Acetylcholine activates a K+ conductance permeable to Cs+ in guinea pig outer hair cells

Short-term plasticity and modulation of synaptic transmission at mammalian inhibitory cholinergic olivocochlear synapses

The organ of Corti, the mammalian sensory epithelium of the inner ear, has two types of mechanoreceptor cells, inner hair cells (IHCs) and outer hair cells (OHCs). In this sensory epithelium, vibrations produced by sound waves are transformed into electrical signals. When depolarized by incoming sounds, IHCs release glutamate and activate auditory nerve fibers innervating them and OHCs, by virtue of their electromotile property, increase the amplification and fine tuning of sound signals. The medial olivocochlear (MOC) system, an efferent feedback system, inhibits OHC activity and thereby reduces the sensitivity and sharp tuning of cochlear afferent fibers. During neonatal development, IHCs fire Ca²⁺ action potentials which evoke glutamate release promoting activity in the immature auditory system in the absence of sensory stimuli. During this period, MOC fibers also innervate IHCs and are thought to modulate their firing rate. Both the MOC-OHC and the MOC-IHC synapses are cholinergic, fast and inhibitory and mediated by the α9α10 nicotinic cholinergic receptor (nAChR) coupled to the activation of calcium-activated potassium channels that hyperpolarize the hair cells. In this review we discuss the biophysical, functional and molecular data which demonstrate that at the synapses between MOC efferent fibers and cochlear hair cells, modulation of transmitter release as well as short term synaptic plasticity mechanisms, operating both at the presynaptic terminal and at the postsynaptic hair-cell, determine the efficacy of these synapses and shape the hair cell response pattern.

A 'calcium capacitor' shapes cholinergic inhibition of cochlear hair cells

Efferent cholinergic neurons project from the brainstem to inhibit sensory hair cells of the vertebrate inner ear. This inhibitory synapse combines the activity of an unusual class of ionotropic cholinergic receptor with that of nearby calcium-dependent potassium channels to shunt and hyperpolarize the hair cell. Postsynaptic calcium signalling is constrained by a thin near-membrane cistern that is co-extensive with the efferent terminal contacts. The postsynaptic cistern may play an essential role in calcium homeostasis, serving as sink or source, depending on ongoing activity and the degree of buffer saturation. Release of calcium from postsynaptic stores leads to a process of retrograde facilitation via the synthesis of nitric oxide in the hair cell. Activity-dependent synaptic modification may contribute to changes in hair cell innervation that occur during development, and in the aged or damaged cochlea.

Biophysical and pharmacological characterization of nicotinic cholinergic receptors in rat cochlear inner hair cells

Before the onset of hearing, a transient efferent innervation is found on inner hair cells (IHCs). This synapse is inhibitory and mediated by a nicotinic cholinergic receptor (nAChR) probably formed by the alpha9 and alpha10 subunits. We analysed the pharmacological and biophysical characteristics of the native nAChR using whole-cell recordings from IHCs in acutely excised apical turns of the rat organ of Corti. Nicotine did not activate but rather blocked the acetylcholine (ACh)-evoked currents with an IC50 of 1 +/- 0.1 microM. Antagonists of non-cholinergic receptors such as strychnine, bicuculline and ICS-205930 blocked ACh-evoked responses with an IC50 of 8.6 +/- 0.8 nM, 59 +/- 4 nM and 0.30 +/- 0.02 microM, respectively. The IHC nAChR was both permeable to (P(Ca)/P(Na) = 8 +/- 0.9) and modulated by external Ca2+. ACh-evoked currents were potentiated by Ca2+ up to 500 microM but were reduced by higher concentrations of this cation. Ba2+ mimicked the effects of Ca2+ whereas Mg2+ only blocked these currents. In addition, elevation of extracellular Ca2+ reduced the amplitude of spontaneous synaptic currents without affecting their time course. The receptor had an EC50 for ACh of 60.7 +/- 2.8 microM in 0.5 mM Ca2+. In the absence of Ca2+, the EC50 for ACh increased, suggesting that potentiation by Ca2+ involves changes in the apparent affinity for the agonist. These pharmacological and biophysical characteristics of the IHC nAChR closely resemble those of the recombinant alpha9alpha10 nAChR, reinforcing the hypothesis that the functional nAChR at the olivocochlear efferent-IHC synapse is composed of both the alpha9 and alpha10 subunits.

