ATP P2X3 receptors and neuronal sensitization

Metabolic network analysis of pre-ASD newborns and 5-year-old children with autism spectrum disorder

Classical metabolomic and new metabolic network methods were used to study the developmental features of autism spectrum disorder (ASD) in newborns (n = 205) and 5-year-old children (n = 53). Eighty percent of the metabolic impact in ASD was caused by 14 shared biochemical pathways that led to decreased anti-inflammatory and antioxidant defenses, and to increased physiologic stress molecules like lactate, glycerol, cholesterol, and ceramides. CIRCOS plots and a new metabolic network parameter,V ° net, revealed differences in both the kind and degree of network connectivity. Of 50 biochemical pathways and 450 polar and lipid metabolites examined, the developmental regulation of the purine network was most changed. Purine network hub analysis revealed a 17-fold reversal in typically developing children. This purine network reversal did not occur in ASD. These results revealed previously unknown metabolic phenotypes, identified new developmental states of the metabolic correlation network, and underscored the role of mitochondrial functional changes, purine metabolism, and purinergic signaling in autism spectrum disorder.

Efficacy and safety of eliapixant in diabetic neuropathic pain and prediction of placebo responders with an exploratory novel algorithm: results from the randomized controlled phase 2a PUCCINI study

ABSTRACT

Phase 2a of the PUCCINI study was a placebo-controlled, double-blind, parallel-group, multicenter, proof-of-concept study evaluating the efficacy and safety of the selective P2X3 antagonist eliapixant in patients with diabetic neuropathic pain (DNP) ( ClinicalTrials.gov NCT04641273). Adults with type 1 or type 2 diabetes mellitus with painful distal symmetric sensorimotor neuropathy of >6 months' duration and neuropathic pain were enrolled and randomized 1:1 to 150 mg oral eliapixant twice daily or placebo for 8 weeks. The primary endpoint was change from baseline in weekly mean 24-hour average pain intensity score at week 8. In total, 135 participants completed treatment, 67 in the eliapixant group and 68 in the placebo group. At week 8, the change from baseline in posterior mean 24-hour average pain intensity score (90% credible interval) in the eliapixant group was -1.56 (-1.95, -1.18) compared with -2.17 (-2.54, -1.80) for the placebo group. The mean treatment difference was 0.60 (0.06, 1.14) in favor of placebo. The use of a model-based framework suggests that various factors may contribute to the placebo-responder profile. Adverse events were mostly mild or moderate in severity and occurred in 51% of the eliapixant group and 48% of the placebo group. As the primary endpoint was not met, the PUCCINI study was terminated after completion of phase 2a and did not proceed to phase 2b. In conclusion, selective P2X3 antagonism in patients with DNP did not translate to any relevant improvement in different pain intensity outcomes compared with placebo. Funding: Bayer AG.

Mass Balance and Metabolic Pathways of Eliapixant, a P2X3 Receptor Antagonist, in Healthy Male Volunteers

BACKGROUND

Overactive adenosine triphosphate signaling via P2X3 homotrimeric receptors is implicated in multiple conditions. To fully understand the metabolism and elimination pathways of eliapixant, a study was conducted to assess the pharmacokinetics, mass balance, and routes of excretion of a single oral dose of the selective P2X3 receptor antagonist eliapixant, in addition to an in vitro characterization.

METHODS

In this single-center open-label non-randomized non-placebo-controlled phase I study, healthy male subjects (n = 6) received a single dose of 50 mg eliapixant blended with 3.7 MBq [14C]eliapixant as a PEG 400-based oral solution. Total radioactivity and metabolites excreted in urine and feces, and pharmacokinetics of total radioactivity, eliapixant, and metabolites in plasma were assessed via liquid scintillation counting and high-performance liquid chromatography-based methods coupled to radiometric and mass spectrometric detection. Metabolite profiles of eliapixant in human in vitro systems and metabolizing enzymes were also investigated.

RESULTS

After administration as an oral solution, eliapixant was rapidly absorbed, reaching maximum plasma concentrations within 2 h. Eliapixant was eliminated from plasma with a mean terminal half-life of 48.3 h. Unchanged eliapixant was the predominant component in plasma (72.6% of total radioactivity area under the curve). The remaining percentage of drug-related components in plasma probably represented the sum of many metabolites, detected in trace amounts. Mean recovery of total radioactivity was 97.9% of the administered dose (94.3-99.4%) within 14 days, with 86.3% (84.8-88.1%) excreted via feces and 11.6% (9.5-13.1%) via urine. Excretion of parent drug was minimal in feces (0.7% of dose) and urine (≈ 0.5%). In feces, metabolites formed by oxidation represented > 90% of excreted total radioactivity. The metabolites detected in the in vitro experiments were similar to those identified in vivo.

CONCLUSION

Complete recovery of administered eliapixant-related radioactivity was observed in healthy male subjects with predominant excretion via feces. Eliapixant was almost exclusively cleared by oxidative biotransformation (> 90% of dose), with major involvement of cytochrome P450 3A4. Excretion of parent drug was of minor importance (~ 1% of dose).

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov: NCT04487431 (registered 27 July 2020)/EudraCT number: 2020-000519-54 (registered 3 February 2020), NCT02817100 (registered 26 June 2016), NCT03310645 (registered 16 October 2017).

Cross-species gene expression patterns of purinergic signaling in the human and mouse trigeminal ganglion

Purinergic signaling system plays a pivotal role in the trigeminal ganglion (TG) which is a primary sensory tissue in vertebrate nervous systems involving orofacial nociception and peripheral sensitization. Despite previous efforts to reveal the expression patterns of purinergic components in the mouse TG, it is still unknown the interspecies differences between human and mouse. In this study, we provide a comprehensive transcriptome profile of the purinergic signaling system across diverse cell types and neuronal subpopulations within the human TG, systematically comparing it with mouse TG. In addition, the evolutionary conservation and species-specific expression patterns of the purinergic components are also discussed. We propose that the data can improve our understanding of purinergic signaling in the peripheral nervous system and facilitate the identification of novel therapeutic targets.

Alterations of Purinergic Receptors Levels and Their Involvement in the Glial Cell Morphology in a Pre-Clinical Model of Autism Spectrum Disorders

Recent data suggest that defects in purinergic signalling are a common denominator of autism spectrum disorders (ASDs), though nothing is known about whether the disorder-related imbalance occurs at the receptor level. In this study, we investigated whether prenatal exposure to valproic acid (VPA) induces changes in purinergic receptor expression in adolescence and whether it corresponds to glial cell activation. Pregnant dams were subjected to an intraperitoneal injection of VPA at embryonic day 12.5. In the hippocampi of adolescent male VPA offspring, we observed an increase in the level of P2X1, with concomitant decreases in P2X7 and P2Y1 receptors. In contrast, in the cortex, the level of P2X1 was significantly reduced. Also, significant increases in cortical P2Y1 and P2Y12 receptors were detected. Additionally, we observed profound alterations in microglial cell numbers and morphology in the cortex of VPA animals, leading to the elevation of pro-inflammatory cytokine expression. The changes in glial cells were partially reduced via a single administration of a non-selective P2 receptor antagonist. These studies show the involvement of purinergic signalling imbalance in the modulation of brain inflammatory response induced via prenatal VPA exposure and may indicate that purinergic receptors are a novel target for pharmacological intervention in ASDs.

Pulsed Radiofrequency Decreases pERK and Affects Intracellular Ca2+ Influx, Cytosolic ATP Level, and Mitochondrial Membrane Potential in the Sensitized Dorsal Root Ganglion Neuron Induced by N-Methyl D-Aspartate

Background

The molecular mechanism of pulsed radiofrequency (PRF) in chronic pain management is not fully understood. Chronic pain involves the activation of specific N-Methyl D-Aspartate receptors (NMDAR) to induce central sensitization. This study aims to determine the effect of PRF on central sensitization biomarker phosphorylated extracellular signal-regulated kinase (pERK), Ca²⁺ influx, cytosolic ATP level, and mitochondrial membrane potential (Δψm) of the sensitized dorsal root ganglion (DRG) neuron following NMDAR activation.

Methods

This study is a true experimental in-vitro study on a sensitized DRG neuron induced with 80 µM NMDA. There are six treatment groups including control, NMDA 80 µM, Ketamine 100 µM, PRF 2Hz, NMDA 80 µM + PRF 2 Hz, and NMDA 80 µM + PRF 2 Hz + ketamine 100 µM. We use PRF 2 Hz 20 ms for 360 seconds. Statistical analysis was performed using the One-Way ANOVA and the Pearson correlation test with α=5%.

