ESTROGEN AND NEUROTRANSMITTER SYSTEMS IN HIPPOCAMPUS

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The neuronal plasticity associated with learning and memory and epileptiform activity depends on precise balances in the activity of two major neurotransmitter systems in the hippocampus: the excitatory glutamate/aspartate neurons and the inhibitory gamma-aminobutyric acid (GABA) neurons. The objective of this research is to test the hypothesis that estradiol and progesterone influence cognitive processes and epileptic seizure activity by interacting with the glutamate and/or GABA neurons or their receptor systems. Estradiol increases the density of NMDA, GABAA, and progesterone receptors and dendritic spines and synapse formation in the CA1 region of the hippocampus. Estradiol may act 1) directly at the receptor by changing conformation and binding properties of the protein, 2) at the gene by increasing transcription and synthesis of receptor subunits, or 3) on neurotransmitter release which in turn affects receptor binding. In order to assess the influence of estradiol on the glutamate and/or GABAA receptor systems, the levels of gene expression of NMDA and GABAA receptor genes will be determined using in situ hybridization histochemistry. The site of action and importance of synaptic transmission will be assessed by implantation of an antiestrogen into the entorhinal cortex, which has a major projection to the hippocampus or into the dorsal hippocampus of estradiol-treated rats and measuring NMDA and GABAA receptor binding sites and mRNA levels. The antiestrogen implants will block the local effects of estradiol. In addition, the effects of estradiol on neurotransmitter synthesis and release will be assessed. The levels of mRNA and the enzymatic activity of glutamic acid decarboxylase (GAD), the synthetic enzyme for GABA, will be measured as an indicator of the magnitude of GABA neuronal activity. Immunocytochemical colocalization of progesterone receptor and GAD peptide will be done to determine whether the mechanism exists for the direct action of progesterone on GAD gene expression. To differentiate the action of progesterone which acts at the intracellular progesterone receptor from the actions of its metabolite, allopregnanolone, which acts at the GABAA receptor, animals will be treated with allopregnanolone. If allopregnanolone mimics the action of progesterone on GAD gene expression, then progesterone is acting at the GABAA receptor. The results of these studies will contribute to our understanding of the cellular mechanisms underlying the effects of estradiol on certain behavioral aspects of learning and memory and epileptiform activity.