delta and kappa opioid receptors are differentially regulated by dynamin-dependent endocytosis when activated by the same alkaloid agonist

Many alkaloid drugs used as analgesics activate multiple opioid receptors. Mechanisms that distinguish the actions of these drugs on the regulation of individual mu, delta, and kappa receptors are not understood. We have observed that individual cloned opioid receptors differ Significantly in their regulation by rapid endocytosis in the presence of alkaloid drug etorphine, a potent agonist of mu, delta, and kappa opioid receptors. Internalization of epitope-tagged delta opioid receptors from the plasma membrane is detectable within 10 min in the presence of etorphine. In contrast, kappa receptors expressed in the same cells remain in the plasma membrane and are not internalized for greater than or equal to 60 min, even when cells are exposed to saturating concentrations of etorphine. The rapid internalization of delta receptors is specifically inhibited in cells expressing K44E mutant dynamin I, suggesting that type-specific internalization of opioid receptors is mediated by clathrin-coated pits. Examination of a series of chimeric mutant kappa/delta receptors indicates that at least two receptor domains, including the highly divergent carboxyl-termindi cytoplasmic tail, determine the type specificity of this endocytic mechanism. We conclude that structurally homologous opioid receptors are differentially sorted by clathrin-mediated endocytosis following-activation by the same agonist ligand. These studies identify a fundamental mechanism of receptor regulation mediating type-specific effects of analgesic drugs that activate more than one type of opioid receptor.