The Estrogen-Induced Advance in the Cyclic LH Surge in the Rat: Dependency on Ovarian Progesterone Secretion1

In the 4-day cycling rat, injection of 20 μg of estradiol benzoate (EB) at 1230 on diestrus day 1 stimulates a 24-hr advance in the cyclic ovulatory LH surge, which then occurs between 1400 and 1900 on diestrus day 2. As a result, ovulation is also advanced 24 hr. Experiments were conducted to determine if this response depends on ovarian hormone secretion. Ovariectomy within one hour of EB injection on day 1 markedly diminished the magnitude of the LH surge on day 2 as compared to sham-ovariectomized controls. However, in similarly estrogenizedovariectomized rats, a supranormal LH surge was elicited by a 2.0 mg progesterone injection at 1200 on day 2. In additional experiments, ovarian progesterone secretion was measured at 1000 and 1300 on day 2 in untreated control rats and in rats receiving the EB injections. Progesterone secretion rates were significantly increased above normal values in the EB-treated rats. Together, these results suggest that the stimulatory feedback effects of estrogen injections on gonadotropin secretion in the cycling rat depend on concurrent ovarian progesterone secretion. Other rats were treated with 10 μg of EB at 1500-1600 on day 1. Ovulation was not advanced in these rats; ovarian progesterone secretion rates at 1000 and 1300 on day 2 were not significantly higher than normal. However, if progesterone levels were raised exogenously on the morning of day 2 by the injection of 2.0 mg of progesterone, advanced ovulation was consistently observed the next day. Such results are consistent with the hypothesis that in the cycling rat, EB-stimulated gonadotropin secretion occurs subsequent to, and is dependent upon, the stimulation of ovarian progesterone secretion. (Endocrinology93: 385, 1973)