Characterization of cultured dorsal root ganglion neuron P2X receptors

Amplified P2X3 pathway activity in muscle afferent dorsal root ganglion neurons and exercise pressor reflex regulation in hindlimb ischaemia-reperfusion

Hindlimb ischaemia-reperfusion (IR) is among the most prominent pathophysiological conditions observed in peripheral artery disease (PAD). An exaggerated arterial blood pressure (BP) response during exercise is associated with an elevated risk of cardiovascular events in individuals with PAD. However, the precise mechanisms leading to this exaggerated BP response are poorly elucidated. The P2X3 signalling pathway, which plays a key role in modifying the exercise pressor reflex (EPR), is the focus of the present study. We determined the regulatory role of P2X3 on the EPR in a rat model of hindlimb IR. In vivo and in vitro approaches were used to determine the expression and functions of P2X3 in muscle afferent nerves and EPR in IR rats. We found that in IR rats there was (1) upregulation of P2X3 protein expression in the L4-6 dorsal root ganglia (DRG); (2) amplified P2X currents in isolated isolectin B4 (IB4)-positive muscle DRG neurons; and (3) amplification of the P2X-mediated BP response. We further verified that both A-317491 and siRNA knockdown of P2X3 significantly decreased the activity of P2X currents in isolated muscle DRG neurons. Moreover, inhibition of muscle afferents' P2X3 receptor using A-317491 was observed to alleviate the exaggerated BP response induced by static muscle contraction and P2X-induced BP response by α,β-methylene ATP injection. P2X3 signalling pathway activity is amplified in muscle afferent DRG neurons in regulating the EPR following hindlimb IR.

Localization of PIP5Kγ selectively in proprioceptive peripheral fields and also in sensory ganglionic satellite cells as well as neuronal cell membranes and their central terminals

Based on a previous study by others reporting that PIP5Kγ (phosphatidylinositol 4-phosphate 5-kinase γ) and its product, phosphatidylinositol 4,5 bisphosphate (PIP2 ), are involved in the regulation of nociception, the present immunohistochemical study examined the localization of PIP5Kγ-immunoreactivity in dorsal root ganglia (DRG) and their peripheral and central terminal fields. PIP5Kγ-immunoreactivity was localized for the first time in the muscle spindles, in which it was found in I-bands of polar regions of intrafusal muscle fibers and also in sensory nerve terminals abutting on equatorial regions of the muscle fibers. This finding indicates the involvement of PIP5Kγ in the proprioception and suggests somehow complicated mechanisms of its involvement because of its heterogeneous localization in intra-I-band structures. In DRG, on the other hand, PIP5Kγ-immunoreactivity was shown to be localized heterogeneously, but not evenly, over apposed plasma membranes of both neurons and ganglionic satellite cells in immune electron microscopy. In addition, no peripheral nerve terminals of DRG showing its distinct immunoreactivity were found in most peripheral fields of nociception and any other sensory perception except for the proprioception through muscle spindles. In contrast, numerous central terminals of DRG in the spinal posterior horn were immunoreactive for it. This finding leads us to consider the possibility that the regulation by PIP5Kγ of nociception is dominantly exerted in DRG and sensory neural tracts central, rather than peripheral, to DRG.

Nociceptive signaling mediated by P2X3, P2X4 and P2X7 receptors

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

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

Current therapies for the treatment of chronic pain provide inadequate relief for millions of suffering patients, demonstrating the need for better therapies that will treat pain effectively and improve the quality of patient's lives. Better understanding of the mechanisms that mediate chronic pain is critical for developing drugs with improved clinical outcomes. Adenosine triphosphate (ATP) is a key modulator in nociceptive pathways. Release of ATP from injured tissue or sympathetic efferents has sensitizing effects on sensory neurons in the periphery, and presynaptic vesicular release of ATP from the central terminals can increase glutamate release thereby potentiating downstream central sensitization mechanisms, a condition thought to underlie many chronic pain conditions. The purinergic receptors on sensory nerves primarily responsible for ATP signaling are P2X3 and P2X2/3. Selective knockdown experiments, or inhibition with small molecules, demonstrate P2X3-containing receptors are key targets to modulate nociceptive signals. Preclinical studies have identified that P2X3-containing receptors are critical for sensory transduction for bladder function, and clinical studies have shown promise in treatment for bladder pain and pain associated with osteoarthritis. Further clinical characterization of antagonists to P2X3-containing receptors may lead to improved therapies in the treatment of chronic pain.

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.

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

BACKGROUND AND PURPOSE

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

EXPERIMENTAL APPROACH

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

KEY RESULTS

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

CONCLUSIONS AND IMPLICATIONS

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

Antihyperalgesic effect of CB1 receptor activation involves the modulation of P2X3 receptor in the primary afferent neuron

Cannabinoid system is a potential target for pain control. Cannabinoid receptor 1 (CB₁) activation play a role in the analgesic effect of cannabinoids once it is expressed in primary afferent neurons. This study investigates whether the anti-hyperalgesic effect of CB₁ receptor activation involves P2X₃ receptor in primary afferent neurons. Mechanical hyperalgesia was evaluated by electronic von Frey test. Cannabinoid effect was evaluated using anandamide or ACEA, a non-selective or a selective CB₁ receptor agonists, respectively; AM251, a CB₁ receptor antagonist, and antisense ODN for CB₁ receptor. Calcium imaging assay was performed to evaluated α,β-meATP-responsive cultured DRG neurons pretreated with ACEA. Anandamide or ACEA administered in peripheral tissue reduced the carrageenan-induced mechanical hyperalgesia. The reduction in the carrageenan-induced hyperalgesia induced by ACEA was completely reversed by administration of AM251 as well as by the intrathecal treatment with antisense ODN for CB₁ receptor. Also, ACEA reduced the mechanical hyperalgesia induced by bradykinin and by α,β-meATP, a P2X₃ receptor non-selective agonist, but not by tumor necrosis factor alpha (TNF-α), interleukin-1 beta (IL-1β) and chemokine-induced chemoattractant-1 (CINC-1). Finally, CB₁ receptors are co-localized with P2X₃ receptors in DRG small-diameter neurons and the treatment with ACEA reduced the number of α,β-meATP-responsive cultured DRG neurons. Our data suggest that the analgesic effect of CB₁ receptor activation is mediated by a negative modulation of the P2X₃ receptor in the primary afferent neurons.

Bradykinin Contributes to Sympathetic and Pressor Responses Evoked by Activation of Skeletal Muscle Afferents P2X in Heart Failure

BACKGROUND/AIMS

Published data suggest that purinergic P2X receptors of muscle afferent nerves contribute to the enhanced sympathetic nervous activity (SNA) and blood pressure (BP) responses during static exercise in heart failure (HF). In this study, we examined engagement of bradykinin (BK) in regulating responses of SNA and BP evoked by P2X stimulation in rats with HF. We further examined cellular mechanisms responsible for BK. We hypothesized that BK potentiates P2X currents of muscle dorsal root ganglion (DRG) neurons, and this effect is greater in HF due to upregulation of BK kinin B2 and P2X3 receptor. As a result, BK amplifies muscle afferents P2X-mediated SNA and BP responses.

METHODS

Renal SNA and BP responses were recorded in control rats and rats with HF. Western Blot analysis and patch-clamp methods were employed to examine the receptor expression and function of DRG neurons involved in the effects of BK.

