Varenicline-mediated cholinergic activation through α4β2 nAChR inhibits microglial neuroinflammation and alleviates neuropathic pain and depression following spinal cord injury

Spinal cord injury (SCI) often leads to neuropathic pain. Chronic pain is frequently comorbid with depression and is often refractory, becoming a focal point in clinical and basic research. Neuroprotective pathways activated by nicotinic acetylcholine receptors (nAChRs) are attractive therapeutic targets for neurodegenerative diseases. The α4β2 nAChR agonist varenicline has been shown to alleviate pain, reduce functional disability, and improve cognitive aspects; however, understanding of its effects and mechanisms on neuropathic pain following SCI remains limited. We hypothesize that varenicline regulates cholinergic activity in microglia by activating α4β2 nAChRs, thereby inhibiting neuroinflammation. We aimed to elucidate this mechanism to provide new therapeutic strategies for alleviating comorbid neuropathic pain and depression following SCI. We used a rat T3 spinal cord compression injury model. Behavioural assessments, immunofluorescence, and transcriptomics were employed to investigate the effects of varenicline on mechanical and cold hypersensitivity as well as changes in negative mood in T3-SCI model rats. We analysed the expression of sensory afferent neurons and cholinergic neurons in the dorsal horn of the spinal cord among the different treatment groups. Additionally, we assessed the impact of varenicline on neuroinflammation and nAChRs following SCI. Following T3-SCI, the expression of nAChRs, including acetylcholine (ACh) neurotransmitter vesicles, ACh transporters, ACh metabolism enzymes, α4β2 nAChRs, and α7 nAChRs, and cholinergic activity were significantly suppressed, accompanied by an increase in microglia-dominated neuroinflammation. Varenicline pretreatment significantly enhanced impaired cholinergic neurotransmission and α4β2 nAChR expression following SCI; furthermore, it simultaneously inhibited the widespread activation and proliferation of microglia/macrophage-mediated neuroinflammation. Varenicline suppressed the sprouting of calcitonin gene-related peptide afferent fibres in the dorsal horn of the spinal cord after SCI. Additionally, varenicline reduced pain hypersensitivity and negative emotional aspects of depression following SCI. Varenicline also improved the Th1/Th2 immune balance after SCI. Varenicline can activate α4β2 nAChRs on neurons and enhance cholinergic activity after SCI. It is also associated with a significant suppression of widespread microglia-mediated neuroinflammation and the promotion of immune balance between Th1 and Th2 cells following injury. Moreover, varenicline appears to alleviate neuropathic pain and the negative emotional components of depression after SCI.