Dynorphin Is a Specific Endogenous Ligand of the κ Opioid Receptor

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Abstract
In the guinea pig ileum myenteric plexus—longitudinal muscle preparation, dynorphin-(1—13) and the prototypical κ agonist ethylketocyclazocine had equally poor sensitivity to naloxone antagonism and showed selective cross protection in receptor inactivation experiments with the alkylating antagonist β-chlornaltrexamine. In binding assays with membranes from guinea pig brain, ethylketocyclazocine and dynorphin-(1—13) amide were more potent in displacing tritium-labeled ethylketocyclazocine than in displacing typical μ and δ opioid receptor ligands. In the two preparations studied, the dynorphin receptor appears to be the same as the κ opioid receptor.