The electrical properties of auditory hair cells in the frog amphibian papilla

Comparative study of ionic currents and exocytosis in hair cells of the basilar and amphibian papilla in bullfrogs

Hearing organs in the peripheral of different vertebrate species are extremely diverse in shape and function. In particular, while the basilar papilla (BP) is elongated and covers the sounds of both low and high frequencies in turtles and birds, it is round and responds to high frequencies only in frogs, leaving the low frequencies to the amphibian papilla (AP). In this study, we performed patch-clamp recordings in hair cells of both hearing organs in bullfrogs and conducted a comparative study of their ionic currents and exocytosis. Compared to hair cells in AP with a large tetraethylammonium (TEA)-sensitive slow-activating K⁺ current (I _K), those in BP exhibited a small 4-aminopyridine (4-AP)-sensitive fast-inactivating K⁺ current (I _A). Furthermore, hair cells in BP exhibited a significantly smaller Ca²⁺ current with a more positive half-activation voltage (V_half) and a slower slope of voltage dependency (k). In response to step depolarization, exocytosis (ΔC_m) in BP hair cells was also significantly smaller, but the Ca²⁺ efficiency, assessed with the ratio between ΔC_m and Ca²⁺ charge (Q_Ca), was comparable to that of AP hair cells. Finally, we applied a paired-step depolarization and varied the interval in between, and we found that the replenishment of synaptic vesicles was significantly slower in BP hair cells. Together, our findings suggest that hair cells tuned to high frequencies in bullfrogs release less synaptic vesicles and recycle synaptic vesicles more slowly, allowing them to cope well with the large DC component found in their receptor potentials in vivo.

DPOAEs and tympanal membrane vibrations reveal adaptations of the sexually dimorphic ear of the concave-eared torrent frog, Odorrana tormota

While most anuran species are highly vocal, few of them seem to be endowed with a complex call repertoire. Odorrana tormota, combines a remarkable vocalization complexity with auditory sensitivity over an extended spectral range spanning from audible to ultrasonic frequencies. This species is also exceptional for its ability to modify its middle ear tuning by closing the Eustachian tubes (ET). Using scanning laser Doppler vibrometry, the tympanal vibrations were measured to investigate if the tuning shift caused by the ET closure contributes to intraspecific acoustic communication. To gain insight into the inner ear frequency selectivity and sensitivity of this species, distortion product otoacoustic emissions were recorded at multiple frequency-level combinations. Our measurements of inner ear responses indicated that in O. tormota each sex is more sensitive to the frequencies of the other sex's vocalizations, female ears are more sensitive to 2-7 kHz, while male ears are more sensitive to 3-15 kHz. We also found that in both sexes the ET closure impacts the sensitivity of the middle and inner ear at frequencies used for communication with conspecifics. This study broadens our understanding of peripheral auditory mechanisms contributing to intraspecific acoustic communication in anurans.

Simultaneous Dual Recordings From Vestibular Hair Cells and Their Calyx Afferents Demonstrate Multiple Modes of Transmission at These Specialized Endings

In the vestibular periphery, transmission via conventional synaptic boutons is supplemented by post-synaptic calyceal endings surrounding Type I hair cells. This review focusses on the multiple modes of communication between these receptors and their enveloping calyces as revealed by simultaneous dual-electrode recordings. Classic orthodromic transmission is accompanied by two forms of bidirectional communication enabled by the extensive cleft between the Type I hair cell and its calyx. The slowest cellular communication low-pass filters the transduction current with a time constant of 10–100 ms: potassium ions accumulate in the synaptic cleft, depolarizing both the hair cell and afferent to potentials greater than necessary for rapid vesicle fusion in the receptor and potentially triggering action potentials in the afferent. On the millisecond timescale, conventional glutamatergic quantal transmission occurs when hair cells are depolarized to potentials sufficient for calcium influx and vesicle fusion. Depolarization also permits a third form of transmission that occurs over tens of microseconds, resulting from the large voltage- and ion-sensitive cleft-facing conductances in both the hair cell and the calyx that are open at their resting potentials. Current flowing out of either the hair cell or the afferent divides into the fraction flowing across the cleft into its cellular partner, and the remainder flowing out of the cleft and into the surrounding fluid compartment. These findings suggest multiple biophysical bases for the extensive repertoire of response dynamics seen in the population of primary vestibular afferent fibers. The results further suggest that evolutionary pressures drive selection for the calyx afferent.

Atypical tuning and amplification mechanisms in gecko auditory hair cells

Geckos are lizards capable of vocalization and can detect frequencies up to 5 kHz, but the mechanism of frequency discrimination is incompletely understood. The gecko’s auditory papilla has a unique arrangement over the high-frequency zone, with rows of mechanically sensitive hair bundles covered with gelatinous sallets. Lower-frequency hair cells are tuned by an electrical resonance employing Ca²⁺-activated K⁺ channels, but hair cells tuned above 1 kHz probably rely on a mechanical resonance of the sallets. The resonance may be boosted by an electromotile force from hair bundles found to be evoked by changes in hair cell membrane potential. This unusual mechanism operates independently of mechanotransduction and differs from mammals which amplify the mechanical input using the motor protein prestin.

The auditory papilla of geckos contains two zones of sensory hair cells, one covered by a continuous tectorial membrane affixed to the hair bundles and the other by discrete tectorial sallets each surmounting a transverse row of bundles. Gecko papillae are thought to encode sound frequencies up to 5 kHz, but little is known about the hair cell electrical properties or their role in frequency tuning. We recorded from hair cells in the isolated auditory papilla of the crested gecko, Correlophus ciliatus, and found that in both the nonsalletal region and part of the salletal region, the cells displayed electrical tuning organized tonotopically. Along the salletal zone, occupying the apical two-thirds of the papilla, hair bundle length decreased threefold and stereociliary complement increased 1.5-fold. The two morphological variations predict a 13-fold gradient in bundle stiffness, confirmed experimentally, which, when coupled with salletal mass, could provide passive mechanical resonances from 1 to 6 kHz. Sinusoidal electrical currents injected across the papilla evoked hair bundle oscillations at twice the stimulation frequency, consistent with fast electromechanical responses from hair bundles of two opposing orientations across the papilla. Evoked bundle oscillations were diminished by reducing Ca²⁺ influx, but not by blocking the mechanotransduction channels or inhibiting prestin action, thereby distinguishing them from known electromechanical mechanisms in hair cells. We suggest the phenomenon may be a manifestation of an electromechanical amplification that augments the passive mechanical tuning of the sallets over the high-frequency region.

Diverse Mechanisms of Sound Frequency Discrimination in the Vertebrate Cochlea

Discrimination of different sound frequencies is pivotal to recognizing and localizing friend and foe. Here, I review the various hair cell-tuning mechanisms used among vertebrates. Electrical resonance, filtering of the receptor potential by voltage-dependent ion channels, is ubiquitous in all non-mammals, but has an upper limit of ~1 kHz. The frequency range is extended by mechanical resonance of the hair bundles in frogs and lizards, but may need active hair-bundle motion to achieve sharp tuning up to 5 kHz. Tuning in mammals uses somatic motility of outer hair cells, underpinned by the membrane protein prestin, to expand the frequency range. The bird cochlea may also use prestin at high frequencies, but hair cells <1 kHz show electrical resonance.

