P2X4 purinoceptor signaling in chronic pain

Neuropathic pain; what we know and what we should do about it

Neuropathic pain can result from injury to, or disease of the nervous system. It is notoriously difficult to treat. Peripheral nerve injury promotes Schwann cell activation and invasion of immunocompetent cells into the site of injury, spinal cord and higher sensory structures such as thalamus and cingulate and sensory cortices. Various cytokines, chemokines, growth factors, monoamines and neuropeptides effect two-way signalling between neurons, glia and immune cells. This promotes sustained hyperexcitability and spontaneous activity in primary afferents that is crucial for onset and persistence of pain as well as misprocessing of sensory information in the spinal cord and supraspinal structures. Much of the current understanding of pain aetiology and identification of drug targets derives from studies of the consequences of peripheral nerve injury in rodent models. Although a vast amount of information has been forthcoming, the translation of this information into the clinical arena has been minimal. Few, if any, major therapeutic approaches have appeared since the mid 1990's. This may reflect failure to recognise differences in pain processing in males vs. females, differences in cellular responses to different types of injury and differences in pain processing in humans vs. animals. Basic science and clinical approaches which seek to bridge this knowledge gap include better assessment of pain in animal models, use of pain models which better emulate human disease, and stratification of human pain phenotypes according to quantitative assessment of signs and symptoms of disease. This can lead to more personalized and effective treatments for individual patients. Significance statement: There is an urgent need to find new treatments for neuropathic pain. Although classical animal models have revealed essential features of pain aetiology such as peripheral and central sensitization and some of the molecular and cellular mechanisms involved, they do not adequately model the multiplicity of disease states or injuries that may bring forth neuropathic pain in the clinic. This review seeks to integrate information from the multiplicity of disciplines that seek to understand neuropathic pain; including immunology, cell biology, electrophysiology and biophysics, anatomy, cell biology, neurology, molecular biology, pharmacology and behavioral science. Beyond this, it underlines ongoing refinements in basic science and clinical practice that will engender improved approaches to pain management.

Neohesperidin Alleviates the Neuropathic Pain Behavior of Rats by Downregulating the P2X4 Receptor

Neuropathic pain (NP) is a type of chronic pain affecting 6-8% of human health as no effective drug exists. The purinergic 2X4 receptor (P2X4R) is involved in NP. Neohesperidin (NH) is a dihydroflavonoside compound, which has anti-inflammatory and antioxidative properties. This study aimed to investigate whether NH has an effect on P2X4R-mediated NP induced by chronic constriction injury (CCI) of the sciatic nerve in rats. In this study, the CCI rat model was established to observe the changes of pain behaviors, P2X4R, and satellite glial cells (SGCs) activation in dorsal root ganglion (DRG) after NH treatment by using RT-PCR, immunofluorescence double labeling and Western blotting. Our results showed CCI rats had mechanical and thermal hyperalgesia with an increased level of P2X4R. Furthermore, SGCs were activated as indicated by increased expression of glial fibrillary acidic protein and increased tumor necrosis factor-alpha receptor 1and interleukin-1β. In addition, phosphorylated extracellular regulated protein kinases and interferon regulatory factor 5 in CCI rats increased. After NH treatment in CCI rats, the levels of above protein decreased, and the pain reduced. Overall, NH can markedly alleviate NP by reducing P2X4R expression and SGCs activation in DRG.

Spinal P2X4 Receptors Involved in Visceral Hypersensitivity of Neonatal Maternal Separation Rats

Recent studies have demonstrated the vital role of P2X4 receptors (a family of ATP-gated non-selective cation channels) in the transmission of neuropathic and inflammatory pain. In this study, we investigated the role of spinal P2X4 receptors in chronic functional visceral hypersensitivity of neonatal maternal separation (NMS) rats. A rat model of irritable bowel syndrome was established by neonatal maternal separation. Visceral sensitivity was assessed by recording the response of the external oblique abdominal muscle to colorectal distension. P2X4 receptor antagonist and agonist were administrated intrathecally. The expression of P2X4 receptor was examined by Western Blot and immunofluorescence. The effect of P2X4 receptor antagonist on expression of brain-derived neurotrophic factor (BDNF) was assessed by Western Blot. We found neonatal maternal separation enhanced visceral hypersensitivity and increased the expression of P2X4 receptor in spinal thoracolumbar and lumbosacral segments of rats. Pharmacological results showed that visceral sensitivity was attenuated after intrathecal injection of P2X4 receptor antagonist, 5-BDBD, at doses of 10 nM or 100 nM, while visceral sensitivity was enhanced after intrathecal injection of P2X4 receptor agonist C5-TDS at doses of 10 μM or 15 μM. In addition, the spinal expression of BDNF significantly increased in NMS rats and intrathecal injection of 5-BDBD significantly decreased the expression of BDNF especially in NMS rats. C5-TDS failed to increase EMG amplitude in the presence of ANA-12 in control rats. Our results suggested the spinal P2X4 receptors played an important role in visceral hypersensitivity of NMS rats through BDNF.

Pathophysiological and Neuroplastic Changes in Postamputation and Neuropathic Pain: Review of the Literature

Despite advancements in surgical and rehabilitation strategies, extremity amputations are frequently associated with disability, phantom limb sensations, and chronic pain. Investigation into potential treatment modalities has focused on the pathophysiological changes in both the peripheral and central nervous systems to better understand the underlying mechanism in the development of chronic pain in persons with amputations.

Methods

Presented in this article is a discussion outlining the physiological changes that occur in the peripheral and central nervous systems following amputation. In this review, the authors examine the molecular and neuroplastic changes occurring in the nervous system, as well as the state-of-the-art treatment to help reduce the development of postamputation pain.

Results

This review summarizes the current literature regarding neurological changes following amputation. Development of both central sensitization and neuronal remodeling in the spinal cord and cerebral cortex allows for the development of neuropathic and phantom limb pain postamputation. Recently developed treatments targeting these pathophysiological changes have enabled a reduction in the severity of pain; however, complete resolution remains elusive.

Conclusions

Changes in the peripheral and central nervous systems following amputation should not be viewed as separate pathologies, but rather two interdependent mechanisms that underlie the development of pathological pain. A better understanding of the physiological changes following amputation will allow for improvements in therapeutic treatments to minimize pathological pain caused by amputation.

