Evidence for P2X3 receptors in the developing rat brain

Coordinated calcium signalling in cochlear sensory and non-sensory cells refines afferent innervation of outer hair cells

Outer hair cells (OHCs) are highly specialized sensory cells conferring the fine‐tuning and high sensitivity of the mammalian cochlea to acoustic stimuli. Here, by genetically manipulating spontaneous Ca²⁺ signalling in mice in vivo, through a period of early postnatal development, we find that the refinement of OHC afferent innervation is regulated by complementary spontaneous Ca²⁺ signals originating in OHCs and non‐sensory cells. OHCs fire spontaneous Ca²⁺ action potentials during a narrow period of neonatal development. Simultaneously, waves of Ca²⁺ activity in the non‐sensory cells of the greater epithelial ridge cause, via ATP‐induced activation of P2X₃ receptors, the increase and synchronization of the Ca²⁺ activity in nearby OHCs. This synchronization is required for the refinement of their immature afferent innervation. In the absence of connexin channels, Ca²⁺ waves are impaired, leading to a reduction in the number of ribbon synapses and afferent fibres on OHCs. We propose that the correct maturation of the afferent connectivity of OHCs requires experience‐independent Ca²⁺ signals from sensory and non‐sensory cells.

Purinergic signalling during development and ageing

Extracellular purines and pyrimidines play major roles during embryogenesis, organogenesis, postnatal development and ageing in vertebrates, including humans. Pluripotent stem cells can differentiate into three primary germ layers of the embryo but may also be involved in plasticity and repair of the adult brain. These cells express the molecular components necessary for purinergic signalling, and their developmental fates can be manipulated via this signalling pathway. Functional P1, P2Y and P2X receptor subtypes and ectonucleotidases are involved in the development of different organ systems, including heart, blood vessels, skeletal muscle, urinary bladder, central and peripheral neurons, retina, inner ear, gut, lung and vas deferens. The importance of purinergic signalling in the ageing process is suggested by changes in expression of A1 and A2 receptors in old rat brains and reduction of P2X receptor expression in ageing mouse brain. By contrast, in the periphery, increases in expression of P2X3 and P2X4 receptors are seen in bladder and pancreas.

Modulation of the neuronal network activity by P2X receptors and their involvement in neurological disorders

ATP is a key energetic molecule, fundamental to cell function, which also has an important role in the extracellular milieu as a signaling molecule, acting as a chemoattractant for immune cells and as a neuro- and gliotransmitter. The ionotropic P2X receptors are members of an ATP-gated ion channels family. These ionotropic receptors are widely expressed through the body, with 7 subunits described in mammals, which are arranged in a trimeric configuration with a central pore permeable mainly to Ca(2+) and Na(+). All 7 subunits are expressed in different brain areas, being present in neurons and glia. ATP, through these ionotropic receptors, can act as a neuromodulator, facilitating the Ca(2+)-dependent release of neurotransmitters, inducing the cross-inhibition between P2XR and GABA receptors, and exercising by this way a modulation of synaptic plasticity. Growing evidence shows that P2XR play an important role in neuronal disorders and neurodegenerative diseases, like Parkinson's and Alzheimer's disease; this role involves changes on P2XR expression levels, activation of key pathways like GSK3β, APP processing, oxidative stress and inflammatory response. This review is focused on the neuromodulatory function of P2XR on pathophysiological conditions of the brain; the recent evidence could open a window to a new therapeutic target.

Targeted delivery of a SNARE protease to sensory neurons using a single chain antibody (scFv) against the extracellular domain of P2X3 inhibits the release of a pain mediator

A single-chain antibody was generated against an extracellular domain of human P2X3 as a targeting moiety. It was conjugated with a pain therapeutic SNARE protease derived from BoNT/A to demonstrate its intracellular delivery into pain-sensing neurons.

Purines as potential morphogens during embryonic development

Components of purinergic signalling are expressed in the early embryo raising the possibility that ATP, ADP and adenosine may contribute to the mechanisms of embryonic development. We summarize the available data from four developmental models—mouse, chick, Xenopus and zebrafish. While there are some notable examples where purinergic signalling is indeed important during development, e.g. development of the eye in the frog, it is puzzling that deletion of single components of purinergic signalling often results in rather minor developmental phenotypes. We suggest that a key step in further analysis is to perform combinatorial alterations of expression of purinergic signalling components to uncover their roles in development. We introduce the concept that purinergic signalling could create novel morphogenetic fields to encode spatial location via the concentration of ATP, ADP and adenosine. We show that using minimal assumptions and the known properties of the ectonucleotidases, complex spatial patterns of ATP and adenosine can be set up. These patterns may provide a new way to assess the potential of purinergic signalling in developmental processes.

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.

Purinergic signaling in embryonic and stem cell development

Nucleotides are of crucial importance as carriers of energy in all organisms. However, the concept that in addition to their intracellular roles, nucleotides act as extracellular ligands specifically on receptors of the plasma membrane took longer to be accepted. Purinergic signaling exerted by purines and pyrimidines, principally ATP and adenosine, occurs throughout embryologic development in a wide variety of organisms, including amphibians, birds, and mammals. Cellular signaling, mediated by ATP, is present in development at very early stages, e.g., gastrulation of Xenopus and germ layer definition of chick embryo cells. Purinergic receptor expression and functions have been studied in the development of many organs, including the heart, eye, skeletal muscle and the nervous system. In vitro studies with stem cells revealed that purinergic receptors are involved in the processes of proliferation, differentiation, and phenotype determination of differentiated cells. Thus, nucleotides are able to induce various intracellular signaling pathways via crosstalk with other bioactive molecules acting on growth factor and neurotransmitter receptors. Since normal development is disturbed by dysfunction of purinergic signaling in animal models, further studies are needed to elucidate the functions of purinoceptor subtypes in developmental processes.

