Ac Receptor Potential Research Articles

Molecular subclassification of breast carcinomas based on aCGH, gene expression, IHC and ploidy – relevance for clinical outcome

Recordings were made from inner hair cells (IHC) at three locations distributed in the apical half of the guinea pig cochlea. Longitudinal variations in ac and dc components of receptor potentials produced in response to single-tone inputs were studied to further understand the ways in which IHCs communicate with their innervating afferent dendrites. While neural synchrony probably depends on the ac receptor potential, discharge rate may be controlled by the dc receptor potential generated by the IHC transducer plus an ac component derived from the phasic receptor potential. The latter reflects low-pass filtering inherent in the hair cell's basolateral membrane and calcium-dependent synaptic processes. By comparing the frequency dependence of ac and dc components in cells with different characteristic frequencies, it may be possible to learn how neural response areas are formed and why their shapes change along the cochlear spiral.

Mathematical model of outer hair cell regulation including ion transport and cell motility

To understand the regulatory processes within the cochlea, and outer hair cells (OHCs) in particular, we have developed a mathematical model of OHC regulation that takes into account their known electrical properties, and includes fast and slow somatic motility. We model how cytosolic Ca 2+ is involved in regulation of (i) the OHC membrane potential, (ii) the operating point of OHC mechano-electrical transduction (MET) channels via slow motility; (iii) basolateral wall K + permeability via Ca 2+-sensitive K + channels; and (iv) cytosolic concentrations of Ca 2+ itself, via Ca 2+-ATPase-mediated sequestration within the OHCs and Ca 2+-induced Ca 2+-release (CICR) from the same intracellular Ca 2+ storage organelles. To account for some aspects of the cochlea’s transient response to experimental perturbations, we have included a putative intracellular second-messenger cascade based on cytosolic Ca 2+. Overall, the OHC basolateral permeability determines the resting membrane potential of the OHCs and their standing current, which influences the endocochlear potential, and also affects the AC receptor potential that drives the prestin-mediated somatic electromotility and active cochlear gain. The model we have developed provides a physiologically-plausible and internally-consistent explanation for the time-courses of the cochlear changes we have observed during a number of different experimental perturbations, including a slow oscillatory behaviour presumed due to oscillations in cytosolic Ca 2+ concentration. We also show how the known ionic mechanisms within OHCs act to regulate membrane potential and hair bundle angle over a very wide range of strial current and intracochlear hydrostatic pressure. Not included in the model are osmotic effects, the nonlinear aspects of prestin’s electromotility, the intracellular role of Cl − in modifying this motility, nor adaptation of MET at the apex of OHCs. Only one Ca 2+ sequestration compartment has been included in this implementation of the model, with the two types of basolateral Ca 2+ cisternae combined into a single compartment. Despite these limitations, the model as presented offers insights into the regulation of OHC membrane potential and MET at the hair cell apex, and is our first step in understanding in a quantitative way the integrated function of the molecular components of ion transport and motility in these cells.

Positional analysis of guinea pig inner hair cell membrane conductances: implications for regulation of the membrane filter.

In mammals, sound transduction by inner hair cells (IHC) generates a receptor potential whose amplitude and phase drive auditory nerve firing. The membrane filter properties that define the input-output function of IHC are derived from membrane conductance and capacitance. These elements of the membrane filter were quantified using whole-cell voltage clamp of IHC from the four turns of the guinea pig cochlea. IHC membrane properties were remarkably constant along the cochlea, in contrast with all other auditory hair cell systems, and suggests that extrinsic processes such as the active filter provided by the outer hair cells are matched to a constant transfer function of the IHC. Two outwardly rectifying K+ currents contribute to the IHC membrane conductance. These combined currents activate at approximately -55 mV. IHC mean input resistance was 140 M ohm and capacitance was 10.0 pF, generating a membrane time constant of 1.4 ms or a corner frequency of approximately 115 Hz, which is consistent with reported low-frequency roll-off of the IHC AC receptor potential in vivo. Approximately 40% of the 313-1 nS total K+ conductance about 0 mV was attributed to charybdotoxin-sensitive K(Ca) channels (also sensitive to cell dialysis with the Ca2+ chelator BAPTA or removal of extracellular Ca2+). The only known ligand-activated conductance in mature IHC, the P2X receptor conductance, averaged 31 nS (activated by 400 microM ATP; about -75 mV) irrespective of cell origin. Thus, regulation of intracellular Ca2+ and activation of P2X receptors by extracellular ATP provide capacity for local dynamic fine-tuning of the IHC membrane filter.

