ENHANCEMENT OF [H-3] D-ASPARTATE RELEASE DURING ISCHEMIA LIKE CONDITIONS IN RAT HIPPOCAMPAL SLICES - SOURCE OF EXCITATORY AMINO-ACIDS

Ischemic neuronal injury is supposed to be caused in part by the extra cellular accumulation of excitatory amino acids (EAA). Neurotransmitte r and metabolic EAA can be released from synaptic vesicles and cytopla sm of neurones and glial cells. In this study the release of the gluta mate analogue [H-3]D-aspartate ([H-3]D-ASP), loaded into 500 mu m slic es of rat hippocampus, was investigated. The efflux of the label was m easured during anoxic - aglycemic (''ischemic'') and normoxic K+ depol arization. To identify the pools from which [H-3]D-ASP is released we have estimated its calcium dependence and the effects of inhibitors of : (1) Na+ - dependent transporter of amino acids (100 mu M L-trans-pyr rolidine-2,4-dicarboxylic acid/L-trans-PDC/), (2) sodium channel (1 mu M tetrodotoxin TTX), and (3) anion channel (1 mM furosemide). [H-3]D- ASP released upon normoxic depolarization was 40% inhibited by TTX, ne arly 40% by L-trans-PDC and over 50% by furosemide. The ''ischemic'' r elease was in 40% calcium dependent, completely TTX independent and in approximately 50% blocked by furosemide treatment. Our data suggest t hat EAA accumulated in the synaptic cleft during ischemia are mainly r eleased from the cytosolic compartment by mechanisms which are connect ed with the ischemic increase of extracellular potassium concentration .