Effects of intracellular stores and extracellular Ca(2+) on Ca(2+)-activated K(+) currents in mature mouse inner hair cells

Ca(2+)-activated K(+) currents were studied in inner hair cells (IHCs) of mature mice. I(K,f), the large-conductance Ca(2+)-activated K(+) current (BK) characteristic of mature IHCs, had a fast activation time constant (0.4 ms at -25 mV at room temperature) and did not inactivate during 170 ms. Its amplitude, measured at -25 mV, and activation time constant were similar between IHCs in the apical and basal regions of the cochlea. I(K,f) was selectively blocked by 30 nm IbTx but was unaffected by superfusion of Ca(2+)-free solution, nifedipine or Bay K 8644, excluding the direct involvement of voltage-gated Ca(2+) channels in I(K,f) activation. Increasing the intracellular concentration of the Ca(2+) chelator BAPTA from 0.1 mm to 30 mm reduced the amplitude of I(K,f) at -25 mV and shifted its activation by 37 mV towards more depolarized potentials. A reduction in the size of I(K,f) and a depolarizing shift of its activation were also seen when either thapsigargin and caffeine or ryanodine were added intracellularly, suggesting that I(K,f) is modulated by voltage-dependent release from intracellular Ca(2+) stores. Mature IHCs had a small additional Ca(2+)-activated K(+) current (I(K(Ca))), activated by Ca(2+) flowing through L-type Ca(2+) channels. This current was still present during superfusion of either IbTx (60 nm) or apamin (300 nm) but was abolished in Cs(+)-based intracellular solution or during superfusion of 5 mm TEA, suggesting the presence of an additional BK-channel type. Current clamp experiments at body temperature show that I(K,f), but not I(K(Ca)), is essential for fast voltage responses of mature IHCs.

Direct interaction of serotonin type 3 receptor ligands with recombinant and native alpha 9 alpha 10-containing nicotinic cholinergic receptors

In the present work, we characterized the effects of serotonin type 3 receptor ligands on recombinant and native alpha 9 alpha 10-containing nicotinic acetylcholine receptors (nAChRs). Our results indicate that the recombinant alpha 9 alpha 10 nAChR shares striking pharmacological properties with 5-HT(3) ligand-gated ion channels. Thus, 5-HT(3) receptor antagonists block ACh-evoked currents in alpha 9 alpha 10-injected Xenopus laevis oocytes with a rank order of potency of tropisetron (IC(50), 70.1 +/- 0.9 nM) > ondansetron (IC(50), 0.6 +/- 0.1 microM) = MDL 72222 (IC(50), 0.7 +/- 0.1 microM). Although serotonin does not elicit responses in alpha 9 alpha 10-injected oocytes, it blocks recombinant alpha 9 alpha 10 receptors in a noncompetitive and voltage-dependent manner (IC(50), 5.4 +/- 0.6 microM). On the other hand, we demonstrate an in vivo correlate of these properties of the recombinant receptor, with those of the alpha 9 alpha 10-containing nAChR of frog saccular hair cells. The possibility that the biogenic amine serotonin might act as a neuromodulator of the cholinergic efferent transmission in the vestibular apparatus and in the organ of Corti is discussed.

The alpha9alpha10 nicotinic acetylcholine receptor is permeable to and is modulated by divalent cations

The native cholinergic receptor that mediates synaptic transmission between olivocochlear fibers and outer hair cells of the cochlea is permeable to Ca(2+) and is thought to be composed of both the alpha 9 and the alpha 10 cholinergic nicotinic subunits. The aim of the present work was to study the permeability of the recombinant alpha 9 alpha 10 nicotinic acetylcholine receptor to Ca(2+), Ba(2+) and Mg(2+) and its modulation by these divalent cations. Experiments were performed, by the two-electrode voltage-clamp technique, in Xenopus laevis oocytes injected with alpha 9 and alpha 10 cRNA. The relative divalent to monovalent cation permeability was high ( approximately 10) for Ca(2+), Ba(2+) and Mg(2+). Currents evoked by acetylcholine (ACh) were potentiated by either Ca(2+) or Ba(2+) up to 500 microM but were blocked by higher concentrations of these cations. Potentiation by Ca(2+) was voltage-independent, whereas blockage was stronger at hyperpolarized than at depolarized potentials. Mg(2+) did not potentiate but it blocked ACh-evoked currents (IC(50)=0.38 mM). In the absence of Ca(2+), the EC(50) for ACh was higher (48 microM) than that obtained with 1.8 mM Ca(2+) (14.3 microM), suggesting that potentiation by Ca(2+) involves changes in the apparent affinity of the alpha 9 alpha 10 receptor for ACh.

