ATP-induced current in isolated outer hair cells of guinea pig cochlea

Diverse identities and sites of action of cochlear neurotransmitters

Accurate encoding of acoustic stimuli requires temporally precise responses to sound integrated with cellular mechanisms that encode the complexity of stimuli over varying timescales and orders of magnitude of intensity. Sound in mammals is initially encoded in the cochlea, the peripheral hearing organ, which contains functionally specialized cells (including hair cells, afferent and efferent neurons, and a multitude of supporting cells) to allow faithful acoustic perception. To accomplish the demanding physiological requirements of hearing, the cochlea has developed synaptic arrangements that operate over different timescales, with varied strengths, and with the ability to adjust function in dynamic hearing conditions. Multiple neurotransmitters interact to support the precision and complexity of hearing. Here, we review the location of release, action, and function of neurotransmitters in the mammalian cochlea with an emphasis on recent work describing the complexity of signaling.

Slow oscillatory changes of DPOAE magnitude and phase after exposure to intense low-frequency sounds

Sensitive sound detection within the mammalian cochlea is performed by hair cells surrounded by cochlear fluids. Maintenance of cochlear fluid homeostasis and tight regulation of intracellular conditions in hair cells are crucial for the auditory transduction process but can be impaired by intense sound stimulation. After a short, intense low-frequency sound, the cochlea shows the previously described "bounce phenomenon," which manifests itself as slow oscillatory changes of hearing thresholds and otoacoustic emissions. In this study, distortion product otoacoustic emissions (DPOAEs) were recorded after Mongolian gerbils were exposed to intense low-frequency sounds (200 Hz, 100 dB SPL) with different exposure times up to 1 h. After all sound exposure durations, a certain percentage of recordings (up to 80% after 1.5-min-long exposure) showed oscillatory DPOAE changes, similar to the bounce phenomenon in humans. Changes were quite uniform with respect to size and time course, and they were independent from sound exposure duration. Changes showed states of hypo- and hyperactivity with either state preceding the other. The direction of changes was suggested to depend on the static position of the cochlear operating point. As assessed with DPOAEs, no indication for a permanent damage after several or long exposure times was detected. We propose that sensitivity changes occur due to alterations of the mechanoelectrical transduction process of outer hair cells. Those alterations could be induced by different challenged homeostatic processes with slow electromotility of outer hair cells being the most plausible source of the bounce phenomenon. NEW & NOTEWORTHY Low-frequency, high-intensity sound can cause slowly cycling activity changes in the mammalian cochlea. We examined the effect of low-frequency sound duration on the degree of these alterations. We found that cochlear changes showed a stereotypical biphasic pattern independent of sound exposure duration, but the probability that significant changes occurred decreased with increasing sound duration. Despite exposure durations of up to 1 h, no permanent or transient impairments of the cochlea were detected.

Developmental regulation of spontaneous activity in the Mammalian cochlea

Neurons in the developing auditory system fire bursts of action potentials before the onset of hearing. This spontaneous activity promotes the survival and maturation of auditory neurons and the refinement of synaptic connections in auditory nuclei; however, the mechanisms responsible for initiating this activity remain uncertain. Previous studies indicate that inner supporting cells (ISCs) in the developing cochlea periodically release ATP, which depolarizes inner hair cells (IHCs), leading to bursts of action potentials in postsynaptic spiral ganglion neurons (SGNs). To determine when purinergic signaling appears in the developing cochlea and whether it is responsible for initiating auditory neuron activity throughout the prehearing period, we examined spontaneous activity from ISCs, IHCs, and SGNs in cochleae acutely isolated from rats during the first three postnatal weeks. We found that ATP was released from ISCs within the cochlea from birth until the onset of hearing, which led to periodic inward currents, Ca(2+) transients, and morphological changes in these supporting cells. This spontaneous release of ATP also depolarized IHCs and triggered bursts of action potentials in SGNs for most of the postnatal prehearing period, beginning a few days after birth as IHCs became responsive to ATP, until the onset of hearing when ATP was no longer released from ISCs. When IHCs were not subject to purinergic excitation, SGNs exhibited little or no activity. These results suggest that supporting cells in the cochlea provide the primary excitatory stimulus responsible for initiating bursts of action potentials in auditory nerve fibers before the onset of hearing.

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

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

ELECTRONIC SUPPLEMENTARY MATERIAL

The online version of this article (doi:10.1007/s11302-010-9193-8) contains supplementary material, which is available to authorized users.

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.

ELECTRONIC SUPPLEMENTARY MATERIAL

The online version of this article (doi:10.1007/s11302-010-9184-9) contains supplementary material, which is available to authorized users.

The postsynaptic function of type II cochlear afferents

The mammalian cochlea is innervated by two classes of sensory neurons. Type I neurons make up 90-95% of the cochlear nerve and contact single inner hair cells (IHCs) to provide acoustic analysis as we know it. In contrast, the far less numerous Type II neurons arborize extensively among outer hair cells (OHCs) 1,2 and supporting cells3,4. Their scarcity, and smaller caliber axons, have made them the subject of much speculation, but little experimental progress for the past 50 years. Here we record from Type II fibers near their terminal arbors under OHCs to show that these receive excitatory glutamatergic synaptic input. The Type II peripheral arbor conducts action potentials, but the small and infrequent glutamatergic excitation implies a requirement for strong acoustic stimulation. Further, we show that Type II neurons are excited by adenosine tri-phosphate (ATP). Exogenous ATP depolarized Type II neurons both directly, and by evoking glutamatergic synaptic input 5. The present results prove that Type II neurons function as cochlear afferents, and can be modulated by ATP. The lesser magnitude of synaptic drive dictates a fundamentally different role in auditory signaling from that of Type I afferents.

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

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

P2X antagonists inhibit styryl dye entry into hair cells

The styryl pyridinium dyes, FM1-43 and AM1-43, are fluorescent molecules that can permeate the mechanotransduction channels of hair cells, the sensory receptors of the inner ear. When these dyes are applied to hair cells, they enter the cytoplasm rapidly, resulting in a readily detectable intracellular fluorescence that is often used as a molecular indication of mechanotransduction channel activity. However, such dyes can also permeate the ATP receptor, P2X(2). Therefore, we explored the contribution of P2X receptors to the loading of hair cells with AM1-43. The chick inner ear was found to express P2X receptors and to release ATP, similar to the inner ear of mammals, allowing for the endogenous stimulation of P2X receptors. The involvement of these receptors was evaluated pharmacologically, by exposing the sensory epithelium of the chick inner ear to 5 microM AM1-43 under different experimental conditions and measuring the fluorescence in hair cells after fixation of the tissue. Pre-exposure of the tissue to 5 mM EGTA for 15 min, which should eliminate most of the gating "tip links" of the mechanotransduction channels, deceased fluorescence by only 44%. In contrast, P2X receptor antagonists (pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid [PPADS], suramin, 2',3'-O-(2,4,6-trinitrophenyl) ATP [TNP-ATP], and d-tubocurarine) had greater effects on dye loading. PPADS, suramin, and TNP-ATP all decreased intracellular AM1-43 fluorescence in hair cells by at least 69% when applied at a concentration of 100 microM. The difference between d-tubocurarine-treated and control fluorescence was statistically insignificant when d-tubocurarine was applied at a concentration that blocks the mechanotransduction channel (200 microM). At a concentration that also blocks P2X(2) receptors (2 mM), d-tubocurarine decreased dye loading by 72%. From these experiments, it appears that AM1-43 can enter hair cells through endogenously activated P2X receptors. Thus, the contribution of P2X receptors to dye entry should be considered when using styryl pyridinium dyes to detect hair cell mechanotransduction channel activity, especially in the absence of explicit mechanical stimulation of stereocilia.

