Localization of mRNA encoding the P2X2 receptor subunit of the adenosine 5?-triphosphate-gated ion channel in the adult and developing rat inner ear by in situ hybridization

Effects of Noise Damage on the Purinergic Signal of Cochlear Spiral Ganglion Cells in Guinea Pigs

To observe the expression changes of P2 protein in cochlear spiral ganglion cells before and after noise injury, and to explore the relationship between the changes of purinergic receptors in spiral ganglion cells and noise-induced hearing loss, so that the signal transduction of purinergic receptors can be used to treat SNHL The target point provides a theoretical basis. The experimental animals were randomly divided into normal and experimental groups. The experimental group was given 120 dB white noise continuous exposure for 10 days and 3 h a day. The auditory brainstem response was measured before and after the noise exposure. After the noise exposure, the two groups of animals were collected. Do immunofluorescence staining, western blot, fluorescence real-time quantitative PCR to observe the expression of P2 protein. The average hearing threshold of the animals in the experimental group increased to 38.75 ± 6.44 dB SPL after 7 days of noise exposure, and the high-frequency hearing loss was lower and severe; the average hearing threshold increased to 54.38 ± 6.80 dB SPL after 10 days of noise exposure, and the hearing loss at 4 k Hz was relatively high. Light; Frozen sections of cochlear spiral ganglion cells and staining of isolated spiral ganglion cells found that P2X2, P2X3, P2X4, P2X7, P2Y2, and P2Y4 proteins were all expressed in cochlear spiral ganglion cells before noise exposure. Among them, P2X3 expression increased and P2X4, the down-regulation of P2Y2 expression was statistically significant (P < 0.05); Western blot and real-time quantitative PCR detection results showed that the expression of P2X3 was significantly increased after noise exposure than before noise exposure (P < 0.05), and P2X4 and P2Y2 were expressed after noise exposure The amount was significantly lower than before noise exposure (P < 0.05). (Figure. 4). After noise exposure, the expression of P2 protein is upregulated or downregulated. By affecting the Ca2+ cycle, the transmission of sound signals to the auditory center is blocked, which provides a theoretical basis for the signal transduction of purinergic receptors to become a target for the treatment of SNHL.

Oncomodulin regulates spontaneous calcium signalling and maturation of afferent innervation in cochlear outer hair cells

Cochlear outer hair cells (OHCs) are responsible for the exquisite frequency selectivity and sensitivity of mammalian hearing. During development, the maturation of OHC afferent connectivity is refined by coordinated spontaneous Ca2+ activity in both sensory and non-sensory cells. Calcium signalling in neonatal OHCs can be modulated by oncomodulin (OCM, β-parvalbumin), an EF-hand calcium-binding protein. Here, we investigated whether OCM regulates OHC spontaneous Ca2+ activity and afferent connectivity during development. Using a genetically encoded Ca2+ sensor (GCaMP6s) expressed in OHCs in wild-type (Ocm+/+ ) and Ocm knockout (Ocm-/- ) littermates, we found increased spontaneous Ca2+ activity and upregulation of purinergic receptors in OHCs from Ocm-/- cochlea immediately following birth. The afferent synaptic maturation of OHCs was delayed in the absence of OCM, leading to an increased number of ribbon synapses and afferent fibres on Ocm-/- OHCs before hearing onset. We propose that OCM regulates the spontaneous Ca2+ signalling in the developing cochlea and the maturation of OHC afferent innervation. KEY POINTS: Cochlear outer hair cells (OHCs) exhibit spontaneous Ca2+ activity during a narrow period of neonatal development. OHC afferent maturation and connectivity requires spontaneous Ca2+ activity. Oncomodulin (OCM, β-parvalbumin), an EF-hand calcium-binding protein, modulates Ca2+ signals in immature OHCs. Using transgenic mice that endogenously expressed a Ca2+ sensor, GCaMP6s, we found increased spontaneous Ca2+ activity and upregulated purinergic receptors in Ocm-/- OHCs. The maturation of afferent synapses in Ocm-/- OHCs was also delayed, leading to an upregulation of ribbon synapses and afferent fibres in Ocm-/- OHCs before hearing onset. We propose that OCM plays an important role in modulating Ca2+ activity, expression of Ca2+ channels and afferent innervation in developing OHCs.

Oncomodulin Regulates Spontaneous Calcium Signaling and Maturation of Afferent Innervation in Cochlear Outer Hair Cells

Cochlear outer hair cells (OHCs) are responsible for the exquisite frequency selectivity and sensitivity of mammalian hearing. During development, the maturation of OHC afferent connectivity is refined by coordinated spontaneous Ca 2+ activity in both sensory and non-sensory cells. Calcium signaling in neonatal OHCs can be modulated by Oncomodulin (OCM, β-parvalbumin), an EF-hand calcium-binding protein. Here, we investigated whether OCM regulates OHC spontaneous Ca 2+ activity and afferent connectivity during development. Using a genetically encoded Ca 2+ sensor (GCaMP6s) expressed in OHCs in wild-type (Ocm +/+ ) and Ocm knockout (Ocm -/- ) littermates, we found increased spontaneous Ca 2+ activity and upregulation of purinergic receptors in OHCs from GCaMP6s Ocm -/- cochlea immediately following birth. The afferent synaptic maturation of OHCs was delayed in the absence of OCM, leading to an increased number of ribbon synapses and afferent fibers on GCaMP6s Ocm -/- OHCs before hearing onset. We propose that OCM regulates the spontaneous Ca 2+ signaling in the developing cochlea and the maturation of OHC afferent innervation.

