KAPPA-OPIOID RECEPTOR ACTIVATES AN INWARDLY RECTIFYING K-PROTEIN-LINKED MECHANISM - COEXPRESSION IN XENOPUS OOCYTES( CHANNEL BY A G)

cRNAs encoding the kappa-opioid receptor and an inwardly rectifying, G protein-coupled, K+ channel were coinjected into Xenopus oocytes. The effects of kappa-opioid receptor agonists and antagonists on the memb rane currents in these oocytes were studied using the two-electrode vo ltage-clamp technique. The kappa-opioid receptor agonists U69593 and d ynorphin A induced a concentration-dependent inward current (EC(50) of similar to 0.3 mu M and similar to 30 nM, respectively) after coinjec tion of both cRNAs, whereas the mu-opioid receptor agonist [D-Ala(2),N -MePhe(4),Gly(5)-ol]enkephalin (10 mu M) and the delta-opioid receptor agonist [D-Pen(2,5)]enkephalin (1 mu M) had no effect. The agonist-in duced inward current was reversible upon washing out of the agonists a nd was inhibited in the presence of the K+ channel blocker Ba2+ (0.1 m M). The specific kappa-opioid receptor antagonist norbinaltorphimine ( 0.1 mu M) and the nonspecific opioid receptor antagonist naloxone (1 m u M) abolished the agonist-induced currents. Furthermore, the agonist- induced currents exhibited rapid desensitization in the continuous pre sence of the agonists or after repeated application. Preincubation of the coinjected oocytes with pertussis toxin (400 ng/ml for 3 days or 1 .5 mu g/ml for 24 hr) abolished most of the agonist-induced activation of the inwardly rectifying K+ current. We therefore conclude that spe cific stimulation of the kappa-opioid receptor can activate the inward ly rectifying K+ channel through a pertussis toxin-sensitive G protein .