Immunoreactive localisation of P2Y1 receptors within the rat and human nodose ganglia and rat brainstem: comparison with [alpha 33P]deoxyadenosine 5'-triphosphate autoradiography

Activation of Intra-nodose Ganglion P2X7 Receptors Elicit Increases in Neuronal Activity

Vagus nerve innervates several organs including the heart, stomach, and pancreas among others. Somas of sensory neurons that project through the vagal nerve are located in the nodose ganglion. The presence of purinergic receptors has been reported in neurons and satellite glial cells in several sensory ganglia. In the nodose ganglion, calcium depletion-induced increases in neuron activity can be partly reversed by P2X7 blockers applied directly into the ganglion. The later suggest a possible role of P2X7 receptors in the modulation of neuronal activity within this sensory ganglion. We aimed to characterize the response to P2X7 activation in nodose ganglion neurons under physiological conditions. Using an ex vivo preparation for electrophysiological recordings of the neural discharges of nodose ganglion neurons, we found that treatments with ATP induce transient neuronal activity increases. Also, we found a concentration-dependent increase in neural activity in response to Bz-ATP (ED₅₀ = 0.62 mM, a selective P2X7 receptor agonist), with a clear desensitization pattern when applied every ~ 30 s. Electrophysiological recordings from isolated nodose ganglion neurons reveal no differences in the responses to Bz-ATP and ATP. Finally, we showed that the P2X7 receptor was expressed in the rat nodose ganglion, both in neurons and satellite glial cells. Additionally, a P2X7 receptor negative allosteric modulator decreased the duration of Bz-ATP-induced maximal responses without affecting their amplitude. Our results show the presence of functional P2X7 receptors under physiological conditions within the nodose ganglion of the rat, and suggest that ATP modulation of nodose ganglion activity may be in part mediated by the activation of P2X7 receptors.

Astrocytes Modulate Baroreflex Sensitivity at the Level of the Nucleus of the Solitary Tract

Maintenance of cardiorespiratory homeostasis depends on autonomic reflexes controlled by neuronal circuits of the brainstem. The neurophysiology and neuroanatomy of these reflex pathways are well understood, however, the mechanisms and functional significance of autonomic circuit modulation by glial cells remain largely unknown. In the experiments conducted in male laboratory rats we show that astrocytes of the nucleus of the solitary tract (NTS), the brain area that receives and integrates sensory information from the heart and blood vessels, respond to incoming afferent inputs with [Ca2+]i elevations. Astroglial [Ca2+]i responses are triggered by transmitters released by vagal afferents, glutamate acting at AMPA receptors and 5-HT acting at 5-HT2A receptors. In conscious freely behaving animals blockade of Ca2+-dependent vesicular release mechanisms in NTS astrocytes by virally driven expression of a dominant-negative SNARE protein (dnSNARE) increased baroreflex sensitivity by 70% (p < 0.001). This effect of compromised astroglial function was specific to the NTS as expression of dnSNARE in astrocytes of the ventrolateral brainstem had no effect. ATP is considered the principle gliotransmitter and is released by vesicular mechanisms blocked by dnSNARE expression. Consistent with this hypothesis, in anesthetized rats, pharmacological activation of P2Y1 purinoceptors in the NTS decreased baroreflex gain by 40% (p = 0.031), whereas blockade of P2Y1 receptors increased baroreflex gain by 57% (p = 0.018). These results suggest that glutamate and 5-HT, released by NTS afferent terminals, trigger Ca2+-dependent astroglial release of ATP to modulate baroreflex sensitivity via P2Y1 receptors. These data add to the growing body of evidence supporting an active role of astrocytes in brain information processing.SIGNIFICANCE STATEMENT Cardiorespiratory reflexes maintain autonomic balance and ensure cardiovascular health. Impaired baroreflex may contribute to the development of cardiovascular disease and serves as a robust predictor of cardiovascular and all-cause mortality. The data obtained in this study suggest that astrocytes are integral components of the brainstem mechanisms that process afferent information and modulate baroreflex sensitivity via the release of ATP. Any condition associated with higher levels of "ambient" ATP in the NTS would be expected to decrease baroreflex gain by the mechanism described here. As ATP is the primary signaling molecule of glial cells (astrocytes, microglia), responding to metabolic stress and inflammatory stimuli, our study suggests a plausible mechanism of how the central component of the baroreflex is affected in pathological conditions.

Release of ATP by pre-Bötzinger complex astrocytes contributes to the hypoxic ventilatory response via a Ca2+ -dependent P2Y1 receptor mechanism

KEY POINTS

The ventilatory response to reduced oxygen (hypoxia) is biphasic, comprising an initial increase in ventilation followed by a secondary depression. Our findings indicate that, during hypoxia, astrocytes in the pre-Bötzinger complex (preBötC), a critical site of inspiratory rhythm generation, release a gliotransmitter that acts via P2Y1 receptors to stimulate ventilation and reduce the secondary depression. In vitro analyses reveal that ATP excitation of the preBötC involves P2Y1 receptor-mediated release of Ca2+ from intracellular stores. By identifying a role for gliotransmission and the sites, P2 receptor subtype, and signalling mechanisms via which ATP modulates breathing during hypoxia, these data advance our understanding of the mechanisms underlying the hypoxic ventilatory response and highlight the significance of purinergic signalling and gliotransmission in homeostatic control. Clinically, these findings are relevant to conditions in which hypoxia and respiratory depression are implicated, including apnoea of prematurity, sleep disordered breathing and congestive heart failure.

