The majority of 5-hydroxytryptamine (5-HT) in the body is found in the gastrointestinal tract, primarily contained within enterochromaffin cells and is released by meals, toxins, and che-motherapeutic agents (161,162). 5-HT is implicated in postinfectious IBS patients by increased numbers of enterochromaffin cells (163), increased mast cell populations (15,164), increased postprandial 5-HT release (162,165), and a decrease in symptoms using serotonergic antagonists (1,166). Metabolism of 5-HT may also be disrupted in both IBS and IBD (167). 5-HT release is well known to activate vagal afferent endings in the upper gastrointestinal tract (168-170). 5-HT also activates cutaneous nociceptive primary afferents contributing to a role in inflammatory pain (171). More recently it was shown that rat colonic LSN afferents also respond to 5-HT (77). Fifty-six percent of LSN afferents responded to 5-HT via both 5-HT3 and non-5-HT3 receptors, which correlates with the percentage of thoracolumbar DRG cell bodies retrogradely labeled from the colon that display 5-HT3 receptors (77). In the rat 5-HT1, 5-HT2 and 5-HT3 receptor subtypes have been demonstrated to modulate responses to noxious colorectal distension (172), and serotonergic activation of visceral sensory neurons may increase their sensitivity to other sensory modalities (158).
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