Effects of estradiol on concentrations of gonadotropin-releasing hormone receptor messenger ribonucleic acid following removal of progesterone

To test the hypothesis that low levels of estradiol are sufficient to increase concentrations of GnRH receptor mRNA in the absence of progesterone, ewes were ovariectomized and immediately treated with estradiol implants for 12 h to achieve circulating concentrations of estradiol typical of the early (n=5) or late (n=4) follicular phase. Five additional ewes underwent lutectomy, and control ewes were untreated. Treatment of ewes with 1/2 or 1 estradiol implant increased concentrations of estradiol in serum to 3.0 ± 0.8 pg/ml or 6.3 ± 0.3 pg/ml, respectively, and concentrations of estradiol in lutectomized ewes (2.4 ± 0.5 pg/ml) were intermediate. Ovariectomy did not alter concentrations of GnRH receptor mRNA or numbers of GnRH receptors. Treatment of ewes with 1 estradiol implant increased concentrations of GnRH receptor mRNA and numbers of GnRH receptors. In ewes treated with 1/2 estradiol implant, concentrations of GnRH receptor mRNA were intermediate between controls and ewes treated with 1 estradiol implant, and numbers of GnRH receptors were greater than controls. Lutectomy increased concentrations of GnRH receptor mRNA but did not affect numbers of GnRH receptors. We conclude that estradiol stimulates expression of the GnRH receptor gene and numbers of GnRH receptors in the absence of progesterone. However, effects of estradiol on expression of the GnRH receptor gene were clearly evident only when concentrations of estradiol were elevated to levels typical of the late follicular phase.