Comparative effects of morphine and nalorphine on colonic motility in the conscious dog

The effects of increased doses (0.05-02 mg/kg) of morphine and nalorphine on colonic motility were investigated in 15-22 h fasted dogs fitted with two strain gauge transducers on the transverse colon at 8 and 15 cm from the ileo-colonic junction. These effects were compared to those obtained after previous administration of naloxone (0.3 mg/kg), atropine (0.1 mg/kg) and methysergide (0.1 mg/kg). Both morphine and nalorphine, at a dose of 0.1 mg/kg increased the colonic motility index by 285 and 248% from 0 to 60 min respectively after their administration, these effects lasting 3.2 and 2.6 h. These stimulatory effects were abolished by previous administration of naloxone (0.3 mg/kg) or atropine (0.1 mg/kg) and limited to 60 min after methysergide (0.1 mg/kg). It was concluded that nalorphine may be considered as a potent morphinomimetic substance upon colonic motility and that cholinergic and serotonergic receptors were involved in the colonic response to morphine.