Active role of glia-like supporting cells in the organ of Corti: Membrane proteins and their roles in hearing

The organ of Corti, located in the cochlea in the inner ear, is one of the major sensory organs involved in hearing. The organ of Corti consists of hair cells, glia-like supporting cells, and the cochlear nerve, which work in harmony to receive sound from the outer ear and transmit auditory signals to the cochlear nucleus in the auditory ascending pathway. In this process, maintenance of the endocochlear potential, with a high potassium gradient and clearance of electrolytes and biochemicals in the inner ear, is critical for normal sound transduction. There is an emerging need for a thorough understanding of each cell type involved in this process to understand the sophisticated mechanisms of the organ of Corti. Hair cells have long been thought to be active, playing a primary role in the cochlea in actively detecting and transmitting signals. In contrast, supporting cells are thought to be silent and function to support hair cells. However, growing lines of evidence regarding the membrane proteins that mediate ionic movement in supporting cells have demonstrated that supporting cells are not silent, but actively play important roles in normal signal transduction. In this review, we summarize studies that characterize diverse membrane proteins according to the supporting cell subtypes involved in cochlear physiology and hearing. This review contributes to a better understanding of supporting cell functions and facilitates the development of potential therapeutic tools for hearing loss.

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.

Developmental GAD2 Expression Reveals Progenitor-like Cells with Calcium Waves in Mammalian Crista Ampullaris

Sense of motion, spatial orientation, and balance in vertebrates relies on sensory hair cells in the inner ear vestibular system. Vestibular supporting cells can regenerate hair cells that are lost from aging, ototoxicity, and trauma, although not all factors or specific cell types are known. Here we report a population of GAD2-positive cells in the mouse crista ampullaris and trace GAD2 progenitor-like cells that express pluripotent transcription factors SOX2, PROX1, and CTBP2. GAD2 progenitor-like cells organize into rosettes around a central branched structure in the eminentia cruciatum (EC) herein named the EC plexus. GCaMP5G calcium indicator shows spontaneous and acetylcholine-evoked whole-cell calcium waves in neonatal and adult mice. We present a hypothetical model that outlines the lineage and potential regenerative capacity of GAD2 cells in the mammalian vestibular neuroepithelium.

To Inhibit or Enhance? Is There a Benefit to Positive Allosteric Modulation of P2X Receptors?

The family of ligand-gated ion channels known as P2X receptors were discovered several decades ago. Since the cloning of the seven P2X receptors (P2X1-P2X7), a huge research effort has elucidated their roles in regulating a range of physiological and pathophysiological processes. Transgenic animals have been influential in understanding which P2X receptors could be new therapeutic targets for disease. Furthermore, understanding how inherited mutations can increase susceptibility to disorders and diseases has advanced this knowledge base. There has been an emphasis on the discovery and development of pharmacological tools to help dissect the individual roles of P2X receptors and the pharmaceutical industry has been involved in pushing forward clinical development of several lead compounds. During the discovery phase, a number of positive allosteric modulators have been described for P2X receptors and these have been useful in assigning physiological roles to receptors. This review will consider the major physiological roles of P2X1-P2X7 and discuss whether enhancement of P2X receptor activity would offer any therapeutic benefit. We will review what is known about identified compounds acting as positive allosteric modulators and the recent identification of drug binding pockets for such modulators.

Calcium signaling in interdental cells during the critical developmental period of the mouse cochlea

The tectorial membrane (TM), a complex acellular structure that covers part of the organ of Corti and excites outer hair cells, is required for normal hearing. It consists of collagen fibrils and various glycoproteins, which are synthesized in embryonic and postnatal development by different cochlear cell types including the interdental cells (IDCs). At its modiolar side, the TM is fixed to the apical surfaces of IDCs, which form the covering epithelium of the spiral limbus. We performed confocal membrane imaging and Ca²⁺ imaging in IDCs of the developing mouse cochlea from birth to postnatal day 18 (P18). Using the fluorescent membrane markers FM 4-64 and CellMask™ Deep Red on explanted whole-mount cochlear epithelium, we identified the morphology of IDCs at different z-levels of the spiral limbus. Ca²⁺ imaging of Fluo-8 AM-loaded cochlear epithelia revealed spontaneous intracellular Ca²⁺ transients in IDCs at P0/1, P4/5, and P18. Their relative frequency was lowest on P0/1, increased by a factor of 12.5 on P4/5 and decreased to twice the initial value on P18. At all three ages, stimulation of IDCs with the trinucleotides ATP and UTP at 1 and 10 μM elicited Ca²⁺ transients of varying amplitude and shape. Before the onset of hearing, IDCs responded with robust Ca²⁺ oscillations. At P18, after the onset of hearing, ATP stimulation either caused Ca²⁺ oscillations or an initial Ca²⁺ peak followed by a plateau while the UTP response was unchanged from that at pre-hearing stage. Parameters of spontaneous and nucleotide-evoked Ca²⁺ transients such as amplitude, decay time and duration were markedly reduced during cochlear development, whereas the kinetics of the Ca²⁺ rise did not show relevant changes. Whether low-frequency spontaneous Ca²⁺ transients are necessary for the formation and maintenance of the tectorial membrane e.g. by regulating gene transcription needs to be elucidated in further studies.

