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 reduces conditioned fear also 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 auricular VNS-induced attenuation of fear. Auricular VNS during prolonged but not during brief re-exposures reduced freezing, indicating selective facilitation of extinction learning. Stimulation during the latter half of prolonged re-exposure, while extinction learning was progressing, reduced fear responses. As a neural circuit mechanism, we focused on projections from the posterior insular cortex (pIC), which receives interoceptive signals, including vagal input, to the medial prefrontal cortex (mPFC), which regulates fear. Auricular VNS increased c-Fos expression in pIC neurons projecting to the mPFC subdivisions: infralimbic and dorsal peduncular cortex (IL/DP). Optogenetic activation of the pIC-IL/DP pathway mimicked the extinction-facilitating effects of auricular VNS, whereas inhibition blocked them. 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.