The structural basis of opioid receptors (OPRs) for the subtype-selective binding of DAMGO, a μ-opioid receptor selective ligand, was investigated using chimeric μ/κ-OPRs. Replacement of the region from the middle of the fifth transmembrane domain to the C-terminal of μ-OPR with the corresponding region of μ-OPR remarkably decreased the binding affinity to DAMGO, while the reciprocal chimera revealed the high affinity to DAMGO. These results indicate that DAMGO distinguishes between μ- and μ-OPRs at the region around the third extracellular loop, different from the case of the distinction between μ-and δ-OPRs in which the region around the first extracellular loop is important. Furthermore, displacement studies revealed that the region around the third extracellular loop is involved in the discrimination between μ- and κ-OPRs not only by peptidic μ- selective ligands but also by non-peptidic ligands, such as morphine and naloxone.
