Effects of estrogen and progesterone on pulsatile discharges of luteinizing hormone in the ovariectomized rat.

The effects of both acute and chronic systemic administration of estrogen and progesterone on pulsatile discharges of luteinizing hormone were observed in ovariectomized rats. Intravenous injection of 10 microgram of 17beta-estradiol resulted in a gradual decrease in plasma luteinizing hormone (LH) concentrations but did not completely block the pulsatile discharges. Subcutaneous implantation of Silastic capsules containing 17beta-estradiol caused similar decreases; plasma LH concentrations decreased for the first 6 h but periodic increases continued, with nearly complete suppression of such pulsatile discharges by 24 h. Such inhibition was maintained for the 14-day course of the experiments. Progesterone alone, whether administered by Silastic capsule or by daily subcutaneous injection in oil, was without apparent effect on either plasma LH concentrations or temporal patterns of the hormone. Further, progesterone combined with 17beta-estradiol was little, if any, more effective in this regard than estradiol alone. These results indicate that estrogen is an effective feedback signal inhibiting pulsatile release of LH in the rat and that progesterone alone has little, if any, influence in this regard.