Low-level laser therapy (LLLT) is widely used in pain control in the field of physical medicine and rehabilitation and is effective for fibromyalgia pain. However, its analgesic mechanism remains unknown. A possible mechanism for the effect of LLLT on fibromyalgia pain is via the antinociceptive signaling of substance P in muscle nociceptors, although the neuropeptide has been known as a neurotransmitter to facilitate pain signals in the spinal cord.

To establish an animal model of LLLT in chronic muscle pain and to determine the role of substance P in LLLT analgesia.

We employed the acid-induced chronic muscle pain model, a fibromyalgia model proposed and developed by Sluka et al., and determined the optimal LLLT dosage.

LLLT with 685 nm at 8 J/cm² was effective to reduce mechanical hyperalgesia in the chronic muscle pain model. The analgesic effect was abolished by pretreatment of NK1 receptor antagonist RP-67580. Likewise, LLLT showed no analgesic effect on Tac1^-/- mice, in which the gene encoding substance P was deleted. Besides, pretreatment with the TRPV1 receptor antagonist capsazepine, but not the ASIC3 antagonist APETx2, blocked the LLLT analgesic effect.

LLLT analgesia is mediated by the antinociceptive signaling of intramuscular substance P and is associated with TRPV1 activation in a mouse model of fibromyalgia or chronic muscle pain. The study results could provide new insight regarding the effect of LLLT in other types of chronic pain.