Serotonin (5-hydroxytryptamine; 5-HT), is a neurotransmitter and hormone that regulates various gut physiological functions. Enterochromaffin (EC) cells are the main producer of gut 5-HT, and tryptophan hydroxylase 1 (Tph1) is the rate-limiting enzyme of 5-HT biosynthesis. There is increasing evidence that gut function as well as pathology rely on interactions with gut microbiota. β-defensins are major class of antimicrobial peptides (AMPs) expressed by colonocytes, maintaining mucosal barrier integrity by shaping gut microbiota composition. Peroxisome proliferator activated receptor-gamma (PPAR-γ), a ligand-inducible transcription factor, has been shown to influence AMP secretion. It is reported that 5-HT inhibit PPAR-γ expression in pulmonary artery smooth muscle cells, providing a crucial link between 5-HT and PPAR-γ regulation. As intestinal epithelial cells express 5-HT receptors, we hypothesize that 5-HT released from EC cells down-regulates PPAR-γ expression, which subsequently inhibits β-defensin production from neighbouring intestinal epithelial cells To determine whether 5-HT inhibits β-defensin production by down-regulating PPAR-γ signalling We measured PPAR-γ, mouse β-defensin (mBD)-1 and mBD-3 levels in colonic tissues of naïve Tph1-/- mice (which have significantly lower 5-HT in gut), wild-type (WT) littermate (Tph1+/+), and Tph1-/- administered with either 5-hydroxytryptophan (5-HTP; precursor of 5-HT) or PPAR-γ antagonist (GW-9662). Colonic epithelial (HT-29) cells were treated with 5-HT to determine direct role of 5-HT in PPAR-γ expression along with subsequent changes in human β-defensin (hBD)-1 and hBD-2 levels. In addition, cells were treated with 5-HT3, 5-HT4, 5-HT7 receptor antagonists and extracellular signal-regulated kinase-1 and -2 (ERK1/2) inhibitor. We observed higher expression of PPAR-γ along with up-regulation of mBD-1 and mBD-3 in Tph1-/- mice, compared with WT littermates; while replenishing 5-HT synthesis by 5-HTP decreased PPAR-γ and β-defensin expression in Tph1-/- mice. Tph1-/- mice administered with GW-9662 showed decreased expression of both mBD-1 and mBD-3. Treatment of HT-29 cells with 5-HT receptor antagonists revealed that 5-HT7 receptor, but not 5-HT3 or 5-HT4 receptor, plays an important role in mediating inhibitory action of 5-HT on PPAR-γ expression and subsequent β-defensin production. ERK1/2 inhibitor (PD98059) restored hBD-1 and hBD-2 production in 5-HT treated cells Our results illustrate 5-HT released from EC cells activates 5-HT7 receptor and down-regulates PPAR-γ expression through ERK1/2-depedent pathway, which subsequently inhibits β-defensin production from intestinal epithelial cells. This study exemplifies novel information on the interaction between 5-HT and PPAR-γ in relation AMP production in the context of intestinal innate response CIHR