Continuous Monitoring of Intracerebral Glutamate Levels in Awake Monkeys Using Microdialysis and Enzyme Fluorometric Detection

A technique for continuous on-line detection of glutamate using brain microdialysis in awake primates is described. The method is based on an enzymatic assay using fluorescence detection of glutamate. The time resolution of the continuous fluorescent readout compares favorably with that of most published studies, which have used standard high-pressure liquid chromatography detection methods for glutamate. Exposure of the system to other amino acids (GABA, aspartate, glutamine, ascorbate, taurine, valine, alanine, and D-glutamate) revealed that this method is highly specific for L-glutamate. In vitro, the system detects reliably glutamate levels as low as 0.5 micromol/l. In vivo testing in the striatum of Rhesus macaques showed that glutamate levels were enhanced after reverse microdialysis with a glutamate uptake blocker. Stimulation with high potassium increased substantially the levels of glutamate, an effect that was calcium-dependent. Glutamate levels were also increased when the microdialysis solution contained the blocker of voltage-gated potassium channels, 4-aminopyridine. This technique effectively detects short-term changes in glutamate levels evoked by physiologic or pharmacologic manipulations in the primate brain.