EXCITOTOXICITY AT BOTH NMDA AND NON-NMDA GLUTAMATE RECEPTORS IS ANTAGONIZED BY AURINTRICARBOXYLIC ACID - EVIDENCE FOR DIFFERING MECHANISMS OF ACTION

In this study, the endonuclease inhibitor aurin-tricarboxylic acid (AT A) was examined for its ability to attenuate both acute and delayed ex citotoxicity mediated through NMDA and non-NMDA glutamate receptors. E x vivo embryonic chick retina, a model system frequently used for stud ies of excitotoxicity, was exposed to either 100 mu M NMDA or kainate (KA) +/- various concentrations of ATA for 60 min, then allowed to rec over for 24 h. Lactate dehydrogenase release into the medium and histo logy were assessed as measures of delayed toxicity. ATA attenuated lac tate dehydrogenase release due to NMDA or KA in a dose-dependent manne r. Histology revealed that ATA decreased the number of pyknotic profil es in response to either glutamate agonist. The mechanism of ATA prote ction was addressed. ATA was found to block NMDA- but not KA-mediated Na-22(+) influx and cyclic GMP formation. In membrane binding studies, ATA was relatively selective for displacement at the NMDA receptor. T he IC50 values for displacement of [H-3]CGS 19755, alpha-[(3)Hlamino-3 -hydroxy-5-methylisoxazole acid ([H-3]AMPA), or [H-3]KA were 29.9 +/- 1.3, 313 +/- 46, and >1,000 mu M +/- SEM, respectively. ATA also fully attenuated NMDA-induced and partially attenuated KA-induced acute exc itotoxicity as monitored histologically by tissue swelling and by the increase in GABA in the medium. Temporal studies of ATA efficacy indic ated that ATA needed to be present during NMDA exposure to afford prot ection but, versus KA, was equally effective if administered immediate ly after KA exposure. Questions regarding the cellular penetration of ATA were raised because incubation with 100 mu M ATA for 60 min had no effect on lactate formation or [H-3]leucine incorporation into trichl oroacetic acid-precipitable material, even though, in cell-free system s, ATA is a potent inhibitor of phosphofructokinase activity and prote in synthesis. These studies demonstrate that ATA can protect against e xcitotoxicity mediated through NMDA or non-NMDA glutamate receptors. T he mechanism of protection versus NMDA is through interruption of NMDA receptor interactions. ATA has no direct effect al the KA receptor; t hus, its mechanism of protection versus KA is distinct from that versu s NMDA and is, at present, unknown.