Results

In the sensitized DRG neuron, there is a significant elevation of pERK. There is a strong correlation between Ca²⁺, cytosolic ATP level, and Δψm with pERK intensity (p<0.05). PRF treatment decreases pERK intensity from 108.48 ± 16.95 AU to 38.57 ± 5.20 AU (p<0.05). PRF exposure to sensitized neurons also exhibits a Ca²⁺ influx, but still lower than in the unexposed neuron. PRF exposure in sensitized neurons has a higher cytosolic ATP level (0.0458 ± 0.0010 mM) than in the unexposed sensitized neuron (0.0198 ± 0.0004 mM) (p<0.05). PRF also decreases Δψm in the sensitized neuron from 109.24 ± 6.43 AU to 33.21 ± 1.769 AU (p<0.05).

Conclusion

PRF mechanisms related to DRG neuron sensitization are by decreasing pERK, altering Ca²⁺ influx, increasing cytosolic ATP level, and decreasing Δψm which is associated with neuron sensitization following NMDAR activation.

Preclinical and Clinical Pharmacokinetics and Bioavailability in Healthy Volunteers of a Novel Formulation of the Selective P2X3 Receptor Antagonist Eliapixant

BACKGROUND AND OBJECTIVES

The potent, selective P2X3 receptor antagonist eliapixant (BAY 1817080) is under development for conditions characterized by neuronal hypersensitization. As prominent food effects and limited bioavailability in the fasted state were observed with immediate-release eliapixant tablets, a novel formulation was needed. Accordingly, several novel eliapixant formulations were assessed by in vitro and animal studies in a structured way. The most promising of the formulations was then investigated in a phase I study designed to assess its pharmacokinetics, food effect, and bioavailability in healthy volunteers.

METHODS

In vitro non-sink dissolution tests were performed with two amorphous solid dispersion (ASD) granule prototypes compared with pure crystalline eliapixant as a surrogate for the immediate-release formulation. Subsequently, the drug exposure of novel eliapixant formulations under fed and fasted conditions in rats and dogs was assessed to confirm improvements in bioavailability versus the suspension-based formulation. A novel Kollidon VA64®-based eliapixant formulation was identified from the preclinical studies and compared with the original tablet formulation in an open-label, partially randomized, threefold, crossover phase I study, in which healthy males received single oral doses (25-400 mg, fasted/fed). Pharmacokinetic parameters, absolute bioavailability (using an intravenous [¹³C₇¹⁵N]-eliapixant microdose), relative bioavailability (novel versus original formulation), effect of food, and adverse events (AEs) were evaluated.

RESULTS

The non-sink dissolution test demonstrated that the two ASD formulations had an improved dissolution rate compared with pure crystalline eliapixant, with a Kollidon VA64-based prototype having the highest dissolution rate. Further testing of this prototype in animal studies confirmed an approximately twofold higher bioavailability compared with the suspension-based formulation. In the phase I study, 30 subjects were randomized. With the novel Kollidon VA64® formulation (400 mg; fasted), area under the concentration-time curve (AUC) and maximum plasma concentration (C_max) were up to 3.1-fold and 1.7-fold higher, respectively, than with the original formulation (fed). AUC increased dose proportionally between 25 and 100 mg, and less than dose proportionally from 100 to 400 mg. Food had no clinically relevant effect on the novel formulation, with AUC increasing 1.3-fold and C_max 2.1-2.4-fold (time to maximum concentration was delayed by 1.5-2.25 h). Absolute bioavailability with the novel formulation (100 mg) was 50%. AEs occurred in 57% of patients; most were mild in severity.

CONCLUSIONS

The novel eliapixant formulation substantially improved bioavailability compared with immediate-release eliapixant and may be administered with/without food.

CLINICAL TRIAL REGISTRATION

Clinicaltrials.gov: NCT03773068 (initial registration: 12 December 2018).

Benzydamine plays a role in limiting inflammatory pain induced by neuronal sensitization

Benzydamine is an active pharmaceutical compound used in the oral care pharmaceutical preparation as NSAID. Beside from its anti-inflammatory action, benzydamine local application effectively reliefs pain showing analgesic and anaesthetic properties. Benzydamine mechanism of action has been characterized on inflammatory cell types and mediators highlighting its capacity to inhibit pro-inflammatory mediators' synthesis and release. On the other hand, the role of benzydamine as neuronal excitability modulator has not yet fully explored. Thus, we studied benzydamine's effect over primary cultured DRG nociceptors excitability and after acute and chronic inflammatory sensitization, as a model to evaluate relative nociceptive response. Benzydamine demonstrated to effectively inhibit neuronal basal excitability reducing its firing frequency and increasing rheobase and afterhyperpolarization amplitude. Its effect was time and dose-dependent. At higher doses, benzydamine induced changes in action potential wavelength, decreasing its height and slightly increasing its duration. Moreover, the compound reduced neuronal acute and chronic inflammatory sensitization. It inhibited neuronal excitability mediated either by an inflammatory cocktail, acidic pH or high external KCl. Notably, higher potency was evidenced under inflammatory sensitized conditions. This effect could be explained either by modulation of inflammatory and/or neuronal sensitizing signalling cascades or by direct modulation of proalgesic and action potential firing initiating ion channels. Apparently, the compound inhibited Nav1.8 channel but had no effect over Kv7.2, Kv7.3, TRPV1 and TRPA1. In conclusion, the obtained results strengthen the analgesic and anti-inflammatory effect of benzydamine, highlighting its mode of action on local pain and inflammatory signalling.

Development of a functional human induced pluripotent stem cell-derived nociceptor MEA system as a pain model for analgesic drug testing

The control of severe or chronic pain has relied heavily on opioids and opioid abuse and addiction have recently become a major global health crisis. Therefore, it is imperative to develop new pain therapeutics which have comparable efficacy for pain suppression but lack of the harmful effects of opioids. Due to the nature of pain, any in vivo experiment is undesired even in animals. Recent developments in stem cell technology has enabled the differentiation of nociceptors from human induced pluripotent stem cells. This study sought to establish an in vitro functional induced pluripotent stem cells-derived nociceptor culture system integrated with microelectrode arrays for nociceptive drug testing. Nociceptors were differentiated from induced pluripotent stem cells utilizing a modified protocol and a medium was designed to ensure prolonged and stable nociceptor culture. These neurons expressed nociceptor markers as characterized by immunocytochemistry and responded to the exogenous toxin capsaicin and the endogenous neural modulator ATP, as demonstrated with patch clamp electrophysiology. These cells were also integrated with microelectrode arrays for analgesic drug testing to demonstrate their utilization in the preclinical drug screening process. The neural activity was induced by ATP to mimic clinically relevant pathological pain and then the analgesics Lidocaine and the opioid DAMGO were tested individually and both induced immediate silencing of the nociceptive activity. This human-based functional nociceptive system provides a valuable platform for investigating pathological pain and for evaluating effective analgesics in the search of opioid substitutes.

Adenosine, Schizophrenia and Cancer: Does the Purinergic System Offer a Pathway to Treatment?

For over a century, a complex relationship between schizophrenia diagnosis and development of many cancers has been observed. Findings from epidemiological studies are mixed, with reports of increased, reduced, or no difference in cancer incidence in schizophrenia patients. However, as risk factors for cancer, including elevated smoking rates and substance abuse, are commonly associated with this patient population, it is surprising that cancer incidence is not higher. Various factors may account for the proposed reduction in cancer incidence rates including pathophysiological changes associated with disease. Perturbations of the adenosine system are hypothesized to contribute to the neurobiology of schizophrenia. Conversely, hyperfunction of the adenosine system is found in the tumor microenvironment in cancer and targeting the adenosine system therapeutically is a promising area of research in this disease. We outline the current biochemical and pharmacological evidence for hypofunction of the adenosine system in schizophrenia, and the role of increased adenosine metabolism in the tumor microenvironment. In the context of the relatively limited literature on this patient population, we discuss whether hypofunction of this system in schizophrenia, may counteract the immunosuppressive role of adenosine in the tumor microenvironment. We also highlight the importance of studies examining the adenosine system in this subset of patients for the potential insight they may offer into these complex disorders.

The Role of Inflammation, Hypoxia, and Opioid Receptor Expression in Pain Modulation in Patients Suffering from Obstructive Sleep Apnea

Obstructive sleep apnea (OSA) is a relatively common disease in the general population. Besides its interaction with many comorbidities, it can also interact with potentially painful conditions and modulate its course. The association between OSA and pain modulation has recently been a topic of concern for many scientists. The mechanism underlying OSA-related pain connection has been linked with different pathophysiological changes in OSA and various pain mechanisms. Furthermore, it may cause both chronic and acute pain aggravation as well as potentially influencing the antinociceptive mechanism. Characteristic changes in OSA such as nocturnal hypoxemia, sleep fragmentation, and systemic inflammation are considered to have a curtailing impact on pain perception. Hypoxemia in OSA has been proven to have a significant impact on increased expression of proinflammatory cytokines influencing the hyperalgesic priming of nociceptors. Moreover, hypoxia markers by themselves are hypothesized to modulate intracellular signal transduction in neurons and have an impact on nociceptive sensitization. Pain management in patients with OSA may create problems arousing from alterations in neuropeptide systems and overexpression of opioid receptors in hypoxia conditions, leading to intensification of side effects, e.g., respiratory depression and increased opioid sensitivity for analgesic effects. In this paper, we summarize the current knowledge regarding pain and pain treatment in OSA with a focus on molecular mechanisms leading to nociceptive modulation.

