Targeting TRP channels for pain relief: A review of current evidence from bench to bedside.

Several decades of research support the involvement of transient receptor potential (TRP) channels in nociception. Despite the disappointments of early TRPV1 antagonist programs, the TRP family remains a promising therapeutic target in pain disorders. High-dose capsaicin patches are already in clinical use to relieve neuropathic pain. At present, localized injections of the side-directed TRPV1 agonist capsaicin and resiniferatoxin are undergoing clinical trials in patients with osteoarthritis and bone cancer pain. TRPA1, TRPM3, and TRPC5 channels are also of significant interest. This review discusses the role of TRP channels in human pain conditions.

Inhibition of Canonical Transient Receptor Potential Channels 4/5 with Highly Selective and Potent Small-Molecule HC-070 Alleviates Mechanical Hypersensitivity in Rat Models of Visceral and Neuropathic Pain.

Transient receptor potential channels C4/C5 are widely expressed in the pain pathway. Here, we studied the putative analgesic efficacy of the highly selective and potent TRPC4/C5 antagonist HC-070 in rats. Inhibitory potency on human TRPC4 was assessed by using the whole-cell manual patch-clamp technique. Visceral pain sensitivity was assessed by the colonic distension test after intra-colonic trinitrobenzene sulfonic acid injection and partial restraint stress. Mechanical pain sensitivity was assessed by the paw pressure test in the chronic constriction injury (CCI) neuropathic pain model. We confirm that HC-070 is a low nanomolar antagonist. Following single oral doses (3-30 mg/kg in male or female rats), colonic hypersensitivity was significantly and dose-dependently attenuated, even fully reversed to baseline. HC-070 also had a significant anti-hypersensitivity effect in the established phase of the CCI model. HC-070 did not have an effect on the mechanical withdrawal threshold of the non-injured paw, whereas the reference compound morphine significantly increased it. Analgesic effects are observed at unbound brain concentrations near the 50% inhibitory concentration (IC50) recorded in vitro. This suggests that analgesic effects reported here are brought about by TRPC4/C5 blocking in vivo. The results strengthen the idea that TRPC4/C5 antagonism is a novel, safe non-opioid treatment for chronic pain.

Targeting TRP Channels for Pain, Itch and Neurogenic Inflammation.

Transient receptor potential (TRP) channels are multifunctional signaling molecules with important roles in health and disease [...].

Spinal TRPA1 Contributes to the Mechanical Hypersensitivity Effect Induced by Netrin-1.

Netrin-1, a chemoattractant expressed by floor plate cells, and one of its receptors (deleted in colorectal cancer) has been associated with pronociceptive actions in a number of pain conditions. Here, we addressed the question of whether spinal TRPC4/C5 or TRPA1 are among the downstream receptors contributing to pronociceptive actions induced by netrin-1. The experiments were performed on rats using a chronic intrathecal catheter for administration of netrin-1 and antagonists of TRPC4/C5 or TRPA1. Pain sensitivity was assessed behaviorally by using mechanical and heat stimuli. Effect on the discharge rate of rostral ventromedial medullary (RVM) pain control neurons was studied in lightly anesthetized animals. Netrin-1, in a dose-related fashion, induced mechanical hypersensitivity that lasted up to three weeks. Netrin-1 had no effect on heat nociception. Mechanical hypersensitivity induced by netrin-1 was attenuated by TRPA1 antagonist Chembridge-5861528 and by the control analgesic compound pregabalin both during the early (first two days) and late (third week) phase of hypersensitivity. TRPC4/C5 antagonist ML-204 had a weak antihypersensitivity effect that was only in the early phase, whereas TRPC4/C5 antagonist HC-070 had no effect on hypersensitivity induced by netrin-1. The discharge rate in pronociceptive ON-like RVM neurons was increased by netrin-1 during the late but not acute phase, whereas netrin-1 had no effect on the discharge rate of antinociceptive RVM OFF-like neurons. The results suggest that spinal TRPA1 receptors and pronociceptive RVM ON-like neurons are involved in the maintenance of submodality-selective pronociceptive actions induced by netrin-1 in the spinal cord.