Inner hair cell response patterns: Implications for low-frequency hearing

Inner hair cell (IHC) responses to tone-burst stimuli were measured from three locations in the apical half of the guinea pig cochlea. In addition to the measurement of ac receptor potentials, average intracellular voltages, reflecting both ac and dc components of the receptor potential, were computed and compared to determine how bandwidth changes with level. Companion phase measures were also obtained and evaluated. Data collected from turn 2, where best frequency (BF) is approximately 4000 Hz, indicate that frequency response functions are asymmetrical with steeper slopes above the best frequency of the cell. However, in turn 4, where BF is around 250 Hz, the opposite behavior is observed and the steepest slopes are measured below BF. The data imply that cochlear filters are generally asymmetrical with steeper slopes above BF. High-pass filtering by the middle ear serves to reduce this asymmetry in turn 3 and to reverse it in turn 4. Apical response patterns are used to assess the degree to which the middle ear transfer function, the IHC's velocity dependence and the shunting effect of the helicotrema influence low-frequency hearing in guinea pigs. Implications for low-frequency hearing in man are also discussed.

Frequency-dependent saturation in inner hair cell responses

Inner hair cell (IHC) recordings are made in guinea pig from cochlear turns 2, 3, and 4 where best frequencies (BF) are approximately 4000, 1000, and 250 Hz, respectively. Input–output functions for the ac receptor potential are measured for inputs at, below, and above the BF of each recording location. Although all responses saturate at high intensities, the response magnitude at which saturation occurs depends on stimulus frequency. For example, in turn 2 more linear responses are observed below BF while in turn 4 a different pattern emerges. At this apical recording site, which responds preferentially to very low frequencies, IHCs exhibit more linear behavior above BF. In other words, response magnitude at saturation exceeds that measured for inputs at and below the BF of the cell. The reasons for this dependence on BF are evaluated by comparing input–output functions for ac responses recorded intracellularly from IHCs and extracellularly from the organ of Corti fluid space. [Work supported by Grant No. 5 R01 DC00089, National Institute on Deafness and Other Communication Disorders, National Institutes of Health.]

Stimulus biasing: A comparison between cochlear hair cell and organ of corti response patterns

Responses from the organ of Corti (OC) fluid space and from individual hair cells are collected for short duration tone pips measured alone and in the presence of a 20 Hz bias tone. Because of the relatively long period of the acoustic bias signal, a probe can be selectively placed at precise locations within a single response period. Since bias effects produced during maximum basilar membrane velocity are negligible, this report documents changes associated with basilar membrane displacements to scala vestibuli and scala tympani. Hair cell response patterns are compared with those measured nearby in the OC to determine the degree to which inner (IHC) and outer hair cells (OHC) contribute to the gross cochlear potentials. It is shown that intracellular OHC and OC dc responses are strongly influenced by the bias while intracellular IHC dc responses are minimally affected. Although an association has been established between the cochlear microphonic and outer hair cell ac receptor potentials, the well correlated changes in the outer hair cell dc receptor potential and the organ of Corti summating potential suggest that gross cochlear potentials reflect dc as well as ac contributions from nearby OHCs. Since IHC responses are not influenced by the bias, they appear to contribute less than OHCs to extracellular potentials. These conclusions, however, are restricted to the moderately high input levels required to produce these effects (Durrant and Dallos, 1974) and to the central region of the cochlea where the recordings are made. Finally, the degree to which bias effects are contaminated by two-tone suppression is discussed.

