CHARACTERIZATION OF K-EVOKED [H-3] D-ASPARTATE OUTFLOW IN THE RAT HIPPOCAMPUS IN-VITRO()

The characteristics of K+-evoked outflow of [H-3]D-aspartate, a glutam ate release marker, were systematically investigated in the rat hippoc ampus, using 35 mM K+-evoked [H-3]noradrenaline outflow as a reference . Elevation of external K+ concentrations increased [H-3]D-aspartate o utflow in a concentration-dependent manner both in slices and synaptos omes. In the absence of external Ca2+, K+-evoked [H-3]D-aspartate outf low was decreased by approx 60% in synaptosomes and 80% in slices. How ever, elimination of external Ca2+ in the presence of 2 mM EGTA signif icantly reduced only 100 mM K+-evoked outflow, both in slices and syna ptosomes. In the absence of external Ca2+, 35 mM K+-evoked [H-3]noradr enaline outflow was abolished even when EGTA was present in the soluti on. Furthermore, the Ca2+-channel blockers omega-conotoxin (10 nM) and nifedipine (0.5 muM) did not significantly reduce K+-evoked [H-3]D-as partate outflow; [H-3]noradrenaline outflow, however, was reduced by m ore than one third by omega-conotoxin. Finally [H-3]D-aspartate overfl ow was insensitive to tetrodotoxin (0.5 muM) both in synaptosomes and in slices; while that of [H-3]noradrenaline was significantly reduced in slices. It is concluded that (1) [H-3]D-aspartate outflow is partly Ca2+-dependent; (2) differences between K+-evoked [H-3]D-aspartate an d [H-3]noradrenaline outflow include sensitivity to stimulation by EGT A, to Ca2+-channel blockers and to tetrodotoxin. Some of these discrep ancies may be ascribed to the existence of a cytosolic, Ca2+-independe nt pool of releasable glutamate and [H-3]D-aspartate. These observatio ns pose some problems as to the experimental approach for the study of Ca2+-dependent [H-3]D-aspartate release.