Advances in TRP channel drug discovery: from target validation to clinical studies.

Transient receptor potential (TRP) channels are multifunctional signalling molecules with many roles in sensory perception and cellular physiology. Therefore, it is not surprising that TRP channels have been implicated in numerous diseases, including hereditary disorders caused by defects in genes encoding TRP channels (TRP channelopathies). Most TRP channels are located at the cell surface, which makes them generally accessible drug targets. Early drug discovery efforts to target TRP channels focused on pain, but as our knowledge of TRP channels and their role in health and disease has grown, these efforts have expanded into new clinical indications, ranging from respiratory disorders through neurological and psychiatric diseases to diabetes and cancer. In this Review, we discuss recent findings in TRP channel structural biology that can affect both drug development and clinical indications. We also discuss the clinical promise of novel TRP channel modulators, aimed at both established and emerging targets. Last, we address the challenges that these compounds may face in clinical practice, including the need for carefully targeted approaches to minimize potential side-effects due to the multifunctional roles of TRP channels.

Spinal mechanisms contributing to the development of pain hypersensitivity induced by sphingolipids in the rat.

BACKGROUND: Earlier studies show that endogenous sphingolipids can induce pain hypersensitivity, activation of spinal astrocytes, release of proinflammatory cytokines and activation of TRPM3 channel. Here we studied whether the development of pain hypersensitivity induced by sphingolipids in the spinal cord can be prevented by pharmacological inhibition of potential downstream mechanisms that we hypothesized to include TRPM3, σ1 and NMDA receptors, gap junctions and D-amino acid oxidase. METHODS: Experiments were performed in adult male rats with a chronic intrathecal catheter for spinal drug administrations. Mechanical nociception was assessed with monofilaments and heat nociception with radiant heat. N,N-dimethylsphingosine (DMS) was administered to induce pain hypersensitivity. Ononetin, isosakuranetin, naringenin (TRPM3 antagonists), BD-1047 (σ1 receptor antagonist), carbenoxolone (a gap junction decoupler), MK-801 (NMDA receptor antagonist) and AS-057278 (inhibitor of D-amino acid oxidase, DAAO) were used to prevent the DMS-induced hypersensitivity, and pregnenolone sulphate (TRPM3 agonist) to recapitulate hypersensitivity. RESULTS: DMS alone produced within 15 min a dose-related mechanical hypersensitivity that lasted at least 24 h, without effect on heat nociception. Preemptive treatments with ononetin, isosakuranetin, naringenin, BD-1047, carbenoxolone, MK-801 or AS-057278 attenuated the development of the DMS-induced hypersensitivity, but had no effects when administered alone. Pregnenolone sulphate (TRPM3 agonist) alone induced a dose-related mechanical hypersensitivity that was prevented by ononetin, isosakuranetin and naringenin. CONCLUSIONS: Among spinal pronociceptive mechanisms activated by DMS are TRPM3, gap junction coupling, the σ1 and NMDA receptors, and DAAO.

Discovery and characterization of ORM-11372, a novel inhibitor of the sodium-calcium exchanger with positive inotropic activity.