RESULTS

BK injected into the arterial blood supply of the hindlimb muscles heightened the reflex SNA and BP responses induced by P2X activation with α,β-methylene ATP to a greater degree in HF rats. In addition, HF upregulated the protein expression of kinin B2 and P2X3 in DRG and the prior application of BK increased the magnitude of α,β-methylene ATP-induced currents in muscle DRG neurons from HF rats.

CONCLUSION

BK plays a facilitating role in modulating muscle afferent P2X-engaged reflex sympathetic and pressor responses. In HF, P2X responsivness is augmented due to increases in expression of kinin B2 and P2X3 receptors and P2X current activity.

Modulation of P2X3 and P2X2/3 Receptors by Monoclonal Antibodies

Purinergic homomeric P2X3 and heteromeric P2X2/3 receptors are ligand-gated cation channels activated by ATP. Both receptors are predominantly expressed in nociceptive sensory neurons, and an increase in extracellular ATP concentration under pathological conditions, such as tissue damage or visceral distension, induces channel opening, membrane depolarization, and initiation of pain signaling. Hence, these receptors are considered important therapeutic targets for pain management, and development of selective antagonists is currently progressing. To advance the search for novel analgesics, we have generated a panel of monoclonal antibodies directed against human P2X3 (hP2X3). We have found that these antibodies produce distinct functional effects, depending on the homomeric or heteromeric composition of the target, its kinetic state, and the duration of antibody exposure. The most potent antibody, 12D4, showed an estimated IC50 of 16 nm on hP2X3 after short term exposure (up to 18 min), binding to the inactivated state of the channel to inhibit activity. By contrast, with the same short term application, 12D4 potentiated the slow inactivating current mediated by the heteromeric hP2X2/3 channel. Extending the duration of exposure to ∼20 h resulted in a profound inhibition of both homomeric hP2X3 and heteromeric hP2X2/3 receptors, an effect mediated by efficient antibody-induced internalization of the channel from the plasma membrane. The therapeutic potential of mAb12D4 was assessed in the formalin, complete Freund's adjuvant, and visceral pain models. The efficacy of 12D4 in the visceral hypersensitivity model indicates that antibodies against P2X3 may have therapeutic potential in visceral pain indications.

Blockade and reversal of spinal morphine tolerance by P2X3 receptor antagonist

In recent years, studies have substantiated the view that P2X3 receptors play a part in the generation and transmission of purinergic signals in inflammatory and chronic neuropathic pain. Data have also been presented to suggest that the process of P2X3 receptor antagonism inhibits inflammatory hyperalgesia, involving the spinal opioid system. The aim of this study was to investigate the effect of the selective P2X3 receptor antagonist A-317491 on the development of antinociceptive tolerance to chronic morphine administration in mice. Daily systemic injection of A-317491 attenuated the morphine-induced antinociceptive tolerance to von Frey and thermal stimuli. Repeated morphine injections alone led to a significant rightward shift in the morphine dose-response curve compared with that with A-317491. A single dose of A-317491 also showed a reversal effect in morphine-tolerant mice. In a withdrawal test, co-administration of A-317491 and morphine also reduced the naloxone-induced withdrawal symptoms compared with the morphine-alone group. Thus, we propose that the P2X3 receptor is involved in the process of morphine antinociceptive tolerance and may be a new therapeutic target in the prevention of tolerance to morphine-induced antinociception.

Small organic molecule disruptors of Cav3.2 - USP5 interactions reverse inflammatory and neuropathic pain

BACKGROUND

Cav3.2 channels facilitate nociceptive transmission and are upregulated in DRG neurons in response to nerve injury or peripheral inflammation. We reported that this enhancement of Cav3.2 currents in afferent neurons is mediated by deubiquitination of the channels by the deubiquitinase USP5, and that disrupting USP5/Cav3.2 channel interactions protected from inflammatory and neuropathic pain.

RESULTS

Here we describe the development of a small molecule screening assay for USP5-Cav3.2 disruptors, and report on two hits of a ~5000 compound screen - suramin and the flavonoid gossypetin. In mouse models of inflammatory pain and neuropathic pain, both suramin and gossypetin produced dose-dependent and long-lasting mechanical anti-hyperalgesia that was abolished or greatly attenuated in Cav3.2 null mice. Suramin and Cav3.2/USP5 Tat-disruptor peptides were also tested in models of diabetic neuropathy and visceral pain, and provided remarkable protection.

CONCLUSIONS

Overall, our findings provide proof of concept for a new class of analgesics that target T-type channel deubiquitination.

Plasma membrane poration by opioid neuropeptides: a possible mechanism of pathological signal transduction

Neuropeptides induce signal transduction across the plasma membrane by acting through cell-surface receptors. The dynorphins, endogenous ligands for opioid receptors, are an exception; they also produce non-receptor-mediated effects causing pain and neurodegeneration. To understand non-receptor mechanism(s), we examined interactions of dynorphins with plasma membrane. Using fluorescence correlation spectroscopy and patch-clamp electrophysiology, we demonstrate that dynorphins accumulate in the membrane and induce a continuum of transient increases in ionic conductance. This phenomenon is consistent with stochastic formation of giant (~2.7 nm estimated diameter) unstructured non-ion-selective membrane pores. The potency of dynorphins to porate the plasma membrane correlates with their pathogenic effects in cellular and animal models. Membrane poration by dynorphins may represent a mechanism of pathological signal transduction. Persistent neuronal excitation by this mechanism may lead to profound neuropathological alterations, including neurodegeneration and cell death.

Phenotypes of ATP-activated current associated with their genotypes of P2X1-6 subunits in neurons innervating tooth-pulp

To explore the association of the phenotype of ATP-activated current with the genotype of P2X1-6 subunits in nociceptors, we developed a method that allows us to label nociceptive neurons innervating tooth-pulp in rat trigeminal ganglion (TG) neurons using a retrograde fluorescence-tracing method, to record ATP-activated current in freshly isolated fluorescence-labeled neurons, and then to conduct single cell immunohistochemical staining for P2X1-6 subunits in the same neuron. We found that fast application of 100 μM ATP to fluorescence-traced TG neurons produced robust inward current in 87% (96/110) of cells tested. The diameter of cells varied from 16 to 56 μm. Three types of ATP-activated current (F, I and S) were recorded with distinct rise times of the current (R10-90, P < 0.05). There was a positive correlation between the cell diameter and the value of R10-90 (P < 0.05): the value of R10-90 increased with increases in the cell diameter. Cells responsive to ATP with the type F current mainly showed positive staining for P2X3 and P2X5, but negative staining for P2X2; cells responsive to ATP with the type I current showed positive staining for P2X1-3 and P2X5, but negative staining for P2X4; and cells responsive to ATP with the type S current showed positive staining for P2X1-5, but negative staining for P2X6. The present findings suggest that in addition to P2X3 subunits, P2X5 subunits are also involved in the generation of the F type of ATP-activated current in small-sized nociceptive neurons. In addition to the P2X2/3 subunit-containing channels, more complex uncharacterized combinations of P2X1-5 subunits exist in native medium-sized nociceptive neurons exhibiting the I and S types of ATP-activated current. In addition, the P2X6 subunit is not a main subunit involved in the nociceptive signal in rat TG neurons innervating tooth-pulp.