Excess intracellular ATP causes neuropathic pain following spinal cord injury

Intractable neuropathic pain following spinal cord injury (NP-SCI) reduces a patient's quality of life. Excessive release of ATP into the extracellular space evokes neuroinflammation via purinergic receptor. Neuroinflammation plays an important role in the initiation and maintenance of NP. However, little is known about whether or not extracellular ATP cause NP-SCI. We found in the present study that excess of intracellular ATP at the lesion site evokes at-level NP-SCI. No significant differences in the body weight, locomotor function, or motor behaviors were found in groups that were negative and positive for at-level allodynia. The intracellular ATP level at the lesion site was significantly higher in the allodynia-positive mice than in the allodynia-negative mice. A metabolome analysis revealed that there were no significant differences in the ATP production or degradation between allodynia-negative and allodynia-positive mice. Dorsal horn neurons in allodynia mice were found to be inactivated in the resting state, suggesting that decreased ATP consumption due to neural inactivity leads to a build-up of intracellular ATP. In contrast to the findings in the resting state, mechanical stimulation increased the neural activity of dorsal horn and extracellular ATP release at lesion site. The forced production of intracellular ATP at the lesion site in non-allodynia mice induced allodynia. The inhibition of P2X4 receptors in allodynia mice reduced allodynia. These results suggest that an excess buildup of intracellular ATP in the resting state causes at-level NP-SCI as a result of the extracellular release of ATP with mechanical stimulation.

The P2X4 Receptor: Cellular and Molecular Characteristics of a Promising Neuroinflammatory Target

The adenosine 5′-triphosphate-gated P2X4 receptor channel is a promising target in neuroinflammatory disorders, but the ability to effectively target these receptors in models of neuroinflammation has presented a constant challenge. As such, the exact role of P2X4 receptors and their cell signalling mechanisms in human physiology and pathophysiology still requires further elucidation. To this end, research into the molecular mechanisms of P2X4 receptor activation, modulation, and inhibition has continued to gain momentum in an attempt to further describe the role of P2X4 receptors in neuroinflammation and other disease settings. Here we provide an overview of the current understanding of the P2X4 receptor, including its expression and function in cells involved in neuroinflammatory signalling. We discuss the pharmacology of P2X4 receptors and provide an overview of P2X4-targeting molecules, including agonists, positive allosteric modulators, and antagonists. Finally, we discuss the use of P2X4 receptor modulators and antagonists in models of neuroinflammatory cell signalling and disease.

Mediators of Neuropathic Pain; Focus on Spinal Microglia, CSF-1, BDNF, CCL21, TNF-α, Wnt Ligands, and Interleukin 1β

Intractable neuropathic pain is a frequent consequence of nerve injury or disease. When peripheral nerves are injured, damaged axons undergo Wallerian degeneration. Schwann cells, mast cells, fibroblasts, keratinocytes and epithelial cells are activated leading to the generation of an “inflammatory soup” containing cytokines, chemokines and growth factors. These primary mediators sensitize sensory nerve endings, attract macrophages, neutrophils and lymphocytes, alter gene expression, promote post-translational modification of proteins, and alter ion channel function in primary afferent neurons. This leads to increased excitability and spontaneous activity and the generation of secondary mediators including colony stimulating factor 1 (CSF-1), chemokine C-C motif ligand 21 (CCL-21), Wnt3a, and Wnt5a. Release of these mediators from primary afferent neurons alters the properties of spinal microglial cells causing them to release tertiary mediators, in many situations via ATP-dependent mechanisms. Tertiary mediators such as BDNF, tumor necrosis factor α (TNF-α), interleukin 1β (IL-1β), and other Wnt ligands facilitate the generation and transmission of nociceptive information by increasing excitatory glutamatergic transmission and attenuating inhibitory GABA and glycinergic transmission in the spinal dorsal horn. This review focusses on activation of microglia by secondary mediators, release of tertiary mediators from microglia and a description of their actions in the spinal dorsal horn. Attention is drawn to the substantial differences in the precise roles of various mediators in males compared to females. At least 25 different mediators have been identified but the similarity of their actions at sensory nerve endings, in the dorsal root ganglia and in the spinal cord means there is considerable redundancy in the available mechanisms. Despite this, behavioral studies show that interruption of the actions of any single mediator can relieve signs of pain in experimental animals. We draw attention this paradox. It is difficult to explain how inactivation of one mediator can relieve pain when so many parallel pathways are available.

Pain-related toxins in scorpion and spider venoms: a face to face with ion channels

Pain is a common symptom induced during envenomation by spiders and scorpions. Toxins isolated from their venom have become essential tools for studying the functioning and physiopathological role of ion channels, as they modulate their activity. In particular, toxins that induce pain relief effects can serve as a molecular basis for the development of future analgesics in humans. This review provides a summary of the different scorpion and spider toxins that directly interact with pain-related ion channels, with inhibitory or stimulatory effects. Some of these toxins were shown to affect pain modalities in different animal models providing information on the role played by these channels in the pain process. The close interaction of certain gating-modifier toxins with membrane phospholipids close to ion channels is examined along with molecular approaches to improve selectivity, affinity or bioavailability in vivo for therapeutic purposes.

Distinct roles of GT1b and CSF-1 in microglia activation in nerve injury-induced neuropathic pain

Although microglia activation plays an important role in the development of nerve injury-induced neuropathic pain, the molecular mechanisms of spinal cord microglia activation in nerve injury are not completely understood. Recently, two injured sensory neuron-derived molecules, colony stimulating factor-1 (CSF-1) and GT1b, were proposed to trigger spinal cord microglia activation, yet their relationship and relative contribution to microglia activation have not been addressed. In the present study, the role of GT1b and CSF-1 in microglia activation and proliferation was characterized. GT1b stimulation upregulated proinflammatory mediators such as IL-1β, TNF-α, and NADPH oxidase 2 (Nox2), without microglia proliferation. Conversely, CSF-1 stimulation induced microglia proliferation with minimal proinflammatory gene induction. Notably, neither GT1b nor CSF-1 induced mechanical hypersensitivity in female mice; however, they induced similar microglial proliferation in both male and female mice. Taken together, our data indicate that injured sensory neuron-derived GT1b and CSF-1 activate spinal cord microglia in concert through distinct activation pathways.

Treatment of chronic neuropathic pain: purine receptor modulation

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

Deciphering the Role of Endolysosomal Ca2+ Channels in Immunity

The role of endolysosomal Ca2+ signalling in immunity has been a subject of increasing interest in recent years. Here, we discuss evolving knowledge relating to the contribution of endolysosomal Ca2+ channels that include TPCs, TRPMLs, and P2X4R in physiological processes related to innate and adaptive immunity-including phagocytosis, inflammation, cytokine/chemokine release, dendritic, natural killer, and T cell activation and migration-and we underscore the paucity of clinical studies in this field. Emerging biomedical and translational data have led to important new insights into the critical roles of these channels in immune cell function and the regulation of innate and adaptive immune responses. The evolving immunological significance of endolysosomal Ca2+ signalling warrants further investigations to better characterize the roles of these channels in immunity in order to expand our knowledge about the pathology of inflammatory and autoimmune diseases and develop endolysosomal Ca2+ channels as viable biomarkers and therapeutic and preventive targets for remodelling the immune response.

