Cochlear efferent neurones and protection against acoustic trauma: protection of outer hair cell receptor current and interanimal variability

Effects of Multisession Anodal Electrical Stimulation of the Auditory Cortex on Temporary Noise-Induced Hearing Loss in the Rat

The protective effect of the efferent system against acoustic trauma (AT) has been shown by several experimental approaches, including damage to one ear, sectioning of the olivocochlear bundle (OCB) in the floor of the IV ventricle, and knock-in mice overexpressing outer hair cell (OHC) cholinergic receptors, among others. Such effects have been related to changes in the regulation of the cholinergic efferent system and in cochlear amplification, which ultimately reverse upon protective hearing suppression. In addition to well-known circuits of the brainstem, the descending corticofugal pathway also regulates efferent neurons of the olivary complex. In this study, we applied our recently developed experimental paradigm of multiple sessions of electrical stimulation (ES) to activate the efferent system in combination with noise overstimulation. ABR thresholds increased 1 and 2 days after AT (8-16 kHz bandpass noise at 107 dB for 90 min) recovering at AT + 14 days. However, after multiple sessions of epidural anodal stimulation, no changes in thresholds were observed following AT. Although an inflammatory response was also observed 1 day after AT in both groups, the counts of reactive macrophages in both experimental conditions suggest decreased inflammation in the epidural stimulation group. Quantitative immunocytochemistry for choline acetyltransferase (ChAT) showed a significant decrease in the size and optical density of the efferent terminals 1 day after AT and a rebound at 14 days, suggesting depletion of the terminals followed by a long-term compensatory response. Such a synthesis recovery was significantly higher upon cortical stimulation. No significant correlation was found between ChAT optical density and size of the buttons in sham controls (SC) and ES/AT + 1day animals; however, significant negative correlations were shown in all other experimental conditions. Therefore, our comparative analysis suggests that cochleotopic cholinergic neurotransmission is also better preserved after multisession epidural stimulation.

The correlation between hair and eye colour and contralateral suppression of otoacoustic emissions

Genetics and environmental factors frequently influence individual's susceptibility to hearing loss. It is postulated that melanin in the inner ear is related to individual's susceptibility to noise induced hearing loss (NIHL). General pigmentation in turn, suspected to be related to the amount of pigmentation in the inner ear. The amount of melanin in the inner ear is said to modulate the endocochlear potential and provide an otoprotective effect.

AIM

The study aimed to determine the relationship between the contralateral suppression of otoacoustic emissions (CSOAE) in individuals with brown eyes and hair, and blue eyes and blond hair, and temporary emission shift (TES) after short-term noise exposure.

SETTING AND DESIGN

The research was conducted using a quantitative research design with a quasi-experimental repeated within the subject design to compare the CSOAE in subjects with different hair and eyes colour with TES after short-term noise exposure. Quantitative research was used to determine the relationship between the measurable variables to predict occurrence.

MATERIAL AND METHOD

The hearing sensitivity of young adults was determined by using pure tone audiometry followed by CSOAE's and distortion product otoacoustic emissions (DPOAE) before listening to music for one hour individually. Pure tone audiometry and DPOAE's were repeated after music exposure to determine the amount of TES and temporary threshold shift (TTS).

STATISTICAL ANALYSIS USED

One-way ANOVA was used during the analysis of the data obtained during this research study, in addition to, two-tailed Wilcoxon Sign Rank test and Friedman's test. In all analyses, a 95% level of significance (P<0.05) was used.

RESULTS

No statistically significant difference between efferent suppression was measured by CSOAE's between the participant groups. A larger TTS at 4000 Hz and TES at 2000 Hz was evident in the blue eyes and blond hair group after short-term music exposure. Conclusion: CSOAE's were unable to predict which group of individuals were more susceptible to NIHL after short-term noise exposure.

Noise-induced chaos and signal detection by the nonisochronous Hopf oscillator

The Hopf oscillator has been shown to capture many phenomena of the auditory and vestibular systems. These systems exhibit remarkable temporal resolution and sensitivity to weak signals, as they are able to detect sounds that induce motion in the angstrom regime. In the present work, we find the analytic response function of a nonisochronous Hopf oscillator to a step stimulus and show that the system is most sensitive in the regime where noise induces chaotic dynamics. We show that this regime also provides a faster response and enhanced temporal resolution. Thus, the system can detect a very brief, low-amplitude pulse. Finally, we subject the oscillator to periodic delta-function forcing, mimicking a spike train, and find the exact analytic expressions for the stroboscopic maps. Using these maps, we find a period-doubling cascade to chaos with increasing force strength.

Musicianship enhances ipsilateral and contralateral efferent gain control to the cochlea

Human hearing sensitivity is easily compromised with overexposure to excessively loud sounds, leading to permanent hearing damage. Consequently, finding activities and/or experiential factors that distinguish "tender" from "tough" ears (i.e., acoustic vulnerability) would be important for identifying people at higher risk for hearing damage. To regulate sound transmission and protect the inner ear against acoustic trauma, the auditory system modulates gain control to the cochlea via biological feedback of the medial olivocochlear (MOC) efferents, a neuronal pathway linking the lower brainstem and cochlear outer hair cells. We hypothesized that a salient form of auditory experience shown to have pervasive neuroplastic benefits, namely musical training, might act to fortify hearing through tonic engagement of these reflexive pathways. By measuring MOC efferent feedback via otoacoustic emissions (cochlear emitted sounds), we show that dynamic ipsilateral and contralateral cochlear gain control is enhanced in musically-trained individuals. Across all participants, MOC strength was correlated with the years of listeners' training suggested that efferent gain control is experience dependent. Our data provide new evidence that intensive listening experience(s) (e.g., musicianship) can strengthen the ipsi/contralateral MOC efferent system and sound regulation to the inner ear. Implications for reducing acoustic vulnerability to damaging sounds are discussed.

The olivocochlear system and protection from acoustic trauma: a mini literature review

Large intersubject variability in the susceptibility to noise-induced hearing loss (NIHL) is known to occur in both humans and animals. It has been suggested that the olivocochlear system (OCS) plays a significant role in protecting the cochlea from exposure to high levels of noise. A mini literature review about the scientific evidence from animal and human studies about the association between the function of the OCS and susceptibility to NIHL was carried out. Animal data consistently show that de-efferented ears exhibit larger temporary threshold shift (TTS) and permanent threshold shift (PTS) than efferented ears. Data from human studies do not consistently show a correlation between the strength of the OCS function and amount of TTS. Further research on human subjects is required to determine how the OCS function could be used to predict susceptibility to NIHL in individual subjects.

Adaptation of distortion product otoacoustic emissions predicts susceptibility to acoustic over-exposure in alert rabbits

A noninvasive test was developed in rabbits based on fast adaptation measures for 2f1-f2 distortion-product otoacoustic emissions (DPOAEs). The goal was to evaluate the effective reflex activation, i.e., "functional strength," of both the descending medial olivocochlear efferent reflex (MOC-R) and the middle-ear muscle reflex (MEM-R) through sound activation. Classically, it is assumed that both reflexes contribute toward protecting the inner ear from cochlear damage caused by noise exposure. The DP-gram method described here evaluated the MOC-R effect on DPOAE levels over a two-octave (oct) frequency range. To estimate the related activation of the middle-ear muscles (MEMs), the MEM-R was measured by monitoring the level of the f1-primary tone throughout its duration. Following baseline measures, rabbits were subjected to noise over-exposure. A main finding was that the measured adaptive activity was highly variable between rabbits but less so between the ears of the same animal. Also, together, the MOC-R and MEM-R tests showed that, on average, DPOAE adaptation consisted of a combined contribution from both systems. Despite this shared involvement, the amount of DPOAE adaptation measured for a particular animal's ear predicted that ear's subsequent susceptibility to the noise over-exposure for alert but not for deeply anesthetized rabbits.

