Mechanical and electromechanical properties of the stereovillar bundles of isolated and cultured hair cells of the chicken

A prestin motor in chicken auditory hair cells: active force generation in a nonmammalian species

Active force generation by outer hair cells (OHCs) underlies amplification and frequency tuning in the mammalian cochlea but whether such a process exists in nonmammals is unclear. Here, we demonstrate that hair cells of the chicken auditory papilla possess an electromechanical force generator in addition to active hair bundle motion due to mechanotransducer channel gating. The properties of the force generator, its voltage dependence and susceptibility to salicylate, as well as an associated chloride-sensitive nonlinear capacitance, suggest involvement of the chicken homolog of prestin, the OHC motor protein. The presence of chicken prestin in the hair cell lateral membrane was confirmed by immunolabeling studies. The hair bundle and prestin motors together create sufficient force to produce fast lateral displacements of the tectorial membrane. Our results imply that the first use of prestin as a motor protein occurred early in amniote evolution and was not a mammalian invention as is usually supposed.

Predicting severity of cochlear hair cell damage in adult chickens using DPOAE input-output functions

Distortion product otoacoustic emissions (DPOAE) were recorded from the ear canal of aged broiler chickens which have been shown to present with age-related cochlear degeneration [Hear. Res. 166 (2002) 82]. We describe the relationship between the shape of the DPOAE input-output (I/O) function and the type of hair cell damage present at and between the cochlear frequency places of the DPOAE primary tones (f1 and f2). The mid stimulus level compressive growth of the mean DPOAE I/O functions is reduced in a graded fashion relative to the severity of hair cell damage. However, individual DPOAE I/O functions within most hair cell damage groups show large variability from this characteristic. Various least squares regression models were used to predict hair cell density from indices derived from the DPOAE I/O function (area, threshold and slope). The results showed that no simple linear relationship exists between hair cell density and the DPOAE I/O function indices. Multivariate binary logistic regression used DPOAE I/O function indices to predict membership in hair cell damage groups. The logistic model revealed that DPOAE threshold can be used to predict the occurrence of severe/total hair cell damage with good specificity though poor sensitivity.

Spontaneous otoacoustic emissions in monitor lizards

Monitors (all of which belong to the genus Varanus) make up a very uniform family of often large lizards. They have a large auditory papilla that is not highly specialized, but is divided into two unequal sub-papillae. All hair cells are covered by a tectorial membrane. Spontaneous otoacoustic emissions (SOAE) were examined in Cape monitor lizards (Varanus exanthematicus) and found between 1.08 and 2.91 kHz (at 32 degrees C) and with levels between -2.8 and 25.8 dB SPL. The frequency of SOAE was temperature dependent, with a maximal shift of 0.07 octaves/degrees C. All SOAE could be suppressed by external tones, most easily by tones near the center frequency and thus suppression tuning curves were V-shaped. In addition, SOAE could be facilitated by external tones, the amplitude increasing up to 10 dB. The most effective tones were generally those between 0.33 and 0.75 octaves above the respective center frequency of the SOAE. External tones could also change the center frequency of SOAE by up to several hundred Hz, most tones causing frequency 'pushing'. Compared to SOAE of other lizards, Varanus SOAE have larger amplitudes and show larger frequency shifts with temperature. Both of these features may be the result of the coupling of large numbers of hair cells via the continuous tectorial membrane.

Auditory function recovery following acoustic overstimulation

OBJECTIVE

To examine electrophysiological data from auditory brainstem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs) in chickens following acoustic overstimulation.

MATERIAL AND METHODS

A total of 15 white 15-day-old Leghorn chickens were used. The animals were anesthetized with Equithensin, and placed with their heads in a special holder. Click stimuli were generated by a Nicolet CA1000 device and ABRs were recorded subcutaneously using three stainless-steel electrodes. An ILO 92/DP analyzer was used to determine DPOAEs. The noise was generated by a Promax GB 212 device. The acoustic exposure was provoked with a 2-kHz pure tone at 120 dB SPL for 24 h. ABRs and DPOAEs were determined before and immediately post-exposure and 5, 15, 21 and 30 days after the traumatic exposure.

