RU486 on an estrogen background blocks the rise in serum follicle-stimulating hormone induced by antiserum to inhibin or ovariectomy

We used passive immunization with an antiserum to the alpha-subunit of inhibin (anti-I) or acute ovariectomy to investigate the relationship between serum inhibin levels and FSH secretion in the presence of the progesterone/glucocorticoid antagonist RU486. We demonstrated previously that 1) anti-I administered at 1700 h causes serum FSH to rise on the morning of estrus, even in the presence of a GnRH antagonist, when the two treatments are delivered on proestrus; and that 2) RU486 given on proestrus (1230 h), a time when serum estradiol levels are high, not only blocks the natural secondary FSH surge, but also suppresses the anti-I-induced rise in serum FSH on the morning of estrus. We have now extended our studies of the relationship between inhibin and RU486 to investigate treatment with RU486 and anti-I on a different day of the cycle, estrus, when serum estradiol levels are low. When both RU486 and anti-I were given on estrus (1230 and 1700 h, respectively), RU486 failed to block the anti-I-induced rise in serum FSH on the next morning of metestrus, in contrast to the blockade seen with RU486 treatment on the day of proestrus. However, pretreatment with estradiol benzoate (50 microgram) on the evening of proestrus, before the RU486 and anti-I treatment on estrus, caused RU486 to suppress the effects of anti-I on serum FSH, as it does when given on proestrus. We then repeated the study, using ovariectomy on proestrus or estrus (1700 h) to raise serum FSH, and assessed the effects of RU486 treatment at proestrus and estrus and estradiol benzoate treatment on proestrus. Our results indicate that treatment with RU486 can block the postovariectomy rise in serum FSH only in the presence of high circulating estradiol levels. We conclude that the inhibitory action of RU486 on FSH secretion after a fall in serum inhibin depends on a precedent estradiol background, probably due to induction of progesterone receptors by estradiol.

Mechanism of the inhibitory action of RU486 on the secondary follicle-stimulating hormone surge

Recent evidence utilizing RU486 has implicated progesterone (P) and glucocorticoids, in addition to a drop in serum inhibin, in the development of the secondary FSH surge on the morning of estrus. To assess the role of these steroids, we treated proestrous female rats with the antiprogestin/antiglucocorticoid RU486 (6 mg/kg sc) at 1230 h, and with dexamethasone (dex; 8.4 or 16.2 mg/kg sc), or with the steroid biosynthesis inhibitor aminoglutethimide (AG; 150 mg/kg ip) at 1030 h, alone or in combination with RU486. The effects of these treatments on uterine ballooning and intraluminal fluid content (an index of P action), ovulation, and serum levels of P, corticosterone (B), FSH, LH, and inhibin-alpha at 1830 h proestrus and 0900 h estrus were examined. In accord with previous work from our laboratory, RU 486 caused uterine intraluminal fluid retention on the morning of estrus and significantly suppressed the preovulatory surges of both FSH and LH, and the secondary surge of FSH without affecting the fall in inhibin-alpha. Treatment with dex alone raised serum FSH at both 1830 h proestrus and 0900 h estrus, coincident with suppression of serum inhibin-alpha. When administered in combination with RU486, dex partially reversed the increased uterine intraluminal fluid retention at 0900 h estrus, but did not modify the inhibitory effect of RU486 on the primary gonadotropin surges or the secondary surge of FSH. AG alone significantly suppressed serum P, B, and gonadotropins (LH to a greater extent than FSH) at 1830 h proestrus and blocked ovulation and uterine intraluminal fluid release at 0900 h estrus; it did not, however, suppress the secondary FSH surge or prevent the fall in serum inhibin-alpha. When administered 2 h before RU486, AG did not prevent the RU486-induced inhibition of the primary gonadotropin surges or the secondary FSH surge. We conclude from these results that development of the secondary FSH surge does not require P or glucocorticoid action and that RU486 suppression of the secondary FSH surge does not involve blockade of binding of these steroids to their receptors. Our data are compatible with ligand-independent activation of the P receptor, susceptible to blockade by RU486, as the mechanism underlying the enhanced secretion of FSH from the gonadotrope on the morning of estrus.