LncRNA UC.360+ shRNA Improves Diabetic Cardiac Sympathetic Dysfunction Mediated by the P2X4 Receptor in the Stellate Ganglion

Diabetic cardiac autonomic neuropathy (DCAN) is a complication that affects more than 60% of diabetic patients. There is evidence for the involvement of P2X4 receptor in DCAN. This study showed that the expression of the long noncoding RNA (lncRNA) UC.360+ was increased in the stellate ganglion (SG) of type 2 diabetes mellitus (DM) rats, and in situ hybridization revealed a clear presence of UC.360+ in SG neurons. The potential roles of UC.360+ in DCAN and its relationship with P2X4 receptor in SG were further explored via application of the short hairpin RNA (shRNA) against lncRNA UC.360+ in DM rats. The abnormal cardiac sympathetic changes in diabetic rats were improved after treatment with lncRNA UC.360+ shRNA. In the SG of these shRNA-treated DM rats, the upregulation of P2X4, tumor necrosis factor-α (TNF-α), interleukin 1β (IL-1β), and phosphorylated ERK1/2 was inhibited. Thus, lncRNA UC.360+ shRNA treatment may improve DCAN mediated by the P2X4 receptor in SG.

Astragalin Alleviates Neuropathic Pain by Suppressing P2X4-Mediated Signaling in the Dorsal Root Ganglia of Rats

Neurologic damage often leads to neuropathic pain, for which there are no effective treatments owing to its complex pathogenesis. The purinergic receptor P2X4 is closely associated with neuropathic pain. Astragalin (AST), a compound that is used in traditional Chinese medicine, has protective effects against allergic dermatitis and neuronal injury, but its mechanism of action is not well understood. The present study investigated whether AST can alleviate neuropathic pain in a rat model established by chronic constriction injury (CCI) to the sciatic nerve. The model rats exhibited pain behavior and showed increased expression of P2X4 and the activated satellite glial cell (SGC) marker glial fibrillary acidic protein in dorsal root ganglia (DRG). AST treatment partly abrogated the upregulation of P2X4, inhibited SGC activation, and alleviated pain behavior in CCI rats; it also suppressed ATP-activated currents in HEK293 cells overexpressing P2X4. These data demonstrate that AST relieves neuropathic pain by inhibiting P2X4 and SGC activation in DRG, highlighting its therapeutic potential for clinical pain management.

Purinergic signaling orchestrating neuron-glia communication

This review discusses the evidence supporting a role for ATP signaling (operated by P₂X and P₂Y receptors) and adenosine signaling (mainly operated by A₁ and A_2A receptors) in the crosstalk between neurons, astrocytes, microglia and oligodendrocytes. An initial emphasis will be given to the cooperation between adenosine receptors to sharpen information salience encoding across synapses. The interplay between ATP and adenosine signaling in the communication between astrocytes and neurons will then be presented in context of the integrative properties of the astrocytic syncytium, allowing to implement heterosynaptic depression processes in neuronal networks. The process of microglia 'activation' and its control by astrocytes and neurons will then be analyzed under the perspective of an interplay between different P₂ receptors and adenosine A_2A receptors. In spite of these indications of a prominent role of purinergic signaling in the bidirectional communication between neurons and glia, its therapeutical exploitation still awaits obtaining an integrated view of the spatio-temporal action of ATP signaling and adenosine signaling, clearly distinguishing the involvement of both purinergic signaling systems in the regulation of physiological processes and in the control of pathogenic-like responses upon brain dysfunction or damage.

Implication of Neuronal Versus Microglial P2X4 Receptors in Central Nervous System Disorders

The P2X4 receptor (P2X4) is an ATP-gated cation channel that is highly permeable to Ca²⁺ and widely expressed in neuronal and glial cell types throughout the central nervous system (CNS). A growing body of evidence indicates that P2X4 plays key roles in numerous central disorders. P2X4 trafficking is highly regulated and consequently in normal situations, P2X4 is present on the plasma membrane at low density and found mostly within intracellular endosomal/lysosomal compartments. An increase in the de novo expression and/or surface density of P2X4 has been observed in microglia and/or neurons during pathological states. This review aims to summarize knowledge on P2X4 functions in CNS disorders and provide some insights into the relative contributions of neuronal and glial P2X4 in pathological contexts. However, determination of the cell-specific functions of P2X4 along with its intracellular and cell surface roles remain to be elucidated before its potential as a therapeutic target in multiple disorders can be defined.

Effect of Paroxetine on the Neuropathic Pain: A Molecular Study

Background

Neuropathic pain, due to peripheral nerve damage, has influenced millions of people living all over the world. It has been shown that paroxetine can relieve neuropathic pain. Recently, the role of certain proteins like brain-derived neurotrophic factor (BDNF), GABAA receptor, and K+-Cl- cotransporter 2 (KCC2) transporter in the occurrence of neuropathic pain has been documented. In the current study, the expression of these proteins affected by paroxetine was evaluated.

Methods

Male Wistar rats were allocated into two main groups of pre- and post-injury. Rats in each main group received paroxetine before nerve injury and at day seven after nerve damage till day 14, respectively. The lumbar spinal cord of animals was extracted to assess the expression of target genes and proteins.

Results

In the preventive study, paroxetine decreased BDNF and increased KCC2 and GABAA gene and protein expression, while in the post-injury paradigm, it decreased BDNF and increased KCC2 genes and protein expression. In this regard, an increase in the protein expression of GABAA was observed.

Conclusion

It seems that paroxetine with a change in the expression of three significant proteins involved in neuropathic pain could attenuate this type of chronic pain.

Pharmacological and genetic characterisation of the canine P2X4 receptor

BACKGROUND AND PURPOSE

P2X4 receptors are emerging therapeutic targets for treating chronic pain and cardiovascular disease. Dogs are well-recognised natural models of human disease, but information regarding P2X4 receptors in dogs is lacking. To aid the development and validation of P2X4 receptor ligands, we have characterised and compared canine and human P2X4 receptors.

EXPERIMENTAL APPROACH

Genomic DNA was extracted from whole blood samples from 101 randomly selected dogs and sequenced across the P2RX4 gene to identify potential missense variants. Recombinant canine and human P2X4 receptors tagged with Emerald GFP were expressed in 1321N1 and HEK293 cells and analysed by immunoblotting and confocal microscopy. In these cells, receptor pharmacology was characterised using nucleotide-induced Fura-2 AM measurements of intracellular Ca²⁺ and known P2X4 receptor antagonists. P2X4 receptor-mediated inward currents in HEK293 cells were assessed by automated patch clamp.

KEY RESULTS

No P2RX4 missense variants were identified in any canine samples. Canine and human P2X4 receptors were localised primarily to lysosomal compartments. ATP was the primary agonist of canine P2X4 receptors with near identical efficacy and potency at human receptors. 2'(3')-O-(4-benzoylbenzoyl)-ATP, but not ADP, was a partial agonist with reduced potency for canine P2X4 receptors compared to the human orthologues. Five antagonists inhibited canine P2X4 receptors, with 1-(2,6-dibromo-4-isopropyl-phenyl)-3-(3-pyridyl)urea displaying reduced sensitivity and potency at canine P2X4 receptors.

CONCLUSION AND IMPLICATIONS

P2X4 receptors are highly conserved across dog pedigrees and display expression patterns and pharmacological profiles similar to human receptors, supporting validation and use of therapeutic agents for P2X4 receptor-related disease onset and management in dogs and humans.