Measurement of ATP-Induced Membrane Potential Changes in IVD cells

Extracellular adenosine-5'-triphosphate (ATP) triggers biological responses in a wide variety of cells and tissues and activates signaling cascades that affect cell membrane potential and excitability. It has been demonstrated that compressive loading promotes ATP production and release by intervertebral disc (IVD) cells, while a high level of extracellular ATP accumulates in the nucleus pulposus (NP) of the IVD. In this study, a noninvasive system was developed to measure ATP-induced changes in the membrane potential of porcine IVD cells using the potential sensitive dye di-8-butyl-amino-naphthyl-ethylene-pyridinium-propyl-sulfonate (di-8-ANEPPS).The responses of NP and annulus fibrosus (AF) cells to ATP were examined in monolayer and 3-dimensional cultures. It was found that the pattern and magnitude of membrane potential change in IVD cells induced by extracellular ATP depended on cell type, culture condition, and ATP dose. In addition, gene expression of P2X₄ purinergic receptor was found in both cell types. Inhibition of the ATP-induced response by pyridoxalphosphate-6-azophenyl-2', 4'-disulfonate (PPADS), a non-competitive inhibitor of P2 receptors, suggests that ATP may modulate the biological activities of IVD cells via P2 purinergic receptors.

Heteromeric assembly of P2X subunits

Transcripts and/or proteins of P2X receptor (P2XR) subunits have been found in virtually all mammalian tissues. Generally more than one of the seven known P2X subunits have been identified in a given cell type. Six of the seven cloned P2X subunits can efficiently form functional homotrimeric ion channels in recombinant expression systems. This is in contrast to other ligand-gated ion channel families, such as the Cys-loop or glutamate receptors, where homomeric assemblies seem to represent the exception rather than the rule. P2XR mediated responses recorded from native tissues rarely match exactly the biophysical and pharmacological properties of heterologously expressed homomeric P2XRs. Heterotrimerization of P2X subunits is likely to account for this observed diversity. While the existence of heterotrimeric P2X2/3Rs and their role in physiological processes is well established, the composition of most other P2XR heteromers and/or the interplay between distinct trimeric receptor complexes in native tissues is not clear. After a description of P2XR assembly and the structure of the intersubunit ATP-binding site, this review summarizes the distribution of P2XR subunits in selected mammalian cell types and the biochemically and/or functionally characterized heteromeric P2XRs that have been observed upon heterologous co-expression of P2XR subunits. We further provide examples where the postulated heteromeric P2XRs have been suggested to occur in native tissues and an overview of the currently available pharmacological tools that have been used to discriminate between homo- and heteromeric P2XRs.

Suppression of ATP-induced excitability in rat small-diameter trigeminal ganglion neurons by activation of GABAB receptor

The aim of the present study was to investigate whether a GABAB receptor agonist could modulate ATP-activated neuronal excitability of nociceptive TRG neurons using perforated whole-cell patch-clamp and immunohistochemical techniques. Immunohistochemical analysis revealed that 86% of P2X3 receptor-immunoreactive, small-diameter TRG neurons co-expressed GABAB receptor. Under voltage-clamp conditions (Vh=-60mV), application of ATP activated the inward current in acutely isolated rat TRG neurons in a dose-dependent manner (10-50 μM) and this current could be blocked by pyridoxal-phosphate-6-azophenyl-27,47-disulfonic acid (PPADS) (10 μM), a selective P2 purinoreceptor antagonist. The peak amplitude of ATP-activated currents was significantly inhibited after application of GABAB receptor agonist, baclofen (10-50 μM), in a concentration-dependent and reversible manner. The baclofen-induced inhibition of ATP-activated current was abolished by co-application of 3-amino-2 (4-chlorophenyl)-2hydroxypropysufonic acid) saclofen, a GABAB receptor antagonist (50 μM). Under current-clamp conditions, application of 20 μM ATP significantly depolarized the membrane potential resulting in increased mean action potential frequencies, and these ATP-induced effects were significantly inhibited by baclofen and these effects were antagonized by co-application of saclofen. Together, the results suggested that GABAB receptor activation could inhibit the ATP-induced excitability of small-diameter TRG neurons activated through the P2X3 receptor. Thus, the interaction between P2X3 and GABAB receptors of small-diameter TRG neuronal cell bodies is a potential therapeutic target for the treatment of trigeminal nociception.

Subtype-specific control of P2X receptor channel signaling by ATP and Mg2+

The identity and forms of activating ligands for ion channels are fundamental to their physiological roles in rapid electrical signaling. P2X receptor channels are ATP-activated cation channels that serve important roles in sensory signaling and inflammation, yet the active forms of the nucleotide are unknown. In physiological solutions, ATP is ionized and primarily found in complex with Mg(2+). Here we investigated the active forms of ATP and found that the action of MgATP(2-) and ATP(4-) differs between subtypes of P2X receptors. The slowly desensitizing P2X2 receptor can be activated by free ATP, but MgATP(2-) promotes opening with very low efficacy. In contrast, both free ATP and MgATP(2-) robustly open the rapidly desensitizing P2X3 subtype. A further distinction between these two subtypes is the ability of Mg(2+) to regulate P2X3 through a distinct allosteric mechanism. Importantly, heteromeric P2X2/3 channels present in sensory neurons exhibit a hybrid phenotype, characterized by robust activation by MgATP(2-) and weak regulation by Mg(2+). These results reveal the existence of two classes of homomeric P2X receptors with differential sensitivity to MgATP(2-) and regulation by Mg(2+), and demonstrate that both restraining mechanisms can be disengaged in heteromeric channels to form fast and sensitive ATP signaling pathways in sensory neurons.

P2X3, but not P2X1, receptors mediate ATP-activated current in neurons innervating tooth-pulp

We developed a method that allows us to label nociceptive neurons innervating tooth-pulp in rat trigeminal ganglion neurons using a retrograde fluorescence-tracing method, to record ATP-activated current in freshly isolated fluorescence-labeled neurons and to conduct single cell immunohistochemical staining for P2X1 and P2X3 subunits in the same neuron. Three types of ATP-activated current in these neurons (F, I and S) were recorded. The cells exhibiting the type F current mainly showed positive staining for P2X3, but negative staining for P2X1. The results provide direct and convincing evidence at the level of single native nociceptive neurons for correlation of the characteristics of ATP-activated currents with their composition of P2X1 and P2X3 subunits and cell size. The results also suggest that the P2X3, but not P2X1, is the main subunit that mediates the fast ATP-activated current in nociceptive neurons.

Augmented P2X response and immunolabeling in dorsal root ganglion neurons innervating skeletal muscle following femoral artery occlusion

The responsiveness of sensory neurons to muscle metabolites is altered under the conditions of insufficient limb blood supply in some diseases, such as peripheral artery disease. The purpose of this study was to examine ATP-induced current with activation of purinergic P2X subtypes P2X₃ and P2X₂/₃ in dorsal root ganglion (DRG) neurons of control limbs and limbs with 24 h of femoral artery occlusion using whole cell patch-clamp methods. Also, dual-labeling immunohistochemistry was employed to determine existence of P2X₃ expression in DRG neurons of thin-fiber afferents. DRG neurons from 4- to 6-wk-old rats were labeled by injecting the fluorescence tracer DiI into the hindlimb muscles 4-5 days before the recording experiments. Transient (P2X₃), mixed (P2X₃ and P2X₂/₃), and sustained (P2X₂/₃) current responses to α,β-methylene ATP (a P2X receptor agonist) are observed in small and medium DRG neurons, and size distribution of DRG neurons is similar in control and occluded limbs. However, the peak current amplitude of DRG neuron induced by stimulation of P2X₃ and/or P2X₂/₃ is larger in occluded limbs than that in control limbs. Moreover, the percentage of DRG neurons with P2X₃ transient currents is greater after arterial occlusion compared with control. In addition, a rapid desensitization was observed in DRG neurons with transient currents, but not with sustained currents in control and occluded groups. Furthermore, results from immunofluorescence experiments show that femoral artery occlusion primarily augments P2X₃ expression within DRG neurons projecting C-fiber afferents. Overall, these findings suggest that 1) greater ATP-induced currents with activation of P2X₃ and P2X₂/₃ are developed when hindlimb arterial blood supply is deficient under ischemic conditions and 2) increased P2X₃ expression is largely observed in C-fibers of DRG neurons after hindlimb vascular insufficiency.

