TRPA1 mediates spinal antinociception induced by acetaminophen and the cannabinoid Δ(9)-tetrahydrocannabiorcol

TRP Channel Cooperation for Nociception: Therapeutic Opportunities

Chronic pain treatment remains a sore challenge, and in our aging society, the number of patients reporting inadequate pain relief continues to grow. Current treatment options all have their drawbacks, including limited efficacy and the propensity of abuse and addiction; the latter is exemplified by the ongoing opioid crisis. Extensive research in the last few decades has focused on mechanisms underlying chronic pain states, thereby producing attractive opportunities for novel, effective and safe pharmaceutical interventions. Members of the transient receptor potential (TRP) ion channel family represent innovative targets to tackle pain sensation at the root. Three TRP channels, TRPV1, TRPM3, and TRPA1, are of particular interest, as they were identified as sensors of chemical- and heat-induced pain in nociceptor neurons. This review summarizes the knowledge regarding TRP channel-based pain therapies, including the bumpy road of the clinical development of TRPV1 antagonists, the current status of TRPA1 antagonists, and the future potential of targeting TRPM3.

Assessing the Effectiveness of Combined Analgesics for Bilateral Ramus Osteotomies

Pain management is important for alleviating patients' suffering and early recovery. Although analgesic combinations are known to be effective, a comparison of the effectiveness of different combinations has never been performed specifically for ramus osteotomy procedures. Therefore, the purpose of this observational retrospective cohort study was to identify an effective combination for pain management throughout the intraoperative and immediate postoperative period for patients undergoing bilateral ramus osteotomy procedures. Inclusion criteria consisted of patients who had undergone bilateral mandibular ramus osteotomies over a 2-year period. The analyzed predictor variables included patient gender, age, body weight, operation, anesthetic method, duration of operation, intraoperative use of fentanyl, nonsteroidal anti-inflammatory drugs (NSAIDs), and intravenous acetaminophen administered in the operating room at the end of the surgery. The outcome variable was the necessity for additional rescue analgesics (yes/no) in the recovery room. Bivariate statistics and multivariate analysis were computed with a p-value of <0.05. The study sample was comprised of 78 patients requiring bilateral mandibular ramus osteotomies. From the multivariate analysis, the combination of NSAIDs-acetaminophen-fentanyl was an independent factor for no additional rescue analgesics during the first 1 hour after bilateral ramus osteotomies, indicating that the combination is significantly effective for bilateral ramus osteotomies compared with the other combinations. Considering that this study consisted of a small sample size, the results of this study suggest that some of the combinations, particularly NSAIDs-acetaminophen-fentanyl, are more effective than NSAIDs alone for postoperative pain control immediately following bilateral ramus osteotomy procedures.

Voltage-dependent modulation of TRPA1 currents by diphenhydramine

Diphenhydramine (DPH) has been broadly used to treat allergy. When used as a topical medicine, DPH temporarily relieves itching and pain. Although transient receptor potential type A1 (TRPA1) channel is known to play roles in both acute and chronic itch and pain, whether DPH affects the activities of TRPA1 remains unclear. Using whole-cell patch clamp recordings, we demonstrated that DPH modulates the voltage-dependence of TRPA1. When co-applied with a TRPA1 agonist, DPH significantly enhanced the inward currents while suppressing the outward currents of TRPA1, converting the channel from outwardly rectifying to inwardly rectifying. This effect of DPH occurred no matter TRPA1 was activated by an electrophilic or non-electrophilic agonist and for both mouse and human TRPA1. The modulation of TRPA1 by DPH was maintained in the L906C mutant, which by itself also causes inward rectification of TRPA1, indicating that additional acting sites are present for the modulation of TRPA1 currents by DPH. Our recordings also revealed that DPH partially blocked capsaicin evoked TRPV1 currents. These data suggest that DPH may exert its therapeutic effects on itch and pain, through modulation of TRPA1 in a voltage-dependent fashion.

Hypersensitivity of Airway Reflexes Induced by Hydrogen Sulfide: Role of TRPA1 Receptors

The activation of capsaicin-sensitive lung vagal (CSLV) afferents can elicit airway reflexes. Hypersensitivity of these afferents is known to contribute to the airway hypersensitivity during airway inflammation. Hydrogen sulfide (H₂S) has been suggested as a potential therapeutic agent for airway hypersensitivity diseases, such as asthma, because of its relaxing effect on airway smooth muscle and anti-inflammatory effect. However, it is still unknown whether H₂S affects airway reflexes. Our previous study demonstrated that exogenous application of H₂S sensitized CSLV afferents and enhanced Ca²⁺ transients in CSLV neurons. The present study aimed to determine whether the H₂S-induced sensitization leads to functional changes in airway reflexes and elevates the electrical excitability of the CSLV neurons. Our results showed that, first and foremost, in anesthetized, spontaneously breathing rats, the inhalation of aerosolized sodium hydrosulfide (NaHS, a donor of H₂S; 5 mg/mL, 3 min) caused an enhancement in apneic response evoked by several stimulants of the CSLV afferents. This enhancement effect was found 5 min after NaHS inhalation and returned to control 30 min later. However, NaHS no longer enhanced the apneic response after perineural capsaicin treatment on both cervical vagi that blocked the conduction of CSLV fibers. Furthermore, the enhancing effect of NaHS on apneic response was totally abolished by pretreatment with intravenous HC-030031 (a TRPA1 antagonist; 8 mg/kg), whereas the potentiating effect was not affected by the pretreatment with the vehicle of HC-030031. We also found that intracerebroventricular infusion pretreated with HC-030031 failed to alter the potentiating effect of NaHS on the apneic response. Besides, the cough reflex elicited by capsaicin aerosol was enhanced by inhalation of NaHS in conscious guinea pigs. Nevertheless, this effect was entirely eliminated by pretreatment with HC-030031, not by its vehicle. Last but not least, voltage-clamp electrophysiological analysis of isolated rat CSLV neurons showed a similar pattern of potentiating effects of NaHS on capsaicin-induced inward current, and the involvement of TRPA1 receptors was also distinctly shown. In conclusion, these results suggest that H₂S non-specifically enhances the airway reflex responses, at least in part, through action on the TRPA1 receptors expressed on the CSLV afferents. Therefore, H₂S should be used with caution when applying for therapeutic purposes in airway hypersensitivity diseases.

Imaging-based vascular-related biomarkers for early detection of acetaminophen-induced liver injury

Acetaminophen (APAP) is the foremost cause of drug-induced liver injury in the Western world. Most studies of APAP hepatotoxicity have focused on the hepatocellular injury, but current hepatocyte-related biomarkers have delayed presentation time and a lack of sensitivity. APAP overdose can induce hepatic microvascular congestion, which importantly precedes the injury of hepatocytes. However, the underlying molecular mechanisms remain unclear. It is imperative to discover and validate sensitive and specific translational biomarkers of APAP-induced liver injury. Methods: In this study, we assessed APAP toxicity in sinusoidal endothelial cells and hepatocytes in mice treated with overdose APAP at different time points. The underlying mechanisms of APAP overdose induced sinusoidal endothelial cell injury were investigated by RT2 Profiler PCR arrays. The impact of APAP overdose on endothelial cell function was assessed by pseudovessel formation of endothelial cells in 2D Matrigel and in vivo hepatic vascular integrity using multiphoton microscopy. Finally, the effects of APAP overdose on oxygen levels in the liver and hepatic microcirculation were evaluated by contrast enhanced ultrasonography. Potential imaging-based vascular-related markers for early detection of APAP induced liver injury were assessed. Results: Our study confirmed that hepatic endothelial cells are an early and direct target for APAP hepatotoxicity. ICAM1-related cellular adhesion pathways played a prominent role in APAP-induced endothelial cell injury, which was further validated in primary human sinusoidal endothelial cells and human livers after APAP overdose. APAP overdose impacted pseudovessel formation of endothelial cells and in vivo hepatic vascular integrity. Use of ultrasound to detect APAP-induced liver injury demonstrated that mean transit time, an imaging-based vascular-related biomarker, was more sensitive and precise for early detection of APAP hepatotoxicity and monitoring the treatment response in comparison with a conventional blood-based biomarker. Conclusion: Imaging-based vascular-related biomarkers can identify early and mild liver injury induced by APAP overdose. With further development, such biomarkers may improve the assessment of liver injury and the efficacy of clinical decision-making, which can be extended to other microvascular dysfunction of deep organs.

