P2X receptor immunoreactivity in the rat cochlea, vestibular ganglion and cochlear nucleus

Purinergic Signalling in the Cochlea

The mammalian cochlea is the sensory organ of hearing with a delicate, highly organised structure that supports unique operating mechanisms. ATP release from the secretory tissues of the cochlear lateral wall (stria vascularis) triggers numerous physiological responses by activating P2 receptors in sensory, supporting and neural tissues. Two families of P2 receptors, ATP-gated ion channels (P2X receptors) and G protein-coupled P2Y receptors, activate intracellular signalling pathways that regulate cochlear development, homeostasis, sensory transduction, auditory neurotransmission and response to stress. Of particular interest is a purinergic hearing adaptation, which reflects the critical role of the P2X₂ receptor in adaptive cochlear response to elevated sound levels. Other P2 receptors are involved in the maturation of neural processes and frequency selectivity refinement in the developing cochlea. Extracellular ATP signalling is regulated by a family of surface-located enzymes collectively known as "ectonucleotidases" that hydrolyse ATP to adenosine. Adenosine is a constitutive cell metabolite with an established role in tissue protection and regeneration. The differential activation of A₁ and A_2A adenosine receptors defines the cochlear response to injury caused by oxidative stress, inflammation, and activation of apoptotic pathways. A₁ receptor agonism, A_2A receptor antagonism, and increasing adenosine levels in cochlear fluids all represent promising therapeutic tools for cochlear rescue from injury and prevention of hearing loss.

Purinergic signaling in the peripheral vestibular system

The inner ear comprises the cochlea and vestibular system, which detect sound and acceleration stimulation, respectively. The function of the inner ear is regulated by ion transport activity among sensory epithelial cells, neuronal cells, non-sensory epithelial cells, and luminal fluid with a unique ionic composition of high [K⁺] and low [Na⁺], which enables normal hearing and balance maintenance. One of the important mechanisms regulating ion transport in the inner ear is purinergic signaling. Various purinergic receptors are distributed throughout inner ear epithelial cells and neuronal cells. To date, most studies have focused on the role of purinergic receptors in the cochlea, and few studies have examined these receptors in the vestibular system. As purinergic receptors play an important role in the cochlea, they would likely do the same in the vestibular system, which is fairly similar to the cochlea in cellular structure and function. Based on available studies performed to date, purinergic signaling is postulated to be involved in the regulation of ion homeostasis, protection of hair cells, otoconia formation, and regulation of electrical signaling from the sensory epithelium to vestibular neurons. In this review, the distribution and roles of purinergic receptors in the peripheral vestibular system are summarized and discussed.

Purinergic Modulation of Activity in the Developing Auditory Pathway

Purinergic P2 receptors, activated by endogenous ATP, are prominently expressed on neuronal and non-neuronal cells during development of the auditory periphery and central auditory neurons. In the mature cochlea, extracellular ATP contributes to ion homeostasis, and has a protective function against noise exposure. Here, we focus on the modulation of activity by extracellular ATP during early postnatal development of the lower auditory pathway. In mammals, spontaneous patterned activity is conveyed along afferent auditory pathways before the onset of acoustically evoked signal processing. During this critical developmental period, inner hair cells fire bursts of action potentials that are believed to provide a developmental code for synaptic maturation and refinement of auditory circuits, thereby establishing a precise tonotopic organization. Endogenous ATP-release triggers such patterned activity by raising the extracellular K⁺ concentration and contributes to firing by increasing the excitability of auditory nerve fibers, spiral ganglion neurons, and specific neuron types within the auditory brainstem, through the activation of diverse P2 receptors. We review recent studies that provide new models on the contribution of purinergic signaling to early development of the afferent auditory pathway. Further, we discuss potential future directions of purinergic research in the auditory system.

Hearing loss mutations alter the functional properties of human P2X2 receptor channels through distinct mechanisms

Activation of P2X2 receptor channels by extracellular ATP is thought to play important roles in cochlear adaptation to elevated sound levels and protection from overstimulation. Each subunit of a trimeric P2X2 receptor is composed of intracellular N and C termini, a large extracellular domain containing the ATP binding site and 2 transmembrane helices (TM1 and TM2) that form a cation permeable pore. Whole-exome sequencing and linkage analysis have identified 3 hP2X2 receptor mutations (V60L, D273Y, and G353R) that cause dominantly inherited progressive sensorineural hearing loss (DFNA41). Available structures of related P2X receptors suggest that these 3 mutations localize to TM1 (V60L), TM2 (G353R), or the β-sheet linking the TMs to the extracellular ATP binding sites (D273Y). Previous studies have concluded that the V60L and G353R mutants are nonfunctional, whereas the D273Y mutant has yet to be studied. Here, we demonstrate that both V60L and G353R mutations do form functional channels, whereas the D273Y mutation prevents the expression of functional channels on the cell membrane. Our results show that the V60L mutant forms constitutively active channels that are insensitive to ATP or the antagonist suramin, suggesting uncoupling of the pore and the ligand binding domains. In contrast, the G353R mutant can be activated by ATP but exhibits alterations in sensitivity to ATP, inward rectification, and ion selectivity. Collectively, our results demonstrate that the loss of functional P2X2 receptors or distinct alterations of its functional properties lead to noise-induced hearing loss, highlighting the importance of these channels in preserving hearing.

Purinergic Signaling and Cochlear Injury-Targeting the Immune System?

Hearing impairment is the most common sensory deficit, affecting more than 400 million people worldwide. Sensorineural hearing losses currently lack any specific or efficient pharmacotherapy largely due to the insufficient knowledge of the pathomechanism. Purinergic signaling plays a substantial role in cochlear (patho)physiology. P2 (ionotropic P2X and the metabotropic P2Y) as well as adenosine receptors expressed on cochlear sensory and non-sensory cells are involved mostly in protective mechanisms of the cochlea. They are implicated in the sensitivity adjustment of the receptor cells by a K⁺ shunt and can attenuate the cochlear amplification by modifying cochlear micromechanics. Cochlear blood flow is also regulated by purines. Here, we propose to comprehend this field with the purine-immune interactions in the cochlea. The role of harmful immune mechanisms in sensorineural hearing losses has been emerging in the horizon of cochlear pathologies. In addition to decreasing hearing sensitivity and increasing cochlear blood supply, influencing the immune system can be the additional avenue for pharmacological targeting of purinergic signaling in the cochlea. Elucidating this complexity of purinergic effects on cochlear functions is necessary and it can result in development of new therapeutic approaches in hearing disabilities, especially in the noise-induced ones.