Association between P2X3 receptors and neuropathic pain: As a potential therapeutic target for therapy

Neuropathic pain is a common clinical symptom of various diseases, and it seriously affects the physical and mental health of patients. Owing to the complex pathological mechanism of neuropathic pain, clinical treatment of pain is challenging. Therefore, there is growing interest among researchers to explore potential therapeutic strategies for neuropathic pain. A large number of studies have shown that development of neuropathic pain is related to nerve conduction and related signaling molecules. P2X3 receptors (P2X3R) are ATP-dependent ion channels that participate in the transmission of neural information and related signaling pathways, sensitize the central nervous system, and play a key role in the development of neuropathic pain. In this paper, we summarized the structure and biological characteristics of the P2X3R gene and discussed the role of P2X3R in the nervous system. Moreover, we outlined the related pathological mechanisms of pain and described the relationship between P2X3R and chronic pain to provide valuable information for development of novel treatment strategies for pain.

First-in-human study of eliapixant (BAY 1817080), a highly selective P2X3 receptor antagonist: tolerability, safety and pharmacokinetics

Aims

Neuronal hypersensitisation due to adenosine triphosphate‐dependent P2X3 receptor signalling plays a significant role in several disorders including chronic cough and endometriosis. This first‐in‐human study of eliapixant (BAY 1817080) investigated the tolerability, safety and pharmacokinetics (PK) of single doses of eliapixant, including the effect of food and coadministration with a CYP3A inhibitor on eliapixant relative bioavailability.

Methods

In this randomised, double‐blind phase I study (NCT02817100), 88 healthy male subjects received single ascending doses of immediate‐release eliapixant (10–800 mg) tablets or placebo under fasted conditions, with food (low‐fat continental or high‐fat American breakfast) or with itraconazole (fasted state). PK parameters, dose proportionality, adverse events and taste assessments (taste strips; dysgeusia questionnaire) were evaluated.

Results

Eliapixant had a long half‐life (23.5–58.9 h [fasted state]; 32.8–43.8 h [high‐fat breakfast]; 38.9–46.0 h [low‐fat breakfast]). Less than dose‐proportional increases in maximum plasma concentrations (C_max) and area under the concentration–time curve from time 0 to infinity (AUC_[0–inf]) were observed with ascending eliapixant doses. We observed a pronounced food effect with the high‐fat breakfast (4.1‐fold increased C_max; 2.7‐fold increased AUC_[0–inf]), a smaller food effect with the low‐fat breakfast and a mild‐to‐moderate effect of itraconazole coadministration on eliapixant (1.1–1.2‐fold increased C_max; 1.7‐fold increased AUC from 0 to 72 h). Eliapixant was well tolerated with minimal impact on taste perception.

Conclusion

The PK profile, particularly the long half‐life, and favourable tolerability with no taste‐related adverse events, supports the further development of eliapixant in disorders with underlying P2X3 receptor‐mediated neuronal hypersensitisation.

Calcium Signalling in Breast Cancer Associated Bone Pain

Calcium (Ca²⁺) is involved as a signalling mediator in a broad variety of physiological processes. Some of the fastest responses in human body like neuronal action potential firing, to the slowest gene transcriptional regulation processes are controlled by pathways involving calcium signalling. Under pathological conditions these mechanisms are also involved in tumoral cells reprogramming, resulting in the altered expression of genes associated with cell proliferation, metastatisation and homing to the secondary metastatic site. On the other hand, calcium exerts a central function in nociception, from cues sensing in distal neurons, to signal modulation and interpretation in the central nervous system leading, in pathological conditions, to hyperalgesia, allodynia and pain chronicization. It is well known the relationship between cancer and pain when tumoral metastatic cells settle in the bones, especially in late breast cancer stage, where they alter the bone micro-environment leading to bone lesions and resulting in pain refractory to the conventional analgesic therapies. The purpose of this review is to address the Ca²⁺ signalling mechanisms involved in cancer cell metastatisation as well as the function of the same signalling tools in pain regulation and transmission. Finally, the possible interactions between these two cells types cohabiting the same Ca²⁺ rich environment will be further explored attempting to highlight new possible therapeutical targets.

WAO-ARIA consensus on chronic cough - Part 1: Role of TRP channels in neurogenic inflammation of cough neuronal pathways

BACKGROUND

Cough features a complex peripheral and central neuronal network. The function of the chemosensitive and stretch (afferent) cough receptors is well described but partly understood. It is speculated that chronic cough reflects a neurogenic inflammation of the cough reflex, which becomes hypersensitive. This is mediated by neuromediators, cytokines, inflammatory cells, and a differential expression of neuronal (chemo/stretch) receptors, such as transient receptor potential (TRP) and purinergic P2X ion channels; yet the overall interaction of these mediators in neurogenic inflammation of cough pathways remains unclear.

OBJECTIVES

The World Allergy Organization/Allergic Rhinitis and its Impact on Asthma (WAO/ARIA) Joint Committee on Chronic Cough reviewed the current literature on neuroanatomy and pathophysiology of chronic cough. The role of TRP ion channels in pathogenic mechanisms of the hypersensitive cough reflex was also examined.

OUTCOMES

Chemoreceptors are better studied in cough neuronal pathways compared to stretch receptors, likely due to their anatomical overabundance in the respiratory tract, but also their distinctive functional properties. Central pathways are important in suppressive mechanisms and behavioral/affective aspects of chronic cough. Current evidence strongly suggests neurogenic inflammation induces a hypersensitive cough reflex marked by increased expression of neuromediators, mast cells, and eosinophils, among others. TRP ion channels, mainly TRP V1/A1, are important in the pathogenesis of chronic cough due to their role in mediating chemosensitivity to various endogenous and exogenous triggers, as well as a crosstalk between neurogenic and inflammatory pathways in cough-associated airways diseases.

Macrophage Activation in the Dorsal Root Ganglion in Rats Developing Autotomy after Peripheral Nerve Injury

Autotomy, self-mutilation of a denervated limb, is common in animals after peripheral nerve injury (PNI) and is a reliable proxy for neuropathic pain in humans. Understanding the occurrence and treatment of autotomy remains challenging. The objective of this study was to investigate the occurrence of autotomy in nude and Wistar rats and evaluate the differences in macrophage activation and fiber sensitization contributing to the understanding of autotomy behavior. Autotomy in nude and Wistar rats was observed and evaluated 6 and 12 weeks after sciatic nerve repair surgery. The numbers of macrophages and the types of neurons in the dorsal root ganglion (DRG) between the two groups were compared by immunofluorescence studies. Immunostaining of T cells in the DRG was also assessed. Nude rats engaged in autotomy with less frequency than Wistar rats. Autotomy symptoms were also relatively less severe in nude rats. Immunofluorescence studies revealed increased macrophage accumulation and activation in the DRG of Wistar rats. The percentage of NF200+ neurons was higher at 6 and 12 weeks in Wistar rats compared to nude rats, but the percentage of CGRP+ neurons did not differ between two groups. Additionally, macrophages were concentrated around NF200-labeled A fibers. At 6 and 12 weeks following PNI, CD4+ T cells were not found in the DRG of the two groups. The accumulation and activation of macrophages in the DRG may account for the increased frequency and severity of autotomy in Wistar rats. Our results also suggest that A fiber neurons in the DRG play an important role in autotomy.

Eliapixant is a selective P2X3 receptor antagonist for the treatment of disorders associated with hypersensitive nerve fibers

ATP-dependent P2X3 receptors play a crucial role in the sensitization of nerve fibers and pathological pain pathways. They are also involved in pathways triggering cough and may contribute to the pathophysiology of endometriosis and overactive bladder. However, despite the strong therapeutic rationale for targeting P2X3 receptors, preliminary antagonists have been hampered by off-target effects, including severe taste disturbances associated with blocking the P2X2/3 receptor heterotrimer. Here we present a P2X3 receptor antagonist, eliapixant (BAY 1817080), which is both highly potent and selective for P2X3 over other P2X subtypes in vitro, including P2X2/3. We show that eliapixant reduces inflammatory pain in relevant animal models. We also provide the first in vivo experimental evidence that P2X3 antagonism reduces neurogenic inflammation, a phenomenon hypothesised to contribute to several diseases, including endometriosis. To test whether eliapixant could help treat endometriosis, we confirmed P2X3 expression on nerve fibers innervating human endometriotic lesions. We then demonstrate that eliapixant reduces vaginal hyperalgesia in an animal model of endometriosis-associated dyspareunia, even beyond treatment cessation. Our findings indicate that P2X3 antagonism could alleviate pain, including non-menstrual pelvic pain, and modify the underlying disease pathophysiology in women with endometriosis. Eliapixant is currently under clinical development for the treatment of disorders associated with hypersensitive nerve fibers.

