N-naphthoyl-β-naltrexamine (NNTA), a highly selective and potent activator of μ/κ-opioid heteromers

Numerous G protein-coupled receptors (GPCRs) have been shown to form heteromeric receptors in cell-based assays. Among the many heteromers reported in the opioid receptor family are mu/kappa, kappa/delta, and mu/delta. However, the in vivo physiological and behavioral relevance for the proposed heteromers have not yet been established. Here we report a unique example of a ligand, N-naphthoyl-beta-naltrexamine (NNTA) that selectively activates heteromeric mu/kappa-opioid receptors in HEK-293 cells and induces potent antinociception in mice. NNTA was an exceptionally potent agonist in cells expressing mu/kappa-opioid receptors. Intriguingly, it was found to be a potent antagonist in cells expressing only mu-receptors. In the mouse tail-flick assay, intrathecal (i.t.) NNTA produced antinociception that was similar to 100-fold greater than by intracerebroventricular (i.c.v.) administration. The.-antagonist, norBNI, decreased the i.t. potency, and the activity was virtually abolished in mu-opioid receptor knockout mice. No tolerance was induced i.t., but marginal tolerance (3-fold) was observed via the i.c.v. route. Moreover, NNTA produced neither significant physical dependence nor place preference in the ED50 dose range. Taken together, this work provides an important pharmacologic tool for investigating the in vivo functional relevance of heteromeric mu/kappa-opioid receptors and suggests an approach to potent analgesics with fewer deleterious side effects.