Purinergic signaling in the organ of Corti: Potential therapeutic targets of sensorineural hearing losses

Purinergic signaling is deeply involved in the development, functions and protective mechanisms of the cochlea. Release of ATP and activation of purinergic receptors on sensory and supporting/epithelial cells play a substantial role in cochlear (patho)physiology. Both the ionotropic P2X and the metabotropic P2Y receptors are widely distributed on the inner and outer hair cells as well as on the different supporting cells in the organ of Corti and on other epithelial cells in the scala media. Among others, they are implicated in the sensitivity adjustment of the receptor cells by a K⁺ shunt and can attenuate the cochlear amplification by modifying cochlear micromechanics acting on outer hair cells and supporting cells. Cochlear blood flow is also regulated by purines. Sensorineural hearing losses currently lack any specific or efficient pharmacotherapy. Decreasing hearing sensitivity and increasing cochlear blood supply by pharmacological targeting of purinergic signaling in the cochlea are potential new therapeutic approaches in these hearing disabilities, especially in the noise-induced ones.

Non-genomic Effects of Glucocorticoids: An Updated View

Glucocorticoid (GC) anti-inflammatory effects generally require a prolonged onset of action and involve genomic processes. Because of the rapidity of some of the GC effects, however, the concept that non-genomic actions may contribute to GC mechanisms of action has arisen. While the mechanisms have not been completely elucidated, the non-genomic effects may play a role in the management of inflammatory diseases. For instance, we recently reported that GCs 'rapidly' enhanced the effects of bronchodilators, agents used in the treatment of allergic asthma. In this review article, we discuss (i) the non-genomic effects of GCs on pathways relevant to the pathogenesis of inflammatory diseases and (ii) the putative role of the membrane GC receptor. Since GC side effects are often considered to be generated through its genomic actions, understanding GC non-genomic effects will help design GCs with a better therapeutic index.

Lysosomal exocytosis of ATP is coupled to P2Y2 receptor in marginal cells in the stria vascular in neonatal rats

Adenosine triphosphate (ATP) is stored as lysosomal vesicles in marginal cells of the stria vascular in neonatal rats, but the mechanisms of ATP release are unclear. Primary cultures of marginal cells from 1-day-old Sprague-Dawley rats were established. P2Y₂ receptor and inositol 1,4,5-trisphosphate (IP3) receptor were immunolabelled in marginal cells of the stria vascular. We found that 30 μM ATP and 30 μM uridine triphosphate (UTP) evoked comparable significant increases in the intracellular Ca²⁺ concentration ([Ca²⁺]_i) in the absence of extracellular Ca²⁺, whereas the response was suppressed by 100 μM suramin, 10 μM 1-(6-(17β-3-methoxyester-1,3,5(10)-trien-17-yl)amino)-hexyl)-1H-pyrrole-2,5-dione(U-73122), 100 μM 2-aminoethoxydiphenyl borate (2-APB) and 5 μM thapsigargin (TG), thus indicating that ATP coupled with the P2Y₂R-PLC-IP3 pathway to evoke Ca²⁺ release from the endoplasmic reticulum (ER). Incubation with 200 μM Gly-Phe-β-naphthylamide (GPN) selectively disrupted lysosomes and caused significant increases in [Ca²⁺]_I; this effect was partly inhibited by P2Y₂R-PLC-IP3 pathway antagonists. After pre-treatment with 5 μM TG, [Ca²⁺]_i was significantly lower than that after treatment with P2Y₂R-PLC-IP3 pathway antagonists under the same conditions, thus indicating that lysosomal Ca²⁺ triggers Ca²⁺ release from ER Ca²⁺ stores. Baseline [Ca²⁺]_i declined after treatment with the Ca²⁺ chelator 50 μM bis-(aminophenolxy) ethane-N,N,N',N'-tetra-acetic acid acetoxyme-thyl ester (BAPTA-AM) and 4 IU/ml apyrase. 30 μM ATP decrease of the number of quinacrine-positive vesicles via lysosome exocytosis, whereas the number of lysosomes did not change. However, lysosome exocytosis was significantly suppressed by pre-treatment with 5 μM vacuolin-1. Release of ATP and β-hexosaminidase both increased after treatment with 200 μM GPN and 5 μM TG, but decreased after incubation with 50 μM BAPTA-AM, 4 IU/ml apyrase and 5 μM vacuolin-1. We suggest that ATP triggers Ca²⁺ release from the ER, thereby contributing to secretion of lysosomal ATP via lysosomal exocytosis. Lysosomal stored Ca²⁺ triggers Ca²⁺ release from the ER directly though the IP3 receptors, and lysosomal ATP evokes Ca²⁺ signals indirectly via the P2Y₂R-PLC-IP3 pathway.

P2X2 Receptor Deficiency in Mouse Vestibular End Organs Attenuates Vestibular Function

P2X₂ receptors are ligand-gated cation channels activated by extracellular ATP that modulate neural transmission in various neuronal systems. Although the function and distribution of P2X₂ receptors in the cochlea portion of the inner ear are well established, their physiological role in the vestibular portion is still not understood. Therefore, we investigated P2X₂ receptor localization in the peripheral vestibular portion, and assessed their physiological function in vivo using P2X₂ receptor knock out (P2X₂-KO) mice. Histological analysis revealed that P2X₂ receptors were localized on the epithelial surface of supporting and transitional cells of the vestibular end organs. To examine vestibular function in P2X₂-KO mice, we conducted behavioral tests and tested the vestibulo-ocular reflex (VOR) during sinusoidal rotations. P2X₂-KO mice exhibited significant motor balance impairment in the balance beam test. VOR gain in P2X₂-KO mice was significantly reduced, with no decrease in the optokinetic response. In conclusion, we showed that P2X₂ receptors are mainly localized in the supporting cells of the vestibular inner ear, and the loss of P2X₂ receptors causes mild vestibular dysfunction. Taken together, our findings suggest that the P2X₂ receptor plays a modulatory role in vestibular function.