ATP activates P2x receptors and requires extracellular Ca(++) participation to modify outer hair cell nonlinear capacitance

Intracochlear ATP is an important mediator in regulating hearing function. ATP can activate ionotropic purinergic (P2x) and metabotropic purinergic (P2y) receptors to influence cell functions. In this paper, we report that ATP can activate P2x receptors directly to modify outer hair cell (OHC) electromotility, which is an active cochlear amplifier determining hearing sensitivity and frequency selectivity in mammals. We found that ATP, but not UTP, a P2y receptor agonist, reduced the OHC electromotility-associated nonlinear capacitance (NLC) and shifted its voltage dependence to the right (depolarizing) direction. Blockage of the activation of P2x receptors by pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), suramin, and 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS) could block the ATP effect. This modification also required extracellular Ca(++) participation. Removal of extracellular Ca(++) abolished the ATP effect. However, chelation of intracellular Ca(++) concentration by a fast calcium-chelating reagent 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA, 10 mM) did not affect the effect of ATP on NLC. The effect is also independent of K(+) ions. Substitution of Cs(+) for intracellular or extracellular K(+) did not affect the ATP effect. Our findings indicate that ATP activates P2x receptors instead of P2y receptors to modify OHC electromotility. Extracellular Ca(++) is required for this modification.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

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.

Photometric recording of transmembrane potential in outer hair cells

Cochlear outer hair cells (OHCs) are polarized epithelial cells that have mechanoelectrical transduction channels within their apical stereocilia and produce electromotile force along their lateral wall. Phase shifts, or time delays, in the transmembrane voltage occurring at different axial locations along the cell may contribute to our understanding of how these cells operate at auditory frequencies. We developed a method to optically measure the phase of the OHC transmembrane potential using the voltage-sensitive dye (VSD) di-8-ANEPPS. The exit aperture of a fibre-optic light source was driven in two dimensions so that a 24 microm spot of excitation light could be positioned along the length of the OHC. We used the whole-cell patch-clamp technique in the current-clamp mode to stimulate the OHC at the base. The photometric response and the voltage response were monitored with a photodetector and patch-clamp amplifier, respectively. The photometric response was used to measure the regional changes in the membrane potential in response to maintained (dc) and sinusoidal (ac) current stimuli applied at the base of the cell. We used a neutral density filter to lower the excitation light intensity and reduce phototoxicity. A sensitive detector and lock-in amplifier were used to measure the small ac VSD signal. This permitted measurements of the ac photometric response below the noise floor of the static fluorescence. The amplitude and phase components of the photometric response were recorded for stimuli up to 800 Hz. VSD data at 400-800 Hz show the presence of a small phase delay between the stimulus voltage at the base of the cell and the local membrane potential measured along the lateral wall. Results are consistent with the hypothesis that OHCs exhibit inhomogeneous membrane potentials that vary with position in analogy with the voltage in nerve axons.

The significance of the calcium signal in the outer hair cells and its possible role in tinnitus of cochlear origin

Finely tuned changes in intracellular Ca(2+) concentration modulate a variety of cellular functions in eukaryotic cells. The cytosolic Ca(2+) concentration is also tightly controlled in the outer hair cells (OHCs), the highly specialized receptor and effector cells in the mammalian auditory epithelium, which are responsible for high sensitivity and sharp frequency discrimination in hearing. OHCs possess a complex system of transporters, pumps, exchangers, channels and binding proteins to develop and to halt the regulatory Ca(2+) signal. The crucial role of elevated intracellular Ca(2+) concentration in OHCs is to increase the efficacy of the electromechanical (electromotile) feedback via remodeling of the cortical cytoskeleton. Anomalies in the Ca(2+) signaling pathway may lead to hypersensitivity of the cochlear amplifier and subsequently trigger tinnitus of cochlear origin. This review describes the dynamics of Ca(2+) signaling in the OHCs and a model that may convey a putative mechanism of development of subjective idiopathic cochlear tinnitus.

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.

Gene-Based Deafness Research: Ion Transport and Hearing

The cochlea is a sensory organ that converts physical (sound) stimulation into electrical signals. This process is fundamentally and substantially based upon the ion transport system. Here, I summarize the physiological and molecular biological aspects of transporters, channels and receptors expressed in the cochlea. With reference to these findings, recent advances in genetic research on hereditary deafness are discussed.

Purinergic modulation of cochlear partition resistance and its effect on the endocochlear potential in the Guinea pig

Introduction of adenosine 5'-triphosphate (ATP) into the endolymphatic compartment of the guinea-pig cochlea decreases the endocochlear potential (EP). To determine if this is due to an ATP-induced change in compartment resistance, the cochlear partition resistance (CoPR) was measured using constant current injections into scala media before, during, and after microinjection of ATP into the same compartment. The CoPR (mean = 3.13 +/- 0.13 kOmega) decreased with ATP in a dose-dependent manner (25.1 +/- 3.0% decrease in relation to baseline values) and this was linearly correlated ( R(2) = 0.91) to the magnitude of the ATP-induced decline in EP (41.6 +/- 7.0% decline in relation to the baseline). Pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS, a P2X receptor antagonist) injected prior to ATP application blocked this ATP-induced reduction in EP and CoPR. This indicates that ATP-gated ion channels (P2X receptors) provide a latent shunt capable of regulating the majority of the electrical potential across the luminal surface of the sensory hair cells, which is necessary for sound transduction. The results suggest a novel sound transduction regulatory mechanism, which, via extracellular ATP, has the capability of adjusting hearing sensitivity.

