Serotonin and the Gastrointestinal Tract

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Serotonin (5-hydroxytryptamine, 5-HT) has long been known to play an integral role in gastrointestinal neurotransmission.39 Serotonin is formed by the hydroxylation and decarboxyla-tion of tryptophan and is converted to its active form in the nerve terminals (Fig. 14.1).

The GI tract contains approximately 95% of the total body serotonin.40 Although 5-HT is found in a subpopulation of myenteric interneu-rons, the vast majority is located in subcellular granules located in the entero chromaffin (EC)

Figure 14.1. Serotonin.

cells. The EC cells are located in the epithelial layer of the mucosa in close contact to the enteric nerve endings. The serotonin in the EC cells is released when the cells are stimulated by chemical or mechanical stimuli such as increased intraluminal pressure or vagal stimulation. This in turn activates nerve endings in the enteric nervous system and initiates the peristaltic reflex.41

Serotonin has a wide array of effects on the GI tract largely due to the presence of multiple subtypes, which appear to be present on several classes of myenteric neurons, smooth muscle cells, and enterocytes. In general, the release of 5-HT has been found to be directly modulated by ligand-gated and G-protein-coupled receptors, which have at least 15 serotonin receptor subtypes that mediate its effect.

The selective, partial 5-HT4 agonist Tegaserod is one of a new class of compounds called the aminoguanidine indoles. Structurally, Tegaserod is very similar to serotonin and stimulates the release of calcitonin gene-related peptide from enteric neurons. This augments the peristaltic reflex, enhances intestinal secretion, and reduces visceral hypersensitivity. Tegaserod interacts with enteric 5-HT4 receptors, resulting in amplification of this process.

In an industry-sponsored, multinational randomized trial, researchers assessed the efficacy of Tegaserod in 1348 adults of whom 90% were females.42 These subjects had complaints of chronic constipation, which was defined as a weekly average of fewer than three spontaneous bowel movements. These patients received 2 mg, 6mg, or placebo twice a day for 12 weeks. The study showed that Zelnorm® (Tegaserod) caused a statistically significant improvement and increased the frequency of spontaneous bowel movements and relieved symptoms including straining, hard stool, incomplete evacuation, and abdominal discomfort.

Recently Tegaserod has been approved for chronic constipation in men.

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