RESULTS

In our control DPgram response, the maximum amplitudes (dB SPL) occurred at 1, 1.5, 2, 3 and 4 kHz and the minimum amplitudes at 0.7, 5 and 6 kHz. Immediately following acoustic overstimulation an amplitude loss in all frequencies was detected (p < 0.001). Five days after noise exposure only the amplitude loss at 3 kHz remained. Three waves with positive and negative peaks appeared in our control ABR recordings. An important threshold shift was detected in the ABR response immediately after acoustic overstimulation. Its complete recovery occurred 15 days after the acoustic trauma.

CONCLUSION

Recovery of the DPgram response was detected 5 days after acoustic overstimulation, whereas the normal ABR threshold appeared on the 15th day.

Hair-bundle movements elicited by transepithelial electrical stimulation of hair cells in the sacculus of the bullfrog

Electrically evoked otoacoustic emission is a manifestation of reverse transduction by the inner ear. We present evidence for a single-cell correlate of this phenomenon, hair-bundle movement driven by transepithelial electrical stimulation of the frog's sacculus. Responses could be observed at stimulus frequencies up to 1 kHz, an order of magnitude higher than the organ's natural range of sensitivity to acceleration or sound. Measurements at high-stimulus frequencies and pharmacological treatments allow us to distinguish two mechanisms that mediate the electrical responses: myosin-based adaptation and Ca(2+)-dependent reclosure of transduction channels. These mechanisms also participate in the active process that amplifies and tunes the mechanical responses of this receptor organ. Transient application of the channel blocker gentamicin demonstrated the crucial role of mechanoelectrical transduction channels in the rapid responses to electrical stimulation. A model for electrically driven bundle motion that incorporates the negative stiffness of the hair bundle as well as its two mechanisms of motility captures the essential features of the measured responses.

Chick hair cells do not exhibit voltage-dependent somatic motility

It is generally believed that mechanical amplification by cochlear hair cells is necessary to enhance the sensitivity and frequency selectivity of hearing. In the mammalian ear, the basis of cochlear amplification is believed to be the voltage-dependent electromotility of outer hair cells (OHCs). The avian basilar papilla contains tall and short hair cells, with the former being comparable to inner hair cells, and the latter comparable to OHCs, based on their innervation patterns. In this study, we sought evidence for somatic electromotility by direct measurements of voltage-dependent length changes in both tall and short hair cells at nanometre resolution. Microchamber and whole-cell voltage-clamp techniques were used. Motility was measured with a photodiode-based measurement system. Non-linear capacitance, an electrical signature of somatic motility, was also measured to complement motility measurement. Significantly, chick hair cells did not exhibit somatic motility nor express non-linear capacitance. The lack of somatic motility suggests that in avian hair cells the active process resides elsewhere, most likely in the hair cell stereocilia.

Acoustic modulation of electrically evoked otoacoustic emission in chickens

Electrically evoked otoacoustic emissions (EEOAEs) can be elicited from the chicken inner ear. Since lesion studies implicate hair cells are the source of EEOAEs, we hypothesized that acoustic stimuli would modulate EEOAE amplitude at cochlear locations where the acoustic and electrical stimuli overlap. To assess this interaction, EEOAEs were measured as the frequency and amplitude of the acoustic stimuli were varied. EEOAEs, evoked by AC current (3-250 microA rms) delivered to the round window had a broad band pass response (1-6 kHz) with a peak between 3 and 4 kHz and maximum amplitude of 27 dB SPL. EEOAE suppression/enhancement tuning curves were measured at 2, 3, 4 and 6 kHz by varying the frequency of a 70 dB SPL tone and measuring the change in EEOAE amplitude. EEOAE tuning curves were characterized by a tip; a narrow range of frequencies where EEOAE amplitude was suppressed by as much as 5 dB, and by sidebands, a range of frequencies above and below the tip where EEOAE amplitude was enhanced by as much as 1.5 dB. The best suppression frequency, or characteristic frequency, was close to the frequency of the EEOAE elicited by the 3- or 4-kHz electric stimulus. However, the characteristic frequency was displaced towards higher frequencies for the 2-kHz electric stimulus, and towards lower frequencies for the 6-kHz electric stimulus. EEOAE suppression increased approximately linearly with acoustic level. These results suggest that EEOAEs evoked by round window stimulation are predominantly generated by hair cells near the 3- to 4-kHz region of the cochlea.

