Correlation between intestinal microbiotal imbalance and 5-HT metabolism, immune inflammation in chronic unpredictable mild stress male rats

To explore the role of intestinal microbiota on the occurrence of depression-like behavior. Twenty male adult Wistar rats were randomly divided into control and experimental groups. Depression-like behavior of the rats was validated using sucrose preference test (SPT) and forced swimming test (FST) after chronic unpredictable mild stress (CUMS) for 3 weeks. Fecal microbiota was analyzed through 16S rRNA sequence analysis. The levels of 5-HT and inflammatory factors in the colon, brain and sera were measured using enzyme-linked immunosorbent assay (ELISA), quantitative PCR (qPCR) and western blotting analyses. The percentage of different types of immune cells in the peripheral blood was determined through flow cytometry. CUMS caused depression-like symptoms, including anhedonia and desperate behavior. Significant differences were found in the structure and abundance of intestinal microbiota. CUMS intervention significantly increased the levels of 5-HT and Tph1 in the colon and decreased the level of Scl6a4. The concentrations of 5-HT and Tph2 in the prefrontal and hippocampal tissues were lower, while IDO1 was higher. Certain cytokines, such as IL-6, IL-1 and TNF-ɑ, were significantly elevated in peripheral blood, while the percentage of CD3⁺ CD4⁺ double-positive cells and CD4⁺/CD8⁺ ratio were downregulated in the CUMS group. Pearson correlation analysis showed that intestinal microbiota was significantly associated with not only the metabolism of 5-HT in intestinal and brain tissues, but also with the proportion of immune cells and certain cytokines. Stress can lead to disturbances in the intestinal microbial structure, which may contribute to depression by interfering with 5-HT metabolism and immune inflammatory responses.