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.