Serotonin regulation of the canine migrating motor complex

A potential role for endogenous serotonin (5-hydroxytryptamine) in the control of the migrating motor complex (MMC) was investigated in 10 conscious dogs. Strain gage force transducers or silver bipolar recording electrodes were sewn along the small intestine to record motor or myoelectrical patterns of activity, respectively. After an 18-hr fast, serotonin stimulated phase II-like circular muscle contractions when administered during phase I of the MMC. Methysergide produced a prolonged atropine-resistant increase in the number of contractions throughout the jejunum and ileum, thus suggesting agonist activity. Phase III contractions were not apparent during the methysergide-induced contractile activity. Cyproheptadine blocked the initiation and the migration of MMCs along the small intestine and significantly increased the period of the MMC. 5-methoxy-N,N-dimethyltryptamine also increased the period of the jejunal MMC. Pretreatment with parachlorophenylalanine reduced qualitatively the contractile amplitude and appearance of phases II and III of the MMC and significantly reduced the MMC period in two of four dogs. In conclusion, endogenous serotonin affects motor activity during MMC phases II and III and appears to be a candidate regulator of the intrinsic mechanisms governing the initiation and propagation of the canine MMC.