We investigated the pharmacological properties and neuroprotective actions of a novel alpha-amino-3-hydroxy-5-methylisoxazole-y-propionate (AMPA)/kainate receptor antagonist, [6-(1H-imidazol-1-yl)-7-nitro-2,3-(1H,4H)-quinoxalinedione hydrochloride (YM90K); formerly YM900], in comparison with those of 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)quinoxaline (NBQX). YM90K selectively displaced [3H]-AMPA binding (Ki = 0.084 microM) and was less potent in inhibiting [3H]-kainate (Ki = 2.2 microM), [3H]-L-glutamate (N-methyl-D-aspartate-sensitive site; Ki > 100 microM) and [3H]-glycine (strychnine-insensitive site; Ki = 37 microM) binding to rat brain membranes. YM90K co-injected with AMPA or kainate into the rat striatum protected cholinergic neurons against AMPA- or kainate-induced neurotoxicity. YM90K showed potent suppressive activity against audiogenic seizure in DBA/2 mice; ED50 values of YM90K and NBQX against tonic seizure were 2.54 and 7.17 mg/kg (i.p.), respectively. The duration of the anticonvulsant effects of YM90K and NBQX was 30 min, indicating that both compounds possess short action. In a global ischemia model, YM90K (15 mg/kg i.p. x 3), NBQX (30 mg/kg i.p. x 3) and CNQX (60 mg/kg i.p. x 3) significantly prevented the delayed neuronal death in the hippocampal CA1 region in Mongolian gerbils when administered 1 h after 5-min ischemia. In addition, the therapeutic time window for the neuroprotective effect of YM90K (30 mg/kg i.p. x 3) was 6 h. In a focal ischemia model, YM90K (30 mg/kg i.v. bolus+10 mg/kg/h for 4 h) reduced the volume of ischemic damage in the cerebral cortex in F344 rats. Thus, YM90K was shown to be a potent and selective antagonist for AMPA/kainate receptors in vitro and in vivo. This compound may provide a therapeutic effect in various neurodegenerative disorders such as ischemic stroke in which glutamate neurotoxicity is thought to play a critical role in neuronal damage.