ABSTRACT

The hypoxic ventilatory response (HVR) is biphasic, consisting of a phase I increase in ventilation followed by a secondary depression (to a steady-state phase II) that can be life-threatening in premature infants who suffer from frequent apnoeas and respiratory depression. ATP released in the ventrolateral medulla oblongata during hypoxia attenuates the secondary depression. We explored a working hypothesis that vesicular release of ATP by astrocytes in the pre-Bötzinger Complex (preBötC) inspiratory rhythm-generating network acts via P2Y1 receptors to mediate this effect. Blockade of vesicular exocytosis in preBötC astrocytes bilaterally (using an adenoviral vector to specifically express tetanus toxin light chain in astrocytes) reduced the HVR in anaesthetized rats, indicating that exocytotic release of a gliotransmitter within the preBötC contributes to the hypoxia-induced increases in ventilation. Unilateral blockade of P2Y1 receptors in the preBötC via local antagonist injection enhanced the secondary respiratory depression, suggesting that a significant component of the phase II increase in ventilation is mediated by ATP acting at P2Y1 receptors. In vitro responses of the preBötC inspiratory network, preBötC inspiratory neurons and cultured preBötC glia to purinergic agents demonstrated that the P2Y1 receptor-mediated increase in fictive inspiratory frequency involves Ca2+ recruitment from intracellular stores leading to increases in intracellular Ca2+ ([Ca2+ ]i ) in inspiratory neurons and glia. These data suggest that ATP is released by preBötC astrocytes during hypoxia and acts via P2Y1 receptors on inspiratory neurons (and/or glia) to evoke Ca2+ release from intracellular stores and an increase in ventilation that counteracts the hypoxic respiratory depression.

Purinergic signaling and the functioning of the nervous system cells

Purinergic signaling in the nervous system has been the focus of a considerable number of studies since the 1970s. The P2X and P2Y receptors are involved in the initiation of purinergic signaling. They are very abundant in the central and peripheral nervous systems, where they are expressed on the surface of neurons and glial cells--microglia, astrocytes, oligodendrocytes and Schwann cells and the precursors of the latter two. Their ligands--extracellular nucleotides--are released in the physiological state by astrocytes and neurons forming synaptic connections, and are essential for the proper functioning of nervous system cells. Purinergic signaling plays a crucial role in neuromodulation, neurotransmission, myelination in the CNS and PNS, intercellular communication, the regulation of ramified microglia activity, the induction of the response to damaging agents, the modulation of synaptic activity and other glial cells by astrocytes, and the induction of astrogliosis. Understanding these mechanisms and the fact that P2 receptors and their ligands are involved in the pathogenesis of diseases of the nervous system may help in the design of drugs with different and more effective mechanisms of action.

P2Y1 receptor-mediated potentiation of inspiratory motor output in neonatal rat in vitro

PreBötzinger complex inspiratory rhythm-generating networks are excited by metabotropic purinergic receptor subtype 1 (P2Y1R) activation. Despite this, and the fact that inspiratory MNs express P2Y1Rs, the role of P2Y1Rs in modulating motor output is not known for any MN pool. We used rhythmically active brainstem-spinal cord and medullary slice preparations from neonatal rats to investigate the effects of P2Y1R signalling on inspiratory output of phrenic and XII MNs that innervate diaphragm and airway muscles, respectively. MRS2365 (P2Y1R agonist, 0.1 mm) potentiated XII inspiratory burst amplitude by 60 ± 9%; 10-fold higher concentrations potentiated C4 burst amplitude by 25 ± 7%. In whole-cell voltage-clamped XII MNs, MRS2365 evoked small inward currents and potentiated spontaneous EPSCs and inspiratory synaptic currents, but these effects were absent in TTX at resting membrane potential. Voltage ramps revealed a persistent inward current (PIC) that was attenuated by: flufenamic acid (FFA), a blocker of the Ca(2+)-dependent non-selective cation current ICAN; high intracellular concentrations of BAPTA, which buffers Ca(2+) increases necessary for activation of ICAN; and 9-phenanthrol, a selective blocker of TRPM4 channels (candidate for ICAN). Real-time PCR analysis of mRNA extracted from XII punches and laser-microdissected XII MNs revealed the transcript for TRPM4. MRS2365 potentiated the PIC and this potentiation was blocked by FFA, which also blocked the MRS2365 potentiation of glutamate currents. These data suggest that XII MNs are more sensitive to P2Y1R modulation than phrenic MNs and that the P2Y1R potentiation of inspiratory output occurs in part via potentiation of TRPM4-mediated ICAN, which amplifies inspiratory inputs.

Neuromodulation: purinergic signaling in respiratory control

The main functions of the respiratory neural network are to produce a coordinated, efficient, rhythmic motor behavior and maintain homeostatic control over blood oxygen and CO2/pH levels. Purinergic (ATP) signaling features prominently in these homeostatic reflexes. The signaling actions of ATP are produced through its binding to a diversity of ionotropic P2X and metabotropic P2Y receptors. However, its net effect on neuronal and network excitability is determined by the interaction between the three limbs of a complex system comprising the signaling actions of ATP at P2Rs, the distribution of multiple ectonucleotidases that differentially metabolize ATP into ADP, AMP, and adenosine (ADO), and the signaling actions of ATP metabolites, especially ADP at P2YRs and ADO at P1Rs. Understanding the significance of purinergic signaling is further complicated by the fact that neurons, glia, and the vasculature differentially express P2 and P1Rs, and that both neurons and glia release ATP. This article reviews at cellular, synaptic, and network levels, current understanding and emerging concepts about the diverse roles played by this three-part signaling system in: mediating the chemosensitivity of respiratory networks to hypoxia and CO2/pH; modulating the activity of rhythm generating networks and inspiratory motoneurons, and; controlling blood flow through the cerebral vasculature.

