Modulation of BzATP and formalin induced nociception: attenuation by the P2X receptor antagonist, TNP-ATP and enhancement by the P2X(3) allosteric modulator, cibacron blue

Meningeal P2X7 Signaling Mediates Migraine-Related Intracranial Mechanical Hypersensitivity

Cortical spreading depolarization (CSD) is a key pathophysiological event that underlies visual and sensory auras in migraine. CSD is also thought to drive the headache phase in migraine by promoting the activation and mechanical sensitization of trigeminal primary afferent nociceptive neurons that innervate the cranial meninges. The factors underlying meningeal nociception in the wake of CSD remain poorly understood but potentially involve the parenchymal release of algesic mediators and damage-associated molecular patterns, particularly ATP. Here, we explored the role of ATP-P2X purinergic receptor signaling in mediating CSD-evoked meningeal afferent activation and mechanical sensitization. Male rats were subjected to a single CSD episode. In vivo, extracellular single-unit recording was used to measure meningeal afferent ongoing activity changes. Quantitative mechanical stimuli using a servomotor force-controlled stimulator assessed changes in the afferent's mechanosensitivity. Manipulation of meningeal P2X receptors was achieved via local administration of pharmacological agents. Broad-spectrum P2X receptor inhibition, selective blockade of the P2X7 receptor, and its related Pannexin 1 channel suppressed CSD-evoked afferent mechanical sensitization but did not affect the accompanying afferent activation response. Surprisingly, inhibition of the pronociceptive P2X2/3 receptor did not affect the activation or sensitization of meningeal afferents post-CSD. P2X7 signaling underlying afferent mechanosensitization was localized to the meninges and did not affect CSD susceptibility. We propose that meningeal P2X7 and Pannexin 1 signaling, potentially in meningeal macrophages or neutrophils, mediates the mechanical sensitization of meningeal afferents, which contributes to migraine pain by exacerbating the headache during normally innocuous physical activities.SIGNIFICANCE STATEMENT Activation and sensitization of meningeal afferents play a key role in migraine headache, but the underlying mechanisms remain unclear. Here, using a rat model of migraine with aura involving cortical spreading depolarization (CSD), we demonstrate that meningeal purinergic P2X7 signaling and its related Pannexin 1 pore, but not nociceptive P2X2/3 receptors, mediate prolonged meningeal afferent sensitization. Additionally, we show that meningeal P2X signaling does not contribute to the increased afferent ongoing activity in the wake of CSD. Our finding points to meningeal P2X7 signaling as a critical mechanism underlying meningeal nociception in migraine, the presence of distinct mechanisms underlying the activation and sensitization of meningeal afferents in migraine, and highlight the need to target both processes for effective migraine therapy.

Pain in chronic prostatitis and the role of ion channels: a brief overview

BACKGROUND

Prostatitis is the third most common urologic condition affecting more than half the male population at some point in their lives. There are different categories of prostatitis, of which approximately 90% of cases can be classified under the National Institute of Health (NIH) type III category (chronic prostatitis/chronic pelvic pain syndrome (CP/CPPS)) with no causative agents identified. CP/CPPS is associated with several symptoms, of which the most prominent being chronic pain. Despite its high incidence, pain management in patients with CP/CPPS has been poor, possibly due to the lack of understanding of aetiological factors and mechanisms underlying pain development.

METHODS

An extensive literature search of published articles on the molecular mechanisms of pain in CP/CPPS was conducted using PubMed and Google Scholar search engines (https://pubmed.ncbi.nlm.nih.gov and https://scholar.google.com). The terms used for the search were: prostatitis, pain mechanism in CP/CPPS, prostatitis pain models, acid-sensing ion channels (ASICs), transient receptor potential vanilloid type 1 (TRPVs), purinergic channels (P2X) in prostatitis pain mechanism and inflammatory mediators in CP/CPPS. The papers were identified based on the title and abstract, and after excluding the articles that did not emphasize the pain mechanism in CP/CPPS. Ninety-five articles (36 review and 59 original research papers) met our criteria and were included in the review.

RESULTS

A number of inflammatory mediator molecules and pain channels, including ASICs, transient receptor potential vanilloid channels (TRPVs) and P2Xs have been investigated for their role in prostatitis pain pathology using various animal models.

CONCLUSION

This review summarizes the pain mechanisms in CP/CPPS focusing on the inflammatory mediators, neurotransmitters, pain-transducing ion channels and small animal models developed for studying prostatitis.

Nociceptive signaling mediated by P2X3, P2X4 and P2X7 receptors

Chronic pain is a debilitating condition that often occurs following peripheral tissue inflammation and nerve injury. This pain, especially neuropathic pain, is a significant clinical problem because of the ineffectiveness of clinically available drugs. Since Burnstock proposed new roles of nucleotides as neurotransmitters, the roles of extracellular ATP and P2 receptors (P2Rs) in pain signaling have been extensively studied, and ATP-P2R signaling has subsequently received much attention as it can provide clues toward elucidating the mechanisms underlying chronic pain and serve as a potential therapeutic target. This review summarizes the literature regarding the role of ATP signaling via P2X3Rs (as well as P2X2/3Rs) in primary afferent neurons and via P2X4Rs and P2X7Rs in spinal cord microglia in chronic pain, and discusses their respective therapeutic potentials.

Action of MK-7264 (gefapixant) at human P2X3 and P2X2/3 receptors and in vivo efficacy in models of sensitisation

BACKGROUND AND PURPOSE

The P2X3 receptor is an ATP-gated ion channel expressed by sensory afferent neurons and is used as a target to treat chronic sensitisation conditions. The first-in-class, selective P2X3 and P2X2/3 receptor antagonist, the diaminopyrimidine MK-7264 (gefapixant), has progressed to Phase III trials for refractory or unexplained chronic cough. We used patch clamp to elucidate the pharmacology and kinetics of MK-7264 and rat models of hypersensitivity and hyperalgesia to test its efficacy on these conditions.

EXPERIMENTAL APPROACH

Whole-cell patch clamp of 1321N1 cells expressing human P2X3 and P2X2/3 receptors was used to determine mode of MK-7264 action, potency, and kinetics. The analgesic efficacy was assessed using paw withdrawal threshold and limb weight distribution in rat models of inflammatory, osteoarthritic, and neuropathic sensitisation.

KEY RESULTS

MK-7264 is a reversible allosteric antagonist at human P2X3 and P2X2/3 receptors. Experiments with the slowly desensitising P2X2/3 heteromer revealed concentration- and state-dependency to wash-on, with faster rates and greater inhibition when applied before agonist compared to during agonist application. The wash-on rate (τ value) for MK-7264 at maximal concentrations was much lower when applied before compared to during agonist application. In vivo, MK-7264 displayed efficacy comparable to naproxen in inflammatory and osteoarthritic sensitisation models and gabapentin in neuropathic sensitisation models, increasing paw withdrawal threshold and decreasing weight-bearing discomfort.

CONCLUSIONS AND IMPLICATIONS

MK-7264 is a reversible and selective P2X3 and P2X2/3 antagonist, exerting allosteric antagonism via preferential activity at closed channels. Its efficacy in rat models supports its clinical investigation for chronic sensitisation conditions.

Overexpression of P2X3 and P2X7 Receptors and TRPV1 Channels in Adrenomedullary Chromaffin Cells in a Rat Model of Neuropathic Pain

We have tested the hypothesis that neuropathic pain acting as a stressor drives functional plasticity in the sympathoadrenal system. The relation between neuropathic pain and adrenal medulla function was studied with behavioral, immunohistochemical and electrophysiological techniques in rats subjected to chronic constriction injury of the sciatic nerve. In slices of the adrenal gland from neuropathic animals, we have evidenced increased cholinergic innervation and spontaneous synaptic activity at the splanchnic nerve–chromaffin cell junction. Likewise, adrenomedullary chromaffin cells displayed enlarged acetylcholine-evoked currents with greater sensitivity to α-conotoxin RgIA, a selective blocker of α9 subunit-containing nicotinic acetylcholine receptors, as well as increased exocytosis triggered by voltage-activated Ca²⁺ entry. Altogether, these adaptations are expected to facilitate catecholamine output into the bloodstream. Last, but most intriguing, functional and immunohistochemical data indicate that P2X3 and P2X7 purinergic receptors and transient receptor potential vanilloid-1 (TRPV1) channels are overexpressed in chromaffin cells from neuropathic animals. These latter observations are reminiscent of molecular changes characteristic of peripheral sensitization of nociceptors following the lesion of a peripheral nerve, and suggest that similar phenomena can occur in other tissues, potentially contributing to behavioral manifestations of neuropathic pain.

