HOW MUCH OF THE GONADOTROPIN-RELEASING-HORMONE (GNRH) SURGE IS REQUIRED FOR GENERATION OF THE LUTEINIZING-HORMONE SURGE IN THE EWE - DURATION OF THE ENDOGENOUS GNRH SIGNAL

The preovulatory LH surge in the sheep is accompanied by a massive and sustained surge of GnRH. The objective of this study was to examine t he duration of the endogenous GnRH signal required to induce and maint ain a LH surge of full amplitude and duration. For this purpose, we as sessed the effect of a competitive GnRH receptor antagonist (Nal-Glu), administered at various times relative to the LH surge, on the develo pment and progression of the surge pattern of LH release. All studies were conducted in a physiological model for the follicular phase of th e estrous cycle (artificial follicular phase). In this model, as durin g the natural follicular phase, the onset of the LH surge is coinciden t with the initiation of a massive and sustained rise in GnRH secretio n. The experimental approach was validated in a preliminary study by d etermination that the GnRH antagonist could block the LH surge without compromising GnRH release, as measured in pituitary portal blood. In the main experiment, 25 ewes were run through five successive artifici al follicular phases, during which the antagonist was not given (contr ol) or was administered before the LH surge, during its ascending limb , or during the descending limb. Treatment with antagonist before the expected time of the surge prevented the LK surge. Treatment during th e ascending limb of the LH surge interrupted the rise in LH and caused a prompt cessation of the surge. Treatment during the descending limb of the LH surge resulted in a faster decline in circulating LH concen trations than in control cycles and caused premature termination of th e LH surge. Our results are consistent with the conclusion that develo pment and progression of the preovulatory LH surge in sheep depend upo n GnRH stimulation throughout its entire time course.