Ultramicronized Palmitoylethanolamide and Paracetamol, a New Association to Relieve Hyperalgesia and Pain in a Sciatic Nerve Injury Model in Rat

Inflammation is known to be an essential trigger of the pathological changes that have a critical impact on nerve repair and regeneration; moreover, damage to peripheral nerves can cause a loss of sensory function and produces persistent neuropathic pain. To date, various potential approaches for neuropathic pain have focused on controlling neuroinflammation. The aim of this study was to investigate the neuroprotective effects of a new association of ultramicronized Palmitoylethanolamide (PEAum), an Autacoid Local Injury Antagonist Amide (ALIAmide) with analgesic and anti-inflammatory properties, with Paracetamol, a common analgesic, in a rat model of sciatic nerve injury (SNI). The association of PEAum-Paracetamol, in a low dose (5 mg/kg + 30 mg/kg), was given by oral gavage daily for 14 days after SNI. PEAum-Paracetamol association was able to reduce hyperalgesia, mast cell activation, c-Fos and nerve growth factor (NGF) expression, neural histological damage, cytokine release, and apoptosis. Furthermore, the analgesic action of PEAum-Paracetamol could act in a synergistic manner through the inhibition of the NF-κB pathway, which leads to a decrease of cyclooxygenase 2-dependent prostaglandin E₂ (COX-2/PGE₂) release. In conclusion, we demonstrated that PEAum associated with Paracetamol was able to relieve pain and neuroinflammation after SNI in a synergistic manner, and this therapeutic approach could be relevant to decrease the demand of analgesic drugs.

Acetaminophen (Paracetamol) Metabolites Induce Vasodilation and Hypotension by Activating Kv7 Potassium Channels Directly and Indirectly

OBJECTIVE

Intravenous acetaminophen/paracetamol (APAP) is well documented to cause hypotension. Since the patients receiving intravenous APAP are usually critically ill, any severe hemodynamic changes, as with those associated with APAP, can be life-threatening. The mechanism underlying this dangerous iatrogenic effect of APAP was unknown. Approach and Results: Here, we show that intravenous APAP caused transient hypotension in rats, which was attenuated by the Kv7 channel blocker, linopirdine. APAP metabolite N-acetyl-p-benzoquinone imine caused vasodilatation of rat mesenteric arteries ex vivo. This vasodilatation was sensitive to linopirdine and also the calcitonin gene-related peptide antagonist, BIBN 4096. Further investigation revealed N-acetyl-p-benzoquinone imine stimulates calcitonin gene-related peptide release from perivascular nerves, causing a cAMP-dependent activation of Kv7 channels. We also show that N-acetyl-p-benzoquinone imine enhances Kv7.4 and Kv7.5 channels overexpressed in oocytes, suggesting that it can activate Kv7.4 and Kv7.5 channels directly, to elicit vasodilatation.

CONCLUSIONS

Direct and indirect activation of Kv7 channels by the APAP metabolite N-acetyl-p-benzoquinone imine decreases arterial tone, which can lead to a drop in blood pressure. Our findings provide a molecular mechanism and potential preventive intervention for the clinical phenomenon of intravenous APAP-dependent transient hypotension.

Regulation of vascular tone by transient receptor potential ankyrin 1 channels

The Ca²⁺-permeable, non-selective cation channel, TRPA1 (transient receptor potential ankyrin 1), is the sole member of the ankyrin TRP subfamily. TRPA1 channels are expressed on the plasma membrane of neurons as well as non-neuronal cell types, such as vascular endothelial cells. TRPA1 is activated by electrophilic compounds, including dietary molecules such as allyl isothiocyanate, a derivative of mustard. Endogenously, the channel is thought to be activated by reactive oxygen species and their metabolites, such as 4-hydroxynonenal (4-HNE). In the context of the vasculature, activation of TRPA1 channels results in a vasodilatory response mediated by two distinct mechanisms. In the first instance, TRPA1 is expressed in sensory nerves of the vasculature and, upon activation, mediates release of the potent dilator, calcitonin gene-related peptide (CGRP). In the second, work from our laboratory has demonstrated that TRPA1 is expressed in the endothelium of blood vessels exclusively in the cerebral vasculature, where its activation produces a localized Ca²⁺ signal that results in dilation of cerebral arteries. In this chapter, we provide an in-depth overview of the biophysical and pharmacological properties of TRPA1 channels and their importance in regulating vascular tone.

Modification of TRPV4 activity by acetaminophen

N-Acetyl-p-aminophenol (APAP/acetaminophen) is a widely used analgesic/antipyretic with weaker inhibitory effects on cyclooxygenase compared to those of non-steroidal anti-inflammatory drugs. The effect of APAP is mediated by its metabolites, N-arachidonoyl-phenolamine and N-acetyl-p-benzoquinone imine, which activate transient receptor potential (TRP) channels, including TRP vanilloid 1 (TRPV1) and TRP ankyrin 1 (TRPA1) or cannabinoid receptor type 1. However, the exact molecular mechanism underlying the cellular actions of APAP remains unclear. Recently, we observed that APAP promotes cell migration through TRPV4; in this study, we examined the effect of APAP on Ca²⁺-channel activity of TRPV4.

In the rat cell line PC12 expressing TRPV4, GSK1016790A (GSK), a TRPV4 agonist, stimulated an increase in [Ca²⁺]_i; these effects were abrogated by HC-067047 treatment. This GSK-induced Ca²⁺ entry through TRPV4 was inhibited by APAP in a dose-dependent manner, whereas APAP alone did not affect [Ca²⁺]_i. The specificity of the effect of APAP on TRPV4 was further confirmed using HeLa cells, which lack endogenous TRPV4 but stably express exogenous TRPV4 (HeLa-mTRPV4). GSK-induced [Ca²⁺]_i elevation was only observed in HeLa-mTRPV4 cells compared to that in the control HeLa cells, indicating the specific action of GSK on TRPV4. APAP dose-dependently suppressed this GSK-induced Ca²⁺ entry in HeLa-mTRPV4. However, it is unlikely that the metabolites of APAP were involved in these effects as the reaction in this study was rapid.

The results suggest that APAP suppresses the newly identified target TRPV4 without being metabolized and exerts antipyretic/analgesic and/or other effects on TRPV4-related phenomena in the body. The effect of APAP on TRPV4 was opposite to that on TRPV1 or TRPA1, as the latter is activated by APAP.

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.

Transient receptor potential ankyrin 1 (TRPA1)-mediated toxicity: friend or foe?

Transient receptor potential (TRP) channels have been widely studied during the last decade. New studies uncover new features and potential applications for these channels. TRPA1 has a huge distribution all over the human body and has been reported to be involved in different physiological and pathological conditions including cold, pain, and damage sensation. Considering its role, many studies have been devoted to evaluating the role of this channel in the initiation and progression of different toxicities. Accordingly, we reviewed the most recent studies and divided the role of TRPA1 in toxicology into the following sections: neurotoxicity, cardiotoxicity, dermatotoxicity, and pulmonary toxicity. Acetaminophen, heavy metals, tear gases, various chemotherapeutic agents, acrolein, wood smoke particulate materials, particulate air pollution materials, diesel exhaust particles, cigarette smoke extracts, air born irritants, sulfur mustard, and plasticizers are selected compounds and materials with toxic effects that are, at least in part, mediated by TRPA1. Considering the high safety of TRPA1 antagonists and their efficacy to resolve selected toxic or adverse drug reactions, the future of these drugs looks promising.

