GONADOTROPIN-RELEASING-HORMONE REQUIREMENTS FOR OVULATION

This article addresses the role of GnRH in ovulation in the context of two general models of GnRH action-deterministic and permissive. Accor ding to the deterministic model, increased GnRH secretion is required to induce the preovulatory LH surge and thus ovulation. The permissive model, in contrast, holds that GnRH secretion need not increase. Rath er, the preovulatory LH surge results from enhanced sensitivity of the pituitary gland to GnRH. Studies in rodents and rabbits support the d eterministic model whereas evidence in primates suggests that GnRH is permissive. Three lines of evidence are presented to support the concl usion that GnRH plays a deterministic role in sheep. First, a large Gn RH surge is secreted together with the preovulatory LH surge. Second, the follicular phase increase in circulating estradiol concentration s timulates this GnRH surge by a positive feedback effect. Third, initia tion of the LH surge requires an abrupt increase in GnRH, and maintena nce of the LH surge requires continued GnRH support. Collectively, the se observations document the fundamental importance of a GnRH surge to ovulation and generation of the estrous cycle of sheep.