Discovery and Characterization of the Potent and Selective P2X4 Inhibitor N-[4-(3-Chlorophenoxy)-3-sulfamoylphenyl]-2-phenylacetamide (BAY-1797) and Structure-Guided Amelioration of Its CYP3A4 Induction Profile

The P2X4 receptor is a ligand-gated ion channel that is expressed on a variety of cell types, especially those involved in inflammatory and immune processes. High-throughput screening led to a new class of P2X4 inhibitors with substantial CYP 3A4 induction in human hepatocytes. A structure-guided optimization with respect to decreased pregnane X receptor (PXR) binding was started. It was found that the introduction of larger and more polar substituents on the ether linker led to less PXR binding while maintaining the P2X4 inhibitory potency. This translated into significantly reduced CYP 3A4 induction for compounds 71 and 73. Unfortunately, the in vivo pharmacokinetic (PK) profiles of these compounds were insufficient for the desired profile in humans. However, BAY-1797 (10) was identified and characterized as a potent and selective P2X4 antagonist. This compound is suitable for in vivo studies in rodents, and the anti-inflammatory and anti-nociceptive effects of BAY-1797 were demonstrated in a mouse complete Freund's adjuvant (CFA) inflammatory pain model.

‬The expression of purinergic P2X4 and P2X7 receptors in selected mesolimbic structures during morphine withdrawal in rats

Morphine is one of the most potent analgesics used in medicine and it's long-term use is associated with the risk of the state of dependence. The cessation of chronic morphine administration leads to withdrawal signs which are associated with neurotransmitter dysregulations within mesolimbic system. Adenosine 5'-triphosphate (ATP) and purinergic system play an important role in the activity of central nervous system (CNS). Purinergic receptors are widely distributed in neurons and glial cells throughout the CNS taking part in integration of functional activity between neurons, glial and vascular cells. In the present study the mRNA and protein expression of purinergic P2X4 and P2X7 receptors in selected mesolimbic structures (striatum, hippocampus and prefrontal cortex) during morphine withdrawal in rats was investigated by RT-PCR and Western Blot analysis. Two experimental models of morphine withdrawal were studied: single and repeated morphine withdrawal. We demonstrated that expression of P2X4 and P2X7 receptors was altered during morphine withdrawal period in rats. These alterations were varied in particular mesolimbic areas depending on the scheme of morphine administration. Our results extend the current knowledge on morphine withdrawal and for the first time high-light interactions between purinergic system and morphine withdrawal. It seems, the purinergic system may be a new, valuable tool in searching for a new strategy of management of opioid dependence.

P2X receptor channels in chronic pain pathways

Chronic pain is a highly prevalent debilitating condition for which treatment options remain limited for many patients. Ionotropic ATP signalling through excitatory and calcium-permeable P2X receptor channels is now rightfully considered as a critical player in pathological pain generation and maintenance; therefore, their selective targeting represents a therapeutic opportunity with promising yet untapped potential. Recent advances in the structural, functional and pharmacological characterization of rodent and human ATP-gated P2X receptor channels have shed brighter light on the role of specific subtypes in the pathophysiology of chronic inflammatory, neuropathic or cancer pain. Here, we will review the contribution of P2X3, P2X4 and P2X7 receptors to chronic pain and discuss the opportunities and challenges associated with the pharmacological manipulation of their function.

LINKED ARTICLES

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

Inflammatory-induced spinal dorsal horn neurons hyperexcitability is mediated by P2X4 receptors

Introduction:

Purinergic ionotropic P2X receptors (P2RX) are involved in normal and pathological pain transmission. Among them, P2X4 are expressed in dorsal root ganglion and in the spinal cord. Their activation during nerve injury or chronic peripheral inflammation modifies pain sensitivity that leads to the phenomenon of allodynia and hyperalgesia.

Objectives:

We study here, in vivo, the role of P2X4 on the excitability of dorsal horn neurons (DHNs) in naive or pathological context.

Methods:

We recorded DHNs in vivo in anesthetized wild-type or P2RX4^−/− mice. We measured nociceptive integration and short-term sensitization by DHNs both in naive and inflamed mice.

Results:

Our results indicate that P2X4 alter neuronal excitability only in the pathological context of peripheral inflammation. Consequently, excitability of DHNs from inflamed P2RX4^−/− mice remains similar to naive animals.

Conclusion:

These results confirm the prominent role of P2X4 in inflammatory pain context and demonstrate that P2X4 are also involved in the hyperexcitability of DHNs.

Fast Green FCF Alleviates Pain Hypersensitivity and Down-Regulates the Levels of Spinal P2X4 Expression and Pro-inflammatory Cytokines in a Rodent Inflammatory Pain Model

Fast Green FCF (FGF), a biocompatible dye, recently drew attention as a potential drug to treat amyloid-deposit diseases due to its effects against amyloid fibrillogenesis in vitro and a high degree of safety. However, its role in inflammatory pain is unknown. Our study aimed to investigate the effect of FGF in the inflammatory pain model induced by complete Freund’s adjuvant (CFA) and to identify the associated mechanisms. We found that systemic administration of FGF reversed mechanical and thermal pain hypersensitivity evoked by CFA in a dose-dependent manner. FGF treatment decreased purinergic spinal P2X4 expression in the spinal cord of CFA-inflamed mice. FGF also down-regulated spinal and peripheral pro-inflammatory cytokines [tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and interleukin-6 (IL-6)], but did not alter the spinal level of nerve growth factor (NGF) or brain-derived neurotrophic factor (BDNF). In conclusion, our results suggest the potential of FGF for controlling the progress of inflammatory pain.

P2rx4 deficiency in mice alleviates allergen-induced airway inflammation

Compelling evidences point out a crucial role for extracellular nucleotides such as adenosine triphosphate (ATP) during inflammatory conditions. Once released into the extracellular space, ATP modulates migration, maturation and function of various inflammatory cells via activating of purinergic receptors of the P2Y- and P2X- family. P2RX4 is an ATP-guided ion channel expressed on structural cells such as alveolar epithelial and smooth muscle cells as well as inflammatory cells including macrophages, dendritic cells (DCs) and T cells. P2RX4 has been shown to interact with P2RX7 and promote NLRP3 inflammasome activation. Although P2RX7 has already been implicated in allergic asthma, the role of P2RX4 in airway inflammation has not been elucidated yet. Therefore, we used a selective pharmacological antagonist and genetic ablation to investigate the role of P2RX4 in an ovalbumin (OVA) driven model of allergen-induced airway inflammation (AAI). Both, P2RX4 antagonist 5-BDBD treatment and P2rx4 deficiency resulted in an alleviated broncho alveolar lavage fluid eosinophilia, peribronchial inflammation, Th2 cytokine production and bronchial hyperresponsiveness. Furthermore, P2rx4-deficient bone marrow derived DCs (BMDCs) showed a reduced IL-1ß production in response to ATP accompanied by a decreased P2rx7 expression and attenuated Th2 priming capacity compared to wild type (WT) BMDCs in vitro. Moreover, mice adoptively transferred with P2rx4-deficient BMDCs exhibit a diminished AAI in vivo. In conclusion our data suggests that P2RX4-signaling contributes to AAI pathogenesis by regulating DC mediated Th2 cell priming via modulating IL-1ß secretion and selective P2RX4-antagonists might be a new therapeutic option for allergic asthma.

