Glutamate-pyruvate transaminase protects against glutamate toxicity in hippocampal slices

Elimination of glutamate through enzymatic degradation is an alternative to glutamate receptor blockade in preventing excitotoxic neuronal injury. Glutamate pyruvate transaminase (GPT) is a highly active glutamate degrading enzyme that requires pyruvate as a co-substrate. This study examined the ability of GPT to protect neurons of the hippocampal slice preparation against glutamate toxicity. Two methods were used to elevate the concentration of glutamate in the peri-neuronal space. In an endogenous release paradigm, slices were incubated with 100-500 microM L-trans-pyrrolidine-2,4-dicarboxylate (PDC), an inhibitor of glutamate re-uptake. One hour of exposure to PDC in normal, pyruvate-free slice maintenance medium caused a dose dependent increase in neuronal death assessed 24 h later by propidium iodide uptake in dead cell nuclei. GPT (10 U/ml) decreased neuronal death caused by exposure to PDC at all PDC concentrations tested. Neuroprotection in this model was not dependent on added or non-physiologic levels of pyruvate. In a different paradigm, glutamate was added directly to the normal, pyruvate-free slice maintenance medium and not rinsed away, exposing the slices to a range of 1-5 mM glutamate for an extended period. Twenty-four hours later, neuronal death was again assessed by propidium iodide uptake. GPT was again neuroprotective, decreasing neuronal death in the range from 3 to 5 mM glutamate. In the setting of incubation with this large load of glutamate, neuroprotection by GPT was enhanced by adding pyruvate to the medium. GPT is an effective neuroprotectant against glutamate excitotoxicity. When exposure is limited to endogenously released glutamate, neuroprotection by GPT is not dependent on added pyruvate.