Molecular physiology of P2X receptors

P2X receptors are membrane ion channels that open in response to the binding of extracellular ATP. Seven genes in vertebrates encode P2X receptor subunits, which are 40-50% identical in amino acid sequence. Each subunit has two transmembrane domains, separated by an extracellular domain (approximately 280 amino acids). Channels form as multimers of several subunits. Homomeric P2X1, P2X2, P2X3, P2X4, P2X5, and P2X7 channels and heteromeric P2X2/3 and P2X1/5 channels have been most fully characterized following heterologous expression. Some agonists (e.g., alphabeta-methylene ATP) and antagonists [e.g., 2',3'-O-(2,4,6-trinitrophenyl)-ATP] are strongly selective for receptors containing P2X1 and P2X3 subunits. All P2X receptors are permeable to small monovalent cations; some have significant calcium or anion permeability. In many cells, activation of homomeric P2X7 receptors induces a permeability increase to larger organic cations including some fluorescent dyes and also signals to the cytoskeleton; these changes probably involve additional interacting proteins. P2X receptors are abundantly distributed, and functional responses are seen in neurons, glia, epithelia, endothelia, bone, muscle, and hemopoietic tissues. The molecular composition of native receptors is becoming understood, and some cells express more than one type of P2X receptor. On smooth muscles, P2X receptors respond to ATP released from sympathetic motor nerves (e.g., in ejaculation). On sensory nerves, they are involved in the initiation of afferent signals in several viscera (e.g., bladder, intestine) and play a key role in sensing tissue-damaging and inflammatory stimuli. Paracrine roles for ATP signaling through P2X receptors are likely in neurohypophysis, ducted glands, airway epithelia, kidney, bone, and hemopoietic tissues. In the last case, P2X7 receptor activation stimulates cytokine release by engaging intracellular signaling pathways.

Voltage-dependent channels in dissociated outer hair cells of the guinea pig

Voltage-dependent channels in outer hair cells (OHCs) dissociated from the guinea pig cochlea were investigated by the use of a whole-cell patch-clamp technique. Two types of K+ current were recorded from OHCs. One was a slowly inactivating K+ current that was activated at a potential more positive than -30 mV. Another is a K+ current that was already activated at resting membrane potential. After suppressing both K+ currents, depolarizing voltage steps elicited a slowly inactivating inward current that was dependent on external Ca2+ and was indicative of an L-type Ca2+ channel in OHCs. Aminoglycoside antibiotics known to be ototoxic selectively inhibited the Ca2+ current.

Ligand-gated purinergic receptors are differentially expressed in the adult rat vestibular periphery

To further characterize the pattern of expression of the ligand-gated purinergic P2X receptors in the peripheral vestibular system, we conducted reverse transcription-polymerase chain reaction amplification of P2X1 and P2X2 messenger RNA extracted from adult rat vestibular ganglia (Scarpa's ganglia) and vestibular end organs. Transcripts encoding P2X1 were found in both Scarpa's ganglia and the end organs, but transcripts encoding P2X2 were found only in the vestibular end organs. These results support previous electrophysiological data, and they provide a more complete understanding of the specific role of purinergic (adenosine-5'-triphosphate) transmission in the vestibular periphery.

Physiological effects of extracellular nucleotides in the inner ear

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

Functional expression of three P2X(2) receptor splice variants from guinea pig cochlea

ATP has been suggested to act as a neurotransmitter or a neuromodulator in the cochlea. The responses to ATP in different cell types of the cochlea vary in terms of the rate of desensitization and magnitude, suggesting that there may be different subtypes of P2X receptors distributed in the cochlea. Recently three ionotropic P2X(2) receptor splice variants, P2X(2-1), P2X(2-2), and P2X(2-3,) were isolated and sequenced from a guinea pig cochlear cDNA library. To test the hypothesis that these different splice variants could be expressed as functional homomeric receptors, the three P2X(2) receptor variants were individually and transiently expressed in human embryonic kidney cells (HEK293). The biophysical and pharmacological properties of these receptors were characterized using the whole cell patch-clamp technique. Extracellular application of ATP induced an inward current in HEK293 cells containing each of the three splice variants in a dose-dependent manner indicating the expression of homomeric receptors. Current-voltage (I-V) relationships for the ATP-gated current show that the three subtypes of the P2X(2) receptor had a similar reversal potential and an inward rectification index (I(50 mV)/I(-50 mV)). However, the ATP-induced currents in cells expressing P2X(2-1) and P2X(2-2) variants were large and desensitized rapidly whereas the current in those cells expressing the P2X(2-3) variant was much smaller and desensitized slower. The order of potency to ATP agonists was 2-MeSATP > ATP > alpha,beta -MeATP for all three expressed splice variants. The ATP receptor antagonists suramin and PPADS reduced the effects of ATP on all three variants. Results demonstrate that three P2X(2) splice variants from guinea pig cochlea, P2X(2-1), P2X(2-2), and P2X(2-3), can individually form nonselective cation receptor channels when these subunits are expressed in HEK293 cells. The distinct properties of these P2X(2) receptor splice variants may contribute to the differences in the response to ATP observed in native cochlear cells.

An interaction between PPADS, an ATP antagonist, and a moderately intense sound in the cochlea

In the organ of Corti ionotropic receptors for ATP (ATPRs) on cells that are bathed by perilymph have been suggested to modulate cochlear mechanics. The purpose of the present study was to test the hypothesis that endogenous extracellular ATP acting through ATPRs is involved in modulating cochlear mechanics during moderately intense sound exposure. Guinea pigs were exposed to either: (1) a perilymphatic administration of pyridoxal-phosphate-6-azophenyl-2', 4'-disulfonic acid (PPADS, 1 mM), an ATP antagonist; (2) a moderately intense sound (6.7 kHz tone, 95 dB SPL, 15 min); or (3) a combination of both the PPADS and the sound. The effects on cochlear potentials (cochlear microphonic, CM; negative summating potential, SP; compound action potential of the auditory nerve, CAP; and N(1) latency) evoked by a 10 kHz tone pip were monitored. PPADS alone reduced the CAP and the SP and increased N(1) latency. The intense sound alone reduced the CAP and SP. The combination of PPADS with the intense tone induced reversible effects on cochlear potentials that were greater than induced by either treatment alone. The effect on N(1) latency and low intensity CM was a potentiation since the effect was greater than a simple addition of the effect of either treatment alone. The effects of the combination treatment on CAP, SP and high intensity CM were not different from additive. Results are consistent with the hypothesis that ATPRs in the organ of Corti are involved in modulating cochlear mechanics during moderately intense sound exposure.

