Ca(v)3.2 calcium channels: the key protagonist in the supraspinal effect of paracetamol

Involvement of cannabinoid receptors in depression of the putative nociceptive response in spinal cord preparations isolated from neonatal rats

A metabolite of acetaminophen, AM404, which is an anandamide transporter inhibitor, induces analgesia mainly via activation of transient receptor potential channel 1 in the spinal cord, although the role of cannabinoid receptors remains to be studied. The ventral root reflex response induced by stimulation of the dorsal root in in vitro preparations of rat spinal cord is useful to assess the effect of analgesics. We analyzed the effects of AM404 and cannabinoid receptor antagonist AM251 on reflex responses in lumbar spinal cord preparations from newborn rats and found that the amplitude of the slow ventral root potential after administration of 10 µM AM404 was not significantly changed, whereas 10 µM AM251 significantly increased the amplitude. Administration of the cannabinoid receptor 1 agonist WIN55,212-2 (10 µM) did not significantly affect the reflex response. We suggest that endogenous cannabinoids in the spinal cord are involved in the antinociceptive mechanism through suppressive effects.

An Updated Review on the Metabolite (AM404)-Mediated Central Mechanism of Action of Paracetamol (Acetaminophen): Experimental Evidence and Potential Clinical Impact

Paracetamol remains the recommended first-line option for mild-to-moderate acute pain in general population and particularly in vulnerable populations. Despite its wide use, debate exists regarding the analgesic mechanism of action (MoA) of paracetamol. A growing body of evidence challenged the notion that paracetamol exerts its analgesic effect through cyclooxygenase (COX)-dependent inhibitory effect. It is now more evident that paracetamol analgesia has multiple pathways and is mediated by the formation of the bioactive AM404 metabolite in the central nervous system (CNS). AM404 is a potent activator of TRPV₁, a major contributor to neuronal response to pain in the brain and dorsal horn. In the periaqueductal grey, the bioactive metabolite AM404 activated the TRPV₁ channel-mGlu5 receptor-PLC-DAGL-CB1 receptor signaling cascade. The present article provides a comprehensive literature review of the centrally located, COX-independent, analgesic MoA of paracetamol and relates how the current experimental evidence can be translated into clinical practice. The evidence discussed in this review established paracetamol as a central, COX-independent, antinociceptive medication that has a distinct MoA from non-steroidal anti-inflammatory drugs (NSAIDs) and a more tolerable safety profile. With the establishment of the central MoA of paracetamol, we believe that paracetamol remains the preferred first-line option for mild-to-moderate acute pain for healthy adults, children, and patients with health concerns. However, safety concerns remain with the high dose of paracetamol due to the NAPQI-mediated liver necrosis. Centrally acting paracetamol/p-aminophenol derivatives could potentiate the analgesic effect of paracetamol without increasing the risk of hepatoxicity. Moreover, the specific central MoA of paracetamol allows its combination with other analgesics, including NSAIDs, with a different MoA. Future experiments to better explain the central actions of paracetamol could pave the way for discovering new central analgesics with a better benefit-to-risk ratio.

Role of T CD4+ cells, macrophages, C-low threshold mechanoreceptors and spinal Cav 3.2 channels in inflammation and related pain-like symptoms in murine inflammatory models

BACKGROUND AND PURPOSE

T-type calcium channels, mainly the Ca_v 3.2 subtype, are important contributors to the nociceptive signalling pathway. We investigated their involvement in inflammation and related pain-like symptoms.

EXPERIMENTAL APPROACH

The involvement of Ca_v 3.2 and T-type channels was investigated using genetic and pharmacological inhibition to assess mechanical allodynia/hyperalgesia and oedema development in two murine inflammatory pain models. The location of Ca_v 3.2 channels involved in pain-like symptoms was studied in mice with Ca_v 3.2 knocked out in C-low threshold mechanoreceptors (C-LTMR) and the use of ABT-639, a peripherally restricted T-type channel inhibitor. The anti-oedema effect of Ca_v 3.2 channel inhibition was investigated in chimeric mice with immune cells deleted for Ca_v 3.2. Lymphocytes and macrophages from either green fluorescent protein-targeted Ca_v 3.2 or KO mice were used to determine the expression of Ca_v 3.2 protein and the functional status of the cells.

KEY RESULTS

Ca_v 3.2 channels contributed to the development of pain-like symptoms and oedema in the two murine inflammatory pain models. Our results provided evidence of the involvement of Ca_v 3.2 channels located on C-LTMRs and spinal cord in inflammatory pain. Ca_v 3.2 channels located in T cells and macrophages contribute to the inflammatory process.

CONCLUSION AND IMPLICATIONS

Ca_v 3.2 channels play crucial roles in inflammation and related pain, implying that targeting of Ca_v 3.2 channels with pharmacological agents could be an attractive and readily evaluable strategy in clinical trials, to relieve chronic inflammatory pain in patients.

Identification of a human estrogen receptor α tetrapeptidic fragment with dual antiproliferative and anti-nociceptive action

The synthetic peptide ERα17p (sequence: PLMIKRSKKNSLALSLT), which corresponds to the 295-311 region of the human estrogen receptor α (ERα), induces apoptosis in breast cancer cells. In mice and at low doses, it promotes not only the decrease of the size of xenografted triple-negative human breast tumors, but also anti-inflammatory and anti-nociceptive effects. Recently, we have shown that these effects were due to its interaction with the seven-transmembrane G protein-coupled estrogen receptor GPER. Following modeling studies, the C-terminus of this peptide (sequence: NSLALSLT) remains compacted at the entrance of the GPER ligand-binding pocket, whereas its N-terminus (sequence: PLMI) engulfs in the depth of the same pocket. Thus, we have hypothesized that the PLMI motif could support the pharmacological actions of ERα17p. Here, we show that the PLMI peptide is, indeed, responsible for the GPER-dependent antiproliferative and anti-nociceptive effects of ERα17p. By using different biophysical approaches, we demonstrate that the NSLALSLT part of ERα17p is responsible for aggregation. Overall, the tetrapeptide PLMI, which supports the action of the parent peptide ERα17p, should be considered as a hit for the synthesis of new GPER modulators with dual antiproliferative and anti-nociceptive actions. This study highlights also the interest to modulate GPER for the control of pain.

Analgesic Action of Acetaminophen via Kv7 Channels

The mechanism of acetaminophen (APAP) analgesia is at least partially unknown. Previously, we showed that the APAP metabolite N-acetyl-p-benzoquinone imine (NAPQI) activated Kv7 channels in neurons in vitro, and this activation of Kv7 channels dampened neuronal firing. Here, the effect of the Kv7 channel blocker XE991 on APAP-induced analgesia was investigated in vivo. APAP had no effect on naive animals. Induction of inflammation with λ-carrageenan lowered mechanical and thermal thresholds. Systemic treatment with APAP reduced mechanical hyperalgesia, and co-application of XE991 reduced APAP's analgesic effect on mechanical pain. In a second experiment, the analgesic effect of systemic APAP was not antagonized by intrathecal XE991 application. Analysis of liver samples revealed APAP and glutathione-coupled APAP indicative of metabolization. However, there were no relevant levels of these metabolites in cerebrospinal fluid, suggesting no relevant APAP metabolite formation in the CNS. In summary, the results support an analgesic action of APAP by activating Kv7 channels at a peripheral site through formation of the metabolite NAPQI.

