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 µ M ) before 500 µ M glutamate, 30 µ M NMDA, or 300 µ 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 m M ) partially reversed the effect of ACEA 1021 on kainate cytotoxicity. Glycine (100 µ M –1 m M ) 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 µ M ACEA 1021 was similar for glutamate and NMDA (18 ± 3 and 29 ± 9 µ 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.