Effects of Progesterone on the Estradiol-Induced Follicle-Stimulating Hormone (FSH) Surge and FSHβMessenger Ribonucleic Acid in the Rat*

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Abstract
This study was designed to investigate the effects of progesterone on the estradiol (E2)-induced FSH surge and FSH.beta. messenger RNA (mRNA) using immature rat models developed previously to demonstrate inhibition or facilitation of the LH surge by progesterone. Twenty-eight day-old rats that received E2 implants at 0900 h had FSH surges about 1700 h on day 29 (32 h). In rats treated with E2 alone, serum FSH was 15.1 .+-. 1.6 ng/ml at this time, while in those animals treated concurrently with E2 and progesterone, serum FSH was significantly suppressed (8.3 .+-. 0.7 ng/ml, P < 0.001). For demonstration of progesterone facilitation, rats were primed for 24 h with E2 before progesterone treatment. This led to premature and enhanced FSH secretion: at 1400 h on day 29 serum FSH was 45.5 .+-. 2.7 ng/ml compared to 6.4 .+-. 0.5 ng/ml in rats treated with E2 alone. To examine the effects of these dual actions of progesterone on FSH synthesis, steady state concentrations of FSH.beta. mRNA were measured by Northern analysis. FSH.beta. mRNA generally increased in parallel with FSH release. Levels of this mRNA were about 1.5-fold higher in rats undergoing E2-induced FSH surges than in rats in which the surge was blocked by progesterone. Also, at the outset of the progesterone-facilitated FSH surge, FSH.beta. mRNA was about 5-fold higher in animals treated with E2 and progesterone than in those treated with E2 only. On the morning after the FSH surge (48 h after E2 treatment) FSH.beta. mRNA was low to undetectable. In contrast, levels of FSH.beta. mRNA were 7- to 8-fold higher at this time in rats in which the surge was blocked by progesterone. Serum inhibin concentrations were significantly elevated (P < 0.05) in animals treated with E2 alone for 32 h (3077 .+-. 260 fmol/ml) or 48 h (2344 .+-. 148 fmol/ml) compared to those treated with E2 and progesterone in the inhibition paradigm (2469 .+-. 106, 1896 .+-. 114 fmol/ml, respectively). After 32 of E2 treatment in the facilitation paradigm, serum inhibin was comparable (P > 0.2) in rats treated for 8 h with blank implants (2592 .+-. 168 fmol/ml) and those treated for 8 h with progesterone (2720 .+-. 188 fmol/ml). In summary, 1)FSH surges are accompanied by increases in FSH.beta. mRNA in these models; 2) the profound decrease in FSH.beta. mRNA at 48 h in surging rats may be due to elevated inhibin levels and/or to increased GnRH secretion at the time of the gonadotropin surge as we showed previously that both inhibin and GnRH down-regulate FSH.beta. mRNA levels in primary cultures of rat pituitary cells.

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