P2X3-Containing Receptors as Targets for the Treatment of Chronic Pain

Research Progress on the Experimental Model and Underlying Mechanistic Studies of Tension-Type Headaches

PURPOSE OF REVIEW

Tension-type headaches (TTH) significantly diminish patients' quality of life and increase absenteeism, thereby imposing a substantial economic burden. Animal models are essential tools for studying disease mechanisms and drug development. However, until now, little focus has been placed on summarizing the animal models of TTH and associated mechanistic studies. This narrative review discusses the current animal models of TTH and related mechanistic studies to provide insights into the pathophysiological mechanisms of and treatments for TTH.

RECENT FINDINGS

The primary method for constructing an animal model of TTH involves injecting a solution of pain relievers, such as adenosine triphosphate, nerve growth factor, or a high concentration of salt solution, into the neck to initiate harmful cervical muscle responses. This model enables the examination of the interaction between peripheral muscles and central sensitization, which is crucial for understanding the pathophysiology of TTH. Mechanistic studies based on this model have investigated the effect of the P2X receptor antagonist, P2X7 receptor blockade, the P2Y1 receptor agonist 2-MESADP, P2Y1 receptor antagonist MRS2179, nitric oxide synthase inhibitors, and acetylsalicylic acid. Despite notable advancements, the current model of TTH has limitations, including surgical complexity and the inability to replicate chronic tension-type headache (CTTH). To gain a more comprehensive understanding and develop more effective treatment methods, future studies should focus on simplifying surgical procedures, examining other predisposing factors, and establishing a model for chronic TTH. This will offer a deeper insight into the pathophysiological mechanism of TTH and pave the way for improved treatment approaches.

Interstitial Cells of Cajal and P2X3 Receptors at Ureteropelvic Junction Obstruction and Their Relationship with Pain Response

Introduction: Etiopathogenesis and the symptomatology of ureteropelvic junction obstruction (UPJO) in the pediatric population has not yet been definitely clarified, suggesting a multifactorial nature of the condition. The aim was to analyze the association between the number of Interstitial Cells of Cajal (ICCs), as well as P2X3 receptors in ureteropelvic junction (UPJ) and the pain response in pediatric patients with hydronephrosis. Methods: 50 patients with congenital hydronephrosis underwent open or laparoscopic pyeloplasty at one of two departments of pediatric surgery and urology in Poland. Patients were divided into two groups according to the pain symptoms before surgery. A total of 50 samples of UPJ were obtained intraoperatively and underwent histopathological and immunohistochemical (IHC) analysis. Quantitative assessment of ICCs was based on the number of CD117(+) cells of adequate morphology in the subepithelial layer and the muscularis propria. Expression of P2X3 receptors was evaluated as the intensity of IHC staining. Results: Patients with hydronephrosis and accompanying pain were on average 60 months older (77 vs. 17 months) than children with asymptomatic hydronephrosis (p = 0.017). Symptomatic children revealed higher numbers of ICCs in both the subepithelial layer and in the lamina muscularis propria. In particular, symptomatic patients aged 2 years or more exhibited significantly higher numbers of ICCs in the subepithelial layer. Significant differences in the distribution of ICCs between the subepithelial layer and the lamina muscularis propria were observed in both groups. Expression of P2X3 receptors was limited to the urothelium and the muscle layer and correlated between these structures. There was no relationship between pain response and the expression of P2X3 receptors. Conclusions: ICCs and P2X3 receptors may participate in the pathogenesis of UPJO and in the modulation of pain response to a dilatation of the pyelocaliceal system. Explanation of the role of ICCs and P2X3 receptors in propagation of ureteral peristaltic wave and the modulation of pain stimuli requires further studies.

CD304 + adipose tissue-derived mesenchymal stem cell abundance in autologous fat grafts highly correlates with improvement of localized pain syndromes

ABSTRACT

Surgery, burns or surgery-free accident are leading causes of scars with altered tissue consistency, a reduced degree of motion and pain. Autologous fat grafting can dramatically improve tissue consistency and elasticity but less frequently results in the reduction of pain. Therefore, we analyzed different cell populations present within the adipose tissue to be engrafted and correlated them with the reduction of pain after surgery. Here, we identify a population of CD3 - CD4 - CD304 + cells present in grafted adipose tissue, whose abundance highly correlates with pain improvement shortly after surgery ( r2 = 0.7243****) as well as persistently over time (3 months later: r2 = 0.6277****, 1 year later: r2 = 0.5346***, and 4 years later: r2 = 0.5223***). These cells are characterized by the absence of the hematopoietic marker CD45, whereas they express CD90 and CD34, which characterize mesenchymal stem cells (MSCs); the concomitant presence of CD10 and CD73 in the plasma membrane supports a function of these cells in pain reduction. We deduce that the enrichment of this adipose tissue-derived MSC subset could enhance the therapeutic properties of adipose grafts and ameliorate localized pain syndromes.

