Abstract
Melatonin in Silastic capsules was implanted subcutaneously to examine the effect on reproductive activities of anestrous ewes under natural photoperiods. The constant supply of melatonin from the capsules increased plasma melatonin levels in a dose dependent manner, and high concentrations exceeding the endogenous night peaks were maintained throughout the day in the ewes carrying two capsules. On May 8 (Day 0) non-lactating ewes (n=23) were divided into 3 groups, and were either implanted with one (MEL 1, n=7) or two (MEL 2, n=6) capsules, or left untreated (control, n=10). Plasma progesterone concentrations in the MEL 2 group rose and became significantly higher than those in the control group from Day 56 onwards. This suggests a stimulatory effect of melatonin on quiescent ovarian function. In the MEL 1 group, however, no apparent effect was observed, probably due to an insufficiency in basal melatonin ele-vation. These results suggest that the continuous presence of high melatonin levels in the circulation nullified the anti-gonadal effect of long days, presumably by blunting the endogenous rhythm of melatonin levels responsible for conveying information of the environmental photoperiod.
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