Transcutaneous auricular vagus nerve stimulation alleviates depression-like behavior by rebalancing septohippocampal cholinergic-microglial signaling.

Dysregulated interactions between stress-sensitive neural circuits and immune signaling contribute to depression. We investigated whether the septohippocampal cholinergic pathway mediates the behavioral and neuroimmune effects of transcutaneous auricular vagus nerve stimulation (taVNS). Male mice were studied using chronic unpredictable mild stress (CUMS) and lipopolysaccharide (LPS) challenge models. Depression-related behaviors were assessed using the sucrose preference, forced swim, tail suspension, open field, and elevated plus maze tests. Cholinergic markers, hippocampal cytokines, microglial morphology, and α7 nicotinic acetylcholine receptor (α7nAChR) expression were examined by Western blotting and immunofluorescence. In a separate LPS cohort, hippocampal extracellular acetylcholine (ACh) was measured by microdialysis-HPLC during awake, freely moving recording. Cre-dependent excitatory DREADDs were used to activate medial septal ChAT neurons during taVNS treatment. CUMS increased medial septal ChAT expression and ChAT/c-Fos co-labeling, reduced hippocampal AChE and α7nAChR expression, and increased hippocampal TNF-α, IL-1β, IL-6, and microglial activation. Three weeks of taVNS improved depression-like behaviors and reversed these cholinergic and neuroimmune changes. In LPS-challenged mice, taVNS reduced hippocampal extracellular ACh and improved inflammation-associated anxiety/depression-related behavior. Chemogenetic activation of medial septal ChAT neurons attenuated the behavioral benefits of taVNS. These findings support septohippocampal cholinergic-microglial signaling as a neuroimmune mechanism through which taVNS improves depression-like behavior.