ESTRADIOL-INDUCED INCREASE IN NUMBER OF GONADOTROPIN-RELEASING-HORMONE RECEPTORS IN CULTURED OVINE PITUITARY-CELLS

Primary cultures of ovine pituitary cells were used to characterize the effects of estradiol on number of GnRH receptors. Number of GnRH receptors was determined by the specific binding of a saturating dose of a radioiodinated GnRH analog. Estradiol stimulated an increase in number of GnRH receptors that was dose-dependent and had a time course similar to that observed in vivo. Estradiol at concentrations from 0.4 to 40 nM increased (p < 0.05) the number of GnRH receptors relative to controls. Numbers of GnRH receptors were increased (p < 0.05) from 8 to 24 h after addition of estradiol, with a maximum occurring at 12 h. Actinomycin D (10 μg/ml) and cycloheximide (25 μg/ml) prevented the estradiol-induced increase in number of GnRH receptors at 12 h, suggesting that both mRNA transcription and protein synthesis are required for this action of estradiol. Cortisol, progesterone, and tamoxifen had no direct effect on number of GnRH receptors. However, tamoxifen blocked the estradiol-induced increase in number of GnRH receptors. There appeared to be some synergism between estradiol and progesterone since the number of receptors in cells treated with both steroids tended (p < 0.1) to be greater than after treatment with estradiol alone. Thus, estradiol elicits an increase in the number of pituitary receptors for GnRH that is time- and dose-dependent, specific, and requires both mRNA transcription and protein synthesis. This effect of estradiol probably plays a role in increasing the sensitivity of the pituitary to GnRH prior to the preovulatory surge of LH.