H 2 S Alleviates Neuropathic Pain in Mice by Nrf2 Signaling Pathway Activation

Neuropathic pain is a chronic pain caused by direct damage to the peripheral or central nervous system, characterized by hyperalgesia, allodynia, and spontaneous pain. Hydrogen sulfide (H 2 S) therapy has been applied for neuropathic pain treatment, although the underlying mechanisms remain unknown. In this study, we sought to ascertain whether H 2 S therapy could alleviate neuropathic pain in a model of chronic constriction injury (CCI) and, if so, the potential mechanism. A CCI model was established in mice through a spinal nerve ligation method. Intrathecal injection of NaHS was used to treat CCI model mice. The thermal paw withdrawal latency (TPWL) and mechanical paw withdrawal threshold (MPWT) were used for pain threshold evaluation in mice. A series of experiments including immunofluorescence, enzyme-linked immunosorbent assay, electrophysiological test, mitochondrial DNA (mtDNA) quantification, measurement of ATP content, demethylase activity, and western blot were performed to investigate the specific mechanism of H2S treatment in neuropathic pain. Mice with CCI exposure exhibited a decrease in MPWT and TPWL, an increase in IL-1β and TNF-α expressions, elevated eEPSP amplitude, an upregulation of mtDNA, and a reduction in ATP production, whereas H 2 S treatment significantly reversed these changes. Furthermore, CCI exposure induced a remarkable increase in vGlut2- and c-fos-positive as well as vGlut2- and Nrf2-positive cells, an increase in Nrf2 located in the nucleus, and an upregulation of H3K4 methylation, and H 2 S treatment further enhanced these changes. In addition, ML385, a selective Nrf2 inhibitor, reversed the neuroprotective effects of H 2 S. H 2 S treatment mitigates CCI-induced neuropathic pain in mice. This protective mechanism is possibly linked to the activation of the Nrf2 signaling pathway in vGlut2-positive cells.