A cyclic pathway of P2 × 7, bradykinin, and dopamine receptor activation induces a sustained articular hyperalgesia in the knee joint of rats

OBJECTIVE

We investigated whether: (1) P2 × 7 receptor activation by its agonist (BzATP) induces articular hyperalgesia in the rat's knee joint via inflammatory mechanisms and (2) activation of P2 × 7 receptors by endogenous ATP contributes to the articular hyperalgesia induced by bradykinin, TNF-α, IL-1β, CINC-1, PGE_2, and dopamine.

METHODS

The articular hyperalgesia was quantified using the rat knee joint incapacitation test. The knee joint inflammation, characterized by the concentration of pro-inflammatory cytokines and by neutrophil migration, was quantified in the synovial lavage fluid by ELISA and myeloperoxidase enzyme activity assay, respectively.

RESULTS

BzATP induced a dose-dependent articular hyperalgesia in the rat's knee joint that was significantly reduced by the selective antagonists for P2 × 7, bradykinin B₁ or B₂ receptors, β₁ or β₂ adrenoceptors, and by pre-treatment with Indomethacin. BzATP induced a local increase of TNF-α, IL-1β, IL-6, and CINC-1 concentration and neutrophil migration into the knee joint. The co-administration of the selective P2 × 7 receptor antagonist A-740003 significantly reduced the articular hyperalgesia induced by bradykinin and dopamine, but not by TNF-α, IL-1β, CINC-1, and PGE₂.

CONCLUSIONS

P2 × 7 receptor activation induces articular hyperalgesia mediated by the previous inflammatory mediator release. P2 × 7 receptor-induced articular hyperalgesia is sustained by the involvement of this purinergic receptor in bradykinin and dopamine-induced hyperalgesia in the knee joint.

P2X7 receptor inhibition attenuated sympathetic nerve sprouting after myocardial infarction via the NLRP3/IL-1β pathway

Mounting evidence supports the hypothesis that inflammation modulates sympathetic sprouting after myocardial infarction (MI). The myeloid P2X₇ signal has been shown to activate the nucleotide-binding and oligomerization domain-like receptor family pyrin domain-containing 3 (NLRP3) inflammasome, a master regulator of inflammation. We investigated whether P2X₇ signal participated in the pathogenesis of sympathetic reinnervation after MI, and whether NLRP3/interleukin-1β (IL-1β) axis is involved in the process. We explored the relationship between P2X₇ receptor (P2X₇ R) and IL-1β in the heart tissue of lipopolysaccharide (LPS)-primed naive rats. 3'-O-(4-benzoyl) benzoyl adenosine 5'-triphosphate (BzATP), a P2X₇ R agonist, induced caspase-1 activation and mature IL-1β release, which was further neutralized by a NLRP3 inhibitor (16673-34-0). MI was induced by coronary artery ligation. Following infarction, a marked increase in P2X₇ R was localized within infiltrated macrophages and observed in parallel with an up-regulation of NLRP3 inflammasome levels and the release of IL-1β in the left ventricle. The administration of A-740003 (a P2X₇ R antagonist) significantly prevented the NLRP3/IL-1β increase. A-740003 and/or Anakinra (an IL-1 receptor antagonist) significantly reduced macrophage infiltration as well as macrophage-based IL-1β and NGF (nerve growth factor) production and eventually blunted sympathetic hyperinnervation, as assessed by the immunofluorescence of tyrosine hydroxylase (TH) and growth-associated protein 43 (GAP 43). Moreover, the use of Anakinra partly attenuated sympathetic sprouting. This indicated that the effect of P2X₇ on neural remodelling was mediated at least partially by IL-1β. The arrhythmia score of programmed electric stimulation was in accordance with the degree of sympathetic hyperinnervation. In vitro studies showed that BzATP up-regulated secretion of nerve growth factor (NGF) in M1 macrophages via IL-1β. Together, these data indicate that P2X₇ R contributes to neural and cardiac remodelling, at least partly mediated by NLRP3/IL-1β axis. Therapeutic interventions targeting P2X₇ signal may be a novel approach to ameliorate arrhythmia following MI.

The protective effect of resveratrol in the transmission of neuropathic pain mediated by the P2X7 receptor in the dorsal root ganglia

The P2X₇ receptor mediates afferent nerve activation and is related to chronic neuropathic pain. Resveratrol (RES) has also been reported to exhibit anti-inflammatory effects. In this study, we investigated the neuroprotective effect of RES on the transmission of neuropathic pain mediated by the P2X₇ receptor. The P2X₇ mRNA and protein expression levels in L4-L5 dorsal root ganglia (DRG)s of the chronic constriction injury (CCI) group were significantly higher than those observed in the Ctrl + NS, Sham + RES and Sham groups. RES increased the threshold of thermal and mechanical hypersensitivity in rats with chronic neuropathic pain. The P2X₇ mRNA and protein expression levels in the CCI + RES group were decreased compared with those in the CCI group. Our results showed that RES inhibited the upregulated co-expression of P2X₇ and glial fibrillary acidic protein (GFAP, a marker of satellite glial cells) in satellite glial cells of DRG in the CCI group. The results demonstrated that the expression of GFAP was increased in the CCI group and that RES inhibited the upregulated expression of GFAP in the rats in the CCI group. In addition, the phosphorylation levels of p38 and extracellular regulated protein kinases (ERK)1/2 in the CCI group were markedly higher than those observed in the Ctrl + NS, Sham + RES and Sham groups, whereas the phosphorylation levels of p38 and ERK1/2 in CCI + RES group were markedly lower than those observed in the CCI group. RES inhibited BzATP-activated currents in DRG non-neurons in the CCI rats. Our data provide evidence that RES may suppress the transmission of neuropathic pain mediated by the P2X₇ receptor in the satellite glial cells of dorsal root ganglia.

Prokineticin 2 facilitates mechanical allodynia induced by α,β-methylene ATP in rats

Prokineticin 2 (PK2), a new chemokine, causes mechanical hypersensitivity in the rat hind paw, but little is known about the molecular mechanism. Here, we have found that ionotropic P2X receptor is essential to mechanical allodynia induced by PK2. First, intraplantar injection of high dose (3 or 10 pmol) of PK2 significantly increased paw withdrawal response frequency (%) to innocuous mechanical stimuli (mechanical allodynia). And the mechanical allodynia induced by PK2 was prevented by co-administration of TNP-ATP, a selective P2X receptor antagonist. Second, although low dose (0.3 or 1 pmol) of PK2 itself did not produce an allodynic response, it significantly facilitated the mechanical allodynia evoked by intraplantar injection of α,β-methylene ATP (α,β-meATP). Third, PK2 concentration-dependently potentiated α,β-meATP-activated currents in rat dorsal root ganglion (DRG) neurons. Finally, PK2 receptors and intracellular signal transduction were involved in PK2 potentiation of α,β-meATP-induced mechanical allodynia and α,β-meATP-activated currents, since the potentiation were blocked by PK2 receptor antagonist PKRA and selective PKC inhibitor GF 109203X. These results suggested that PK2 facilitated mechanical allodynia induced by α,β-meATP through a mechanism involved in sensitization of cutaneous P2X receptors expressed by nociceptive nerve endings.

Participation of peripheral P2Y1, P2Y6 and P2Y11 receptors in formalin-induced inflammatory pain in rats

Metabotropic P2Y receptors subfamily consists of eight functional mammalian receptors. Specifically, P2Y1, P2Y6 and P2Y11 receptors have been described in the sensory nervous system, but their participation, at peripheral level, in behavioral pain models is scarcely understood. This study assessed the role of peripheral P2Y1, P2Y6 and P2Y11 receptors in formalin-induced inflammatory pain. Ipsilateral, but not contralateral peripheral pre-treatment with the endogenous P2Y1 (ADP, 100-1000nmol/paw), P2Y6 (UDP, 180-300nmol/paw) and P2Y11 (ATP, 100-1000nmol/paw), or selective P2Y1 (MRS2365, 0.1-10nmol/paw), P2Y6 (PSB0474, 0.1-0.10pmol/paw) and P2Y11 (NF546, 0.3-3nmol/paw) receptor agonists increased 0.5% formalin-induced flinching behavior. Concordantly, peripheral pre-treatment with the selective P2Y1 (MRS2500, 0.01-10pmol/paw), P2Y6 (MRS2578, 3-30nmol/paw) and P2Y11 (NF340, 1-10nmol/paw) receptor antagonists significantly decreased 1% formalin-induced flinching behavior. Furthermore, the pronociceptive effect of ADP (100nmol/paw) or MRS2365 (10nmol/paw), UDP (300nmol/paw) or PSB0474 (10pmol/paw) and ATP (1000nmol/paw) or NF546 (3nmol/paw) was blocked by the selective P2Y1 (MRS2500, 0.01nmol/paw), P2Y6 (MRS2578, 3nmol/paw), and P2Y11 (NF340, 1nmol/paw) receptor antagonists, respectively. Western blot analysis confirmed the presence of P2Y1 (66kDa), P2Y6 (36kDa) and P2Y11 (75kDa) receptors in dorsal root ganglia (DRG) and sciatic nerve. Results suggest that peripheral activation of P2Y1, P2Y6 and P2Y11 receptors plays a pronociceptive role in formalin-induced pain.

