The origin of the 900 Hz spectral peak in spontaneous and sound-evoked round-window electrical activity

Estimation of Cochlear Frequency Selectivity Using a Convolution Model of Forward-Masked Compound Action Potentials

PURPOSE

Frequency selectivity is a fundamental property of the peripheral auditory system; however, the invasiveness of auditory nerve (AN) experiments limits its study in the human ear. Compound action potentials (CAPs) associated with forward masking have been suggested as an alternative to assess cochlear frequency selectivity. Previous methods relied on an empirical comparison of AN and CAP tuning curves in animal models, arguably not taking full advantage of the information contained in forward-masked CAP waveforms.

METHODS

To improve the estimation of cochlear frequency selectivity based on the CAP, we introduce a convolution model to fit forward-masked CAP waveforms. The model generates masking patterns that, when convolved with a unitary response, can predict the masking of the CAP waveform induced by Gaussian noise maskers. Model parameters, including those characterizing frequency selectivity, are fine-tuned by minimizing waveform prediction errors across numerous masking conditions, yielding robust estimates.

RESULTS

The method was applied to click-evoked CAPs at the round window of anesthetized chinchillas using notched-noise maskers with various notch widths and attenuations. The estimated quality factor Q10 as a function of center frequency is shown to closely match the average quality factor obtained from AN fiber tuning curves, without the need for an empirical correction factor.

CONCLUSION

This study establishes a moderately invasive method for estimating cochlear frequency selectivity with potential applicability to other animal species or humans. Beyond the estimation of frequency selectivity, the proposed model proved to be remarkably accurate in fitting forward-masked CAP responses and could be extended to study more complex aspects of cochlear signal processing (e.g., compressive nonlinearities).

Computational modeling of the human compound action potential

The auditory nerve (AN) compound action potential (CAP) is an important tool for assessing auditory disorders and monitoring the health of the auditory periphery during surgical procedures. The CAP has been mathematically conceptualized as the convolution of a unit response (UR) waveform with the firing rate of a population of AN fibers. Here, an approach for predicting experimentally recorded CAPs in humans is proposed, which involves the use of human-based computational models to simulate AN activity. CAPs elicited by clicks, chirps, and amplitude-modulated carriers were simulated and compared with empirically recorded CAPs from human subjects. In addition, narrowband CAPs derived from noise-masked clicks and tone bursts were simulated. Many morphological, temporal, and spectral aspects of human CAPs were captured by the simulations for all stimuli tested. These findings support the use of model simulations of the human CAP to refine existing human-based models of the auditory periphery, aid in the design and analysis of auditory experiments, and predict the effects of hearing loss, synaptopathy, and other auditory disorders on the human CAP.

Neural Contributions to the Cochlear Summating Potential: Spiking and Dendritic Components

Using electrocochleography, the summating potential (SP) is a deflection from baseline to tones and an early rise in the response to clicks. Here, we use normal hearing gerbils and gerbils with outer hair cells removed with a combination of furosemide and kanamycin to investigate cellular origins of the SP. Round window electrocochleography to tones and clicks was performed before and after application of tetrodotoxin to prevent action potentials, and then again after kainic acid to prevent generation of an EPSP. With appropriate subtractions of the response curves from the different conditions, the contributions to the SP from outer hair cells, inner hair cell, and neural "spiking" and "dendritic" responses were isolated. Like hair cells, the spiking and dendritic components had opposite polarities to tones - the dendritic component had negative polarity and the spiking component had positive polarity. The magnitude of the spiking component was larger than the dendritic across frequencies and intensities. The onset to tones and to clicks followed a similar sequence; the outer hair cells responded first, then inner hair cells, then the dendritic component, and then the compound action potential of the spiking response. These results show the sources of the SP include at least the four components studied, and that these have a mixture of polarities and magnitudes that vary across frequency and intensity. Thus, multiple possible interactions must be considered when interpreting the SP for clinical uses.

Assessment of cochlear electrophysiology in typically developing children and children with auditory processing disorder

OBJECTIVE

Children with auditory processing disorder (APD) are reported to have abnormal auditory brainstem responses (ABR) but little is understood about their cochlear integrity. Poor cochlear integrity can affect neural responses. In this study, cochlear and auditory brainstem integrity was investigated in children with APD.

