Effects of estrogen and progesterone on luteinizing hormone-releasing hormone messenger ribonucleic acid levels: consideration of temporal and neuroanatomical variables.

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Abstract
We examined temporal effects of estradiol (E2) and progesterone on cellular levels of LHRH messenger RNA (mRNA) in several brain regions. Female rats were ovariectomized and implanted with SILASTIC brand capsules of oil or E2 1 week later (day 0). Progesterone capsules were implanted on the morning of day 2. Using in situ hybridization histochemistry, we found that on day 2, E2 induced a complex temporal pattern of changes in LHRH mRNA levels. Levels in ovariectomized E2-treated animals were lower than control values in the morning, then increased before the LH surge and declined again as the surge waned. The magnitude of these changes was greatest in neurons of the rostral preoptic area/organum vasculosum of the lamina terminalis, but a similar pattern was detected in neurons of the medial preoptic area. No changes were seen in neurons of the diagonal band of Broca. Our finding that the effect of E2 on LHRH mRNA levels depends on the time of day and brain region examined largely reconciles discrepancies among previous studies. Progesterone triggered events that advanced the onset of and augmented the LH surge, but had no effect on LHRH mRNA levels. These findings support the hypothesis that the mechanism by which E2 induces region-specific changes in LHRH mRNA levels before the surge is separate from the progesterone-amplified mechanism that induces LHRH release.

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