Involvement of the Hsp70/TLR4/IL-6 and TNF-α pathways in delayed-onset muscle soreness

Delayed-onset muscle soreness (DOMS) is a very common condition in athletes and individuals not accustomed to physical activity that occurs after moderate/high-intensity exercise sessions. The activation of microglial Toll-like receptor 4 (TLR4) in the spinal cord has been described to be important for the induction and maintenance of persistent pain. Based on that, we hypothesize that 70 kilodalton heat-shock protein (Hsp70), a mediator released by exercise, could activate microglial TLR4 in the spinal cord, releasing proinflammatory cytokines and contributing to the start of DOMS. In fact, we found that the knockout of TLR4, myeloid differentiation primary response 88 (MyD88), interleukin-6 (IL-6), or both tumor necrosis factor-α (TNF-α) receptor 1 and TNF-α receptor 2 in mice prevented the development of DOMS following acute aerobic exercise in contrast to the findings in male C57BL/6 wild-type mice. Furthermore, DOMS in exercised wild-type mice was also prevented after pre-treatment with microglia inhibitor, TLR4 antagonist, and anti-Hsp70 antibody. During exercise-induced DOMS, Hsp70 mRNA, TLR4 mRNA, and protein levels, as well as Iba-1 (a microglial marker), IL-6, and TNF-α protein levels, were increased in the muscle and/or spinal cord. Together, these findings suggest that Hsp70 released during exercise-induced DOMS activates the microglial TLR4/IL-6/TNF-α pathway in the spinal cord. Thus, the blockade of TLR4 activation may be a new strategy to prevent the development of DOMS before intense exercise.