METHOD

Twenty children with APD, sixteen typically developing children and twenty adults participated in this study. Click evoked electrocochleography (ECochG) and ABRs were recorded from all the participants. Cochlear responses were analyzed using a) latency and amplitude of summating potential; action potential, b) transmission time between summating potential and action potential, c) summating potential/action potential amplitude ratio and d) action potential latency difference to condensation and rarefaction polarity. Amplitude in the ABR components was examined.

RESULTS

Children with APD showed similar cochlear function to the typically developing children. There were no significant differences in wave I amplitude between children with APD and typically developing children. However, wave V amplitude was significantly reduced in children with APD compared to typically developing children.

CONCLUSION

In the absence of any functional differences in the cochlea, children with APD can show poor amplitude in the later components of the ABR. The ABR anomalies observed in children with APD arise due to poor neural processing, possibly after the first auditory synapse.

Age-related decline in cochlear ribbon synapses and its relation to different metrics of auditory-nerve activity

Loss of inner hair cell-auditory nerve fiber synapses is considered to be an important early stage of neural presbyacusis. Mass potentials, recorded at the cochlear round window, can be used to derive the neural index (NI), a sensitive measure for pharmacologically-induced synapse loss. Here, we investigate the applicability of the NI for measuring age-related auditory synapse loss in young-adult, middle-aged, and old Mongolian gerbils. Synapse loss, which was progressively evident in the 2 aged groups, correlated weakly with NI when measured at a fixed sound level of 60 dB SPL. However, the NI was confounded by decreases in single-unit firing rates at 60 dB SPL. NI at 30 dB above threshold, when firing rates were similar between age groups, did not correlate with synapse loss. Our results show that synapse loss is poorly reflected in the NI of aged gerbils, particularly if further peripheral pathologies are present. The NI may therefore not be a reliable clinical tool to assess synapse loss in aged humans with peripheral hearing loss.

Noise-Induced Hearing Loss in Gerbil: Round Window Assays of Synapse Loss

Previous work in animals with recovered hearing thresholds but permanent inner hair cell synapse loss after noise have suggested initial vulnerability of low spontaneous rate (SR) auditory nerve fibers (ANF). As these fibers have properties of response that facilitate robust sound coding in continuous noise backgrounds, their targeted loss would have important implications for function. To address the issue of relative ANF vulnerabilities after noise, we assessed cochlear physiologic and histologic consequences of temporary threshold shift-producing sound over-exposure in the gerbil, a species with well-characterized distributions of auditory neurons by SR category. The noise exposure targeted a cochlear region with distributed innervation (low-, medium- and high-SR neurons). It produced moderate elevations in outer hair cell-based distortion-product otoacoustic emission and whole nerve compound action potential thresholds in this region, with accompanying reductions in suprathreshold response amplitudes, quantified at 24 h. These parameters of response recovered well with post-exposure time. Chronic synapse loss was maximum in the frequency region initially targeted by the noise. Cochlear round window recorded mass potentials (spontaneous neural noise and sound-driven peri-stimulus time responses, PSTR) reflected parameters of the loss not detected by the conventional assays. Spontaneous activity was acutely reduced. Steady-state (PSTR plateau) activity was correlated with synapse loss in frequency regions with high concentrations of low-SR neurons, whereas the PSTR onset peak and spontaneous round window noise, both dominated by high-SR fiber activity, were relatively unaltered across frequency in chronic ears. Together, results suggest that acute targets of noise were of mixed SR subtypes, but chronic targets were predominantly low-SR neurons. PSTRs captured key properties of the auditory nerve response and vulnerability to injury that should yield important diagnostic information in hearing loss etiologies producing cochlear synaptic and neural loss.

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.

On the Origin of the 1,000 Hz Peak in the Spectrum of the Human Tympanic Electrical Noise

The spectral analysis of the spontaneous activity recorded with an electrode positioned near the round window of the guinea pig cochlea shows a broad energy peak between 800 and 1,000 Hz. This spontaneous electric activity is called round window noise or ensemble background activity. In guinea pigs, the proposed origin of this peak is the random sum of the extracellular field potentials generated by action potentials of auditory nerve neurons. In this study, we used a non-invasive method to record the tympanic electric noise (TEN) in humans by means of a tympanic wick electrode. We recorded a total of 24 volunteers, under silent conditions or in response to stimuli of different modalities, including auditory, vestibular, and motor activity. Our results show a reliable peak of spontaneous activity at ~1,000 Hz in all studied subjects. In addition, we found stimulus-driven responses with broad-band noise that in most subjects produced an increase in the magnitude of the energy band around 1,000 Hz (between 650 and 1,200 Hz). Our results with the vestibular stimulation were not conclusive, as we found responses with all caloric stimuli, including 37°C. No responses were observed with motor tasks, like eye movements or blinking. We demonstrate the feasibility of recording neural activity from the electric noise of the tympanic membrane with a non-invasive method. From our results, we suggest that the 1,000 Hz component of the TEN has a mixed origin including peripheral and central auditory pathways. This research opens up the possibility of future clinical non-invasive techniques for the functional study of auditory and vestibular nerves in humans.