Effects of spectral and temporal disruption on cortical encoding of gerbil vocalizations

Animal communication sounds contain spectrotemporal fluctuations that provide powerful cues for detection and discrimination. Human perception of speech is influenced both by spectral and temporal acoustic features but is most critically dependent on envelope information. To investigate the neural coding principles underlying the perception of communication sounds, we explored the effect of disrupting the spectral or temporal content of five different gerbil call types on neural responses in the awake gerbil's primary auditory cortex (AI). The vocalizations were impoverished spectrally by reduction to 4 or 16 channels of band-passed noise. For this acoustic manipulation, an average firing rate of the neuron did not carry sufficient information to distinguish between call types. In contrast, the discharge patterns of individual AI neurons reliably categorized vocalizations composed of only four spectral bands with the appropriate natural token. The pooled responses of small populations of AI cells classified spectrally disrupted and natural calls with an accuracy that paralleled human performance on an analogous speech task. To assess whether discharge pattern was robust to temporal perturbations of an individual call, vocalizations were disrupted by time-reversing segments of variable duration. For this acoustic manipulation, cortical neurons were relatively insensitive to short reversal lengths. Consistent with human perception of speech, these results indicate that the stable representation of communication sounds in AI is more dependent on sensitivity to slow temporal envelopes than on spectral detail.

Toluene effect on the olivocochlear reflex

Animal studies have shown that toluene can cause hearing loss and can exacerbate the effects of noise by inhibiting the middle ear acoustic reflex. In this investigation, carried out in Long-Evans rats, the tensor tympani tendon was cutoff and the stapedius muscle was electrocoagulated in one or both middle ears. Rat hearing was evaluated by measuring cubic distortion otoacoustic emissions (2f1-f2; f1 = 8000 Hz; f2 = 9600 Hz; f1/f2 = 1.2) prior to, during, and after activation of the olivocochlear (OC) reflex. A band noise centered at 4 kHz was used as suppressor noise. It was delivered contralaterally to decrease 2f1-f2 amplitude. The strength of the inner ear acoustic reflex was tested by increasing contralateral noise intensity, and toluene injected into the carotid artery was used to study physiological efficacy. Results showed that the protective effect of the OC reflex is intensity dependent. In addition, the OC reflex was found to be less sensitive to toluene than the middle ear acoustic reflex. This may be because the efferent neurons involved in inner ear and middle ear reflexes are located differently. In conclusion, the synergistic effects on hearing of co-exposure to noise and aromatic solvents are because of solvents depressing the central nuclei, which mainly drive the middle ear acoustic reflex.

An adaptive model simulating the somatic motility and the active hair bundle motion of the OHC

The outer hair cells (OHC) of the mammalian inner ear change the sensitivity and frequency selectivity of the filtering system of the cochlea using two kinds of mechanical activity: the somatic motility and the active hair bundle motion. We designed a non-linear adaptive model of the OHC employing both mechanisms of the mechanical activity. The modeling results show that the high sensitivity and frequency selectivity of the filtering system of the cochlea depend on the somatic motility of the OHC. However, both mechanisms of mechanical activity are involved in the adaptation to sound intensity and efferent-synaptic influence. The fast (alternating) component (AC) of the mechanical-electrical transduction signal controls the motor protein prestin and fast changes in axial length of the cell. The slow (direct) component (DC) appearing at high signal intensity affects the axial stiffness, the cell length and the position of the hair bundle. The efferent influence is realized by the same mechanism.

Contralateral suppression of distortion product otoacoustic emissions: effect of the primary frequency in Dpgrams

The amplitude of the 2f1-f2 distortion product otoacoustic emission (DPOAE) can be suppressed by presenting contralateral acoustic stimulation. To test the hypothesis that DPOAE contralateral suppression is influenced by the primary frequency in DPgrams, DPgrams were recorded at resolutions of 1, 8, and 17 pts/octave, in the absence and presence of contralateral broadband noise (BBN). Participants were 20 normal-hearing human adults. In DPgrams with higher frequency resolutions, DPOAE suppression at amplitude peaks in DPgrams (8 pts/octave: Mean = - 0.92 dB, SD = 0.71 for BBN at 60 dB SPL; 17 pts/octave: Mean = - 0.25 to -1.44 dB, SD = 0.51 to 0.86 for BBN at 40 to 70 dB SPL, respectively) was larger than the suppression at the dips in DPgrams (8 pts/octave: Mean = - 0.13 dB, SD = 1.00; 17 pts/octave: Mean = - 0.03 to -0.73 dB, SD = 0.55 to 0.91). A larger intersubject variability in DPOAE contralateral suppression was observed at the dips. The results suggest that measuring DPOAE contralateral suppression at the primary frequencies corresponding to the peaks in DPgrams with higher frequency resolutions may improve the assessment of the efferent system function.

Bandwidth determines modulatory effects of centrifugal pathways on cochlear hearing desensitization caused by loud sound

Centrifugal olivocochlear (OC) pathways modulate cochlear hearing losses induced in cats by loud sounds varying in bandwidth from tones to clicks and noise bands, in a variety of conditions. The general effect, always to reduce hearing damage, can be a net effect resulting from complex interactions between OC subcomponents (crossed and uncrossed OC pathways). The interactions between these subcomponents vary with type of loud sound, suggesting that sound bandwidth may be important in determining how OC pathways modulate loud sound-induced hearing loss. This dependency was examined and here it is reported that OC pathways do not alter cochlear hearing losses caused by loud noise with a 2-kHz-wide bandwidth intermediate between the loud sounds of previous studies. Increasing stimulus bandwidth even slightly more, to use a loud 3.5-kHz-wide bandwidth noise as the damaging sound, once again revealed OC modulation of cochlear hearing loss. The fact that OC pathways do not modulate cochlear hearing losses induced by loud 2-kHz-wide noise was demonstrated in three very different test conditions in which OC pathways modulate hearing losses caused by narrower or broader bandwidth sounds. This confirmed that the absence of centrifugal modulation of hearing loss to this particular sound was a robust phenomenon not related to test condition. The absence of overall centrifugal effects was also true at the level of subcomponent pathways; neither crossed nor uncrossed OC pathways individually modulated cochlear hearing losses to the loud 2-kHz-wide noise. This surprising frequency dependency has general implications for centrifugal modulation of cochlear responses.

Audio-vocal interaction in the pontine brainstem during self-initiated vocalization in the squirrel monkey

The adjustment of the voice by auditory input happens at several brain levels. The caudal pontine brainstem, though rarely investigated, is one candidate area for such audio-vocal integration. We recorded neuronal activity in this area in awake, behaving squirrel monkeys (Saimiri sciureus) during vocal communication, using telemetric single-unit recording techniques. We found audio-vocal neurons at locations not described before, namely in the periolivary region of the superior olivary complex and the adjacent pontine reticular formation. They showed various responses to external sounds (noise bursts) and activity increases (excitation) or decreases (inhibition) to self-produced vocalizations, starting prior to vocal onset and continuing through vocalizations. In most of them, the responses to noise bursts and self-produced vocalizations were similar, with the only difference that neuronal activity started prior to vocal onset. About one-third responded phasically to noise bursts, independent of whether they increased or decreased their activity to vocalization. The activity of most audio-vocal neurons correlated with basic acoustic features of the vocalization, such as call duration and/or syllable structure. Auditory neurons near audio-vocal neurons showed significantly more frequent phasic response patterns than those in areas without audio-vocal activity. Based on these findings, we propose that audio-vocal neurons showing similar activity to external acoustical stimuli and vocalization play a role in olivocochlear regulation. Specifically, audio-vocal neurons with a phasic response to external auditory stimuli are candidates for the mediation of basal audio-vocal reflexes such as the Lombard reflex. Thus, our findings suggest that complex audio-vocal integration mechanisms exist in the ventrolateral pontine brainstem.

Olivocochlear activity and temporary threshold shift-susceptibility in humans

STUDY OBJECTIVES

Animal studies (guinea pig, cat, chinchilla) have shown that activity of the medial olivocochlear efferents can exert noise-protective effects on the cochlea. It is not yet known whether such effects are also existent in humans. Olivocochlear activity can be estimated indirectly by contralateral suppression (CS) of otoacoustic emissions (OAE).

