MECHANISM OF MCH ACTION ON CHROMATOPHORES OF TELEOST FISH

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The primary structure of melanin-concentrating hormone (MCH) purified from chum salmon pituitary was determined in 1983. In melanophores, MCH caused pigment aggregation in all teleosts studied, whereas pigment within erythrophores was not always aggregated. For example, swordtail erythrophores responded to MCH,but erythrophores present in tail fin of Nile tilapia did not at all. Moreover, the minimum effective does of MCH in swordtail erythrophores was rather higher than that in melanophores.Five analogues of MCH were tested for their melanin-aggregating activities in the in vitro fish skin (Corydoras paleatus) bioassay. Four analogues were fragments of the native sequence : Asp-Thr-Met-Arg-Cys-Met-Val-Cly-Arg-Val-Tyr-Arg-Pro-Cys-Trp-Glu-Val with sequential elimination of substituents from both the carboxy- and amino- termini. Their relative effectiveness as follows : intact MCH > fragment (5-17) * fragment (1-15) > fragment (5-15) > fragment (1-14). Linear fragment (1-14) did not have the melanin-aggregating activity at all. From these results, we suggest the importance of S-S binding and the Trp^<15> residue for interaction with MCH receptors mediating melanosome aggregation. When intact MCH at concentrations higher than 1 muM was applied, the aggregation of melanosomes was followed by dispersion of them, suggesting that another type of MCH receptor mediating melanosome dispersion is also present on the melanophore membrane.Radio Immuno Assay indicated that the intracellular concentration of cAMP was decreased to about 55% of control value, when melanophores were treated with 10 nM MCH for 10 min, whereas intracellular level of cAMP was increased to about 126% when melanosomes were redispersed after the maximum aggregation by the treatment with 10 muM MCH.Moreover, it was shown in the present experiments, that Protein kinase C may be partly involved in the pigment aggregation, although calcium ions may not take part in the response.