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

Abstract

The effect of 25 and 50 μ g 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· and 37.2 ± 0.3· , respectively; the daily fluctuation between these was 17 ± 0.4· . Melatonin was injected (ip) 6 days a week for 31 days and frogs were killed on the 32nd day. Treatment with 25 or 50 μg 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 μg and 62.5% of the 50 μg melatonin-injected frogs, there was significant reduction in the number of both MSGP and LSGP oocytes. Frogs which received melatonin (25 and 50 μg) 11 hr after the onset of natural photophase exhibited a significant (P < 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 μg melatonin. The mean diameter of the largest oocytes decreased significantly in 50 μg melatonin-injected frogs. Frogs injected with 25 μg melatonin thrice a day, morning, afternoon, and evening (MAE), showed no change in GSI and number of LSGP oocytes, although MSGP oocytes were reduced significantly. Further, there was a significant increase in AF and FGP oocytes in the MAE group. The cell length of gonadotrophs decreased significantly (P < 0.05) in the evening-treated frogs receiving either dose of melatonin but not in the MAE group. The nuclear diameter of these cells was unaffected. The cytoplasmic granulation was slightly greater in both evening-injected groups receiving melatonin compared to the controls. The administration of 25 or 50 μ g melatonin in mid (A) and later (E) photophase caused significant reduction in the percentage weight of oviducts. In the MAE group also, there was a significant decrease in oviduct weight. The present findings suggest that melatonin is antigonadal in R. cyanophlyctis and from midlight phase onward, and toward the end of the light phase, the effects become more pronounced, affecting vitellogenic growth of oocytes and causing increased follicular atresia. Further, the findings suggest that melatonin has no antigonadal effect when it is continually available (MAE group). The down-regulation theory of melatonin receptors suggested in mammals may occur in the frog.