Onset kinetics of noise-induced purinergic adaptation of the 'cochlear amplifier'

A major component of slowly reversible hearing loss which develops with sustained exposure to noise has been attributed to release of ATP in the cochlea activating P2X2 receptor (P2X2R) type ATP-gated ion channels. This purinergic humoral adaptation is thought to enable the highly sensitive hearing organ to maintain function with loud sound, protecting the ear from acoustic overstimulation. In the study that established this hearing adaptation mechanism as reported by Housley et al. (Proc Natl Acad Sci U S A 110:7494-7499, 2013), the activation kinetics were determined in mice from auditory brainstem response (ABR) threshold shifts with sustained noise presentation at time points beyond 10 min. The present study was designed to achieve finer resolution of the onset kinetics of purinergic hearing adaptation, and included the use of cubic (2f1-f2) distortion product otoacoustic emissions (DPOAEs) to probe whether the active mechanical outer hair cell 'cochlear amplifier' contributed to this process. We show that the ABR and DPOAE threshold shifts were largely complete within the first 7.5 min of moderate broadband noise (85 dB SPL) in wildtype C57Bl/6J mice. The ABR and DPOAE adaptation rates were both best fitted by a single exponential function with ~ 3 min time constants. ABR and DPOAE threshold shifts with this noise were minimal in mice null for the P2rx2 gene encoding the P2X2R. The findings demonstrate a considerably faster purinergic hearing adaptation to noise than previously appreciated. Moreover, they strongly implicate the outer hair cell as the site of action, as the DPOAEs stem from active cochlear electromotility.

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.

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.

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.

Non-synonymous single nucleotide polymorphisms in the P2X receptor genes: association with diseases, impact on receptor functions and potential use as diagnosis biomarkers

P2X receptors are Ca²⁺-permeable cationic channels in the cell membranes, where they play an important role in mediating a diversity of physiological and pathophysiological functions of extracellular ATP. Mammalian cells express seven P2X receptor genes. Single nucleotide polymorphisms (SNPs) are widespread in the P2RX genes encoding the human P2X receptors, particularly the human P2X7 receptor. This article will provide an overview of the non-synonymous SNPs (NS-SNPs) that have been associated with or implicated in altering the susceptibility to pathologies or disease conditions, and discuss the consequences of the mutations resulting from such NS-SNPs on the receptor functions. Disease-associated NS-SNPs in the P2RX genes have been valuable in understanding the disease etiology and the receptor function, and are promising as biomarkers to be used for the diagnosis and development of stratified therapeutics.

Permeation of fluorophore-conjugated phalloidin into live hair cells of the inner ear is modulated by P2Y receptors

Phalloidin, a toxin isolated from the death cap mushroom, Amanita phalloides, binds to filamentous actin with high affinity, and this has made fluorophore-conjugated phalloidin a useful tool in cellular imaging. Hepatocytes take up phalloidin via the liver-specific organic anion transporting polypeptide 1b2, but phalloidin does not permeate most living cells. Rapid entry of styryl dyes into live hair cells has been used to evaluate function, but the usefulness of those fluorescence dyes is limited by broad and fixed absorption spectra. Since phalloidin can be conjugated to fluorophores with various spectra, we investigated whether it would permeate living hair cells. When we incubated mouse utricles in 66 nM phalloidin-CF488A and followed that by washes in phalloidin-free medium, we observed that it entered a subset of hair cells and labeled entire hair bundles fluorescently after 20 min. Incubations of 90 min labeled nearly all the hair bundles. When phalloidin-treated utricles were cultured for 24 h after washout, the label disappeared from the hair cells and progressively but heterogeneously labeled filamentous actin in the supporting cells. We investigated how phalloidin may enter hair cells and found that P2 receptor antagonists, pyridoxalphosphate-6-azophenyl-2', 4'-disulfonic acid and suramin, blocked phalloidin entry, while the P2Y receptor ligands, uridine-5'-diphosphate and uridine-5'-triphosphaste, stimulated uptake. Consistent with that, the P2Y6 receptor antagonist, MRS 2578, decreased phalloidin uptake. The results show that phalloidin permeates live hair cells through a pathway that requires metabotropic P2Y receptor signaling and suggest that phalloidin can be transferred from hair cells to supporting cells in culture.

ATP-gated ion channels mediate adaptation to elevated sound levels

The sense of hearing is remarkable for its auditory dynamic range, which spans more than 10(12) in acoustic intensity. The mechanisms that enable the cochlea to transduce high sound levels without damage are of key interest, particularly with regard to the broad impact of industrial, military, and recreational auditory overstimulation on hearing disability. We show that ATP-gated ion channels assembled from P2X2 receptor subunits in the cochlea are necessary for the development of temporary threshold shift (TTS), evident in auditory brainstem response recordings as sound levels rise. In mice null for the P2RX2 gene (encoding the P2X2 receptor subunit), sustained 85-dB noise failed to elicit the TTS that wild-type (WT) mice developed. ATP released from the tissues of the cochlear partition with elevation of sound levels likely activates the broadly distributed P2X2 receptors on epithelial cells lining the endolymphatic compartment. This purinergic signaling is supported by significantly greater noise-induced suppression of distortion product otoacoustic emissions derived from outer hair cell transduction and decreased suprathreshold auditory brainstem response input/output gain in WT mice compared with P2RX2-null mice. At higher sound levels (≥95 dB), additional processes dominated TTS, and P2RX2-null mice were more vulnerable than WT mice to permanent hearing loss due to hair cell synapse disruption. P2RX2-null mice lacked ATP-gated conductance across the cochlear partition, including loss of ATP-gated inward current in hair cells. These data indicate that a significant component of TTS represents P2X2 receptor-dependent purinergic hearing adaptation that underpins the upper physiological range of hearing.

