Sini San ameliorates symptoms of depression by modulating gut microbiota structure, Tryptophan metabolism, and short-chain fatty acid levels.

backgroundSini San (SNS) is a traditional Chinese medicinal formulation originating from the "Treatise on Febrile Diseases", an ancient text that has been historically employed for treating depression. However, the underlying mechanisms by which SNS potentially ameliorates symptoms of depression remain unclear. This study aimed to elucidate the role of gut microbiota in tryptophan metabolism and examine the involvement of short-chain fatty acids (SCFAs) in the putative antidepressant effects of SNS.

methodsA rat model of depression using chronic unpredictable mild stress (CUMS) was established through simultaneous modeling and treatment for six consecutive weeks. The possible therapeutic effect of SNS on CUMS-induced rats was thoroughly assessed using body weight measurements, open-field tests, and sucrose preference tests. Alterations in rat gut microbiota were analyzed using full-length 16 S rRNA third-generation sequencing. Furthermore, metabolomic analysis was performed on fecal samples. An enzyme-linked immunosorbent assay (ELISA) was used to quantify tryptophan (Trp), serotonin (5-HT), and kynurenine (Kyn) in the hippocampal and colonic tissues. Reverse transcription quantitative PCR (RT-qPCR) was used to measure the mRNA levels of TPH-1 and TPH-2 in hippocampal and colonic tissues. Immunofluorescence was used to assess hippocampal IDO-1 and IBA-1 expression. Finally, gas chromatography-mass spectrometry (GC-MS) was used to analyze the SCFAs content in fecal samples.

resultsAdministration of SNS significantly alleviated depressive symptoms induced by CUMS, as demonstrated by increased body weight, improved sucrose preference, and a statistically significant decrease in immobility time during the forced swim test (P < 0.05). The Shannon and Simpson indices were elevated following SNS treatment, signifying an enhancement in both the quantity and variety of the gut microbiota. Analysis using 16 S rRNA gene sequencing revealed that SNS influenced the composition of the gut microbiome, favorably increasing the presence of beneficial bacteria, such as Firmicutes, Lactobacillaceae, Ruminococcaceae, and Oscillospiraceae, while simultaneously decreasing the levels of potentially detrimental bacteria, such as Muribaculaceae, Prevotellaceae, Lachnospiraceae, and Alloprevotella. Fecal metabolomic studies identified 87 metabolites, of which 57 were differentially expressed, most notably l-tryptophan and phenylalanine. These metabolites were primarily linked to 15 metabolic pathways, with notable enrichment in the biosynthetic pathways of phenylalanine, tyrosine, and tryptophan, as well as the metabolic pathways of arachidonic acid and phenylalanine. Metabolomic profiling indicated that SNS stimulated the kynurenine pathway, leading to increased levels of metabolites with antidepressant properties, such as Trp, 5-HT, and Kyn, in both hippocampal and colonic tissues. In addition, SNS promoted the synthesis of SCFAs, particularly butyrate and propionate, which are known for their neuroprotective properties.

conclusionsThe present study suggests that the antidepressant effects of SNS can be attributed to its ability to modulate the structure of the gut microbiota, consequently regulating tryptophan metabolism and SCFA levels, which ultimately ameliorate symptoms of depression. These findings provide experimental evidence supporting the concept of the brain-gut axis in depression treatment and lay a foundation for further investigation of the underlying antidepressant mechanisms of the SNS.