Involvement of neutrophils and interleukin-18 in nociception in a mouse model of muscle pain.

Muscle pain is a common condition that relates to various pathologies. Muscle overuse induces muscle pain, and neutrophils are key players in pain production. Neutrophils also play a central role in chronic pain by secreting interleukin (IL)-18. The aim of this study was to investigate the involvement of neutrophils and IL-18 in a mouse model of muscle pain. The right hind leg muscles of BALB/c mice were stimulated electrically to induce excessive muscle contraction. The left hind leg muscles were not stimulated. The pressure pain threshold, number of neutrophils, and IL-18 levels were investigated. Furthermore, the effects of the IL-18-binding protein and Brilliant Blue G on pain were investigated. In stimulated muscles, pressure pain thresholds decreased, and neutrophil and IL-18 levels increased compared with that in non-stimulated muscles. The administration of IL-18-binding protein and Brilliant Blue G attenuated hyperalgesia caused by excessive muscle contraction. These results suggest that increased IL-18 secretion from larger numbers of neutrophils elicits mechanical hyperalgesia.