Predicting vulnerability to acoustic injury with a noninvasive assay of olivocochlear reflex strength

Permanent noise-induced damage to the inner ear is a major cause of hearing impairment, arising from exposures occurring during both work- and pleasure-related activities. Vulnerability to noise-induced hearing loss is highly variable: some have tough, whereas others have tender ears. This report documents, in an animal model, the efficacy of a simple nontraumatic assay of normal ear function in predicting vulnerability to acoustic injury. The assay measures the strength of a sound-evoked neuronal feedback pathway to the inner ear, the olivocochlear efferents, by examining otoacoustic emissions created by the normal ear, which can be measured with a microphone in the external ear. Reflex strength was inversely correlated with the degree of hearing loss after subsequent noise exposure. These data suggest that one function of the olivocochlear efferent system is to protect the ear from acoustic injury. This assay, or a simple modification of it, could be applied to human populations to screen for individuals most at risk in noisy environments.

Acoustic injury in mice: 129/SvEv is exceptionally resistant to noise-induced hearing loss

129/SvEv is an inbred mouse strain popular for use in genetic knockout studies. Here, we compare normal auditory function and vulnerability to acoustic injury in wild-type mice of the 129/SvEv vs. CBA/CaJ strains. Compound action potentials (CAPs) and distortion product otoacoustic emissions (DPOAEs) showed slightly higher thresholds for 129/SvEv re CBA/CaJ, especially at frequencies >20 kHz. Middle-ear motion (i.e. umbo velocity) was similar in the two strains; although frequencies >20 kHz could not be evaluated. Permanent threshold shift (PTS) and hair cell losses, measured 1 week after high-intensity exposure to an 8-16 kHz noise band, were smaller in129/SvEv at all exposure levels and durations from 97 dB SPLx2 h to 106 dB SPLx8 h. Furthermore, PTS growth with increasing exposure energy was slower in 129/SvEv (<2 dB/dB) than CBA/CaJ (9 dB/dB). These data suggest that the vulnerability differences lie in the inner ear, not the middle ear. Several 129/Sv substrains show age-related hearing loss (AHL): 129/SvEv has not yet been evaluated (Zheng, Q.Y., Johnson, K. R., Erway, L.C., 1999. Assessment of hearing in 80 inbred strains of mice by ABR threshold analyses. Hear. Res. 130, 94-107). Thus, although other strains with AHL, e.g. C57Bl/6J, show increased vulnerability to noise-induced hearing loss (NIHL), pairing of AHL and NIHL vulnerabilities may not be obligatory.

Gentamicin blocks both fast and slow effects of olivocochlear activation in anesthetized guinea pigs

The medial olivocochlear (MOC) efferent system, which innervates cochlear outer hair cells, suppresses cochlear responses. MOC-mediated suppression includes both slow and fast components, with time courses differing by three orders of magnitude. Pharmacological studies in anesthetized guinea pigs suggest that both slow and fast effects on cochlear responses require an initial acetylcholine activation of alpha-9 nicotinic receptors on outer hair cells and that slow effects require additional intracellular events downstream from those mediating fast effects. Gentamicin, an aminoglycoside antibiotic, has been reported to block fast effects of sound-evoked OC activation following intramuscular injection in unanesthetized guinea pigs, without changing slow effects. In the present study, we show that electrically evoked fast and slow effects in the anesthetized guinea pig are both blocked by either intramuscular or intracochlear gentamicin, with similar time courses and/or dose-response curves. We suggest that sound-evoked slow effects in unanesthetized animals are fundamentally different from electrically evoked slow effects in anesthetized animals, and that the former may arise from effects of the lateral OC system.

