Opioid-induced tolerance and dependence in mice is modulated by the distance between pharmacophores in a bivalent ligand series
Given the mounting evidence for involvement of δ opioid receptors in the tolerance and physical dependence of μ opioid receptor agonists, we have investigated the possible physical interaction between μ and δ opioid receptors by using bivalent ligands. Based on reports of suppression of antinociceptive tolerance by the δ antagonist naltrindole (NTI), bivalent ligands [μ-δ agonist-antagonist (MDAN) series] that contain different length spacers, and pharmacophores derived from NTI and the μ agonist oxymorphone, have been synthesized and evaluated by intracerebroventricular (i.c.v.) administration in the tail-flick test in mice. In acute i.c.v. studies, the bivalent ligands functioned as agonists with potencies ranging from 1.6- to 45-fold greater than morphine. In contrast, the monovalent μ agonist analogues were substantially more potent than the MDAN congeners and were essentially equipotent with one another and oxymorphone. Pretreatment with NTI decreased the ED50 values for MDAN-19 to a greater degree than for MDAN-16 but had no effect on MDAN-21. Chronic i.c.v. studies revealed that MDAN ligands whose spacer was 16 atoms or longer produced less dependence than either morphine or μ monovalent control MA-19. On the other hand, both physical dependence and tolerance were suppressed at MDAN spacer lengths of 19 atoms or greater. These data suggest that physical interaction between the μ and δ opioid receptors modulates μ-mediated tolerance and dependence. Because MDAN-21 was found to be 50-fold more potent than morphine by the i.v. route (i.v.), it offers a previously uncharacterized approach for the development of analgesics devoid of tolerance and dependence.