Differences in anticonvulsant potency and adverse effects between dextromethorphan and dextrorphan in amygdala-kindled and non-kindled rats.

The anticonvulsant and adverse effects of dextromethorphan, a non-opioid antitussive, and its metabolite dextrorphan were examined in amygdala-kindled rats. Both drugs have repeatedly been proposed to be functional non-competitive N-methyl-D-aspartate (NMDA) receptor antagonists, but they also exert effects distinct from antagonism at NMDA receptors, such as blockade of voltage-gated calcium channels and sigma-site mediated actions. Since recent data have demonstrated that kindled rats are more susceptible to the adverse effects of NMDA receptor antagonists than non-kindled rats, the time course, characteristics and severity of adverse effects of dextromethorphan and dextrorphan were also determined in non-kindled animals. Dextromethorphan dose dependently increased the focal seizure threshold (i.e. the threshold for induction of afterdischarges recorded from the amygdala) in fully kindled rats. This anticonvulsant effect was found at relatively low doses (7.5-15 mg/kg i.p.) which were almost free of any adverse effects. At higher doses, dextromethorphan induced motor impairment and seizures, but no phenyclidine (PCP)-like adverse effects, such as hyperlocomotion or stereotypies. In contrast, such adverse effects were seen after dextrorphan, although only infrequently. Dextrorphan was less potent in inducing anticonvulsant but more potent in inducing motor impairing effects than dextromethorphan in kindled rats. In non-kindled rats, the motor impairment induced by dextrorphan was significantly less severe than in kindled rats, whereas no marked differences between kindled and non-kindled rats were found for dextromethorphan. The data indicate that dextromethorphan and dextrorphan differ in their mechanisms of action. Only dextrorphan exerts effects which are characteristic for NMDA receptor antagonism, whereas the potent anticonvulsant effect of dextromethorphan in presumably unrelated to the NMDA receptor complex.