Role of TRPV1 and ASIC3 in formalin-induced secondary allodynia and hyperalgesia

Mechanisms of Action of the Peptide Toxins Targeting Human and Rodent Acid-Sensing Ion Channels and Relevance to Their In Vivo Analgesic Effects

Acid-sensing ion channels (ASICs) are voltage-independent H⁺-gated cation channels largely expressed in the nervous system of rodents and humans. At least six isoforms (ASIC1a, 1b, 2a, 2b, 3 and 4) associate into homotrimers or heterotrimers to form functional channels with highly pH-dependent gating properties. This review provides an update on the pharmacological profiles of animal peptide toxins targeting ASICs, including PcTx1 from tarantula and related spider toxins, APETx2 and APETx-like peptides from sea anemone, and mambalgin from snake, as well as the dimeric protein snake toxin MitTx that have all been instrumental to understanding the structure and the pH-dependent gating of rodent and human cloned ASICs and to study the physiological and pathological roles of native ASICs in vitro and in vivo. ASICs are expressed all along the pain pathways and the pharmacological data clearly support a role for these channels in pain. ASIC-targeting peptide toxins interfere with ASIC gating by complex and pH-dependent mechanisms sometimes leading to opposite effects. However, these dual pH-dependent effects of ASIC-inhibiting toxins (PcTx1, mambalgin and APETx2) are fully compatible with, and even support, their analgesic effects in vivo, both in the central and the peripheral nervous system, as well as potential effects in humans.

Inhibiting BDNF/TrkB.T1 receptor improves resiniferatoxin-induced postherpetic neuralgia through decreasing ASIC3 signaling in dorsal root ganglia

BACKGROUND

Postherpetic neuralgia (PHN) is a devastating complication after varicella-zoster virus infection. Brain-derived neurotrophic factor (BDNF) has been shown to participate in the pathogenesis of PHN. A truncated isoform of the tropomyosin receptor kinase B (TrkB) receptor TrkB.T1, as a high-affinity receptor of BDNF, is upregulated in multiple nervous system injuries, and such upregulation is associated with pain. Acid-sensitive ion channel 3 (ASIC3) is involved in chronic neuropathic pain, but its relation with BDNF/TrkB.T1 in the peripheral nervous system (PNS) during PHN is unclear. This study aimed to investigate whether BDNF/TrkB.T1 contributes to PHN through regulating ASIC3 signaling in dorsal root ganglia (DRGs).

METHODS

Resiniferatoxin (RTX) was used to induce rat PHN models. Mechanical allodynia was assessed by measuring the paw withdrawal thresholds (PWTs). Thermal hyperalgesia was determined by detecting the paw withdrawal latencies (PWLs). We evaluated the effects of TrkB.T1-ASIC3 signaling inhibition on the behavior, neuronal excitability, and inflammatory response during RTX-induced PHN. ASIC3 short hairpin RNA (shRNA) transfection was used to investigate the effect of exogenous BDNF on inflammatory response in cultured PC-12 cells.

RESULTS

RTX injection induced mechanical allodynia and upregulated the protein expression of BDNF, TrkB.T1, ASIC3, TRAF6, nNOS, and c-Fos, as well as increased neuronal excitability in DRGs. Inhibition of ASIC3 reversed the abovementioned effects of RTX, except for BDNF and TrkB.T1 protein expression. In addition, inhibition of TrkB.T1 blocked RTX-induced mechanical allodynia, activation of ASIC3 signaling, and hyperexcitability of neurons. RTX-induced BDNF upregulation was found in both neurons and satellite glia cells in DRGs. Furthermore, exogenous BDNF activated ASIC3 signaling, increased NO level, and enhanced IL-6, IL-1β, and TNF-α levels in PC-12 cells, which was blocked by shRNA-ASIC3 transfection.

CONCLUSION

These findings demonstrate that inhibiting BDNF/TrkB.T1 reduced inflammation, decreased neuronal hyperexcitability, and improved mechanical allodynia through regulating the ASIC3 signaling pathway in DRGs, which may provide a novel therapeutic target for patients with PHN.

Therapeutic Effect of Tetrapanax papyriferus and Hederagenin on Chronic Neuropathic Pain of Chronic Constriction Injury of Sciatic Nerve Rats Based on KEGG Pathway Prediction and Experimental Verification

Background

Hederagenin is one of the main components of Tetrapanax papyriferus, and Tetrapanax papyriferus is one of the ingredients of Danggui Sini decoction. To explore whether Tetrapanax papyriferus and hederagenin can alleviate mechanical pain, thermal hyperalgesia, and cold pain at the same time, we comprehensively investigated the effects of two drugs on the levels of p38 MAPK phosphorylation, TRP proteins, and IL1β, IL6, and TNF-α in serum.