Novel drugs affecting diabetic peripheral neuropathy

Diabetic peripheral neuropathy (DPN) poses a significant threat, affecting half of the global diabetic population and leading to severe complications, including pain, impaired mobility, and potential amputation. The delayed manifestation of diabetic neuropathy (DN) makes early diagnosis challenging, contributing to its debilitating impact on individuals with diabetes mellitus (DM). This review examines the multifaceted nature of DPN, focusing on the intricate interplay between oxidative stress, metabolic pathways, and the resulting neuronal damage. It delves into the challenges of diagnosing DN, emphasizing the critical role played by hyperglycemia in triggering these cascading effects. Furthermore, the study explores the limitations of current neuropathic pain drugs, prompting an investigation into a myriad of pharmaceutical agents tested in both human and animal trials over the past decade. The methodology scrutinizes these agents for their potential to provide symptomatic relief for DPN. The investigation reveals promising results from various pharmaceutical agents tested for DPN relief, showcasing their efficacy in ameliorating symptoms. However, a notable gap persists in addressing the underlying problem of DPN. The results underscore the complexity of DPN and the challenges in developing therapies that go beyond symptomatic relief. Despite advancements in treating DPN symptoms, there remains a scarcity of options addressing the underlying problem. This review consolidates the state-of-the-art drugs designed to combat DPN, highlighting their efficacy in alleviating symptoms. Additionally, it emphasizes the need for a deeper understanding of the diverse processes and pathways involved in DPN pathogenesis.

Overlap of pain-related and general measures of disability among adults with chronic pain

PURPOSE

Chronic pain is known to be correlated with disability. We aimed to determine the overlap between a general self-reported measure of disability and a measure of disability due to pain problems among adults with chronic pain.

MATERIALS AND METHODS

We used data from the National Health Interview Survey (NHIS) and analyzed respondents with chronic pain in the past 3 months. General disability was defined as being limited in the kind or amount of work one can do due to any physical, mental, or emotional problem. Pain-related disability was defined as pain limiting one's activity on "most days" or "every day."

RESULTS

Based on a sample of 6874 respondents with chronic pain, 58% had either kind of disability, including 9% who reported only pain-related, but not general disability; and 27% who reported both types of disability. Respondents reporting only pain-related, but not general disability tended to be younger and had lower rates of obesity, smoking, diabetes, and hypertension than respondents reporting both pain-related and general disability.

DISCUSSION

Among people with chronic pain, most people with disability are experiencing limitations related to pain problems. Assessment of disability without addressing pain interference has likely underestimated the disability burden in this population.

Acute pulpitis promotes purinergic signaling to induce pain in rats via P38MAPK/NF-κB signaling pathway

Toothache is one of the most common types of pain, but the mechanisms underlying pulpitis-induced pain remain unknown. The ionotropic purinergic receptor family (P2X) is reported to mediate nociception in the nervous system. This study aims to investigate the involvement of P2X3 in the sensitisation of the trigeminal ganglion (TG) and the inflammation caused by acute pulpitis. An acute tooth inflammation model was established by applying LPS to the pulp of SD rats. We found that the increased expression of P2X3 was induced by acute pulpitis. A selective P2X3 inhibitor (A-317491) reduced pain-like behavior in the maxillofacial region of rats and depressed the activation of neurons in the trigeminal ganglion induced by pulpitis. The upregulated MAPK signaling (p-p38, p-ERK1/2) expression in the ipsilateral TG induced by pulpitis could also be depressed by the application of the P2X3 inhibitor. Furthermore, the expression of markers of inflammatory processes, such as NF-κB, TNF-α and IL-1β, could be induced by acute pulpitis and deduced by the intraperitoneal injection of P2X3 antagonists. Our findings demonstrate that purinergic P2X3 receptor signaling in TG neurons contributes to pulpitis-induced pain in rats and that P2X3 signaling may be a potential therapeutic target for tooth pain.

Electroacupuncture Alleviates Dry Eye Ocular Pain Through TNF-ɑ Mediated ERK1/2/P2X3R Signaling Pathway in SD Rats

Purpose

This study aimed to examine electroacupuncture's influence on ocular pain and its potential modulation of the TNF-ɑ mediated ERK1/2/P2X3R signaling pathway in dry eye-induced rat models.

