Protection by U-50,488H against beta-chlornaltrexamine antagonism of nitrous oxide antinociception in mice.

Nitrous oxide antinociception in the abdominal constriction test was significantly reduced in mice pretreated intracerebroventricularly (i.c.v.) with beta-chlornaltrexamine (beta-CNA). However, this antagonism was reversed when the beta-CNA was co-administered i.c.v. with the kappa-opioid ligand U-50,488H but not the mu-opioid ligand CTOP. These findings further demonstrate that nitrous oxide antinociception in the mouse abdominal constriction paradigm is mediated by kappa- but not mu-opioid receptors.