Cholinergic innervation of the chick basilar papilla

Antibodies directed against choline acetyltransferase (ChAT), the synthesizing enzyme for acetylcholine (ACh) and a specific marker of cholinergic neurons, were used to label axons and nerve terminals of efferent fibers that innervate the chick basilar papilla (BP). Two morphologically distinct populations of cholinergic fibers were labeled and classified according to the region of the BP they innervated. The inferior efferent system was composed of thick fibers that coursed radially across the basilar membrane in small fascicles, gave off small branches that innervated short hair cells with large cup-like endings, and continued past the inferior edge of the BP to ramify extensively in the hyaline cell area. The superior efferent system was made up of a group of thin fibers that remained in the superior half of the epithelium and innervated tall hair cells with bouton endings. Both inferior and superior efferent fibers richly innervated the basal two thirds of the BP. However, the apical quarter of the chick BP was virtually devoid of efferent innervation except for a few fibers that gave off bouton endings around the peripheral edges. The distribution of ChAT-positive efferent endings appeared very similar to the population of efferent endings that labeled with synapsin antisera. Double labeling with ChAT and synapsin antibodies showed that the two markers colocalized in all nerve terminals that were identified in BP whole-mounts and frozen sections. These results strongly suggest that all of the efferent fibers that innervate the chick BP are cholinergic.

Tetraethylammonium ions block the nicotinic cholinergic receptors of cochlear outer hair cells

Cochlear outer hair cells (OHCs) express nicotinic acetylcholine receptors (nAChRs) whose calcium permeability allow the activation of co-localized Ca(2+)-sensitive K+ channels (SK-type). The large organic cation tetraethylammonium (TEA) is known to block at millimolar concentration voltage-gated and Ca(2+)-activated K+ currents in OHCs. In the present study, we show that extracellular TEA blocked much more efficiently, at micromolar concentrations, ACh-evoked K+ currents in isolated guinea pig OHCs. The dose-inhibition curve indicated an IC(50) of 60 microM, a value two orders of magnitude lower than the one reported on SK or BK channels. The site of the blocking action was on the extracellular side of the plasma membrane since 10 mM intracellular TEA did not prevent or change the characteristics of the ACh-evoked K+ current. The block of this K+ current in OHCs was mainly explained by a direct action of TEA at the nAChRs. Indeed, we demonstrated that extracellular TEA inhibited directly the ionotropic cation current flowing through the nAChRs (IC(50)=30 microM). This study demonstrated for the first time that extracellular TEA is an effective blocker of the OHCs' nAChRs.

Directional rectification of gap junctional voltage gating between dieters cells in the inner ear of guinea pig

Deiters cells (DCs) are the cochlear supporting cells in inner ear and contain multiple gap junction connexin genes, which when mutated can induce hearing loss. In the present study, the gap junctions between DCs were investigated by a double voltage clamp technique. Besides asymmetric responses to the polarities of transjunctional voltage (V(j)) and transmembrane potential (V(m)), the channels were also sensitive to which cell side was stimulated in a cell pair, i.e. voltage gating had directional dependence. The direction-dependent voltage gating could result in asymmetric current flow between the cells and influenced K(+) passage. Multiple connexins may constitute non-homotypic channels with directional dependence of voltage gating to mediate functional gap junction pathways in the cochlea. This may explain how a single connexin mutation can produce hearing loss.

Two types of voltage-dependent potassium channels in outer hair cells from the guinea pig cochlea