Methods

Male Sprague-Dawley rats with induced dry eye, achieved through extraorbital lacrimal gland removal, were treated with electroacupuncture. Comprehensive metrics such as the corneal mechanical perception threshold, palpebral fissure height, eyeblink frequency, eye wiping duration, behavioral changes in the open field test, and the forced swimming test were employed. Additionally, morphological changes in microglia and neurons were observed. Expression patterns of key markers, TNF-ɑ, TNFR1, p-ERK1/2, and P2X3R, in the trigeminal ganglion (TG) and spinal trigeminal nucleus caudalis (SpVc) regions, were studied with etanercept serving as a control to decipher the biochemistry of electroacupuncture's therapeutic effects.

Results

Electroacupuncture treatment demonstrated a notable decrease in the corneal mechanical perception threshold, improvement in palpebral fissure height, and significant reductions in both eyeblink frequency and eye wiping duration. Moreover, it exhibited a promising role in anxiety alleviation. Notably, the technique effectively diminished ocular pain by curbing microglial and neuronal activation in the TG and SpVc regions. Furthermore, it potently downregulated TNF-ɑ, TNFR1, p-ERK1/2, and P2X3R expression within these regions.

Conclusion

Electroacupuncture attenuated damage to sensory nerve pathways, reduced pain, and eased anxiety in dry eye-afflicted rats. The findings suggest a crucial role of TNF-ɑ mediated ERK1/2/P2X3R signaling pathway inhibition by electroacupuncture in these benefits.

The ethanol extract of Scutellaria baicalensis Georgi attenuates complete Freund's adjuvant (CFA)-induced inflammatory pain by suppression of P2X3 receptor

ETHNOPHARMACOLOGICAL RELEVANCE

Scutellaria baicalensis Georgi (SBG) is a perennial herb with anti-inflammatory, antibacterial, and antioxidant activities, which is traditionally used to treat inflammation of respiratory tract and gastrointestinal tract, abdominal cramps, bacterial and viral infections. Clinically, it is often used to treat inflammatory-related diseases. Research has shown that the ethanol extract of Scutellaria baicalensis Georgi (SGE) has anti-inflammatory effect, and its main components baicalin and baicalein have analgesic effects. However, the mechanism of SGE in relieving inflammatory pain has not been deeply studied.

AIM OF THE STUDY

This study aimed to evaluate the analgesic effect of SGE on complete Freund's adjuvant (CFA)-induced inflammatory pain rats, and to investigate whether its effect on relieving inflammatory pain is associated with regulation of P2X3 receptor.

MATERIALS AND METHODS

The analgesic effects of SGE on CFA-induced inflammatory pain rats were evaluated by measuring mechanical pain threshold, thermal pain threshold, and motor coordination ability. The mechanisms of SGE in relieving inflammatory pain were explored by detecting inflammatory factors levels, NF-κB, COX-2 and P2X3 expression, and were further verified by addition of P2X3 receptor agonist (α, β me-ATP).

RESULTS

Our results revealed that SGE can notably increase the mechanical pain threshold and thermal pain threshold of CFA-induced inflammatory pain rats, and markedly alleviate the pathological damage in DRG. SGE could suppress the release of inflammatory factors including IL-1β, IL-6, TNF-α and restrain the expression of NF-κB, COX-2 and P2X3. Moreover, α, β me-ATP further exacerbated the inflammatory pain of CFA-induced rats, while SGE could markedly raise the pain thresholds and relieve inflammatory pain. SGE could attenuate the pathological damage, inhibit P2X3 expression, inhibit the elevation of inflammatory factors caused by α, β me-ATP. SGE can also inhibit NF-κB and ERK1/2 activation caused by α, β me-ATP, and inhibit the mRNA expression of P2X3, COX-2, NF-κB, IL-1β, IL-6 and TNF-α in DRG of rats induced by CFA coupled with α, β me-ATP.

CONCLUSIONS

In summary, our research indicated that SGE could alleviate CFA-induced inflammatory pain by suppression of P2X3 receptor.

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.

DT-0111: a novel P2X3 receptor antagonist

Extracellular adenosine 5'-triphosphate (ATP) acts as an autocrine and paracrine agent, the actions of which on affected cells are mediated by P2 receptors (P2R), which include trans cell-membrane cationic channels (P2XRs), and G protein coupled receptors (P2YRs). The mammalian P2X receptors form homotrimeric or heterotrimeric cationic channels, each of which contains three ATP-binding sites. There are seven homotrimeric P2X receptors (P2X1-7) and three heteromeric (P2X2/P2X3, P2X4/P2X6, P2X1/P2X5). In the lungs and airways, ATP activates P2X3 and P2X2/3 receptors (P2X3R, P2X2/3R, respectively) localized on vagal sensory nerve terminals resulting in bronchoconstriction, and cough, and probably also localized release of pro-inflammatory neuropeptides via the axon reflex. Currently, several P2X3R and P2X2/3R antagonists are being developed as drug-candidates for the treatment of chronic cough. This report presents the receptor affinity data of a novel water-soluble small molecule, DT-0111, that acts as a selective P2X3R antagonist.

