Human κ opiate receptor second extracellular loop elevates dynorphin's affinity for human μ/κ chimeras

To investigate roles of second extracellular loop sequences in peptide and nonpeptide ligand recognition by human opiate receptors, we have constructed a chimeric receptor in which this domain of the human μ opiate receptor has been replaced with that of the human κ opiate receptor. The chimeric opiate receptor displays dramatically increased affinity for dynorphin peptides. Affinities for dynorphin A-(1-17), dynorphin A-(1-13), and α-neoendorphin increase by up to 250-fold when compared with the wild-type human μ opiate receptor. The chimera maintains recognition of the μ-selective ligands morphine and [D-Ala²,MePhe⁴,Gly-ol⁵]enkephalin and displays no significant changes in affinity for the κ-selective small molecule ligand U50,488. The chimeric opiate receptor displays evidence for effective G-protein coupling; 100 nM dynorphin A-(1-17) is as effective as 100 nM morphine at inhibiting forskolin-stimulated adenyl cyclase activity through actions at the chimeric receptor. These data suggest that the putative second extracellular loop contributes substantially to the κ receptor's selectivity in dynorphin ligand recognition.