Pharmacological properties of TRK-820 on cloned mu-, delta- and kappa-opioid receptors and nociceptin receptor.

We analyzed the pharmacological properties of 17-cyclopropylmethyl-3,14beta-dihydroxy-4,5alpha-epoxy-6b eta-[N-methyl-trans-3-(3-furyl)acrylamido]morphinan hydrochloride (TRK-820) using Chinese hamster ovary (CHO) cells expressing cloned rat mu-, delta- and kappa-opioid receptors and human nociceptin receptor. TRK-820 showed high affinity for the kappa-opioid receptor, with a Ki value of 3.5 +/- 0.9 nM. In CHO cells expressing kappa-opioid receptors, TRK-820 inhibited forskolin-stimulated cAMP accumulation, and the maximal inhibitory effect was equivalent to that of (+)-(5alpha,7alpha,8beta)-N-methyl-N-[7-(1-pyrrolidiny l)-1-oxaspiro-(4,5)dec-8-yl]benzeneacetamide (U69,593), a full agonist of kappa-opioid receptor. In CHO cells expressing mu-opioid receptors, TRK-820 inhibited cAMP accumulation, but the maximal inhibitory effect was significantly smaller than that of [D-Ala2, N-MePhe4, Gly-ol5]enkephalin (DAMGO), a full agonist of mu-opioid receptor. In CHO cells expressing delta-opioid receptor, the inhibitory effect of TRK-820 on cAMP accumulation was very weak. Using site-directed mutagenesis, the high affinity of TRK-820 for the kappa-opioid receptor was revealed to require Glu297. TRK-820 bound to the nociceptin receptor with a Ki value of 380 +/- 50 nM. TRK-820 by itself had no effect on cAMP accumulation in CHO cells expressing nociceptin receptors, but significantly antagonized the nociceptin (10 nM)-mediated inhibition of cAMP accumulation at high concentrations. These results indicate that TRK-820 acts as a full agonist for the kappa-opioid receptor, a partial agonist for the mu-opioid receptor and a low-affinity antagonist for the nociceptin receptor.