Purinergic transmission and transglial signaling between neuron somata in the dorsal root ganglion

Most dorsal root ganglion neuronal somata (NS) are isolated from their neighbours by a satellite glial cell (SGC) sheath. However, some NS are associated in pairs, separated solely by the membrane septum of a common SGC to form a neuron-glial cell-neuron (NGlN) trimer. We reported that stimulation of one NS evokes a delayed, noisy and long-duration inward current in both itself and its passive partner that was blocked by suramin, a general purinergic antagonist. Here we test the hypothesis that NGlN transmission involves purinergic activation of the SGC. Stimulation of the NS triggered a sustained current noise in the SGC. Block of transmission through the NGlN by reactive blue 2 or thapsigargin, a Ca(2+) store-depletion agent, implicated a Ca(2+) store discharge-linked P2Y receptor. P2Y2 was identified by simulation of the NGlN-like transmission by puffing UTP onto the SGC and by immunocytochemical localization to the SGC membrane septum. Block of the UTP effect by BAPTA, an intracellular Ca(2+) scavenger, supported the involvement of SGC Ca(2+) stores in the signaling pathway. We infer that transmission through the NGlN trimer involves secretion of ATP from the NS and triggering of SGC Ca(2+) store discharge via P2Y2 receptors. Presumably, cytoplasmic Ca(2+) elevation leads to the release of an as-yet unidentified second transmitter from the glial cell to complete transmission. Thus, the two NS of the NGlN trimer communicate via a 'sandwich synapse' transglial pathway, a novel signaling mechanism that may contribute to information transfer in other regions of the nervous system.

Analgesic effect of electroacupuncture on chronic neuropathic pain mediated by P2X3 receptors in rat dorsal root ganglion neurons

Adenosine 5'-triphosphate disodium (ATP) gated P2X receptors, especially the subtype P2X(3), play a key role in transmission of pain signals in neuropathic pain, ATP has been documented to play a significant role in the progression of pain signals, suggesting that control of these pathways through electroacupuncture (EA) is potentially an effective treatment for chronic neuropathic pain. EA has been accepted to effectively manage chronic pain by applying the stimulating current to acupoints through acupuncture needles. To determine the significance of EA on neuropathic pain mediated by P2X(3) receptors in the dorsal root ganglion (DRG) neurons, mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) were recorded, and the expression of P2X(3) receptors in the DRG neurons was assessed by immunohistochemistry (IHC) and in situ hybridization (ISH). In addition, the currents which were evoked in DRG neurons isolated from rats following chronic constriction injury (CCI) by the P2X(3) receptors agonists i.e. ATP and α,β-methylen-ATP (α,β-meATP) were examined through the experimental use of whole cell patch clamp recording. The present study demonstrates that EA treatment can increase the MWT and TWL values and decrease the expression of P2X(3) receptors in DRG neurons in CCI rats. Simultaneously, EA treatment attenuates the ATP and α,β-meATP evoked currents. EA may be expected to induce an apparent induce analgesic effect by decreasing expression and inhibiting P2X(3) receptors in DRG neurons of CCI rats. There is a similar effect on analgesic effect between rats with contralateral EA and those with ipsilateral EA.

Prenatal expression of purinergic receptor P2X3 in human dorsal root ganglion

The dorsal root ganglion (DRG) is consisted of neurons that relay multiple types of spinal sensory stimuli to the central nervous system. Several neuroactive molecules may be involved in sensory modulation especially pain processing at the DRG, including the purinergic receptor P2X3 and calcitonin-gene-related peptide (CGRP). P2X3 receptor has been considered a promising pharmaceutical target for the development of new pain medicine. Currently, litter is known about the expression of P2X3 in the human DRG. The present study characterized the localization of P2X3 in prenatal human DRG obtained from fetuses at 4-8 gestational months, by comparing to CGRP expression as well as binding pattern of isolectin-B4 (IB4), a marker of small DRG neurons presumably relevant to nociception. P2X3 immunoreactivity (IR) appeared in most neuron-like perikarya, with their numerical density reduced during the gestational period studied. P2X3 IR was co-labeled very commonly with IB4 binding and infrequently with CGRP IR and was not colocalized with IR for the gliocyte marker glutamine synthetase. Together, the data show an early and broad expression of P2X3 in prenatal human DRG neurons, pointing to a biological role of purinergic signaling during the development of spinal sensory system.

Neuronal soma-satellite glial cell interactions in sensory ganglia and the participation of purinergic receptors

It has been known for some time that the somata of neurons in sensory ganglia respond to electrical or chemical stimulation and release transmitters in a Ca2+-dependent manner. The function of the somatic release has not been well delineated. A unique characteristic of the ganglia is that each neuronal soma is tightly enwrapped by satellite glial cells (SGCs). The somatic membrane of a sensory neuron rarely makes synaptic contact with another neuron. As a result, the influence of somatic release on the activity of adjacent neurons is likely to be indirect and/or slow. Recent studies of neuron-SGC interactions have demonstrated that ATP released from the somata of dorsal root ganglion neurons activates SGCs. They in turn exert complex excitatory and inhibitory modulation of neuronal activity. Thus, SGCs are actively involved in the processing of afferent information. In this review, we summarize our understanding of bidirectional communication between neuronal somata and SGCs in sensory ganglia and its possible role in afferent signaling under normal and injurious conditions. The participation of purinergic receptors is emphasized because of their dominant roles in the communication.

Differential purinergic signaling in bladder sensory neurons of naïve and bladder-inflamed mice

This study explored purinergic signaling in lumbosacral (LS) and thoracolumbar (TL) dorsal root ganglion neurons innervating the urinary bladder. In naïve mice, a greater proportion of LS (93%) than that of TL (77%) bladder neurons responded to purinergic agonists. Three types of purinergic currents were identified: 'sustained' (homomeric P2X2) currents were detected only in LS neurons, rapidly activating, 'slow' deactivating (heteromeric P2X2/3) currents predominated in both LS and TL neurons, and 'fast' activating/de-activating (homomeric P2X3) currents were detected only in TL neurons. Relative to TL bladder neurons, slow current density was greater in LS neurons, which also had a more negative action potential threshold and generated more action potentials in response to purinergic agonists (suggesting greater excitability of LS neurons). Single cell nested PCR documented P2X2 and P2X3 subunit expression in both TL and LS bladder neurons. Relative to saline treatment, bladder wall thickness and weight increased after cyclophosphamide (CYP) treatment. Both LS and TL neuron excitability increased (rheobase was decreased and responses to purinergic agonists increased) after CYP treatment. The proportion of sustained currents in LS bladder neurons increased fourfold after CYP bladder inflammation. Although proportions of slow and fast purinergic currents in TL neurons were unchanged by CYP treatment, the fast current density was greater than in saline-treated mice. These results in mouse, as previously described in rat, reveal differential purinergic signaling in TL and LS bladder neurons. The predominant currents and significant changes after inflammation, however, occur in different ganglia/sensory pathways in mouse and rat.

