PHOTOPERIODIC SYNCHRONIZATION OF A CIRCANNUAL REPRODUCTIVE RHYTHM IN SHEEP - IDENTIFICATION OF SEASON-SPECIFIC TIME CUES

Seasonal reproduction in the ewe is generated by an endogenous circann ual rhythm of reproductive neuroendocrine activity. Exposure to as few as 70 days of photoperiodic information a year is sufficient to synch ronize the rhythm. The present study was conducted to identify which p ortions of the photoperiodic cycle are utilized for synchronization. F or this purpose, we used pinealectomized ewes that could not respond r eproductively to changes in day length. Selected photoperiodic informa tion was provided via infusion of melatonin, a hormone that provides t he neuroendocrine code for day length in this species. Melatonin was d elivered according to circadian patterns. The infusion patterns were t ailored to mimic those of melatonin secretion in pineal-intact ewes du ring one of the four seasons: winter, spring, summer, or autumn. The i nfusions were provided for 90 days a year during each of the three yea rs following pinealectomy. The ewes were ovariectomized and treated wi th constant-release Silastic capsules containing estradiol; reproducti ve neuroendocrine activity was monitored by measurement of serum conce ntrations of LH. In the absence of exogenous melatonin, most (19 of 24 ) pinealectomized controls exhibited circannual LH cycles that were no t in synchrony. indicating that the rhythm was free-running. Melatonin synchronized the rhythm (such that the period was 365 days and the st ages of the rhythm were both concurrent among animals and in appropria te phase with the geophysical year), but not all melatonin patterns we re equally effective in this regard. The most effective melatonin patt erns mimicked those of secretion during summer. Spring and autumn mela tonin patterns were less effective, and winter melatonin patterns were ineffective. These results support the concept that there is a season al specificity with regard to the photoperiodic cues that synchronize the circannual rhythm of reproductive neuroendocrine activity in the e we. The rhythm is synchronized most effectively by long-day photoperio dic cues perceived on or around the summer solstice.