In vivo patch-clamp analysis of the antinociceptive actions of TRPA1 activation in the spinal dorsal horn

Remote neurostimulation through an endogenous ion channel using a near-infrared light-activatable nanoagonist

The development of noninvasive approaches to precisely control neural activity in mammals is highly desirable. Here, we used the ion channel transient receptor potential ankyrin-repeat 1 (TRPA1) as a proof of principle, demonstrating remote near-infrared (NIR) activation of endogenous neuronal channels in mice through an engineered nanoagonist. This achievement enables specific neurostimulation in nongenetically modified mice. Initially, target-based screening identified flavins as photopharmacological agonists, allowing for the photoactivation of TRPA1 in sensory neurons upon ultraviolet A/blue light illumination. Subsequently, upconversion nanoparticles (UCNPs) were customized with an emission spectrum aligned to flavin absorption and conjugated with flavin adenine dinucleotide, creating a nanoagonist capable of NIR activation of TRPA1. Following the intrathecal injection of the nanoagonist, noninvasive NIR stimulation allows precise bidirectional control of nociception in mice through remote activation of spinal TRPA1. This study demonstrates a noninvasive NIR neurostimulation method with the potential for adaptation to various endogenous ion channels and neural processes by combining photochemical toolboxes with customized UCNPs.

Methylglyoxal activates transient receptor potential A1/V1 via reactive oxygen species in the spinal dorsal horn

Methylglyoxal (MGO), a highly reactive dicarbonyl metabolite of glucose primarily formed during the glycolytic pathway, is a precursor of advanced glycation end-products (AGEs). Recently, numerous studies have shown that MGO accumulation can cause pain and hyperalgesia. However, the mechanism through which MGO induces pain in the spinal dorsal horn remains unclear. The present study investigated the effect of MGO on spontaneous excitatory postsynaptic currents (sEPSC) in rat spinal dorsal horn neurons using blind whole-cell patch-clamp recording. Perfusion of MGO increased the frequency and amplitude of sEPSC in spinal horn neurons in a concentration-dependent manner. Additionally, MGO administration increased the number of miniature EPSC (mEPSC) in the presence of tetrodotoxin, a sodium channel blocker. However, 6-cyano-7-nitroqiunocaline-2,3-dione (CNQX), an AMPA/kainate receptor antagonist, blocked the enhancement of sEPSC by MGO. HC-030031, a TRP ankyrin-1 (TRPA1) antagonist, and capsazepine, a TRP vanilloid-1 (TRPV1) antagonist, inhibited the action of MGO. Notably, the effects of MGO were completely inhibited by HC-030031 and capsazepine. MGO generates reactive oxygen species (ROS) via AGEs. ROS also potentially induce pain via TRPA1 and TRPV1 in the spinal dorsal horn. Furthermore, we examined the effect of MGO in the presence of N-tert-butyl-α-phenylnitrone (PBN), a non-selective ROS scavenger, and found that the effect of MGO was completely inhibited. These results suggest that MGO increases spontaneous glutamate release from the presynaptic terminal to spinal dorsal horn neurons through TRPA1, TRPV1, and ROS and could enhance excitatory synaptic transmission.

Transient receptor potential ankyrin 1 in the knee is involved in osteoarthritis pain

Transient receptor potential families play important roles in the pathology of osteoarthritis (OA) of the knee. While transient receptor potential ankyrin 1 (TRPA1) is also an essential component of the pathogenesis of various arthritic conditions, its association with pain is controversial. Thus, we researched whether TRPA1 is involved in knee OA pain by in vivo patch-clamp recordings and evaluated the behavioral responses using CatWalk gait analysis and pressure application measurement (PAM). Injection of the Trpa1 agonist, allyl isothiocyanate (AITC), into the knee joint significantly increased spontaneous excitatory synaptic current (sEPSC) frequency in the substantia gelatinosa of rats with knee OA, while injection of the Trpa1 antagonist, HC-030031, significantly decreased the sEPSC. Meanwhile, AITC did not affect the sEPSC in sham rats. In the CatWalk and PAM behavioral tests, AITC significantly decreased pain thresholds, but no difference between HC-030031 and saline injections was observed. Our results indicate that Trpa1 mediates knee OA-induced pain. We demonstrated that Trpa1 is activated in the knee joints of rats with OA, and Trpa1 activity enhanced the pain caused by knee OA.