Heat stress and protection from permanent acoustic injury in mice

The inner ear can be permanently damaged by overexposure to high-level noise; however, damage can be decreased by previous exposure to moderate level, nontraumatic noise (). The mechanism of this "protective" effect is unclear, but a role for heat shock proteins has been suggested. The aim of the present study was to directly test protective effects of heat stress in the ear. For physiological experiments, CBA/CaJ mice were exposed to an intense octave band of noise (8-16 kHz) at 100 dB SPL for 2 hr, either with or without previous whole-body heat stress (rectal temperature to 41. 5 degrees C for 15 min). The interval between heat stress and sound exposure varied in different groups from 6 to 96 hr. One week later, inner ear function was assessed in each animal via comparison of compound action potential thresholds to mean values from unexposed controls. Permanent threshold shifts (PTSs) were approximately 40 dB in the group sound-exposed without previous heat stress. Heat-stressed animals were protected from acoustic injury: mean PTS in the group with 6 hr heat-stress-trauma interval was reduced to approximately 10 dB. This heat stress protection disappeared when the treatment-trauma interval surpassed 24 hr. A parallel set of quantitative PCR experiments measured heat-shock protein mRNA in the cochlea and showed 100- to 200-fold increase over control 30 min after heat treatment, with levels returning to baseline at 6 hr after treatment. Results are consistent with the idea that upregulation of heat shock proteins protects the ear from acoustic injury.

Divergent roles for thyroid hormone receptor beta isoforms in the endocrine axis and auditory system

Thyroid hormone receptors (TRs) modulate various physiological functions in many organ systems. The TR alpha and TR beta isoforms are products of 2 distinct genes, and the beta 1 and beta 2 isoforms are splice variants of the same gene. Whereas TR alpha 1 and TR beta 1 are widely expressed, expression of the TR beta 2 isoform is mainly limited to the pituitary, triiodothyronine-responsive TRH neurons, the developing inner ear, and the retina. Mice with targeted disruption of the entire TR beta locus (TR beta-null) exhibit elevated thyroid hormone levels as a result of abnormal central regulation of thyrotropin, and also develop profound hearing loss. To clarify the contribution of the TR beta 2 isoform to the function of the endocrine and auditory systems in vivo, we have generated mice with targeted disruption of the TR beta 2 isoform. TR beta 2-null mice have preserved expression of the TR alpha and TR beta 1 isoforms. They develop a similar degree of central resistance to thyroid hormone as TR beta-null mice, indicating the important role of TR beta 2 in the regulation of the hypothalamic-pituitary-thyroid axis. Growth hormone gene expression is marginally reduced. In contrast, TR beta 2-null mice exhibit no evidence of hearing impairment, indicating that TR beta 1 and TR beta 2 subserve divergent roles in the regulation of auditory function.

Long-term sound conditioning enhances cochlear sensitivity

Sound conditioning, by chronic exposure to moderate-level sound, can protect the inner ear (reduce threshold shifts and hair cell damage) from subsequent high-level sound exposure. To investigate the mechanisms underlying this protective effect, the present study focuses on the physiological changes brought on by the conditioning exposure itself. In our guinea-pig model, 6-h daily conditioning exposure to an octave-band noise at 85 dB SPL reduces the permanent threshold shifts (PTSs) from a subsequent 4-h traumatic exposure to the same noise band at 109 dB SPL, as assessed by both compound action potentials (CAPs) and distortion product otoacoustic emissions (DPOAEs). The frequency region of maximum threshold protection is approximately one-half octave above the upper frequency cutoff of the exposure band. Protection is also evident in the magnitude of suprathreshold CAPs and DPOAEs, where effects are more robust and extend to higher frequencies than those evident at or near threshold. The conditioning exposure also enhanced cochlear sensitivity, when evaluated at the same postconditioning time at which the traumatic exposure would be delivered in a protection study. Response enhancements were seen in both threshold and suprathreshold CAPs and DPOAEs. The frequency dependence of the enhancement effects differed, however, by these two metrics. For CAPs, effects were maximum in the same frequency region as those most protected by the conditioning. For DPOAEs, enhancements were shifted to lower frequencies. The conditioning exposure also enhanced both ipsilaterally and contralaterally evoked olivocochlear (OC) reflex strength, as assessed using DPOAEs. The frequency and level dependence of the reflex enhancements were consistent with changes seen in sound-evoked discharge rates in OC fibers after conditioning. However, comparison with the frequency range and magnitude of conditioning-related protection suggests that the protection cannot be completely explained by amplification of the OC reflex and the known protective effects of OC feedback. Rather, the present results suggest that sound conditioning leads to changes in the physiology of the outer hair cells themselves, the peripheral targets of the OC reflex.

