The tricyclic antidepressants amitriptyline (CAS 549-18-8), amitriptylinoxide (CAS 4317-14-0) and doxepine (CAS 1229-29-4) as well as the atypical neuroleptic clozapine (CAS 5786-21-0)--all of them substances with well-established clinical efficacy--were investigated in order to elucidate their effect on N-methyl-D-aspartate (NMDA) receptor-mediated events. Modulation of NMDA receptor function was studied using the model of NMDA-evoked [3H]-acetylcholine ([3H]-ACh) release in slices of rat caudatoputamen. All substances reduced [3H]-ACh release in a concentration dependent manner. Significant inhibition occurred in the low micromolar range with the exception of amitriptylinoxide which was less potent in vitro (amitriptylinoxide is not being metabolized in vitro). Amitriptyline and clozapine at 10 mumol/l both decreased the maximum effect of NMDA by around 17%, but left its EC50 unchanged. This suggests a "classical" non-competitive antagonism and excludes an uncompetitive or "use-dependent" antagonism. Considering the important role of NMDA receptor-mediated effects in spinal nociception the analgesic properties of tricyclic antidepressants may partly be explained by their inhibitory action on spinal NMDA receptors, in addition to their enhancement of monoaminergic transmissions in the dorsal horn.