Introduction and perspective, historical note

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

Neurologic bases for comorbidity of balance disorders, anxiety disorders and migraine: neurotherapeutic implications

The comorbidity among balance disorders, anxiety disorders and migraine has been studied extensively from clinical and basic research perspectives. From a neurological perspective, the comorbid symptoms are viewed as the product of sensorimotor, interoceptive and cognitive adaptations that are produced by afferent interoceptive information processing, a vestibulo-parabrachial nucleus network, a cerebral cortical network (including the insula, orbitofrontal cortex, prefrontal cortex and anterior cingulate cortex), a raphe nuclear-vestibular network, a coeruleo-vestibular network and a raphe-locus coeruleus loop. As these pathways overlap extensively with pathways implicated in the generation, perception and regulation of emotions and affective states, the comorbid disorders and effective treatment modalities can be viewed within the contexts of neurological and psychopharmacological sites of action of current therapies.

P2X receptors in health and disease

Seven P2X receptor subunits have been cloned which form functional homo- and heterotrimers. These are cation-selective channels, equally permeable to Na(+) and K(+) and with significant Ca(2+) permeability. The three-dimensional structure of the P2X receptor is described. The channel pore is formed by the α-helical transmembrane spanning region 2 of each subunit. When ATP binds to a P2X receptor, the pore opens within milliseconds, allowing the cations to flow. P2X receptors are expressed on both central and peripheral neurons, where they are involved in neuromuscular and synaptic neurotransmission and neuromodulation. They are also expressed in most types of nonneuronal cells and mediate a wide range of actions, such as contraction of smooth muscle, secretion, and immunomodulation. Changes in the expression of P2X receptors have been characterized in many pathological conditions of the cardiovascular, gastrointestinal, respiratory, and urinogenital systems and in the brain and special senses. The therapeutic potential of P2X receptor agonists and antagonists is currently being investigated in a range of disorders, including chronic neuropathic and inflammatory pain, depression, cystic fibrosis, dry eye, irritable bowel syndrome, interstitial cystitis, dysfunctional urinary bladder, and cancer.

Rizatriptan reduces vestibular-induced motion sickness in migraineurs

A previous pilot study suggested that rizatriptan reduces motion sickness induced by complex vestibular stimulation. In this double-blind, randomized, placebo-controlled study we measured motion sickness in response to a complex vestibular stimulus following pretreatment with either rizatriptan or a placebo. Subjects included 25 migraineurs with or without migraine-related dizziness (23 females) aged 21–45 years (31.0 ± 7.8 years). Motion sickness was induced by off-vertical axis rotation in darkness, which stimulates both the semicircular canals and otolith organs of the vestibular apparatus. Results indicated that of the 15 subjects who experienced vestibular-induced motion sickness when pretreated with placebo, 13 showed a decrease in motion sickness following pretreatment with rizatriptan as compared to pretreatment with placebo (P < 0.02). This significant effect was not seen when subjects were exposed to more provocative vestibular stimulation. We conclude that the serotonin agonist, rizatriptan, reduces vestibular-induced motion sickness by influencing serotonergic vestibular-autonomic projections.

Damage-induced cell-cell communication in different cochlear cell types via two distinct ATP-dependent Ca waves

Intercellular Ca(2+) waves can coordinate the action of large numbers of cells over significant distances. Recent work in many different systems has indicated that the release of ATP is fundamental for the propagation of most Ca(2+) waves. In the organ of hearing, the cochlea, ATP release is involved in critical signalling events during tissue maturation. ATP-dependent signalling is also implicated in the normal hearing process and in sensing cochlear damage. Here, we show that two distinct Ca(2+) waves are triggered during damage to cochlear explants. Both Ca(2+) waves are elicited by extracellular ATP acting on P2 receptors, but they differ in their source of Ca(2+), their velocity, their extent of spread and the cell type through which they propagate. A slower Ca(2+) wave (14 mum/s) communicates between Deiters' cells and is mediated by P2Y receptors and Ca(2+) release from IP(3)-sensitive stores. In contrast, a faster Ca(2+) wave (41 mum/s) propagates through sensory hair cells and is mediated by Ca(2+) influx from the external environment. Using inhibitors and selective agonists of P2 receptors, we suggest that the faster Ca(2+) wave is mediated by P2X(4) receptors. Thus, in complex tissues, the expression of different receptors determines the propagation of distinct intercellular communication signals.

ELECTRONIC SUPPLEMENTARY MATERIAL

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Purinergic signaling in cochleovestibular hair cells and afferent neurons

Purinergic signaling in the mammalian cochleovestibular hair cells and afferent neurons is reviewed. The scope includes P2 and P1 receptors in the inner hair cells (IHCs) of the cochlea, the type I spiral ganglion neurons (SGNs) that convey auditory signals from IHCs, the vestibular hair cells (VHCs) in the vestibular end organs (macula in the otolith organs and crista in the semicircular canals), and the vestibular ganglion neurons (VGNs) that transmit postural and rotatory information from VHCs. Various subtypes of P2X ionotropic receptors are expressed in IHCs as well as P2Y metabotropic receptors that mobilize intracellular calcium. Their functional roles still remain speculative, but adenosine 5'-triphosphate (ATP) could regulate the spontaneous activity of the hair cells during development and the receptor potentials of mature hair cells during sound stimulation. In SGNs, P2Y metabotropic receptors activate a nonspecific cation conductance that is permeable to large cations as NMDG(+) and TEA(+). Remarkably, this depolarizing nonspecific conductance in SGNs can also be activated by other metabotropic processes evoked by acetylcholine and tachykinin. The molecular nature and the role of this depolarizing channel are unknown, but its electrophysiological properties suggest that it could lie within the transient receptor potential channel family and could regulate the firing properties of the afferent neurons. Studies on the vestibular partition (VHC and VGN) are sparse but have also shown the expression of P2X and P2Y receptors. There is still little evidence of functional P1 (adenosine) receptors in the afferent system of the inner ear.

