Membrane properties specialize mammalian inner hair cells for frequency or intensity encoding

Ca2+ regulation of glutamate release from inner hair cells of hearing mice

In our hearing organ, sound is encoded at ribbon synapses formed by inner hair cells (IHCs) and spiral ganglion neurons (SGNs). How the underlying synaptic vesicle (SV) release is controlled by Ca2+ in IHCs of hearing animals remained to be investigated. Here, we performed patch-clamp SGN recordings of the initial rate of release evoked by brief IHC Ca2+-influx in an ex vivo cochlear preparation from hearing mice. We aimed to closely mimic physiological conditions by perforated-patch recordings from IHCs kept at the physiological resting potential and at body temperature. We found release to relate supralinearly to Ca2+-influx (power, m: 4.3) when manipulating the [Ca2+] available for SV release by Zn2+-flicker-blocking of the single Ca2+-channel current. In contrast, a near linear Ca2+ dependence (m: 1.2 to 1.5) was observed when varying the number of open Ca2+-channels during deactivating Ca2+-currents and by dihydropyridine channel-inhibition. Concurrent changes of number and current of open Ca2+-channels over the range of physiological depolarizations revealed m: 1.8. These findings indicate that SV release requires ~4 Ca2+-ions to bind to their Ca2+-sensor of fusion. We interpret the near linear Ca2+-dependence of release during manipulations that change the number of open Ca2+-channels to reflect control of SV release by the high [Ca2+] in the Ca2+-nanodomain of one or few nearby Ca2+-channels. We propose that a combination of Ca2+ nanodomain control and supralinear intrinsic Ca2+-dependence of fusion optimally links SV release to the timing and amplitude of the IHC receptor potential and separates it from other IHC Ca2+-signals unrelated to afferent synaptic transmission.

From the outer ear to the nerve: A complete computer model of the peripheral auditory system

Computer models of the individual components of the peripheral auditory system - the outer, middle, and inner ears and the auditory nerve - have been developed in the past, with varying level of detail, breadth, and faithfulness of the underlying parameters. Building on previous work, we advance the modeling of the ear by presenting a complete, physiologically justified, bottom-up computer model based on up-to-date experimental data that integrates all of these parts together seamlessly. The detailed bottom-up design of the present model allows for the investigation of partial hearing mechanisms and their defects, including genetic, molecular, and microscopic factors. Also, thanks to the completeness of the model, one can study microscopic effects in the context of their implications on hearing as a whole, enabling the correlation with neural recordings and non-invasive psychoacoustic methods. Such a model is instrumental for advancing quantitative understanding of the mechanism of hearing, for investigating various forms of hearing impairment, as well as for devising next generation hearing aids and cochlear implants.

Cochlear aging disrupts the correlation between spontaneous rate- and sound-level coding in auditory nerve fibers

The spiking activity of auditory nerve fibers (ANFs) transmits information about the acoustic environment from the cochlea to the central auditory system. Increasing age leads to degeneration of cochlear tissues, including the sensory hair cells and stria vascularis. Here, we aim to identify the functional effects of such age-related cochlear pathologies of ANFs. Rate-level functions (RLFs) were recorded from single-unit ANFs of young adult (n = 52, 3-12 months) and quiet-aged (n = 24, >36 months) Mongolian gerbils of either sex. RLFs were used to determine sensitivity and spontaneous rates (SRs) and were classified into flat-saturating, sloping-saturating, and straight categories, as previously established. A physiologically based cochlear model, adapted for the gerbil, was used to simulate the effects of cochlear degeneration on ANF physiology. In ANFs tuned to low frequencies (<3.5 kHz), SR was lower in those of aged gerbils, while an age-related loss of low-SR fibers was evident in ANFs tuned to high frequencies. These changes in SR distribution did not affect the typical SR versus sensitivity correlation. The distribution of RLF types among low-SR fibers, however, shifted toward that of high-SR fibers, specifically showing more fast-saturating and fewer sloping-saturating RLFs. A modeled striatal degeneration, which affects the combined inner hair cell and synaptic output, reduced SR but left RLF type unchanged. An additional reduced basilar membrane gain, which decreased sensitivity, explained the changed RLF types. Overall, the data indicated age-related changes in the characteristics of single ANFs that blurred the established relationships between SR and RLF types.NEW & NOTEWORTHY Auditory nerve fibers, which connect the cochlea to the central auditory system, change their encoding of sound level in aged gerbils. In addition to a general shift to higher levels, indicative of decreased sensitivity, level coding was also differentially affected in fibers with low- and high-spontaneous rates. Loss of low-spontaneous rate fibers, combined with a general decrease of spontaneous rate, further blurs the categorization of auditory nerve fiber types in the aged gerbil.