Stereociliary anomaly in the guinea pig: effects of hair bundle rotation on cochlear sensitivity

Histological analysis of cochleas from 100 albino guinea pigs (Hartley strain) obtained from Charles River Laboratories revealed an apparently congenital anomaly in 24% of animals, with roughly equal prevalence in males and females. In affected animals, 15-50% of the first-row outer hair cells (OHCs) showed distinctly abnormal orientation of the W-shaped stereociliary array. These abnormal hair bundles could be rotated by up to 180 degrees from the normal quasi-radial orientation. Second- and third-row OHCs appeared normal in all cases. Cochlear sensitivity was assayed in a subset of animals via compound action potentials (CAPs): CAP thresholds in affected animals were, on average, elevated by 5-10 dB with respect to normal controls. If the contributions of individual OHCs to cochlear 'amplification' add linearly, and if the total OHC contribution corresponds to approximately 45 dB of 'gain', a quantitative correlation of the degree of stereociliary rotation and the degree of threshold shift in these ears suggests that first-row OHCs make a larger contribution to the cochlear amplifier than either of the other OHC rows.

Role of alpha9 nicotinic ACh receptor subunits in the development and function of cochlear efferent innervation

Cochlear outer hair cells (OHCs) express alpha9 nACh receptors and are contacted by descending, predominately cholinergic, efferent fibers originating in the CNS. Mice carrying a null mutation for the nACh alpha9 gene were produced to investigate its role(s) in auditory processing and development of hair cell innervation. In alpha9 knockout mice, most OHCs were innervated by one large terminal instead of multiple smaller terminals as in wild types, suggesting a role for the nACh alpha9 subunit in development of mature synaptic connections. Alpha9 knockout mice also failed to show suppression of cochlear responses (compound action potentials, distortion product otoacoustic emissions) during efferent fiber activation, demonstrating the key role alpha9 receptors play in mediating the only known effects of the olivocochlear system.

Feedback control of the auditory periphery: anti-masking effects of middle ear muscles vs. olivocochlear efferents

Both MEM and MOC systems are sound-evoked reflexes to the auditory periphery which can be elicited by sound in either ear. Both MEM and MOC systems can increase thresholds in the auditory periphery: the MEM system acts by stiffening the ossicular chain, the MOC system by decreasing outer hair cell amplification of sound-induced motion in the inner ear. MEM-induced attenuations are largest for low frequency stimuli, MOC-induced attenuations are largest for mid- to high-frequency sounds. Both MEM and MOC systems can have anti-masking effects. The MEM reflex can decrease the masking of high-frequency signals by low-frequency noise (i.e., the upward spread of masking). The MOC reflex is complementary in that it minimizes masking of high-frequency transient signals by high-frequency continuous noise. MEM anti-masking arises by reducing suppressive masking and can improve masked thresholds at high frequencies. MOC anti-masking arises by counteracting excitatory masking. It does not improve masked thresholds, but can improve the detectability of small suprathreshold intensity increments. Anti-masking effects of both MEM and MOC systems should be reduced in cases of sensorineural hearing loss.

Dense innervation of Deiters' and Hensen's cells persists after chronic deefferentation of guinea pig cochleas

Innervation of Deiters' and Hensen's cells has been described in the organ of Corti of several mammalian species and has been suggested to arise from the olivocochlear (OC) efferent system (Wright and Preston [1976] Acta Otolaryngol. 82:41-47). In the present study, antineurofilament immunostaining was used to reveal these outer supporting cell fibers (OSCFs) in the normal guinea pig. In control ears, OSCFs were absent in the basal half of the cochlea but increased in number steadily toward the apex, peaking at values of over 1,200 fibers/mm. These values indicate a far more profuse innervation of supporting cells than has been described previously, suggesting that most OSCFs were not stained in previous immunohistochemical studies. Chronic cochlear deefferentation was used to test whether OSCFs are part of the OC system. The OC bundle was transected unilaterally, and the animals were allowed to survive for 4-8 weeks. Completeness of deefferentation was assessed by using acetylcholinesterase staining of the brainstem and measurement of the density of OC fascicles in the cochlea. By using these metrics, unilateral deefferentation was nearly complete in three animals. In successfully deefferented cases, the OSCF innervation density was not statistically different from control values. We conclude that the vast majority of OSCFs are not of OC origin. We speculate that they may be branches of type II afferent fibers to outer hair cells and that a smaller population of OSCFs with different morphology and immunoreactivity may arise from the OC system.

