Regulatory role of estrogen in the ovulatory discharge of luteinizing hormone

The positive feedback role of estrogen in facilitating the ovulatory discharge of LH has been investigated. It has been shown that estrogen secretion increases in 4-day cyclic rats sometime late in diestrus-2 or early proestrus and reaches peak concentrations at 1000 hours proestrus. LH release begins at 1400 hours and peaks at 1600 hours. Simultaneous with changes in estrogen, peripheral plasma progesterone, of adrenal origin, exhibits diurnal rhythms being elevated between 0100-0500 hours, decreasing to nadir at 1400-1600 hours each day. Consequently, since both sex steroids' plasma levels are fluctuating prior to the ovulatory LH surge, the ratio of estrogen/progesterone may prove to be more important in altering the hypothalamic thresholds of excitability required for LH release than estrogen acting alone. Four-day cyclic rats were ovariectomized (ovx) and/or adrenalectomized (adx) on diestrus-2 at 1000 hours. In ovx rats, replacement of endogenous estrogen by injection of 10 μg 17β-estradiol (E₂) resulted in a more or less normal LH surge at proestrus. When progesterone was administered to such ovxestrogen-primed rats, at 0930 hours proestrus, the timing of the LH surge was synchronized, and all rats exhibited peak LH values at 1400 hours. Ovx-adx rats treated with E₂ failed to release LH and only did so if estrogen were combined with progesterone. It is suggested from these studies that, in the rat, both steroids, acting synergistically, are essential for the ovulatory LH surge to occur.