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

Functional P2X7 Receptors in the Auditory Nerve of Hearing Rodents Localize Exclusively to Peripheral Glia

P2X₇ receptors (P2X₇Rs) are associated with numerous pathophysiological mechanisms, and this promotes them as therapeutic targets for certain neurodegenerative conditions. However, the identity of P2X₇R-expressing cells in the nervous system remains contentious. Here, we examined P2X₇R functionality in auditory nerve cells from rodents of either sex, and determined their functional and anatomic expression pattern. In whole-cell recordings from rat spiral ganglion cultures, the purinergic agonist 2',3'-O-(4-benzoylbenzoyl)-ATP (BzATP) activated desensitizing currents in spiral ganglion neurons (SGNs) but non-desensitizing currents in glia that were blocked by P2X₇R-specific antagonists. In imaging experiments, BzATP gated sustained Ca²⁺ entry into glial cells. BzATP-gated uptake of the fluorescent dye YO-PRO-1 was reduced and slowed by P2X₇R-specific antagonists. In rats, P2X₇Rs were immuno-localized predominantly within satellite glial cells (SGCs) and Schwann cells (SCs). P2X₇R expression was not detected in the portion of the auditory nerve within the central nervous system. Mouse models allowed further exploration of the distribution of cochlear P2X₇Rs. In GENSAT reporter mice, EGFP expression driven via the P2rx7 promoter was evident in SGCs and SCs but was undetectable in SGNs. A second transgenic model showed a comparable cellular distribution of EGFP-tagged P2X₇Rs. In wild-type mice the discrete glial expression was confirmed using a P2X₇-specific nanobody construct. Our study shows that P2X₇Rs are expressed by peripheral glial cells, rather than by afferent neurons. Description of functional signatures and cellular distributions of these enigmatic proteins in the peripheral nervous system (PNS) will help our understanding of ATP-dependent effects contributing to hearing loss and other sensory neuropathies.SIGNIFICANCE STATEMENT P2X₇ receptors (P2X₇Rs) have been the subject of much scrutiny in recent years. They have been promoted as therapeutic targets in a number of diseases of the nervous system, yet the specific cellular location of these receptors remains the subject of intense debate. In the auditory nerve, connecting the inner ear to the brainstem, we show these multimodal ATP-gated channels localize exclusively to peripheral glial cells rather than the sensory neurons, and are not evident in central glia. Physiologic responses in the peripheral glia display classical hallmarks of P2X₇R activation, including the formation of ion-permeable and also macromolecule-permeable pores. These qualities suggest these proteins could contribute to glial-mediated inflammatory processes in the auditory periphery under pathologic disease states.

Properties of ATP-gated ion channels assembled from P2X2 subunits in mouse cochlear Reissner's membrane epithelial cells

In the cochlea, Reissner's membrane separates the scala media endolymphatic compartment that sustains the positive endocochlear potential and ion composition necessary for sound transduction, from the scala vestibuli perilymphatic compartment. It is known that with sustained elevated sound levels, adenosine 5'-triphosphate (ATP) is released into the endolymph and ATP-gated ion channels on the epithelial cells lining the endolymphatic compartment shunt the electrochemical driving force, contributing to protective purinergic hearing adaptation. This study characterises the properties of epithelial cell P2X(2)-type ATP-activated membrane conductance in the mouse Reissner's membrane, which forms a substantial fraction of the scale media surface. The cells were found to express two isoforms (a and b) of the P2X(2) subunit arising from alternative splicing of the messenger RNA (mRNA) transcript that could contribute to the trimeric subunit assembly. The ATP-activated conductance demonstrated both immediate and delayed desensitisation consistent with incorporation of the combination of P2X(2) subunit isoforms. Activation by the ATP analogue 2meSATP had equipotency to ATP, whereas α,β-meATP and adenosine 5'-diphosphate (ADP) were ineffective. Positive allosteric modulation of the P2X(2) channels by protons was profound. This native conductance was blocked by the P2X(2)-selective blocker pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) and the conductance was absent in these cells isolated from mice null for the P2rX2 gene encoding the P2X(2) receptor subunit. The activation and desensitisation properties of the Reissner's membrane epithelial cell ATP-gated P2X(2) channels likely contribute to the sensitivity and kinetics of purinergic control of the electrochemical driving force for sound transduction invoked by noise exposure.