BACKGROUND AND PURPOSE: The lack of selective sodium-calcium exchanger (NCX) inhibitors has hampered the exploration of physiological and pathophysiological roles of cardiac NCX 1.1. We aimed to discover more potent and selective drug like NCX 1.1 inhibitor. EXPERIMENTAL APPROACH: A flavan series-based pharmacophore model was constructed. Virtual screening helped us identify a novel scaffold for NCX inhibition. A distinctively different NCX 1.1 inhibitor, ORM-11372, was discovered after lead optimization. Its potency against human and rat NCX 1.1 and selectivity against other ion channels was assessed. The cardiovascular effects of ORM-11372 were studied in normal and infarcted rats and rabbits. Human cardiac safety was studied ex vivo using human ventricular trabeculae. KEY RESULTS: ORM-11372 inhibited human NCX 1.1 reverse and forward currents; IC50 values were 5 and 6 nM respectively. ORM-11372 inhibited human cardiac sodium 1.5 (INa ) and hERG KV 11.1 currents (IhERG ) in a concentration-dependent manner; IC50 values were 23.2 and 10.0 µM. ORM-11372 caused no changes in action potential duration; short-term variability and triangulation were observed for concentrations of up to 10 µM. ORM-11372 induced positive inotropic effects of 18 ± 6% and 35 ± 8% in anaesthetized rats with myocardial infarctions and in healthy rabbits respectively; no other haemodynamic effects were observed, except improved relaxation at the lowest dose. CONCLUSION AND IMPLICATIONS: ORM-11372, a unique, novel, and potent inhibitor of human and rat NCX 1.1, is a positive inotropic compound. NCX inhibition can induce clinically relevant improvements in left ventricular contractions without affecting relaxation, heart rate, or BP, without pro-arrhythmic risk.

Fadolmidine - Favourable adverse effects profile for spinal analgesia suggested by in vitro and in vivo models.

Fadolmidine is an α2-adrenoceptor full agonist developed for spinal analgesia with a local mode of action. The purpose of this study was to demonstrate the safety of fadolmidine on known α2-adrenoceptor-related effects: kidney function, urodynamics and cardiovascular variables. Furthermore, the binding affinity of fadolmidine for the 5-HT3 receptor prompted functional studies on 5-HT3. According to the binding affinity data, fadolmidine demonstrated partial agonism on the 5-HT3 receptor in transfected cells and in guinea pig ileum preparation. However, intravenous (IV) fadolmidine did not produce any 5-HT3-related hemodynamic effects in anaesthetised rats. In urodynamic studies, intrathecal (IT) fadolmidine interrupted volume-evoked voiding cycles and induced overflow incontinence at high concentrations in anaesthetised rats; however, at the analgesic dose range, the effects were mild. The effects of fadolmidine on kidney function were studied in conscious rats after IV and IT dosing. While IT fadolmidine increased dose-dependent urine output, sodium ion concentration, IV doses increased only sodium ion concentration The effects of IT fadolmidine on heart rate (HR), mean arterial pressure (MAP) and sedation were evaluated in the home cage and in the open field using a telemetry system. In resting conditions, fadolmidine decreased HR dose-dependently and increased initial MAP, whereas in actively moving rats, there were no effects at analgesic doses. The results suggest that at anticipated analgesic clinical doses, IT fadolmidine provides analgesia without significant adverse effects on sedation, MAP or HR and with only modest effects on kidney function and urodynamics.

Amygdaloid administration of tetrapentylammonium attenuates development of pain and anxiety-like behavior following peripheral nerve injury.

BACKGROUND: The central amygdaloid nucleus (CeA) is involved in processing and descending regulation of pain. Amygdaloid mechanisms underlying pain processing and control are poorly known. Here we tested the hypothesis that perioperative CeA administration of tetrapentylammonium (TPA), a non-selective THIK-1 channel blocker and thereby inhibitor of microglia, attenuates development of chronic neuropathic pain and comorbid anxiety-like behavior. METHODS: Rats with a spared nerve injury (SNI) model of neuropathy or sham operation had a chronic cannula for drug microinjections into the CeA or a control injection site. Monofilament test was used to evaluate pain, and light-dark box (LDB) to assess anxiety. RESULTS: Perioperative CeA treatment with TPA (30 µg/day up to the third postoperative day, D3) significantly attenuated the development of pain and anxiety-like behavior. In the late phase (> D14), CeA administration of TPA (3-30 µg) failed to influence pain. Perioperative minocycline (microglia inhibitor; 25 µg), MK-801 (an N-Methyl-D-aspartate receptor antagonist; 0.1 µg), vehicle or TPA in a control injection site failed to attenuate pain development. CONCLUSIONS: Perioperative treatment of the CeA with TPA delayed development of neuropathic pain and comorbid anxiety-like behavior, while TPA treatment failed to influence maintenance of established neuropathic pain. The failures to attenuate pain development with CeA administrations of minocycline or MK-801 do not support the hypothesis that the TPA-induced prophylactic effect was due to inhibition of amygdaloid microglia or N-methyl-D-aspartate receptors. While TPA in the CeA proved to have a prophylactic effect on SNI-induced pain behavior, the underlying mechanism still remains to be studied.

