Fertility impairment after mifepristone treatment to rats at proestrus. Actions on the hypothalamic-hypophyseal-ovarian axis.

Accumulated evidence indicates that the antigestagen mifepristone affects the reproductive axis acting on hypothalamic, pituitary, ovarian, and uterine tissues. The purpose of this study was to further investigate which reproductive functions are impaired by the antagonist, critically compromising the reproductive process, leading to unsuccessful pregnancy. Circulating pituitary and ovarian hormones, sexual receptivity, ovulation, and implantation rates were studied in cycling rats receiving a single dose of mifepristone (1 or 10 mg/kg subcutaneously) at 12:00 proestrus, before luteinizing hormone (LH) stimulation of the ovulatory process. Mifepristone-treated rats had decreased preovulatory surges of LH and prolactin (PRL), and hypersecretion of LH, PRL, and progesterone at estrus. The sexual receptivity was dramatically affected by the antagonist as indicated by the profound decrease in the lordosis response evaluated on the night of proestrus. The number of ovulating animals and the number of oocytes recovered from the oviduct on the morning of estrus were not affected by mifepristone. The low number of rats that succeeded in mating with potent males became pregnant. However, they delivered an average of only two pups at parturition, indicating a failure in the implantation of the fertilized ova, as ovulation was not affected by the antagonist at the dose used. We conclude that a dramatic inhibition of the sexual receptivity and unsuccessful implantation, preceded by a reduction on LH and PRL secretion, are the major components leading to fertility impairment after a single dose of mifepristone administered before the preovulatory surge of LH.