Encoding of inflammatory hyperalgesia in mouse spinal cord

ABSTRACT

Inflammation modifies the input-output properties of peripheral nociceptive neurons such that the same stimulus produces enhanced nociceptive firing. This increased nociceptive output enters the superficial dorsal spinal cord (SDH), an intricate neuronal network composed largely of excitatory and inhibitory interneurons and a small percentage of projection neurons. The SDH network comprises the first central nervous system network integrating noxious information. Using in vivo calcium imaging and a computational approach, we characterized the responsiveness of the SDH network in mice to noxious stimuli in normal conditions and investigated the changes in SDH response patterns after acute burn injury-induced inflammation. We show that the application of noxious heat stimuli to the hind paw of naïve mice results in an overall increase in SDH network activity. Single-cell response analysis reveals that 70% of recorded neurons increase or suppress their activity, while ∼30% of neurons remain nonresponsive. After acute burn injury and the development of inflammatory hyperalgesia, application of the same noxious heat stimuli leads to the activation of previously nonresponding neurons and desuppression of suppressed neurons. We further demonstrate that an increase in afferent activity mimics the response of the SDH network to noxious heat stimuli under inflammatory conditions. Using a computational model of the SDH network, we predict that the changes in SDH network activity result in overall increased activity of excitatory neurons, amplifying the output from SDH to higher brain centers. We suggest that during acute local peripheral inflammation, the SDH network undergoes dynamic changes promoting hyperalgesia.

Paclitaxel Regulates TRPA1 Function and Expression Through PKA and PKC

Paclitaxel (PTX) is a frequently used anticancer drug that causes peripheral neuropathy. Transient receptor potential ankyrin 1 (TRPA1), a plasma membrane calcium channel, has been associated with PTX toxicity and with other chemotherapy agents such as oxaliplatin and vincristine. However, the effect of PTX on the functional expression and calcium currents of TRPA1 has not been determined. The present study shows the effect of PTX on TRPA1 activity in a neuronal cell line (SH-SY5Y). The effect of PTX on the expression of TRPA1 was assessed through quantitative PCR and Western blot analyses to determine the relative mRNA and protein expression levels. To assess the effect on calcium flux and currents, cells were exposed to PTX; simultaneously, a specific agonist and antagonist of TRPA1 were added to evaluate the differential response in exposed versus control cells. To assess the effect of PKA, PKC and PI3K on PTX-induced TRPA1 increased activity, selective inhibitors were added to these previous experiments. PTX increased the mRNA and protein expression of TRPA1 as well as the TRPA1-mediated Ca2+ currents and intracellular Ca2+ concentrations. This effect was dependent on AITC (a selective specific agonist) and was abolished with HC-030031 (a selective specific antagonist). The inhibition of PKA and PKC reduced the effect of PTX on the functional expression of TRPA1, whereas the inhibition of PI3K had no effects. PTX-induced neuropathy involves TRPA1 activity through an increase in functional expression and is regulated by PKA and PKC signaling. These findings support the role of the TRPA1 channel in the mechanisms altered by PTX, which can be involved in the process that lead to chemotherapy-induced neuropathy.

Lack of transient receptor potential ankyrin 1 (TRPA1) retards cutaneous wound healing in mice: A preliminary study

Wound healing is an important process in various diseases, and elucidating the underlying mechanism is essential for developing therapeutic strategies. We investigated whether the loss of transient receptor potential ankyrin 1 (TRPA1) affects the cutaneous wound healing process in mice. We assessed the formation of granulation tissue by myofibroblasts and macrophages, re-epithelialization, and related gene expression. TRPA1-null (KO) and wild-type (WT) C57BL/6 mice were used for establishing the wound model. Two round full-thickness excision wounds (diameter, 5.0 mm) were produced in the dorsal skin of mice under general anesthesia. After specific intervals, healing was evaluated using macroscopic observation, histology, immunohistochemistry, and real-time reverse transcription-polymerase chain reaction (RT-PCR). TRPA1 KO retarded the formation of granulation tissue and re-epithelialization in the healing of cutaneous wound. Furthermore, TRPA1 KO suppressed the appearance of myofibroblasts, macrophage infiltration, and mRNA expression of αSMA, F4/80, and Col-1α2. These findings indicate that TRPA1 is required for cutaneous wound healing in mice. The lack of TRPA1 retards macrophage infiltration and the subsequent fibrotic tissue formation, which might further impair the fibrogenic behavior of fibroblasts.

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Transient receptor potential ankyrin 1 (TRPA1) is essential for wound healing.

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Early cutaneous wound healing was delayed in TRPA-1-deficient mice.

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Loss of TRPA1 retards formation of new granulation tissue in the wound.

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Lack of TRPA1 inhibits macrophage infiltration and myofibroblast formation.

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TRPA1 deficiency inhibits expression of TGF-β1, αSMA, F4/80, and collagen 1α2 mRNA.

