Electrophysiological experiments were performed in vitro and in vivo to characterize the inhibitory effects of 6,7-dichloro-5-nitro-1,4-dihydro-2,3-quinoxalinedione (ACEA 1021; licostinel) on rat brain glutamate receptors. In vitro, ACEA 1021 was tested on N-methyl-D-aspartate (NMDA)-induced depolarizations in the neocortical slice preparation and on epileptiform activity in Mg2+-free hippocampal slices, which is known to be NMDA receptor mediated. In both in vitro models, ACEA 1021 exhibited antagonistic effects on the NMDA receptor-mediated responses. Selectivity tests in the neocortical slice preparation, using NMDA, kainate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) showed that 10 microM ACEA 1021 reduced NMDA and kainate responses to 27.9 and 79.9% of the control value, respectively, whereas responses to AMPA were increased by 2.4% above the control value, thus showing that at this concentration ACEA 1021 acts preferentially at NMDA receptors. However, at 30 microM, all the NMDA-, AMPA- and kainate-induced responses were reduced. In vivo, ACEA 1021 was tested on NMDA-induced excitation in the CA1 region. After systemic administration of ACEA 1021, central effects were observed at 10 mg/kg i.v. in the CA1 region. These results indicate that ACEA 1021 is centrally active and inhibits NMDA receptor-mediated responses. Interestingly, selectivity tests in the CA1 region did not show clear differences in the action of ACEA 1021 on NMDA- and AMPA-induced excitations. Furthermore, ACh-induced excitations were also reduced. Thus, at low concentrations, ACEA 1021 seems to be a selective antagonist at the strychnine-insensitive glycine site of the NMDA receptor. However, at 30 microM in vitro and at 10 mg/kg in vivo, non-NMDA receptor-mediated actions of ACEA 1021 are observed. Our results suggest that these additional effects of ACEA 1021 may contribute to its anticonvulsive properties in mice as well as to its neuroprotective properties in animal models of cerebral ischemia.