Pharmacological properties of TRK-820 on cloned μ-, δ- and κ-opioid receptors and nociceptin receptor

Abstract

We analyzed the pharmacological properties of 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6β-[ N-methyl- trans-3-(3-furyl)acrylamido]morphinan hydrochloride (TRK-820) using Chinese hamster ovary (CHO) cells expressing cloned rat μ-, δ- and κ-opioid receptors and human nociceptin receptor. TRK-820 showed high affinity for the κ-opioid receptor, with a K i value of 3.5±0.9 nM. In CHO cells expressing κ-opioid receptors, TRK-820 inhibited forskolin-stimulated cAMP accumulation, and the maximal inhibitory effect was equivalent to that of (+)-(5α,7α,8β)- N-methyl- N-[7-(1-pyrrolidinyl)-1-oxaspiro-(4,5)dec-8-yl]benzeneacetamide (U69,593), a full agonist of κ-opioid receptor. In CHO cells expressing μ-opioid receptors, TRK-820 inhibited cAMP accumulation, but the maximal inhibitory effect was significantly smaller than that of [ d-Ala 2, N-MePhe 4, Gly-ol 5]enkephalin (DAMGO), a full agonist of μ-opioid receptor. In CHO cells expressing δ-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 κ-opioid receptor was revealed to require Glu 297. TRK-820 bound to the nociceptin receptor with a K i 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 κ-opioid receptor, a partial agonist for the μ-opioid receptor and a low-affinity antagonist for the nociceptin receptor.