TRPA1 Antagonists for Pain Relief.

Here, we review the literature assessing the role of transient receptor potential ankyrin 1 (TRPA1), a calcium-permeable non-selective cation channel, in various types of pain conditions. In the nervous system, TRPA1 is expressed in a subpopulation of nociceptive primary sensory neurons, astroglia, oligodendrocytes and Schwann cells. In peripheral terminals of nociceptive primary sensory neurons, it is involved in the transduction of potentially harmful stimuli and in their central terminals it is involved in amplification of nociceptive transmission. TRPA1 is a final common pathway for a large number of chemically diverse pronociceptive agonists generated in various pathophysiological pain conditions. Thereby, pain therapy using TRPA1 antagonists can be expected to be a superior approach when compared with many other drugs targeting single nociceptive signaling pathways. In experimental animal studies, pharmacological or genetic blocking of TRPA1 has effectively attenuated mechanical and cold pain hypersensitivity in various experimental models of pathophysiological pain, with only minor side effects, if any. TRPA1 antagonists acting peripherally are likely to be optimal for attenuating primary hyperalgesia (such as inflammation-induced sensitization of peripheral nerve terminals), while centrally acting TRPA1 antagonists are expected to be optimal for attenuating pain conditions in which central amplification of transmission plays a role (such as secondary hyperalgesia and tactile allodynia caused by various types of peripheral injuries). In an experimental model of peripheral diabetic neuropathy, prolonged blocking of TRPA1 has delayed the loss of nociceptive nerve endings and their function, thereby promising to provide a disease-modifying treatment.

Regulation of neuropathic pain behavior by amygdaloid TRPC4/C5 channels.

Pain per se may increase anxiety and conversely, anxiety may increase pain. Therefore, a positive feedback loop between anxiety and pain possibly contributes to pain and suffering in some pathophysiological pain conditions, such as that induced by peripheral nerve injury. Recent results indicate that transient receptor channels 4 and 5 (TRPC4/C5) in the amygdala have anxiogenic effects in rodents, while their role in chronic pain conditions is not known. Here, we studied whether the amygdaloid TRPC4/C5 that are known to have anxiogenic properties contribute to the maintenance of sensory or affective aspects of pain in an experimental model of peripheral neuropathy. Rats with a spared nerve injury (SNI) model of neuropathy in the left hind limb had a chronic cannula for microinjections of drugs into the right amygdala or the internal capsule (a control site). Sensory pain was assessed by determining mechanical hypersensitivity with calibrated monofilaments and affective pain by determining aversive place-conditioning. Amygdaloid treatment with ML-204, a TRPC4/C5 antagonist, produced a dose-related (5-10 µg) antihypersensitivity effect, without obvious side-effects. Additionally, amygdaloid administration of ML-204 reduced affective-like pain behavior. In the internal capsule, ML-204 had no effect on hypersensitivity or affective-like pain in SNI animals. In healthy controls, amygdaloid administration of ML-204 failed to influence pain behavior induced by mechanical stimulation or noxious heat. The results indicate that the amygdaloid TRPC4/C5 contribute to maintenance of pain hypersensitivity and pain affect in neuropathy.

TRPA1: a transducer and amplifier of pain and inflammation.

