text: In contrast, combined pretreatment with beta-funaltrexamine (mu-receptor antagonist; 20 mg/kg, s.c.) and << norbinaltorphimine >> ([[ kappa-receptor ]] antagonist; 20 mg/kg, s.c.), at doses that inhibited the antitussive activity of mu- and kappa-receptor agonists, respectively, was without effect on the antitussive response of SB 227122 (20 mg/kg, i.p.).