Nucleotides control the excitability of sensory neurons via two P2Y receptors and a bifurcated signaling cascade

Satellite glial cells in sensory ganglia play a wider role in chronic pain via multiple mechanisms

Satellite glial cells are unique glial cells that surround the cell body of primary sensory neurons. An increasing body of evidence suggests that in the presence of inflammation and nerve damage, a significant number of satellite glial cells become activated, thus triggering a series of functional changes. This suggests that satellite glial cells are closely related to the occurrence of chronic pain. In this review, we first summarize the morphological structure, molecular markers, and physiological functions of satellite glial cells. Then, we clarify the multiple key roles of satellite glial cells in chronic pain, including gap junction hemichannel Cx43, membrane channel Pannexin1, K channel subunit 4.1, ATP, purinergic P2 receptors, and a series of additional factors and their receptors, including tumor necrosis factor, glutamate, endothelin, and bradykinin. Finally, we propose that future research should focus on the specific sorting of satellite glial cells, and identify genomic differences between physiological and pathological conditions. This review provides an important perspective for clarifying mechanisms underlying the peripheral regulation of chronic pain and will facilitate the formulation of new treatment plans for chronic pain.

Selective KCNQ2/3 Potassium Channel Opener ICA-069673 Inhibits Excitability in Mouse Vagal Sensory Neurons

Heightened excitability of vagal sensory neurons in inflammatory visceral diseases contributes to unproductive and difficult-to-treat neuronally based symptoms such as visceral pain and dysfunction. Identification of targets and modulators capable of regulating the excitability of vagal sensory neurons may lead to novel therapeutic options. KCNQ1-KCNQ5 genes encode KV7.1-7.5 potassium channel α-subunits. Homotetrameric or heterotetrameric KV7.2-7.5 channels can generate the so-called M-current (IM) known to decrease the excitability of neurons including visceral sensory neurons. This study aimed to address the hypothesis that KV7.2/7.3 channels are key regulators of vagal sensory neuron excitability by evaluating the effects of KCNQ2/3-selective activator, ICA-069673, on IM in mouse nodose neurons and determining its effects on excitability and action potential firings using patch clamp technique. The results showed that ICA-069673 enhanced IM density, accelerated the activation, and delayed the deactivation of M-channels in a concentration-dependent manner. ICA-069673 negatively shifted the voltage-dependent activation of IM and increased the maximal conductance. Consistent with its effects on IM, ICA-069673 induced a marked hyperpolarization of resting potential and reduced the input resistance. The hyperpolarizing effect was more pronounced in partially depolarized neurons. Moreover, ICA-069673 caused a 3-fold increase in the minimal amount of depolarizing current needed to evoke an action potential, and significantly limited the action potential firings in response to sustained suprathreshold stimulations. ICA-069673 had no effect on membrane currents when Kcnq2 and Kcnq3 were deleted. These results indicate that opening KCNQ2/3-mediated M-channels is sufficient to suppress the excitability and enhance spike accommodation in vagal visceral sensory neurons. SIGNIFICANCE STATEMENT: This study supports the hypothesis that selectively activating KCNQ2/3-mediated M-channels is sufficient to suppress the excitability and action potential firings in vagal sensory neurons. These results provide evidence in support of further investigations into the treatment of various visceral disorders that involve nociceptor hyperexcitability with selective KCNQ2/3 M-channel openers.

The Similar and Distinct Roles of Satellite Glial Cells and Spinal Astrocytes in Neuropathic Pain

Preclinical studies have identified glial cells as pivotal players in the genesis and maintenance of neuropathic pain after nerve injury associated with diabetes, chemotherapy, major surgeries, and virus infections. Satellite glial cells (SGCs) in the dorsal root and trigeminal ganglia of the peripheral nervous system (PNS) and astrocytes in the central nervous system (CNS) express similar molecular markers and are protective under physiological conditions. They also serve similar functions in the genesis and maintenance of neuropathic pain, downregulating some of their homeostatic functions and driving pro-inflammatory neuro-glial interactions in the PNS and CNS, i.e., "gliopathy". However, the role of SGCs in neuropathic pain is not simply as "peripheral astrocytes". We delineate how these peripheral and central glia participate in neuropathic pain by producing different mediators, engaging different parts of neurons, and becoming active at different stages following nerve injury. Finally, we highlight the recent findings that SGCs are enriched with proteins related to fatty acid metabolism and signaling such as Apo-E, FABP7, and LPAR1. Targeting SGCs and astrocytes may lead to novel therapeutics for the treatment of neuropathic pain.

Peripheral mechanisms of chronic pain

Acutely, pain serves to protect us from potentially harmful stimuli, however damage to the somatosensory system can cause maladaptive changes in neurons leading to chronic pain. Although acute pain is fairly well controlled, chronic pain remains difficult to treat. Chronic pain is primarily a neuropathic condition, but studies examining the mechanisms underlying chronic pain are now looking beyond afferent nerve lesions and exploring new receptor targets, immune cells, and the role of the autonomic nervous system in contributing chronic pain conditions. The studies outlined in this review reveal how chronic pain is not only confined to alterations in the nervous system and presents findings on new treatment targets and for this debilitating disease.