Differentiation of the 50B11 dorsal ganglion cells into NGF and GDNF responsive nociceptor subtypes

The embryonic rat dorsal root ganglion (DRG) neuron-derived 50B11 cell line is a promising sensory neuron model expressing markers characteristic of NGF and GDNF-dependent C-fibre nociceptors. Whether these cells have the capacity to develop into distinct nociceptive subtypes based on NGF- or GDNF-dependence has not been investigated. Here we show that by augmenting forskolin (FSK) and growth factor supplementation with NGF or GDNF, 50B11 cultures can be driven to acquire differential functional responses to common nociceptive agonists capsaicin and ATP respectively. In addition, to previous studies, we also demonstrate that a differentiated neuronal phenotype can be maintained for up to 7 days. Western blot analysis of nociceptive marker proteins further demonstrates that the 50B11 cells partially recapitulate the functional phenotypes of classical NGF-dependent (peptidergic) and GDNF-dependent (non-peptidergic) neuronal subtypes described in DRGs. Further, 50B11 cells differentiated with NGF/FSK, but not GDNF/FSK, show sensitization to acute prostaglandin E2 treatment. Finally, RNA-Seq analysis confirms that differentiation with NGF/FSK or GDNF/FSK produces two 50B11 cell subtypes with distinct transcriptome expression profiles. Gene ontology comparison of the two subtypes of differentiated 50B11 cells to rodent DRG neurons studies shows significant overlap in matching or partially matching categories. This transcriptomic analysis will aid future suitability assessment of the 50B11 cells as a high-throughput nociceptor model for a broad range of experimental applications. In conclusion, this study shows that the 50B11 cell line is capable of partially recapitulating features of two distinct types of embryonic NGF and GDNF-dependent nociceptor-like cells.

Purinergic Receptors of the Central Nervous System: Biology, PET Ligands, and Their Applications

Purinergic receptors play important roles in central nervous system (CNS). These receptors are involved in cellular neuroinflammatory responses that regulate functions of neurons, microglial and astrocytes. Based on their endogenous ligands, purinergic receptors are classified into P1 or adenosine, P2X and P2Y receptors. During brain injury or under pathological conditions, rapid diffusion of extracellular adenosine triphosphate (ATP) or uridine triphosphate (UTP) from the damaged cells, promote microglial activation that result in the changes in expression of several of these receptors in the brain. Imaging of the purinergic receptors with selective Positron Emission Tomography (PET) radioligands has advanced our understanding of the functional roles of some of these receptors in healthy and diseased brains. In this review, we have accumulated a list of currently available PET radioligands of the purinergic receptors that are used to elucidate the receptor functions and participations in CNS disorders. We have also reviewed receptors lacking radiotracer, laying the foundation for future discoveries of novel PET radioligands to reveal these receptors roles in CNS disorders.

Overexpressed Na V 1.7 Channels Confer Hyperexcitability to in vitro Trigeminal Sensory Neurons of Ca V 2.1 Mutant Hemiplegic Migraine Mice

Trigeminal sensory neurons of transgenic knock-in (KI) mice expressing the R192Q missense mutation in the α1A subunit of neuronal voltage-gated Ca_V2.1 Ca²⁺ channels, which leads to familial hemiplegic migraine type 1 (FHM1) in patients, exhibit a hyperexcitability phenotype. Here, we show that the expression of Na_V1.7 channels, linked to pain states, is upregulated in KI primary cultures of trigeminal ganglia (TG), as shown by increased expression of its α1 subunit. In the majority of TG neurons, Na_V1.7 channels are co-expressed with ATP-gated P2X3 receptors (P2X3R), which are important nociceptive sensors. Reversing the trigeminal phenotype with selective Ca_V2.1 channel inhibitor ω-agatoxin IVA inhibited Na_V1.7 overexpression. Functionally, KI neurons revealed a TTX-sensitive inward current of larger amplitude that was partially inhibited by selective Na_V1.7 blocker Tp1a. Under current-clamp condition, Tp1a raised the spike threshold of both wild-type (WT) and KI neurons with decreased firing rate in KI cells. Na_V1.7 activator OD1 accelerated firing in WT and KI neurons, a phenomenon blocked by Tp1a. Enhanced expression and function of Na_V1.7 channels in KI TG neurons resulted in higher excitability and facilitated nociceptive signaling. Co-expression of Na_V1.7 channels and P2X3Rs in TGs may explain how hypersensitivity to local stimuli can be relevant to migraine.

Treatment of chronic neuropathic pain: purine receptor modulation

Extracellular nucleosides and nucleotides have widespread functions in responding to physiological stress. The "purinome" encompasses 4 G-protein-coupled receptors (GPCRs) for adenosine, 8 GPCRs activated by nucleotides, 7 adenosine 5'-triphosphate-gated P2X ion channels, as well as the associated enzymes and transporters that regulate native agonist levels. Purinergic signaling modulators, such as receptor agonists and antagonists, have potential for treating chronic pain. Adenosine and its analogues potently suppress nociception in preclinical models by activating A1 and/or A3 adenosine receptors (ARs), but safely harnessing this pathway to clinically treat pain has not been achieved. Both A2AAR agonists and antagonists are efficacious in pain models. Highly selective A3AR agonists offer a novel approach to treat chronic pain. We have explored the structure activity relationship of nucleoside derivatives at this subtype using a computational structure-based approach. Novel A3AR agonists for pain control containing a bicyclic ring system (bicyclo [3.1.0] hexane) in place of ribose were designed and screened using an in vivo phenotypic model, which reflected both pharmacokinetic and pharmacodynamic parameters. High specificity (>10,000-fold selective for A3AR) was achieved with the aid of receptor homology models based on related GPCR structures. These A3AR agonists are well tolerated in vivo and highly efficacious in models of chronic neuropathic pain. Furthermore, signaling molecules acting at P2X3, P2X4, P2X7, and P2Y12Rs play critical roles in maladaptive pain neuroplasticity, and their antagonists reduce chronic or inflammatory pain, and, therefore, purine receptor modulation is a promising approach for future pain therapeutics. Structurally novel antagonists for these nucleotide receptors were discovered recently.

Functional Coupling of Slack Channels and P2X3 Receptors Contributes to Neuropathic Pain Processing

The sodium-activated potassium channel Slack (K_Na1.1, Slo2.2, or Kcnt1) is highly expressed in populations of sensory neurons, where it mediates the sodium-activated potassium current (I_KNa) and modulates neuronal activity. Previous studies suggest that Slack is involved in the processing of neuropathic pain. However, mechanisms underlying the regulation of Slack activity in this context are poorly understood. Using whole-cell patch-clamp recordings we found that Slack-mediated I_KNa in sensory neurons of mice is reduced after peripheral nerve injury, thereby contributing to neuropathic pain hypersensitivity. Interestingly, Slack is closely associated with ATP-sensitive P2X3 receptors in a population of sensory neurons. In vitro experiments revealed that Slack-mediated I_KNa may be bidirectionally modulated in response to P2X3 activation. Moreover, mice lacking Slack show altered nocifensive responses to P2X3 stimulation. Our study identifies P2X3/Slack signaling as a mechanism contributing to hypersensitivity after peripheral nerve injury and proposes a potential novel strategy for treatment of neuropathic pain.

Platelet-rich plasma in umbilical cord blood reduces neuropathic pain in spinal cord injury by altering the expression of ATP receptors

BACKGROUND

Neuropathic pain following injury or dysfunction of the peripheral or CNS is one of the most important medical challenges to treat. Humane platelet-rich plasma (HPRP), which is a rich source of growth factors, may be able to treat and reduce pain caused by spinal cord injury (SCI). In this study, the effect of HPRP on neuropathic pain caused by SCI was investigated.