Soft Tissue Conduction: Review, Mechanisms, and Implications

Soft tissue conduction (STC) is a recently explored mode of auditory stimulation, complementing air (AC) and bone (BC) conduction stimulation. STC can be defined as the hearing induced when vibratory stimuli reach skin and soft tissue sites not directly overlying skull bone such as the head, neck, thorax, and body. Examples of STC include the delivery of vibrations to the skin of parts of the body by a clinical bone vibrator, hearing underwater sounds and free field air sounds, while AC hearing is attenuated by earplugs. The vibrations induced in the soft tissues are apparently transmitted along soft tissues, reaching, and exciting the ear. Further research is required to determine whether the mechanism of the final stage of STC hearing involves the excitation of the ear by eliciting inner ear fluid pressures that activate the hair cells directly, by the induction of skull bone vibrations, or by a combination of both mechanisms, depending on the magnitude of each mechanism.

Sensory Hair Cells: An Introduction to Structure and Physiology

Sensory hair cells are specialized secondary sensory cells that mediate our senses of hearing, balance, linear acceleration, and angular acceleration (head rotation). In addition, hair cells in fish and amphibians mediate sensitivity to water movement through the lateral line system, and closely related electroreceptive cells mediate sensitivity to low-voltage electric fields in the aquatic environment of many fish species and several species of amphibian. Sensory hair cells share many structural and functional features across all vertebrate groups, while at the same time they are specialized for employment in a wide variety of sensory tasks. The complexity of hair cell structure is large, and the diversity of hair cell applications in sensory systems exceeds that seen for most, if not all, sensory cell types. The intent of this review is to summarize the more significant structural features and some of the more interesting and important physiological mechanisms that have been elucidated thus far. Outside vertebrates, hair cells are only known to exist in the coronal organ of tunicates. Electrical resonance, electromotility, and their exquisite mechanical sensitivity all contribute to the attractiveness of hair cells as a research subject.

Probing electrical tuning of hair cells with a Zap current method in the intact amphibian papilla of bullfrogs

Most, if not all, modern vertebrate species have evolved exquisite inner ears to discriminate acoustic signals of different frequencies, through a process called frequency tuning. For non-mammalian species, at least part of frequency tuning has been attributed to intrinsic electrical properties of hair cells, i.e. electrical tuning. Since it was first discovered, the traditional method to assess electrical tuning has been to inject step current into hair cells and examine dampened membrane voltage oscillation. However, this method is not applicable for hair cells that do not oscillate. In this study, we developed a Zap current method that can be unbiasedly applied to all hair cells regardless of their oscillating behavior. Similar to a chirp sound in acoustic stimulation, a Zap current is a sinusoidal current with the frequency increased linearly with time. We first validated this new method with the traditional step current method on hair cells with dampened membrane voltage oscillation, and then applied it to all hair cells in the intact amphibian papilla of bullfrogs. We found that while hair cells with dampened membrane voltage oscillation are sharply tuned, non-oscillating hair cells are broadly tuned. In addition, we found a third type of hair cells, which oscillate continuously and are extremely sharply tuned, with multiple peaks that are reminiscent of harmonics in the mammalian cochlea. In conclusion, the new Zap current method provides an unbiased way to assess electrical tuning, and it reveals an underappreciated heterogeneity of electrical tuning in the bullfrog amphibian papilla.

BK Channels in the Vertebrate Inner Ear

The perception of complex acoustic stimuli begins with the deconstruction of sound into its frequency components. This spectral processing occurs first and foremost in the inner ear. In vertebrates, two very different strategies of frequency analysis have evolved. In nonmammalian vertebrates, the sensory hair cells of the inner ear are intrinsically electrically tuned to a narrow band of acoustic frequencies. This electrical tuning relies on the interplay between BK channels and voltage-gated calcium channels. Systematic variations in BK channel density and kinetics establish a gradient in electrical resonance that enables the coding of a broad range of acoustic frequencies. In contrast, mammalian hair cells are extrinsically tuned by mechanical properties of the cochlear duct. Even so, mammalian hair cells also express BK channels. These BK channels play critical roles in various aspects of mammalian auditory signaling, from developmental maturation to protection against acoustic trauma. This review summarizes the anatomical localization, biophysical properties, and functional contributions of BK channels in vertebrate inner ears. Areas of future research, based on an updated understanding of the biology of both BK channels and the inner ear, are also highlighted. Investigation of BK channels in the inner ear continues to provide fertile research grounds for examining both BK channel biophysics and the molecular mechanisms underlying signal processing in the auditory periphery.

Phase-locking precision is enhanced by multiquantal release at an auditory hair cell ribbon synapse

Sound-evoked spikes in the auditory nerve can phase-lock with submillisecond precision for prolonged periods of time. However, the synaptic mechanisms that enable this accurate spike firing remain poorly understood. Using paired recordings from adult frog hair cells and their afferent fibers, we show here that during sine-wave stimuli, synaptic failures occur even during strong stimuli. However, exclusion of these failures leads to mean excitatory postsynaptic current (EPSC) amplitudes that are independent of Ca(2+) current. Given the intrinsic jitter in spike triggering, evoked synaptic potentials and spikes had surprisingly similar degrees of synchronization to a sine-wave stimulus. This similarity was explained by an unexpected finding: large-amplitude evoked EPSCs have a significantly larger synchronization index than smaller evoked EPSCs. Large EPSCs therefore enhance the precision of spike timing. The hair cells' unique capacity for continuous, large-amplitude, and highly synchronous multiquantal release thus underlies its ability to trigger phase-locked spikes in afferent fibers.

Exocytosis in the frog amphibian papilla

Using whole-cell patch-clamp recordings, we measured changes in membrane capacitance (ΔC (m)) in two subsets of hair cells from the leopard frog amphibian papilla (AP): the low-frequency (100-500 Hz), rostral hair cells and the high-frequency (500-1200 Hz), caudal hair cells, in order to investigate tonotopic differences in exocytosis. Depolarizations of both rostral and caudal hair cells evoked robust ΔC (m) responses of similar amplitude. However, the calcium dependence of release, i.e., the relationship between ΔC (m) relative to the amount of calcium influx (Q (Ca) (2+)), was found to be linear in rostral hair cells but supra-linear in caudal hair cells. In addition, the higher numbers of vesicles released at caudal hair cell active zones suggests increased temporal precision of caudal hair cell exocytosis. ΔC (m) responses were also obtained in response to sinusoidal stimuli of varying frequency, but neither rostral nor caudal hair cell ΔC (m) revealed any frequency selectivity. While all AP hair cells express both otoferlin and synaptotagmin IV (SytIV), we obtained evidence of a tonotopic distribution of the calcium buffer calretinin which may further increase temporal resolution at the level of the hair cell synapse. Our findings suggest that the low (rostral) and high (caudal) frequency hair cells apply different mechanisms for fine-tuning exocytosis.

