Melatonin effects on the melanophores in adults and tadpoles of Rana cyanophlyctis (Schneider)

Effects of melatonin (MT) were comparatively examined on melanophores of isolated skin in adults and tadpole’s tailfin of a frog Rana cyanophlyctis. MT is generally considered as a potent melanophores aggregating hormone besides regulating the sleep wake cycle in vertebrates.
Melanophore size index (MSI) was chosen as a recording parameter of the responses. Concentration-response curve was obtained by application of MT to the frog skin. Against this MT, antagonists were employed to observe their blocking effects on aggregatory responses of frog melanophores.
MT has induced aggregation in a wide dose-range on spotted and non-spotted regions in adults as well as in the tailfin of tadpoles. MT induced aggregation was somewhat independent to the applied concentrations of MT and beyond the dose 4.31 × 10-8 M of MT, aggregation of melanophores was decreased. Phenomenons of auto-desensitization and auto-antagonism have been observed. For tadpoles, the sensitivity to MT was higher than that of the adult skin melanophores as evident with the lowest threshold dose of MT to induce a discernible response. MT induced aggregatory responses were effectively inhibited by the specific MT antagonists luzindole and K-185 and also by the Ca channel blocker verapamil. Seasonal variation in inhibition of MT receptors by K-185 is being reported in this species.
Tadpole melanophores of Rana cyanophlyctis were more sensitive towards MT aggregation than their adult counterparts. Seasonal variations and auto-desensitization are all expressed through the specific MT1, MT2 receptors and Ca channels.