Disrupted Association of Sensory Neurons With Enveloping Satellite Glial Cells in Fragile X Mouse Model

Among most prevalent deficits in individuals with Fragile X syndrome (FXS) is hypersensitivity to sensory stimuli and somatosensory alterations. Whether dysfunction in peripheral sensory system contributes to these deficits remains poorly understood. Satellite glial cells (SGCs), which envelop sensory neuron soma, play critical roles in regulating neuronal function and excitability. The potential contributions of SGCs to sensory deficits in FXS remain unexplored. Here we found major structural defects in sensory neuron-SGC association in the dorsal root ganglia (DRG), manifested by aberrant covering of the neuron and gaps between SGCs and the neuron along their contact surface. Single-cell RNAseq analyses demonstrated transcriptional changes in both neurons and SGCs, indicative of defects in neuronal maturation and altered SGC vesicular secretion. We validated these changes using fluorescence microscopy, qPCR, and high-resolution transmission electron microscopy (TEM) in combination with computational analyses using deep learning networks. These results revealed a disrupted neuron-glia association at the structural and functional levels. Given the well-established role for SGCs in regulating sensory neuron function, altered neuron-glia association may contribute to sensory deficits in FXS.

A peptide encoded within a 5' untranslated region promotes pain sensitization in mice

ABSTRACT

Translational regulation permeates neuronal function. Nociceptors are sensory neurons responsible for the detection of harmful stimuli. Changes in their activity, termed plasticity, are intimately linked to the persistence of pain. Although inhibitors of protein synthesis robustly attenuate pain-associated behavior, the underlying targets that support plasticity are largely unknown. Here, we examine the contribution of protein synthesis in regions of RNA annotated as noncoding. Based on analyses of previously reported ribosome profiling data, we provide evidence for widespread translation in noncoding transcripts and regulatory regions of mRNAs. We identify an increase in ribosome occupancy in the 5' untranslated regions of the calcitonin gene-related peptide (CGRP/Calca). We validate the existence of an upstream open reading frame (uORF) using a series of reporter assays. Fusion of the uORF to a luciferase reporter revealed active translation in dorsal root ganglion neurons after nucleofection. Injection of the peptide corresponding to the calcitonin gene-related peptide-encoded uORF resulted in pain-associated behavioral responses in vivo and nociceptor sensitization in vitro. An inhibitor of heterotrimeric G protein signaling blocks both effects. Collectively, the data suggest pervasive translation in regions of the transcriptome annotated as noncoding in dorsal root ganglion neurons and identify a specific uORF-encoded peptide that promotes pain sensitization through GPCR signaling.

Nucleotides Cytidine and Uridine Associated with Vitamin B12 vs B-Complex Vitamins in the Treatment of Low Back Pain: The NUBES Study

Purpose

We report the results of low back pain treatment using a combination of nucleotides, uridine (UTP), cytidine (CMP) and vitamin B₁₂, vs a combination of vitamins B₁, B₆, and B₁₂.

Patients and Methods

Randomized, double-blind, controlled trial, of a 60-day oral treatment: Group A (n=317) receiving nucleotides+B₁₂ and Group B (n=317) receiving B vitamins. The primary endpoint was the percentage of subjects in each group presenting adverse events (AEs). Secondary endpoints were visual analog scale (VAS) pain scores at Visit 2 (day 30) and Visit 3 (day 60) in relation to pretreatment values, Roland–Morris Questionnaire (RMQ) scores and finger-to-floor distance (FFD) (percentage of subjects per group presenting improvement ≥5 points and ≥3cm, respectively).

Results

Seventy-five (24%) and 105 (33%) subjects (P=0.21) presented 133 and 241 AEs, with 3159% of subjects presenting ≥2 AEs (P=0.0019) in Group A and Group B, respectively. Twenty-four subjects in Group B were discontinued due to AEs, while no AE-related discontinuations occurred in Group A (P<0.0001). VAS score reduction after 30 and 60 days of treatment was statistically significant (P<0.0001) in both groups, with Group A showing greater reduction at Visit 2 (P<0.0001). RMQ score improvement ≥5 points occurred in 99% of subjects from each group, and FFD improvement ≥3 cm occurred in all subjects.

Conclusion

Treatment with nucleotides+B₁₂ was associated with a lower number of total AEs, fewer AEs per subject, and no AE-related treatment discontinuation. Pain intensity (VAS) reduction was superior at 30 days of treatment in the nucleotides+B₁₂ group and equivalent between groups at 60 days of treatment. Improvements in efficacy measures RMQ and FFD were observed in both groups at treatment days 30 and 60.

P2Y2 Receptors Mediate Masseter Muscle Mechanical Hypersensitivity in Rats

Purpose

P₂Y₂ receptors (P₂Y₂Rs) are among the various receptors that play an important role in nociception. The goal of this research was to investigate possible P₂Y₂R expression changes in the trigeminal ganglion (TRG) in bilateral masseter muscle (MM) hypersensitivity following unilateral MM inflammation. The impact of unilateral intramasseteric administration of P₂Y₂R antagonist on bilateral MM hypersensitivity was also explored.

Materials and Methods

Bilateral MM hypersensitivity was provoked by unilateral intramasseteric injection of complete Freund’s adjuvant (CFA). The head withdrawal threshold (HWT) was assessed bilaterally 4 days later. Bilateral TRG and MM isolation were followed, and quantitative real-time polymerase chain reaction (qRT-PCR) and histopathological analysis were carried out on these tissues, respectively. The involvement of P₂Y₂Rs in nocifensive behavior was evaluated by administering two doses of P₂Y₂R antagonist AR-C118925 (0.2 or 1 mg/100 μL) in inflamed MM 4 days post-CFA administration. Bilateral HWT was assessed at different time points following antagonist injection.

Results

qRT-PCR analysis demonstrated P₂Y₂R up-regulation in TRG ipsilateral to the site of CFA administration. Compared to the controls, both doses of AR-C118925 injected ipsilateral to the TRG increased the bilateral HWT at 30, 60, 90, and 120 minutes after antagonist administration.

Conclusion

The findings suggest that P₂Y₂Rs may affect MM inflammatory hypersensitivity owing to its up-regulation in the TRG in MM inflammatory pain states.

