Acid mediates a prolonged antinociception via substance P signaling in acid-induced chronic widespread pain
© Chen and Chen; licensee BioMed Central Ltd. 2014
Received: 20 March 2014
Accepted: 10 April 2014
Published: 21 May 2014
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.
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.
Acid could mediate a prolonged antinociceptive signaling via the release of substance P from muscle afferent neurons in a non-ASIC3, non-TRPV1 manner.
KeywordsAntinociception ASIC3 TRPV1 Muscle pain Nociceptor
Substance P (SP) is a neuropeptide released from nociceptors to mediate pain transmission centrally and neurogenic inflammation peripherally [1, 2]. Our recent study showed that release of SP from muscle nociceptive nerve fibers was antinociceptive in a mouse model of acid-induced chronic widespread pain . The unexpected antinociceptive role of SP was found exclusively in acid-sensing ion channel 3 (ASIC3)-expressing muscle nociceptors: SP acts on neurokinin 1 (NK1) receptors to reduce acid-induced depolarization via activation of an M-type potassium channel that is G protein-independent but tyrosine kinase-dependent. The inhibitory effect of SP could prevent the development of chronic widespread pain induced by repeated intramuscular acid insults.
Acid is effective in causing chronic muscle pain via activation of ASIC3 or transient receptor potential V1 (TRPV1) in muscle nociceptors [4–8]. The mouse model of acid-induced chronic widespread muscle pain was developed to mimic the clinical symptoms of fibromyalgia, exhibiting both chronic widespread pain and autonomic dysfunction [4, 9]. In this model, 2 injections (separated by 2 to 5 days) of pH4.0 acid saline to one side of the gastrocnemius muscle caused bilateral, long-lasting referred hyperalgesia in mouse hind paws. The first acid injection produces only transient hyperalgesia that diminished in 24 h, and the second acid injection 2 to 5 days later to the same muscle causes long-lasting hyperalgesia that lasts for more than 4 weeks. In mice lacking SP signaling by genetic ablation or pharmacological blockade in muscle, a single intramuscular acid injection is enough to produce long-lasting hyperalgesia . In contrast, an additional boost of SP with the second acid injection prevents the development of chronic widespread pain in wild-type mice that received the first acid injection.
The physiological mechanism underlying the SP-mediated inhibition of ASIC3 signaling might be therapeutically useful, because application of an NK1 agonist prevented the development of long-lasting hyperalgesia induced by a second acid injection; however, the beneficial effect of endogenous SP release was lost during the second acid challenge .
Here, we aimed to probe possible ways to trigger and enhance SP analgesia in the mouse model of chronic widespread pain.
Although acid is an effective algogen causing pain, especially muscle pain [16, 17], we report here an antinocieptive role for acid in a mouse model of chronic widespread pain. This acid-mediated antinociceptive pathway might act via a subset of acid-sensitive muscle afferent neurons that are neither ASIC3- nor TRPV1-positive to release SP. This finding expands our recent knowledge of SP antinociceptive signaling, which could have a prolonged inhibitory effect, lasting for 2 days, on muscle nociceptors.
Previously, we found that SP activated M-type potassium channels in muscle nociceptors in a G protein-independent manner and thus inhibited the ASIC3-induced inward current. The inhibitory effect of SP was reproduced in vivo, with abolished acid-induced chronic widespread pain on co-injection of SP with acid saline into the gastrocnemius muscle . In the acid-induced chronic widespread pain model, acid signaling seems to dominantly activate ASIC3 or TRPV1 to evoke a referred hyperalgesia in mouse hind paws and nociceptor priming [4, 8]. The results of the current study suggest that acid also activates receptors or ion channels in muscle afferent neurons that express neither ASIC3 nor TRPV1 to elicit the release of SP and limit the hyperalgesia to a transient phase. The possible acid sensors might be other members of the ASIC family (e.g., ASIC1a and ASIC2), which are also expressed in muscle afferent neurons . Alternatively, acid might activate a group of proton-sensing G protein-coupled receptors (e.g., G2A, GPR4, OGR1, and TDAG8) that are functionally coupled with ASIC3 [18–20]. Future studies should identify the molecular identity of the antinociceptive acid sensor(s), which will shed new insight for the development of analgesic drugs targeting chronic widespread muscle pain, such as in fibromyalgia.
We found that muscular SP signaling, endogenously released from muscle afferent neurons by acid challenge or exogenously injected, could silence muscle nociceptors from further firing in 2 days. How the G-protein-independent signaling with SP antinociception could last for 2 days remains unknown. The G protein-independent SP/NK1 signaling in muscle nociceptors might be as complicated as its G protein-dependent pathway in other cell types [21, 22]. Nevertheless, this knowledge is clinically useful because we can then develop strategies to prevent the development of chronic muscle pain caused by repeated muscle injury or ischemic insult.
Material and methods
We used male C57/BL6 mice (8 to 12 weeks old). All procedures were approved by the Institutional Animal Care and Use Committee of Academia Sinica and followed the Guide for the Use of Laboratory Animals (National Academy Press, Washington, DC). Mice lacking tachykinin 1, the gene encoding SP (Tac1 −/− ), were generated as previously described . Tac1 +/+ and Tac1 −/− mice were offspring of congenic C57BL6 Tac1 +/− intercrosses.
The mouse model of acid-induced chronic widespread pain induced by repeated intramuscular acid injection was as described , with modification, and described previously . Briefly, mice received an intramuscular injection in the gastrocnemius muscle of acid containing 20 μL acid saline (pH 4.0) with or without APETx2 (20 pmol; Alomone, Jerusalem, Israel) and capsazepine (1 nmol; Torcis, Avonmouth, UK). Mechanical hyperalgesia of hind paws was measured by application of a 0.2-mN von Frey filament. The experimenter conducting the von Frey test had no information about the mouse genotypes or drug injections. In some studies, mice received pretreatment with an intramuscular injection of pH 7.4 saline or 40 μM [Sar9,Met(O2)11]-substance P (SM-SP; Sigma, St. Louis, MO) at 1 or 5 days before the acid injection.
The Mann–Whitney U test was used to compare withdrawal responses to the von Frey filament application in mice before acid or pH 7.4 saline injection (baseline) and at each time after intramuscular injection of acid (with or without drugs). P < 0.05 was considered statistically significant.
Acid-sensing ion channel 3
Transient receptor potential V1.
This work was supported by Institute of Biomedical Sciences, Academia Sinica, and the Ministry of Science and Technology, Taiwan (NSC102-2325-001-042, NSC102-2321-B-001-056, and NSC102-2320-B-001-021-MY3).
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