κ-Opioid receptor activates an inwardly rectifying K⁺ channel by a G protein-linked mechanism: Coexpression in Xenopus oocytes

cRNAs encoding the κ-opioid receptor and an inwardly rectifying, G protein-coupled, K⁺ channel were coinjected into Xenopus oocytes. The effects of κ-opioid receptor agonists and antagonists on the membrane currents in these oocytes were studied using the two-electrode voltage-clamp technique. The κ-opioid receptor agonists U69593 and dynorphin A induced a concentration-dependent inward current (EC⁵⁰ of ~0.3 μM and ~30 nM, respectively) after coinjection of both cRNAs, whereas the μ-opioid receptor agonist [D-Ala²,N-MePhe⁴,Gly⁵-ol]en-kephalin (10 μM) and the δ-opioid receptor agonist [D-Pen^2,5]enkephalin (1 μM) had no effect. The agonist- induced inward current was reversible upon washing out of the agonists and was inhibited in the presence of the K⁺ channel blocker Ba²⁺ (0.1 mM). The specific κ-opioid receptor antagonist norbinaltorphimine (0.1 μM) and the nonspecific opioid receptor antagonist naloxone (1 μM) abolished the agonist-induced currents. Furthermore, the agonist-induced currents exhibited rapid desensitization in the continuous presence of the agonists or after repeated application. Preincubation of the coinjected oocytes with pertussis toxin (400 ng/ml for 3 days or 1.5 μg/ml for 24 hr) abolished most of the agonist-induced activation of the inwardly rectifying K⁺ current. We therefore conclude that specific stimulation of the κ-opioid receptor can activate the inwardly rectifying K⁺ channel through a pertussis toxin- sensitive G protein.