Unaccustomed strenuous exercise that includes lengthening contraction (LC) often causes delayed-onset muscle soreness (DOMS), a kind of muscular mechanical hyperalgesia. The substances that induce this phenomenon are largely unknown. Peculiarly, DOMS is not perceived during and shortly after exercise, but rather is first perceived after ϳ1 d. Using B 2 bradykinin receptor antagonist HOE 140, we show here that bradykinin released during exercise plays a pivotal role in triggering the process that leads to muscular mechanical hyperalgesia. HOE 140 completely suppressed the development of muscular mechanical hyperalgesia when injected before LC, but when injected 2 d after LC failed to reverse mechanical hyperalgesia that had already developed. B 1 antagonist was ineffective, regardless of the timing of its injection. Upregulation of nerve growth factor (NGF) mRNA and protein occurred in exercised muscle over a comparable time course (12 h to 2 d after LC) for muscle mechanical hyperalgesia. Antibodies to NGF injected intramuscularly 2 d after exercise reversed muscle mechanical hyperalgesia. HOE 140 inhibited the upregulation of NGF. In contrast, shortening contraction or stretching induced neither mechanical hyperalgesia nor NGF upregulation. Bradykinin together with shortening contraction, but not bradykinin alone, reproduced lasting mechanical hyperalgesia. We also showed that rat NGF sensitized thin-fiber afferents to mechanical stimulation in the periphery after 10 -20 min. Thus, NGF upregulation through activation of B 2 bradykinin receptors is essential (though not satisfactory) to mechanical hyperalgesia after exercise. The present observations explain why DOMS occurs with a delay, and why lengthening contraction but not shortening contraction induces DOMS.

Key points• Unaccustomed strenuous exercise that includes lengthening contraction often causes delayed onset muscle soreness (DOMS), characterised as muscular mechanical hyperalgesia.• It has been reported that bradykinin triggers upregulation of nerve growth factor in exercised muscle, sensitizing nociceptors and resulting in DOMS, but additional mechanism(s) may be involved.• We showed that pretreatment with cyclooxygenase (COX)-2 inhibitors completely suppressed the development of DOMS, but treatment 2 days after lengthening contraction failed to reverse existing mechanical hyperalgesia.• We demonstrated that COX-2 induced upregulation of glial cell line-derived neurotrophic factor (GDNF) and that intramuscularly injected anti-GDNF antibody reduced muscle mechanical hyperalgesia after exercise.• These results suggest that upregulation of GDNF through COX-2 activation is essential to mechanical hyperalgesia after exercise, and is another pathway alongside the bradykinin-nerve growth factor pathway that is involved in DOMS development.Abstract Unaccustomed strenuous exercise that includes lengthening contraction (LC) often causes delayed onset muscle soreness (DOMS), characterised as muscular mechanical hyperalgesia. Previously we reported that a bradykinin-like substance released from the muscle during exercise plays a pivotal role in triggering the process of muscular mechanical hyperalgesia by upregulating nerve growth factor (NGF) in exercised muscle of rats. We show here that cyclooxygenase (COX)-2 and glial cell line-derived neurotrophic factor (GDNF) are also involved in DOMS. COX-2 inhibitors but not COX-1 inhibitors given orally before LC completely suppressed the development of DOMS, but when given 2 days after LC they failed to reverse the mechanical hyperalgesia. COX-2 mRNA and protein in exercised muscle increased six-to 13-fold in mRNA and 1.7-2-fold in protein 0-12 h after LC. COX-2 inhibitors did not suppress NGF upregulation after LC. Instead, we found GDNF mRNA was upregulated seven-to eight-fold in the exercised muscle 12 h-1 day after LC and blocked by pretreatment of COX-2 inhibitors. In situ hybridisationThe authors E. Terazawa and K. Hirate passed away from illness during the course of this experiment (on 13 studies revealed that both COX-2 and GDNF mRNA signals increased at the periphery of skeletal muscle cells 12 h after LC. The accumulation of COX-2 mRNA signals was also observed in small blood vessels. Intramuscular injection of anti-GDNF antibody 2 days after LC partly reversed DOMS. Based on these findings, we conclude that GDNF upregulation through COX-2 activation is essential to mechanical hyperalgesia after exercise.

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