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

Calcium flux through predominantly independent purinergic ATP and nicotinic acetylcholine receptors

Ligand-gated nicotinic acetylcholine receptors (nAChRs) and purinergic ATP receptors are often expressed in the same peripheral and central neurons, and ATP and acetylcholine (ACh) are stored together in some synaptic vesicles. Evidence has suggested that nAChRs and ATP receptors are not independent and that some agonists strongly cross-activate and desensitize both receptor types. Rat sympathetic neurons and nAChRs expressed in Xenopus oocytes were studied to determine the significance of the interactions caused by the two agonist types. Current amplitudes included with separate or combined applications of ATP and nicotine are > 90% additive and independent. Half of all neurons tested responded to either ATP or nicotine but not to both, indicating differences in the expression of the two receptors. In neurons that expressed both receptors types, the nAChRs were inhibited by the activity-dependent open-channel blocker chlorisondamine. If the purinergic and nicotinic receptors were significantly dependent and coactivated, then blocking the ion channels opened by a nicotinic agonist should diminish the current activated by a purinergic agonist. That result was not seen; rather, complete open-channel block of nAChRS with chlorisondamine did not significantly alter the amplitude or kinetics of ATP-induced currents in the same neurons. Finally, when cloned nAChR subunits were expressed in oocytes, ATP activated only very small currents compared with the current activated by Ach. For the 13 different nAChR subunit combinations that were studied, ATP (50-500 microM) activated a current that ranged from 0 to 4% of the size of the current activated by 100 microM ACh. In summary, we find that there is little cross reactivity, and nAChRs and purinergic ATP receptors are predominantly independent, acting with separable physiological characteristics. Therefore the quantitative Ca2+ flux could be separately determined for nAChRs and ATP receptors. The fraction of total current that is carried by Ca2+ was quantitatively determined by simultaneously measuring the whole cell current and the associated change in intracellular Ca2+ with fura-2. For sympathetic neurons bathed in 2.5 mM Ca2+ at a holding potential of -50 mV, Ca2+ carries 4.8 +/- 0.3% (mean +/- SE) of the inward current through neuronal nAChRs and 6.5 +/- 0.1% of the current through purinergic ATP receptors. In conclusion, activity-dependent Ca2+ influx through predominately independent populations of nAChRS and ATP neurons can produce different intracellular signals at purinergic and cholinergic synapses.

Variation in expression of the outer hair cell P2X receptor conductance along the guinea-pig cochlea

1. Whole-cell patch-clamp recordings were used to determine the variation in the P2X receptor conductance, activated by extracellular ATP, in outer hair cells (OHCs) isolated from each of the four turns of the guinea-pig cochlea. 2. In standard solution (containing 1.5 mM Ca2+) slope conductances were determined in OHCs of known origin from current-voltage relationships obtained from voltage ramps applied between -100 and +50 mV. Membrane conductance throughout this voltage range was greatest in OHCs originating from the basal (high frequency encoding) region of the cochlea. This gradient in OHC conductance from apex to base of the cochlea can be attributed to variation in expression of both a negatively activated K+ conductance and a TEA-sensitive outwardly rectifying K+ conductance. OHC slope conductance measured about a membrane potential of -75 mV increased from a mean of 33.5 nS in the apical region (turn 4) to 96.8 nS in the basal region (turn 1) of the cochlea. 3. Removal of external Ca2+ reduced OHC conductance by an average of 25%, reflecting a Ca2+ dependence of the background conductances in these cells. In zero external Ca2+ the mean slope conductance measured at -75 mV in the apical turn was 25.0 nS compared with 73.8 nS in the basal turn. 4. In Ca(2+)-free solution both 2 mM and 4 microM ATP produced inward currents that were progressively larger in OHCs originating from more basal regions of the cochlea. The steady-state inward current elicited by 2 mM extracellular ATP increased from -1.44 to -3.26 nA for turns 4 and 1, respectively. 5. The P2X receptor conductance was determined between -100 and +50 mV by comparing voltage ramps in the presence and absence of extracellular ATP in Ca(2+)-free solution. The conductance was inwardly rectifying with a reversal potential close to 0 mV. Measured close to the resting membrane potential of the cells (-75 mV), 2 mM ATP elicited an average 300% increase in conductance in parallel with the systematic increase in background conductance which occurs in OHCs originating from the more basal regions of the cochlea. The conductance at -75 mV activated by 2 mM ATP increased from a mean of 59.6 nS in turn 4 OHCs to a mean of 166.2 nS in turn 1 OHCs. The conductance activated by 4 microM ATP was also greater in the basal turn OHCs (45.3 nS) than in the apical region OHCs (5.9 nS). 6. The number of ATP-gated ion channels on individual OHCs, presumed to be localized to the stereocilia, increases from approximately 6000 in turn 4 cells to 16,500 in turn 1 cells, based on estimates of unitary conductance and average maximum ATP-activated OHC conductance (2 mM ATP).

An ATP-gated cation channel with some P2Z-like characteristics in gastric smooth muscle cells of toad

1. Whole-cell and single-channel currents elicited by extracellular ATP were studied in freshly dissociated smooth muscle cells from the stomach of the toad Bufo marinus using standard patch clamp and microfluorimetric techniques. 2. This ATP-gated cation channel shares a number of pharmacological and functional properties with native rat myometrium receptors, certain native P2Z purinoceptors and the recently cloned P2X7 purinoceptor. But, unlike the last two, the ATP-gated channel does not mediate the formation of large non-specific pores. Thus, it may represent a novel member of the P2X or P2Z class. 3. Extracellular application of ATP (> or = 150 microM) elicited an inward whole-cell current at negative holding potentials that was inwardly rectifying and showed no sign of desensitization. Na+, Cs+ and, to a lesser degree, the organic cation choline served as charge carriers, but Cl- did not. Ratiometric fura-2 measurements indicated that the current is carried in part by Ca2+. The EC50 for ATP was 700 microM in solutions with a low divalent cation concentration. 4. ATP (> or = 100 microM) at the extracellular surface of cell-attached or excised patches elicited inwardly rectifying single-channel currents with a 22 pS conductance. Cl- did not serve as a charge carrier but both Na+ and Cs+ did, as did choline to a lesser extent. The mean open time of the channel was quite long, with a range in hundreds of milliseconds at a holding potential of -70 mV. 5. Mg2+ and Ca2+ decreased the magnitude of the ATP-induced whole-cell currents. Mg2+ decreased both the amplitude and the activity of ATP-activated single-channel currents. 6. ADP, UTP, P1, P5-di-adenosine pentaphosphate (AP5A), adenosine and alpha, beta-methylene ATP (alpha, beta-Me-ATP) did not induce significant whole-cell current. ATP-gamma-S and 2-methylthio ATP (2-Me-S-ATP) were significantly less effective than ATP in inducing whole-cell currents, whereas benzoylbenzoyl ATP (BzATP) was more effective. BzATP, alpha, beta-Me-ATP, ATP-gamma-S and 2-Me-S-ATP induced single-channel currents, but a higher concentration of alpha, beta-Me-ATP was required. 7. BzATP did not induce the formation of large non-specific pores, as assayed using mag-fura-2 as a high molecular mass probe.

Ionic permeability of, and divalent cation effects on, two ATP-gated cation channels (P2X receptors) expressed in mammalian cells

1. Complementary DNAs for the ATP-gated ion channel subunits P2X1 (from human bladder) and P2X2 (from rat phaeochromocytoma (PC12) cells) were used to express the receptors in human embryonic kidney cells by stable transfection, and in Chinese hamster ovary cells by viral infection. 2. Membrane currents evoked by ATP were recorded by the whole-cell patch clamp method. The reversal potential of the current was measured with various intracellular and extracellular solutions and used to compute the relative permeability of the P2X receptor channels. 3. There was no difference between the two receptors with respect to their permeability to monovalent organic cations. The relative permeabilities (PX/PNa) were 2.3, 1.0, 1.0, 0.95, 0.72, 0.5, 0.29, 0.16, 0.04 and 0.03 for guanidinium, potassium, sodium, methylamine, caesium, dimethylamine, 2-methylethanolamine, tris(hydroxymethyl)-aminomethane, tetraethylammonium and N-methyl-D-glucamine, respectively (values for P2X2 receptor). 4. The calcium permeability of P2X1 receptors was greater than that of P2X2 receptors. Under biionic conditions (112 mM calcium outside, 154 mM sodium inside), PCa/PNa values were 3.9 and 2.2, respectively (corrected for ionic activities). 5. ATP-evoked currents in cells expressing the P2X2 receptor were strongly inhibited when the extracellular calcium concentration was increased (0.3-30 mM); the action of ATP could be restored by increasing the ATP concentration. ATP-evoked currents in cells expressing the P2X1 receptor were not inhibited by such increases in the extracellular calcium concentration.