Detecting sweet and umami tastes in the gastrointestinal tract

Information about nutrients is a critical part of food selection in living creatures. Each animal species has developed its own way to safely seek and obtain the foods necessary for them to survive and propagate. Necessarily, humans and other vertebrates have developed special chemosensory organs such as taste and olfactory organs. Much attention, recently, has been given to the gastrointestinal (GI) tract as another chemosensory organ. Although the GI tract had been considered to be solely for digestion and absorption of foods and nutrients, researchers have recently found taste-signalling elements, including receptors, in this tissue. Further studies have revealed that taste cells in the oral cavity and taste-like cells in the GI tract appear to share common characteristics. Major receptors to detect umami, sweet and bitter are found in the GI tract, and it is now proposed that taste-like cells reside in the GI tract to sense nutrients and help maintain homeostasis. In this review, we summarize recent findings of chemoreception especially through sweet and umami sensors in the GI tract. In addition, the possibility of purinergic transmission from taste-like cells in the GI tract to vagus nerves is discussed.

Redefining the components of central CO2 chemosensitivity--towards a better understanding of mechanism

Abstract

The field of CO₂ chemosensitivity has developed considerably in recent years. There has been a mounting number of competing nuclei proposed as chemosensitive along with an ever increasing list of potential chemosensory transducing molecules. Is it really possible that all of these areas and candidate molecules are involved in the detection of chemosensory stimuli? How do we discriminate rigorously between molecules that are chemosensory transducers at the head of a physiological reflexversusthose that just happen to display sensitivity to a chemosensory stimulus? Equally, how do we differentiate between nuclei that have a primary chemosensory function, versusthose that are relays in the pathway? We have approached these questions by proposing rigorous definitions for the different components of the chemosensory reflex, going from the salient molecules and ions, through the components of transduction to the identity of chemosensitive cells and chemosensitive nuclei. Our definitions include practical and rigorous experimental tests that can be used to establish the identity of these components. We begin by describing the need for central CO₂ chemosensitivity and the problems that the field has faced. By comparing chemosensory mechanisms to those in the visual system we suggest stricter definitions for the components of the chemosensory pathway. We then, considering these definitions, re-evaluate current knowledge of chemosensory transduction, and propose the ‘multiple salient signal hypothesis’ as a framework for understanding the multiplicity of transduction mechanisms and brain areas seemingly involved in chemosensitivity.

P2Y receptors and pain transmission

It is widely accepted that the most important ATP receptors involved in pain transmission belong to the P2X₃ and P2X_2/3 subtypes, selectively expressed in small diameter dorsal root ganglion (DRG) neurons. However, several types of the metabotropic ATP (P2Y) receptors have also been found in primary afferent neurons; P2Y₁ and P2Y₂ receptors are typically expressed in small, nociceptive cells. Here we review the results available on the involvement of P2Y receptors in the modulation of pain transmission.

Involvement of the purinergic system in central cardiovascular modulation at the level of the nucleus ambiguus of anaesthetized rats

Anatomical studies have demonstrated the existence of purinergic P2 receptors in the nucleus ambiguus (NA), a site containing cardiac vagal motoneurons. However, very little is known about the functional role of these receptors in central cardiac vagal regulation. The aims of our study were to evaluate the following: (1) the blood pressure and heart rate responses following purinoceptor activation within the NA; (2) the role of purinoceptors and excitatory amino acid (EAA) receptors in mediating the cardiovascular responses evoked by ATP and L-glutamate stimulation of NA; and (3) the role of NA purinoceptors in mediating the cardiovascular responses of the Bezold-Jarisch reflex. In anaesthetized rats, microinjection of L-glutamate (5.0 nmol/50 nl) into the NA induced a marked and immediate onset bradycardia with minimal change in arterial pressure. Microinjection of ATP into the NA induced a dose-dependent (0.31-6.0 nmol/50 nl) bradycardia and pressor responses. It is noteworthy that the bradycardia occurred either before or simultaneously with a pressor response (when present), indicating that it was not a baroreceptor reflex mediated response due to the rise in arterial pressure. The pressor response was prevented by α(1)-adrenergic blockade with prazosin, whereas muscarinic blockade with methyl-atropine abolished the evoked bradycardia. Ipsilateral microinjection of PPADS (a P2 receptor antagonist; 500 pmol/100 nl) into the NA significantly attenuated the ATP-induced bradycardia but spared the pressor response. In contrast, PPADS in the NA had no effect on the L-glutamate-evoked bradycardic response. Ipsilateral injection of kynurenic acid (a non-selective EAA receptor antagonist; 10 nmol/50 nl) into the NA totally blocked the bradycardia induced by l-glutamate and partly attenuated the ATP induced bradycardia. Finally, both the depressor and the bradycardic responses of the Bezold-Jarisch reflex were attenuated significantly (P < 0.01 and P < 0.05, respectively) following bilateral microinjection of PPADS into the NA. These results identify ATP and purinergic P2 receptors within the ventrolateral medulla as excitatory to cardiovagal neurons. Additionally, our data show that P2 receptors within the ventrolateral medulla are integral to the cardiovascular responses of the Bezold-Jarisch reflex.

