Auditory nerve fibers (ANFs) transmit acoustic information from the sensory hair cells to the cochlear nuclei. In experimental and clinical audiology, probing the whole ANF population remains a ...difficult task, as the ANFs differ greatly in their threshold and onset response to sound. Thus, low spontaneous rate (SR) fibers, which have rather higher thresholds, delay and larger jitter in their first spike latency are not detectable in the far-field compound action potential of the auditory nerve. Here, we developed a new protocol of acoustic stimulation together with electrophysiological signal processing to track the steady state activity of ANFs. Mass potentials at the round window were recorded in response to repetitive 300-ms bursts of 1/3 octave band noise centered on a frequency probe. Analysis was assessed during the last 200-ms of the response to capture the steady-state response of ANFs. To eliminate the microphonic component reflecting the sensory cells activity, repetitive pairs of sounds of opposite polarities were used. The spectral analysis was calculated on the average of two consecutive responses, and the neural gain was calculated by dividing point-by-point the spectrum to sound over unstimulated condition. In response to low-sound-level stimulation, neural gain predominated in the low-frequency cochlear regions, while a second component of responses centered on higher cochlear frequency regions appeared beyond 30 dB SPL. At 60 dB SPL, neural gain showed a bimodal shape, with a notch near 5.6 kHz. In addition to correlate with the functional mapping of ANFs along the tonotopic axis, the deletion of low-SR fibers leads to a reduction in the high-frequency response, where the low-SR fibers are preferentially located. Thus, mass potentials at the round window may provide a useful tool to probe the SR-based distribution of ANFs in humans and in other species in which direct single-unit recordings are difficult to achieve or not feasible.
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Glutamate is the neurotransmitter released from hair cells. Its clearance from the synaptic cleft can shape neurotransmission and prevent excitotoxicity. This may be particularly important in the ...inner ear and in other sensory organs where there is a continually high rate of neurotransmitter release. In the case of most cochlear and type II vestibular hair cells, clearance involves the diffusion of glutamate to supporting cells, where it is taken up by EAAT1 (GLAST), a glutamate transporter. A similar mechanism cannot work in vestibular type I hair cells as the presence of calyx endings separates supporting cells from hair-cell synapses. Because of this arrangement, it has been conjectured that a glutamate transporter must be present in the type I hair cell, the calyx ending, or both. Using whole-cell patch-clamp recordings, we demonstrate that a glutamate-activated anion current, attributable to a high-affinity glutamate transporter and blocked by DL-TBOA, is expressed in type I, but not in type II hair cells. Molecular investigations reveal that EAAT4 and EAAT5, two glutamate transporters that could underlie the anion current, are expressed in both type I and type II hair cells and in calyx endings. EAAT4 has been thought to be expressed almost exclusively in the cerebellum and EAAT5 in the retina. Our results show that these two transporters have a wider distribution in mice. This is the first demonstration of the presence of transporters in hair cells and provides one of the few examples of EAATs in presynaptic elements.
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Glutamate is thought to be the main neurotransmitter at the synapse between the type I vestibular hair cell and its cognate
calyx afferent. The present study was designed to identify the type of ...glutamate receptors involved in neurotransmission at
this unusual synapse. Immunocytochemistry showed that AMPA GluR2, NMDA NR1 and NR2A/B subunits of the glutamate receptors
were confined to the synaptic contact. We then examined the electrical activity at calyx terminals using direct electrophysiological
recordings from intact dendritic terminals in explanted turtle posterior crista. We found that sodium-based action potentials
support a background discharge that could be modulated by the mechanical stimulation of the hair bundle of the sensory cells.
These activities were prevented by blocking both the mechano-electrical transduction channels and L-type voltage-gated Ca 2+ channels involved in synaptic transmission. Although pharmacological analysis revealed that NMDA receptors could operate,
our results show that AMPA receptors are mainly involved in synaptic neurotransmission. We conclude that although both AMPA
and NMDA glutamate receptor subunits are present at the calyx synapse, only AMPA receptors appear to be involved in the synaptic
transmission between the type I vestibular hair cell and the calyx afferent.