Nucleotide signaling and cutaneous mechanisms of pain transduction

Sensory neurons that innervate the skin provide critical information about physical contact between the organism and the environment, including information about potentially-damaging stimuli that give rise to the sensation of pain. These afferents also contribute to the maintenance of tissue homeostasis, inflammation and wound healing, while sensitization of sensory afferents after injury results in painful hypersensitivity and protective behavior. In contrast to the traditional view of primary afferent terminals as the sole site of sensory transduction, recent reports have lead to the intriguing idea that cells of the skin play an active role in the transduction of sensory stimuli. The search for molecules that transduce different types of sensory stimuli (mechanical, heat, chemical) at the axon terminal has yielded a wide range of potential effectors, many of which are expressed by keratinocytes as well as neurons. Emerging evidence underscores the importance of nucleotide signaling through P2X ionotropic and P2Y metabotropic receptors in pain processing, and implicates nucleotide signaling as a critical form of communication between cells of the skin, immune cells and sensory neurons. It is of great interest to determine whether pathological changes in these mechanisms contribute to chronic pain in human disease states such as complex regional pain syndrome (CRPS). This review discusses recent advances in our understanding of communication mechanisms between cells of the skin and sensory axons in the transduction of sensory input leading to pain.

Functional up-regulation of P2X 3 receptors in the chronically compressed dorsal root ganglion

P2X receptors on dorsal root ganglion (DRG) neurons have been strongly implicated in pathological nociception after peripheral nerve injuries or inflammation. However, nothing is known of a role for purinergic receptors in neuropathic pain produced by a chronic compression of DRG (CCD) - an injury that may accompany an intraforaminal stenosis, a laterally herniated disc or other disorders of the spine leading to radicular pain. In a rat model of DRG compression, hyperexcitable neurons retain functioning axonal connections with their peripheral targets. It is unknown whether such hyperexcitability might enhance chemically mediated nociceptive stimulation of the skin. In this study, CCD facilitated the nocifensive behavior and mechanical hyperalgesia-induced by the P2X 3 agonist, alpha,beta-methylene ATP (alpha,beta-meATP). An injection of alpha,beta-meATP into the hind paw of CCD rats resulted in a significantly greater decrease in the mean threshold to von Frey stimuli and a greater duration of paw lifts than in sham-operated control rats. CCD also increased the levels of P2X 3 receptor protein and the number of P2X 3 immunoreactive, small diameter DRG neurons in the compressed ganglion. P2X 3 receptors were co-labeled with the isolectin IB4, consistent with a role in nociception. In addition, a alpha,beta-meATP induced significantly larger fast-inactivating currents in CCD- than in sham-operated acutely dissociated DRG neurons. These currents were accompanied by the generation of action potentials - but only in the CCD neurons. U0126, a specific inhibitor of the MEK1/2, greatly down-regulated the enhanced current. Taken together, these observations suggest that enhanced purinergic responses after CCD are mediated by P2X 3 receptors.

Roles of peripheral P2X and P2Y receptors in the development of melittin-induced nociception and hypersensitivity

A recent report from our laboratory shows that subcutaneous (s.c.) injection of melittin could induce persistent spontaneous nociception (PSN) and primary thermal or mechanical hyperalgesia. However, the exact peripheral mechanisms underlying melittin-induced multiple pain-related behaviors remain unclear. In this study, behavioral tests combined with pharmacological manipulations were used to explore potential roles of local P2X and P2Y receptors in melittin-induced inflammatory pain and hyperalgesia. Post-treatment of the primary injury site with s.c. injection of A-317491 (a potent P2X(3)/P2X(2/3) receptor antagonist) and Reactive Blue 2 (a potent P2Y receptor antagonist) could significantly suppress the development of melittin-evoked PSN and hypersensitivity (thermal and mechanical). Our control experiments demonstrated that local administration of either antagonist into the contralateral hindpaw produced no significant effect on any kind of pain-associated behaviors. Taken together, these data indicate that activation of P2X and P2Y receptors might be essential to the maintenance of melittin-induced primary thermal and mechanical hyperalgesia as well as on-going pain.

Cutaneous sensory neurons expressing the Mrgprd receptor sense extracellular ATP and are putative nociceptors

Sensory neurons expressing the Mrgprd receptor are known to innervate the outermost living layer of the epidermis, the stratum granulosum. The sensory modality that these neurons signal and the stimulus that they respond to are not established, although immunocytochemical data suggest they could be nonpeptidergic nociceptors. Using patch clamp of dissociated mouse dorsal root ganglion (DRG) neurons, the present study demonstrates that Mrgprd+ neurons have several properties typical of nociceptors: long-duration action potentials, TTX-resistant Na(+) current, and Ca(2+) currents that are inhibited by mu opioids. Remarkably, Mrgprd+ neurons respond almost exclusively to extracellular ATP with currents similar to homomeric P2X3 receptors. They show little or no sensitivity to other putative nociceptive agonists, including capsaicin, cinnamaldehyde, menthol, pH 6.0, or glutamate. These properties, together with selective innervation of the stratum granulosum, indicate that Mrgprd+ neurons are nociceptors in the outer epidermis and may respond indirectly to external stimuli by detecting ATP release in the skin.

Neutralization of nerve growth factor induces plasticity of ATP-sensitive P2X3 receptors of nociceptive trigeminal ganglion neurons

The molecular mechanisms of migraine pain are incompletely understood, although migraine mediators such as NGF and calcitonin gene-related peptide (CGRP) are believed to play an algogenic role. Although NGF block is proposed as a novel analgesic approach, its consequences on nociceptive purinergic P2X receptors of trigeminal ganglion neurons remain unknown. We investigated whether neutralizing NGF might change the function of P2X3 receptors natively coexpressed with NGF receptors on cultured mouse trigeminal neurons. Treatment with an NGF antibody (24 h) decreased P2X3 receptor-mediated currents and Ca2+ transients, an effect opposite to exogenously applied NGF. Recovery from receptor desensitization was delayed by anti-NGF treatment without changing desensitization onset. NGF neutralization was associated with decreased threonine phosphorylation of P2X3 subunits, presumably accounting for their reduced responses and slower recovery. Anti-NGF treatment could also increase the residual current typical of heteromeric P2X2/3 receptors, consistent with enhanced membrane location of P2X2 subunits. This possibility was confirmed with cross-linking and immunoprecipitation studies. NGF neutralization also led to increased P2X2e splicing variant at mRNA and membrane protein levels. These data suggest that NGF controlled plasticity of P2X3 subunits and their membrane assembly with P2X2 subunits. Despite anti-NGF treatment, CGRP could still enhance P2X3 receptor activity, indicating separate NGF- or CGRP-mediated mechanisms to upregulate P2X3 receptors. In an in vivo model of mouse trigeminal pain, anti-NGF pretreatment suppressed responses evoked by P2X3 receptor activation. Our findings outline the important contribution by NGF signaling to nociception of trigeminal sensory neurons, which could be counteracted by anti-NGF pretreatment.

