EFFECTS OF ESTROGEN AND PROGESTERONE ON LUTEINIZING-HORMONE-RELEASINGHORMONE MESSENGER-RIBONUCLEIC-ACID LEVELS - CONSIDERATION OF TEMPORALAND NEUROANATOMICAL VARIABLES

We examined temporal effects of estradiol (E(2)) and progesterone on c ellular levels of LHRH messenger RNA (mRNA) in several brain regions. Female rats were ovariectomized and implanted with SILASTIC brand caps ules of oil or E(2) 1 week later (day 0). Progesterone capsules were i mplanted on the morning of day 2. Using in situ hybridization histoche mistry, we found that on day 2, E(2) induced a complex temporal patter n of changes in LHRH mRNA levels. Levels in ovariectomized E(2)-treate d animals were lower than control values in the morning, then increase d before the LH surge and declined again as the surge waned. The magni tude of these changes was greatest in neurons of the rostral preoptic area/organum vasculosum of the lamina terminalis, but a similar patter n 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 e ffect of E(2) on LHRH mRNA levels depends on the time of day and brain region examined largely reconciles discrepancies among previous studi es. Progesterone triggered events that advanced the onset of and augme nted the LH surge, but had no effect on LHRH mRNA levels. These findin gs support the hypothesis that the mechanism by which E(2) induces reg ion-specific changes in LHRH mRNA levels before the surge is separate from the progesterone-amplified mechanism that induces LHRH release.