Mass Potentials Recorded at the Round Window Enable the Detection of Low Spontaneous Rate Fibers in Gerbil Auditory Nerve

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.

Assessment of the limits of neural phase-locking using mass potentials

In the diverse mechanosensory systems that animals evolved, the waveform of stimuli can be encoded by phase locking in spike trains of primary afferents. Coding of the fine structure of sounds via phase locking is thought to be critical for hearing. The upper frequency limit of phase locking varies across species and is unknown in humans. We applied a method developed previously, which is based on neural adaptation evoked by forward masking, to analyze mass potentials recorded on the cochlea and auditory nerve in the cat. The method allows us to separate neural phase locking from receptor potentials. We find that the frequency limit of neural phase locking obtained from mass potentials was very similar to that reported for individual auditory nerve fibers. The results suggest that this is a promising approach to examine neural phase locking in humans with normal or impaired hearing or in other species for which direct recordings from primary afferents are not feasible.

Effects of furosemide on cochlear neural activity, central hyperactivity and behavioural tinnitus after cochlear trauma in guinea pig

Cochlear trauma causes increased spontaneous activity (hyperactivity) to develop in central auditory structures, and this has been suggested as a neural substrate for tinnitus. Using a guinea pig model we have previously demonstrated that for some time after cochlear trauma, central hyperactivity is dependent on peripheral afferent drive and only later becomes generated intrinsically within central structures. Furosemide, a loop diuretic, reduces spontaneous firing of auditory afferents. We investigated in our guinea pig model the efficacy of furosemide in reducing 1) spontaneous firing of auditory afferents, using the spectrum of neural noise (SNN) from round window recording, 2) hyperactivity in inferior colliculus, using extracellular single neuron recordings and 3) tinnitus at early time-points after cochlear trauma. Tinnitus was assessed using gap prepulse inhibition of acoustic startle (GPIAS). Intraperitoneal furosemide, but not saline, caused a marked decrease in both SNN and central hyperactivity. Intracochlear perfusion with furosemide similarly reversed central hyperactivity. In animals in which GPIAS measurements suggested the presence of tinnitus (reduced GPIAS), this could be reversed with an intraperitoneal injection with furosemide but not saline. The results are consistent with furosemide reducing central hyperactivity and behavioural signs of tinnitus by acting peripherally to decrease spontaneous firing of auditory afferents. The data support the notion that hyperactivity may be involved in the generation of tinnitus and further suggest that there may be a therapeutic window after cochlear trauma using drug treatments that target peripheral spontaneous activity.

Comparison of salicylate- and quinine-induced tinnitus in rats: development, time course, and evaluation of audiologic correlates

BACKGROUND

Salicylate and quinine have been shown to reliably induce short-term tinnitus when administered at high doses. The present study compared salicylate and quinine-induced tinnitus in rats using the gap prepulse inhibition of acoustic startle (GPIAS).

METHODS

Twenty-four rats were divided into 2 groups; the first group (n = 12) was injected with salicylate (300 mg kg d), whereas the second (n = 12) was treated with quinine orally at a dose of 200 mg kg d. Animals were treated daily for 4 consecutive days. All rats were tested for tinnitus and hearing loss before and 2, 24, 48, 72, and 96 hours after the first drug administration. Tinnitus was assessed using GPIAS; hearing function was measured with distortion product otoacoustic emissions (DPOAEs) and auditory brainstem response.

RESULTS

Salicylate treatment induced transient tinnitus with a pitch near 16 kHz starting 2 hours posttreatment, persisting over the 4-day treatment period and disappearing 24 hours later. Animals in the quinine group showed GPIAS changes at a higher pitch (20 kHz); however, changes were more variable among animals, and the mean data were not statistically significant. Hearing function varied across treatments. In the salicylate group, high-level DPOAEs were slightly affected; most changes occurred 2 hours posttreatment. Low-level DPOAEs were affected at all frequencies with a progressive dose-dependent effect. In the quinine group, only high-level DPOAEs were affected, mainly at 16 kHz.