A new function for ATP: activating cardiac sympathetic afferents during myocardial ischemia

Myocardial ischemia activates cardiac sympathetic afferents leading to chest pain and reflex cardiovascular responses. Brief myocardial ischemia leads to ATP release in the interstitial space. Furthermore, exogenous ATP and α,β-methylene ATP (α,β-meATP), a P2X receptor agonist, stimulate cutaneous group III and IV sensory nerve fibers. The present study tested the hypothesis that endogenous ATP excites cardiac afferents during ischemia through activation of P2 receptors. Nerve activity of single unit cardiac sympathetic afferents was recorded from the left sympathetic chain or rami communicates (T(2)-T(5)) in anesthetized cats. Single fields of 45 afferents (conduction velocities = 0.25-4.92 m/s) were identified in the left ventricle with a stimulating electrode. Five minutes of myocardial ischemia stimulated 39 of 45 cardiac afferents (8 Aδ, 37 C fibers). Epicardial application of ATP (1-4 μmol) stimulated six ischemically sensitive cardiac afferents in a dose-dependent manner. Additionally, epicardial ATP (2 μmol), ADP (2 μmol), a P2Y agonist, and α,β-meATP (0.5 μmol) significantly activated eight other ischemically sensitive afferents. Third, pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid, a P2 receptor antagonist, abolished the responses of six afferents to epicardial ATP (2 μmol) and attenuated the ischemia-related increase in activity of seven other afferents by 37%. In the absence of P2 receptor blockade, cardiac afferents responded consistently to repeated application of ATP (n = 6) and to recurrent myocardial ischemia (n = 6). Finally, six ischemia-insensitive cardiac spinal afferents did not respond to epicardial ATP (2-4 μmol), although these afferents did respond to epicardial bradykinin. Taken together, these data indicate that, during ischemia, endogenously released ATP activates ischemia-sensitive, but not ischemia-insensitive, cardiac spinal afferents through stimulation of P2 receptors likely located on the cardiac sensory neurites.

Roles of gastro-oesophageal afferents in the mechanisms and symptoms of reflux disease

Oesophageal pain is one of the most common reasons for physician consultation and/or seeking medication. It is most often caused by acid reflux from the stomach, but can also result from contractions of the oesophageal muscle. Different forms of pain are evoked by oesophageal acid, including heartburn and non-cardiac chest pain, but the basic mechanisms and pathways by which these are generated remain to be elucidated. Both vagal and spinal afferent pathways are implicated by basic research. The sensitivity of afferent fibres within these pathways may become altered after acid-induced inflammation and damage, but the severity of symptoms in humans does not necessarily correlate with the degree of inflammation. Gastro-oesophageal reflux disease (GORD) is caused by transient relaxations of the lower oesophageal sphincter, which are triggered by activation of gastric vagal mechanoreceptors. Vagal afferents are therefore an emerging therapeutic target for GORD. Pain in the absence of excess acid reflux remains a major challenge for treatment.

Deletion of P2X3 receptors blunts gastro-oesophageal sensation in mice

Prior studies have demonstrated P2X receptor expression in the majority of nodose neurons. Immunoreactivity for P2X receptors has also been seen in putative gastric mechanoreceptors, the intraganglionic laminar endings. We therefore hypothesized that deletion of P2X3 receptors will blunt responses to gastric distension in vagal sensory neurons. Using wildtype and P2X3(-/-) mice, we examined responses to purinergic agonists in retrogradely labelled gastric sensory neurons with patch-clamp techniques. Activation of gastro-oesophageal neurons by fluid distension was studied with intracellular electrodes. Distension-evoked ATP release into the gastric lumen was determined with the luciferase assay and intake and gastric emptying of a solid meal was assessed. ATP triggered inward currents in 80% of gastric nodose neurons. In P2X3(-/-) mice, the peak current density was lower compared to controls. Ten of 14 controls but none of 30 neurons from P2X3(-/-) mice responded to alpha,beta-metATP. Gastro-oesophageal sensory neurons of P2X3(-/-) mice showed a blunted response to fluid distension of oesophagus and stomach. This difference was not explained by differences in distension-evoked ATP release, which did not differ between knockout mice and controls. Food intake during a 3-h period was lower in P2X3(-/-) mice. Gastric emptying of a solid meal was slightly faster in knockout mice after 1.5 h, but did not differ between groups at 3 h. Our data support a role of purinergic signalling in gastric vagal afferents. Considering the role of vagal input in sensations of fullness or nausea, P2X receptors may be interesting treatment targets for dyspeptic symptoms.

ATP in central respiratory control: a three-part signaling system

The landmark demonstrations in 2005 that ATP released centrally during hypoxia and hypercapnia contributes to the respective ventilatory responses validated a decade-old hypothesis and ignited interest in the potential significance of P2 receptor signaling in central respiratory control. Our objective in this review is to provide a non-specialist overview of ATP signaling from the perspective that it is a three-part system where the net effects are determined by an interaction between the signaling actions of ATP and adenosine at P2 and P1 receptors, respectively, and a family of enzymes (ectonucleotidases) that breakdown ATP into adenosine. We review the rationale for the original interest in P2 signaling in respiratory control, the evolution of this hypothesis, and the mechanisms by which ATP might affect respiratory behaviour. The potential significance of P2 receptor, P1 receptor and ectonucleotidase diversity for the different compartments of the respiratory control system is also considered. We conclude with a look to future questions and technical challenges.

