CENTRAL REGULATION OF PULSATILE GONADOTROPIN-RELEASING-HORMONE (GNRH)SECRETION BY ESTRADIOL DURING THE PERIOD LEADING UP TO THE PREOVULATORY GNRH SURGE IN THE EWE

An experiment was conducted to investigate the central regulatory effe cts of estradiol on GnRH secretion leading up to the preovulatory LH s urge in the ewe. Midluteal phase ewes were ovariectomized, treated wit h steroid implants to maintain luteal phase concentrations of progeste rone and estradiol, and fitted with an apparatus for collection of hyp ophyseal portal blood. After simulated luteolysis (removal of progeste rone implants), the ewes were allocated to one of three groups: estrad iol withdrawn, estradiol maintained at a luteal phase level, or estrad iol raised from a luteal phase level to a peak follicular phase level in two increments. The results demonstrated that during the interval b etween luteolysis and the preovulatory gonadotropin surge, estradiol e xerts a dose-dependent suppression of GnRH secretion from the hypothal amus. This effect reflects a suppression of GnRH pulse size and occurs despite a stimulatory action of estradiol on GnRH pulse frequency. Th e suppressive effect of estradiol on GnRH secretion, however, was dela yed relative to that on LH. We conclude that during the period leading up to the preovulatory surge in the ewe, estradiol acts centrally, re ducing GnRH secretion by suppressing GnRH pulse size.