METHODS

Sixty adult male Wistar rats were randomly divided into 6 groups: control, sham, SCI, vehicle (SCI+platelet-poor plasma), SCI+ PRP2day (injection 48 hrs after SCI) and SCI+PRP14day (injection 14 days after SCI). SCI was induced at the T12-T13 level. Behavioral tests were conducted weekly after injury for six weeks. Allodynia and hyperalgesia were assessed using acetone drops, plantar test and von Frey filament. Cavity size and the number of fibroblasts were determined by H&E stain, and the expression of mTOR, p-mTOR, P₂×₃R and P₂Y₄R were determined using the western blot technique. Data were analyzed using PRISM & SPSS software.

RESULTS

PRP injection showed a higher pain threshold in mechanical allodynia (p<0.0001), cold allodynia (p<0.0001) and thermal hyperalgesia (p<0.0001) than those in the spinal. Animals treated with PRP also reduced cavity size, fibroblast number, p-mTOR/mTOR ratio, and P₂×₃R expression, and increased P₂Y₄R expression. The difference between the two groups was not statistically significant.

CONCLUSIONS

The results showed that PRP reduced SCI-induced allodynia and hyperalgesia by regulating ATP signaling. Using HPRP can open a new window in the treatment of pain caused by damage to the nervous system.

State-of-the-art pharmacotherapy for diabetic neuropathy

INTRODUCTION

The global epidemic of diabetes has led to an epidemic of diabetes complications. Diabetic neuropathy is the most common microvascular complication, of which diabetic peripheral neuropathy (DPN) and autonomic neuropathy (AN) are the most prevalent, affecting ~50% of patients. DPN results in pain with a poor quality of life and a loss of sensation with an increased risk of foot ulceration. Autonomic neuropathy can cause significant morbidity in a minority and is associated with increased mortality. The cornerstone of treatment to prevent or limit the progression of DPN/AN is multifactorial risk factor modification including treatment of glycemia, lipids and blood pressure. Whilst, there are no FDA-approved disease-modifying therapies, there are a number of therapies to relieve symptoms in DPN and AN.

AREAS COVERED

The authors discuss current approved therapies for painful diabetic neuropathy and autonomic neuropathy. They also address the potential role of improving risk factors to limit the development and progression of diabetic neuropathy and new pathogenetic and pain-relieving treatments.

EXPERT OPINION

The FDA-approved Pregabalin and Duloxetine over 25 years ago and Tapentadol, 6 years ago for painful diabetic neuropathy. There are currently no FDA-approved disease-modifying treatments for diabetic neuropathy which has been attributed to inappropriate models of the disease with limited translational capacity and major limitations of trial designs and endpoints in clinical trials.

Peripheral Inflammatory Hyperalgesia Depends on P2X7 Receptors in Satellite Glial Cells

Peripheral inflammatory hyperalgesia depends on the sensitization of primary nociceptive neurons. Inflammation drives molecular alterations not only locally but also in the dorsal root ganglion (DRG) where interleukin-1 beta (IL-1β) and purinoceptors are upregulated. Activation of the P2X7 purinoceptors by ATP is essential for IL-1β maturation and release. At the DRG, P2X7R are expressed by satellite glial cells (SGCs) surrounding sensory neurons soma. Although SGCs have no projections outside the sensory ganglia these cells affect pain signaling through intercellular communication. Therefore, here we investigated whether activation of P2X7R by ATP and the subsequent release of IL-1β in DRG participate in peripheral inflammatory hyperalgesia. Immunofluorescent images confirmed the expression of P2X7R and IL-1β in SGCs of the DRG. The function of P2X7R was then verified using a selective antagonist, A-740003, or antisense for P2X7R administered in the L5-DRG. Inflammation was induced by CFA, carrageenan, IL-1β, or PGE₂ administered in rat's hind paw. Blockage of P2X7R at the DRG reduced the mechanical hyperalgesia induced by CFA, and prevented the mechanical hyperalgesia induced by carrageenan or IL-1β, but not PGE₂. It was also found an increase in P2X7 mRNA expression at the DRG after peripheral inflammation. IL-1β production was also increased by inflammatory stimuli in vivo and in vitro, using SGC-enriched cultures stimulated with LPS. In LPS-stimulated cultures, activation of P2X7R by BzATP induced the release of IL-1β, which was blocked by A-740003. In summary, our data suggest that peripheral inflammation leads to the activation of P2X7R expressed by SGCs at the DRG. Then, ATP-induced activation of P2X7R mediates the release of IL-1β from SGC. This evidence places the SGC as an active player in the establishment of peripheral inflammatory hyperalgesia and highlights the importance of the events in DRG for the treatment of inflammatory diseases.

Update on novel purinergic P2X3 and P2X2/3 receptor antagonists and their potential therapeutic applications

Introduction: Purinergic P2X3-P2X2/3 receptors are placed in nociceptive neurons' strategic location and show unique desensitization properties; hence, they represent an attractive target for many pain-related diseases. Therefore, a broad interest from academic and pharmaceutical scientists has focused on the search for P2X3 and P2X2/3 receptor ligands and has led to the discovery of numerous new selective antagonists. Some of them have been studied in clinical trials for the treatment of pathological conditions such as bladder disorders, gastrointestinal and chronic obstructive pulmonary diseases.Areas covered: This review provides a summary of the patents concerning the discovery of P2X3 and/or P2X2/3 receptor antagonists published between 2015 and 2019 and their potential clinical use. Thus, the structures and biological data of the most representative molecules are reported.Expert opinion: The 2016 publication of the crystallographic structure of the human P2X3 receptor subtype gave an improvement of published patents in 2017. Hence, a great number of small molecules with dual antagonist activity on P2X3-P2X2/3 receptors, a favorable pharmacokinetic profile, and reasonable oral bioavailability was discovered. The most promising compounds are the phenoxy-diaminopyrimidines including gefapixant (AF-219), and the imidazo-pyridines like BLU-5937, which are in phase III and phase II clinical trials, respectively, for refractory chronic cough.

P2X3 receptors contribute to muscle pain induced by static contraction by a mechanism dependent on neutrophil migration

P2X3 receptors are involved with several pain conditions. Muscle pain induced by static contraction has an important socioeconomic impact. Here, we evaluated the involvement of P2X3 receptors on mechanical muscle hyperalgesia and neutrophil migration induced by static contraction in rats. Also, we evaluated whether static contraction would be able to increase muscle levels of TNF-α and IL-1β. Male Wistar rats were pretreated with the selective P2X3 receptor antagonist, A-317491, by intramuscular or intrathecal injection and the static contraction-induced mechanical muscle hyperalgesia was evaluated using the Randall-Selitto test. Neutrophil migration was evaluated by measurement of myeloperoxidase (MPO) kinetic-colorimetric assay and the cytokines TNF-α and IL-1β by enzyme-linked immunosorbent assay. Intramuscular or intrathecal pretreatment with A-317491 prevented static contraction-induced mechanical muscle hyperalgesia. In addition, A-317491 reduced static contraction-induced mechanical muscle hyperalgesia when administered 30 and 60 min of the beginning of static contraction, but not after 30 and 60 min of the end of static contraction. Intramuscular A-317491 also prevented static contraction-induced neutrophil migration. In a period of 24 h, static contraction did not increase muscle levels of TNF-α and IL-1β. These findings demonstrated that mechanical muscle hyperalgesia and neutrophil migration induced by static contraction are modulated by P2X3 receptors expressed on the gastrocnemius muscle and spinal cord dorsal horn. Also, we suggest that P2X3 receptors are important to the development but not to maintenance of muscle hyperalgesia. Therefore, P2X3 receptors can be pointed out as a target to musculoskeletal pain conditions induced by daily or work-related activities.

CGRP Receptor Signalling Pathways

Calcitonin gene-related peptide (CGRP) is a promiscuous peptide, similar to many other members of the calcitonin family of peptides. The potential of CGRP to act on many different receptors with differing affinities and efficacies makes deciphering the signalling from the CGRP receptor a challenging task for researchers.Although it is not a typical G protein-coupled receptor (GPCR), in that it is composed not just of a GPCR, the CGRP receptor activates many of the same signalling pathways common for other GPCRs. This includes the family of G proteins and a variety of protein kinases and transcription factors. It is now also clear that in addition to the initiation of cell-surface signalling, GPCRs, including the CGRP receptor, also activate distinct signalling pathways as the receptor is trafficking along the endocytic conduit.Given CGRP's characteristic of activating multiple GPCRs, we will first consider the complex of calcitonin receptor-like receptor (CLR) and receptor activity-modifying protein 1 (RAMP1) as the CGRP receptor. We will discuss the discovery of the CGRP receptor components, the molecular mechanisms controlling its internalization and post-endocytic trafficking (recycling and degradation) and the diverse signalling cascades that are elicited by this receptor in model cell lines. We will then discuss CGRP-mediated signalling pathways in primary cells pertinent to migraine including neurons, glial cells and vascular smooth muscle cells.Investigation of all the CGRP- and CGRP receptor-mediated signalling cascades is vital if we are to fully understand CGRP's role in migraine and will no doubt unearth new targets for the treatment of migraine and other CGRP-driven diseases.

