The long-term objective of this research is to determine the mechanism(s) by which estradiol (E2) and progesterone (P4) regulate LHRH biosynthesis and surge release. E2 and P4 differentially regulate two intracellular markers of changes in LHRH neuronal activity in a subpopulation of LHRH neurons--an E2-induced increase in LHRH gene transcription before the onset of LHRH surge release and a P4-dependent increase in Fos expression at the onset of the surge. Because few, if any, LHRH neurons contain estrogen receptors (ER) or progestin receptors (PR), these intracellular events must be mediated by afferent neuronal systems. Recent indirect evidence suggests that noradrenergic (NA) and GABAergic may be these afferent systems. Therefore, in the proposed studies we will test the novel hypothesis that sequential changes in NA and GABAergic signalling directly regulate steroid-specific changes in LHRH synthesis and release and alter intracellular markers of neuronal function. We will first determine whether E2 induces changes in NA release around the time of increased LHRH gene expression. To do this we will assess changes in the activity of brainstem neurons that supply LHRH neurons, and changes in NA turnover rates in the region containing LHRH neurons. We will use dual-label in situ hybridization to determine whether increases in LHRH gene transcription occur preferentially in neurons with ARs and whether specific AR antagonists can block transcription in these neurons. Finally, we will determine whether ligand-independent activation of PR decreases GABAergic signalling to LHRH neurons, and whether this signal is amplified by administration of P4 and marked by Fos expression. To accomplish this goal, we will test whether RU486 blocks the decline in GABA turnover rates and in levels of glutamic acid decarboxylase (GAD) mRNA previously observed before LHRH surge release, whether P4 furthers these declines, whether changes in GAD mRNA occur preferentially in neurons that also express PR, and whether GABA receptor agonists block the appearance of Fos expression in LHRH neurons. These studies will provide important new information on the identity of the afferent neuronal systems that transduce steroid signals to LHRH neurons. In addition, they win form the basis for future studies on the intracellular mechanisms regulating LHRH biosynthesis and release. This information will be critical for under- standing the neuroendocrine mechanisms controlling ovulation, as well as alterations in these control mechanism that result in precocious puberty, hypothalamic infertility and menopause. Thus, this information will be important for developing safer and more effective contraceptives and therapeutic modalities.
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