were isolated from the skin of three fish species and treated with 10 nM or 10 microM MCH, indicate that MCH receptors mediating aggregation may be coupled with Gi protein, whereas MCH receptors that mediate dispersion may be linked to Gs. The response of erythrophores, xanthophores and leucophores to MCH at various concentrations was also examined, and the results suggest that the distribution patterns of the two types of MCH receptors may differ among fish species and among types of chromatophore in the same fish.

beta-Adrenergic receptor subtypes in melanophores of the marine gobies Tridentiger trigonocephalus and Chasmichthys gulosus

The subtype of beta-adrenergic receptors in melanophores of the marine gobies Tridentiger trigonocephalus and Chasmichthys gulosus was studied. Pigment of denervated melanophores in isolated, split caudal fins was preliminarily aggregated by incubating the specimens in a physiological saline containing 10 microM phentolamine and 30-100 microM verapamil or 2-10 nM melatonin, and the responses of the melanophores to a beta-adrenergic agonist added to the incubating medium were recorded photoelectrically. The beta-adrenergic agonists noradrenaline, adrenaline, isoproterenol, salbutamol and, dobutamine were all effective in evoking a dispersion of melanophore pigment in the presence of phentolamine and verapamil or melatonin. The pigment-dispersing effect of noradrenaline (beta 1-selective agonist) was inhibited by metoprolol (beta 1-selective antagonist), propranolol,- and butoxamine. Whereas, the effect of salbutamol (beta 2-selective agonist) was hardly inhibited by metoprolol, though it was considerably inhibited by propranolol and ICI-118551. It was estimated that beta 1- and beta 2-adrenergic receptors coexist at ratios of 8.6:91.4, in the melanophore of Tridentiger trigonocephalus, and 25:75, in the melanophore of Chasmichthys gulosus, through the analyses of Hofstee plots of the effects of the beta-adrenergic drugs. It was suggested that the relation between the pigment-dispersing effect of a beta-adrenergic agonist on the melanophores and the concentration of the drug follows mass action kinetics, when the effect is mainly caused by the activation of beta 2-adrenergic receptors of the melanophores. However, when it is mainly caused by the activation of beta 1-adrenergic receptors of the melanophores, the relation does not follow mass action kinetics.

Proopiomelanocortin, its derived peptides, and the skin

Proopiomelanocortin (POMC) is a protein synthesized predominately in the pituitary gland but also in a variety of other tissues, including the skin. Through enzyme-mediated cleavage that varies among cell types, POMC can give rise to at least eight distinct peptides whose biologic roles are incompletely delineated. Although blood-borne pituitary-derived bioactivity for the skin was first recognized 80 years ago and the responsible neuropeptides isolated 20-40 years ago, our understanding of POMC-derived peptides in skin is still rapidly evolving. In particular, recent work in cultured human and murine skin-derived cells has demonstrated POMC gene expression as well as modulation of POMC and many of its derived peptides in response to physiologic signals including ultraviolet irradiation and cytokines. Immunoreactivity for these peptides has also been detected in normal skin and hair follicles, strongly suggesting cutaneous synthesis in vivo. Candidate autocrine or paracrine functions include enhanced melanogenesis, immunomodulation, and effects on cell cycle regulation and differentiated function in both the epidermis and its appendages. This article reviews recent data concerning POMC gene expression and regulation, protein processing, signal transduction, and biologic functions relevant to cutaneous biology.

The measurement of melanin-concentrating hormone in trout blood

Two methods are described for measuring the titres of melanin-concentrating hormone (MCH) in trout plasma. One involves the extraction of MCH from 1-ml plasma onto C18 Sep Pak cartridges, after which the eluted peptide is measured by conventional radioimmunoassay. In the alternative method, antibodies are bound onto immunobeads which are added to 0.5 ml plasma. After incubation for 24 hr, the beads are washed to remove the plasma and are incubated with 125I-labelled MCH; the following day, the labelled beads are separated by centrifugation, washed, and counted. The relative advantages of each method is discussed. Using these two methods, it is shown that the plasma concentration of the hormone is significantly higher in fish from white tanks (greater than 50 pmol/litre) than in fish from black tanks (approximately 10 pmol/litre) or those kept in the dark (approximately 5 pmol/litre). The plasma concentration of MCH changes rapidly when trout are moved from one coloured background to another, indicating its involvement in physiological colour change.

Melanin concentrating hormone. II. Structure and biosynthesis of melanin-concentrating hormone

Melanin-concentrating hormone is a neuropeptide produced in teleost hypothalami and transferred to the neurohypophysis. Salmon MCH was a novel cyclic heptadecapeptide capable of inducing melanin aggregation of integumentary melanophores at picoto nano-molar concentrations in all teleosts tested. The MCH gene is intronless and the exon encodes a 132 amino acid precursor protein, in which the heptadecapeptide of MCH locates at the C-terminal end. Immunohistochemical surveys with anti-salmon MCH antiserum strongly suggest that an MCH-like peptide is present in the hypothalami of higher vertebrates. Biological effects of salmon MCH on other vertebrates are found to be versatile.