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Sound-evoked compound action potential (CAP), which captures the synchronous activation of the auditory nerve fibers (ANFs), is commonly used to probe deafness in experimental and clinical settings. ...All ANFs are believed to contribute to CAP threshold and amplitude: low sound pressure levels activate the high-spontaneous rate (SR) fibers, and increasing levels gradually recruit medium- and then low-SR fibers. In this study, we quantitatively analyze the contribution of the ANFs to CAP 6 days after 30-min infusion of ouabain into the round window niche. Anatomic examination showed a progressive ablation of ANFs following increasing concentration of ouabain. CAP amplitude and threshold plotted against loss of ANFs revealed three ANF pools: 1) a highly ouabain-sensitive pool, which does not participate in either CAP threshold or amplitude, 2) a less sensitive pool, which only encoded CAP amplitude, and 3) a ouabain-resistant pool, required for CAP threshold and amplitude. Remarkably, distribution of the three pools was similar to the SR-based ANF distribution (low-, medium-, and high-SR fibers), suggesting that the low-SR fiber loss leaves the CAP unaffected. Single-unit recordings from the auditory nerve confirmed this hypothesis and further showed that it is due to the delayed and broad first spike latency distribution of low-SR fibers. In addition to unraveling the neural mechanisms that encode CAP, our computational simulation of an assembly of guinea pig ANFs generalizes and extends our experimental findings to different species of mammals. Altogether, our data demonstrate that substantial ANF loss can coexist with normal hearing threshold and even unchanged CAP amplitude.
Infrared laser irradiation has been established as an appropriate stimulus for primary sensory neurons under conditions where sensory receptor cells are impaired or lost. Yet, development of clinical ...applications has been impeded by lack of information about the molecular mechanisms underlying the laser-induced neural response. Here, we directly address this question through pharmacological characterization of the biological response evoked by midinfrared irradiation of isolated retinal and vestibular ganglion cells from rodents. Whole cell patch-clamp recordings reveal that both voltage-gated calcium and sodium channels contribute to the laser-evoked neuronal voltage variations (LEVV). In addition, selective blockade of the LEVV by micromolar concentrations of ruthenium red and RN 1734 identifies thermosensitive transient receptor potential vanilloid channels as the primary effectors of the chain reaction triggered by midinfrared laser irradiation. These results have the potential to facilitate greatly the design of future prosthetic devices aimed at restoring neurosensory capacities in disabled patients.
Auditory nerve fibers (ANFs) encode pure tones through two modes of coding, spike time and spike rate, depending on the tone frequency. In response to a low-frequency tone, ANF firing is phase locked ...to the sinusoidal waveform. Because time coding vanishes with an increase in the tone frequency, high-frequency tone coding relies on the spike rate of the ANFs. Adding a continuous broadband noise to a tone compresses the rate intensity function of ANFs and shifts its dynamic range toward higher intensities. Therefore, the ANFs with high-threshold/low-spontaneous rate (SR) are thought to contribute to behavioral tone detection in noise. However, this theory relies on the discharge rate of the ANFs. The direct comparison with the masking threshold through spike timing, irrespective of the spontaneous rate, has not so far been investigated. Taking advantage of a unique proxy to quantify the spike synchrony (i.e., the shuffle autocorrelogram), we show in female gerbils that high-SR ANFs are more adapted to encode low-frequency thresholds through temporal code, giving them a strong robustness in noise. By comparing behavioral thresholds measured using prepulse inhibition of the acoustical startle reflex with population thresholds calculated from ANFs pooled per octave band, we show that threshold-based spike timing provides a better estimate of behavioral thresholds in the low-frequency range, whereas the high-frequency behavioral thresholds rely on the spiking rate, particularly in noise. This emphasizes the complementarity of temporal and rate modes to code tone-in-noise thresholds over a large range of frequencies.