ATP sensitivity of preBötzinger complex neurones in neonatal rat in vitro: mechanism underlying a P2 receptor-mediated increase in inspiratory frequency

P2 receptor (R) signalling plays an important role in the central ventilatory response to hypoxia. The frequency increase that results from activation of P2Y(1)Rs in the preBötzinger complex (preBötC; putative site of inspiratory rhythm generation) may contribute, but neither the cellular nor ionic mechanism(s) underlying these effects are known. We applied whole-cell recording to rhythmically-active medullary slices from neonatal rat to define, in preBötC neurones, the candidate cellular and ionic mechanisms through which ATP influences rhythm, and tested the hypothesis that putative rhythmogenic preBötC neurones are uniquely sensitive to ATP. ATP (1 mm) evoked inward currents in all non-respiratory neurones and the majority of respiratory neurons, which included inspiratory, expiratory and putative rhythmogenic inspiratory neurones identified by sensitivity to substance P (1 microM) and DAMGO (50 microM) or by voltage-dependent pacemaker-like activity. ATP current densities were similar in all classes of preBötC respiratory neurone. Reversal potentials and input resistance changes for ATP currents in respiratory neurones suggested they resulted from either inhibition of a K(+) channel or activation of a mixed cationic conductance. The P2YR agonist 2MeSADP (1 mm) evoked only the latter type of current in inspiratory and pacemaker-like neurones. In summary, putative rhythmogenic preBötC neurones were sensitive to ATP. However, this sensitivity was not unique; ATP evoked similar currents in all types of preBötC respiratory neurone. The P2Y(1)R-mediated frequency increase is therefore more likely to reflect activation of a mixed cationic conductance in multiple types of preBötC neurone than excitation of one, highly sensitive group.

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.

International Union of Pharmacology LVIII: update on the P2Y G protein-coupled nucleotide receptors: from molecular mechanisms and pathophysiology to therapy

There have been many advances in our knowledge about different aspects of P2Y receptor signaling since the last review published by our International Union of Pharmacology subcommittee. More receptor subtypes have been cloned and characterized and most orphan receptors de-orphanized, so that it is now possible to provide a basis for a future subdivision of P2Y receptor subtypes. More is known about the functional elements of the P2Y receptor molecules and the signaling pathways involved, including interactions with ion channels. There have been substantial developments in the design of selective agonists and antagonists to some of the P2Y receptor subtypes. There are new findings about the mechanisms underlying nucleotide release and ectoenzymatic nucleotide breakdown. Interactions between P2Y receptors and receptors to other signaling molecules have been explored as well as P2Y-mediated control of gene transcription. The distribution and roles of P2Y receptor subtypes in many different cell types are better understood and P2Y receptor-related compounds are being explored for therapeutic purposes. These and other advances are discussed in the present review.

Purinergic P2 receptors as targets for novel analgesics

Following hints in the early literature about adenosine 5'-triphosphate (ATP) injections producing pain, an ion-channel nucleotide receptor was cloned in 1995, P2X3 subtype, which was shown to be localized predominantly on small nociceptive sensory nerves. Since then, there has been an increasing number of papers exploring the role of P2X3 homomultimer and P2X2/3 heteromultimer receptors on sensory nerves in a wide range of organs, including skin, tongue, tooth pulp, intestine, bladder, and ureter that mediate the initiation of pain. Purinergic mechanosensory transduction has been proposed for visceral pain, where ATP released from epithelial cells lining the bladder, ureter, and intestine during distension acts on P2X3 and P2X2/3, and possibly P2Y, receptors on subepithelial sensory nerve fibers to send messages to the pain centers in the brain as well as initiating local reflexes. P1, P2X, and P2Y receptors also appear to be involved in nociceptive neural pathways in the spinal cord. P2X4 receptors on spinal microglia have been implicated in allodynia. The involvement of purinergic signaling in long-term neuropathic pain and inflammation as well as acute pain is discussed as well as the development of P2 receptor antagonists as novel analgesics.

Functions of neuronal P2Y receptors

Within the last 15 years, at least eight different G protein-coupled nucleotide receptors, i.e., P2Y receptors, have been characterized by molecular means. While ionotropic P2X receptors are mainly involved in fast synaptic neurotransmission, P2Y receptors rather mediate slower neuromodulatory effects. This P2Y receptor-dependent neuromodulation relies on changes in synaptic transmission via either pre- or postsynaptic sites of action. At both sites, the regulation of voltage-gated or transmitter-gated ion channels via G protein-linked signaling cascades has been identified as the predominant underlying mechanisms. In addition, neuronal P2Y receptors have been found to be involved in neurotoxic and neurotrophic effects of extracellular adenosine 5-triphosphate. This review provides an overview of the most prominent actions mediated by neuronal P2Y receptors and describes the signaling cascades involved.

[32P]2-iodo-N6-methyl-(N)-methanocarba-2'-deoxyadenosine-3',5'-bisphosphate ([32P]MRS2500), a novel radioligand for quantification of native P2Y1 receptors

