Abstract

The opioid agonist properties of (±)-methadone are ascribed almost entirely to the (−)-methadone enantiomer. To extend our knowledge of the pharmacological actions of methadone at ligand-gated ion channels, we investigated the effects of the two enantiomers of methadone and its metabolitesR-(+)-2-ethyl-1,5-dimethyl-3,3-diphenylpyrrolinium perchlorate (EDDP) andR-(+)-2-ethyl-5-methyl-3,3-diphenyl-1-pyrroline hydrochloride (EMDP), as well as structural analogs of methadone, including (−)-α-acetylmethadol hydrochloride (LAAM) and (+)-α-propoxyphene, on rat α3β4 neuronal nicotinic acetylcholine receptors (nAChRs) stably expressed in a human embryonic kidney 293 cell line, designated KXα3β4R2. (±)-Methadone inhibited nicotine-stimulated ⁸⁶Rb⁺ efflux from the cells in a concentration-dependent manner with an IC₅₀ value of 1.9 ± 0.2 μM, indicating that it is a potent nAChR antagonist. The (−)- and (+)-enantiomers of methadone have similar inhibitory potencies on nicotine-stimulated ⁸⁶Rb⁺ efflux, with IC₅₀ values of approximately 2 μM. EDDP, the major metabolite of methadone, is even more potent, with an IC₅₀value of approximately 0.5 μM, making it one of the most potent nicotinic receptor blockers reported. In the presence of (±)-methadone, EDDP, or LAAM, the maximum nicotine-stimulated⁸⁶Rb⁺ efflux was markedly decreased, but the EC₅₀ value for nicotine stimulation was altered only slightly, if at all, indicating that these compounds block α3β4 nicotinic receptor function by a noncompetitive mechanism. Consistent with a noncompetitive mechanism, (±)-methadone, its metabolites, and structural analogs have very low affinity for nicotinic receptor agonist binding sites in membrane homogenates from KXα3β4R2 cells. We conclude that both enantiomers of methadone and its metabolites as well as LAAM and (+)-α-propoxyphene are potent noncompetitive antagonists of α3β4 nAChRs.