Effects of prolactins on the chromatophores of the tilapia, Oreochromis niloticus.

Using isolated scales and split-fin preparations of the tilapia Oreochromis niloticus, the effects of a pair of prolactins of the tilapia Oreochromis mossambicus (tPRL177 and tPRL188) and of ovine prolactin (oPRL) on chromatophores were studied in vitro. These peptides caused melanosome aggregation and dispersion of xanthosomes, especially in the split preparations. Their relative effectiveness was as follows: tPRL177 > oPRL > tPRL188. Moreover, tPRL177 at 100 nM induced a high level of pigment dispersion in cultured xanthophores and erythrophores, but tPRL188 at the same concentration did not have this effect. We also examined the responses of chromatophores to oPRL in primary cell culture and found that xanthophores and erythrophores respond to the peptide by pigment dispersion in a dose-dependent manner, whereas cultured melanophores showed little aggregation of pigment. In denervated melanophores in the split-fin preparations, tPRL177 failed to induce aggregation of pigment. From these results, it was concluded that prolactin affects brightly pigmented cells of the tilapia directly, but affects melanophores indirectly. Norepinephrine which might leak from varicosities of chromatic nerve fibers by virtue of the action of prolactin molecules may be responsible for melanosome aggregation.