Direct effect of prolactin on fish chromatophores and possible role of the hormone in change in body color

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Pigment-dispersing action of tilapia prolactin 177 (tPRL177) and ovine prolactin (oPRL) on erythrophores and xanthophores was found in some fish species. The action was direct on such cells. PRL had no effect on cultured melanophores in any teleost species examined. Therefore, we conclude that the pigment-dispersing effect of PRL on erythrophores and xanthophores and the inefficacy of PRL on melanophores are general properties in teleosts. In fish species possessing many erythrophores and/or xanthophores in their skin throughout the year, the cells responded to PRL all the time. However, in species that reveal the nuptial coloration in the spawning season, erythrophores responded better to PRL in spring and summer as compared with late fall and winter. The seasonal chane in responsiveness of erythrophores suggests the involvement of PRL in the nuptial coloration and an augmentation of red color by pigment dispersion in the spawning season. Next, plasma levels of tPRL177 and tPRL188 in Oreochromis niloticus in freshwater were ditermined : tPRL177 55.7(]SY+-[)13.2 ng/ml, tPRL188 3.2(]SY+-[)1.0 ng/ml. In O.niloticus, the relative effectiveness on erythrophores and xanthophores was as follows : tPRL177>oPRL>tPRL188. Therefore, it is likely that tPRL177 actually affects the skin color of the fish species. The plasma levels of tPRLs in O.niloticus assuming the nuptial coloration will be determined in near future. Although prolactin and androgen were injected into abdorminal cavity of O.niloticus every other day for 2 weeks, the nuptial coloration did not appear. Further experiments will be required. Signal transduction of PRL was examined in erythrophores and xanthophores isolated from some fish species. Cyclic AMP was found to be to the intracellular second messenger.