Promiscuous G-Protein-Coupled Receptor Inhibition of Transient Receptor Potential Melastatin 3 Ion Channels by Gβγ Subunits

Transient receptor potential melastatin 3 (TRPM3) is a nonselective cation channel that is inhibited by Gβγ subunits liberated following activation of Gαi/o protein-coupled receptors. Here, we demonstrate that TRPM3 channels are also inhibited by Gβγ released from Gαs and Gαq Activation of the Gs-coupled adenosine 2B receptor and the Gq-coupled muscarinic acetylcholine M1 receptor inhibited the activity of TRPM3 heterologously expressed in HEK293 cells. This inhibition was prevented when the Gβγ sink βARK1-ct (C terminus of β-adrenergic receptor kinase-1) was coexpressed with TRPM3. In neurons isolated from mouse dorsal root ganglion (DRG), native TRPM3 channels were inhibited by activating Gs-coupled prostaglandin-EP2 and Gq-coupled bradykinin B2 (BK2) receptors. The Gi/o inhibitor pertussis toxin and inhibitors of PKA and PKC had no effect on EP2- and BK2-mediated inhibition of TRPM3, demonstrating that the receptors did not act through Gαi/o or through the major protein kinases activated downstream of G-protein-coupled receptor (GPCR) activation. When DRG neurons were dialyzed with GRK2i, which sequesters free Gβγ protein, TRPM3 inhibition by EP2 and BK2 was significantly reduced. Intraplantar injections of EP2 or BK2 agonists inhibited both the nocifensive response evoked by TRPM3 agonists, and the heat hypersensitivity produced by Freund's Complete Adjuvant (FCA). Furthermore, FCA-induced heat hypersensitivity was completely reversed by the selective TRPM3 antagonist ononetin in WT mice and did not develop in Trpm3-/- mice. Our results demonstrate that TRPM3 is subject to promiscuous inhibition by Gβγ protein in heterologous expression systems, primary neurons and in vivo, and suggest a critical role for this ion channel in inflammatory heat hypersensitivity.SIGNIFICANCE STATEMENT The ion channel TRPM3 is widely expressed in the nervous system. Recent studies showed that Gαi/o-coupled GPCRs inhibit TRPM3 through a direct interaction between Gβγ subunits and TRPM3. Since Gβγ proteins can be liberated from other Gα subunits than Gαi/o, we examined whether activation of Gs- and Gq-coupled receptors also influence TRPM3 via Gβγ. Our results demonstrate that activation of Gs- and Gq-coupled GPCRs in recombinant cells and sensory neurons inhibits TRPM3 via Gβγ liberation. We also demonstrated that Gs- and Gq-coupled receptors inhibit TRPM3 in vivo, thereby reducing pain produced by activation of TRPM3, and inflammatory heat hypersensitivity. Our results identify Gβγ inhibition of TRPM3 as an effector mechanism shared by the major Gα subunits.

Targeting TRP Channels - Valuable Alternatives to Combat Pain, Lower Urinary Tract Disorders, and Type 2 Diabetes?

Transient receptor potential (TRP) channels are a family of functionally diverse and widely expressed cation channels involved in a variety of cell signaling and sensory pathways. Research in the last two decades has not only shed light on the physiological roles of the 28 mammalian TRP channels, but also revealed the involvement of specific TRP channels in a plethora of inherited and acquired human diseases. Considering the historical successes of other types of ion channels as therapeutic drug targets, small molecules that target specific TRP channels hold promise as treatments for a variety of human conditions. In recent research, important new findings have highlighted the central role of TRP channels in chronic pain, lower urinary tract disorders, and type 2 diabetes, conditions with an unmet medical need. Here, we discuss how these advances support the development of TRP-channel-based pharmacotherapies as valuable alternatives to the current mainstays of treatment.

TRPA1 Channel as a Regulator of Neurogenic Inflammation and Pain: Structure, Function, Role in Pathophysiology, and Therapeutic Potential of Ligands

TRPA1 is a cation channel located on the plasma membrane of many types of human and animal cells, including skin sensory neurons and epithelial cells of the intestine, lungs, urinary bladder, etc. TRPA1 is the major chemosensor that also responds to thermal and mechanical stimuli. Substances that activate TRPA1, e.g., allyl isothiocyanates (pungent components of mustard, horseradish, and wasabi), cinnamaldehyde from cinnamon, organosulfur compounds from garlic and onion, tear gas, acrolein and crotonaldehyde from cigarette smoke, etc., cause burning, mechanical and thermal hypersensitivity, cough, eye irritation, sneezing, mucus secretion, and neurogenic inflammation. An increased activity of TRPA1 leads to the emergence of chronic pruritus and allergic dermatitis and is associated with episodic pain syndrome, a hereditary disease characterized by episodes of debilitating pain triggered by stress. TRPA1 is now considered as one of the targets for developing new anti-inflammatory and analgesic drugs. This review summarizes information on the structure, function, and physiological role of this channel, as well as describes known TRPA1 ligands and their significance as therapeutic agents in the treatment of inflammation-associated pain.

TRPA1/NOX in the soma of trigeminal ganglion neurons mediates migraine-related pain of glyceryl trinitrate in mice

Glyceryl trinitrate is administered as a provocative test for migraine pain. Glyceryl trinitrate causes prolonged mechanical allodynia in rodents, which temporally correlates with delayed glyceryl trinitrate-evoked migraine attacks in patients. However, the underlying mechanism of the allodynia evoked by glyceryl trinitrate is unknown. The proalgesic transient receptor potential ankyrin 1 (TRPA1) channel, expressed by trigeminal nociceptors, is sensitive to oxidative stress and is targeted by nitric oxide or its by-products. Herein, we explored the role of TRPA1 in glyceryl trinitrate-evoked allodynia. Systemic administration of glyceryl trinitrate elicited in the mouse periorbital area an early and transient vasodilatation and a delayed and prolonged mechanical allodynia. The systemic, intrathecal or local administration of selective enzyme inhibitors revealed that nitric oxide, liberated from the parent drug by aldehyde dehydrogenase 2 (ALDH2), initiates but does not maintain allodynia. The central and the final phases of allodynia were respectively associated with generation of reactive oxygen and carbonyl species within the trigeminal ganglion. Allodynia was absent in TRPA1-deficient mice and was reversed by TRPA1 antagonists. Knockdown of neuronal TRPA1 by intrathecally administered antisense oligonucleotide and selective deletion of TRPA1 from sensory neurons in Advillin-Cre; Trpa1^fl/fl mice revealed that nitric oxide-dependent oxidative and carbonylic stress generation is due to TRPA1 stimulation, and resultant NADPH oxidase 1 (NOX1) and NOX2 activation in the soma of trigeminal ganglion neurons. Early periorbital vasodilatation evoked by glyceryl trinitrate was attenuated by ALDH2 inhibition but was unaffected by TRPA1 blockade. Antagonists of the calcitonin gene-related peptide receptor did not affect the vasodilatation but partially inhibited allodynia. Thus, although both periorbital allodynia and vasodilatation evoked by glyceryl trinitrate are initiated by nitric oxide, they are temporally and mechanistically distinct. While vasodilatation is due to a direct nitric oxide action in the vascular smooth muscle, allodynia is a neuronal phenomenon mediated by TRPA1 activation and ensuing oxidative stress. The autocrine pathway, sustained by TRPA1 and NOX1/2 within neuronal cell bodies of trigeminal ganglia, may sensitize meningeal nociceptors and second order trigeminal neurons to elicit periorbital allodynia, and could be of relevance for migraine-like headaches evoked by glyceryl trinitrate in humans.

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.

