Protein kinase inhibitor potentiates opioid δ-receptor currents in Xenopus oocytes

In Xenopus oocytes injected with RNA coding for the delta-opioid receptor a small chloride current was evoked by [D-Ala2,Ser5]leucine-enkephaline-Thr6 (DSLET), a delta 2-opioid agonist. The evoked currents were rapidly reduced upon repeated challenges of DSLET. When Gil alpha RNA was co-injected into the oocyte, the evoked currents were increased 3.8-fold and became constant after at least three repeated challenges. In oocytes injected with RNAs coding for delta-receptor and Gil alpha, pretreatment with K-252a, a potent inhibitor of protein kinases, further potentiated the delta-receptor-mediated current responses, compared with those without the inhibitor. These results suggest that signalling involving the delta-opioid receptor is inactivated through in vivo phosphorylation in the Xenopus oocyte.