Inhibitory effects of dextrorotatory morphinans on the human 5-HT3A receptor expressed in Xenopus oocytes: Involvement of the N-terminal domain of the 5-HT3A receptor

We previously developed a series of dextromethorphan (DM, 3-methoxy-17-methylmorphinan) analogs modified at positions 3 and 17 of the morphinan ring system. Recent reports have shown that DM attenuates abdominal pain caused by irritable bowel syndrome, and multidrug regimens that include DM prevent nausea/vomiting following cancer surgery. However, little is known regarding the molecular mechanisms underlying the beneficial effects of DM. Here, we investigated the effects of DM, 3 of its analogs (AM, 3-allyloxy-17-methoxymorphian; CM, 3-cyclopropyl-17-methoxymorphinan; and DF, 3-methyl-17-methylmorphinan), and 1 of its metabolites (HM, 3-methoxymorphinan) on the activity of the human 5-HT3A receptor channel expressed in Xenopus laevis oocytes, using the 2-microelectrode voltage clamp technique. We found that intra-oocyte injection of human 5-HT3A receptor cRNAs elicited an inward current (I5-HT) in the presence of 5-HT. Cotreatment with AM, CM, DF, DM, or HM inhibited I5-HT in a dose-dependent, voltage-independent, and reversible manner. The IC50 values for AM, CM, DF, DM, and HM were 24.5 ± 1.4, 21.5 ± 4.2, 132.6 ± 35.8, 181.3 ± 23.5, and 191.3 ± 31.5 μM, respectively. The IC50 values of AM and CM were 7-fold lower than that of DM, and mechanistic analysis revealed that DM, DF, HM, AM, and CM were competitive inhibitors of I5-HT. Point mutations of Arg241 in the N-terminal, but not amino acids in the pore region, to other amino acid residues attenuated or abolished DM- and DM-analog-induced inhibition of I5-HT. Together, these results demonstrated that dextrorotatory morphinans might regulate 5-HT3A receptor channel activity via interaction with its N-terminal domain.