The transduction channel filter in auditory hair cells

Frequency-dependent properties of a fluid jet stimulus: calibration, modeling, and application to cochlear hair cell bundles

The investigation of small physiological mechano-sensory systems, such as hair cells or their accessory structures in the inner ear or lateral line organ, requires mechanical stimulus equipment that allows spatial manipulation with micrometer precision and stimulation with amplitudes down to the nanometer scale. Here, we describe the calibration of a microfluid jet produced by a device that was designed to excite individual cochlear hair cell bundles or cupulae of the fish superficial lateral line system. The calibration involves a precise definition of the linearity and time- and frequency-dependent characteristics of the fluid jet as produced by a pressurized fluid-filled container combined with a glass pipette having a microscopically sized tip acting as an orifice. A procedure is described that can be applied during experiments to obtain a fluid jet’s frequency response, which may vary with each individual glass pipette. At small orifice diameters (<15 μm), the fluid velocity of the jet is proportional to the displacement of the piezoelectric actuator pressurizing the container’s volume and is suitable to stimulate the hair bundles of sensory hair cells. With increasing diameter, the fluid jet velocity becomes proportional to the actuator’s velocity. The experimentally observed characteristics can be described adequately by a dynamical model of damped fluid masses coupled by elastic components.

Mechanoelectric transduction of adult inner hair cells

Inner hair cells (IHCs) are the true sensory receptors in the cochlea; they transmit auditory information to the brain. IHCs respond to basilar membrane (BM) vibration by producing a transducer current through mechanotransducer (MET) channels located at the tip of their stereocilia when these are deflected. The IHC MET current has not been measured from adult animals. We simultaneously recorded IHC transducer currents and BM motion in a gerbil hemicochlea to examine relationships between these two variables and their variation along the cochlear length. Results show that although maximum transducer currents of IHCs are uniform along the cochlea, their operating range is graded and is narrower in the base. The MET current displays adaptation, which along with response magnitude depends on extracellular calcium concentration. The rate of adaptation is invariant along the cochlear length. We introduce a new method of measuring adaptation using sinusoidal stimuli. There is a phase lead of IHC transducer currents relative to sinusoidal BM displacement, reflecting viscoelastic coupling of their cilia and their adaptation process.

Hair cell mechanotransduction: the dynamic interplay between structure and function

Hair cells are capable of detecting mechanical vibrations of molecular dimensions at frequencies in the 10s to 100s of kHz. This remarkable feat is accomplished by the interplay of mechanically gated ion channels located near the top of a complex and dynamic sensory hair bundle. The hair bundle is composed of a series of actin-filled stereocilia that has both active and passive mechanical components as well as a highly active turnover process, whereby the components of the hair bundle are rapidly and continually recycled. Hair bundle mechanical properties have significant impact on the gating of the mechanically activated channels, and delineating between attributes intrinsic to the ion channel and those imposed by the channel's microenvironment is often difficult. This chapter describes what is known and accepted regarding hair-cell mechanotransduction and what remains to be explored, particularly, in relation to the interplay between hair bundle properties and mechanotransducer channel response. The interplay between hair bundle dynamics and mechanotransduction are discussed.

Mechanosensitive ion channels of spiders: mechanical coupling, electrophysiology, and synaptic modulation

Arthropods have provided several important mechanoreceptor models because of the relatively large size and accessibility of their primary sensory neurons. Three types of spider receptors- tactile hairs, trichobothria, and slit sensilla-have given important information about the coupling of external mechanical stimuli to the neuronal membrane, transduction of mechanical force into receptor current, encoding of afferent action potentials, and efferent modulation of peripheral sensory receptors. Slit sensilla, found only in spiders, are particularly important because they allow intracellular recording from sensory neurons during mechanical stimulation. Experiments on slit sensilla have shown that their mechanosensitive ion channels are sodium selective, blocked by amiloride, and open more at low pH. This evidence suggests that the channels are members of the same molecular family as degenerins, acid-sensitive ion channels, and epithelial sodium channels. Slit sensilla have also yielded evidence about the location, density, single-channel conductance, and dynamic properties of the mechanosensitive channels. Spider mechanoreceptors are modulated in the periphery by efferent neurons and possibly by circulating chemicals. Mechanisms of modulation, intracellular signaling, and role of intracellular calcium are areas of active investigation.

Steady-state adaptation of mechanotransduction modulates the resting potential of auditory hair cells, providing an assay for endolymph [Ca2+]

The auditory hair cell resting potential is critical for proper translation of acoustic signals to the CNS, because it determines their filtering properties, their ability to respond to stimuli of both polarities, and, because the hair cell drives afferent firing rates, the resting potential dictates spontaneous transmitter release. In turtle auditory hair cells, the filtering properties are established by the interactions between BK calcium-activated potassium channels and an L-type calcium channel (electrical resonance). However, both theoretical and in vitro recordings indicate that a third conductance is required to set the resting potential to a point on the I(Ca) and I(BK) activation curves in which filtering is optimized like that found in vivo. Present data elucidate a novel mechanism, likely universal among hair cells, in which mechanoelectric transduction (MET) and its calcium-dependent adaptation provide the depolarizing current to establish the hair cell resting potential. First, mechanical block of the MET current hyperpolarized the membrane potential, resulting in broadband asymmetrical resonance. Second, altering steady-state adaptation by altering the [Ca2+] bathing the hair bundle changed the MET current at rest, the magnitude of which resulted in membrane potential changes that encompassed the best resonant voltage. The Ca2+ sensitivity of adaptation allowed for the first physiological estimate of endolymphatic Ca2+ near the MET channel (56 +/- 11 microM), a value similar to bulk endolymph levels. These effects of MET current on resting potential were independently confirmed using a theoretical model of electrical resonance that included the steady-state MET conductance.

