The neuroprotective actions of remacemide and its anticonvulsant metabolite 1,2-diphenyl-2-propylamine monohydrochloride (desglycinylremacemide; DGR) a low-affinity NMDA receptor antagonist, were investigated using primary rat cortical neuronal cultures. Exposure of cortical cultures to NMDA (100 microM) for 15 min killed 85% of the neurons during the next 24 h. This neurotoxicity was blocked in a concentration-dependent manner by adding DGR (5-20 microM), but not its remacemide precursor (10-100 microM), to the cultures during the time of NMDA exposure. This suggests that the neuroprotective, as well as the anticonvulsant, activity of remacemide is mediated by DGR. Neuroprotective concentrations of DGR also inhibited two of the principal acute effects of NMDA. DGR (5-20 microM) prevented the loss of membrane-associated protein kinase C (PKC) activity that developed by 4 h after transient exposure to 100 microM NMDA and reduced the NMDA-triggered increases in intracellular free Ca2+ concentration ([Ca2+]i) by up to 70%. By contrast, remacemide (50 and 100 microM) did not prevent the NMDA-induced loss of PKC activity or reduce the [Ca2+]i responses. These data suggest that DGR protection against NMDA-mediated toxicity in cultured cortical neurons is associated with a reduction of NMDA-triggered [Ca2+]i surges and a prevention of the loss of membrane-associated PKC activity. In addition, the inhibition of NMDA-triggered [Ca2+]i responses by DGR was qualitatively different from the inhibition of these responses by the high-affinity NMDA-receptor antagonists MK-801 and phencyclidine. This may be a consequence of DGR's lower affinity for the NMDA receptor.