Activity of ionotropic glutamate receptors in retinal cells: effect of ascorbate/Fe(2+)-induced oxidative stress

The effect of oxidative stress induced by the oxidant pair ascorbate/Fe2+ on the activity of ionotropic glutamate receptors was studied in cultured chick retina cells. The release of [3H]GABA and the increase of the intracellular free Na+ concentration ([Na+]i), evoked by glutamate receptor agonists, were used as functional assays for the activity of the receptors. The results show that the maximal release of [3H]GABA evoked by kainate (KA; approximately 20% of the total) or AMPA (approximately 11% of the total) was not different in control and peroxidized cells, whereas the EC50 values determined for peroxidized cells (33.6 +/- 1.7 and 8.0 +/- 2.0 microM for KA and AMPA, respectively) were significantly lower than those determined under control conditions (54.1 +/- 6.6 and 13.0 +/- 2.2 microM for KA and AMPA, respectively). The maximal release of [3H]GABA evoked by NMDA under K+ depolarization was significantly higher in peroxidized cells (7.5 +/- 0.5% of the total) as compared with control cells (4.0 +/- 0.2% of the total), and the effect of oxidative stress was significantly reduced by a phospholipase A2 inhibitor or by fatty acid-free bovine serum albumin. The change in the intracellular [Na+]i evoked by saturating concentrations of NMDA under depolarizing conditions was significantly higher in peroxidized cells (8.9 +/- 0.6 mM) than in control cells (5.9 +/- 1.0 mM). KA, used at a subsaturating concentration (35 microM), evoked significantly greater increases of the [Na+]i in peroxidized cells (11.8 +/- 1.7 mM) than in control cells (7.1 +/- 0.8 mM). A saturating concentration (150 microM) of this agonist triggered similar increases of the [Na+]i in control and peroxidized cells. Accordingly, the maximal number of binding sites for (+)-5-[3H]methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10- imine 11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate ([3H]MK-801) was increased after peroxidation, whereas the maximal number of binding sites for [3H]KA was not affected by oxidative stress. These data suggest that under oxidative stress the activity of the ionotropic glutamate receptors is increased, with the NMDA receptor being the most affected by peroxidation.