Melatonin counteracts the stimulatory effects of blinding or exposure to red light on reproduction in the skipper frog Rana cyanophlyctis.

An earlier study reported that blinding or exposure to red light stimulates reproduction in R. cyanophlyctis. The present study investigates whether melatonin counteracted blinding- or red-light-induced ovarian stimulation. The study consisted of two experiments carried out during the prebreeding (March/April) and breeding phase (August/September) of the reproductive cycle of this species. In experiment 1, exposure to red light for 30 days increased (P<0.01) the gonadosomatic index (GSI) and oviductal weights, whereas exposure to continuous darkness for the same duration decreased (P<0.01) the GSI. Data on follicular kinetics revealed an increase in vitellogenic oocytes and decrease in previtellogenic oocyte numbers following exposure to red lights. Exposure to continuous dark on the other hand increased previtellogenic oocyte and decreased vitellogenic oocyte numbers. In experiment 2, the GSI and the number of vitellogenic oocytes increased in blinded and red-light-exposed frogs. However, treatment with melatonin (15 microg sc) for 30 days to both these groups of frogs reduced the GSI, oviductal weights, and the number of vitellogenic oocytes. The results both confirm earlier finding that blinding and exposure to red light stimulate ovarian growth and also demonstrate that melatonin counteracts blinding or red-light-induced stimulation of ovarian activity. This apart, the findings from experiment 1 demonstrate for the first time that exposure to continuous dark inhibits ovarian growth in this species. Ocular melatonin seems to play an important role in regulation of reproduction in this species.