Desensitization of the δ‐Opioid Receptor Correlates with Its Phosphorylation in SK‐N‐BE Cells: Involvement of a G Protein‐Coupled Receptor Kinase

: Phosphorylation of G protein‐coupled receptors is considered an important step during their desensitization. In SK‐N‐BE cells, recently presented as a pertinent model for the studies of the human δ‐opioid receptor, pretreatment with the opioid agonist etorphine increased time‐dependently the rate of phosphorylation of a 51‐kDa membrane protein. Immunological characterization of this protein with an antibody, raised against the amino‐terminal region of the cloned human δ‐opioid receptor, revealed that it corresponded to the δ‐opioid receptor. During prolonged treatment with etorphine, phosphorylation increased as early as 15 min to reach a maximum within 1 h. Phosphorylation and desensitization of adenylyl cyclase inhibition paralleled closely and okadaic acid inhibited the resensitization, a result strongly suggesting that phosphorylation of the δ‐opioid receptor plays a prominent role in its rapid desensitization. The increase in phosphorylation of the δ‐opioid receptor, as well as its desensitization, was not affected by H7, an inhibitor of protein kinase A and protein kinase C, but was drastically reduced by heparin or Zn2+, known to act as G protein‐coupled receptor kinase (GRK) inhibitors. These results are the first to show, on endogenously expressed human δ‐opioid receptor, that a close link exists between receptor phosphorylation and agonist‐promoted desensitization and that desensitization involves a GRK.