DOES ESTRADIOL INDUCE THE PREOVULATORY GONADOTROPIN-RELEASING-HORMONE(GNRH) SURGE IN THE EWE BY INDUCING A PROGRESSIVE CHANGE IN THE MODE OF OPERATION OF THE GNRH NEUROSECRETORY-SYSTEM

Estradiol profoundly influences GnRH secretion during the follicular p hase of the estrous cycle of the sheep. Estradiol not only regulates t he frequency and amplitude of GnRH pulses, but also produces qualitati ve changes in its pattern of release and induces a sustained GnRH surg e during which discrete pulses are not readily evident. In this study, we tested the hypothesis that qualitative changes in GnRH secretion a re an integral part of an estradiol-induced change in the mode of oper ation of the GnRH neurosecretory system that leads to generation of th e GnRH surge. This was achieved by the measurement of GnRH in samples of pituitary portal blood collected at 1-min intervals for an 11-h per iod encompassing the pre- and early surge periods in an artificial fol licular phase model. In each of the seven ewes studied, a highly chara cteristic alteration in the moment to moment pattern of GnRH was obser ved. This consisted of a progressive change from a strictly episodic p attern of GnRH release to one containing both episodic and nonepisodic components and, after amplification of both components, a period of e xtremely high values during which individual episodic increases were n o longer readily recognizable. Preliminary mathematical modeling of th e data suggested that these patterns could be produced by a change in GnRH from a predominantly low to a mixture of low and high amplitude i nputs. Similar changes in minute to minute patterns of GnRH secretion were observed during the natural follicular phase. These findings are consistent with the hypothesis that estradiol induces the GnRH surge b y altering the mode of neurosecretion, rather than by merely causing q uantitative changes in the episodic pattern of release.