Purinergic P2X receptors: structural models and analysis of ligand-target interaction

The purinergic P2X receptors are ligand-gated cation channels activated by the endogenous ligand ATP. They assemble as homo- or heterotrimers from seven cloned subtypes (P2X1-7) and all trimer subunits present a common topology consisting in intracellular N- and C- termini, two transmembrane domains and a large extracellular domain. These membrane proteins are present in virtually all mammalian tissues and regulate a large variety of responses in physio- and pathological conditions. The development of ligands that selectively activate or block specific P2X receptor subtypes hence represents a promising strategy to obtain novel pharmacological tools for the treatment of pain, cancer, inflammation, and neurological, cardiovascular, and endocrine diseases. The publication of the crystal structures of zebrafish P2X4 receptor in inactive and ATP-bound active forms provided structural data for the analysis of the receptor structure, the interpretation of mutagenesis data, and the depiction of ligand binding and receptor activation mechanism. In addition, the availability of ATP-competitive ligands presenting selectivity for P2X receptor subtypes supports the design of new potent and selective ligands with possibly improved pharmacokinetic profiles, with the final aim to obtain new drugs. This study describes molecular modelling studies performed to develop structural models of the human and rat P2X receptors in inactive and active states. These models allowed to analyse the role of some non-conserved residues at ATP binding site and to study the receptor interaction with some non-specific or subtype selective agonists and antagonists.

Peripheral P2X7 receptor-induced mechanical hyperalgesia is mediated by bradykinin

P2X7 receptors play an important role in inflammatory hyperalgesia, but the mechanisms involved in their hyperalgesic role are not completely understood. In this study, we hypothesized that P2X7 receptor activation induces mechanical hyperalgesia via the inflammatory mediators bradykinin, sympathomimetic amines, prostaglandin E2 (PGE2), and pro-inflammatory cytokines and via neutrophil migration in rats. We found that 2'(3')-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate triethylammonium salt (BzATP), the most potent P2X7 receptor agonist available, induced a dose-dependent mechanical hyperalgesia that was blocked by the P2X7 receptor-selective antagonist A-438079 but unaffected by the P2X1,3,2/3 receptor antagonist TNP-ATP. These findings confirm that, although BzATP also acts at both P2X1 and P2X3 receptors, BzATP-induced hyperalgesia was mediated only by P2X7 receptor activation. Co-administration of selective antagonists of bradykinin B1 (Des-Arg(8)-Leu(9)-BK (DALBK)) or B2 receptors (bradyzide), β1 (atenolol) or β2 adrenoceptors (ICI 118,551), or local pre-treatment with the cyclooxygenase inhibitor indomethacin or the nonspecific selectin inhibitor fucoidan each significantly reduced BzATP-induced mechanical hyperalgesia in the rat hind paw. BzATP also induced the release of the pro-inflammatory cytokines tumor necrosis factor α (TNF-α), interleukin (IL)-1β, IL-6 and cytokine-induced neutrophil chemoattractant-1 (CINC-1), an effect that was significantly reduced by A-438079. Co-administration of DALBK or bradyzide with BzATP significantly reduced BzATP-induced IL-1β and CINC-1 release. These results indicate that peripheral P2X7 receptor activation induces mechanical hyperalgesia via inflammatory mediators, especially bradykinin, which may contribute to pro-inflammatory cytokine release. These pro-inflammatory cytokines in turn may mediate the contributions of PGE2, sympathomimetic amines and neutrophil migration to the mechanical hyperalgesia induced by local P2X7 receptor activation.

Role of spinal P2Y6 and P2Y11 receptors in neuropathic pain in rats: possible involvement of glial cells

Background

The participation of spinal P2X receptors in neuropathic pain is well recognized. However, the role of P2Y receptors has been less studied. The purpose of this study was to investigate the contribution of spinal P2Y_6,11 receptors following peripheral nerve damage induced by spinal nerve ligation. In addition, we determined the expression of P2Y_6,11 receptors in the dorsal spinal cord in presence of the selective P2Y_6,11 receptors antagonists. Furthermore, we evaluated the participation of spinal microglia and astrocytes in the pronociceptive role of P2Y_6,11 receptors.

Results

Spinal administration of the selective P2Y₆ (MRS2578, 10–100 μM) and P2Y₁₁ (NF340, 0.3–30 μM) receptor antagonists reduced tactile allodynia in spinal nerve ligated rats. Nerve injury increased the expression of P2Y_6,11 receptors at 7, 14 and 21 days after injury. Furthermore, intrathecal administration of MRS2578 (100 μM/day) and NF340 (30 μM/day) for 3 days significantly reduced spinal nerve injury-induced increase in P2Y₆,₁₁ receptors expression, respectively. Spinal treatment (on day 14 after injury) with minocycline (100 μg/day) or fluorocitrate (1 nmol/day) for 7 days reduced tactile allodynia and spinal nerve injury-induced up-regulation in Iba-1 and GFAP, respectively. In addition, minocycline reduced nerve injury-induced up-regulation in P2Y_6,11 receptors whereas that fluorocitrate diminished P2Y₁₁, but not P2Y₆, receptors up-regulation. Intrathecal treatment (on day 21 after injury) with the selective P2Y₆ (PSB0474, 3–30 μM) and P2Y₁₁ (NF546, 1–10 μM) receptor agonists produced remarkable tactile allodynia in nerve ligated rats previously treated with minocycline or fluorocitrate for 7 days.

Conclusions

Our data suggest that spinal P2Y₆ is present in spinal microglia while P2Y₁₁ receptors are present in both spinal microglia and astrocytes, and both receptors are up-regulated in rats subjected to spinal nerve injury. In addition, our data suggest that the spinal P2Y₆ and P2Y₁₁ receptors participate in the maintenance of neuropathic pain.

Calcium-permeable ion channels in pain signaling

The detection and processing of painful stimuli in afferent sensory neurons is critically dependent on a wide range of different types of voltage- and ligand-gated ion channels, including sodium, calcium, and TRP channels, to name a few. The functions of these channels include the detection of mechanical and chemical insults, the generation of action potentials and regulation of neuronal firing patterns, the initiation of neurotransmitter release at dorsal horn synapses, and the ensuing activation of spinal cord neurons that project to pain centers in the brain. Long-term changes in ion channel expression and function are thought to contribute to chronic pain states. Many of the channels involved in the afferent pain pathway are permeable to calcium ions, suggesting a role in cell signaling beyond the mere generation of electrical activity. In this article, we provide a broad overview of different calcium-permeable ion channels in the afferent pain pathway and their role in pain pathophysiology.

P2X7 Cell Death Receptor Activation and Mitochondrial Impairment in Oxaliplatin-Induced Apoptosis and Neuronal Injury: Cellular Mechanisms and In Vivo Approach

Limited information is available regarding the cellular mechanisms of oxaliplatin-induced painful neuropathy during exposure of patients to this drug. We therefore determined oxidative stress in cultured cells and evaluated its occurrence in C57BL/6 mice. Using both cultured neuroblastoma (SH-SY5Y) and macrophage (RAW 264.7) cell lines and also brain tissues of oxaliplatin-treated mice, we investigated whether oxaliplatin (OXA) induces oxidative stress and apoptosis. Cultured cells were treated with 2–200 µM OXA for 24 h. The effects of pharmacological inhibitors of oxidative stress or inflammation (N-acetyl cysteine, ibuprofen, acetaminophen) were also tested. Inhibitors were added 30 min before OXA treatment and then in combination with OXA for 24 h. In SH-SY5Y cells, OXA caused a significant dose-dependent decrease in viability, a large increase in ROS and NO production, lipid peroxidation and mitochondrial impairment as assessed by a drop in mitochondrial membrane potential, which are deleterious for the cell. An increase in levels of negatively charged phospholipids such as cardiolipin but also phosphatidylserine and phosphatidylinositol, was also observed. Additionally, OXA caused concentration-dependent P2X7 receptor activation, increased chromatin condensation and caspase-3 activation associated with TNF-α and IL-6 release. The majority of these toxic effects were equally observed in Raw 264.7 which also presented high levels of PGE2. Pretreatment of SH-SY5Y cells with pharmacological inhibitors significantly reduced or blocked all the neurotoxic OXA effects. In OXA-treated mice (28 mg/kg cumulated dose) significant cold hyperalgesia and oxidative stress in the tested brain areas were shown. Our study suggests that targeting P2X7 receptor activation and mitochondrial impairment might be a potential therapeutic strategy against OXA-induced neuropathic pain.

