Inhibitory action of morphine and chlorpromazine on defecation in mice

A plastic tube (10*1.8 mm) was evacuated immediately after having put into the rectum of mice. Inhibition of the tube-evacuation from the rectum was used as a criterion of the effects of drugs on defecation. Morphine hydrochloride (Mor, 2.5-5 mg/kg, sc) and chlorpromazine hydrochloride (CPZ, 2-5 mg/kg, sc) produced an inhibition of the defecation reflex in mice. Intracerebroventricular injection of Mor 2.5-5 microg and CPZ 2-4 microg also inhibited such a reflex. Inhibition of the defecation reflex caused by Mor 5 mg/kg sc was antagonized by naloxone (1 mg/kg, sc or 2 microg, icv) and by amphetamine 5 mg/kg sc. However, the inhibition caused by CPZ 2 mg/kg sc was only antagonized by amphetamine 10 mg/kg sc and not affected by naloxone 1-4 mg/kg sc.