The two faces of pannexins: new roles in inflammation and repair

Pannexins belong to a family of ATP-release channels expressed in almost all cell types. An increasing body of literature on pannexins suggests that these channels play dual and sometimes contradictory roles, contributing to normal cell function, as well as to the pathological progression of disease. In this review, we summarize our understanding of pannexin "protective" and "harmful" functions in inflammation, regeneration and mechanical signaling. We also suggest a possible basis for pannexin's dual roles, related to extracellular ATP and K⁺ levels and the activation of various types of P2 receptors that are associated with pannexin. Finally, we speculate upon therapeutic strategies related to pannexin using eyes, lacrimal glands, and peripheral nerves as examples of interesting therapeutic targets.

Long noncoding RNA MRAK009713 is a novel regulator of neuropathic pain in rats

Long noncoding RNAs have been implicated in neuropathy. Here, we identify and validate a long noncoding RNA, MRAK009713, as the primary regulator of neuropathic pain in chronic constriction injury (CCI) rats. MRAK009713 expression was markedly increased in CCI rats associated with enhanced pain behaviors, and small interfering RNA against MRAK009713 significantly reduced both mechanical and thermal hyperalgesia in the CCI rats. MRAK009713 is predicted to interact with the nociceptive P2X3 receptor by CatRAPID, a bioinformatics technology. Overexpression of MRAK009713 markedly increased expression of P2X3 in the dorsal root ganglia of the control rats, and MRAK009713 small interfering RNA significantly inhibited the P2X3 expression in the dorsal root ganglia of the CCI rats. MRAK009713 directly interacted with the P2X3 protein heterologously expressed in the human embryonic kidney (HEK) 293 cells and potentiated P2X3 receptor function. Thus, MRAK009713 is a novel positive regulator of neuropathic pain in rats through regulating the expression and function of the P2X3 receptor.

Exchange factor directly activated by cAMP-PKCε signalling mediates chronic morphine-induced expression of purine P2X3 receptor in rat dorsal root ganglia

BACKGROUND AND PURPOSE

The P2X3 receptor is a major receptor in the processing of nociceptive information in dorsal root ganglia. We investigated the role of the P2X3 receptor and the detailed mechanisms underlying chronic morphine-induced analgesic tolerance in rats.

EXPERIMENTAL APPROACH

Repeated i.t. morphine treatment was used to induce anti-nociceptive tolerance. The expression of spinal P2X3 receptor, phosphorylated PKCε and exchange factor directly activated by cAMP (Epac) were evaluated. Effects of A-317491 (P2X3 antagonist), ε-V1-2 (PKCε inhibitor) and ESI-09 (Epac inhibitor) on mechanical pain thresholds and tail-flick latency after chronic morphine treatment were determined. Co-localization of P2X3 receptor with NeuNs (marker of neuron), IB4 (marker of small DRG neurons), peripherin, PKCε and Epac were performed by double immunofluorescence staining.

KEY RESULTS

Chronic morphine time-dependently increased the expression of P2X3 receptor, phosphorylated PKCε and Epac in DRGs. ε-V1-2 prevented chronic morphine-induced expression of P2X3 receptor. ESI-09 decreased the phosphorylation of PKCε and up-regulated expression of Epac after chronic morphine exposure. Mechanical pain thresholds and tail-flick latency showed that A317491, ε-V1-2 and ESI-09 significantly attenuated the loss of morphine's analgesic potency. Morphine-induced P2X3 receptor expression mainly occurred in neurons staining for IB4 and peripherin. Co-localization of P2X3 receptor with PKCε and Epac was demonstrated in the same neurons.

CONCLUSIONS AND IMPLICATIONS

Chronic morphine exposure increased the expression of P2X3 receptor, and i.t. P2X3 receptor antagonists attenuated the loss of morphine's analgesic effect. Inhibiting Epac/PKCε signalling was shown to play a significant inhibitory role in chronic morphine-induced P2X3 receptor expression and attenuate morphine-induced tolerance.

Pannexin 1 Modulates Axonal Growth in Mouse Peripheral Nerves

The pannexin family of channels consists of three members—pannexin-1 (Panx1), pannexin-2 (Panx2), and pannexin-3 (Panx3) that enable the exchange of metabolites and signaling molecules between intracellular and extracellular compartments. Pannexin-mediated release of intracellular ATP into the extracellular space has been tied to a number of cellular activities, primarily through the activity of type P2 purinergic receptors. Previous work indicates that the opening of Panx1 channels and activation of purinergic receptors by extracellular ATP may cause inflammation and apoptosis. In the CNS (central nervous system) and PNS (peripheral nervous system), coupled pannexin, and P2 functions have been linked to peripheral sensitization (pain) pathways. Purinergic pathways are also essential for other critical processes in the PNS, including myelination and neurite outgrowth. However, whether such pathways are pannexin-dependent remains to be determined. In this study, we use a Panx1 knockout mouse model and pharmacological inhibitors of the Panx1 and the ATP-mediated signaling pathway to fill gaps in our understanding of Panx1 localization in peripheral nerves, roles for Panx1 in axonal outgrowth and myelination, and neurite extension. Our data show that Panx1 is localized to axonal, myelin, and vascular compartments of the peripheral nerves. Knockout of Panx1 gene significantly increased axonal caliber in vivo and axonal growth rate in cultured dorsal root ganglia (DRG) neurons. Furthermore, genetic knockout of Panx1 or inhibition of components of purinergic signaling, by treatment with probenecid and apyrase, resulted in denser axonal outgrowth from cultured DRG explants compared to untreated wild-types. Our findings suggest that Panx1 regulates axonal growth in the peripheral nervous system.

P2X3 receptor involvement in endometriosis pain via ERK signaling pathway

The purinergic receptor P2X ligand-gated ion channel 3 (P2X3) is crucially involved in peripheral nociceptive processes of somatic and visceral pain. Endometriosis pain is considered as a kind of inflammatory and neuropathic pain. However, whether P2X3 is involved in endometriosis pain has not been reported up to date. Here, we aimed to determine whether P2X3 expression in endometriotic lesions is involved in endometriosis pain, which is regulated by inflammatory mediators through extracellular regulated protein kinases (ERK) signalling pathway. We found that P2X3 expressions in endometriosis endometrium and endometriotic lesions were both significantly higher as compared with control endometrium (P<0.05), and both positively correlated with pain (P<0.05). The expression levels of phosphorylated –ERK (p-ERK), phosphorylated-cAMP-response element binding protein (p-CREB), and P2X3 in endometriotic stromal cells (ESCs) were all significantly increased in comparison to the initial levels after treated with interleukin (IL)-1β (P<0.05) or adenosine triphosphate (ATP) (P<0.05), respectively, and did not increase after the ESCs were pre-treated with ERK1/2 inhibitor. Additionally, P2X3 and calcitonin gene related peptide (CGRP) were co-expressed in endometriotic lesions. These obtained results suggest that P2X3 might be involved in endometriosis pain signal transduction via ERK signal pathway.

Activation of P2X3 receptors in the cerebrospinal fluid-contacting nucleus neurons reduces formalin-induced pain behavior via PAG in a rat model

The cerebrospinal fluid (CSF)-contacting nucleus is implicated in the descending inhibitory pathway in pain processing, whereas the cellular and molecular mechanisms underpinning CSF-contacting nucleus regulating pain signals remains largely elusive. ATP is evidenced to inhibit pain transmission at supraspinal level by the mediation of the receptor P2X, wherein its subtype P2X3 is identified as the most potent. Our present experiment investigated the functionality of P2X3 receptors in CSF-contacting nucleus in the formalin-evoked inflammatory pain. Immunofluorescence and western blot revealed the expression of P2X3 receptors in the CSF-contacting nucleus and their upregulated expression subsequent to administration of formalin in rat model. ATP (a P2X3 receptor agonist, 100nmol/5µl) by intracerebroventricular (i.c.v.) administration ameliorated pain behaviors and enhanced c-Fos immunoreactivity in the neurons of the periaqueductal gray (PAG), both of which were discounted by pre-administration of A-317491 (a selective P2X3 receptor antagonist, 25nmol/5µl). After the CSF-contacting nucleus was ablated by cholera toxin subunit B-saporin, ATP failed to induce analgesia, with the c-Fos immunoreactivity in the PAG neurons remaining intact. Our results validated that P2X3 receptors in the CSF-contacting nucleus are pivotal in inflammatory pain processing via the activation of PAG neurons.

