Melatonin alleviates diarrhea and visceral hypersensitivity in rats with diarrhea-predominant irritable bowel syndrome by modulating of the TLR4/MyD88/NF-κB pathway

This study aimed to investigate the effects of melatonin on diarrhea and visceral hypersensitivity in rats with diarrhea-predominant irritable bowel syndrome (IBS-D) and to explore its potential mechanisms through modulation of the TLR4/MyD88/NF-κB pathway. Adult male Sprague-Dawley (SD) rats were used to establish an IBS-D model through a combination of chronic and acute stress. The rats were randomly divided into four groups: healthy control (HC), IBS-D, IBS-D + melatonin 5 mg/kg (M-L), and IBS-D + melatonin 10 mg/kg (M-H), with six rats in each group. Visceral sensitivity was assessed using the abdominal withdrawal reflex (AWR). The expression levels of tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) in colon tissue were measured using enzyme-linked immunosorbent assay (ELISA). Western blotting and immunohistochemistry were employed to detect the expression of TLR4, MyD88, and NF-κB proteins in colon tissue. Additionally, 16 S rRNA sequencing was used to analyze the composition of the intestinal microbiota. Compared to the HC group, the IBS-D group exhibited colonic inflammatory injury, increased AWR scores, elevated levels of TNF-α, IL-6, TLR4, MyD88, and NF-κB in the colon, and altered intestinal microbiota composition. Melatonin treatment reduced colonic inflammatory injury, decreased AWR scores, and lowered the levels of TNF-α, IL-6, TLR4, MyD88, and NF-κB in a dose-dependent manner. The intestinal microbiota composition in melatonin-treated groups showed a trend towards that of the HC group. Melatonin improved diarrhea and visceral hypersensitivity in IBS-D rats, potentially through the modulation of the TLR4/MyD88/NF-κB pathway and partial restoration of the intestinal microbiota.