Observations on facilitation of the preovulatory rise of LH by estrogen

Systemic levels of estradiol (E2) during the 4-day estrous cycle rise in a biphasic manner; slow elevations during diestrus II are followed by rapid increases during the early morning of proestrus (Kalra and Kalra, 1973). The relation of these alterations in systemic E2 levels to the proestrus surge of LH was investigated. Administration of progesterone (P 4 mg/rat) to rats at 2200 hr of diestrus II blocked ovulation and the surge of LH in the afternoon of proestrus. Injection of estradiol benzoate (EB, 5 mug/rat) 2 hr prior to, together with or following P treatment at 0300 hr of proestrus restored ovulation and the surge of LH. However, blockade of ovulation was not reversed if EB was administered at 0500 or 0800 hr of proestrus following P treatment on diestrus II. Similarly, bilateral ovariectomy at 2300 hr of diestrus II but not at 0300 hr or later on proestrus abolished the proestrus surge of LH. Apparently, the rapid rise in systemic E2 between 2300 hr of diestrus II and 0300 hrs of proestrus is crucial and serves to activate the neural "trigger" of preovulatory release of LH. The role of ovarian E2 secretion prior to 2300 hr of diestrus II in facilitation of neural "trigger" was studied. In the absence of endogenous E2 priming produced by ovariectomy at 0800 hr of diestrus II, EB injections at 2300 hr of the same day failed to restore the proestrus surge of LH. However, when ovariectomy was delayed to allow normal E2 priming through diestrus II, administration of EB shortly after ovariectomy at 2300 hr of diestrus II elicited the normal proestrus surge of LH. These results indicated that following the initial "priming" of central site(s) with low levels of circulating E2 during diestrus II, rapid elevations in ovarian estrogen secretion between 2300 hr of diestrus II and 0300 hr of proestrus facilitated the neural "trigger" of pituitary LH release during the critical period on proestrus.