CONCLUSION

The present study highlights the similarities and differences in the frequency and the time course of tinnitus and hypoacusis induced by salicylate and quinine. Transient tinnitus was reliably induced pharmacologically with salicylate, whereas hearing loss remained subclinical with only minor changes in DPOAEs.

Evidence that the compound action potential (CAP) from the auditory nerve is a stationary potential generated across dura mater

We have investigated the generation of the compound action potential (CAP) from the auditory nerve of guinea pigs. Responses to acoustic tone-bursts were recorded from the round window (RW), throughout the cochlear fluids, from the surface of the cochlear nucleus, from the central end of the auditory nerve after removal of the cochlear nucleus, from the scalp vertex, and from the contralateral ear. Responses were compared before, during and after experimental manipulations including pharmacological blockade of the auditory nerve, section of the auditory nerve, section of the efferent nerves, removal of the cochlear nucleus, and focal cooling of the cochlear nerve and/or cochlear nucleus. Regardless of the waveform changes occurring with these manipulations, the responses were similar in waveform but inverted polarity across the internal auditory meatus. The CAP waveforms were very similar before and after removal of the cochlear nucleus, apart from transient changes that could last many minutes. This suggests that the main CAP components are generated entirely by the eighth nerve. Based on previous studies and a clear understanding of the generation of extracellular potentials, we suggest that the early components in the responses recorded from the round window, from the cochlear fluids, from the surface of the cochlear nucleus, or from the scalp are a far-field or stationary potential, generated when the circulating action currents associated with each auditory neurone encounters a high extracellular resistance as it passes through the dura mater.

The influence of noise exposure on the parameters of a convolution model of the compound action potential

The influence of noise exposure on the parameters of a convolution model of the compound action potential (CAP) was examined. CAPs were recorded in normal-hearing gerbils and in gerbils exposed to a 117 dB SPL 8 kHz band of noise for various durations. The CAPs were fitted with an analytic CAP to obtain the parameters representing the number of nerve fibers (N), the probability density function [P(t)] from a population of nerve fibers, and the single-unit waveform [U(t)]. The results showed that the analytic CAP fitted the physiologic CAPs well with correlations of approximately 0.90. A subsequent analysis using hierarchical linear modeling quantified the change in the parameters as a function of both signal level and hearing threshold. The results showed that noise exposure caused some of the parameter-level functions to simply shift along the signal level axis in proportion to the amount of hearing loss, whereas others shifted along the signal level axis and steepened. Significant changes occurred in the U(t) parameters, but they were not related to hearing threshold. These results suggest that noise exposure alters the physiology underlying the CAP, some of which can be explained by a simple lack of gain, whereas others may not.

Salicylate enables cochlear arachidonic-acid-sensitive NMDA receptor responses

Currently, many millions of people treated for various ailments receive high doses of salicylate. Consequently, understanding the mechanisms by which salicylate induces tinnitus is an important issue for the research community. Behavioral testing in rats have shown that tinnitus induced by salicylate or mefenamate (both cyclooxygenase blockers) are mediated by cochlear NMDA receptors. Here we report that the synapses between the sensory inner hair cells and the dendrites of the cochlear spiral ganglion neurons express NMDA receptors. Patch-clamp recordings and two-photon calcium imaging demonstrated that salicylate and arachidonate (a substrate of cyclooxygenase) enabled the calcium flux and the neural excitatory effects of NMDA on cochlear spiral ganglion neurons. Salicylate also increased the arachidonate content of the whole cochlea in vivo. Single-unit recordings of auditory nerve fibers in adult guinea pig confirmed the neural excitatory effect of salicylate and the blockade of this effect by NMDA antagonist. These results suggest that salicylate inhibits cochlear cyclooxygenase, which increased levels of arachidonate. The increased levels of arachidonate then act on NMDA receptors to enable NMDA responses to glutamate that inner hair cells spontaneously release. This new pharmacological profile of salicylate provides a molecular mechanism for the generation of tinnitus at the periphery of the auditory system.