ATP enhances repair of hair bundles in sea anemones

Hair bundle mechanoreceptors of sea anemones are similar to those of the acousticolateralis system of vertebrates (Watson, Mire and Hudson, 1997, Hear. Res. 107, 53-63). Anemone hair bundles are repaired by 'repair proteins' secreted following a complete loss of structural integrity and loss of function caused by 1 h exposure to calcium free seawater. Exogenously supplied repair proteins (RP) restore structural integrity to hair bundles and restore vibration sensitivity in 7-8 min (Watson, Mire and Hudson, 1998, Hear. Res. 115, 119-128). We here report that exogenously supplied ATP enhances the rate by which RP restore vibration sensitivity. A bimodal dose response to ATP indicates maximal enhancement at picomolar and micromolar concentrations of ATP. At these concentrations of ATP, vibration sensitivity is restored in 2 min. These data suggest that at least two ATPases exhibiting different binding affinities for ATP are involved in the repair process. Whereas the higher affinity site is specific for ATP, the lower affinity site does not discriminate between ATP and ADP. Nucleotidase cytochemistry localizes ATPase activity in isolated repair proteins. In the absence of exogenously added RP, sea anemones secrete and consume ATP during the 4 h recovery period after 1 h exposure to calcium free seawater. In the presence of exogenously added RP, ATP is secreted and then consumed within 10 min. Quinacrine cytochemistry localizes possible stores of ATP in the apical cytoplasm of sensory neurons located at the center of the hair bundle. According to our model, ATP is secreted by the sensory neuron after its hair bundle loses structural integrity. Hydrolysis of ATP by repair proteins is essential to the repair process.

ATP-Induced Ca(2+) release in cochlear outer hair cells: localization of an inositol triphosphate-gated Ca(2+) store to the base of the sensory hair bundle

We used a high-performance fluorescence imaging system to visualize rapid changes in intracellular free Ca(2+) concentration ([Ca(2+)](i)) evoked by focal applications of extracellular ATP to the hair bundle of outer hair cells (OHCs): the sensory-motor receptors of the cochlea. Simultaneous recordings of the whole-cell current and Calcium Green-1 fluorescence showed a two-component increase in [Ca(2+)](i). After an initial entry of Ca(2+) through the apical membrane, a second and larger, inositol triphosphate (InsP(3))-gated, [Ca(2+)](i) surge occurred at the base of the hair bundle. Electron microscopy of this intracellular Ca(2+) release site showed that it coincides with the localization of a unique system of endoplasmic reticulum (ER) membranes and mitochondria known as Hensen's body. Using confocal immunofluorescence microscopy, we showed that InsP(3) receptors share this location. Consistent with a Ca(2+)-mobilizing second messenger system linked to ATP-P2 receptors, we also determined that an isoform of G-proteins is present in the stereocilia. Voltage-driven cell shape changes and nonlinear capacitance were monitored before and after ATP application, showing that the ATP-evoked [Ca(2+)](i) rise did not interfere with the OHC electromotility mechanism. This second messenger signaling mechanism bypasses the Ca(2+)-clearance power of the stereocilia and transiently elevates [Ca(2+)](i) at the base of the hair bundle, where it can potentially modulate the action of unconventional myosin isozymes involved in maintaining the hair bundle integrity and potentially influence mechanotransduction.

Single channel properties of P2X2 purinoceptors

The single channel properties of cloned P2X2 purinoceptors expressed in human embryonic kidney (HEK) 293 cells and Xenopus oocytes were studied in outside-out patches. The mean single channel current-voltage relationship exhibited inward rectification in symmetric solutions with a chord conductance of approximately 30 pS at -100 mV in 145 mM NaCl. The channel open state exhibited fast flickering with significant power beyond 10 kHz. Conformational changes, not ionic blockade, appeared responsible for the flickering. The equilibrium constant of Na+ binding in the pore was approximately 150 mM at 0 mV and voltage dependent. The binding site appeared to be approximately 0.2 of the electrical distance from the extracellular surface. The mean channel current and the excess noise had the selectivity: K+ > Rb+ > Cs+ > Na+ > Li+. ATP increased the probability of being open (Po) to a maximum of 0.6 with an EC50 of 11.2 microM and a Hill coefficient of 2.3. Lowering extracellular pH enhanced the apparent affinity of the channel for ATP with a pKa of approximately 7.9, but did not cause a proton block of the open channel. High pH slowed the rise time to steps of ATP without affecting the fall time. The mean single channel amplitude was independent of pH, but the excess noise increased with decreasing pH. Kinetic analysis showed that ATP shortened the mean closed time but did not affect the mean open time. Maximum likelihood kinetic fitting of idealized single channel currents at different ATP concentrations produced a model with four sequential closed states (three binding steps) branching to two open states that converged on a final closed state. The ATP association rates increased with the sequential binding of ATP showing that the binding sites are not independent, but positively cooperative. Partially liganded channels do not appear to open. The predicted Po vs. ATP concentration closely matches the single channel current dose-response curve.

Functional and molecular diversity of purinergic ion channel receptors

P2X receptors are membrane ion channels gated by extracellular adenosine 5'-triphosphate (ATP); nucleotides also activate a family of seven transmembrane G protein-coupled receptors (P2Y). P2X receptors are widely expressed on mammalian cells, where they can be broadly differentiated into three groups. The first group is almost equally well activated by ATP and its analog alpha beta methyleneATP (alpha beta meATP), whereas a second group is not activated by alpha beta meATP. A third-group type of receptor (termed P2Z) is distinguished by the fact that the channel opening is followed by cell permeabilization and lysis if the agonist application is continued for more than a few seconds. Seven cDNAs have been cloned that encode P2X receptor subunits. When expressed individually in heterologous systems, P2X1 and P2X3 subunits form channels activated by ATP or alpha beta meATP; whereas P2X2, P2X4, and P2X5 form channels activated by ATP but not alpha beta meATP. P2X6 receptors do not express readily, and P2X7 receptors correspond closely in their properties to P2Z. Further phenotypes can be produced when two subunits are coexpressed, indicating hetero-multimerization. This chapter compares the properties of the native P2X receptors with those of the cloned and expressed subunits.

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.

Evidence for three additional P2X2 purinoceptor isoforms produced by alternative splicing in the adult rat vestibular end-organs

P2X2 receptors are ligand-gated ion channels that are activated by extracellular ATP. To characterize the expression of P2X2 purinoceptor in the adult rat vestibular periphery, reverse transcription-polymerase chain reaction (RT-PCR) was used. No transcript for P2X2 receptor was found in the vestibular primary afferent neurons (Scarpa's ganglia); however, partial cDNAs encoding four splice variants of the P2X2 receptor were isolated from vestibular end-organs. In all four cDNAs, the deletions were of different lengths but started at the same position on the P2X2 gene (Val-370 codon) located toward the intracellular carboxyl terminus. One of these receptor isoforms was identical in sequence to the recently published P2X2(b) receptor (Simon et al., 1997, Mol. Pharmacol. 52, 237-248) (also known as P2X2-2, in the nomenclature of Brändle et al., 1997, FEBS Lett. 404, 294-298). The remaining three novel splice variants of the P2X2 receptor were designated P2X2(e), P2X2(f) and P2X2(g) (GenBank accession numbers AF028603, AF028604 and AF028605, respectively). The functional significance of these three splice variants remains to be determined. Pituitary and cerebellum were used as survey tissues and only the P2X2(b) receptor cDNA was found.

