IMPORTANCE OF THE GONADOTROPIN-RELEASING-HORMONE (GNRH) SURGE FOR INDUCTION OF THE PREOVULATORY LUTEINIZING-HORMONE SURGE OF THE EWE - DOSE-RESPONSE RELATIONSHIP AND EXCESS OF GNRH

The preovulatory LH surge in the ewe is stimulated by a large sustaine d surge of GnRH. We have previously demonstrated that the duration of this GnRH signal exceeds that necessary to initiate and sustain the LH surge. The objective of the present study was to determine whether a similar excess exists for amplitude of the GnRH surge. Experiments wer e performed using an animal model in which GnRH secretion was blocked by progesterone, which in itself does not block the pituitary response to GnRH. To assess the amplitude of the GnRH surge needed to induce t he LH surge, we introduced artificial GnRH surges of normal contour an d duration but varying amplitudes. Twelve ewes were run through 3 succ essive artificial follicular phases (total of SG). Sit: of these artif icial follicular phases were positive controls, in which progesterone was removed, the estradiol stimulus was provided, and vehicle was infu sed. In these control cycles, animals generated endogenous LH surges. In the remaining artificial follicular phases, progesterone was not wi thdrawn, the estradiol stimulus was provided, and either vehicle (nega tive control) or GnRH solutions of varying concentrations (experimenta l) were infused. The circulating GnRH concentrations achieved by infus ion were monitored. No LH surges were observed in negative controls, w hereas LH surges were induced in experimental cycles provided a suffic ient dose of GnRH was infused. A highly significant dose-response rela tionship was observed between the amplitude of the GnRH surge and both the amplitude of the LH surge and the area under the curve describing the LH response. but no such relationship existed between the amplitu de of the GnRH surge and the duration of the LH response. In numerous cases, LII surges similar to those in the positive control animals res ulted from infusion of amounts of GnRH estimated to be considerably le ss than those delivered to the pituitary during the endogenously gener ated GnRH/LH surge. These findings indicate that, in the ewe, increase d GnRH secretion drives the preovulatory LW surge in a dose-dependent fashion, and they provide evidence that the amplitude of the GnRH surg e may exceed that needed to generate the LH surge.