Role of P2X4 Receptor in Mouse Voiding Function

Purinergic signalling plays an important role in the regulation of bladder smooth muscle (BSM) contractility, and P2X₄ receptor is expressed in the bladder wall, where it may act by forming heteromeric receptors with P2X₁, the major purinergic force-generating muscle receptor. To test this hypothesis, we examined mouse BSM contractile properties in the absence and presence of selective P2X₁ (NF449 & NF279) and P2X₄ antagonists (5-BDBD). These drugs inhibited BSM purinergic contraction only partially, suggesting the possibility of a heteromeric receptor. However, carefully controlled co-immunoprecipitation experiments indicated that P2X₁ and P2X₄ do not form physically linked heteromers. Furthermore, immunofluorescence staining showed that P2X₄ is not present in mouse BSM per se, but in an unknown cellular structure among BSM bundles. To investigate whether deletion of P2X₄ could impact voiding function in vivo, P2X₄ null mice were characterized. P2X₄ null mice had normal bladder weight and morphology, normal voiding spot size and number by voiding spot assay, normal voiding interval, pressure and compliance by cystometrogram, and normal BSM contractility by myography. In conclusion, these data strongly suggest that P2X₄ is not present in mouse BSM cells, does not affect smooth muscle contractility and that mice null for P2X₄ exhibit normal voiding function.

Microglia in health and pain: impact of noxious early life events

NEW FINDINGS

What is the topic of this review? This review discusses the origins and development of microglia, and how stress, pain or inflammation in early life disturbs microglial function during critical developmental periods, leading to altered pain sensitivity and/or increased risk of chronic pain in later life. What advances does it highlight? We highlight recent advances in understanding how disrupted microglial function impacts the developing nervous system and the consequences for pain processing and susceptibility for development of chronic pain in later life. The discovery of microglia is accredited to Pío del Río-Hortega, who recognized this 'third element' of CNS cells as being morphologically distinct from neurons and astrocytes. For decades after this finding, microglia were altogether ignored or relegated as simply being support cells. Emerging from virtual obscurity, microglia have now gained notoriety as immune cells that assume a leading role in the development, maintenance and protection of a healthy CNS. Pioneering studies have recently shed light on the origins of microglia, their role in the developing nervous system and the complex roles they play beyond the immune response. These studies reveal that altered microglial function can have a profoundly negative impact on the developing brain and may be a determinant in a range of neurodevelopmental disorders and neurodegenerative diseases. The realization that aberrant microglial function also critically underlies chronic pain, a debilitating disorder that afflicts over 1.5 billion people worldwide, was a major conceptual leap forward in the pain field. Adding to this advance is emerging evidence that early life noxious experiences can have a long-lasting impact on central pain processing and adult pain sensitivity. With microglia now coming of age, in this review we examine the association between adverse early life events, such as stress, injury or inflammation, and the influence of sex differences, on the role of microglia in pain physiology in adulthood.

Basic/Translational Development of Forthcoming Opioid- and Nonopioid-Targeted Pain Therapeutics

Opioids represent an efficacious therapeutic modality for some, but not all pain states. Singular reliance on opioid therapy for pain management has limitations, and abuse potential has deleterious consequences for patient and society. Our understanding of pain biology has yielded insights and opportunities for alternatives to conventional opioid agonists. The aim is to have efficacious therapies, with acceptable side effect profiles and minimal abuse potential, which is to say an absence of reinforcing activity in the absence of a pain state. The present work provides a nonexclusive overview of current drug targets and potential future directions of research and development. We discuss channel activators and blockers, including sodium channel blockers, potassium channel activators, and calcium channel blockers; glutamate receptor-targeted agents, including N-methyl-D-aspartate, α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid, and metabotropic receptors. Furthermore, we discuss therapeutics targeted at γ-aminobutyric acid, α2-adrenergic, and opioid receptors. We also considered antagonists of angiotensin 2 and Toll receptors and agonists/antagonists of adenosine, purine receptors, and cannabinoids. Novel targets considered are those focusing on lipid mediators and anti-inflammatory cytokines. Of interest is development of novel targeting strategies, which produce long-term alterations in pain signaling, including viral transfection and toxins. We consider issues in the development of druggable molecules, including preclinical screening. While there are examples of successful translation, mechanistically promising preclinical candidates may unexpectedly fail during clinical trials because the preclinical models may not recapitulate the particular human pain condition being addressed. Molecular target characterization can diminish the disconnect between preclinical and humans' targets, which should assist in developing nonaddictive analgesics.

Blockade of P2X4 Receptors Inhibits Neuropathic Pain-Related Behavior by Preventing MMP-9 Activation and, Consequently, Pronociceptive Interleukin Release in a Rat Model

Neuropathic pain is still an extremely important problem in today's medicine because opioids, which are commonly used to reduce pain, have limited efficacy in this type of pathology. Therefore, complementary therapy is needed. Our experiments were performed in rats to evaluate the contribution of the purinergic system, especially P2X4 receptor (P2X4R), in the modulation of glia activation and, consequently, the levels of nociceptive interleukins after chronic constriction injury (CCI) of the right sciatic nerve, a rat model of neuropathic pain. Moreover, we studied how intrathecal (ith.) injection of a P2X4R antagonist Tricarbonyldichlororuthenium (II) dimer (CORM-2) modulates nociceptive transmission and opioid effectiveness in the CCI model. Our results demonstrate that repeated ith. administration of CORM-2 once daily (20 μg/5 μl, 16 and 1 h before CCI and then daily) for eight consecutive days significantly reduced pain-related behavior and activation of both spinal microglia and/or astroglia induced by CCI. Moreover, even a single administration of CORM-2 on day 7 after CCI attenuated mechanical and thermal hypersensitivity as efficiently as morphine and buprenorphine. In addition, using Western blot, we have shown that repeated ith. administration of CORM-2 lowers the CCI-elevated level of MMP-9 and pronociceptive interleukins (IL-1β, IL-18, IL-6) in the dorsal L4-L6 spinal cord and/or DRG. Furthermore, in parallel, CORM-2 upregulates spinal IL-1Ra; however, it does not influence other antinociceptive factors, IL-10 and IL-18BP. Additionally, based on our biochemical results, we hypothesize that p38MAPK, ERK1/2 and PI3K/Akt but not the NLRP3/Caspase-1 pathway are partly involved in the CORM-2 analgesic effects in rat neuropathic pain. Our data provide new evidence that P2X4R may indeed play a significant role in neuropathic pain development by modulating neuroimmune interactions in the spinal cord and DRG, suggesting that its blockade may have potential therapeutic utility.