Approaches for designing and discovering purinergic drugs for gastrointestinal diseases

INTRODUCTION

Purines finely modulate physiological motor, secretory, and sensory functions in the gastrointestinal tract. Their activity is mediated by the purinergic signaling machinery, including receptors and enzymes regulating their synthesis, release, and degradation. Several gastrointestinal dysfunctions are characterized by alterations affecting the purinergic system.

AREAS COVERED

The authors provide an overview on the purinergic receptor signaling machinery, the molecules and proteins involved, and a summary of medicinal chemistry efforts aimed at developing novel compounds able to modulate the activity of each player involved in this machinery. The involvement of purinergic signaling in gastrointestinal motor, secretory, and sensory functions and dysfunctions, and the potential therapeutic applications of purinergic signaling modulators, are then described.

EXPERT OPINION

A number of preclinical and clinical studies demonstrate that the pharmacological manipulation of purinergic signaling represents a viable way to counteract several gastrointestinal diseases. At present, the paucity of purinergic therapies is related to the lack of receptor-subtype-specific agonists and antagonists that are effective in vivo. In this regard, the development of novel therapeutic strategies should be focused to include tools able to control the P1 and P2 receptor expression as well as modulators of the breakdown or transport of purines.

A Role for The P2Y1 Receptor in Nonsynaptic Cross-depolarization in the Rat Dorsal Root Ganglia

Non-synaptic transmission is pervasive throughout the nervous system. It appears especially prevalent in peripheral ganglia, where non-synaptic interactions between neighboring cell bodies have been described in both physiological and pathological conditions, a phenomenon referred to as cross-depolarization (CD) and thought to play a role in sensory processing and chronic pain. CD has been proposed to be mediated by a chemical agent, but its identity has remained elusive. Here, we report that in the rat dorsal root ganglion (DRG), the P2Y1 purinergic receptor (P2RY1) plays an important role in regulating CD. The effect of P2RY1 is cell-type specific: pharmacological blockade of P2RY1 inhibited CD in A-type neurons while enhancing it in C-type neurons. In the nodose ganglion of the vagus, CD requires extracellular calcium in a large percentage of cells. In contrast, we show that in the DRG extracellular calcium appears to play no major role, pointing to a mechanistic difference between the two peripheral ganglia. Furthermore, we show that DRG glial cells also play a cell-type specific role in CD regulation. Fluorocitrate-induced glial inactivation had no effect on A-cells but enhanced CD in C-cells. These findings shed light on the mechanism of CD in the DRG and pave the way for further analysis of non-synaptic neuronal communication in sensory ganglia.

Corneal pain and experimental model development

The cornea is a valuable tissue for studying peripheral sensory nerve structure and regeneration due to its avascularity, transparency, and dense innervation. Somatosensory innervation of the cornea serves to identify changes in environmental stimuli at the ocular surface, thereby promoting barrier function to protect the eye against injury or infection. Due to regulatory demands to screen ocular safety of potential chemical exposure, a need remains to develop functional human tissue models to predict ocular damage and pain using in vitro-based systems to increase throughput and minimize animal use. In this review, we summarize the anatomical and functional roles of corneal innervation in propagation of sensory input, corneal neuropathies associated with pain, and the status of current in vivo and in vitro models. Emphasis is placed on tissue engineering approaches to study the human corneal pain response in vitro with integration of proper cell types, controlled microenvironment, and high-throughput readouts to predict pain induction. Further developments in this field will aid in defining molecular signatures to distinguish acute and chronic pain triggers based on the immune response and epithelial, stromal, and neuronal interactions that occur at the ocular surface that lead to functional outcomes in the brain depending on severity and persistence of the stimulus.

Nociceptor Signalling through ion Channel Regulation via GPCRs

The prime task of nociceptors is the transformation of noxious stimuli into action potentials that are propagated along the neurites of nociceptive neurons from the periphery to the spinal cord. This function of nociceptors relies on the coordinated operation of a variety of ion channels. In this review, we summarize how members of nine different families of ion channels expressed in sensory neurons contribute to nociception. Furthermore, data on 35 different types of G protein coupled receptors are presented, activation of which controls the gating of the aforementioned ion channels. These receptors are not only targeted by more than 20 separate endogenous modulators, but can also be affected by pharmacotherapeutic agents. Thereby, this review provides information on how ion channel modulation via G protein coupled receptors in nociceptors can be exploited to provide improved analgesic therapy.

The paracetamol metabolite N-acetylp-benzoquinone imine reduces excitability in first- and second-order neurons of the pain pathway through actions on KV7 channels

Paracetamol (acetaminophen, APAP) is one of the most frequently used analgesic agents worldwide. It is generally preferred over nonsteroidal anti-inflammatory drugs because it does not cause typical adverse effects resulting from the inhibition of cyclooxygenases, such as gastric ulcers. Nevertheless, inhibitory impact on these enzymes is claimed to contribute to paracetamols mechanisms of action which, therefore, remained controversial. Recently, the APAP metabolites N-arachidonoylaminophenol (AM404) and N-acetyl-p-benzoquinone imine (NAPQI) have been detected in the central nervous system after systemic APAP administration and were reported to mediate paracetamol effects. In contrast to nonsteroidal anti-inflammatory drugs that rather support seizure activity, paracetamol provides anticonvulsant actions, and this dampening of neuronal activity may also form the basis for analgesic effects. Here, we reveal that the APAP metabolite NAPQI, but neither the parent compound nor the metabolite AM404, reduces membrane excitability in rat dorsal root ganglion (DRG) and spinal dorsal horn (SDH) neurons. The observed reduction of spike frequencies is accompanied by hyperpolarization in both sets of neurons. In parallel, NAPQI, but neither APAP nor AM404, increases currents through KV7 channels in DRG and SDH neurons, and the impact on neuronal excitability is absent if KV7 channels are blocked. Furthermore, NAPQI can revert the inhibitory action of the inflammatory mediator bradykinin on KV7 channels but does not affect synaptic transmission between DRG and SDH neurons. These results show that the paracetamol metabolite NAPQI dampens excitability of first- and second-order neurons of the pain pathway through an action on KV7 channels.