Acetaminophen-induced liver injury is mediated by the ion channel TRPV4

Overdosing of the analgesic acetaminophen (APAP) is one of the most common causes for acute liver failure in modern countries. Although the exact molecular mechanisms mediating hepatocellular necrosis are still elusive, it is preceded by oxidative stress triggered by excessive levels of the metabolite N-acetyl-para-benzoquinone imine (NAPQI). Here, we describe the role of the redox-sensitive transient receptor potential (TRP) ion channel TRP vanilloid 4 (TRPV4) for APAP-induced hepatoxicity. Both pharmacological inhibition and genetic deletion of TRPV4 ameliorate APAP-induced necrosis in mouse and human hepatocytes in vitro. Liver injury caused by a systemic overdose of APAP is reduced in TRPV4-deficient mice and in wild-type mice treated with a TRPV4 inhibitor. The reduction of hepatotoxicity accomplished by systemic TRPV4 inhibition is comparable to the protective effects of the antioxidant N-acetyl-cysteine. Although TRPV4 does not modulate intrahepatic levels of glutathione, both its inhibition and genetic deletion attenuate APAP-induced oxidative and nitrosative stress as well as mitochondrial membrane depolarization. NAPQI evokes a calcium influx by activating heterologously expressed TRPV4 channels and endogenous TRPV4 channels in hepatoma cells but not in primary mouse hepatocytes. Taken together, our data suggest that TRPV4 mediates APAP-induced hepatotoxicity and thus may be a suitable target for treatment of this critical side effect.-Echtermeyer, F., Eberhardt, M., Risser, L., Herzog, C., Gueler, F., Khalil, M., Engel, M., Vondran, F., Leffler, A. Acetaminophen-induced liver injury is mediated by the ion channel TRPV4.

The role of Nav1.7 and methylglyoxal-mediated activation of TRPA1 in itch and hypoalgesia in a murine model of type 1 diabetes

Methylglyoxal (MGO), an endogenous reactive carbonyl compound, plays a key role in the pathogenesis of diabetic neuropathy. The aim of this study is to investigate the role of MGO in diabetic itch and hypoalgesia, two common symptoms associated with diabetic neuropathy.

Methods: Scratching behavior, mechanical itch (alloknesis), and thermal hypoalgesia were quantified after intradermal (i.d.) injection of MGO in naïve mice or in diabetic mice induced by intraperitoneal (i.p.) injection of streptozotocin (STZ). Behavioral testing, patch-clamp recording, transgenic mice, and gene expression analysis were used to investigate the mechanisms underlying diabetic itch and hypoalgesia in mice.

Results: I.d. injection of MGO evoked dose-dependent scratching in normal mice. Addition of MGO directly activated transient receptor potential ankyrin 1 (TRPA1) to induce inward currents and calcium influx in dorsal root ganglia (DRG) neurons or in TRPA1-expressing HEK293 cells. Mechanical itch, but not spontaneous itch was developed in STZ-induced diabetic mice. Genetic ablation of Trpa1 (Trpa1^-/-), pharmacological blockade of TRPA1 and Na_v1.7, antioxidants, and mitogen-activated protein kinase kinase enzyme (MEK) inhibitor U0126 abrogated itch induced by MGO or in STZ-induced diabetic mice. Thermal hypoalgesia was induced by intrathecal (i.t.) injection of MGO or in STZ-induced diabetic mice, which was abolished by MGO scavengers, intrathecal injection of TRPA1 blockers, and in Trpa1^-/-mice.

Conclusion: This study revealed that Na_v1.7 and MGO-mediated activation of TRPA1 play key roles in itch and hypoalgesia in a murine model of type 1 diabetes. Thereby, we provide a novel potential therapeutic strategy for the treatment of itch and hypoalgesia induced by diabetic neuropathy.

Dark Classics in Chemical Neuroscience: Δ9-Tetrahydrocannabinol

Cannabis ( Cannabis sativa) is the most widely used illicit drug in the world, with an estimated 192 million users globally. The main psychoactive component of cannabis is (-)- trans-Δ⁹-tetrahydrocannabinol (Δ⁹-THC), a compound with a diverse range of pharmacological actions. The unique and distinctive intoxication caused by Δ⁹-THC primarily reflects partial agonist action at central cannabinoid type 1 (CB₁) receptors. Δ⁹-THC is an approved therapeutic treatment for a range of conditions, including chronic pain, chemotherapy-induced nausea and vomiting, and multiple sclerosis, and is being investigated in indications such as anorexia nervosa, agitation in dementia, and Tourette's syndrome. It is available as a regulated pharmaceutical in products such as Marinol, Sativex, and Namisol as well as in an ever-increasing range of unregistered medicinal and recreational cannabis products. While cannabis is an ancient medicament, contemporary use is embroiled in legal, scientific, and social controversy, much of which relates to the potential hazards and benefits of Δ⁹-THC itself. Robust contemporary debate surrounds the therapeutic value of Δ⁹-THC in different diseases, its capacity to produce psychosis and cognitive impairment, and the addictive and "gateway" potential of the drug. This review will provide a profile of the chemistry, pharmacology, and therapeutic uses of Δ⁹-THC as well as the historical and societal import of this unique, distinctive, and ubiquitous psychoactive substance.

Active metabolites of dipyrone induce a redox-dependent activation of the ion channels TRPA1 and TRPV1

Introduction:

The nonopioid analgesic and antipyretic dipyrone (metamizol) is frequently used worldwide. Dipyrone is a prodrug, and the metabolites 4-N-methylaminoantipyrine (MAA) and 4-aminoantipyrine (AA) seem to induce analgesia and antipyresia in part by inhibiting cyclooxygenase. In mice, however, the analgesic effect of dipyrone also seems to depend on the ion channel TRPA1. In this study, we explored the effects of dipyrone and its active metabolites on recombinant and native TRPA1 and TRPV1 channels.

Methods:

Constructs human (h) TRPA1 and TRPV1 were expressed in HEK293 cells, and dorsal root ganglion neurons were isolated from adult mice. Effects of dipyrone, MAA, and AA were explored by means of whole-cell patch clamp recordings and ratiometric calcium imaging.

Results:

Dipyrone failed to activate both hTRPA1 and hTRPV1. However, both MAA and AA evoked small outwardly rectifying membrane currents and an increase of intracellular calcium in cells expressing hTRPA1 or hTRPV1. MAA also sensitized both channels and thus potentiated inward currents induced by carvacrol (hTRPA1) and protons (hTRPV1). MAA-induced activation was inhibited by the antioxidant 10-mM glutathione included in the pipette, and the mutant constructs hTRPA1-C621/C641/C665S and hTRPV1-C158A/C391S/C767S were insensitive to both MAA and AA. Mouse dorsal root ganglion neurons exhibited a marginal calcium influx when challenged with MAA.

Conclusion:

The metabolites MAA and AA, but not dipyrone itself, activate and sensitize the nociceptive ion channels TRPA1 and TRPV1 in a redox-dependent manner. These effects may be relevant for dipyrone-induced analgesia and antipyresia.

The paracetamol metabolite N-acetylp-benzoquinone imine reduces excitability in first- and second-order neurons of the pain pathway through actions on KV7 channels

Paracetamol (acetaminophen, APAP) is one of the most frequently used analgesic agents worldwide. It is generally preferred over nonsteroidal anti-inflammatory drugs because it does not cause typical adverse effects resulting from the inhibition of cyclooxygenases, such as gastric ulcers. Nevertheless, inhibitory impact on these enzymes is claimed to contribute to paracetamols mechanisms of action which, therefore, remained controversial. Recently, the APAP metabolites N-arachidonoylaminophenol (AM404) and N-acetyl-p-benzoquinone imine (NAPQI) have been detected in the central nervous system after systemic APAP administration and were reported to mediate paracetamol effects. In contrast to nonsteroidal anti-inflammatory drugs that rather support seizure activity, paracetamol provides anticonvulsant actions, and this dampening of neuronal activity may also form the basis for analgesic effects. Here, we reveal that the APAP metabolite NAPQI, but neither the parent compound nor the metabolite AM404, reduces membrane excitability in rat dorsal root ganglion (DRG) and spinal dorsal horn (SDH) neurons. The observed reduction of spike frequencies is accompanied by hyperpolarization in both sets of neurons. In parallel, NAPQI, but neither APAP nor AM404, increases currents through KV7 channels in DRG and SDH neurons, and the impact on neuronal excitability is absent if KV7 channels are blocked. Furthermore, NAPQI can revert the inhibitory action of the inflammatory mediator bradykinin on KV7 channels but does not affect synaptic transmission between DRG and SDH neurons. These results show that the paracetamol metabolite NAPQI dampens excitability of first- and second-order neurons of the pain pathway through an action on KV7 channels.