P2X3 receptor involvement in pain states

The understanding of how pain is processed at each stage in the peripheral and central nervous system is the precondition to develop new therapies for the selective treatment of pain. In the periphery, ATP can be released from various cells as a consequence of tissue injury or visceral distension and may stimulate the local nociceptors. The highly selective distribution of P2X(3) and P2X(2/3) receptors within the nociceptive system has inspired a variety of approaches to elucidate the potential role of ATP as a pain mediator. Depolarization by ATP of neurons in pain-relevant neuronal structures such as trigeminal ganglion, dorsal root ganglion, and spinal cord dorsal horn neurons are well investigated. P2X receptor-mediated afferent activation appears to have been implicated in visceral and neuropathic pain and even in migraine and cancer pain. This article reviews recently published research describing the role that ATP and P2X receptors may play in pain perception, highlighting the importance of the P2X(3) receptor in different states of pain.

ATP and pain

Many decades have passed since the pain-producing properties of ATP were demonstrated in both animals and humans. However, the more recent discovery of a family of ion channels for which ATP is a ligand and which are expressed by nociceptive neurons, has led to a resurgence of interest into the physiological and pathophysiological actions of ATP. This article considers the extent to which available evidence supports the notion that ATP receptors might be important novel analgesic targets. The hypothesis that ATP is a pain mediator is considered in terms of: the distribution of ATP receptors (specifically the P2X ion channel family); whether ATP release occurs under appropriate conditions; the evidence that ATP is capable of initiating pain in humans and pain-related behaviour in animals; and, lastly, the analgesic effects of pharmacological or molecular block of ATP receptors.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Characterization of three types of ATP-activated current in relation to P2X subunits in rat trigeminal ganglion neurons

In the present study, ATP-activated currents (I(ATP)s) recorded from rat trigeminal ganglion (TG) neurons using whole-cell patch clamp technique are classified into three types (F, I and S) based on the characteristics of their activation and desensitization. The time of rising phase (R(10-90)) of types F, I and S of I(ATP) is measured to be 33.6+/-4.5, 62.2+/-9.9 and 302.1+/-62.0 ms respectively, and positively correlated to cell size. The time of decaying phases (D(10-90)) of types F and S is 399.4+/-58.2 and >1500 ms, respectively. The dose-response curves for the three types of I(ATP) show that their EC(50) values are close (3.44 x 10(-5), 4.89 x 10(-5) and 4.14 x 10(-5) M for types F, I and S respectively, P>0.05). Their reversal potentials are basically the same, varying from +4 to +10 mV. In addition, using whole-cell patch clamp technique in combination with single cell immunohistochemical staining for P2X receptor subunits, our results suggest that the type distinction of ATP-activated current was associated with cell size and P2X receptor subunits: small-sized cells with type F of I(ATP) express only P2X1 and/or P2X3 subunits, while cells with types S and I of I(ATP) express P2X2 or P2X4 in addition to P2X1 and P2X3.

Activation and sensitization of C and Adelta afferent fibers mediated by P2X receptors in rat dorsal skin

The present study investigated the activation and sensitization effects of local injection of P2X receptor agonist alpha,beta-methylene ATP (alphabeta-meATP) into the receptive fields of afferent fibers innervating dorsal hairy skin in anesthetized rats. Single unit activities of afferent fibers were recorded by means of isolation of the fiber filaments from the dorsal cutaneous nerve branch. A total of 237 fibers were obtained. Of these, 67 were classed as C fibers, 104 as Adelta fibers and 66 as Abeta fibers. When alphabeta-meATP (0.1-100 microM, 10 microl) was injected subcutaneously into the receptive fields of these units, C and Adelta fibers demonstrated a dose-related increase in the discharge rates of the response. The activated proportion of C and Adelta fibers with a response to the drug also increased with dose. However, Abeta fibers did not exhibit significant activation. Furthermore, injection of alphabeta-meATP (10 microl) at a concentration of 100 microM resulted in a significant decrease of mechanical thresholds in C and Adelta fibers compared with pre-injection baseline (P < 0.05). In control experiments, injection of the vehicle phosphate-buffered saline (PBS, 10 microl) had no effect on all units tested. alphabeta-meATP (100 microM, 10 microl) followed by pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), a P2X receptor antagonist, successfully blocked the activation and sensitization effects of alphabeta-meATP on C and Adelta fibers tested. These results suggest that peripheral P2X receptors are involved in mediating peripheral excitation of C and Adelta fibers.

Pharmacology of P2X channels

Significant progress in understanding the pharmacological characteristics and physiological importance of homomeric and heteromeric P2X channels has been achieved in recent years. P2X channels, gated by ATP and most likely trimerically assembled from seven known P2X subunits, are present in a broad distribution of tissues and are thought to play an important role in a variety of physiological functions, including peripheral and central neuronal transmission, smooth muscle contraction, and inflammation. The known homomeric and heteromeric P2X channels can be distinguished from each other on the basis of pharmacological differences when expressed recombinantly in cell lines, but whether this pharmacological classification holds true in native cells and in vivo is less well-established. Nevertheless, several potent and selective P2X antagonists have been discovered in recent years and shown to be efficacious in various animal models including those for visceral organ function, chronic inflammatory and neuropathic pain, and inflammation. The recent advancement of drug candidates targeting P2X channels into human trials, confirms the medicinal exploitability of this novel target family and provides hope that safe and effective medicines for the treatment of disorders involving P2X channels may be identified in the near future.

Experimental and modeling studies of desensitization of P2X3 receptors

The function of ATP-activated P2X3 receptors involved in pain sensation is modulated by desensitization, a phenomenon poorly understood. The present study used patch-clamp recording from cultured rat or mouse sensory neurons and kinetic modeling to clarify the properties of P2X3 receptor desensitization. Two types of desensitization were observed, a fast process (t1/2 = 50 ms; 10 microM ATP) following the inward current evoked by micromolar agonist concentrations, and a slow process (t1/2 = 35 s; 10 nM ATP) that inhibited receptors without activating them. We termed the latter high-affinity desensitization (HAD). Recovery from fast desensitization or HAD was slow and agonist-dependent. When comparing several agonists, there was analogous ranking order for agonist potency, rate of desensitization and HAD effectiveness, with 2-methylthioadenosine triphosphate the strongest and beta,gamma-methylene-ATP the weakest. HAD was less developed with recombinant (ATP IC50 = 390 nM) than native P2X3 receptors (IC50 = 2.3 nM). HAD could also be induced by nanomolar ATP when receptors seemed to be nondesensitized, indicating that resting receptors could express high-affinity binding sites. Desensitization properties were well accounted for by a cyclic model in which receptors could be desensitized from either open or closed states. Recovery was assumed to be a multistate process with distinct kinetics dependent on the agonist-dependent dissociation rate from desensitized receptors. Thus, the combination of agonist-specific mechanisms such as desensitization onset, HAD, and resensitization could shape responsiveness of sensory neurons to P2X3 receptor agonists. By using subthreshold concentrations of an HAD-potent agonist, it might be possible to generate sustained inhibition of P2X3 receptors for controlling chronic pain.

