DIRECT EFFECTS OF LUTEAL REGRESSION ON ANTERIOR-PITUITARY RESPONSE TOGNRH

The preovulatory period of the ewe is marked by a dramatic decrease in concentrations of progesterone in serum during the late luteal phase, followed by elevated luteinizing hormone (LH) secretion, final follic ular maturation and ovulation. This experiment was designed to ascerta in the extent to which removal of endogenous progesterone negative fee dback at the anterior pituitary gland, independent of effects at the h ypothalamus, promotes increased secretion of LH in the hours immediate ly after induction of luteolysis. Estrus was synchronized in ovary-int act ewes with two injections of prostaglandin F(2)alpha (PGF(2)alpha) analog given 10 d apart (Day 0 = second day after the second PGF(2)alp ha injection). Ewes were subjected to hypothalamic-pituitary disconnec tion (HPD; n = 6) on Day 3 and were pulsed with gonadotropin-releasing hormone (GnRH). Ewes were used during the estrous cycle or received a pproximately 400 IU pregnant mare serum gonadotropin (PMSG) on Day 2 t o stimulate ovulation; there was no difference (P < 0.10) in ovulation rate or progesterone production between these two groups. Luteal regr ession was induced by injection of PGF(2)alpha analog on approximately Day 10 of the estrous cycle. Blood samples were collected around exog enous GnRH pulses before and at 2- or 4-hr intervals after PGF(2)alpha administration and concentrations of LH and progesterone determined. At 4, 12 and 24 hr after PGF(2)alpha administration, mean serum proges terone levels in all ewes had decreased by 54.7%, 66.2% and 89.4%, res pectively (P < 0.05) from pre-injection levels. The decrease in proges terone was associated with an increase (P < 0.01) in LH pulse amplitud e with means at 4-hr post-PGF(2)alpha ranging from 190% to 288% of pre -PGF(2)alpha values. Mean serum LH levels were also increased (P < 0.0 1) within 4 hr of PGF(2)alpha administration and remained elevated at all but the 24-hr time point. The timing of this increase (within 3 hr ) indicates that it is independent of changes in serum estradiol conce ntrations, which do not increase for at least 16 hr after induction of luteolysis. Thus, removal of endogenous progesterone negative feedbac k at the anterior pituitary gland in the hours immediately after induc tion of luteolysis seems to play a role in facilitating LH release ind ependently of hypothalamic action. (C) Elsevier Science Inc. 1998.