Differential expression of P2Y receptors in the rat cochlea during development

Purinergic signaling has broad physiological significance to the hearing organ, involving signal transduction via ionotropic P2X receptors and metabotropic G-protein-coupled P2Y and P1 (adenosine), alongside conversion of nucleotides and nucleosides by ecto-nucleotidases and ecto-nucleoside diphosphokinase. In addition, ATP release is modulated by acoustic overstimulation or stress and involves feedback regulation. Many of these principal elements of the purinergic signaling complex have been well characterized in the cochlea, while the characterization of P2Y receptor expression is emerging. The present study used immunohistochemistry to evaluate the expression of five P2Y receptors, P2Y(1), P2Y(2), P2Y(4), P2Y(6), and P2Y(12), during development of the rat cochlea. Commencing in the late embryonic period, the P2Y receptors studied were found in the cells lining the cochlear partition, associated with establishment of the electrochemical environment which provides the driving force for sound transduction. In addition, early postnatal P2Y(2) and P2Y(4) protein expression in the greater epithelial ridge, part of the developing hearing organ, supports the view that initiation and regulation of spontaneous activity in the hair cells prior to hearing onset is mediated by purinergic signaling. Sub-cellular compartmentalization of P2Y receptor expression in sensory hair cells, and diversity of receptor expression in the spiral ganglion neurons and their satellite cells, indicates roles for P2Y receptor-mediated Ca(2+)-signaling in sound transduction and auditory neuron excitability. Overall, the dynamics of P2Y receptor expression during development of the cochlea complement the other elements of the purinergic signaling complex and reinforce the significance of extracellular nucleotide and nucleoside signaling to hearing.

ATP-induced morphological changes in supporting cells of the developing cochlea

The developing cochlea of mammals contains a large group of columnar-shaped cells, which together form a structure known as Kölliker's organ. Prior to the onset of hearing, these inner supporting cells periodically release adenosine 5'-triphosphate (ATP), which activates purinergic receptors in surrounding supporting cells, inner hair cells and the dendrites of primary auditory neurons. Recent studies indicate that purinergic signaling between inner supporting cells and inner hair cells initiates bursts of action potentials in auditory nerve fibers before the onset of hearing. ATP also induces prominent effects in inner supporting cells, including an increase in membrane conductance, a rise in intracellular Ca(2+), and dramatic changes in cell shape, although the importance of ATP signaling in non-sensory cells of the developing cochlea remains unknown. Here, we review current knowledge pertaining to purinergic signaling in supporting cells of Kölliker's organ and focus on the mechanisms by which ATP induces changes in their morphology. We show that these changes in cell shape are preceded by increases in cytoplasmic Ca(2+), and provide new evidence indicating that elevation of intracellular Ca(2+) and IP(3) are sufficient to initiate shape changes. In addition, we discuss the possibility that these ATP-mediated morphological changes reflect crenation following the activation of Ca(2+)-activated Cl(-) channels, and speculate about the possible functions of these changes in cell morphology for maturation of the cochlea.

Role of nitric oxide on purinergic signalling in the cochlea

In the inner ear, there is considerable evidence that extracellular adenosine 5'-triphosphate (ATP) plays an important role in auditory neurotransmission as a neurotransmitter or a neuromodulator, although the potential role of adenosine signalling in the modulation of auditory neurotransmission has also been reported. The activation of ligand-gated ionotropic P2X receptors and G protein-coupled metabotropic P2Y receptors has been reported to induce an increase of intracellular Ca(2+) concentration ([Ca(2+)](i)) in inner hair cells (IHCs), outer hair cells (OHCs), spiral ganglion neurons (SGNs), and supporting cells in the cochlea. ATP may participate in auditory neurotransmission by modulating [Ca(2+)](i) in the cochlear cells. Recent studies showed that extracellular ATP induced nitric oxide (NO) production in IHCs, OHCs, and SGNs, which affects the ATP-induced Ca(2+) response via the NO-cGMP-PKG pathway in those cells by a feedback mechanism. A cross-talk between NO and ATP may therefore exist in the auditory signal transduction. In the present article, I review the role of NO on the ATP-induced Ca(2+) signalling in IHCs and OHCs. I also consider the possible role of NO in the ATP-induced Ca(2+) signalling in SGNs and supporting cells.

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.

ATP-mediated potassium recycling in the cochlear supporting cells

Gap junction-mediated K(+) recycling in the cochlear supporting cell has been proposed to play a critical role in hearing. However, how potassium ions enter into the supporting cells to recycle K(+) remains undetermined. In this paper, we report that ATP can mediate K(+) sinking to recycle K(+) in the cochlear supporting cells. We found that micromolar or submicromolar levels of ATP could evoke a K(+)-dependent inward current in the cochlear supporting cells. At negative membrane potentials and the resting membrane potential of -80 mV, the amplitude of the ATP-evoked inward current demonstrated a linear relationship to the extracellular concentration of K(+), increasing as the extracellular concentration of K(+) increased. The inward current also increased as the concentration of ATP was increased. In the absence of ATP, there was no evoked inward current for extracellular K(+) challenge in the cochlear supporting cells. The ATP-evoked inward current could be inhibited by ionotropic purinergic (P2X) receptor antagonists. Application of pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS, 50 microM) or pre-incubation with an irreversible P2X7 antagonist oxidized ATP (oATP, 0.1 mM) completely abolished the ATP-evoked inward current at the negative membrane potential. ATP also evoked an inward current at cell depolarization, which could be inhibited by intracellular Cs(+) and eliminated by positive holding potentials. Our data indicate that ATP can activate P2X receptors to recycle K(+) in the cochlear supporting cells at the resting membrane potential under normal physiological and pathological conditions. This ATP-mediated K(+) recycling may play an important role in the maintenance of cochlear ionic homeostasis.

