Orexinergic activation in the ventrolateral periaqueductal gray contributes to voluntary exercise-induced hypoalgesia in neuropathic pain model mice

Voluntary exercise is known to alleviate chronic pain, yet the underlying neural mechanisms, including the descending pain inhibitory system, remain incompletely understood. As orexin is recognized as a neuromodulator involved in pain regulation and projects widely throughout the central nervous system, we investigated the role of orexinergic signaling in the ventrolateral periaqueductal gray (vlPAG) in the induction of exercise-induced hypoalgesia (EIH) using a neuropathic pain mouse model. Mice with free access to running wheels exhibited significantly improved mechanical and thermal pain thresholds compared to sedentary mice, and this pain relief positively correlated with running distance. Immunohistochemical analyses revealed increased orexin immunoreactivity and neuronal activation in the vlPAG following voluntary running (VR). Although dopaminergic neuron numbers in the vlPAG were low, their activation was significantly enhanced after VR, whereas GABAergic neurons showed only minimal activation. These findings suggest that VR may promote EIH via activation of orexinergic projections from the lateral hypothalamus to the vlPAG, potentially engaging descending pain inhibitory pathways. Our results indicate a novel neural mechanism underlying EIH via orexin signaling and support voluntary exercise as a promising non-pharmacological strategy for managing chronic neuropathic pain.