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DAMGO, a mu-opioid receptor selective ligand, distinguishes between mu-and kappa-opioid receptors at a different region from that for the distinction between mu- and delta-opioid receptors.

The structural basis of opioid receptors (OPRs) for the subtype-selective binding of DAMGO, a mu-opioid receptor selective ligand, was investigated using chimeric mu/kappa-OPRs. Replacement of the region from the middle of the fifth transmembrane domain to the C-terminal of mu-OPR with the corresponding region of mu-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 mu- and mu-OPRs at the region around the third extracellular loop, different from the case of the distinction between mu-and delta-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 mu- and kappa-OPRs not only by peptidic mu- selective ligands but also by non-peptidic ligands, such as morphine and naloxone.