Effect of melatonin on oocyte growth and recruitment, hypophyseal gonadotrophs, and oviduct of the frog Rana cyanophlyctis maintained under natural photoperiod during the prebreeding phase.

The effect of 25 and 50 micrograms melatonin on oocyte growth and recruitment, hypophyseal gonadotrophs, and oviduct was studied in Rana cyanophlyctis maintained under natural photoperiod (12.36 L/11.24 D) during the prebreeding phase (April). During the experiment minimum and maximum temperature was 20 +/- 0.2 degrees and 37.2 +/- 0.3 degrees, respectively; the daily fluctuation between these was 17 +/- 0.4 degrees. Melatonin was injected (ip) 6 days a week for 31 days and frogs were killed on the 32nd day. Treatment with 25 or 50 micrograms melatonin 2 hr after the beginning of natural photophase had no effect on the gonadosomatic index (GSI) or on the percentage and number of oocytes at each stage of development. Injections 7 hr after the beginning of the natural photophase caused a marked decrease in the percentage of medium second growth phase (MSGP) and large second growth phase (LSGP) oocytes, while the percentage of the first growth phase (FGP) oocytes increased. However, in 50% of the 25 micrograms and 62.5% of the 50 micrograms melatonin-injected frogs, there was significant reduction in the number of both MSGP and LSGP oocytes. Frogs which received melatonin (25 and 50 micrograms) 11 hr after the onset of natural photophase exhibited a significant (P less than 0.05) decrease in the number of MSGP and LSGP oocytes and GSI. Atretic follicles (AF) increased in all the melatonin-treated groups, but a significant increase was seen only in evening-injected frogs receiving 50 micrograms melatonin. The mean diameter of the largest oocytes decreased significantly in 50 micrograms melatonin-injected frogs.(ABSTRACT TRUNCATED AT 250 WORDS)