The transient receptor potential ankyrin 1 (TRPA1) ion channel on peripheral terminals of nociceptive primary afferent nerve fibres contributes to the transduction of noxious stimuli to electrical signals, while on central endings in the spinal dorsal horn, it amplifies transmission to spinal interneurons and projection neurons. The centrally propagating nociceptive signal that is induced and amplified by TRPA1 not only elicits pain sensation but also contributes to peripheral neurogenic inflammation through a peripheral axon reflex or a centrally mediated back propagating dorsal root reflex that releases vasoactive agents from sensory neurons in the periphery. Endogenous TRPA1 agonists that are generated under various pathophysiological conditions both in the periphery and in the spinal cord have TRPA1-mediated pro-nociceptive and pro-inflammatory effects. Among endogenous TRPA1 agonists that have been shown to play a role in the pathogenesis of pain and inflammatory conditions are, for example, methylglyoxal, 4-hydroxynonenal, 12-lipoxygenase-derived hepoxilin A3, 5,6-epoxyeicosatrienoic acid and reactive oxygen species, while mustard oil and cinnamaldehyde are most commonly used exogenous TRPA1 agonists in experimental studies. Among selective TRPA1 antagonists are HC-030031, A-967079, AP-14 and Chembridge-5861528. Recent evidence indicates that TRPA1 plays a role also in transition of acute to chronic pain. Due to its location on a subpopulation of pain-mediating primary afferent nerve fibres, blocking the TRPA1 channel is expected to have antinociceptive, antiallodynic and anti-inflammatory effects.

Pharmacological characterisation of a structurally novel α2C-adrenoceptor antagonist ORM-10921 and its effects in neuropsychiatric models.

The α2-adrenoceptors (ARs) are important modulators of a wide array of physiological responses. As only a few selective compounds for the three α2-AR subtypes (α2A , α2B and α2C ) have been available, the pharmacological profile of a new α2C-selective AR antagonist ORM-10921 is reported. Standard in vitro receptor assays and antagonism of α2, and α1-AR agonist-evoked responses in vivo were used to demonstrate the α2C-AR selectivity for ORM-10921 which was tested in established behavioural models related to schizophrenia and cognitive dysfunction with an emphasis on pharmacologically induced hypoglutamatergic state by phencyclidine or MK-801. The Kb values of in vitro α2C-AR antagonism for ORM-10921 varied between 0.078-1.2 nM depending on the applied method. The selectivity ratios compared to α2A-AR subtype and other relevant receptors were 10-100 times in vitro. The in vivo experiments supported its potent α2C-antagonism combined with only a weak α2A-antagonism. In the pharmacodynamic microdialysis study, ORM-10921 was found to increase extracellular dopamine levels in prefrontal cortex in the baseline conditions. In the behavioural tests, ORM-10921 displayed potent antidepressant and antipsychotic-like effects in the forced swimming test and prepulse-inhibition models analogously with the previously reported results with structurally different α2C-selective AR antagonist JP-1302. Our new results also indicate that ORM-10921 alleviated the NMDA-antagonist-induced impairments in social behaviour and watermaze navigation. This study extends and further validates the concept that α2C -AR is a potential therapeutic target in CNS disorders such as schizophrenia or Alzheimer's disease and suggests the potential of α2C-antagonism to treat such disorders.

Dissociated modulation of conditioned place-preference and mechanical hypersensitivity by a TRPA1 channel antagonist in peripheral neuropathy.

Transient receptor potential ankyrin 1 (TRPA1) channel antagonists have suppressed mechanical hypersensitivity in peripheral neuropathy, while their effect on ongoing neuropathic pain is not yet known. Here, we assessed whether blocking the TRPA1 channel induces place-preference, an index for the relief of ongoing pain, in two experimental rat models of peripheral neuropathy. Diabetic neuropathy was induced by streptozotocin and spared nerve injury (SNI) model of neuropathy by ligation of two sciatic nerve branches. Conditioned place-preference (CPP) paradigm involved pairing of the drug treatment with one of the chambers of a CPP device once or four times, and the time spent in each chamber was recorded after conditioning sessions to reveal place-preference. The mechanical antihypersensitivity effect was assessed by the monofilament test immediately after the conditioning sessions. Intraperitoneally (30mg/kg; diabetic and SNI model) or intrathecally (10µg; diabetic model) administered Chembridge-5861528 (CHEM) was used as a selective TRPA1 channel antagonist. In diabetic and SNI models of neuropathy, CHEM failed to induce CPP at a dose that significantly attenuated mechanical hypersensitivity, independent of the route of drug administration or number of successive conditioning sessions. Intrathecal clonidine (an α2-adrenoceptor agonist; 10µg), in contrast, induced CPP in SNI but not control animals. The results indicate that ongoing pain, as revealed by CPP, is less sensitive to treatment by the TRPA1 channel antagonist than mechanical hypersensitivity in peripheral neuropathy.