Acetaminophen ingestion improves muscle activation and performance during a 3-min all-out cycling test

Acute acetaminophen (ACT) ingestion has been shown to enhance cycling time-trial performance. The purpose of this study was to assess whether ACT ingestion enhances muscle activation and critical power (CP) during maximal cycling exercise. Sixteen active male participants completed two 3-min all-out tests against a fixed resistance on an electronically braked cycle ergometer 60 min after ingestion of 1 g of ACT or placebo (maltodextrin, PL). CP was estimated as the mean power output over the final 30 s of the test and W′ (the curvature constant of the power–duration relationship) was estimated as the work done above CP. The femoral nerve was stimulated every 30 s to measure membrane excitability (M-wave) and surface electromyography (EMGRMS) was recorded continuously to infer muscle activation. Compared with PL, ACT ingestion increased CP (ACT: 297 ± 32 W vs. PL: 288 ± 31 W, P < 0.001) and total work done (ACT: 66.4 ± 6.5 kJ vs. PL: 65.4 ± 6.4 kJ, P = 0.03) without impacting W′ (ACT: 13.1 ± 2.9 kJ vs. PL: 13.6 ± 2.4 kJ, P = 0.19) or the M-wave amplitude (P = 0.66) during the 3-min all-out cycling test. Normalised EMGRMS amplitude declined throughout the 3-min protocol in both PL and ACT conditions; however, the decline in EMGRMS amplitude was attenuated in the ACT condition, such that the EMGRMS amplitude was greater in ACT compared with PL over the last 60 s of the test (P = 0.04). These findings indicate that acute ACT ingestion might increase performance and CP during maximal cycling exercise by enhancing muscle activation.

Paracetamol and Pain Modulation by TRPV1, UGT2B15, SULT1A1 Genotypes: A Randomized Clinical Trial in Healthy Volunteers

Background

The influence of the genetic polymorphism of enzymes and receptors involved in paracetamol metabolism and mechanism of action has not been investigated. This trial in healthy volunteers investigated the link between paracetamol pain relief and the genetic polymorphism of 23 enzymes and receptors.

Design

This randomized double-blind crossover controlled pilot study took place in the Clinical Pharmacology Department, University Hospital, Clermont-Ferrand, France. Forty-seven Caucasian volunteers were recruited. The trial consisted of two randomized sessions one week apart with oral paracetamol or placebo, and pain changes were evaluated with mechanical pain stimuli. The genetic polymorphism of 23 enzymes and receptors was studied, and correlations were made with pain relief. All tests are two-sided with a type I error at 0.05.

Results

Paracetamol was antinociceptive compared with placebo (222 ± 482 kPaxmin vs 23 ± 431 kPaxmin; P = 0.0047), and the study showed 30 paracetamol responders and 17 paracetamol nonresponders. Responders were characterized by TRPV1rs224534 A allele, UGT2B15rs1902023 TT genotype, and SULT1A1rs9282861 GG genotype (P &lt; 0.05 for all). These findings confirm for the first time the involvement of a specific TRPV1 rs224534 variant in paracetamol antinociception. They also reveal a new antinociceptive role for specific variants of hepatic phase II enzymes associated with paracetamol metabolism.

Conclusions

The study warrants larger clinical trials on these potential genomic markers of paracetamol analgesia in patients. Confirmation of the present findings would open the way to effective individualized pain treatment with paracetamol, the most commonly used analgesic worldwide.

The Role of Toxins in the Pursuit for Novel Analgesics

Chronic pain is a major medical issue which reduces the quality of life of millions and inflicts a significant burden on health authorities worldwide. Currently, management of chronic pain includes first-line pharmacological therapies that are inadequately effective, as in just a portion of patients pain relief is obtained. Furthermore, most analgesics in use produce severe or intolerable adverse effects that impose dose restrictions and reduce compliance. As the majority of analgesic agents act on the central nervous system (CNS), it is possible that blocking pain at its source by targeting nociceptors would prove more efficient with minimal CNS-related side effects. The development of such analgesics requires the identification of appropriate molecular targets and thorough understanding of their structural and functional features. To this end, plant and animal toxins can be employed as they affect ion channels with high potency and selectivity. Moreover, elucidation of the toxin-bound ion channel structure could generate pharmacophores for rational drug design while favorable safety and analgesic profiles could highlight toxins as leads or even as valuable therapeutic compounds themselves. Here, we discuss the use of plant and animal toxins in the characterization of peripherally expressed ion channels which are implicated in pain.

Modulators of Transient Receptor Potential (TRP) Channels as Therapeutic Options in Lung Disease

The lungs are essential for gas exchange and serve as the gateways of our body to the external environment. They are easily accessible for drugs from both sides, the airways and the vasculature. Recent literature provides evidence for a role of Transient Receptor Potential (TRP) channels as chemosensors and essential members of signal transduction cascades in stress-induced cellular responses. This review will focus on TRP channels (TRPA1, TRPC6, TRPV1, and TRPV4), predominantly expressed in non-neuronal lung tissues and their involvement in pathways associated with diseases like asthma, cystic fibrosis, chronic obstructive pulmonary disease (COPD), lung fibrosis, and edema formation. Recently identified specific modulators of these channels and their potential as new therapeutic options as well as strategies for a causal treatment based on the mechanistic understanding of molecular events will also be evaluated.

N-acetylcysteine prevents glutathione decrease and does not interfere with paracetamol antinociceptive effect at therapeutic dosage: a randomized double-blind controlled trial in healthy subjects

Paracetamol (APAP) may lead to hepatic changes even at therapeutic dosages. Glutathione (GSH) plays a pivotal role in APAP metabolism as it allows the detoxification of a toxic metabolite. N-Acetylcysteine (NAC) is APAP antidote, is also largely used as a mucoactive drug and is often associated with APAP. This study aims at evaluating if 1- NAC modifies APAP pain efficacy and 2- NAC prevents glutathione depletion with APAP at therapeutic doses. This double-blind randomized controlled study (NCT02206178) was carried out in 24 healthy volunteers. APAP was given for 4 days (1 g ×4 daily) with NAC or with placebo. Thermal pain tests, whole blood GSH, and hepatic enzymes (ASAT, ALAT) were measured before (D0) and after (D4) oral APAP-NAC or APAP-placebo intake. anova for repeated measures adapted to cross-overdesign was performed and a two-tailed type I error was fixed at 5%. The primary endpoint was the area under the curve (0-240 min) of pain intensity (Numerical Scale) after thermal pain stimulation using Pathway-Medoc^® . APAP antinociceptive effect was similar in both groups. GSH was maintained to its baseline value in the APAP/NAC group but diminished in the APAP/placebo group (P = 0.033). This study shows for the first time that APAP antinociceptive effectiveness is not influenced by NAC. It also shows that the effect of APAP at therapeutic dosage on GSH may be counteracted by NAC. These issues are particularly important for patients as APAP is often prescribed for years as a first-line pain treatment and further trials in patients are now warranted.

