Effects of progesterone on the secondary surge of follicle-stimulating hormone in the rat.

In the cyclic rat, the secondary surge of FSH on estrus appears to depend on the LH surge-induced fall in serum concentrations of inhibin. To investigate the involvement of progesterone in the regulation of the secondary surge of FSH, 4-day cyclic rats were treated on proestrus with an antagonist of LHRH (LHRHant) and with an ovulatory dose of ovine (o) LH, progesterone, the antiprogestin RU486, or the combination of RU486 and oLH. Serum concentrations of gonadotropins and inhibin at 1830 h on proestrus and at 0030 h on estrus were determined, and the expression of inhibin/activin subunit mRNAs in the ovary at 0030 h on estrus was analyzed by in situ hybridization. Rats receiving saline showed low expression of alpha-, beta(A)-, and beta(B)-subunit mRNAs in the ovary and low serum levels of inhibin in conjunction with the elevated serum concentrations of FSH on estrus. Administration of LHRHant blocked the decrease in the synthesis and secretion of inhibin and abolished the FSH secondary surge, whereas the injection of oLH prevented these effects. Exogenous progesterone, compared with LHRHant injection, increased alpha-, beta(A)-, and beta(B)-subunit mRNA hybridization intensity in the ovary and serum inhibin immunoreactivity, and also restored, in part, the surge of FSH on estrus. The antiprogestin RU486 did not modify the effect of oLH on either inhibin/ activin subunit mRNAs in the ovary or serum levels of inhibin, but blocked the FSH surge. These results indicate that, in the cyclic rat, 1) the secretion of progesterone on proestrous afternoon, induced by the LH surge, is not involved in the fall of ovarian inhibin synthesis and secretion; and 2) in combination with a drop in serum inhibin, a stimulatory action of progesterone on another factor, possibly pituitary activin, could be necessary to elicit a complete secondary surge of FSH.