P2Y1R is involved in visceral hypersensitivity in rats with experimental irritable bowel syndrome

AIM

To evaluate the role of P2Y1R in visceral hypersensitivity in rats with experimental irritable bowel syndrome.

METHODS

A rat model of irritable bowel syndrome was generated by intra-colonic administration of acetic acid (AA) and assessed by histology and myeloperoxidase (MPO) activity assay. Then P2Y1R expression in the colonic tissue was detected by Western blot. In order to explore the regulatory role of P2Y1R in visceral hypersensitivity, an agonist (MRS2365) and an antagonist (MRS2179) of P2Y1R were intra-colonically administered and effects were tested through a colorectal distension test. The abdominal withdrawal reflex and abdominal electromyography were tested during the course.

RESULTS

Model assessment tests showed an obvious inflammatory reaction that appeared on the 2^nd d after the AA injection, and the inflammatory reaction gradually recovered and almost disappeared on the 7^th d. The model finished on day 8 and showed a clear feature of IBS that had no organic lesion. The average expression of P2Y1R was significantly higher in the AA group than in the naïve group (0.319 ± 0.02 vs 0.094 ± 0.016, P < 0.001). MRS2365 could effectively raise the colonic hypersensitivity status at intervention doses of 10 (AUC value from 0.30 ± 0.089 to 1.973 ± 0.127 mv·s, P < 0.01) and 100 μmol/L (AUC value from 0.290 ± 0.079 to 1.983 ± 0.195 mv·s, P < 0.01); MRS2179 could effectively reduce the hypersensitivity status at intervention dose of 100 μmol/L (from a mean baseline AUC value of 1.587 ± 0.099 mv·s to 0.140 ± 0.089 mv·s, P < 0.0001). Differences between the MRS2179 group (1.88 ± 1.45) and either the MRS2365 group (3.96 ± 0.19) or the combined treatment (MRS2179 and MRS2365) group (3.28 ± 0.11) were significant (P < 0.01).

CONCLUSION

P2Y1R plays a regulatory role in visceral hypersensitivity in rats with experimental IBS. Specific antagonists of P2Y1R may have potential therapeutic value in treating abdominal pain in IBS.

Control of sensory neuron excitability by serotonin involves 5HT2C receptors and Ca(2+)-activated chloride channels

Serotonin (5HT) is a constituent of the so-called “inflammatory soup” that sensitizes nociceptors during inflammation. Nevertheless, receptors and signaling mechanisms that mediate an excitation of dorsal root ganglion (DRG) neurons by 5HT remained controversial. Therefore, capsaicin-sensitive nociceptive neurons dissociated from rat DRGs were used to investigate effects of 5HT on membrane excitability and currents through ligand- as well as voltage-gated ion channels. In 58% of the neurons tested, 5HT increased action potential firing, an effect that was abolished by the 5HT2 receptor antagonist ritanserin, but not by the 5HT3 antagonist tropisetron. Unlike other algogenic mediators, such as PGE2 and bradykinin, 5HT did not affect currents through TTX-resistant Na⁺ channels or Kv7 K⁺ channels. In all neurons investigated, 5HT potentiated capsaicin-evoked currents through TRPV1 channels, an effect that was attenuated by antagonists at 5HT2A (4 F 4 PP), 5HT2B (SB 204741), as well as 5HT2C (RS 102221) receptors. 5HT triggered slowly arising inward Cl⁻ currents in 53% of the neurons. This effect was antagonized by the 5HT2C receptor blocker only, and the current was prevented by an inhibitor of Ca²⁺-activated chloride channels (CaCC). The 5HT-induced increase in action potential firing was also abolished by this CaCC blocker and by the TRPV1 inhibitor capsazepine. Amongst the subtype selective 5HT2 antagonists, only RS 102221 (5HT2C-selectively) counteracted the rise in action potential firing elicited by 5HT. These results show that 5HT excites DRG neurons mainly via 5HT2C receptors which concomitantly mediate a sensitization of TRPV1 channels and an opening of CaCCs.

P2Y Receptors Sensitize Mouse and Human Colonic Nociceptors

Activation of visceral nociceptors by inflammatory mediators contributes to visceral hypersensitivity and abdominal pain associated with many gastrointestinal disorders. Purine and pyrimidine nucleotides (e.g., ATP and UTP) are strongly implicated in this process following their release from epithelial cells during mechanical stimulation of the gut, and from immune cells during inflammation. Actions of ATP are mediated through both ionotropic P2X receptors and metabotropic P2Y receptors. P2X receptor activation causes excitation of visceral afferents; however, the impact of P2Y receptor activation on visceral afferents innervating the gut is unclear. Here we investigate the effects of stimulating P2Y receptors in isolated mouse colonic sensory neurons, and visceral nociceptor fibers in mouse and human nerve-gut preparations. Additionally, we investigate the role of Nav1.9 in mediating murine responses. The application of UTP (P2Y2 and P2Y4 agonist) sensitized colonic sensory neurons by increasing action potential firing to current injection and depolarizing the membrane potential. The application of ADP (P2Y1, P2Y12, and P2Y13 agonist) also increased action potential firing, an effect blocked by the selective P2Y1 receptor antagonist MRS2500. UTP or ADP stimulated afferents, including mouse and human visceral nociceptors, in nerve-gut preparations. P2Y1 and P2Y2 transcripts were detected in 80% and 56% of retrogradely labeled colonic neurons, respectively. Nav1.9 transcripts colocalized in 86% of P2Y1-positive and 100% of P2Y2-positive colonic neurons, consistent with reduced afferent fiber responses to UTP and ADP in Na(v)1.9(-/-) mice. These data demonstrate that P2Y receptor activation stimulates mouse and human visceral nociceptors, highlighting P2Y-dependent mechanisms in the generation of visceral pain during gastrointestinal disease.

