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

0013.7227/96/$03.00/O Endocrinology Copyright 0 1996 by The Endocrine Vol 137, No. 4 Prrnted in U.S.A. Society RU486 on an Estrogen Background Blocks the Rise in Serum Follicle-Stimulating Hormone Induced by Antiserum to Inhibin or Ovariectomy* KERRY L. KNOX, SONIA SARA SUTANDI, NEENA Department of Neurobiology J. RINGSTROM, B. SCHWARTZ and Physiology, MARTA Northwestern SZABO, CHAD lJniversit.y, Evanston, ABSTRACT A. PERLYN, Illinois 60208 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 pg) on the evening of proestrus, before the RU486 and anti-1 treatment on estrus, caused RU486 to suppress the effects of anti-1 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. (Endocrinology 137: 1226-1232, 1996) We used passive immunization with an antiserum to the a-subunit of inhibin (anti-11 or acute ovariectomy to investigate the relationship between serum inhibin levels and FSH secretion in the presence ofthe progesteroneiglucocorticoid antagonist RU486. We demonstrated previously that 1) anti-1 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-1 on a different day of the cycle, estrus, when serum estradiol levels are low. When both RU486 and anti-1 were given on estrus (1230 and 1700 h, respectively), RU486 failed to block the T under conditions where the level of serum estradiol on estrus was raised by estrogen administration on proestrus. The results of the study demonstrate that the inhibitory effects of RU486 on FSH secretion are cycle stage dependent and that RU486’s ability to antagonize the rise in serum FSH in response to a drop in circulating inhibin due to immunoneutralization or ovariectomy requires an estrogen background. HE ANTIPROGESTERONE /antiglucocorticoid RU486 given on proestrus, a day when ambient estradiol levels are high (l), blocks the naturally occurring secondary FSH surge as well as the rise in serum FSH induced by immunoneutralization with an antiserum to inhibin-a (anti-0 (2). Recently, we showed that this suppression of the FSH rise on estrus by treatment with RU486 on proestrus (2,3) probably does not involve blockade of the action of either progesterone or corticosterone (4). We wanted to probe further the effect of RU486 on serum FSH on a different day of the estrous cycle, because progesterone per SE has different effects (stimulatory or inhibitory) depending on when it is given during the cycle (5-8). Administration of anti-1 on proestrus (2) or diestrus (9) increases serum FSH levels in adult female rats. The natural preovulatory and secondary FSH surges, susceptible to suppression by RU486, are triggered on proestrus, when ambient estrogen levels are high (1). The objective of the present study was to test the effect of RU486 on the rise in serum FSH induced by a fall in serum inhibin on estrus when serum estradiol is normally low (1). We lowered circulating inhibin, in addition to anti-I, by acute ovariectomy. We then retested the effects of RU486 on elevated serum FSH Materials and Methods Animals Female Sprague-Dawley rats (55-60 days old) were obtained from Charles River (Portage, MI). Animals were housed under a 14-h light, 10-h dark schedule, with lights on at 0500 h, and were provided with standard rat chow and tap water ali lihit~rm. Estrous cyclicity was monitored by daily vaginal cytology; only rats that showed at least two consecutive 4-day cstrous cycles were used. Drug treatments RU486 (6 mg/kg) was dissolved in sesame oil with slight warming and injected SC; the oil vehicle alone served as a control. Sheep antirat inhibin-a-(1-26) serum 795 (0.5 ml) was injected into the tail vein under light metophane anesthesia. Normal sheep serum (NSS; 0.5 ml; ICN ImmunoBiologicals, Costa Mesa, CA) served as the control. Estradiol benzoate (EB; 50 wg; Sigma Chemical Co., St. Louis, MO) was dissolved in sesame oil and injected SC; oil vehicle served as the control. Received November 6, 1995. Address all correspondence and requests for reprints to: Dr. Neena B. Schwartz, Department of Neurobiology and Physiology, Northwestern University, 2153 North Campus Drive, Evanston, Illinois 60208.3520. * This work was supported in part by NIH Grants POl-HD-21921, P30-HD-28048, and ROl-HD-07504 (to N.B.S.) and T32-HD-07068 (to K.L.K.). Surgery After cleansing the skin with ethanol, rats were bilaterally ovariectomized under metophane anesthesia; sham surgery consisted of making the 1226 RU486, appropriate incisions proved by the animal Collection ESTROGEN, AND FSH SECRETION and exposing the ovaries. These protocols were apcare and use committee of Northwestern University. of tissue and serum " E 10 P G2 5 RIAs 0 Serum FSH and LH were determined by double antibody RIA, as previously described (10); reagents were supplied by the NIDDK. Serum inhibin-a was determined by a homologous double antibody RIA, as previously described (11). The intra- and interassay coefficients of variation were 7.6% and lO.l%, respectively. Serum progesterone and estradiol were measured using kits from ICN Biomedicals (Irvine, CA) and Diagnostics Products Corp. (Los Angeles, CA), respectively. 12 p3 c F -I OiI/NSS Oil/Anti-l RWNSS RU/Anti-I 4 protocols Exp 1: effects of RU486 and anti-l administered on estrus on gonadotropin secretion on metestrus. Rats were injected at 1230 h on estrus with oil or RU488. At 1700 h on the same day, half the rats in each group were treated with anti-I, and the other half were treated with NSS. Rats were killed the next morning at 0800 h on metestrus, and blood was collected for the measurement of serum LH and FSH by RIA. Exp 11: effects of pretreatment with EB on proestrus on gonadotropin secretion in rats treated with RU486 and anti-l on estrus. Rats were injected SC with oil or EB at 1700 h proestrus. Half of the animals in each group (...truncated)