A Possible Cause for the Differing Responses of the Luteinizing Hormone Surge Mechanism of Ovariectomized Rats to Short Term Exposure to Estradiol*

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Abstract
Subcutaneous placement of Silastic implants of 17β-estradiol (E2) for 33 h induces a surge of LH on only 1 day in rats acutely ovariectomized (ovx) on the morning of placement but produces daily surges of LH in rats ovx 14 days previously. A possible explanation for this difference in response of acute and long term ovx rats to E2 challenge is provided in this study. Rats which were exposed to endogenous levels of E2 on diestrus-2 (day 0) and progesterone (P) on proestrus (day 1) were ovx on the following diestrus-2 (day 4). The surge of LH in response to sc implantation of E2 for 33 h on day 4 or 8 was limited to a single day (days 5 and 9, respectively) and did not return on subsequent days. However, sc implantation of E2 for 33 h on day 12 induced daily surges of LH on days 13, 14, and 15. Long term (≥14 days) ovx rats received sc implants of E2 from 0800 h on day 0 (simulated diestrus-2) to 1700 h on day 1 (simulated proestrus) and implants of P from 1400 h on day 1 until 0300 h on day 2 (simulated estrus). As in the acutely ovx rat, when challenged with E2 4 or 8 days later, these animals responded with a surge of LH on day 5 or 9, respectively, but not on subsequent days. In contrast, implantation of E2 on day 12 in similarly pretreated long term ovx rats resulted in LH surges on each of days 13, 14, and 15. Thus, the differing sensitivities of acute and long term ovx rats to an estradiol stimulus is due, at least in part, to retention of the inhibitory effect of progesterone on the daily LH release mechanism for at least 8 days. (Endocrinology106: 1677, 1980)