Transient receptor potential ankyrin 1 (TRPA1) ion channel in the pathophysiology of peripheral diabetic neuropathy.

Background Transient receptor potential ankyrin 1 (TRPA1) is a non-selective cation channel permeable to calcium that is expressed on pain-mediating primary afferent nerve fibers. Here we review recent experimental evidence supporting the hypothesis that activation of the TRPA1 channel by reactive compounds generated in diabetes mellitus, such as 4-hydroxynonenal and methylglyoxal, exerts an important role in the pathophysiology of peripheral diabetic neuropathy (PDN). The hypothesis includes development of the early diabetic pain hypersensitivity and the later loss of cutaneous nerve endings of pain fibers and their dysfunction, which are hallmarks of peripheral diabetic neuropathy (PDN). Methods The evidence for a role of the TRPA1 channel in PDN consists of in vitro patch clamp and calcium imaging data and assessments of pain behavior, axon reflex measurements, and immunohistochemical analyses of cutaneous innervation in an experimental animal model of diabetes. The experiments were combined with blocking the TRPA1 channel with selective antagonists Chembridge-5861528 or A-967079. Results In vitro studies indicate that under physiological concentration of Ca2+, methylglyoxal and 4-hydroxynonenal produce sustained activation of the TRPA1 channel and sustained inflow of calcium. In vivo studies indicate that diabetic pain hypersensitivity is maintained by the TRPA1 channel as indicated by the antihypersensitivity effect induced by acute blocking of the TRPA1 channel. Moreover, TRPA1 channel is involved in the development of diabetic hypersensitivity as indicated by prevention of the development of pain hypersensitivity in diabetic animals treated daily with Chembridge-5861528. The diabetes-induced loss of substance P-like cutaneous innervation and that of the TRPA1 channel-mediated cutaneous axon reflex function during the later phase of diabetes were also prevented or delayed by prolonged blocking of the TRPA1 channel. No motor impairment or other obvious side-effects were observed following block of the TRPA1 channel. Conclusions Together the in vitro and in vivo results indicate that reactive compounds generated in diabetes exert, through action on the TRPA1 channel, an important role in the pathophysiology of PDN. Sustained activation of the TRPA1 channel is a plausible mechanism that contributes to the early diabetic pain hypersensitivity and the later loss of cutaneous pain fiber endings and their dysfunction with prolonged diabetes. Implications Blocking the TRPA1 channel with a selective antagonist provides a promising disease-modifying treatment for PDN, with only minor, if any, side-effects.

Transient receptor potential ankyrin 1 ion channel contributes to guarding pain and mechanical hypersensitivity in a rat model of postoperative pain.

