THE COMPETITIVE TRANSPORT INHIBITOR L-TRANS-PYRROLIDINE-2,4-DICARBOXYLATE TRIGGERS EXCITOTOXICITY IN RAT CORTICAL NEURON-ASTROCYTE COCULTURES VIA GLUTAMATE RELEASE RATHER THAN UPTAKE INHIBITION

We studied the early and late effects of L-trans-pyrrolidine-2,4-dicar boxylate (PDC), a competitive inhibitor of glutamate uptake with low a ffinity for glutamate receptors, in co-cultures of rat cortical neuron s and glia expressing spontaneous excitatory amino acid (EAA) neurotra nsmission. At 100 or 200 mu M, PDC induced different patterns of elect rical changes: 100 mu M prolonged tetrodotoxin-sensitive excitation tr iggered by synaptic glutamate release; 200 mu M produced sustained, te trodotoxin-insensitive and EAA-mediated neuronal depolarization, overw helming synaptic activity. At 200 mu M, but not at 100 mu M, PDC cause d rapid elevation of the glutamate concentration ([Glu](o)) in the cul ture medium, resulting in NMDA receptor-mediated excitotoxic death of neurons 24 h later. The increase in [Glu](o) was largely insensitive t o tetrodotoxin, independent of extracellular Ca2+, and present also in astrocyte-pure cultures. By the use of glutamate transporters functio nally reconstituted in liposomes, we showed directly that PDC activate s carrier-mediated release of glutamate via heteroexchange. Glutamate release and delayed neurotoxicity in our cultures were suppressed if P DC was applied in a Na+-free medium containing Li+. However, replaceme nt of Na+ with choline instead of Li+ did not result in an identical e ffect, suggesting that Li+ does not act simply as an external Na+ subs titute. In conclusion, our data indicate that alteration of glutamate transport by PDC has excitotoxic consequences and that active release of glutamate rather than just uptake inhibition is responsible for the generation of neuronal injury.