BN-9, a chimeric peptide with mixed opioid and neuropeptide FF receptor agonistic properties, produces nontolerance-forming antinociception in mice

BACKGROUND AND PURPOSE Neuropeptide FF (NPFF) behaves as an endogenous opioid-modulating peptide. In the present study, the opioid and NPFF pharmacophore-containing chimeric peptide BN-9 was synthesized and pharmacologically characterized. EXPERIMENTAL APPROACH Agonist activities of BN-9 at opioid and NPFF receptors were characterized in in vitro cAMP assays. Antinociceptive activities of BN-9 were evaluated in the mouse tail-flick and formalin tests. Furthermore, its side effects were investigated in rotarod, antinociceptive tolerance, reward and gastrointestinal transit tests. KEY RESULTS BN-9 acted as a novel multifunctional agonist at mu, delta, kappa, NPFF1 and NPFF2 receptors in cAMP assays. In the tail-flick test, BN-9 produced dose-related antinociception and was approximately equipotent to morphine; this antinociception was blocked by mu and kappa receptor antagonists, but not by the d receptor antagonist. In the formalin test, supraspinal administration of BN-9 produced significant analgesia. Notably, repeated administration of BN-9 produced analgesia without loss of potency over 8 days. In contrast, repeated i.c.v. co-administration of BN-9 with the NPFF receptor antagonist RF9 produced significant antinociceptive tolerance. Furthermore, i.c.v. BN-9 induced conditioned place preference. When given by the same routes, BN-9 had a more than eightfold higher ED50 value for gastrointestinal transit inhibition compared with the ED50 values for antinociception. CONCLUSIONS AND IMPLICATIONS BN-9 produced a robust, nontolerance-forming analgesia with limited inhibition of gastrointestinal transit. As BN-9 is able to activate both opioid and NPFF systems, this provides an interesting approach for the development of novel analgesics with minimal side effects.