MATERIAL AND METHODS

We measured Input/Output functions of distortion products of OAE (DPOAE), with and without contralateral acoustic stimulation by white noise, in 94 normal hearing young male subjects. Seven stimuli with L2 between 20 and 60 dB SPL and L1 = 39 dB + 0.4 L2 ("scissor paradigm") were used at f2 = 2, 3, 4, 5, and 6 kHz. The measurement was repeated 2 weeks later. In 83 subjects of the same group, pure tone audiometry was registered before and 6 minutes after shooting exercises to evaluate individual susceptibility to develop a temporary threshold shift (TTS).

RESULTS

Test-retest repeatability of CS was generally good. CS averaged 0.98 dB SPL (SD 1.19 dB, median 0.56 dB). As expected, CS was greatest at low stimulus levels (median 1.06 dB at L2 = 20 dB, as compared with 0.33 dB at L2 = 60 dB). The smallest average CS was found at 4 kHz, and the greatest CS appeared at 2 kHz. A TTS occurred in 7 of 83 (8.5%) subjects. Statistical analysis did not reveal any correlation between the amount of CS and individual TTS susceptibility.

CONCLUSIONS AND OUTLOOK

1) Measurement of CS of DPOAE using an extensive measurement paradigm revealed good test-retest repeatability, confirming the reliability of this audiologic tool. 2) CS of DPOAE does not predict individual susceptibility to mild TTS induced by impulse noise in humans. Possible explanations for the missing association are discussed. Future perspectives include longitudinal studies to further elucidate the association between medial olivocochlear bundle-activity and permanent threshold shift in humans. The goal is to develop a diagnostic tool for the prediction of individual noise vulnerability in humans, thereby preventing noise-induced hearing loss.

Compression, adaptation and efferent control in a revised outer hair cell functional model

In the cochlea of the inner ear, outer hair cells (OHC) together with the local passive structures of the tectorial and basilar membranes comprise non-linear resonance circuits with the local and central (afferent-efferent) feedback. The characteristics of these circuits and their control possibilities depend on the mechanomotility of the OHC. The main element of our functional model of the OHC is the mechanomotility circuit with the general transfer characteristic y=ktanh(x-a). The parameter k of this characteristic reflects the axial stiffness of the OHC, and the parameter a working position of the hair bundle. The efferent synaptic signals act on the parameter k directly and on the parameter a indirectly through changes in the membrane potential. The dependences of the sensitivity and selectivity on changes in the parameters a and k are obtained by the computer simulation. Functioning of the model at low-level input signals is linear. Due to the non-linearity of the transfer characteristic of the mechanomotility circuit the high-level signals are compressed. For the adaptation and efferent control, however, the transfer characteristic with respect to the initial operating point should be asymmetrical (a>0). The asymmetry relies on the deflection of the hair bundle from the axis of the OHC.

Effects of Quinine, Quinidine, and Chloroquine on α9α10 Nicotinic Cholinergic Receptors

In this study, we report the effects of the quinoline derivatives quinine, its optical isomer quinidine, and chloroquine on alpha9alpha10-containing nicotinic acetylcholine receptors (nAChRs). The compounds blocked acetylcholine (ACh)-evoked responses in alpha9alpha10-injected Xenopus laevis oocytes in a concentration-dependent manner, with a rank order of potency of chloroquine (IC50 = 0.39 microM) > quinine (IC50 = 0.97 microM) approximately quinidine (IC50= 1.37 microM). Moreover, chloroquine blocked ACh-evoked responses on rat cochlear inner hair cells with an IC50 value of 0.13 microM, which is within the same range as that observed for recombinant receptors. Block by chloroquine was purely competitive, whereas quinine inhibited ACh currents in a mixed competitive and noncompetitive manner. The competitive nature of the blockage produced by the three compounds was confirmed by equilibrium binding experiments using [3H]methyllycaconitine. Binding affinities (Ki values) were 2.3, 5.5, and 13.0 microM for chloroquine, quinine, and quinidine, respectively. Block by quinine was found to be only slightly voltage-dependent, thus precluding open-channel block as the main mechanism of interaction of quinine with alpha9alpha10 nAChRs. The present results add to the pharmacological characterization of alpha9alpha10-containing nicotinic receptors and indicate that the efferent olivocochlear system that innervates the cochlear hair cells is a target of these ototoxic antimalarial compounds.

Diverse responses of single auditory afferent fibres to electrical stimulation of the inferior colliculus in guinea-pig

Medial olivocochlear (MOC) neurons in the auditory brainstem project to the cochlea and inhibit cochlear neural output by their action on the cochlear outer hair cells. The function of the lateral olivocochlear (LOC) neurons, projecting to the auditory primary afferents is still under debate. Recent studies have suggested that the olivocochlear system can have frequency-specific, spatially restricted effects within the cochlea. It has been shown that the inferior colliculus (IC) projects to the MOC neurons in a tonotopic manner and that electrical stimulation of the IC can activate the MOC system, suppressing cochlear gross potentials. In addition, it has been shown that stimulation of the IC may be able to activate the LOC neurons. We investigated the effect of IC stimulation on single units in the cochlea of guinea-pigs and searched for evidence of spatially restricted effects of the MOC system and effects of the LOC system. We found a variety of effects on single units. About 40% of units were unchanged whereas others (53%) showed inhibitory effects, reflected in a rightward shift of their rate-level function, sometimes accompanied by a suppression of the spontaneous rate. About 18% of the inhibited neurons showed an increased spontaneous rate. In 5% of the units we observed an excitatory effect of IC stimulation, resulting in a leftward shift of the rate-level functions. We also found that the effect could vary greatly between units of the same and adjacent frequencies within a single animal. These results imply an involvement of another regulatory system besides the MOC system, possibly the LOC system, which acts directly on the primary afferents. These data also demonstrate that the olivocochlear system is capable of eliciting highly localized effects on different frequency regions in the cochlea.

Sound-evoked efferent effects on cochlear mechanics of the mustached bat

The influence of the crossed medial efferent system on cochlear mechanics of the mustached bat was tested by measuring delayed evoked otoacoustic emissions (DEOAEs), cochlear microphonics, distortion product otoacoustic emissions (DPOAEs) and stimulus frequency otoacoustic emissions. Contralaterally delivered sinusoids, broadband noise and bat echolocation calls were used for acoustic stimulation of the efferent system. With all four measures we found a level-dependent suppression under stimulation with both broadband noise and echolocation calls. In addition, the sharply tuned cochlear resonance of the mustached bat which is involved in processing echolocation signals at 61 kHz shifted upward in frequency by several 100 Hz. Presentation of sinusoids did not have any significant effect. DEOAEs and DPOAEs were in some cases enhanced during contralateral presentation of the bat calls at moderate intensities. The most important function of the efferent system in the mustached bat might be the control of the extraordinarily fine-tuned resonator of this species, which is close to instability as evident from the very pronounced evoked otoacoustic emissions which sometimes convert into spontaneous otoacoustic emissions of high level.

Assessment of medial olivocochlear system function in pre-term and full-term newborns using a rapid test of transient otoacoustic emissions

This study was conducted to investigate maturation of the medial olivocochlear efferent system (MOCS) in pre- and full-term neonates using Quickscreen (Otodynamics Ltd) and to confirm previous findings on transient otoacoustic emission (TEOAE) suppression in neonates. MOCS maturation was investigated in 46 neonates born at the Chaim Sheba Medical Center, Tel Hashomer, Israel, using Quickscreen. All neonates were normal with no family history of general or auditory disease and no risk factors for hearing impairment. MOCS function appears gradually in human pre-term neonates and is considered to reach maturity shortly after term birth. The clinical value of MOCS testing in specific populations of newborns at risk for hearing and/or brainstem function can be legitimately raised as activation of the MOCS clearly alters cochlear output. The present results can be interpreted to support the testing of infants at risk of developing abnormal MOCS function using a commercially available rapid TEOAE measurement system.