ATP activates a cation conductance and Ca(2+)-dependent Cl- conductance in Hensen cells of guinea pig cochlea

Simultaneous whole cell patch-clamp and indo 1 fluorescence measurements were used to characterize ATP-evoked membrane currents and intracellular Ca2+ concentration ([Ca2+]i) changes in isolated Hensen cells of the guinea pig organ of Corti. At negative holding potential, ATP activated a biphasic inward current and a concomitant increase in [Ca2+]i. The initial current activated within < 50 ms, showed a reversal potential near 0 mV and was reversibly inhibited by 30 microM suramin, suggesting this conductance was mediated by ATP-gated nonselective cation channels. The delayed ATP-activated current was mainly carried by Cl- as indicated by its shift in reversal potential when intracellular Cl- was replaced by gluconate. This Cl- conductance appeared to be Ca(2+)-activated secondarily to Ca2+ influx, since it required the presence of extracellular Ca2+ and was suppressed when an intracellular solution containing 10 mM 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid was used. In the absence of extracellular Ca2+, ATP still increased [Ca2+]i concomitant with a monophasic inward cation current, indicating Ca2+ release from intracellular stores. We conclude that Hensen cells have ionotropic and metabotropic P2 purinoceptors. They also have Ca(2+)-activated Cl- channels that can be activated by extracellular ATP, suggesting that purinoceptors in Hensen cells could play a regulatory role in ion and water balance of cochlear fluids.

P2X purinoceptors in cultured myenteric neurons of guinea-pig small intestine

1. Fast excitatory postsynaptic currents (fEPSCs) and responses to exogenously applied purinoceptor agonists were studied in primary cultures of myenteric neurons from guinea-pig small intestine. Whole-cell and outside-out configurations of the patch clamp technique were used. Hexamethonium (100 microM) partly inhibited fEPSCs in 28% of neurons. Hexamethonium-resistant fEPSCs were inhibited by 97 +/- 2% by the P2X receptor antagonist, pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 10 microM). 2. ATP caused two types of inward currents. In 92% of neurons (n = 123), ATP caused a slowly desensitizing current that declined with a double exponential time course (tau 1 = 7.1 +/- 2.0 s; tau 2 = 57 +/- 7.4 s, n = 4). The rank order potency for purinoceptor agonists in these neurons was ATP > 2-methylthio-ATP (2-MeSATP) > > alpha, beta-methylene ATP (alpha, beta-me ATP) > beta, gamma-meATP > ADP. The EC50 values for ATP and 2-MeSATP were 40 and 65 microM, respectively. alpha, beta-MeATP acted as a partial agonist at these receptors. In 8% of neurons (n = 11), ATP-induced currents desensitized rapidly with a double exponential time course (tau 1 = 0.13 +/- 0.015 s; tau 2 = 2.2 +/- 1.3 s, n = 4); alpha, beta-meATP caused similar responses in these cells. Both types of ATP-induced current were associated with an increased conductance and an inwardly rectifying I-V relationship (Erev = 10 mV). Halving [Na+]o shifted the reversal potential of ATP currents by -22 +/- 6 mV. 3. ATP activated single channel currents in outside-out patches. The single channel I-V relationship was linear between -120 and 60 mV (Erev approximately 0 mV). Single channel conductance between -100 and -60 mV was 25 +/- 2 pS. Single channel open probability was voltage dependent and decreased from 0.05 +/- 0.01 at -100 mV to 0.007 +/- 0.002 at +40 mV. 4. These data show that P2X purinoceptors mediate some fEPSCs in cultured myenteric neurons. Myenteric neurons express the fast-desensitizing alpha, beta-me ATP-sensitive subtype of P2X receptor that has the properties of cloned P2X1 receptors and is similar to native receptors in smooth muscle cells. Myenteric neurons also express a P2X receptor that desensitized slowly and was alpha, beta-meATP-insensitive. This receptor has the properties of cloned P2X2 or P2X5 receptors and is similar to native receptors found in PC-12 cells and superior cervical ganglion neurons. The known distribution of P2X2 and P2X5 receptors suggests that myenteric neurons are likely to express predominantly P2X2 receptors.

Ectonucleotidase activity in the perilymphatic compartment of the guinea pig cochlea

It has been clearly demonstrated that extracellular adenosine 5'-triphosphate (ATP) exerts a potent modulatory activity in the cochlea through its interaction with P2 purinoceptors. However, little is known regarding the metabolism of extracellular ATP in cochlear tissues via ectonucleotidases. This study provides evidence for the presence of ectonucleotidases in the perilymphatic compartment of the guinea pig cochlea. Using microperfusion, ATP (500 microM) was introduced into the cochlear perilymph through the basal turn scala tympani, and effluent was collected from the basal turn scala vestibuli. Samples were subsequently analysed for the presence of adenine metabolites using high performance liquid chromatography (HPLC). Cell viability was evaluated by the activity of the intracellular enzyme lactate dehydrogenase (LDH) in the perfusate. ATP was degraded to 122.8 +/- 9.9 microM (25.0 +/- 5.8%) during the passage through the cochlear perilymphatic compartment. Breakdown of ATP resulted in the formation of adenosine 5'-diphosphate (41.5 +/- 9.0 microM), adenosine 5'-monophosphate (201.3 +/- 15.5 microM), adenosine (108.6 +/- 8.3) and inosine (15.0 +/- 1.5 microM). The degradation of ATP was significantly (P < 0.001, Student's t-test) inhibited in the absence of divalent cations, Ca2+ and Mg2+ in the perfusate. In control experiments, no spontaneous degradation of ATP was observed in vitro. LDH activity was similar during ATP perfusions (2.9 +/- 0.9%) to control perfusions with artificial perilymph (4.2 +/- 1.0%) indicating well preserved cell integrity in the cochlear perilymphatic compartment. The degradation of extracellular ATP in the presence of intact tissues and its inhibition in the absence of divalent cations, a cofactor for ectonucleotidases, provides evidence for ectonucleotidase activity in the perilymphatic fluid space of the cochlea.

Chronic low-level noise exposure alters distortion product otoacoustic emissions

Chen et al. (1995) recently reported an altered response to the application of ATP in outer hair cells (OHC) isolated from guinea pigs continuously exposed for 10 or 11 days to a 65 dB SPL (A-scale) narrow-band noise (1.1-2.0 kHz). The primary goal of the present study was to test the hypothesis that the continuous low-level noise used by Chen et al. (1995) alters cochlear function. Cubic (2f1-f2) and quadratic (f2-f1) DPOAEs, as well as, the amount of contralateral suppression of DPOAE amplitudes were chosen for study. Responses were recorded in urethane-anesthetized guinea pigs with sectioned middle ear muscles. The animals had either been exposed to the low-level noise for 3 or 11 days or not exposed at all (n = 13 animals per group). Results demonstrate that this noise induces frequency-dependent and very localized reductions in 2f1-f2 DPOAE input/output (I/O) functions. However, the f2-f1 DPOAE I/O functions appear to be insensitive to the noise exposure. No noise-related changes were found in the amount of contralateral suppression between the different exposure groups, with the exception of one unexplainable data point (f2-f1 DPOAE = 0.5 kHz; day 3) where it was reduced. The 2f1-f2 DPOAE amplitude alterations lend support to the conclusions of Chen et al. (1995) that chronic low-level noise exposure induces molecular changes in the OHCs which may, in turn, alter cochlear function.

Modulation by purines of calcium-activated non-selective cation channels in the outer hair cells of the guinea-pig cochlea

1. The cell-attached and cell-free configurations of the patch-clamp technique were used to investigate whether external ATP and its derivatives modulate channel activity in outer hair cells freshly isolated from the guinea-pig cochlea. 2. Submicromolar concentrations of ATP stimulated a non-selective cation channel with a conductance of about 25 pS. The ATP-elicited stimulation was partly blocked by the membrane-permeant blocker 3',5-dichlorodiphenylamine-2-carboxylic acid (DCDPC), and mimicked by the calcium ionophore, ionomycin, suggesting that the channel activated by ATP is identical to a previously reported calcium-activated non-selective (CAN) cation channel. 3. The P2x agonist beta, gamma-methylene-ATP (beta, gamma-MeATP, 10 microM) and the P2Y agonist 2-methyl-thio-ATP (2-MeSATP, 1 microM) both activated CAN channels. The effect of ATP was inhibited by the P2 antagonist suramin but not by the P2Y antagonist Reactive Blue 2. These results suggest that both purinergic receptors are involved in the ATP-evoked response and that internal calcium acts as a second messenger for opening CAN channels. 4. In contrast, adenosine inhibited CAN channels. This effect was reproduced by the A2 agonist 5'-N-ethylcarboxyamidoadenosine (NECA) and the permeant cAMP analogue 8-bromo-adenosine 3',5'-cyclic monophosphate (8-Br-cAMP), but not by the A1 agonist N6-cyclo-hexyladenosine (CHA). CAN channels were also inhibited when the catalytic subunit of protein kinase A was applied internally on inside-out patches, suggesting that adenosine A2 receptor downregulates CAN channels via a cAMP-dependent phosphorylation.

