EFFECT OF SYSTEMIC ADMINISTRATION OF N-METHYL-D-ASPARTIC ACID ON EXTRACELLULAR TAURINE LEVEL MEASURED BY MICRODIALYSIS IN THE HIPPOCAMPAL CA1 FIELD AND STRIATUM OF RATS

The extracellular concentrations of amino acids in the hippocampal CA1 field and striatum of conscious freely moving rats were monitored sim ultaneously by in vivo brain microdialysis using HPLC with electrochem ical detection. Under basal conditions, aspartate, glutamate, glutamin e, glycine, taurine, and alanine were detected, but gamma-aminobutyric acid was undetectable in both regions. Intraperitoneal injection of N -methyl-D-aspartic acid (NMDA; 10 mg/kg) caused a significant increase (three- to fivefold) in the taurine concentration in the dialysate ob tained from both the hippocampal CA1 and striatum, whereas other amino acids (aspartate, glutamate, and alanine) did not show significant ch anges. Local application of NMDA (300 muM) to both regions via the dia lysis probes also caused a similar increase (three- to fivefold) in bo th regions. Under infusion of hypertonic Ringer's solution containing 150 mM sucrose, the eff ect of NMDA on the level of taurine in both th e regional dialysates was not affected. The effect of NMDA was totally reduced by intraperitoneal administration of MK-801 (0.3-1.0 mg/kg), a noncompetitive antagonist of NMDA receptors. Continuous infusion Of DL-2-amino-5-phosphonovaleric acid (1.0 mM), a competitive antagonist of NMDA receptors, via the dialysis probes completely inhibited the ef fect of NMDA. These findings suggest that systemic administration of N MDA is effective as well as local administration into the brain and th at NMDA receptors might be involved in the regulation of the extracell ular taurine level in the brain without dependence on cell swelling.