Inner ear morphological correlates of ultrasonic hearing in frogs

Three species of anuran amphibians (Odorrana tormota, Odorrana livida and Huia cavitympanum) have recently been found to detect ultrasounds. We employed immunohistochemistry and confocal microscopy to examine several morphometrics of the inner ear of these ultrasonically sensitive species. We compared morphological data collected from the ultrasound-detecting species with data from Rana pipiens, a frog with a typical anuran upper cut-off frequency of ∼3 kHz. In addition, we examined the ears of two species of Lao torrent frogs, Odorrana chloronota and Amolops daorum, that live in an acoustic environment approximating those of ultrasonically sensitive frogs. Our results suggest that the three ultrasound-detecting species have converged on small-scale functional modifications of the basilar papilla (BP), the high-frequency hearing organ in the frog inner ear. These modifications include: 1. reduced BP chamber volume, 2. reduced tectorial membrane mass, 3. reduced hair bundle length, and 4. reduced hair cell soma length. While none of these factors on its own could account for the US sensitivity of the inner ears of these species, the combination of these factors appears to extend their hearing bandwidth, and facilitate high-frequency/ultrasound detection. These modifications are also seen in the ears of O. chloronota, suggesting that this species is a candidate for high-frequency hearing sensitivity. These data form the foundation for future functional work probing the physiological bases of ultrasound detection by a non-mammalian ear.

Structural and functional effects of acoustic exposure in goldfish: evidence for tonotopy in the teleost saccule

BACKGROUND

Mammalian and avian auditory hair cells display tonotopic mapping of frequency along the length of the cochlea and basilar papilla. It is not known whether the auditory hair cells of fishes possess a similar tonotopic organization in the saccule, which is thought to be the primary auditory receptor in teleosts. To investigate this question, we determined the location of hair cell damage in the saccules of goldfish (Carassius auratus) following exposure to specific frequencies. Subjects were divided into six groups of six fish each (five treatment groups plus control). The treatment groups were each exposed to one of five tones: 100, 400, 800, 2000, and 4000 Hz at 176 dB re 1 μPa root mean squared (RMS) for 48 hours. The saccules of each fish were dissected and labeled with phalloidin in order to visualize hair cell bundles. The hair cell bundles were counted at 19 specific locations in each saccule to determine the extent and location of hair cell damage. In addition to quantification of anatomical injury, hearing tests (using auditory evoked potentials) were performed on each fish immediately following sound exposure. Threshold shifts were calculated by subtracting control thresholds from post-sound exposure thresholds.

RESULTS

All sound-exposed fish exhibited significant hair cell and hearing loss following sound exposure. The location of hair cell loss varied along the length of the saccule in a graded manner with the frequency of sound exposure, with lower and higher frequencies damaging the more caudal and rostral regions of the saccule, respectively. Similarly, fish exposed to lower frequency tones exhibited greater threshold shifts at lower frequencies, while high-frequency tone exposure led to hearing loss at higher frequencies. In general, both hair cell and hearing loss declined as a function of increasing frequency of exposure tone, and there was a significant linear relationship between hair cell loss and hearing loss.

CONCLUSIONS

The pattern of hair cell loss as a function of exposure tone frequency and saccular rostral-caudal location is similar to the pattern of hearing loss as a function of exposure tone frequency and hearing threshold frequency. This data suggest that the frequency analysis ability of goldfish is at least partially driven by peripheral tonotopy in the saccule.

Imaging electrical resonance in hair cells

The mechanosensory hair cells of many auditory receptor organs are tuned by an electrical resonance that increases their responses to stimulation over a narrow band of frequencies. The small oscillations of membrane potential characteristic of this phenomenon have previously been detectable only through intracellular electrode measurements, which are laborious and preclude analysis at the level of an entire sensory organ. We used a voltage-sensitive dye to image hair-cell electrical resonance in an intact preparation of the bullfrog's sacculus, a receptor organ sensitive to low-frequency seismic and auditory stimuli. Imaging revealed distinct populations of hair cells whose resonant response varied with the frequency of transepithelial electrical stimulation. Most of the hair cells in the saccular epithelium in vitro were electrically tuned to stimulation at 25-50 Hz. The frequency dependence of the fluorescence signal was sensitive to pharmacological blockade of large-conductance Ca(2+)-sensitive K(+) channels and to enzymatic digestion. At an elevated concentration of Ca(2+), we observed transient fluorescence signals that probably represented action potentials. The stroboscopic imaging and analysis techniques described here present a general approach for studying subthreshold oscillations in electrically excitable cells.

Input-output characteristics of the tectorial membrane in the frog basilar papilla

The basilar papilla (BP) in the frog inner ear is a relatively simple auditory receptor. Its hair cells are embedded in a stiff support structure, with the stereovilli connecting to a flexible tectorial membrane (TM). Acoustic energy passing the papilla presumably causes displacement of the TM, which in turn deflects the stereovilli and stimulates the hair cells. In this paper we present optical measurements of the mechanical response of the TM to various stimulus levels. Results were obtained from 3 specimens (4 ears). The phase of the displaced area of the TM was constant across stimulus levels. Phase differences between the orthogonal spatial motion components were either close to 0 degrees or 180 degrees. These findings were consistent with a TM motion along the epithelium surface. The TM response was linear for stimulus levels up to -30 dB (re. 1 microm) at the operculum. This amplitude was estimated to exceed that at which neural responses saturate. Apparently, saturation of the neural response in the frog inner ear is not based on saturation of the mechanical response of the tectorial membrane.

Long-term consequences of Sox9 depletion on inner ear development

The transcription factor Sox9 has been implicated in inner ear formation in several species. To investigate the long-term consequences of Sox9 depletion on inner ear development we analyzed the inner ear architecture of Sox9-depleted Xenopus tadpoles generated by injection of increasing amounts of Sox9 morpholino antisense oligonucleotides. We found that Sox9-depletion resulted in major defects in the development of vestibular structures, semicircular canals and utricle, while the ventrally located saccule was less severely affected in these embryos. Consistent with this phenotype, we observed a specific loss of the dorsal expression of Wnt3a expression in the otic vesicle of Sox9 morphants, associated with an increase in cell death and a reduction in cell proliferation in the region of the presumptive otic epithelium. We propose that, in addition to its early role in placode specification, Sox9 is also required for the maintenance of progenitors in the otic epithelium.

Mechanics of the frog ear

The frog inner ear contains three regions that are sensitive to airborne sound and which are functionally distinct. (1) The responses of nerve fibres innervating the low-frequency, rostral part of the amphibian papilla (AP) are complex. Electrical tuning of hair cells presumably contributes to the frequency selectivity of these responses. (2) The caudal part of the AP covers the mid-frequency portion of the frog's auditory range. It shares the ability to generate both evoked and spontaneous otoacoustic emissions with the mammalian cochlea and other vertebrate ears. (3) The basilar papilla functions mainly as a single auditory filter. Its simple anatomy and function provide a model system for testing hypotheses concerning emission generation. Group delays of stimulus-frequency otoacoustic emissions (SFOAEs) from the basilar papilla are accounted for by assuming that they result from forward and reverse transmission through the middle ear, a mechanical delay due to tectorial membrane filtering and a rapid forward and reverse propagation through the inner ear fluids, with negligible delay.