Modulation of potassium conductances optimizes fidelity of auditory information

Potassium channels in auditory neurons are rapidly modified by changes in the auditory environment. In response to elevated auditory stimulation, short-term mechanisms such as protein phosphorylation and longer-term mechanisms such as accelerated channel synthesis increase the amplitude of currents that promote high-frequency firing. It has been suggested that this allows neurons to fire at high rates in response to high sound levels. We have carried out simple simulations of the response to postsynaptic neurons to patterns of neurotransmitter release triggered by auditory stimuli. These demonstrate that the amplitudes of potassium currents required for optimal encoding of a low-amplitude auditory signal differ from those for louder sounds. Specifically, the cross-correlation of the output of a neuron with an auditory stimulus is improved by increasing potassium currents as sound amplitude increases. Temporal fidelity for low-frequency stimuli is improved by increasing potassium currents that activate at negative potentials, while that for high-frequency stimuli requires increases in currents that activate at positive membrane potentials. These effects are independent of the firing rate. Moreover, levels of potassium currents that maximize the fidelity of the output of an ensemble of neurons differ from those that maximize fidelity for a single neuron. This suggests that the modulatory mechanisms must coordinate channel activity in groups of neurons or an entire nucleus. The simulations provide an explanation for the modulation of the intrinsic excitability of auditory brainstem neurons by changes in environmental sound levels, and the results may extend to information processing in other neural systems.

Lack of neural contributions to the summating potential in humans with Meniere's disease

Objective

To investigate the electrophysiology of the cochlear summating potential (SP) in patients with Meniere's disease (MD). Although long considered a purely hair cell potential, recent studies show a neural contribution to the SP. Patients with MD have an enhanced SP compared to those without the disease. Consequently, this study was to determine if the enhancement of the SP was in whole or part due to neural dysfunction.

Design

Study participants included 41 adults with MD and 53 subjects with auditory neuropathy spectrum disorder (ANSD), undergoing surgery where the round window was accessible. ANSD is a condition with known neural dysfunction, and thus represents a control group for the study. The ANSD subjects and 17 of the MD subjects were undergoing cochlear implantation (CI) surgery; the remaining MD subjects were undergoing either endolymphatic sac decompression or labyrinthectomy to alleviate the symptoms of MD. Electrocochleography was recorded from the round window using high intensity (90 dB nHL) tone bursts. The SP and compound action potential (CAP) were measured to high frequencies (> = 2 kHz) and the SP, cochlear microphonic (CM) and auditory nerve neurophonic (ANN) to low frequencies. Linear mixed models were used to assess differences between MD and ANSD subjects.

Results

Across frequencies, the MD subjects had smaller alternating current (AC) response than the ANSD subjects (F = 31.61,534, p < 0.001), but the SP magnitudes were larger (F = 94.31,534, p < 0.001). For frequencies less than 4 kHz the SP magnitude in the MD group was significantly correlated with the magnitude of the CM (p's < 0.001) but not in the ANSD group (p's > 0.05). Finally, the relative proportions of both ANN and CAP were greater in MD compared to ANSD subjects. The shapes of the waveforms in the MD subjects showed the presence of multiple components contributing to the SP, including outer and inner hair cells and neural activity.

Conclusion

The results support the view that the increased negative polarity SP in MD subjects is due to a change in the operating point of hair cells rather than a loss of neural contribution. The steady-state SP to tones in human subjects is a mixture of different sources with different polarities.

Signal transmission in mature mammalian vestibular hair cells

The maintenance of balance and gaze relies on the faithful and rapid signaling of head movements to the brain. In mammals, vestibular organs contain two types of sensory hair cells, type-I and type-II, which convert the head motion-induced movement of their hair bundles into a graded receptor potential that drives action potential activity in their afferent fibers. While signal transmission in both hair cell types involves Ca2+-dependent quantal release of glutamate at ribbon synapses, type-I cells appear to also exhibit a non-quantal mechanism that is believed to increase transmission speed. However, the reliance of mature type-I hair cells on non-quantal transmission remains unknown. Here we investigated synaptic transmission in mammalian utricular hair cells using patch-clamp recording of Ca2+ currents and changes in membrane capacitance (ΔCm). We found that mature type-II hair cells showed robust exocytosis with a high-order dependence on Ca2+ entry. By contrast, exocytosis was approximately 10 times smaller in type-I hair cells. Synaptic vesicle exocytosis was largely absent in mature vestibular hair cells of CaV1.3 (CaV1.3−/−) and otoferlin (Otof−/−) knockout mice. Even though Ca2+-dependent exocytosis was small in type-I hair cells of wild-type mice, or absent in CaV1.3−/− and Otof−/−mice, these cells were able to drive action potential activity in the postsynaptic calyces. This supports a functional role for non-quantal synaptic transmission in type-I cells. The large vesicle pools in type-II cells would facilitate sustained transmission of tonic or low-frequency signals. In type-I cells, the restricted vesicle pool size, together with a rapid non-quantal mechanism, could allow them to sustain high-frequency phasic signal transmission at their specialized large calyceal synapses.