BACKGROUND: The transient receptor potential ankyrin 1 (TRPA1) ion channel is expressed on nociceptive primary afferent nerve fibers. On the distal ending, it is involved in transduction of noxious stimuli, and on the proximal ending (within the spinal dorsal horn), it regulates transmission of nociceptive signals. Here we studied whether the cutaneous or spinal TRPA1 ion channel contributes to mechanical hypersensitivity or guarding, an index of spontaneous pain, in an experimental model of postoperative pain in the rat. METHODS: A skin plus deep-tissue incision was performed under general anesthesia in the plantar skin of one hind paw, after which the incised skin was closed with sutures. Postoperative pain and hypersensitivity were assessed 24-48 h after the operation. Guarding pain was assessed by scoring the hind-paw position. Mechanical hypersensitivity was assessed with a calibrated series of monofilaments applied to the wound area in the operated paw or the contralateral control paw. Chembridge-5861528, a TRPA1 channel antagonist, was administered intaperitoneally (10-30 mg/kg), intraplantarly (10-30 µg), or intrathecally (10 µg) in attempts to suppress guarding and hypersensitivity. RESULTS: Intraperitoneal or ipsi- but not contralateral intraplantar treatment with Chembridge-5861528 reduced mechanical hypersensitivity and guarding in the operated limb. Intrathecal treatment attenuated hypersensitivity but not guarding. Intraplantar Chembridge-5861528 suppressed preferentially mechanical hyperalgesia and intrathecal Chembridge-5861528 tactile allodynia. CONCLUSIONS: The TRPA1 channel in the skin contributes to sustained as well noxious mechanical stimulus-evoked postoperative pain, whereas the spinal TRPA1 channel contributes predominantly to innocuous mechanical stimulus-evoked postoperative pain.

Inhibiting TRPA1 ion channel reduces loss of cutaneous nerve fiber function in diabetic animals: sustained activation of the TRPA1 channel contributes to the pathogenesis of peripheral diabetic neuropathy.

Peripheral diabetic neuropathy (PDN) is a devastating complication of diabetes mellitus (DM). Here we test the hypothesis that the transient receptor potential ankyrin 1 (TRPA1) ion channel on primary afferent nerve fibers is involved in the pathogenesis of PDN, due to sustained activation by reactive compounds generated in DM. DM was induced by streptozotocin in rats that were treated daily for 28 days with a TRPA1 channel antagonist (Chembridge-5861528) or vehicle. Laser Doppler flow method was used for assessing axon reflex induced by intraplantar injection of a TRPA1 channel agonist (cinnamaldehyde) and immunohistochemistry to assess substance P-like innervation of the skin. In vitro calcium imaging and patch clamp were used to assess whether endogenous TRPA1 agonists (4-hydroxynonenal and methylglyoxal) generated in DM induce sustained activation of the TRPA1 channel. Axon reflex induced by a TRPA1 channel agonist in the plantar skin was suppressed and the number of substance P-like immunoreactive nerve fibers was decreased 4 weeks after induction of DM. Prolonged treatment with Chembridge-5861528 reduced the DM-induced attenuation of the cutaneous axon reflex and loss of substance P-like immunoreactive nerve fibers. Moreover, in vitro calcium imaging and patch clamp results indicated that reactive compounds generated in DM (4-hydroxynonenal and methylglyoxal) produced sustained activations of the TRPA1 channel, a prerequisite for adverse long-term effects. The results indicate that the TRPA1 channel exerts an important role in the pathogenesis of PDN. Blocking the TRPA1 channel provides a selective disease-modifying treatment of PDN.

Model for long QT syndrome type 2 using human iPS cells demonstrates arrhythmogenic characteristics in cell culture.

Long QT syndrome (LQTS) is caused by functional alterations in cardiac ion channels and is associated with prolonged cardiac repolarization time and increased risk of ventricular arrhythmias. Inherited type 2 LQTS (LQT2) and drug-induced LQTS both result from altered function of the hERG channel. We investigated whether the electrophysiological characteristics of LQT2 can be recapitulated in vitro using induced pluripotent stem cell (iPSC) technology. Spontaneously beating cardiomyocytes were differentiated from two iPSC lines derived from an individual with LQT2 carrying the R176W mutation in the KCNH2 (HERG) gene. The individual had been asymptomatic except for occasional palpitations, but his sister and father had died suddenly at an early age. Electrophysiological properties of LQT2-specific cardiomyocytes were studied using microelectrode array and patch-clamp, and were compared with those of cardiomyocytes derived from control cells. The action potential duration of LQT2-specific cardiomyocytes was significantly longer than that of control cardiomyocytes, and the rapid delayed potassium channel (I(Kr)) density of the LQT2 cardiomyocytes was significantly reduced. Additionally, LQT2-derived cardiac cells were more sensitive than controls to potentially arrhythmogenic drugs, including sotalol, and demonstrated arrhythmogenic electrical activity. Consistent with clinical observations, the LQT2 cardiomyocytes demonstrated a more pronounced inverse correlation between the beating rate and repolarization time compared with control cells. Prolonged action potential is present in LQT2-specific cardiomyocytes derived from a mutation carrier and arrhythmias can be triggered by a commonly used drug. Thus, the iPSC-derived, disease-specific cardiomyocytes could serve as an important platform to study pathophysiological mechanisms and drug sensitivity in LQT2.

