A new type of K-agonist, 17-cyclopropylmethyl-3, 14β-dihydroxy-4, 5α-epoxy-6β-[N-methyl- trans-3-(3-furyl) acrylamido] morphinan hydrochloride (1, TRK-820), was discoverd by a new working hypothesis. The" message-address concept" for opioid antagonists and the" accessory site" for general antagonists were applied to design TRK-820. A unique structural feature of TRK-820, which is different from other prototypical K-opioid receptor agonists, is ...