Signatures of cochlear processing in neuronal coding of auditory information

The vertebrate ear is endowed with remarkable perceptual capabilities. The faintest sounds produce vibrations of magnitudes comparable to those generated by thermal noise and can nonetheless be detected through efficient amplification of small acoustic stimuli. Two mechanisms have been proposed to underlie such sound amplification in the mammalian cochlea: somatic electromotility and active hair-bundle motility. These biomechanical mechanisms may work in concert to tune auditory sensitivity. In addition to amplitude sensitivity, the hearing system shows exceptional frequency discrimination allowing mammals to distinguish complex sounds with great accuracy. For instance, although the wide hearing range of humans encompasses frequencies from 20 Hz to 20 kHz, our frequency resolution extends to one-thirtieth of the interval between successive keys on a piano. In this article, we review the different cochlear mechanisms underlying sound encoding in the auditory system, with a particular focus on the frequency decomposition of sounds. The relation between peak frequency of activation and location along the cochlea - known as tonotopy - arises from multiple gradients in biophysical properties of the sensory epithelium. Tonotopic mapping represents a major organizational principle both in the peripheral hearing system and in higher processing levels and permits the spectral decomposition of complex tones. The ribbon synapses connecting sensory hair cells to auditory afferents and the downstream spiral ganglion neurons are also tuned to process periodic stimuli according to their preferred frequency. Though sensory hair cells and neurons necessarily filter signals beyond a few kHz, many animals can hear well beyond this range. We finally describe how the cochlear structure shapes the neural code for further processing in order to send meaningful information to the brain. Both the phase-locked response of auditory nerve fibers and tonotopy are key to decode sound frequency information and place specific constraints on the downstream neuronal network.

Fast adaptation of cooperative channels engenders Hopf bifurcations in auditory hair cells

Since the pioneering work of Thomas Gold, published in 1948, it has been known that we owe our sensitive sense of hearing to a process in the inner ear that can amplify incident sounds on a cycle-by-cycle basis. Called the active process, it uses energy to counteract the viscous dissipation associated with sound-evoked vibrations of the ear's mechanotransduction apparatus. Despite its importance, the mechanism of the active process and the proximate source of energy that powers it have remained elusive, especially at the high frequencies characteristic of amniote hearing. This is partly due to our insufficient understanding of the mechanotransduction process in hair cells, the sensory receptors and amplifiers of the inner ear. It has been proposed previously that cyclical binding of Ca2+ ions to individual mechanotransduction channels could power the active process. That model, however, relied on tailored reaction rates that structurally forced the direction of the cycle. Here we ground our study on our previous model of hair-cell mechanotransduction, which relied on cooperative gating of pairs of channels, and incorporate into it the cyclical binding of Ca2+ ions. With a single binding site per channel and reaction rates drawn from thermodynamic principles, the current model shows that hair cells behave as nonlinear oscillators that exhibit Hopf bifurcations, dynamical instabilities long understood to be signatures of the active process. Using realistic parameter values, we find bifurcations at frequencies in the kilohertz range with physiological Ca2+ concentrations. The current model relies on the electrochemical gradient of Ca2+ as the only energy source for the active process and on the relative motion of cooperative channels within the stereociliary membrane as the sole mechanical driver. Equipped with these two mechanisms, a hair bundle proves capable of operating at frequencies in the kilohertz range, characteristic of amniote hearing.

Dynamic Heterogeneity Shapes Patterns of Spiral Ganglion Activity

Neural response properties that typify primary sensory afferents are critical to fully appreciate because they establish and, ultimately represent, the fundamental coding design used for higher-level processing. Studies illuminating the center-surround receptive fields of retinal ganglion cells, for example, were ground-breaking because they determined the foundation of visual form detection. For the auditory system, a basic organizing principle of the spiral ganglion afferents is their extensive electrophysiological heterogeneity establishing diverse intrinsic firing properties in neurons throughout the spiral ganglion. Moreover, these neurons display an impressively large array of neurotransmitter receptor types that are responsive to efferent feedback. Thus, electrophysiological diversity and its neuromodulation are a fundamental encoding mechanism contributed by the primary afferents in the auditory system. To place these features into context, we evaluated the effects of hyperpolarization and cAMP on threshold level as indicators of overall afferent responsiveness in CBA/CaJ mice of either sex. Hyperpolarization modified threshold gradients such that distinct voltage protocols could shift the relationship between sensitivity and stimulus input to reshape resolution. This resulted in an "accordion effect" that appeared to stretch, compress, or maintain responsivity across the gradient of afferent thresholds. cAMP targeted threshold and kinetic shifts to rapidly adapting neurons, thus revealing multiple cochleotopic properties that could potentially be independently regulated. These examples of dynamic heterogeneity in primary auditory afferents not only have the capacity to shift the range, sensitivity, and resolution, but to do so in a coordinated manner that appears to orchestrate changes with a seemingly unlimited repertoire.SIGNIFICANCE STATEMENT How do we discriminate the more nuanced qualities of the sound around us? Beyond the basics of pitch and loudness, aspects, such as pattern, distance, velocity, and location, are all attributes that must be used to encode acoustic sensations effectively. While higher-level processing is required for perception, it would not be unexpected if the primary auditory afferents optimized receptor input to expedite neural encoding. The findings reported herein are consistent with this design. Neuromodulation compressed, expanded, shifted, or realigned intrinsic electrophysiological heterogeneity to alter neuronal responses selectively and dynamically. This suggests that diverse spiral ganglion phenotypes provide a rich substrate to support an almost limitless array of coding strategies within the first neural element of the auditory pathway.