Functional ligand-gated purinergic receptors (P2X) in rat vestibular ganglion neurons

The expression of purinergic receptors (P2X) on rat vestibular ganglion neurons (VGNs) was examined using whole-cell patch-clamp recordings. An application of adenosine 5'-triphosphate (ATP; 100microM) evoked inward currents in VGNs at a holding potential of -60mV. The decay time constant of the ATP-evoked currents was 2-4s, which is in between the values for rapidly desensitizing subgroups (P2X1 and P2X3) and slowly desensitizing subgroups (P2X2, P2X4, etc.), suggesting the heterogeneous expression of P2X receptors. A dose-response experiment showed an EC(50) of 11.0microM and a Hill's coefficient of 0.82. Suramin (100microM) reversibly inhibited the ATP-evoked inward currents. Alpha, beta-methylene ATP (100microM), a P2X-specific agonist, also evoked inward currents but less extensively than ATP. An application of adenosine 5'-dihosphate (ADP; 100microM) evoked similar, but much smaller, currents. The current-voltage relationship of the ATP-evoked conductance showed pronounced inward rectification with a reversal potential more positive than 0mV, suggesting non-selective cation conductance. However, the channel was not permeable to a large cation (N-methyl-d-glucamine) and acidification (pH 6.3) had little effect on the ATP-evoked conductance. RT-PCR confirmed the expression of five subtypes (P2X2-P2X6) in VGNs. The physiological role of P2X receptors includes the modulation of excitability at the synapses between hair cells and dendrites and/or trophic support (or also neuromodulation) from supporting cells surrounding the VGNs.

P2 receptor-mediated signaling in spherical bushy cells of the mammalian cochlear nucleus

Purinoreceptors of the P2 family contribute strongly to signaling in the cochlea, but little is known about the effects of purinergic neurotransmission in the central auditory system. Here we examine P2 receptor-mediated signaling in the large spherical bushy cells (SBCs) of Mongolian gerbils around the onset of acoustically evoked signal processing (P9-P14). Brief adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS) application evoked inward current, membrane depolarization, and somatic Ca2+ signals. Moreover, ATPgammaS changed the SBCs firing pattern from phasic to tonic, when the application was synchronized with depolarizing current injection. This bursting discharge activity was dependent on [Ca2+]i and Ca2+-dependent protein kinase (PKC) activity and is presumably caused by modulation of low-threshold K+ conductance. Activation of P2Y1 receptors could not evoke these changes per se, thus it was concluded that the involvement of P2X receptors seems to be necessary. Ca2+ imaging data showed that both P2X and P2Y1 receptors mediate Ca2+ signals in SBCs where P2Y1 receptors most likely activate the PLC-IP3 (inositol trisphosphate) pathway and release Ca2+ from internal stores. Immunohistochemical staining confirmed the expression of P2X2 and P2Y1 receptor proteins in SBCs, providing additional evidence for the involvement of both receptors in signal transduction in these neurons. Purinergic signaling might modulate excitability of SBCs and thereby contribute to regulation of synaptic strength. Functionally, the increase in firing rate mediated by P2 receptors could reduce temporal precision of the postsynaptic firing, e.g., phase locking, which has an immediate effect on signal processing related to sound localization. This might provide a mechanism for adaptation to the ambient acoustic environment.

Purinergic signaling in the inner ear

Epithelial cells of the inner ear coordinate their ion transport activity through a number of mechanisms. One important mechanism is the autocrine and paracrine signaling among neighboring cells in the ear via nucleotides, such as adenosine, ATP and UTP. This review summarizes observations on the release, detection and degradation of nucleotides by epithelial cells of the inner ear. Purinergic signaling is thought to be important for endolymph ion homeostasis and for protection from acoustic over-stimulation.

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.

P2X receptor signaling inhibits BDNF-mediated spiral ganglion neuron development in the neonatal rat cochlea

Type I and type II spiral ganglion neurons (SGN) innervate the inner and outer hair cells of the cochlea, respectively. This neural system is established by reorganization of promiscuous innervation of the hair cells, immediately before hearing is established. The mechanism for this synaptic reorganization is unresolved but probably includes regulation of trophic support between the hair cells and the neurons. We provide evidence that P2X receptors (ATP-gated ion channels) contribute such a mechanism in the neonatal rat cochlea. Single-cell quantitative RT-PCR identified the differential expression of two P2X receptor subunits, splice variant P2X(2)(-3) and P2X(3), in a 1:2 transcript ratio. Downregulation of this P2X(2-3/3) receptor coincided with maturation of the SGN innervation of the hair cells. When the P2X(2-3) and P2X(3) subunits were co-expressed in Xenopus oocytes, the resultant P2X receptor properties corresponded to the SGN phenotype. This included enhanced sensitivity to ATP and extended agonist action. In P4 spiral ganglion explants, activation of the P2X receptor signaling pathway by ATPgammaS or alpha,betaMeATP inhibited BDNF-induced neurite outgrowth and branching. These findings indicate that P2X receptor signaling provides a mechanism for inhibiting neurotrophin support of SGN neurites when synaptic reorganization is occurring in the cochlea.

Age-related changes in cochlear gene expression in normal and shaker 2 mice

The vertebrate cochlea is a complex organ optimized for sound transduction. Auditory hair cells, with their precisely arranged stereocilia bundles, transduce sound waves to electrical signals that are transmitted to the brain. Mutations in the unconventional myosin XV cause deafness in both human DFNB3 families and in shaker 2 (sh2) mice as a result of defects in stereocilia. In these mutant mice, hair cells have relatively normal spatial organization of stereocilia bundles but lack the graded, stair-step organization. We used sh2 mice as an experimental model to investigate the molecular consequences of the sh2 mutation in the Myo15 gene. Gene expression profiling with Affymetrix GeneChips in deaf homozygous (sh2/sh2) mice at 3 weeks and 3 months of age, and in age-matched, normal-hearing heterozygotes (+/sh2) identified only a few genes whose expression was affected by genotype, but a large number with age-associated changes in expression in both normal mice and sh2/sh2 homozygotes. Microarray data analyzed using Robust Multiarray Average identified Aim1, Dbi, and Tm4sf3 as genes with increased expression in sh2/sh2 homozygotes. These increases were confirmed by quantitative reverse transcription-polymerase chain reaction. Genes exhibiting altered expression with age encoded collagens and proteins involved in collagen maturation, extracellular matrix, and bone mineralization. These results identified potential cellular pathways associated with myosin XV defects, and age-associated molecular events that are likely to be involved in maturation of the cochlea and auditory function.

Pathophysiology and therapeutic potential of purinergic signaling

The concept of a purinergic signaling system, using purine nucleotides and nucleosides as extracellular messengers, was first proposed over 30 years ago. After a brief introduction and update of purinoceptor subtypes, this article focuses on the diverse pathophysiological roles of purines and pyrimidines as signaling molecules. These molecules mediate short-term (acute) signaling functions in neurotransmission, mechanosensory transduction, secretion and vasodilatation, and long-term (chronic) signaling functions in cell proliferation, differentiation, and death involved in development and regeneration. Plasticity of purinoceptor expression in pathological conditions is frequently observed, including an increase in the purinergic component of autonomic cotransmission. Recent advances in therapies using purinergic-related drugs in a wide range of pathological conditions will be addressed with speculation on future developments in the field.