Distribution of high-conductance calcium-activated potassium channels in rat vestibular epithelia

Voltage- and calcium-activated potassium channels (BK) are important regulators of neuronal excitability. BK channels seem to be crucial for frequency tuning in nonmammalian vestibular and auditory hair cells. However, there are a paucity of data concerning BK expression in mammalian vestibular hair cells. We therefore investigated the localization of BK channels in mammalian vestibular hair cells, specifically in rat vestibular neuroepithelia. We find that only a subset of hair cells in the utricle and the crista ampullaris express BK channels. BK-positive hair cells are located mainly in the medial striolar region of the utricle, where they constitute at most 12% of hair cells, and in the central zone of the horizontal crista. A majority of BK-positive hair cells are encapsulated by a calretinin-positive calyx defining them as type I cells. The remainder are either type I cells encapsulated by a calretinin-negative calyx or type II hair cells. Surprisingly, the number of BK-positive hair cells in the utricle peaks in juvenile rats and declines in early adulthood. BK channels were not found in vestibular afferent dendrites or somata. Our data indicate that BK channel expression in the mammalian vestibular system differs from the expression pattern in the mammalian auditory and the nonmammalian vestibular system. The molecular diversity of vestibular hair cells indicates a functional diversity that has not yet been fully characterized. The predominance of BK-positive hair cells within the medial striola of juvenile animals suggests that they contribute to a scheme of highly lateralized coding of linear head movements during late development.

Monitoring transient Ca2+ dynamics with large-conductance Ca2+-dependent K+ channels at active zones in frog saccular hair cells

Neurotransmitter release from the basolateral surface of auditory and vestibular hair cells is mediated by Ca(2+) influx through voltage-gated Ca(2+) channels. Co-localization of large-conductance Ca(2+)-activated K(+) (BK) channels at the active zones of these cells affords them with an optimal location to act as reporters of the Ca(2+) concentration changes at active zones of transmitter release. In this report we use BK channels in frog (Rana pipiens) hair cells to monitor dynamic changes in intracellular Ca(2+) concentration during transient influxes of Ca(2+), showing that BK current magnitude and delay to onset are correlated with the rate and duration of Ca(2+) entry through Ca(2+) channels. We also show that BK channels exhibit a much higher Ca(2+) binding affinity in the open state than in the closed state.

Multifrequency forcing of a Hopf oscillator model of the inner ear

In response to a sound stimulus, the inner ear emits sounds called otoacoustic emissions. While the exact mechanism for the production of otoacoustic emissions is not known, active motion of individual hair cells is thought to play a role. Two possible sources for otoacoustic emissions, both localized within individual hair cells, include somatic motility and hair bundle motility. Because physiological models of each of these systems are thought to be poised near a Hopf bifurcation, the dynamics of each can be described by the normal form for a system near a Hopf bifurcation. Here we demonstrate that experimental results from three-frequency suppression experiments can be predicted based on the response of an array of noninteracting Hopf oscillators tuned at different frequencies. This supports the idea that active motion of individual hair cells contributes to active processing of sounds in the ear. Interestingly, the model suggests an explanation for differing results recorded in mammals and nonmammals.

Mechanics of the exceptional anuran ear

The anuran ear is frequently used for studying fundamental properties of vertebrate auditory systems. This is due to its unique anatomical features, most prominently the lack of a basilar membrane and the presence of two dedicated acoustic end organs, the basilar papilla and the amphibian papilla. Our current anatomical and functional knowledge implies that three distinct regions can be identified within these two organs. The basilar papilla functions as a single auditory filter. The low-frequency portion of the amphibian papilla is an electrically tuned, tonotopically organized auditory end organ. The high-frequency portion of the amphibian papilla is mechanically tuned and tonotopically organized, and it emits spontaneous otoacoustic emissions. This high-frequency portion of the amphibian papilla shows a remarkable, functional resemblance to the mammalian cochlea.

Kvbeta1.1 associates with Kvalpha1.4 in Chinese hamster ovary cells and pigeon type II vestibular hair cells and enhances the amplitude, inactivation and negatively shifts the steady-state inactivation range

Although A-type potassium currents are found in type II hair cells in the inner ear of most species, the molecular mechanisms for activation and inactivation of the A-type potassium current (I(A)) remain unknown. In frog semicircular canal hair cells, for example, there appear to be two classes of currents having either fast or slow inactivation [Norris CH, Ricci AJ, Housley GD, Guth PS (1992) The inactivating potassium currents of hair cells isolated from the crista ampullaris of the frog. J Neurophysiol 68:1642-1653; Russo G, Calzi D, Martini M, Rossi ML, Fesce R, Prigioni I (2007) Potassium currents in the hair cells of vestibular epithelium: position-dependent expression of two types of A channels. Eur J Neurosci 25:695-704]. It has been suggested that somehow the "ball and chain" mechanism (NH(3) (N) terminus motif) is modified by alternative splicing to account for the two classes of inactivation. To examine other possibilities, we cloned alpha and beta subunits that comprise the A-type potassium channel complex in adult and embryonic pigeon brain, cochlea and labyrinth. By sequence homology, we concluded that the subunits present were Kvalpha1.4 and Kvbeta1.1. The sequence of the open reading frame for Kvalpha1.4 contained the N-terminus, pore and COOH (C) terminus motifs for N-and C-type inactivation. The sequence for Kvbeta1.1 displayed amino acids consistent with assembly and association with Kvalpha1.4 alpha subunits. Kvalpha1.4 and Kvbeta1.1 were transfected either singly or in combination into Chinese hamster ovary (CHO) cells. These cells and native hair cells from the pigeon utricle were patch clamped and the inactivation properties of the A-type current were studied. In the native hair cells, the A-type current was identified by its pharmacological (4-aminopyridine (4-AP); IC(50)=11 microM) and voltage dependent inactivation properties. A comparison of the mean time constants from best-fitted single exponential and sum of two exponential equations to the ionic current inactivation revealed the following. In CHO cells when Kvalpha1.4 was expressed alone, the mean time constant (tau(1)=107 ms+/-19, N=32) was significantly (P<0.001) longer and the mean peak amplitude (2.28 nA+/-0.39, N=32) was smaller than when Kvalpha1.4 and Kvbeta1.1 were expressed in CHO cells. Moreover, the co-transfection of Kvalpha1.4 and Kvbeta1.1 into CHO cells caused a shift in the steady state inactivation curve parameter Vo 30 mV in the hyperpolarized direction relative to CHO cells expressing only Kvalpha1.4. Similarly, Kvalpha1.4-transfected CHO cells produced longer time constants and smaller amplitudes than those found for native utricular hair cells. These data lead us to conclude that while the amino acid motifs are present in Kvalpha1.4 and Kvbeta1.1 to suggest N-and C-type inactivation, co-assembly and association of Kvalpha1.4 and Kvbeta1.1 may also produce changes in the time dependent inactivation properties of vestibular hair cells.

