Mechanisms of glial swelling induced by glutamate

The mechanisms of glutamate-induced glial swelling have been studied using an in vitro model that permits detection of cell volume changes with high accuracy. The model allows for a close control of the extracellular environment to study in isolation the effect of defined extracellular alterations occurring in brain under pathophysiologic conditions. Glutamate was applied in concentrations between 50 microM and 10 mM to either C6 glioma cells or astrocytes from primary culture. Glutamate uptake was assessed by HPLC measurements of amino acids in the extracellular medium. Glutamate at all concentrations tested caused glial swelling, which, however, was moderate, with maximal average volume increases between 5.0 +/- 1.92 and 18.38 +/- 1.6% of control at 50 microM and 5 mM glutamate, respectively. Swelling was concentration dependent and correlated with glutamate uptake. After removal of all extracellular glutamate by glial uptake, cell volume spontaneously normalized. Pretreatment of the cells for 90 min with ouabain (1 mM) to abolish the extracellular/intracellular Na+ gradient, prevented glutamate-induced swelling. It is concluded that while glial cells readily accumulate glutamate from the extracellular environment to protect neurons from excitotoxic effects, swelling results from the increase of intracellular osmotic activity due to the uptake of Na+ and glutamate.