Protection from acoustic trauma is not a primary function of the medial olivocochlear efferent system

The medial olivocochlear (MOC) efferent system is an important component of an active mechanical outer hair cell system in mammals. An extensive neurophysiological literature demonstrates that the MOC system attenuates the response of the cochlea to sound by reducing the gain of the outer hair cell mechanical response to stimulation. Despite a growing understanding of MOC physiology, the biological role of the MOC system in mammalian audition remains uncertain. Some evidence suggests that the MOC system functions in a protective role by acting to reduce receptor damage during intense acoustic exposure. For the MOC system to have evolved as a protective mechanism, however, the inner ears of mammals must be exposed to potentially damaging sources of noise that can elicit MOC-mediated protective effects under natural conditions. In this review, we evaluate the possibility that the MOC system evolved to protect the inner ear from naturally occurring environmental noise. Our survey of nonanthropogenic noise levels shows that while sustained sources of broadband noise are found in nearly all natural acoustic environments, frequency-averaged ambient noise levels in these environments rarely exceed 70 dB SPL. Similarly, sources reporting ambient noise spectra in natural acoustic environments suggest that noise levels within narrow frequency bands are typically low in intensity (<40 dB SPL). Only in rare instances (e.g., during frog choruses) are ambient noise levels sustained at moderately high intensities (~70-90 dB SPL). By contrast, all experiments in which an MOC-mediated protective effect was demonstrated used much higher sound intensities to traumatize the cochlea (100-150 dB SPL). This substantial difference between natural ambient noise levels and the experimental conditions necessary to evoke MOC-mediated protection suggests that even the noisiest natural acoustic environments are not sufficiently intense to have selected for the evolution of the MOC system as a protective mechanism. Furthermore, although relatively intense noise environments do exist in nature, they are insufficiently distributed to account for the widespread distribution of the MOC system in mammals. The paucity of high-intensity noise and the near ubiquity of low-level noise in natural environments supports the hypothesis that the MOC system evolved as a mechanism for "unmasking" biologically significant acoustic stimuli by reducing the response of the cochlea to simultaneous low-level noise. This suggested role enjoys widespread experimental support.

Intracochlear injection of pseudorabies virus labels descending auditory and monoaminerg projections to olivocochlear cells in guinea pig

Pseudorabies virus was used to label transneuronally descending auditory projections following intracochlear injections. At different time points after injection, virus-infected cells were detected immunohistochemically in the central nervous system. Initially (25 h), virus was transported retrogradely to olivocochlear cells in the pons. At 32-72 h after injection, labelling occurred in higher order auditory brainstem nuclei as well as in the locus coeruleus and pontine dorsal raphe. At 90-108 h, virus-infected neurons were found bilaterally in the medial geniculate body and in layer V of the auditory cortex. Viral transneuronal labelling in the auditory cortex after intracochlear application confirms the existence of a continuous descending chain of neurons from the auditory cortex to the cochlea, via the medial and lateral olivocochlear systems. The transneuronal labelling of the locus coeruleus and pontine dorsal raphe suggests that noradrenergic and serotonergic inputs may substantially influence the activity of olivocochlear cells, and thus the cochlea.

A non-linear circuit for simulating OHC of the cochlea

In the present paper, referring to known characteristics of the outer hair cells functioning in the cochlea of the inner ear, a functional model of the outer hair cells is constructed. It consists of a linear feed-forward circuit and a non-linear positive feedback circuit. The feed-forward circuit reflects the contribution of local basilar and tectorial membrane areas and passive outer hair cells' physical parameters to the forming of low-selectivity resonance characteristics. The non-linear positive feedback circuit reflects the non-linear outer hair cell signal transduction processes and the active role of efferents from the medial superior olive in altering circuit sensitivity and selectivity. Referring to an analytical description of the circuit model and computer simulation results, an explanation is given over the biological meaning of the outer hair cells' non-linearities in signal transduction processes and the role of the non-linearities in achieving the following: signal compression, the dependency of circuit sensitivity and frequency selectivity upon the input signal amplitude, the compatibility of high-frequency selectivity and short transient response of the biological filtering circuits.

Similar half-octave TTS protection of the cochlea by xylazine/ketamine or sympathectomy

Cochlear efferents, sympathetic control and stress conditions have been shown to influence sound-induced hearing loss. These factors are also known to be modified by sedation/anesthesia. We tested here the effect of sedation/anesthesia on temporary threshold shift (TTS) compared to that in the same awake animals. The effect of sympathectomy was also tested. We employed awake guinea pigs with a chronically implanted electrode on the round window of each of the cochleae. Each ear was tested for its sensitivity to TTS induced by a 1 min or a 10 min exposure to an 8 kHz pure tone at 96 dB sound pressure level. After an intramuscular injection of xylazine or ketamine together with xylazine, TTS at half-octave frequencies was reduced compared to that in awake animals. The second half-octave frequencies were less affected. This specific pattern of protection was also observed here after surgical ablation of a superior cervical ganglion. The data lead to the speculation that protection from TTS under sedation/anesthesia might be due to diminished sympathetic influence. Xylazine is a pre-synaptic alpha2-adrenoreceptor agonist which blocks noradrenaline release from the sympathetic system. Ketamine is a N-methyl-D-aspartic acid receptor antagonist which could reduce glutamate excitotoxicity as well as reduce sympathetic activity.

Variation in inter-animal susceptibility to noise damage is associated with alpha 9 acetylcholine receptor subunit expression level

Large intersubject variabilities in acoustic injury are known to occur in both humans and animals; however, the mechanisms underlying such differences are poorly understood. The olivocochlear efferent system has been hypothesized to play a significant role in protecting the cochlea from noise overexposure. In this study, we demonstrate that a newly developed test for determining average efferent system strength can predict intersubject variations in acoustic injury. In addition, the intersubject variability in cochlear expression of the alpha9 subunit of the nicotinic acetylcholine receptor was found to be proportional to an animals average efferent strength. Therefore, the inter-animal variability in the alpha9-containing acetylcholine receptor expression may be one mechanism contributing to the inter-animal variability in acoustic injury.

Inputs from the cochlea and the inferior colliculus converge on olivocochlear neurones

Medial olivocochlear (MOC) neurones, located in the superior olivary complex, can suppress cochlear gain by their action on the cochlear outer hair cells. Inputs from the contralateral cochlea and the inferior colliculus (IC) have been separately shown to increase activity of MOC neurones. In this study we have investigated in guinea-pigs under barbiturate anaesthesia the interactions between these two inputs by combining electrical stimulation of the IC with acoustic stimulation of the contralateral cochlea. Electrical stimulation of the IC resulted in a significant suppression of the amplitude of the compound action potential (CAP) of the auditory nerve to test tones. This suppression was equivalent to an average decrease in sound intensity of 5.7 dB and 3.7 dB for contralateral and ipsilateral stimulation, respectively. Acoustic stimulation of the contralateral cochlea with broadband noise produced no detectable change in the amplitude of the CAP in the test cochlea in all but one animal. However, simultaneous electrical stimulation of the IC and acoustic stimulation of the contralateral cochlea resulted in a reduction in CAP amplitude that was markedly larger than that produced by IC stimulation alone. The suppression with the addition of contralateral noise was equivalent to a mean reduction in sound intensity of 8.7 dB with contralateral and 5.7 dB with ipsilateral IC stimulation. We hypothesise that excitatory input from the contralateral cochlea converges with excitatory input from the IC on the MOC neurones and in this way augments the activity of these neurones, resulting in a larger peripheral effect.

