Neuroprotective effects of NMDA receptor glycine recognition site antagonism: Dependence on glycine concentration

High-affinity NMDA receptor glycine recognition site antagonists protect brain tissue from ischemic damage. The neuroprotective effect of 5-nitro-6,7-dichloro-2,3-quinoxalinedione (ACEA 1021), a selective NMDA receptor antagonist with nanomolar affinity for the glycine binding site, was examined in rat cortical mixed neuronal/glial cultures. ACEA 1021 alone did not alter spontaneous lactate dehydrogenase (LDH) release. Treatment with ACEA 1021 (0.1-10 mu M) before 500 mu M glutamate, 30 mu M NMDA, or 300 mu M kainate exposure was found to reduce LDH release in a concentration-dependent fashion. These effects were altered by adding glycine to the medium. Glycine (1 mM) partially reversed the effect of ACEA 1021 on kainate cytotoxicity. Glycine (100 mu M-1 mM) completely blocked the effects of ACEA 1021 on glutamate and NMDA cytotoxicity, The glycine concentration that produced a half-maximal potentiation of excitotoxin-induced LDH release in the presence of 1.0 mu M ACEA 1021 was Similar for glutamate and NMDA (18 +/- 3 and 29 +/- 9 mu M, respectively). ACEA 1021 also reduced kainate toxicity in cultures treated with MK-801. The effects of glycine and ACEA 1021 on glutamate-induced LDH release were consistent with a model of simple competitive interaction for the strychnine-insensitive NMDA receptor glycine recognition site, although nonspecific effects at the kainate receptor may be of lesser importance.