Abstract

Tricyclic antidepressants (TCAs) have been reported to interact with the opioid system, but their pharmacological activity at opioid receptors has not yet been elucidated. In the present study, we investigated the actions of amoxapine, amitriptyline, nortriptyline, desipramine, and imipramine at distinct cloned and native opioid receptors. In Chinese hamster ovary (CHO) cells expressing δ-opioid receptors (CHO/DOR), TCAs displaced [3H]naltrindole binding and stimulated guanosine 5′-O-(3-[³⁵S]thio)triphosphate ([³⁵S]GTPγS) binding at micromolar concentrations with amoxapine displaying the highest potency and efficacy. Amoxapine and amitriptyline inhibited cyclic AMP formation and induced the phosphorylation of signaling molecules along the extracellular signal-regulated kinase 1/2 (ERK1/2) and phosphatidylinositol-3 kinase pathways. Amoxapine also activated δ-opioid receptors in rat dorsal striatum and nucleus accumbens and human frontal cortex. In CHO cells expressing κ-opioid receptors (CHO/KOR), TCAs, but not amoxapine, exhibited higher receptor affinity and more potent stimulation of [³⁵S]GTPγS binding than in CHO/DOR and effectively inhibited cyclic AMP accumulation. Amitriptyline regulated ERK1/2 phosphorylation and activity in CHO/KOR and C6 glioma cells endogenously expressing κ-opioid receptors, and this effect was attenuated by the κ-opioid antagonist nor-binaltorphimine. In rat nucleus accumbens, amitriptyline slightly inhibited adenylyl cyclase activity and counteracted the inhibitory effect of the full κ agonist trans-(−)-3,4dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide (U50,488). At the cloned μ-opioid receptor, TCAs showed low affinity and no significant agonist activity. These results show that TCAs differentially regulate opioid receptors with a preferential agonist activity on either δ or κ subtypes and suggest that this property may contribute to their therapeutic and/or side effects.