ATP activates non-selective cation channels and calcium release in inner hair cells of the guinea-pig cochlea

1. ATP-evoked currents and Ca2+ signals were simultaneously recorded in isolated inner hair cells (IHC) of guinea-pig cochlea by combining conventional whole-cell or perforated patch clamp recording with indo-1 dual emission microfluorometry. 2. In most IHCs, voltage clamped near resting membrane potential (-40 mV), extracellular ATP evoked a rapid inward current (time constant, 150 ms). This current was concomitant with a slow rise in [Ca2+]i (time constant, 5 s). The ATP-evoked inward currents could be repeated several times with only a small run-down in amplitude (< 10%), while the ATP-evoked Ca2+ responses showed a rapid run-down (> 80% at the third ATP application). 3. The current-voltage relationship of ATP-evoked currents showed a reversal potential at -11 +/- 6 mV (n = 8), suggesting that ATP essentially activated a non-specific cationic conductance. On the contrary, the amplitude of the ATP-evoked Ca2+ responses did not show significant dependence on holding membrane potential. 4. The Ca2+ response showed an apparent Kd for ATP (EC50, 1.8 +/- 0.3 microM; Hill coefficient, 1.0 +/- 0.1) eightfold smaller than for the evoked currents (EC50, 13.7 +/- 3.0 microM; Hill coefficient, 2.0 +/- 0.7). 5. Perfusion with high extracellular Ca2+ solution (10 mM CaCl2) reduced the amplitude of the ATP-evoked currents by 90%, while perfusion with zero Ca2+ solution increased it by more than 100%. However, similar variations in external Ca2+ concentration did not change the amplitude of the ATP-evoked Ca2+ responses. Furthermore, intracellular heparin (1 mg mL-1), a potent inhibitor of InsP3 receptors, did not significantly change the amplitude of ATP-evoked currents but reduced the ATP-evoked Ca2+ response, suggesting again that the latter is related to Ca2+ release from intracellular stores. 6. The results suggested that two types of P2-purinergic receptor are expressed in IHCs: ATP-gated ion channels and ATP-activated metabotropic receptors. At submicromolar ATP concentrations, the metabotropic receptors raising intracellular [Ca2+] would hyperpolarize IHCs via Ca(2+)-sensitive K+ channels. The ATP-gated ion channels activated at higher ATP concentrations would mainly have a depolarizing effect on IHCs.

Cytoplasmic Ca2+ concentrations in intact Merkel cells of an isolated, functioning rat sinus hair preparation

An isolated, functioning sinus hair preparation was developed to investigate cytoplasmic Ca2+ concentrations in intact Merkel cells using microfluorimetric techniques. Intracellular Ca2+ levels were monitored by means of photon counters in small groups of Merkel cells loaded with the calcium fluorescent indicators fura-2 or fluo-3. Mechanical stimulation of Merkel cells with fine glass rods resulted in small transient increases in intracellular Ca2+ levels (by about 20%) in the group of Merkel cells around the stimulating probe. A rise in Ca2+ is presumed to be essential for the postulated synaptic transmission to the afferent nerve terminal. Depolarization with a high concentration of potassium chloride (100 mM) caused increases in intracellular Ca2+ concentrations in Merkel cells (by about 70%) only in the presence of extracellular Ca2+, indicating an influx of Ca2+ through voltage-gated channels. The Ca2+ response was abolished neither by (+)-BayK8644 nor omega-conotoxin, suggesting that the Ca2+ channels are different from the classical L- or N-type channels. Extracellular application of ATP (10 microM to 5 mM) caused dose-dependent increases in intracellular Ca2+ levels in Merkel cells of up to sevenfold from the basal level of about 100 nM. Similar responses to ATP were also measured during superfusion with Ca(2+)-free medium, suggesting intracellular stores as the main Ca2+ source. Pre-incubation of Merkel cells with the purinoceptor antagonist suramin (100 microM) for 30 min reduced the Ca2+ responses to ATP by about 50% compared with control conditions. In conclusion, the results have demonstrated that a rise in intracellular Ca2+ in Merkel cells can be evoked by mechanical stimulation, membrane depolarization and chemical stimulation by ATP. These observations strongly suggest a possible contribution of Ca2+ to the normal responsiveness of Merkel cell mechanoreceptors, in turn supporting the hypothesis that Merkel cells are involved in the mechano-electric transduction process in sinus hair type I mechanoreceptors.

Chemical synaptic transmission in the cochlea

The last two decades have witnessed major progress in the understanding of cochlear mechanical functioning, and in the emergence of cochlear neurochemistry and neuropharmacology. Recent models describe active processes within the cochlea that amplify and sharpen the mechanical response to sound. Although it is widely accepted that outer hair cells (OHCs) contribute to these processes, the nature of the medial efferent influence on cochlear mechanics needs further clarification. Acetylcholine (ACh) is the major transmitter released onto OHCs during the stimulation of these efferents. The inhibitory influence of this system is mediated by post- and presynaptic nicontinic and muscarinic receptors and the role of other neuroactive substances [gamma-aminobutyric acid (GABA), calcitonin gene-related peptide (CGRP), adenosine 5'-triphosphate (ATP) or nitric oxide (NO)] remains to be determined. The inner hair cells (IHCs) that transduce the mechanical displacements into neural activity, release glutamate on receptor-activated channels of AMPA, kainate, and NMDA types. This synapse is in turn controlled and/or regulated by the lateral efferents containing a cocktail of neuroactive substances (ACh, GABA, dopamine, enkephalins, dynorphin, CGRP). This glutamatergic nature of the IHCs is responsible for the acute destruction of the nerve endings and subsequently for neuronal death, damage usually described in various cochlear diseases (noise-induced hearing losses, neural presbycusis and certain forms of sudden deafness or peripheral tinnitus). These pathologies also include a regrowth of new dendritic processes by surviving neurons up to IHCs. Understanding the subtle molecular mechanisms which underly the control of neuronal excitability, synaptic plasticity and neuronal death in cochlear function and disease is a very important issue for the development of future therapies.

Suppression of a nonselective cation conductance by substance P in cochlear outer hair cells

Substance P (SP) hyperpolarizes outer hair cells (OHCs) of guinea pig cochlea. The cellular mechanisms of the SP response were investigated with the whole cell patch-clamp technique. SP induced outward currents in a dose-dependent manner at a holding potential of -60 mV in a concentration range between 3 x 10(-6) and 10(-4) M. SP decreased slope conductance between -60 and +20 mV. Ion substitutions in the external medium revealed that SP suppresses nonselective cation conductance with high permeability for Ca2+. The relative ion permeability of the channel modulated by SP was as follows: Ca2+ > Li+ approximately Cs+ approximately Na+ > Tris+. The potency of the agonist action was as follows: SP >> neurokinin A > neurokinin B. Peptide antagonists induced currents similar to those of SP. CP-96345, a selective nonpeptide antagonist for the neurokinin type 1 receptor, did not inhibit the SP-induced current. Intracellular dialysis of guanosine 5'-O-(2-thiodiphosphate) and pertussis toxin (PTX) suggests that a PTX-insensitive G protein is involved in the SP response. Neither the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid nor staurosporine (10(-6) M) affects the SP response. Local application of SP by a puffer pipette indicates that the SP receptors are distributed along the side of the OHC. These results suggest the possibility that the action of SP on the OHCs may not be mediated by the tachykinin receptors but rather by a tachykinin receptor-independent pathway. It is proposed that SP suppresses the nonselective cation conductance in the lateral wall of OHCs via a PTX-insensitive G protein.

Extracellular adenosine 5'-triphosphate (ATP) in the endolymphatic compartment influences cochlear function

There is strong evidence for the presence of P2 purinoceptors on cochlear tissues, but the role of extracellular ATP in cochlear function is still unclear. Our previous studies have determined the presence of ATP in the cochlear fluids and indicated that the purinoceptors are substantially localized to the tissues lining the endolymphatic compartment. This implies that extracellular ATP may have an humoral role confined to the endolymphatic space. In order to study the influence of extracellular ATP in the endolymphatic space, a series of studies were undertaken in which ATP (10 microM to 10 mM) in artificial endolymph (EL) (test solution: 2-12.5 nl) was injected into the scala media and the effect on the cochlear microphonic (CM) and endocochlear potential (EP) evaluated. A double-barrelled pipette, with one barrel containing the test solution and the other artificial EL (control solution) was inserted into scala media of the third turn of the guinea-pig cochlea. A known volume (2-12.5 nl) of test or control solution was then pressure-injected into the space. ATP had a significant dose-dependent suppressive effect on both EP and CM with a threshold of approximately 2 x 10(-14) mol; the response was readily reversible, also in a dose-dependent fashion. Artificial EL of the same volume had no effect on EP and CM. The ATP effect on EP was blocked by the P2 purinoceptor antagonists suramin and reactive blue 2 (RB2). Neither adenosine (2 x 10(-13) to 2 x 10(-11) mol) nor suramin or RB2 on their own had any effect on EP and CM. This study provides the first evidence for an effect of extracellular ATP in the endolymphatic compartment on cochlear function which is mediated via P2 purinoceptors. This provides supporting evidence for an humoral role for extracellular ATP in the modulation of cochlear function.

