FOLLICULAR AND LUTEAL PROGESTERONE PLAY DIFFERENT ROLES SYNCHRONIZINGPITUITARY AND OVARIAN EVENTS IN THE 4-DAY CYCLIC RAT

Administration of 4 mg of the antiprogestagen RU486 to 4-day cyclic ra ts during proestrus induced a 1-day shortening of the ovarian cycle, a reduction in ovulatory rate that was reversed by an injection of exog enous human (h)FSH in the evening of proestrus, and the absence of the LH-inhibiting effect of exogenous estradiol resulting in a 24-h advan cement of the preovulatory LH surge. These effects were not present wh en RU486 was injected during estrus, RU486 injected during either proe strus or estrus increased serum levels of LH and estradiol-17 beta in diestrus and reduced the magnitude of the preovulatory surge of gonado tropins. Only rats treated with RU486 during estrus showed increased f ollicular size and acceleration of oocyte maturation on proestrous aft ernoon. These results demonstrate that in 4-day cyclic rats receiving an injection of RU486 during proestrus, the low ovulatory rate is a co nsequence of a reduced secondary FSH surge-induced follicular recruitm ent in the afternoon of estrus and that the shortening of the estrous cycle is the result of an advanced desensitization to the negative fee dback of estradiol on LH secretion. Furthermore, since the administrat ion of RU486 during proestrus blocks both follicular and luteal proges terone actions whereas injection during estrus blocks only luteal prog esterone actions, we suggest that, in 4-day cyclic rats, the actions o f progesterone during diestrus retard maturation of follicles via the lowering of serum LH concentrations and that the actions of progestero ne in proestrous evening delay the desensitization to the negative est rogen feedback on LH secretion.