Opioid Peptides/pharmacology; Protein Binding/drug effects; Receptors, Opioid; Recombinant Proteins/metabolism; Amino Acid Sequence; Animals; Brain Chemistry; Cloning, Molecular; DNA, Complementary; Gene Expression; Gills/chemistry; Guanosine 5'-O-(3-Thiotriphosphate)/pharmacology; HEK293 Cells; Humans; Kinetics; Liver/chemistry; Models, Animal; Molecular Sequence Data; Opioid Peptides/metabolism; Phylogeny; Protein Binding/physiology; Receptors, Opioid/genetics; Receptors, Opioid/isolation & purification; Receptors, Opioid/metabolism; Receptors, Opioid, kappa/genetics; Receptors, Opioid, kappa/metabolism; Recombinant Proteins/genetics; Sequence Alignment; Zebrafish/genetics; Zebrafish/metabolism; Nociceptin Receptor; Nociceptin