A convolutional neural-network framework for modelling auditory sensory cells and synapses

In classical computational neuroscience, analytical model descriptions are derived from neuronal recordings to mimic the underlying biological system. These neuronal models are typically slow to compute and cannot be integrated within large-scale neuronal simulation frameworks. We present a hybrid, machine-learning and computational-neuroscience approach that transforms analytical models of sensory neurons and synapses into deep-neural-network (DNN) neuronal units with the same biophysical properties. Our DNN-model architecture comprises parallel and differentiable equations that can be used for backpropagation in neuro-engineering applications, and offers a simulation run-time improvement factor of 70 and 280 on CPU or GPU systems respectively. We focussed our development on auditory neurons and synapses, and show that our DNN-model architecture can be extended to a variety of existing analytical models. We describe how our approach for auditory models can be applied to other neuron and synapse types to help accelerate the development of large-scale brain networks and DNN-based treatments of the pathological system.

Drakopoulos et al developed a machine-learning and computational-neuroscience approach that transforms analytical models of sensory neurons and synapses into deep-neural-network (DNN) neuronal units with the same biophysical properties. Focusing on auditory neurons and synapses, they showed that their DNN-model architecture could be extended to a variety of existing analytical models and to other neuron and synapse types, thus potentially assisting the development of large-scale brain networks and DNN-based treatments.

A simplified physiological model of rate-level functions of auditory-nerve fibers

Several approaches have been used to describe the rate-level functions of auditory-nerve fibers (ANFs). One approach uses descriptive models that can be fitted easily to data. Another derives rate-level functions from comprehensive physiological models of auditory peripheral processing. Here, we seek to identify the minimal set of components needed to provide a physiologically plausible account of rate-level functions. Our model consists of a first-order Boltzmann mechanoelectrical transducer function relating the instantaneous stimulus pressure to an instantaneous output, followed by a lowpass filter that eliminates the AC component, followed by an exponential synaptic transfer function relating the DC component to the mean spike rate. This is perhaps the simplest physiologically plausible model capable of accounting for rate-level functions under the assumption that the model parameters for a given ANF and stimulus frequency are level-independent. We find that the model typically accounts well for rate-level functions from cat ANFs for all stimulus frequencies. More complicated model variants having saturating synaptic transfer functions do not perform significantly better, implying the system operates far away from synaptic saturation. Rate saturation in the model is caused by saturation of the DC component of the filter output (e.g., the receptor potential), which in turn is due to the saturation of the transducer function. The maximum mean spike rate is approximately constant across ANFs, such that the slope parameter of the exponential synaptic transfer function decreases with increasing spontaneous rate. If the synaptic parameters for a given ANF are assumed to be constant across stimulus frequencies, then frequency- and level-dependent input nonlinearities are derived that are qualitatively similar to those reported in the literature. Contrary to assumptions in the literature, such nonlinearities are obtained even for ANFs having high spontaneous rates. Finally, spike-rate adaptation is examined and found to be accounted for by a decrease in the slope parameter of the synaptic transfer function over time following stimulus onset.

Encoding sound in the cochlea: from receptor potential to afferent discharge

Ribbon-class synapses in the ear achieve analog to digital transformation of a continuously graded membrane potential to all-or-none spikes. In mammals, several auditory nerve fibres (ANFs) carry information from each inner hair cell (IHC) to the brain in parallel. Heterogeneity of transmission among synapses contributes to the diversity of ANF sound-response properties. In addition to the place code for sound frequency and the rate code for sound level, there is also a temporal code. In series with cochlear amplification and frequency tuning, neural representation of temporal cues over a broad range of sound levels enables auditory comprehension in noisy multi-speaker settings. The IHC membrane time constant introduces a low-pass filter that attenuates fluctuations of the receptor potential above 1-2 kHz. The ANF spike generator adds a high-pass filter via its depolarization-rate threshold that rejects slow changes in the postsynaptic potential and its phasic response property that ensures one spike per depolarization. Synaptic transmission involves several stochastic subcellular processes between IHC depolarization and ANF spike generation, introducing delay and jitter that limits the speed and precision of spike timing. ANFs spike at a preferred phase of periodic sounds in a process called phase-locking that is limited to frequencies below a few kilohertz by both the IHC receptor potential and the jitter in synaptic transmission. During phase-locking to periodic sounds of increasing intensity, faster and facilitated activation of synaptic transmission and spike generation may be offset by presynaptic depletion of synaptic vesicles, resulting in relatively small changes in response phase. Here we review encoding of spike-timing at cochlear ribbon synapses.