P2 receptor expression in the dopaminergic system of the rat brain during development

Extracellular ATP facilitates the release of dopamine via P2 receptor activation in parts of the mesolimbic system. To characterize P2X/Y receptor subtypes in the developing dopaminergic system, their expression in organotypic slice co-cultures including the ventral tegmental area/substantia nigra (VTA/SN) complex and the prefrontal cortex (PFC) was studied in comparison to the receptor expression in 3-5 day-old and adult rats. Reverse transcriptase-polymerase chain reaction (RT-PCR) with specific primers for the P2X(1,2,3,4,6,7) and P2Y(1) receptors in the tissue extracts of organotypic co-cultures revealed the presence of the P2X and P2Y receptor mRNAs investigated. Multiple immunofluorescence labeling of the P2X/Y receptor protein indicated differences in the regional expression in the organotypic co-cultures after 10 days of cultivation (VTA/SN, P2X(1,2,3,4,6,7), P2Y(1,6,12); PFC, P2X(1,3,4,6,7), P2Y(1,2,4,6,12)). At postnatal days 3-5, an immunofluorescence mostly comparable to that of adult rats was observed (VTA/SN and PFC: P2X(1,2,3,4,6,7), P2Y(1,2,4,6,12)). There was one important exception: the P2X(7) receptor immunocytochemistry was not found in adult tissue, suggesting a potential role of this receptor in the development. Only few P2 receptors (e.g. P2X(1), P2Y(1)) were expressed at fibers interconnecting the dopaminergic VTA/SN with the PFC in the organotypic co-cultures. The treatment of the cultures with the ATP analogues 2-methylthio-ATP and alpha,beta-methylene-ATP induced an increase in axonal outgrowth and fiber density, which could be inhibited by pre-treatment with the P2X/Y receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid. The co-localization of the dopamine-(D1) receptor with the P2X(1) receptor in organotypic slice cultures was evident. In the PFC of the co-cultures, and that of young but not adult rats, a number of tyrosine hydroxylase (TH)-positive cells also possessed P2Y(1)-immunoreactivity (IR). Additionally, a strong P2Y(1)-IR was observed on astrocytes. The present results show a time-, region- and cell type-dependent in vitro and in vivo expression pattern of different P2 receptor subtypes in the dopaminergic system indicating the involvement of ATP and its receptors in neuronal development and growth.

New insights into purinergic receptor signaling in neuronal differentiation, neuroprotection, and brain disorders

Ionotropic P2X and metabotropic P2Y purinergic receptors are expressed in the central nervous system and participate in the synaptic process particularly associated with acetylcholine, GABA, and glutamate neurotransmission. As a result of activation, the P2 receptors promote the elevation of free intracellular calcium concentration as the main signaling pathway. Purinergic signaling is present in early stages of embryogenesis and is involved in processes of cell proliferation, migration, and differentiation. The use of new techniques such as knockout animals, in vitro models of neuronal differentiation, antisense oligonucleotides to induce downregulation of purinergic receptor gene expression, and the development of selective inhibitors for purinergic receptor subtypes contribute to the comprehension of the role of purinergic signaling during neurogenesis. In this review, we shall discuss the participation of purinergic receptors in developmental processes and in brain physiology, including neuron-glia interactions and pathophysiology.

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.

An antibody to the beta-secretase cleavage site on amyloid-beta-protein precursor inhibits amyloid-beta production

Proteolytic cleavage of amyloid-beta-protein precursor (AbetaPP) by beta- and gamma-secretases results in production of the amyloid-beta peptide (Abeta) that accumulates in the brains of sufferers of Alzheimer's disease (AD). We have developed a monoclonal antibody, 2B12, which binds in the vicinity of the beta-secretase cleavage site on AbetaPP but does not bind within the Abeta region. We hypothesised that this antibody, directed against the substrate rather than the enzyme, could inhibit cleavage of AbetaPP by beta-secretase via steric hindrance and thus reduce downstream production of Abeta. The antibody would enter cells by binding to AbetaPP when it is at the cell surface and then be internalised with the protein. We subsequently demonstrated that, after addition of 2B12 to standard growth media, this antibody was indeed capable of inhibiting Abeta40 production in neuroblastoma and astrocytoma cells expressing native AbetaPP, as measured by an ELISA. This inhibition was both concentration- and time-dependent and was specific to 2B12. We were only able to inhibit approximately 50% of Abeta40 production suggesting that not all AbetaPP is trafficked to the cell surface. We propose that this antibody could be used as a novel, putative therapy for the treatment of AD.

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.

Nucleotide signaling in nervous system development

The development of the nervous system requires complex series of cellular programming and intercellular communication events that lead from the early neural induction to the formation of a highly structured central and peripheral nervous system. Neurogenesis continuously takes place also in select regions of the adult mammalian brain. During the past years, a multiplicity of cellular control mechanisms has been identified, ranging from differential transcriptional mediators to inducers or inhibitors of cell specification or neurite outgrowth. While the identification of transcription factors typical for the stage-specific progression has been a topic of key interest for many years, less is known concerning the potential multiplicity of relevant intercellular signaling pathways and the fine tuning of epigenetic gene regulation. Nucleotide receptors can induce a multiplicity of cellular signaling pathways and are involved in multiple molecular interactions, thus opening the possibility of cross talk between several signaling pathways, including growth factors, cytokines, and extracellular matrix components. An increasing number of studies provides evidence for a role of nucleotide signaling in nervous system development. This includes progenitor cell proliferation, cell migration, neuronal and glial cellular interaction and differentiation, and synaptic network formation.