P2X receptors as drug targets

The study of P2X receptors has long been handicapped by a poverty of small-molecule tools that serve as selective agonists and antagonists. There has been progress, particularly in the past 10 years, as cell-based high-throughput screening methods were applied, together with large chemical libraries. This has delivered some drug-like molecules in several chemical classes that selectively target P2X1, P2X3, or P2X7 receptors. Some of these are, or have been, in clinical trials for rheumatoid arthritis, pain, and cough. Current preclinical research programs are studying P2X receptor involvement in pain, inflammation, osteoporosis, multiple sclerosis, spinal cord injury, and bladder dysfunction. The determination of the atomic structure of P2X receptors in closed and open (ATP-bound) states by X-ray crystallography is now allowing new approaches by molecular modeling. This is supported by a large body of previous work using mutagenesis and functional expression, and is now being supplemented by molecular dynamic simulations and in silico ligand docking. These approaches should lead to P2X receptors soon taking their place alongside other ion channel proteins as therapeutically important drug targets.

Differential expression and pharmacology of native P2X receptors in rat and primate sensory neurons

Evidence suggesting the involvement of P2X2 and P2X3 in chronic pain has been obtained mostly from rodent models. Here we show that rodents may be poor predictors of P2X3 pharmacology in human. We demonstrate that monkey and human dorsal root ganglion (DRG) neurons do not express appreciable levels of P2X2 subunit, contrary to rat sensory neurons. Additionally, we report functional P2X3 activity in monkey DRG neurons and confirm the absence of functional P2X2/3 receptors. Interestingly, native P2X3 receptors in rat and monkey DRGs show similar agonist potency, but different antagonist potencies for TNP-ATP [2-O-(2,4,6-trinitrophenyl)-ATP] and RO51. This unexpected difference in antagonist potency was confirmed by comparing rat and human P2X3 receptors in HEK293 cells. Mutagenesis studies reveal that two extracellular residues, A197 and T202, are synergistically responsible for the potency drop in primate P2X3 receptors. These results uncover species-specific P2X3 pharmacology and identify key mechanisms impacting the translatability of potential analgesics targeting P2X3 receptors.

P2X3 antagonists: novel therapeutics for afferent sensitization and chronic pain

SUMMARY Despite decades of innovation and effort, the pharmaceutical needs of countless patients with chronic pain remain underserved. Effective and safe treatments must clearly come from novel approaches, yet targets and molecules selected hitherto have returned little benefit. Antagonism of P2X3 purinoceptors on pain-conveying nerves is a highly novel approach, and compounds from this class are advancing into patient studies. P2X3 channels are found in C- and Aδ-primary afferent neurons in most tissues, and are strikingly specific to pain detection. P2X3 antagonists block peripheral activation of these fibers via ATP, released from most cells by inflammation, injury, stress and distension, and clearly provide an alternative pharmacological mechanism to attenuate pain signals. P2X3 is also expressed presynaptically at central spinal terminals of afferent neurons, where ATP further sensitizes painful signals en route to the brain. The selectivity of P2X3 expression allows hope of a lower potential for adverse effects in brain, gut and cardiovascular tissues – limiting factors for most analgesics. P2X3 receptor-mediated sensitization has been implicated in rodent models in inflammatory, visceral, neuropathic and cancer pain states, as well as in airways hyper-reactivity, migraine and visceral organ irritability. Although we are often reminded that the effects of new medicines can translate poorly into clinical effectiveness, the broad efficacy seen following P2X3 inhibition in rodent models strengthens the prospect that an unprecedented mechanism to counter sensitization of afferent pathways may offer some merciful relief to millions of patients struggling daily with persistent discomfort and pain.

Electrophysiological characterization of recombinant and native P2X receptors

ATP acts as a fast neurotransmitter by activating a family of ligand-gated ion channels, the P2X receptors. Functional homomeric P2X(3) and heteromeric P2X(2/3) receptors are highly localized on primary sensory afferent neurons that transmit nociceptive sensory information. Activation of these P2X(3)-containing channels may provide a specific mechanism whereby ATP, released via synaptic transmission or by cellular injury, elicits pain. The experimental procedures described in this unit are useful for the electorphysiological characterization of P2X receptors. In addition, these protocols provide methods for the evaluation of ligands that interact with P2X receptors that are either natively expressed on excitable cells or cloned and expressed in heterologous cell systems. These methods are derived from standard electrophysiological principles and procedures that are applicable to a wide variety of ligand-gated ion channels. Specific attention is given here to the reliable electrophysiological measurement of both quickly (P2X(3)) and more slowly (P2X(2) and P2X(2/3)) desensitizing receptors.

Spontaneous firing and evoked responses of spinal nociceptive neurons are attenuated by blockade of P2X3 and P2X2/3 receptors in inflamed rats

P2X3 and P2X2/3 receptors are selectively expressed on primary afferent nociceptors and have been implicated in modulating nociception in different models of pathological pain, including inflammatory pain. In an effort to delineate further the role of P2X3 receptors (homomeric and heteromeric) in the modulation of nociceptive transmission after a chronic inflammation injury, A-317491, a potent and selective P2X3-P2X2/3 antagonist, was administered to CFA-inflamed rats in order to examine its effects on responses of spinal dorsal horn neurons to mechanical and thermal stimulation. Systemic injection of A-317491 (30 μmol/kg, i.v.) reduced the responses of wide-dynamic-range (WDR) and nociceptive specific (NS) neurons to both high-intensity mechanical (pinch) and heat (49°C) stimulation. A-317491 also decreased low-intensity (10 g von Frey hair) mechanically evoked activity of WDR neurons but did not alter WDR neuronal responses to cold stimulation (5°C). Spontaneous firing of WDR neurons in CFA-inflamed rats was also significantly attenuated by A-317491 injection. By using immunohistochemistry, P2X3 receptors were demonstrated to be enhanced in lamina II of the spinal dorsal horn after inflammation. In summary, blockade of P2X3 and P2X2/3 receptors dampens mechanical- and heat-related signaling, as well as nonevoked activity of key classes of spinal nociceptive neurons in inflamed animals. These data suggest that P2X3 and/or P2X2/3 receptors have a broad contribution to somatosensory/nociceptive transmission in rats with a chronic inflammatory injury and are consistent with previous behavioral data demonstrating antiallodynic and antihyperalgesic effects of receptor antagonists.

In pursuit of P2X3 antagonists: novel therapeutics for chronic pain and afferent sensitization

Treating pain by inhibiting ATP activation of P2X3-containing receptors heralds an exciting new approach to pain management, and Afferent's program marks the vanguard in a new class of drugs poised to explore this approach to meet the significant unmet needs in pain management. P2X3 receptor subunits are expressed predominately and selectively in so-called C- and Aδ-fiber primary afferent neurons in most tissues and organ systems, including skin, joints, and hollow organs, suggesting a high degree of specificity to the pain sensing system in the human body. P2X3 antagonists block the activation of these fibers by ATP and stand to offer an alternative approach to the management of pain and discomfort. In addition, P2X3 is expressed pre-synaptically at central terminals of C-fiber afferent neurons, where ATP further sensitizes transmission of painful signals. As a result of the selectivity of the expression of P2X3, there is a lower likelihood of adverse effects in the brain, gastrointestinal, or cardiovascular tissues, effects which remain limiting factors for many existing pain therapeutics. In the periphery, ATP (the factor that triggers P2X3 receptor activation) can be released from various cells as a result of tissue inflammation, injury or stress, as well as visceral organ distension, and stimulate these local nociceptors. The P2X3 receptor rationale has aroused a formidable level of investigation producing many reports that clarify the potential role of ATP as a pain mediator, in chronic sensitized states in particular, and has piqued the interest of pharmaceutical companies. P2X receptor-mediated afferent activation has been implicated in inflammatory, visceral, and neuropathic pain states, as well as in airways hyperreactivity, migraine, itch, and cancer pain. It is well appreciated that oftentimes new mechanisms translate poorly from models into clinical efficacy and effectiveness; however, the breadth of activity seen from P2X3 inhibition in models offers a realistic chance that this novel mechanism to inhibit afferent nerve sensitization may find its place in the sun and bring some merciful relief to the torment of persistent discomfort and pain. The development philosophy at Afferent is to conduct proof of concept patient studies and best identify target patient groups that may benefit from this new intervention.

