Accumulation of extracellular glutamate and neuronal death in astrocyte-poor cortical cultures exposed to glutamine.

The function of astrocytes in cerebral cortex may be studied by comparing the properties of conventional, astrocyte-rich cultures with astrocyte-poor cultures in which astrocyte proliferation has been stringently suppressed. Exposure of astrocyte-poor, but not astrocyte-rich, cultures to fresh medium containing 2 mM glutamine resulted in the death of most neurons within 24 h. This study was undertaken to understand the basis for the apparent toxicity of glutamine in astrocyte-poor cultures. The toxicity of glutamine was found to be mediated by glutamate, which demonstrated an LD50 as a neurotoxin in astrocyte-poor cultures of 2 microM. Exposure of astrocyte-poor (but not astrocyte-rich) cultures to fresh medium containing glutamine for 17.5-24 h resulted in the accumulation of substantial quantities of glutamate (255 +/- 158 microM; mean +/- standard deviation) coincident with the death of neurons in the cultures. Exposure of astrocyte-poor cultures to glutamate in the absence of glutamine did not result in the accumulation of extracellular glutamate. Both the neuronal death and the extracellular glutamate accumulation in astrocyte-poor cultures exposed to glutamine could be blocked by N-methyl-D-aspartate (NMDA) antagonists. These observations suggest that astrocytes as well as glutamine may play an important role in the pathogenesis of glutamate neurotoxicity in the central nervous system.