Human temporal lobe epilepsy is associated with marked neurodegeneration of the hippocampus. AMPA (alpha - amino -3- hydroxy - 5 - methyl -4- isoxazole-propionic acid)-type glutamate receptors lacking the GluR2 subunit are highly permeable to Ca2+. Although reduced GluR2 expression may increase AMPA-mediated CA2+ permeability and contribute to seizure- induced damage (the """"""""GLUR2 Hypothesis""""""""), compelling data to support the GluR2 Hypothesis has not yet been obtained. This is due to the difficulty in separating glutamate-mediated increases in Ca2+ levels via voltage-gated channels from AMPA-mediated Ca2+ currents. Here, a particular emphasis will be put on developmental aspects of this hypothesis since both antisense knockdown and genetic knockout approaches reveal age-dependent epileptogenesis and selective CA3 hippocampal neurodegeneration,, suggesting that the GluR2 subunit has specific functions at different ages. In situ hybridization, immunohistochemistry, and Western analyses will determine whether selective changes in AMP receptor subunit expression are achieved in the hippocampus following antisense knockdown. AMPA receptor mutant mice will be used to examine seizure susceptibility and hippocampal damage when GluR2 or GluR 1 genes are ablated. EEG recordings from AMPA receptor knockdown rats and mutant mice will allow us to define the physiological relevance of the deprived GluR2 circuit in vivo. Pilot data show unilateral hippocampal down regulation of GluR2 subunits by hippocampal infusion of GluR2 antisense oligodeoxynucleotides (AS-ODNs) may provide a novel partial seizure model in young but not adult rats. If a shift in the GluR1/GluR2 ratio contributes to enhanced vulnerability, then GluR1 knockdown prior to induction of status epilepticus should prevent potential formation of toxic CA2+ permeable GluR1 homomers that may assemble after a seizure. Spider toxins and polyamine toxin synthetic analogues can block AMPA receptor assemblies that lack the GluR2 subunit at a higher affinity than other receptor subtypes and may be neuroprotective in the proposed knockdown model. The specificity of the antisense approach and genetic strategies combined may elucidate the relevance of the GluR2 Hypothesis and provide new clinical trials in epilepsy.
