Regulation of anterior pituitary gonadotropin subunit mRNA levels during the preovulatory gonadotropin surge: A physiological role of progesterone in regulating LH-β and FSH-β mRNA levels

In a previous study we demonstrated that in the ovariectomized estrogen-primed immature rat, progesterone induced a gonadotropin surge while the gonadotropin mRNA subunit levels were either suppressed or unaltered. This observation has now been confirmed using more frequent time points. Progesterone administered at 0900 h was found to suppress LH-β mRNA levels at 1300, 1400, and 0800 h the next day, with no subsequent effects at 1000, 1200 or 1600 h. FSH-β mRNA levels were unaffected by progesterone except for a slight elevation at 1400 h and a suppression at 0800 h. Progesterone was either suppressive or had no effect on alpha mRNA levels. Since elevations in LH-β and FSH-β mRNA levels were observed in the cycling rat, the observed differences in the ovariectomized estrogen-primed rat could be due to a higher basal synthesis occurring due to ovariectomy. This was indeed the case because LH-β and FSH-β mRNA levels were 3.7- and 42.7-fold higher in such animals as compared to intact estrogen-primed rats. In contrast to the ovariectomized estrogen-primed rats, in intact estrogen-primed rats LH-β mRNA levels were increased at 1000 h and FSH-β mRNA levels were increased at 1000, 1200 and 1300 h after the administration of progesterone. In pregnant mare's serum gonadotropin-primed immature rats, LH-β, FSH-β and α-subunit mRNA levels were significantly elevated at 1800 and 2000 h, paralleling the serum LH and FSH surge. The progesterone antagonist RU486 (0.2 and 1.0 mg) significantly reduced serum LH and FSH levels at 2000 h. The lower dose reduced LH-β and α-subunit mRNA levels at 2000 h and FSH-β mRNA levels at 1800 h. The higher dose caused an increase in LH-β mRNA levels at 1200 and 1800 h and a decrease in FSH-β mRNA levels at 1800 and 2000 h. In conclusion, the present study provides evidence that preovulatory progesterone plays an important role in the increase in FSH-β mRNA levels as well as the release of LH and FSH during the normal preovulatory gonadotropin surge. This relationship appears to be dependent on the ongoing rate of synthesis because this does not occur in the ovariectomized estrogen-primed rat in which synthesis is at a high basal level. Furthermore, the correlation with FSH appears to be tighter as compared to LH.