Origin of the noradrenergic innervation of the superior olivary complex in the rat

In the rat, the superior olivary complex contains lateral and medial olivocochlear neurones, which respectively innervate two separate targets within the cochlea; the auditory afferents contacting the inner hair cells and the outer hair cells themselves. Previous double label immunohistochemical studies have shown that both lateral and medial olivocochlear neurones are contacted by noradrenergic nerve endings, and electrophysiological studies on in-vitro rat brain slices have demonstrated that noradrenaline exerts a direct, predominantly excitatory effect on medial olivocochlear neurones. In this paper, we have investigated the origin of the noradrenergic input to the superior olivary complex (SOC). A retrograde tracer, Fluorogold, was used to map the inputs to the SOC, and this was combined with immunofluorescent staining for dopamine-beta-hydroxylase (DbetaH) to identify which of the afferent inputs was noradrenergic. These experiments showed small numbers of neurones double-stained for both Fluorogold and DbetaH in the A6 cell group (the locus coeruleus). In the A7 cell group, within and medial to the lateral lemniscus, numerous Fluorogold labelled and DbetaH positive neurones were found, but no neurones were seen that were double-labelled. In none of the other major noradrenergic cell groups were labelled Fluorogold neurones ever detected. To confirm the results obtained by retrograde tracer injections, anterograde tracer injections with biotinylated dextran amine were made in the locus coeruleus. This resulted in labelled fibres within all subdivisions of the superior olivary complex. These experiments indicate that the noradrenergic input to the olivocochlear neurones originates solely from the locus coeruleus. The small numbers of double-labelled neurones found in the locus coeruleus indicate a very divergent non-selective noradrenergic input to the SOC.

Cochlear de-efferentation and impulse noise-induced acoustic trauma in the chinchilla

The olivocochlear bundle (OCB) has been shown to protect the ear from acoustic trauma induced by continuous noise or tones. The present study examines the OCB's role in the ear's response to impulse noise (150 dB pSPL, 100 impulses, 50 s total exposure duration). Successful section of the OCB was achieved through a posterior parafloccular fossa approach for the right ears of six out of 15 adult chinchillas. The left ears from the same animals served as efferent-innervated controls. Measurements of inferior colliculus evoked potentials (ICPs) showed that the de-efferented ears incurred similar temporary and permanent threshold shifts as the control ears. Twenty days after noise exposure, depressed ICP amplitudes had virtually recovered to pre-values in the control ears whereas those in the de-efferented ears remained significantly depressed. Greater loss of inner hair cells was seen in the de-efferented ears than in the control ears. Both control and de-efferented ears incurred large loss of outer hair cells, with no statistically significant differences between groups. The current data are intriguing, yielding tentative evidence to suggest that inner hair cells of de-efferented ears are more susceptible to impulse noise than those in efferented control ears. In contrast, outer hair cell vulnerability to impulse noise appears to be unaffected by de-efferentation.

Morphological relationships of peptidergic and noradrenergic nerve terminals to olivocochlear neurones in the rat

In the rat, the outer hair cells in the cochlea receive direct synaptic input from neurones in the ventral nucleus of the trapezoid body. These so-called medial olivocochlear neurones exert an inhibitory influence on the cochlear neural output. Electrophysiological in vitro studies suggest that the activity of medial olivocochlear neurones may be affected by a variety of neuropeptides as well as noradrenaline, but anatomical confirmation of direct synaptic input is still lacking. We have investigated, at the light microscopical level, the morphological relationships between terminals containing noradrenaline, substance P, cholecystokinin and leu-enkephalin, and medial olivocochlear neurones in the rat. A retrograde tracer was injected into the cochlea to label medial olivocochlear neurones and a double labelling immunocytochemical method was used to visualise the retrograde tracer as well as the neurotransmitters within each brain section. Light microscopical analysis revealed nerve endings containing substance P, cholecystokinin and leu-enkephalin in close apposition to the dendrites of medial olivocochlear neurones, and nerve endings containing dopamine-beta-hydroxylase, a marker for noradrenaline, in close contact with the somata as well as dendrites of medial olivocochlear neurones. Although the technique cannot prove the existence of functional synaptic contacts, the results are broadly consistent with electrophysiological data and suggest a direct input to medial olivocochlear neurones from substance P, cholecystokinin, leu-enkephalin and noradrenaline-containing neural pathways. Differences in the densities and spatial distribution of the various neuropharmacological inputs suggest differences in the relative strengths and possible roles of these diverse inputs to the olivocochlear system.

Monitoring noise susceptibility: sensitivity of otoacoustic emissions and subjective audiometry

The capacity of different audiological methods to detect a high noise susceptibility was examined in 20 normally hearing and 26 especially noise-susceptible subjects. The latter were selected from 422 soldiers in field studies: they had shown a temporary threshold shift (TTS) in pure tone audiometry (PTA) after regular training with firearms. In laboratory experiments, the TTS-positive soldiers were re-examined using greatly reduced sound intensities, which caused no TTS in a control subject group. Before and after acoustic stimulation, different subjective (PTA, high frequency audiometry (HFA), upper limit of hearing (ULH)) and objective (transiently evoked otoacoustic emissions (TEOAE), distortion products (DPOAE)) audiological tests were performed. After exposure to low impact noise in the laboratory, in both PTA and HFA, a TTS was observed in 11.5% (N = 3) of the noise-susceptible group (compared to 0% in the control group). In the TTS-positive group, deterioration of the ULH occurred in 28% (N = 7) (compared to 15% (N = 3) in the control group). An ULH improvement occurred in only one subject (3.8%) (compared to 25% (N = 5) in the control group). Significant alterations of click-evoked OAE-amplitudes were found in 26.9% (N = 7) of the selected groups, whereas stable emissions were observed in all but one subject (5%) of the control group. However, DPOAE alterations were seen in 19.2% (N = 5) of the TTS-positive soldiers but also in 25% (N = 5) of the control group. These results suggest that TEOAE provides a more sensitive and more objective method of detecting a subtle noise-induced disturbance of cochlear function than do PTA or DPOAE.

Maturation of medial efferent system function in humans

Otoacoustic emissions are typically reduced in amplitude when broadband noise is presented to the contralateral ear. This contralateral suppression is attributed to activation of the medial olivocochlear system, which has an inhibitory effect on outer hair-cell activity. By studying the effects of contralateral noise on cochlear output at different stages of auditory maturation in human neonates, it is possible to describe the timecourse for development of medial efferent system function in humans. The present study recorded 2 f1-f2 distortion product otoacoustic emissions (DPOAE) in human adults, term and premature neonates at three f2 frequencies: 1500, 3000, and 6000 Hz, using fixed primary tone frequency ratio (f2/f1 = 1.2) and level separation (10 dB, L1 > L2). Average DPOAE growth functions were recorded with and without contralateral broadband noise. Results indicate that contralateral suppression of DPOAEs is absent at 6000 Hz, but present at 1500 and 3000 Hz for all ages. However, DPOAE amplitude from premature neonates was not altered by noise in an adult-like manner; in this age group, DPOAE amplitude was equally likely to by suppressed or enhanced by noise presented contralaterally. Contralateral enhancement may reflect a temporary stage of immaturity in outer hair cell-medial efferent fiber synapses just prior to term birth.

Exponential onset and recovery of temporary threshold shift after loud sound: evidence for long-term inactivation of mechano-electrical transduction channels

The onset and recovery of temporary threshold shift (TTS) in one human subject (the author) has been studied during and after pure-tone overstimulation lasting between minutes and days. Under the conditions of these experiments the time courses appeared reproducible, and thresholds always recovered to normal within 3 days. The onset and recovery followed a multiple-exponential time course, with the time constants for the onset being 6.5 and 800 min, and the recovery time constants being 30, 240 and 800 min. The observed time courses were consistent with data previously reported in humans, and with the view that the threshold elevation was due to an inactivation and reactivation of the stretch-activated channels at the apex of the outer hair cells of the cochlea. The time constants of the multi-exponential onset and recovery do not appear to depend on the duration of the overstimulation, but the exponential coefficients do. A simple kinetic model of the onset and recovery is described (for more detail see Patuzzi (1998)). It is suggested that the rapid recovery in the first 5 min after exposure is due to a short-lived disruption of the synapses between the inner hair cells and the primary afferent neurones. Intermittent exposures were found to produce much less TTS than continuous tones, and this reduction was found to be inconsistent with the Equal Energy Hypothesis, in that the TTS produced by intermittent tones was much less than predicted using the Equal Energy model, and the recovery time course was also different from that expected from a shorter exposure to a continuous tone of equal energy.