Cell-attached and cell-free configurations of the patch-clamp technique were used to investigate the conductive properties and regulation of the major K(+) channels in the basolateral membrane of outer hair cells freshly isolated from the guinea pig cochlea. There were two major voltage-dependent K(+) channels. A Ca(2+)-activated K(+) channel with a high conductance (220 pS, P(K)/P(Na) = 8) was found in almost 20% of the patches. The inside-out activity of the channel was increased by depolarizations above 0 mV and increasing the intracellular Ca(2+) concentration. External ATP or adenosine did not alter the cell-attached activity of the channel. The open probability of the excised channel remained stable for several minutes without rundown and was not altered by the catalytic subunit of protein kinase A (PKA) applied internally. The most frequent K(+) channel had a low conductance and a small outward rectification in symmetrical K(+) conditions (10 pS for inward currents and 20 pS for outward currents, P(K)/P(Na) = 28). It was found significantly more frequently in cell-attached and inside-out patches when the pipette contained 100 microM acetylcholine. It was not sensitive to internal Ca(2+), was inhibited by 4-aminopyridine, was activated by depolarization above -30 mV, and exhibited a rundown after excision. It also had a slow inactivation on ensemble-averaged sweeps in response to depolarizing pulses. The cell-attached activity of the channel was increased when adenosine was superfused outside the pipette. This effect also occurred with permeant analogs of cAMP and internally applied catalytic subunit of PKA. Both channels could control the cell membrane voltage of outer hair cells.

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.

Aminoglycoside ototoxicity and the medial efferent system: II. Comparison of acute effects of different antibiotics

Gentamicin (GM) has been shown to reversibly reduce the ability of contralateral noise to suppress ipsilateral cochlear activity, in a dose-dependent manner. However, during chronic administration of lower doses (60 mg/kg) the involvement of medial efferents could not be demonstrated. The purposes of the present study were to determine whether other aminoglycosides would display the same acute effects as GM and whether there was any correlation between their specificity and degree of cochlear and vestibular toxicity and their potency of blockade of the medial efferent system. Thus, we observed changes in ipsilateral ensemble background activity (EBA) of the VIIIth nerve without and with contralateral low level (55 dB SPL) broadband noise stimulation, in awake guinea pigs (GPs), before and after one single high-dose intramuscular injection of different aminoglycoside antibiotics (AAs) (gentamicin, amikacin, neomycin, netilmicin, streptomycin, tobramycin). For comparison, the effects of strychnine, a known antagonist of the efferent transmission and of cisplatin, an antineoplastic agent with cochleotoxic properties were also studied. Netilmicin displayed blocking properties similar to GM, although less pronounced, while amikacin and neomycin had no effect on medial efferent function. With tobramycin and streptomycin a decrease in suppression was usually associated with a reduction of the EBA measured without acoustic stimulation. However, with cisplatin, suppression was still effective when EBA was severely decreased. We could not observe specific effects of strychnine on medial efferent function. In conclusion, no correlation was found between specificity and degree of AA ototoxicity and their action on the medial efferent system.

Effects of protein kinase and phosphatase inhibitors on slow shortening of guinea pig cochlear outer hair cells

The intracellular mechanisms of slow shortening in isolated guinea pig cochlear outer hair cells were investigated using inhibitors and/or an activator of protein kinases and protein phosphatases. The slow shortening was induced by tetanic electrical field stimulation, and changes in the cell length, volume and intracellular Cl- concentration were microscopically monitored using a chloride-sensitive fluorescent dye. The slow shortening was inhibited by a calmodulin inhibitor, W-7, and a calcium calmodulin-dependent protein kinase II (CaMKII) inhibitor, KN-62. The inhibition by W-7 or KN-62, was abolished by the supplemented conductance of K+ with valinomycin. Among the protein phosphatase inhibitors tested, a type 1 and 2A protein phosphatase inhibitor, calyculin A, inhibited the slow shortening. The inhibition by calyculin A was abolished by the increased Cl- permeability, but neither by the increased K+ conductance with valinomycin nor by the increased Ca2+ conductance with A23187. A protein serine/threonine phosphatase activator, N-acetylsphingosine, inhibited the shortening, which was abolished by either valinomycin or a type 2A protein phosphatase inhibitor, okadaic acid, but not by calyculin A. These findings suggest the following signaling mechanisms in the slow shortening of outer hair cells; the K+ channel opening is facilitated through protein phosphorylation by CaMKII and suppressed via okadaic acid-sensitive dephosphorylation, and the Cl- channel opening depends on calyculin A-sensitive protein phosphatase activity.