Evidence for opening of hair-cell transducer channels after tip-link loss

The mechanosensitive transducer channels of hair cells have long been proposed to be gated directly by tension in the tip links. These are thin, elastic extracellular elements connecting the tips of adjacent stereocilia located on the apical surface of the cell. If this hypothesis is true, the channels should close after destruction of tip links. The hypothesis was tested pharmacologically using receptor currents obtained in response to mechanical stimulation of the stereociliary bundle of outer hair cells isolated from the adult guinea pig cochlea. Application of elastase (20 U/ml) or 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetra-acetic acid (BAPTA; 5 mM), both of which are known to disrupt tip links in other hair-cell preparations, led to the expected irreversible loss of receptor currents. However, the cells then displayed a maintained inward current, implying that channels were left permanently open. This current was similar in magnitude to the receptor current before treatment and was reduced reversibly by known blockers of mechanosensitive channels, namely, dihydrostreptomycin (100 microM), amiloride (300 microM), and gadolinium ions (1 mM). These observations suggest that the maintained current flows through the mechanosensitive channels. Electron microscopical analysis of isolated hair cells, exposed to the same concentrations of elastase or BAPTA as in the electrophysiological experiments, demonstrated an almost total loss of tip links in hair bundles that showed no evidence of other mechanical damage. It is concluded that although the tip links are required for mechanoelectrical transduction, the channels are not gated directly by the tip links.

Outward rectifying potassium currents are the dominant voltage activated currents present in Deiters' cells

Supporting cells in the cochlea are thought to maintain the homeostasis of the organ of Corti and contribute to the electrical and micromechanical environment of the hair cells. Of the different types of supporting cells, Deiters' cells form a structure that holds the outer hair cells (OHCs) at their base and apex. This structure may play an important role in modifying cochlear mechanics by influencing the force produced by sound induced motion of the OHCs which in turn may be modulated by ATP acting on ligand gated cation channels on the Deiters' cells. Also, a glia-like role of buffering external K+ concentration for the Deiters' cells has been suggested. We studied Deiters' cells' electrical properties and ion conductances using the whole cell variant of the patch clamp technique since they must play an important role in the function of these cells. It was found that isolated Deiters' cells possess a large voltage activated, outwardly rectifying K+ selective conductance. Voltage activated Ca2+ currents and non-selective currents were not detected and voltage activated inward currents were very small. The outward K+ currents were found to be dependent on voltage but not on Ca2+ for their activation. Nimodipine and 4-aminopyridine (4-AP) were shown to interact directly with the K+ channels in a voltage dependent manner. It is suggested that the K+ selective channels in Deiters' cells may be similar to the Kv1.5 type channel. However, based on the voltage dependence of the channels that was described by double Boltzmann equation and on the alteration of that dependence by 4-AP, it is possible that more than one type of K+ selective channel exists.

ATP in endolymph enhances electrically-evoked oto-acoustic emissions from the guinea pig cochlea

ATP was iontophoresed into the scala media of the guinea pig cochlea. A reversible increase in the amplitude of electrically-evoked oto-acoustic emissions (EEOAEs), and reductions in the endocochlear potential (EP) and the cochlear microphonic (CM) were observed. These effects were consistent with an action of ATP on P2X receptors on outer hair cells (OHC). The results confirm that endogenous endolymphatic ATP, acting via P2X receptors on OHCs, may serve a regulatory function in the cochlea.

Pharmacological characterization of ATP receptors in ampulla from frog semicircular canal

Phosphoinositidase C activities sensitive to purine and pyrimidine nucleotides have been identified earlier in ampulla from Rana ridibunda semicircular canal. The aim of this study was to characterize the pharmacological properties of other P2 receptors borne by this structure. A microassay was developed to measure the binding of [35S]adenosine 5'-O-(2-thiodiphosphate) ([35S]ADPbetaS) to a few ampullas microdissected from frog semicircular canals. When determined at 4 degrees C in the absence of divalent cations, [35S]ADPbetaS binding was saturable with incubation time and reversible after elimination of free radioligand. The dissociation kinetics were biphasic and comprised a major component that was rapidly reversible and a minor component that dissociated slowly. [35S]ADPbetaS binding was competitively inhibited by unlabeled ADPbetaS with an apparent dissociation constant of 0.48 +/- 0.09 microM and a Hill coefficient of 0.70 +/- 0.06, and Scatchard analysis revealed a minor class of high-affinity binding sites (RT1 = 52 +/- 11 fmol [35S]ADPbetaS bound/ampulla and Kd1 = 0.15 +/- 0.04 microM) and a major class of low-affinity binding sites (RT2 = 436 +/- 79 fmol [35S]ADPbetaS bound/ampulla and Kd2 = 2.0 +/- 0.8 microM). The pattern of stereospecificity for recognition of unlabeled structural ATP analogs was ADPbetaS >/= alpha, beta-methyleneadenosine 5'-triphosphate = ADP = adenosine 5'-O-(3-thiotriphosphate) > ATP = diadenosine tetraphosphate = AMP > 2'- and 3'-O-(4-benzoylbenzoyl)-adenosine 5'-triphosphate >/= 2-methylthioadenosine 5'-triphosphate > 2-desoxythymidine 5'-triphosphate = guanosine 5'-triphosphate = inosine-5'-triphosphate = xanthosine 5'-triphosphate = cytosine 5'-triphosphate = uridine 5'-triphosphate = uridine-5'-diphosphate, whereas cAMP and adenosine were devoid of activity. For antagonists, suramin revealed competitive inhibitor potencies, whereas reactive blue 2 and DIDS acted as pure noncompetitive inhibitors. Results suggest that the population of labeled receptors is heterogeneous and contains a low number of P2Y-like receptors and a large number of P2X-like receptors whose molecular subtypes and functions in endolymph homeostasis remain to be defined.