P2X receptor antagonists for pain management: examination of binding and physicochemical properties

Enhanced sensitivity to noxious stimuli and the perception of non-noxious stimuli as painful are hallmark sensory perturbations associated with chronic pain. It is now appreciated that ATP, through its actions as an excitatory neurotransmitter, plays a prominent role in the initiation and maintenance of chronic pain states. Mechanistically, the ability of ATP to drive nociceptive sensitivity is mediated through direct interactions at neuronal P2X3 and P2X2/3 receptors. Extracellular ATP also activates P2X4, P2X7, and several P2Y receptors on glial cells within the spinal cord, which leads to a heightened state of neural-glial cell interaction in ongoing pain states. Following the molecular identification of the P2 receptor superfamilies, selective small molecule antagonists for several P2 receptor subtypes were identified, which have been useful for investigating the role of specific P2X receptors in preclinical chronic pain models. More recently, several P2X receptor antagonists have advanced into clinical trials for inflammation and pain. The development of orally bioavailable blockers for ion channels, including the P2X receptors, has been traditionally difficult due to the necessity of combining requirements for target potency and selectivity with suitable absorption distribution, metabolism, and elimination properties. Recent studies on the physicochemical properties of marketed orally bioavailable drugs, have identified several parameters that appear critical for increasing the probability of achieving suitable bioavailability, central nervous system exposure, and acceptable safety necessary for clinical efficacy. This review provides an overview of the antinociceptive pharmacology of P2X receptor antagonists and the chemical diversity and drug-like properties for emerging antagonists of P2X3, P2X2/3, P2X4, and P2X7 receptors.

Characterization of three diaminopyrimidines as potent and selective antagonists of P2X3 and P2X2/3 receptors with in vivo efficacy in a pain model

BACKGROUND AND PURPOSE

P2X3 and P2X2/3 receptors are highly localized on the peripheral and central pathways of nociceptive signal transmission. The discovery of A-317491 allowed their validation as chronic inflammatory and neuropathic pain targets, but this molecule has a very limited oral bioavailability and CNS penetration. Recently, potent P2X3 and P2X2/3 blockers with a diaminopyrimidine core group and better bioavailability were synthesized and represent a new opportunity for the validation of P2X3-containing receptors as targets for pain. Here we present a characterization of three representative diaminopyrimidines.

EXPERIMENTAL APPROACH

The activity of compounds was evaluated in intracellular calcium flux and electrophysiological recordings from P2X receptors expressed in mammalian cells and in a in vivo model of inflammatory pain (complete Freund's adjuvant (CFA) in rat paws).

KEY RESULTS

Compound A potently blocked P2X3 (pIC(50)= 7.39) and P2X2/3 (pIC(50)=6.68) and showed no detectable activity at P2X1, P2X2, P2X4 and P2X7 receptors (pIC(50)< 4.7). Whole-cell voltage clamp electrophysiology confirmed these results. Compounds showed good selectivities when tested against a panel of different classes of target. In the CFA model, compound B showed significant anti-nociceptive effects (57% reversal at 3mg·kg(-1) ).

CONCLUSIONS AND IMPLICATIONS

The diaminopyrimidines were potent and selective P2X3 and P2X2/3 receptor antagonists, showing efficacy in vivo and represent useful tools to validate these receptors as targets for inflammatory and neuropathic pain and provide promising progress in the identification of therapeutic tools for the treatment of pain-related disorders.

Increased local concentration of complement C5a contributes to incisional pain in mice

BACKGROUND

In our previous study, we demonstrated that local injection of complement C5a and C3a produce mechanical and heat hyperalgesia, and that C5a and C3a activate and sensitize cutaneous nociceptors in normal skin, suggesting a contribution of complement fragments to acute pain. Other studies also have shown that the complement system can be activated by surgical incision, and the systemic blockade of C5a receptor (C5aR) reduces incision-induced pain and inflammation. In this study, we further examined the possible contribution of wound area C5a to incisional pain.

METHODS

Using of a hind paw incisional model, the effects of a selective C5aR antagonist, PMX53, on nociceptive behaviors were measured after incision in vivo. mRNA levels of C5 and C5aR in skin, dorsal root ganglia (DRG) and spinal cord, and C5a protein levels in the skin were quantified after incision. The responses of nociceptors to C5a were also evaluated using the in vitro skin-nerve preparation.

RESULTS

Local administration of PMX53 suppressed heat hyperalgesia and mechanical allodynia induced by C5a injection or after hind paw incision in vivo. mRNA levels of C5 and C5aR in the skin, but not DRG and spinal cord, were dramatically increased after incision. C5a protein in the skin was also increased after incision. In vitro C5a did not increase the prevalence of fibers with ongoing activity in afferents from incised versus control, unincised skin. C5a sensitized C-fiber afferent responses to heat; however, this was less evident in afferents adjacent to the incision. PMX53 blocked sensitization of C-fiber afferents to heat by C5a but did not by itself influence ongoing activity or heat sensitivity in afferents innervating control or incised skin. The magnitude of mechanical responses was also not affected by C5a in any nociceptive fibers innervating incised or unincised skin.

CONCLUSIONS

This study demonstrates that high locally generated C5a levels are present in wounds for at least 72 hours after incision. In skin, C5a contributes to hypersensitivity after incision, but increased responsiveness of cutaneous nociceptors to C5a was not evident in incised skin. Thus, high local concentrations of C5a produced in wounds likely contribute to postoperative pain.

Activation and regulation of purinergic P2X receptor channels

Mammalian ATP-gated nonselective cation channels (P2XRs) can be composed of seven possible subunits, denoted P2X1 to P2X7. Each subunit contains a large ectodomain, two transmembrane domains, and intracellular N and C termini. Functional P2XRs are organized as homomeric and heteromeric trimers. This review focuses on the binding sites involved in the activation (orthosteric) and regulation (allosteric) of P2XRs. The ectodomains contain three ATP binding sites, presumably located between neighboring subunits and formed by highly conserved residues. The detection and coordination of three ATP phosphate residues by positively charged amino acids are likely to play a dominant role in determining agonist potency, whereas an AsnPheArg motif may contribute to binding by coordinating the adenine ring. Nonconserved ectodomain histidines provide the binding sites for trace metals, divalent cations, and protons. The transmembrane domains account not only for the formation of the channel pore but also for the binding of ivermectin (a specific P2X4R allosteric regulator) and alcohols. The N- and C- domains provide the structures that determine the kinetics of receptor desensitization and/or pore dilation and are critical for the regulation of receptor functions by intracellular messengers, kinases, reactive oxygen species and mercury. The recent publication of the crystal structure of the zebrafish P2X4.1R in a closed state provides a major advance in the understanding of this family of receptor channels. We will discuss data obtained from numerous site-directed mutagenesis experiments accumulated during the last 15 years with reference to the crystal structure, allowing a structural interpretation of the molecular basis of orthosteric and allosteric ligand actions.

Regulation of GABA(A) receptor dynamics by interaction with purinergic P2X(2) receptors

γ-Aminobutyric acid type A receptors (GABA(A)Rs) in the spinal cord are evolving as an important target for drug development against pain. Purinergic P2X(2) receptors (P2X(2)Rs) are also expressed in spinal cord neurons and are known to cross-talk with GABA(A)Rs. Here, we investigated a possible "dynamic" interaction between GABA(A)Rs and P2X(2)Rs using co-immunoprecipitation and fluorescence resonance energy transfer (FRET) studies in human embryonic kidney (HEK) 293 cells along with co-localization and single particle tracking studies in spinal cord neurons. Our results suggest that a significant proportion of P2X(2)Rs forms a transient complex with GABA(A)Rs inside the cell, thus stabilizing these receptors and using them for co-trafficking to the cell surface, where P2X(2)Rs and GABA(A)Rs are primarily located extra-synaptically. Furthermore, agonist-induced activation of P2X(2)Rs results in a Ca(2+)-dependent as well as an apparently Ca(2+)-independent increase in the mobility and an enhanced degradation of GABA(A)Rs, whereas P2X(2)Rs are stabilized and form larger clusters. Antagonist-induced blocking of P2XRs results in co-stabilization of this receptor complex at the cell surface. These results suggest a novel mechanism where association of P2X(2)Rs and GABA(A)Rs could be used for specific targeting to neuronal membranes, thus providing an extrasynaptic receptor reserve that could regulate the excitability of neurons. We further conclude that blocking the excitatory activity of excessively released ATP under diseased state by P2XR antagonists could simultaneously enhance synaptic inhibition mediated by GABA(A)Rs.