Temporary hearing loss influences post-stimulus time histogram and single neuron action potential estimates from human compound action potentials

An analytic compound action potential (CAP) obtained by convolving functional representations of the post-stimulus time histogram summed across auditory nerve neurons [P(t)] and a single neuron action potential [U(t)] was fit to human CAPs. The analytic CAP fit to pre- and postnoise-induced temporary hearing threshold shift (TTS) estimated in vivo P(t) and U(t) and the number of neurons contributing to the CAPs (N). The width of P(t) decreased with increasing signal level and was wider at the lowest signal level following noise exposure. P(t) latency decreased with increasing signal level and was shorter at all signal levels following noise exposure. The damping and oscillatory frequency of U(t) increased with signal level. For subjects with large amounts of TTS, U(t) had greater damping than before noise exposure particularly at low signal levels. Additionally, U(t) oscillation was lower in frequency at all click intensities following noise exposure. N increased with signal level and was smaller after noise exposure at the lowest signal level. Collectively these findings indicate that neurons contributing to the CAP during TTS are fewer in number, shorter in latency, and poorer in synchrony than before noise exposure. Moreover, estimates of single neuron action potentials may decay more rapidly and have a lower oscillatory frequency during TTS.

Effect of intracochlear perfusion of vanilloids on cochlear neural activity in the guinea pig

Recent findings show that the vanilloid receptor subtype 1 (TRPV1) is expressed by cochlear outer hair cells and spiral ganglion cells, and that its expression is up-regulated in ganglion cells after aminoglycoside treatment. This study tested the hypothesis that agents that act on TRPV1 receptors affect the spectrum of ensemble background activity (EBA). Consecutive intracochlear perfusions of the TRPV1 agonist, capsaicin (CAP 0.1, 1, and 10 parts per million), as well as its antagonist capsazepine (CZP), were used to test effects of TRPV1 activation on EBA recorded from the cochlear base. Perfusion with CAP alone produced a dose-dependent increase of the 900-Hz peak ratio (power normalized re the overall spectrum) of the EBA. The CAP effect was attenuated during concurrent perfusion with CZP. These findings are consistent with the hypothesis that TRPV1 activation increases background activity of spiral ganglion cells and support a role of TRPV1 in gating spontaneous and evoked auditory nerve excitability.

An integrative model of developing tinnitus based on recent neurobiological findings

Brain imaging studies suggest that the functional connectivity of various limbic, prefrontal, and temporal brain structures can be understood as the basis for particularly intense experiences of impairment and many secondary symptoms of decompensated tinnitus. Results to date suggest cortical plasticity as a likely mechanism to be involved in the chronic progression of tinnitus. Relationships between the processing of auditory signals and neural networks associated with somatosensory, attentional, cognitive and emotional processes are relevant for the genesis of the pathology. However, the neural bases of subjective impairment in tinnitus patients are unknown. An integrative model of developing tinnitus is presented here, based on the most recent neurophysiological data in current discussion. We assume the involvement of altered brain functions in the development and maintenance of perceptions of tinnitus and outline various possibilities which could contribute to decompensation and chronic progression. The discussions surrounding present models of the generation of tinnitus and its reinforcement to the point of decompensation make it clear that a unidimensional approach in clinical interventions is insufficient. Patients with decompensated tinnitus suffer from a complex somatic and psychological disorder. The interactive processes involving emotions, behaviour and symptoms, as well as the high co-morbidity with affective and psychosomatic illnesses and the important influence of psychosocial factors (distress), make the use of stage-appropriate interdisciplinary treatments necessary.

Spectrum of neural electrical activity in guinea pig cochlea: Effects of anaesthesia regimen, body temperature and ambient noise

Spectral analysis of electric noise recorded from the round window of the cochlea is thought to represent the summed spontaneous activity of the auditory nerve. It has been postulated that it could provide a possible tinnitus index. Because experimental conditions could change this neural activity, the effect of anaesthesia regimen, body temperature and ambient noise on the spectrum of spontaneous neural noise (SNN) were investigated in guinea pig cochlea. SNN was studied in awake guinea pigs and after anaesthesia with pentobarbital (P), xylazine/ketamine (XK) or xylazine/tiletamine-zolazepam (XTZ). Body temperature varied gradually from 33 to 41 degrees C under XK regimen. In awake animals, broadband noise was generated with intensity varying from 0 to 50 dB. The SNN consisted in a broad peak at approximately 900 Hz. With ambient broadband noise, it increased exponentially with the sound level with no shift in frequency. Soon after anaesthetic induction, the lowest frequencies were constantly decreased, and gradually the 900 Hz peak either increased moderately (P) or dropped steeply (XTZ) or remained unchanged (XK). Peak frequency increased linearly with body temperature whereas the amplitude reached a maximum at around 39.5 degrees C. In conclusion, these data indicate that experimental conditions such as anaesthesia regimen, body temperature and ambient noise modify the spontaneous neural outflow of the cochlea and must be taken into account when studying SNN.

