Abstract

Homomeric 5-hydroxytryptamine (5-HT)_3A and heteromeric 5-HT_3AB receptors mediate rapid excitatory responses to serotonin in the central and peripheral nervous systems. The alkaloid morphine, in addition to being a μ-opioid receptor agonist, is a potent competitive inhibitor of 5-HT₃ receptors. We examined whether methadone, an opioid often used to treat morphine dependence, also exhibited 5-HT₃ receptor antagonist properties. Racemic (R/S)-methadone inhibited currents mediated by human homomeric 5-HT_3A receptors (IC₅₀ = 14.1 ± 2.5 μM). Incorporation of the 5-HT_3B subunit into heteromeric 5-HT_3AB receptors reduced the potency of inhibition by (R/S)-methadone (IC₅₀ = 41.1 ± 0.9 μM). (R/S)-Methadone also increased apparent desensitization of both 5-HT₃ receptor subtypes. The inhibition of the 5-HT_3A receptor was competitive; however, incorporation of the 5-HT_3B subunit caused the appearance of inhibition that was insurmountable by 5-HT. In the absence of rapid desensitization, when dopamine was used as an agonist of 5-HT_3AB receptors, the inhibition by (R/S)-methadone was voltage-dependent. The antagonist and desensitization-enhancing effects of (R/S)-methadone were shared by pure (R)- and (S)-methadone enantiomers, which had similar actions on 5-HT-evoked currents mediated by 5-HT₃ receptors. However, (R)-methadone exhibited a larger voltage-dependent inhibition of dopamine-evoked currents mediated by 5-HT_3AB receptors than did (S)-methadone. Inhibition of 5-HT_3A receptors by (R/S)-methadone was not influenced by voltage. Thus, methadone displays multimodal subunit-dependent antagonism of 5-HT₃ receptors.