Pharmacological characterization of KT-90 using cloned μ-, δ- and κ-opioid receptors

Abstract

We analyzed the pharmacological characteristics of (−)-3-acetyl-6β-acetylthio- N-cyclopropylmethyl-normorphine (KT-90) using Chinese hamster ovary (CHO) cells expressing cloned rat μ-, δ- and κ-opioid receptors. KT-90 displaced the specific binding of the following radiolabeled ligands selective to the μ-, δ- and κ-opioid receptors, [ 3H][ d-Ala 2,MePhe 4,Gly(ol) 5]enkephalin (DAMGO), [ 3H][ d-Pen 2, d-Pen 5]enkephalin (DPDPE), [ 3H] (+)-(5α,7α,8β)- N-methyl- N-[7-(1-pyrrolidinyl)-1-oxaspiro-4,5)dec-8-yl]benzeneacetamide (U69,593), with K i values of 3.3 ± 0.7, 22.8 ± 1.5 and 1.9 ± 0.3 nM, respectively. In CHO cells expressing the μ-, δ- and κ-opioid receptors, KT-90 inhibited forskolin (10 μM)-induced cyclic AMP accumulation in a concentration-dependent manner with IC 50 values of 2337 ± 750, 17.3 ± 4.6 and 2.0 ± 0.1 nM, respectively. The maximal inhibitory effects of KT-90 in the cells expressing μ-, δ- and κ-opioid receptors were significantly lower than those of the type-selective agonists DAMGO, DPDPE and U69,593, respectively. These results indicated that KT-90 acts as a partial agonist on μ-, δ and κ-opioid receptors. KT-90 (10 and 100 nM), when added with morphine, produced a rightward shift of the concentration-response curve of morphine to inhibit the cyclic AMP accumulation in CHO cells expressing μ-, but not δ- or κ-, opioid receptors. This finding is consistent with the findings that lower doses of KT-90 antagonize morphine analgesia in vivo.