Tinnitus: neurobiological substrates

Tinnitus is an auditory phantom sensation of ringing in the ears that is experienced when no external sound is present. It is a prevalent disorder that is frequently caused by insults to the peripheral auditory and somatosensory systems, especially in the elderly. This creates an imbalance between inhibitory and excitatory transmitter actions in the midbrain, auditory cortex and brainstem (where neural activity from somatosensory and auditory stimuli interact). This imbalance causes hyperexcitability often leading to the perception of phantom sounds. Although changes in transmitter-receptor systems have become better documented, there are currently no proven drug treatments for humans. Methods for preventing tinnitus have been demonstrated in animal studies.

Noradrenergic modulation of brainstem nuclei alters cochlear neural output

The peripheral auditory sense organ, the cochlea, receives innervation from lateral and medial olivocochlear neurons in the brainstem. These neurons are able to modulate cochlear neural output. Anatomical studies have shown that one of the neurotransmitters which is present in varicosities surrounding the olivocochlear neurons in the brainstem is noradrenaline and previous work on brainstem slices has demonstrated a generally excitatory effect of noradrenaline on medial olivocochlear neurons. In order to assess in vivo the function of the noradrenergic inputs to olivocochlear neurons, we injected noradrenaline in the brainstem of anaesthetised guinea pigs and recorded ipsilateral cochlear electrical activity. Injections of noradrenaline close to the lateral olivocochlear neurons evoked increases in the sound-driven neural activity from the cochlea, measured as compound action potential (CAP) amplitude, as well as in the spontaneous activity, measured as amplitude of the 900 Hz peak of the spectrum of the neural noise in the cochlear fluids. In contrast, noradrenaline in the vicinity of the medial olivocochlear neurons evoked inhibitory effects on both the CAP amplitude and 900 Hz peak. These results indicate most likely an excitatory action of noradrenaline on both the lateral and medial olivocochlear neurons in the brainstem, and show that such noradrenergic inputs can modulate cochlear function.

Effects of a calcium channel blocker on spontaneous neural noise and gross action potential waveforms in the guinea pig cochlea

The effects of the L-type Ca2+ channel blocker nimodipine on the spectrum of the spontaneous neural noise (SNN) and the waveform of the gross sound-evoked compound action potential (CAP) were investigated by perilymphatic perfusion in the guinea pig cochlea. Both the SNN and the CAP were reversibly suppressed by nimodipine. The percentage reduction in SNN was dose-dependent in a manner very similar to the results obtained with the measures of CAP threshold changes. The reduction in the peak SNN caused by 10 microM nimodipine was the same as that caused by 500 microM kainic acid, which totally eliminated any neural responses. For 1 microM nimodipine there was an apparent dissociation between the SNN and CAP changes such that the SNN could be markedly suppressed with only very small changes in CAP thresholds. These results imply that spontaneous release of neurotransmitter from the inner hair cell is more sensitive to block of calcium channels than evoked release. There was no evidence for any marked shift caused by nimodipine, in the position of the main (900 Hz) spectral peak in the SNN. Comparison of the CAP waveform before and after nimodipine perfusion showed that the CAP waveforms were unchanged despite the change in sensitivity. These data do not support the notion of any significant postsynaptic site of action of nimodipine. The data hence provide further support for an exclusively presynaptic role for L-type Ca2+ channels in the regulation of both evoked and spontaneous neurotransmitter release from inner hair cells.

Determinants of the spectrum of the neural electrical activity at the round window: transmitter release and neural depolarisation

In this paper we summarise the changes we have observed in the electrical activity at the round window (RW) of guinea pigs during transient cooling of the RW or cochlear nucleus (CN), transient hypoxia, low frequency acoustic biasing, ablation of the CN, and DC current injection into the basal cochlear turn. We have measured the compound action potential (CAP), the spectrum of the average CAP waveform (SAW) evoked by brief tone-bursts, and the spectrum of the neural noise (SNN). We discuss how the changes we have observed can be understood in terms of changes in transmitter release from inner hair cells (that controls stochastic neural firing), or changes in the membrane potential of the primary afferent neurones (that controls the neural firing waveshape and the spectral content of the SAW and SNN). We note that changes in sound intensity produce a simple increase in the stochastic release of transmitter from inner hair cells, without much change in the waveform of the neural response, but manipulations of the auditory brainstem, cooling and current injection all appear to alter neural firing rate and the neural response waveform, producing a baseline shift in the CAP and changes in 1000 Hz peak and low frequency content of the SAW and SNN. We also discuss the use of the CAP, SAW and SNN as an indication of cochlear and auditory brainstem neural activity.