Estrogen modulation of peripheral pain signal transduction: involvement of P2X(3) receptors

There is evidence that gonadal hormones may affect the perception of painful stimulation, although the underlying mechanisms remain unclear. This investigation was undertaken to determine whether the adenosine 5'-triphosphate (ATP) receptor subunit, P2X(3), is involved in the modulatory action of estrogen in peripheral pain signal transduction in dorsal root ganglion (DRG). The mechanical pain behavior test, real-time quantitative reverse transcription-polymerase chain reaction analysis, and Western blot methods were used to determine the mean relative concentrations and functions of P2X(3) receptors in DRG in sham, ovariectomized (OVX), and estradiol replacement (OVX+E(2)) female rats and in sham and orchiectomized male rats. The mechanical hyperalgesia appeared after ovariectomy, which was subsequently reversed after estradiol replacement, whereas it was not observed after orchiectomy in male rats. Plantar injection of 2'(3')-O-(2,4,6-trinitrophenyl) ATP (TNP-ATP), a P2X(3) and P2X(2/3) receptor antagonist, resulted in an increase of the pain threshold force in OVX rats while had no effect on sham rats. Furthermore, A-317491, a selective P2X(3)/P2X(2/3) receptor antagonist, significantly reversed the hyperalgesia of OVX rats. Injection of ATP into the plantars also caused a significant increase of the paw withdrawal duration in OVX rats compared with that seen in the sham group, which became substantially attenuated by TNP-ATP. P2X(3) receptors expressed in DRG were significantly increased in both mRNA and protein levels after ovariectomy and then reversed after estrogen replacement, while a similar increase was not observed after orchiectomy in male rats. Furthermore, P2X(3) mRNA was significantly decreased 24 h after the application of 17β-estradiol in a concentration-dependent manner in cultured DRG neurons. ICI 182,780, an estrogen receptor antagonist, blocked the reduction in the protein level. These results suggest that the female gonadal hormone, 17β-estradiol, might participate in the control of peripheral pain signal transduction by modulating P2X(3) receptor-mediated events in primary sensory neurons, probably through genomic mechanisms.

ATP and P2X purinoceptors in urinary tract disorders

The pharmacological concept of specifically targeting purinoceptors (receptors for ATP and related nucleotides) has emerged over the last two decades in the quest for novel, differentiated therapeutics. Investigations from many laboratories have established a prominent role for ATP in the functional regulation of most tissue and organ systems, including the urinary tract, under normal and pathophysiological conditions. In the particular case of the urinary tract, ATP signaling via P2X1 receptors participates in the efferent control of detrusor smooth muscle excitability, and this function may be heightened in disease and aging. Perhaps of greater interest, ATP also appears to be involved in bladder sensation, operating via activation of P2X3-containing receptors on sensory afferent neurones, both on peripheral terminals within the urinary tract tissues (e.g., ureters, bladder) and on central synapses in the dorsal horn of the spinal cord. Such findings are based on results from classical pharmacological and localization studies in nonhuman and human tissues, gene knockout mice, and studies using recently identified pharmacological antagonists - some of which have progressed as candidate drug molecules. Based on recent advances in this field, it is apparent that the development of selective antagonists for these receptors will occur that could lead to therapies offering better relief of storage, voiding, and sensory symptoms for patients, while minimizing the systemic side effects that curb the clinical effectiveness of current urologic medicines.

AF-353, a novel, potent and orally bioavailable P2X3/P2X2/3 receptor antagonist

BACKGROUND AND PURPOSE

Purinoceptors containing the P2X3 subunit (P2X3 homotrimeric and P2X2/3 heterotrimeric) are members of the P2X family of ion channels gated by ATP and may participate in primary afferent sensitization in a variety of pain-related diseases. The current work describes the in vitro pharmacological characteristics of AF-353, a novel, orally bioavailable, highly potent and selective P2X3/P2X2/3 receptor antagonist.

EXPERIMENTAL APPROACH

The antagonistic potencies (pIC(50)) of AF-353 for rat and human P2X3 and human P2X2/3 receptors were determined using methods of radioligand binding, intracellular calcium flux and whole cell voltage-clamp electrophysiology.

KEY RESULTS

The pIC(50) estimates for these receptors ranged from 7.3 to 8.5, while concentrations 300-fold higher had little or no effect on other P2X channels or on an assortment of receptors, enzymes and transporter proteins. In contrast to A-317491 and TNP-ATP, competition binding and intracellular calcium flux experiments suggested that AF-353 inhibits activation by ATP in a non-competitive fashion. Favourable pharmacokinetic parameters were observed in rat, with good oral bioavailability (%F = 32.9), reasonable half-life (t(1/2) = 1.63 h) and plasma-free fraction (98.2% protein bound).

CONCLUSIONS AND IMPLICATIONS

The combination of a favourable pharmacokinetic profile with the antagonist potency and selectivity for P2X3 and P2X2/3 receptors suggests that AF-353 is an excellent in vivo tool compound for study of these channels in animal models and demonstrates the feasibility of identifying and optimizing molecules into potential clinical candidates, and, ultimately, into a novel class of therapeutics for the treatment of pain-related disorders.

P2X3 and P2X2/3 receptors mediate mechanical hyperalgesia induced by bradykinin, but not by pro-inflammatory cytokines, PGE₂ or dopamine

Activation of peripheral P2X3 and P2X2/3 receptors by endogenous ATP is essential to the development of inflammatory hyperalgesia. We have previously demonstrated that this essential role of P2X3 and P2X2/3 receptors in the development of mechanical hyperalgesia induced by the inflammatory agent carrageenan is mediated by an indirect sensitization of the primary afferent nociceptors dependent on the previous release of tumor necrosis factor alpha (TNF-α) and by a direct sensitization of the primary afferent nociceptors. Therefore, in this study we asked whether activation of P2X3 and P2X2/3 receptors contribute to the mechanical hyperalgesia induced by the inflammatory mediators involved in carrageenan-induced mechanical hyperalgesia, such as bradykinin, tumor necrosis factor alpha (TNF-α), interleukin-1 beta (IL-1β), interleukin-6 (IL-6), chemokine-induced chemoattractant-1 (CINC-1), prostaglandin E₂ (PGE₂) and dopamine. Co-administration of the non-selective P2X3 receptor antagonist TNP-ATP or the selective P2X3 and P2X2/3 receptor antagonist A-317491 with bradykinin, but not with TNF-α, IL-1β, IL-6, CINC-1, PGE₂ or dopamine, prevented in a dose-dependent manner the mechanical hyperalgesia. We also verified whether the activation of P2X3 and P2X2/3 receptors by endogenous ATP contributes to bradykinin-induced mechanical hyperalgesia via neutrophil migration and/or cytokine release. Co-administration of TNP-ATP or A-317491 did not affect either neutrophil migration or the increased concentration of TNF-α, IL-1β, IL-6 and CINC-1 induced by bradykinin. These findings demonstrate that the activation of P2X3 and P2X2/3 receptors by endogenous ATP mediates bradykinin-induced mechanical hyperalgesia by a mechanism that does not depend on neutrophil migration or cytokines release.

VEGF and its receptor-2 involved in neuropathic pain transmission mediated by P2X₂(/)₃ receptor of primary sensory neurons

The pathogenesis of neuropathic pain is complex. P2X₂(/)₃ receptor plays a crucial role in nociception transduction of chronic pain. VEGF inhibitors are effective for pain relief. The present study investigated the effects of VEGF and VEGF receptor-2 (VEGFR2) on the pain transmission in neuropathic pain states that mediated by P2X₂(/)₃ receptor in primary sensory neurons. Chronic constriction injury (CCI) model was used as neuropathic pain model. Sprague-Dawley rats had been randomly divided into sham group, CCI group and CCI rats treated with anti-rVEGF antibody group. Mechanical withdrawal threshold and thermal withdrawal latency were measured. Expressions of VEGF, VEGFR2 and P2X₂(/)₃ in L4-6 dorsal root ganglia (DRG) were detected by immunohistochemistry, RT-PCR and western blot analysis. The mechanical withdrawal threshold and thermal withdrawal latency in CCI group were lower than those in sham group and CCI rats treated with anti-rVEGF antibody group (p<0.05), while VEGF, VEGFR2 and P2X₂(/)₃ receptors' expressions of L4-6 DRG in CCI group were higher than those in the other two groups (p<0.05). The expressions of VEGF, VEGFR2 and P2X₂(/)₃ in L4-6 DRG of CCI rats treated with anti-rVEGF antibody group were decreased compared with those in CCI group (p<0.05). The results show that VEGF and VEGFR2 are involved in the pathogenesis of neuropathic pain and VEGF primarily potentiates pain responses mediated by P2X₂(/)₃ receptor on DRG neurons. Anti-rVEGF treatment in CCI rats may alleviate chronic neuropathic pain by decreasing the expressions of VEGFR2 and P2X₂(/)₃ receptors on DRG neurons to inhibit the transmission of neuropathic pain signaling.

