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. According 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. Rather, the preovulatory LH surge results from enhanced sensitivity of the pituitary gland to GnRH. Studies in rodents and rabbits support the deterministic model whereas evidence in primates suggests that GnRH is permissive. Three lines of evidence are presented to support the conclusion that GnRH plays a deterministic role in sheep. First, a large GnRH surge is secreted together with the preovulatory LH surge. Second, the follicular phase increase in circulating estradiol concentration stimulates this GnRH surge by a positive feedback effect. Third, initiation of the LH surge requires an abrupt increase in GnRH, and maintenance of the LH surge requires continued GnRH support. Collectively, these observations document the fundamental importance of a GnRH surge to ovulation and generation of the estrous cycle of sheep.