Ensemble spontaneous activity in the guinea-pig cochlear nerve

Spectral analysis of electrical noise recorded from the round window (RW) of the cochlea is referred to as the ensemble spontaneous activity (ESA) of the cochlear nerve. The ESA is considered to represent the summed spontaneous activity of single fibers of the auditory nerve and changes in the spectral characteristics of the ESA have been observed in humans with tinnitus. Experiments were undertaken to determine the relationship of the ESA to auditory neurotransmission. The ESA consisted of energy centered at approximately 900 Hz, similar to the spectral peak of single auditory neuron discharges. The amplitude of the ESA was correlated with good auditory sensitivity in the 12-30 kHz region of the cochlea. Constant pure tones of 12-22 kHz suppressed the ESA reducing its amplitude in a frequency and intensity dependent manner implying that the ESA recorded at the RW is generated or dominated by neurons in the basal region of the cochlea. The ESA was significantly suppressed by round window perfusion of the P2X receptor agonist adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS) (10 mM) the glutamate receptor antagonist 6-7-dinitroquinoxaline-2,3-dione (DNQX) (1 mM), and the sodium channel antagonist tetrodotoxin (TTX) (20 microM). Following intravenous furosemide injection (40 mg/kg) reduction and recovery of the ESA correlated with similar changes in the endocochlear potential (EP). Following DNQX and ATPgammaS an additional spectral peak at 200 Hz was often observed. This peak has been postulated to be a correlate of tinnitus in humans but had not previously been observed in a guinea-pig model of tinnitus. These data confirm the spectral characteristics of the ESA in guinea-pigs and show it is dependent on the sensitivity of the auditory nerve and intact auditory neurotransmission. In addition these experiments support the view that the ESA represents summed spontaneous neural activity in the cochlea and provide a platform for studies of the influence of ototoxic compounds on the spontaneous neural outflow of the cochlea as a model of tinnitus.

Transient focal cooling at the round window and cochlear nucleus shows round window CAP originates from cochlear neurones alone

We have measured the compound action potential (CAP) evoked by very brief high-frequency tone-bursts (20 kHz, 1/4 ms) at the round window (RW) and at the surface of the cochlear nucleus (CN) in guinea pigs before, during and after a localised chilling of either the cochlea or CN, with a non-toxic 'freeze spray'. CN chilling almost abolished the negative-going component of the CAP measured in the CN (generated by the CN and here called the cochlear nucleus response or CNR), leaving a positive-going localised response from the cochlear neurones as they leave the internal auditory meatus. Within 3 min, the CNR recovered to control values. During that time, the N(1) component of the RW CAP was slightly increased and the P(1) was larger, even though the CNR was abolished, indicating that the P(1) was not due to electrotonic spread of current from the CN. The N(2) and successive peaks at the RW were also abolished, but returned after 30 s. When the cochlea was chilled, the RW CAP was initially reduced in amplitude, presumably due to a drop in the number of cochlear neurones spiking in response to sound, but recovered within 3 min to be larger than the control waveform, with a more prominent N(1) peak which was delayed slightly, making the CAP more monophasic. At the same time, the CNR was smaller, presumably due to fewer cochlear neurones responding, but overall the CN CAP was altered little in waveshape. These experiments indicate that that RW CAP is generated almost solely by cochlear neurones. We also suggest that some of the changes in the RW CAP during the chills were due to changes in the firing of the lateral olivo-cochlear system of efferent neurons.