Electrically evoked otoacoustic emissions from the chicken ear

The outer hair cell electromotile response is believed to underlie the sharp tuning and exquisite sensitivity of the mammalian inner ear, and contribute to the production of electrically evoked otoacoustic emissions (EEOAEs) and sound-evoked otoacoustic emissions (OAEs). Avian ears are also sharply tuned, extremely sensitive and generate spontaneous and sound-evoked OAEs, but avian hair cells do not exhibit somatic electromotility. However, stereocilia bundle movements have been observed in avian and amphibian hair cells suggesting that EEOAEs might arise from electrically evoked bundle movements. Here, we demonstrate for the first time that AC current applied to the round window of the chicken evokes EEOAE of up to 18 dB SPL. The EEOAE produces a bandpass response with maximum amplitude in the 1000-3000 Hz range; the response drops off rapidly above 4000 Hz and below 500 Hz. The impulse response to current pulses is characterized by a large peak sometimes followed by a damped oscillation with a frequency around 2000 Hz. EEOAEs decreased significantly after anoxia and paraformaldehyde damage of the cochlea. Kanamycin-induced hair cell loss also caused a significant reduction in EEOAE and distortion product OAE; these emissions showed only a small recovery at long recovery times, when most hair cells should have regenerated. These results suggest that the EEOAE has a biological origin in the cochlea, which could presumably involve electrically evoked stereocilia bundle movements.

Effect of inner and outer hair cell lesions on electrically evoked otoacoustic emissions

When the cochlea is stimulated by a sinusoidal current, the inner ear emits an acoustic signal at the stimulus frequency, termed the electrically evoked otoacoustic emission (EEOAE). Recent studies have found EEOAEs in birds lacking outer hair cells (OHCs), raising the possibility that other types of hair cells, including inner hair cells (IHCs), may generate EEOAEs. To determine the relative contribution of IHCs and OHCs to the generation of the EEOAE, we measured the amplitude of EEOAEs, distortion product otoacoustic emissions (DPOAEs), the cochlear microphonic (CM) and the compound action potential (CAP) in normal chinchillas and chinchillas with IHC lesions or IHC plus OHC lesions induced by carboplatin. Selective IHC loss had little or no effect on CM amplitude and caused a slight reduction in mean DPOAE amplitude. However, IHC loss resulted in a massive reduction in CAP amplitude. Importantly, selective IHC lesions did not reduce EEOAE amplitude, but instead, EEOAE amplitude increased at high frequencies. When both IHCs and OHCs were destroyed, the amplitude of the CM, DPOAE and EEOAE all decreased. The increase in EEOAE amplitude seen with IHC loss may be due to (1) loss of tonic efferent activity to the OHCs, (2) change in the mechanical properties of the cochlea or (3) elimination of EEOAEs produced by IHCs in phase opposition to those from OHCs.