Analysis of the P2Y family of nucleotide-activated G-protein-coupled receptors has been compromised by the lack of selective high-affinity, high-specific-radioactivity radioligands. We have pursued quantification of the P2Y1 receptor through the development of a series of selective P2Y1 receptor antagonists. Recently, we synthesized 2-iodo-N6-methyl-(N)-methanocarba-2'-deoxyadenosine 3',5'-bisphosphate (MRS2500), a selective, competitive antagonist that exhibits a Ki of 0.8 nM in competition-binding assays with [3H]MRS2279. A 3'-monophosphate precursor molecule, MRS2608, was radiolabeled at the 5' position with 32P using polynucleotide kinase and [gamma32P]ATP to yield [32P]MRS2500. [32P]MRS2500 bound selectively to Sf9 insect cell membranes expressing the human P2Y1 receptor (Sf9-P2Y1), but did not detectably bind membranes expressing other P2Y receptors. P2Y1 receptor binding to [32P]MRS2500 was saturable with a KD of 1.2 nM. Agonists and antagonists of the P2Y1 receptor inhibited [32P]MRS2500 binding in Sf9-P2Y1 membranes with values in agreement with those observed in functional assays of the P2Y1 receptor. A high-affinity binding site for [32P]MRS2500 (KD=0.33 nM) was identified in rat brain, which exhibited the pharmacological selectivity of the P2Y1 receptor. Distribution of this binding site varied among rat tissues, with the highest amount of binding appearing in lung, liver, and brain. Among brain regions, distribution of the [32P]MRS2500 binding site varied by six-fold, with the highest and lowest amounts of sites detected in cerebellum and cortex, respectively. Taken together, these data illustrate the synthesis and characterization of a novel P2Y1 receptor radioligand and its utility for examining P2Y1 receptor expression in native mammalian tissues.

Major classes of sensory neurons to the urinary bladder

A novel in vitro bladder preparation was used to examine effect of various stimuli (stretch, von Frey hair compression, stroking of receptive fields, applications of chemical stimuli to the mucosa) on electrophysiological recordings from guinea pig bladder afferents in vitro. Several functionally distinct classes of bladder sensory neurons were distinguished. These include stretch-sensitive afferents-muscle mechanoreceptors which behaved as "in-series tension receptors" and tension-mucosal mechanoreceptors, which could be activated by stretch, mucosal stroking with light von Frey hair (0.1-2 mN) and by hypertonic solutions (1 M mannitol and 490-850 mM NaCl) applied locally to their receptive fields in the mucosa. In addition, we have recorded stretch-insensitive afferents-mucosal mechanoreceptors and chemoreceptors. The non-selective P2X/P2Y purinoreceptor antagonist, PPADS (30 microM) did not affect stretch-induced firing by low threshold muscle mechanoreceptors but significantly inhibited alpha,beta-methylene ATP (30 microM)-induced contractions and associated afferent firing. Transduction by low threshold stretch-sensitive muscle mechanoreceptors does not appear to involve exocytotic synaptic transmission since it occurs in Ca2+-free (with 1 mM EDTA and 6 mM Mg2+) Krebs solution. The data suggest that the endogenous transmitter ATP is not involved in mechanotransduction by this specific class of low threshold muscle mechanoreceptors in the guinea pig bladder; rather they appear to transduce mechanical stimuli directly, possibly via stretch-activated ion channels. Mechanisms of activation of other classes of mechanoreceptors and chemoreceptors remain to be established.

Immunohistochemical localization of GAD67-expressing neurons and processes in the rat brainstem: subregional distribution in the nucleus tractus solitarius

The role of gamma-aminobutyric acid (GABA) in homeostatic control in the brainstem, in particular, in the nucleus tractus solitarius (NTS), is well established. However, to date, there is no detailed description of the distribution of GABAergic neurons within the NTS. The goal of the current study was to reexamine the efficacy of immunohistochemical localization of glutamic acid decarboxylase (GAD) protein, specifically the 67-kDa isoform (GAD67), as a marker for GABAergic neurons in the medulla and to provide a detailed map of GAD67-immunoreactive (-ir) cells within rat NTS by using a recently developed mouse monoclonal antibody. We describe a distribution of GAD67-ir cells in the medulla similar to that reported previously from in situ hybridization study. GAD67-ir cells were localized in regions known to contain high GABA content, including the ventrolateral medulla, raphe nuclei, and area postrema, but were absent from all motor nuclei, although dense terminal labeling was discerned in these regions. In the NTS, GAD67-ir was localized in all subregions. Semiquantitative analysis of the GAD67-ir distribution in the NTS revealed greater numbers of GAD67-ir cells medial to the solitary tract. Finally, dense GAD67 terminal labeling was found in the medial, central, intermediate, commissural, and subpostremal subregions, whereas sparse labeling was observed in the ventral subregion. Our findings support the use of immunohistochemistry for GAD67 as a marker for the localization of GABAergic cells and terminal processes in the rat brainstem. Furthermore, the reported heterogeneous distribution of GAD67-ir in the NTS suggests differential inhibitory modulation of sensory processing.

Purinoceptors are involved in the induction of an osmolyte permeability in malaria-infected and oxidized human erythrocytes

In human erythrocytes, infection by the malaria parasite Plasmodium falciparum or oxidative stress induces a new organic osmolyte and anion permeability. To examine a role for autocrine purinoceptor signaling during this induction process, erythrocytic purinoceptor expression, and ATP release were determined. Furthermore, using pharmacological and genetic approaches the dependence on purinoceptor signaling of osmolyte permeability and Plasmodium development, both in vitro and in vivo, were assessed. Extracellular ATP did not induce an osmolyte permeability in non-infected or non-oxidized erythrocytes. ATP and other purinoceptor agonists increased the induction of osmolyte permeability during infection or oxidation as measured by isosmotic hemolysis and patch-clamp recording. Purinoceptor antagonists and apyrase decreased the induced permeability. The observed pharmacology suggested the involvement of P2Y purinoceptors. Accordingly, human erythrocytes expressed P2Y1 protein. Moreover, P2Y1-deficient mouse erythrocytes exhibited a delayed appearance of the osmolyte permeability during P. berghei infection- or oxidation compared with wild-type erythrocytes. Furthermore, the nonspecific purinoceptor antagonist suramin decreased in vitro growth and DNA/RNA amplification of P. falciparum in human erythrocytes and decreased in vivo growth of P. berghei. P. berghei developed slower in P2Y1-deficient mice in vivo compared with wild-type animals. In conclusion, induction of the osmolyte permeability in Plasmodium-infected erythrocytes involves autocrine purinoceptor signaling.