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Developmentally regulated expression of ectonucleotidases NTPDase5 and NTPDase6 and UDP-responsive P2Y receptors in the rat cochlea

Ectonucleoside triphosphate diphosphohydrolases (E-NTPDases) regulate complex extracellular P2 receptor signalling pathways in mammalian tissues by hydrolysing extracellular nucleotides to the respective nucleosides. All enzymes from this family (NTPDase1-8) are expressed in the adult rat cochlea. This study reports the changes in expression of NTPDase5 and NTPDase6 in the developing rat cochlea. These two intracellular members of the E-NTPDase family can be released in a soluble form and show preference for nucleoside 5'-diphosphates, such as UDP and GDP. Here, we demonstrate differential spatial and temporal patterns for NTPDase5 and NTPDase6 expression during cochlear development, which are indicative of both cytosolic and extracellular action via pyrimidines. NTPDase5 is noted during the early postnatal period in developing sensory hair cells and supporting Deiters' cells of the organ of Corti, and primary auditory neurons located in the spiral ganglion. In contrast, NTPDase6 is confined to the embryonic and early postnatal hair cell bundles. NTPDase6 immunolocalisation in the developing cochlea underpins its putative role in hair cell bundle development, probably via cytosolic action, whilst NTPDase5 may have a broader extracellular role in the development of sensory and neural tissues in the rat cochlea. Both NTPDase5 and NTPDase6 colocalize with UDP-preferring P2Y(4), P2Y(6) and P2Y(14) receptors during cochlear development, but this strong association was lost in the adult cochlea. Spatiotemporal topographic expression of NTPDase5 and NTPDase6 and P2Y receptors in adult and developing cochlear tissues provide strong support for the role of pyrimidinergic signalling in cochlear development.

A corticotropin-releasing factor system expressed in the cochlea modulates hearing sensitivity and protects against noise-induced hearing loss

Noise-induced hearing loss is a highly prevalent occupational injury, yet little is known concerning the signals controlling normal cochlear sensitivity and susceptibility to noise-induced trauma. While the corticotropin-releasing factor (CRF) system is involved in activation of the classic hypothalamic-pituitary-adrenal axis, it is also involved in local physiological responses to stress in many tissues, and is expressed in the inner ear. We demonstrate that mice lacking the CRF receptor CRFR2 exhibit a significantly lower auditory threshold than wild type mice, but this gain of function comes at the price of increased susceptibility to acoustic trauma. We further demonstrate that glutamatergic transmission, purinergic signaling, and activation of Akt (PKB) pathways within the cochlea are misregulated, which may underlie the enhanced sensitivity and trauma susceptibility observed in CRFR2(-/-) mice. Our data suggest that CRFR2 constitutively modulates hearing sensitivity under normal conditions, and thereby provides protection against noise-induced hearing loss.

Changes in P2Y4 receptor expression in rat cochlear outer sulcus cells during development

Extracellular adenosine triphosphate (ATP) released from cellular sources plays an important role in variety of the cochlear physiologic processes. The primary purinergic receptor subtype in the cochlea is the P2X2 receptor, which is a subtype of P2X receptor. This receptor appears to mediate a protective decrease in the electrical driving force in response to acoustic overstimulation. Outer sulcus cells (OSCs) in the cochlear lateral wall appear to maintain an adequate K+ concentration in the cochlear endolymph in response to varying intensities of auditory stimulation. However, little is known about developing OSCs. The purpose of this study was to investigate subtypes of purinergic receptors in developing rat OSCs using a voltage-sensitive vibrating probe. Results showed that only two P2 receptors (P2Y4 and P2X2) contributed to the regulation of short circuit currents in neonatal OSCs. ATP increased cation absorption via apical nonselective cation channels after activating P2Y4 receptors in early neonatal OSCs. P2Y4 expression rapidly declined postnatally and reached near adult levels on postnatal day 14. P2X2 was co-expressed with P2Y4 in early neonatal OSCs. Temporal changes in P2Y4 during OSC development might be involved in the establishment of the endolymphatic ion composition needed for normal auditory transduction and/or specific cellular differentiation.

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.

Reactive blue 2, an antagonist of rat P2Y4, increases K+ secretion in rat cochlea strial marginal cells

Extracellular ATP decreases K+ secretion in strial marginal cells via apical P2Y4 receptors. We investigated the effect of reactive blue 2 (RB-2), an antagonist of rat P2Y4, on rat strial marginal cells using a voltage-sensitive vibrating probe. The application of RB-2 increased K+ secretion in a dose-dependent manner, and this increase was characterized as a peak followed by a partial relaxation to a steady-state. Moreover, this response was similar to that caused by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS). Suramin had no similar effect, except at high concentration. Thus, we tested the effects of these chemicals on P2Y4 receptors in strial marginal cells. Both RB-2 and DIDS had antagonistic activities at P2Y4, and the antagonist potency at P2Y4 paralleled the potency of K+ secretion. Interestingly, 2'- and 3'-O-(4-benzoyl-benzoyl)adenosine 5'-triphosphate (BzATP) exhibited an agonistic effect at P2Y4 receptor, which was blocked by RB-2, but not by pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS). Based on these results, we speculate that direct and/or indirect inhibitory mechanisms between P2Y4 and KENQ1/KCNE1 K+ channels exist in strial marginal cell.

Hair Cells – Beyond the Transducer

OVERVIEW

This review considers the "tween twixt and twain" of hair cell physiology, specifically the signaling elements and membrane conductances which underpin forward and reverse transduction at the input stage of hair cell function and neurotransmitter release at the output stage. Other sections of this review series outline the advances which have been made in understanding the molecular physiology of mechanoelectrical transduction and outer hair cell electromotility. Here we outline the contributions of a considerable array of ion channels and receptor signaling pathways that define the biophysical status of the sensory hair cells, contributing to hair cell development and subsequently defining the operational condition of the hair cells across the broad dynamic range of physiological function.

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.

