Gb. Digregorio et Tm. Nett, ESTRADIOL AND PROGESTERONE INFLUENCE THE SYNTHESIS OF GONADOTROPINS IN THE ABSENCE OF GONADOTROPIN-RELEASING-HORMONE IN THE EWE, Biology of reproduction, 53(1), 1995, pp. 166-172
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.