P2X3 receptor-mediated visceral hyperalgesia and neuronal sensitization following exposure to PTSD-like stress in the dorsal root ganglia of rats

BACKGROUND

Patients with posttraumatic stress disorder (PTSD) often share co-morbidity with chronic pain conditions. Recent studies suggest a role of P2X3 receptors and ATP signaling in pain conditions. However, the underlying mechanisms of visceral hyperalgesia following exposure to PTSD-like stress conditions remain unclarified.

METHODS

The behavior and hormones relevant for PTSD were studied. Visceromotor responses (VMR) and the abdominal withdrawal reflexes (AWR) to colorectal distention (CRD) were recorded to determine P2X3-receptor-mediated alteration of hyperalgesia following single-prolonged stress (SPS) exposure. Immunofluorescence, Western blotting, and patch-clamp were used.

KEY RESULTS

The escape latency, adrenocorticotropic hormone and cortisol were increased on days 7-14. Visceromotor responses and AWR was reduced at day 1 in SPS rats but increased to higher levels than in controls after exposure to day 7. Intrathecal administration of the P2X3-receptor antagonist TNP-ATP abolished the CRD response. Based on immunofluorescence and Western blotting analysis, SPS-treated rats exhibited reduced P2X3 expression in dorsal root ganglia (DRG) after day 1 compared with controls. P2X3 expression in DRG was enhanced on day 7 after SPS and the increase of the P2X3 expression was maintained on day 14 and 21 compared with controls. The P2X3-receptor agonist α,β-me ATP (10 μM) induced a fast desensitizing inward current in DRG neurons of both control and SPS-treated rats. The average peak current densities in SPS-treated group were increased 3.6-fold. TNP-ATP (100 nM) markedly blocked all fast α,β-me ATP-induced inward currents in the DRG neurons both in control and SPS-treated rats.

CONCLUSIONS & INFERENCES

The data indicate an important role of P2X3 signaling in visceral hyperalgesia following PTSD-like stress.

CD73 Controls Extracellular Adenosine Generation in the Trigeminal Nociceptive Nerves

Purinergic signaling is involved in pain generation and modulation in the nociceptive sensory nervous system. Adenosine triphosphate (ATP) induces pain via activation of ionotropic P2X receptors while adenosine mediates analgesia via activation of metabotropic P1 receptors. These purinergic signaling are determined by ecto-nucleotidases that control ATP degradation and adenosine generation. Using enzymatic histochemistry, we detected ecto-AMPase activity in dental pulp, trigeminal ganglia (TG) neurons, and their nerve fibers. Using immunofluorescence staining, we confirmed the expression of ecto-5'-nucleotidase (CD73) in trigeminal nociceptive neurons and their axonal fibers, including the nociceptive nerve fibers projecting into the brainstem. In addition, we detected the existence of CD73 and ecto-AMPase activity in the nociceptive lamina of the trigeminal subnucleus caudalis (TSNC) in the brainstem. Furthermore, we demonstrated that incubation with specific anti-CD73 serum significantly reduced the ecto-AMPase activity in the nociceptive lamina in the brainstem. Our results indicate that CD73 might participate in nociceptive modulation by affecting extracellular adenosine generation in the trigeminal nociceptive pathway. Disruption of TG neuronal ecto-nucleotidase expression and axonal terminal localization under certain circumstances such as chronic inflammation, oxidant stress, local constriction, and injury in trigeminal nerves may contribute to the pathogenesis of orofacial neuropathic pain.

The impact of inflammation on respiratory plasticity

Breathing is a vital homeostatic behavior and must be precisely regulated throughout life. Clinical conditions commonly associated with inflammation, undermine respiratory function may involve plasticity in respiratory control circuits to compensate and maintain adequate ventilation. Alternatively, other clinical conditions may evoke maladaptive plasticity. Yet, we have only recently begun to understand the effects of inflammation on respiratory plasticity. Here, we review some of common models used to investigate the effects of inflammation and discuss the impact of inflammation on nociception, chemosensory plasticity, medullary respiratory centers, motor plasticity in motor neurons and respiratory frequency, and adaptation to high altitude. We provide new data suggesting glial cells contribute to CNS inflammatory gene expression after 24h of sustained hypoxia and inflammation induced by 8h of intermittent hypoxia inhibits long-term facilitation of respiratory frequency. We also discuss how inflammation can have opposite effects on the capacity for plasticity, whereby it is necessary for increases in the hypoxic ventilatory response with sustained hypoxia, but inhibits phrenic long term facilitation after intermittent hypoxia. This review highlights gaps in our knowledge about the effects of inflammation on respiratory control (development, age, and sex differences). In summary, data to date suggest plasticity can be either adaptive or maladaptive and understanding how inflammation alters the respiratory system is crucial for development of better therapeutic interventions to promote breathing and for utilization of plasticity as a clinical treatment.

Neuronal NTPDase3 Mediates Extracellular ATP Degradation in Trigeminal Nociceptive Pathway

ATP induces pain via activation of purinergic receptors in nociceptive sensory nerves. ATP signaling is terminated by ATP hydrolysis mediated by cell surface-localized ecto-nucleotidases. Using enzymatic histochemical staining, we show that ecto-ATPase activity is present in mouse trigeminal nerves. Using immunofluorescence staining, we found that ecto-NTPDase3 is expressed in trigeminal nociceptive neurons and their projections to the brainstem. In addition, ecto-ATPase activity and ecto-NTPDase3 are also detected in the nociceptive outermost layer of the trigeminal subnucleus caudalis. Furthermore, we demonstrate that incubation with anti-NTPDase3 serum reduces extracellular ATP degradation in the nociceptive lamina of both the trigeminal subnucleus caudalis and the spinal cord dorsal horn. These results are consistent with neuronal NTPDase3 activity modulating pain signal transduction and transmission by affecting extracellular ATP hydrolysis within the trigeminal nociceptive pathway. Thus, disruption of trigeminal neuronal NTPDase3 expression and localization to presynaptic terminals during chronic inflammation, local constriction and injury may contribute to the pathogenesis of orofacial neuropathic pain.

Loss of inhibition by brain natriuretic peptide over P2X3 receptors contributes to enhanced spike firing of trigeminal ganglion neurons in a mouse model of familial hemiplegic migraine type-1

Purinergic P2X3 receptors (P2X3Rs) play an important role in pain pathologies, including migraine. In trigeminal neurons, P2X3Rs are constitutively downregulated by endogenous brain natriuretic peptide (BNP). In a mouse knock-in (KI) model of familial hemiplegic migraine type-1 with upregulated calcium CaV2.1 channel function, trigeminal neurons exhibit hyperexcitability with gain-of-function of P2X3Rs and their deficient BNP-mediated inhibition. We studied whether the absent BNP-induced control over P2X3Rs activity in KI cultures may be functionally expressed in altered firing activity of KI trigeminal neurons. Patch-clamp experiments investigated the excitability of wild-type and KI trigeminal neurons induced by either current or agonists for P2X3Rs or transient receptor potential vanilloid-1 (TRPV1) receptors. Consistent with the constitutive inhibition of P2X3Rs by BNP, sustained pharmacological block of BNP receptors selectively enhanced P2X3R-mediated excitability of wild-type neurons without affecting firing evoked by the other protocols. This effect included increased number of action potentials, lower spike threshold and shift of the firing pattern distribution toward higher spiking activity. Thus, inactivation of BNP signaling transformed the wild-type excitability phenotype into the one typical for KI. BNP receptor block did not influence excitability of KI neurons in accordance with the lack of BNP-induced P2X3R modulation. Our study suggests that, in wild-type trigeminal neurons, negative control over P2X3Rs by the BNP pathway is translated into tonic suppression of P2X3Rs-mediated excitability. Lack of this inhibition in KI cultures results in a hyperexcitability phenotype and might contribute to facilitated trigeminal pain transduction relevant for migraine.

RNA Sequencing of Trigeminal Ganglia in Rattus Norvegicus after Glyceryl Trinitrate Infusion with Relevance to Migraine

INTRODUCTION

Infusion of glyceryl trinitrate (GTN), a donor of nitric oxide, induces immediate headache in humans that in migraineurs is followed by a delayed migraine attack. In order to achieve increased knowledge of mechanisms activated during GTN-infusion this present study aims to investigate transcriptional responses to GTN-infusion in the rat trigeminal ganglia.

METHODS

Rats were infused with GTN or vehicle and trigeminal ganglia were isolated either 30 or 90 minutes post infusion. RNA sequencing was used to investigate transcriptomic changes in response to the treatment. Furthermore, we developed a novel method for Gene Set Analysis Of Variance (GSANOVA) to identify gene sets associated with transcriptional changes across time.

