GLUTAMATE-RECEPTOR (GAMMA-AMINO-3-HYDROXY-5-METHYL-4-ISOXAZOLE PROPIONATE AND KAINATE SUBTYPE) ACTIVITY ENHANCED BY DIZOCILPINE (MK801) IN RAT HIPPOCAMPUS - RAPID CHEMICAL KINETIC MEASUREMENTS OF SODIUM FLUX AND RECEPTOR DESENSITIZATION WITH NATIVE MEMBRANES

L-Glutamate mediated transmembrane influx of Na-22(+) into a native membrane vesicle preparation from rat hippocampus was resolved into components due to glutamate receptor and Na+-glutamate symport, by kinetic analysis and pharmacological specificity. Measurements made with a quench-flow technique showed that receptor-mediated cation exchange proceeded in two phases, the faster phase progressively attenuated by a desensitization process with a half time of approximately 90 ms. The influx of Na-22(+) continued in the second phase with a half time of 6 s. Receptor-mediated Na-22(+) influx was replicated with the glutamate mimetics, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate and kainate but not with N-methyl-D-aspartate and was inhibited by 6- cyano-7-nitroquinoxaline-2,3-dione but not by 2-amino-5-phosphonovaleric acid. Receptor-mediated Na-22(+) influx did not require glycine and was pH de pendent; influx was inhibited al pH values less than pH 5.0. Thus, the receptor-mediated activity was of the gamma-amino-3-hydroxy-5-methyl-4-isoxazole propionate-kainate subtype. The N-methyl-D-aspartate receptor inhibitor 5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (dizocilpine or MK801), increased receptor-mediated transmembrane Na-22(+) flux in both phases, but did not accelerate receptor desensitization. Receptor-mediated radiotracer exchange was the major contribution to Na-22(+) influx in times less than 5 s and the receptor-mediated responses could be measured with a small correction for sodium-glutamate symport or in the presence of a glutamate-uptake inhibitor. A third phase of Na-22(+) influx accessed a different internal volume and proceeded with the same first-order rate constant as [H-3]glutamate uptake when measured simultaneously. This was shown to be mediated by the glutamate uptake mechanism, with a half time of 20-40 s varying with the preparation. The Na+-glutamate symport became the largest cause of Na-22(+) influx after at least 5 s, the final magnitude being, on average, twofold larger than the receptor-mediated Na-22(+) influx.