Potassium currents in the hair cells of vestibular epithelium: position-dependent expression of two types of A channels

The complement of voltage-dependent K+ currents was investigated in hair cells of the frog crista ampullaris. The currents were recorded in transversal slices of the peripheral, intermediate and central regions of the crista by applying the patch clamp technique to cells located at different positions in the slices. Voltage-clamp recordings confirmed that cells located in each region have a distinctive complement of K+ channels. Detailed investigation of the currents in each region revealed that the complement of K+ channels in intermediate and central regions showed no variations among cells, whereas peripheral hair cells differed in the expression of two classes of A-type currents. These currents showed different kinetics of inactivation as well as steady-state inactivation properties. We termed these currents fast I(A) and slow I(A) based on their inactivation speed. The magnitude of both currents exhibited a significant gradient along the transversal axis of the peripheral regions. Fast I(A) magnitude was maximal in cells located in the external zone of the crista slice and decreased gradually to become very small in the median zone (centre) of the section, while the gradient of slow I(A) magnitude was reversed. A-type currents appear to act as a transient buffer that opposes hair cell depolarization induced by positive current injections. However, fast I(A) is partially active at the cell resting potential, while slow I(A) can be recruited only following large hyperpolarizations. Thus, two types of A currents are differentially distributed in vestibular hair cells and have different roles in shaping receptor potential.

Stimulus frequency otoacoustic emissions in the Northern leopard frog, Rana pipiens pipiens: implications for inner ear mechanics

Otoacoustic emissions (OAEs) are weak sounds that originate from the inner ear which are traditionally classified/named based on their evoking stimulus. Recently, it has been argued that such a classification, at least for mammals, misrepresents the underlying mechanisms of emission-generation. As an alternative classification, it has been suggested to recognize that OAEs arise either via nonlinear distortion or linear coherent reflection. For non-mammalian vertebrates, data on evoked OAEs that arise via the latter mechanism are largely missing. Here, we present the first measurements of stimulus frequency OAEs (SFOAEs), which are emissions thought to arise via linear coherent reflection, from an amphibian (the Northern leopard frog, Rana pipiens pipiens). Their properties as a function of the evoking stimulus frequencies and levels are described and subsequently compared with the previously reported properties of distortion product OAEs (DPOAEs) from the same frog species.

Has the evolution of complexity in the amphibian papilla influenced anuran speciation rates?

For anurans, increasing complexity of the inner ear has been correlated with speciation rates. The evolution of a complex amphibian papilla (AP) is thought to have facilitated speciation by extending the range of frequencies over which mating calls may diverge. Although this example has been proposed to represent a key innovation, the mechanism by which the AP is thought to promote speciation makes the questionable assumption that anurans generally use the AP for detection of their mating calls. This study uses mating calls from 852 species to test this assumption. Surprisingly, the calls of most species are not detected by the AP but by a second organ, the basilar papilla (BP). This refutes the role of AP complexity in facilitating call divergence and hence, speciation. Future research into the evolution of acoustically mediated reproductive isolation should focus instead on the BP as it may play a more critical role in anuran speciation.

Transfer characteristics of the hair cell's afferent synapse

The sense of hearing depends on fast, finely graded neurotransmission at the ribbon synapses connecting hair cells to afferent nerve fibers. The processing that occurs at this first chemical synapse in the auditory pathway determines the quality and extent of the information conveyed to the central nervous system. Knowledge of the synapse's input-output function is therefore essential for understanding how auditory stimuli are encoded. To investigate the transfer function at the hair cell's synapse, we developed a preparation of the bullfrog's amphibian papilla. In the portion of this receptor organ representing stimuli of 400-800 Hz, each afferent nerve fiber forms several synaptic terminals onto one to three hair cells. By performing simultaneous voltage-clamp recordings from presynaptic hair cells and postsynaptic afferent fibers, we established that the rate of evoked vesicle release, as determined from the average postsynaptic current, depends linearly on the amplitude of the presynaptic Ca(2+) current. This result implies that, for receptor potentials in the physiological range, the hair cell's synapse transmits information with high fidelity.

Ion channel regulation of the dynamical instability of the resting membrane potential in saccular hair cells of the green frog (Rana esculenta)

AIMS

We investigated the ion channel regulation of the resting membrane potential of hair cells with the aim to determine if the resting membrane potential is poised close to instability and thereby a potential cause of the spontaneous afferent spike activity.

METHODS

The ionic mechanism and the dynamic properties of the resting membrane potential were examined with the whole-cell patch clamp technique in dissociated saccular hair cells and in a mathematical model including all identified ion channels.

RESULTS

In hair cells showing I/V curves with a low membrane conductance flanked by large inward and outward rectifying potassium conductances, the inward rectifier (K(IR)), the delayed outward rectifier (K(V)) and the large conductance, calcium-sensitive, voltage-gated potassium channel (BK(Ca)) were all activated at rest. Under current clamp conditions, the outward current through these channels balanced the inward current through mechano-electrical transduction (MET) and Ca2+ channels. In 45% (22/49) of the cells, the membrane potential fluctuated spontaneously between two voltage levels determined by the voltage extent of the low membrane conductance range. These fluctuations were not influenced by blocking the MET channels but could be reversibly stopped by increasing [K+]o or by blocking of K(IR) channels. Blocking the BK(Ca) channels induced regular voltage oscillations.

CONCLUSIONS

Two intrinsic dynamical instabilities of V(m) are present in hair cells. One of these is observed as spontaneous voltage fluctuations by currents through K(IR), K(V) and h-channels in combination with a steady current through MET channels. The other instability shows as regenerative voltage changes involving Ca2+ and K(V) channels. The BK(Ca) channels prevent the spontaneous voltage fluctuations from activating the regenerative system.

Effects of cinnarizine on calcium and pressure-dependent potassium currents in guinea pig vestibular hair cells

In vestibular hair cells, K+ currents induced by rises in hydrostatic pressure have recently been demonstrated. These currents are inhibited by charybdotoxin, a blocker of Ca2+-dependent K+ channels. On the other hand, cinnarizine is a blocker of voltage-gated Ca2+ currents in hair cells and is used as a drug in conditions with vestibular vertigo. Our aim was to test in patch-clamp experiments (conventional whole-cell mode) whether cinnarizine, by reducing Ca2+ influx, inhibited Ca2+ and pressure-sensitive K+ currents in vestibular type-II hair cells of guinea pigs. A quantitatively similar inhibition of K+ currents was evoked by extracellular Ca2+ removal, cinnarizine (0.5 microM), and the L-type Ca2+ channel blocker nifedipine (3 microM). Cinnarizine abrogated increases of K+ currents induced by increases in the hydrostatic pressure (from 0.2 to 0.5 cm H2O). At a higher concentration (1 microM), cinnarizine elicited K+ current inhibitions larger than those elicited by Ca2+ removal. Moreover, it reduced K+ currents in the absence of Ca2+, in contrast to nifedipine. However, charybdotoxin abolished these effects of cinnarizine. We thus conclude that cinnarizine inhibits, by two mechanisms, pressure-induced currents that are sensitive to charybdotoxin and Ca2+. It reduces Ca2+ influx and exerts a Ca2+-independent inhibition, with a lower IC50 than that required for Ca2+ channel blockade. These two actions may importantly contribute to its therapeutic effects.