There is a general agreement that high-threshold/low-spontaneous rate (SR) auditory nerve fibers (ANFs) are of prime importance for tone detection in noise. However, this theory is based on the discharge rate of the fibers. Comparing the behavioral thresholds and single ANF thresholds shows that this is only true in the high-frequency range of tone stimulations. In the low-frequency range of tones (up to 2.7 kHz in the gerbil), the most sensitive ANFs (high-SR fibers) carry neural information through a spike-timing mode, even for noise in which tones do not induce a noticeable increment in the spike rate. This emphasizes the interplay between spike-time and spike-rate modes in the auditory nerve to encode tone-in-noise threshold over a large range of tone frequencies.
Sound-level coding in the auditory nerve is achieved through the progressive recruitment of auditory nerve fibers (ANFs) that differ in threshold of activation and in the stimulus level at which the ...spike rate saturates. To investigate the functional state of the ANFs, the electrophysiological tests routinely used in clinics only capture the first action potentials firing in synchrony at the onset of the acoustic stimulation. Assessment of other properties (e.g., spontaneous rate and adaptation time constants) requires single-fiber recordings directly from the nerve, which for ethical reasons is not allowed in humans. By combining neuronal activity measurements at the round window and signal-processing algorithms, we constructed a peristimulus time response (PSTR), with a waveform similar to the peristimulus time histograms (PSTHs) derived from single-fiber recordings in young adult female gerbils. Simultaneous recordings of round-window PSTR and single-fiber PSTH provided models to predict the adaptation kinetics and spontaneous rate of the ANFs tuned at the PSTR probe frequency. The predictive model derived from gerbils was then validated in female mice and finally applied to humans by recording PSTRs from the auditory nerve in normal-hearing patients who underwent cerebellopontine angle surgeries. A rapid adaptation time constant of ∼3 ms and a mean spontaneous rate of ∼22 spikes/s in the 4 kHz frequency range were found. This study offers a promising diagnostic tool to map the human auditory nerve, thus opening new avenues to better understanding auditory neuropathies, tinnitus, and hyperacusis.
Neural adaptation in auditory nerve fibers corresponds to the reduction in the neuronal activity to prolonged or repeated sound stimulation. For obvious ethical reasons, single-fiber recordings from the auditory nerve are not feasible in humans, creating a critical gap in extending data obtained using animal models to humans. Using electrocochleography in rodents, we inferred adaptation kinetics and spontaneous discharge rates of the auditory nerve fibers in humans. Routinely used in basic and clinical laboratories, this tool will provide a better understanding of auditory disorders such as neuropathies, tinnitus, and hyperacusis, and will help to improve hearing-aid fittings.
Gerbils possess a very specialized cochlea in which the low-frequency inner hair cells (IHCs) are contacted by auditory nerve fibers (ANFs) having a high spontaneous rate (SR), whereas high frequency ...IHCs are innervated by ANFs with a greater SR-based diversity. This specificity makes this animal a unique model to investigate, in the same cochlea, the functional role of different pools of ANFs. The distribution of the characteristic frequencies of fibers shows a clear bimodal shape (with a first mode around 1.5 kHz and a second around 12 kHz) and a notch in the histogram near 3.5 kHz. Whereas the mean thresholds did not significantly differ in the two frequency regions, the shape of the rate-intensity functions does vary significantly with the fiber characteristic frequency. Above 3.5 kHz, the sound-driven rate is greater and the slope of the rate-intensity function is steeper. Interestingly, high-SR fibers show a very good synchronized onset response in quiet (small first-spike latency jitter) but a weak response under noisy conditions. The low-SR fibers exhibit the opposite behavior, with poor onset synchronization in quiet but a robust response in noise. Finally, the greater vulnerability of low-SR fibers to various injuries including noise- and age-related hearing loss is discussed with regard to patients with poor speech intelligibility in noisy environments. Together, these results emphasize the need to perform relevant clinical tests to probe the distribution of ANFs in humans, and develop appropriate techniques of rehabilitation.