Preparation and Functional Identification of a Novel Conotoxin QcMNCL-XIII0.1 from Conus quercinus

Conotoxins are tools used by marine Conus snails to hunt and are a significant repository for marine drug research. Conotoxins highly selectively coordinate different subtypes of various ion channels, and a few have been used in pain management. Although more than 8000 conotoxin genes have been found, the biological activity and function of most have not yet been examined. In this report, we selected the toxin gene QcMNCL-XIII0.1 from our previous investigation and studied it in vitro. First, we successfully prepared active recombinant QcMNCL-XIII0.1 using a TrxA (Thioredoxin A)-assisted folding expression vector based on genetic engineering technology. Animal experiments showed that the recombinant QcMNCL-XIII0.1 exhibited nerve conduction inhibition similar to that of pethidine hydrochloride. With flow cytometry combined fluorescent probe Fluo-4 AM, we found that 10 ng/μL recombinant QcMNCL-XIII0.1 inhibited the fluorescence intensity by 31.07% in the 293T cell model transfected with Cav3.1, implying an interaction between α1G T-type calcium channel protein and recombinant QcMNCL-XIII0.1. This toxin could be an important drug in biomedical research and medicine for pain control.

Anti-Inflammatory and Anti-Oxidative Effects of AM404 in IL-1β-Stimulated SK-N-SH Neuroblastoma Cells

An emerging number of studies address the involvement of neuroinflammation and oxidative stress in the pathophysiology of central nervous system (CNS) disorders such as depression, schizophrenia, anxiety, and neurodegenerative diseases. Different cytokines and molecules, such as prostaglandin (PG) E₂, are associated with neuroinflammatory processes. The active acetaminophen metabolite AM404 has been shown to prevent inflammation and neuroinflammation in primary microglia and organotypic hippocampal slice cultures. However, its effects on pathophysiological conditions in the CNS and especially on neurons are still poorly understood. In this study, we therefore evaluated the effects of AM404 and acetaminophen on the arachidonic acid cascade and oxidative stress induced by interleukin (IL)-1β in human SK-N-SH neuronal cells. We observed that AM404 and acetaminophen significantly and concentration-dependent inhibited IL-1β-induced release of PGE₂, independent of cyclooxygenases (COX)-1 and COX-2 enzymatic activity as well as COX-2 mRNA and protein levels in SK-N-SH-cells. The reduction of IL-1β-induced PGE₂-release by AM404 and acetaminophen treatment might be mediated by the 8-iso-PGF_2α pathway since IL-1β-induced synthesis of this free radical marker is dose-dependently reduced by both compounds, respectively. Therefore, understanding of the potential therapeutic properties of AM404 in neuroinflammation and oxidative stress might lead to future treatment options of different neurological disorders.

Painful diabetic neuropathy leads to functional CaV3.2 expression and spontaneous activity in skin nociceptors of mice

Painful diabetic neuropathy occurs in approximately 20% of diabetic patients with underlying pathomechanisms not fully understood. We evaluated the contribution of the CaV3.2 isoform of T-type calcium channel to hyperglycemia-induced changes in cutaneous sensory C-fiber functions and neuropeptide release employing the streptozotocin (STZ) diabetes model in congenic mouse strains including global knockouts (KOs). Hyperglycemia established for 3-5 weeks in male C57BL/6J mice led to major reorganizations in peripheral C-fiber functions. Unbiased electrophysiological screening of mechanosensitive single-fibers in isolated hairy hindpaw skin revealed a relative loss of (polymodal) heat sensing in favor of cold sensing. In healthy CaV3.2 KO mice both heat and cold sensitivity among the C-fibers seemed underrepresented in favor of exclusive mechanosensitivity, low-threshold in particular, which deficit became significant in the diabetic KOs. Diabetes also led to a marked increase in the incidence of spontaneous discharge activity among the C-fibers of wildtype mice, which was reduced by the specific CaV3.2 blocker TTA-P2 and largely absent in the KOs. Evaluation restricted to the peptidergic class of nerve fibers - measuring KCl-stimulated CGRP release - revealed a marked reduction in the sciatic nerve by TTA-P2 in healthy but not diabetic wildtypes, the latter showing CGRP release that was as much reduced as in healthy and, to the same extent, in diabetic CaV3.2 KOs. These data suggest that diabetes abrogates all CaV3.2 functionality in the peripheral nerve axons. In striking contrast, diabetes markedly increased the KCl-stimulated CGRP release from isolated hairy skin of wildtypes but not KO mice, and TTA-P2 reversed this increase, strongly suggesting a de novo expression of CaV3.2 in peptidergic cutaneous nerve endings which may contribute to the enhanced spontaneous activity. De-glycosylation by neuraminidase showed clear desensitizing effects, both in regard to spontaneous activity and stimulated CGRP release, but included actions independent of CaV3.2. However, as diabetes-enhanced glycosylation is decisive for intra-axonal trafficking, it may account for the substantial reorganizations of the CaV3.2 distribution. The results may strengthen the validation of CaV3.2 channel as a therapeutic target of treating painful diabetic neuropathy.

Systematic Review of Systemic and Neuraxial Effects of Acetaminophen in Preclinical Models of Nociceptive Processing

Acetaminophen (APAP) in humans has robust effects with a high therapeutic index in altering postoperative and inflammatory pain states in clinical and experimental pain paradigms with no known abuse potential. This review considers the literature reflecting the preclinical actions of acetaminophen in a variety of pain models. Significant observations arising from this review are as follows: 1) acetaminophen has little effect upon acute nociceptive thresholds; 2) acetaminophen robustly reduces facilitated states as generated by mechanical and thermal hyperalgesic end points in mouse and rat models of carrageenan and complete Freund’s adjuvant evoked inflammation; 3) an antihyperalgesic effect is observed in models of facilitated processing with minimal inflammation (eg, phase II intraplantar formalin); and 4) potent anti-hyperpathic effects on the thermal hyperalgesia, mechanical and cold allodynia, allodynic thresholds in rat and mouse models of polyneuropathy and mononeuropathies and bone cancer pain. These results reflect a surprisingly robust drug effect upon a variety of facilitated states that clearly translate into a wide range of efficacy in preclinical models and to important end points in human therapy. The specific systems upon which acetaminophen may act based on targeted delivery suggest both a spinal and a supraspinal action. Review of current targets for this molecule excludes a role of cyclooxygenase inhibitor but includes effects that may be mediated through metabolites acting on the TRPV1 channel, or by effect upon cannabinoid and serotonin signaling. These findings suggest that the mode of action of acetaminophen, a drug with a long therapeutic history of utilization, has surprisingly robust effects on a variety of pain states in clinical patients and in preclinical models with a good therapeutic index, but in spite of its extensive use, its mechanisms of action are yet poorly understood.