Effects of potassium channel blockers on the acetylcholine-induced currents in dissociated outer hair cells of guinea pig cochlea

Much physiological evidence is available to show that acetylcholine (ACh) hyperpolarizes the outer hair cells (OHCs) of guinea pig cochlea and induces Ca2+-activated K+ currents. In this study, using the nystatin perforated patch-clamp technique, we investigated the effects of various K+ channel blockers on the ACh-induced currents (I[ACh]) in dissociated OHCs of guinea pig cochlea. The I(ACh) were inhibited by apamin in a concentration-dependent manner. The half-maximal inhibitory concentration for apamin on the ACh-induced response was 1.59 x 10(-9) M. Charybdotoxin and iberiotoxin had no inhibitory effect on the I(ACh) The inhibitory potency of the various K+ channel blockers on the I(ACh) was as follows: apamin >> quinine approximately quinidine approximately d-tubocurarine > tetraethylammonium chloride > 4-aminopyridine approximately Ba2+ > Cs2+. It is proposed that the Ca2+-activated K+ channels of mammalian cochlear OHCs should be classified as small conductance Ca2+-activated K+ (SK) channels according to their pharmacological properties.

Outwardly rectifying currents in guinea pig outer hair cells

Studies of K+ conductances in hair cells report that big-conductance Ca(2+)-dependent K+ (BK) channels carry parts of the outwardly rectifying currents. Lin et al. (1995) suggested that in guinea pig outer hair cells (OHCs) a portion of these currents is carried via a voltage-dependent and Ca(2+)-independent K+ channel. The present study tests the hypothesis that there are two separable current components of the outwardly rectifying currents by using patch clamp methods in OHCs to characterize the voltage dependence and sensitivity of the outwardly rectifying currents to channel blockers. Lowering of external Ca2+ caused no change in the currents while charybdotoxin (ChTx; 100 nM), a BK K+ channel blocker, and Cd2+ (200 microM), and L-type calcium channel blocker, abolished about 50% of the currents. Both ChTx and Cd2+ caused a depolarizing shift in the half-activation voltage paralleled by a decrease in the voltage sensitivity. 4-Aminopyridine (4-AP, 0.01 mM), an A-type and delayed rectifier type channel blocker, abolished about 50% of the currents and caused a hyperpolarizing shift in the half-activation voltage together with an increase in the voltage sensitivity. The outwardly rectifying currents were more sensitive to block by 4-AP at membrane voltages around 40 mV compared to voltages around -20 mV. The differences in the current characteristics may be due to two separate channel types, one of which is similar to the delayed rectifier type channels while the other may be similar to the BK Ca(2+)-dependent K+ channels. In addition, the largest outwardly rectifying currents were present in long OHCs with the smallest present in short OHCs.

Acetylcholine response in guinea pig outer hair cells. I. Properties of the response

The properties of the ACh (acetylcholine) response in guinea pig outer hair cells (OHCs) are not well understood. It has been shown that the response to ACh involves the activation of a Ca2+ dependent K+ selective conductance (referred to as Ksub where sub stands for suberyldicholine). In the present study, we examined the voltage dependence, the time dependence, and the desensitization of the ACh response. In addition, we examined the K+ selectivity of K(sub). These properties are important for aiding in the determination of the type of K+ channels activated by ACh. Patch-clamp technique in the whole-cell mode was used to record from single OHCs isolated from adult pigmented guinea pigs. ACh (100 microM) was applied to the voltage-clamped OHCs and the ACh induced currents (IACh) were measured. A voltage dependence of the ACh response was found with the ACh induced currents decaying monoexponentially at potentials positive to -30 mV. The decay of the ACh induced currents was faster soon after establishing the whole-cell mode of recording when compared to the decay of the currents some time later. This effect, referred to as the time dependence, was different from the desensitization of the response upon prolonged application of ACh. The desensitization of the ACh induced currents was about 50% after 2 min of continuous application of 100 microM ACh. The examined characteristics of the ACh response in guinea pig OHCs indicate a voltage and time dependence of the response and strong K+ selectivity of the Ksub.

Acetylcholine response in guinea pig outer hair cells. II. Activation of a small conductance Ca(2+)-activated K+ channel