Comparative analysis of P2X1, P2X2, P2X3, and P2X4 receptor subunits in rat nodose ganglion neurons

Nodose ganglion (NG) neurons are visceral primary sensory neurons. The transmission and regulation of visceral sensation is mediated mainly by the P2X purinoceptor (P2X receptor). Although the characteristics of different P2X receptor subunits in the NG have been studied previously, comprehensive analyses have not been performed. In this study, we used immunohistochemistry, immunocytochemistry, and whole cell patch clamp techniques to compare the expression and function of P2X1, P2X2, P2X3, and P2X4 receptor subunits in adult rat NG neurons. Polyclonal antibodies against the four P2X subunits labeled different subpopulations of NG neurons. P2X1 and P2X3 were expressed mainly in small-to-medium sized NG neurons, whereas P2X2 and P2X4 were located mostly in medium- and larger-sized NG neurons. Over 36% of NG neurons were P2X3 positive, which was higher than the other three P2X subunits. In addition, different types of currents were recorded from neurons expressing different P2X subunits. The fast type of ATP current was recorded from neurons containing P2X1–4 subunits, the intermediate type of current was recorded from neurons containing the P2X1, P2X3, and P2X4 subunits, the slow type was recorded from neurons expressing P2X1–3, and/or P2X4 subunits, whereas the very slow type was recorded from neurons containing the P2X2 and P2X3 subunits. These comparative results provide an anatomical verification of the different subunits in NG neurons, and offer direct support for the idea that various functional NG populations have distinct responses to ATP, which might be in part due to the different expression profiles of diverse P2X subunits.

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.

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.

Signaling at purinergic P2X receptors

P2X receptors are membrane cation channels gated by extracellular ATP. Seven P2X receptor subunits (P2X(1-7)) are widely distributed in excitable and nonexcitable cells of vertebrates. They play key roles in inter alia afferent signaling (including pain), regulation of renal blood flow, vascular endothelium, and inflammatory responses. We summarize the evidence for these and other roles, emphasizing experimental work with selective receptor antagonists or with knockout mice. The receptors are trimeric membrane proteins: Studies of the biophysical properties of mutated subunits expressed in heterologous cells have indicated parts of the subunits involved in ATP binding, ion permeation (including calcium permeability), and membrane trafficking. We review our current understanding of the molecular properties of P2X receptors, including how this understanding is informed by the identification of distantly related P2X receptors in simple eukaryotes.

The origin of spontaneous activity in the developing auditory system

Spontaneous activity in the developing auditory system is required for neuronal survival as well as the refinement and maintenance of tonotopic maps in the brain. However, the mechanisms responsible for initiating auditory nerve firing in the absence of sound have not been determined. Here we show that supporting cells in the developing rat cochlea spontaneously release ATP, which causes nearby inner hair cells to depolarize and release glutamate, triggering discrete bursts of action potentials in primary auditory neurons. This endogenous, ATP-mediated signalling synchronizes the output of neighbouring inner hair cells, which may help refine tonotopic maps in the brain. Spontaneous ATP-dependent signalling rapidly subsides after the onset of hearing, thereby preventing this experience-independent activity from interfering with accurate encoding of sound. These data indicate that supporting cells in the organ of Corti initiate electrical activity in auditory nerves before hearing, pointing to an essential role for peripheral, non-sensory cells in the development of central auditory pathways.

Otopetrin 1 activation by purinergic nucleotides regulates intracellular calcium

Otopetrin1 (Otop1) is a multitransmembrane domain protein required for the formation of otoconia in the vertebrate inner ear. Otoconia are complex calcium carbonate (CaCO(3)) biominerals that are required for the sensation of gravity. Examination of the phenotypes of animals with mutations or deficiencies in Otop1 suggests a direct role for Otop1 in the initiation of extracellular biomineralization, possibly through the regulation of intracellular Ca(2+). Here, we demonstrate that Otop1 overexpression can modulate purinergic-mediated Ca(2+) homeostasis in transfected cell lines. These experiments define a unique set of biochemical activities of Otop1, including depletion of endoplasmic reticulum Ca(2+) stores, specific inhibition of the purinergic receptor P2Y, and regulation of the influx of extracellular Ca(2+) in response to ATP, ADP, and UDP. These activities can be inhibited by the polyanion suramin in a rapidly reversible manner. This first characterization of the consequences of Otop1 overexpression indicates a profound effect on cellular Ca(2+) regulation. In a physiologic setting, these activities could direct the formation and growth of otoconia and regulate other biomineralization processes.

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.

