Differential effects of estrogen and progesterone on levels of POMC mRNA levels in the arcuate nucleus: relationship to the timing of LH surge release.

Beta-endorphin is thought to be an important inhibitor of LHRH neuronal activity and also to play a role in conveying information about changes in steroid levels to LHRH neurons. We have previously shown that the mRNA encoding the precursor of beta-endorphin, proopiomelanocortin (POMC), fluctuates during the estrous cycle with the most dramatic changes occurring on proestrus. POMC mRNA levels decline before the onset of LH surge release but then dramatically rise and remain elevated during the surge. In the present studies we tested the hypothesis that the decline in POMC mRNA levels immediately before the proestrus LH surge is mediated by estrogen and the rise during the surge by progesterone. To test this hypothesis, we compared changes in POMC mRNA levels between ovariectomized (OVX) and OVX estrogen (E2)-treated rats and between OVX E2-treated rats with and without progesterone. Animals were examined at hourly intervals after the administration of progesterone, then at every 4 h during the LH surge. Using in situ hybridization histochemistry, we found that E2 decreased POMC mRNA levels in OVX rats before the onset of the LH surge and further suppressed levels during the surge. Compared to animals treated with E2 alone, progesterone advanced the time at which both the LH surge began and the time at which POMC mRNA levels declined. After a transient decline, POMC mRNA levels rose in these progesterone-treated animals and remained elevated throughout the period of the LH surge. These results support the hypothesis that progesterone times the LH surge and limits its appearance to one day be exerting a biphasic effect on the activity of beta-endorphinergic neurons of the arcuate nucleus.