Melatonin and photorefractoriness: loss of response to the melatonin signal leads to seasonal reproductive transitions in the ewe.

Experiments were conducted to examine whether the refractoriness of the Suffolk ewe to the reproductive effects of day length is associated with a deficit in the generation of the circadian rhythm of melatonin secretion or in the postpineal processing of this photoperiodic message. Using serum luteinizing hormone (LH) concentrations in ovariectomized ewes bearing constant-release estradiol implants as a marker of reproductive induction, ewes with intact pineal glands were found to become unresponsive to fixed artificial photoperiods that initially had been either inductive (short days) or inhibitory (long days). The loss of the photoperiodic response was not associated with notable changes in the 24-h secretory pattern of melatonin, which remained characteristically low throughout the day and rose at night. In pinealectomized ewes, nightly infusion of a stimulatory pattern of melatonin (simulating that seen on short days) initially provoked reproductive induction; this response then lessened over much the same time course that pineal intact ewes became refractory to short days. These results support the hypothesis that photorefractoriness reflects a deficit in the postpineal processing of the photoperiodic message. Further, in view of recent evidence that photorefractoriness normally leads to both onset and cessation of the breeding season in Suffolk ewes maintained outdoors, these findings suggest that the loss of response to the melatonin signal contributes to at least one of these reproductive transitions, the cessation of the breeding season, under natural environmental conditions.