Synergistic interaction between amitriptyline and paracetamol in persistent and neuropathic pain models: An isobolografic analysis

The current study was designed to evaluate the transient antinociceptive interaction between amitriptyline and paracetamol in the formalin test. In addition, considering other long-term neuroprotective mechanisms of these drugs, we hypothesized that this combination might exert some synergistic effects on neuropathic pain linked with its possible ability to prevent Wallerian degeneration (WD). The effects of individual and fixed-ratio of 1:1 combinations of orally administered amitriptyline and paracetamol were assayed in the two phases of the formalin test and in the chronic constriction injury (CCI) model in rats. Isobolographic analysis was employed to characterize the synergism produced by the combinations. Amitriptyline, paracetamol, and fixed-ratio amitriptyline-paracetamol combinations produced dose-dependent antinociceptive effects mainly on the inflammatory tonic phase. Repeated doses of individual drugs and their combination decreased CCI-induced mechanical allodynia in a dose-dependent manner. ED₃₀ (formalin) and ED₅₀ (CCI) values were estimated for the individual drugs, and isobolograms were constructed. Theoretical ED_30/50 values for the combination estimated from the isobolograms were 16.5 ± 3.9 mg/kg and 26.0 ± 7.2 mg/kg for the single and repeated doses in persistent and neuropathic pain models, respectively. These values were significantly higher than the actually observed ED_30/50 values, which were 0.39 ± 0.1 mg/kg and 8.2 ± 0.8 mg/kg in each model, respectively, indicating a synergistic interaction. Remarkably, CCI-induced sciatic nerve WD-related histopathological changes were prevented by this combination compared to either drug administered alone.

Treating osteoarthritis pain: mechanisms of action of acetaminophen, nonsteroidal anti-inflammatory drugs, opioids, and nerve growth factor antibodies

Osteoarthritis (OA) is a common difficult-to-treat condition where the goal, in the absence of disease-modifying treatments, is to alleviate symptoms such as pain and loss of function. Acetaminophen, nonsteroidal anti-inflammatory drugs (NSAIDs), and opioids are common pharmacologic treatments for OA. Antibodies directed against nerve growth factor (NGF-Abs) are a new class of agents under clinical investigation for the treatment of OA. This narrative review describes (and uses schematics to visualize) nociceptive signaling, chronification of pain, and the mechanisms of action (MOAs) of these different analgesics in the context of OA-related pain pathophysiology. Further, the varying levels of efficacy and safety of these agents observed in patients with OA is examined, based on an overview of published clinical data and/or treatment guidelines (when available), in the context of differences in their MOAs.

Paracetamol (acetaminophen): A familiar drug with an unexplained mechanism of action

Paracetamol (acetaminophen) is undoubtedly one of the most widely used drugs worldwide. As an over-the-counter medication, paracetamol is the standard and first-line treatment for fever and acute pain and is believed to remain so for many years to come. Despite being in clinical use for over a century, the precise mechanism of action of this familiar drug remains a mystery. The oldest and most prevailing theory on the mechanism of analgesic and antipyretic actions of paracetamol relates to the inhibition of CNS cyclooxygenase (COX) enzyme activities, with conflicting views on the COX isoenzyme/variant targeted by paracetamol and on the nature of the molecular interactions with these enzymes. Paracetamol has been proposed to selectively inhibit COX-2 by working as a reducing agent, despite the fact that in vitro screens demonstrate low potency on the inhibition of COX-1 and COX-2. In vivo data from COX-1 transgenic mice suggest that paracetamol works through inhibition of a COX-1 variant enzyme to mediate its analgesic and particularly thermoregulatory actions (antipyresis and hypothermia). A separate line of research provides evidence on potentiation of the descending inhibitory serotonergic pathway to mediate the analgesic action of paracetamol, but with no evidence of binding to serotonergic molecules. AM404 as a metabolite for paracetamol has been proposed to activate the endocannabinoid and the transient receptor potential vanilloid-1 (TRPV1) systems. The current review gives an update and in some cases challenges the different theories on the pharmacology of paracetamol and raises questions on some of the inadequately explored actions of paracetamol. List of Abbreviations: AM404, N-(4-hydroxyphenyl)-arachidonamide; CB1R, Cannabinoid receptor-1; Cmax, Maximum concentration; CNS, Central nervous system; COX, Cyclooxygenase; CSF, Cerebrospinal fluid; ED50, 50% of maximal effective dose; FAAH, Fatty acid amidohydrolase; IC50, 50% of the maximal inhibitor concentration; LPS, Lipopolysaccharide; NSAIDs, Non-steroidal anti-inflammatory drugs; PGE2, Prostaglandin E2; TRPV1, Transient receptor potential vanilloid-1.

Paracetamol - An old drug with new mechanisms of action

Paracetamol (acetaminophen) is the most commonly used over-the-counter (OTC) drug in the world. Despite its popularity and use for many years, the safety of its application and its mechanism of action are still unclear. Currently, it is believed that paracetamol is a multidirectional drug and at least several metabolic pathways are involved in its analgesic and antipyretic action. The mechanism of paracetamol action consists in inhibition of cyclooxygenases (COX-1, COX-2, and COX-3) and involvement in the endocannabinoid system and serotonergic pathways. Additionally, paracetamol influences transient receptor potential (TRP) channels and voltage-gated Kv7 potassium channels and inhibits T-type Cav3.2 calcium channels. It also exerts an impact on L-arginine in the nitric oxide (NO) synthesis pathway. However, not all of these effects have been clearly confirmed. Therefore, the aim of our paper was to summarize the current state of knowledge of the mechanism of paracetamol action with special attention to its safety concerns.

The Antitumor Peptide ERα17p Exerts Anti-Hyperalgesic and Anti-Inflammatory Actions Through GPER in Mice

Persistent inflammation and persistent pain are major medical, social and economic burdens. As such, related pharmacotherapy needs to be continuously improved. The peptide ERα17p, which originates from a part of the hinge region/AF2 domain of the human estrogen receptor α (ERα), exerts anti-proliferative effects in breast cancer cells through a mechanism involving the hepta-transmembrane G protein-coupled estrogen receptor (GPER). It is able to decrease the size of xenografted human breast tumors, in mice. As GPER has been reported to participate in pain and inflammation, we were interested in exploring the potential of ERα17p in this respect. We observed that the peptide promoted anti-hyperalgesic effects from 2.5 mg/kg in a chronic mice model of paw inflammation induced by the pro-inflammatory complete Freund's adjuvant (CFA). This action was abrogated by the specific GPER antagonist G-15, leading to the conclusion that a GPER-dependent mechanism was involved. A systemic administration of a Cy5-labeled version of the peptide allowed its detection in both, the spinal cord and brain. However, ERα17p-induced anti-hyperalgesia was detected at the supraspinal level, exclusively. In the second part of the study, we have assessed the anti-inflammatory action of ERα17p in mice using a carrageenan-evoked hind-paw inflammation model. A systemic administration of ERα17p at a dose of 2.5 mg/kg was responsible for reduced paw swelling. Overall, our work strongly suggests that GPER inverse agonists, including ERα17p, could be used to control hyperalgesia and inflammation.