Adenosine 5'-triphosphate (ATP) concentrations in the endolymph and perilymph of the guinea-pig cochlea

The concentration of adenosine 5'-triphosphate (ATP) in endolymph (EL), perilymph (PL) and cerebrospinal fluid (CSF), collected from anesthetized guinea pigs was determined using the luciferase-luciferin reaction. The cochlea was exposed by a ventrolateral approach and the bone overlying scala media of the third turn (EL) or scala vestibuli of the first turn (PL) was shaved to a thin layer and a small fenestrum made. For EL sampling, a double-barrelled pipette was inserted through the spiral ligament-stria vascularis complex. One barrel was filled with 150 mM KCl to record the endocochlear potential (EP) and upon the appearance of the positive EP, 0.12-1.22 microliter of fluid was aspirated into the other barrel by gentle negative pressure. For PL sampling, a single-barrelled pipette was advanced into scala vestibuli and 0.3-1.6 microliter of fluid was collected by capillarity. CSF (0.36-1.75 microliter) was obtained from the cisterna magna. The cochleae were removed and processed for light microscopy to determine the extent of tissue damage from the sampling procedure. ATP concentrations (mean +/- SEM, nM) for EL, PL and CSF were 12.95 +/- 2.4 (n = 10), 10.5 +/- 3.9 (n = 11) and 16.1 +/- 5.4 (n = 11) respectively. Differences in ATP concentrations among fluids were not statistically significant. To test the effect of hypoxia on ATP levels, a group of guinea pigs was subjected to a 90 s period of respiratory anoxia prior to sampling of EL, PL or CSF. ATP concentrations were 14.4 +/- 3.5 (n = 11), 20.7 +/- 4.1 (n = 10) and 13.5 +/- 4.6 (n = 4) for EL, PL and CSF, respectively; only PL ATP concentrations were statistically different (P = 0.018, Wilcoxon rank sum test) to basal conditions. This is the first study which demonstrates the presence of free ATP in cochlear fluids. The results indicate that ATP is present in cochlear fluids at concentrations close to those known to cause hair cell depolarization in vitro.

Quinacrine staining of marginal cells in the stria vascularis of the guinea-pig cochlea: a possible source of extracellular ATP?

There is accumulating evidence for a purinergic humoral system involved in the control of cochlear function. Evidence of specific P2 purinoceptors on cochlear tissues implies a role for extracellular adenosine triphosphate (ATP) in the cochlea. To further this hypothesis a study was undertaken to determine if there was any specific source of purine compounds in cochlear tissues. Cochlear tissues (the sensory epithelium and lateral wall) from the guinea pig were incubated with the acridine derivative quinacrine dihydrochloride (5 x 10(-6) M in phosphate-buffered saline for 30 min at room temperature) which fluoresces on binding to high concentrations of ATP. Most cochlear tissues showed a diffuse green fluorescence slightly above the background level. However, a region of the marginal cells of the stria vascularis showed a specific punctate fluorescence. Optical sectioning of these cells by confocal microscopy revealed that the fluorescent structures in these marginal cells was confined to a region up to 10 microns from their endolymphatic surface. Similar cells studied by transmission electron microscopy showed membrane-bound vesicles located in the same region of the cell. These data imply that purine compounds are localized in discrete structures, perhaps vesicles, within the marginal cells which could serve as a source of extracellular ATP in the cochlea.

Noise exposure alters the response of outer hair cells to ATP

The outer hair cells (OHCs) are one target of noise-induced effects. To date there are few studies which examine changes in the function of OHCs induced by noise exposure. There is increasing evidence that ATP may be a neuromodulator acting on OHCs. Therefore, we examined the possibility that the response to ATP may be altered by low-level noise exposure. ATP was tested on cation currents recorded from outer hair cells (OHCs) isolated from chronic noise-exposed guinea pigs and compared to currents recorded from normal control animals. The whole-cell variant of the patch-clamp technique was used. The incidence of response to 100 microM ATP was decreased in OHCs from noise-exposed animals as compared to controls when normal internal and external solutions were employed. When K+ was substituted by N-methyl-glucamine (NMG+) in the pipette solution, there were significant differences in the magnitudes of ATP-evoked currents between cells from noise-exposed and control animals. This was observed in both normal and 20 mM Ba2+ external solutions. In addition, the response to ATP exhibited a dependency on OHC length. In short OHCs (< 65 microns) from noise-exposed animals the magnitude of the response to ATP was significantly reduced. By contrast, the response in long OHCs (> 65 microns) from noise-exposed animals was increased. Results suggest that low-level noise exposure induces changes in OHCs which affect the response of the cell to ATP.

ATP modulation of L-type calcium channel currents in guinea pig outer hair cells

Ca2+ channel currents and their modulation by adenosine 5'-triphosphate (ATP) in acutely isolated guinea pig outer hair cells (OHCs) were investigated using the whole-cell patch-clamp technique. The current-voltage (I-V) relation of OHCs indicated that the Ca2+ channel opened near -30 mV, and the current reached a maximum at +10 and 0 mV in 20 mM Ca2+ and Ba2+ external solutions, respectively. BayK 8644 (BayK, 2 microM) caused a 3.5-fold increase in peak Ca2+ currents and shifted the I-V curves toward more negative potentials. These results suggest that the majority of Ca2+ channels in OHCs have L-type characteristics. The effects of ATP on Ca2+ channels of OHCs were heterogenous. ATP (100 microM) decreased Ca2+ channel currents by 31.7 +/- 5.6% at 0 mV and shifted Ca2+ tail activation curves toward more depolarized potentials in some cells (N = 6). By contrast, in others, ATP enhanced the currents by 43.5 +/- 12.5% at +10 mV (N = 6). In the presence of BayK, however, ATP-induced inhibition or enhancement of Ca2+ channel currents was attenuated. In addition, 100 microM ATP produced little effect on Ca2+ channel currents in another subpopulation of cells (N = 12). This heterogenous neuromodulation of Ca2+ channel currents by ATP may reflect a functional diversity among OHCs.

P2X receptors bring new structure to ligand-gated ion channels

P2X receptors are cation-selective ion channels that open on binding to extracellular ATP; they play a role in fast synaptic transmission between neurones, and from autonomic nerves to smooth muscles. Isolation of cDNAs that encode P2X receptors in the smooth muscle of vas deferens and in phaeochromocytoma cells indicates that the receptors are not related to other ligand-gated ion channels. Their overall structure resembles more closely that of epithelial Na+ channels and the proteins that are thought to form mechanosensitive channels in Caenorhabditis elegans. The type of P2X RNA that is found in vas deferens is expressed preferentially by apoptotic thymocytes, and the type of P2X RNA that is found in PC12 cells is abundant in the pituitary gland, suggesting hitherto unsuspected roles for ATP-gated channels in endocrine and immune function.

Caffeine rapidly decreases potassium conductance of dissociated outer hair cells of guinea-pig cochlea

The effects of caffeine on the outer hair cells (OHCs) freshly dissociated from guinea-pig cochlea were investigated with the whole-cell patch-clamp technique, in both the conventional and the nystatin perforated patch-clamp configurations under voltage-clamp condition. Application of caffeine (> 1 mM for 10-30 s) induced an inward current (Icaffeine) with decrease of conductance in a dose-dependent manner at a holding potential (VH) of -60 mV. The reversal potential of Icaffeine (Ecaffeine) was close to the K+ equilibrium potential. The Icaffeine was not affected by Ca(2+)-free external solution. The internal perfusion of the Ca2+ chelator BAPTA had no effect on Icaffeine. The Icaffeine was not modulated by the external application of H-8 or staurosporine and by the internal perfusion of GDP-beta S. The amplitude of Icaffeine was the largest at the basal region of OHCs when caffeine was locally applied by the 'puffer' method. These results suggest that caffeine induces a decrease in membrane potassium conductance of the OHCs mainly at the basal region without mediating the intracellular signaling pathway.

Autoradiographic labelling of P2 purinoceptors in the guinea-pig cochlea

Two different radioligands were used to identify extracellular ATP binding sites specific to P2 purinoceptors in guinea-pig cochlear tissue. Deoxyadenosine 5'-(alpha-[35S]thio)triphosphate ([35S]dATP alpha S; 10 nM) provided a high activity probe for the P2y purinoceptor subtype on the basis of selective block by 2-methylthio-ATP (2MeSATP; 100 microM). [3H]alpha, beta-methylene-ATP (10 nM), a high affinity probe for a P2x purinoceptor subtype was selectively blocked by inclusion of the related compound beta, gamma-methylene-ATP (100 microM). Both probes labelled the organ of Corti, stria vascularis and spiral prominence regions. The P2x purinoceptor probe also bound to lateral wall tissue below the spiral prominence and insertion point of the basilar membrane within the scala tympani compartment, a region which failed to show significant binding using [35S]dATP alpha S. Frozen sections of whole cochlea permitted analysis of radioligand binding to the cell body region (spiral ganglion in Rosenthal's canal) of the primary auditory afferents and the auditory nerve itself, which lies within the central region of the modiolus of the cochlea. Both these regions exhibited 2MeSATP blockable [35S]dATP alpha S binding whereas specific [3H]alpha, beta-methylene-ATP binding was absent from spiral ganglion and minimal in the auditory nerve region. These results demonstrate a mixed P2 purinoceptor distribution in cochlear tissues and suggest that complex purine-mediated neurohumoral mechanisms may influence cochlear function at a number of sites.

Current state of purinoceptor research

It is hoped that this summary of the history and current status of purinoceptors will convince readers that receptors for purines are now established alongside other well-known extracellular messenger systems. These receptors are primitive, widespread and serve many different systems. Receptors of adenosine (P1-purinoceptors) are clearly different from receptors of ATP (P2-purinoceptors). As for other major transmitters such as acetylcholine, GABA, glutamate and 5-HT, receptors of two major families are activated by ATP, one (the P2X-purinoceptor family) mediates fast responses via ligand-gated ion channels, while the other (the P2Y-purinoceptor family) mediates slower responses via G-proteins (see Table 3). Subclasses of these two families have been suggested on the basis of recent molecular biology studies and the development of new selective agonists and antagonists (Abbracchio and Burnstock, 1994). It would indeed be helpful if the work on purinoceptors could be extended to studies of their chemical structure employing crystallography.

