Neurons depend on astrocytes in a coculture system for protection from glutamate toxicity

Glutamate can be toxic to neurons although it is a neurotransmitter. Regulation of extracellular glutamate levels is essential for prevention of glutamate neurotoxicity. Astrocytes play a major role in clearance of glutamate released by neurons. A coculture system combining cerebellar cells and astrocytes was employed to investigate the astrocytic control of glutamate toxicity. Coculture of astrocytes with cerebellar neurons enhanced uptake of glutamate by astrocytes. Inhibition of glutamate uptake in a coculture system led to death of cerebellar cells. This toxicity could be inhibited by MK801. However, in the presence of the glutamate uptake inhibitor, there was no increase in glutamate in the cultures compared to when the neurons were not cocultured. This indicated that neurons become more susceptible to glutamate toxicity in the presence of astrocytes and thus become dependent on astrocytes for prevention of glutamate toxicity. Astrocytes treated with conditioned medium from cerebellar cells did not show an increase in glutamate uptake but medium from astrocytes exposed to neuron conditioned medium was toxic to cerebellar cells. This toxicity was due to glutamate present in the medium. This suggests that a soluble factor released by neurons signals to astrocytes that neurons are present and stimulates a signal back to neurons which causes an increased sensitivity to glutamate toxicity.