TRPA1: a molecular view

The transient receptor potential ankyrin 1 (TRPA1) ion channel is expressed in pain-sensing neurons and other tissues and has become a major target in the development of novel pharmaceuticals. A remarkable feature of the channel is its long list of activators, many of which we are exposed to in daily life. Many of these agonists induce pain and inflammation, making TRPA1 a major target for anti-inflammatory and analgesic therapies. Studies in human patients and in experimental animals have confirmed an important role for TRPA1 in a number of pain conditions. Over the recent years, much progress has been made in elucidating the molecular structure of TRPA1 and in discovering binding sites and modulatory sites of the channel. Because the list of published mutations and important molecular sites is steadily growing and because it has become difficult to see the forest for the trees, this review aims at summarizing the current knowledge about TRPA1, with a special focus on the molecular structure and the known binding or gating sites of the channel.

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.

Long-term adverse effects of paracetamol - a review

Paracetamol (acetaminophen) is the most commonly used drug in the world, with a long record of use in acute and chronic pain. In recent years, the benefits of paracetamol use in chronic conditions has been questioned, notably in the areas of osteoarthritis and lower back pain. Over the same period, concerns over the long-term adverse effects of paracetamol use have increased, initially in the field of hypertension, but more recently in other areas as well. The evidence base for the adverse effects of chronic paracetamol use consists of many cohort and observational studies, with few randomized controlled trials, many of which contradict each other, so these studies must be interpreted with caution. Nevertheless, there are some areas where the evidence for harm is more robust, and if a clinician is starting paracetamol with the expectation of chronic use it might be advisable to discuss these side effects with patients beforehand. In particular, an increased risk of gastrointestinal bleeding and a small (~4 mmHg) increase in systolic blood pressure are adverse effects for which the evidence is particularly strong, and which show a degree of dose dependence. As our estimation of the benefits decreases, an accurate assessment of the harms is ever more important. The present review summarizes the current evidence on the harms associated with chronic paracetamol use, focusing on cardiovascular disease, asthma and renal injury, and the effects of in utero exposure.

TRP Channels as Novel Targets for Endogenous Ligands: Focus on Endocannabinoids and Nociceptive Signalling

Background:

Chronic pain is a significant clinical problem and a very complex pathophysiological phenomenon. There is growing evidence that targeting the endocannabinoid system may be a useful approach to pain alleviation. Classically, the system includes G protein-coupled receptors of the CB1 and CB2 subtypes and their endogenous ligands. More recently, several subtypes of the large superfamily of cation TRP channels have been coined as “ionotropic cannabinoid receptors”, thus highlighting their role in cannabinoid signalling. Thus, the aim of this review was to explore the intimate connection between several “painful” TRP channels, endocannabinoids and nociceptive signalling.

Methods:

Research literature on this topic was critically reviewed allowing us not only summarize the existing evidence in this area of research, but also propose several possible cellular mechanisms linking nociceptive and cannabinoid signaling with TRP channels.

Results:

We begin with an overview of physiology of the endocannabinoid system and its major components, namely CB1 and CB2 G protein-coupled receptors, their two most studied endogenous ligands, anandamide and 2-AG, and several enzymes involved in endocannabinoid biosynthesis and degradation. The role of different endocannabinoids in the regulation of synaptic transmission is then discussed in detail. The connection between the endocannabinoid system and several TRP channels, especially TRPV1-4, TRPA1 and TRPM8, is then explored, while highlighting the role of these same channels in pain signalling.

Conclusion:

There is increasing evidence implicating several TRP subtypes not only as an integral part of the endocannabinoid system, but also as promising molecular targets for pain alleviation with the use of endo- and phytocannabinoids, especially when the function of these channels is upregulated under inflammatory conditions.

An insight into paracetamol and its metabolites using molecular docking and molecular dynamics simulation

Paracetamol is a relatively safe analgesia/antipyretic drug without the risks of addiction, dependence, tolerance, and withdrawal when used alone. However, when administrated in an opioid/paracetamol combination product, which often contains a large quantity of paracetamol, it can be potentially dangerous due to the risk of hepatotoxicity. Paracetamol is known to be metabolized into N-(4-hydroxyphenyl)-arachidonamide (AM404) via fatty acid amide hydrolase (FAAH) and into N-acetyl-p-benzoquinone imine (NAPQI) via cytochrome P450 (CYP) enzymes. However, the underlying mechanism of paracetamol is still unclear. In addition, paracetamol has the potential to interact with other drugs that are also involved with CYP family enzymes (inducer/inhibitor/substrate), an example being illicit drugs. In our present work, we looked into the relationship between paracetamol and its metabolites (AM404 and NAPQI) using molecular docking and molecular dynamics (MD) simulations. We first carried out a series of molecular docking studies between paracetamol/AM404/NAQPI and their reported targets, including CYP 2E1, FAAH, TRPA1, CB1, and TRPV1. Subsequently, we performed MD simulations and energy decomposition for CB1-AM404, TRPV1-AM404, and TRPV1-NAPQI for further investigation of the dynamics interactions. Finally, we summarized and discussed the reported drug-drug interactions between paracetamol and central nervous system drugs, especially illicit drugs. Overall, we are able to provide new insights into the structural and functional roles of paracetamol and its metabolites that can inform the potential prevention and treatment of paracetamol overdose. Graphical abstract Paracetamol and its metabolites.

N,N'-Bis(2-mercaptoethyl)isophthalamide Binds Electrophilic Paracetamol Metabolites and Prevents Paracetamol-Induced Liver Toxicity

Paracetamol overdosing may cause liver injury including fulminant liver failure due to generation of the toxic metabolites, N-acetyl-p-benzoquinone imine (NAPQI) and p-benzoquinone (p-BQ). Herein, the chelating agent, N,N'-Bis(2-mercaptoethyl)isophthalamide (NBMI), was examined for its potential ability to entrap NAPQI and p-BQ and to prevent paracetamol-induced liver injury. Both NBMI and the conventional paracetamol antidote N-acetylcysteine (NAC) were investigated with regard to their abilities to scavenge the NAPQI and p-BQ in a Transient Receptor Potential Ankyrin 1-dependent screening assay. Stoichiometric evaluations indicated that NBMI was able to entrap these metabolites more efficiently than NAC. Furthermore, oral administration of either NBMI (680 mg/kg) or NAC (680 mg/kg) prevented the development of the characteristic liver necrosis and elevation of serum alanine aminotransferase in a mouse model for paracetamol-induced liver injury. In summary, these results show that NBMI is able to entrap the toxic metabolites NAPQI and p-BQ and to prevent paracetamol-induced liver injury in mice.

Structure-Activity Relationship of Cannabis Derived Compounds for the Treatment of Neuronal Activity-Related Diseases

Cannabis sativa active compounds are extensively studied for their therapeutic effects, beyond the well-known psychotropic activity. C. Sativa is used to treat different medical indications, such as multiple sclerosis, spasticity, epilepsy, ulcerative colitis and pain. Simultaneously, basic research is discovering new constituents of cannabis-derived compounds and their receptors capable of neuroprotection and neuronal activity modulation. The function of the various phytochemicals in different therapeutic processes is not fully understood, but their significant role is starting to emerge and be appreciated. In this review, we will consider the structure-activity relationship (SAR) of cannabinoid compounds able to bind to cannabinoid receptors and act as therapeutic agents in neuronal diseases, e.g., Parkinson’s disease.