Mechanisms of mechanotransduction by specialized low-threshold mechanoreceptors in the guinea pig rectum

The guinea pig rectum, but not the colon, is innervated by a specialized class of distension-sensitive mechanoreceptors that have transduction sites corresponding to rectal intraganglionic laminar endings (rIGLEs). Rectal mechanoreceptors recorded in vitro had low threshold to circumferential stretch, adapted slowly, and could respond within 2 ms to mechanical stimulation by a piezo-electric probe. Antagonists to ionotropic N-methyl-D-aspartate (NMDA; CGS 19755, memantine) and non-NMDA (6,7-dinitroquinoxaline-2,3-dione) glutamate receptors did not affect mechanotransduction. In normal Krebs solution, the P2X purinoreceptor agonist alpha,beta-methylene ATP reduced mechanoreceptor firing evoked by distension but simultaneously relaxed circular smooth muscle and inhibited stretch-induced contractions. Neither ATP nor alpha,beta-methylene ATP affected mechanotransduction when transduction sites were directly compressed with von Frey hairs. The P2 purinoreceptor antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid did not affect stretch-induced firing but reduced the inhibitory effect of alpha,beta-methylene ATP on stretch-induced firing. Under isometric conditions, blocking synaptic transmission in Ca2+-free solution reduced stretch-evoked firing but not when basal tension was restored to control levels. Under isotonic condition, Ca2+-free solution did not significantly affect load-evoked firing. The blockers of mechanogated and/or transient receptor potential channels, benzamil, Gd3+, SKF 96365, and ruthenium red inhibited stretch-induced firing but, in parallel, significantly reduced stretch-induced contractions. Benzamil and SKF 96365 were able to inhibit mechanotransduction when transduction sites were compressed with von Frey hairs. The results show that mechanotransduction is rapid but does not depend on fast exocytotic release of mediators. It is likely that stretch-activated ion channels on rIGLEs are involved in direct, physical mechanotransduction by rectal low-threshold mechanoreceptors.

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.

Multiple actions of extracellular ATP and adenosine on calcium currents mediated by various purinoceptors in neurons of nucleus tractus solitarius

Whole-cell patch-clamp recordings were performed on freshly dissociated nucleus tractus solitarius (NTS) of rat to determine the action of extracellular adenosine 5'-triphosphate (ATP) and adenosine (ADO) on voltage-dependent calcium channel (VDCC) currents (I(Ca)). Application of ATP and ATP-analog inhibited I(Ca). The rank order of potency of inhibition of I(Ca) was 2-methylthioATP (2-MeSATP) > ATP > adenosine 5'-diphosphate (ADP) >> alpha,beta-methylene ATP (alpha,beta-MeATP) = uridine 5'-triphosphate (UTP). Application of ADO receptor agonists also inhibited I(Ca). The rank order of potency of inhibition of I(Ca) was N(6)-cyclohexyladenosine (CHA) > ADO > 2-(4-(2-carboxyethyl)phenylethylamino)adenosine-5'-N-ethylcarboxamideadenosine (CGS-21680) > N(6)-2-(4-aminophenyl)ethyladenosine (APNEA). Application of prepulse attenuated these inhibition. Both intracellular dialysis of guanosin 5'-O-(2-thiodiphosphate) (GDP-beta-S) and anti-G(i) antibody also attenuated these inhibition. L-, N- and P/Q-type VDCCs were inhibited by ATP. In contrast, N- and P/Q-type VDCCs were inhibited by ADO. In addition to inhibition, application of 100 microM ATP facilitated I(Ca). Intracellular dialysis of GDP-beta-S did not attenuate these facilitations. In conclusion, activation of P2Y purinoceptors inhibits L-, N- and P/Q-types VDCCs via G(i)-protein betagamma subunits. Activation of A(1) and/or A(2) receptors inhibit N- and P/Q-types VDCCs via G(i)-protein betagamma subunits. Activation of P2X purinoceptors facilitates Ca(2+) entry in NTS.

Extracellular ATP both inhibits and facilitates calcium channel currents in acutely dissociated rat nucleus tractus solitarius

The postsynaptic actions of exogenously applied adenosine 5'-triphosphate (ATP) were investigated in nucleus tractus solitarius (NTS) of the rat. Whole cell patch-clamp recordings were used to examine the regulation of voltage-dependent Ca2+ channels (VDCCs) currents (I(Ca)) by ATP in freshly dissociated NTS. Application of ATP inhibited I(Ca) from -905 pA to -741 pA. In addition to this inhibition, application of ATP facilitated I(Ca) from -941 pA to -1,094 pA in other neurons. The data presented here demonstrate for the first time that ATP has both inhibitory and facilitative effects on I(Ca) in NTS. It can be considered that ATP acts as a neurotransmitter in the NTS by having multiple regulatory effects on VDCCs.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

ATP modulates intracellular Ca2+ and firing rate through a P2Y1 purinoceptor in cane toad pacemaker cells

