Acid mediates a prolonged antinociception via substance P signaling in acid-induced chronic widespread pain

Abstract

Background: Substance P is an important neuropeptide released from nociceptors to mediate pain signals. We recently revealed antinociceptive signaling by substance P in acid-sensing ion channel 3 (ASIC3)-expressing muscle nociceptors in a mouse model of acid-induced chronic widespread pain. However, methods to specifically trigger the substance P antinociception were still lacking.

Results: Here we show that acid could induce antinociceptive signaling via substance P release in muscle. We prevented the intramuscular acid-induced hyperalgesia by pharmacological inhibition of ASIC3 and transient receptor potential V1 (TRPV1). The antinociceptive effect of non-ASIC3, non-TRPV1 acid signaling lasted for 2 days. The non-ASIC3, non-TRPV1 acid antinociception was largely abolished in mice lacking substance P. Moreover, pretreatment with substance P in muscle mimicked the acid antinociceptive effect and prevented the hyperalgesia induced by next-day acid injection.

Conclusions: Acid could mediate a prolonged antinociceptive signaling via the release of substance P from muscle afferent neurons in a non-ASIC3, non-TRPV1 manner.

Figure 1

Acid-induced prolonged antinociceptive signaling in muscle. The withdrawal responses of mouse hind paws to a 0.2-mN bending force in mice before and after intramuscular acid injection in the gastrocnemius muscle. (A) In the dual acid injection scheme, mice showed transient hyperalgesia after the first intramuscular acid injection and chronic hyperalgesia after a second acid injection spaced 5 days apart. (B) A mix of acid saline with APETx2 and capsazepine abolished the transient hyperalgesia with the first injection and prevented the development of chronic hyperalgesia induced by the second acid injection 5 days later. (C,D) With the first acid injection, acid induced a prolonged antinociceptive effect lasting for 2 days, with ASIC3 and TRPV1 blocked by APETx2 and capsazepine, respectively, and a second acid injection at day 2 could not induce any hyperalgesic effect. Black arrows indicate when mice received the intramuscular acid injection. Red arrows indicate when mice received the co-injection of acid with APETx2 and capsazepine. B, baseline on day 0; D, day; WT, wild-type mice; CZP, capsazepine. *P < 0.05 compared with the response at baseline.

Figure 2

Substance P mediates the acid-induced prolonged antinociception in acid-induced chronic widespread pain model. (A,B) APETx2 and capsazepine prevented acid-induced chronic hyperalgesia in tachykinin 1-positive (Tac1^+/+) mice (A) but not Tac1^−/− mice (B) when the second acid injection was given the next day. (C) Pretreatment with pH 7.4 saline did not affect the acid-induced hyperalgesia in wild-type mice. (D) Pretreatment with SM-SP abolished the acid-induced transient hyperalgesia the next day and prevented the development of chronic hyperalgesia induced by a second acid injection 5 days later. (E) Pretreatment with SM-SP 5 days before the acid injection did not affect the acid-induced transient and chronic hyperalgesia in the dual acid injection scheme. Black arrows indicate when mice received the intramuscular acid injection. Red arrows indicate when mice received the co-injection of acid with APETx2 and capsazepine. Green arrow indicates when mice received a pretreatment of pH 7.4 saline 1 day before the acid injection. Blue arrows indicate when mice received a pretreatment of SM-SP 1 or 5 days before acid injection. B, baseline on day 0; D, day; WT, wild-type mice; CZP, capsazepine; SM-SP, [Sar⁹,Met(O₂)¹¹]-substance P; Tac1, tachykinin 1. *P < 0.05 compared with the response at baseline.