Intestinal motility was measured by the transit of charcoal meal through the small intestine of the rat. 2. Morphine given subcutaneously induced a reduction in the gastrointestinal transit (GIT), and the response was dose-dependent (0.1-2.5 mg/kg). 3. The inhibitory effect of morphine was antagonized by prior subcutaneous administration of naloxone (1 mg/kg). 4. Depletion of serotonergic neurons by para-chlorophenylalanine (100 mg/kg, ip, 3 days) completely abolished the inhibitory action of morphine. 5. Methysergide (5 mg/kg, sc) and ketanserin (5 mg/kg, sc) also reduced the morphine effect. 6. Destruction of serotonergic neurones by intracerebroventricular injection of 5,7-dihydroxytryptamine (20 mu/kg) abolished the effect of morphine. 7. These observations provide some evidence for the involvement of a central serotonergic system in the effect of morphine on gastrointestinal motility.
