Recent studies have shown that mucosal serotonin transporter (SERT) expression is decreased in animal models of colitis, as well as in the colonic mucosa of humans with ulcerative colitis (UC) and irritable bowel syndrome (IBS). Altered serotonin transporter function or expression may underlie the altered motility, secretion, and sensation seen in these inflammatory gut disorders. In an effort to elucidate possible mediators of SERT down-regulation, we treated cultured colonic epithelial cells (Caco2) with conditioned media from activated human lymphocytes. Application of the conditioned media caused a decrease in fluoxetine-sensitive [³H]5-HT uptake. Individual pro-inflammatory agents were then tested for their ability to affect uptake. Cells were treated for 48 to 72 hours with PGE2 (10uM), IFN- (500 ng/ml), TNF- (50 ng/ml), IL-12 (50 ng/ml) or the nitric oxide releasing agent GSNO (100 µM). [³H]5-HT uptake was then measured. Neither PGE nor IL-12 had any effect on [³H]5-HT uptake, and GSNO increased uptake. However, after 3 days incubation, both TNF- and IFN- elicited significant decreases in SERT function. Neither TNF- nor IFN- were cytotoxic when used for this period of time and at these concentrations. These two cytokines also induced decreases in SERT mRNA and protein levels. By altering SERT expression, TNF- and IFN- could contribute to the altered motility and expression seen in vivo in UC or IBS.