Interactions between oestradiol and the progesterone antagonist RU-486 in establishing and maintaining female rats' sexual responsiveness: central versus peripheral effects

Dose-dependent effects of the antiprogestin, RU486, on sexual behavior of naturally cycling Fischer rats

Regularly cycling Fischer female rats were treated with either a low (5mg/kg) or high (5mg/RAT; approximately 30mg/kg) dose of the antiprogestin, RU486, before the morning of proestrus or on the morning of proestrus. The emergence of sexual behavior after treatment with RU486 was examined in a mating test with a sexually active male rat. Lordosis behavior was remarkably resistant to the effects of RU486. Only the high dose of RU486 given the evening before proestrus, approximately 22h before mating, reduced lordosis behavior. Independent of dose or time of treatment, proceptivity was reduced and resistance to the male's attempts to mount was increased by RU486 treatment. In addition, the effect of a 5min restraint stress on sexual behavior was examined. In contrast to the relative resistance of lordosis behavior of unrestrained rats to RU486 treatment, RU486 treated rats showed a significant decline in lordosis behavior after restraint. These findings allow the suggestion that the emergence of lordosis behavior is relatively resistant to the antiprogestin while the maintenance of lordosis behavior after restraint may require participation of intracellular progesterone receptors.

The display of sexual behaviors by female rats administered ICI 182,780

ICI 182,780 (ICI) is a pure antiestrogen that when administered systemically does not cross the blood-brain barrier, thus its actions are limited to the periphery. Four experiments were conducted to test the effects of ICI on the display of sexual behaviors in ovariectomized rats. Experiment 1 examined the effects of three doses of ICI (250, 500, and 750 micro g/rat) on sexual receptivity and paced mating behavior in rats primed with estradiol benzoate (EB) in combination with progesterone (P). Experiments 2 and 3 compared the display of sexual behaviors in rats primed with EB+P or EB alone and administered either 250 micro g ICI (Experiment 2) or 500 micro g ICI (Experiment 3). Experiment 4 tested the effects of ICI (250 and 500 micro g) on the expression of estrogen-induced progestin receptors in the uterus. ICI did not affect the display of sexual receptivity in any experiment. In rats primed with EB+P, paced mating behavior was altered by the 500 and 750 micro g, but not the 250 micro g, doses of ICI. The lowest (250 micro g) dose of ICI did alter paced mating behavior in rats primed with EB alone. The effects of ICI on paced mating behavior were manifested by a substantial lengthening of contact-return latencies following intromissions and ejaculations. The percentage of exits were not affected by ICI. Estrogen stimulation of uterine weight and induction of uterine progestin receptors was suppressed by ICI (250 and 500 micro g). ICI effects on paced mating behavior in hormone-primed female rats are likely to reflect antiestrogenic actions in the periphery, including interference with the estrogen induction of progestin receptors.

A single dose of mifepristone induces ovulation in pseudopregnant rats

We used mifepristone (M) to evaluate the role of progesterone in maintaining pseudopregnancy. Cycling rats were made pseudopregnant (psp) by cervico-vaginal stimulation (CVS) on the day of estrus (day 0) and received 10 mg/kg M or vehicle (control groups) on day 3. Blood samples were taken at 06.00 h on days 4, 6 or 7 or at 18.00 h on days 3, 4, 6 or 10. M induced proestrus 2 days later (day 5), estrus on day 6, and a second prolonged diestrus afterwards. Prolactin and progesterone levels were similar in the control and M treated groups excepting on day 6, when both were reduced in the M-treated animals, and these rats were in estrus, suggesting a temporary impairment of luteal function. To demonstrate activated corpora lutea the endometrium was scratched on the fourth day of the first or second diestrus in additional control and M-treated groups. The deciduomal response was seen in the control and M groups after scratching the endometrium on day 4 of the first or second diestrus, respectively, but M blocked the deciduomal response in the first diestrus. Ovulation was confirmed by finding that 66.7% of the M-treated rats showed ova in the Fallopian tubes on the M-induced estrus and 4 out of 10 of the M rats placed with males on the M-induced proestrus showed spermatozoa in the vaginal smears. Half of these became pregnant, delivering 2 pups each. The results show that M can induce ovulation in psp rats, demonstrating that the anovulation observed after CVS is dependent on progesterone, yet luteal function persists after M in pseudopregnancy. Progesterone may act either by suppressing LH secretion or by permitting prolactin secretion, or both. Moreover, progesterone is required to maintain endometrial responsiveness.