RESULTS

15 genes displayed significant changes in transcription levels in response to GTN-infusion. Ten of these genes showed either sustained up- or down-regulation in the 90-minute period after infusion. The GSANOVA analysis demonstrate enrichment of pathways pointing towards an increase in immune response, signal transduction, and neuroplasticity in response to GTN-infusion. Future functional in-depth studies of these mechanisms are expected to increase our understanding of migraine pathogenesis.

Inefficient constitutive inhibition of P2X3 receptors by brain natriuretic peptide system contributes to sensitization of trigeminal sensory neurons in a genetic mouse model of familial hemiplegic migraine

BACKGROUND

On trigeminal ganglion neurons, pain-sensing P2X3 receptors are constitutively inhibited by brain natriuretic peptide via its natriuretic peptide receptor-A. This inhibition is associated with increased P2X3 serine phosphorylation and receptor redistribution to non-lipid raft membrane compartments. The natriuretic peptide receptor-A antagonist anantin reverses these effects. We studied whether P2X3 inhibition is dysfunctional in a genetic familial hemiplegic migraine type-1 model produced by introduction of the human pathogenic R192Q missense mutation into the mouse CACNA1A gene (knock-in phenotype). This model faithfully replicates several properties of familial hemiplegic migraine type-1, with gain-of-function of CaV2.1 Ca(2+) channels, raised levels of the algogenic peptide calcitonin gene-related peptide, and enhanced activity of P2X3 receptors in trigeminal ganglia.

RESULTS

In knock-in neurons, anantin did not affect P2X3 receptor activity, membrane distribution, or serine phosphorylation level, implying ineffective inhibition by the constitutive brain natriuretic peptide/natriuretic peptide receptor-A pathway. However, expression and functional properties of this pathway remained intact together with its ability to downregulate TRPV1 channels. Reversing the familial hemiplegic migraine type-1 phenotype with the CaV2.1-specific antagonist, ω-agatoxin IVA restored P2X3 activity to wild-type level and enabled the potentiating effects of anantin again. After blocking calcitonin gene-related peptide receptors, P2X3 receptors exhibited wild-type properties and were again potentiated by anantin.

CONCLUSIONS

P2X3 receptors on mouse trigeminal ganglion neurons are subjected to contrasting modulation by inhibitory brain natriuretic peptide and facilitatory calcitonin gene-related peptide that both operate via complex intracellular signaling. In the familial hemiplegic migraine type-1 migraine model, the action of calcitonin gene-related peptide appears to prevail over brain natriuretic peptide, thus suggesting that peripheral inhibition of P2X3 receptors becomes insufficient and contributes to trigeminal pain sensitization.

Brain natriuretic peptide constitutively downregulates P2X3 receptors by controlling their phosphorylation state and membrane localization

BACKGROUND

ATP-gated P2X3 receptors are important transducers of nociceptive stimuli and are almost exclusively expressed by sensory ganglion neurons. In mouse trigeminal ganglion (TG), P2X3 receptor function is unexpectedly enhanced by pharmacological block of natriuretic peptide receptor-A (NPR-A), outlining a potential inhibitory role of endogenous natriuretic peptides in nociception mediated by P2X3 receptors. Lack of change in P2X3 protein expression indicates a complex modulation whose mechanisms for downregulating P2X3 receptor function remain unclear.

RESULTS

To clarify this process in mouse TG cultures, we suppressed NPR-A signaling with either siRNA of the endogenous agonist BNP, or the NPR-A blocker anantin. Thus, we investigated changes in P2X3 receptor distribution in the lipid raft membrane compartment, their phosphorylation state, as well as their function with patch clamping. Delayed onset of P2X3 desensitization was one mechanism for the anantin-induced enhancement of P2X3 activity. Anantin application caused preferential P2X3 receptor redistribution to the lipid raft compartment and decreased P2X3 serine phosphorylation, two phenomena that were not interdependent. An inhibitor of cGMP-dependent protein kinase and siRNA-mediated knockdown of BNP mimicked the effect of anantin.

CONCLUSIONS

We demonstrated that in mouse trigeminal neurons endogenous BNP acts on NPR-A receptors to determine constitutive depression of P2X3 receptor function. Tonic inhibition of P2X3 receptor activity by BNP/NPR-A/PKG pathways occurs via two distinct mechanisms: P2X3 serine phosphorylation and receptor redistribution to non-raft membrane compartments. This novel mechanism of receptor control might be a target for future studies aiming at decreasing dysregulated P2X3 receptor activity in chronic pain.

p38 MAPK mediates glial P2X7R-neuronal P2Y1R inhibitory control of P2X3R expression in dorsal root ganglion neurons

Background

We have previously shown that endogenously active purinergic P2X7 receptors (P2X7Rs) in satellite glial cells of dorsal root ganglia (DRGs) stimulate ATP release. The ATP activates P2Y1Rs located in the enwrapped neuronal somata, resulting in down-regulation of P2X3Rs. This P2X7R-P2Y1-P2X3R inhibitory control significantly reduces P2X3R-mediated nociceptive responses. The underlying mechanism by which the activation of P2Y1Rs inhibits the expression of P2X3Rs remains unexplored.

Results

Examining the effect of the activation of p38 mitogen-activated protein kinase on the expression of P2X3Rs in DRGs, we found that the p38 activator, anisomycin (Anis), reduced the expression of P2X3Rs. Blocking the activity of SGCs by the glial Krebs cycle inhibitor, fluorocitrate, did not change the effect of Anis. These results suggest that neuronal p38 plays a major role in the inhibition of P2X3R expression. Western blotting analyses showed that inhibiting P2Y1Rs by MRS2179 (MRS) or blocking P2X7Rs by either oxATP or A740003 reduced pp38 and increased P2X3R expression in DRGs. These results are further supported by the immunohistochemical study showing that P2X7R and P2Y1R antagonists reduce the percentage of pp38-positive neurons. These observations suggest that activation of P2X7Rs and P2Y1Rs promotes p38 activity to exert inhibitory control on P2X3R expression. Since activation of p38 by Anis in the presence of either A740003 or MRS could overcome the block of P2X7R-P2Y1R inhibitory control, p38 in DRG neurons is downstream of P2Y1Rs. In addition, inhibition of p38 by SB202190 was found to prevent the P2X7R and P2Y1R block of P2X3R expression and increase P2X3R-mediated nociceptive flinch behaviors.

Conclusions

p38 in DRG neurons downstream of P2Y1R is necessary and sufficient for the P2X7R-P2Y1R inhibitory control of P2X3R expression.

Constellation pharmacology: a new paradigm for drug discovery

Constellation pharmacology is a cell-based high-content phenotypic-screening platform that utilizes subtype-selective pharmacological agents to elucidate the cell-specific combinations (constellations) of key signaling proteins that define specific cell types. Heterogeneous populations of native cells, in which the different individual cell types have been identified and characterized, are the foundation for this screening platform. Constellation pharmacology is useful for screening small molecules or for deconvoluting complex mixtures of biologically active natural products. This platform has been used to purify natural products and discover their molecular mechanisms. In the ongoing development of constellation pharmacology, there is a positive feedback loop between the pharmacological characterization of cell types and screening for new drug candidates. As constellation pharmacology is used to discover compounds with novel targeting-selectivity profiles, those new compounds then further help to elucidate the constellations of specific cell types, thereby increasing the content of this high-content platform.

Sustained release carrier for adenosine triphosphate as signaling molecule

Adenosine triphosphate (ATP) is a molecule with a fascinating variety of intracellular and extracellular biological functions that go far beyond energy metabolism. Due to its limited passive diffusion through biological membranes, controlled release systems may allow to interact with ATP-mediated extracellular processes. In this study, two release systems were explored to evaluate the capacity for either long-term or short-term release: (i) Poly[(rac-lactide)-co-glycolide] (PLGA) implant rods were capable of ATP release over days to weeks, depending on the PLGA molecular weight and end-group capping, but were also associated with partial hydrolytic degradation of ATP to ADP and AMP, but not adenosine. (ii) Thermosensitive methylcellulose hydrogels with a gelation occurring at body temperature allowed combining adjustable loading levels and the capacity for injection, with injection forces less than 50N even for small 27G needles. Finally, a first in vitro study illustrated purinergic-triggered response of primary murine microglia to ATP released from hydrogels, demonstrating the potential relevance for biomedical applications.

Neuronal calcium signaling in chronic pain

Acute physiological pain, the unpleasant sensory response to a noxious stimulus, is essential for animals and humans to avoid potential injury. Pathological pain that persists after the original insult or injury has subsided, however, not only results in individual suffering but also imposes a significant cost on society. Improving treatments for long-lasting pathological pain requires a comprehensive understanding of the biological mechanisms underlying pain perception and the development of pain chronicity. In this review, we aim to highlight some of the major findings related to the involvement of neuronal calcium signaling in the processes that mediate chronic pain.