Excitatory amino acids such as glutamate have been implicated in the pathogenesis of ischemic neurocellular injury. In previous studies, we have shown that infarct size and neurologic deficits in the rat can be attenuated by blockade of N-methyl-D-aspartate (NMDA)-preferring excitatory amino acid receptors with drugs such as kynurenate, AP-7, and CGS 19755. We have now developed a new and highly reproducible technique of middle cerebral artery occlusion in the adult rat, and used in vivo microdialysis to quantify the changes in extracellular amino acids during the evolution of the ischemic process. We have studied the time course of glucose hypermetabolism during the evolution of infarction in this model and have documented the attenuation of this ischemic hypermetabolism with pharmacologic blockade of the NMDA receptor. Further we have shown that the pattern of hypermetabolism during the evolution of ischemia predicts the eventual anatomy of the infarction. We have also identified heat shock protein as a possible marker of neuronal injury or neuroprotection in ischemia. Finally, we have used a variety of in vitro techniques to identify drugs and metabolic factors that modify intracellular calcium levels and may thereby influence ischemic cell death. Our preliminary studies suggest that drugs affecting presynaptic glutamate release are also neuroprotective in stroke. These compounds may avoid the clinical side effects and apparent neurotoxicity (vacuolization) of the currently available postsynaptic inhibitors which are delaying clinical trials. Further we have shown that a nonpharmacologic technique - hypercarbic ventilation rapidly lowers brain pH to levels recently shown to block the NMDA receptor and is neuroprotective in vitro thus offering a possible non pharmacologic method for acute neuroprotection in ischemia. Accordingly, the current specific aims are: to investigate additional approaches to reducing excitotoxic ischemic brain injury, including drugs that inhibit excitatory amino acid release and non-NMDA (kainate- or quisqualate- preferring) excitatory amino acid receptor antagonists; to determine how excitotoxic and ischemic injury are affected by acidosis; and to compare representative neuroprotective agents in a single well studied model of focal irreversible ischemia. The methods used will include induction of permanent, focal ischemia by middle cerebral artery occlusion in the adult rat; in vivo microdialysis; and histologic studies of ischemic rat brain. The broad, long-term objective of this work is to identify pharmacologic and biochemical modifiers of excitotoxic ischemic neuronal injury that might ultimately be useful in the clinical treatment of stroke.
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