Effects of hyperbaric oxygen therapy on neuropathic pain via mitophagy in microglia

AbstractPurpose: Hyperbaric oxygen (HBO) therapy has been suggested to palliate neuropathic pain, but the mechanisms involvedare not well understood. This study explored the involvement of microglial mitophagy via HBO relative to neuropathic paintherapy.Materials and methods: A total of 80 male Sprague Dawley rats were randomly divided into two groups: a normal group(n ¼ 40) and a mitophagy inhibitor group (n ¼ 40) in which the mitophagy inhibitor cyclosporin A (CsA) was administratedprior to chronic constriction injury (CCI). Groups (n ¼ 10 rats per group) consisted of the following: control (C), shamoperation (S), sciatic nerve with chronic constriction injury (CCI), and a CCI plus HBO treatment (CCI þ HBO). Pain-relatedbehaviors were evaluated using mechanical withdraw tendency and thermal withdraw latency analysis. Mitochondrial mem brane potential was measured, and Western blot was employed to assess expression of NIX and BNIP3.Immunofluorescence changes in neuron protein (NESTIN) and mitochondria inner or outer layer proteins (TIM23,TOM20) were examined.Results: HBO significantly ameliorated pain-related behaviors, which were downregulated by mitophagy inhibitors (P < 0.05). Mitochondrial membrane potential indexes were decreased after HBO therapy, but were reversed in the mito phagy inhibitor group (P < 0.05). HBO upregulated NIX and BNIP3 expression, which did not occur in the CCI group (P < 0.05). However, expression was reduced when mitophagy inhibitors were administered. Immunofluorescence examin ation showed that mitophagy in microglia was induced by CCI, which was upregulated after HBO treatment. This phenom enon was not observed in the mitophagy inhibitor group.Conclusions: HBO therapy palliated CCI-induced neuropathic pain in rats by upregulating microglial mitophagy. These results could serve as guidelines to improve neuropathic pain therapy using HBO to maximize therapeutic efficiency.