Changes in cochlear mechanics during vocalization: evidence for a phasic medial efferent effect

The mustached bat, Pteronotus p. parnellii, has a finely tuned cochlea that rings at its resonant frequency in response to an acoustic tone pip. The decay time (DT) and frequency of these damped oscillations can be measured from the cochlear microphonic potential (CM) to study changes in cochlear mechanics. In this report, we describe phasic changes that occur in synchrony with communication sound vocalizations of the bat. Three animals with chronically implanted electrodes were studied. During the experiments, 1-2 ms tone pips were emitted from a speaker every 200 ms. This triggered a computer analysis of the resulting CM to determine the DT and cochlear resonance frequency (CRF) of the ringing. The time relative to vocalizations was determined by monitoring the output of a microphone placed near a bat's mouth. Similar results were obtained from all three bats tested. In a representative case, the average DT was 2.33 +/- 0.25 ms while the bat was quiet, but it decreased by 46% to 1.26 +/- 0.75 during vocalizations, which indicates a greater damping of the cochlear partition. Sometimes, DT started decreasing immediately before the bat vocalized. After the end of a vocalization, the return to baseline values varied from rapid (milliseconds) to gradual (1-2 seconds). The CRF also changed from baseline values during vocalization, although the amount and direction of change were not predictable. When gentamicin was administered to block the action of medial olivocochlear (MOC) efferents, DT reduction was still evident during vocalization but less pronounced. We conclude that phasic changes in damping occur in synchrony with vocalization, and that the MOC system plays a role in causing suppression. Since suppression can begin prior to vocalization, this may be a synkinetic effect, mediated by neural outflow to the ear in synchrony with neural outflow to the middle ear muscles and the muscles used for vocalization.

Substance P-sensitive neurones in the rat auditory brainstem: possible relationship to medial olivocochlear neurones

Using in vitro techniques, intracellular microelectrode recordings were made from the regions of origin of medial olivocochlear neurones in the rat auditory brainstem. Cells were characterised according to their action potential shape and their excitatory response to bath application of micromolar concentrations of the peptide neurotransmitter substance P and were filled with biocytin by injection through the intracellular microelectrode for subsequent morphological reconstruction. Cells with a rapid component to the after-hyperpolarising phase of their action potentials (AHP2 cells) were most likely to show excitatory effects of substance P. Histological reconstruction showed that these cells were stellate with numerous large, slowly tapering dendrites exhibiting small. scattered spines. In examples in which the major axon was not cut near the cell body, the axons ascended dorsally out of the superior olivary complex, in a manner that was consistent with the trajectory of axons of medial olivocochlear neurones. These features differed from other cells in the ventral nucleus of the trapezoid body that were unresponsive to substance P. In a further series of experiments, medial olivocochlear cells in the same region were retrogradely labeled by prior intracochlear injection with fast blue and recordings were made under direct visual observation using either microelectrode impalement or whole-cell patch methods. These data support the view that medial olivocochlear neurones are substance P-sensitive and exhibit a characteristic spike shape. These data strongly suggest that medial olivocochlear neurones possess receptors for substance P and may therefore receive excitatory input from a substance P-utilising neural pathway.

Two types of actions of norepinephrine on identified auditory efferent neurons in rat brain stem slices

Whole cell voltage-clamp recordings were performed on auditory olivocochlear neurons in the ventral nucleus of the trapezoid body (VNTB) of brain stem slices from neonatal rats. Each neuron was identified by retrograde labeling with Fast Blue injected into the cochlea. Bath application of norepinephrine (NE; 1-10 microM) reversibly induced an inward current in 26 of 38 labeled neurons that were voltage clamped at -75 mV. This was responsible for the membrane depolarization to NE observed in current-clamp mode. The NE-induced inward current appeared to be more prominent at -55 mV than at -75 mV and reversed at around -100 mV. It was attenuated but not prevented by 20 mM tetraethylammonium, and it persisted when the perfusate contained 2 mM Cs+ or 100 microM Cd2+. However, the NE-induced inward current was attenuated to varying degrees in a zero-Ca2+ solution. Current-voltage plots revealed that NE caused a decrease in membrane K+ conductance. A suppression of voltage-gated Ca2+ currents by NE was also observed. The excitatory action of NE was blocked by the alpha-adrenoreceptor antagonist phentolamine. The alpha1-adrenoreceptor agonist phenylephrine had an effect similar to that of NE. In 6 of 38 labeled neurons, an inhibitory action of NE (1-10 microM) was observed that appeared to be due to an activation of an inwardly rectified K+ current, which caused hyperpolarization of resting membrane potentials in current-clamp mode. This inhibitory response was independent of external Ca2+ and was abolished by 2-5 mM Cs+ or 0.5 mM Ba2+ applied in the perfusate. The receptors involved in the inhibitory actions of NE are not clear. The effect was partially and reversibly blocked by propranolol (10 microM), a beta-adrenoreceptor antagonist. However, isoprenaline (10 microM), a beta-adrenoreceptor agonist, failed to induce any effect. On the other hand, the inhibitory effect was irreversibly blocked by pretreatment with phentolamine (5-10 microM). Phenylephrine (5-10 microM) had no effect.

Chronic strychnine administration into the cochlea potentiates permanent threshold shift following noise exposure

To investigate whether elimination of the medial efferent system influences permanent threshold shift following noise exposure, we developed an animal model in which strychnine was chronically delivered into the cochlea via an osmotic pump. Pigmented female guinea pigs were allocated into three groups: group I was treated with strychnine (50 microM, 0.5 microl/h, 14 days) in the left ear and exposed to noise (105 dB SPL broadband, 3 h) 3 weeks after the cessation of the strychnine perfusion; group II received strychnine in the left ear but no noise exposure; group III was treated with Ringer's solution in the left ear and exposed to noise. Animals in group II developed no hearing loss after the strychnine perfusion. The operated ears in group I demonstrated greatest hearing threshold shift 3 h after noise exposure. Hearing recovered during 2 weeks after noise exposure in both operated and non-operated ears in groups I and III. Two weeks after noise exposure, the operated ears in group I showed significantly greater threshold shift at 12, 16, and 20 kHz compared to the operated ears in group III and non-operated ears in groups I and III. These findings suggest that chronic strychnine administration into the cochlea inactivates the medial efferents without changing hearing threshold and that the medial efferents help to protect against permanent threshold shift following noise exposure.

The influence of the cochlear efferent system on chronic acoustic trauma

The role of the olivocochlear bundle (OCB) in modulating noise-induced permanent injury to the auditory periphery was studied by completely sectioning the OCB fibers in chinchillas and exposing the animals while awake to a broad-band noise at 105 dB SPL for 6 h. Outer hair cell (OHC) function was assessed by measuring 2f1-f2 distortion product otoacoustic emissions (DPOAE) at frequencies from 1.2 to 9.6 kHz and cochlear microphonics (CM) at frequencies from 1 to 8 kHz. As a result of de-efferentation, the CM was decreased but the DPOAEs were unchanged in de-efferented ears as compared with efferented control and sham-operated ears. Following noise exposure, the ears that were de-efferented showed significantly more depression of DPOAE input/output functions and greater decrement of CM amplitude. The differences between de-efferented and efferent-innervated ears were evident across all the frequencies. The cochlear lesions of the OHCs reflected by traditional cytocochleograms, however, were minimal in both efferented and de-efferented ears. The results indicate that cochlear de-efferentation decreases the CM in chinchilla and increases the ear's susceptibility to noise-induced permanent hearing damage. More importantly, de-efferentation increases susceptibility at low frequencies as well as high frequencies.