A large-conductance calcium-selective mechanotransducer channel in mammalian cochlear hair cells

Sound stimuli are detected in the cochlea by opening of hair cell mechanotransducer (MT) channels, one of the few ion channels not yet conclusively identified at a molecular level. To define their performance in situ, we measured MT channel properties in inner hair cells (IHCs) and outer hair cells (OHCs) at two locations in the rat cochlea tuned to different characteristic frequencies (CFs). The conductance (in 0.02 mM calcium) of MT channels from IHCs was estimated as 260 pS at both low-frequency and mid-frequency positions, whereas that from OHCs increased with CFs from 145 to 210 pS. The combination of MT channel conductance and tip link number, assayed from scanning electron micrographs, accounts for variation in whole-cell current amplitude for OHCs and its invariance for IHCs. Channels from apical IHCs and OHCs having a twofold difference in unitary conductance were both highly calcium selective but were distinguishable by a small but significant difference in calcium permeability and in their response to lowering ionic strength. The results imply that the MT channel has properties possessed by few known candidates, and its diversity suggests expression of multiple isoforms.

Mechanisms underlying the temporal precision of sound coding at the inner hair cell ribbon synapse

Our auditory system is capable of perceiving the azimuthal location of a low frequency sound source with a precision of a few degrees. This requires the auditory system to detect time differences in sound arrival between the two ears down to tens of microseconds. The detection of these interaural time differences relies on network computation by auditory brainstem neurons sharpening the temporal precision of the afferent signals. Nevertheless, the system requires the hair cell synapse to encode sound with the highest possible temporal acuity. In mammals, each auditory nerve fibre receives input from only one inner hair cell (IHC) synapse. Hence, this single synapse determines the temporal precision of the fibre. As if this was not enough of a challenge, the auditory system is also capable of maintaining such high temporal fidelity with acoustic signals that vary greatly in their intensity. Recent research has started to uncover the cellular basis of sound coding. Functional and structural descriptions of synaptic vesicle pools and estimates for the number of Ca(2+) channels at the ribbon synapse have been obtained, as have insights into how the receptor potential couples to the release of synaptic vesicles. Here, we review current concepts about the mechanisms that control the timing of transmitter release in inner hair cells of the cochlea.

Active hair bundle movements in auditory hair cells

The frequency selectivity of mammalian hearing depends on not only the passive mechanics of the basilar membrane but also an active amplification of the mechanical stimulus by the cochlear hair cells. The common view is that amplification stems from the somatic motility of the outer hair cells (OHCs), changes in their length impelled by voltage-dependent transitions in the membrane protein prestin. Whether this voltage-controlled mechanism, whose frequency range may be limited by the membrane time constant, has the band width to cover the entire auditory range of mammals is uncertain. However, there is ample evidence for an alternative mode of force generation by hair cells of non-mammals, such as frogs and turtles, which probably lack prestin. The latter process involves active motion of the hair bundle underpinned by conformational changes in the mechanotransducer (MT) channels and activation of one or more isoforms of myosin. This review summarizes evidence for active hair bundle motion and its connection to MT channel adaptation. Key factors for the hair bundle motor to play a role in the mammalian cochlea include the size and speed of force production.

Mechano-electrical transduction: new insights into old ideas

The gating-spring theory of hair cell mechanotransduction channel activation was first postulated over twenty years ago. The basic tenets of this hypothesis have been reaffirmed in hair cells from both auditory and vestibular systems and across species. In fact, the basic findings have been reproduced in every hair cell type tested. A great deal of information regarding the structural, mechanical, molecular and biophysical properties of the sensory hair bundle and the mechanotransducer channel has accumulated over the past twenty years. The goal of this review is to investigate new data, using the gating spring hypothesis as the framework for discussion. Mechanisms of channel gating are presented in reference to the need for a molecular gating spring or for tethering to the intra- or extracellular compartments. Dynamics of the sensory hair bundle and the presence of motor proteins are discussed in reference to passive contributions of the hair bundle to gating compliance. And finally, the molecular identity of the channel is discussed in reference to known intrinsic properties of the native transducer channel.

The sensory and motor roles of auditory hair cells

Cochlear hair cells respond with phenomenal speed and sensitivity to sound vibrations that cause submicron deflections of their hair bundle. Outer hair cells are not only detectors, but also generate force to augment auditory sensitivity and frequency selectivity. Two mechanisms of force production have been proposed: contractions of the cell body or active motion of the hair bundle. Here, we describe recently identified proteins involved in the sensory and motor functions of auditory hair cells and present evidence for each force generator. Both motor mechanisms are probably needed to provide the high sensitivity and frequency discrimination of the mammalian cochlea.

Hair-cell mechanotransduction and cochlear amplification

In the inner ear, sensory hair cells not only detect but also amplify the softest sounds, allowing us to hear over an extraordinarily wide intensity range. This amplification is frequency specific, giving rise to exquisite frequency discrimination. Hair cells detect sounds with their mechanotransduction apparatus, which is only now being dissected molecularly. Signal detection is not the only role of this molecular network; amplification of low-amplitude signals by hair bundles seems to be universal in hair cells. "Fast adaptation," the rapid closure of transduction channels following a mechanical stimulus, appears to be intimately involved in bundle-based amplification.