Absence of mechanical hyperalgesia after exercise (delayed onset muscle soreness) in neonatally capsaicin-treated rats

Delayed onset muscle soreness (DOMS) appears with some delay after unaccustomed, strenuous exercise, especially after lengthening contraction (LC). It is characterized by tenderness and movement related pain, namely muscular mechanical hyperalgesia. To clarify the involvement of C-fibers in this mechanical hyperalgesia, we examined whether DOMS could be induced in rats treated neonatally with capsaicin. We confirmed that a large portion of unmyelinated afferent fibers were lost in capsaicin treated rats. In these animals, LC failed to induce muscular mechanical hyperalgesia. mRNA of nerve growth factor (NGF) in the muscle, which plays a pivotal role in maintaining mechanical hyperalgesia, was upregulated in the capsaicin treated animals similar to the vehicle treated animals. These results demonstrate that C-fiber afferents are essential in transmitting the nociceptive information from exercised muscle in DOMS.

► We examine C-fiber afferent involvement in delayed onset muscle soreness (DOMS). ► Upregulated nerve growth factor (NGF) sensitizes muscle nociceptors ending in DOMS. ► C-fiber afferents are largely destroyed in neonatally capsaicin-treated rats. ► DOMS is not induced in these rats despite intact upregulation of NGF. ► C-fiber afferents are essential in transmitting DOMS.