Apical membrane P2Y4 purinergic receptor controls K+ secretion by strial marginal cell epithelium

Background

It was previously shown that K⁺ secretion by strial marginal cell epithelium is under the control of G-protein coupled receptors of the P2Y family in the apical membrane. Receptor activation by uracil nucleotides (P2Y₂, P2Y₄ or P2Y₆) leads to a decrease in the electrogenic K⁺ secretion. The present study was conducted to determine the subtype of the functional purinergic receptor in gerbil stria vascularis, to test if receptor activation leads to elevation of intracellular [Ca²⁺] and to test if the response to these receptors undergoes desensitization.

Results

The transepithelial short circuit current (Isc) represents electrogenic K⁺ secretion and was found to be decreased by uridine 5'-triphosphate (UTP), adenosine 5'-triphosphate (ATP) and diadenosine tetraphosphate (Ap4A) but not uridine 5'-diphosphate (UDP) at the apical membrane of marginal cells of the gerbil stria vascularis. The potencies of these agonists were consistent with rodent P2Y₄ and P2Y₂ but not P2Y₆ receptors. Activation caused a biphasic increase in intracellular [Ca²⁺] that could be partially blocked by 2-aminoethoxy-diphenyl borate (2-APB), an inhibitor of the IP3 receptor and store-operated channels. Suramin (100 μM) did not inhibit the effect of UTP (1 μM). The ineffectiveness of suramin at the concentration used was consistent with P2Y₄ but not P2Y₂. Transcripts for both P2Y₂ and P2Y₄ were found in the stria vascularis. Sustained exposure to ATP or UTP for 15 min caused a depression of Isc that appeared to have two components but with apparently no chronic desensitization.

Conclusion

The results support the conclusion that regulation of K⁺ secretion across strial marginal cell epithelium occurs by P2Y₄ receptors at the apical membrane. The apparent lack of desensitization of the response is consistent with two processes: a rapid-onset phosphorylation of KCNE1 channel subunit and a slower-onset of regulation by depletion of plasma membrane PIP₂.

Noise exposure induces up-regulation of ecto-nucleoside triphosphate diphosphohydrolases 1 and 2 in rat cochlea

Extracellular ATP acting via P2 receptors in the inner ear initiates a variety of signaling pathways that may be involved in noise-induced cochlear injury. Nucleoside triphosphate diphosphohydrolase (NTPDase)1/CD39 and NTPDase2/CD39L1 are key elements for regulation of extracellular nucleotide concentrations and P2 receptor signaling in the cochlea. This study characterized the effect of noise exposure on regulation of NTPDase1 and NTPDase2 expression in the cochlea using a combination of real-time RT-PCR, immunohistochemistry and functional studies. Adult Wistar rats were exposed to broad band noise at 90 dB and 110 dB sound pressure level (SPL) for 72 h. Exposure to 90 dB SPL induced a small and temporary change of auditory thresholds (temporary threshold shift), while exposure to 110 dB SPL induced a robust and permanent change of auditory thresholds (permanent threshold shift). NTPDase1 and NTPDase2 mRNA transcripts were upregulated in the cochlea exposed to 110 dB SPL, while mild noise (90 dB SPL) altered only NTPDase1 mRNA expression levels. Changes in NTPDases expression did not correlate with levels of circulating corticosterone, implying that the up-regulation of NTPDases expression was not stress-related. Semi-quantitative immunohistochemistry in the cochlea exposed to 110 dB SPL localized the increased NTPDase1 and NTPDase2 immunostaining in the stria vascularis and up-regulation of NTPDase2 in the intraganglionic spiral bundle. In contrast, NTPDase1 was down-regulated in the cell bodies of the spiral ganglion neurones. Distribution of NTPDases was not altered in the cochlea exposed to 90 dB SPL. Functional studies revealed increased ectonucleotidase activities in the cochlea after exposure to 110 dB SPL, consistent with up-regulation of NTPDases. The changes in NTPDases expression may reflect adaptive response of cochlear tissues to limit ATP signaling during noise exposure.

Purinergic receptors in auditory neurotransmission

The effects of ATP (adenosine 5' triphosphate) analogs on gross cochlear potentials and single primary afferent discharge properties were studied by intracochlear perfusion in anesthetized guinea pigs. ATP-gamma-S was most potent, with betagammamethylene-ATP and Bz-ATP being significantly less effective. These data are consistent with the notion that purinergic receptors activated by scala tympani perfusion contain subunits of the P2X(2) variant. The relative ineffectiveness of Bz-ATP (a P2X(7) agonist) suggests that while this variant has been reported to be expressed in the cochlea, it may not play a major functional role under normal conditions. Changes in the threshold of the gross DC receptor potential (summating potential, SP) and the compound action potential (CAP) were consistent with a combination of effects on both early and final stages of the transduction process, as reported by previous workers. Effects of ATP-gamma-S on single-neuron spontaneous firing rates varied according to the initial spontaneous rate of each primary afferent. Effects on single-neuron tuning curves were consistent with an action mainly on the outer hair cell transduction with betagammamethylene-ATP (elevation of tuning curve tips), but with ATP-gamma-S changes in sensitivity across the full extent of the tuning curve indicated an additional action on inner hair cell-afferent neurotransmission. In agreement with previous reports on ATP-gamma-S, it was found that all ATP analogs produced significant increases in the DC potential in scala media (endocochlear potential, EP). However, the relationship between changes in EP (a major component of the driving force on ions through hair cells) and the alterations in gross and single unit measures of cochlear activity was not clear.

