Effect of constant-release implants of melatonin on seasonal cycles in reproduction, prolactin secretion and moulting in rams

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Abstract
Seasonal cycles in the size of the testes, blood plasma concentrations of testosterone, FSH and prolactin, intensity of the sexual skin flush, timing of rutting behavior and moulting of the body coat were recorded in Soay rams after s.c. implantation of melatonin contained in a Silastic envelope which increased the circulating blood levels of melatonin to 200-600 pg/ml for many months. Two groups of 8 adult rams were held under alternating periods of short days (8L [light]:16D [darkness]) and long days (16L:8D) to drive the seasonal cycles and the treatments with melatonin were initiated during the long or short days, and 1 group of 8 ram lambs was kept out of doors and given implants during the long days of summer (4 melatonin-implanted and 4 control (empty implants) rams per group). Melatonin implants during exposure to long days resulted in a rapid ''switch on'' of reproductive redevelopment similar to that produced by exposure to short days. Melatonin implants prevented the rams from showing the normal responses to changes in the prevailing photoperiod rendering them nonphotoperiodic. Long-term cyclic changes in testicular activity, prolactin secretion and other characteristics occurred in the melatonin-implanted rams; the pattern was similar to that previously observed in rams exposed to prolonged periods of short days. Melatonin is apparently the physiological hormone that relays the effects of changing photoperiod on reproduction and other seasonal features and continuous exogenous melatonin from an implant interferes with the normal ''signal'' and produces an over-riding short-day response.

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