Novel variant of the P2X2 ATP receptor from the guinea pig organ of Corti

ATP functions as a neurotransmitter and a neuromodulator in various tissues by acting on metabotropic (P2Y) and ionotropic (P2X) receptors. Evidence suggests that ATP activates P2X receptors on several cell types in the organ of Corti of guinea pig including outer hair cells (OHCs), Deiters' cells, Hensen's cells, pillar cells and inner hair cells (IHCs). Determining the sequence and structure of P2X receptors in guinea pig organ of Corti is important for understanding the function of ATP in the cochlea. We screened a guinea pig organ of Corti cDNA library for P2X2 ATP receptors using rat P2X2 cDNA as a probe. We sequenced three P2X2 variants which were found to be abundant in this library. One is a novel P2X2 isoform (P2X2-3) created by a retained intron coding for an additional 27 amino acids (81 bp) in the putative extracellular domain. We have also sequenced a variant (P2X2-2) that lacks both the 81-bp sequence and a 192-bp sequence in the 3' intracellular domain. A third variant (P2X2-1) contains the intracellular 192-bp sequence but not the extracellular 81-bp sequence found in P2X2-3. The multiple transcripts arise from alternative intron and exon splicing events. In situ hybridization with a probe common to the three variants localized P2X2 to many of the cells of the organ of Corti.

Fluorescence imaging of Na+ influx via P2X receptors in cochlear hair cells

The adenosine 5'-triphosphate (ATP)-activated membrane conductance, mediated by P2X receptors, was examined in isolated guinea-pig cochlear inner and outer hair cells. Photo-activated release of caged-ATP elicted a 30-ms latency inwardly rectifying non-selective cation conductance, blocked by the P2X receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS; 10-100 microM), consistent with the direct activation of ATP-gated ion channels. A K(Ca) conductance in the inner hair cells (IHC), activated by the entry of Ca2+ through the ATP-gated ion channels, was blocked by including 10 mM 1,2-his(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) in the internal solution. Real-time confocal slit-scanning fluorescence imaging of Na+ influx through the ATP-gated ion channels was performed using the dye Sodium Green with simultaneous whole-cell recording of membrane currents. The Na+ entry was localized to the endolymphatic surface, with the increase in [Na+]i detected within approximately 200 ms of the onset of the inward current response. Within 600 ms Na+ had diffused throughout the cell cytoplasm with the exception of the subnuclear region of the outer hair cells. Correlation of voltage-clamp measurements of Na+ entry with regional increases in Na+-induced fluorescence demonstrated ATP-induced increases in intracellular Na+ in excess of 45 mM within 4 s. These data provide direct evidence for the Na+ permeability of the ATP-gated ion channels as well as independent evidence for the localization of P2X receptors at the endolymphatic surface of the sensory hair cells. The localization of the ATP-gated ion channels to the apical surface of the hair cells supports an ATP-mediated modulation of 'silent' K+ current across the cochlear partition which could regulate hearing sensitivity by controlling the transcellular driving force for both mechanoelectrical and electromechanical transduction in hair cells.

P2X receptors in cochlear Deiters' cells

1. The ionotropic purinoceptors in isolated Deiters' cells of guinea-pig cochlea were characterized by use of the whole-cell variant of the patch-clamp technique. 2. Extracellular application of adenosine 5'-triphosphate (ATP) induced a dose-dependent inward current when the cells were voltage-clamped at -80 mV. The ATP-induced current showed desensitization and had a reversal potential around -4 mV. 3. Increasing intracellular free Ca2+ by decreasing the concentration of EGTA in the pipette solution reduced the amplitude of the ATP-gated current. 4. The order of agonist potency was: 2-methylthioATP (2-meSATP)>ATP>benzoylbenzoyl-ATP (BzATP)>alpha,beta-methyleneATP (alpha,beta,meATP>adenosine 5'-diphosphate (ADP)>uridine 5'-triphosphate (UTP)>adenosine 5'-monophosphate (AMP)=adenosine (Ad). 5. Pretreatment with forskolin (10 microM), 8-bromoadenosine-3',5'-cyclophosphate (8-Br-cyclic AMP, 1 mM), 3-isobutyl-1-methylxanthine (IBMX, 1 mM) or phorbol-12-myristate-13-acetate (PMA, 1 microM) reversibly reduced the ATP-induced peak current. 6. The results are consistent with molecular biological data which indicate that P2X2 purinoceptors are present in Deiters' cells. In addition, the reduction of the ATP-gated current by activators of protein kinase A and protein kinase C indicates that these P2X2 purinoceptors can be functionally modulated by receptor phosphorylation.

How well do we understand the cochlea?

As sensory cells, hair cells within the mammalian inner ear convert sounds into receptor potentials when their projecting stereocilia are deflected. The organ of Corti of the cochlea contains two types of hair cell, inner and outer hair cells, which differ in function. It has been appreciated for over two decades that although inner hair cells act as the primary receptor cell for the auditory system, the outer hair cells can also act as motor cells. Outer hair cells respond to variation in potential, and change length at rates unequalled by other motile cells. The forces generated by outer hair cells are capable of altering the delicate mechanics of the cochlear partition, increasing hearing sensitivity and frequency selectivity. The discovery of such hair-cell motility has modified the view of the cochlea as a simple frequency analyser into one where it is an active non-linear filter that allows only the prominent features of acoustic signals to be transmitted to the acoustic nerve by the inner hair cells. In this view, such frequency selectivity arises through the suppression of adjacent frequencies, a mechanical effect equivalent to lateral inhibition in neural structures. These processes are explained by the interplay between the hydrodynamic interactions among different parts of the cochlear partition and the effective non-linear behaviour of the cell motor.

Additional pharmacological evidence that endogenous ATP modulates cochlear mechanics

In the cochlea, outer hair cells (OHCs) generate the active cochlear mechanics whereas the supporting cells, such as Deiters' cells and Hensen's cells, may play a role in both the active and passive cochlear mechanics. The presence of receptors for adenosine triphosphate (ATP) on OHCs, Deiters' cells and Hensen's cells indicates that endogenous ATP may have a role in cochlear mechanics. To explore this possibility, the effects of the ATP antagonist, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), were studied in guinea pig both in vitro on isolated OHCs, Deiters' cells, Hensen's cells and pillar cells using the whole-cell configuration of the patch-clamp technique, and in vivo on sound evoked cochlear potentials (cochlear microphonic, CM; summating potential, SP; compound action potential, CAP) and distortion product otoacoustic emissions (DPOAEs) using cochlear perilymphatic perfusion. Results show that PPADS (100 microM) reduced the inward current evoked by 5-10 microM ATP in OHCs, Deiters' cells, Hensen's cells and pillar cells. This effect of PPADS was slow in onset and was slowly reversed to a varying degree in the different cell types. In vivo application of PPADS in increasing concentrations reduced the sound evoked CAP, SP and increased N1 latency starting at about 0.33 mM (SP) and 1 mM (CAP and N1 latency). PPADS (0.33-1 mM) reversibly suppressed the initial value of the quadratic DPOAE and reversed the 'slow decline' in the quadratic DPOAE that occurs during continuous stimulation with moderate level primaries. These results, together with the similar effects of the ATP antagonist suramin reported previously (Skellett et al., 1997), may be evidence that endogenous ATP acting on cells in the organ of Corti alters cochlear mechanics.