Inhibition mechanism of the recombinant rat P2X(2) receptor in glial cells by suramin and TNP-ATP

P2X receptors play an important role in communication between cells in the nervous system. Therefore, understanding the mechanisms of inhibition of these receptors is important for the development of new tools for drug discovery. Our objective has been to determine the pharmacological activity of the antagonist suramin, the most important antagonist of purinergic receptor function, as well as to demonstrate its noncompetitive inhibition and confirm a competitive mechanism between ATP and TNP-ATP in 1321N1 glial cells stably transfected with the recombinant rat P2X(2) receptor. A radioligand binding assay was employed to determine whether suramin, TNP-ATP, and ATP compete for the same binding site on the receptor. TNP-ATP displaced [alpha-32P]ATP, whereas suramin did not interfere with [alpha-32P]ATP-receptor binding. To determine the inhibition mechanism relevant for channel opening, currents obtained in fast kinetic whole-cell recording experiments, following stimulation of cells by ATP in the presence of suramin, were compared to those obtained by ATP in the presence of TNP-ATP. Supported by a mathematical model for receptor kinetics [Breitinger, H. G., Geetha, N., and Hess, G. P. (2001) Biochemistry 40, 8419-8429], the inhibition factors were plotted as functions of inhibitor or agonist concentrations. Analysis of the data indicated a competitive inhibition mechanism for TNP-ATP and a noncompetitive inhibition for suramin. Taken together, both data support a noncompetitive inhibition mechanism of the rat recombinant P2X(2) receptor by suramin, confirm the competitive inhibition by TNP-ATP, and allow the prediction of a model for P2X(2) receptor inhibition.

Molecular properties of P2X receptors

P2X receptors for adenosine tri-phosphate (ATP) are a distinct family of ligand-gated cation channels with two transmembrane domains, intracellular amino and carboxy termini and a large extracellular ligand binding loop. Seven genes (P2X(1-7)) have been cloned and the channels form as either homo or heterotrimeric channels giving rise to a wide range of phenotypes. This review aims to give an account of recent work on the molecular basis of the properties of P2X receptors. In particular, to consider emerging information on the assembly of P2X receptor subunits, channel regulation and desensitisation, targeting, the molecular basis of drug action and the functional contribution of P2X receptors to physiological processes.

Comparison of P2X and TRPV1 receptors in ganglia or primary culture of trigeminal neurons and their modulation by NGF or serotonin

BACKGROUND

Cultured sensory neurons are a common experimental model to elucidate the molecular mechanisms of pain transduction typically involving activation of ATP-sensitive P2X or capsaicin-sensitive TRPV1 receptors. This applies also to trigeminal ganglion neurons that convey pain inputs from head tissues. Little is, however, known about the plasticity of these receptors on trigeminal neurons in culture, grown without adding the neurotrophin NGF which per se is a powerful algogen. The characteristics of such receptors after short-term culture were compared with those of ganglia. Furthermore, their modulation by chronically-applied serotonin or NGF was investigated.

RESULTS

Rat or mouse neurons in culture mainly belonged to small and medium diameter neurons as observed in sections of trigeminal ganglia. Real time RT-PCR, Western blot analysis and immunocytochemistry showed upregulation of P2X(3) and TRPV1 receptors after 1-4 days in culture (together with their more frequent co-localization), while P2X(2) ones were unchanged. TRPV1 immunoreactivity was, however, lower in mouse ganglia and cultures. Intracellular Ca(2+) imaging and whole-cell patch clamping showed functional P2X and TRPV1 receptors. Neurons exhibited a range of responses to the P2X agonist alpha, beta-methylene-adenosine-5'-triphosphate indicating the presence of homomeric P2X(3) receptors (selectively antagonized by A-317491) and heteromeric P2X(2/3) receptors. The latter were observed in 16 % mouse neurons only. Despite upregulation of receptors in culture, neurons retained the potential for further enhancement of P2X(3) receptors by 24 h NGF treatment. At this time point TRPV1 receptors had lost the facilitation observed after acute NGF application. Conversely, chronically-applied serotonin selectively upregulated TRPV1 receptors rather than P2X(3) receptors.

CONCLUSION

Comparing ganglia and cultures offered the advantage of understanding early adaptive changes of nociception-transducing receptors of trigeminal neurons. Culturing did not prevent differential receptor upregulation by algogenic substances like NGF or serotonin, indicating that chronic application led to distinct plastic changes in the molecular mechanisms mediating pain on trigeminal nociceptors.

P2X receptors: targets for novel analgesics?

The ability of adenosine 5'-triphosphate (ATP) to evoke acute pain has been known for many years, but its role in nociceptive signaling is only now becoming clear. ATP acts via P2X and P2Y receptors, and of particular importance here is the P2X(3) receptor. It is expressed selectively at high levels in nociceptive sensory neurons, where it forms functional receptors on its own and in combination with the P2X(2) receptor. Recent reports using gene knockout methods; antisense oligonucleotide and small, interfering RNA technologies; and a novel, selective P2X(3) antagonist, A-317491, show that P2X(3) receptors are involved in chronic inflammatory and neuropathic pain. The mRNA for other P2X subunits is also found in sensory neurons, and there is evidence for functional P2X(1/5) or P2X(2/6) heteromers in some of these. These data support the possibility that P2X receptors, particularly the P2X(3) subtype, could be targeted in the search for new, effective analgesics.

The P2X3 antagonist P1, P5-di[inosine-5'] pentaphosphate binds to the desensitized state of the receptor in rat dorsal root ganglion neurons

P2X3 purinergic receptors are predominantly expressed in dorsal root ganglion (DRG) neurons and play an important role in pain sensation. P2X3-specific antagonists are currently being sought to ameliorate pain in several indications. Understanding how antagonists interact with the P2X3 receptor can aid in the discovery and development of P2X3-specific antagonists. We studied the activity of the noncompetitive antagonist P1, P5-di[inosine-5'] pentaphosphate (IP5I) at the P2X3 receptor, compared with the well studied competitive antagonist TNP-ATP, using a whole-cell voltage-clamp technique in dissociated rat DRG neurons. IP5I blocked alphabeta-methylene ATP (alphabeta-meATP)-evoked P2X3 responses in a concentration-dependent manner (IC50 = 0.6 +/- 0.1 microM). IP5I effectively inhibited P2X3 currents when pre-exposed to desensitized but not unbound receptors. Furthermore, IP5I equally blocked 1 and 10 microM alphabeta-meATP-evoked currents and had no effect on the desensitization rate constant of these currents. This supports the action of IP5I as a noncompetitive antagonist that interacts with the desensitized state of the P2X3 receptor. In contrast, TNP-ATP inhibited the current evoked by 1 microM alphabeta-meATP significantly more than the one evoked by 10 microM alphabeta-meATP. It also significantly slowed down the desensitization rate constant of the current. These results suggest that TNP-ATP acts as a competitive antagonist and competes with alphabeta-meATP at the P2X3 agonist binding site. These findings may help to explain why IP5I acts selectively at the fast-desensitizing P2X1 and P2X3 subtypes of the P2X purinoceptor, while having much less potency at slow-desensitizing P2X2 and P2X(2/3) subtypes that lack the fast desensitized conformational state.