P2 receptor-stimulation influences axonal outgrowth in the developing hippocampus in vitro

Extracellular ATP might act as a trophic factor on growing axons during development of the CNS via P2 receptors. In the present study the postnatal presence of selected P2 receptor subtypes was analyzed and their putative trophic capacity in entorhino-hippocampal slice co-cultures of mouse brain was tested. The effect of the P2 receptor ligands 2-methylthioadenosine-5'-triphosphate (P2X/Y receptor agonist) and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (P2X/Y receptor antagonist) on axonal growth and fiber density of biocytin-labeled hippocampal projections was compared both with untreated cultures and with cultures treated with artificial cerebrospinal fluid. After 10 days in vitro, double immunofluorescence labeling revealed the expression of P2X(1), P2X(2), P2X(4) as well as P2Y(1) and P2Y(2) receptors in the examined regions of entorhinal fiber termination. Further, quantitative analysis of identified biocytin-traced entorhinal fibers showed a significant increase in fiber density in the dentate gyrus after incubation of the slices with the P2 receptor agonist 2-methylthioadenosine-5'-triphosphate. This neurite outgrowth promoting effect was completely abolished by the P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid. Our in vitro data indicate that ATP via its P2X and P2Y receptors can shape hippocampal connectivity during development.

P2X purinoceptor subtypes on paraventricular nucleus neurones projecting to the rostral ventrolateral medulla in the rat

The rostral ventrolateral medulla (RVLM) is essential for the generation of sympathetic nerve activity. The RVLM receives a substantial innervation from the hypothalamic paraventricular nucleus (PVN). Activation of P2X purinoceptors via ATP has been shown to mediate fast excitatory synaptic neurotransmission. There is mounting evidence to suggest the presence of P2X purinoceptors in hypothalamic nuclei, including the PVN. In this study, we determined whether P2X1-P2X6 purinoceptor subtypes were present on PVN neurones that projected to the RVLM. Injection of the retrogradely transported tracer, rhodamine-tagged microspheres, into the pressor region of the RVLM was used to identify the neurones in the PVN that innervated the RVLM. P2X1-P2X6 purinoceptors were detected by immunohistochemistry. Double-labelled neurones were quantified and expressed as a proportion of the retrogradely labelled neurones. The proportions of double-labelled neurones for each of the P2X purinoceptor subtypes varied, on average, from 14 to 29%. The P2X3 purinoceptor subtype was found to be the dominant purinoceptor subtype present on PVN neurones projecting to the RVLM. Additionally it was apparent that more than one P2X purinoceptor subtype was present on the PVN neurones projecting to the RVLM, since the sum of the average percentages of double-labelled neurones for each P2X purinoceptor subtype exceeded 100%. These findings highlight the presence of the P2X1-P2X6 purinoceptors on PVN neurones projecting to the RVLM. The results suggest a potential role for ATP in the PVN in the regulation of sympathetic nerve activity.

Developmentally regulated expression of the P2X3 receptor in the mouse cochlea

ATP-gated non-selective cation channels assembled from P2X(3) receptor subunits contribute to transduction and neurotransmitter signaling in peripheral sensory systems and also feature prominently in the development of the central nervous system. In this study, P2X(3) receptor expression was characterized in the mouse cochlea from embryonic day 18 (E18) using confocal immunofluorescence. From E18 to P6, spiral ganglion neuron cell bodies and peripheral neurites projecting to the inner and outer hair cells were labeled. The inner spiral plexus associated with the inner hair cell synapses had a stronger fluorescence signal than outer spiral bundle fibers which provide the afferent innervation to the outer hair cells. Labeling in the cell bodies and peripheral neurites diminished around P6, and was no longer detected after the onset of hearing (P11, P17, adult). In opposition to the axiom that P2X(3) expression is neuron-specific, inner and outer sensory hair cells were labeled in the base and mid turn region at E18, but at P3 only the outer hair cells in the most apical region of the cochlea continued to express the protein. These data suggest a role for P2X(3) receptor-mediated purinergic signaling in cochlear synaptic reorganization, and establishment of neurotransmission, which occurs just prior to the onset of hearing function.

Developmental downregulation of P2X3 receptors in motoneurons of the compact formation of the nucleus ambiguus

Motoneurons of the compact division of the nucleus ambiguus (cNA) are the final output neurons of the swallowing pattern generator. Thus, their normal function is critical to neonatal survival. To explore the role of purinergic signaling in modulating the excitability of these motoneurons during development, immunohistochemical and whole-cell recording techniques were used to characterize expression patterns of ionotropic P2X receptors and the effects of ATP on cNA motoneurons. Medullary slices containing the cNA were prepared from neonatal (P0-4) and juvenile (P15-21) rats. In neonatal cNA motoneurons, local application of 1 mM ATP produced a large (-133 +/- 17 pA; n = 78), desensitizing, inward current that was mimicked by 1 mM alpha,beta meATP and 2meSATP, and inhibited by the P2 antagonist, PPADS (5 microM), and the P2X3 antagonist, A-317481 (0.1-1 mM). In juvenile cNA motoneurons, 1 mM ATP produced negligible currents, while 10 mM ATP produced small (-59 +/- 14 pA; n = 42), primarily non-desensitizing currents. Immunohistochemistry demonstrated that in the neonate, the expression of P2X3 was robust, P2X2 and P2X5 moderate, P2X4 and P2X6 weak, and P2X1 absent. In the juvenile cNA, only low levels of P2X5 and P2X6 labeling were detected. These data indicate that P2X receptors in cNA motoneurons are profoundly downregulated during the first two postnatal weeks, and suggest a role for the purinoceptor system, particularly P2X3 receptors, in the control of esophageal motor networks during early postnatal periods.