Single olivocochlear neurons in the guinea pig. II. Response plasticity due to noise conditioning

Previous studies have shown that daily, moderate-level sound exposure, or conditioning, can reduce injury from a subsequent high-level noise exposure. We tested the hypothesis that this conditioning produces an increased activity in the olivocochlear efferent reflex, a reflex known to provide protection to the cochlea. Guinea pigs were conditioned by a 10-day intermittent exposure to 2-4 kHz noise at 85 dB sound pressure level. This conditioning is known to reduce damage from a subsequent high-level exposure to the same noise band. Responses to monaural and binaural sound were recorded from single medial olivocochlear (MOC) efferent neurons, and data from conditioned animals were compared with those obtained from unexposed controls. MOC neurons were classified by their response to noise bursts in the ipsilateral or contralateral ears as ipsi units, contra units, or either-ear units. There were no significant differences in the distributions of these unit types between control and conditioned animals. There were also no differences in other responses to monaural stimuli, including the distribution of characteristic frequencies (CFs), the sharpness of tuning, or thresholds at the CF. For binaural sound at high levels, particularly relevant to sound-evoked activation of the MOC reflex during acoustic overstimulation, the firing rates of MOC neurons with CFs just above the conditioning band showed slight (but statistically significant) elevations relative to control animals. Frequency regions just above the conditioning band also demonstrated maximum conditioning-related protection; thus protection could be due, in part, to long-term changes in MOC discharge rates. For binaural sound at low levels, MOC firing rates in conditioned animals also were increased significantly relative to controls. Again, increases were largest for neurons with CFs just above the conditioning band. For equivalent monaural sound, rates were not significantly increased; thus, conditioning appears to increase binaural facilitation by opposite-ear sound. These data indicate that MOC neurons show long-term plasticity in acoustic responsiveness that is dependent on their acoustic history.

Long-term effects of sectioning the olivocochlear bundle in neonatal cats

The olivocochlear bundle (OCB) was cut in neonatal cats to evaluate its role in the development of normal cochlear function. Approximately 1 year after deefferentation, acute auditory nerve fiber (ANF) recordings were made from lesioned animals, lesion shams, and normal controls. The degree of deefferentation was quantified via light microscopic evaluation of the density of OCB fascicles in the tunnel of Corti, and selected cases were analyzed via electron microscopy. In the most successful cases, the deefferentation was virtually complete. ANFs from successfully lesioned animals exhibited significant pathophysiology compared with normals and with other animals in which the surgery failed to interrupt the OCB. Thresholds at the characteristic frequency (CF), the frequency at which ANFs are most sensitive, were elevated across the CF range, with maximal effects for CFs in the 10 kHz region. Frequency threshold or tuning curves displayed reduction of tip-to-tail ratios (the difference between CF and low-frequency "tail" thresholds) and decreased sharpness of tuning. These pathological changes are generally associated with outer hair cell (OHC) damage. However, light microscopic histological analysis showed minimal hair cell loss and no significant differences between normal and deefferented groups. Spontaneous discharge rates (SRs) were lower than normal; however, those fibers with the highest SRs remained more sensitive than those with lower SRs. Findings suggest that the interaction between OC efferents and OHCs early in development may be critical for full expression of active mechanical processes.

Conditioning-related protection from acoustic injury: effects of chronic deefferentation and sham surgery

The inner ear can be made less vulnerable to acoustic injury by a "conditioning" treatment involving exposure to a moderate-level acoustic stimulus before the acoustic overexposure. The present study was designed to explore the role of the olivocochlear (OC) system in this "protection." Guinea pigs were divided into a number of groups: some (trauma-only) were exposed to a traumatic noise for 4 h at 109 dB SPL; others (condition/trauma) were conditioned by daily exposure to the same noise at 85 dB SPL before the traumatic exposure. In OC-intact animals, the condition/trauma group showed significantly less permanent threshold shift (PTS) than the trauma-only group as measured via compound action potentials and distortion-product otoacoustic emissions (DPOAEs). Other animals with identical noise-exposure regimens underwent deefferentation surgery before the start of conditioning: the OC bundle (OCB) was cut in the brain stem, either at the midline (cutting the crossed OCB to both ears) or at the sulcus limitans (cutting all OC fibers to 1 side). Lesion success was quantified by measuring OC fascicles to the outer hair cell region in each ear. The results from the surgical groups showed that total loss of the OCB significantly increased the noise-induced PTS, whereas loss of the COCB only did not; that the conditioning exposure in deefferented animals increased, rather than decreased, the PTS from the traumatic exposure; and that animals undergoing sham surgery (brain stem cuts that failed to transect the OCB) appeared protected whether or not they received the conditioning noise exposure. The latter result suggests that conditioning-related protection may arise from a generalized stress response, which can be elicited by noise exposure, brain surgery, or a variety of other means. The former results make an OC role in the conditioning process, per se, difficult to assess, given the large effects of OC activity on general acoustic vulnerability.