Characterization of multiple P2X receptors in cultured normal human epidermal keratinocytes

ATP-gated ion channels (P2X) are expressed in human epidermis and cultured keratinocytes. The aim of this study was to characterize native P2X receptors in normal human epidermal keratinocytes (NHEK) using whole-cell patch clamp technique, RT-PCR, and determination of intracellular Ca(2+) concentration ([Ca(2+)](i)). Application of ATP resulted in an inward current with a reversal potential of 0 mV. Response to ATP showed two types of currents: the slowly desensitizing response and the rapidly desensitizing response. The slowly desensitizing response was blocked by iso-pyridocaphosphate-6-azophenyl-2', 5' disulfonic acid (PPADS), a P2X receptor antagonist. We found that the expression of multiple P2X(2), P2X(3), P2X(5), and P2X(7) receptor subtype mRNA was increased in differentiated cells. On the other hand, the expression of G-protein-coupled P2Y(2) mRNA was downregulated in differentiated cells. Increases in [Ca(2+)](i) evoked by alphabeta-methylene ATP (alphabeta-meATP) and 2', 3'-O-(4-benzoylbenzoyl) ATP (BzATP) were elevated, whereas elevation of [Ca(2+)](i) evoked by uridine 5'-triphosphate (UTP) was decreased in differentiated cells. Application of ATP or UVB radiation increased the expression of P2X(1), P2X(2), P2X(3), and P2X(7) receptors in NHEK. Changes in the expression levels and cation influx via multiple P2X receptors might be involved in the regulation of differentiation and one of the epidermal external sensors.

Gastric ulcers evoke hyperexcitability and enhance P2X receptor function in rat gastric sensory neurons

Tissue inflammation contributes to the development of hyperalgesia, which is at least in part due to altered properties of primary afferent neurons. We hypothesized that gastric ulcers enhance the excitability of gastric sensory neurons and increase their response to purinergic agonists. The rat stomach was surgically exposed, and a retrograde tracer [1.1'-dioctadecyl-3,3,3,'3-tetramethylindocarbocyanine methanesulfonate (DiI)] was injected into the wall of the distal stomach. Kissing ulcers (KUs) were produced by a single injection of acetic acid (0.1 ml for 45 s; 60%) into the clamped gastric lumen. Saline injection served as control. Gastric nodose ganglion (NG) or dorsal root ganglion (DRG) cells were harvested 7 days later and acutely dissociated for whole cell recordings. Based on whole cell capacitance, gastric DRG neurons exhibited larger cell size than NG neurons. Significantly more control gastric DRG neurons compared with NG counterparts had TTX-resistant action potentials. Almost all control NG neurons (90%) compared with significantly less DRG neurons (< or =38%) responded to ATP or alpha,beta-metATP. Whereas none of the control cells exhibited spontaneous activity, about 20% of the neurons from KU animals generated spontaneous action potentials. KUs enhanced excitability as shown by a decrease in threshold for action potential generation, which was in part due to an increased input resistance. This was associated with an increase in the fraction of neurons with TTX-resistant action potentials and cells responding to capsaicin and purinergic agonists. KU doubled the current density evoked by the P2X receptor agonist alpha,beta-metATP and slowed decay of the slowly desensitizing component of the current without affecting the concentration dependence of the response. These data show that KU sensitizes vagal and spinal gastric afferents by affecting both voltage- and ligand-gated channels, thereby potentially contributing to the development of dyspeptic symptoms.

Inhibitory, facilitatory, and excitatory effects of ATP and purinergic receptor agonists on the activity of rat cutaneous nociceptors in vitro

Pathological pain is often associated with changed sympathetic nerve activities. It is known that sympathetic nerve endings release ATP as a co-transmitter of norepinephrine, but the effect of this ATP on the nociceptive system has not been properly studied in that the concentration range used in the previous studies was much higher than is expected in the surroundings of nociceptor terminals. We examined the effects of ATP, especially at low concentration (10(-5) M or less), on C-fiber polymodal receptor (CPR) activity using a rat skin-nerve preparation in vitro. We found for the first time that ATP inhibited the heat response of CPRs at low concentration (10(-5) M), but facilitated it at high concentration (10(-3) M). The former effect was mimicked by a P2X3 agonist, alpha,beta-methylene ATP, at 10(-5) M, while the latter was mimicked by 2-methylthio ADP (a P2Y1 agonist) or UTP (a P2Y2 agonist) at 10(-3) M, suggesting that the former is mediated by P2X receptors and the latter by P2Y receptors. After repetitive heat stimuli, ATP-induced CPR excitation was increased (10(-5) to 10(-3) M), but none of the purinergic agonists induced CPR excitation in a magnitude comparable to that by ATP. Possible mechanisms for these effects were discussed.

A-317491, a selective P2X3/P2X2/3 receptor antagonist, reverses inflammatory mechanical hyperalgesia through action at peripheral receptors in rats

The effect of A-317491 (5-([(3-Phenoxybenzyl)[(1S)-1,2,3,4-tetrahydro-1-naphthalenyl]amino]carbonyl)-1,2,4-benzenetricarboxylic acid), a recently described selective P2X3 and P2X(2/3) receptor antagonist, on inflammatory mechanical hyperalgesia was examined. In the rat Freund's complete adjuvant model of inflammatory pain, s.c. administration of A-317491 dose-dependently reversed mechanical hyperalgesia. Maximum percent reversal (72%) was seen 3 h after administration at 10 mg/kg. Substantial plasma concentrations were measured for A-317491 after s.c. dosing 3, 10 and 30 mg/kg. However, the brain-to-plasma concentration ratio, determined 1 h after a 10 mg/kg s.c. dose, indicated limited penetration of A-317491 into the central nervous system. As revealed by neural activity recorded from single C-fiber nociceptive afferent in a Freund's complete adjuvant-inflamed rat skin-nerve preparation, topical application of A-317491 completely blocked afferent activation and mechanical sensitization induced by alpha,beta-methylene ATP, a P2X agonist. These results suggest that A-317491 is a peripherally acting P2X blocker. Its efficacy demonstrates the importance of peripheral P2X3/P2X(2/3) receptors in mediating ATP-associated mechanical hyperalgesia following inflammation, confirming previous suggestions of a significant role for P2X(2/3).

Effects of pentobarbital on purinergic P2X receptors of rat dorsal root ganglion neurons

Purinergic P2X receptors are ligand-gated ion channels that are activated by extracellular adenosine triphosphate (ATP) and are widely expressed not only in the central and peripheral nervous system but also in tissues throughout the body, playing an important role in the transfer of nociceptive information. Since the influence of barbiturates on P2X receptor subtypes is not known, we studied the effects of pentobarbital sodium (PB) on ATP responses in dorsal root ganglion (DRG) neurons. DRG neurons were dissected from 10- to 14-day-old rats and dissociated after enzyme treatment. Electrical measurements were performed using the nystatin-perforated patch recording mode under voltage-clamp conditions. Drugs were applied using the Y-tube method. ATP evoked three types of inward current at -60 mV: fast desensitizing, slow desensitizing, and mixed. The fast-type current was attributed to activation of P2X3 subtype and the slow type to the P2X2 subtype. PB suppressed the fast-type current in a concentration-dependent manner, while the slow type was slightly reduced. A noncompetitive inhibition was suggested by a downward shift of the ATP concentration-response curves. The current-voltage relationships showed inward rectification, and the extent of suppression was not affected by the holding potential. The reduction was greater in external solutions of higher pH. PB had subtype-specific effects on P2X receptors. The ionized form is likely to be responsible for the suppression of the P2X3 receptor current, which may result in a reduction of the excitability of central and peripheral neurons and may contribute to the anesthetic and analgesic actions of the agent.