Inhibition of Astrocyte Glutamate Uptake by Reactive Oxygen Species: Role of Antioxidant Enzymes

Olivier Sorg, Thomas F. W. Horn, Naichen Yu, Donna L. Gruol, Floyd E. Bloom

The recent literature suggests that free radicals and reactive oxygen species may account for many pathologies, including those of the nervous system. The influence of various reactive oxygen species on the rate of glutamate uptake by astrocytes was investigated on monolayers of primary cultures of mouse cortical astrocytes. Hydrogen peroxide and peroxynitrite inhibited glutamate uptake in a concentration-dependent manner. Addition of copper ions and ascorbate increased the potency and the efficacy of the hydrogen peroxide effect, supporting the potential neurotoxicity of the hydroxyl radical. The free radical scavenger dimethylthiourea effectively eliminated the inhibitory potential of a mixture containing hydrogen peroxide, copper sulphate, and ascorbate on the rate of glutamate transport into astrocytes. The inhibitory effect of hydrogen peroxide on glutamate uptake was not altered by the inhibition of glutathione peroxidase, whereas the inhibition of catalase by sodium azide clearly potentiated this effect. Superoxide and nitric oxide had no effect by themselves on the rate of glutamate uptake by astrocytes. The absence of an effect of nitric oxide is not due to an inability of astrocytes to respond to this substance, since the same cultures did respond to nitric oxide with a sustained increase in cytoplasmic free calcium. These results confirm that reactive oxygen species have a potential neurotoxicity by means of impairing glutamate transport into astrocytes, and they suggest that preventing the accumulation of hydrogen peroxide in the extracellular space of the brain, especially during conditions that favor hydroxyl radical formation, could be therapeutic.