The Contribution of Serotonergic Receptors and Nitric Oxide Systems in the Analgesic Effect of Acetaminophen: An Overview of the Last Decade

Acetaminophen is a widely used analgesic and antipyretic agent. It is also available in over the counter formulations, which has increased its wide use. There have been many studies to date that have aimed to evaluate the mechanism of the analgesic action of acetaminophen. Additional to the inhibition of the cyclooxygenase pathway in the central nervous system, the involvement of opioidergic, cannabinoidergic, dopaminergic, cholinergic, and nitrergic systems as well as the contribution of descending pain inhibitory systems like the bulbospinal serotonergic pathway has been proposed as possible mechanisms of the analgesic action of acetaminophen. In this review, we aimed to collect the data from studies revealing the contribution of the central serotonergic system and the role of central nervous system-located serotonergic receptor subtypes in the analgesic effect of acetaminophen. While doing this, we mainly focused on the research that has been performed in the last ten years and tried to link the previous data with the lately added results. In addition to serotonergic system involvement, we also reviewed the role of nitric oxide in the analgesic action of acetaminophen, especially with the new findings reported over the last decade.

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.

Paracetamol is a centrally acting analgesic using mechanisms located in the periaqueductal grey

BACKGROUND AND PURPOSE

We previously demonstrated that paracetamol has to be metabolised in the brain by fatty acid amide hydrolase enzyme into AM404 (N-(4-hydroxyphenyl)-5Z,8Z,11Z,14Z-eicosatetraenamide) to activate CB₁ receptors and TRPV1 channels, which mediate its analgesic effect. However, the brain mechanisms supporting paracetamol-induced analgesia remain unknown.

EXPERIMENTAL APPROACH

The effects of paracetamol on brain function in Sprague-Dawley rats were determined by functional MRI. Levels of neurotransmitters in the periaqueductal grey (PAG) were measured using in vivo ¹ H-NMR and microdialysis. Analgesic effects of paracetamol were assessed by behavioural tests and challenged with different inhibitors, administered systemically or microinjected in the PAG.

KEY RESULTS

Paracetamol decreased the connectivity of major brain structures involved in pain processing (insula, somatosensory cortex, amygdala, hypothalamus, and the PAG). This effect was particularly prominent in the PAG, where paracetamol, after conversion to AM404, (a) modulated neuronal activity and functional connectivity, (b) promoted GABA and glutamate release, and (c) activated a TRPV1 channel-mGlu₅ receptor-PLC-DAGL-CB₁ receptor signalling cascade to exert its analgesic effects.

CONCLUSIONS AND IMPLICATIONS

The elucidation of the mechanism of action of paracetamol as an analgesic paves the way for pharmacological innovations to improve the pharmacopoeia of analgesic agents.

KLHL1 Controls CaV3.2 Expression in DRG Neurons and Mechanical Sensitivity to Pain

Dorsal root ganglion (DRG) neurons process pain signaling through specialized nociceptors located in their peripheral endings. It has long been established low voltage-activated (LVA) CaV3.2 calcium channels control neuronal excitability during sensory perception in these neurons. Silencing CaV3.2 activity with antisense RNA or genetic ablation results in anti-nociceptive, anti-hyperalgesic and anti-allodynic effects. CaV3.2 channels are regulated by many proteins (Weiss and Zamponi, 2017), including KLHL1, a neuronal actin-binding protein that stabilizes channel activity by recycling it back to the plasma membrane through the recycling endosome. We explored whether manipulation of KLHL1 levels and thereby function as a CaV3.2 modifier can modulate DRG excitability and mechanical pain transmission or sensitivity to pain. We first assessed the mechanical sensitivity threshold and DRG properties in the KLHL1 KO mouse model. KO DRG neurons exhibited smaller T-type current density compared to WT without significant changes in voltage dependence, as expected in the absence of its modulator. Western blot analysis confirmed CaV3.2 but not CaV3.1, CaV3.3, CaV2.1, or CaV2.2 protein levels were significantly decreased; and reduced neuron excitability and decreased pain sensitivity were also found in the KLHL1 KO model. Analogously, transient down-regulation of KLHL1 levels in WT mice with viral delivery of anti-KLHL1 shRNA also resulted in decreased pain sensitivity. These two experimental approaches confirm KLHL1 as a physiological modulator of excitability and pain sensitivity, providing a novel target to control peripheral pain.

Neuroprotective Effect of AM404 Against NMDA-Induced Hippocampal Excitotoxicity

Different studies have demonstrated that inflammation and alterations in glutamate neurotransmission are two events contributing to the pathophysiology of neurodegenerative or neurological disorders. There are evidences that N-arachidonoylphenolamine (AM404), a cannabinoid system modulator and paracetamol metabolite, modulates inflammation and exerts neuroprotective effects on Huntington's (HD) and Parkinson's diseases (PD), and ischemia. However, the effects of AM404 on the production of inflammatory mediators and excitotoxicity in brain tissue stimulated with N-methyl-D-aspartic acid (NMDA) are not elucidated. In this present study, we investigated the effects of AM404 on the production of inflammatory mediators and neuronal cell death induced by NMDA in organotypic hippocampal slices cultures (OHSC) using qPCR, western blot (WB), and immunohistochemistry. Moreover, to comprehend the mechanism of excitotoxicity, we evaluated the effects of AM404 on glutamate release in hippocampal synaptosomes and the NMDA-induced calcium responses in acute hippocampal slices. Our results showed that AM404 led to a significant decrease in cell death induced by NMDA, through a mechanism possibly involving the reduction of glutamate release and the calcium ions responses. Furthermore, it decreased the expression of the interleukin (IL)-1β. This study provides new significant insights about the anti-inflammatory and neuroprotection effects of AM404 on NMDA-induced excitotoxicity. To understand the effects of AM404 in these processes might contribute to the therapeutic potential of AM404 in diseases with involvement of neuroinflammation and neurodegeneration and might lead to a possible future treatment of neurodegenerative diseases.

Ethosuximide improves chronic pain-induced anxiety- and depression-like behaviors

Chronic pain is a heavy burden disease. Current treatments are generally weakly effective or associated with adverse effects. New therapeutic approaches are therefore needed. Recent studies have suggested T-type calcium channels as an attractive target for the treatment of chronic pain. In this perspective, it was decided to perform a preclinical evaluation of the efficacy of ethosuximide, a T-type channel blocker used clinically as an antiepileptic, as a novel pharmacological treatment for chronic pain. Assessment of the effect of ethosuximide was thus made in both nociception and pain-related comorbidities as anxiety and depression are frequently encountered in chronic pain patients. Our results show that such symptoms occurred in three animal models of chronic pain designed to reflect traumatic neuropathic, chemotherapy-induced neuropathic and inflammatory pain conditions. Administration of ethosuximide reduced both chronic pain and comorbidities with a marked intensity ranging from partial reduction to a complete suppression of symptoms. These results make ethosuximide, and more broadly the inhibition of T-type calcium channels, a new strategy for the management of uncontrolled chronic pain, likely to improve not only pain but also the accompanying anxiety and depression.