Involvement of temporomandibular joint P2X3 and P2X2/3 receptors in carrageenan-induced inflammatory hyperalgesia in rats

The aim of this study was to investigate the role of P2X3, P2X2/3 and P2X7 receptors in the development of TMJ hyperalgesia induced by carrageenan. We also investigated the expression of mRNA of P2X7 receptors in the trigeminal ganglia and the existence of functional P2X7 receptors in the rat's TMJ. The P2X1, P2X3 and P2X2/3 receptor antagonist TNP-ATP, but not the selective P2X7 receptor antagonist A-438079, significantly reduced carrageenan-induced TMJ inflammatory hyperalgesia. The qPCR assay showed that mRNA of P2X7 receptors are expressed in the trigeminal ganglia but this expression is not increased by the inflammation induced by carrageenan in the TMJ region. The P2X7 receptor agonist BzATP induced TMJ inflammatory hyperalgesia that was significantly reduced by pretreatment with dexamethasone. These results indicate that P2X3 and P2X2/3 but not P2X7 receptors are involved in carrageenan-induced TMJ inflammatory hyperalgesia. However, functional P2X7 receptors are expressed in the TMJ region. The activation of these receptors by BzATP sensitizes the primary afferent nociceptors in the TMJ through the previous release of inflammatory mediators. The findings of this study point out P2X3 and P2X2/3 receptors, but not P2X7 receptors, as potential targets for the development of new analgesic drugs to control TMJ inflammatory pain.

P2X receptor ligands and pain

P2X receptors belong to a superfamily of ligand-gated ion channels that conduct the influx of Ca(2+), Na(+) and K(+) cations following activation by extracellular nucleotides such as ATP. Molecular cloning studies have identified seven subunits, namely P2X(1-7), that share approximately 40 - 50% identity in amino acid sequences within the subfamily. Using gene-silencing, pharmacological and electrophysiological approaches, recent studies have revealed roles for P2X(2), P2X(3), P2X(4) and P2X(7) receptors in nociceptive signalling. Homomeric P2X(3) and heteromeric P2X(2/3) receptors are highly localised in the peripheral sensory afferent neurons that conduct nociceptive sensory information to the spinal chord and brain. The discovery of A-317491, a selective and potent non-nucleotide P2X(3) antagonist, provided a pharmacological tool to determine the site and mode of action of P2X(3)-containing receptors in different pain behaviours, including neuropathic, inflammatory and visceral pain. Other P2X receptors (P2X(4) and P2X(7)) that are predominantly expressed in microglia, macrophages and cells of immune origin can trigger the release of cytokines, such as IL-1-beta and TNF-alpha. Genetic disruption of P2X(4) and P2X(7) signalling has been demonstrated to reduce inflammatory and neuropathic pain, suggesting that these two receptors might serve as integrators of neuroinflammation and pain. This article provides an overview of recent scientific literature and patents focusing on P2X(3), P2X(4) and P2X(7) receptors, and the identification of small molecule ligands for the potential treatment of neuropathic and inflammatory pain.

Antagonist selective modulation of adenosine A1 and A3 receptor pharmacology by the food dye Brilliant Black BN: evidence for allosteric interactions

Allosteric binding sites on the adenosine receptor family represent potential therapeutic targets for a number of conditions involving metabolic stress. This study has identified Brilliant Black BN as a novel allosteric modulator of the adenosine A(1) and A(3) receptors. In addition to being a food dye and pharmaceutical excipient, Brilliant Black BN is commonly used within calcium mobilization assays to quench extracellular fluorescence. Brilliant Black BN (5-500 microM) had no significant effect on the calcium mobilization stimulated by the nonselective adenosine receptor agonist 5'-(N-ethylcarboxamido)adenosine in Chinese hamster ovary cells stably transfected with the human adenosine A(1) or A(3) receptor. Likewise, calcium mobilization and radioligand binding assays found that Brilliant Black BN (5-500 microM) did not significantly influence the antagonism mediated by 8-cyclopentyl-1,3-dipropylxanthine (100 nM) at the A(1) receptor. In contrast, the affinity of N-[9-chloro-2-(2-furanyl)[1,2,4]-triazolo[1,5-c]quinazolin-5-yl]benzene acetamide (MRS1220) at the A(3) receptor and xanthine amine congener (XAC) and XAC-X-BY630 at the A(1) and A(3) receptors was significantly decreased in the presence of 500 muM Brilliant Black BN. A reduction in XAC potency at the A(1) and A(3) receptor was achieved within 1 min of Brilliant Black BN addition, despite receptors having been pre-equilibrated with antagonist. Dissociation kinetics of the fluorescent XAC derivative, XAC-X-BY630, revealed that the decrease in affinity is probably due to a significant increase in dissociation rate of the antagonist in the presence of Brilliant Black BN. Taken together, these results suggest that Brilliant Black BN can act allosterically to modify ligand affinity at A(1) and A(3) receptors.

Comparison of the peripheral mediator background of heat injury- and plantar incision-induced drop of the noxious heat threshold in the rat

AIMS

Previously we described the drop of the noxious heat threshold in response to mild heat injury or plantar incision. While mild heat injury elicits an immediate and short-lasting thermal hyperalgesia, surgical incision leads to a delayed and sustained heat hyperalgesia. Only very few peripheral mediators of these phenomena have been identified. Therefore the present study aimed at comparing the peripheral mediator background of heat hyperalgesia evoked by mild heat injury or surgical incision.

MAIN METHODS

Heat hyperalgesia was assessed by measuring the behavioural noxious heat threshold in conscious rats employing an increasing-temperature water bath.

KEY FINDINGS

The heat threshold drop evoked by a mild heat injury and measured 10min afterwards was reduced by intraplantarly applied HOE 140, a bradykinin B(2) receptor antagonist, NDGA, a non-selective lipoxygenase inhibitor, L-NOARG, a non-selective nitric oxide synthase inhibitor, TNP-ATP, a P2X purinoceptor antagonist and AMG9810, an antagonist of the transient receptor potential vanilloid type 1 (TRPV1) receptor. The heat threshold drop evoked by plantar incision and measured 18h later was reduced by intraplantarly applied HOE 140, [des-Arg(10)]-HOE 140, a bradykinin B(1) receptor antagonist, L-NOARG, TNP-ATP and the TRPV1 receptor antagonist SB-366791.

SIGNIFICANCE

Only small differences have been revealed between the examined peripheral mediators of the acute heat hyperalgesia evoked by mild heat injury and the sustained increase in heat responsiveness induced by surgical incision. The B(2) and B(1) bradykinin receptor, P2X purinoceptors, TRPV1 receptor, nitric oxide synthase and lipoxygenase(s) are involved in at least one of these hyperalgesia models.

The neural-glial purinergic receptor ensemble in chronic pain states

Chronic pain is characterized by enhanced sensory neurotransmission that underlies increased sensitivity to noxious stimuli and the perception of non-noxious stimuli as painful. Evidence from neurophysiological and pharmacological studies demonstrates that ATP produces pain by directly enhancing neuronal excitability via the activation of specific ligand-gated ion channels, the P2X3 and P2X2/3 receptors. In addition, ATP activates CNS glial cells (e.g. microglia) in response to persistent nociceptive stimulation. This latter effect involves several distinct receptor-mediated signaling pathways linked to the P2X4, P2X7 and P2Y(12) receptors. This review summarizes new data that places these purinergic signaling events in a mechanistic context that illustrates the ability of ATP to initiate and maintain states of heightened sensory neuron excitability associated with persistent pain.

Estrogen altered visceromotor reflex and P2X(3) mRNA expression in a rat model of colitis

P2X(3) and P2X(2/3) receptors are expressed in peripheral tissues and dorsal root ganglia (DRG) and participate in peripheral pain. However, the mechanisms underlying P2X receptor-mediated nociception at different ovarial hormone levels has not been examined. In this study, 24 female rats were randomly divided into sham-operated (sham), ovariectomized (OVX), estrogen-treated, and estrogen-progesterone-treated groups with colitis. In each group, the visceromotor reflex (VMR) to colorectal distension was tested and the DRG were harvested for a real-time PCR analysis of P2X(3) and P2X(2) receptor mRNA. In OVX rats with colitis we found that the VMR to colorectal distension and P2X(3) receptor mRNA in DRG were both significantly decreased. Estrogen replacement reversed the decrease. However, neither the VMR nor the P2X(3) mRNA level in DRG from OVX colitis rats was reversed by the complex of estrogen and progesterone. Patch-clamp recording showed that in colitis rats, estradiol rapidly potentiated the sustained and transient currents evoked by ATP to 336+/-49% and 122+/-12% of controls, respectively, in a subpopulation of DRG neurons, which were blocked by ICI 182, 780, an antagonist of the estrogen receptor. Whereas progesterone rapidly inhibited the transient currents induced by ATP to 67+/-10% of control and had no effect on the sustained currents evoked by the same agonist. These results indicate that P2X(3) receptors are likely to be an important contributor to the altered colonic functions in colitis rats, where the underlying mechanisms are closely related to endogenous estrogen modulation.

