NEUROTOXICITY OF ACUTE GLUTAMATE TRANSPORT BLOCKADE DEPENDS ON COACTIVATION OF BOTH NMDA AND AMPA KAINATE RECEPTORS IN ORGANOTYPIC HIPPOCAMPAL CULTURES/

Excessive activation of glutamate receptors is neurotoxic, contributin g to brain injury caused by cerebral ischemia. The pharmacology of glu tamate neurotoxicity is difficult to study in animals because it is ef ficiently cleared from the extracellular space by a family of glutamat e transporters. We have investigated the receptor specificity of endog enous glutamate's toxic effects in organotypic cultures of the hippoca mpus by acute blockade of these transporters. The organotypic cultures used in these experiments preserve the intrinsic connections and regi onal differentiation of the hippocampus in long term culture and may m ore closely reproduce the pharmacology of the mature brain region. Mem brane injury was measured with digital fluorescence imaging of the vit al dye, propidium iodide, 24 h after a 30-min exposure to glutamate re ceptor agonists or to antagonists of glutamate transport. Confirming o ur previous results, bath-applied, exogenous glutamate caused dose-dep endent neuronal injury. Glutamate was less potent than the selective a gonists NMDA, AMPA, and quisqualate. Blockade of glutamate transport w ith the selective antagonists threo-hydroxy-aspartate and pyrrolidine- dicarboxylic acid also caused dose-dependent neuronal injury. Endogeno us or exogenous glutamate toxicity was caused by a coactivation of bot h NMDA and AMPA/kainate receptors; blockade of either was sufficient t o substantially prevent neuronal injury. Protective effects of combine d application of antagonists were generally less than additive. We con clude that AMPA/kainate receptors play a more prominent role in glutam ate neurotoxicity in organotypic cultures than in dissociated cortical or hippocampal cultures, acting together with NMDA receptors to cause neuronal injury. (C) 1995 Academic Press, Inc.