Differential expression of ryanodine receptors in the rat cochlea

This study examined the localization and functional expression of ryanodine receptors (RyR) within the cochlea using a combination of reverse transcription-polymerase chain reaction, immunolabeling techniques, and confocal Ca2+ imaging. All three RyR isoform mRNA transcripts were detected in the adult rat cochlea. Immunoperoxidase and immunofluorescence labeling showed that the three isoforms were differentially expressed. The most pronounced RyR protein expression, involving all three isoforms, occurred in the cell bodies of the spiral ganglion neurons. RyR3 labeling extended to the synaptic terminals innervating the inner and outer hair cells. RyR2 expression also occurred in the inner hair cells and supporting cells of the organ of Corti, while cells associated with ion homeostasis in the cochlea, such as the interdental cells of the spiral limbus (RyR1), and the epithelial cells of the spiral prominence and basal cells of the stria vascularis (RyR2 and RyR3), were also immunopositive. The functionality of RyR-gated Ca2+ stores in the spiral ganglion neurons was shown by confocal calcium imaging of fluo-4 fluorescence in rat cochlear slices. Caffeine (5 mM) evoked an increase in intracellular Ca2+ concentration in the cell bodies of the spiral ganglion neurons which occurred inthe absence of external Ca2+. Ryanodine (50 nm-1 microM) evoked comparable increases in intracellular Ca2+ concentration. These findings suggest that RyR-mediated Ca2+ release may be involved in auditory neurotransmission, sound transduction, and cochlear electrochemical homeostasis.

Developmentally regulated expression of the P2X3 receptor in the mouse cochlea

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

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.

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.

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.

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.

Membrane properties of type II spiral ganglion neurones identified in a neonatal rat cochlear slice

Neuro-anatomical studies in the mammalian cochlea have previously identified a subpopulation of approximately 5 % of primary auditory neurones, designated type II spiral ganglion neurones (sgnII). These neurones project to outer hair cells and their supporting cells, within the 'cochlear amplifier' region. Physiological characterization of sgnII has proven elusive. Whole-cell patch clamp of spiral ganglion neurones in P7-P10 rat cochlear slices provided functional characterization of sgnII, identified by biocytin or Lucifer yellow labelling of their peripheral neurite projections (outer spiral fibres) subsequent to electrophysiological characterisation. SgnII terminal fields comprised multiple outer hair cells and supporting cells, located up to 370 mum basal to their soma. SgnII firing properties were defined by rapidly inactivating A-type-like potassium currents that suppress burst firing of action potentials. Type I spiral ganglion neurones (sgnI), had shorter radial projections to single inner hair cells and exhibited larger potassium currents with faster activation and slower inactivation kinetics, compatible with the high temporal firing fidelity seen in auditory nerve coding. Based on these findings, sgnII may be identified in future by the A-type current. Glutamate-gated somatic currents in sgnII were more potentiated by cyclothiazide than those in sgnI, suggesting differential AMPA receptor expression. ATP-activated desensitising inward currents were comparable in sgn II and sgnI. These data support a role for sgnII in providing integrated afferent feedback from the cochlear amplifier.

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.

Allosteric modulation of native cochlear P2X receptors: insights from comparison with recombinant P2X2 receptors

Extracellular adenosine 5'-triphosphate (ATP)-gated ion channels assembled from P2X receptor subunits exhibit subunit-selective allosteric modulation by protons and divalent cations. In voltage-clamped guinea-pig cochlear outer hair cells (OHC) and Deiters' cells (DC), H(+) and Cu(2+), but not Zn(2+), enhanced the P2X receptor-mediated inward currents. Acid pH (6.5) potentiated OHC ATP-gated currents by 45%. Co-application of Cu(2+) (1-40 microM) with ATP increased the response by 20%. In DCs, ATP-gated currents were potentiated 85% by acid pH, and 70% by Cu(2+). Alkaline pH inhibited ATP-gated inward currents by 73% in OHCs and 85% in DCs. Zn(2+) was either ineffective (1-10 microM) or inhibitory (40-400 microM). Recombinant rat P2X(2) receptor-mediated inward currents in XENOPUS oocytes displayed allosteric modulation that was different from the native guinea-pig cochlear P2X receptors. The oocyte ATP-gated inward current was potentiated 450% by shifting from pH 7.5 to pH 6.5, and 130% with 40 microM Cu(2+). The enhanced response to ATP with acid pH and Cu(2+) is a signature of the P2X(2) subunit. In contrast to native guinea-pig cochlear cells, extracellular Zn(2+) (40 microM) increased the recombinant ATP-gated inward current by 200% in oocytes. These results suggest that the positive allosteric modulation of cochlear OHC and DC ATP-gated ion channels by protons and Cu(2+) arises in part from the P2X(2) receptor subunit, with additional regulatory elements.

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.