Expression of P2X receptors on immune cells in the rat liver during postnatal development

Single and double-labeling immunofluorescence and RT-PCR expression of P2X receptor proteins and mRNAs were used in a study of the liver of postnatal rats. OX62 and ED1 were used as markers for dendritic and macrophage (Kupffer) cells respectively. The results showed that the P2X6 receptor subunit was up-regulated by 15-fold on hepatic sinusoid cells during postnatal days P1 to P60. Subpopulations of Kupffer cells co-expressed P2X4 and P2X6 receptor subunits and dendritic cells co-expressed P2X4 and P2X7 receptor subunits. Lipopolysaccharide (endotoxin) injected into the peritoneal cavity led to increased expression of the P2X6 receptor on Kupffer cells, suggesting that the P2X6 receptor subunit may be up-regulated by endotoxin. This study presents the first evidence that P2X receptors are widely distributed in the rat liver immune system and that activation of Kupffer and dendritic cells in the rat liver might be regulated by extracellular ATP.

Acute effects of glucocorticoids on ATP-induced Ca2+ mobilization and nitric oxide production in cochlear spiral ganglion neurons

Rapid, non-genomic effects of glucocorticoids on extracellular adenosine 5'-triphosphate (ATP)-induced intracellular Ca(2+) concentration ([Ca(2+)](i)) changes and nitric oxide (NO) production were investigated in type I spiral ganglion neurons (SGNs) of the guinea-pig cochlea using the Ca(2+)-sensitive dye fura-2 and the NO-sensitive dye 4,5-diaminofluorescein (DAF-2). Pretreatment of SGNs with 1 microM dexamethasone for 10 min, a synthetic glucocorticoid hormone, enhanced the ATP-induced [Ca(2+)](i) increase in SGNs. RU 38486, a competitive glucocorticoid receptor antagonist eliminated the effects of dexamethasone on the ATP-induced [Ca(2+)](i) increase in SGNs. These acute effects of dexamethasone were dependent on the presence of extracellular Ca(2+), thereby suggesting that dexamethasone may rapidly enhance the Ca(2+) influx through the activation of ionotropic P2X receptors which may interact with glucocorticoid-mediated membrane receptors. Extracellular ATP increased the intensity of DAF-2 fluorescence, indicating NO production in SGNs. The ATP-induced NO production was mainly due to the Ca(2+) influx through the activation of P2 receptors. S-nitroso-N-acetylpenicillamine, a NO donor, enhanced the ATP-induced [Ca(2+)](i) increase in SGNs while L-N(G)-nitroarginine methyl ester (L-NAME), a NO synthesis inhibitor, inhibited it. Dexamethasone enhanced the ATP-induced NO production in SGNs. The augmentation of dexamethasone on ATP-induced NO production was abolished in the presence of l-NAME. It is concluded that the ATP-induced [Ca(2+)](i) increase induces NO production which enhances a [Ca(2+)](i) increase in SGNs by a positive-feedback mechanism. Dexamethasone enhances the ATP-induced [Ca(2+)](i) increase in SGNs which results in the augmentation of NO production. The present study suggests that NO may play an important role in auditory signal transduction. Our results also indicate that glucocorticoids may rapidly affect auditory neurotransmission due to a novel non-genomic mechanism.

ATP receptors in pain sensation: Involvement of spinal microglia and P2X(4) receptors

There is abundant evidence that extracellular ATP and other nucleotides have an important role in pain signaling at both the periphery and in the CNS. At first, it was thought that ATP was simply involved in acute pain, since ATP is released from damaged cells and excites directly primary sensory neurons by activating their receptors. However, neither blocking P2X/Y receptors pharmacologically nor suppressing the expression of P2X/Y receptors molecularly in sensory neurons or in the spinal cord had an effect on acute physiological pain. The focus of attention now is on the possibility that endogenous ATP and its receptor system might be activated in pathological pain states, particularly in neuropathic pain. Neuropathic pain is often a consequence of nerve injury through surgery, bone compression, diabetes or infection. This type of pain can be so severe that even light touching can be intensely painful; unfortunately, this state is generally resistant to currently available treatments. An important advance in our understanding of the mechanisms involved in neuropathic pain has been made by a recent work demonstrating the crucial role of ATP receptors (i.e., P2X₃ and P2X₄ receptors). In this review, we summarize the role of ATP receptors, particularly the P2X₄ receptor, in neuropathic pain. The expression of P2X₄ receptors in the spinal cord is enhanced in spinal microglia after peripheral nerve injury, and blocking pharmacologically and suppressing molecularly P2X₄ receptors produce a reduction of the neuropathic pain behaviour. Understanding the key roles of ATP receptors including P2X₄ receptors may lead to new strategies for the management of neuropathic pain.

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.

Differential expression of purinergic receptor subtypes in the outer hair cells of the guinea pig

ATP acts as a neuro-modulator through purinoceptors in many different tissues. Many subtypes of these receptors have been identified in the inner ear, but so far only two types have been shown to be present in the membrane of the isolated outer hair cells (OHCs). The aim of this study was to detect and visualize the existence and distribution of purinoceptor subtypes as well as to study the [Ca(2+)](i) response of these cells in response to stimulation with ATP. Four P2X and three P2Y receptor subtypes were identified with different expression pattern in the membrane of guinea pig outer hair cells. Whereas intense labeling was observed for P2X1, P2X2, P2X4, P2Y1, P2Y2, and P2Y4, the labeling for the subtype P2X7 was weak. There was a marked difference in the distribution of the receptors along the surface of the cells with a homogenous distribution in cases of P2X1, P2X4, and P2Y1. In contrast, P2X2 and P2Y2 receptor density was high mainly at the apical, while P2X7 and P2Y4 at the basal pole of the cells. Similarly a heterogeneity was observed in the ATP-induced transient elevation in [Ca(2+)](i), which had either fast kinetics without desensitization or slow rise with desensitization.

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.

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.

