Papers of the Week

Therapeutic ultrasound (tUS) is widely used in chronic muscle pain control. However, its analgesic molecular mechanism is still not known. Our objective is to reveal the mechanism of the tUS-induced analgesia in mouse models of fibromyalgia. We applied tUS in mice that have developed chronic hyperalgesia induced by intramuscular acidification and determined the tUS frequency at 3MHz, dosage at 1W/cm (measured output as 6.3 mW/cm) and 100% duty cycle for 3 min having the best analgesic effect. Pharmacological and genetic approaches were used to probe the molecular determinants involved in tUS-mediated analgesia. A second mouse model of fibromyalgia induced by intermittent cold stress (ICS) was further used to validate the mechanism underlying the tUS-mediated analgesia. The tUS-mediated analgesia was abolished by a pretreatment of NK1 receptor antagonist-RP-67580 or knockout of substance P (Tac1). Besides, the tUS-mediated analgesia was abolished by ASIC3-selective antagonist APETx2 but not TRPV1-selective antagonist capsazepine, suggesting a role for ASIC3. Moreover, the tUS-mediated analgesia was attenuated by ASIC3-selective non-steroid anti-inflammation drugs (NSAIDs) – aspirin and diclofenac but not by ASIC1a-selective ibuprofen. We next validated the anti-nociceptive role of substance P signaling in the model induced by ICS, in which tUS-mediated analgesia was abolished in mice lacking substance P, NK1R, Asic1a, Asic2b, or Asic3 gene. tUS treatment could activate ASIC3-containing channels in muscle afferents to release substance P intramuscularly and exert an analgesic effect in mouse models of fibromyalgia. NSAIDs should be cautiously used or avoided in the tUS treatment. PERSPECTIVE: Therapeutic ultrasound showed analgesic effects against chronic mechanical hyperalgesia in the mouse model of fibromyalgia through the signaling pathways involving substance P and ASIC3-containing ion channels in muscle afferents. NSAIDs should be cautiously used during tUS treatment.