NMDA RECEPTOR REGULATION IN LIMBIC EPILEPSY

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The long term goal of this laboratory is to understand the pathophysiology of limbic epilepsy in molecular terms. The two objectives of this proposal represent related but distinct approaches to this goal Kindling is the most widely studied animal model of limbic epilepsy. Enhanced function of excitatory synapses using the NMDA subtype of glutamate receptor may contribute to the expression of the enduring hyperexcitability of kindling. CA3 pyramidal cells of the kindled hippocampus exhibit a selective and longlasting increased sensitivity to NMDA as evident in an NMDA-evoked depolarization. At least part of the molecular basis of this increased sensitivity appears to be a strikingly increased density (greater than 10% 1) of a "novel" NMDA receptor (NMDARk), identified during the present funding period. We think that NMDARk is part of the molecular basis of the lasting hyperexcitability of the kindled brain. Our first objective is to elucidate the molecular basis of NMDARk. The molecular cloning of multiple cDNAs encoding NMDA receptor subunits has provided both reagents and insights with which to pursue this inquiry. Since the affinity of ligands for neurotransmitter receptors can be differentially modulated by alterations of either subunit composition or phosphorylation, we hypothesize that NMDARk reflects a receptor bearing one or both of these molecular modifications. We will test these alternative hypotheses using antibodies specific to distinct subunits of the NMDA receptor expressed in the hippocampus. Understanding the molecular nature of NMDARk should facilitate elucidating how NMDARk affects the excitability of CA3 pyramidal cells, where (in addition to CA3) in a kindled brain NMDARk is present, how nMDARk contributes to the hyperexcitability of a kindled animal and, ultimately, of human limbic epilepsy. We have unexpectedly-discovered that transgenic mice carrying a null mutation of the alpha-subunit of calcium calmodulin kinase II gene exhibit limbic epilepsy. This model recapitulates a number of aspects of human limbic epilepsy including spontaneous seizures, axonal sprouting of hippocampal granule cell axons in the supragranular region of the dentate, and dispersion of the granule cell layer. The link to a single, identified gene focuses the search for the underlying mechanisms. The second objective of this proposal is to characterize this new model and thereby obtain information helpful for mechanistic analyses to be pursued by Drs. Dingledine and Nadler.