Analgesic transient receptor potential vanilloid-1-active compounds inhibit native and recombinant T-type calcium channels

BACKGROUND AND PURPOSE

T-type calcium (Ca_v 3) and transient receptor potential vanilloid-1 (TRPV1) channels play central roles in the control of excitability in the peripheral nervous system and are regarded as potential therapeutic pain targets. Modulators that either activate or inhibit TRPV1-mediated currents display analgesic properties in various pain models despite opposing effects on their connate target, TRPV1. We explored the effects of TRPV1-active compounds on Ca_v 3-mediated currents.

EXPERIMENTAL APPROACH

Whole-cell patch clamp recordings were used to examine the effects of TRPV1-active compounds on rat dorsal root ganglion low voltage-activated calcium currents and recombinant Ca_v 3 isoforms in expression systems.

KEY RESULTS

The classical TRPV1 agonist capsaicin as well as TRPV1 antagonists A-889425, BCTC, and capsazepine directly inhibited Ca_v 3 channels. These compounds altered the voltage-dependence of activation and inactivation of Ca_v 3 channels and delayed their recovery from inactivation, leading to a concomitant decrease in T-type current availability. The TRPV1 antagonist capsazepine potently inhibited Ca_v 3.1 and 3.2 channels (K_D  < 120 nM), as demonstrated by its slow off rate. In contrast, neither the TRPV1 agonists, Palvanil and resiniferatoxin, nor the TRPV1 antagonist AMG9810 modulated Ca_v 3-mediated currents.

CONCLUSIONS AND IMPLICATIONS

Analgesic TRPV1-active compounds inhibit Ca_v 3 currents in native and heterologous systems. Hence, their analgesic effects may not be exclusively attributed to their actions on TRPV1, which has important implications in the current understanding of nociceptive pathways. Importantly, our results highlight the need for attention in the experimental design used to address the analgesic properties of Ca_v 3 channel inhibitors.

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.

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.

Cav3.2 T-type calcium channels shape electrical firing in mouse Lamina II neurons

The T-type calcium channel, Cav3.2, is necessary for acute pain perception, as well as mechanical and cold allodynia in mice. Being found throughout sensory pathways, from excitatory primary afferent neurons up to pain matrix structures, it is a promising target for analgesics. In our study, Cav3.2 was detected in ~60% of the lamina II (LII) neurons of the spinal cord, a site for integration of sensory processing. It was co-expressed with Tlx3 and Pax2, markers of excitatory and inhibitory interneurons, as well as nNOS, calretinin, calbindin, PKCγ and not parvalbumin. Non-selective T-type channel blockers slowed the inhibitory but not the excitatory transmission in LII neurons. Furthermore, T-type channel blockers modified the intrinsic properties of LII neurons, abolishing low-threshold activated currents, rebound depolarizations, and blunting excitability. The recording of Cav3.2-positive LII neurons, after intraspinal injection of AAV-DJ-Cav3.2-mcherry, showed that their intrinsic properties resembled those of the global population. However, Cav3.2 ablation in the dorsal horn of Cav3.2^GFP-Flox KI mice after intraspinal injection of AAV-DJ-Cav3.2-Cre-IRES-mcherry, had drastic effects. Indeed, it (1) blunted the likelihood of transient firing patterns; (2) blunted the likelihood and the amplitude of rebound depolarizations, (3) eliminated action potential pairing, and (4) remodeled the kinetics of the action potentials. In contrast, the properties of Cav3.2-positive neurons were only marginally modified in Cav3.1 knockout mice. Overall, in addition to their previously established roles in the superficial spinal cord and in primary afferent neurons, Cav3.2 channel appear to be necessary for specific, significant and multiple controls of LII neuron excitability.

Pharmacogenetics in Pain Treatment

Pain is an unpleasant feeling usually resulting from tissue damage that can persist along weeks, months, or even years after the injury, turning into pathological chronic pain, the leading cause of disability. Currently, pharmacology interventions are usually the first-line therapy but there is a highly variable analgesic drug response. Pharmacogenetics (PGx) offers a means to identify genetic biomarkers that can predict individual analgesic response opening doors to precision medicine. PGx analyze the way in which the presence of variations in the DNA sequence (single-nucleotide polymorphisms, SNPs) could be responsible for portions of the population reaching different levels of pain relief (phenotype) due to gene interference in the drug mechanism of action (pharmacodynamics) and/or its concentration at the place of action (pharmacokinetics). SNPs in the cytochrome P450 enzymes genes (CYP2D6) influence metabolism of codeine, tramadol, hydrocodone, oxycodone, and tricyclic antidepressants. Blood concentrations of some NSAIDs depend on CYP2C9 and/or CYP2C8 activity. Additional candidate genes encode for opioid receptors, transporters, and other molecules important for pharmacotherapy in pain management. However, PGx studies are often contradictory, slowing the uptake of this information. This is likely due, in large part, to a lack of robust evidence demonstrating clinical utility and to its polygenic response modulated by other exogenous or epigenetics factors. Novel therapies, including targeting of epigenetic changes and gene therapy-based approaches, broaden future options to improve understanding of pain and the treatment of people who suffer it.

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.

Pharmacogenomics in pain treatment

The experience of chronic pain is one of the commonest reasons for seeking medical attention, being a major issue in clinical practice. While pain is a universal experience, only a small proportion of people who felt pain develop pain syndromes. In addition, painkillers are associated with wide inter-individual variability in the analgesic response. This may be partly explained by the presence of single nucleotide polymorphisms in genes encoding molecular entities involved in pharmacodynamics and pharmacokinetics. However, uptake of this information has been slow due in large part to the lack of robust evidences demonstrating clinical utility. Furthermore, novel therapies, including targeting of epigenetic changes and gene therapy-based approaches are further broadening future options for the treatment of chronic pain. The aim of this article is to review the evidences behind pharmacogenetics (PGx) to individualize therapy (boosting the efficacy and minimizing potential toxicity) and genes implicated in pain medicine, in two parts: (i) genetic variability with pain sensitivity and analgesic response; and (ii) pharmacological concepts applied on PGx.

Reactive metabolites of acetaminophen activate and sensitize the capsaicin receptor TRPV1

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

Acetaminophen Metabolite N-Acylphenolamine Induces Analgesia via Transient Receptor Potential Vanilloid 1 Receptors Expressed on the Primary Afferent Terminals of C-fibers in the Spinal Dorsal Horn

BACKGROUND

The widely used analgesic acetaminophen is metabolized to N-acylphenolamine, which induces analgesia by acting directly on transient receptor potential vanilloid 1 or cannabinoid 1 receptors in the brain. Although these receptors are also abundant in the spinal cord, no previous studies have reported analgesic effects of acetaminophen or N-acylphenolamine mediated by the spinal cord dorsal horn. We hypothesized that clinical doses of acetaminophen induce analgesia via these spinal mechanisms.