Membrane coordination of receptors and channels mediating the inhibition of neuronal ion currents by ADP

ADP and other nucleotides control ion currents in the nervous system via various P2Y receptors. In this respect, Cav2 and Kv7 channels have been investigated most frequently. The fine tuning of neuronal ion channel gating via G protein coupled receptors frequently relies on the formation of higher order protein complexes that are organized by scaffolding proteins and harbor receptors and channels together with interposed signaling components. However, ion channel complexes containing P2Y receptors have not been described. Therefore, the regulation of Cav2.2 and Kv7.2/7.3 channels via P2Y1 and P2Y12 receptors and the coordination of these ion channels and receptors in the plasma membranes of tsA 201 cells have been investigated here. ADP inhibited currents through Cav2.2 channels via both P2Y1 and P2Y12 receptors with phospholipase C and pertussis toxin-sensitive G proteins being involved, respectively. The nucleotide controlled the gating of Kv7 channels only via P2Y1 and phospholipase C. In fluorescence energy transfer assays using conventional as well as total internal reflection (TIRF) microscopy, both P2Y1 and P2Y12 receptors were found juxtaposed to Cav2.2 channels, but only P2Y1, and not P2Y12, was in close proximity to Kv7 channels. Using fluorescence recovery after photobleaching in TIRF microscopy, evidence for a physical interaction was obtained for the pair P2Y12/Cav2.2, but not for any other receptor/channel combination. These results reveal a membrane juxtaposition of P2Y receptors and ion channels in parallel with the control of neuronal ion currents by ADP. This juxtaposition may even result in apparent physical interactions between receptors and channels.

Chronic inflammatory pain upregulates expression of P2Y2 receptor in small-diameter sensory neurons

Roles of ionotropic purinergic (P2X) receptors in chronic pain have been intensively investigated. However, the contribution of metabotropic purinergic (P2Y) receptors to pathological pain is controversial. In the present study, using single cell RT-PCR (reverse transcription-polymerase chain reaction) and single cell nested-PCR techniques, we examined the expression of P2X(2), P2X(3), P2Y(1) and P2Y(2) mRNA transcripts in retrogradely labeled cutaneous sensory neurons from mouse lumber dorsal root ganglia (DRGs) following peripheral inflammation. The percentage of cutaneous sensory neurons expressing P2Y(2) mRNA transcripts increased after complete Freund's adjuvant (CFA) treatment. Particularly, the P2Y(2) mRNA transcripts were more frequently detected in small-diameter cutaneous neurons from CFA-treated mice than those from control mice. Coexpression of P2Y(2) and P2X (P2X(2) or P2X(3)) mRNAs was more frequently observed in cutaneous sensory neurons from CFA-treated mice relative to controls. Pain behavioral tests showed that the blockade of P2Y receptors by suramin attenuated mechanical allodynia evoked either by CFA or uridine triphosphate (UTP), an endogenous P2Y(2) and P2Y(4) agonist. These results suggest that chronic inflammatory pain enhances expression of P2Y(2) receptor in peripheral sensory neurons that innervate the injured tissue and the activation of P2Y receptors contributes to mechanical allodynia following inflammation.

Enhancement of acid-sensing ion channel activity by metabotropic P2Y UTP receptors in primary sensory neurons

Peripheral purinergic signaling plays an important role in nociception. Increasing evidence suggests that metabotropic P2Y receptors are also involved, but little is known about the underlying mechanism. Herein, we report that selective P2Y receptor agonist uridine 5'-triphosphate (UTP) can exert an enhancing effect on the functional activity of acid-sensing ion channels (ASICs), key sensors for extracellular protons, in rat dorsal root ganglia (DRG) neurons. First, UTP dose-dependently increased the amplitude of ASIC currents. UTP also shifted the concentration-response curve for proton upwards, with a 56.6 ± 6.4% increase of the maximal current response to proton. Second, UTP potentiation of proton-gated currents can be mimicked by adenosine 5'-triphosphate (ATP), but not by P2Y1 receptor agonist ADP. Potentiation of UTP was blocked by P2Y receptor antagonist suramin and by inhibition of intracellular G protein, phospholipase C (PLC), protein kinase C (PKC), or protein interacting with C-kinase 1 (PICK1) signaling. Third, UTP altered acidosis-evoked membrane excitability of DRG neurons and caused a significant increase in the amplitude of the depolarization and the number of spikes induced by acid stimuli. Finally, UTP dose-dependently exacerbated nociceptive responses to injection of acetic acid in rats. These results suggest that UTP enhanced ASIC-mediated currents and nociceptive responses, which reveal a novel peripheral mechanism underlying UTP-sensitive P2Y2 receptor involvement in hyperalgesia by sensitizing ASICs in primary sensory neurons.

