A daily signal for the LH surge in the rat.

We have demonstrated that a brief step-like increment in circulating estradiol concentrations to approximately 100 pg/ml, achieved by SC insertion of Silastic capsules containing estradiol-17 beta and their withdrawal after 29 1/2 hr, elicited a daily LH surge on 4 consecutive afternoons in rats ovariectomized 2 weeks previously. Since the duration of this stimulus was similar to that of the preovulatory increment in serum estradiol concentrations, it was postulated that the endogenous estrogen signal in intact rats might also trigger repetitive LH surges if ovulation and formation of corpora lutea were prevented. To test this hypothesis, rats were ovariectomized at 10:00 AM on proestrus (day 1) and blood samples were obtained at 12:30 and 5:00 PM on days 1 to 4. Although an LH surge occurred on proestrus, no subsequent LH discharges were observed. The absence of an LH surge on consecutive days could not be attributed to a difference between the endogenous estradiol stimulus and the exogenous stimulus which elicited repetitive LH surges in long-term ovariectomized rats. Rather, it was determined that in recently ovariectomized rats, in contrast to long-term ovariectomized rats, a daily LH surge occurred only if elevated serum estradiol concentrations were maintained. Thus, by leaving implants in place, an LH discharge was elicited on 10 consecutive days. These results support the concept that a neural signal for the LH surge is emitted each day throughout the estrous cycle of the rat, and that prolonged maintenance of elevated circulating estradiol concentrations is essential for the expression of these signals.