ESTRADIOL AND PROGESTERONE INFLUENCE THE SYNTHESIS OF GONADOTROPINS IN THE ABSENCE OF GONADOTROPIN-RELEASING-HORMONE IN THE EWE

Acute actions of estradiol and progesterone on synthesis and secretion of gonadotropins without the confounding effects of endogenous steroi ds and GnRH were examined by using the ovariectomized (OVX), hypothala mic-pituitary-disconnected (HPD) ewe. Thirty-two OVX ewes were subject ed to HPD and randomly assigned to eight groups (4 ewes/group). An add itional four OVX awes served as controls, Each of 12 OVX-HPD ewes rece ived 4 s.c. implants of estradiol 24 h after HPD (implantation was des ignated as Time 0 h). Sixteen OVX-HPD ewes received no treatment and s erved as contemporary controls. The remaining four OVX-HPD ewes receiv ed implants of estradiol for 48 h and were administered 16 mg progeste rone i.m. twice daily in the last 24 h of estradiol treatment. Blood s amples and pituitary glands were collected from OVX-HPD control and st eroid-treated ewes at 0, 12, 24, and 48 h. Amounts of gonadotropin sub unit mRNAs and serum concentrations of gonadotropins were decreased at 24 h after HPD and remained unchanged thereafter in OVX-HPD controls. Although treatment with estradiol decreased pituitary content of LH a nd steady-state levels of mRNA for LH beta-subunit (p < 0.05) compared to contemporary controls, serum concentrations of LH were not affecte d. Serum concentrations and pituitary content of FSH decreased (p < 0. 05) 24 h after initiating treatment with estradiol, but steady-state l evels of FSH beta-subunit mRNA were unchanged. There was a transient d ecrease in pituitary content of FSH at 24 h of estradiol treatment. St eady-state levels of mRNA for alpha-subunit were unaffected by estradi ol. Administration of progesterone did not influence the amount of mRN A for LH beta-subunit but increased pituitary content of LH (p < 0.05) . Moreover, the amount of mRNA for FSH beta-subunit decreased (p < 0.0 5) in progesterone-treated ewes but pituitary content of FSH increased . There was a modest reduction in the amount of mRNA for a-subunit aft er administration of progesterone, but serum concentrations of gonadot ropins were not affected. Collectively, from these data we conclude th at estradiol and progesterone can act directly at the pituitary gland in the ewe to regulate steady-state levels of gonadotropin subunit mRN As independent of GnRH.