Extinction of the estrogen-induced daily signal for LH release in the rat: a role for the proestrous surge of progesterone.

The heightened secretion of estrogen on diestrus-2 is required for the release of an ovulatory amount of LH on the following day, proestrus. Though these surges occur once every 4-5 days in cycling rats, the treatment of ovariectomized rats with a single injection of estrogen results in daily proestrus-like surges of LH. The present study was designed to test the possibility that progesterone, secreted on proestrus, prevents the daily expression of LH surges in cycling animals. The administration of estradiol benzoate (EB, 50 mug) to ovariectomized rats resulted in daily surges of LH secretion for 3 consecutive days. These surges were similar in the timing of onset and duration to the preovulatory surge of LH on proestrus. Serum LH concentrations were not increased on either the second or the third day when 5 mg of progesterone was given at 1600 h on the day of the first surge. These data suggest that progesterone blocks the expression of daily LH surges induced by estrogen. To determine whether this relationship obtains during the estrous cycle, groups of rats were treated with sodium pentobarbital (PB) at 1345 h during proestrus. This treatment postponed the proestrous surges of LH and progesterone for 24 h. When the surge of progesterone was simulated in PB-treated rats by the administration of 5 mg of the steroid at 1400 h during proestrus, a preovulatory surge of LH was not detected during either proestrous, estrus,, or diestrus-1. These data, taken together, suggest that estrogen turns on a "memory center" for the expression of daily LH surges and that one function of the increased secretion of progesterone on proestrus is to limit the expression of the "memory center" to this day.