Triggering of Major Brain Disorders by Protons and ATP: The Role of ASICs and P2X Receptors

Adenosine triphosphate (ATP) is well-known as a universal source of energy in living cells. Less known is that this molecule has a variety of important signaling functions: it activates a variety of specific metabotropic (P2Y) and ionotropic (P2X) receptors in neuronal and non-neuronal cell membranes. So, a wide variety of signaling functions well fits the ubiquitous presence of ATP in the tissues. Even more ubiquitous are protons. Apart from the unspecific interaction of protons with any protein, many physiological processes are affected by protons acting on specific ionotropic receptors-acid-sensing ion channels (ASICs). Both protons (acidification) and ATP are locally elevated in various pathological states. Using these fundamentally important molecules as agonists, ASICs and P2X receptors signal a variety of major brain pathologies. Here we briefly outline the physiological roles of ASICs and P2X receptors, focusing on the brain pathologies involving these receptors.

Role of purinergic receptors in cardiac sympathetic nerve injury in diabetes mellitus

Diabetic cardiac autonomic neuropathy is a common and serious chronic complication of diabetes, which can lead to sympathetic and parasympathetic nerve imbalance and a relative excitation of the sympathetic nerve. Purinergic receptors play a crucial role in this process. Diabetic cardiac sympathetic nerve injury affects the expression of purinergic receptors, and activated purinergic receptors affect the phosphorylation of different signaling pathways and the regulation of inflammatory processes. This paper introduces the abnormal changes of sympathetic nerve in diabetes mellitus and summarizes the recently published studies on the role of several purinergic receptor subtypes in diabetic cardiac sympathetic nerve injury. These studies suggest that purinergic receptors as novel drug targets are of great significance for the treatment of diabetic autonomic neuropathy. This article is part of the Special Issue on "Purinergic Signaling: 50 years".

Discovery of 4-(1,2,4-Oxadiazol-5-yl)azepan-2-one Derivatives as a New Class of Cannabinoid Type 2 Receptor Agonists for the Treatment of Inflammatory Pain

Selectively targeting the cannabinoid receptor CB2 is an attractive therapeutic strategy for the treatment of inflammatory pain without psychiatric side effects mediated by the cannabinoid receptor CB1. Herein, we report the discovery of 4-(1,2,4-oxadiazol-5-yl)azepan-2-one derivatives as a new class of CB2 agonists. Systematic structure-activity relationship investigations resulted in the identification of the most potent compound 25r. This compound displayed high selectivity for CB2 against CB1 (CB2 EC₅₀ = 21.0 nM, E_max = 87%, CB1 EC₅₀ > 30 μM, ratio CB1/CB2 > 1428) with favorable pharmacokinetic properties. Especially, 25r demonstrated significant efficacy in the analgesic model of rodent inflammatory pain. All the results suggest that compound 25r could serve as a lead compound for treating inflammatory pain and deserves further in-depth studies.

Dorsal root ganglia P2X4 and P2X7 receptors contribute to diabetes-induced hyperalgesia and the downregulation of electroacupuncture on P2X4 and P2X7

Diabetic neuropathic pain (DNP) is highly common in diabetes patients. P2X receptors play critical roles in pain sensitization. We previously showed that elevated P2X3 expression in dorsal root ganglion (DRG) contributes to DNP. However, the role of other P2X receptors in DNP is unclear. Here, we established the DNP model using a single high-dose streptozotocin (STZ) injection and investigated the expression of P2X genes in the DRG. Our data revealed elevated P2X2, P2X4, and P2X7 mRNA levels in DRG of DNP rats. The protein levels of P2X4 and P2X7 in DNP rats increased, but the P2X2 did not change significantly. To study the role of P2X4 and P2X7 in diabetes-induced hyperalgesia, we treated the DNP rats with TNP-ATP (2',3'-O-(2,4,6-trinitrophenyl)-adenosine 5'-triphosphate), a nonspecific P2X1-7 antagonist, and found that TNP-ATP alleviated thermal hyperalgesia in DNP rats. 2 Hz electroacupuncture is analgesic against DNP and could downregulate P2X4 and P2X7 expression in DRG. Our findings indicate that P2X4 and P2X7 in L4-L6 DRGs contribute to diabetes-induced hyperalgesia, and that EA reduces thermal hyperalgesia and the expression of P2X4 and P2X7.

Electroacupuncture Reduces Ocular Surface Neuralgia in Dry-Eyed Guinea Pigs by Inhibiting the Trigeminal Ganglion and Spinal Trigeminal Nucleus Caudalis P2X3R-PKC Signaling Pathway

PURPOSE

To observe the effects of electroacupuncture on ocular surface neuralgia and the P2X₃R-PKC signaling pathway in guinea pigs with dry eye.

