Nontransportable inhibitors attenuate reversal of glutamate uptake in synaptosomes following a metabolic insult

Na+-dependent, high-affinity glutamate transporters in the central nervous system are generally credited with regulating extracellular levels of L-glutamate and maintaining concentrations below those that would induce excitotoxic injury. Under pathological conditions, however, it has been suggested that these same transporters may contribute to excitotoxic injury by sewing as sites of efflux for cellular L-glutamate. In this study, we examine the efflux of [H-3]D-aspartate from synaptosomes in response to both alternative substrates (i.e., heteroexchange), such as L-glutamate, and a metabolic insult (5 mM potassium cyanide and 1 mM iodoacetate). Exposure of synaptosomes containing [H-3]D-aspartate to either L-glutamate or metabolic inhibitors increased the efflux of the radiolabeled substrate to over 200% of control values. Two previously identified competitive transport inhibitors (L-trans-2,3-pyrrolidine dicarboxylate and dihydrokainate) failed to stimulate [H-3]D-aspartate efflux but did inhibit: glutamate-mediated heteroexchange, consistent with the action of nontransportable inhibitors. These compounds also attenuated the efflux of [H-3]D-aspartate from synaptosomes exposed to the metabolic inhibitors. These results add further strength to the model of central nervous system injury-induced efflux of L-glutamate through its high-affinity transporters and identify a novel strategy to attenuate this process.