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

Introduction: 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. Methods: 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. Results: 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. Conclusion: 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.