Possible involvement of mu 2-mediated mechanisms in mu-mediated antitussive activity in the mouse

The effect of pretreatment with naloxonazine on mu-opioid agonist-mediated antitussive effects was studied in mice. The antitussive effects of [D-Ala2, MePhe4, Gly-ol5]enkephalin (DAMGO) and morphine were significantly antagonized by naloxone pretreatment, 1 mg/kg given i.p. 5 min earlier, but not by naloxonazine pretreatment, 35 mg/kg given s.c. 24 h earlier. In contrast, the antinociceptive effects of these mu agonists, as determined by the tail-flick method, were significantly reduced by pretreatment with both naloxone and naloxonazine. These results suggest that mu 2 rather than mu 1 mechanisms are involved in mu-mediated antitussive effects.