The pharmacology and kinetics of ecto-nucleotidases in the perilymphatic compartment of the guinea-pig cochlea

This study investigated the characteristics of ecto-nucleotidases in tissues lining the perilymphatic cavity of the cochlea. The perilymphatic space of the isolated guinea-pig cochlea was maintained with oxygenated artificial perilymph (AP) perfused at a rate of 100 microl/min. Following AP perfusion, either adenosine triphosphate (ATP), adenosine diphosphate (ADP) or adenosine monophosphate (AMP) was introduced into scala tympani, and perfusion arrested for 2 min for substrate incubation with cochlear tissues. Effluent collected from the cochlea was assayed for adenine nucleotide metabolites by reverse-phase high-performance liquid chromatography (RP-HPLC). Extracellular ATP and ADP were rapidly and sequentially hydrolysed to adenosine by Ca2+/Mg2+-dependent and Ca2+/Mg2+-independent enzymatic mechanisms. The degradation of extracellular ATP, ADP and AMP occurred in the presence of intact tissues, as demonstrated by the limited lactate dehydrogenase (LDH) activity (0-2.2%). ATPase activity was not affected by inhibitors of intracellular ATPases (oligomycin, ouabain, N-ethylmaleimide, 100 microM NaN3) and non-specific alkaline phosphatase (beta-glycerophosphate). The hydrolysis of ATP was inhibited by 5 mM NaN3, suramin, ATPgammaS, La3+ and CTP, the hydrolysis of ADP by beta,gamma-imidoATP, and AMP degradation by alpha,beta-methyleneADP. Ecto-ATPase, ecto-ADPase and ecto-5'-nucleotidase followed Michaelis-Menten hyperbolic kinetics, with estimated Km values of 2282 microM, 6619 microM and 881 microM, respectively. Our results indicate the presence of considerable ecto-nucleotidase activity within scala tympani of the cochlea, and support its role as the terminating mechanism for P2 receptor signalling known to occur in the cochlea. A competition plot is consistent with ATP and ADP degradation mediated by the same enzyme (ecto-ADP diphosphohydrolase) with two different catalytic sites.

Extracellular nucleotide signaling in the inner ear

Extracellular nucleotides, particularly adenosine 5'-triphosphate (ATP), act as signaling molecules in the inner ear. Roles as neurotransmitters, neuromodulators, and as autocrine or paracrine humoral factors are evident. The diversity of the signaling pathways for nucleotides, which include a variety of ATP-gated ion channels (assembled from different subtypes of P2X-receptor subunit) and also different subtypes of G protein-coupled nucleotide receptors (P2Y receptors) supports a major physiological role for ATP in the regulation of hearing and balance. Almost invariably both P2X and P2Y receptor expression is apparent in the complex tissue structures associated with the inner-ear labyrinth. However P2X-receptor expression, commonly associated with fast neurotransmission, is apparent not only with the cochlear and vestibular primary afferent neurons, but also appears to mediate humoral signaling via ATP-gated ion channel localization to the endolymphatic surface of the cochlear sensory epithelium (organ of Corti). This is the site of the sound-transduction process and recent data, including both electrophysiological, imaging, and immunocytochemistry, has shown that the ATP-gated ion channels are colocalized here with the mechano-electrical transduction channels of the cochlear hair cells. In contrast to this direct action of extracellular ATP on the sound-transduction process, an indirect effect is apparent via P2Y-receptor expression, prevalent on the marginal cells of the stria vascularis, a tissue that generates the standing ionic and electrical gradients across the cochlear partition. The site of generation of these gradients, including the dark-cell epithelium of the vestibular labyrinth, may be under autocrine or paracrine regulation mediated by P2Y receptors sensitive to both purines (ATP) and pyrimidines such as UTP. There is also emerging evidence that the nucleoside adenosine, formed as a breakdown product of ATP by the action of ectonucleotidases and acting via P1 receptors, is also physiologically significant in the inner ear. P1-receptor expression (including A1, A2, and A3 subtypes) appear to have roles associated with stress, acting alongside P2Y receptors to enhance cochlear blood flow and to protect against the action of free radicals and to modulate the activity of membrane conductances. Given the positioning of a diverse range of purinergic-signaling pathways within the inner ear, elevations of nucleotides and nucleosides are clearly positioned to affect hearing and balance. Recent data clearly supports endogenous ATP- and adenosine-mediated changes in sensory transduction via a regulation of the electrochemical gradients in the cochlea, alterations in the active and passive mechanical properties of the cells of the sensory epithelium, effects on primary afferent neurons, and control of the blood supply. The field now awaits conclusive evidence linking a physiologically-induced modulation of extracellular nucleotide and nucleoside levels to altered inner ear function.

P2 purinoceptor of the globular substance in the otoconial membrane of the guinea pig inner ear

The biological characteristics of the globular substance, a precursor of otoconia, are unclear. In the present study, the ATP-induced internal free Ca2+ concentration ([Ca2+]i) changes of the globular substance and the ATP distribution in the vestibular organ were investigated using a Ca2+ indicator, fluo 3, and an adenine nucleotide-specific fluorochrome, quinacrine, by means of confocal laser scanning microscopy. [Ca2+]i showed a rapid and dose-dependent increase in response to ATP with a 50% effective concentration (EC50) of 16.7 microM. This reaction was independent of external Ca2+, indicating the presence of an internal Ca2+ reservoir. Neither adenosine, alpha, beta-methylene-ATP, 3'-O-(4-benzoylbenzoyl)-ATP, ADP, nor UTP evoked this reaction, whereas 2-methylthio-ATP induced an increase of [Ca2+]i with an EC50 of 14.4 microM. Moreover, P2 antagonists, reactive blue 2 and suramin, and a phospholipase C inhibitor, U-73122, inhibited the ATP-induced [Ca2+]i increase. These findings indicate the presence of a P2Y purinoceptor on the globular substance. In addition, granular fluorescence was observed in the quinacrine-stained macular sensory epithelium, indicating the presence of ATP-containing granules in this tissue. These results suggest that a paracrine mechanism involving ATP may exist in the macula and that this mechanism regulates the biological behavior of the globular substance.