METHODS

A dry eye guinea pig model was established by subcutaneous injection of scopolamine hydrobromide. Guinea pigs were monitored for body weight, palpebral fissure height, number of blinks, corneal fluorescein staining score, phenol red thread test, and corneal mechanical perception threshold. Histopathological changes and mRNA expression of P2X₃R and protein kinase C in the trigeminal ganglion and spinal trigeminal nucleus caudalis were observed. We performed a second part of the experiment, which involved the P2X₃R-specific antagonist A317491 and the P2X₃R agonist ATP in dry-eyed guinea pigs to further validate the involvement of the P2X₃R-protein kinase C signaling pathway in the regulation of ocular surface neuralgia in dry eye. The number of blinks and corneal mechanical perception threshold were monitored before and 5 min after subconjunctival injection and the protein expression of P2X₃R and protein kinase C was detected in the trigeminal ganglion and spinal trigeminal nucleus caudalis of guinea pigs.

RESULTS

Dry-eyed guinea pigs showed pain-related manifestations and the expression of P2X₃R and protein kinase C in the trigeminal ganglion and spinal trigeminal nucleus caudalis was upregulated. Electroacupuncture reduced pain-related manifestations and inhibited the expression of P2X₃R and protein kinase C in the trigeminal ganglion and spinal trigeminal nucleus caudalis. Subconjunctival injection of A317491 attenuated corneal mechanoreceptive nociceptive sensitization in dry-eyed guinea pigs, while ATP blocked the analgesic effect of electroacupuncture.

CONCLUSIONS

Electroacupuncture reduced ocular surface sensory neuralgia in dry-eyed guinea pigs, and the mechanism of action may be associated with the inhibition of the P2X₃R-protein kinase C signaling pathway in the trigeminal ganglion and spinal trigeminal nucleus caudalis by electroacupuncture.

Botulinum Toxin Therapy for Bladder Pain Syndrome/Interstitial Cystitis

Purpose of Review

Bladder pain syndrome (BPS)/interstitial cystitis (IC) can also be classified as either non-ulcerative or ulcerative, corresponding to the characteristic cystoscopic findings under hydrodistention. Promising therapeutic effects, including decreased bladder pain, have been reported from recent clinical trials using botulinum toxin A (BoNTA) for the treatment of BPS/IC. This review summarizes the current state of the literature on the underlying mechanisms of BoNTA therapy in BPS/IC as well as new forms of its application.

Recent Findings

BoNTA has its effect in the central nervous system in the afferent nerves as well as in the bladder wall. Besides the well-known effects of BoNTA in the nervous system, pain control as well as reduction of urinary urgency in BPS patients could be achieved by mast cell stabilization effecting histamine release as well as modulation of TRPV and PGE2 pathways, among other systems. In addition, new forms of BoNTA administration have focused on intravesical instillation of the drug in order to circumvent bladder wall injections. Hyperthermia, intravesical hydrogel, and lysosomes have been studied as new ways of BoNTA application in BPS/IC patients. From the available studies, bladder instillation of BoNTA in combination with EMDA is the most promising and effective novel approach.

Summary

The most promising novel application methods for BoNTA in patient with BPS/IC are bladder instillations. Future research needs to point out if bladder instillations with BoNTA with some form of bladder absorption enhancement such as hyperthermia or EMDA would be able to replace BoNTA injections in patients with BPS/IC

Tuina for peripherally-induced neuropathic pain: A review of analgesic mechanism

Peripherally-induced neuropathic pain (pNP) is a kind of NP that is common, frequent, and difficult to treat. Tuina, also known as massage and manual therapy, has been used to treat pain in China for thousands of years. It has been clinically proven to be effective in the treatment of pNP caused by cervical spondylosis, lumbar disc herniation, etc. However, its analgesic mechanism is still not clear and has been the focus of research. In this review, we summarize the existing research progress, so as to provide guidance for clinical and basic studies. The analgesic mechanism of tuina is mainly manifested in suppressing peripheral inflammation by regulating the TLR4 pathway and miRNA, modulating ion channels (such as P2X3 and piezo), inhibiting the activation of glial cells, and adjusting the brain functional alterations. Overall, tuina has an analgesic effect by acting on different levels of targets, and it is an effective therapy for the treatment of pNP. It is necessary to continue to study the mechanism of tuina analgesia.