Co-administration of the neurotrophic ACTH(4-9) analogue, ORG 2766, may reduce the cochleotoxic effects of cisplatin

In this study the effect of the neurotrophic ACTH(4-9) analogue, ORG 2766, on cisplatin cochleotoxicity was investigated with both light- and transmission electron microscopy. Guinea pigs were treated with either cisplatin+ORG 2766 (n = 11) or cisplatin + physiological saline (n = 9). All animals treated with cisplatin + physiological saline showed complete loss of outer hair cells (OHC) and degeneration of the organ of Corti in the basal cochlear turns, while partial OHC loss was found in the middle and apical turns. The inner hair cells (IHC) and other cochlear tissues were not affected. Eight animals from the group treated with cisplatin + ORG 2766 demonstrated similar pathological changes, but to a lesser degree, especially in the middle turns. The three remaining animals demonstrated no cochlear alterations at all, light-microscopically, and only minor subcellular changes in the OHCs at the ultrastructural level. Electrophysiologically, these three animals showed normals compound action potential (CAP) amplitudes at stimulus frequencies from 0.5 to 16 kHz and normal cochlear microphonics (CM) in the frequency range from 0.5 to 8 kHz. The other animals treated with cisplatin + ORG 2766 showed a severe loss in their CAPs and CM, except for one showing intermediate loss. All animals from the group treated with cisplatin alone showed a severe loss in their CAPs and CM. Endolymphatic hydrops was present in all animals from the cisplatin- and the cisplatin + ORG 2766-treated groups. These data indicate that daily, concomitant administration of ORG 2766 may reduce OHC loss and subsequent degeneration of the organ of Corti in cisplatin-treated guinea pig cochleas.

The role of the cochlear efferent system in acquired resistance to noise-induced hearing loss

Previous work has shown that the cochlear efferent system may play a role in protecting the ear from noise-induced temporary threshold shifts (TTS) following exposures to a single tone or series of moderate-level noises ('toughening'). However, whether the olivocochlear bundle (OCB) is important in decreasing noise-induced permanent threshold shifts (PTS) remains an open question. The importance of the OCB in decreasing the ear's susceptibility to noise, as reflected by 2f1-f2 distortion product otoacoustic emissions, was assessed by sectioning both the ipsilateral and contralateral divisions of the efferent system and exposing chinchillas while awake to an octave band noise (4 kHz) at a low level (85 dB SPL) for 10 days (6 h/day) and then at a high level (95 dB SPL) for 48 h. Complete de-efferentation was verified by cochlear acetylcholinesterase staining. The ears that were de-efferent showed substantially more TTS, greater PTS and larger cochlear lesions of outer hair cells. The results suggest that the efferent system may influence the ear's ability to develop resistance to noise trauma.

Low-frequency 'conditioning' provides long-term protection from noise-induced threshold shifts in chinchillas

Studies have shown that loss of auditory sensitivity caused by exposure to high-level acoustic stimuli can be significantly reduced by pre-exposing the subject to moderate-level acoustic stimuli. Although the protective effects of such 'conditioning' exposures have been well documented, very little is known about the persistence of conditioning-induced protection, or about the biological mechanisms underlying it. In the present study, the persistence of conditioning-induced protection was examined in chinchillas by imposing either a 30- or 60-day recovery period between conditioning (10 days of exposure to 0.5 kHz noise at 90 or 95 dB, 6 h/day) and high-level (0.5 kHz noise at 106 dB for 48 h) exposures. Comparisons of threshold shifts between conditioned animals and control animals exposed only to high-level noise indicated that conditioning provided significant protection from noise-induced threshold shifts for at least 2 months. Conditioned animals sustained outer hair cell losses similar to controls, ranging from 15 to 30% in the apical half of the cochlea. The results suggest that low-frequency conditioning can trigger long-lasting changes in cochlear homeostasis rather than temporary changes in physiology or reductions in susceptibility to hair cell loss in chinchillas.

The "toughening" phenomenon in rat's auditory organ

In audiological "toughening" or "conditioning" phenomenon prior exposure to moderate noise reduces the extent of hearing deterioration caused by the subsequent exposure to traumatic test noise known to cause inner ear damage. "Toughening" has been demonstrated in many mammalian laboratory animals such as guinea pig and chinchilla but not in rat or mouse. Our aim was to study the occurrence of this phenomenon in the rat. Ninety-one white male Wistar rats were divided into four groups: unexposed control group (U, n = 10), "conditioning" only (C, n = 32), "conditioning" plus test noise (C + T, n = 36) and test noise only (T, n = 13). Groups C and C + T were "conditioned" for 10 hours with 4.0 kHz OBN between 55 and 95 dB sound pressure levels (SPLs). After 10 hours rest groups C + T and T were exposed to the same noise at 105 dB SPL for 13 hours. The hearing thresholds were determined by auditory brainstem response audiometry (ABR) either immediately after or 3 weeks after the exposures. After that the animals were sacrificed. The cochleas were removed and perilymphatically fixed and further processed for quantitative cytocochleograms. Both the temporary (TTS) and the permanent threshold shifts (PTS) were smaller in animals which had been "conditioned" prior exposure to traumatic noise. Yet only 95 dB SPL "conditioning" gave statistically significant difference (p < 0.05) in PTS. From our results we conclude that "conditioning" effect seems to be present also in the rat. However to confirm this, further experiments are needed. The mechanisms behind "conditioning" are still unknown and also to clarify them, further efforts are needed.

The course and distribution of medial efferent fibers in the cochlea of the mustached bat

The course and distribution of medial olivocochlear (MOC) nerve fibers were studied in the cochlea of the mustached bat. This animal is of interest because of the very sharp tuning of the ear and fine frequency resolution in small frequency bands near 60 and 90 kHz. The MOC fibers arise from about 400 cells in the dorsomedial periolivary (DMPO) nucleus and they are distributed to approximately 4500 outer hair cells (OHCs), resulting in an average OHC unit size of 11.25. Individual fibers appear to have a small number of branches and each branch entering the tunnel of Corti terminates on a patch of OHCs. The patch size is typically 1-3 OHCs with the smallest average patch sizes in the regions tuned to 60 and 90 kHz. The majority of the MOC terminals are derived from the contralateral DMPO. Contralateral vs. ipsilateral projecting fibers are not preferentially distributed within any of the three rows of OHCs or within specific regions throughout most of the cochlea. It can be concluded that the main differences between the mustached bat's MOC system and that of most other mammals are: (1) origin from a single nucleus; (2) relatively small sizes of the patches; (3) a single terminal on each OHC; (4) a gradient in the size of the terminals but not in the number of terminals from row to row or from base to apex.

The effects of moderate and low levels of acoustic overstimulation on stereocilia and their tip links in the guinea pig

Guinea pigs were exposed to pure tones of 10 kHz at intensities between 98 and 115 dB SPL for 5-30 min, to produce varying degrees of acoustic trauma. Changes in auditory thresholds were measured electrophysiologically, and the animals were immediately fixed for scanning electron microscopy. Correlation between morphological changes to the hair bundle and losses in threshold, showed that with the smallest degrees of trauma (98 dB SPL for 15 min, mean maximum threshold loss of 22 dB), damage was confined to a small stretch of inner hair cells (IHC), with only subtle changes to the stereocilia of the outer hair cells (OHC). At exposure intensities greater than 102 dB SPL (duration: 15 min) the IHC stereocilia in the centre of the lesion were always substantially disarrayed. Substantial damage to the OHC bundles was seen only with exposures above 110 dB SPL (duration: > or = 5 min), producing threshold losses of 50 dB or more. Tip links were lost only where the stereocilia were disarrayed. It is concluded that the tip links are not the most vulnerable components of the cochlear hair cell, but that relatively low levels of acoustic stimulation can cause significant damage to the stereociliary bundle of the IHCs.

Suppression of stimulus frequency otoacoustic emissions by contralateral noise

The effect of different bands of contralaterally presented noise at low and moderate intensities on stimulus frequency otoacoustic emissions (SFOAE) from human ears is examined. A SFOAE evoked by a continuous stimulus tone and suppressed by a second tone to produce an SFOAE residual was chosen as the probe to determine the effect of the efferent input. At low levels of contralateral noise, a band centred on the ipsilateral stimulus frequency was the most effective suppressor of the SFOAE residual. For higher levels of the contralateral stimulus, noise bands containing higher frequency components produced most reductions in the SFOAE residual. Small changes in the phase of the SFOAE residual during the contralateral noise were also recorded. Increases in the SFOAE residual onset latency were also found to be small, being around 1 ms. In some cases increases in the level of the SFOAE residual produced by low-frequency suppressors were recorded during the contralateral noise presentation. The results are discussed in the context of current knowledge of the functioning of the auditory efferent innervation, and it is suggested that the method of evoking SFOAEs presents a viable method for determining the effect of efferent stimulation on cochlear mechanics which also allows possible artifact contamination to be readily identified.

