Potentiation of excitotoxic injury by high concentrations of extracellular reduced glutathione

Abstract

Glutathione is present in the central nervous system in millimolar concentrations, and is a predominant intracellular antioxidant and detoxicant. In addition, glutathione is released into the extracellular space via a depolarization-enhanced process. Although the role of extracellular glutathione has not been precisely defined, a growing body of experimental evidence suggests that it has multifaceted electrophysiological effects. At low micromolar concentrations, glutathione depolarizes neurons by binding to its own receptors and modulates glutamatergic excitatory neurotransmission by displacing glutamate from its ionotropic receptors. At higher concentrations, reduced glutathione may increase N-methyl- d-aspartate receptor responses by interacting with its redox sites. In this study, the effect of extracellular glutathione on excitotoxic neuronal injury was quantitatively assessed in murine cortical cell cultures. Neuronal death due to 20–25 h exposure to 6–9 μM N-methyl- d-aspartate was not altered by 10–100 μM reduced glutathione but was markedly enhanced by 300–1000 μM reduced glutathione; kainate neurotoxicity was unaffected. Two related compounds that lack a sulfhydryl group, oxidized glutathione and S-hexylglutathione, had no significant effect on N-methyl- d-aspartate neurotoxicity alone but completely blocked the effect of reduced glutathione. Mercaptoethanol, a sulfhydryl reducing agent that increases N-methyl- d-aspartate receptor responses by interacting with redox sites, increased N-methyl- d-aspartate neurotoxicity to a degree comparable to that of reduced glutathione; this effect was also blocked by equimolar S-hexylglutathione or oxidized glutathione. Addition of reduced glutathione to mercaptoethanol did not further increase N-methyl- d-aspartate-induced neuronal death. These results suggest that release of reduced glutathione from central nervous system cells that are subjected to traumatic or ischemic insults may enhance excitotoxic neuronal loss. Although multiple mechanisms may account for this phenomenon, the high concentrations required suggest that it is at least partly mediated by reduction of N-methyl- d-aspartate receptor redox sites.