The kappa-opioid receptor: evidence for the different subtypes.

Classification of drugs acting on the kappa-opioid receptors seems to be difficult, since some of these ligands are also sigma agonists and/or display non-opioid actions as well. Furthermore, certain benzomorphans having kappa-agonistic character, are shown to be mu-antagonists too. Therefore the classification of the kappa-opioid receptor has to be presently restricted to two subclasses that also have physiological meaning. Dynorphin and Met-enkephalin-Arg6-Phe7 are proposed as endogenous peptide ligands for kappa-receptors. Nonpeptide agonists are benzeneacetamides interacting with the kappa1 receptor. Benzomorphans bind to both subtypes of kappa-receptors. No selective nonpeptide ligand for the kappa2 receptor exists as yet. Nor-binaltorphimine, a specific kappa-antagonist also inhibits both kappa-subtypes. Further research for kappa2 selective drugs is necessary for clear distinction between the two kappa-opioid binding sites. Molecular cloning of opioid receptors including their subtypes are expected to provide direct proof of their existence.