DIRECT MEASUREMENT OF GLUTAMATE RELEASE IN THE BRAIN USING A DUAL ENZYME-BASED ELECTROCHEMICAL SENSOR

The in vivo measurement of the rapid changes in the extracellular conc entrations of L-glutamic acid in the mammalian brain during normal neu ronal activity or following excessive release due to episodes of anoxi a or ischemia has not been possible to this date. Current techniques f or the measurement of the release of endogenous glutamate into the ext racellular space of the central nervous system are relatively slow and do not measure the actual concentration of free glutamate in the extr acellular space. An enzyme-based electrode with rapid response times ( about 1 s) and high degree of sensitivity (less than 2 mu M) and selec tivity for L-glutamic acid is described in this paper. This electrode has both L-glutamate and ascorbate oxidase immobilized on its surface. The latter enzyme removes almost completely any interferences produce d by the high levels of extracellular ascorbate present in brain tissu e. The response of the electrode to glutamate and other potentially in terfering substances was fully characterized in vitro and its selectiv ity, sensitivity and rapidity in responding to a rise in extracellular glutamate concentrations was also demonstrated in vivo. Placement of the electrode in the dentate gyrus of the hippocampus led to the detec tion of both KCl-induced release of L-glutamic acid and the release in duced by stimulation of the axons in the perforant pathway. The develo pment of this selective, sensitive and rapidly responding glutamate se nsor should make it now possible to measure the dynamic events associa ted with glutamate neurotransmission in the central nervous system.