Involvement of P2 receptors in the growth and survival of neurons in the CNS

Extracellular adenosine 5'-triphosphate (ATP) has been recognized as a ubiquitous, unstable signalling molecule, acting as a fast neurotransmitter and modulator of transmitter release and neuronal excitability. Recent findings have demonstrated that ATP is a growth factor participating in differentiation, cell proliferation, and survival, as well as a toxic agent that mediates cellular degeneration and death. Potential sources of extracellular purines in the nervous system include neurons, glia, endothelium, and blood. A complex family of ectoenzymes rapidly hydrolyzes or interconverts extracellular nucleotides, thereby either terminating their signalling action or producing an active metabolite of altered purinoceptor selectivity. Most effects are mediated through the 2 main subclasses of specific cell surface receptors, P2X and P2Y. Members of these P2X/Y receptor families are widely expressed in the central nervous system (CNS) and are involved in glia-glia and glia-neuron communications, whereby they play important physiological and pathophysiological roles in a variety of biological processes. After different kinds of "acute" CNS injury (e.g., ischemia, hypoxia, mechanical stress, axotomy), extracellular ATP can reach high concentrations, up to the millimolar range, flowing out from cells into the extracellular space, exocytotically, via transmembrane transport, or as a result of cell damage. In this review, P2 receptor activation as a cause or a consequence of neuronal cell activation or death and/or glial activation is described. The involvement of P2 receptors is also described under different "chronic" pathological conditions, such as pain, epilepsia, toxic influence of ethanol or amphetamine, retinal diseases, Alzheimer's disease (AD), and possibly, Parkinson's disease. The relationship between changes in P2 receptor expression and the specific response of different cell types to injury is extremely complex and can be related to detrimental and/or beneficial effects. The present review therefore considers ATP acting via P2 receptors as a potent regulator of normal physiological and pathological processes in the brain, with a focus on pathophysiological implications of P2 receptor functions.

Differential chemosensory function and receptor expression of splanchnic and pelvic colonic afferents in mice

Lumbar splanchnic (LSN) and sacral pelvic (PN) nerves convey different mechanosensory information from the colon to the spinal cord. Here we determined whether these pathways also differ in their chemosensitivity and receptor expression. Using an in vitro mouse colon preparation, individual primary afferents were tested with selective P2X and transient receptor potential vanilloid receptor 1 (TRPV1) receptor ligands. Afferent cell bodies in thoracolumbar and lumbosacral dorsal root ganglia (DRG) were retrogradely labelled from the colon and analysed for P2X3- and TRPV1-like immunoreactivity (LI). Forty per cent of LSN afferents responded to alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-meATP; 1 mm), an effect that was concentration dependent and reversed by the P2X antagonist pyridoxyl5-phosphate 6-azophenyl-2',4'-disulphonic acid (PPADS) (100 microm). Significantly fewer PN afferents (7%) responded to alpha,beta-meATP. Correspondingly, 36% of colonic thoracolumbar DRG neurones exhibited P2X3-LI compared with only 19% of colonic lumbosacral neurones. Capsaicin (3 microm) excited 61% of LSN afferents and 47% of PN afferents; 82% of thoracolumbar and 50% of lumbosacral colonic DRG neurones displayed TRPV1-LI. Mechanically insensitive afferents were recruited by alpha,beta-meATP or capsaicin, and were almost exclusive to the LSN. Capsaicin-responsive LSN afferents displayed marked mechanical desensitization after responding to capsaicin, which did not occur in capsaicin-responsive PN afferents. Therefore, colonic LSN and PN pathways differ in their chemosensitivity to known noxious stimuli and their corresponding receptor expression. As these pathways relay information that may relate to symptoms in functional gastrointestinal disease, these results may have implications for the efficacy of therapies targeting receptor modulation.

Changes in expression of P2X purinoceptors in rat cerebellum during postnatal development

Changes in expression of P2X receptors (P2X1-7) during postnatal development of the rat cerebellum are described. At P3, immunoreactivity (ir) to all the P2X receptors, except for P2X3 receptors, was found in Purkinje cells and deep cerebellar nuclei, P2X5-ir being most prominent. Granular and microglial cells were labeled for P2X5 (weakly) and P2X4 receptors, respectively. At P7, expression of all the P2X receptors (with the exception of P2X3) was up-regulated, P2X5 and P2X6 receptors being most prominent. Scattered P2X receptor-ir in unipolar brush cells in the granular cell layer and P2X1- and P2X7-ir of microglial cells was also present. At P14, the dendritic trees of Purkinje cells were intensely labeled by P2X1-7 receptor antibodies, except for P2X3, while P2X1, P2X4 and P2X7 receptor immunostaining in microglial cells and P2X5 receptor immunostaining in granular cells was up-regulated. At P21, expression of all P2X receptors (except P2X3) was down-regulated in the Purkinje cells and deep cerebellar nuclei; P2X1, P2X4 and P2X7 receptors-ir was present in microglial cells. In contrast, expression of P2X5-ir in granular cells was up-regulated. At P60, expression levels of all the P2X receptors (except P2X3) were similar with those at P21. In double-labeling experiments, almost all the P2X-ir Purkinje cells were immunoreactive for calbindin-D28k, while 60-80% of P2X-ir cells in the granular cell layer were immunoreactive for calretinin. The possible short- and long-term functional significance of the changes in expression of P2X receptors during postnatal development is discussed.

