Sinisan Alleviates Stress-Induced Intestinal Dysfunction and Depressive-like Behaviors in Mice with Irritable Bowel Syndrome by Enhancing the Intestinal Barrier and Modulating Central 5-Hydroxytryptamine

Irritable bowel syndrome (IBS) is a common chronic functional bowel disorder and is strongly associated with an increased risk of depression and anxiety. The brain–gut axis plays an important role in the pathophysiologic changes in IBS, yet effective treatments for IBS are still lacking. Sinisan, originating from the Treatise on Typhoid Fever by the medical sage Zhang Zhongjing, is a classic formula in the Eight Methods of Traditional Chinese Medicine (TCM) that focuses on dispersing the liver and regulating the spleen, relieving depression and transmitting evils, and has been widely used in the treatment of liver-depression and spleen-deficiency, diarrhea, and related liver and stomach disorders. However, the therapeutic effect of sinisan in IBS has not been clarified. The aim of this study was to investigate the effects of sinisan on stress-induced intestinal dysfunction and depressive behavior in IBS mice. We established a diarrhea-predominant irritable bowel syndrome (IBS-D) mouse model using a 4% acetic acid enema combined with restraint stress, and analyzed the results using behavioral tests, relevant test kits, hematoxylin-eosin (HE) staining, immunofluorescence (IF), Western blot (WB), and quantitative fluorescence polymerase chain reaction (qRT-PCR). The results showed that sinisan administration significantly alleviated intestinal dysfunction and depressive-like behaviors in IBS-D mice, improved mild colonic inflammation and intestinal mucosal permeability, up-regulated the expression of tight junction proteins ZO-1 and occludin. Sinisan significantly alleviated intestinal dysfunction and depressive-like behaviors in IBS-D mice by decreasing the expression of TNF-α, promoting the expression of tight junction proteins (occludin, ZO-1) expression, and inhibiting the Tlr4/Myd88 signaling pathway, thereby attenuating the inflammatory response, protecting the intestinal barrier, and alleviating symptoms in the IBS-D mouse model. Taken together, sinisan may ameliorate intestinal inflammation and the intestinal barrier by regulating 5-HT expression and the Tlr4/Myd88 pathway, thereby alleviating stress-induced intestinal dysfunction and depressive behaviors in IBS-D mice.