We used northern and western blotting to measure the quantity of glutamate and GABA transporters mRNA and their proteins within the hippocampal tissue of rats with epileptogenesis. Chronic seizures were induced by amygdalar injection of kainic acid 60 days before death. We found that expression of the mRNA and protein of the glial glutamate transporters GLAST and GLT-1 were down-regulated in the kainic acid-administered group. In contrast, EAAC-1 and GAT-3 mRNA and their proteins were increased, while GAT-1 mRNA and protein were not changed. We performed in vivo microdialysis in the freely moving state. During the interictal state, the extracellular glutamate concentration was increased, whereas the GABA level was decreased in the kainic acid group. Following potassium-induced depolarization, glutamate over¯ow was higher and the recovery time to the basal release was prolonged in the kainic acid group relative to controls. Our data suggest that epileptogenesis in rats with kainic acid-induced chronic seizures is associated with the collapse of extracellular glutamate regulation caused by both molecular down-regulation and functional failure of glutamate transport.

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Kainic acid (KA) induces seizures and degeneration in CA1 of the ventral hippocampus, though its mechanism of action is unknown. We used KA to induce seizures in freely moving rats prepared for in vivo microdialysis with probe placement, and then measured extracellular glutamate with an online fluorometric detector. Generation of free radicals was monitored by electron paramagnetic resonance (EPR) spectroscopy coupled with perfusion of the spin-trapping agent, alpha-(4-pyridyl- N-oxide)- N- tert-butylnitrone (POBN). Regional antioxidant efficacy was measured by observing the eliminating ratio of nitroxide radicals, using 3-carbamoyl-2, 2, 5, 5-tetramethylpyrrolidine-1-oxyl (carbamoyl-PROXYL) applied exogenously from the probe. Increased levels of extracellular glutamate observed at the initiation of KA-induced seizures appear to be associated with generation of lipid free radicals and with a decrease in residual antioxidant effects. These data suggest that collapse of the redox state in the hippocampus, the region most vulnerable to injury from seizure activity, may be critical in the regional injury induced by seizures. Further, we propose that the functional failure of glutamate transporters due to oxidative stress results in high levels of extracellular glutamate during sustained generalized seizures induced with KA.

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