Pharmacological and transcutaneous auricular vagal targeting of endoplasmic reticulum stress in the trigeminal ganglion alleviates migraine-like behaviors

BACKGROUND: Endoplasmic reticulum (ER) stress has emerged as a key player in the development of diverse chronic pain disorders; however, its specific contribution to migraine pathophysiology has yet to be fully elucidated. Although accumulating evidence indicates that transcutaneous auricular vagus nerve stimulation (taVNS) mitigates chronic pain, the precise molecular mechanisms governing this effect remain poorly understood. This study investigates the functional contribution of ER stress and its downstream signaling mechanisms in a nitroglycerin (NTG)-induced mouse model of chronic migraine, and whether taVNS exerts analgesic effects by modulating ER stress within trigeminal ganglion (TG) neurons. METHODS: Chronic migraine was induced in mice via repeated intraperitoneal injection of NTG. Facial mechanical hypersensitivity was assessed using Von Frey filaments. Light-aversive and anxiety-like behaviors were evaluated through open field testing and light/dark box testing. mRNA and protein expression were analyzed using quantitative PCR (qPCR) and Western blot, respectively. The cellular localization of alpha7 nicotinic acetylcholine receptor (α7nAChR) receptors in the TG was determined by immunofluorescence staining. TG neuronal excitability was recorded using whole-cell patch-clamp electrophysiology. taVNS was delivered via the auricular branch of the vagus nerve using varying parameters. RESULTS: NTG injection induced migraine-like behaviors characterized by increased facial mechanical allodynia, anxiety-like behaviors and photophobia. Significant activation of ER stress-related marker proteins was observed in the TG of NTG-treated mice, and intra-TG injection of the ER stress inhibitor tauroursodeoxycholic acid (TUDCA) alleviated migraine-like behaviors and reduced neuronal excitability of TG neurons. Transcriptomic sequencing revealed that TUDCA treatment reduced the expression of genes related to the PI3K-AKT signaling pathway and cytokine-cytokine receptor interaction. Western blot confirmed that phosphorylated AKT (p-AKT) expression was also reduced by TUDCA. qPCR showed that NTG-induced upregulation of inflammatory mediators (including Ccl6, Ccl8, Ccl11 and Il-16) was also decreased by TUDCA. Additionally, daily treatment with 20 Hz taVNS mitigated ER stress, neuronal hyperexcitability, and inflammatory mediators' production, which were reversed by an inhibitor of α7nAChR. CONCLUSIONS: ER stress serves as an upstream driver of neuronal hypersensitivity and neuroinflammation in the TG, leading to pain sensitization, while taVNS targets this core organellar stress state through α7nAChR activation to exert therapeutic effects. These findings provide a novel mechanistic perspective and identify potential therapeutic targets for migraine treatment.