METHODS

We assessed our hypothesis in a rat model using behavioral measures. We also used in vivo and in vitro whole cell patch-clamp recordings of dorsal horn neurons to assess excitatory synaptic transmission.

RESULTS

Intravenous acetaminophen decreased peripheral pinch-induced excitatory responses in the dorsal horn (53.1 ± 20.7% of control; n = 10; P < 0.01), while direct application of acetaminophen to the dorsal horn did not reduce these responses. Direct application of N-acylphenolamine decreased the amplitudes of monosynaptic excitatory postsynaptic currents evoked by C-fiber stimulation (control, 462.5 ± 197.5 pA; N-acylphenolamine, 272.5 ± 134.5 pA; n = 10; P = 0.022) but not those evoked by stimulation of Aδ-fibers. These phenomena were mediated by transient receptor potential vanilloid 1 receptors, but not cannabinoid 1 receptors. The analgesic effects of acetaminophen and N-acylphenolamine were stronger in rats experiencing an inflammatory pain model compared to naïve rats.

CONCLUSIONS

Our results suggest that the acetaminophen metabolite N-acylphenolamine induces analgesia directly via transient receptor potential vanilloid 1 receptors expressed on central terminals of C-fibers in the spinal dorsal horn and leads to conduction block, shunt currents, and desensitization of these fibers.

TRPV1 channels are critical brain inflammation detectors and neuropathic pain biomarkers in mice

The capsaicin receptor TRPV1 has been widely characterized in the sensory system as a key component of pain and inflammation. A large amount of evidence shows that TRPV1 is also functional in the brain although its role is still debated. Here we report that TRPV1 is highly expressed in microglial cells rather than neurons of the anterior cingulate cortex and other brain areas. We found that stimulation of microglial TRPV1 controls cortical microglia activation per se and indirectly enhances glutamatergic transmission in neurons by promoting extracellular microglial microvesicles shedding. Conversely, in the cortex of mice suffering from neuropathic pain, TRPV1 is also present in neurons affecting their intrinsic electrical properties and synaptic strength. Altogether, these findings identify brain TRPV1 as potential detector of harmful stimuli and a key player of microglia to neuron communication.

Clinical update on benefit versus risks of oral paracetamol alone or with codeine: still a good option?

BACKGROUND

After decades of worldwide use of paracetamol/acetaminophen as a popular and apparently safe prescription and over-the-counter medicine, the future role of this poorly understood analgesic has been seriously questioned by recent concerns about prenatal, cardiovascular (CV) and hepatic safety, and also about its analgesic efficacy. At the same time the usefulness of codeine in combination products has come under debate.

METHODS

Based on a PubMed database literature search on the terms efficacy, safety, paracetamol, acetaminophen, codeine and their combinations up to and including June 2016, this clinical update reviews the current evidence of the benefit and risks of oral paracetamol alone and with codeine for mild-to-moderate pain in adults, and compares the respective efficacy and safety profiles with those of nonsteroidal anti-inflammatory drugs (NSAIDs).

RESULTS

Whereas there is a clear strong association of NSAID use and gastrointestinal (GI) and CV morbidity and mortality, evidence for paracetamol with and without codeine supports the recommended use even in most vulnerable individuals, such as the elderly, pregnant women, alcoholics, and compromised GI and CV patients. The controversies and widespread misconceptions about the complex hepatic metabolism and potential hepatotoxicity have been corrected by recent reviews, and paracetamol remains the first-line nonopioid analgesic in patients with liver diseases if notes of caution are applied.

CONCLUSION

Due to its safety and tolerability profile paracetamol remained a first-line treatment in many international guidelines. Alone and with codeine it is a safe and effective option in adults, whilst NSAIDs are obviously less safe as alternatives, given the risk of potentially fatal GI and CV adverse effects.

Activity-dependent regulation of T-type calcium channels by submembrane calcium ions

Voltage-gated Ca²⁺ channels are involved in numerous physiological functions and various mechanisms finely tune their activity, including the Ca²⁺ ion itself. This is well exemplified by the Ca²⁺-dependent inactivation of L-type Ca²⁺ channels, whose alteration contributes to the dramatic disease Timothy Syndrome. For T-type Ca²⁺ channels, a long-held view is that they are not regulated by intracellular Ca²⁺. Here we challenge this notion by using dedicated electrophysiological protocols on both native and expressed T-type Ca²⁺ channels. We demonstrate that a rise in submembrane Ca²⁺ induces a large decrease in T-type current amplitude due to a hyperpolarizing shift in the steady-state inactivation. Activation of most representative Ca²⁺-permeable ionotropic receptors similarly regulate T-type current properties. Altogether, our data clearly establish that Ca²⁺ entry exerts a feedback control on T-type channel activity, by modulating the channel availability, a mechanism that critically links cellular properties of T-type Ca²⁺ channels to their physiological roles.

DOI:http://dx.doi.org/10.7554/eLife.22331.001

Neurons, muscle cells and many other types of cells use electrical signals to exchange information and coordinate their behavior. Proteins known as calcium channels sit in the membrane that surrounds the cell and can generate electrical signals by allowing calcium ions to cross the membrane and enter the cell during electrical activities. Although calcium ions are needed to generate these electrical signals, and for many other processes in cells, if the levels of calcium ions inside cells become too high they can be harmful and cause disease.

Cells have a “feedback” mechanism that prevents calcium ion levels from becoming too high. This mechanism relies on the calcium ions that are already in the cell being able to close the calcium channels. This feedback mechanism has been extensively studied in two types of calcium channel, but it is not known whether a third group of channels – known as Cav3 channels – are also regulated in this way.

Cav3 channels are important in electrical signaling in neurons and have been linked with epilepsy, chronic pain and various other conditions in humans. Cazade et al. investigated whether calcium ions can regulate the activity of human Cav3 channels. The experiments show that these channels are indeed regulated by calcium ions, but using a distinct mechanism to other types of calcium channels. For the Cav3 channels, calcium ions alter the gating properties of the channels so that they are less easily activated . As a result, fewer Cav3 channels are “available” to provide calcium ions with a route into the cell.

The next steps following on from this work will be to identify the molecular mechanisms underlying this new feedback mechanism. Another challenge will be to find out what role this calcium ion-driven feedback plays in neurological disorders that are linked with altered Cav3 channel activity.

DOI:http://dx.doi.org/10.7554/eLife.22331.002

Assessment of the effectiveness and safety of Ethosuximide in the Treatment of non-Diabetic Peripheral Neuropathic Pain: EDONOT-protocol of a randomised, parallel, controlled, double-blinded and multicentre clinical trial

INTRODUCTION

Currently available analgesics are ineffective in 30-50% of patients suffering from neuropathic pain and often induce deleterious side effects. T-type calcium channel blockers (mibefradil, ethosuximide, NNC 55-0396) are of great interest for the development of new symptomatic treatments of neuropathic pain, due to their various effects on pain perception. Interestingly, ethosuximide, which has already been approved for treating epilepsy, is available on the European market for clinical use. Despite numerous preclinical data demonstrating an antinociceptive effect of ethosuximide in various animal models of neuropathic pain, no clinical studies have been published to date on the analgesic efficacy of ethosuximide in patients with neuropathic pain.

