Vol. 299, Issue 1, 366-371, October 2001

Blockade of Rat 34 Nicotinic Receptor Function by Methadone, Its Metabolites, and Structural Analogs

Yingxian Xiao, Richard D. Smith, Frank S. Caruso and Kenneth J. Kellar

Department of Pharmacology, Georgetown University School of Medicine, Washington, DC (Y.X., K.J.K.); and Endo Pharmaceuticals, Inc., Neptune, New Jersey (R.S.-C., F.S.C.)

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 metabolites R-(+)-2-ethyl-1,5-dimethyl-3,3-diphenylpyrrolinium perchlorate (EDDP) and R-(+)-2-ethyl-5-methyl-3,3-diphenyl-1-pyrroline hydrochloride (EMDP), as well as structural analogs of methadone, including ()--acetylmethadol hydrochloride (LAAM) and (+)--propoxyphene, on rat 34 neuronal nicotinic acetylcholine receptors (nAChRs) stably expressed in a human embryonic kidney 293 cell line, designated KX34R2. (±)-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 IC₅₀ value 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-stimulated ⁸⁶Rb⁺ efflux was markedly decreased, but the EC₅₀ value for nicotine stimulation was altered only slightly, if at all, indicating that these compounds block 34 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 KX34R2 cells. We conclude that both enantiomers of methadone and its metabolites as well as LAAM and (+)--propoxyphene are potent noncompetitive antagonists of 34 nAChRs.