Cytotoxicity and mitogenicity of adenosine triphosphate in the cochlea

Evidence is accumulating to indicate that extracellular adenosine 5'-triphosphate (ATP) may function as a neurotransmitter, neuromodulator, cytotoxin and mitogen. Many of the cells in the cochlea have ATP receptors, however, their function is unknown. The purpose of the present study was to test whether ATP may act as a cytotoxin in the cochlea. ATP was applied to acutely isolated outer hair cells (OHCs) and their shape changes monitored. In addition, ATP was applied into the cochlea by perfusion of the perilymph compartment for 2 h and the animals allowed to survive 3-4 weeks post drug application. At this time, sound-evoked cochlear potentials and distortion product otoacoustic emissions (DPOAEs) were monitored and the cochleas evaluated histologically. Results indicate that when applied to isolated OHCs, ATP (3-30 mM) induced a bleb formation in the infracuticular region of the cell that burst within a few minutes. Short OHCs were more sensitive to this effect of ATP than long OHCs. 3-4 weeks after the perilymph perfusion of ATP (60 mM; 2 h) cochlear potentials and DPOAEs were abolished, and histologically, cells in the organ of Corti and the stria vascularis were found to have been destroyed. In addition, there was loss of spiral ganglion cells and proliferating connective tissue filled varying proportions of the scala tympani and vestibuli. Application of sodium gluconate, a control, at the same concentrations had no effect either on the isolated OHCs or when applied in vivo. Results suggest that extracellular ATP or a metabolic product may act as a cytotoxin to some epithelial and neural elements in the cochlea and possibly as a mitogen to mesenchymal cells or fibrocytes.

ATP-induced cochlear blood flow changes involve the nitric oxide pathway

Although control mechanisms of cochlear blood flow (CBF) have been intensively studied since laser Doppler flowmetry was introduced for CBF measurement in animals and humans, the role of adenosine 5'-triphosphate (ATP) in CBF regulation is not known. Since ATP is a potent vasoactive agent in other organs, the aim of this study is to examine ATP-induced changes in CBF and to test whether the nitric oxide pathway is involved in ATP-induced CBF changes. The anterior inferior cerebellar artery (AICA) of anesthetized pigmented guinea pigs was exposed, and ATP was perfused into the AICA. For CBF measurement, the bulla was opened and the 0.7 mm laser probe of a Perimed PF2B flowmeter was positioned on the basal turn of the cochlea. AICA perfusion of an ATP solution caused dose-dependent transient CBF increases. The maximum CBF increase induced was 220% of the baseline. In some animals, CBF showed a dual effect; a transient decrease followed by a longer-lasting increase. The perfusions of sodium nitroprusside (SNP) also resulted in dose-dependent CBF changes. The intravenous application of N(omega)-nitro-L-arginine methyl ester (L-NAME) significantly attenuated ATP-induced CBF increases, and enhanced ATP-induced decreases, but did not affect SNP-induced CBF changes. The ATP-induced CBF responses indicate that ATP plays a role in CBF regulation. The biphasic characteristic of the ATP-induced CBF change suggests the involvement of both P2x- and P2y-subtype purinoceptors. That L-NAME caused attenuation of the ATP-induced CBF increase implies that the ATP-induced CBF increase is mediated by the release of endothelium-derived relaxing factor, nitric oxide, following activation of endothelial P2y-purinoceptors in the cochlear vascular bed and/or cochlear supplying vessels.

Mechanically and ATP-induced currents of mouse outer hair cells are independent and differentially blocked by d-tubocurarine

Mechano-electrical transducer channels (MET) and ATP-gated ion channels (P2X receptors) of hair cells have several properties in common: they share the same location at the apex of the cell, both channels are non-selective for cations and blocked by aminoglycosides and pyrazinecarboxamides (amiloride-related compounds). In this study, we test the relationship and possible identity of these two channel types. Using whole-cell patch-clamp recordings of outer hair cells (OHCs) of the cultured neonatal mouse cochlea and a fluid jet to stimulate their hair bundles mechanically, we show that d-tubocurarine, a blocker of P2X2 receptors, blocks MET channels with a half-blocking concentration of 2.3 microM. In contrast, the KD for the P2X2 receptors was 90 microM and 84 microM measured in hair cells and Xenopus oocytes, respectively. When hair bundles of OHCs were simultaneously stimulated with saturating mechanical stimuli and superfused by 100-300 microM ATP, transducer currents and ATP-activated currents were elicited simultaneously. Their amplitudes were additive, however. We conclude that MET- and ATP-activated currents are mediated by two distinct channel populations in hair cells.

Pharmacological evidence that endogenous ATP modulates cochlear mechanics

In the cochlea, outer hair cells (OHCs) and Deiters' cells most likely contribute to the generation of active cochlear mechanics. The presence of ATP receptors on these cells indicates that endogenous ATP may have a role in cochlear mechanics. To explore this possibility, the effects of ATP antagonists were studied both in vivo on distortion product otoacoustic emissions (DPOAEs) using cochlear perfusion and in vitro on isolated OHCs and Deiters' cells using the whole-cell configuration of the patch-clamp technique. Results show that extracellular application of 5-10 microM ATP to OHCs and Deiters' cells induced an inward current that was reduced by both suramin (100 microM) and cibacron (100 microM). Cibacron reduced the voltage gated currents in Deiters' cells and increased them in OHCs, while suramin had no effect. In addition, cibacron induced a hyperpolarizing shift of the half activation voltage of the whole cell currents in Deiters' cells. Suramin (0.1-1 mM) reversibly suppressed the 'slow decline' in the quadratic DPOAE that occurs during continuous stimulation with moderate level primaries. This effect of suramin may be evidence that endogenous ATP alters active cochlear mechanics.

Differences in the distribution of responses to ATP and acetylcholine between outer hair cells of rat and guinea pig

Adenosine 5' triphosphate (ATP) and acetylcholine (ACh) are neurotransmitters (ACh) and/or modulators (ATP) in the mammalian cochlea. In guinea pig, it appears that both neurotransmitters have a similar response distribution, with larger responses being evoked by the ligands in short hair cells compared to long hair cells (e.g., Chen et al., 1995b. Noise exposure alters the response of outer hair cells to ATP. Hear. Res. 88, 215-221.; Erostegui et al., 1994. In vitro pharmacologic characterization of a cholinergic receptor on outer hair cells. Hear. Res. 74, 135 147). The purpose of the present study was to test whether the distribution of responses to ACh and ATP in the OHCs of rat is the same as guinea pig. The ligand-induced current was monitored using the whole-cell configuration of the patch-clamp technique. Results show that in guinea pig OHCs, extracellular application of 100 microM ATP induced a current response in a majority of the same cells that responded to the application of 100 microM ACh. In contrast in rat OHCs, 100 microM ATP did not induce a current in the majority of cells that responded to the application of 100 microM ACh. N-methyl-glucamine (NMG+) substituted for K+ in the pipette solution failed to unmask an ATP-evoked inward current in rat OHCs. In addition, no response was produced in rat or guinea pig OHCs by adenosine, adenosine 5'-monophosphate (AMP) or adenosine 5'-diphosphate (ADP) at 100 microM. Results suggest that in guinea pig ACh-gated channels are present on most of the same OHCs that have ATP-gated ion channels, whereas in rat ACh-gated ion channels are present without ATP-gated channels on some OHCs.