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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L-701,324 (7-chloro-4-hydroxy-3-(3-phenoxy) phenyl-2-(1H)-quinolone) is a novel, orally active antagonist at the N-methyl-D-aspartate (NMDA) receptor glycine site. As NMDA receptor antagonism is generally associated with anaesthetic effects, we have examined the electroencephalographic alterations produced by doses of L-701,324 that effectively reduce NMDA-evoked responses in vivo. Microdialysis probes incorporating an electrode were implanted in the striatum of rats and perfused with artificial cerebrospinal fluid (ACSF). Under light halothane anaesthesia, 12 consecutive depolarizations were elicited by switching to ACSF containing 200 microM NMDA for 2 or 3 min, every 20 min. NMDA-evoked depolarizations and EEG were recorded with the microdialysis electrode. L-701,324 (5 or 10 mg kg-1 i.v.) or vehicle were administered 5 min after the 3rd NMDA stimulus. L-701,324 dose-dependently inhibited NMDA-evoked depolarizations, with 10 mg kg-1 reducing these responses by 50% for at least 3 h. The average amplitude of the EEG in the window 0.25-6 Hz (low frequencies) and 6-21 Hz (high frequencies) did not change in the control group. At the higher dose of 10 mg kg-1 L-701,324 transiently increased the amplitude of low frequencies by around 20%. In contrast, both 5 and 10 mg kg-1 significantly reduced the high frequencies to around 70% of control, and this action was sustained with the higher dose. Analysis of the relative EEG power spectra confirmed a small, but persistent shift from high to low EEG frequencies. Our results suggest that L-701,324 slightly strengthened halothane anaesthesia at doses inhibiting effectively NMDA receptor function. Accordingly, the resulting anticonvulsant and neuroprotective actions of L-701,324 may not be associated with marked anaesthesia-like side-effects.

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