The photoneuroendocrine control of seasonal breeding in the ewe.

The major environmental parameter controlling seasonal breeding in ewes is photoperiod. Short days stimulate, and long days inhibit breeding activity. One of the more intriguing enigmas of neuroendocrinology is the endocrine mechanism whereby a mere change in daylength initiates or prevents estrous cycles. Recent experiments have begun to solve this problem by demonstrating that in ewes, photoperiod governs response of the hypothalamopituitary axis to the negative feedback action of estradiol. In long days, estradiol is a potent inhibitor of gonadotropin secretion, whereas in short days it is relatively ineffective in this regard. These photoperiod-induced changes in estradiol feedback are proposed to permit or prevent estrous cycles by controlling the occurrence of a crucial step in the sequence of events leading to ovulation, namely a sustained, preovulatory rise in LH. Thus, estrous cycles cease in long days because an increase in estradiol negative feedback prevents the sustained rise in LH. In short days, estrous cycles resume because a decrease in estradiol feedback permits the sustained rise in LH required for ovulation. Even more puzzling, at present, than the mechanism of transduction of photoperiodic information into an endocrine event controlling seasonal breeding is the problem of transmission of photoperiodic information from the environment to the hypothalamo-pituitary axis. In sheep, early investigations of the transmission pathway have elicited provocative results, only some of which are similar to those obtained in other species. Among the questions which remain to be answered are: what is the location of the photoreceptors, and what, if any, are the roles of the suprachiasmatic nuclei and the pineal in photoperiodic control of seasonal breeding? These issues promise to provide a tantalizing challenge for future investigations into the photoneuroendocrine control of seasonal breeding.