Importance of Duration of Nocturnal Melatonin Secretion in Determining the Reproductive Response to Inductive Photoperiod in the Ewe 1

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Abstract
The pineal gland, through its nocturnal melatonin secretion, mediates the effects of inhibitory (long) and stimulatory (short) photoperiod on reproduction in female sheep. Earlier studies revealed that duration of the nighttime melantonin rise is important in determining the inhibitory effects of day length on reproduction in the ewe. Whether the duration is also important in mediating the inductive response to short days was tested. Pinealectomized ewes, housed under long days, received a short-day melantonin infusion (16 h duration) for 90 days. Reproductive status was monitored form the response to estradiol negative feedbck of luteinizing hormone (LH) secretion. This short-day melatonin pattern led to unambiguous reproductive induction, despite the exposure to inhibitory long days. The increase in serum LH was comparable, in terms of latency and magnitude, to that in penalectomized controls receiving the same short-day melatonin pattern under short days and in peak-intact controls transferred from long to short days. Since the reproductive status conformed to the length of time that melatonin was elevated each day rather than to photoperiod, these results support the conclusion that duration of the nighttime melatonin rise mediates the reproductive response of the ewe to an inductive photoperiod. In all, the melatonin rhythm is considered an integral component of the physiologic mechanism measuring day length; through duration of its nocturnal secretion, melatonin encodes both inhibitory and sitmulatory photoperiods.

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