5-HT receptor-dependent intestinal neurite outgrowth contributes to visceral hypersensitivity in irritable bowel syndrome.

Irritable bowel syndrome (IBS) is characterized by visceral hypersensitivity (VH) associated with abnormal serotonin/5-hydroxytryptamine (5-HT) metabolism and neurotrophin-dependent mucosal neurite outgrowth. The underlying mechanisms of VH remain poorly understood. We investigated the role of 5-HT receptor in mucosal innervation and intestinal hyperalgesia. A high density of mucosal nerve fibres stained for 5-HT was observed in colonoscopic biopsy specimens from IBS patients compared with those from healthy controls. Staining of 5-HT and 5-HT receptors was observed mainly in colonic epithelia with comparable levels between IBS and controls. Visceromotor responses to colorectal distension were evaluated in two mouse models, one postinfectious with Giardia and subjected to water avoidance stress (GW) and the other postinflammatory with trinitrobenzene sulfonic acid-induced colitis (PT). Increased VH was associated with higher mucosal density of 5-HT-expressing nerve fibres and elevated neurotrophin and neurotrophin receptor levels in the GW and PT mice. The increased VH was inhibited by intraperitoneal injection of SB-269970 (a selective 5-HT antagonist). Peroral multiple doses of CYY1005 (a novel 5-HT ligand) decreased VH and reduced mucosal density of 5-HT-expressing nerve fibres in mouse colon. Human neuroblastoma SH-SY5Y cells incubated with bacteria-free mouse colonic supernatant, 5-HT, nerve growth factor, or brain-derived neurotrophic factor exhibited nerve fibre elongation, which was inhibited by 5-HT antagonists. Gene silencing of HTR7 also reduced the nerve fibre length. Activation of 5-HT upregulated NGF and BDNF gene expression, while stimulation with neurotrophins increased the levels of tryptophan hydroxylase 2 and 5-HT in neurons. A positive-feedback loop was observed between serotonin and neurotrophin pathways via 5-HT activation to aggravate fibre elongation, whereby 5-HT and 5-HT had no roles. In conclusion, 5-HT-dependent mucosal neurite outgrowth contributed to VH. A novel 5-HT antagonist could be used as peroral analgesics for IBS-related pain.