Contribution of P2X purinergic receptor in cerebral ischemia injury

The development of cerebral ischemia involves brain damage and abnormal changes in brain function, which can cause neurosensory and motor dysfunction, and bring serious consequences to patients. P2X purinergic receptors are expressed in nerve cells and immune cells, and are mainly expressed in microglia. The P2X4 and P2X7 receptors in the P2X purinergic receptors play a significant role in regulating the activity of microglia. Moreover, ATP-P2X purine information transmission is involved in the progression of neurological diseases, including the release of pro-inflammatory factors, driving factors and cytokines after cerebral ischemia injury, inducing inflammation, and aggravating cerebral ischemia injury. P2X receptors activation can mediate the information exchange between microglia and neurons, induce neuronal apoptosis, and aggravate neurological dysfunction after cerebral ischemia. However, inhibiting the activation of P2X receptors, reducing their expression, inhibiting the activation of microglia, and has the effect of protecting nerve function. In this paper, we discussed the relationship between P2X receptors and nervous system function and the role of microglia activation inducing cerebral ischemia injury. Additionally, we explored the potential role of P2X receptors in the progression of cerebral ischemic injury and their potential pharmacological targets for the treatment of cerebral ischemic injury.

Activation of GDNF-ERK-Runx1 signaling contributes to P2X3R gene transcription and bone cancer pain

Bone cancer pain is a common symptom in cancer patients with bone metastases and its underlying mechanisms remain unknown. Here, we report that Runx1 directly upregulates the transcriptional activity of P2X3 receptor (P2X3R) gene promoter in PC12 cells. Knocking down Runx1 in dorsal root ganglion (DRG) neurons suppresses the functional upregulation of P2X3R, attenuates neuronal hyperexcitability and pain hypersensitivity in tumor-bearing rats, whereas overexpressing Runx1 promotes P2X3R gene transcription in DRG neurons, induces neuronal hyperexcitability and pain hypersensitivity in naïve rats. Activation of GDNF-GFRα1-Ret-ERK signaling is required for Runx1-mediated P2X3R gene transcription in DRG neurons, and contributes to neuronal hyperexcitability and pain hypersensitivity in tumor-bearing rats. These findings indicate that the Runx1-mediated P2X3R gene transcription resulted from activation of GDNF-GFRα1-Ret-ERK signaling contributes to the sensitization of DRG neurons and pathogenesis of bone cancer pain. Our findings identify a potentially targetable mechanism that may cause bone metastasis-associated pain in cancer patients.

•

Runx1 directly upregulates the transcriptional activity of P2X3R gene promoter

•

Upregulation of Runx1-mediated P2X3R gene transcription underlies bone cancer pain

•

Involvement of GDNF-Ret-ERK signaling in Runx1-mediated P2X3R gene transcription

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.

The State-of-the-art Pharmacotherapeutic Options for the Treatment of Chronic Non-Cancer Pain

INTRODUCTION

Pharmacotherapeutic options continue to expand for the treatment of chronic non-cancer pain. There has been an increasing emphasis on multimodal analgesia. This strategy employs use of multiple analgesic medications each with a distinct mechanism of action, which when administered concomitantly may provide profound analgesia.

AREAS COVERED

The authors describe evidence from randomized controlled trials and systematic reviews on a variety of established medications including anti-inflammatory agents, opioids, anti-convulsants, anti-depressants, N-methyl-D-aspartate receptor antagonists, sodium channel blockers, cannabinoids, and alpha-2-receptor blockers. Furthermore, they provide developing evidence on more novel pharmacotherapeutics including alpha lipoic acid, acetyl-L-carnitine, low dose naltrexone, calcitonin gene-related peptide antagonists, targeted toxin therapy, Nav1.7 inhibitors, neurotensin agonists, purinoceptor antagonists, and sigma-1 receptor antagonists. Furthermore, the authors review the safety and adverse effect profile for these agents.

EXPERT OPINION

In this era of the opioid epidemic, clinicians should first offer non-opioid analgesics and employ a multimodal analgesic strategy. Current guidelines recommend a personalized approach to the chronic pain treatment, in each case accounting for type, location, severity, and chronicity of pain. Clinicians should also carefully consider the risk-to-benefit ratio to the patient based on the drug side effect profile, patient age, and comorbidities.

SZAP exerts analgesic effects on rheumatalgia in CIA rats by suppressing pain hyperalgesia and inhibiting TRPV1 and P2X3

ETHNOPHARMACOLOGICAL RELEVANCE

ShexiangZhuifeng Analgesic Plaster (SZAP) is a traditional Chinese medicine and transdermal formulation composed of many Chinese herbs and active compounds. SZAP was recently approved by the China Food and Drug Administration for the treatment of pain associated with osteoarticular diseases and is preferred by most rheumatoid arthritis patients in China. However, its mechanism has not been elucidated in detail.

AIM OF THE STUDY

We sought to determine the analgesic effect of SZAP in collagen-induced arthritis (CIA) rats and explore the underlying mechanisms of pain transmission, such as via the TRPV1 and P2X3 receptors.