Methods

Firstly, we obtained pain-related targets and performed KEGG pathway enrichment on these targets. Then, 42 SD rats were separated randomly into six groups: sham operation group, CCI group, pregabalin group, mecobalamin group, Tetrapanax papyriferus group, and hederagenin group. All drugs were given orally. Rats in the sham operation group and CCI group were gavaged with saline. Rats in the pregabalin group were given pregabalin, while rats in the mecobalamin group were given mecobalamin. Rats in the Tetrapanax papyriferus group were given Tetrapanax papyriferus, while rats in the hederagenin group were given hederagenin. Besides, we conducted behavioral tests including acetone test, hot plate experiment, and von Frey filaments, and then dorsal root ganglion neurons were taken out on the 21st day after operation. Then, western blot, ELISA, and hematoxylin-eosin staining were conducted.

Results

Rats in the CCI group were more sensitive to hyperalgesia and allodynia to mechanical and thermal stimuli, as well as cold pain. All four drugs could relieve these pains. Pregabalin, mecobalamin, and Tetrapanax papyriferus can reduce the levels of IL1β, IL6, and TNF-α in serum compared to those of the CCI group. The expression of TRPM8, TRPA1, TRPV1, TRPV4, and phosphorylated p38 MAPK in DRG increased evidently on the 21st day after the operation in the CCI group. All four drugs could reduce the expressions of TRPM8, TRPA1, TRPV1, TRPV4, and phosphorylated p38 MAPK in dorsal root ganglion compared to those of the CCI group.

Conclusion

Tetrapanax papyriferus and hederagenin relieved sciatica by reducing inflammation levels, inhibiting p38 MAPK phosphorylation, and decreasing the levels of dorsal root ganglion proteins.

KChIP3 N-Terminal 31-50 Fragment Mediates Its Association with TRPV1 and Alleviates Inflammatory Hyperalgesia in Rats

Potassium voltage-gated channel interacting protein 3 (KChIP3), also termed downstream regulatory element antagonist modulator (DREAM) and calsenilin, is a multifunctional protein belonging to the neuronal calcium sensor (NCS) family. Recent studies revealed the expression of KChIP3 in dorsal root ganglion (DRG) neurons, suggesting the potential role of KChIP3 in peripheral sensory processing. Herein, we show that KChIP3 colocalizes with transient receptor potential ion channel V1 (TRPV1), a critical molecule involved in peripheral sensitization during inflammatory pain. Furthermore, the N-terminal 31-50 fragment of KChIP3 is capable of binding both the intracellular N and C termini of TRPV1, which substantially decreases the surface localization of TRPV1 and the subsequent Ca²⁺ influx through the channel. Importantly, intrathecal administration of the transmembrane peptide transactivator of transcription (TAT)-31-50 remarkably reduces Ca²⁺ influx via TRPV1 in DRG neurons and alleviates thermal hyperalgesia and gait alterations in a complete Freund's adjuvant-induced inflammatory pain model in male rats. Moreover, intraplantar injection of TAT-31-50 attenuated the capsaicin-evoked spontaneous pain behavior and thermal hyperalgesia, which further strengthened the regulatory role of TAT-31-50 on TRPV1 channel. In addition, TAT-31-50 could also alleviate inflammatory thermal hyperalgesia in kcnip3^-/- rats generated in our study, suggesting that the analgesic effect mediated by TAT-31-50 is independent of endogenous KChIP3. Our study reveals a novel peripheral mechanism for the analgesic function of KChIP3 and provides a potential analgesic agent, TAT-31-50, for the treatment of inflammatory pain.SIGNIFICANCE STATEMENT Inflammatory pain arising from inflamed or injured tissues significantly compromises the quality of life in patients. This study aims to elucidate the role of peripheral potassium channel interacting protein 3 (KChIP3) in inflammatory pain. Direct interaction of the KChIP3 N-terminal 31-50 fragment with transient receptor potential ion channel V1 (TRPV1) was demonstrated. The KChIP3-TRPV1 interaction reduces the surface localization of TRPV1 and thus alleviates heat hyperalgesia and gait alterations induced by peripheral inflammation. Furthermore, the transmembrane transactivator of transcription (TAT)-31-50 peptide showed analgesic effects on inflammatory hyperalgesia independently of endogenous KChIP3. This work reveals a novel mechanism of peripheral KChIP3 in inflammatory hyperalgesia that is distinct from its classical role as a transcriptional repressor in pain modulation.

Peripheral and spinal TRPA1 channels contribute to formalin-induced long-lasting mechanical hypersensitivity

Background

Transient receptor potential ankyrin 1 (TRPA1) is a non-selective cation channel expressed by a subset of nociceptive neurons that acts as a multimodal receptor. Its activity contributes to modulate nociceptive transmission in acute inflammatory pain. However, the role of this channel in chronic pain has been less studied. The purpose of this study was to investigate the local peripheral and spinal participation of TRPA1 channels in formalin-induced long-lasting hypersensitivity.

Materials and methods

Formalin (1%)-induced chronic hypersensitivity was determined by the application of von Frey filaments to ipsilateral and contralateral paws and through pharmacological testing using a selective TRPA1 blocker (A-967079). TRPA1 expression in the dorsal root ganglion (DRG) and spinal cord was analyzed by Western blotting.