Optimized Tuning of Auditory Inner Hair Cells to Encode Complex Sound through Synergistic Activity of Six Independent K+ Current Entities

Auditory inner hair cells (IHCs) convert sound vibrations into receptor potentials that drive synaptic transmission. For the precise encoding of sound qualities, receptor potentials are shaped by K⁺ conductances tuning the properties of the IHC membrane. Using patch-clamp and computational modeling, we unravel this membrane specialization showing that IHCs express an exclusive repertoire of six voltage-dependent K⁺ conductances mediated by K_v1.8, K_v7.4, K_v11.1, K_v12.1, and BK_Ca channels. All channels are active at rest but are triggered differentially during sound stimulation. This enables non-saturating tuning over a far larger potential range than in IHCs expressing fewer current entities. Each conductance contributes to optimizing responses, but the combined activity of all channels synergistically improves phase locking and the dynamic range of intensities that IHCs can encode. Conversely, hypothetical simpler IHCs appear limited to encode only certain aspects (frequency or intensity). The exclusive channel repertoire of IHCs thus constitutes an evolutionary adaptation to encode complex sound through multifaceted receptor potentials.

Exocytosis in mouse vestibular Type II hair cells shows a high-order Ca2+ dependence that is independent of synaptotagmin-4

Mature hair cells transduce information over a wide range of stimulus intensities and frequencies for prolonged periods of time. The efficiency of such a demanding task is reflected in the characteristics of exocytosis at their specialized presynaptic ribbons. Ribbons are electron-dense structures able to tether a large number of releasable vesicles allowing them to maintain high rates of vesicle release. Calcium entry through rapidly activating, non-inactivating CaV 1.3 (L-type) Ca2+ channels in response to cell depolarization causes a local increase in Ca2+ at the ribbon synapses, which is detected by the exocytotic Ca2+ sensors. The Ca2+ dependence of vesicle exocytosis at mammalian vestibular hair cell (VHC) ribbon synapses is believed to be linear, similar to that observed in mature cochlear inner hair cells (IHCs). The linear relation has been shown to correlate with the presence of the Ca2+ sensor synaptotagmin-4 (Syt-4). Therefore, we studied the exocytotic Ca2+ dependence, and the release kinetics of different vesicle pool populations, in Type II VHCs of control and Syt-4 knockout mice using patch-clamp capacitance measurements, under physiological recording conditions. We found that exocytosis in mature control and knockout Type II VHCs displayed a high-order dependence on Ca2+ entry, rather than the linear relation previously observed. Consistent with this finding, the Ca2+ dependence and release kinetics of the ready releasable pool (RRP) of vesicles were not affected by an absence of Syt-4. However, we did find that Syt-4 could play a role in regulating the release of the secondary releasable pool (SRP) in these cells. Our findings show that the coupling between Ca2+ influx and neurotransmitter release at mature Type II VHC ribbon synapses is faithfully described by a nonlinear relation that is likely to be more appropriate for the accurate encoding of low-frequency vestibular information, consistent with that observed at low-frequency mammalian auditory receptors.

Phase Locking of Auditory Nerve Fibers: The Role of Lowpass Filtering by Hair Cells

Phase locking of auditory-nerve-fiber (ANF) responses to the temporal fine structure of acoustic stimuli, a hallmark of the auditory system's temporal precision, is important for many aspects of hearing. Previous work has shown that phase-locked period histograms are often well described by exponential transfer functions relating instantaneous stimulus pressure to instantaneous spike rate, with no observed clipping of the histograms. The operating points and slopes of these functions change with stimulus level. The mechanism underlying this apparent gain control is unclear but is distinct from mechanical compression, is independent of refractoriness and spike-rate adaptation, and is apparently instantaneous. Here we show that these findings can be accounted for by a model consisting of a static Boltzmann transducer function yielding a clipped output, followed by a lowpass filter and a static exponential transfer function. Using responses to tones of ANFs from cats of both sexes, we show that, for a given ANF, the period histograms obtained at all stimulus levels for a given stimulus frequency can be described using one set of level-independent model parameters. The model also accounts for changes in the maximum and minimum instantaneous spike rates with changes in stimulus level. Notably, the estimated cutoff frequency is lower for low- than for high-spontaneous-rate ANFs, implying a synapse-specific contribution to lowpass filtering. These findings advance our understanding of ANF phase locking by highlighting the role of peripheral filtering mechanisms in shaping responses of individual ANFs.SIGNIFICANCE STATEMENT Phase locking of auditory-nerve-fiber responses to the temporal fine structure of acoustic stimuli is important for many aspects of hearing. Period histograms typically retain an approximately sinusoidal shape across stimulus levels, with the peripheral auditory system operating as though its overall transfer function is an exponential function whose slope decreases with increasing stimulus level. This apparent gain control can be accounted for by a static saturating transducer function followed by a lowpass filter. In addition to attenuating the AC component, the filter approximately recovers the sinusoidal waveform of the stimulus. The estimated cutoff frequency varies with spontaneous rate, revealing a synaptic contribution to lowpass filtering. These findings highlight the significant impact of peripheral filtering mechanisms on phase locking.