Current and Future Issues in the Development of Spinal Agents for the Management of Pain

Targeting analgesic drugs for spinal delivery reflects the fact that while the conscious experience of pain is mediated supraspinally, input initiated by high intensity stimuli, tissue injury and/or nerve injury is encoded at the level of the spinal dorsal horn and this output informs the brain as to the peripheral environment. This encoding process is subject to strong upregulation resulting in hyperesthetic states and downregulation reducing the ongoing processing of nociceptive stimuli reversing the hyperesthesia and pain processing. The present review addresses the biology of spinal nociceptive processing as relevant to the effects of intrathecally-delivered drugs in altering pain processing following acute stimulation, tissue inflammation/injury and nerve injury. The review covers i) the major classes of spinal agents currently employed as intrathecal analgesics (opioid agonists, alpha 2 agonists; sodium channel blockers; calcium channel blockers; NMDA blockers; GABA A/B agonists; COX inhibitors; ii) ongoing developments in the pharmacology of spinal therapeutics focusing on less studied agents/targets (cholinesterase inhibition; Adenosine agonists; iii) novel intrathecal targeting methodologies including gene-based approaches (viral vectors, plasmids, interfering RNAs); antisense, and toxins (botulinum toxins; resniferatoxin, substance P Saporin); and iv) issues relevant to intrathecal drug delivery (neuraxial drug distribution), infusate delivery profile, drug dosing, formulation and principals involved in the preclinical evaluation of intrathecal drug safety.

sec-Butylpropylacetamide (SPD), a new amide derivative of valproic acid for the treatment of neuropathic and inflammatory pain

Chronic pain is a multifactorial disease comprised of both inflammatory and neuropathic components that affect ∼20% of the world's population. sec-Butylpropylacetamide (SPD) is a novel amide analogue of valproic acid (VPA) previously shown to possess a broad spectrum of anticonvulsant activity. In this study, we defined the pharmacokinetic parameters of SPD in rat and mouse, and then evaluated its antinociceptive potential in neuropathic and acute inflammatory pain models. In the sciatic nerve ligation (SNL) model of neuropathic pain, SPD was equipotent to gabapentin and more potent than its parent compound VPA. SPD also showed either higher or equal potency to VPA in the formalin, carrageenan, and writhing tests of inflammatory pain. SPD showed no effects on compound action potential properties in a sciatic nerve preparation, suggesting that its mechanism of action is distinct from local anesthetics and membrane stabilizing drugs. SPD's activity in both neuropathic and inflammatory pain warrants its development as a potential broad-spectrum anti-nociceptive drug.

Animal models of chronic pain: Advances and challenges for clinical translation

Chronic pain is a global problem that has reached epidemic proportions. An estimated 20% of adults suffer from pain, and another 10% are diagnosed with chronic pain each year (Goldberg and McGee, ). Despite the high prevalence of chronic pain (an estimated 1.5 billion people are afflicted worldwide), much remains to be understood about the underlying causes of this condition, and there is an urgent requirement for better pain therapies. The discovery of novel targets and the development of better analgesics rely on an assortment of preclinical animal models; however, there are major challenges to translating discoveries made in animal models to realized pain therapies in humans. This review discusses common animal models used to recapitulate clinical chronic pain conditions (such as neuropathic, inflammatory, and visceral pain) and the methods for assessing the sensory and affective components of pain in animals. We also discuss the advantages and limitations of modeling chronic pain in animals as well as highlighting strategies for improving the predictive validity of preclinical pain studies. © 2016 Wiley Periodicals, Inc.

Nociceptive transmission and modulation via P2X receptors in central pain syndrome

Painful sensations are some of the most frequent complaints of patients who are admitted to local medical clinics. Persistent pain varies according to its causes, often resulting from local tissue damage or inflammation. Central somatosensory pathway lesions that are not adequately relieved can consequently cause central pain syndrome or central neuropathic pain. Research on the molecular mechanisms that underlie this pathogenesis is important for treating such pain. To date, evidence suggests the involvement of ion channels, including adenosine triphosphate (ATP)-gated cation channel P2X receptors, in central nervous system pain transmission and persistent modulation upon and following the occurrence of neuropathic pain. Several P2X receptor subtypes, including P2X2, P2X3, P2X4, and P2X7, have been shown to play diverse roles in the pathogenesis of central pain including the mediation of fast transmission in the peripheral nervous system and modulation of neuronal activity in the central nervous system. This review article highlights the role of the P2X family of ATP receptors in the pathogenesis of central neuropathic pain and pain transmission. We discuss basic research that may be translated to clinical application, suggesting that P2X receptors may be treatment targets for central pain syndrome.

Pain and Poppies: The Good, the Bad, and the Ugly of Opioid Analgesics

Treating pain is one of the most difficult challenges in medicine and a key facet of disease management. The isolation of morphine by Friedrich Sertürner in 1804 added an essential pharmacological tool in the treatment of pain and spawned the discovery of a new class of drugs known collectively as opioid analgesics. Revered for their potent pain-relieving effects, even Morpheus the god of dreams could not have dreamt that his opium tincture would be both a gift and a burden to humankind. To date, morphine and other opioids remain essential analgesics for alleviating pain. However, their use is plagued by major side effects, such as analgesic tolerance (diminished pain-relieving effects), hyperalgesia (increased pain sensitivity), and drug dependence. This review highlights recent advances in understanding the key causes of these adverse effects and explores the effect of chronic pain on opioid reward.

SIGNIFICANCE STATEMENT

Chronic pain is pervasive and afflicts >100 million Americans. Treating pain in these individuals is notoriously difficult and often requires opioids, one of the most powerful and effective classes of drugs used for controlling pain. However, their use is plagued by major side effects, such as a loss of pain-relieving effects (analgesic tolerance), paradoxical pain (hyperalgesia), and addiction. Despite the potential side effects, opioids remain the pharmacological cornerstone of modern pain therapy. This review highlights recent breakthroughs in understanding the key causes of these adverse effects and explores the cellular control of opioid systems in reward and aversion. The findings will challenge traditional views of the good, the bad, and the ugly of opioids.

Purinergic Mechanisms and Pain

There is a brief introductory summary of purinergic signaling involving ATP storage, release, and ectoenzymatic breakdown, and the current classification of receptor subtypes for purines and pyrimidines. The review then describes purinergic mechanosensory transduction involved in visceral, cutaneous, and musculoskeletal nociception and on the roles played by receptor subtypes in neuropathic and inflammatory pain. Multiple purinoceptor subtypes are involved in pain pathways both as an initiator and modulator. Activation of homomeric P2X3 receptors contributes to acute nociception and activation of heteromeric P2X2/3 receptors appears to modulate longer-lasting nociceptive sensitivity associated with nerve injury or chronic inflammation. In neuropathic pain activation of P2X4, P2X7, and P2Y12 receptors on microglia may serve to maintain nociceptive sensitivity through complex neural-glial cell interactions and antagonists to these receptors reduce neuropathic pain. Potential therapeutic approaches involving purinergic mechanisms will be discussed.