Extracellular ATP as a signaling molecule for epithelial cells

The charge of this invited review is to present a convincing case for the fact that cells release their ATP for physiological reasons. Many of our "purinergic" colleagues as well as ourselves have experienced resistance to this concept, because it is teleologically counter-intuitive. This review serves to integrate the three main tenets of extracellular ATP signaling: ATP release from cells, ATP receptors on cells, and ATP receptor-driven signaling within cells to affect cell or tissue physiology. First principles will be discussed in the Introduction concerning extracellular ATP signaling. All possible cellular mechanisms of ATP release will then be presented. Use of nucleotide and nucleoside scavengers as well as broad-specificity purinergic receptor antagonists will be presented as a method of detecting endogenous ATP release affecting a biological endpoint. Innovative methods of detecting released ATP by adapting luciferase detection reagents or by using "biosensors" will be presented. Because our laboratory has been primarily interested in epithelial cell physiology and pathophysiology for several years, the role of extracellular ATP in regulation of epithelial cell function will be the focus of this review. For ATP release to be physiologically relevant, receptors for ATP are required at the cell surface. The families of P2Y G protein-coupled receptors and ATP-gated P2X receptor channels will be introduced. Particular attention will be paid to P2X receptor channels that mediate the fast actions of extracellular ATP signaling, much like neurotransmitter-gated channels versus metabotropic heptahelical neurotransmitter receptors that couple to G proteins. Finally, fascinating biological paradigms in which extracellular ATP signaling has been implicated will be highlighted. It is the goal of this review to convert and attract new scientists into the exploding field of extracellular nucleotide signaling and to convince the reader that extracellular ATP is indeed a signaling molecule.

Lighting up the senses: FM1-43 loading of sensory cells through nonselective ion channels

We describe a novel mechanism for vital fluorescent dye entry into sensory cells and neurons: permeation through ion channels. In addition to the slow conventional uptake of styryl dyes by endocytosis, small styryl dyes such as FM1-43 rapidly and specifically label hair cells in the inner ear by entering through open mechanotransduction channels. This labeling can be blocked by pharmacological or mechanical closing of the channels. This phenomenon is not limited to hair cell transduction channels, because human embryonic kidney 293T cells expressing the vanilloid receptor (TRPV1) or a purinergic receptor (P2X2) rapidly take up FM1-43 when those receptor channels are opened and not when they are pharmacologically blocked. This channel permeation mechanism can also be used to label many sensory cell types in vivo. A single subcutaneous injection of FM1-43 (3 mg/kg body weight) in mice brightly labels hair cells, Merkel cells, muscle spindles, taste buds, enteric neurons, and primary sensory neurons within the cranial and dorsal root ganglia, persisting for several weeks. The pattern of labeling is specific; nonsensory cells and neurons remain unlabeled. The labeling of the sensory neurons requires dye entry through the sensory terminal, consistent with permeation through the sensory channels. This suggests that organic cationic dyes are able to pass through a number of different sensory channels. The bright and specific labeling with styryl dyes provides a novel way to study sensory cells and neurons in vivo and in vitro, and it offers new opportunities for visually assaying sensory channel function.

A voltage- and Ca2+-dependent big conductance K channel in cochlear spiral ligament fibrocytes

Evidence is accruing that spiral ligament fibrocytes (SLFs) play an important role in cochlear K(+) homeostasis, but little direct physiological data is available to support this concept. Here we report the presence and characterization of a voltage- and Ca(2+)-dependent big-conductance K (BK) channel in type I SLFs cultured from the gerbil cochlea. A single-channel conductance of 298+/-5.6 pS (n=28) was measured under symmetrical K(+). Membrane potentials for half-maximal open probability (P(o)) were -67, -45 and 85 mV with cytosolic free-Ca(2+) levels of 0.7 mM, 10 microM and 1 microM, respectively (n=8-14). The Hill coefficient for Ca(2+) affinity was 1.9 at a membrane potential of 60 mV (n=6). The BK channel showed very low activity (P(o)=0.0019, n=5) under normal physiological conditions, suggesting a low resting intracellular free [Ca(2+)]. Pharmacological results fit well with the profile of classic BK channels. The estimated half-maximal inhibitory concentration and Hill coefficient for tetraethylammonium were 0.086+/-0.021 mM and 0.99, respectively (n=4-9). In whole cell recordings, the voltage-activated outward K current was inhibited 85.7+/-4.5% (n=6) by 0.1 microM iberiotoxin. A steady-state kinetic model with two open and two closed stages best described the BK gating process (tau(o1) 0.23+/-0.08 ms, tau(o2) 1.40+/-0.32 ms; tau(c1) 0.26+/-0.09 ms, tau(c2) 3.10+/-1.2 ms; n=11). RT-PCR analyses revealed a splice variant of the BK channel alpha subunit in cultured type I SLFs and freshly isolated spiral ligament tissues. The BK channel is likely to play a major role in regulating the membrane potential of type I SLFs, which may in turn influence K(+) recycling dynamics in the mammalian cochlea.

ATP-gated ion channels assembled from P2X2 receptor subunits in the mouse cochlea

Extracellular ATP has several neuro-humoral actions on cochlear physiology, many of which involve P2X receptor-mediated signal transduction. The present study extends the molecular physiology of P2X receptor gene expression in the cochlea to the principal platform for transgenic studies, the mouse model. P2X receptor subunits, which assemble to form ATP-gated ion channels, were localised in cryosections and whole-mount tissues from the adult mouse cochlea using a specific antiserum and immunoperoxidase histochemistry. Whole-cell voltage clamp recordings functionally correlated immunolocalisation of ATP-gated ion channels in isolated hair cells and supporting cells. P2X immunoreactivity was widespread throughout the epithelial lining of the cochlea (except vascular stria); spiral ganglion neurons, organ of Corti supporting cells, and outer hair cell (OHC) stereocilia exhibited strong P2X immunolabelling. Localisation of ATP-gated ion channels on the endolymphatic surface (cuticular plates and stereocilia) of outer hair cells was confirmed electrophysiologically. In contrast, Deiters' cells exhibited an even distribution of both immunolabelling over the whole cell membrane and inward currents could be evoked by localised ATP application anywhere on these cells. In both OHC and Deiters' cells, the slowly-desensitising inward currents were blocked by the P2X-selective antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), compatible with P2X subunits contributing to the ATP-gated ion channels. Our immunohistochemical and functional localisation of P2X receptors in the mouse cochlea extends previous studies to verify and characterise extracellular ATP signalling in the cochlea and extends support for P2X receptor-mediated regulation of endolymphatic ionic homeostasis, sound transduction, auditory neurotransmission and cochlear mechanics.

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.