Response to ‘‘Comment on ‘Two-tone suppression of inner hair cell and basilar membrane responses in the guinea pig’ ’’ [J. Acoust. Soc. Am. 94, 3509–3510 (1993)

This letter provides additional data concerning the nature of two-tone suppression when an above characteristic frequency (CF) tone suppresses the responses of hair cells (in the basal turn of the cochlea) stimulated by lower than characteristic frequency tones. It is shown that the phase of the ac receptor potential of the below CF probe tone undergoes a phase lag during suppression. This is the opposite direction from that which would occur during attenuation of the probe. These data are consistent with the changes that have been reported [Cheatham and Dallos, J. Acoust. Soc. Am. 94 (1993)] in turns two and three of the guinea pig cochlea. Other additional data are provided showing that the phase change of a probe tone having a frequency higher than CF is a function of whether the high-side supressor is excitatory or nonexcitatory for the hair cell. Nonexcitatory high-side suppression is attenuation-like, whereas the mutual suppression of two (close in frequency) excitatory tones are not like attenuation.

Longitudinal comparisons of IHC ac and dc receptor potentials recorded from the guinea pig cochlea

Recordings were made from inner hair cells (IHC) at three locations distributed in the apical half of the guinea pig cochlea. Longitudinal variations in ac and dc components of receptor potentials produced in response to single-tone inputs were studied to further understand the ways in which IHCs communicate with their innervating afferent dendrites. While neural synchrony probably depends on the ac receptor potential, discharge rate may be controlled by the dc receptor potential generated by the IHC transducer plus an ac component derived from the phasic receptor potential. The latter reflects low-pass filtering inherent in the hair cell's basolateral membrane and calcium-dependent synaptic processes. By comparing the frequency dependence of ac and dc components in cells with different characteristic frequencies, it may be possible to learn how neural response areas are formed and why their shapes change along the cochlear spiral.

Intermodulation distortion (F2-F1) in inner hair cell and basilar membrane responses.

Round window (RW) recordings in guinea pigs show a large response at the frequency of the quadratic difference tone (QDT) at F2-F1 for high-frequency (> 8 kHz) tone pairs. The magnitude of the RW response is dependent on frequency and level of the primaries. The QDT cochlear microphonic produced is largest for primaries with a frequency separation of approximately 900 Hz. Its source is probably the local activity of hair cells in basal cochlear locations where the high-frequency tones interact. QDT measurements from locations inside the organ of Corti show that the magnitude of the QDT is largest in the inner hair cell (IHC) region and monotonically increases as the difference in the frequency of the primaries decreases. Measured as IHC intracellular ac receptor potential, the QDT appears to be the result of a mechanical stimulus to the cell rather than as an inherent property of the nonlinear transduction in the IHC. However, QDT of the same form is not evident in the velocity responses of the basilar membrane. These results suggest that the outer hair cells (OHC) produce a strong quadratic distortion product mechanical force to stimulate inner hair cells. This mechanical drive may not be present, or is a weak component, in basilar membrane motion.

Two-tone suppression of inner hair cell and basilar membrane responses in the guinea pig

Recordings of receptor potentials from inner hair cells (IHCs) and the basilar membrane (BM) motion were made in pigmented guinea pigs. The acoustic stimuli were single tones near best frequency (BF) and two-tone complexes. Single tone input/output (I/O) functions had a saturating growth for the magnitude and their phase shifts were strongly dependent on the tone frequency relative to BF. For IHCs, a BF tone stimulus produced no phase shift in the ac receptor potential response. Phase lag or lead occurred for tones below or above BF, respectively. BM velocity I/O functions were not as compressively saturating as IHC ac I/O curves. BM phase shifts (in relation to BF) were similar to those of the IHCs. Two-tone suppression was observed in both IHC and BM response measures. Suppressor tones on the low-frequency side of BF produced complex suppression results, which were inconsistent with a simple attenuation model for suppression. The growth of suppression was faster than the attenuation from equivalent level reductions of the probe tone, and phase shifts were phase lead. Depending upon experimental conditions, phase change with suppression may be in the opposite direction from phase change observed from pure attenuation of the probe tone. High-frequency suppressors (relative to BF) are consistent with an attenuation model of suppression for the IHCs of the current study. High side suppression of basilar membrane velocity, however, differed from the IHCs in a systematic way. The phase change caused by suppression of BM velocity was always smaller than that of an equivalent reduction in the probe tone level.

