Evoked-potential thresholds and cubic distortion product otoacoustic emissions in the chinchilla following carboplatin treatment and noise exposure

Distortion Product Otoacoustic Emission Test is Not the Test to Use in Nonclinical Safety Assessment

Ototoxicity and ocular toxicity screening are but two examples of specialty product lines that are often employed as Tier II or III nonclinical safety/hazard screening assessments. Compared to the regulatory guidelines that govern over standard toxicology or neurotoxicology programs, there is a paucity of regulatory strategies to address these specialized product lines. With respect to ototoxicity testing, we argue for the inclusion of the "least burdensome principles" adopted by the US FDA in providing the most pragmatic, efficient, and directed identification of potential harm to auditory function in the nonclinical safety arena. We argue for the exclusive use of the auditory brainstem response and the exclusion of the distortion product otoacoustic emissions (DPOAEs) in these Tiered II safety assessment programs. The inclusion of both are a burden on operational staff and, due to the extended episodes of anesthesia required to conduct both assays, this strategy poses a health and welfare concern for the selected animal species to be used. The DPOAE does not provide any sufficiently valid or reliable data above and beyond the gold standard ABR data, followed by complete oto-histopathology and cytocochleogram combination designs.

Auditory Threshold Variability in the SAMP8 Mouse Model of Age-Related Hearing Loss: Functional Loss and Phenotypic Change Precede Outer Hair Cell Loss

Age-related hearing loss (ARHL) is the most common sensory deficit in aging society, which is accompanied by increased speech discrimination difficulties in noisy environments, social isolation, and cognitive decline. The audiometric degree of ARHL is largely correlated with sensory hair cell loss in addition to age-related factors not captured by histopathological analysis of the human cochlea. Previous studies have identified the senescence-accelerated mouse prone strain 8 (SAMP8) as a model for studying ARHL and age-related modifications of the cochlear redox environment. However, the SAMP8 population exhibits a large variability in auditory function decline over age, whose underlying cause remains unknown. In this study, we analyzed auditory function of SAMP8 mice by measuring auditory brainstem response (ABR) thresholds at the age of 6 weeks (juvenile), 12 weeks (young adult), and 24 weeks (adult). Consistent with previous studies, SAMP8 mice exhibit an early progressive, age-related decline of hearing acuity. However, a spatiotemporal cytohistological analysis showed that the significant increase in threshold variability was not concurrently reflected in outer hair cell (OHC) loss observed in the lower and upper quartiles of the ABR threshold distributions over age. This functional loss was found to precede OHC loss suggesting that age-related phenotypic changes may be contributing factors not represented in cytohistological analysis. The expression of potassium channels KCNQ4 (K_V7.4), which mediates the current I_K,n crucial for the maintenance of OHC membrane potential, and KCNQ1 (K_V7.1), which is an essential component in potassium circulation and secretion into the endolymph generating the endocochlear potential, showed differences between these quartiles and age groups. This suggests that phenotypic changes in OHCs or the stria vascularis due to variable oxidative deficiencies in individual mice may be predictors of the observed threshold variability in SAMP8 mice and their progressive ARHL. In future studies, further phenotypic predictors affected by accumulated metabolic challenges over age need to be investigated as potentially underlying causes of ARHL preceding irreversible OHC loss in the SAMP8 mouse model.

DPOAE-Grams in Patients with Acute Tonal Tinnitus

OBJECTIVE: To investigate cochlear outer hair cell function in patients with acute tonal tinnitus and normal or near-normal hearing threshold.

STUDY DESIGN AND SETTING: Prospective controlled study in an academic tertiary health center. Distortion products of otoacoustic emissions (DPOAE)-grams of 32 ears with acute tonal tinnitus and normal hearing or minimal hearing loss were compared with those of 17 healthy nontinnitus ears.

RESULTS: Tinnitus ears exhibited relatively increased amplitudes of DPOAE at high frequencies (4-6.3 kHz) when compared with the group of healthy ears and relatively decreased DPOAE amplitudes at middle frequencies (1650-2400 Hz). Statistically significant ( P < 0.01) increased mean values of DPOAE amplitudes were observed only at a frequency of f2 equal to 4.9 kHz.

CONCLUSIONS AND SIGNIFICANCE: These findings suggest an altered functional state of the outer hair cells at a selected high-frequency region of the cochlea in ears with acute tonal tinnitus and normal or near-normal hearing threshold.

Effect of tinnitus on distortion product otoacoustic emissions varies with hearing loss

PURPOSE

The aim of this study was to measure the effect of tinnitus, while accounting for the effect of hearing loss and aging, on distortion product otoacoustic emissions (DPOAEs).

METHOD

DPOAEs were measured twice in both ears in 5 groups of participants: young adults with normal hearing, middle-age adults with normal hearing, adults with high-frequency sensorineural hearing loss, age-matched adults with similar hearing loss and tinnitus, and adults with normal hearing and chronic tinnitus.

RESULTS

Multivariate analysis revealed a main effect of hearing loss and age, but no effect of tinnitus, across all 5 groups. Separate tests revealed significant effects of age and tinnitus in the normal-hearing groups and hearing loss in adults with or without tinnitus, but no effect of tinnitus in those with hearing loss.

CONCLUSION

DPOAE levels in the group of adults with hearing loss and tinnitus were diminished, but those in the group with normal hearing and tinnitus were enhanced, relative to DPOAE levels in the controls. Outer hair cell function, as indexed by DPOAEs, exhibits a complex association with tinnitus, and this has implications in the use of DPOAEs as a tool both for testing for tinnitus presence and for creating a model of neural mechanisms underlying tinnitus.