Indirect evidence for the presence and physiological role of endogenous extracellular ATP in the semicircular canal

Previous studies have shown that low concentrations of exogenous ATP, added to the perilymphatic fluid, could modify the bioelectrical activity of the isolated semicircular canal of the frog (Rana pipiens). To test the hypothesis that ATP is endogenously present and active in the perilymphatic fluid, the influence of two ATP-purinoceptor antagonists, Reactive Blue 2 and suramin, and of the enzyme, nucleotide pyrophosphatase, were examined. When applied by perilymphatic bath substitution, the three compounds reduced, in a dose-dependent manner, the firing of the afferent fibers monitored in the absence of mechanically-applied stimulation. The response of the afferent fibers, recorded when the sensory cells were mechanically inhibited, was also reduced. No modification of the response of the excitatory phase of the mechanical stimulus was observed in the presence of the two antagonists. In contrast, the signal was significantly reduced by the enzyme. None of the three compounds exhibited an influence on the transepithelial potential, or its variation in response to mechanical stimulation. The ATP-induced modification of the firing rate of the afferent fibers, monitored in the absence of mechanical stimulation, was reduced in the presence of the three drugs. No influence of Reactive Blue 2 and suramin was observed on the increase of the spontaneous firing induced by carbachol. In contrast, the effect of carbachol was decreased by nucleotide pyrophosphatase. The excitatory influence of glutamate on the spontaneous firing was not modified by Reactive Blue 2, while it was slightly increased by suramin and nucleotide pyrophosphatase.(ABSTRACT TRUNCATED AT 250 WORDS)

Nucleotides as extracellular signalling molecules

There is now wide acceptance that ATP and other nucleotides are ubiquitous extracellular chemical messengers. ATP and diadenosine polyphosphates can be released from synaptosomes. They act on a large and diverse family of P2 purinoceptors, four of which have been cloned. This receptor family can be divided into two distinct classes: ligand-gated ion channels for P2X receptors and G protein-coupled receptors for P2Y, P2U, P2T and P2D receptors. The P2Y, P2U and P2D receptors have a fairly wide tissue distribution, while the P2X receptor is mainly found in neurons and muscles and the P2T and P2Z receptors confined to platelets and immune cells, respectively. Inositol phosphate and calcium signalling appear to be the predominant mechanisms for transducing the G-protein linked P2 receptor signals. Multiple P2 receptors are expressed by neurons and glia in the CNS and also in neuroendocrine cells. ATP and other nucleotides may therefore have important roles not only as a neurotransmitter but also as a neuroendocrine regulatory messenger.

ATP responses in the embryo chick ciliary ganglion cells

The ATP responses in ciliary neurons acutely dissociated from chick ciliary ganglia were investigated using the nystatin perforated patch recording mode. ATP induced a transient inward current at a holding potential of -50 mV in a concentration-dependent manner. The half-maximum effective concentration of ATP was 8.2 microM and the Hill coefficient was 0.9. The current-voltage relationship of the ATP response revealed a strong inward rectification at potentials more positive than -30 mV and the reversal potential was near 0 mV. The relative potencies of purinoceptor agonists were in the order of ATP > 2-methylthio-ATP > ADP. Neither adenosine, AMP nor alpha, beta-methylene-ATP induced any response. The ATP-induced inward current was blocked by suramin, a selective P2 purinoceptor antagonist, in a concentration-dependent manner. The half-maximum inhibitory concentration was 4.5 microM. The cytosolic Ca2+ concentration was increased by ATP and suramin inhibited the increase in a concentration-dependent manner. These results suggest that ATP operates non-selective cation channels by acting on P2y purinoceptors and has a role in the excitation of ciliary neurons.

Ionic properties of IK,n in outer hair cells of guinea pig cochlea

The ionic properties of voltage-dependent K+ current activated at the resting membrane potential (IK,n) of outer hair cells (OHC) isolated from the guinea pig cochlea were studied using a patch-clamp technique in a whole-cell recording mode. The reversal potential of IK,n indicated a high selectivity for K+, and the relative permeability ratios for various monovalent cations were K+:Rb+:NH4+ = 1:1.21:0.13. Decrease in extracellular Cl- inhibited the IK,n. IK,n was blocked by Cs+ and Ba2+, although the inhibitory manner of Cs+ and Ba2+ were voltage-dependent and voltage-independent, respectively. By the use of puff-application method, the local application of Ba2+ to basolateral surface of OHC shifted the holding current level in an inward direction, whereas the application to apex and hair showed little change. Indicating that the IK,n channels preferentially locate at the basolateral region of cell membrane.

Neuronal responses to purinoceptor agonists in the leech central nervous system

Extracellular nucleotides like ATP and its derivatives are possible chemical messengers in vertebrate nervous systems. In invertebrate nervous system, however, little is known about their role in neurotransmission. We have studied the response of identified neurones of the leech Hirudo medicinalis to the purinoceptor agonist ATP, ADP, AMP, and adenosine using conventional intracellular microelectrodes and whole-cell patch-clamp recording. Bath application of the agonists depolarized the different neurons, but not the neuropil glial cells. The most effective responses (up to 10 mV) were observed with ATP (100 microM) or ADP (100 microM) in the noxious and touch cells. In most neurons the nonhydrolyzable ATP derivative ATP-gamma-S (5 microM) induced larger depolarizations than 100 microM ATP, indicating that most of the potency of ATP is lost presumably due to its degradation by ectonucleotidases. In medial noxious cells, ATP (100 microM) induced an inward current of 1.7 +/- 1.1 nA at a holding potential of -60 mV. The ATP-induced current-voltage relationship showed an inward rectification and a reversal potential close to 0 mV. In a Na+-free extracellular solution, the ATP-induced inward current decreased and in a Na+- and Ca(2+)-free saline only a small residual current persisted. The possible P2 purinoceptor antagonist suramin did not antagonize the ATP-induced current, but itself evoked an inward current and a conductance increase. We conclude that ATP activates nonselective cation channels in medial noxious cells of the leech with the order of potency of purinoceptor agonists ATP > or = ADP > AMP. The results suggest that these cells express purinoceptors of the P2 type.

Influence of ATP and ATP agonists on the physiology of the isolated semicircular canal of the frog (Rana pipiens)

In the present study, the influence of extracellular ATP and ATP agonists in the physiology of the vestibular organs was examined, using the in vitro model of the isolated semicircular canal of the frog (Rana pipiens). The firing activity of the afferent nerve, the d.c. nerve potential and the transepithelial potential were measured in the absence and presence of mechanical stimulation of the sensory epithelium. Administration of ATP into the perilymphatic compartment, from 10(-12) to 10(-3) M, increased the firing rate of the afferent fibers recorded in the absence of mechanical stimulation. Recordings of the d.c. nerve potential indicated that the afferent fibers were hyperpolarized. The presence of the purine also modified the transepithelial potential. During mechanical stimulation of the sensory epithelium, both the evoked afferent firing and the evoked variation of the d.c. nerve potential were reduced in the presence of ATP. However, ATP did not effect the evoked modulation of the transepithelial potential, evoked by the mechanical stimulation. Administration of the P2x purinoceptor agonists, alpha, beta-methylene-ATP and beta, gamma-methylene-ATP, at concentrations between 10(-12) and 10(-3) M, did not significantly modify the different bioelectrical activities investigated. In contrast, 2-methylthio-ATP, a P2y purinoceptor agonist, more potent and efficacious than ATP in its effect on the spontaneous firing. Concurrently, no modification of the d.c. nerve potential, the transepithelial potential and their variation during mechanical stimulation was observed. In opposition to the ATP effect, the total amplitude of the evoked firing was increased in the presence of 2-methylthio-ATP. These data suggest that extracellular ATP, present in the perilymphatic compartment, may act as a neuromodulator in the vestibular physiology. The effects of the purine appear to be mediated by the activation of a P2y subtype of purinoceptor. The absence of an effect of ATP and 2-methylthio-ATP on the evoked variation of the transepithelial potential suggest that the purine did not affect the processes responsible for the generation of the receptor potential but more likely modified the mechanisms involved in the release of the neurotransmitter from the hair cells and/or acted on the afferent endings.

ATP antagonists cibacron blue, basilen blue and suramin alter sound-evoked responses of the cochlea and auditory nerve

The P2-purinergic receptor antagonists suramin, cibacron blue and basilen blue, the latter two being isomers of reactive blue 2, were studied for their effects on sound-evoked responses from the cochlea (cochlear microphonic, CM; summating potential, SP; distortion product otoacoustic emissions, DPOAE) and auditory nerve (compound action potential, CAP). Local application of these compounds (10-1000 microM) into the cochlear perilymph was associated with concentration-dependent response alterations. Effects of suramin on cochlear responses were minimal: High-intensity SP was reduced slightly at concentrations > or = 330 microM without significant alterations in CM or DPOAEs. The amplitude of the auditory nerve CAP was suppressed and its latency increased at drug concentrations > or = 100 microM. Cibacron blue and basilen blue were of greater potency in their effects on cochlear and auditory nerve responses. DPOAEs were generally reduced, low-intensity SP was reduced and high-intensity SP was increased and CM was little affected at drug concentrations 100-1000 microM. The CAP was suppressed and its latency increased at concentrations > or = 33 microM. Effects of suramin were largely reversible; those associated with cibacron blue and basilen blue generally were not. To the extent that these drugs acted selectively as antagonists of ATP receptor-mediated activity, results support the hypothesis that endogenous ATP exerts profound actions at the level of the cochlea and the auditory nerve.