Differential Gene Expression Patterns Between Apical and Basal Inner Hair Cells Revealed by RNA-Seq

Tonotopic differences in the structure and physiological function, e.g., synapse number, membrane properties, Ca²⁺ channels, Ca²⁺ dependence of exocytosis and vesicle pool replenishment of inner hair cells (IHCs) along the longitudinal cochlear axis have recently been discovered, suggesting different gene expression patterns of IHCs. To determine whether IHCs present different gene expression patterns along the longitudinal cochlear axis, apical and basal IHCs were collected separately using the suction pipette technique from adult mouse cochleae for RNA-seq and genome-wide transcriptome analysis. We found 689 annotated genes showed more than 2-fold increase in expression. Interestingly, 93.4% of the differentially expressed genes (DEGs) was upregulated in apical IHCs. Although a subset of genes that related to IHC machinery and deafness were found to be differentially expressed, a gradient of gene expression was indeed detected in Ocm, Pvalb, Prkd1, Fbxo32, Nme2, and Sncg, which may play putative roles in the Ca²⁺ buffering and survival regulation. The expression of these genes was validated by real-time quantitative PCR (RT-qPCR) or immunostaining. We conclude that IHCs from different mouse cochlear longitudinal position have different gene expression profiles. Our data might serve as a valuable resource for exploring the molecular mechanisms underlying different biological properties as well as the survival regulation of IHCs.

Hair Cell Afferent Synapses: Function and Dysfunction

To provide a meaningful representation of the auditory landscape, mammalian cochlear hair cells are optimized to detect sounds over an incredibly broad range of frequencies and intensities with unparalleled accuracy. This ability is largely conferred by specialized ribbon synapses that continuously transmit acoustic information with high fidelity and sub-millisecond precision to the afferent dendrites of the spiral ganglion neurons. To achieve this extraordinary task, ribbon synapses employ a unique combination of molecules and mechanisms that are tailored to sounds of different frequencies. Here we review the current understanding of how the hair cell's presynaptic machinery and its postsynaptic afferent connections are formed, how they mature, and how their function is adapted for an accurate perception of sound.

De novo sequencing and initial annotation of the Mongolian gerbil (Meriones unguiculatus) genome

The Mongolian gerbil (Meriones unguiculatus) is a member of the rodent family that displays several features not found in mice or rats, including sensory specializations and social patterns more similar to those in humans. These features have made gerbils a valuable animal for research studies of auditory and visual processing, brain development, learning and memory, and neurological disorders. Here, we report the whole gerbil annotated genome sequence, and identify important similarities and differences to the human and mouse genomes. We further analyze the chromosomal structure of eight genes with high relevance for controlling neural signaling and demonstrate a high degree of homology between these genes in mouse and gerbil. This homology increases the likelihood that individual genes can be rapidly identified in gerbil and used for genetic manipulations. The availability of the gerbil genome provides a foundation for advancing our knowledge towards understanding evolution, behavior and neural function in mammals. ACCESSION NUMBER: The Whole Genome Shotgun sequence data from this project has been deposited at DDBJ/ENA/GenBank under the accession NHTI00000000. The version described in this paper is version NHTI01000000. The fragment reads, and mate pair reads have been deposited in the Sequence Read Archive under BioSample accession SAMN06897401.

Hair cell and neural contributions to the cochlear summating potential

The cochlear summating potential (SP) to a tone is a baseline shift that persists for the duration of the burst. It is often considered the most enigmatic of cochlear potentials because its magnitude and polarity vary across frequency and level and its origins are uncertain. In this study, we used pharmacology to isolate sources of the SP originating from the gerbil cochlea. Animals either had the full complement of outer and inner hair cells (OHCs and IHCs) and an intact auditory nerve or had systemic treatment with furosemide and kanamycin (FK) to remove the outer hair cells. Responses to tone bursts were recorded from the round window before and after the neurotoxin kainic acid (KA) was applied. IHC responses were then isolated from the post-KA responses in FK animals, neural responses were isolated from the subtraction of post-KA from pre-KA responses in NH animals, and OHC responses were isolated by subtraction of post-KA responses in FK animals from post-KA responses in normal hearing (NH) animals. All three sources contributed to the SP; OHCs with a negative polarity and IHCs and the auditory nerve with positive polarity. Thus the recorded SP in NH animals is a sum of contributions from different sources, contributing to the variety of magnitudes and polarities seen across frequency and intensity. When this information was applied to observations of the SP recorded from the round window in human cochlear implant subjects, a strong neural contribution to the SP was confirmed in humans as well as gerbils. NEW & NOTEWORTHY Of the various potentials produced by the cochlea, the summating potential (SP) is typically described as the most enigmatic. Using combinations of ototoxins and neurotoxins, we show contributions to the SP from the auditory nerve and from inner and outer hair cells, which differ in polarity and vary in size across frequency and level. This complexity of sources helps to explain the enigmatic nature of the SP.

