Involvement of interaction of Cav3.2 and nociceptive TRPA1 in pathological pain transmission

T-type Ca2+ channels and TRPA1 expressed in sensory neurons are involved in pain. We previously demonstrated a functional interaction of these channels under physiological conditions. Here we investigated the possible involvement of these channels in inflammatory pain condition. We also evaluated the relationship of these channels endogenously expressed in RIN-14B, a rat pancreatic islet tumor cell line. In dorsal root ganglion (DRG) neurons innervated inflammatory side, [Ca2+]i increases induced by 15 mM KCl (15K) were enhanced in neurons responded to AITC. This enhancement was not observed in genetically TRPA1-deficient neurons. The T-type and AITC-induced currents were larger in neurons of the inflammatory side than in those of the control one. In DRGs of the inflammatory side, the protein expression of Cav3.2, but not TRPA1, was increased. In RIN-14B, 15K-induced [Ca2+]i increases were decreased by blockers of T-type Ca2+ channel and TRPA1, and by TRPA1-silencing. Immunoprecipitation suggested the coexistent of these channels in sensory neurons and RIN-14B. In mice with inflammation, mechanical hypersensitivity was suppressed by blockers of both channels. These data suggest that the interaction of Cav3.2 with TRPA1 in sensory neurons is enhanced via the augmentation of the activities of both channels under inflammatory conditions, indicating that both channels are therapeutic targets for inflammatory pain.

Slack Potassium Channels Modulate TRPA1-Mediated Nociception in Sensory Neurons

The transient receptor potential (TRP) ankyrin type 1 (TRPA1) channel is highly expressed in a subset of sensory neurons where it acts as an essential detector of painful stimuli. However, the mechanisms that control the activity of sensory neurons upon TRPA1 activation remain poorly understood. Here, using in situ hybridization and immunostaining, we found TRPA1 to be extensively co-localized with the potassium channel Slack (K_Na1.1, Slo2.2, or Kcnt1) in sensory neurons. Mice lacking Slack globally (Slack^−/−) or conditionally in sensory neurons (SNS-Slack^−/−) demonstrated increased pain behavior after intraplantar injection of the TRPA1 activator allyl isothiocyanate. By contrast, pain behavior induced by the TRP vanilloid 1 (TRPV1) activator capsaicin was normal in Slack-deficient mice. Patch-clamp recordings in sensory neurons and in a HEK cell line transfected with TRPA1 and Slack revealed that Slack-dependent potassium currents (I_KS) are modulated in a TRPA1-dependent manner. Taken together, our findings highlight Slack as a modulator of TRPA1-mediated, but not TRPV1-mediated, activation of sensory neurons.

The effects of vanilloid analogues structurally related to capsaicin on the transient receptor potential vanilloid 1 channel

Transient receptor potential vanilloid 1 (TRPV1) is known as a receptor of capsaicin, a spicy ingredient of chili peppers. It is also sensitive to a variety of pungent compounds and is involved in nociception. Here, we focused on the structural characteristics of capsaicin, and investigated whether vanillylmanderic acid (VMA), vanillic acid (VAcid), vanillyl alcohol (VAlc), vanillyl butyl ether (VBE), and vanillin, containing a vanillyl skeleton similar to capsaicin, affected the TRPV1 activities. For detection of TRPV1 activity, intracellular Ca²⁺ concentration ([Ca²⁺]_i) was measured in HEK 293 cells heterologously expressing mouse TRPV1 (mTRPV1-HEK) and in mouse sensory neurons. Except for vanillin, four vanilloid analogues dose-dependently increased [Ca²⁺]_i in mTRPV1-HEK. The solutions that dissolved VMA, VAcid and vanillin at high concentrations were acidic, whereas those of VAlc and VBE were neutral. Neutralized VAcid evoked [Ca²⁺]_i increases but neutralized VMA did not. Mutation of capsaicin-sensing sites diminished [Ca²⁺]_i responses to VAcid, VAlc and VBE. VAcid, VMA, and vanillin suppressed the activation of TRPV1 induced by capsaicin. VAcid and VMA also inhibited the acid-induced TRPV1 activation. In sensory neurons, VMA diminished TRPV1 activation by capsaicin or acids. The present data indicate that these structural characteristics of chemical compounds on TRPV1 may provide strategies for the development of novel analgesic drugs targeting nociceptive TRPV1.

Inhibitory effect of polysulfide, an endogenous sulfur compound, on oxidative stress-induced TRPA1 activation

Polysulfide (PS), an endogenous sulfur compound, generated by oxidation of hydrogen sulfide, has a stimulatory action on the nociceptive TRPA1 channel. TRPA1 is also activated by reactive oxygen species such as hydrogen peroxide (H₂O₂) produced during inflammation. Here, we examined the effect of PS on H₂O₂-induced responses in native and heterologously expressed TRPA1 using a cell-based calcium assay. We also carried out behavioral experiments in vivo. In mouse sensory neurons, H₂O₂ elicited early TRPA1-dependent and late TRPA1-independent increases of [Ca²⁺]_i. The former was suppressed by the pretreatment with PS. In cells heterologously expressed TRPA1, PS suppressed [Ca²⁺]_i responses to H₂O₂. Simultaneous measurement of [Ca²⁺]_i and the intracellular PS level revealed that scavenging effect of PS was not related to the inhibitory effect. Removal of extracellular Ca²⁺, a calmodulin inhibitor and dithiothreitol attenuated the inhibitory effect of PS. Pretreatment with PS diminished nociceptive behaviors induced by H₂O₂. The present data suggest that PS suppresses oxidative stress-induced TRPA1 activation due to cysteine modification and Ca²⁺/calmodulin signaling. Thus, endogenous sulfurs may have regulatory roles in nociception via functional changes in TRPA1 under inflammatory conditions.