P2X receptor subtype-specific modulation of excitatory and inhibitory synaptic inputs in the rat brainstem

The role of P2 receptors in synaptic transmission to the rat medial nucleus of the trapezoid body (MNTB) was studied in an in vitro brain slice preparation. Whole-cell patch recordings were made and spontaneous synaptic responses studied under voltage clamp during application of P2X receptor agonists. ATPgammaS (100 microm) had no effect on holding current, but facilitated spontaneous excitatory postsynaptic current (sEPSC) frequency in 41% of recordings and facilitated spontaneous inhibitory postsynaptic currents (sIPSCs) in 20% of recordings. These were blocked by the P2 receptor antagonist suramin (100 microm). alpha,beta-meATP also facilitated sEPSC and sIPSC frequency, while l-beta,gamma-meATP facilitated only sIPSCs. The sEPSC facilitation by ATPgammaS was blocked by TTX (but did not block facilitation of sIPSCs). sEPSC facilitation was blocked by PPADS (30 microm) and the selective P2X(3) receptor antagonist A-317491 (3 microm), suggesting that modulation of sEPSCs involves P2X(3) receptor subunits. alpha,beta-meATP-facilitated sIPSCs were also recorded in wild-type mouse MNTB neurones, but were absent in the MNTB from P2X(1) receptor-deficient mice demonstrating a functional role for P2X(1) receptors in the CNS.

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.

Noise induces up-regulation of P2X2 receptor subunit of ATP-gated ion channels in the rat cochlea

Regulation of P2X(2) receptor (P2X(2)R) expression in the rat cochlea in response to noise was analysed. Sustained loud sound (90-120 dB white noise, > 6 h), increased P2X(2)R mRNA and protein levels in rat organ of Corti and spiral ganglion (primary auditory neurones). P2X(2)R expression by the type I spiral ganglion neurones, which innervate the inner hair cells via the inner spiral plexus, was confirmed by confocal immunofluorescence. This also revealed increased P2X(2)R labelling of outer hair cell (OHC) stereocilia and cuticular plates, reflecting trafficking of greater numbers of ATP-gated ion channels assembled with P2X(2)R subunits to the transducer site. Whole-cell voltage clamp of OHC confirmed the noise-induced up-regulation of ATP-gated inward currents. These data indicate that regulation of P2X(2) receptor gene expression in the cochlea is adaptive, with sustained loud sound promoting increased transcription and translation specifically at sites regulating hearing sensitivity and auditory neurotrans-mission.

Ecto-ATPase activity sites in vestibular tissues: an ultracytochemical study in frog semicircular canals

The present study describes the localization and distribution of putative ecto-nucleoside-triphosphate-diphosphohydrolases in the frog semicircular canals. These enzymes provide the terminating mechanism of adenosine-5'-triphosphate (ATP) signalling. The localization of the ATP hydrolysis was mapped ultracytochemically using a one-step cerium citrate reaction. Electron-dense precipitates, indicating ecto-adenosine-triphosphatase (ecto-ATPase) activity, were found at the outer surface of plasma membranes of crista hair cells and supporting cells of the sensory epithelium, transitional cells and undifferentiated cells of the ampullar wall and dark cells constituting the secretory epithelium. Non-sensory cells of the ampulla usually exhibited reaction deposits at the level of both apical and basolateral membranes coming into contact with the endolymph and the perilymph respectively, while cells constituting the sensory epithelium showed evident differences in relation to their position. Hair cells and supporting cells of the peripheral regions exhibited clear reaction products both at the level of apical and basolateral membranes, while those of the isthmus region showed abundant reactivity only at the level of their apical membranes. Of particular interest was the observation that hair cell stereocilia exhibited an abundant ecto-ATPase activity, thus suggesting a possible colocalization of enzymatic sites with purinergic receptors and mechanotransduction channels. This strategic expression of ecto-ATPase sites could provide a rapid mechanism of ATP removal able to rapidly restore the sensitivity of transduction channels. In conclusion, the widespread distribution of ecto-ATPase sites at the level of sensory and non-sensory cells of the frog semicircular canals suggests that ATP may have a key role in controlling vestibular function.

NTPDase1 and NTPDase2 immunolocalization in mouse cochlea: implications for regulation of p2 receptor signaling

Cellular, molecular, and physiological studies have demonstrated an important signaling role for ATP and related nucleotides acting via P2 receptors in the cochlea of the inner ear. Signal modulation is facilitated by ectonucleotidases, a heterologous family of surface-located enzymes involved in extracellular nucleotide hydrolysis. Our previous studies have implicated CD39/NTPDase1 and CD39L1/NTPDase2, members of the ectonucleoside triphosphate diphosphohydrolase (E-NTPDase) family, as major ATP-hydrolyzing enzymes in the tissues lining the cochlear endolymphatic and perilymphatic compartments. NTPDase1 hydrolyzes both nucleoside triphosphates and diphosphates. In contrast, NTPDase2 is a preferential nucleoside triphosphatase. This study characterizes expression of these E-NTPDases in the mouse cochlea by immunohistochemistry. NTPDase1 can be immunolocalized to the cochlear vasculature and neural tissues (primary auditory neurons in the spiral ganglion). In contrast, NTPDase2 immunolabeling was principally localized to synaptic regions of the sensory inner and outer hair cells, stereocilia and cuticular plates of the outer hair cells, supporting cells of the organ of Corti (Deiters' cells and inner border cells), efferent nerve fibers located in the intraganglionic spiral bundle, and in the outer sulcus and root region of the spiral ligament. This differential expression of NTPDase1 and 2 in the cochlea suggests spatial regulation of P2 receptor signaling, potentially involving different nucleotide species and hydrolysis kinetics.

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.

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.

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.

