Exploring brain-gut interaction mechanisms in Transcutaneous auricular Vagus Nerve stimulation for Major Depressive Disorder

The gut microbiota is intricately implicated in the pathogenesis of Major Depressive Disorder (MDD), with the vagus nerve serving as a key regulatory bridge. Transcutaneous Auricular Vagus Nerve Stimulation (taVNS) has emerged as a promising non-invasive therapeutic strategy for MDD by modulating the gut-brain axis, yet the precise brain-gut interaction mechanisms underlying its antidepressant effects remain poorly characterized. This study is a registered clinical trial (ChiCTR2200059591; Registered 4 May 2022; https://www.chictr.org.cn ). This study aimed to verify the clinical efficacy of taVNS for MDD and elucidate the underlying brain-gut crosstalk mechanisms, by integrating comprehensive clinical assessments, resting-state functional magnetic resonance imaging (rs-fMRI) neuroimaging data and gut metagenomic profiling. Ninety-five patients diagnosed with MDD were randomly allocated at a 1:1 ratio to either the active taVNS group (auricular concha stimulation) or the sham taVNS group (superior concha of mid-helix stimulation). Eighty patients (40 per group) completed the entire intervention course and were included in the final statistical analysis. All participants underwent 30-minute stimulation twice daily (4/20 Hz, 3–8 mA) for 8 consecutive weeks (5 days per week). Standardized clinical assessments were administered at baseline and post-intervention, including the 17-item Hamilton Depression Rating Scale (HAMD-17), 14-item Hamilton Anxiety Rating Scale (HAMA-14), and Gastrointestinal Symptom Rating Scale (GSRS). Rs-fMRI was performed to quantify core neural activity metrics, including amplitude of low-frequency fluctuation (ALFF), fractional ALFF (fALFF), regional homogeneity (ReHo), and degree centrality (DC); fecal samples were collected for high-throughput metagenomic analysis. Spearman correlation analysis and mediation analysis were further conducted to dissect the interactive relationships between brain neural activity and gut microbiota. The active taVNS group achieved significantly superior clinical efficacy relative to the sham group, with a HAMD-17 response rate of 62.50% and remission rate of 35.00%, versus 30.00% and 2.50% in the sham group (all P < 0.05). Rs-fMRI analyses revealed significant group×time interaction effects on neural activity: decreased ALFF in the right calcarine sulcus; altered fALFF in the right inferior temporal gyrus, left cuneus, right superior frontal gyrus (SFG) and right angular gyrus; reduced ReHo in the right calcarine sulcus and bilateral insula; and increased DC in the right caudate nucleus and left anterior cingulate gyrus. Gut microbiota profiling identified anaerobic butyrate-producing bacteria and Faecalibacterium prausnitzii as potential biomarkers linked to taVNS therapeutic effects. HAMD-17 scores were negatively correlated with Faecalibacterium prausnitzii abundance (r=-0.566, P < 0.01) and positively correlated with anaerobic butyrate-producing bacteria abundance (r = 0.406, P < 0.01). Mediation analysis suggested that fALFF values in the right SFG may indirectly modulate depressive symptoms via regulating Faecalibacterium prausnitzii abundance (indirect effect 95% CI: 0.3039–2.4466), with a significant partial mediation effect observed, though future studies controlling for dietary and other confounding variables are needed to confirm this relationship. taVNS effectively alleviates depressive symptoms in MDD patients via dual complementary pathways: directly modulating neural activity in the right SFG to regulate depression-related brain function, and indirectly maintaining gut microbiota homeostasis by enriching beneficial taxa such as Faecalibacterium prausnitzii. These findings provide novel mechanistic insights into the brain-gut interaction underlying the antidepressant effects of taVNS, laying a theoretical foundation for its clinical application in MDD management.