Reduced glutamate uptake by retinal glial cells under ischemic/hypoxic conditions

The high-affinity uptake of glutamate by glial cells and neurons of the central nervous system, including the retina, serves to inactivate synaptically released glutamate and maintains glutamate at low concentrations in the extracellular space. This uptake prevents accumulation of glutamate extracellularly and thus minimizes the possibility of glutamate neurotoxicity secondary to ischemic insult. One mechanism whereby glutamate neurotoxicity may occur in ischemic/hypoxic insult is through increased extracellular K+ reversing the electrogenic glutamate uptake into retinal glial (Müller) cells. We investigated glial uptake of the amino acids glutamate, GABA, and D-aspartate in the intact isolated rat retina under high extracellular K+ conditions and under conditions simulating ischemia. Immunocytochemical findings showed that uptake of glutamate and GABA by MIller cells in the intact isolated rat retina continues under conditions simulating ischemia and high extracellular K+ conditions, and uptake of D-aspartate also continues under high K+ conditions. However, under high K+ conditions, the glutamate uptake system saturates at a lower concentration of exogenous glutamate than in the normal K+ condition. These findings provide evidence that disruption of glutamate uptake by Müller cells is likely to be a significant contributing factor to excess glutamate accumulation in the extracellular space which can lead to neurotoxicity.