Peripheral mechanisms underlying the essential role of P2X3,2/3 receptors in the development of inflammatory hyperalgesia

Activation of P2X3,2/3 receptors by endogenous ATP contributes to the development of inflammatory hyperalgesia. Given the clinical importance of mechanical hyperalgesia in inflammatory states, we hypothesized that the activation of P2X3,2/3 receptors by endogenous ATP contributes to carrageenan-induced mechanical hyperalgesia and that this contribution is mediated by an indirect and/or a direct sensitization of the primary afferent nociceptors. Co-administration of the selective P2X3,2/3 receptors antagonist A-317491, or the non-selective P2X3 receptor antagonist, TNP-ATP, with carrageenan blocked the mechanical hyperalgesia induced by carrageenan, and significantly reduced the increased concentration of tumor necrosis factor alpha (TNF-alpha) and chemokine-induced chemoattractant-1 (CINC-1) but not of interleukin-1 beta (IL-1 beta) induced by carrageenan. Co-administration of the selective P2X3,2/3 receptors antagonist A-317491 with carrageenan did not affect the neutrophil migration induced by carrageenan. Intrathecal administration of oligonucleotides antisense against P2X3 receptors for seven days significantly reduced the expression of P2X3 receptors in the saphenous nerve and significantly reduced the mechanical hyperalgesia induced by carrageenan. We concluded that the activation of P2X3,2/3 receptors by endogenous ATP is essential to the development of the mechanical hyperalgesia induced by carrageenan. Furthermore, we showed that this essential role of P2X3,2/3 receptors in the development of carrageenan-induced mechanical hyperalgesia is mediated by an indirect sensitization of the primary afferent nociceptors dependent on the previous release of TNF-alpha and by a direct sensitization of the primary afferent nociceptors.

Behavioural responses and expression of P2X3 receptor in trigeminal ganglion after experimental tooth movement in rats

OBJECTIVE

To explore the role of P2X(3) receptor in pain induced by experimental tooth movement.

DESIGN

Male Sprague-Dawley rats weighing 200-300g were used. P2X(3) receptor distribution in the caudal one-third portion of the trigeminal ganglion (TG) was studied by IHC. Next, the changes of P2X(3) expression were detected by Western blotting 4h, 1d, 2d, 3d, 5d, 7d, 14d after tooth movement. We then developed a behaviour pain model associated with directed mouth wiping. Finally, the effect of TNP-ATP on nociceptive-like behaviour was evaluated.

RESULTS

Our results showed that P2X(3) receptors were expressed mainly in small- and medium-sized cells and experimental tooth movement led to an increase in staining of mandibular P2X(3) receptors. In addition, following experimental tooth movement, the expression of P2X(3) receptor in TG was statistically significantly up-regulated from days 1 to 5, with a peak on day 3. It was also found that the time spent on directed mouth wiping was dramatically increased by experimental tooth movement from days 1 to 7. The rhythm change of P2X(3) receptor expression in TG and the mouth wiping behaviour were in concert with the initial orthodontic pain responses. The directed mouth wiping behaviour was modulated in a force-dependent manner and could be attenuated by peripheral and systemic morphine. Furthermore, peripherally administered TNP-ATP could exert an analgesic effect on this pain model.

CONCLUSION

These results suggest that directed mouth wiping behaviour can be a reliable measurement of pain following experimental tooth movement in rats. The P2X(3) receptor is important in the development and maintenance of tooth movement pain and thus may be peripheral targets for analgesics in orthodontic pain control.

Role of sodium ferulate in the nociceptive sensory facilitation of neuropathic pain injury mediated by P2X(3) receptor

Neuropathic pain usually is persistent and no effective treatment. ATP plays an important role in the initiation of pain. P2X(3) receptors are localized in the dorsal root ganglion (DRG) neurons and activated by extracellular ATP. Sodium ferulate (SF) is an active principle from Chinese herbal medicine and has anti-inflammatory activities. This study observed the effects of SF on the nociceptive facilitation of the primary sensory afferent after chronic constriction injury (CCI) mediated by P2X(3) receptor. In this study, the content of ATP in DRG neurons was measured by high-performance liquid chromatography (HPLC). P2X(3) agonist-activated currents in DRG neurons was recorded by the whole-cell patch-clamp skill. The expression of P2X(3) mRNA in DRG neurons was analyzed by in situ hybridization. The ATP content of DRG was increased after CCI. In CCI rats treated with SF, the content of ATP in DRG neurons was reduced. SF decreased the increment of P2X(3) agonist-activated currents and P2X(3) mRNA expression in DRG neurons during CCI. SF may inhibit the initiation of pain and primary afferent sensitization mediated by P2X(3) receptor during CCI.

Involvement of P2Y1 and P2Y11 purinoceptors in parasympathetic inhibition of colonic smooth muscle

Purinergic signaling was first recognized in the guinea pig (Cavia porcellus) taenia coli, where relaxation of smooth muscle by nerve-released ATP may involve the activation of P2Y(1) and P2Y(11) receptors, and where transcripts for both genes have been found. A partial sequence for P2Y(11) protein was identified; the full-length P2Y(1) sequence has already been described. P2Y(1) and P2Y(11) proteins were localized by immunohistochemistry in smooth muscle cells. P2X(2) and P2X(3) proteins were also localized in motoneurons of the myenteric plexus. alphabeta-Methylene-ATP (alphabetameATP) and dibenzoyl-ATP (BzATP) evoked fast relaxations in the taenia, and they were inhibited by the P2Y(1) receptor antagonist 2'-deoxy-N(6)-methyladenosine 3',5'-bisphosphate (MRS2179). However, alphabetameATP and BzATP may stimulate neuronal P2X receptors to release ATP, which then acts on P2Y(1) receptors. In accordance, fast relaxations evoked by alphabetameATP and BzATP were inhibited by the P2X(3) and P2X(2/3) receptor antagonist 5-({[3-phenoxybenzyl][(1S)-1,2,3,4-tetrahydro-1-naphthalenyl] amino} carbonyl)-1,2,4-benzene-tricarboxylic acid (A317491). When P2Y(1), P2X(3), and P2X(2/3) receptors were blocked and adenosine was removed enzymatically, alphabetameATP and BzATP evoked slow relaxations that were inhibited by Reactive Red. Fast and slow relaxations involve small and large conductance calcium-activated potassium channels; the latter are dependent on intracellular cyclic AMP levels, which altered the duration and amplitude of relaxations. alphabetameATP and BzATP were confirmed as agonists, and Reactive Red as an antagonist, of human P2Y(11) receptors. In summary, G(q)-coupled P2Y(1) receptors are involved mainly in fast relaxations, whereas G(q)and G(s)-coupled P2Y(11) receptors are involved in both fast and slow relaxations. These P2Y receptor subtypes, plus neuronal P2X receptors, may explain the phenomenon of parasympathetic inhibition first described by Langley (1898).

Painful purinergic receptors

Multiple P2 receptor-mediated mechanisms exist by which ATP can alter nociceptive sensitivity following tissue injury. Evidence from a variety of experimental strategies, including genetic disruption studies and the development of selective antagonists, has indicated that the activation of P2X receptor subtypes, including P2X(3), P2X(2/3), P2X(4) and P2X(7), and P2Y (e.g., P2Y(2)) receptors, can modulate pain. For example, administration of a selective P2X(3) antagonist, A-317491, has been shown to effectively block both hyperalgesia and allodynia in different animal models of pathological pain. Intrathecally delivered antisense oligonucleotides targeting P2X(4) receptors decrease tactile allodynia following nerve injury. Selective antagonists for the P2X(7) receptor also reduce sensitization in animal models of inflammatory and neuropathic pain, providing evidence that purinergic glial-neural interactions are important modulators of noxious sensory neurotransmission. Furthermore, activation of P2Y(2) receptors leads to sensitization of polymodal transient receptor potential-1 receptors. Thus, ATP acting at multiple purinergic receptors, either directly on neurons (e.g., P2X(3), P2X(2/3), and P2Y receptors) or indirectly through neural-glial cell interactions (P2X(4) and P2X(7) receptors), alters nociceptive sensitivity. The development of selective antagonists for some of these P2 receptors has greatly aided investigations into the nociceptive role of ATP. This perspective highlights some of the recent advances to identify selective P2 receptor ligands, which has enhanced the investigation of ATP-related modulation of pain sensitivity.