The effect of extracellular ATP (10-100 microM) on intracellular Ca2+ concentration ([Ca2+]i) and firing rate has been studied in single pacemaker cells isolated from the sinus venosus of cane toads. In spontaneously firing cells, ATP initially increased peak [Ca2+]i by 43 +/- 5 %, increased diastolic [Ca2+]i by 20 + 3 % and increased the firing rate by 58 +/- 8 %. These early effects were followed by a late phase in which both the peak [Ca2+]i and the firing rate declined. Adenosine, and UTP (respectively, P1- and P2Y2,4,6-selective agonists) caused no significant change in [Ca2+]i or firing rate, while alphabeta-methylene ATP (a P2X1,3 agonist) caused a small increase in firing rate but no changes in [Ca2+]i. In contrast the P2Y1-selective agonist 2-MesADP (1 microM) mimicked the biphasic effects of ATP and these effects were inhibited by the purinoceptor antagonists suramin and PPADS and by the P2Y1-selective antagonist MRS 2179. Immunohistochemistry established that P2Y1 purinoceptors were present on the cell surface. Western blotting analysis demonstrated that the P2Y1 antibody recognised a 57 kDa protein. After sarcoplasmic reticulum Ca2+ release was prevented with caffeine or ryanodine, ATP no longer had any effect on [Ca2+]i or firing rate. Furthermore, the SR Ca2+ store content was decreased during the late phase of 2-MesADP application. The effect of ATP was coupled to phospholipase C (PLC) activity because the PLC inhibitor U-73122 eliminated the effects of ATP. Our study shows that in toad pacemaker cells, the biphasic effects of ATP on pacemaker activity are mainly through P2Y1 purinoceptors, which are able to modulate Ca2+ release from the SR Ca2+ store.

Crossing the pain barrier: P2 receptors as targets for novel analgesics

In 1995 the P2X3 receptor was found to be expressed at high levels in nociceptive sensory neurones, consistent with earlier reports that ATP induced pain in humans and animals. At first it was thought that ATP was most likely to play a role in acute pain, following its release from damaged or stressed cells and since then a wide variety of experimental techniques and approaches have been used to study this possibility. Whilst it is clear that exogenous and endogenous ATP can indeed acutely stimulate sensory neurones, more recent reports using gene knockout and antisense oligonucleotide technologies, and a novel, selective P2X3 antagonist, A-317491, all indicate that ATP and P2X3 receptors are more likely to be involved in chronic pain conditions, particularly chronic inflammatory and neuropathic pain. These reports indicate that P2X3 receptors on sensory nerves may be tonically activated by ATP released from nearby damaged or stressed cells, or perhaps from the sensory nerves themselves. This signal, when transmitted to the CNS, will be perceived consciously as chronic pain. In addition, it is now clear that several subtypes of P2Y receptor are also expressed in sensory neurones. Although their distribution and functions have not been as widely studied as P2X receptors, the effects that they mediate indicate that they might also be considered as therapeutic targets in the treatment of pain. Although our ability to treat persistent painful conditions, such as chronic inflammatory and neuropathic pain, has improved in recent years, these conditions are often resistant to currently available therapies, such as opioids or non-steroidal anti-inflammatory drugs. This reflects a limited understanding of the underlying pathophysiology. It is now clear that the development and maintenance of chronic pain are mediated by multiple factors, but many of these factors, and the receptors and mechanisms through which they act, remain to be identified. Chronic pain is debilitating and can greatly decrease quality of life, not just due to the pain per se, but also because of the depression that can often ensue. Thus a greater understanding of the mechanisms that underlie chronic pain will help identify new targets for novel analgesics, which will be of great therapeutic benefit to many people.

Mechanotransduction by intraganglionic laminar endings of vagal tension receptors in the guinea-pig oesophagus

Vagal mechanoreceptors to the guinea-pig oesophagus, recorded extracellularly, in vitro, fired spontaneously at 3.3 +/- 0.2 Hz, (n = 75, from 57 animals), and had low thresholds to circumferential stretch. In this study, we have investigated whether mechanotransduction by intraganglionic laminar endings (IGLEs) directly relies on mechano-gated ion channels, or whether it is due to chemical activation by neurotransmitters (glutamate or ATP) released from other cells during mechanical distortion. Rapid distortion of focal transduction sites (IGLEs) evoked action potentials with a latency of < 10 ms. Antagonists to ionotropic (AP5, memantine and 6,7-dinitroquinoxaline-2,3-dione (DNQX)) and metabotropic glutamate receptors (N-phenyl-7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxamide (PHCCC) and (RS)-a-methyl-4-phosphono-phenylglycine (MPPG)) did not affect mechano-transduction. Glutamate, NMDA and the selective mGluR group II and III agonists, (2R, 4R)-APDC and L-AP4, had no effect on spontaneous or stretch-induced firing. The P2X purinoreceptor agonist, alpha,beta-methylene ATP, caused concentration-dependent excitation of vagal mechanoreceptors (EC50 = 22.2 microM) which was blocked by the non-selective P2 antagonist PPADS (30 microM). On its own, PPADS affected neither stretch-induced firing nor spontaneous firing. Neither Ca(2+)-free solution (1 mM EDTA, 3.6 mM Mg(2+)) solution nor Cd(2+) (100 microM) blocked stretch-induced firing. Thus chemical transmission is not involved in activation of vagal mechanoreceptors. The blocker of stretch-activated channels, Gd(3+) (300 microM), did not inhibit stretch-induced firing. However, benzamil (100 microM) significantly inhibited spontaneous and distension-evoked firing in a stretch-dependent manner; proportionally greater inhibition was seen with larger stretches. The results suggest that IGLEs of vagal tension receptors directly transduce mechanical stimuli probably via benzamil-sensitive, Gd3+-insensitive, stretch-activated ion channels, and that chemical transmission is not involved in transduction.