EFFECT OF OXIDATIVE STRESS ON THE RELEASE OF [H-3] GABA IN CULTURED CHICK RETINA CELLS

The effect of ascorbate (1.5 mM)/Fe2+ (7.5 mu M)-induced oxidative stress on the release of pre-accumulated [H-3]gamma-aminobutyric acid ([H-3]GABA) from cultured chick retina cells was studied. Depolarization of control cells with 50 mM K+ increased the release of [H-3]GABA by 1.01 +/- 0.16% and 2.5 +/- 0.3% of the total, in the absence and in the presence of Ca2+, respectively. Lipid peroxidation increased the release of [H-3]GABA to 2.07 +/- 0.31% and 3.6 +/- 0.39% of the total, in Ca2+-free or in Ca2+-containing media, respectively. The inhibitor of the GABA carrier, 1-(2-(((diphenylmethylene)amino)oxy)ethyl)-1,2,5,6-tetrahydro-3-pyridine-carboxylic acid hydrochloride (NNC-711) blocked almost completely the release of [H-3]GABA due to K+-depolarization in the absence of Ca2+, but only 65% of the release occurring in the presence of Ca2+ in control and peroxidized cells. Under oxidative stress retina cells release more [H-3]GABA than control cells, being the Ca2+-independent mechanism, mediated by the reversal of the Na+/GABA carrier, the most affected. MK-801 (1 mu M), a non-competitive antagonist of the NMDA receptor-channel complex, blocked by 80% the release of [H-3]GABA in peroxidized cells, whereas in control cells the inhibitory effect was of 48%. The non-selective blocker of the non-NMDA glutamate receptors, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), inhibited the release of [H-3]GABA by 30% and 70% in control and peroxidized cells, respectively. Glycine (5 mu M) stimulated [H-3]GABA release evoked by 50 mM K+-depolarization in control but not in peroxidized cells. The release of D-[H-3]aspartate (a non-metabolized analog of L-glutamate) evoked by 50 mM K+, in the absence of Ca2+, was significantly higher in peroxidized cells (6.76 +/- 0.64% of the total) than in control cells (3.79 +/- 0.27% of the total). The results suggest that oxidative stress induced by ascorbate/Fe2+ causes an excessive release of endogenous excitatory amino acids upon K+-depolarization. The glutamate released may activate NMDA and non-NMDA receptors, raising the intracellular Na+ concentration and consequently stimulating the release of [H-3]GABA by reversal of the Na+/GABA carrier.