Serotonin and colonic motility

The role of serotonin (5‐hydroxytryptamine [5‐ HT ]) in gastrointestinal motility has been studied for over 50 years. Most of the 5‐ HT in the body resides in the gut wall, where it is located in subsets of mucosal cells (enterochromaffin cells) and neurons (descending interneurons). Many studies suggest that 5‐ HT is important to normal and dysfunctional gut motility and drugs affecting 5‐ HT receptors, especially 5‐ HT 3 and 5‐ HT 4 receptors, have been used clinically to treat motility disorders; however, cardiovascular side effects have limited the use of these drugs. Recently studies have questioned the importance and necessity of 5‐ HT in general and mucosal 5‐ HT in particular for colonic motility. Recent evidence suggests the importance of 5‐ HT 3 and 5‐ HT 4 receptors for initiation and generation of one of the key colonic motility patterns, the colonic migrating motor complex ( CMMC ), in rat. The findings suggest that 5‐ HT 3 and 5‐ HT 4 receptors are differentially involved in two different types of rat CMMC s: the long distance contraction ( LDC ) and the rhythmic propulsive motor complex (RPMC). The understanding of the role of serotonin in colonic motility has been influenced by the specific motility pattern(s) studied, the stimulus used to initiate the motility (spontaneous vs induced), and the route of administration of drugs. All of these considerations contribute to the understanding and the controversy that continues to surround the role of serotonin in the gut.