Anti-allodynic and promotive effect on inhibitory synaptic transmission of riluzole in rat spinal dorsal horn

Riluzole (2-amino-6-(trifluoromethoxy)benzothiazole) is a drug known for its inhibitory effect on glutamatergic transmission and its anti-nociceptive and anti-allodynic effects in neuropathic pain rat models. Riluzole also has an enhancing effect on GABAergic synaptic transmission. However, the effect on the spinal dorsal horn, which plays an important role in modulating nociceptive transmission, remains unknown. We investigated the ameliorating effect of riluzole on mechanical allodynia using the von Frey test in a rat model of neuropathic pain and analyzed the synaptic action of riluzole on inhibitory synaptic transmission in substantia gelatinosa (SG) neurons using whole-cell patch clamp recordings. We found that single-dose intraperitoneal riluzole (4 mg/kg) administration effectively attenuated mechanical allodynia in the short term in a rat model of neuropathic pain. Moreover, 300 μM riluzole induced an outward current in rat SG neurons. The outward current induced by riluzole was not suppressed in the presence of tetrodotoxin. Furthermore, we found that the outward current was suppressed by simultaneous bicuculline and strychnine application, but not by strychnine alone. Altogether, these results suggest that riluzole enhances inhibitory synaptic transmission monosynaptically by potentiating GABAergic synaptic transmission in the rat spinal dorsal horn.

Emerging Perspectives on Pain Management by Modulation of TRP Channels and ANO1

Receptor-type ion channels are critical for detection of noxious stimuli in primary sensory neurons. Transient receptor potential (TRP) channels mediate pain sensations and promote a variety of neuronal signals that elicit secondary neural functions (such as calcitonin gene-related peptide [CGRP] secretion), which are important for physiological functions throughout the body. In this review, we focus on the involvement of TRP channels in sensing acute pain, inflammatory pain, headache, migraine, pain due to fungal infections, and osteo-inflammation. Furthermore, action potentials mediated via interactions between TRP channels and the chloride channel, anoctamin 1 (ANO1), can also generate strong pain sensations in primary sensory neurons. Thus, we also discuss mechanisms that enhance neuronal excitation and are dependent on ANO1, and consider modulation of pain sensation from the perspective of both cation and anion dynamics.

Action potentials and subthreshold potentials of dorsal horn neurons in a rat model of myositis: a study employing intracellular recordings in vivo

Intracellular in vivo recordings from rat dorsal horn neurons were made to study the contribution of microglia to the central sensitization of spinal synapses induced by a chronic muscle inflammation. To block microglia activation, minocycline was continuously administered intrathecally during development of the inflammation. The aim was to test whether an inflammation-induced sensitization of dorsal horn neurons is mediated by changes in synaptic strength or other synaptic changes and how activated microglia influence these processes. Intracellular recordings were used to measure subthreshold excitatory postsynaptic potentials (EPSPs) and suprathreshold action potentials (APs). The muscle inflammation significantly increased the proportion of dorsal horn neurons responding with APs or EPSPs to electrical stimulation of the muscle nerve from 27 to 56% (P < 0.01) and to noxious muscle stimulation (3 vs. 44%, P < 0.01). Neurons showing spontaneous ongoing AP or EPSP activity increased from 28 to 74% (P < 0.01). Generally, the increases in suprathreshold AP responses did not occur at the expense of subthreshold EPSPs, because EPSP-only responses also increased. Intrathecal minocycline prevented the inflammation-induced increase in responsiveness to electrical (24%, P < 0.02) and mechanical stimulation (14%, P < 0.02); the effect was stronger on suprathreshold APs than on subthreshold EPSPs. The increase in ongoing activity was only partly suppressed. These data suggest that the myositis-induced hypersensitivity of the dorsal horn neurons to peripheral input and its prevention by intrathecal minocycline treatment were due to both an increase in the number of active synapses and an increased synaptic strength.NEW & NOTEWORTHY During a chronic muscle inflammation (myositis), activated microglia controls both the increase in the number of active synapses and the increase in synaptic strength.

