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 dep end on the LH surge-induced fall in serum concentrations of inhibin. T o investigate the involvement of progesterone in the regulation of the secondary surge of FSH, 4-day cyclic rats were treated on proestrus w ith an antagonist of LHRH (LHRHant) and with an ovulatory dose of ovin e to) LH, progesterone, the antiprogestin RU486, or the combination of RU486 and oLH. Serum concentrations of gonadotropins and inhibin at 1 830 h on proestrus and at 0030 h on estrus were determined, and the ex pression of inhibin/activin subunit mRNAs in the ovary at 0030 h on es trus was analyzed by in situ hybridization. Rats receiving saline show ed low expression of alpha-, beta(A)-, and beta(B)-subunit mRNAs in th e ovary and low serum levels of inhibin in conjunction with the elevat ed serum concentrations of FSH on estrus. Administration of LHRHant bl ocked the decrease in the synthesis and secretion of inhibin and aboli shed the FSH secondary surge, whereas the injection of oLH prevented t hese effects. Exogenous progesterone, compared with LHRHant injection, increased alpha-, beta(A)-, and beta(B)-subunit mRNA hybridization in tensity in the ovary and serum inhibin immunoreactivity, and also rest ored, 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. Th ese results indicate that, in the cyclic rat, 1) the secretion of prog esterone on proestrous afternoon, induced by the LH surge, is not invo lved in the fall of ovarian inhibin synthesis and secretion; and 2) in combination with a drop in serum inhibin, a stimulatory action of pro gesterone on another factor, possibly pituitary activin, could be nece ssary to elicit a complete secondary surge of FSH.