Effect of Melatonin Feeding on Serum Prolactin and Gonadotropin Levels and the Onset of Seasonal Estrous Cyclicity in Sheep*

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Abstract
The effects of melatonin feeding on serum PRL and gonadotropin concentrations and on the reproductive activity of seasonally anestrous sheep were examined. Ten maiden Border Leicester X Merino ewes (18 months) were acclimatized to 16 h of light daily (from 0300–1900 h) for 4 weeks. Five of the animals were then fed pellets treated with melatonin (2 mg/ day); the remainder received untreated pellets daily at 1100 h for 103 days. Seventeen days after the commencement of treatment, serum PRL levels were found to be significantly lower between 1500–1900 h and 2400–0800 h (P < 0.05) in the melatonin- fed group compared to untreated animals. By day 31 and on all subsequent occasions tested up to day 99, serum PRL levels were lower (P < 0.01) in the melatonin-fed animals throughout the entire 8-h sampling period (0900–1700 h). The feeding of melatonin had no apparent effect on the baseline serum levels of either FSH or LH, the frequency and amplitude of the LH peaks, or the changes evoked in serum PRL after the administration of 10 μg TRH. However, melatonin may have affected the hypothalamo-pituitary response to the positive feedback effect of estradiol. When tested after 52 days, the LH and FSH responses of the melatonin-treated animals to an injection of 12.5 μg 17β-estradiol were significantly diminished (P < 0.05) compared with controls. After the administration of estradiol (day 53) and the introduction of a ram (day 71), the five melatonin- fed ewes commenced cyclic ovarian activity on days 65,65, 73, 81, and 88, respectively, compared to days 78 and 97, after day 103, after day 103, and after day 103 in the five untreated control ewes, respectively. These results indicate that melatonin administered orally to seasonally anestrous sheep induced changes in circulating PRL and influenced the time of onset of the breeding season.