Nature and bioactivity of gonadotropin-releasing hormone (GnRH) secreted during the GnRH surge.

Previous studies have demonstrated a neural action of estradiol in inducing a surge of GnRH in the ewe. However, although the GnRH and LH surges began concurrently, the GnRH surge consistently continued well beyond the surge of LH. Three experiments were conducted to test the hypothesis that the termination of the LH surge results from the secretion of a relatively inactive variant of GnRH during the later phases of the GnRH surge. In the first experiment, hypophyseal portal blood collected during an estrogen-induced LH surge was analyzed for GnRH immunoreactivity using two antibodies having specificity for the N- or C-terminal portion of the GnRH molecule. The duration, amplitude, and time course of the GnRH surge were found to be similar irrespective of the antisera used. In a second experiment, a competitive GnRH antagonist was administered at the beginning of the estrogen-induced GnRH/LH surge at a dose capable of blocking pituitary responsiveness for approximately half the duration of the GnRH surge. Antagonist treatment did not result in any change in the time of onset of the GnRH surge, but there was no increase in LH that naturally occurs coincident with onset of the GnRH surge. Rather, a persistent increase in LH secretion was observed during the latter stages of the GnRH surge, indicating that the GnRH molecules secreted at this time were biologically active. Finally, a sensitive and specific ovine pituitary cell bioassay was used to test bioactivity of GnRH in hypophyseal portal blood during different phases of the GnRH surge. GnRH bioactivity in samples collected early in the GnRH surge was greater than that before the onset of the GnRH surge but no greater than that collected during the descending limb of the surge. The results of all three experiments fail to support the hypothesis that the LH surge ends because of a change in the nature of the GnRH secreted. Rather they show that GnRH secreted throughout the surge is biologically active. Thus, the termination of the LH surge before that of the GnRH surge occurs for reasons other than lack of a bioactive GnRH signal.