Intracellular labeling of auditory nerve fibers in guinea pig: central and peripheral projections

Auditory-nerve fibers (ANFs) in the cat have been subdivided according to spontaneous rate (SR), with high-SR fibers showing the lowest thresholds. Cochlear terminals of the three SR groups differ in caliber and synaptic position around the inner hair cell (Liberman [1982b] Science 216:1239-1241); central terminals differ in degree of branching and in which subregions of the cochlear nucleus (CN) are targeted (Liberman [1991] J. Comp. Neurol. 313:240-258). The present study investigates whether these SR-based differences in ANF connections are unique to the cat. Thirty ANFs from 15 guinea pigs were intracellularly labeled after measuring characteristic frequency, threshold, and SR. Labeled cochlear projections showed significant SR-based differences in axonal caliber, with low- and medium-SR fibers 20-40% thinner than those of high-SR fibers for both peripheral and central (modiolar) axons. Spatial segregation in the inner hair cell area could not be assessed; however, the peripheral axons in the osseous spiral lamina showed the same SR-based organization reported for the cat (Kawase and Liberman [1992] J. Comp. Neurol. 319:312-318). Labeled central projections also showed significant SR-based differences. Low- and medium-SR fibers: 1) were more highly branched, 2) sent significantly more terminals to the small-cell cap region of the CN, and 3) produced endbulb terminals (on spherical cells) that were significantly more complex than high-SR fibers. All of these SR-based trends for both central and peripheral projections are analogous to those reported in the cat, and, thus, may represent a fundamental organizational principle of the mammalian ear.

Morphological subclasses of lateral olivocochlear terminals? Ultrastructural analysis of inner spiral bundle in cat and guinea pig

The lateral olivocochlear efferent pathway terminates in vesicle-filled swellings in the inner spiral bundles under inner hair cells (IHCs) and has been suggested to include at least two chemically distinct subclasses (see, e.g., Vetter et al. [1991] Synapse 7:21-43). In the present study, the ultrastructure and peripheral targets of vesicle-filled swellings in the IHC area of the cat and guinea pig cochleas were quantitatively analyzed to determine 1) whether morphological subclasses could be defined based on swelling size or on the density, size or shape of clear and dense-cored vesicles and 2) whether swellings with different postsynaptic targets differed morphologically. In both cat and guinea pig, all swellings contained large, round, clear vesicles and a variable number of dense-core vesicles. Although evidence of clear-cut subclasses was not compelling, the smallest swellings tended to be rich in dense-core and poor in clear vesicles and rarely formed synaptic contacts. Most of the larger swellings, which tended to contain few dense-core vesicles and a rich complement of clear round vesicles, formed synapses with radial afferent fibers. However, there were no morphological differences between swellings contacting afferents originating on the modiolar vs. pillar sides of the IHC (the source of afferents with low and high spontaneous discharge rates, respectively). We conclude that 1) if distinct gamma-amino butyric acid (GABA)ergic and cholinergic subclasses of lateral olivocochlear (LOC) fibers exist, then the vesicle morphology of their terminals does not differ as it does in the central nervous system and that 2) if peptide neurotransmitters, such as calcitonin gene-related peptide and enkephalins, are packaged in dense-core vesicles, then the LOC terminals synapsing with IHC afferent fibers are not particularly rich in these peptides.