Evidence for an active process and a cochlear amplifier in nonmammals

The last two decades have produced a great deal of evidence that in the mammalian organ of Corti outer hair cells undergo active shape changes that are part of a "cochlear amplifier" mechanism that increases sensitivity and frequency selectivity of the hearing epithelium. However, many signs of active processes have also been found in nonmammals, raising the question as to the ancestry and commonality of these mechanisms. Active movements would be advantageous in all kinds of sensory hair cells because they help signal detection at levels near those of thermal noise and also help to overcome fluid viscosity. Such active mechanisms therefore presumably arose in the earliest kinds of hair cells that were part of the lateral line system of fish. These cells were embedded in a firm epithelium and responded to relative motion between the hair bundle and the hair cell, making it highly likely that the first active motor mechanism was localized in the hair-cell bundle. In terrestrial nonmammals, there are many auditory phenomena that are best explained by the presence of a cochlear amplifier, indicating that in this respect the mammalian ear is not unique. The latest evidence supports siting the active process in nonmammals in the hair-cell bundle and in intimate association with the transduction process.

Effects of AC and DC stimulation on chinchilla SOAE amplitude and frequency

The effects of AC and DC current on spontaneous otoacoustic emissions (SOAEs) were studied in normal chinchillas and chinchillas with selective inner hair cell (IHC) loss. Electrical stimulation was delivered through an electrode on the round window or through an electrode in scala media. SOAE frequencies ranged from 4 to 11 kHz and amplitudes ranged from 13 to 51 dB SPL. AC simulation suppressed SOAE amplitude. The suppression contours had a narrowly tuned, low-threshold tip located above the frequency of the SOAE. AC suppression contours were similar to acoustic suppression contours except that the AC suppression contours lacked a high-threshold, low frequency tail. The lowest threshold of the AC suppression contour was 3.9 microA rms whereas the lowest acoustic suppression threshold was 19 dB SPL. AC stimulation, which induced an electrically evoked otoacoustic emission, interacted with the SOAE to generate distortion product otoacoustic emissions (DPOAEs) of up to 26 dB SPL at 2f(S)-f(AC) (f(S)=SOAE). DPOAE amplitude increased with AC current, but saturated at high levels. DC current steps affected both SOAE frequency and amplitude. Positive current at the round window decreased SOAE amplitude and frequency whereas negative current increased SOAE frequency, but had little effect on amplitude. The effects of AC and DC current on SOAEs in animals with IHC loss were similar to those in normal chinchillas.

Induction of spontaneous otoacoustic emissions in chinchillas from carboplatin-induced inner hair cell loss

Fifteen chinchillas were evaluated for spontaneous otoacoustic emissions (SOAEs) before and after administering carboplatin (126-200 mg/kg), an anti-neoplastic drug that selectively destroys inner hair cells (IHCs) in this species. SOAEs were absent from all animals prior to carboplatin treatment, but at 1 week post-treatment, 47% of the animals and 30% of the ears had developed SOAEs. SOAE frequencies were clustered between 5 and 10 kHz and SOAE intensity ranged from 10 to 32 dB SPL. All of the ears with SOAEs had IHC lesions exceeding 60% throughout most of the cochlea and two ears had outer hair cell lesions of 25-60% at a cochlear place associated with the frequency of the SOAE. Thus, high doses of carboplatin that cause IHC loss can be used to create an animal model with SOAEs.

Mechanisms of hair cell tuning

Mechanosensory hair cells of the vertebrate inner ear contribute to acoustic tuning through feedback processes involving voltage-gated channels in the basolateral membrane and mechanotransduction channels in the apical hair bundle. The specific number and kinetics of calcium-activated (BK) potassium channels determine the resonant frequency of electrically tuned hair cells. Kinetic variation among BK channels may arise through alternative splicing of slo gene mRNA and combination with modulatory beta subunits. The number of transduction channels and their rate of adaptation rise with hair cell response frequency along the cochlea's tonotopic axis. Calcium-dependent feedback onto transduction channels may underlie active hair bundle mechanics. The relative contributions of electrical and mechanical feedback to active tuning of hair cells may vary as a function of sound frequency.

