Alteration in the glial cell metabolism of glutamate by kainate and N-methyl-D-aspartate.

Incubation of coronal slices of rat brain with neurotoxic concentrations of kainate (300 microM) and N-methyl-D-aspartate (NMDA; 500 microM) for 40 min reduced the activity of the glial enzyme, glutamine synthetase, by 33% and 21%, respectively. The immunoreactivity of the neuronal enzyme, gamma gamma-enolase (neuron-specific enolase), was also decreased, but to a lesser extent than glutamine synthetase. Pre-incubation of the slices with L-methionine-S-sulphoximine (500 microM), an irreversible inhibitor of both glutamine synthetase and gamma-glutamylcysteine synthetase, before addition of either kainate or NMDA produced a supra-additive reduction in the activity of the enzyme in both cases. Neither kainate nor NMDA directly inhibited the activity of glutamine synthetase, but kainate did inhibit gamma-glutamylcysteine synthetase, a rate-limiting enzyme of the gamma-glutamyl cycle, which is responsible for maintaining glutathione levels within cells. Pre-incubation of the slices with L-NG-nitroarginine, a competitive inhibitor of nitric oxide synthase, effectively prevented the NMDA-induced reduction in glutamine synthetase and neuron specific enolase, but did not diminish the kainate-induced decrease in the activity of either enzyme. These results provide evidence that NMDA, as well as kainate, indirectly affects the activity of glutamine synthetase in brain slices, yet does so by a different mechanism from kainate. The results are discussed in terms of the possible mode of action of each toxin in inhibiting the glial cell metabolism of glutamate.