Quantal nature of synaptic transmission at the cytoneural junction in the frog labyrinth

1. The mechanism of transmitter release at the cytoneural junction of the frog posterior canal was investigated by recording intracellularly subthreshold postsynaptic potentials (EPSPs), and performing a statistical analysis of time intervals and peak amplitudes. In single units EPSPs display highly variable size, so it is not clear whether they are generated by the release of single quanta of transmitter and whether large ones represent giant events, multiquantal events, or the random summation of independent unitary events. 2. In units with low resting EPSP rates, peak amplitudes and time intervals between EPSPs were measured directly. Peak amplitude histograms were continuous, unimodal and well fitted by log normal distributions. Time-interval histograms were well described by single exponentials. 3. At high EPSP rates (either at rest or during experimental treatments), where single events overlapped extensively, peak amplitude histograms were skewed markedly towards high values. Under these conditions, the EPSP waveform was estimated by autoregressive fit to the autocorrelation of the recorded signal. The fit was used to build a Wiener filter, for sharpening the original signal, before computing time-interval and peak amplitude histograms. This yielded consistent log normal peak amplitude distributions with no 'excess' skewness, similar to those obtained with low resting rates. 4. After sharpening by the Wiener filter, shoulders or small second peaks in amplitude distributions were observed only at the highest EPSP rates (> 300 s-1). The number of 'multiquantal' events was reduced by Wiener filtering, and was in general consistent with the expectation that more than one independent event occurred within the duration of the single event. This suggests that the events are uniquantal, random and independent, i.e. miniature EPSPs (mEPSPs). 5. In general, peak amplitude distributions obtained with modified external Ca2+ concentration ([Ca2+]o) and/or during mechanical stimulation or under efferent activation were not significantly altered with respect to those obtained in the same units at rest. Time-interval histograms were generally mono-exponential at rest as well as during mechanical or efferent stimulation, and irrespective of [Ca2+]o. Resting mEPSP rate was slightly increased by elevated [Ca2+]o and reduced by low [Ca2+]o. The increase in mEPSP rate produced by mechanical excitation was depressed by both high and low [Ca2+]o, whereas both conditions enhanced mechanical inhibition. Efferent inhibition was little affected. High [Ca2+]o hastened adaptation during efferent facilitation. Low [Ca2+]o reduced peak response during facilitation, but suppressed its warning. 6. In the presence of ATP a consistent though transient increase in resting mEPSP rate was observed in about 50% of units.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of adenosine 5'-triphosphate and related agonists on cochlear function

Several lines of evidence implicate a neurotransmitter/modulator role for ATP in the cochlea. Most of the work supporting such a notion has been accomplished using in vitro preparations of sensory hair cells or other cochlear tissues. Little is known regarding the functional consequences of ATP receptor activation in vivo. In the present experiments, we tested ATP and related agonist analogs for their effects on sound-evoked responses of the cochlea (cochlear microphonic, CM; summating potential, SP; distortion product otoacoustic emissions, DPOAE) and auditory nerve (compound action potential, CAP) in vivo and on outer hair cell (OHC) currents and cell length in vitro. In vivo, local application of these compounds was associated with concentration- and intensity-dependent response alterations. The slowly-hydrolyzable P2y agonist, ATP-gamma-S, was clearly of greatest in vivo potency: At low to moderate stimulus intensities, micromolar concentrations of this drug reduced all responses, in particular CAP and DPOAEs, which fell to the level of the noise floor. At high intensities, response suppression was smaller and SP was increased. In vivo effects of ATP, ATP-alpha-S and 2-Me-S-ATP were qualitatively similar to, but smaller in magnitude and requiring higher concentrations than those observed for ATP-gamma-S. Adenosine was without significant effect on responses of the cochlea and auditory nerve. In vitro, effects of ATP-gamma-S and ATP were similar: both induced inward currents in OHCs held at -60 mV without producing observable (> 0.3 micron) changes in OHC length. Results suggest that endogenous ATP influences cochlear function through receptors at several sites in the cochlea. Results suggest further that these response alterations are mediated, at least in part, by receptors of the P2y subtype.

A calcium-activated nonselective cationic channel in the basolateral membrane of outer hair cells of the guinea-pig cochlea

The patch-clamp technique was used to investigate ion channels in the basolateral perilymph-facing membrane of freshly isolated outer hair cells (OHCs) from the guinea-pig cochlea. These sensory cells probably determine, via their motile activity, the fine tuning of sound frequencies and the high sensitivity of the inner ear. A Ca(2+)-activated nonselective cationic channel was found in excised inside-out membrane patches. The current/voltage relationship was linear with a unit conductance of 26.3 +/- 0.3 pS (n = 15) under symmetrical inger conditions. The channel excluded anions (PNa/PCl = 18 where PNa/PCl denotes the relative permeability of Na to Cl); it was equally permeant to the Na+ and K+ ions and exhibited a low permeability to N-methyl-D-glucamine and Ba2+ or Ca2+. Channel opening required a free Ca2+ concentration of about 10(-6) mol/l on the internal side of the membrane and the open probability (Po) was maximal at 10(-3) mol/l (Po = 0.72 +/- 0.06, n = 12). Adenosine 5'mono-, tri- and di-phosphate reduced Po to 29 +/- 14 (n = 5), 42 +/- 10 (n = 8) and 51 +/- 12 (n = 5) % of control Po, respectively, when they were added at a concentration of 10(-3) mol/l to the internal side. The channel was partially blocked by flufenamic acid (10(-4) mol/l) and 3',5'-dichlorodiphenylamine-2-carboxylic acid (DCDPC, 10(-5) mol/l). This type of channel, together with Ca(2+)-activated K+ channels, might participate in the control of membrane potential and modulate the motility of OHCs.

In vitro pharmacologic characterization of a cholinergic receptor on outer hair cells

Acetylcholine (ACh) is the major neurotransmitter released from the efferent fibers in the cochlea onto the outer hair cells (OHCs). The type of ACh receptor on OHCs and the events subsequent to receptor activation are unclear. Therefore we studied the effect of agonists and antagonists of the ACh receptor on isolated OHCs from the guinea pig. OHCs were recorded from in whole cell voltage and current clamp configuration. ACh induced an increase in outward K+ current (IACh) which hyperpolarized the OHCs. No desensitization to ACh application was observed. Cs+ replaced K+ in carrying the IACh. The IACh is Ca(2+)-dependent, time and voltage sensitive, and different from the IKCa induced by depolarization of the membrane potential. When tested at 100 microM, several agonists also induced outward current responses (acetylcholine > suberyldicholine > or = carbachol > DMPP) whereas nicotine, cytisine and muscarine did not. The IACh response to 10 microM ACh was blocked by low concentrations of traditional and non-traditional-nicotinic antagonists (strychnine > curare > bicuculline > alpha-bungarotoxin > thimethaphan) and by higher concentrations of muscarinic antagonists (atropine > 4-DAMP > AF-DX 116 > pirenzepine). Pharmacologically, the ACh receptor on OHCs is nicotinic.

Purinoceptors: are there families of P2X and P2Y purinoceptors?

There has been an exponential growth in interest in purinoceptors since the potent effects of purines were first reported in 1929 and purinoceptors defined in 1978. A distinction between P1 (adenosine) and P2 (ATP/ADP) purinoceptors was recognized at that time and later, A1 and A2, as well as P2x and P2y subclasses of P1 and P2 purinoceptors were also defined. However, in recent years, many new subclasses have been claimed, particularly for the receptors to nucleotides, including P2t, P2z, P2u(n) and P2D, and there is some confusion now about how to incorporate additional discoveries concerning the responses of different tissues to purines. The studies beginning to appear defining the molecular structure of P2-purinoceptor subtypes are clearly going to be important in resolving this problem, as well as the introduction of new compounds that can discriminate pharmacologically between subtypes. Thus, in this review, on the basis of this new data and after a detailed analysis of the literature, we propose that: (1) P2X(ligand-gated) and P2Y(G-protein-coupled) purinoceptor families are established; (2) four subclasses of P2X-purinoceptor can be identified (P2X1-P2X4) to date; (3) the variously named P2-purinoceptors that are G-protein-coupled should be incorporated into numbered subclasses of the P2Y family. Thus: P2Y1 represents the recently cloned P2Y receptor (clone 803) from chick brain; P2Y2 represents the recently cloned P2u (or P2n) receptor from neuroblastoma, human epithelial and rat heart cells; P2Y3 represents the recently cloned P2Y receptor (clone 103) from chick brain that resembles the former P2t receptor; P2Y4-P2Y6 represent subclasses based on agonist potencies of newly synthesised analogues; P2Y7 represents the former P2D receptor for dinucleotides. This new framework for P2 purinoceptors would be fully consistent with what is emerging for the receptors to other major transmitters, such as acetylcholine, gamma-aminobutyric acid, glutamate and serotonin, where two main receptor families have been recognised, one mediating fast receptor responses directly linked to an ion channel, the other mediating slower responses through G-proteins. We fully expect discussion on the numbering of the different receptor subtypes within the P2X and P2Y families, but believe that this new way of defining receptors for nucleotides, based on agonist potency order, transduction mechanisms and molecular structure, will give a more ordered and logical approach to accommodating new findings. Moreover, based on the extensive literature analysis that led to this proposal, we suggest that the development of selective antagonists for the different P2-purinoceptor subtypes is now highly desirable, particularly for therapeutic purposes.

