EXTRACELLULAR GLUTAMATE DURING FOCAL CEREBRAL-ISCHEMIA IN RATS - TIME-COURSE AND CALCIUM DEPENDENCY

The time course of changes in extracellular glutamic acid levels and t heir Ca2+ dependency were studied in the rat striatum during focal cer ebral ischaemia, using microdialysis. Ischaemia-induced changes were c ompared 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 fluori metry or biosensor. The Ca2+ dependency of glutamate changes was exami ned by perfusing the probe with Ca2+-free medium. With normal artifici al CSF, ischaemia produced a biphasic increase in extracellular glutam ate, which started from the onset of ischaemia. During the first phase lasting similar to 10 min, dialysate glutamate level increased from 5 .8 +/- 0.9 mu M.min(-1) to 35.8 +/- 6.2 mu M where it stabilized for s imilar to 3 min. During the second phase dialysate glutamate increased progressively to its maximum (82 +/- 8 mu M), reached after 55 min of ischaemia, where it remained for as long as it was recorded (3 h). Th e overall changes in extracellular glutamate were similar when Ca2+ wa s omitted from the perfusion medium, except that the first phase was n o longer detectable and, early in ischaemia, extracellular glutamate i ncreased at a significantly slower rate than in the control group (2.2 +/- 1 mu M.min(-1); p <0.05). On the basis of these data, we propose that most of the glutamate released in the extracellular space in seve re ischaemia is of metabolic origin, probably originating from both ne urons and glia, and caused by altered glutamate uptake mechanisms. Com parison with high K+-induced glutamate release did not suggest that gl utamate ''exocytosis,'' early after middle cerebral artery occlusion, was markedly limited by deficient ATP levels.