Developmental regulation of neuron-specific P2X3 receptor expression in the rat cochlea

ATP-gated ion channels assembled from P2X3 receptor (P2X3R) subunits contribute to neurotransmission and neurotrophic signaling, associated with neurite development and synaptogenesis, particularly in peripheral sensory neurons. Here, P2X3R expression was characterized in the rat cochlea from embryonic day 16 (E16) to adult (P49-56), using RT-PCR and immunohistochemistry. P2X3R mRNA was strongly expressed in the cochlea prior to birth, declined to a minimal level at P14, and was absent in adult tissue. P2X3R protein expression was confined to spiral ganglion neurons (SGN) within Rosenthal's canal of the cochlea. At E16, immunolabeling was detected in the SGN neurites, but not the distal neurite projection within the developing sensory epithelium (greater epithelial ridge). From E18, the immunolabeling was observed in the peripheral neurites innervating the inner hair cells but was reduced by P6. However, from P2-8, immunolabeling of the SGN neurites extended to include the outer spiral bundle fiber tract beneath the outer hair cells. This labeling of type II SGN afferent fiber declined after P8. By P14, all synaptic terminal immunolabeling in the organ of Corti was absent, and SGN cell body labeling was minimal. In adult cochlear tissue, P2X3R immunolabeling was not detected. Noise exposure did not induce P2X3R expression in the adult cochlea. These data indicate that ATP-gated ion channels incorporating P2X3R subunit expression are specifically targeted to the afferent terminals just prior to the onset of hearing, and likely contribute to the neurotrophic signaling which establishes functional auditory neurotransmission.

Molecular physiology of P2 receptors in the central nervous system

Neurons of the central nervous system (CNS) are endowed with ATP-sensitive receptors belonging to the P2X (ligand-gated cationic channels) and P2Y (G protein-coupled receptors) types. Whereas a number of P2X receptors mediate fast synaptic responses to the transmitter ATP, P2Y receptors mediate either slow changes of the membrane potential in response to non-synaptically released ATP or the interaction with receptors for other transmitters. To date seven P2X and seven P2Y receptors of human origin have been molecularly identified and functionally characterized. P2X subunits may occur as homooligomers or as heterooligomeric assemblies of more than one subunit. P2X(7) subunits do not form heterooligomeric assemblies and are unique in mediating apoptosis and necrosis of glial cells and possibly also of neurons. The P2X(2), P2X(4), P2X(4)/P2X(6) and P2Y(1) receptors appear to be the predominant neuronal types. The localisation of these receptors may be at the somato-dendritic region (postsynaptic) or at the nerve terminals (presynaptic). Postsynaptic P2 receptors appear to be mostly excitatory, while presynaptic P2 receptors may be either excitatory (P2X) or inhibitory (P2Y). Since in the CNS the stimulation of a single neuron may activate multiple networks, a concomitant stimulation of facilitatory and inhibitory circuits as a result of ATP release is also possible. Finally, the enzymatic degradation of ATP may lead to the local generation of adenosine which can modulate via A(1) or A(2A) receptor-activation the ATP effect.

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.

P2X2 and P2X3 receptor expression in human bladder urothelium and changes in interstitial cystitis

OBJECTIVE

To investigate whether the expression of P2X(3) receptors (implicated in the pathophysiology of pain) is altered in human bladder urothelium from patients with interstitial cystitis (IC, a major symptom of which is pain), and as P2X(2) receptors can be co-expressed with P2X(3) receptors, to assess their expression also.

PATIENTS AND METHODS

Bladder tissue samples were collected from patients undergoing cystectomy or radical prostatectomy. Patients with IC were diagnosed using the international criteria. RNA protein expression levels of both receptors were evaluated using reverse transcription-polymerase chain reaction (PCR), real-time quantitative PCR and Western blot analysis.

RESULTS

P2X(2) was expressed in the human urothelium, in a glycosylated form. There was less gene expression of P2X(3) in IC urothelium, whereas P2X(2) gene expression was unchanged. This contrasted with the protein expression, which was increased for both P2X(2) and P2X(3).

CONCLUSION

This is the first report of the expression of the P2X(2) receptor in human bladder urothelium. There was greater protein expression of both P2X(2) and P2X(3) in IC bladder urothelium which did not directly correlate with the gene expression. Changes in expression of P2X(2) and P2X(3) receptors may contribute to the pain that patients with IC have, and might provide novel drug targets.

P2X3receptor localizes into lipid rafts in neuronal cells

P2X receptors are a family of seven (P2X(1-7)) cation channels gated by extracellular ATP, widely expressed in neurons and nonneuronal cells. Lipid rafts are cholesterol/sphingolipid-rich membrane domains, involved in many cellular processes, including transmembrane receptor signaling, vesicle traffic, and protein sorting. We provide direct biochemical evidence that P2X3 receptor localizes into lipid rafts, in primary cultures of cerebellar granule neurons as well as in brain and dorsal root ganglia extracts. We show that P2X3 exhibits all the characteristics distinctive of a protein associated with lipid rafts. These characteristics include resistance to detergent extraction at 4 degrees C, solubility after extraction of cholesterol from membranes with either saponin or methyl-beta-cyclodextrin, and partitioning to low buoyant density fractions after sucrose gradient centrifugation in both detergent-containing and detergent-free conditions. Furthermore, P2X3 localizes in raft-containing fractions in transiently transfected SH-SY5Y neuroblastoma cells. The present finding contributes to the characterization of the functional localization of P2X3 in neurons and provides a novel potential mechanism for correct targeting and dynamic activation of this receptor.

