Auricular vagus nerve stimulation facilitates contextual fear extinction via insular-mPFC pathway without affecting reconsolidation

Cervical vagus nerve stimulation (VNS) suppresses conditioned fear in rodents and may serve as an adjunct to exposure-based therapy for anxiety disorders. Non-invasive auricular VNS also reduces conditioned fear in humans, although the underlying mechanisms remain unclear. Using a contextual fear conditioning paradigm, in which brief re-exposure induces reconsolidation and prolonged re-exposure promotes extinction learning in mice, we examined whether auricular VNS modulates these distinct memory processes. Auricular VNS during prolonged, but not brief, re-exposure reduced subsequent freezing, indicating facilitation of extinction learning without affecting reconsolidation. Stimulation restricted to the latter half of prolonged re-exposure, when extinction learning was underway, was sufficient to reduce subsequent fear responses. As a potential neural circuit mechanism, we focused on projections from the posterior insular cortex (pIC) to the medial prefrontal cortex (mPFC). The pIC receives interoceptive signals, including vagal input, whereas the mPFC plays a key role in fear regulation. Auricular VNS during extinction learning increased c-Fos expression in pIC neurons projecting to the infralimbic and dorsal peduncular cortex (IL/DP) within the mPFC. Optogenetic activation of the pIC-IL/DP pathway mimicked the extinction-facilitating effect of auricular VNS, whereas inhibition blocked it. These results demonstrate that auricular VNS promotes extinction learning of contextual fear memory by activating the pIC-IL/DP circuit, providing insight into how vagal afferent stimulation engages insular-prefrontal networks to modulate fear.