Distribution of ectonucleoside triphosphate diphosphohydrolases 1 and 2 in rat cochlea

Extracellular ATP and other extracellular nucleotides acting via P2 receptors in the inner ear initiate a wide variety of signalling pathways important for regulation of hearing and balance. Ectonucleotidases are extracellular nucleotide-metabolising enzymes that modulate purinergic signalling in most tissues. Major ectonucleotidases in the cochlea are likely members of the ectonucleoside triphosphate diphosphohydrolase (E-NTPDase) family. In this study, we provide a detailed description of NTPDase1 and NTPDase2 distribution in cochlear tissues using immunocytochemistry. E-NTPDase immunoreactivity was not equally distributed in the tissues bordering scala media. It was observed in the organ of Corti, including sensory and supporting cells, but was notably absent from Reissner's membrane and most of the marginal cells of the stria vascularis. NTPDase1 expression was most prominent in the cochlear vasculature and cell bodies of the spiral ganglion neurones, whereas considerable NTPDase2 immunoreactivity was detected in the stria vascularis. Both E-NTPDases were expressed in the cuticular plates of the sensory hair cells and nerve fibres projecting from the synaptic area underneath the inner and outer hair cells. E-NTPDase localisation corresponds to the reported distribution of some P2X receptor subunits (P2X(2) in particular) in sensory, supporting and neural cells and also P2Y receptor distribution in the vasculature and secretory tissues of the lateral wall. The role for E-NTPDases in purinergic signalling is most likely to regulate extracellular nucleoside triphosphate and diphosphate levels and thus provide termination for extracellular ATP signalling that has been linked to control of cochlear blood flow, electrochemical regulation of sound transduction and to neurotransmission in the cochlea.

ATP-gated currents in rat primary auditory neurones in situ arise from a heteromultimetric P2X receptor subunit assembly

Spiral ganglion neurones provide the primary afferent innervation to sensory hair cells within the mammalian cochlea. Recent evidence suggests that their function may be modulated by purinergic signalling mechanisms, associated with release of adenosine 5'-triphosphate (ATP). Utilising a newly developed slice preparation of the neonatal rat cochlea, we have investigated the response of neurones in situ, to purinergic agonists and antagonists using whole-cell voltage clamp recordings. In cells identified as type I spiral ganglion neurones on the basis of morphology and voltage-dependent conductances, pressure-applied ATP, alpha,beta-methyleneATP (alpha,beta-meATP), 2-methylthioATP (2-MeSATP) and adenosine 5'-diphosphate (ADP) elicited a consistent phenotype of desensitising, inwardly rectifying current. The ATP-activated currents were reversibly blocked by the P2X receptor antagonists pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 10 microM), and 2',3'-O-(2,4,6-trinitrophenyl)-ATP (TNP-ATP; IC(50) 407 nM). Neurones were more sensitive to ATP at low pH. The EC(50) value for ATP shifted from 18 microM at pH 7.3, to 1 microM at pH 6.3, with Hill coefficients of approximately 1. The results indicate that ATP-gated ion channels in spiral ganglion neurones arise from a specific heteromultimeric assembly of P2X receptor subunits which has no correspondence with present recombinant P2X receptor models.

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.

Heteromultimeric P2X(1/2) receptors show a novel sensitivity to extracellular pH

Rat P2X(1) and P2X(2) subunits were coexpressed in defolliculated Xenopus oocytes and the resultant P2X receptors studied under voltage-clamp conditions. Extracellular ATP elicited biphasic inward currents, involving an initial rapidly inactivating (P2X(1)-like) component and a later slowly inactivating (P2X(2)-like) component. The maximum amplitude of P2X(1)-like ATP responses was increased in some cells by lowering extracellular pH (from 7.5 to 6.5), whereas P2X(2)-like responses and those of homomeric rP2X(1) and rP2X(2) receptors were not changed by this treatment. Concentration-response (C/R) curves for ATP for pH-enhanced P2X(1)-like responses were biphasic, and clearly distinct from monophasic ATP C/R curves for homomeric rP2X(1) and rP2X(2) receptors. Under acidic (pH 5.5 and 6.5) and alkaline (pH 8.5) conditions, ATP C/R curves for P2X(1)-like responses showed increases in agonist potency and efficacy, compared with data at pH 7.5, but the same was not true of homomeric rP2X(1) and rP2X(2) receptors. ATP C/R curves for P2X(2)-like responses overlay C/R curves for homomeric rP2X(2) receptors, and determinations of agonist potency and efficacy were identical for P2X(2)-like and P2X(2) responses at all pH levels tested. Our results show that P2X(1)-like responses possessed the kinetics of homomeric P2X(1) receptors but an acid sensitivity different from homomeric P2X(1) and P2X(2) receptors. In contrast, the P2X(2)-like responses exactly matched the profile expected of homomeric P2X(2) receptors. Thus, coexpression of P2X(1) and P2X(2) subunits yielded a mixed population of homomeric and heteromeric P2X receptors, with a subpopulation of novel pH-sensitive P2X receptors showing identifiably unique properties that indicated the formation of heteromeric P2X(1/2) ion channels.

P2X(2), P2X(2-2) and P2X(5) receptor subunit expression and function in rat thoracolumbar sympathetic neurons

The present study investigated the pharmacological properties of excitatory P2X receptors and P2X(2) and P2X(5) receptor subunit expression in rat-cultured thoracolumbar sympathetic neurons. In patch-clamp recordings, ATP (3-1000 microM; applied for 1 s) induced inward currents in a concentration-dependent manner. Pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate (PPADS; 30 microM) counteracted the ATP response. In contrast to ATP, alpha,beta-meATP (30 microM; for 1 s) was virtually ineffective. Prolonged application of ATP (100 microM; 10 s) induced receptor desensitization in a significant proportion of sympathetic neurons in a manner typical for P2X(2-2) splice variant-mediated responses. Using single-cell RT-PCR, P2X(2), P2X(2-2) and P2X(5) mRNA expression was detectable in individual tyrosine hydroxylase-positive neurons; coexpression of both P2X(2) isoforms was not observed. Laser scanning microscopy revealed both P2X(2) and P2X(5) immunoreactivity in virtually every TH-positive neuron. P2X(2) immunoreactivity was largely distributed over the cell body, whereas P2X(5) immunoreactivity was most distinctly located close to the nucleus. In summary, the present study demonstrates the expression of P2X(2), P2X(2-2) and P2X(5) receptor subunits in rat thoracolumbar neurons. The functional data in conjunction with a preferential membranous localization of P2X(2)/P2X(2-2) compared with P2X(5) suggest that the excitatory P2X responses are mediated by P2X(2) and P2X(2-2) receptors. Apparently there exist two types of P2X(2) receptor-bearing sympathetic neurons: one major population expressing the unspliced isoform and another minor population expressing the P2X(2-2) splice variant.

