Antinociceptive interactions of mu- and kappa-opioid agonists in the colorectal distension assay in rats

Interactions of opioid agonists, fentanyl and oxymorphone (mu-selective) and spiradoline and enadoline (kappa-selective), were examined for additive, sub-additive, or supra-additive antinociception in the colorectal distension (CRD) assay. Single-dose values (mg/kg, 0.006–0.016 for fentanyl, 0.25–1.26 for spiradoline, etc.) were summed to formulate theoretical additive-dose plots for comparison with actual combined-dose effects. Combined fentanyl and spiradoline yielded additive (low-dose levels) or supra-additive (high-dose levels) effects. Single and combined doses of fentanyl (0.012 mg/kg) and spiradoline (0.3 mg/kg) were tested after pretreatment with saline, beta-funaltrexamine (b-FNA, mu-selective antagonist), or nor-binaltorphimine (n-BNI, kappa-selective antagonist). Supra-additive effects of combined agonists were attenuated by either antagonist (greater with n-BNI). But paradoxical patterns of antagonism of single-dose effects occurred: the fentanyl antinociception was not antagonized by b-FNA, whereas the spiradoline antinociception was. The results indicate complex interactions of agonists in this visceral pain model and potential for combined agonists to improve pain relief with decreased side effects.