METHODS AND ANALYSIS

The Ethosuximide in the Treatment of non-Diabetic Peripheral Neuropathic Pain (EDONOT) trial is a randomised, parallel, controlled, double-blinded, multicentre clinical study. It is the first clinical trial to evaluate the efficacy and safety of ethosuximide in the treatment of non-diabetic peripheral neuropathic pain. Adult patients exhibiting peripheral neuropathic pain (Numeric Rating Scale (NRS) ≥4 and Douleur Neuropathique 4 (DN4)≥4) for at least 3 months and under stable analgesic treatment for at least 1 month will be included. Patients (n=220) will be randomly assigned to receive either ethosuximide or control treatment for 6 weeks following a 1 week run-in period. The primary end point is the intensity of neuropathic pain, assessed by NRS (0-10) before and after 6 weeks of treatment. The secondary end points are safety (adverse events are collected during the study: daily by the patient on the logbook and during planned phone calls by investigators), the intensity and features of neuropathic pain (assessed by Brief Pain Inventory (BPI) and Neuropathic Pain Symptom Inventory (NPSI) questionnaires) and health-related quality of life (assessed by Medical Outcome Study Short Form 12 (MOS SF-12) and Leeds questionnaires).

ETHICS AND COMMUNICATION

The study was approved by an independent ethics committee (CPP Sud-Est VI, France, IRB00008526) and registered by the French competent authority (Agence nationale de sécurité du médicament (ANSM)).

TRIAL REGISTRATION NUMBER

NCT02100046, Recruiting.

Paracetamol sharpens reflection and spatial memory: a double-blind randomized controlled study in healthy volunteers

Background

Acetaminophen (APAP, paracetamol) mechanism for analgesic and antipyretic outcomes has been largely addressed, but APAP action on cognitive function has not been studied in humans. Animal studies have suggested an improved cognitive performance but the link with analgesic and antipyretic modes of action is incomplete. This study aims at exploring cognitive tests in healthy volunteers in the context of antinociception and temperature regulation. A double-blind randomized controlled study (NCT01390467) was carried out from May 30, 2011 to July 12, 2011.

Methods

Forty healthy volunteers were included and analyzed. Nociceptive thresholds, core temperature (body temperature), and a battery of cognitive tests were recorded before and after oral APAP (2 g) or placebo: Information sampling task for predecisional processing, Stockings of Cambridge for spatial memory, reaction time, delayed matching of sample, and pattern recognition memory tests. Analysis of variance for repeated measures adapted to crossover design was performed and a two-tailed type I error was fixed at 5%.

Results

APAP improved information sampling task (diminution of the number of errors, latency to open boxes, and increased number of opened boxes; all P<0.05). Spatial planning and working memory initial thinking time were decreased (P=0.04). All other tests were not modified by APAP. APAP had an antinociceptive effect (P<0.01) and body temperature did not change.

Conclusion

This study shows for the first time that APAP sharpens decision making and planning strategy in healthy volunteers and that cognitive performance and antinociception are independent of APAP effect on thermogenesis. We suggest that cognitive performance mirrors the analgesic rather than thermic cascade of events, with possibly a central role for serotonergic and cannabinoid systems that need to be explored further in the context of pain and cognition.

Capsaicin: Current Understanding of Its Mechanisms and Therapy of Pain and Other Pre-Clinical and Clinical Uses

In this review, we discuss the importance of capsaicin to the current understanding of neuronal modulation of pain and explore the mechanisms of capsaicin-induced pain. We will focus on the analgesic effects of capsaicin and its clinical applicability in treating pain. Furthermore, we will draw attention to the rationale for other clinical therapeutic uses and implications of capsaicin in diseases such as obesity, diabetes, cardiovascular conditions, cancer, airway diseases, itch, gastric, and urological disorders.

Neonatal paracetamol treatment reduces long-term nociceptive behaviour after neonatal procedural pain in rats

BACKGROUND

Pain from skin penetrating procedures (procedural pain) during infancy in the neonatal intensive care unit (NICU) may result in changes of nociceptive sensitivity in later life. This supports the need for pain management during such vulnerable periods in life. This study, therefore, analyses the short- and long-term consequences of neonatal paracetamol (acetaminophen) treatment on pain behaviour in an experimental rat model of neonatal procedural pain.

METHODS

A repetitive needle-prick model was used, in which neonatal rats received four needle pricks into the left hind paw per day from postnatal day 0 to day 7 (P0-P7). Paracetamol (50 mg/kg/day s.c.) was administered daily (P0-P7), and sensitivity to mechanical stimuli was compared with a needle-prick/saline-treated group and to a tactile control group. At 8 weeks of age, all animals underwent an ipsilateral paw-incision, modelling postoperative pain, and the duration of hypersensitivity was assessed.

RESULTS

Neonatal paracetamol administration had no effect upon short-term mechanical hypersensitivity during the first postnatal week or upon long-term baseline sensitivity from 3 to 8 weeks. However, neonatal paracetamol administration significantly reduced the postoperative mechanical hypersensitivity in young adults, caused by repetitive needle pricking.

CONCLUSION

Paracetamol administration during neonatal procedural pain does not alter short-term or long-term effects on mechanical sensitivity, but does reduce the duration of increased postoperative mechanical hypersensitivity in a clinically relevant neonatal procedural pain model.

WHAT DOES THIS STUDY ADD

Paracetamol can be used safely in neonatal rats. Neonatal paracetamol treatment had no effect upon short-term mechanical hypersensitivity during the first postnatal week, nor upon long-term baseline sensitivity from 3 to 8 weeks. Paracetamol treatment during the first postnatal week significantly reduced the postoperative mechanical hypersensitivity in young adult rats.

Endogenous opiates and behavior: 2014

This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants). This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants).

A Randomized, Double-Blind, Placebo-Controlled, Crossover Study of the T-Type Calcium Channel Blocker ABT-639 in an Intradermal Capsaicin Experimental Pain Model in Healthy Adults

OBJECTIVE

This randomized, double-blind, placebo-controlled, crossover trial evaluated the pharmacodynamic effects of a single 100-mg dose of ABT-639, a peripherally active, selective T-type Ca_v3.2 channel blocker, with the intradermal capsaicin pain model using pregabalin 300 mg as a positive control.

SUBJECTS

Healthy adult males (aged 21 to 55 years) were randomly assigned to receive single oral doses of ABT-639, pregabalin, and placebo.

METHODS

Serial measurements for area (cm²) of hyperalgesia, allodynia, and flare response were performed over a 20-minute period after each capsaicin injection at 1 and 4 hours post-dose. Capsaicin injections were administered in different arms as determined by random assignment. Serial measurements for spontaneous pain and elicited pain were performed over a 60-minute period at 1 and 4 hours post-dose using a 100-mm visual analog scale. Standard safety evaluations were performed.

