Ischemia induces release of glutamate in regions spared from histopathologic damage in the rat

Excessive release of glutamate is thought to play a major role in the susceptibility of neurons to ischemia. In the present study, we evaluated whether differences in the magnitude of glutamate release resulted in some regions being vulnerable to ischemia, but others being spared from irreversible histopathologic damage. Specifically, we compared the temporal profile of ischemia-induced changes in extracellular levels of glutamate in a region selectively vulnerable to 10 minutes of transient ischemia (CA1 sector of the hippocampus) to the changes occurring in regions that, although rendered ischemic, are usually unaffected by a 10-minute insult (i.e., thalamus, cortex, and dorsolateral striatum). In an attempt to correlate the regional changes in glutamate release to the magnitude of the ischemic insult, the degree of ischemia (e.g., ATP depletion, lactate accumulation, and local cerebral blood flow reduction) and the final histopathologic outcome were also evaluated in these regions. Blood flow reduction and energy depletion were severe and uniform in all regions. However, the histopathologic outcome illustrated a different pattern. Although the CA1 sector of the hippocampus was severely damaged, all other brain regions were unaffected by the 10-minute insult. Extracellular glutamate levels, measured by microdialysis, were significantly elevated during ischemia in all four regions. These levels continued to increase during the early recirculation period and gradually returned to baseline by 30 minutes of reperfusion, with a similar temporal changes in all four brain structures. These results, taken with our previous findings, demonstrate that elevated intraischemic glutamate levels are insufficient to independently engender ischemic damage.(ABSTRACT TRUNCATED AT 250 WORDS)