Targeting nociceptive transient receptor potential channels to treat chronic pain: current state of the field

Control of chronic pain is frequently inadequate and/or associated with intolerable adverse effects, prompting a frantic search for new therapeutics and new therapeutic targets. Nearly two decades of preclinical and clinical research supports the involvement of transient receptor potential (TRP) channels in temperature perception, nociception and sensitization. Although there has been considerable excitement around the therapeutic potential of this channel family since the cloning and identification of TRPV1 cation channels as the capsaicin receptor more than 20 years ago, only modulators of a few channels have been tested clinically. TRPV1 channel antagonists have suffered from side effects related to the channel's role in temperature sensation; however, high dose formulations of capsaicin have reached the market and shown therapeutic utility. A number of potent, small molecule antagonists of TRPA1 channels have recently advanced into clinical trials for the treatment of inflammatory and neuropathic pain, and TRPM8 antagonists are following closely behind for cold allodynia. TRPV3, TRPV4, TRPM2 and TRPM3 channels have also been of significant interest. This review discusses the preclinical promise and status of novel analgesic agents that target TRP channels and the challenges that these compounds may face in development and clinical practice.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

The antipyretic effect of paracetamol occurs independent of transient receptor potential ankyrin 1-mediated hypothermia and is associated with prostaglandin inhibition in the brain

The mode of action of paracetamol (acetaminophen), which is widely used for treating pain and fever, has remained obscure, but may involve several distinct mechanisms, including cyclooxygenase inhibition and transient receptor potential ankyrin 1 (TRPA1) channel activation, the latter being recently associated with paracetamol's propensity to elicit hypothermia at higher doses. Here, we examined whether the antipyretic effect of paracetamol was due to TRPA1 activation or cyclooxygenase inhibition. Treatment of wild-type and TRPA1 knockout mice rendered febrile by immune challenge with LPS with a dose of paracetamol that did not produce hypothermia (150 mg/kg) but is known to be analgetic, abolished fever in both genotypes. Paracetamol completely suppressed the LPS-induced elevation of prostaglandin E₂ in the brain and also reduced the levels of several other prostanoids. The hypothermia induced by paracetamol was abolished in mice treated with the electrophile-scavenger N-acetyl cysteine. We conclude that paracetamol's antipyretic effect in mice is dependent on inhibition of cyclooxygenase activity, including the formation of pyrogenic prostaglandin E₂, whereas paracetamol-induced hypothermia likely is mediated by the activation of TRPA1 by electrophilic metabolites of paracetamol, similar to its analgesic effect in some experimental paradigms.-Mirrasekhian, E., Nilsson, J. L. Å., Shionoya, K., Blomgren, A., Zygmunt, P. M., Engblom, D., Högestätt, E. D., Blomqvist, A. The antipyretic effect of paracetamol occurs independent of transient receptor potential ankyrin 1-mediated hypothermia and is associated with prostaglandin inhibition in the brain.

Oxidative Stress and Inflammation Induced by Environmental and Psychological Stressors: A Biomarker Perspective

SIGNIFICANCE

The environment can elicit biological responses such as oxidative stress (OS) and inflammation as a consequence of chemical, physical, or psychological changes. As population studies are essential for establishing these environment-organism interactions, biomarkers of OS or inflammation are critical in formulating mechanistic hypotheses. Recent Advances: By using examples of stress induced by various mechanisms, we focus on the biomarkers that have been used to assess OS and inflammation in these conditions. We discuss the difference between biomarkers that are the result of a chemical reaction (such as lipid peroxides or oxidized proteins that are a result of the reaction of molecules with reactive oxygen species) and those that represent the biological response to stress, such as the transcription factor NRF2 or inflammation and inflammatory cytokines.

CRITICAL ISSUES

The high-throughput and holistic approaches to biomarker discovery used extensively in large-scale molecular epidemiological exposome are also discussed in the context of human exposure to environmental stressors.

FUTURE DIRECTIONS

We propose to consider the role of biomarkers as signs and to distinguish between signs that are just indicators of biological processes and proxies that one can interact with and modify the disease process. Antioxid. Redox Signal. 28, 852-872.

Hypertrophic osteoarthropathy: estrogens, prostaglandinE2, prostaglandin A2, and the inflammatory reflex

It has been claimed that hyperestrogenism occurs in hypertrophic osteoarthropathy (HOA), but not in simple clubbing. However, one of our patients had simple clubbing and hyperestrogenism. We therefore measured estrogens, androgens, sex hormone-binding globulin (SHBG), and gonadotropins in five patients with HOA and in 18 patients with simple clubbing. Of the patients with HOA, 80% had a high urinary estriol concentration. In their serum, 80% had high estrone, 0% high estradiol, and 40% high SHBG. Of the patients with simple clubbing, 89% had a high urinary estriol concentration. In their serum, 76% had high estrone, 6% high estradiol, and 31% high SHBG. In all patients, urinary estriol concentration correlated positively with the degree of clubbing. Serum concentration of androstenedione, testosterone, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) was mostly normal, but androstenedione concentration correlated positively with the degree of clubbing. Spider angiomas were present in 74%, palmar erythema in 39%, and gynecomastia in 9%. Urinary creatinine concentration was low in 48% and correlated positively with the degree of clubbing. We reject the claim that hyperestrogenism occurs in HOA, but not in simple clubbing. Hyperestrogenism occurs both in HOA and in simple clubbing. Our results also support earlier reports that clubbing and HOA are associated with spider angiomas, palmar erythema, gynecomastia, adrenal cortical hyperfunction, muscle atrophy, and water retention. These results led to a new hypothesis on the pathogenesis of HOA, involving estrogens, prostaglandin E₂, prostaglandin A₂, and the inflammatory reflex.

TRPA1 polymorphisms in chronic and complete spinal cord injury patients with neuropathic pain: a pilot study

Study design

Pilot study.

Objectives

Single-nucleotide polymorphisms (SNPs) in TRPA1 gene are related to the etiology of chronic pain. The study is a pilot study with the primary objective of analyzing these SNPs in Spanish patients with chronic and complete spinal cord injury (SCI) and neuropathic pain (NPP).

Setting

Asepeyo Hospital Department of Chronic and Complete SCI.

Methods

Twelve patients with chronic and complete SCI and NPP, and 12 patients with chronic and complete SCI with no pain were reviewed. International Spinal Cord Injury Pain Classification (LANSS) and visual analog score (VAS) were chosen to classify pain syndrome. SNPs were identified by melting analysis after DNA amplification with real-time fluorescence PCR.

Results

There were differences in rs11988795 variant: GG homozygous (p = 0.01) and G allele (p = 0.001) were more frequent in SCI patients with no pain. There were differences in rs13255063 variant: TT homozygous were prevalent (p = 0.03) in patients with NPP.

Conclusions

Until now this is the first study to show a description of TRPA1 SNPs in Spanish patients with chronic and complete SCI and NPP. These results suggest that GG genotype in rs11988795 variant and G allele could be protective factors against NPP. TT genotype in rs13255063 variant could be a risk factor for NPP. Neuropathic pain after spinal cord injuries may have genetic contributions.

Anxiety- and activity-related effects of paracetamol on healthy and neuropathic rats

Paracetamol has recently been suggested to affect emotion processing in addition to alleviating pain in humans. We investigated in adult male Hannover-Wistar rats whether acute intraperitoneally administrated paracetamol affects behavior in tests measuring anxiety, mood, motor activity, and memory. Unoperated rats received saline or a low (50 mg/kg) or high (300 mg/kg) dose of paracetamol, while rats with a spared nerve injury (SNI) model of neuropathy and sham-operated rats received saline or the low dose of paracetamol. Rats were tested on open-field (OFT), elevated plus-maze (EPM), light-dark box (LDB), novel-object recognition (NOR), sucrose preference, rotarod, and monofilament tests. In unoperated rats, both the low and high dose of paracetamol reduced line crossings, and grooming time in the OFT, and novel preference in NOR. The high dose of paracetamol increased the time spent in the closed arm in EPM, reduced the number of rearings and leanings in OFT, the time spent in the light box in LDB, and sucrose preference. Paracetamol had no significant effect on the rotarod test measuring motor activity. The low dose of paracetamol suppressed mechanical pain hypersensitivity in SNI rats, without influencing pain behavior in sham-operated rats. Saline- but not paracetamol-treated SNI rats spent more time than sham-operated rats in the closed arm in the EPM test. Together the results suggest that a high dose of paracetamol increases anxiety-like and anhedonic behavior, and impairs recognition memory in unoperated controls, while in neuropathy, a low dose of paracetamol reduces nerve injury-associated anxiety probably by reducing neuropathic pain.

