Possible involvement of mu 2-opioid receptor-mediated mechanisms in morphine-induced enhancement of the pentobarbital-induced loss of the righting reflex in the mouse.

The effects of pretreatment with selective mu-, delta- and kappa-opioid receptor antagonists on the enhancement of the pentobarbital-induced loss of the righting reflex by morphine were examined in mice. Mice injected with pentobarbital (50 mg/kg, i.p.) showed an increased duration for the loss of the righting reflex after administration of morphine. This enhancement of the pentobarbital-induced loss of the righting reflex produced by morphine (10 mg/kg, s.c.) was significantly antagonized by pretreatment with beta-funaltrexamine, a selective mu-opioid receptor antagonist, but not by pretreatment with naloxonazine, a selective mu 1-opioid receptor antagonist. Furthermore, neither naltrindole, a selective delta-opioid receptor antagonist, nor nor-binaltorphimine, a selective kappa-opioid receptor antagonist, had a significant effect on the enhancement of the pentobarbital-induced loss of the righting reflex by morphine. These results suggest that mu 2-, rather than mu 1-, delta- and kappa-, opioid receptor-mediated mechanisms are involved in the morphine-induced enhancement of the pentobarbital-induced loss of the righting reflex.