Extrinsic Fabry-Perot interferometer for measuring the stiffness of ciliary bundles on hair cells

We have developed an extrinsic Fabry-Perot interferometer (EFPI) to measure displacements of microscopic, living organelles in the inner ear. The EFPI is an optical phase-shifted instrument that can be used to measure nanometer displacements. The instrument transmits a coherent light signal to the end of a single glass optical fiber where the measurement is made. As the coherent light reaches the end of the fiber, part of this incident signal is reflected off the internal face of the fiber end (reference reflection) and part is transmitted through the end of the fiber. This transmitted light travels a short distance and is reflected off the surface whose displacement is to be measured (the target). This sensing reflection then reenters the fiber where it interferes with the reference reflection. The resulting interference signal then travels up the same optical fiber to a detector, where it is converted into a voltage that can be read from an oscilloscope. When the target moves, the phase relation between reference and sensing reflections changes, and the detector receives a modulated signal proportional to the target movement. Reflections of as little as 1% at both the sensor tip and target surfaces produce good results with this system. We use the EFPI in conjunction with fine glass whiskers to measure the stiffness (force per unit deflection) of stereociliary bundles on hair cells of the inner ear. The forces generated are in the tenths of picoNewton range and the displacements are tens of nanometers. Here we describe the EFPI and its development as a method for measuring displacements of microscopic organelles in a fluid medium. We also report experiments to validate the accuracy of the EFPI output and preliminary measurements of ciliary bundle stiffness in the posterior semicircular canal.

The tip link's role in asymmetric stereocilia motion of chick cochlear hair cells

The symmetry of chick cochlear hair bundle motion was examined in this study. Isolated segments from the basilar papilla were incubated in vitro in either normal or low calcium medium, which is known to disrupt tip links. Stereociliary bundles, stimulated with an oscillating water microjet, were oriented in profile and viewed in slow motion at high magnification with stroboscopic illumination. The displacement of the tallest hair in the bundle was fixed to 20 degrees peak-to-peak (P-P) motion. The angular deflections of the shortest and tallest hairs were then measured in both the positive (towards the tallest hair) and negative (towards the shortest) directions with respect to the non-stimulated position of the hair. The tallest hairs exhibited nearly symmetric motion in medium containing normal and low calcium. The shortest hairs, in normal calcium, displayed considerable asymmetry with angular deflections in the positive direction significantly larger than in the negative direction. This asymmetric motion disappeared after incubation in low calcium. The shortest hair angular displacement in the negative direction, however, was the same in both normal and low calcium conditions. These results indicated that the tallest and shortest hairs moved with equal angular deflection in the negative direction, while in the positive direction the shortest hair moved through a significantly greater angular deflection than the tallest hair. The implication of this finding is that the tip links contributed significantly to hair bundle motion in the positive direction only.

Quantitative relationship of carboplatin dose to magnitude of inner and outer hair cell loss and the reduction in distortion product otoacoustic emission amplitude in chinchillas

The outer hair cells (OHCs) are thought to be the dominant source of distortion product otoacoustic emissions (DPOAEs) in the mammalian cochlea; however, little is known about the quantitative relationship between reduction in DPOAE amplitude and the degree of inner hair cell (IHC) and OHC loss. To examine this relationship, we measured the DPOAE input/output functions in the chinchilla before and after destroying the IHCs and/or OHCs with carboplatin. Low-to-moderate doses (38-150 mg/kg, i.p.) of carboplatin selectively destroyed some or all of the IHCs along the entire length of the cochlea while sparing the OHCs. Selective loss of all the IHCs had little effect on DPOAE amplitude as long as the OHCs were present. With high doses of carboplatin (200 mg/kg, i.p.), there was complete destruction of IHCs plus massive OHC loss that decreased from the base towards the apex of the cochlea. OHC loss resulted in a large decrease in DPOAE amplitude. DPOAE amplitude at 9.6 kHz decreased at the rate of 4.1 dB for every 10% loss of OHCs. At 7.2 and 4.8 kHz, DPOAE amplitude decreased 3.1 dB and 2.4 dB per 10% OHC loss, respectively. These results indicate that OHCs are the dominant source of DPOAEs.