Chronic administration of estradiol produces a triphasic effect on serum concentrations of gonadotropins and messenger ribonucleic acid for gonadotropin subunits, but not on pituitary content of gonadotropins, in ovariectomized ewes

To determine the acute and chronic effects of estradiol on synthesis and secretion of LH and FSH, ovariectomized ewes were administered estradiol via silastic capsules for 0 h, 12 h, 1 day, 2 days, 4 days, 8 days, 16 days, or 32 days (n = 5/group). Concentrations of GnRH in the median eminence began to decrease within 12 h and were lower (p less than 0.05) than in control ewes from 1 to 4 days after estradiol administration was begun. Serum concentrations of LH were decreased relative to pretreatment control levels from 1 to 10 h, elevated during a preovulatory-like surge from 11 to 22 h, and then decreased and remained below 1 ng/ml for the duration of the experiment. Serum concentrations of FSH followed a pattern similar to those for LH except that the magnitude of change was smaller. Treatment with estradiol initially (12 h) reduced (p less than 0.05) quantities of mRNA for alpha-, LH beta-, and FSH beta-subunits, after which the quantities of mRNA for the subunits returned to near or above control levels by Day 2. After 8 days of treatment the amounts of mRNAs for gonadotropin subunits were again less (p less than 0.05) than those of controls, and they remained suppressed through Day 32. Pituitary concentrations of LH and FSH decreased (p less than 0.05) during the first day of treatment and remained suppressed for the duration of the experiment. Thus, estradiol had a triphasic effect on secretion of gonadotropins and steady-state levels of mRNA for the gonadotropin subunits, but not on pituitary content of gonadotropins.(ABSTRACT TRUNCATED AT 250 WORDS)