Localisation of functional muscarinic receptors in the rat cochlea: evidence for efferent presynaptic autoreceptors

In the rat cochlea, the activation of muscarinic receptors stimulates the hydrolysis of phosphoinositides but the importance of this muscarinic effect is still unknown. In order to find out about the role of the muscarinic receptors in the cochlea, we examined their functional distribution within this organ. This was achieved by measuring the formation of [3H]inositol phosphates induced by carbachol (1 mM) in two regions of the cochlea: the modiolus and the organ of Corti. At both sites, carbachol enhanced the accumulation of inositol phosphates in an atropine-sensitive way. These stimulations were completely antagonised by 4-diphenylacetoxy-N-methyl piperidine methiodide (1 microM) but unchanged by pirenzepine (1 microM). In cochleas depleted of outer hair cells by a treatment with amikacin, the carbachol-induced formation of inositol phosphates is not altered with respect to control, undamaged cochleas. Conversely, when the medial cholinergic axons which form synapses with the outer hair cells are destroyed by the section of the crossed olivocochlear bundle the carbachol-stimulated inositol phosphates response is reduced by 35% in the organ of Corti. This section has no effect in the modiolus, despite the degeneration of some modiolar fibers. Our results show that functional muscarinic receptors are distributed both in the organ of Corti and in the modiolus. These two structures contain presumably the same class of cholinoceptor. The effects of selective destruction clearly demonstrate that a population of muscarinic receptors is located on presynaptic membranes at the level of the medial axon-outer hair cell contacts. They also point to spiral ganglion neurons and/or the Schwann cells as sites for the functional cholinoceptors in the modiolus.

Effects of extracellular ATP on hair cells isolated from the guinea-pig semicircular canals

Externally applied ATP (100 microM) induced membrane currents in type I and type II vestibular hair cells enzymatically isolated from guinea-pig semicircular canals. In whole-cell voltage-clamp and with 140 mM K+ in the pipette solution, ATP evoked an inwardly directed current in 58% of the cells when held at potentials below -40 mV. In the remainder, external ATP produced an outward current. After block of the K currents, an inward current activated by ATP was revealed at -50 mV. Intracellular Ca2+ levels were monitored using the Ca2+ indicator Fura-2 and were found to rise in both hair cell types in response to ATP. These results strongly suggest that ATP directly controls the entry of Ca2+ into crista hair cells which can then further modulate K+ currents.

Receptor-mediated release of inositol phosphates in the cochlear and vestibular sensory epithelia of the rat

Various neurotransmitters, hormones and other modulators involved in intercellular communication exert their biological action at receptors coupled to phospholipase C (PLC). This enzyme catalyzes the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PtdInsP2) to inositol 1,4,5-trisphosphate (InsP3) and 1,2-diacylglycerol (DG) which act as second messengers. In the organ of Corti of the guinea pig, the InsP3 second messenger system is linked to muscarinic cholinergic and P2y purinergic receptors. However, nothing is known about the InsP3 second messenger system in the vestibule. In this study, the receptor-mediated release of inositol phosphates (InsPs) in the vestibular sensory epithelia was compared to that in the cochlear sensory epithelia of Fischer-344 rats. After preincubation of the isolated intact tissues with myo-[3H]inositol, stimulation with the cholinergic agonist carbamylcholine or the P2 purinergic agonist ATP-gamma-S resulted in a concentration-dependent increase in the formation of [3H]InsPs in both epithelia. Similarly, the muscarinic cholinergic agonist muscarine enhanced InsPs release in both organs, while the nicotinic cholinergic agonist dimethylphenylpiperadinium (DMPP) was ineffective. The muscarinic cholinergic antagonist atropine completely suppressed the InsPs release induced by carbamylcholine, while the nicotinic cholinergic antagonist mecamylamine was ineffective. Potassium depolarization did not alter unstimulated or carbamylcholine-stimulated release of InsPs in either organ. In both tissues, the P2 purinergic agonist alpha,beta-methylene ATP also increased InsPs release, but the P1 purinergic agonist adenosine did not. These results extend our previous observations in the organ of Corti of the guinea pig to the rat and suggest a similar control of the InsP3 second messenger system in the vestibular sensory epithelia.(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular mechanism of acetylcholine-induced response in dissociated outer hair cells of guinea-pig cochlea

1. The acetylcholine (ACh)-induced currents (IACh) in dissociated outer hair cells (OHCs) of guinea-pig cochlea were investigated using the whole-cell patch-clamp technique, in both conventional and nystatin perforated-patch configurations. 2. ACh and carbamylcholine (CCh) induced outward currents at a holding potential (VH) of -60 mV in the perforated-patch configuration. The IACh increased in a sigmoidal fashion over the concentration range between 3 x 10(-6) and 10(-3) M. The dissociation constant (KD) was 1.7 x 10(-5) M and the Hill coefficient (n) was 2.7. The KD and n for CCh were 8.7 x 10(-5) M and 2.2, respectively. Neither nicotine nor muscarine induced any detectable current up to a concentration of 10(-3) M. 3. Various muscarinic agonists such as oxotremorine-M, McN-A-343 and oxotremorine could also induce the outward currents, although these current amplitudes were about one-third that of ACh, indicating that they were partial agonists. 4. The muscarinic antagonists atropine, 4-DAMP, AF-DX 116 and pirenzepine inhibited the IACh in a concentration-dependent manner. The half-inhibitory concentrations (IC50) for atropine, 4-DAMP, AF-DX 116 and pirenzepine were 4.8 x 10(-6), 6.2 x 10(-6), 2.1 x 10(-5) and 2.9 x 10(-4) M, respectively. 5. When the extracellular Ca2+ concentration ([Ca2+])o) was reduced to lower than 1 mM, the amplitude of IACh, abruptly decreased. In a nominally Ca(2+)-free external solution ACh did not induce any current. The increase of [Ca2+]o beyond 1 mM did not change the IACh. 6. When OHCs were perfused intracellularly with a pipette solution containing 10 mM BAPTA in the conventional whole-cell mode, ACh could not induce outward K+ currents. The Ca2+ ionophore A23187 induced an outward current. These results indicate that intracellular Ca2+ is involved in the ACh response. 7. Calmodulin inhibitors such as chlorpromazine, W-7 and trifluoperazine inhibited the IACh in a concentration-dependent manner. 8. When OHCs were dialysed with either 100 microM GDP beta S or 1 micrograms/ml pertussis toxin (PTX) through the patch pipette at a VH of -60 mV, the IACh diminished within 10 min, whereas the IACh of the control remained steady for over 20 min, suggesting that a PTX-sensitive G-protein is involved in the ACh response.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of Ca2+ signals generated by extracellular nucleotides in supporting cells of the organ of Corti

ATP has been demonstrated to act as a co-transmitter or neuromodulator in various physiological processes. There is recent evidence that ATP receptors, characterized as P2 purinergic receptors, are expressed in the sensory hair cells of the auditory organ. The aim of the present study was to know whether other cell types of the organ of Corti, the supporting cells, were also sensitive to external ATP. In both types of supporting cells considered in this study, Deiters' cells (DCs) and Hensen's cells (HEs), extracellular ATP at sub-micromolar concentrations evoked a transient increase in [Ca2+]i as monitored with fluorescence microscopy using the calcium probe Indo-1. An apparent Kd of 0.5 and 0.9 microM was determined for DCs and HEs, respectively. Furthermore, we demonstrated that ATP stimulated Ca2+ release from internal stores in DCs, but not in HEs. Dynamic calcium imaging by confocal laser scanning microscopy of ATP induced Ca2+ mobilization demonstrated a calcium wave propagation in the cell body of DCs which originated in the phalangeal processes, suggesting a functional organization of Ca2+ sequestering stores in DCs.

Block by calcium of ATP-activated channels in pheochromocytoma cells

We have investigated the effects of Ca2+ on Na+ influx through ATP-activated channels in pheochromocytoma PC12 cells using single channel current recordings. Under cell-attached patch-clamp conditions with 150 mM Na+ and 2 mM Ca2+ in the pipette, the unitary current activity showed an open level of about -4.3 pA at -150 mV. The channel opening was interrupted by flickery noise as well as occasional transition to a subconducting state of about -1.7 pA at -150 mV. The open level was decreased with increased external Ca2+, suggesting that external Ca2+ blocks Na+ permeation. We assessed the block by Ca2+ as the mean amplitude obtained with heavy filtration according to Pietrobon et al. (Pietrobon, D., B. Prod'hom, and P. Hess, 1989. J. Gen. Physiol. 94:1-21). The block was concentration dependent with a Hill coefficient of 1 and a half-maximal concentration of approximately 6 mM. A similar block was observed with other divalent cations, and the order of potency was Cd2+ > Mn2+ > Mg2+ not equal to Ca2+ > Ba2+. High Ca2+, Mg2+ and Ba2+ did not block completely, probably because they can carry current in the channel. The block by external Ca2+ did not exhibit voltage dependence between -100 and -210 mV. In the inside-out patch-clamp configuration, the amplitude of inward channel current obtained with 150 mM external Na+ was reduced by increased internal Ca2+. The reduction was observed at lower concentrations than that by external Ca2+. Internal Ba2+ and Cd2+ induced similar reduction in current amplitude. This inhibitory effect of internal Ca2+ was voltage dependent; the inhibition was relieved with hyperpolarization. The results suggest that both external and internal Ca2+ can block Na+ influx through the ATP-activated channel. A simple one-binding site model with symmetric energy barriers is not sufficient to explain the Ca2+ block from both sides.