Phase Locking of Auditory-Nerve Fibers Reveals Stereotyped Distortions and an Exponential Transfer Function with a Level-Dependent Slope

Phase locking of auditory-nerve-fiber (ANF) responses to the fine structure of acoustic stimuli is a hallmark of the auditory system's temporal precision and is important for many aspects of hearing. Period histograms from phase-locked ANF responses to low-frequency tones exhibit spike-rate and temporal asymmetries, but otherwise retain an approximately sinusoidal shape as stimulus level increases, even beyond the level at which the mean spike rate saturates. This is intriguing because apical cochlear mechanical vibrations show little compression, and mechanoelectrical transduction in the receptor cells is thought to obey a static sigmoidal nonlinearity, which might be expected to produce peak clipping at moderate and high stimulus levels. Here we analyze phase-locked responses of ANFs from cats of both sexes. We show that the lack of peak clipping is due neither to ANF refractoriness nor to spike-rate adaptation on time scales longer than the stimulus period. We demonstrate that the relationship between instantaneous pressure and instantaneous rate is well described by an exponential function whose slope decreases with increasing stimulus level. Relatively stereotyped harmonic distortions in the input to the exponential can account for the temporal asymmetry of the period histograms, including peak splitting. We show that the model accounts for published membrane-potential waveforms when assuming a power-of-three, but not a power-of-one, relationship to exocytosis. Finally, we demonstrate the relationship between the exponential transfer functions and the sigmoidal pseudotransducer functions obtained in the literature by plotting the maxima and minima of the voltage responses against the maxima and minima of the stimuli.SIGNIFICANCE STATEMENT Phase locking of auditory-nerve-fiber responses to the temporal fine structure of acoustic stimuli is important for many aspects of hearing, but the mechanisms underlying phase locking are not fully understood. Intriguingly, period histograms retain an approximately sinusoidal shape across sound levels, even when the mean rate has saturated. We find that neither refractoriness nor spike-rate adaptation is responsible for this behavior. Instead, the peripheral auditory system operates as though it contains an exponential transfer function whose slope changes with stimulus level. The underlying mechanism is distinct from the comparatively weak cochlear mechanical compression in the cochlear apex, and likely resides in the receptor cells.

Water Waves to Sound Waves: Using Zebrafish to Explore Hair Cell Biology

Although perhaps best known for their use in developmental studies, over the last couple of decades, zebrafish have become increasingly popular model organisms for investigating auditory system function and disease. Like mammals, zebrafish possess inner ear mechanosensory hair cells required for hearing, as well as superficial hair cells of the lateral line sensory system, which mediate detection of directional water flow. Complementing mammalian studies, zebrafish have been used to gain significant insights into many facets of hair cell biology, including mechanotransduction and synaptic physiology as well as mechanisms of both hereditary and acquired hair cell dysfunction. Here, we provide an overview of this literature, highlighting some of the particular advantages of using zebrafish to investigate hearing and hearing loss.

The effects of the activation of the inner-hair-cell basolateral K+ channels on auditory nerve responses

The basolateral membrane of the mammalian inner hair cell (IHC) expresses large voltage and Ca²⁺ gated outward K⁺ currents. To quantify how the voltage-dependent activation of the K⁺ channels affects the functionality of the auditory nerve innervating the IHC, this study adopts a model of mechanical-to-neural transduction in which the basolateral K⁺ conductances of the IHC can be made voltage-dependent or not. The model shows that the voltage-dependent activation of the K⁺ channels (i) enhances the phase-locking properties of the auditory fiber (AF) responses; (ii) enables the auditory nerve to encode a large dynamic range of sound levels; (iii) enables the AF responses to synchronize precisely with the envelope of amplitude modulated stimuli; and (iv), is responsible for the steep offset responses of the AFs. These results suggest that the basolateral K⁺ channels play a major role in determining the well-known response properties of the AFs and challenge the classical view that describes the IHC membrane as an electrical low-pass filter. In contrast to previous models of the IHC-AF complex, this study ascribes many of the AF response properties to fairly basic mechanisms in the IHC membrane rather than to complex mechanisms in the synapse.

