AURINTRICARBOXYLIC ACID PREVENTS NMDA-MEDIATED EXCITOTOXICITY - EVIDENCE FOR ITS ACTION AS AN NMDA RECEPTOR ANTAGONIST

The effect of the endonuclease inhibitor aurintricarboxylic acid (ATA) versus NMDA-mediated delayed cell death was examined in an ex vivo chick retinal preparation. Transient exposure to 100 muM NMDA for 60 min followed by a 24-h recovery period resulted in a sevenfold increase in lactate dehydrogenase (LDH) release into the medium. ATA at 100 mum significantly reduced NMDA-mediated LDH release by 60%. In clarifying the mechanism of protection versus NMDA, ATA was found to inhibit several acute NMDA-mediated effects: ATA attenuated NMDA-mediated GABA release in a dose-dependent manner (IC50 = 29.5 muM), prevented NMDA-stimulated cyclic GMP formation, and blocked NMDA-mediated Na-22+ influx. These acute inhibitory effects of ATA were overcome by increasing the NMDA concentration, which suggested a competitive interaction between NMDA and ATA. In a binding assay using membranes prepared from adult rat forebrain, ATA displaced the competitive NMDA receptor ligand [H-3]CGS 19755 with an IC50 of 26.9 muM. Maximal displacement was 88% with 100 muM ATA. These studies demonstrate that ATA protected neurons from NMDA-mediated cell death upstream of endonuclease inhibition, i.e., by antagonizing NMDA receptor activity in a manner consistent with competitive antagonism.