Receptor-induced internalization of selective peptidic mu and delta opioid ligands

The binding and internalization of radioiodinated and fluorescent mu and delta opioid peptides in mammalian cells were quantitatively studied by biochemical techniques and directly visualized by confocal microscopy. The labeled peptides were prepared by inserting either a I-125-Bolton-Hunter group or a fluorescent probe into the C-terminal part of 5-aminopentylamide derivatives of deltorphin-I and [Lys(7)]dermorphin. The purified derivatives kept most of their specificity and selectivity toward delta and mu opioid receptors, respectively. Biochemical and confocal microscopy data showed that both mu and delta opioid peptides were internalized in mammalian cells transfected with the corresponding opioid receptor according to a receptor-mediated mechanism. The internalization process was time- and temperature- dependent and was completely blocked by the endocytosis inhibitor phenylarsine oxyde. Internalization of both delta and mu ligands occurred from a single large cap at one pole of the cell, indicating that polymerization of ligand-receptor complexes preceeded internalization. Finally, green and red fluorescent analogues of deltorphin-I and [Lys(7)]dermorphin, respectively, were found to internalize through partly distinct endocytic pathways in cells co-transfected with mu and delta receptors, suggesting that each of these receptors interacts with distinct proteins mediating intracellular sorting and trafficking.