Lack of effect of a P2Y6 receptor antagonist on neuropathic pain behavior in mice

Accumulating evidence indicates that various subtypes of purinergic receptors (P2X and P2Y receptor families) play an essential role in the development and the maintenance of neuropathic pain. However, there is only limited data available about the role of P2Y6 receptors in pain processing. Here we detected P2Y6 receptor immunoreactivity in primary afferent neurons of mice and observed an upregulation in response to peripheral nerve injury. However, systemic and intrathecal administration of the P2Y6 receptor antagonist MRS2578 failed to affect the injury-induced neuropathic pain behavior. Our results suggest that P2Y6 receptors, in contrast to other purinergic receptor subtypes, are not critically involved in nerve injury-induced neuropathic pain processing in mice.

Inhibition of G protein-coupled P2Y2 receptor induced analgesia in a rat model of trigeminal neuropathic pain

BACKGROUNDS

ATP and P2X receptors play important roles in the modulation of trigeminal neuropathic pain, while the role of G protein-coupled P2Y₂ receptors and the underlying mechanisms are less clear. The threshold and frequency of action potentials, fast inactivating transient K+ channels (IA) are important regulators of membrane excitability in sensory neurons because of its vital role in the control of the spike onset. In this study, pain behavior tests, QT-RT-PCR, immunohistochemical staining, and patch-clamp recording, were used to investigate the role of P2Y₂ receptors in pain behaviour.

RESULTS

In control rats: 1) UTP, an agonist of P2Y₂/P2Y₄ receptors, caused a significant decrease in the mean threshold intensities for evoking action potentials and a striking increase in the mean number of spikes evoked by TG neurons. 2) UTP significantly inhibited IA and the expression of Kv1.4, Kv3.4 and Kv4.2 subunits in TG neurons, which could be reversed by the P2 receptor antagonist suramin and the ERK antagonist U0126. In ION-CCI (chronic constriction injury of infraorbital nerve) rats: 1) mRNA levels of Kv1.4, Kv3.4 and Kv4.2 subunits were significantly decreased, while the protein level of phosphorylated ERK was significantly increased. 2) When blocking P2Y₂ receptors by suramin or injection of P2Y2R antisense oligodeoxynucleotides both led to a time- and dose-dependent reverse of allodynia in ION-CCI rats. 3) Injection of P2Y₂ receptor antisense oligodeoxynucleotides induced a pronounced decrease in phosphorylated ERK expression and a significant increase in Kv1.4, Kv3.4 and Kv4.2 subunit expression in trigeminal ganglia.

CONCLUSIONS

Our data suggest that inhibition of P2Y₂ receptors leads to down-regulation of ERK-mediated phosphorylation and increase of the expression of I(A)-related Kv channels in trigeminal ganglion neurons, which might contribute to the clinical treatment of trigeminal neuropathic pain.

Acid evoked thermal hyperalgesia involves peripheral P2Y1 receptor mediated TRPV1 phosphorylation in a rodent model of thrombus induced ischemic pain

Background

We previously developed a thrombus-induced ischemic pain (TIIP) animal model, which was characterized by chronic bilateral mechanical allodynia without thermal hyperalgesia (TH). On the other hand we had shown that intraplantar injection of acidic saline facilitated ATP-induced pain, which did result in the induction of TH in normal rats. Because acidic pH and increased ATP are closely associated with ischemic conditions, this study is designed to: (1) examine whether acidic saline injection into the hind paw causes the development of TH in TIIP, but not control, animals; and (2) determine which peripheral mechanisms are involved in the development of this TH.

Results

Repeated intraplantar injection of pH 4.0 saline, but not pH 5.5 and 7.0 saline, for 3 days following TIIP surgery resulted in the development of TH. After pH 4.0 saline injections, protein levels of hypoxia inducible factor-1α (HIF-1α) and carbonic anhydrase II (CA II) were elevated in the plantar muscle indicating that acidic stimulation intensified ischemic insults with decreased tissue acidity. At the same time point, there were no changes in the expression of TRPV1 in hind paw skin, whereas a significant increase in TRPV1 phosphorylation (pTRPV1) was shown in acidic saline (pH 4.0) injected TIIP (AS-TIIP) animals. Moreover, intraplantar injection of chelerythrine (a PKC inhibitor) and AMG9810 (a TRPV1 antagonist) effectively alleviated the established TH. In order to investigate which proton- or ATP-sensing receptors contributed to the development of TH, amiloride (an ASICs blocker), AMG9810, TNP-ATP (a P2Xs antagonist) or MRS2179 (a P2Y1 antagonist) were pre-injected before the pH 4.0 saline. Only MRS2179 significantly prevented the induction of TH, and the increased pTRPV1 ratio was also blocked in MRS2179 injected animals.

Conclusion

Collectively these data show that maintenance of an acidic environment in the ischemic hind paw of TIIP rats results in the phosphorylation of TRPV1 receptors via a PKC-dependent pathway, which leads to the development of TH mimicking what occurs in chronic ischemic patients with severe acidosis. More importantly, peripheral P2Y1 receptors play a pivotal role in this process, suggesting a novel peripheral mechanism underlying the development of TH in these patients.

Purinergic stimulation of K+-dependent Na+/Ca2+ exchanger isoform 4 requires dual activation by PKC and CaMKII

K⁺-dependent Na⁺/Ca²⁺-exchanger isoform 4 (NCXK4) is one of the most broadly expressed members of the NCKX (K⁺-dependent Na⁺/Ca²⁺-exchanger) family. Recent data indicate that NCKX4 plays a critical role in controlling normal Ca²⁺ signal dynamics in olfactory and other neurons. Synaptic Ca²⁺ dynamics are modulated by purinergic regulation, mediated by ATP released from synaptic vesicles or from neighbouring glial cells. Previous studies have focused on modulation of Ca²⁺ entry pathways that initiate signalling. Here we have investigated purinergic regulation of NCKX4, a powerful extrusion pathway that assists in terminating Ca²⁺ signals. NCKX4 activity was stimulated by ATP through activation of the P2Y receptor signalling pathway. Stimulation required dual activation of PKC (protein kinase C) and CaMKII (Ca²⁺/calmodulin-dependent protein kinase II). Mutating T312, a putative PKC phosphorylation site on NCKX4, partially prevented purinergic stimulation. These data illustrate how purinergic regulation can shape the dynamics of Ca²⁺ signalling by activating a signal damping and termination pathway.

