The follicular phase rise in circulating estradiol causes a suppression of GnRH pulse amplitude and an increase in pulse frequency before stimulation of the preovulatory GnRH surge in the ewe. Studies in which samples were collected at 10-min intervals were found unsuitable to determine whether GnRH secretion is exclusively pulsatile at this time or whether estradiol induces additional qualitative changes in the pattern of GnRH secretion that, combined with the quantitative changes, result in generation of the surge. To address these issues, GnRH was measured in pituitary portal blood and LH in peripheral blood collected at 1- and 10-min intervals, respectively, in ewes receiving no estradiol (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 falling phases of each pulse. These findings lead to the conclusion that during the presurge period in the ewe, estradiol induces qualitative change in the pattern of GnRH release in addition to stimulating GnRH pulse frequency and reducing pulse amplitude.
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