Acid-sensing ion channel 3 in macrophages, but not sensory neurons, mediates development of activity-induced muscle pain.

Activity-induced pain is a significant symptom for people with chronic pain and reduces participation in daily activities and effective exercise programs. We previously showed that pharmacological blockade of ASIC3 locally in muscle, but not downregulation of ASIC3 in sensory neurons, prevents development of activity-induced muscle pain. Further, depleting macrophages from muscle prevents activity-induced pain. We therefore hypothesized that ASIC3 expression in muscle macrophages is necessary for development of activity-induced muscle pain by promoting release of pro-inflammatory cytokines. We developed a conditional ASIC3 knockout mouse (ASIC3fl/fl) to test if removal of ASIC3 from macrophages prevents development of activity-induced pain. Cx3cr1Cre+/ASIC3fl/fl mice do not develop activity-induced pain; however, AdvCre+/ASIC3fl/fl had no effect on activity-induced pain. In contrast, carrageenan-induced muscle pain was prevented in both Cx3cr1Cre+/ASIC3fl/fl and AdvCre+/ASIC3fl/fl mice. Further, removal of ASIC3 from muscle macrophages, using a lentivirus expressing CRE injected into muscle from ASIC3fl/fl mice, attenuated activity-induced pain. Flow cytometry revealed the proportion of muscle macrophages in muscle was decreased in Cx3cr1Cre+/ASIC3fl/fl compared to controls (Cx3cr1Cre-/ASIC3fl/fl), despite overall increases in myeloid cells and T-cells. Further in Cx3cr1Cre+/ASIC3fl/fl there was an attenuated release of the inflammatory cytokines GM-csf and TNFα from cultured macrophages treated with ATP and pH 6.5 when compared to controls. Together these data show that ASIC3 in muscle macrophages plays a critical role in development of activity-induced muscle pain by modulating release of inflammatory cytokines.