Antagonism of phosphoramidon-induced antinociception in mice by mu- but not kappa-opioid receptor blockers

Intracerebroventricular (i.c.v.) administration of the neutral endopeptidase 24.11-inhibitor phosphoramidon evoked a dose-dependent antinociceptive effect in the mouse acetic acid abdominal constriction test. The present study was conducted to identify the opioid receptor subtype(s) that mediate phosphoramidon antinociception in this paradigm. Mice were pretreated with different opioid antagonists prior to being challenged with phosphoramidon, i.c.v., the mu-opioid agonist sufentanil, s.c., or the kappa-opioid agonist U-50,488H, s.c. Naltrexone significantly attenuated phosphoramidon-induced antinociception at an i.c.v. dose that also blocked both sufentanil and U-50,488H. The mu-opioid antagonist beta-funaltrexamine (beta-FNA) blocked phosphoramidon and sufentanil at an i.c.v. dose that did not block U-50,488H. The kappa-opioid antagonist nor-binaltorphimine (nor-BNI) produced dose-related effects. A low dose (10 microg) of nor-BNI had no effect on either phosphoramidon or sufentanil but did reduce U-50,488H antinociception. A higher dose (30 microg) of nor-BNI blocked phosphoramidon, sufentanil, and U-50,488H, suggesting a loss of kappa-opioid receptor selectivity at this dose. These findings suggest that mu- but not kappa-opioid receptors mediate phosphoramidon-induced antinociception in the abdominal constriction test.