METHODS

After CIA was established, rats were treated with SZAP for 7 days. Paw thickness, arthritis score, and haematoxylin and eosin staining were used to evaluate the effectiveness of SZAP. Paw withdrawal threshold (PWT) and tail-flick latency (TFL) were used to estimate the analgesic effect of SZAP. The levels of PGE₂, BK, 5-HT, SP, and CGRP in the serum and synovium were determined using ELISA kits, and ATP in the synovium was measured using HPLC. The expression of TRPV1 and P2X3 in the DRG was detected using western blotting and immunofluorescence. TRPV1 and P2X3 agonists were further used to determine the analgesic effects of SZAP on CIA rats based on PWT and TFL.

RESULTS

SZAP not only significantly ameliorated arthritis scores and paw thickness by improving the pathological damage of synovial joints, but also remarkably alleviated pain in CIA rats. Further, treatment with SZAP significantly reduced peripheral 5-HT, PGE₂ BK, SP, CGRP, and ATP. Additionally, the expression of TRPV1 and P2X3 in the DRG was markedly downregulated by SZAP. Interestingly, the analgesic effect of SZAP was weakened (reduction of PWT and TFL) when TRPV1 and P2X3 were activated by capsaicin or α,β-meATP, respectively.

CONCLUSION

SZAP ameliorates rheumatalgia by suppressing hyperalgesia and pain transmission through the inhibition of TRPV1 and P2X3 in the DRG of CIA rats.

Pharmacotherapy of Painful Diabetic Neuropathy: A Clinical Update

The rising prevalence of diabetes mellitus (DM) leads on to an increase in chronic diabetic complications. Diabetic peripheral neuropathies (DPNs) are common chronic complications of diabetes. Distal symmetric polyneuropathy is the most prevalent form. Most patients with DPN will remain pain-free; however, painful DPN (PDPN) occurs in 6-34% of all DM patients and is associated with reduced health-related-quality-of-life and substantial economic burden. Symptomatic treatment of PDPN and diabetic autonomic neuropathy is the key treatment goals. Using certain patient related characteristics, subjects with PDPN can be stratified and assigned targeted therapies to produce better pain outcomes. The aim of this review is to discuss the various pathogenetic mechanisms of DPN with special reference to the mechanisms leading to PDPN and the various pharmacological and non-pharmacological therapies available for its management. Recommended pharmacological therapies include anticonvulsants, antidepressants, opioid analgesics, and topical medications.

Glycosides for Peripheral Neuropathic Pain: A Potential Medicinal Components

Neuropathic pain is a refractory disease that occurs across the world and pharmacotherapy has limited efficacy and/or safety. This disease imposes a significant burden on both the somatic and mental health of patients; indeed, some patients have referred to neuropathic pain as being ‘worse than death’. The pharmacological agents that are used to treat neuropathic pain at present can produce mild effects in certain patients, and induce many adverse reactions, such as sedation, dizziness, vomiting, and peripheral oedema. Therefore, there is an urgent need to discover novel drugs that are safer and more effective. Natural compounds from medical plants have become potential sources of analgesics, and evidence has shown that glycosides alleviated neuropathic pain via regulating oxidative stress, transcriptional regulation, ion channels, membrane receptors and so on. In this review, we summarize the epidemiology of neuropathic pain and the existing therapeutic drugs used for disease prevention and treatment. We also demonstrate how glycosides exhibit an antinociceptive effect on neuropathic pain in laboratory research and describe the antinociceptive mechanisms involved to facilitate the discovery of new drugs to improve the quality of life of patients experiencing neuropathic pain.

Recommended tool compounds and drugs for blocking P2X and P2Y receptors

This review article presents a collection of tool compounds that selectively block and are recommended for studying P2Y and P2X receptor subtypes, investigating their roles in physiology and validating them as future drug targets. Moreover, drug candidates and approved drugs for P2 receptors will be discussed.

Targeting P2 receptors in purinergic signaling: a new strategy of active ingredients in traditional Chinese herbals for diseases treatment

Adenosine triphosphate (ATP) and its metabolites adenosine diphosphate, adenosine monophosphate, and adenosine in purinergic signaling pathway play important roles in many diseases. Activation of P2 receptors (P2R) channels and subsequent membrane depolarization can induce accumulation of extracellular ATP, and furtherly cause kinds of diseases, such as pain- and immune-related diseases, cardiac dysfunction, and tumorigenesis. Active ingredients of traditional Chinese herbals which exhibit superior pharmacological activities on diversified P2R channels have been considered as an alternative strategy of disease treatment. Experimental evidence of potential ingredients in Chinese herbs targeting P2R and their pharmacological activities were outlined in the study.