Model-based estimation of the frequency tuning of the inner-hair-cell stereocilia from neural tuning curves

This study proposes that the frequency tuning of the inner-hair-cell (IHC) stereocilia in the intact organ of Corti can be derived from the responses of the auditory fibers (AFs) using computational tools. The frequency-dependent relationship between the AF threshold and the amplitude of the stereocilia vibration is estimated using a model of the IHC-mediated mechanical to neural transduction. Depending on the response properties of the considered AF, the amplitude of stereocilia deflection required to drive the simulated AF above threshold is 1.4 to 9.2 dB smaller at low frequencies (≤500 Hz) than at high frequencies (≥4 kHz). The estimated frequency-dependent relationship between ciliary deflection and neural threshold is employed to derive constant-stereocilia-deflection contours from previously published AF recordings from the chinchilla cochlea. This analysis shows that the transduction process partially accounts for the observed differences between the tuning of the basilar membrane and that of the AFs.

The Coupling between Ca2+ Channels and the Exocytotic Ca2+ Sensor at Hair Cell Ribbon Synapses Varies Tonotopically along the Mature Cochlea

The cochlea processes auditory signals over a wide range of frequencies and intensities. However, the transfer characteristics at hair cell ribbon synapses are still poorly understood at different frequency locations along the cochlea. Using recordings from mature gerbils, we report here a surprisingly strong block of exocytosis by the slow Ca²⁺ buffer EGTA (10 mM) in basal hair cells tuned to high frequencies (∼30 kHz). In addition, using recordings from gerbil, mouse, and bullfrog auditory organs, we find that the spatial coupling between Ca²⁺ influx and exocytosis changes from nanodomain in low-frequency tuned hair cells (∼<2 kHz) to progressively more microdomain in high-frequency cells (∼>2 kHz). Hair cell synapses have thus developed remarkable frequency-dependent tuning of exocytosis: accurate low-latency encoding of onset and offset of sound intensity in the cochlea's base and submillisecond encoding of membrane receptor potential fluctuations in the apex for precise phase-locking to sound signals. We also found that synaptic vesicle pool recovery from depletion was sensitive to high concentrations of EGTA, suggesting that intracellular Ca²⁺ buffers play an important role in vesicle recruitment in both low- and high-frequency hair cells. In conclusion, our results indicate that microdomain coupling is important for exocytosis in high-frequency hair cells, suggesting a novel hypothesis for why these cells are more susceptible to sound-induced damage than low-frequency cells; high-frequency inner hair cells must have a low Ca²⁺ buffer capacity to sustain exocytosis, thus making them more prone to Ca²⁺-induced cytotoxicity.

SIGNIFICANCE STATEMENT In the inner ear, sensory hair cells signal reception of sound. They do this by converting the sound-induced movement of their hair bundles present at the top of these cells, into an electrical current. This current depolarizes the hair cell and triggers the calcium-induced release of the neurotransmitter glutamate that activates the postsynaptic auditory fibers. The speed and precision of this process enables the brain to perceive the vital components of sound, such as frequency and intensity. We show that the coupling strength between calcium channels and the exocytosis calcium sensor at inner hair cell synapses changes along the mammalian cochlea such that the timing and/or intensity of sound is encoded with high precision.

In vivo physiological recording from the lateral line of juvenile zebrafish

KEY POINTS

Zebrafish provide a unique opportunity to investigate in vivo sensory transduction in mature hair cells. We have developed a method for studying the biophysical properties of mature hair cells from the lateral line of juvenile zebrafish. The method involves application of the anaesthetic benzocaine and intubation to maintain ventilation and oxygenation through the gills. The same approach could be used for in vivo functional studies in other sensory and non-sensory systems from juvenile and adult zebrafish.

ABSTRACT

Hair cells are sensory receptors responsible for transducing auditory and vestibular information into electrical signals, which are then transmitted with remarkable precision to afferent neurons. The zebrafish lateral line is emerging as an excellent in vivo model for genetic and physiological analysis of hair cells and neurons. However, research has been limited to larval stages because zebrafish become protected from the time of independent feeding under European law (from 5.2 days post-fertilization (dpf) at 28.5°C). In larval zebrafish, the functional properties of most of hair cells, as well as those of other excitable cells, are still immature. We have developed an experimental protocol to record electrophysiological properties from hair cells of the lateral line in juvenile zebrafish. We found that the anaesthetic benzocaine at 50 mg l(-1) was an effective and safe anaesthetic to use on juvenile zebrafish. Concentrations up to 300 mg l(-1) did not affect the electrical properties or synaptic vesicle release of juvenile hair cells, unlike the commonly used anaesthetic MS-222, which reduces the size of basolateral membrane K(+) currents. Additionally, we implemented a method to maintain gill movement, and as such respiration and blood oxygenation, via the intubation of > 21 dpf zebrafish. The combination of benzocaine and intubation provides an experimental platform to investigate the physiology of mature hair cells from live zebrafish. More generally, this method would allow functional studies involving live imaging and electrophysiology from juvenile and adult zebrafish.