Level dependence of distortion product otoacoustic emissions in the leopard frog, Rana pipiens pipiens

The inner ear of frogs holds two papillae specialized in detecting airborne sound, the amphibian papilla (AP) and the basilar papilla (BP). We measured input-output (I/O) curves of distortion product otoacoustic emissions (DPOAEs) from both papillae, and compared their properties. As in other vertebrates, DPOAE I/O curves showed two distinct segments, separated by a notch or kneepoint. The slope of the low-level segment was conspicuously different between the AP and the BP. For DPOAE I/O curves from the AP, slopes were < or = 1 dB/dB, similar to what is found in mammals, birds and some lizards. For DPOAE I/O curves from the BP these slopes were much steeper (approximately 2 dB/dB). Slopes found at high stimulus levels were similar in the AP and the BP (approximately 2 dB/dB). This quantitative difference between the low-level slopes for DPOAEs from the AP and the BP may signify the involvement of different mechanisms in low-level DPOAE generation for the two papillae, respectively.

Differences between the negatively activating potassium conductances of Mammalian cochlear and vestibular hair cells

Cochlear and type I vestibular hair cells of mammals express negatively activating potassium (K(+)) conductances, called g(K,n) and g(K,L) respectively, which are important in setting the hair cells' resting potentials and input conductances. It has been suggested that the channels underlying both conductances include KCNQ4 subunits from the KCNQ family of K(+) channels. In whole-cell recordings from rat hair cells, we found substantial differences between g(K,n) and g(K,L) in voltage dependence, kinetics, ionic permeability, and stability during whole-cell recording. Relative to g(K,L), g(K,n) had a significantly broader and more negative voltage range of activation and activated with less delay and faster principal time constants over the negative part of the activation range. Deactivation of g(K,n) had an unusual sigmoidal time course, while g(K,L) deactivated with a double-exponential decay. g(K,L), but not g(K,n), had appreciable permeability to Cs(+). Unlike g(K,L), g(K,n)'s properties did not change ("wash out") during the replacement of cytoplasmic solution with pipette solution during ruptured-patch recordings. These differences in the functional expression of g(K,n) and g(K,L) channels suggest that there are substantial differences in their molecular structure as well.

Molecular characterization of an inward rectifier channel (IKir) found in avian vestibular hair cells: cloning and expression of pKir2.1

A fast inwardly rectifying current has been observed in some of the sensory cells (hair cells) of the inner ear of several species. While the current was presumed to be an IKir current, contradictory evidence existed as to whether the cloned channel actually belonged to the Kir2.0 subfamily of potassium inward rectifiers. In this paper, we report for the first time converging evidence from electrophysiological, biochemical, immunohistochemical, and genetic studies that show that the Kir2.1 channel carries the fast inwardly rectifying currents found in pigeon vestibular hair cells. Following cytoplasm extraction from single type II and multiple pigeon vestibular hair cells, mRNA was reverse transcribed, amplified, and sequenced. The open reading frame (ORF), consisting of a 1,284-bp nucleotide sequence, showed 94, 85, and 83% identity with Kir2.1 subunit sequences from chick lens, Kir2 sequences from human heart, and a mouse macrophage cell line, respectively. Phylogenetic analyses revealed that pKir2.1 formed an immediate node with hKir2.1 but not with hKir2.2-2.4. Hair cells (type I and type II) and supporting cells in the sensory epithelium reacted positively with a Kir2.1 antibody. The whole cell current recorded in oocytes and CHO cells, transfected with pigeon hair cell Kir2.1 (pKir2.1), demonstrated blockage by Ba2+ and sensitivity to changing K+ concentration. The mean single-channel linear slope conductance in transfected CHO cells was 29 pS. The open dwell time was long (approximately 300 ms at -100 mV), and the closed dwell time was short (approximately 34 ms at -100 mV). Multistates ranging from 3-6 were noted in some single-channel responses. All of the above features have been described for other Kir2.1 channels. Current clamp studies of native pigeon vestibular hair cells illustrated possible physiological roles of the channel and showed that blockage of the channel by Ba2+ depolarized the resting membrane potential by approximately 30 mV. Negative currents hyperpolarized the membrane approximately 20 mV before block but approximately 60 mV following block. RT-PCR studies revealed that the pKir2.1 channels found in pigeon vestibular hair cells were also present in pigeon vestibular nerve, vestibular ganglion, lens, neck muscle, brain (brain stem, cerebellum and optic tectum), liver, and heart.

Voltage-gated outward K currents in frog saccular hair cells

A biophysical analysis of the voltage-gated K (Kv) currents of frog saccular hair cells enzymatically isolated with bacterial protease VIII was carried out, and their contribution to the cell electrical response was addressed by a modeling approach. Based on steady-state and kinetic properties of inactivation, two distinct Kv currents were found: a fast inactivating IA and a delayed rectifier IDRK. IA exhibited a strongly hyperpolarized inactivation V(1/2) (-83 mV), a relatively rapid single exponential recovery from inactivation (taurec of approximately 100 ms at -100 mV), and fast activation and deactivation kinetics. IDRK showed instead a less-hyperpolarized inactivation V(1/2) (-48 mV), a slower, double-exponential recovery from inactivation (taurec1 approximately 490 ms and taurec2 approximately 4,960 ms at -100 mV), and slower activation and deactivation kinetics. Steady-state activation gave a V(1/2) and a k of -46.2 and 8.2 mV for IA and -48.3 and 4.2 mV for IDRK. Both currents were not appreciably blocked by bath application of 10 mM TEA, but were inhibited by 4-AP, with IDRK displaying a higher sensitivity. IDRK also showed a relatively low affinity to linopirdine, being half blocked at approximately 50 microM. Steady-state and kinetic properties of IDRK and IA were described by 2nd- and 3rd-order Hodgkin-Huxley models, respectively. The goodness of our quantitative description of the Kv currents was validated by including IA and IDRK in a theoretical model of saccular hair cell electrical activity and by comparing the simulated responses with those obtained experimentally. This thorough description of the IDRK and IA will contribute toward understanding the role of these currents in the electrical response on this preparation.

Voltage-gated calcium channel currents in type I and type II hair cells isolated from the rat crista

When studied in vitro, type I hair cells in amniote vestibular organs have a large, negatively activating K+ conductance. In type II hair cells, as in nonvestibular hair cells, outwardly rectifying K+ conductances are smaller and more positively activating. As a result, type I cells have more negative resting potentials and smaller input resistances than do type II cells; large inward currents fail to depolarize type I cells above -60 mV. In nonvestibular hair cells, afferent transmission is mediated by voltage-gated Ca2+ channels that activate positive to -60 mV. We investigated whether Ca2+ channels in type I cells activate more negatively so that quantal transmission can occur near the reported resting potentials. We used the perforated patch method to record Ca2+ channel currents from type I and type II hair cells isolated from the rat anterior crista (postnatal days 4-20). The activation range of the Ca2+ currents of type I hair cells differed only slightly from that of type II cells or nonvestibular hair cells. In 5 mM external Ca2+, currents in type I and type II cells were half-maximal at -41.1 +/- 0.5 (SE) mV (n = 10) and -37.2 +/- 0.2 mV (n = 10), respectively. In physiological external Ca2+ (1.3 mM), currents in type I cells were half-maximal at -46 +/- 1 mV (n = 8) and just 1% of maximal at -72 mV. These results lend credence to suggestions that type I cells have more positive resting potentials in vivo, possibly through K+ accumulation in the synaptic cleft or inhibition of the large K+ conductance. Ca2+ channel kinetics were also unremarkable; in both type I and type II cells, the currents activated and deactivated rapidly and inactivated only slowly and modestly even at large depolarizations. The Ca2+ current included an L-type component with relatively low sensitivity to dihydropyridine antagonists, consistent with the alpha subunit being CaV1.3 (alpha1D). Rat vestibular epithelia and ganglia were probed for L-type alpha-subunit expression with the reverse transcription-polymerase chain reaction. The epithelia expressed CaV1.3 and the ganglia expressed CaV1.2 (alpha1C).

