Effect of melatonin on photoperiod responses, ovarian secretion of oestrogen, and coital responses in the domestic cat.

Ovarian activity was studied in cats treated with melatonin under different photoperiods and during folliculogenesis. The effect of melatonin on coital responses of LH release and sexual refractoriness was also studied. Some cats were treated with melatonin (5 mg every other day) while under a photoperiod stimulatory (24L:0D) or inhibitory (8L:16D) for ovarian activity. Ovarian activity was determined by oestrogen analysis of plasma and sexual receptivity by interaction with a male. Melatonin suppressed ovarian follicle development in animals exposed to 24L:0D during a 60-day photoperiod regimen. However, melatonin did not block perception of the 24L:0D regimen since there was increased oestrogen synthesis during ovarian follicular cycles immediately after the end of treatment (compared with the 45 days normally required for a response to 24L:0D). The administration of melatonin to animals kept in 8L:16D resulted in 3/4 animals becoming sexually receptive with normal follicular growth patterns beginning about 70 days into the treatment period. In a second experiment, the administration of 5 mg melatonin on the 2nd day of follicle growth resulted in suppression of oestrogen synthesis. In a third experiment, melatonin administration tended to decrease the amount of LH released in response to coitus on the 3rd day of oestrus although the difference was not significant. Melatonin administration increased the duration of sexual refractoriness after coitus.