Two-tone suppression in inner hair cell responses: Correlates of rate suppression in the auditory nerve

Inner hair cell (IHC) recordings were made from second turn of the guinea pig cochlea where characteristic frequencies are approximately 4000 Hz. In order to compare IHC responses with rate suppression measured in the auditory nerve, suppressors were introduced that produced little or no response in the hair cell. The effects of a variable-frequency suppressor on a constant-frequency probe, placed near characteristic frequency, were also investigated since this paradigm is commonly used in single unit experiments. Resulting magnitude changes were measured in the fundamental component of the ac receptor potential and/or in the total de produced in the region of temporal overlap between the two stimulus inputs. This latter component is escecially important when considering how changes in IHC responses relate to decreases in discharge rate in single auditorynnerve fibers. Since the ac receptor potential is filtered by the hair cell's basolateral membrane, the dc component probably controls transmitter release at the characteristic frequency of these second-turn IHCs. Based on results from these and previous experiments, a proposal is advanced to explain the evolution of two-tone suppression in the peripheral auditory system. The paper also discusses the use of excitatory versus non-excitatory suppressors and includes a description of two-tone suppression areas at the mechanical, IHC and single unit levels. The explanation of low-side suppression areas is of special interest since hitherto they have been difficult to model (Kim, 1985).

Modulation of hair cell voltage responses to tones by low-frequency biasing of the basilar membrane in the guinea pig cochlea

Inner (IHC) and outer (OHC) hair cell receptor potentials were recorded during stimulation with combinations of high-frequency (HF) tones and a 100 Hz tone burst of constant level (80 dB SPL). For frequencies at and below characteristic frequency (CF), OHC AC receptor potentials were suppressed by the 100 Hz tone at levels of the HF tone below about 70 dB SPL (the initial steep part of the AC/level function) and at levels that were frequency specific for frequencies above CF. Suppression was associated with a phase lead for frequencies at and close to the CF. For frequencies above CF, the OHC AC response was either suppressed or augmented at levels of the HF tone both below and above 70 dB SPL, depending on the frequency. The action of the 100 Hz tone on the AC response/level functions was to change nonmonotonic functions into monotonic functions or vice versa. IHC DC receptor potentials were suppressed maximally at the CF and at levels and frequencies where suppression of the OHC AC response and the appearance of the IHC DC response overlapped. For levels of the HF tone above 70 dB SPL, the amplitude of the responses of both IHCs and OHCs to the 100 Hz tone are suppressed and become more symmetrical through selective attenuation of the depolarizing phase of the IHC response and the hyperpolarizing phase of the OHC response. In IHCs from insensitive preparations, the response to the 100 Hz tone is augmented in the presence of the HF tone, which may indicate a shift in the operating point of transduction. At frequencies about one-half an octave below the CF, the phase of the 100 Hz voltage response of OHCs but not IHCs is inverted for levels of the HF tone above about 90 dB SPL. It is proposed that amplitude and phase changes in the response to the HF tone due to the presence of the 100 Hz tone are the result of changes in OHC feedback processes and in the mode of movement of the interface between OHC stereocilia and tectorial membrane. The interaction between the responses to HF and 100 Hz tones indicates that feedback contributes significantly to the voltage responses of OHCs throughout the frequency response range.

Relations between dc and ac mechanical responses of the outer hair cell.

Whole cell voltage clamp and displacement measuring photodiode techniques were used to study electrophysiological and mechanical properties of the guinea pig OHC. OHCs demonstrate a voltage-to-movement (V-M) function which can be fit by a two-state Boltzmann relation, where the cell normally rests near the hyperpolarizing saturation region (−70 to −90 mV). The voltage at half-maximal length change (Vh;−40 to −20 mV) is depolarized relative to the resting potential. Mechanical responses can be as large as 30 nm/mV. The form of the V–M function ensures that for symmetrical sinusoidal voltage stimulation about the resting potential, ac and dc mechanical responses will be produced. Analysis of OHC motility using pure tone voltage bursts from 11 to 3200 Hz demonstrates both ac and dc mechanical responses; ac responses can be as large as 3–4 μm, and decrease in magnitude roughly linearly with voltage. However, dc mechanical responses which can be as large as 1/4 of the ac response with large ac voltage stimuli, decrease nonlinearly with ac voltage. The net effect is that the mechanical dc/ac ratio approaches zero for small ac voltages. This is predicted from the form of the V–M function, whose nonlinearity is a consequence of the resting potential being displaced from Vh. Invivo, an ac receptor potential at auditory threshold at 15 μV, would produce a dc mechanical response 2500 times smaller than the ac response. Measures of the ac mechanical frequency response have shown 3-dB break points approaching 1 kHz, with the phase of the fundamental component (obtained by FFT) corresponding to the phase of the transmembrane voltage and not that of current, as would be expected for a voltage driven process. Maximum velocity induced by voltage step is 6.5 mm/s. The invivo speed of OHC motility will be limited by the cell’s RC time constant, which itself is dynamic due to voltage dependent conductances and capacitance in the OHC membrane. [Work supported by NIDCD DC00273.]

