Microbial dysbiosis alters serotonin signaling in a postinflammatory murine model of visceral pain

Serotonin (5-HT) is a multifunctional signaling molecule in the gastrointestinal (GI) tract. 5-HT synthesis is regulated by the gut microbiota. Microbial dysbiosis has been implicated in visceral pain and persistent alterations in gut function that occur following inflammation. Here, we tested the hypothesis that alterations in gut microbiota in a postinflammatory model of visceral pain contribute to dysregulated 5-HT signaling. We used mice treated with dextran sodium sulfate (DSS) 42 days earlier (postcolitis) or untreated mice as donors for fecal microbiota transplants (FMTs) into germ-free mice to explore changes in enterochromaffin (EC) cell populations, expression of 5-HT synthesis, transport, and degradation genes, levels of 5-HT and its major metabolite, 5-hydroxyindoleacetic acid (5-HIAA), and 5-HT release. Significant differences were observed in EC cells, <i>Tph1</i>, <i>Slc6a4</i>, and <i>Maoa</i> gene expression, 5-HT and 5-HIAA levels, and 5-HT release between germ-free mice and mice receiving an FMT from either control or postcolitis donor mice. We observed no differences in the total number of EC cells, <i>Tph1</i>, or <i>Slc6a4</i> gene expression of mice after FMT from postcolitis or control mice. However, there was a significant increase in <i>Maoa</i> gene expression in the terminal ileum, an increased 5-HIAA/5-HT ratio in the proximal colon, and reduced 5-HT release to mechanical and chemical stimulation in the proximal and distal colon after FMT from postcolitis mice. Collectively, these findings provide additional evidence that the gut microbiota regulates 5-HT signaling. Moreover, they reveal functional changes in EC cell sensitivity in the presence of an altered microbiota after recovery from inflammation. <b>NEW & NOTEWORTHY</b> The gut microbiota regulates serotonin biosynthesis in enterochromaffin cells. Here, we show that a dysbiotic gut microbiota that occurs after recovery from inflammation alters serotonin signaling and produces functional changes in enterochromaffin cell sensitivity.