Novel Approaches, Drug Candidates, and Targets in Pain Drug Discovery

Because of the problems associated with opioids, drug discovery efforts have been employed to develop opioids with reduced side effects using approaches such as biased opioid agonism, multifunctional opioids, and allosteric modulation of opioid receptors. Receptor targets such as adrenergic, cannabinoid, P2X3 and P2X7, NMDA, serotonin, and sigma, as well as ion channels like the voltage-gated sodium channels Nav1.7 and Nav1.8 have been targeted to develop novel analgesics. Several enzymes, such as soluble epoxide hydrolase, sepiapterin reductase, and MAGL/FAAH, have also been targeted to develop novel analgesics. In this review, old and recent targets involved in pain signaling and compounds acting at these targets are summarized. In addition, strategies employed to reduce side effects, increase potency, and efficacy of opioids are also elaborated. This review should aid in propelling drug discovery efforts to discover novel analgesics.

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.

Current standard of care in treatment of bladder pain syndrome/interstitial cystitis

Bladder pain syndrome/interstitial cystitis (BPS/IC) is a debilitating, systemic pain syndrome with a cardinal symptom of bladder related pain with associated systemic symptoms. It is characterized by an inflammation that partially or completely destroys the mucus membrane and can extend into the muscle layer; however, the etiology and pathogenesis is still enigmatic. It has been suggested that mast cell activation, defects in the glycosaminoglycan layer, non-functional proliferation of bladder epithelial cells, neurogenic inflammation, microvascular abnormalities in the submucosal layer, autoimmunity and infectious causes may cause BPS/IC. Available treatment options include general relaxation techniques, patient education, behavioral treatments, physical therapy, multimodal pain therapy, oral (amitriptyline, cimetidine, hydroxyzine) and intravesical treatments (heparin, lidocaine, hyaluronic acid and chondroitin sulfate), hydrodistension and other more invasive treatments. Available treatments are mostly not based on a high level of evidence. Lack of understanding of disease mechanisms has resulted in lack of targeted therapies on this area and a wealth of empirical approaches with usually inadequate efficacy. The aim of this article is to review the available evidence on the pathophysiological mechanisms of BPS/IC as they relate to available treatment options.

Update of P2X receptor properties and their pharmacology: IUPHAR Review 30

The known seven mammalian receptor subunits (P2X1–7) form cationic channels gated by ATP. Three subunits compose a receptor channel. Each subunit is a polypeptide consisting of two transmembrane regions (TM1 and TM2), intracellular N- and C-termini, and a bulky extracellular loop. Crystallization allowed the identification of the 3D structure and gating cycle of P2X receptors. The agonist-binding pocket is located at the intersection of two neighbouring subunits. In addition to the mammalian P2X receptors, their primitive ligand-gated counterparts with little structural similarity have also been cloned. Selective agonists for P2X receptor subtypes are not available, but medicinal chemistry supplied a range of subtype-selective antagonists, as well as positive and negative allosteric modulators. Knockout mice and selective antagonists helped to identify pathological functions due to defective P2X receptors, such as male infertility (P2X1), hearing loss (P2X2), pain/cough (P2X3), neuropathic pain (P2X4), inflammatory bone loss (P2X5), and faulty immune reactions (P2X7).

Intracolonic Mustard Oil Induces Visceral Pain in Mice by TRPA1-Dependent and -Independent Mechanisms: Role of Tissue Injury and P2X Receptors

Both TRPA1 and purinergic P2X receptors have been proposed as potential targets for the treatment of visceral pain. We found that the intracolonic administration of a low dose mustard oil (0.5%), a well-known TRPA1 agonist, produced nociceptive responses and abdominal wall referred mechanical hyperalgesia, without inducing apparent tissue damage. Both nociceptive responses and referred hyperalgesia were abolished by the ablation of TRPV1-expressing neurons (and the consequent ablation of TRPA1+ nociceptors) by resiniferatoxin (RTX) treatment, and by the TRPA1 antagonist AP18. However, a higher dose of mustard oil (2.5%) damaged the colonic epithelium and induced pERK activation in the spinal cord, and these processes were clearly independent of TRPV1-expressing neurons ablated by RTX. This higher dose of mustard oil induced nociceptive responses and referred mechanical hyperalgesia which were insensitive or only slightly sensitive to resiniferatoxin or AP18, but were markedly reduced by the P2X antagonist TNP-ATP, which is known to inhibit nociceptive actions induced by ATP released from injured tissues. In conclusion, whereas a low dose of intracolonic mustard oil induces visceral pain in a manner fully dependent on TRPA1 actions, when a high dose of this chemical irritant is used, visceral pain becomes mostly independent of TRPA1 activation but clearly enhanced by ATP purportedly released by the damaged colonic epithelium. Therefore, TRPA1 inhibition is not sufficient to substantially decrease visceral pain during tissue injury, whereas purinergic antagonism appears to be a more effective strategy.