Activity of the K⁺-dependent Na⁺/Ca²⁺-exchanger, NCKX4, is stimulated by purinergic signals that depend on dual activation of two protein kinase pathways. This regulation provides a novel mechanism to shape Ca²⁺ signaling and thus to have important impact on neuronal processes.

Blockade of peripheral P2Y1 receptors prevents the induction of thermal hyperalgesia via modulation of TRPV1 expression in carrageenan-induced inflammatory pain rats: involvement of p38 MAPK phosphorylation in DRGs

Although previous reports have suggested that P2Y1 receptors (P2Y1Rs) are involved in cutaneous nociceptive signaling, it remains unclear how P2Y1Rs contribute to peripheral sensitization. The current study was designed to delineate the role of peripheral P2Y1Rs in pain and to investigate potential linkages to mitogen-activated protein kinase (MAPK) in DRGs and Transient Receptor Potential Vanilloid 1 (TRPV1) expression in a rodent inflammatory pain model. Following injection of 2% carrageenan into the hind paw, expressions of P2Y1 and TRPV1 and the phosphorylation rates of both p38 MAPK and ERK but not JNK were increased and peaked at day 2 post-injection. Blockade of peripheral P2Y1Rs by the P2Y1R antagonist, MRS2500 injection (i.pl, D0 to D2) significantly reduced the induction of thermal hyperalgesia, but not mechanical allodynia. Simultaneously, MRS2500 injections suppressed upregulated TRPV1 expression and DRG p38 phosphorylation, while pERK signaling was not affected. Furthermore, inhibition of p38 activation in the DRGs by SB203580 (a p38 inhibitor, i.t, D0 to D2) prevented the upregulation of TRPV1 and a single i.t injection of SB203580 reversed the established thermal hyperalgesia, but not mechanical allodynia. Lastly, to identify the mechanism of action of P2Y1Rs, we repeatedly injected the P2Y1 agonist, MRS2365 into the naïve rat's hind paw and observed a dose-dependent increase in TRPV1 expression and p38 MAPK phosphorylation. These data demonstrate a sequential role for P2Y1R, p38 MAPK and TRPV1 in inflammation-induced thermal hyperalgesia; thus, peripheral P2Y1Rs activation modulates p38 MAPK signaling and TRPV1 expression, which ultimately leads to the induction of thermal hyperalgesia.

Communication between neuronal somata and satellite glial cells in sensory ganglia

Studies of the structural organization and functions of the cell body of a neuron (soma) and its surrounding satellite glial cells (SGCs) in sensory ganglia have led to the realization that SGCs actively participate in the information processing of sensory signals from afferent terminals to the spinal cord. SGCs use a variety ways to communicate with each other and with their enwrapped soma. Changes in this communication under injurious conditions often lead to abnormal pain conditions. "What are the mechanisms underlying the neuronal soma and SGC communication in sensory ganglia?" and "how do tissue or nerve injuries affect the communication?" are the main questions addressed in this review.

The role of purinergic pathways in the pathophysiology of gut diseases: pharmacological modulation and potential therapeutic applications

Gut homeostasis results from complex neuro-immune interactions aimed at triggering stereotypical and specific programs of coordinated mucosal secretion and powerful motor propulsion. A prominent role in the regulation of this highly integrated network, comprising a variety of immune/inflammatory cells and the enteric nervous system, is played by purinergic mediators. The cells of the digestive tract are literally plunged into a "biological sea" of functionally active nucleotides and nucleosides, which carry out the critical task of driving regulatory interventions on cellular functions through the activation of P1 and P2 receptors. Intensive research efforts are being made to achieve an integrated view of the purinergic system, since it is emerging that the various components of purinergic pathways (i.e., enzymes, transporters, mediators and receptors) are mutually linked entities, deputed to finely modulating the magnitude and the duration of purinergic signaling, and that alterations occurring in this balanced network could be intimately involved in the pathophysiology of several gut disorders. This review article intends to provide a critical appraisal of current knowledge on the purinergic system role in the regulation of gastrointestinal functions, considering these pathways as a whole integrated network, which is capable of finely controlling the levels of bioactive nucleotides and nucleosides in the biophase of their respective receptors. Special attention is paid to the mechanisms through which alterations in the various compartments of the purinergic system could contribute to the pathophysiology of gut disorders, and to the possibility of counteracting such dysfunctions by means of pharmacological interventions on purinergic molecular targets.

Control of P2X3 channel function by metabotropic P2Y2 utp receptors in primary sensory neurons

Purinergic signaling contributes significantly to pain mechanisms, and the nociceptor-specific P2X3 ATP receptor channel is considered a target in pain therapeutics. Recent findings suggesting the coexpression of metabotropic P2Y receptors with P2X3 implies that ATP release triggers the activation of both ionotropic and metabotropic purinoceptors, with strong potential for functional interaction. Modulation of native P2X3 function by P2Y receptor activation was investigated in rat dorsal root ganglia (DRG) neurons using whole cell patch-clamp recordings. Application of the selective P2Y receptor agonist UTP decreased peak amplitudes of α,β-meATP-evoked homomeric P2X3-mediated currents, but had no effect on heteromeric P2X2/3-mediated currents. Treatment with phospholipase C inhibitor U73122 significantly reversed P2X3 current inhibition induced by UTP-sensitive P2Y receptor activation. We previously reported the modulation of P2X receptors by phospholipids in DRG neurons and injection of exogenous phosphatidylinositol-4,5-bisphosphate (PIP(2)) fully reverses UTP-mediated regulation of P2X3 channel activity. Pharmacological as well as functional screening of P2Y receptor subtypes indicates the predominant involvement of P2Y2 receptor in P2X3 inhibition, and immunolocalization confirms a significant cellular coexpression of P2X3 and P2Y2 in rat DRG neurons. In summary, the function of P2X3 ATP receptor can be inhibited by P2Y2-mediated depletion of PIP(2). We propose that expression of P2Y2 purinoceptor in nociceptive sensory neurons provides an homeostatic mechanism to prevent excessive ATP signaling through P2X3 receptor channels.