Ultrastructural differences among afferent synapses on cochlear hair cells: correlations with spontaneous discharge rate

The major class of cochlear afferent fibers, the type-I or radial-fiber (RF) population, has been subdivided into three functional groups according to spontaneous discharge rate (SR): those with low SR have the highest acoustic thresholds, high SR fibers have the lowest thresholds and medium SR fibers are of intermediate sensitivity (Liberman [1978] J. Acoust. Soc. Amer. 63:442-455). Existing evidence from intracellular labeling studies at the light microscopic level (Liberman [1982a] Science 216:1239-1241) suggests that a single cochlear inner hair cell makes synaptic contact with representatives of all three functional groups; however, low and medium SR fibers are spatially segregated from high SR fibers around the hair cell circumference, and low and medium SR fibers are smaller in caliber than those with high SR. The present study extends to the ultrastructural level the structure-function correlations available via intracellular labeling. Analysis is based on serial section reconstruction of the synaptic contacts between 11 radial fibers of known SR and their target hair cells. Results suggest systematic differences in synaptic ultrastructure among fibers of the three SR groups: with decreasing SR, the size and complexity of the synaptic body (a presynaptic specialization characteristic of the peripheral afferent synapses in all hair cell systems and some other peripheral receptors) tend to increase, as does the associated number of synaptic vesicles. The possible functional significance of these trends is discussed in the context of other known structural and functional differences among the three SR groups.

The ipsilaterally evoked olivocochlear reflex causes rapid adaptation of the 2f1-f2 distortion product otoacoustic emission

The onset behavior of the distortion product otoacoustic emission (DPOAE) at 2f1-f2 in anesthetized cats was measured with temporal resolution finer than 70 ms. The amplitude of the DPOAE adapts after onset of the primary tones by as much as 6 dB for monaural stimulation and 10 dB when the primaries are presented binaurally. DPOAE adaptation consists of a large, rapid component, with a time constant of roughly 100 ms, and a small, slower component with a time constant of roughly 1000 ms. The rapid component disappears when only the crossed olivocochlear bundle (OCB) is cut, whereas the slow adaptation persists after complete OCB section. The loss of rapid adaptation upon OC section is accompanied by a concomitant increase in the steady-state amplitude of the DPOAE. Thus an intact OC reflex can significantly alter DPOAEs obtained during routine measurement. Rapid adaptation of the monaurally evoked 2f1-f2 DPOAE is probably mediated by reflex activity in ipsilaterally responsive OC neurons innervating outer hair cells. The effects of this ipsilateral reflex on DPOAE amplitudes are typically twice as large as those of the contralateral reflex, presumably because there are twice as many ipsilaterally responsive OC neurons. Tests for the ipsilateral OC reflex based on the phenomenon of rapid adaptation should be both feasible and useful in human subjects.

Olivocochlear reflex assays: effects of contralateral sound on compound action potentials versus ear-canal distortion products

The strength of the olivocochlear reflex has been assayed by comparing ipsilateral cochlear responses with and without contralateral sound. In humans, ipsilateral cochlear responses have usually been inferred by measuring otoacoustic emissions (OAEs), whereas, in animal work, they have been assessed by measuring compound action potentials (CAPs). Thus reports that the reflex strength is smaller in humans than in animals cannot be interpreted until the differences between the two tests are better understood. The present study directly compares reflex assays using distortion-product (DP) OAE and CAP measures in the same animals. For ipsilateral frequencies of 2-8 kHz and levels from 25 to 80 dB SPL, efferent reflex strength was computed from the CAP or DPOAE amplitude-versus-level curves measured with and without contralateral noise. The "effective attenuation" produced by efferent activation was, with few exceptions, greater when measured with the CAP than with the DPOAE assay. Differences between the two measures increased as frequency increased, with differences as large as 10 dB observed. These results, coupled with previous measurements on humans and animals, suggest that the efferent reflex is at least as strong in humans as has been shown in animal experiments.

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.

A novel cholinergic "slow effect" of efferent stimulation on cochlear potentials in the guinea pig

