A dual modulatory effect of progesterone on the LHRH-induced LH release

The role of progesterone on the release of LH induced by 25 or 50 ng of LHRH was studied in proestrus rats in which spontaneous preovulatory release of LH was prevented by sodium pentobarbitone. After the s.c. administration of progesterone (5 mg) at 18.00 h of diestrus day 2 or at 12.00 h of proestrus, serum LH was not detectable at 17.15 h of proestrus. Injections of 25 or 50 ng of LHRH at 17.00 h of proestrus induced a dose response release of LH 15 min after. However, the LH response to LHRH administration increased significantly when progesterone was injected at 12.00 h of proestrus. The potentiating effect of progesterone seems to be exerted at pituitary level. The effect of LHRH and the enhanced response of the pituitary after progesterone treatment was prevented by the administration of the antiestrogen Tamoxifen in diestrus day 2. The release of LH induced by 50 ng of LHRH on proestrus day was blocked by the previous injection of progesterone on diestrus day 2. The inhibition was maintained even though a second dose of progesterone was given at 12.00 h of proestrus. The simultaneous administration of estrogen and progesterone on diestrus day 2 did not prevent the inhibitory effect of progesterone. It is concluded that the facilitatory or inhibitory effect of progesterone on the release of LH induced by LHRH is dependent upon the previous sensitization of the pituitary to estrogen.