RESULTS

Nineteen participants were randomized and included in the analysis. No significant differences were observed between ABT-639 and placebo in spontaneous pain, elicited pain, and areas of allodynia, hyperalgesia, and flare after intradermal capsaicin injection at 1 and 4 hours post-dose. In contrast, pregabalin demonstrated significant reductions in spontaneous pain at 1 and 4 hours post-dose, and elicited pain and areas of allodynia and hyperalgesia at 4 hours post-dose compared with placebo. ABT-639 demonstrated acceptable safety and tolerability; somnolence and euphoric mood were the most commonly reported adverse events.

CONCLUSIONS

These data indicate that a single 100-mg dose of ABT-639 had no effect on experimental pain induced by intradermal capsaicin injection.

The brain signature of paracetamol in healthy volunteers: a double-blind randomized trial

BACKGROUND

Paracetamol's (APAP) mechanism of action suggests the implication of supraspinal structures but no neuroimaging study has been performed in humans.

METHODS AND RESULTS

This randomized, double-blind, crossover, placebo-controlled trial in 17 healthy volunteers (NCT01562704) aimed to evaluate how APAP modulates pain-evoked functional magnetic resonance imaging signals. We used behavioral measures and functional magnetic resonance imaging to investigate the response to experimental thermal stimuli with APAP or placebo administration. Region-of-interest analysis revealed that activity in response to noxious stimulation diminished with APAP compared to placebo in prefrontal cortices, insula, thalami, anterior cingulate cortex, and periaqueductal gray matter.

CONCLUSION

These findings suggest an inhibitory effect of APAP on spinothalamic tracts leading to a decreased activation of higher structures, and a top-down influence on descending inhibition. Further binding and connectivity studies are needed to evaluate how APAP modulates pain, especially in the context of repeated administration to patients with pain.

Acetaminophen: old drug, new issues

INTRODUCTION

The purpose of this review was to discuss new issues related to safety, labeling, dosing, and a better understanding of the analgesic effect of acetaminophen.

METHODS

The MEDLINE, Embase, Cochrane, and PubMed databases were searched. Additionally, the bibliography of all relevant articles and textbooks were manually searched. Two reviewers independently selected the relevant articles.

RESULTS

Concerns about acetaminophen overdose and related liver failure have led the US Food and Drug Administration to mandate new labeling on acetaminophen packaging. In addition, large-scale epidemiologic studies increasingly report evidence for second-generation adverse effects of acetaminophen. Prenatal exposure to acetaminophen is associated with neurodevelopmental and behavioral disorders. Recent studies also suggest that acetaminophen is a hormone disrupter (ie, it interferes with sex and thyroid hormone function essential for normal brain development) and thus may not be considered a safe drug during pregnancy. Finally, emerging evidence suggests that although the predominant mechanism by which acetaminophen exerts its therapeutic effect is by inhibition of cyclooxygenase, multiple other mechanisms also contribute to its analgesic effect.

CONCLUSIONS

Available evidence suggests that indiscriminate usage of this drug is not warranted. and its administration to a pregnant patient should be considered with great caution.

Contributions of peripheral, spinal, and supraspinal actions to analgesia

Pain signaling involves several main compartments that can be considered as potential sites for analgesic drug actions. When drugs are given systemically, they can act at spinal, supraspinal and peripheral sites, and several methods have been developed for identifying where they act. These include (1) localized delivery of drugs to specific sites (via intracerebral, intrathecal, and intraplantar injections), (2) systemic delivery of drugs with localized delivery of antagonists for the receptor on which the drug acts or for a system recruited by the drug, (3) use of peripherally restricted analogs, and (4) use of conditional knockout technology to selectively deplete receptors on nociceptors. Delivery of drugs simultaneously to several sites (spinal/supraspinal, peripheral/spinal, and peripheral/supraspinal) reveals "self-synergy" between sites for some agents. Knowledge of peripheral contributions to drug actions is important because of the potential to develop peripherally restricted analgesics (with a diminished side effect profile due to not entering the central nervous system), the potential to deliver drugs peripherally (e.g. topically) to act on sensory nerve endings and adjacent tissue (with a diminished side effect profile due to limited systemic absorption), and the potential to use combinations of topical and oral drug regimens to obtain improved pain relief (without increasing the side effect burden). This review considers methods used for compartmental analysis, and results of such site analysis for several major classes of analgesic drugs that are in current use.

T-type channel-mediated neurotransmitter release

Besides controlling a wide variety of cell functions, T-type channels have been shown to regulate neurotransmitter release in peripheral and central synapses and neuroendocrine cells. Growing evidence over the last 10 years suggests a key role of Cav3.2 and Cav3.1 channels in controlling basal neurosecretion near resting conditions and sustained release during mild stimulations. In some cases, the contribution of low-voltage-activated (LVA) channels is not directly evident but requires either the activation of coupled presynaptic receptors, block of ion channels, or chelation of metal ions. Concerning the coupling to the secretory machinery, T-type channels appear loosely coupled to neurotransmitter and hormone release. In neurons, Cav3.2 and Cav3.1 channels mainly control the asynchronous appearance of "minis" [miniature inhibitory postsynaptic currents (mIPSCs) and miniature excitatory postsynaptic currents (mEPSCs)]. The same loose coupling is evident from membrane capacity and amperometric recordings in chromaffin cells and melanotropes where the low-threshold-driven exocytosis possesses the same linear Ca(2+) dependence of the other voltage-gated Ca(2+) channels (Cav1 and Cav2) that is strongly attenuated by slow calcium buffers. The intriguing issue is that, despite not expressing a consensus "synprint" site, Cav3.2 channels do interact with syntaxin 1A and SNAP-25 and, thus, may form nanodomains with secretory vesicles that can be regulated at low voltages. In this review, we discuss all the past and recent issues related to T-type channel-secretion coupling in neurons and neuroendocrine cells.

T-type calcium channels in chronic pain: mouse models and specific blockers

Pain is a quite frequent complaint accompanying numerous pathologies. Among these pathological cases, neuropathies are retrieved with identified etiologies (chemotherapies, diabetes, surgeries…) and also more diffuse syndromes such as fibromyalgia. More broadly, pain is one of the first consequences of the majority of inherited diseases. Despite its importance for the quality of life, current pain management is limited to drugs that are either old or with a limited efficacy or that possess a bad benefit/risk ratio. As no new pharmacological concept has led to new analgesics in the last decades, the discovery of medications is needed, and to this aim the identification of new druggable targets in pain transmission is a first step. Therefore, studies of ion channels in pain pathways are extremely active. This is particularly true with ion channels in peripheral sensory neurons in dorsal root ganglia (DRG) known now to express unique sets of these channels. Moreover, both spinal and supraspinal levels are clearly important in pain modulation. Among these ion channels, we and others revealed the important role of low voltage-gated calcium channels in cellular excitability in different steps of the pain pathways. These channels, by being activated nearby resting membrane potential have biophysical characteristics suited to facilitate action potential generation and rhythmicity. In this review, we will review the current knowledge on the role of these channels in the perception and modulation of pain.