This article is part of a Special Issue entitled <Annual Reviews 2016>.
•The high-spontaneous rate (SR) fibers show a very good synchronized response in quiet but saturate rapidly under noisy conditions.•Inversely, the synchronized response of low-SR fibers is weaker compared to high-SR fibers, but more robust in noise.•Patients with a specific degeneration of low-SR fibers may have poor speech intelligibility in noisy environments.
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Calretinin is a calcium-binding protein of the EF-hand family. It has been previously identified in particular cell types of adult guinea pig, rat, and chinchilla inner ear. Development of calretinin ...immunoreactivity in the mouse inner ear was investigated from embryonic day 13 (E13) to the adult stage. In the adult mouse vestibule, calretinin immunoreactivity was present in the same structures as described for the rat and guinea pig: the population of afferent fibers forming calyx units and a small number of ganglion neurons. The earliest immunoreactivity was found at E17 in vestibular hair cells (VHCs), then, at E19, in afferent fibers entering the sensory epithelia and in rare ganglion neurons. At postnatal day 4 (P4), a few vestibular nerve fibers and ganglion neurons were reactive. From this stage until P14, immunoreactivity developed in the calyx units and disappeared from VHCs. At P14, immunostaining was adult-like. In the adult mouse cochlea, immunoreactivity was present in the same cell populations as described in the rat: the inner hair cells (IHCs) and most of Corti's ganglion neurons. Calretinin immunoreactivity appeared at E19-P0 in IHCs and ganglion neurons of the basal turn. At P1, outer hair cells (OHCs) of the basal turn were positive. Calretinin immunoreactivity then appeared in IHCs, OHCs, and ganglion neurons of the medial turn, then of the apical turn. At P4, all IHCs and OHCs and most of the ganglion neurons were immunostained. Immunoreactivity gradually disappeared from the OHCs starting at P10 and, at P22, only IHCs and ganglion neurons were positive. The sequences of appearance of calretinin were specific to each cell type of the inner ear and paralleled their respective maturation. Calretinin was transiently expressed in VHCs and OHCs.
Previous experimental data indicates the hyperpolarization-activated cation (I sub(h)) current, in the inner ear, consists of two components different hyperpolarization-activated cyclic ...nucleotide-gated (HCN) subunits which are impossible to pharmacologically isolate. To confirm the presence of these two components in vestibular ganglion neurons we have applied a parameter identification algorithm which is able to discriminate the parameters of the two components from experimental data. Using simulated data we have shown that this algorithm is able to identify the parameters of two populations of non-inactivated ionic channels more accurately than a classical method. Moreover, the algorithm was demonstrated to be insensitive to the key parameter variations. We then applied this algorithm to I sub(h) current recordings from mouse vestibular ganglion neurons. The algorithm revealed the presence of a high-voltage-activated slow component and a low-voltage-activated fast component. Finally, the electrophysiological significance of these two I sub(h) components was tested individually in computational vestibular ganglion neuron models (sustained and transient), in the control case and in the presence of cAMP, an intracellular cyclic nucleotide that modulates HCN channel activity. The results suggest that, first, the fast and slow components modulate differently the action potential excitability and the excitatory postsynaptic potentials in both sustained and transient vestibular neurons and, second, the fast and slow components, in the control case, provide different information about characteristics of the stimulation and this information is significantly modified after modulation by cAMP. Previous experimental data indicates that the Ih current, in the inner ear, consists of two components which are impossible to pharmacologically isolate. We propose an algorithm able to discriminate the parameters of both components from voltage clamp data from vestibular ganglion cells. In computational neuron models, we test the effect of the Ih components on the response to synaptic inputs from hair cells to afferent terminals, and on rebound firing of afferents following hyperpolarisation.
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