Glutamate Neurotoxicity in Culture Depends on the Presence of Glutamine: Implications for the Role of Glial Cells in Normal and Pathological Brain Development

Glutamate toxicity was studied in neuronal (SC9), glial (WC5), and neuroblastoma-glioma hybrid cell lines. In all three cell types, glutamate had a dual effect, depending on the concentration of glutamine in the culture medium. An expected dose-dependent cytotoxicity of the amino acid was observed when cells were cultured in medium containing the standard glutamine concentration (1-4 mM), but when the culture's glutamine content was decreased to 0.15-0.5 mM, glutamate had an apparent opposite, growth-promoting effect. The specificity of glutamate effect was indicated by the following: (a) it was stereospecific, with the L and not the D isomer being active; (b) monosodium aspartate was inactive in the presence of either high or low glutamine; and (c) monosodium glutamate and monopotassium glutamate had a similar dual effect. Furthermore, the glutamate receptor antagonist gamma-glutamylglycine blocked the amino acid cytotoxicity in a dose-dependent fashion. As glial cells are a major source of glutamine in the brain, neuronal-glial co-cultures were used to analyze the possible role of glial cells in glutamate neurotoxicity. It was found that SC9 cells were more sensitive to glutamate when co-cultured with WC5 cells. Continuous depolarization of the SC9 cells with KCl decreased cell number, but glutamate had no additive neurotoxic effect when added with KCl. We suggest that glutamine, glial cells, and neuronal activation play roles in modulating glutamate neurotoxicity, in developing as well as aged brains. It is tempting to speculate also that alterations in the glutamate/glutamine ratio under pathological conditions may take part in the etiology of some neurodegenerative diseases.