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

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Abstract
To investigate the effects of clozapine, an atypical antipsychotic, on the cloned μ‐, Δ‐ and κ‐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. In the oocytes co‐injected with either the Δ‐ or κ‐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 Ba2+, which blocks the GIRK channel. Since the opioid receptors functionally couple to the GIRK channel, these results indicate that clozapine activates the Δ‐ and κ‐opioid receptors and that the inward‐current responses are mediated by the GIRK channel. The action of clozapine at the Δ‐opioid receptor was more potent and efficacious than that at the κ‐opioid receptor. In the oocytes co‐injected with the μ‐opioid receptor and GIRK1 mRNAs, application of clozapine (100 μM) did not induce an inward current, suggesting that clozapine could not activate the μ‐opioid receptor. 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. To test the antagonism of clozapine for the μ‐ and κ‐opioid receptors, we applied clozapine together with each selective opioid agonist to the oocytes co‐injected with either the μ‐ or κ‐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 significant antagonist effect on the μ‐ and κ‐opioid receptors. We conclude that clozapine acts as a Δ‐ and κ‐agonist and as a GIRK channel blocker. Our results suggest that the efficacy and side effects of clozapine under clinical conditions may be partly due to activation of the Δ‐opioid receptor and blockade of the GIRK channel. British Journal of Pharmacology (1998) 123, 421–426; doi:10.1038/sj.bjp.0701621

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