Results

Formalin (1%) injection produced acute flinching behavior (1 h) as well as secondary allodynia and hyperalgesia (12 days). Local peripheral pretreatment (10 min before) or posttreatment (6 days later) with A-967079 (1–100 µM) partially prevented and reversed, respectively, in a dose-dependent manner, long-lasting secondary mechanical allodynia and hyperalgesia evoked by 1% formalin. Likewise, intrathecal pretreatment or posttreatment with A-967079 partially prevented and reversed, respectively, formalin-induced long-lasting hypersensitivity. A-967079 (100 µM) completely abolished the pro-nociceptive effect of formalin (adjusted to pH 7.4). Finally, formalin injection increased TRPA1 protein expression in the DRG and spinal cord.

Conclusion

Results indicate that TRPA1 expressed in the DRG and spinal cord plays a relevant role in formalin-induced long-lasting secondary nociceptive hypersensitivity.

Effects of creatine supplementation on nociception in young male and female mice

BACKGROUND

The objective of this study was to evaluate creatine as an anti-nociceptive compound in an animal model of thermal and inflammatory pain. Creatine has the structural potential to interact with acid-sensing ion channels (ASIC), which have been involved in pain sensation modulation. The hypothesis evaluated in this study was that creatine will interact with ASICs leading to decreased nociception.

METHODS

Male and female C57BL/6J mice were fed with either a control diet or the control diet supplemented with creatine (6.25 g/kg diet). After one week on the diet, the mice were tested for thermal hyperalgesia and inflammatory pain response.

RESULTS

The latency to withdraw the tail during the thermal hyperalgesia test was unaffected by sex or diet. During the formalin test, males and females responded differently to the stimulus, and the female mice supplemented with creatine seemed to recover faster than the controls. To determine whether ASICs mediate the action of creatine, GMQ, an ASIC3 agonist, was injected in one paw and pain response was quantified. Females responded more strongly to GMQ injections, and all mice fed creatine had a decreased response to GMQ.

CONCLUSIONS

These preliminary data suggest a potential effect of creatine on inflammation-based nociception that may be mediated via ASIC3. While preliminary, this study warrants further research on the potential of creatine as an analgesic and can serve as a stepping stone for the development of ASIC-based therapeutics.

Analgesic Effects of Danggui-Shaoyao-San on Various "Phenotypes" of Nociception and Inflammation in a Formalin Pain Model

Danggui-Shaoyao-San (DSS) is a traditional Chinese medicine, which has long been used for pain treatment and has been demonstrated to possess anti-oxidative, cognitive enhancement, and anti-depressant effects. In the present study, the effects of aqueous extracts of DSS on spontaneous pain behaviors and long-term hyperalgesia were examined to investigate the anti-nociceptive effects and underlying mechanisms. Single pretreatment of DSS dose-dependently reduced spontaneous flinches/licking time in the second, rather than the first, phase after subcutaneous injection of 5 % formalin into one hindpaw, in doses of 2.4 and 9.6 g/kg. DSS also dose-dependently inhibited FOS and cyclooxygenase-2 (COX-2) expression in both superficial and deep layers within the spinal dorsal horn. Further, DSS reduced hypoalgesia in the injected paw from 1 to 3 days and produced anti-hyperalgesic actions in both the injected paw after 3 days and non-injected paw. These data suggest involvement of enhancement of descending pain inhibition by suppression of 5-HTT levels in the spinal dorsal horn and reduction of peripheral long-term inflammation, including paw edema and ulcers. These findings suggest that DSS may be a useful therapeutic agent for short- and long-term inflammation induced pain, through both anti-inflammatory and suppression of central sensitization mechanisms.

Blockade of transient receptor potential cation channel subfamily V member 1 promotes regeneration after sciatic nerve injury

The transient receptor potential cation channel subfamily V member 1 (TRPV1) provides the sensation of pain (nociception). However, it remains unknown whether TRPV1 is activated after peripheral nerve injury, or whether activation of TRPV1 affects neural regeneration. In the present study, we established rat models of unilateral sciatic nerve crush injury, with or without pretreatment with AMG517 (300 mg/kg), a TRPV1 antagonist, injected subcutaneously into the ipsilateral paw 60 minutes before injury. At 1 and 2 weeks after injury, we performed immunofluorescence staining of the sciatic nerve at the center of injury, at 0.3 cm proximal and distal to the injury site, and in the dorsal root ganglia. Our results showed that Wallerian degeneration occurred distal to the injury site, and neurite outgrowth and Schwann cell regeneration occurred proximal to the injury. The number of regenerating myelinated and unmyelinated nerve clusters was greater in the AMG517-pretreated rats than in the vehicle-treated group, most notably 2 weeks after injury. TRPV1 expression in the injured sciatic nerve and ipsilateral dorsal root ganglia was markedly greater than on the contralateral side. Pretreatment with AMG517 blocked this effect. These data indicate that TRPV1 is activated or overexpressed after sciatic nerve crush injury, and that blockade of TRPV1 may accelerate regeneration of the injured sciatic nerve.