Characterization of the primary spinal afferent innervation of the mouse colon using retrograde labelling

Visceral pain is the most common form of pain produced by disease and is thus of interest in the study of gastrointestinal (GI) complaints such as irritable bowel syndrome, in which sensory signals perceived as GI pain travel in extrinsic afferent neurones with cell bodies in the dorsal root ganglia (DRG). The DRG from which the primary spinal afferent innervation of the mouse descending colon arises are not well defined. This study has combined retrograde labelling and immunohistochemistry to identify and characterize these neurones. Small to medium-sized retrogradely labelled cell bodies were found in the DRG at levels T8-L1 and L6-S1. Calcitonin gene-related peptide (CGRP)- and P2X3-like immunoreactivity (LI) was seen in 81 and 32%, respectively, of retrogradely labelled cells, and 20% bound the Griffonia simplicifolia-derived isolectin IB4. CGRP-LI and IB4 were co-localized in 22% of retrogradely labelled cells, whilst P2X3-LI and IB4 were co-localized in 7% (vs 34% seen in the whole DRG population). Eighty-two per cent of retrogradely labelled cells exhibited vanilloid receptor 1-like immunoreactivity (VR1-LI). These data suggest that mouse colonic spinal primary afferent neurones are mostly peptidergic CGRP-containing, VR1-LI, C fibre afferents. In contrast to the general DRG population, a subset of neurones exist that are P2X3 receptor-LI but do not bind IB4.

P2X(3) receptor expression at early stage of mouse embryogenesis

In this study we examined the expression of P2X(3) receptor in mouse embryos from E9.5 to E14.5 using immunohistochemistry. We found a uniform labeling in the developing trigeminal and dorsal root ganglia (DRG), while adult DRG and trigeminal ganglia expressed P2X(3) only in small-diameter neurons. In the brainstem, the mesencephalic trigeminal and facial motor nuclei were immunoreactive for P2X(3). P2X(3) was also transiently expressed in the developing brain, and precursors of spinal motor neurons. We also detected immunolabeling in the paravertebral sympathetic chain ganglia, in the sympathoadrenal cells and in non-neural tissues including testis, epidermis, wall of the aorta, as well as in subepidermal structures and mesenchymal tissues of limbs, branchial arches and tail.

Analgesic profile of intrathecal P2X(3) antisense oligonucleotide treatment in chronic inflammatory and neuropathic pain states in rats

Extracellular adenosine triphosphate (ATP), acting at P2X ionotropic receptors, is implicated in numerous sensory processes. Exogenous ATP has been shown to be algogenic in both animals and humans. Research focus has been directed towards the P2X(3) receptor, as it is preferentially expressed on nociceptive C-fibers and its implication in pain processing is supported by an altered nociceptive phenotype in P2X(3) knock-out mice. In order to further characterize the role of P2X(3) receptor activation in nociception, we evaluated the effects of continuous intrathecal administration of P2X(3) antisense oligonucleotides for 7 days in the rat. P2X(3) receptor antisense oligonucleotide treatment significantly decreased nociceptive behaviors observed after injection of complete Freund's adjuvant (CFA), formalin or alphabeta-methylene ATP into the rat's hind paw. The anti-hyperalgesic effects of the antisense treatment in the CFA model of inflammatory pain were dose related. Similar effects were observed with two distinct P2X(3) antisense oligonucleotides. These behavioral effects were significantly correlated with a decrease in P2X(3) receptor protein expression in the dorsal root ganglia (DRG). In contrast, a decrease in P2X(3) receptor protein expression in the DRG did not affect nociceptive behavior in the carrageenan model of acute thermal hyperalgesia. P2X(3) receptor antisense oligonucleotide treatment also significantly reduced mechanical allodynia observed after spinal nerve ligation. Overall, the present data demonstrate that activation of P2X(3) receptors contribute to the expression of chronic inflammatory and neuropathic pain states and that relief form these forms of chronic pain might be achieved by selective blockade of P2X(3 )receptor expression or activation.

Stimulation of Ca(2+) influx through ATP receptors on rat brain synaptosomes: identification of functional P2X(7) receptor subtypes

1. ATP receptors of the P2X class have previously been identified on autonomic nerve endings and on a limited population of CNS neurons. 2. In the present study P2X receptors on mammalian cortical synaptosomes have been identified by a variety of functional and biochemical studies. In choline buffer ATP analogues caused concentration/time dependent Ca(2+) influx. Relative to the effects caused by ATP, benzoylbenzoyl ATP (BzATP) was about seven times more active than ATP while 2-me-S-ATP and ATPgammaS were much less active. alpha,beta-me- ATP and beta,gamma-me-ATP were virtually inactive. In sucrose buffer, relative to choline buffer, the activity of BzATP was more than doubled while activity in sodium buffer was reduced. Moreover, the P2X antagonists PPADS or Brilliant Blue G both significantly attenuated influx. These observations suggest the presence of P2X receptors on synaptosomes which subserve Ca(2+) influx. This activity profile of the ATP analogues and the response to blocking agents are characteristic of responses of P2X(7) receptors. 3. Influx was unaffected by the VSCC inhibitors omega-CTx-MVIIC and (-) 202 - 791, indicating that ATP induced Ca(2+) influx occurred primarily through P2X receptors. 4. P2X(7) receptor protein was identified by Western blotting and immunohistochemical staining. Purified preparations were devoid of significant concentrations of GFAP or the microglial marker OX-42 but contained greatly enriched amounts of syntaxin and SNAP 25. 5. The various pharmacological and biochemical studies were all consistent with the presence of functional P2X(7) receptors.