Rapid correction of vestibular imbalance by intracochlear administration of ATP in a guinea pig model of unilateral peripheral vestibular disorder

The effect of inner ear administration of adenosine triphosphate (ATP) on vestibular function was investigated in guinea pigs with vestibular disorder. The right lateral semicircular canal was cut surgically. Animals were then treated with saline, 5 mM ATP, 50 mM ATP, or 50 mM ATP+10 mM pyridoxal-phosphate-6-azophenyl-2', 4'-disulfonic acid (PPADS), a P2X receptor antagonist, administered directly into the scala tympani by osmotic pump. Before treatment, and at 3, 5 and 7 days after treatment, trapezoid rotation tests were performed on all animals, and the post-rotatory nystagmus (PRN) ratio (number of nystagmus beats after counterclockwise rotation/number of nystagmus beats after clockwise rotation) was calculated and compared between groups. The PRN ratio was statistically greater at 5 days after treatment in the 50 mM ATP group than in the saline group. A statistical difference was also observed in animals treated with 50 mM ATP+10 mM PPADS. Our results indicate that ATP plays an important role in the vestibular periphery to correct vestibular imbalance and that this action may not occur via P2X receptors.

Transient expression of P2X(1) receptor subunits of ATP-gated ion channels in the developing rat cochlea

The expression pattern of the ATP-gated ion channel P2X(1) receptor subunit was studied in the developing rat cochlea by riboprobe in situ hybridisation and immunohistochemistry. Embryonic (E12, E14, E16 and E18) and postnatal (P0, P2, P4, P6, P10 and adult) rat cochleae were examined. Both mRNA and protein localisation techniques demonstrated comparable P2X(1) receptor expression from E16 until P6 but this expression was absent at later developmental stages. P2X(1) receptor mRNA expression was localised within the otic capsule and associated mesenchyme (from E16 to P6), spiral limbus (from P0 to P6) and within the spiral ligament adjacent to the insertion of Reissner's membrane (from P2 to P6). P2X(1) receptor protein had a similar distribution based upon immunoperoxidase localisation. P2X(1) receptor-like immunoreactivity was detected in the otic capsule and the surrounding mesenchyme (from E16 to P6), spiral limbus (from P0) and epithelial cells of Reissner's membrane (from P2 to P6). The spiral ganglion neurones showed the earliest P2X(1) receptor expression (from E16 to P6). This became associated with immunolabelling of their afferent neurite projections to the base of the developing inner and outer hair cells (observed from E18 and peaking at P2). Immunolabelling of the efferent nerve fibres of the intraganglionic spiral bundle (from E18 to P6) within the spiral ganglion was also observed. The results suggest that ATP-gated ion channels assembled from P2X(1) receptor subunits provide a signal transduction pathway for development of afferent and efferent innervation of the sensory hair cells and purinergic influence on cochlear morphogenesis.

Physiological effects of extracellular nucleotides in the inner ear

1. Electrochemical homeostasis, sound transduction and auditory neurotransmission in the cochlea are influenced by extracellular purines and pyrimidines. 2. Evidence that ATP and related nucleotides influence inner ear function arises from a considerable number of cellular, molecular and physiological studies in vitro and in vivo. 3. With a full understanding of these processes, which include ionotropic (P2X receptor) and metabotropic (P2Y receptor) signal transduction pathways, signal termination involving ecto-nucleotidases and recycling via nucleoside transporters, exciting possibilities emerge for treating hearing disorders, such as Meniere's disease, tinnitus and sensorineural deafness.

Purinergic signalling: an experimental perspective

Investigation of the multiple roles of extracellular nucleotides in the cochlea has developed from analysis of ATP-activated conductances in single sensory hair cells. Molecular probes such as radiolabelled ATP analogues and radiolabelled mRNA for ATP-gated ion channel subunits (P2X receptors) rapidly revealed the extensive nature of ATP signalling in this sensory organ. This has provided a foundation for physiological investigations which put extracellular nucleotides at the centre of homeostatic regulation of the driving force for sound transduction, modulation of mechanical tuning, control of cochlear blood flow and auditory neurotransmission. The purinergic signal transduction pathways associated with these processes have several novel features of significance to the broader field of purinergic neuroscience. In turn, these studies have benefited from the recent experimental advances in the field of purinergic signalling, a significant component of which is associated with the work of Professor Geoffrey Burnstock.

Immunohistochemical localization of adenosine 5'-triphosphate-gated ion channel P2X(2) receptor subunits in adult and developing rat cochlea

Substantial in vitro and in vivo data support a role for extracellular adenosine 5;-triphosphate (ATP) and associated P2 receptors in cochlear function. However, the precise spatiotemporal distribution of the involved receptor protein(s) has not been determined. By using a specific antiserum and immunoperoxidase labeling, the tissue distribution of the P2X(2) subunit of the ATP-gated ion channel was investigated. Here, we describe the first extensive immunohistochemical mapping of P2X(2) receptor subunits in the adult and developing rat cochlea. In the adult, immunoreactivity was observed in most cells bordering on the endolymphatic compartment (scala media), particularly in the supporting cells. Hair cells were not immunostained by the P2X(2) antiserum, except for outer hair cell stereocilia. In addition, weak immunolabeling was observed in some spiral ganglion neurons. P2X(2) receptor subunit protein expression during labyrinthine ontogeny was detected first on embryonic day 19 in the spiral ganglion and in associated nerve fibers extending to the inner hair cells. Immunostaining also was observed underneath outer hair cells, and, by postnatal day 6 (P6), intense immunolabeling was seen in the synaptic regions of both types of hair cell. Supporting cells of the sensory epithelium were labeled at P0. This labeling became most prominent from the onset of cochlear function (P8-P12). Conversely, expression in the vascular stria declined from this time. By P21, the pattern of immunolabeling was similar to that found in the adult. The localization and timing of P2X(2) immunoreactivity suggest involvement of extracellular ATP and associated ATP-gated ion channels in important physiological events, such as inner ear ontogeny, sound transduction, cochlear micromechanics, electrochemical homeostasis, and auditory neurotransmission.