TRPA1 ion channel in the spinal dorsal horn as a therapeutic target in central pain hypersensitivity and cutaneous neurogenic inflammation.

Transient receptor potential ankyrin 1 (TRPA1) is a non-selective, calcium permeable cation channel expressed by a subpopulation of primary afferent nociceptive nerve fibers. On peripheral nerve endings, TRPA1 channel contributes to transduction of chemical and physical stimuli, whereas on the central endings in the spinal dorsal horn, which is the topic of this brief review, it regulates glutamatergic transmission. Blockade of the spinal TRPA1 channel has attenuated mechanical pain hypersensitivity particularly to low-intensity stimulation in various pathophysiological conditions, whereas blockade of the TRPA1 channel-mediated regulation of transmission failed to influence baseline pain behavior in healthy control animals. Additionally, blockade of the spinal TRPA1 channel reduced cutaneous neurogenic inflammation, presumably by decreasing drive of spinal interneurons that induce a proinflammatory dorsal root reflex. The spinal TRPA1 channel provides a promising target for development of a selective disease-modifying therapy for central pain hypersensitivity. Blockade of the spinal TRPA1 channel-mediated regulation of transmission may also attenuate cutaneous neurogenic inflammation.

Spinal transient receptor potential ankyrin 1 channel contributes to central pain hypersensitivity in various pathophysiological conditions in the rat.

The transient receptor potential ankyrin 1 (TRPA1) ion channel is expressed on nociceptive primary afferent neurons. On the proximal nerve ending within the spinal dorsal horn, TRPA1 regulates transmission to spinal interneurons, and thereby pain hypersensitivity. Here we assessed whether the contribution of the spinal TRPA1 channel to pain hypersensitivity varies with the experimental pain model, properties of test stimulation or the behavioral pain response. The antihypersensitivity effect of intrathecally (i.t.) administered Chembridge-5861528 (CHEM; a selective TRPA1 channel antagonist; 5-10µg) was determined in various experimental models of pain hypersensitivity in the rat. In spinal nerve ligation and rapid eye movement (REM) sleep deprivation models, i.t. CHEM attenuated mechanical hypersensitivity. Capsaicin-induced secondary (central) but not primary (peripheral) mechanical hypersensitivity was also reduced by i.t. administration of CHEM or A-967079, another TRPA1 channel antagonist. Formalin-induced secondary mechanical hypersensitivity, but not spontaneous pain, was suppressed by i.t. CHEM. Moreover, mechanical hypersensitivity induced by cholekystokinin in the rostroventromedial medulla was attenuated by i.t. pretreatment with CHEM. Independent of the model, the antihypersensitivity effect induced by i.t. CHEM was predominant on responses evoked by low-intensity stimuli (⩽6g). CHEM (10µg i.t.) failed to attenuate pain behavior in healthy controls or mechanical hypersensitivities induced by i.t. administrations of a GABA(A) receptor antagonist, or NMDA or 5-HT(3) receptor agonists. Conversely, i.t. administration of a TRPA1 channel agonist, cinnamon aldehyde, induced mechanical hypersensitivity. The results indicate that the spinal TRPA1 channel exerts an important role in secondary (central) pain hypersensitivity to low-intensity mechanical stimulation in various pain hypersensitivity conditions. The spinal TRPA1 channel provides a promising target for the selective attenuation of a central mechanism contributing to pathophysiological pain.