Muller cell swelling, glutamate uptake, and excitotoxic neurodegeneration in the isolated rat retina

We characterized morphological effects of the endogenous excitotoxin, glutamate in ex vivo retinal segments prepared from 30-day-old rats. Initial changes induced by glutamate consisted of reversible, sodium-dependent Muller cell swelling. This glial swelling was mimicked by glutamate transport substrates but not by ionotropic glutamate receptor agonists. Only very high concentrations of exogenous glutamate (3,000 μM) produced excitotoxic neuronal damage. The neuronal damage was accompanied by severe glial swelling and was blocked by an antagonist of non-N-methyl-D-aspartate (NMDA) receptors but not by an NMDA receptor antagonist. Because glutamate uptake can be influenced by changes in cellular energy levels, we studied the effects of oxidative and glycolytic energy depletion on glutamate-mediated Muller cell swelling. Oxygen deprivation produced little morphological change and did not alter either glutamate-mediated Muller cell swelling or glutamate-induced excitotoxicity. In contrast, inhibition of glycolysis by iodoacetate produced severe neuronal damage without Muller cell swelling. In the presence of iodoacetate, exogenous glutamate failed to cause glial swelling. The neuronal damage produced by iodoacetate was inhibited by pyruvate, a substrate that sustains oxidative energy pathways. In the presence of iodoacetate plus pyruvate, glutamate failed to cause Muller cell swelling but became neurotoxic at low concentrations through activation of non-NMDA receptors. These results indicate that glycolytic energy metabolism plays a critical role in sustaining ionic balances required for Muller cell glutamate uptake and glial uptake helps to prevent glutamate-mediated excitotoxicity.