This report documents slow changes in cochlear responses produced by electrical stimulation of the olivocochlear bundle (OCB), which provides efferent innervation to the hair cells of the cochlea. These slow changes have time constants of 25-50 sec, three orders of magnitude slower than those reported previously. Such "slow effects" are similar to classically described "fast effects" in that (1) they comprise a suppression of the compound action potential (CAP) of the auditory nerve mirrored by an enhancement of the cochlear microphonic potential (CM) generated largely by the outer hair cells; (2) the magnitude of suppression decreases as the intensity of the acoustic stimulus increases; (3) they share the same dependence on OCB stimulation rate; (4) both are extinguished upon cutting the OCB; and (5) both are blocked with similar concentrations of a variety of cholinergic antagonists as well as with strychnine and bicuculline. These observations suggest that both fast and slow effects are mediated by the same receptor and are produced by conductance changes in outer hair cells. Slow effects differ from fast effects in that (1) fast effects are greatest for acoustic stimulus frequencies between 6 and 10 kHz, whereas slow effects peak for frequencies from 12 to 16 kHz, and (2) fast effects persist over long periods of OCB stimulation, whereas slow effects diminish after 60 sec of stimulation. The time course of the slow effects can be described mathematically by assuming that each shock-burst produces, in addition to a fast effect, a small decrease in CAP amplitude that decays exponentially with a time constant that is long relative to the intershock interval. The long time constant of the slow effect compared to the fast effect suggests that it may arise from a distinct intracellular mechanism, possibly mediated by second-messenger systems.

Efferent-mediated protection from acoustic overexposure: relation to slow effects of olivocochlear stimulation

1. The present study attempts to resolve discrepancies in the reported role of olivocochlear (OC) efferent activation in protecting the inner ear from acoustic overstimulation: in previous studies, activating the OC system in guinea pigs reduced the threshold shift caused by 1 min monaural exposure to a 10-kHz tone; whereas unilateral OC activation in cats had no effect on threshold shifts following binaural exposure to a 10 min 6-kHz tone. 2. In this study, anesthetized and curarized guinea pigs were exposed either monaurally or binaurally to tones of different duration (1-5 min), frequency (6 to 10 kHz) and intensity (105-118 dB SPL). For each exposure condition, threshold shifts were compared among ears with different levels of OC activation: in some cases, the OC bundle (OCB) was electrically stimulated during (and/or before) the acoustic overexposure; in others, the OCB was cut before the exposure; in control cases, the OCB was neither cut nor electrically stimulated. 3. Electrical stimulation of the OCB delivered simultaneously with acoustic overstimulation produced significant reductions in threshold shift only for acoustic exposures at higher frequencies (8 and 10 kHz) and shorter durations (1 and 2 min). The protective effects on 1-min exposures could be extinguished by prior stimulation of the OCB, i.e., if the OC stimulation was turned on 4 min before the acoustic overexposure.(ABSTRACT TRUNCATED AT 250 WORDS)

Antimasking effects of the olivocochlear reflex. II. Enhancement of auditory-nerve response to masked tones

1. The antimasking effects of olivocochlear (OC) efferent feedback were studied in anesthetized or decerebrate cats by comparing responses of single auditory-nerve fibers (ANFs) to tone bursts in continuous masking noise seen with and without addition of a moderate-level contralateral noise known to activate the OC reflex. Responses were measured as a function of tone-burst intensity, tone-burst frequency, and masker intensity and were analyzed so as to allow quantitative estimates of the detectability of the tone bursts against the noise background. 2. Addition of the contralateral OC elicitor both increased the maximum discharge rates to the masked tone bursts and decreased the rates to the ipsilateral masker. The rate increases to the tone bursts could be explained on the basis of a decrease in adaptation caused by decreasing the steady response to the masker. The result is a steepening of the rate-versus-level function for masked tone bursts and a concomitant increase in the estimated discriminability of small increments of tone-burst intensity. 3. For tone bursts at the fiber's characteristic frequency (CF), the OC effects on detection threshold for the masked tone bursts depended on masker level, with small increases in threshold for low masker levels and somewhat larger decreases in threshold for higher masker levels. For tone bursts below CF, OC effects, when present, always decreased the detection threshold. 4. The largest antimasking effects were seen for fibers with CFs between 6 and 12 kHz and for masker levels within 20 dB of the fiber's threshold to the masker. These trends appeared to hold for fibers of all spontaneous rates (SRs). 5. Enhancement of the response to unmasked tone bursts and concomitant decrease in the "spontaneous rate" was elicited by OC activation in fibers if threshold sensitivity approached -10 dB SPL. This "enhancement-in-quiet" appears to arise when an animal-generated noise produces a continuous response (in the absence of purposely applied sound) that is suppressed by OC activity. This finding raises questions as to the range of "true" spontaneous rates in the cat. 6. The results highlight two important distinctions between the effects of OC feedback in quiet versus those in noise. In quiet, the effects are predominately suppressive and are restricted to stimuli at frequencies near a fiber's CF and at intensities within its dynamic range. In continuous background noise, the OC reflex can enhance the responses to transient stimuli. Such effects are seen throughout the fiber's response area.(ABSTRACT TRUNCATED AT 400 WORDS)

