PROJECT SUMMARY/ABSTRACT Irritable bowel syndrome (IBS) is a highly prevalent disorder characterized by visceral pain and dysmotility. IBS causes substantial morbidity in children and adults and current therapy is inadequate. Serotonin (5-HT) signaling plays roles in pain and motility, but the efficacy of modifying 5-HT signaling to treat IBS is limited and fraught with adverse effects. A greater understanding of how enteric 5-HT contributes to IBS pathophysiology may therefore provide for novel and effective treatments for the condition. Enterochromaffin (EC) cells in the gastrointestinal (GI) epithelium produce most of the 5-HT in the gut, which is thought to stimulate extrinsic primary afferent neuron (ExPAN) and intrinsic primary afferent neuron (IPAN) terminals to promote sensory and motor signaling, respectively. The serotonin reuptake transporter (SERT), present throughout epithelial cells, rapidly inactivates 5-HT. Selective serotonin reuptake inhibitors (SSRIs) inhibit SERT and thus increase 5-HT availability for IPAN and ExPAN stimulation. Despite their use for pediatric IBS, SSRIs are often ineffective and plagued by adverse GI effects, which may be due to their effects at sites other than the GI epithelium. My prior and preliminary data strongly suggest that epithelial-restricted 5-HT modulation may limit unwanted effects and thus improve therapy. My data also show a novel visceral pain mechanism involving SERT regulation of mucosal 5- HT. In the current proposal, I will investigate the effects of epithelial 5-HT on GI motility and visceral nociception using optogenetic tools that induce or inhibit EC cell secretion, mouse lines that either lack mucosal 5-HT or SERT, and pharmacological interventions that alter mucosal 5-HT signaling. The proposed research strategy will allow me to test the hypotheses that 1) 5-HT released from EC cells and 2) SERT-mediated regulation of mucosal 5-HT availability modulate visceral nociception and GI motility.