Activation of the capsaicin-receptor TRPV1 by the acetaminophen metabolite N-arachidonoylaminophenol results in cytotoxicity

AIMS

The anandamide reuptake inhibitor N-arachidonoylaminophenol (AM404) and the reactive substance N-acetyl-p-benzoquinone imine (NAPQI) are both metabolites of acetaminophen and may contribute to acetaminophen-induced analgesia by acting at TRPV1 expressed in the peripheral or central nervous system. While NAPQI slowly sensitizes and activates TRPV1 by interacting with distinct intracellular cysteine residues, detailed properties of AM404 as an agonist of TRPV1 have not yet been reported on. We explored the effects of AM404 on recombinant human TRPV1 and in rodent dorsal root ganglion (DRG) neurons.

MATERIALS AND METHODS

HEK 293 cells expressing different isoforms of recombinant TRPV1 and rodent DRG neurons were employed for patch clamp and calcium imaging experiments. Cytotoxicity was assessed by propidium iodide and Annexin V staining on TRPV1-HEK 293 cells and with trypan blue staining on DRG neurons.

KEY FINDINGS

AM404 activates hTRPV1 at concentrations >1μM and in a concentration-dependent manner. AM404 also potentiates TRPV1-mediated currents evoked by heat and anandamide. Moreover, AM404-evoked currents are potentiated by NAPQI. While the partly capsaicin-insensitive rabbit (o) TRPV1 fails to respond to AM404, AM404-sensitivity is restored by insertion of the capsaicin binding-domain of rat TRPV1 into oTRPV1. In DRG neurons, AM404-evoked calcium influx as well as cell death is mediated by TRPV1.

SIGNIFICANCE

AM404 gates TRPV1 by interacting with the vanilloid-binding site, and TRPV1 is the main receptor for AM404 in DRG neurons. While direct activation of TRPV1 requires high concentrations of AM404, it is possible that synergistic effects of AM404 with further TRPV1-agonists may occur at clinically relevant concentrations.

Prediction of metabolism-induced hepatotoxicity on three-dimensional hepatic cell culture and enzyme microarrays

Human liver contains various oxidative and conjugative enzymes that can convert nontoxic parent compounds to toxic metabolites or, conversely, toxic parent compounds to nontoxic metabolites. Unlike primary hepatocytes, which contain myriad drug-metabolizing enzymes (DMEs), but are difficult to culture and maintain physiological levels of DMEs, immortalized hepatic cell lines used in predictive toxicity assays are easy to culture, but lack the ability to metabolize compounds. To address this limitation and predict metabolism-induced hepatotoxicity in high-throughput, we developed an advanced miniaturized three-dimensional (3D) cell culture array (DataChip 2.0) and an advanced metabolizing enzyme microarray (MetaChip 2.0). The DataChip is a functionalized micropillar chip that supports the Hep3B human hepatoma cell line in a 3D microarray format. The MetaChip is a microwell chip containing immobilized DMEs found in the human liver. As a proof of concept for generating compound metabolites in situ on the chip and rapidly assessing their toxicity, 22 model compounds were dispensed into the MetaChip and sandwiched with the DataChip. The IC₅₀ values obtained from the chip platform were correlated with rat LD₅₀ values, human C _max values, and drug-induced liver injury categories to predict adverse drug reactions in vivo. As a result, the platform had 100% sensitivity, 86% specificity, and 93% overall predictivity at optimum cutoffs of IC₅₀ and C _max values. Therefore, the DataChip/MetaChip platform could be used as a high-throughput, early stage, microscale alternative to conventional in vitro multi-well plate platforms and provide a rapid and inexpensive assessment of metabolism-induced toxicity at early phases of drug development.

Acetaminophen Relieves Inflammatory Pain through CB1 Cannabinoid Receptors in the Rostral Ventromedial Medulla

Acetaminophen (paracetamol) is a widely used analgesic and antipyretic drug with only incompletely understood mechanisms of action. Previous work, using models of acute nociceptive pain, indicated that analgesia by acetaminophen involves an indirect activation of CB₁ receptors by the acetaminophen metabolite and endocannabinoid reuptake inhibitor AM 404. However, the contribution of the cannabinoid system to antihyperalgesia against inflammatory pain, the main indication of acetaminophen, and the precise site of the relevant CB₁ receptors have remained elusive. Here, we analyzed acetaminophen analgesia in mice of either sex with inflammatory pain and found that acetaminophen exerted a dose-dependent antihyperalgesic action, which was mimicked by intrathecally injected AM 404. Both compounds lost their antihyperalgesic activity in CB₁^-/- mice, confirming the involvement of the cannabinoid system. Consistent with a mechanism downstream of proinflammatory prostaglandin formation, acetaminophen also reversed hyperalgesia induced by intrathecal prostaglandin E₂ To distinguish between a peripheral/spinal and a supraspinal action, we administered acetaminophen and AM 404 to hoxB8-CB₁^-/- mice, which lack CB₁ receptors from the peripheral nervous system and the spinal cord. These mice exhibited unchanged antihyperalgesia indicating a supraspinal site of action. Accordingly, local injection of the CB₁ receptor antagonist rimonabant into the rostral ventromedial medulla blocked acetaminophen-induced antihyperalgesia, while local rostral ventromedial medulla injection of AM 404 reduced hyperalgesia in wild-type mice but not in CB₁^-/- mice. Our results indicate that the cannabinoid system contributes not only to acetaminophen analgesia against acute pain but also against inflammatory pain, and suggest that the relevant CB₁ receptors reside in the rostral ventromedial medulla.SIGNIFICANCE STATEMENT Acetaminophen is a widely used analgesic drug with multiple but only incompletely understood mechanisms of action, including a facilitation of endogenous cannabinoid signaling via one of its metabolites. Our present data indicate that enhanced cannabinoid signaling is also responsible for the analgesic effects of acetaminophen against inflammatory pain. Local injections of the acetaminophen metabolite AM 404 and of cannabinoid receptor antagonists as well as data from tissue-specific CB₁ receptor-deficient mice suggest the rostral ventromedial medulla as an important site of the cannabinoid-mediated analgesia by acetaminophen.

TRPA1 and TRPV1 are required for lidocaine-evoked calcium influx and neuropeptide release but not cytotoxicity in mouse sensory neurons

Background

Local anaesthetics (LA) reduce neuronal excitability by inhibiting voltage-gated Na⁺ channels. When applied at high concentrations in the direct vicinity of nerves, LAs can also induce relevant irritation and neurotoxicity via mechanisms involving an increase of intracellular Ca²⁺. In the present study we explored the role of the Ca²⁺-permeable ion channels TRPA1 and TRPV1 for lidocaine-induced Ca²⁺-influx, neuropeptide release and neurotoxicity in mouse sensory neurons.

Methods

Cultured dorsal root ganglion (DRG) neurons from wildtype and mutant mice lacking TRPV1, TRPA1 or both channels were explored by means of calcium imaging, whole-cell patch clamp recordings and trypan blue staining for cell death. Release of calcitonin gene-related peptide (CGRP) from isolated mouse peripheral nerves was determined with ELISA.

Results

Lidocaine up to 10 mM induced a concentration-dependent reversible increase in intracellular Ca²⁺ in DRG neurons from wildtype and mutant mice lacking one of the two receptors, but not in neurons lacking both TRPA1 and TRPV1. 30 mM lidocaine also released Ca²⁺ from intracellular stores, presumably from the endoplasmic reticulum. While 10 mM lidocaine evoked an axonal CGRP release requiring expression of either TRPA1 or TRPV1, CGRP release induced by 30 mM lidocaine again mobilized internal Ca²⁺ stores. Lidocaine-evoked cell death required neither TRPV1 nor TRPA1.

Summary

Depending on the concentration, lidocaine employs TRPV1, TRPA1 and intracellular Ca²⁺ stores to induce a Ca²⁺-dependent release of the neuropeptide CGRP. Lidocaine-evoked cell death does not seem to require Ca²⁺ influx through TRPV1 or TRPV1.