The effects of hyperbaric oxygen therapy on neuropathic pain via mitophagy in microglia

Purpose: Hyperbaric oxygen (HBO) therapy has been suggested to palliate neuropathic pain, but the mechanisms involved are not well understood. This study explored the involvement of microglial mitophagy via HBO relative to neuropathic pain therapy.

Materials and methods: A total of 80 male Sprague Dawley rats were randomly divided into two groups: a normal group (n = 40) and a mitophagy inhibitor group (n = 40) in which the mitophagy inhibitor cyclosporin A (CsA) was administrated prior to chronic constriction injury (CCI). Groups (n = 10 rats per group) consisted of the following: control (C), sham operation (S), sciatic nerve with chronic constriction injury (CCI), and a CCI plus HBO treatment (CCI + HBO). Pain-related behaviors were evaluated using mechanical withdraw tendency and thermal withdraw latency analysis. Mitochondrial membrane 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 mitophagy 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 examination showed that mitophagy in microglia was induced by CCI, which was upregulated after HBO treatment. This phenomenon 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.