Effect of beta-funaltrexamine on opioid side-effects produced by morphine and U-50, 488H

Pretreatment of rats with the irreversible mu-opioid receptor antagonist, beta-funaltrexamine (beta-FNA), 20-40 mg kg-1 s.c., produced a dose-related antagonism of the reduction in respiratory rate, gastrointestinal (GI) propulsion, rotarod reaction latencies and body temperature produced by morphine administration 24 h later, suggesting that these effects are mediated via mu-opioid receptors. The kappa-receptor agonist, U-50,488H, was without effect on respiratory rate at the doses tested, but produced hypothermia, sedation and low maximum inhibition of GI propulsion. These effects of U-50,488H were not blocked by beta-FNA suggesting that they are mediated via kappa-receptors.