Potassium currents in vestibular type II hair cells activated by hydrostatic pressure

An elevated hydrostatic pressure in the endolymphatic space of the inner ear is discussed as pathophysiological factor in hydrops-related diseases of the inner ear. An increase in pressure by fractions of 1 cm H(2)O is sufficient to induce vertigo-like symptoms in animal models. To establish a link between hydrostatic pressure and the function of vestibular hair cells, we studied potassium currents in isolated vestibular type II hair cells from guinea-pig utricles when the hydrostatic pressure was increased by raising the height of the bath from 0.2-0.5, 0.7 or 1.0 cm. Elevated pressure enhanced K(+) currents significantly; a rise in pressure from 0.2-0.5 cm H(2)O increased the total K(+) current at +40 mV by 22+/-14% (+/-S.D.). The pressure-sensitive current I(K,p) was non-inactivating during depolarizing pulses. It was maintained when the pressure was kept elevated for several minutes and receded promptly after return to a pressure of 0.2 cm H(2)O. Voltage-gated Ca(2+) currents, in contrast, were not altered by hydrostatic pressure. A pharmacological characterization of I(K,p) revealed that tetraetylammonium (100 mM) abolished all outward currents including I(K,p). I(K,p) was partly and reversibly inhibited by 4-aminopyridine. Dihydrostreptomycin, a blocker of the transduction channel, left I(K,p) unaffected. Charybdotoxin (100 nM), a blocker of Ca(2+)-dependent K(+) channels, completely yet reversibly abolished I(K,p). We conclude that small elevations in hydrostatic pressure evoke a charybdotoxin-sensitive, probably Ca(2+)-dependent K(+) current in vestibular hair cells. This is likely to alter their frequency response and may be a relevant mechanism how hydrostatic pressure disturbs transduction.

Effects of permeant ion concentrations on the gating of L-type Ca2+ channels in hair cells

We determined the gating and permeation properties of single L-type Ca(2+) channels, using hair cells and varying concentrations (5-70 mM) of the charge carriers Ba(2+) and Ca(2+). The channels showed distinct gating modes with high- and low-open probability. The half-activation voltage (V(1/2)) shifted in the hyperpolarizing direction from high to low permeant ion concentrations consistent with charge screening effects. However, the differences in the slope of the voltage shifts (in VM(-1)) between Ca(2+) (0.23) and Ba(2+) (0.13), suggest that channel-ion interaction may also contribute to the gating of the channel. We examined the effect of mixtures of Ba(2+) and Ca(2+) on the activation curve. In 5 mM Ca(2+), the V(1/2) was, -26.4 +/- 2.0 mV compared to Ba(2+), -34.7 +/- 2.9 mV, as the charge carrier. However, addition of 1 mM Ba(2+) in 4 mM Ca(2+), a molar ratio, which yielded an anomalous-mole fraction effect, was sufficient to shift the V(1/2) to -34.7 +/- 1.5 mV. Although Ca(2+)-dependent inactivation of the L-type channels in hair cells can yield the present findings, we provide evidence that the anomalous gating of the channel may stem from the closed interaction between ion permeation and gating.

Functional interaction of auxiliary subunits and synaptic proteins with Ca(v)1.3 may impart hair cell Ca2+ current properties

We assessed the functional determinants of the properties of L-type Ca(2+) currents in hair cells by co-expressing the pore-forming Ca(V)1.3alpha(1) subunit with the auxiliary subunits beta(1A) and/or alpha(2delta). Because Ca(2+) channels in hair cells are poised to interact with synaptic proteins, we also co-expressed the Ca(V)1.3alpha(1) subunit with syntaxin, vesicle-associated membrane protein (VAMP), and synaptosome associated protein of 25 kDa (SNAP25). Expression of the Ca(V)1.3alpha(1) subunit in human embryonic kidney cells (HEK 293) produced a dihydropyridine (DHP)-sensitive Ca(2+) current (peak current density -2.0 +/- 0.2 pA/pF; n = 11). Co-expression with beta(1A) and alpha(2delta) subunits enhanced the magnitude of the current (peak current density: Ca(V)1.3alpha(1) + beta(1A) = -4.3 +/- 0.8 pA/pF, n = 10; Ca(V)1.3alpha(1) + beta(1A) + alpha(2delta) = -4.1 +/- 0.6 pA/pF, n = 9) and produced a leftward shift of approximately 9 mV in the voltage-dependent activation of the currents. Furthermore, co-expression of Ca(V)1.3alpha(1) with syntaxin/VAMP/SNAP resulted in at least a twofold increase in the peak current density (-4.7 +/- 0.2 pA/pF; n = 11) and reduced the extent of inactivation of the Ca(2+) currents. Botulinum toxin, an inhibitor of syntaxin, accelerated the inactivation profile of Ca(2+) currents in hair cells. Immunocytochemical data also indicated that the Ca(2+) channels and syntaxin are co-localized in hair cells, suggesting there is functional interaction of the Ca(V)1.3alpha(1) with auxiliary subunits and synaptic proteins, that may contribute to the distinct properties of the DHP-sensitive channels in hair cells.

Identification of genes expressed in the Xenopus inner ear

Recent studies indicate that hearing loss in humans has strong hereditary components associated with expression of specific genes in the auditory apparatus of the inner ear. However, the inner ear poses challenges for molecular research because the amount of tissue that can be isolated is limited, and extraction procedures yield small quantities of RNA and protein. To begin to identity genes essential for auditory function, we synthesized a cDNA library using an RT-PCR protocol and total RNA isolated from eight Xenopus laevis inner ears. Sequence analysis of randomly selected clones demonstrated expression of both identified (calmodulin, SNARE protein, syndecan-2) and unidentified genes, and confirmed synthesis of full length transcripts. Confocal and scanning electron microscopy (SEM) were used to examine the structure of inner ear organs that serve as auditory receptors in amphibians: the sacculus, the amphibian papilla and the basilar papilla. SEM images illustrate the heterogeneity of bundle morphology and demonstrate the continuous appearance of stereociliary bundles in the X. laevis amphibian papilla during larval development and adult life. Investigations of gene expression in Xenopus auditory organs using clones recovered from inner ear cDNA libraries should provide insight regarding the molecular basis of hearing.

