Effects of clozapine on the δ‐ and κ‐opioid receptors and the G‐protein‐activated K⁺ (GIRK) channel expressed in Xenopus oocytes

Abstract

1 To investigate the e ects of clozapine, an atypical antipsychotic, on the cloned m-, d-and k-opioid receptors and G-protein-activated inwardly rectifying K + (GIRK) channel, we performed the Xenopus oocyte functional assay with each of the three opioid receptor mRNAs and/or the GIRK1 mRNA. 2 In the oocytes co-injected with either the d-or k-opioid receptor mRNA and the GIRK1 mRNA, application of clozapine induced inward currents which were attenuated by naloxone, an opioid-receptor antagonist, and blocked by Ba 2+ , which blocks the GIRK channel. Since the opioid receptors functionally couple to the GIRK channel, these results indicate that clozapine activates the d-and k-opioid receptors and that the inward-current responses are mediated by the GIRK channel. The action of clozapine at the d-opioid receptor was more potent and e cacious than that at the k-opioid receptor. In the oocytes co-injected with the m-opioid receptor and GIRK1 mRNAs, application of clozapine (100 mM) did not induce an inward current, suggesting that clozapine could not activate the m-opioid receptor. 3 Application of clozapine caused a reduction of the basal inward current in the oocytes injected with the GIRK1 mRNA alone, but caused no current response in the uninjected oocytes. These results indicate that clozapine blocks the GIRK channel. 4 To test the antagonism of clozapine for the m-and k-opioid receptors, we applied clozapine together with each selective opioid agonist to the oocytes co-injected with either the m-or k-opioid receptor mRNA and the GIRK1 mRNA. Each of the peak currents induced by each selective opioid agonist together with clozapine was almost equal to the responses to a selective opioid agonist alone. These results indicate that clozapine has no signi®cant antagonist e ect on the m-and k-opioid receptors. 5 We conclude that clozapine acts as a d-and k-agonist and as a GIRK channel blocker. Our results suggest that the e cacy and side e ects of clozapine under clinical conditions may be partly due to activation of the d-opioid receptor and blockade of the GIRK channel.

N-Desmethylclozapine, a Major Clozapine Metabolite, Acts as a Selective and Efficacious δ-Opioid Agonist at Recombinant and Native Receptors

“…Thus, clozapine affected that displayed by DPDPE, suggesting that the drug activates the d-opioid receptor with a relatively low efficiency. The finding that clozapine possesses agonist activity at the dopioid receptor is in agreement with a previous study by Kobayashi et al (1998), who reported that in Xenopus oocytes co-injected with the G-protein-activated inwardly rectifying potassium channel and either the d-or k-opioid receptor mRNA, clozapine stimulated the d-opioid receptor more potently and efficaciously (EC 50 ¼ 4.56 mM; 60% of the DPDPE response) than the k-opioid receptor (EC 50 ¼ 30.2 mM; 20% of the U-50,488H response).…”

“…Studies in vitro have shown that clozapine inhibited [ 3 H]Met-enkephalin binding to synaptosomeenriched fractions of rat whole brain and hippocampus with a micromolar potency (Somoza et al, 1981). In Xenopus oocytes co-injected with either the d-or k-opioid receptor mRNA and the G protein-activated inwardly rectifying potassium channel, Kobayashi et al (1998) reported that clozapine displayed both d-and k-receptor agonist activity and suggested that this effect may contribute to the antinociceptive and proconvulsant effect of the drug. Although early studies suggested possible links between opioids and schizophrenia (Pickar et al, 1982;Schmauss and Emrich, 1985;Brizer et al, 1985;Schmauss et al, 1987;Gulya, 1990), the involvement of the opioid system in the therapeutic actions of clozapine has been generally overlooked, mostly because of the low affinity of clozapine for the opioid receptors.…”

Molecular Pharmacology ofδ-Opioid Receptors

“…The ability of DOPrs to activate these Gbg effectors has been demonstrated in vitro (Ikeda et al, 1995;Kovoor et al, 1997;Kobayashi et al, 1998) as well as in vivo (Svoboda and Lupica, 1998;Williams et al, 2001;Vaughan et al, 2003), and the role of Kir3 channels (or G protein-activated K + channels) in opioid analgesia has been clearly established using null mice for different channel subunits (Mitrovic et al, 2003;Marker et al, 2005). Moreover, Kir3 contribution to the clinical response of opioid analgesics has been confirmed in association studies showing that genetic variations of different Kir3 subunits influence opioid dose requirements for pain management (Lötsch et al, 2010;Bruehl et al, 2013).…”

Inhibition of G Protein-Activated Inwardly Rectifying K+ Channels by Various Antidepressant Drugs

“…Therefore, modulators of GIRK channel activity may affect many brain and cardiac functions. Using the Xenopus oocyte expression system, we demonstrated that various antipsychotic drugs including thioridazine and clozapine and an SSRI antidepressant drug, fluoxetine, inhibited GIRK channels (Kobayashi et al, 1998. Imipramine, a tricyclic antidepressant of the dibenzazepines, is chemically similar to the antipsychotic drugs thioridazine, a phenothiazine, and clozapine, a dibenzodiazepine.…”