The long-term objectives of these experiments are to explore the biphasic effects of 15beta-estradiol (E2) on synaptic transmission that results in both inhibition and excitation of gonadotropin releasing hormone (GnRH) neurons. Our hypothesis is that prolonged exposure to preovulatory levels of E2 enhances excitatory synaptic input and attenuates inhibitory synaptic input onto GnRH neurons, which ultimately facilitates bursting activity and peptide release. The first experiments will test the hypothesis that E2 will decrease the inhibitory input and increase the excitatory input to GnRH neurons after a period of at least 24 h. Tissues will be prepared from ovariectomized, estrogen- and oil-treated females at 1, 24, 36, and 40 h after treatment. Inhibitory neurotransmitter agonists (and antagonists) selective for mu-opioid and GABAB receptors will be tested on POA neurons and the underlying conductances which they activate characterized using sharp electrode recording. Excitatory neurotransmitter agonists (and antagonists) selective for alpha1-noradrenergic and glutamate receptors will be tested and the underlying conductances which they activate characterized in POA (GnRH) neurons. Focal electrical stimulation of afferent pathways into the POA will be done to identify inhibitory and excitatory synaptic currents using whole-cell patch recording. The second experiments will test the hypothesis that preovulatory levels of E2 will increase the intrinsic conductances underlying phasic bursting activity in GnRH neurons. Tissues will be prepared from ovariectomized, estrogen- and oil-treated females at 42 h after treatment. The expression of calcium T-current and the small conductance, calcium-dependent K+ (SK) current will be measured using single-electrode voltage clamp and in situ hybridization. In addition, the expression of the hyperpolarization-activated, non-selective cation current (Ih) will be measured using single-electrode voltage clamp. Lastly, the biocytin-injected neurons will be analyzed using histochemical techniques combined with confocal microscopy to elucidate the transmitter phenotype of the biocytin-injected neurons and their anatomical interaction with other neurons. The results from these studies will provide important new information about the mechanism by which estrogen alters the responsiveness of hypothalamic (GnRH) neurons, and how estrogens in general modify synaptic plasticity in the mammalian brain.
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