Perforated-patch method reveals extracellular ATP-induced K+ conductance in dissociated rat nucleus solitarii neurons

Extracellular adenosine 5'-triphosphate (ATP)-induced responses were investigated in acutely dissociated rat nucleus tractus solitarii (NTS) neurons, using nystatin perforated-patch and conventional whole-cell patch-clamp recordings. Extracellular application of ATP could evoke the inward currents with both methods. An additional outward current was observed only with the perforated patch method. The outward current resulted in an increase of K+ conductance which was activated by Ca2+ influx through the ATP receptor-channel complex.

Delayed shortening and shrinkage of cochlear outer hair cells

Slow shortening of cochlear outer hair cells has been speculated to modify cochlear sensitivity. Tetanic electrical field stimulation of isolated outer hair cells from guinea pigs shortened the cells for 2-3 min. Electrical stimulation reduced cell length and volume (-13.5 +/- 1.5 and -37.3 +/- 3.0% of initial values, respectively, n = 16) and decreased the intracellular Cl- concentration. Cytochalasin B (100 microM) inhibited electrical stimulation-induced shortening but not volume reduction. The following chemicals or manipulations inhibited the responses: 10 microM furosemide, 0.1 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), 1 mM anthracene-9-carboxylic acid (AC9), 25 mM tetraethylammonium, 2.3 microM charybdotoxin (ChTX), 250 nM omega-conotoxin, and Ca(2+)-free medium. These findings suggest that both electrical stimulation-induced shortening and shrinkage of outer hair cells result not only from an actin-mediated contractile force, but also from Cl- efflux through furosemide-, DIDS-, and AC9-sensitive Cl- channels, and K+ efflux through ChTX-sensitive K+ channels.

Membrane potential measurement in isolated outer hair cells of the guinea pig cochlea using conventional microelectrodes

Membrane potential of the isolated outer hair cells (OHCs) from the guinea pig cochlea was measured using conventional microelectrodes filled with 200 mM KCl. The resting membrane potential during superfusion with the standard physiological saline solution containing 3.5 mM K+ was -47.3 +/- 1.4 mV (N = 72), which was higher than those previously reported for isolated OHCs studied by using microelectrodes. Addition of ouabain (10(-5)-10(-3) M), the specific Na+, K+ ATPase inhibitor, depolarized the cell slowly and progressively, indicating the presence of low but definite Na+, K+ ATPase activity in the plasma membrane of OHCs. The magnitude of membrane potential was mainly dependent on the extracellular K+ concentration ([K+]O). A ten-fold increase of [K+]O depolarized the membrane potential by 49.6 +/- 1.0 mV (N = 58). A decrease of [Na+]O to one tenth of the control hyperpolarized the membrane potential by about 2 mV. Decreasing extracellular Cl- from 131.3 mM to 27.5 mM did not cause a significant change in the membrane potential. Using the Goldman-Hodgkin-Katz equation, assuming a negligible contribution of Cl- to the membrane potential and total monovalent cat ion concentration of the cytosol similar to the extracellular fluid, we calculated the permeability ratio of K+ versus Na+ to 131 +/- 19 and intracellular K+ concentration to 33.3 +/- 1.9 mM.

Filtering properties of hair cells

Three questions were asked about filtering properties of hair cells. First, it was asked if hair cells in different receptors showed different filtering properties to match the receptor response to the particular characteristics of the stimulus? It seems that the answer is yes, since for example, the resonant frequencies of pigeon semicircular hair cells at membrane potentials around the RMP are in a range (12-280 Hz) that could match angular head frequencies. Since critical frequency tuning of one frequency probably is not necessary for the semicircular canals it is quite reasonable that the largest quality factor (Qe) of resonance in pigeon semicircular canal hair cells for membrane potentials around the RMP is 4 times lower than the largest Qe in bullfrog saccular hair cells and 10 times lower than Qe in the turtle basilar papilla. Second, it was asked if hair cells of the same morphological type but in different regions of the neuroepithelium have different filtering properties. It seems that this question needs more careful study since most available data on hair cells that show different filtering properties depending on their location in the neuroepithelium show subtle morphological differences and therefore the hair cells could be classified as different types. Finally, it was asked if different hair cell types (based on morphology and innervation) have different filtering properties. The currents in pigeon and guinea pig type I and type II semicircular canal hair cells, guinea pig inner and outer cochlear hair cells, goldfish short and tall saccule hair cells and chick short and tall cochlea hair cells suggest that the answer to this question is yes. The challenge is to continue to precisely specify the filtering properties of different types of hair cells in different places on the neuroepithelium of different receptors.

Effects of Ca2+ antagonists and aminoglycoside antibiotics on Ca2+ current in isolated outer hair cells of guinea pig cochlea

The effects of various Ca2+ antagonists and aminoglycoside antibiotics on the Ca2+ channel in isolated outer hair cells of the guinea pig were investigated using a whole-cell patch-clamp technique. The inhibitory action was in the order of La3+ much greater than Cd2+ much greater than Ni2+ greater than Co2+ for inorganic Ca2+ antagonists, and flunarizine = nicardipine greater than omega-conotoxin greater than methoxyverapamil = diltiazem much greater than amiloride for organic ones. Aminoglycoside antibiotics also had antagonistic effects on the Ca2+ channel.

Na(+)-Ca2+ exchange in the isolated cochlear outer hair cells of the guinea-pig studied by fluorescence image microscopy

The outer hair cell isolated from the guinea-pig was superfused in vitro and the cytosolic calcium concentration ([Ca2+]i) and sodium concentration ([Na+]i) were measured using fluorescence indicators. Under the resting condition, [Ca2+]i and [Na+]i were 91 +/- 9 nM (n = 51) and 110 +/- 5 mM (n = 12), respectively. Removal of external Na+ by replacing with N-methyl-D-glucamine (NMDG+) increased [Ca2+]i by 270 +/- 79% (n = 27) and decreased [Na+]i by 23 +/- 4 mM (n = 6). Both changes in [Ca2+]i and [Na+]i were totally reversible on returning external Na+ to the initial value and were inhibited by addition of 0.1 mM La3+ or 100 microM amiloride 5-(N,N-dimethyl) hydrochloride. Elevation of external Ca2+ ions to 20 mM reversibly decreased [Na+]i by 8 +/- 6 mM (n = 5). Moreover, the chelation of the intracellular Ca2+ with 1,2-bis (2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA) exerted an inhibitory action on the NMDG(+)-induced reduction in [Na+]i. Exposure to 5 mM NaCN for 2 min significantly and reversibly increased [Ca2+]i by 290 +/- 37% (n = 5), but did not affect the [Ca2+]i elevation induced by the NMDG+ solution. The rise in [Ca2+]i induced by the NMDG+ solution was not enhanced by ouabain pretreatment. Addition of ouabain did not alter the [Na+]i. The present results are best explained by the presence of an Na(+)-Ca2+ exchanger in cell membrane and indicate that the activity of Na+/K+ pump is poor in outer hair cells.

Ionic currents of outer hair cells isolated from the guinea-pig cochlea

1. Whole-cell currents were measured in outer hair cells isolated from each turn of the organ of Corti of the guinea-pig. 2. The slope input conductances at -70 mV of the cells ranged from 3.6 to 51 nS depending on the length of the cell. Shorter cells from the basal turns of the cochlea had the highest values. The membrane time constant of the cells varied from 3 to 0.2 ms from the apex to the base. 3. Irrespective of the position of the cells along the cochlea, three distinct currents were found. Each type of current was found in approximately the same proportion in all cells. 4. An outward K+ current was present which activated at potentials more positive than -35 mV. The current was sensitive to tetraethylammonium (30 mM), quinidine (100 microM) and nifedipine (50 microM). It could be removed by replacing external Ca2+ with Ba2+ or Mg2+. The current was also removed by substituting Nai+ or Csi+ for Ki+ pipette solution. This outwardly rectifying current appears similar to the calcium-activated K+ current described in other hair cells. 5. The main current present at membrane potentials from -90 mV to -50 mV was a second voltage-activated K+ current. It was 50% activated at -80 mV, and relaxed with a time constant of 20-40 ms on hyperpolarization to -120 mV. Near rest the kinetics were essentially time-dependent , but depended upon the external K+ concentration. The current was blocked by 5 mM external Cs+. 6. This current was highly selective for K+. Measured from reversal of the tail currents, the permeability ratio PK:PNa was approximately 30:1. Depolarization of the cell, presumed to lead to an elevation of intracellular calcium, produced a prolonged activation of the current. 7. A third current found in the cells was a cation current. By external ion replacement, the selectivity sequence was determined to be Ca2+ greater than Na+ approximately equal to K+ greater than choline+ greater than NMDG+ (respective permeabilities relative to Na: 2.9, 1.0, 0.99, 0.63 and 0.37). This current was reduced by external Ba2+ (3 mM) and by nifedipine (50 microM). The activation of this current appeared to depend upon raised levels of Cai2+. 8. These currents account for reported in vivo properties of cochlear outer hair cells as cells permeable to potassium at large negative resting potentials. The consequences for sound detection in the cochlea are briefly discussed.