THE 3RD EXTRACELLULAR LOOP OF THE MU-OPIOID RECEPTOR IS IMPORTANT FORAGONIST SELECTIVITY

To investigate the interaction between the mu opioid receptor and its ligands, we compared the binding of mu-selective ligands to two mu/kap pa chimeric opioid receptors and to mu and kappa receptors. The two ch imeras were constructed from cloned rat mu and kappa receptors in whic h a segment from the middle of the third intracellular loop to the C t erminus was exchanged. When this portion of the kappa receptor was rep laced by that of the mu receptor, affinities of mu selective agonists, DAMGO (Tyr-D-Ala-Gly-NMePhe-Gly-ol), PL017 (Tyr-Pro-NMePhe-D-Pro-NH2) , sufentanil, and morphine, were greatly increased as compared to thos e for the kappa receptor. Conversely, when this region of the gamma re ceptor was substituted by that of the kappa receptor, affinities for t hese agonists were substantially decreased as compared with those of t he mu receptor. Unlike selective agonists, the mu-selective antagonist , CTAP -Phe-Cys-Tyr-D-Trp-Arg-Thr-penicillamine-Thr-NH2), displayed a low affinity for both chimeric receptors, similar to that of the kappa receptor. Thus, the region from the middle of the third intracellular loop to the C terminus of the mu receptor is important for the bindin g of selective agonists. Conversely, the determinants for selective bi nding of the antagonist CTAP reside in a more extended region of the r eceptor.