ESTRADIOL INDUCES BOTH QUALITATIVE AND QUANTITATIVE CHANGES IN THE PATTERN OF GONADOTROPIN-RELEASING-HORMONE SECRETION DURING THE PRESURGE PERIOD IN THE EWE

The follicular phase rise in circulating estradiol causes a suppressio n of GnRH pulse amplitude and an increase in pulse frequency before st imulation of the preovulatory GnRH surge in the ewe. Studies in which samples were collected at 10-min intervals were found unsuitable to de termine whether GnRH secretion is exclusively pulsatile at this time o r whether estradiol induces additional qualitative changes in the patt ern of GnRH secretion that, combined with the quantitative changes, re sult in generation of the surge. To address these issues, GnRH was mea sured in pituitary portal blood and LH in peripheral blood collected a t 1- and 10-min intervals, respectively, in ewes receiving no estradio l (n = 5), a luteal phase level of circulating estradiol (n = 5), or a peak follicular phase concentration of estradiol (n = 5). The results provide evidence that, in a dose-dependent manner, estradiol induces the secretion of significant amounts of GnRH between pulses and alters the shape of GnRH pulses by reducing the slope of the rising and fall ing phases of each pulse. These findings lead to the conclusion that d uring the presurge period in the ewe, estradiol induces qualitative ch ange in the pattern of GnRH release in addition to stimulating GnRH pu lse frequency and reducing pulse amplitude.