EVIDENCE OF A PERMISSIVE EFFECT OF EXTRAOVARIAN STEROIDS ON THE RELEASE OF FSH AT EARLY ESTRUS IN RATS LACKING INHIBIN SECRETION OR ACTION

The administration (sc) of 1 mg of a LHRH antagonist (LRRHa) (Organon, Oss, The Netherlands) to 4-day cyclic rats at 0900 h in proestrus blo cked the preovulatory (proestrous 1830 h) release of gonadotropins (LH and FSH) and abolished the secondary (estrous 0200 h) release of FSH. The administration (sc) of 4 mg of either antiprogestagen RU486 (Rous sel-Uclaf, Romainville, France) or antiprogestagen ZK299 (Schering, Be rlin, Germany) at 0900 h in proestrus blunted the preovulatory release of gonadotropins and abolished the secondary release of FSH. The effe ct of the LHRHa on the secondary surge of FSH in cyclic rats was total ly reversed by an ovulatory (sc) injection (10 IU) of hCG at 1700 h in proestrus. Injections of 3, 5 or 10 mg (SC) Of progesterone at 1500 h in proestrus or of 0.5 mi (iv) of an anti-inhibin serum at 1900 h in proestrus done reversed, although only in part, the effect of LHRHa on the serum concentration of FSH at 0200 h in estrus. The combination o f progesterone and anti-inhibin serum injections to LHRHa-treated rats completely restored the secretion of FSH at early estrus. The removal of ovarian progesterone and inhibin by ovariectomy (OVX) at 1500 h on proestrus did not affect the serum concentration of FSH at 0200 h on estrus either in oil- or RU486-treated rats, The injection of progeste rone to OVX-rats did not affect the serum concentrations of FSH or LH at 0200 h in estrus. These combined observations suggest that, in the rat, the preovulatory LH-dependent drop in ovarian inhibin secretion, together with the actions of steroids, which can be blocked by the adm inistration of either RU486 or ZK299, during proestrous afternoon and evening allow the secondary release of FSH during early estrus. These steroids (progesterone and/or glucocorticoids) come from extraovarian tissues (most probably the adrenal glands).