Paradoxical convulsant action of a novel non-competitive N-methyl-D-aspartate (NMDA) antagonist, tiletamine

Intracerebroventricular (i.c.v.) injection of tiletamine, 0.001 mumol, a presumed non-competitive antagonist of N-methyl-D-aspartate (NMDA) receptors, protected mice from convulsions induced by NMDA and quinolinate, but not from those induced by excitatory amino acids interacting preferentially with non-NMDA receptors. At higher doses, however, tiletamine induced convulsions by itself. Tiletamine-induced convulsions were antagonized by the broad spectrum excitatory amino acid antagonist, gamma-D-glutamylamino-methylsulphonate (gamma-D-GAMS), and were potentiated by the competitive NMDA antagonist, 2-amino-7-phosphonohepatanoate (AP7). Intrathecal (i.t.) injection of tiletamine, 0.01-1.0 mumol, dose-dependently suppressed spinal flexor reflexes. Tiletamine, 0.01 and 0.1 mumol, failed to affect spinal Hoffman- (H-) reflexes, whereas tiletamine, 1.0 mumol, led to a 50% increase of the H-reflex amplitude. It is concluded that the anticonvulsant and reflex suppressant action of tiletamine are due to antagonism of NMDA receptor-mediated excitation. The convulsant effect of tiletamine and its excitatory effect on spinal H-reflexes at higher doses, however, appear to be mediated by non-NMDA receptors.