Electrical responses from the chicken basilar papilla

A surgical approach to the basilar papilla of the chicken cochlea has been developed which allows recordings from within the hair cells and supporting cells in vivo. The frequency tuning curves for the AC receptor potentials measured in extracellular space immediately outside hair cells, within the hair cells and in adjacent supporting cells were similar, with best frequencies ranging from 600 Hz to 2000 Hz, and Q 10 dB values between 0.6 and 2.5. In cells classified as hair cells there was no evidence of spontaneous oscillations in the membrane potentials, nor of ringing of the membrane potential in response to injected current pulses. Moreover, displacement of the papilla with the microelectrodes could modulate the hair cell membrane potential over the range 0 to −90 mV, suggesting that the current-voltage relationship in these cells was essentially linear. This view was supported by preliminary investigations of cell properties with current injection. We interpret these observations as evidence that the hair cells impaled were not electrically tuned under our experimental conditions, unlike the hair cells of the turtle cochlea. These observations, taken together with the electrode angles used, fluorescent dye-marking and previous measurements of chicken hair cells in vitro [Fuchs, P.A., Nagai, T. and Evans, M.G. (1988) J. Neuro. Sci. 8, 2460–2467], suggest that the hair cells we have impaled were the short hair cells of the papilla, and that the tuning of the AC receptor potentials we have observed was due solely to a tuned mechanical drive to their hair bundles.

Inner hair cell responses to the 2f1-f2 intermodulation distortion product

Recordings of dc and ac receptor potentials from pigmented guinea pig inner hair cells indicate strong responses to the 2f1-f2 intermodulation tone when f1 and f2 are greater than the hair cell characteristic frequency and do not cause a response when given individually. The effective magnitude of this cubic distortion product (CDP) was about 25-30 dB below equal sound level primaries over a 20-30-dB range of their sound levels. The relative strength of the CDP declined at a rate greater than 180-dB/oct separation of the primaries. When magnitude of f1 or f2 was held constant, the growth of CDP was nonmonotonic, exhibiting a distinct maximum. With a constant level of f1 or f2, optimal CDP was produced when the level of f2 was 10-15 dB greater than f1. Strong two-tone suppression from the primaries has a role in shaping the CDP growth. The ac receptor potentials of the CDP show a 150 degrees-200 degrees phase shift when the primaries are increased over a 50-dB range. These results support the hypothesis of a propagated CDP in the cochlea and are consistent with the major features of related studies of human psychoacoustic experiments, afferent nerve neural rate functions, and ear canal distortion products.

Two-tone interactions in inner hair cell receptor potentials: AC versus DV effects

Two-tone suppression was studied in both ac and dc receptor potentials recorded from inner hair cells in the third turn of the guinea pig cochlea. Frequency response functions for the ac component obtained at moderate intensities indicate that frequency selectivity is enhanced when a high-side suppressor is added to the stimulus. This occurs because the largest reductions in magnitude take place well above and below the characteristic frequency (CF) of the cell. Changes near CF are relatively small. In contrast, frequency response functions for the dc receptor potential become broader in the presence of an excitatory suppressor. The significance of these findings for the processing of complex stimuli is considered.