On the immunohistochemical distribution of ionotropic P2X receptors in the nucleus tractus solitarius of the rat

ATP has been shown to excite neurones of the nucleus tractus solitarius (NTS) via the activation of P2X receptors. In the present study, the distribution of six P2X receptors (P2X(1)-P2X(6)) within the rat NTS was investigated by peroxidase immunohistochemistry. Immunopositive neurones for P2X receptor subtypes were detected in all divisions of the NTS, although the staining densities differed according to receptor subtype and sub-nuclei. P2X(1)-immunopositive cells were distributed throughout the rostro-caudal extent of the NTS, while varicose fibres were mainly located along the postremal border. P2X(2) immunoreactivity was present in neurones and fibres located throughout the NTS. In the commissural NTS intense staining was observed medial of the solitary tract while in the sub-postremal NTS neurones were observed along the postremal border. A high density of P2X(3)-positive neurones and fibres was observed in the sub-postremal NTS along the border of the area postrema and in the rostral NTS in the medial subdivision. In comparison to the staining observed with the other receptor antibodies, there was considerably reduced P2X(4) receptor immunoreactivity. P2X(4)-positive neurones tended to be more sparsely distributed, and found mainly in the intermediate portion of the commissural NTS, and along the postremal border. In contrast, we observed dense staining for the P2X(5) receptor subtype in a majority of regions within the NTS. The most striking staining was observed in the intermediate subdivision at the level of the sub-postremal NTS and the medial portion of the rostral NTS. P2X(6) immunoreactive neurones were observed in the medial commissural NTS, along the postremal border and in the dorso-medial and medial subdivisions of the rostral NTS.Taken together, our findings confirm the presence of six P2X receptor subtypes within the NTS of the rat, consistent with a neurotransmitter role for ATP in the rat NTS. These results indicate the need for more extensive functional studies to elucidate the roles of the individual and heterodimeric assemblies of P2X receptor subtypes within the NTS.

Localization of P2X3 receptors and coexpression with P2X2 receptors during rat embryonic neurogenesis

It is well known that extracellular ATP mediates rapid excitatory signaling by means of the ionotropic P2X receptors. One of its subunits, the P2X(3) receptor, is well documented to be associated with sensory innervation in adult animals. It is speculated that the P2X(3) receptor may have already been present in the early sensory system. The aim of this study was to investigate the distribution of the P2X(3) receptor during neurogenesis by using immunohistochemistry on rat embryos from embryonic day (E)9.5-18.5. The P2X(3) receptor was first identified in the hindbrain neural tube and the sensory ganglia in E11-11.5 embryos. At E14.5, the optic tract and retina, nucleus tractus solitarius, mesencephalic trigeminal nucleus, and sensory nerves in both respiratory and digestive tract showed positive staining. The facial nucleus, the prepositus hypoglossal nucleus, and the sympathetic ganglia also showed P2X(3) immunoreactivity, even though these are not sensory associated. P2X(3) immunoreactivity was detected in the vestibular nucleus, the nerves in mesentery, bladder, and kidney in E16.5 and in nerves in vibrissae in E18.5. P2X(3) immunoreactivity in the facial nucleus, spinal trigeminal tract, the mesencephalic trigeminal nucleus, and the vestibular nucleus were undetectable in postnatal day 16 rat brainstem. The P2X(3) receptor was coexpressed with the P2X(2) receptor in nucleus tractus solitarius, dorsal root ganglion, nodose ganglion, and the taste bud in E16.5 embryo, which was 5 days later than the first appearance of the native P2X(3) receptor. In summary, we present a detailed expression pattern of the P2X(3) receptor during neurogenesis and report that P2X(3) immunoreactivity is down-regulated in early postnatal brainstems.

Antinociceptive effects of intracerebroventricularly administered P2 purinoceptor agonists in the rat

We examined the effects of adenosine 5'-triphosphate (ATP) and its analogues administered intracerebroventricularly on nociceptive thresholds in rats. Intracerebroventricular (i.c.v.) administration of ATP (10 and 100 nmol/rat), alpha,beta-methylene-ATP (1-30 nmol/rat) and 2', 3'-O-(4-benzoylbenzoyl)-ATP (1-30 nmol/rat) dose-dependently elevated the mechanical nociceptive threshold in the paw pressure test. These antinociceptive effects were rapid and short-lasting, peaking at 5 min and disappearing by 20 min after the administration. However, i.c.v. administration of beta,gamma-methylene-ATP (1-30 nmol/rat) and UTP (10 and 100 nmol/rat) had no significant effects on the mechanical nociceptive threshold. In other tests, i.c.v. administration of alpha,beta-methylene-ATP (10 and 30 nmol/rat) prolonged the thermal nociceptive latency in the hot plate test, but only a higher dose (30 nmol/rat) of alpha,beta-methylene-ATP prolonged the latency in the tail flick test. alpha,beta-Methylene-ATP produced no motor deficit in the inclined plane test. These results suggest that P2X purinoceptors play an inhibitory role in nociception at the supraspinal level.