K+ currents produce P1 in the RW CAP: evidence from DC current bias, K+ channel blockade and recordings from cochlea and brainstem

Tone-burst-evoked compound action potentials (CAP) from the guinea pig round window (RW) are altered by DC current injection through the RW. The CAP waveform consists of a series of interleaved negative and positive peaks (N(1), P(1), N(2), P(2) etc.) of decreasing amplitude. During positive DC current injection (around +50 microA) the positive peaks are depressed substantially and there is an overall negative baseline shift of the waveform following the N(1). Negative current injection (around -50 microA) increased the positive peaks, in particular P(1), and produced an overall positive baseline shift following the N(1) peak. Results support our hypothesis that the first and dominant N(1) peak in the RW CAP is due to depolarising Na(+) currents into the primary afferent dendrites and axons within the cochlea, and that the P(1) potential is largely due to the exit of the hyperpolarising K(+) currents in the same cells. We have reached this conclusion on the basis of the sign and latency of the N(1) and P(1) components at the RW, beneath the myelin layers around the spiral ganglion cells, at the internal auditory meatus (IAM) within the brain case, and on the basis of the differential susceptibility of the various peaks to perfusion of lidocaine in the cochlear nucleus, sectioning of the cochlear nerve at the IAM, application of the K(+) channel blockers 4-amino-pyridine and tetraethylammonium within the cochlea, and DC current biasing at the RW.

Modulation of cochlear afferent response by the lateral olivocochlear system: activation via electrical stimulation of the inferior colliculus

The olivocochlear (OC) efferent innervation of the mammalian inner ear consists of two subdivisions, medial (MOC) and lateral (LOC), with different peripheral terminations on outer hair cells and cochlear afferent terminals, respectively. The cochlear effects of electrically activating MOC efferents are well known, i.e., response suppression effected by reducing outer hair cells' contribution to cochlear amplification. LOC peripheral effects are unknown, because their unmyelinated axons are difficult to electrically stimulate. Here, stimulating electrodes are placed in the inferior colliculus (IC) to indirectly activate the LOC system, while recording cochlear responses bilaterally from anesthetized guinea pigs. Shocks at some IC sites produced novel cochlear effects attributable to activation of the LOC system: long-lasting (5-20 min) enhancement or suppression of cochlear neural responses (compound action potentials and round window noise), without changes in cochlear responses dominated by outer hair cells (otoacoustic emissions and cochlear microphonics). These novel effects also differed from classic MOC effects in their lack of dependence on the level and frequency of the acoustic stimulus. These effects disappeared on sectioning the entire OC bundle, but not after selective lesioning of the MOC tracts or the cochlea's autonomic innervation. We conclude that the LOC pathway comprises two functional subdivisions, capable of inducing slow increases or decreases in response magnitudes in the auditory nerve. Such a system may be useful in maintaining accurate binaural comparisons necessary for sound localization in the face of slow changes in interaural sensitivity.

Primary afferent and cochlear nucleus contributions to extracellular potentials during tone-bursts

Gross electrical responses to tone bursts were measured in the guinea pig with electrodes located in scala tympani (ST) and scala vestibuli (SV) of the cochlea, on the central portion of the VIIIth nerve fibres in the internal auditory meatus, and on the surface of the cochlear nuclear complex (CN). Intracochlear perfusion of pharmacological blockers of neural and postsynaptic activity as well as aspiration of parts or all of the CN were used to dissect the origin of the many components of the gross responses. It was shown that single-ended recordings from either ST or SV or those derived from the sum of the ST and SV responses not only contain mixed responses from the auditory nerve fibres and cochlear hair cells, but are contaminated or modified by neural activity central to the internal auditory meatus, probably in various parts of the CN. Differential recordings between ST and SV were relatively uncontaminated by such activity. Recordings from central locations were largely uncontaminated by potentials from cochlear hair cells. These results suggest that a revised and extended system of nomenclature for the different components of the gross cochlear potentials is necessary, and interpretation of such potentials needs to take into account multiple central as well as peripheral generators.

Mechanisms of tinnitus

The generation of tinnitus is a topic of much scientific enquiry. This chapter reviews possible mechanisms of tinnitus, whilst noting that the heterogeneity observed within the human population with distressing tinnitus means that there may be many different mechanisms by which tinnitus can occur. Indeed, multiple mechanisms may be at work within one individual. The role of the cochlea in tinnitus is considered, and in particular the concept of discordant damage between inner and outer hair cells is described. Biochemical models of tinnitus pertaining to the cochlea and the central auditory pathway are considered. Potential mechanisms for tinnitus within the auditory brain are reviewed, including important work on synchronised spontaneous activity in the cochlear nerve. Whilst the number of possible mechanisms of tinnitus within the auditory system is considerable, the identification of the physiological substrates underlying tinnitus is a crucial element in the design of novel and effective therapies.