N-naphthoyl-beta-naltrexamine (NNTA), a highly selective and potent activator of µ/kappa-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 µ/κ, κ/δ, and µ/δ. 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-ß-naltrexamine (NNTA) that selectively activates heteromeric µ/κ-opioid receptors in HEK-293 cells and induces potent antinociception in mice. NNTA was an exceptionally potent agonist in cells expressing µ/κ-opioid receptors. Intriguingly, it was found to be a potent antagonist in cells expressing only µ-receptors. In the mouse tail-flick assay, intrathecal (i.t.) NNTA produced antinociception that was ~100-fold greater than by intracerebroventricular (i.c.v.) administration. The κ-antagonist, norBNI, decreased the i.t. potency, and the activity was virtually abolished in µ-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 µ/κ-opioid receptors and suggests an approach to potent analgesics with fewer deleterious side effects.

A κ Opioid Pharmacophore Becomes a Spinally Selective κ-δ Agonist When Modified with a Basic Extender Arm.

We have explored the concept of a molecular extender arm attached to a κ opioid agonist pharmacophore 3 (ICI-199,441) in an effort to potentially interact with a complementary group on a neighboring opioid receptor. The molecular arm containing a terminal amine group was lengthened incrementally from 11 up to 18 atoms. Increasing the number of atoms in the arm produced virtually no change in the mouse intracerebroventricular (i.c.v.) antinociceptive potency. In contrast, the intrathecal (i.t.) potency of 6 (KDA-16) with a 16-atom arm was dramatically increased, as reflected by its antinociceptive i.c.v./i.t. ED50 ratio of ∼130. Further lengthening led to a decreased ED50 ratio. In vivo selective antagonist studies of KDA-16 revealed that κ and δ opioid receptors were responsible for the greatly enhanced i.t. potency. Calcium release experiments in HEK-293 cells suggested that KDA-16 selectively activate κ-δ heteromers. These data are consistent with the reported possible presence of heteromeric κ-δ opioid receptors in mouse spinal cord but not in the brain. The use of a molecular extender arm may be useful for developing spinally selective analgesics.