Distinct Localization of P2X receptors at excitatory postsynaptic specializations

ATP mediates fast excitatory synaptic transmission in some regions of the central nervous system through activation of P2X receptors. Nonetheless, the functional significance of ATP-mediated neurotransmission is not yet understood. Using postembedding immunocytochemistry, we describe the distribution of P2X(2), P2X(4), and P2X(6) subunits in cerebellum and in the CA1 region of the hippocampus. Dendritic spines of cerebellar Purkinje cells showed immunogold labeling for all three subunits when apposed to parallel fiber (PF) terminals. In contrast, no immunogold labeling was observed on dendritic spines or cell bodies receiving inputs from climbing fibers and basket cells, respectively. In CA1 pyramidal cells, postsynaptic membranes apposed to terminals of Schaffer collaterals were immunogold-labeled for P2X(2), P2X(4), and P2X(6) subunits. Immunolabeling was also observed perisynaptically and intracellularly in relation to membranes of the endoplasmic reticulum. The analysis of the tangential distribution of gold particles showed that they were preferentially located at the peripheral portion of the postsynaptic specialization of both parallel fiber and Schaffer collateral synapses. By double imunogold labeling using antibodies for P2X receptor subunits and GluR2/3 subunits of the AMPA glutamate receptors, we show that synapses expressing P2X receptors are also glutamatergic. The present study shows for the first time qualitatively and quantitatively the precise localization of P2X receptors in brain. Moreover, our data indicate that P2X receptors may play a significant role at glutamatergic synapses.

Warm-coding deficits and aberrant inflammatory pain in mice lacking P2X3 receptors

ATP activates damage-sensing neurons (nociceptors) and can evoke a sensation of pain. The ATP receptor P2X3 is selectively expressed by nociceptors and is one of seven ATP-gated, cation-selective ion channels. Here we demonstrate that ablation of the P2X3 gene results in the loss of rapidly desensitizing ATP-gated cation currents in dorsal root ganglion neurons, and that the responses of nodose ganglion neurons to ATP show altered kinetics and pharmacology resulting from the loss of expression of P2X(2/3) heteromultimers. Null mutants have normal sensorimotor function. Behavioural responses to noxious mechanical and thermal stimuli are also normal, although formalin-induced pain behaviour is reduced. In contrast, deletion of the P2X3 receptor causes enhanced thermal hyperalgesia in chronic inflammation. Notably, although dorsal-horn neuronal responses to mechanical and noxious heat application are normal, P2X3-null mice are unable to code the intensity of non-noxious 'warming' stimuli.

Adenosine 5'-tetraphosphate (Ap(4)), a new agonist on rat midbrain synaptic terminal P2 receptors

The aim of this study was to see whether the compound adenosine 5'-tetraphosphate (Ap(4)) is active in the central nervous system by examining its effect on isolated rat brain synaptic terminals. Ap(4) proved to be more resistant to ecto-enzymatic hydrolysis than adenosine triphosphate (ATP), showing only 2% hydrolysis after a 2-min incubation, compared to 75% for ATP. In addition, Ap(4) was able to produce concentration-dependent increases in intracellular Ca(2+) when applied extracellularly. This action was dependent upon the presence of extracellular calcium. Ap(4) acts through ionotropic ATP receptors (P2X receptors) and not through diadenosine polyphosphate receptors, since ATP abolished the response elicited by Ap(4) whereas Ap(5)A did not. Ap(4), ATP and ATP-gamma-S were of similar potency (EC(50) approximately 20 microM) while 2MeSATP, alpha,beta-meATP and ADP-beta-S possessed slightly lower potency (EC(50) approximately 50 microM). The P2-purinoceptor antagonists suramin and PPADS blocked the Ap(4) effect. The IC(50) values for these compounds were 35.5 and 7.8 microM respectively. Diinosine polyphosphates and inosine tetraphosphate inhibited the response elicited by Ap(4) with IC(50) values that varied between approximately 40 and 50 microM. These results show that Ap(4) is as good an agonist as ATP on synaptosomal P2X receptors, being more resistant to extracellular hydrolysis by ecto-nucleotidases.

Distribution of P2X3 receptors in the rat trigeminal ganglion after inferior alveolar nerve injury

The ATP-gated cation channel receptor P2X3 is associated with nociceptive primary sensory neurons. We have, using immunohistochemistry, examined the expression of P2X3 in rat trigeminal ganglia 4-22 days after ligation/section or chronic constriction of the mandibular inferior alveolar nerve. In the normal trigeminal ganglion the anti-P2X3 receptor antibody labeled 37-58% of all neurons. Double labeling demonstrated that about 70-95% of the small neurons that bind the isolectin I-B4 displayed P2X3-immunoreactivity, and that about 40% of larger RT97-positive nerve cells were P2X3 receptor-immunoreactive. At 4 and 10 days after inferior alveolar nerve injury, the proportion of P2X3-immunoreactive neurons had increased to about 65% (range 52-78%). Examinations at the injury sites showed an intense P2X3 receptor-immunoreactivity in nerve endings. At longer survival stages the proportion of P2X3 receptor-positive sensory neurons had returned to control values. These results show that the P2X3 receptor is transiently upregulated and anterogradely transported in trigeminal primary sensory neurons after nerve injury. Since the receptor is accumulated in injured nerve endings, it may be associated with abnormal impulse propagation from these sites.