P2Y purinergic receptors: new targets for analgesic and antimigraine drugs

Millions of individuals worldwide suffer from acute and, more severely, chronic pain conditions (e.g., neuropathic pain, and migraine). The latter bear tremendous personal, familial, and social costs, since sufferers and their relatives undergo a complete turnaround of their lives with the search of relief from pain becoming their pivotal thought. Sadly, to date no effective pharmacological approaches are available which can alleviate chronic pain significantly or in the long run in all patients. The current central strategy for the development of new and effective painkillers lies in the hypothesis that cellular and/or molecular players in nociception must exists that are not targeted by "classical" analgesics, and therefore researchers have put tremendous efforts into the in-depth analysis of the pathways leading to pain development and maintenance over time. In this complex scenario, two outsiders are now taking the center stage: glial cells in sensory ganglia and in the central nervous system, thanks to their ability to communicate with neurons and to modulate their firing, and the purinergic system. Extracellular purine and pyrimidine nucleotides are involved in the physiology of virtually every body district, and their extracellular concentrations massively increase under pathological situations, suggesting that they might represent potential targets for the modulation of disease-associated symptoms, like pain. Here, we provide an overview of the present knowledge of the role of nucleotides in nociception, with a particular emphasis on G protein-coupled P2Y receptors and their involvement in the communication between first- and second-order neurons in sensory nerve pathways and surrounding glial cells.

Concomitant facilitation of GABAA receptors and KV7 channels by the non-opioid analgesic flupirtine

BACKGROUND AND PURPOSE

Flupirtine is a non-opioid analgesic that has been in clinical use for more than 20 years. It is characterized as a selective neuronal potassium channel opener (SNEPCO). Nevertheless, its mechanisms of action remain controversial and are the purpose of this study.

EXPERIMENTAL APPROACH

Effects of flupirtine on native and recombinant voltage- and ligand-gated ion channels were explored in patch-clamp experiments using the following experimental systems: recombinant K(IR)3 and K(V)7 channels and α3β4 nicotinic acetylcholine receptors expressed in tsA 201 cells; native voltage-gated Na(+), Ca(2+), inward rectifier K(+), K(V)7 K(+), and TRPV1 channels, as well as GABA(A), glycine, and ionotropic glutamate receptors expressed in rat dorsal root ganglion, dorsal horn and hippocampal neurons.

KEY RESULTS

Therapeutic flupirtine concentrations (≤10 µM) did not affect voltage-gated Na(+) or Ca(2+) channels, inward rectifier K(+) channels, nicotinic acetylcholine receptors, glycine or ionotropic glutamate receptors. Flupirtine shifted the gating of K(V)7 K(+) channels to more negative potentials and the gating of GABA(A) receptors to lower GABA concentrations. These latter effects were more pronounced in dorsal root ganglion and dorsal horn neurons than in hippocampal neurons. In dorsal root ganglion and dorsal horn neurons, the facilitatory effect of therapeutic flupirtine concentrations on K(V)7 channels and GABA(A) receptors was comparable, whereas in hippocampal neurons the effects on K(V)7 channels were more pronounced.

CONCLUSIONS AND IMPLICATIONS

These results indicate that flupirtine exerts its analgesic action by acting on both GABA(A) receptors and K(V)7 channels.

P2Y1 purinoceptor inhibition reduces extracellular signal-regulated protein kinase 1/2 phosphorylation in spinal cord and dorsal root ganglia: implications for cancer-induced bone pain

It remains unclear as to whether P2Y1 purinergic receptor (P2Y1R) and the molecules that act downstream, such as extracellular signal-regulated protein kinase 1/2 (ERK1/2), are involved in the development of cancer-induced bone pain (CIBP) in vivo. Here, we investigated the role of the P2Y1R in the modulation of CIBP-associated nociception in spinal cord and dorsal root ganglia (DRG). A CIBP model was established by inoculating Walker 256 gland carcinoma cells into the tibia of female rats. Tactile allodynia and spontaneous pain were assessed using von Frey filaments and ambulatory scores. The results showed that both the paw withdrawal latency to tactile allodynia and the ambulatory score to spontaneous pain were significantly different between the CIBP group and the sham group on days 7-9 post-inoculation (P< 0.01). Furthermore, rats in the CIBP group also showed a progressive increase in ambulatory score, which is different from the sham group (P< 0.01). Furthermore, P2Y1R mRNA and phosphorylated ERK1/2 (p-ERK1/2) protein expression levels were increased in the spinal dorsal horn and DRG of the CIBP group relative to the sham group. However, intrathecal injection of the P2Y1R antagonist MRS2179 decreased P2Y1R mRNA and p-ERK1/2 protein expression in the spinal dorsal horn and DRG (P< 0.01). These results provide evidence that the inhibition of P2Y1R-mediated ERK1/2 phosphorylation in the spinal dorsal horn and DRG can attenuate nociception transmission.