Vagus nerve-dependent antidepressant effects of ineupatorolide B via gut microbiota modulation in a mouse model of depression.

Ineupatorolide B (InB), a sesquiterpene lactone with anti-inflammatory and anti-tumor properties, has not previously been evaluated for antidepressant effects. Given the central role of the gut-brain axis in stress-related mood disorders, we investigated whether InB alleviates depression-like phenotypes in mice exposed to chronic social defeat stress (CSDS) through microbiota- and vagus-dependent mechanisms. CSDS-susceptible mice received vehicle, fluoxetine, or InB and were assessed using behavioral paradigms, plasma inflammatory cytokine measurements, and analyses of synaptic-related protein expression in the medial prefrontal cortex (mPFC). CSDS induced robust depression-like behaviors, elevated interleukin-6 and tumor necrosis factor-α, caused splenomegaly, and reduced PSD-95 and BDNF expression in the mPFC. Repeated InB administration significantly reversed these behavioral, inflammatory, and synaptic abnormalities. 16S rRNA gene sequencing revealed that CSDS disrupted gut microbial α- and β-diversity, whereas InB treatment largely normalized these alterations and restored depression-associated short-chain fatty acids, including isobutyric and valeric acid, consistent with reestablishment of microbial metabolic homeostasis. Importantly, subdiaphragmatic vagotomy abolished the antidepressant-like, anti-inflammatory, and synaptic effects of InB, demonstrating that its efficacy requires intact vagus nerve-mediated gut-brain signaling. Together, these findings identify vagus-dependent modulation of the microbiota-immune-synaptic axis as a mechanistic pathway through which InB exerts antidepressant effects and highlight this compound as a potential microbiota-targeted therapeutic strategy for depression.