Reactive metabolites of acetaminophen activate and sensitize the capsaicin receptor TRPV1

The irritant receptor TRPA1 was suggested to mediate analgesic, antipyretic but also pro-inflammatory effects of the non-opioid analgesic acetaminophen, presumably due to channel activation by the reactive metabolites parabenzoquinone (pBQ) and N-acetyl-parabenzoquinonimine (NAPQI). Here we explored the effects of these metabolites on the capsaicin receptor TRPV1, another redox-sensitive ion channel expressed in sensory neurons. Both pBQ and NAPQI, but not acetaminophen irreversibly activated and sensitized recombinant human and rodent TRPV1 channels expressed in HEK 293 cells. The reducing agents dithiothreitol and N-acetylcysteine abolished these effects when co-applied with the metabolites, and both pBQ and NAPQI failed to gate TRPV1 following substitution of the intracellular cysteines 158, 391 and 767. NAPQI evoked a TRPV1-dependent increase in intracellular calcium and a potentiation of heat-evoked currents in mouse spinal sensory neurons. Although TRPV1 is expressed in mouse hepatocytes, inhibition of TRPV1 did not alleviate acetaminophen-induced hepatotoxicity. Finally, intracutaneously applied NAPQI evoked burning pain and neurogenic inflammation in human volunteers. Our data demonstrate that pBQ and NAQPI activate and sensitize TRPV1 by interacting with intracellular cysteines. While TRPV1 does not seem to mediate acetaminophen-induced hepatotoxicity, our data identify TRPV1 as a target of acetaminophen with a potential relevance for acetaminophen-induced analgesia, antipyresia and inflammation.

TRPA1 channels: molecular sentinels of cellular stress and tissue damage

TRPA1 is a non-selective cation channel expressed in mammalian peripheral pain receptors, with a major role in chemonociception. TRPA1 has also been implicated in noxious cold and mechanical pain sensation. TRPA1 has an ancient origin and plays important functions in lower organisms, including thermotaxis, mechanotransduction and modulation of lifespan. Here we highlight the role of TRPA1 as a multipurpose sensor of harmful signals, including toxic bacterial products and UV light, and as a sensor of stress and tissue damage. Sensing roles span beyond the peripheral nervous system to include major barrier tissues: gut, skin and lung. Tissue injury, environmental irritants and microbial pathogens are danger signals that can threaten the health of organisms. These signals lead to the coordinated activation of the nociceptive and the innate immune system to provide a homeostatic response trying to re-establish physiological conditions including tissue repair. Activation of TRPA1 participates in protective neuroimmune interactions at multiple levels, sensing ROS and bacterial products and triggering the release of neuropeptides. However, an exaggerated response to danger signals is maladaptive and can lead to the development of chronic inflammatory conditions.

Differential Activation of TRP Channels in the Adult Rat Spinal Substantia Gelatinosa by Stereoisomers of Plant-Derived Chemicals

Activation of TRPV1, TRPA1 or TRPM8 channel expressed in the central terminal of dorsal root ganglion (DRG) neuron increases the spontaneous release of l-glutamate onto spinal dorsal horn lamina II (substantia gelatinosa; SG) neurons which play a pivotal role in regulating nociceptive transmission. The TRP channels are activated by various plant-derived chemicals. Although stereoisomers activate or modulate ion channels in a distinct manner, this phenomenon is not fully addressed for TRP channels. By applying the whole-cell patch-clamp technique to SG neurons of adult rat spinal cord slices, we found out that all of plant-derived chemicals, carvacrol, thymol, carvone and cineole, increase the frequency of spontaneous excitatory postsynaptic current, a measure of the spontaneous release of l-glutamate from nerve terminals, by activating TRP channels. The presynaptic activities were different between stereoisomers (carvacrol and thymol; (-)-carvone and (+)-carvone; 1,8-cineole and 1,4-cineole) in the extent or the types of TRP channels activated, indicating that TRP channels in the SG are activated by stereoisomers in a distinct manner. This result could serve to know the properties of the central terminal TRP channels that are targets of drugs for alleviating pain.

Emerging Role of Spinal Cord TRPV1 in Pain Exacerbation

TRPV1 is well known as a sensor ion channel that transduces a potentially harmful environment into electrical depolarization of the peripheral terminal of the nociceptive primary afferents. Although TRPV1 is also expressed in central regions of the nervous system, its roles in the area remain unclear. A series of recent reports on the spinal cord synapses have provided evidence that TRPV1 plays an important role in synaptic transmission in the pain pathway. Particularly, in pathologic pain states, TRPV1 in the central terminal of sensory neurons and interneurons is suggested to commonly contribute to pain exacerbation. These observations may lead to insights regarding novel synaptic mechanisms revealing veiled roles of spinal cord TRPV1 and may offer another opportunity to modulate pathological pain by controlling TRPV1. In this review, we introduce historical perspectives of this view and details of the recent promising results. We also focus on extended issues and unsolved problems to fully understand the role of TRPV1 in pathological pain. Together with recent findings, further efforts for fine analysis of TRPV1's plastic roles in pain synapses at different levels in the central nervous system will promote a better understanding of pathologic pain mechanisms and assist in developing novel analgesic strategies.