Studies of NMDA- and non-NMDA-mediated neurotoxicity in cultured neurons

The neurotoxic effects of various glutamate agonists were studied using whole fetal rat brain cultures. The results showed that L-glutamate (L-glu) and N-methyl-D-aspartate (NMDA) were the most potent agonists for inducing neurotoxicity, producing significant toxicity at 0.10 and 0.01 mM concentrations, respectively. Kainic acid (KA) and quisqualic acid (QA) also produced neurotoxicity, but only at a relatively high concentration (1.0 mM). No other glutamate agonist tested produced neurotoxicity in the cultures following brief incubations. The effects of each agonist were found to be Ca2+ dependent, and the selective NMDA Ca2+ channel agonist, (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,1 0-imine hydrogen maleate (MK-801), blocked the toxicity produced by all the glutamate agonists. Thus, the results of this study found little or no evidence for a direct non-NMDA receptor mediated neurotoxicity. These results suggest that the neurotoxicity produced by the non-NMDA agonists may be due to one of the following mechanisms: (i) non-specific binding of non-NMDA agonists to NMDA receptor; (ii) release of L-glu via non-NMDA agonists induced depolarization of cell membrane and subsequent activation of NMDA receptor by released L-glu; (iii) inhibition of L-glu uptake by non-NMDA agonists resulting in activation of L-glu receptors including NMDA receptors.