Paroxetine attenuates the development and existing pain in a rat model of neurophatic pain

BACKGROUND

P2X4 receptor (P2X4R), a purinoceptor expressed in activated spinal microglia, plays a key role in the pathogenesis of neuropathic pain. Spinal nerve injury induces up-regulation of P2X4R on activated microglia in the spinal cord, and blockade of this receptor can reduce neuropathic pain. The present study was undertaken to determine whether paroxetine, an inhibitor of P2X4R, could attenuate allodynia and hyperalgesia in chronic constriction injury (CCI) model of neuropathic pain when used preemptively or after the sciatic nerve injury.

METHODS

Male Wistar rats (150-200 g, n = 6) were divided into 3 different groups: 1- CCI vehicle-treated group, 2- Sham group, and 3- CCI paroxetine-treated group. Paroxetine (10 mg/kg, i.p.) was administered 1 h before surgery and continued daily until day 14. In other part of the study, paroxetine (10 mg/kg, i.p.) was administered at day 7 post injury and continued daily until day 14. von Frey filaments for mechanical allodynia and analgesia meter for thermal hyperalgesia were used to assay pain behavior.

RESULTS

In a preventive paradigm, paroxetine significantly attenuated both mechanical allodynia and thermal hyperalgesia (P<0.001). A significant decrease in pain behavior was seen with paroxetine on existing allodynia (P<0.001) and hyperalgesia (P<0.01) when initiated at day 7 post injury.

CONCLUSION

It seems that paroxetine can attenuate pain behavior when administered before and also after sciatic nerve injury in CCI model of neuropathic pain.

P2X4 receptor regulates alcohol-induced responses in microglia

Mounting evidence indicates that alcohol-induced neuropathology may result from multicellular responses in which microglia cells play a prominent role. Purinergic receptor signaling plays a key role in regulating microglial function and, more importantly, mediates alcohol-induced effects. Our findings demonstrate that alcohol increases expression of P2X4 receptor (P2X4R), which alters the function of microglia, including calcium mobilization, migration and phagocytosis. Our results show a significant up-regulation of P2X4 gene expression as analyzed by real-time qPCR (***p < 0.002) and protein expression as analyzed by flow cytometry (**p < 0.004) in embryonic stem cell-derived microglial cells (ESdM) after 48 hours of alcohol treatment, as compared to untreated controls. Calcium mobilization in ethanol treated ESdM cells was found to be P2X4R dependent using 5-BDBD, a P2X4R selective antagonist. Alcohol decreased migration of microglia towards fractalkine (CX3CL1) by 75 % following 48 h of treatment compared to control (***p < 0.001). CX3CL1-dependent migration was confirmed to be P2X4 receptor-dependent using the antagonist 5-BDBD, which reversed the effects as compared to alcohol alone (***p < 0.001). Similarly, 48 h of alcohol treatment significantly decreased phagocytosis of microglia by 15 % compared to control (*p < 0.05). 5-BDBD pre-treatment prior to alcohol treatment significantly increased microglial phagocytosis (***p < 0.001). Blocking P2X4R signaling with 5-BDBD decreased the level of calcium mobilization compared to ethanol treatment alone. These findings demonstrate that P2X4 receptor may play a role in modulating microglial function in the context of alcohol abuse.

Central P2Y12 receptor blockade alleviates inflammatory and neuropathic pain and cytokine production in rodents

In this study the role of P2Y₁₂ receptors (P2Y₁₂R) was explored in rodent models of inflammatory and neuropathic pain and in acute thermal nociception. In correlation with their activity to block the recombinant human P2Y₁₂R, the majority of P2Y₁₂R antagonists alleviated mechanical hyperalgesia dose-dependently, following intraplantar CFA injection, and after partial ligation of the sciatic nerve in rats. They also caused an increase in thermal nociceptive threshold in the hot plate test. Among the six P2Y₁₂R antagonists evaluated in the pain studies, the selective P2Y₁₂ receptor antagonist PSB-0739 was most potent upon intrathecal application.

P2Y₁₂R mRNA and IL-1β protein were time-dependently overexpressed in the rat hind paw and lumbar spinal cord following intraplantar CFA injection. This was accompanied by the upregulation of TNF-α, IL-6 and IL-10 in the hind paw. PSB-0739 (0.3 mg/kg i.t.) attenuated CFA-induced expression of cytokines in the hind paw and of IL-1β in the spinal cord. Subdiaphragmatic vagotomy and the α7 nicotinic acetylcholine receptor antagonist MLA occluded the effect of PSB-0739 (i.t.) on pain behavior and peripheral cytokine induction. Denervation of sympathetic nerves by 6-OHDA pretreatment did not affect the action of PSB-0739. PSB-0739, in an analgesic dose, did not influence motor coordination and platelet aggregation. Genetic deletion of the P2Y₁₂R in mice reproduced the effect of P2Y₁₂R antagonists on mechanical hyperalgesia in inflammatory and neuropathic pain models, on acute thermal nociception and on the induction of spinal IL-1β.

Here we report the robust involvement of the P2Y₁₂R in inflammatory pain. The anti-hyperalgesic effect of P2Y₁₂R antagonism could be mediated by the inhibition of both central and peripheral cytokine production and involves α7-receptor mediated efferent pathways.

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Pharmacological blockade of P2Y₁₂ receptors alleviates inflammatory and neuropathic pain.

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Central inhibition of P2Y₁₂ receptors attenuates cytokine production in the spinal cord.

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Central P2Y₁₂ receptor inhibition attenuates cytokine production in the inflamed hind paw.

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α7-Receptors mediate the effect of P2Y₁₂ receptor blockade on hyperalgesia and cytokine level.

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Genetic deletion of P2Y₁₂ receptors alleviates inflammatory, neuropathic and acute pain.

Purinergic signalling: pathophysiology and therapeutic potential

The article begins with a review of the main conceptual steps involved in the development of our understanding of purinergic signalling, including non-adrenergic, non-cholinergic (NANC) neurotransmission; identification of ATP as a NANC transmitter; purinergic cotransmission; recognition of two families of purinoceptors [P1 (adenosine) and P2 (ATP/ADP)]; and, later, cloning and characterisation of P1 (G protein-coupled), P2X (ion channel) and P2Y (G protein-coupled) receptor subtypes. Further studies have established the involvement of ATP in synaptic neurotransmission in both ganglia and in the central nervous system; long-term (trophic) purinergic signalling in cell proliferation, differentiation and death occurring in development and regeneration; and short-term purinergic signalling in neurotransmission, neuromodulation and secretion. ATP is released from most cell types in response to gentle mechanical stimulation and is rapidly degraded to adenosine by ecto-nucleotidases. This review then focuses on the pathophysiology of purinergic signalling in a wide variety of systems, including urinogenital, cardiovascular, airway, musculoskeletal and gastrointestinal. Consideration is also given to the involvement of purinoceptors in pain, cancer and diseases of the central nervous system. Purinergic therapeutic approaches for the treatment of some of these diseases are discussed.