Two-tone suppression in inner hair cell responses

In an attempt to characterize certain aspects of two-tone suppression (2TS), ac receptor potentials were recorded from mammalian inner hair cells (IHC) in the third turn of the guinea pig cochlea. By comparing magnitude and phase changes occurring during suppression with predictions made on the basis of level-dependent responses to single-tone inputs, it is possible to determine whether 2TS is mimicked by simply attenuating stimulus intensity. Results indicate that the effects of suppression are not simulated by simple input attenuation for low probe levels which produce responses below saturation. In these situations, the suppressor causes a decrease in the magnitude of the ac receptor potential with the largest deviations measured at the characteristic frequency (CF) of the cell. Thus, frequency response functions become broader. Response phase goes through a lag/lead transition at CF, also opposite to the results expected by simply decreasing input to the cell. At higher probe levels, within the saturation region, the magnitude reductions produced during 2TS are largest for stimulus frequencies well below and well above CF. This effect partially reverses the broadening of frequency response functions seen at moderate intensities with possible benefits for the processing of complex stimuli at conversational levels. Although the magnitude data obtained at high probe levels are consistent with the attenuation hypothesis, the companion phase measures did not show the expected lead/lag transition through CF since phase changes were generally lags. Consequently, the high-level suppression data suggest that 2TS may reduce input to the IHC but in a way which is not equivalent to the attenuation of a single-input stimulus.

The low-frequency response of inner hair cells in the guinea pig cochlea: Implications for fluid coupling and resonance of the stereocilia

AC receptor potentials within the inner hair cells of the basal turn of the guinea pig cochlea have been recorded for stimuli in the frequency range 20 Hz to 3200 Hz. Comparison of these potentials with potentials recorded in scala media suggests that the stereocilia of many inner hair cells are stimulated by the transverse velocity of the cochlear partition for very low frequency, but above a transition frequency in the range 400 Hz to 1000 Hz they become entrained with partition displacement. It is suggested that such a transition is probably a simple consequence of the fluid coupling that drives these cells, and that mechanical resonance of the free-standing stereocilia of the inner hair cells does not occur in the basal turn of the guinea pig. These results do not, however, preclude the possibility of mechanical resonance involving the stereocilia of the outer hair cells. The results also indicate that the bodies of these cells low-pass filter the intracellular receptor potential, with a cutoff frequency of approximately 1000 Hz.

Membrane potential and response changes in mammalian cochlear hair cells during intracellular recording

In the preceding paper (Dallos, P. (1985) J. Neurosci. 5: 1591-1608) some fundamental properties of cochlear inner and outer hair cells were considered. On the basis of intracellular recording from the low frequency region of the guinea pig cochlea, such measures as membrane potentials, the dependence of AC and DC receptor potential magnitude on stimulus level and frequency, and tuning characteristics relative to gross responses were discussed. During prolonged recording various changes in membrane and receptor potentials were observed. In the present paper the relationship between these variations is described and evaluated in the context of a simple hair cell model.

Dynamic aspects of guinea pig inner hair cell receptor potentials with transient asphyxia.

DC and AC receptor potentials of cochlear inner hair cells in response to tone bursts of various frequencies and intensities were continuously measured during and following periods of transient asphyxia. The effects of asphyxia were most pronounced for low sound pressure level (SPL) acoustic stimuli near the characteristic frequency (CF) of the inner hair cell, leading to vulnerability of the 'tip' of the cell's frequency tuning curve (FTC). The resulting changes in the shape of the FTC are, first, a reduction in tip criterion sensitivity of 10-20 dB without significant loss in sharpness of tuning. Later, when the full effect of 30-45 s asphyxia occurs, tip sensitivity loss between 30 and 65 dB is accompanied by greatly broadened tuning and a shift downward in frequency of the CF by greater than 1/4 octave. The CF shift is due to a progressive loss of high frequency sensitivity. The linear segment of the input-output (intensity) function, plotted as log DC receptor potential versus SPL (at the original CF), becomes longer during the early phase asphyxia, and the slope of the segment declines by 50%. At high SPLs, for all frequencies, the time course of the receptor potential change was similar in shape to that exhibited by the endocochlear potential (EP). In particular, for high sound levels, the recovery of response matches the EP while for low level tip frequency sounds recovery is protracted. No difference between the decline of the AC and DC receptor potentials at CF was observed. Inner hair cell resting membrane potential (Em) hyperpolarized during asphyxia by 2-6 mV, correlating with the change in EP according to a ratio of 1/10 (Em/EP).