Do NMDA antagonists protect against cerebral ischemia: are clinical trials warranted?
An accumulation of experimental data suggests that N-methyl-D-aspartate (NMDA) receptor antagonists will prevent ischemic neuronal injury following transient global ischemia and reduce infarct volumes following focal ischemic insults. The excitotoxic hypothesis states that the excitatory amino acid neurotransmitter L-glutamate has neurotoxic properties that can be attenuated by antagonism of the NMDA receptor. In vitro work has shown that a variety of NMDA antagonists will prevent the death of neurons grown in culture and subsequently exposed to either brief periods of hypoxia or glutamate exposure. In vivo it has been shown that glutamate is released following ischemia, that the NMDA receptors remain functional both during and following ischemia, and that the concentration of NMDA receptors is highest in those regions that are most sensitive to ischemic neuronal injury. Once stimulated, these receptors mediate a lethal influx of calcium. Experiments with global ischemia have reported a cytoprotective effect by either prior removal of glutamate afferents or pretreatment with either competitive or noncompetitive receptor antagonists. Some of these data have been challenged and one suggestion that has been made is that the observed pharmacoprotection may be the result of coincidental drug-induced hypothermia. Numerous studies using a variety of models of focal ischemia have shown that the volume of a cortical infarct can be reduced with NMDA antagonists given either before or after an ischemic insult. These data are more consistent than those achieved for models of global ischemia and have led to proposals for clinical trials. Novel compounds that antagonize the NMDA receptor are now the subject of phase I clinical studies that are envisaged as a prelude to randomized acute stroke trials. The hypothesis that blockade of excitatory amino acid receptors will prevent neuronal death presages a new era in acute stroke treatment.