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.

Sensitization of cutaneous neuronal purinergic receptors contributes to endothelin-1-induced mechanical hypersensitivity

Endothelin (ET-1), an endogenous peptide with a prominent role in cutaneous pain, causes mechanical hypersensitivity in the rat hind paw, partly through mechanisms involving local release of algogenic molecules in the skin. The present study investigated involvement of cutaneous ATP, which contributes to pain in numerous animal models. Pre-exposure of ND7/104 immortalized sensory neurons to ET-1 (30nM) for 10min increased the proportion of cells responding to ATP (2μM) with an increase in intracellular calcium, an effect prevented by the ETA receptor-selective antagonist BQ-123. ET-1 (3nM) pre-exposure also increased the proportion of isolated mouse dorsal root ganglion neurons responding to ATP (0.2-0.4μM). Blocking ET-1-evoked increases in intracellular calcium with the IP3 receptor antagonist 2-APB did not inhibit sensitization to ATP, indicating a mechanism independent of ET-1-mediated intracellular calcium increases. ET-1-sensitized ATP calcium responses were largely abolished in the absence of extracellular calcium, implicating ionotropic P2X receptors. Experiments using quantitative polymerase chain reaction and receptor-selective ligands in ND7/104 showed that ET-1-induced sensitization most likely involves the P2X4 receptor subtype. ET-1-sensitized calcium responses to ATP were strongly inhibited by broad-spectrum (TNP-ATP) and P2X4-selective (5-BDBD) antagonists, but not antagonists for other P2X subtypes. TNP-ATP and 5-BDBD also significantly inhibited ET-1-induced mechanical sensitization in the rat hind paw, supporting a role for purinergic receptor sensitization in vivo. These data provide evidence that mechanical hypersensitivity caused by cutaneous ET-1 involves an increase in the neuronal sensitivity to ATP in the skin, possibly due to sensitization of P2X4 receptors.

Introduction and perspective, historical note

P2 nucleotide receptors were proposed to consist of two subfamilies based on pharmacology in 1985, named P2X and P2Y receptors. Later, this was confirmed following cloning of the receptors for nucleotides and studies of transduction mechanisms in the early 1990s. P2X receptors are ion channels and seven subtypes are recognized that form trimeric homomultimers or heteromultimers. P2X receptors are involved in neuromuscular and synaptic neurotransmission and neuromodulation. They are also expressed on many types of non-neuronal cells to mediate smooth muscle contraction, secretion, and immune modulation. The emphasis in this review will be on the pathophysiology of P2X receptors and therapeutic potential of P2X receptor agonists and antagonists for neurodegenerative and inflammatory disorders, visceral and neuropathic pain, irritable bowel syndrome, diabetes, kidney failure, bladder incontinence and cancer, as well as disorders if the special senses, airways, skin, cardiovascular, and musculoskeletal systems.

Hypoxia Attenuates Purinergic P2X Receptor-Induced Inflammatory Gene Expression in Brainstem Microglia

Hypoxia and increased extracellular nucleotides are frequently coincident in the brainstem. Extracellular nucleotides are potent modulators of microglial inflammatory gene expression via P2X purinergic receptor activation. Although hypoxia is also known to modulate inflammatory gene expression, little is known about how hypoxia or P2X receptor activation alone affects inflammatory molecule production in brainstem microglia, nor how hypoxia and P2X receptor signaling interact when they occur together. In the study reported here, we investigated the ability of a brief episode of hypoxia (2 hours) in the presence and absence of the nonselective P2X receptor agonist 2′(3′)-O-(4-benzoylbenzoyl)adenosine-5′-triphosphate (BzATP) to promote inflammatory gene expression in brainstem microglia in adult rats. We evaluated inducible nitric oxide synthase (iNOS), tumor necrosis factor alpha (TNFα), and interleukin (IL)-6 messenger RNA levels in immunomagnetically isolated brainstem microglia. While iNOS and IL-6 gene expression increased with hypoxia and BzATP alone, TNFα expression was unaffected. Surprisingly, BzATP-induced inflammatory effects were lost after hypoxia, suggesting that hypoxia impairs proinflammatory P2X-receptor signaling. We also evaluated the expression of key P2X receptors activated by BzATP, namely P2X1, P2X4, and P2X7. While hypoxia did not alter their expression, BzATP upregulated P2X4 and P2X7 mRNAs; these effects were ablated in hypoxia. Although both P2X4 and P2X7 receptor expression correlated with increased microglial iNOS and IL-6 levels in microglia from normoxic rats, in hypoxia, P2X7 only correlated with IL-6, and P2X4 correlated only with iNOS. In addition, correlations between P2X7 and P2X4 were lost following hypoxia, suggesting that P2X4 and P2X7 receptor signaling differs in normoxia and hypoxia. Together, these data suggest that hypoxia suppresses P2X receptor-induced inflammatory gene expression, indicating a potentially immunosuppressive role of extracellular nucleotides in brainstem microglia following exposure to hypoxia.

Natural Products as a Source for New Anti-Inflammatory and Analgesic Compounds through the Inhibition of Purinergic P2X Receptors

Natural products have reemerged in traditional medicine as a potential source of new molecules or phytomedicines to help with health disorders. It has been established that members of the P2X subfamily, ATP-gated ion channels, are crucial to the inflammatory process and pain signalization. As such, several preclinical studies have demonstrated that P2X2R, P2X3R, P2X4R and P2X7R are promising pharmacological targets to control inflammatory and pain disorders. Several studies have indicated that natural products could be a good source of the new specific molecules needed for the treatment of diseases linked to inflammation and pain disorders through the regulation of these receptors. Herein, we discuss and give an overview of the applicability of natural products as a source to obtain P2X receptors (P2XR) selective antagonists for use in clinical treatment, which require further investigation.

Purinergic mechanisms and pain--an update

There is a brief summary of the background literature about purinergic signalling. The review then considers purinergic mechanosensory transduction involved in visceral, cutaneous and musculoskeletal nociception and on the roles played by P2X3, P2X2/3, P2X4, P2X7 and P2Y₁₂ receptors in neuropathic and inflammatory pain. Current developments of compounds for the therapeutic treatment of both visceral and neuropathic pain are discussed.

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.

Neuromodulation by extracellular ATP and P2X receptors in the CNS

Extracellular adenosine 5' triphosphate (ATP) is a widespread cell-to-cell signaling molecule in the brain, where it activates cell surface P2X and P2Y receptors. P2X receptors define a protein family unlike other neurotransmitter-gated ion channels in terms of sequence, subunit topology, assembly, and architecture. Within milliseconds of binding ATP, they catalyze the opening of a cation-selective pore. However, recent data show that P2X receptors often underlie neuromodulatory responses on slower time scales of seconds or longer. Herein, we review these findings at molecular, cellular and systems levels. We propose that, while P2X receptors are fast ligand-gated cation channels, they are most adept at mediating slow neuromodulatory functions that are more widespread and more physiologically utilized than fast ATP synaptic transmission in the CNS.