Chronic cochlear de-efferentation and susceptibility to permanent acoustic injury

The question of whether olivocochlear (OC) efferent feedback can decrease permanent damage from acoustic overexposure was investigated by comparing the chronic threshold shifts and cochlear histopathology in guinea pigs either surgically de-efferented or sham-operated and then exposed (awake and unrestrained) to a 109- or 112-dB narrow-band noise centered at 10 kHz for 2 h. Threshold shifts were estimated using compound action potentials; hair cell loss and stereocilia condition were evaluated via light-microscopic examination of plastic-embedded surface preparations, and the degree of de-efferentation was assessed by measuring OC fascicles in the tunnel of Corti. Among animals exposed to 109-dB noise, the mean permanent threshold shift (PTS) was less than 25 dB, and there were no significant differences between normal and de-efferented animals with respect to either physiological or histological measures of acoustic injury. Among animals exposed to 112 dB, the mean peak PTS was roughly 50 dB. There was a small (but statistically significant) increase in PTS for de-efferented animals, especially at frequencies above the region of peak threshold shift; however, the patterns of hair cell loss and stereocilia damage were statistically indistinguishable. Thus, for these particular exposure conditions, sound-evoked activity in the OC system does not play a major protective role in the auditory periphery, except perhaps for the extreme basal regions of the cochlea.

The effect of contralateral stimulation on cochlear resonance and damping in the mustached bat: the role of the medial efferent system

In the unanesthetized mustached bat, stimulation of the ear with an acoustic transient produces damped oscillations which are evident in the cochlear microphonic potential. In this report we demonstrate how the decay time of these oscillations is affected by broadband noise presented to the contralateral ear (CLN). In the absence of CLN, the mean decay time was 1.94 +/- 0.23 ms, but during the presentation of CLN the decay time consistently decreased. The changes were finely graded, the higher the CLN, the greater the change. The effect could be maintained at a constant level for extended periods of time and this was evident when the CLN exceeded 40 dB SPL. The latency of the reflex for 64 dB noise was about 11 ms and near maximum changes occurred within 15 ms of CLN onset. Sectioning medial efferent nerve fibers in the floor of the fourth ventricle or the administration of a single dose of gentamicin eliminated changes produced by CLN. The prominence of CM responses to damped oscillations and the robust changes in response to CLN make the mustached bat an excellent model for studying the influence of the medial efferent system on cochlear mechanics.

Acute and chronic effects of unilateral elimination of auditory nerve activity on susceptibility to temporary deafness induced by loud sound in the guinea pig

The involvement of crossed cochlear pathways in modulating the deafening effects of loud sound was investigated in the anaesthetized guinea pig. Auditory nerve activity was blocked unilaterally, either by surgical cochlear destruction or intracochlear perfusion of lignocaine, and the effect of a standard loud sound exposure in the untreated ear was then assessed using the compound action potential (CAP) audiogram technique. It was found that both cochlear destruction or lignocaine perfusion reduced the amount of threshold elevation in the untreated ear. The effect of lignocaine perfusion was significantly greater than acute cochlear destruction. In animals allowed to survive for 24 h and one week post-cochlear destruction before loud sound exposure, the protective effect was still present and was significantly greater than immediately post-destruction. This long-term protective effect of contralateral cochlear destruction was blocked by administering strychnine prior to the loud sound exposure. The results of lignocaine perfusion and chronic destruction make it unlikely that protection immediately post-destruction is the result of a transient barrage of primary afferent activity. We conclude that elimination of auditory nerve input can alter the effectiveness of brainstem circuitry responsible for protection (possibly the olivocochlear system). Since acoustic stimulation of the contralateral ear also has acute protective effects thought to be mediated by olivocochlear efferents, the circuitry responsible for protection appears to be subject to a complex balance between excitatory and inhibitory influences.

Input from the inferior colliculus to medial olivocochlear neurons in the rat: a double label study with PHA-L and cholera toxin

The inferior colliculus provides a strong descending influence capable of modulating the excitability levels of olivocochlear neurons (Rajan, 1990). In an attempt to anatomically demonstrate this pathway in rats, an experimental paradigm was designed by which anterogradely transported Phaseolus vulgaris-leucoagglutinin (PHA-L), which delineates axonal arbors, and retrogradely transported cholera toxin B subunit alone (CT-B) or conjugated to horseradish peroxidase (CT-HRP), which delineate dendritic arbors, are visualized in the same brainstem sections. PHA-L was injected unilaterally into the central nucleus of the inferior colliculus of adult rats 5-9 days prior to injection of CT-B or CT-HRP into either the contralateral or the ipsilateral cochlea. Descending collicular axons labeled with PHA-L densely innervate the ventral nucleus of the trapezoid body (VNTB), which contains neurons of the medial olivocochlear system (MOCS), but do not enter the lateral superior olive, where the neurons of the lateral olivocochlear system (LOCS) are found. The collicular projection to VNTB is largely ipsilateral and supplies mostly the ventral half of the nucleus. Within VNTB, the collicular fibers intermingle with dendrites and, to a lesser extent, cell bodies of MOCS. Collicular boutons, predominantly of the en passant type, are often observed in close apposition to dendrites and, less frequently, cell bodies of both crossed and uncrossed MOCS. These light microscopic results suggest the existence of direct, synaptic contacts between descending collicular axons and ipsilateral crossed and uncrossed MOCS. Numerous collicular boutons were also seen at a distance from MOCS, suggesting that they establish synapses with other neuron types of the VNTB that do not send their axons to the cochlea.

Additivity of threshold losses produced by acute acoustic trauma

We have previously [Patuzzi and Rajan, Hear. Res. 60, 165-177, 1992] formulated a model to describe how the threshold elevations produced by a variety of independent, short-term cochlear manipulations add when the manipulations are combined. The manipulations were presumed to affect only the 'active process' in the cochlea. The present report applied this model to the effects observed after acute acoustic trauma in normal-hearing guinea pigs and in guinea pigs with idiopathic threshold losses. Successive loud pure-tone exposures were presented to the normal-hearing guinea pigs, while only a single exposure was presented to the guinea pigs with idiopathic hearing losses. Various parameters of exposure and inter-exposure delays were used to create a variety of threshold elevations, and the total hearing losses observed in the various groups were compared to the total hearing losses predicted by the model. In most cases a statistically-valid 1:1 relationship was obtained between the predicted values and the observed values. In cases where the model's predictions were found not to fit the data, this appeared to be due to inclusion of data previously defined to be outside the scope of the model. When such data were excluded, there was good agreement between the model's predictions and the observed data. The model was further tested by comparing its predictions with data obtained in studies of acute noise trauma in chinchillas and humans by other researchers. The model's predictions were found to agree with these data as well. Thus, across a number of different types and conditions of exposures, the model appears to provide a very good description of the additivity of threshold losses produced by acute acoustic trauma. The generality of and constraints on the model are discussed.

Tone-induced stereocilia lesions as a function of exposure level and duration in the hamster cochlea

The present study presents an atlas of the effects of 10 kHz tone exposures of different levels and durations on cochlear hair cells and their stereocilia in the Syrian golden hamster. Animals were sound exposed while under anesthesia. The exposure conditions were varied over an intensity range of 90-129 dB SPL; at the highest levels (126-129 dB SPL) the exposure periods were varied over a range of 30 min to 4 h. In animals with mild damage the lesions were commonly restricted to either the inner hair cells and/or the first row of outer hair cells; the order of damage susceptibility was IHC, OHC1, OHC2, OHC3. Damage to the second and third rows of outer hair cells were found only in animals with the severest lesions. Possible mechanisms underlying the row-specific distributions of these lesions and relative susceptibilities of the 4 rows of hair cells are discussed.