Molecular basis of mechanosensory transduction

Mechanotransduction - a cell's conversion of a mechanical stimulus into an electrical signal - reveals vital features of an organism's environment. From hair cells and skin mechanoreceptors in vertebrates, to bristle receptors in flies and touch receptors in worms, mechanically sensitive cells are essential in the life of an organism. The scarcity of these cells and the uniqueness of their transduction mechanisms have conspired to slow molecular characterization of the ensembles that carry out mechanotransduction. But recent progress in both invertebrates and vertebrates is beginning to reveal the identities of proteins essential for transduction.

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.

Cryoembedding and sectioning of cochleas for immunocytochemistry and in situ hybridization

Current emphasis on biochemical and molecular aspects of cochlear anatomy underscores the necessity for high quality cryostat sections of the inner ear. The large volume of fluid space within the cochlea makes cryoembedding and sectioning of the organ more problematic than that of other, more homogeneous tissues. Our method for cryoembedding of cochleas for immunocytochemistry and in situ hybridization uses slow infiltration with increasing concentrations of sucrose followed by degassed embedding medium before final orientation and freezing. This method permits high quality cryosections to be cut which preserve overall structure and cellular resolution.

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.

P2X receptor-mediated changes in cochlear potentials arising from exogenous adenosine 5'-triphosphate in endolymph

Our previous studies have determined the presence of adenosine 5'-triphosphate (ATP) in the cochlear fluids and shown that extracellular ATP introduced into the endolymphatic compartment of the guinea pig cochlea has a significant dose-dependent suppressive effect on both endocochlear potential (EP) and cochlear microphonic (CM), which is mediated via P2 receptors. In the present study, the influence of P2 receptor agonists and antagonists on this suppressive effect was investigated to characterise the subtypes of P2 receptor mediating the ATP-induced effect on cochlear function. Using a double-barreled pipette attached to a pressure injector, small volumes (2-10 nl) of ATP (0.01-1 mM) and P2 receptor agonists or P2 receptor antagonists in artificial endolymph were introduced into the scala media of the first (basal) and third turns of the guinea pig cochlea, while the EP and CM were monitored. ATP and P2 receptor agonists (5x10(-14)-1x10(-11)cibacron blue. Neither adenosine nor uridine 5'-triphosphate (2x10(-13)-2x10(-11) moles) nor the P2 receptor antagonists on their own had any effect on EP and CM. The ATP effect on the potentials was greater at the third cochlear turn when compared to the first turn. These results provide evidence that in the endolymphatic compartment of the guinea pig, the extracellular ATP effect on cochlear function is likely mediated through an interaction with P2 receptors which assemble as ATP-gated ion channels.

Gene expression of P2X-receptors in the developing inner ear of the rat

Reverse transcription-polymerase chain reaction (RT-PCR) was used to characterize the expression of P2X receptor subunits (P2X1-P2X7) in different inner ear tissues. The present study revealed the presence of P2X2, P2X3, P2X4 and P2X7-mRNA in rat organ of Corti, vestibular organ and spiral ganglion at different postnatal developmental stages (PD1-PD16), with slight differences in the onset of expression. Expression of P2X1, P2X5 and P2X6-mRNA was not detectable in the inner ear tissues. In addition, single cell RT-PCR experiments with outer hair cells (OHC) revealed the expression of either the P2X2 or the P2X2-2 splice variant or coexpression of both isoforms in individual cells. Our data suggest that extracellular adenosine-5'-triphosphate (ATP) may play an important role in signal transduction in the inner ear.

ATP-gated ion channel expression in primary auditory neurones

Extracellular ATP acts via ionotropic P2X receptors to mediate fast neurotransmission in the central and autonomic nervous systems. Recent data, including identification of P2X2 receptor mRNA expression by spiral ganglion neurones, suggests that purinergic signalling may influence auditory neurotransmission via ATP-gated ion channels assembled from these subunits. Expression of the P2X2 receptor was localized to the region of the spiral ganglion neurone synapses with the inner hair cells using a P2X2 receptor specific antiserum. Whole-cell patch clamping of neurones cultured from post-natal day 3-5 spiral ganglia demonstrated a heterogeneity of ATP-activated conductances, consistent with the functional expression of P2X2 receptor subunit isoforms along with possible co-expression of additional P2X receptor subunits. These data provide substantive support for a purinergic transmission element at the peripheral auditory synapse.

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

P2X receptors have been reported to be involved in neurotransmission in both central and peripheral nerves. In the present study, polyclonal antibodies against P2X1, P2X2, P2X3, P2X4, P2X5, and P2X6 were used to study the distribution of P2X receptors in rat cochlea and vestibulocochlear nerve pathways. It was found that in the vestibular ganglion all six types of antibodies stained the neurons to different intensities. The strongest signal was obtained with the P2X2 antibodies, followed by P2X3 antibodies. The other four antibodies produced weak signals, of approximately the same intensity. In the spiral ganglion, the six types of antibodies also stained almost all neurons. However, the rank order of intensity was different from that in the vestibular ganglion: the strongest signal was still obtained with P2X2 antibodies, followed by P2X4, P2X1, and P2X3 antibodies. The immunolabelling was much weaker with P2X5, and P2X6 antibodies compared with the other four types of antibodies. In the cochlea, besides the spiral ganglion neurons, other tissues such as stria vascularis, the organ of Corti and the tectorial membrane were labelled intensively with P2X2 antibodies only. High density P2X2 immunoreactivity was also observed in the vestibulocochlear nerve fibres. In the cochlear nucleus, neurons and nerve fibres were stained with the P2X2 antibodies, as were the neurons in the trapezoid body. These results suggested that P2 receptors, especially the P2X2 receptors, may play important roles in the signal transduction involved in the perception of sound and balance.