Antimasking effects of the olivocochlear reflex. I. Enhancement of compound action potentials to masked tones

1. The effects of olivocochlear (OC) feedback on signal processing in the cochlea were studied by comparing responses seen with and without a contralateral noise or by comparing responses seen before and after cutting the OC bundle (OCB). Adding and subtracting a contralateral noise is a convenient, reversible way of changing the level of OC feedback; however, it fully reveals only the contribution of the contralaterally responsive efferent fibers. Cutting the OCB can reveal the full contribution of all fibers in the OCB; however, the manipulation can only be performed once per experiment. 2. The amplitude of the compound action potential (CAP), recorded from anesthetized or decerebrate cats in response to tone pips, could be increased by addition of contralateral noise at moderate sound pressure levels. These enhancement phenomena were most easily demonstrable when the tone pips were masked by ipsilateral broadband noise; however, in some animals CAP enhancement was seen in the absence of ipsilateral maskers. Enhancement-in-quiet may arise because of internal masking from animal-generated noise. All contralateral-noise enhancement disappeared when the OCB was cut. 3. Enhancement effects of contralateral noise could be seen in both simultaneous and forward-masking paradigms. Enhancement was largest for high-frequency tone pips (8-16 kHz) and could be demonstrated over a wide range of tone-pip levels and ipsilateral-masker levels. Suppression of CAP by the contralateral noise was often seen for lower tone-pip frequencies (2-8 kHz) and lower tone-pip intensities. These trends may be understood in the context of known properties of OC peripheral effects and known properties of physiological masking. 4. Cutting the OCB resulted in a decrease in CAP amplitudes to masked tone pips. When CAP was measured to tone pips presented in equilevel, binaural noise, OCB section resulted in a decrease in CAP amplitudes equivalent to at least a 6-dB decrease in signal-to-noise ratio. Such antimasking effects of an intact OCB were seen in both simultaneous and forward-masking paradigms. 5. Present evidence suggests that all these antimasking effects can be explained on the basis of activation of the medial OC fibers to the outer hair cells. By suppressing responses to continuous noise backgrounds, the OC reflex may enhance responses to transient masked stimuli by decreasing the level of adaptation in auditory nerve fibers. Such effects of the OC reflex should improve discrimination of transient signals presented in a continuous noise background.

Central projections of auditory nerve fibers of differing spontaneous rate, II: Posteroventral and dorsal cochlear nuclei

Response properties of auditory nerve fibers (ANFs), including threshold sensitivity, vary systematically with spontaneous discharge rate (SR) (Liberman, M.C.: J. Acoust. Soc Amer. 63:442-455, 1978). Thus, an understanding of the mechanisms underlying signal transformation in the cochlear nucleus (CN) must include a description of any SR-based difference in ANF projections. This study is the second of a pair describing the CN projections of intracellularly labeled ANFs of known SR, the first of which summarized projection to the anteroventral CN (Liberman, M.C.: J. Comp. Neurol. 313:240-258, 1991). For each swelling from each labeled fiber, the position (within CN subdivisions), the size, and the type of cell contacted (if determinable) was noted: roughly one in four labeled swellings appeared in intimate contact with the soma or proximal dendrites of a CN cell. In all such cases, cell size and swelling size were measured. As reported for auteroventral cochlear nucleus, the ANF innervation of the small-cell regions of posteroventral CN (PVCN) was almost exclusively by low- and medium-SR fibers. Other significant SR-based trends in ANF projections included 1) a tendency for high-SR fibers to contact larger cells in PVCN, 2) a meager projection of low- and medium-SR fibers to octopus cells, and 3) a tendency in the dorsal CN (DCN) for low-SR terminals to end closer to the fusiform cell layer than high-SR terminals. There were no significant SR-based difference in ANF swelling sizes in any subdivision. A consideration of the average cell sizes, ANF swelling sizes and estimated numbers of ANFs of different CF and SR converging on each CN cell help explain some of the differences in response transformation associated with different cell types in the CN.