Mechanism of glutamate release from rat hippocampal slices during in vitro ischemia

There was a large release of endogenous glutamate and of pre-accumulated [3H]-d-aspartate from rat hippocampal slices during deprivation of oxygen and glucose (in vitro ischemia). The role of Na+-dependent glutamate transporters in this process was investigated. The release of both glutamate and [3H]-d-aspartate was largely blocked by two competitive substrate analogues of the Na+-dependent glutamate transporters (l-trans-pyrrolidine-2, 4-dicar☐ylate and d,l-threo-B-hydroxyaspartate) if the substrate analogues were intracellularly loaded prior to the ischemia. The pre-loaded analogue, d,l-threo-B-hydroxyaspartate, did not block exocytotic release of glutamate, induced by high-potassium. Dihydrokainate, an inhibitor of a subset of the Na+-dependent transporters, did not inhibit ischemia-induced release of glutamate or [3H]-d-aspartate However, it did block release induced by veratridine, which was also blocked by the pre-loaded substrate analogues. Dihydrokainate could still inhibit veratridine-induced release during ischemia, showing that conditions during ischemia did not reduce its efficacy. It is concluded that release of glutamate during ischemia is largely via reversal of the Na+-dependent glutamate transport system. The differential effects of dihydrokainate and the competitive substrate analogues on ischemia-induced release indicate that this release occurs via a subset of the glutamate transporters that are present in the hippocampus.