Extracellular Glutamate During Focal Cerebral Ischaemia in Rats: Time Course and Calcium Dependency

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Abstract
The time course of changes in extracellular glutamic acid levels and their Ca2+ dependency were studied in the rat striatum during focal cerebral ischaemia, using microdialysis. Ischaemia-induced changes were compared with those produced by high K+-evoked local depolarization. To optimize time resolution, glutamate was analysed continuously as the dialysate emerged from the microdialysis probe by either enzyme fluorimetry or biosensor. The Ca2+ dependency of glutamate changes was examined by perfusing the probe with Ca2+-free medium. With normal artificial CSF, ischaemia produced a biphasic increase in extracellular glutamate, which started from the onset of ischaemia. During the first phase lasting ∼10 min, dialysate glutamate level increased from 5.8 ± 0.9 µM· min−1 to 35.8 ± 6.2 µM where it stabilized for ∼3 min. During the second phase dialysate glutamate increased progressively to its maximum (82 ± 8 µM), reached after 55 min of ischaemia, where it remained for as long as it was recorded (3 h). The overall changes in extracellular glutamate were similar when Ca2+ was omitted from the perfusion medium, except that the first phase was no longer detectable and, early in ischaemia, extracellular glutamate increased at a significantly slower rate than in the control group (2.2 ± 1 µM· min−1; p < 0.05). On the basis of these data, we propose that most of the glutamate released in the extracellular space in severe ischaemia is of metabolic origin, probably originating from both neurons and glia, and caused by altered glutamate uptake mechanisms. Comparison with high K+-induced glutamate release did not suggest that glutamate “exocytosis,” early after middle cerebral artery occlusion, was markedly limited by deficient ATP levels.

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