Evidence of a Hopf bifurcation in frog hair cells

The membrane potential of hair cells in the low-frequency hearing organ of the bullfrog, the amphibian papilla, sinusoidally oscillates at small amplitude in the absence of acoustical input. We stimulate the cell with a series of periodic currents close to this natural frequency and observe that its current-to-voltage transfer function is compressively nonlinear, having a large gain for small stimuli and a smaller gain for larger currents. Along with the spontaneous oscillation, this implies that the cell is poised close to a dynamical instability such as a Hopf bifurcation, because distant from the instability the transfer function becomes linear. The cell's frequency selectivity is enhanced for small stimuli. Simulations show that the cell's membrane capacitance is effectively reduced due to a current gain provided by this dynamical instability. We propose that the Hopf resonance is widely used by transducer cells on the sensory periphery to achieve small-signal amplification.

Direct measurement of single-channel Ca(2+) currents in bullfrog hair cells reveals two distinct channel subtypes

1. To confer their acute sensitivity to mechanical stimuli, hair cells employ Ca(2+) ions to mediate sharp electrical tuning and neurotransmitter release. We examined the diversity and properties of voltage-gated Ca(2+) channels in bullfrog saccular hair cells by means of perforated and cell-attached patch-clamp techniques. Whole-cell Ca(2+) current records provided hints that hair cells express L-type as well as dihydropyridine-insensitive Ca(2+) currents. 2. Single Ca(2+) channel records confirmed the presence of L-type channels, and a distinct Ca(2+) channel, which has sensitivity towards omega-conotoxin GVIA. Despite its sensitivity towards omega-conotoxin GVIA, the non-L-type channel cannot necessarily be considered as an N-type channel because of its distinct voltage-dependent gating properties. 3. Using 65 mM Ca(2+) as the charge carrier, the L-type channels were recruited at about -40 mV and showed a single-channel conductance of 13 pS. Under similar recording conditions, the non-L-type channels were activated at approximately -60 mV and had a single-channel conductance of approximately 16 pS. 4. The non-L-type channel exhibited at least two fast open time constants (tau(o) = 0.2 and 5 ms). In contrast, the L-type channels showed long openings (tau(o) = approximately 23 ms) that were enhanced by Bay K 8644, in addition to the brief openings (tau(o) = 0.3 and 10 ms). 5. The number of functional channels observed in patches of similar sizes suggests that Ca(2+) channels are expressed singly, in low-density clusters (2-15 channels) and in high-density clusters (20-80 channels). Co-localization of the two channel subtypes was observed in patches containing low-density clusters, but was rare in patches containing high-density clusters. 6. Finally, we confirmed the existence of two distinct Ca(2+) channel subtypes by using immunoblot and immunohistochemical techniques.

Distortion product otoacoustic emissions in the tree frog Hyla cinerea

The frog inner ear contains two hearing organs: the amphibian and the basilar papilla. The amphibian papilla is sensitive to low- and mid-frequency stimuli (0.1--0.5 and 0.5--1.3 kHz, respectively, in Hyla cinerea), while the basilar papilla is sensitive to high-frequency stimuli (2.8--3.9 kHz in H. cinerea). Distortion product otoacoustic emissions (DPOAE) were recorded from the ear of the tree frog H. cinerea. In each of six ears investigated, a cubic distortion product (DP) at 2f(1)--f(2) was present when the primary frequencies f(1) and f(2) and the DP frequency were close to either the mid- or the high-frequency range. At frequencies between the sensitive ranges of both papillae, no emissions were observed. For the basilar papilla, the dependence of DP level on the primary tone frequency ratio f(2)/f(1) showed a pattern characteristic of the response of a single nonlinear resonator. Thus, in agreement with neural data, DPOAE from the basilar papilla reflect the contribution of a single auditory filter to emission generation.

Regional distribution of calcium currents in frog semicircular canal hair cells

In the present work we studied the regional expression of voltage-dependent Ca channels in hair cells from the frog semicircular canals, employing whole-cell patch-clamp on isolated and in situ hair cells. Although Ca channels are thought to play a major role in afferent transmission, up to now no data were available regarding their distribution in vestibular organs. The problem appears of interest, especially in the light of recent results showing the presence of multiple Ca current components in semicircular canal hair cells. Our data suggest the presence, in all regions of the crista ampullaris, of two classes of cells, one displaying an inactivating Ca current (R1) and one lacking it. In the former cells, Ca current amplitude decreased from the central to the peripheral zone (the maximal currents being observed in the intermediate zone). Only L-type and R2 current components displayed regional differences in expression, whereas the size and properties of R1, although variable among cells, were not regionalized. However, in cells lacking R1, Ca current amplitudes were similar regardless of cell shape and location. The possible contributions of this Ca current distribution to afferent discharge properties are discussed.

Hair cells, hearing and hopping: a field guide to hair cell physiology in the frog

For more than four decades, hearing in frogs has been an important source of information for those interested in auditory neuroscience, neuroethology and the evolution of hearing. Individual features of the frog auditory system can be found represented in one or many of the other vertebrate classes, but collectively the frog inner ear represents a cornucopia of evolutionary experiments in acoustic signal processing. The mechano-sensitive hair cell, as the focal point of transduction, figures critically in the encoding of acoustic information in the afferent auditory nerve. In this review, we provide a short description of how auditory signals are encoded by the specialized anatomy and physiology of the frog inner ear and examine the role of hair cell physiology and its influence on the encoding of sound in the frog auditory nerve. We hope to demonstrate that acoustic signal processing in frogs may offer insights into the evolution and biology of hearing not only in amphibians but also in reptiles, birds and mammals, including man.

Potassium currents in auditory hair cells of the frog basilar papilla

The whole-cell patch-clamp technique was used to identify and characterize ionic currents in isolated hair cells of the leopard frog basilar papilla (BP). This end organ is responsible for encoding the upper limits of a frog's spectral sensitivity (1.25-2.0 kHz in the leopard frog). Isolated BP hair cells are the smallest hair cells in the frog auditory system, with spherical cell bodies typically less than 20 microm in diameter and exhibiting whole-cell capacitances of 4-7 pF. Hair cell zero-current resting potentials (Vz) varied around a mean of -65 mV. All hair cells possessed a non-inactivating, voltage-dependent calcium current (I(Ca)) that activates above a threshold of -55 mV. Similarly all hair cells possessed a rapidly activating, outward, calcium-dependent potassium current (I(K)(Ca)). Most hair cells also possessed a slowly activating, outward, voltage-dependent potassium current (I(K)), which is approximately 80% inactive at the hair cell Vz, and a fast-activating, inward-rectifying potassium current (I(K1)) which actively contributes to setting Vz. In a small subset of cells I(K) was replaced by a fast-inactivating, voltage-dependent potassium current (I(A)), which strongly resembled the A-current observed in hair cells of the frog sacculus and amphibian papilla. Most cells have very similar ionic currents, suggesting that the BP consists largely of one homogeneous population of hair cells. The kinetic properties of the ionic currents present (in particular the very slow I(K)) argue against electrical tuning, a specialized spectral filtering mechanism reported in the hair cells of birds, reptiles, and amphibians, as a contributor to frequency selectivity of this organ. Instead BP hair cells reflect a generalized strategy for the encoding of high-frequency auditory information in a primitive, mechanically tuned, terrestrial vertebrate auditory organ.