The type of K+ channel involved in the acetylcholine (ACh) evoked response (Ksub; sub stands for suberyldicholine) in guinea pig outer hair cells (OHCs) is still uncertain. The present study tests the hypotheses that Ksub is one of the following: a big conductance Ca(2+)-dependent K+ channel (BK), a small conductance Ca(2+)-dependent K+ channel (SK), a KA type of K+ channel, or a Kn type of K+ channel. Patch-clamp technique in the whole-cell mode was used to record from single guinea pig OHCs. ACh (100 microM) was applied to voltage-clamped OHCs and the ACh-induced currents (IACh) were measured. Charybdotoxin (100 nM) had no effect on IACh, while apamin (1 microM) blocked more than 90% of IACh. Lowering the external Ca2+ concentration caused a hyperpolarizing shift of the IACh monitored as a function of the prepulse voltage. Increasing internal Mg2+ (Mgi2+) concentration caused a reduction in the outward IACh without affecting the inward IACh. The Ksub channel was found to be permeable to Cs+. In Cs+ solutions, IACh was 45% of the IACh in K+ solutions. The block of IACh by apamin, the dependence on extracellular Ca2+, the incomplete block of IACh by Cs+, and the ACh-induced Cs+ currents favor the hypothesis that Ksub belongs to the SK type of channels. An ionotropic/nicotinic nature of the ACh mechanism of action is favored. It is suggested that, in vivo, the amplitude of the ACh-induced hyperpolarization may depend on the Ca2+/Mg2+ ratio inside and outside the cell.

Differences in cholinergic responses from outer hair cells of rat and guinea pig

A cholinergic receptor on outer hair cells (OHC) in guinea pig cochlea induces a K+ current when it is activated by acetylcholine and suberyldicholine but not by nicotine or muscarine (Bobbin, 1995). This unusual receptor may contain an alpha 9-subunit. However, the pharmacology of the alpha 9-subunit cloned from rat and expressed in Xenopus oocytes does not completely match that obtained for the ACh receptor in guinea pig OHCs. The response to 1,1-dimethyl-4-phenylpiperazinium (DMPP) is large in guinea pig OHCs and small in oocytes containing receptors of the alpha 9-subunit. Therefore, we compared the effects of cholinergic receptor agonists in rat and guinea pig OHCs using the whole-cell variant of the patch-clamp technique. ACh caused the largest outward K+ current in OHCs from both rat and guinea pig. Carbachol- and suberyldicholine-induced responses were similar in magnitude in OHCs of rat and guinea pig. However, DMPP produced a small response in OHCs from rat and a large response in OHCs from guinea pig. At a concentration of 100 microM, muscarine, oxotremorine M, nicotine and cytisine induced little response in guinea pig OHCs and none in rat OHCs. Results suggest that the ACh receptor on rat OHCs is similar to the alpha 9-subunit-containing receptor expressed in oocytes but different from the ACh receptor on guinea pig OHCs.

Monitoring calcium in turtle hair cells with a calcium-activated potassium channel

1. An apamin-sensitive Ca(2+)-activated K+ channel was characterized in turtle hair cells and utilized to monitor submembranous intracellular Ca2+ and to evaluate the concentration of the mobile endogenous calcium buffer. 2. Isolated hair cells were voltage clamped with whole-cell patch electrodes filled with a Cs(+)-based intracellular solution to block the large-conductance Ca(2+)-activated K+ (BK) channel. Ca2+ currents evoked by depolarization were followed by inward tail currents lasting several hundred milliseconds. Both the Ca2+ current and slow tail current were abolished by nifedipine. 3. The tail current was carried by K+ and Cs+ (relative permeabilities PCa/PK = 0.22), and was fully blocked by 0.1 microM apamin and half blocked by 5 mM external TEA. These properties suggest the tail current flows through a Ca(2+)-activated K+ channel distinct from the BK channels. 4. Intracellular Ca2+ was imaged with a confocal microscope in hair cells filled with the indicator Calcium Green-5N introduced via the patch pipette. Increases in Ca2+ evoked by depolarization were localized to hotspots on the basolateral surface of the cell. The time course of the tail current closely matched the fast component of the fluorescenece monitored at a hotspot. 5. Ca(2+)-ATPase pump inhibitors thapsigargin, 2,4-di-(t-butyl)hydroquinone (BHQ) and vanadate, which are known to influence calcium regulation in turtle hair cells, prolonged the time course of the tail current, supporting the idea that the channel monitors cytoplasmic Ca2+. 6. The mobile endogenous buffer was estimated by combining perforated-patch and whole-cell recordings on a single cell. After recording tail currents with an amphotericin-perforated patch, the patch was ruptured to obtain the whole-cell mode, thus allowing washout of soluble cytoplasmic proteins and exchange with pipette buffers. By varying the concentration of Ca2+ buffer in the pipette, the mobile endogenous buffer was found to be equivalent to about 1 mM BAPTA.