T-type calcium channel blockade induces apoptosis in C2C12 myotubes and skeletal muscle via endoplasmic reticulum stress activation

Loss of T‐type calcium channel (TCC) function has been reported to result in decreased cell viability and impaired muscle regeneration, but the underlying mechanisms remain largely unknown. We previously found that expression of TCC is reduced in aged pelvic floor muscle of multiple vaginal delivery mice, and this is related to endoplasmic reticulum stress (ERS) activation and autophagy flux blockade. In the present work, we further investigated the effects of TCC function loss on C2C12 myotubes and skeletal muscle, which is mediated by promotion of ERS and ultimately contributes to mitochondrial‐related apoptotic cell death. We found that application of a TCC inhibitor induced mitochondria‐related apoptosis in a dose‐dependent manner and also reduced mitochondrial transmembrane potential (MMP), induced mito‐ROS generation, and enhanced expression of mitochondrial apoptosis proteins. Functional inhibition of TCC induced ERS, resulting in disorder of Ca²⁺ homeostasis in endoplasmic reticulum, and ultimately leading to cell apoptosis in C2C12 myotubes. Tibialis anterior muscles of T‐type α1H channel knockout mice displayed a smaller skeletal muscle fiber size and elevated ERS‐mediated apoptosis signaling. Our data point to a novel mechanism whereby TCC blockade leads to ERS activation and terminal mitochondrial‐related apoptotic events in C2C12 myotubes and skeletal muscles.

Peripheral Pain Modulation of Chrysaora pacifica Jellyfish Venom Requires Both Ca2+ Influx and TRPA1 Channel Activation in Rats

The venom of jellyfish triggers severe dermal pain along with inflammation and tissue necrosis, and occasionally, induces internal organ dysfunction. However, the basic mechanisms underlying its cytotoxic effects are still unknown. Here, we report one of the mechanisms involved in peripheral pain modulation associated with inflammatory and neurotoxic oxidative signaling in rats using the venom of jellyfish, Chrysaora pacifica (CpV). This jellyfish is identified by brown tentacles carrying nematocysts filled with cytotoxic venom that induces severe pain, pruritus, tentacle marks, and blisters. The subcutaneous injection of CpV into rat forepaws in behavioral tests triggered nociceptive response with a decreased threshold for mechanical pain perception. These responses lasted up to 48 h and were completely blocked by verapamil and TTA-P2, T-type Ca²⁺ channel blockers, or HC030031, a transient receptor potential cation ankyrin 1 (TRPA1) channel blocker, while another Ca²⁺ channel blocker, nimodipine, was ineffective. Also, treatment with Ca²⁺ chelators (EGTA and BaptaAM) significantly alleviated the CpV-induced pain response. These results indicate that CpV-induced pain modulation may require both Ca²⁺ influx through the T-type Ca²⁺ channels and activation of TRPA1 channels. Furthermore, CpV induced Ca²⁺-mediated oxidative neurotoxicity in the dorsal root ganglion (DRG) and cortical neurons dissociated from rats, resulting in decreased neuronal viability and increased intracellular levels of ROS. Taken together, CpV may activate Ca²⁺-mediated oxidative signaling to produce excessive ROS acting as an endogenous agonist of TRPA1 channels in the peripheral terminals of the primary afferent neurons, resulting in persistent inflammatory pain. These findings provide strong evidence supporting the therapeutic effectiveness of blocking oxidative signaling against pain and cytotoxicity induced by jellyfish venom.

Mammalian Transient Receptor Potential TRPA1 Channels: From Structure to Disease

The transient receptor potential ankyrin (TRPA) channels are Ca²⁺-permeable nonselective cation channels remarkably conserved through the animal kingdom. Mammals have only one member, TRPA1, which is widely expressed in sensory neurons and in non-neuronal cells (such as epithelial cells and hair cells). TRPA1 owes its name to the presence of 14 ankyrin repeats located in the NH₂ terminus of the channel, an unusual structural feature that may be relevant to its interactions with intracellular components. TRPA1 is primarily involved in the detection of an extremely wide variety of exogenous stimuli that may produce cellular damage. This includes a plethora of electrophilic compounds that interact with nucleophilic amino acid residues in the channel and many other chemically unrelated compounds whose only common feature seems to be their ability to partition in the plasma membrane. TRPA1 has been reported to be activated by cold, heat, and mechanical stimuli, and its function is modulated by multiple factors, including Ca²⁺, trace metals, pH, and reactive oxygen, nitrogen, and carbonyl species. TRPA1 is involved in acute and chronic pain as well as inflammation, plays key roles in the pathophysiology of nearly all organ systems, and is an attractive target for the treatment of related diseases. Here we review the current knowledge about the mammalian TRPA1 channel, linking its unique structure, widely tuned sensory properties, and complex regulation to its roles in multiple pathophysiological conditions.