Insensitivity of the audiogram to carboplatin induced inner hair cell loss in chinchillas

Noise trauma, aging, and ototoxicity preferentially damage the outer hair cells of the inner ear, leading to increased hearing thresholds and poorer frequency resolution. Whereas outer hair cells make synaptic connections with less than 10% of afferent auditory nerve fibers (type-II), inner hair cells make connections with over 90% of afferents (type-I). Despite these extensive connections, little is known about how selective inner hair cell loss impacts hearing. In chinchillas, moderate to high doses of the anticancer compound carboplatin produce selective inner hair cell and type-I afferent loss with little to no effect on outer hair cells. To determine the effects of carboplatin-induced inner hair cell loss on the most widely used clinical measure of hearing, the audiogram, pure-tone thresholds were determined behaviorally before and after 75 mg/kg carboplatin. Following carboplatin treatment, small effects on audiometric thresholds were observed even with extensive inner hair cell losses that exceed 80%. These results suggest that conventional audiometry is insensitive to inner hair cell loss and that only small populations of inner hair cells appear to be necessary for detecting tonal stimuli in a quiet background.

Local cochlear damage reduces local nonlinearity and decreases generator-type cochlear emissions while increasing reflector-type emissions

Distortion product otoacoustic emissions (DPOAEs) originate in cochlear nonlinearity and emerge into the ear canal as an apparent sum of emission types, one of which (generator) travels directly out and the other (reflector) travels out following linear reflection. The present study explores intracochlear sources of DPOAEs via simultaneous ear canal and intracochlear pressure measurements in gerbils. A locally damaged cochlea was produced with reduced local intracochlear nonlinearity and significant elevation of the compound action potential thresholds at frequencies represented within the damaged region. In the DPOAE the comparison of healthy to locally damaged cochleae showed the following: (1) In the broad frequency region corresponding to the locally damaged best frequency, DPOAEs evoked by wider f(2)/f(1) stimuli decreased, consistent with the reduction in local nonlinearity. (2) DPOAEs evoked by narrow f(2)/f(1) stimuli often had a bimodal change, decreasing in a lower frequency band and increasing in a band just adjacent and higher, and the DPOAE phase-vs-frequency slope steepened. These changes confirm the complex nature of the DPOAE.

Evaluation of inner hair cell and nerve fiber loss as sufficient pathologies underlying auditory neuropathy

Auditory neuropathy is a hearing disorder characterized by normal function of outer hair cells, evidenced by intact cochlear microphonic (CM) potentials and otoacoustic emissions (OAEs), with absent or severely dys-synchronized auditory brainstem responses (ABRs). To determine if selective lesions of inner hair cells (IHCs) and auditory nerve fibers (ANFs) can account for these primary clinical features of auditory neuropathy, we measured physiological responses from chinchillas with large lesions of ANFs (about 85%) and IHCs (45% loss in the apical half of the cochlea; 73% in the basal half). Distortion product OAEs and CM potentials were significantly enhanced, whereas summating potentials and compound action potentials (CAPs) were significantly reduced. CAP threshold was elevated by 7.5dB, but response synchrony was well preserved down to threshold levels of stimulation. Similarly, ABR threshold was elevated by 5.6dB, but all waves were present and well synchronized down to threshold levels in all animals. Thus, large lesions of IHCs and ANFs reduced response amplitudes but did not abolish or severely dys-synchronize CAPs or ABRs. Pathologies other than or in addition to ANF and IHC loss are likely to account for the evoked potential dys-synchrony that is a clinical hallmark of auditory neuropathy in humans.

Relation of distortion product otoacoustic emission with tinnitus

OBJECTIVE

To investigate cochlear outer hair cell function based on distortion product otoacoustic emission (DPOAE) in patients with tinnitus.

STUDY DESIGN

This is a case control study.

SUBJECTS AND METHODS

The subjects are patients who attended the Otorhinolaryngology Clinic in Hospital Universiti Kebangsaan Malaysia over a period of 19 months from April 2005 until October 2006. All patients underwent a full ENT assessment and had tympanometry, pure tone audiometry, and DPOAE tests. The UKM Research and Ethics Committee reviewed and approved the study proposal prior to commencement of this study.

RESULTS

The study population included 49 patients. They consisted of 16 patients (32 ears) with tinnitus and reduced hearing, 13 patients (26 ears) with tinnitus and normal hearing, 7 patients (13 ears) without tinnitus with reduced hearing, and 13 patients (26 ears) without tinnitus with normal hearing. Statistical analysis showed significant differences (P = .00) of mean DPOAE levels between the four groups of patients.

CONCLUSION

Our results suggest that reduced outer hair cell activity, as detected by reduced DPOAE levels, may manifest as tinnitus even before there is a shift on hearing threshold. We also postulate that further reduction of cochlear outer hair cell activity, as shown by further reduced DPOAE levels, may actually terminate the source of tinnitus.

Electrophysiological correlates of progressive sensorineural pathology in carboplatin-treated chinchillas

Carboplatin produces progressive damage to auditory nerve fibers, spiral ganglion neurons (SGNs) and inner hair cells (IHC) in the chinchilla cochlea but leaves outer hair cells intact. Within 1 h after injection, many afferent terminals beneath IHCs and myelin lamellae surrounding SGN processes are vacuolated. One day after injection, approximately half of the nerve fibers are missing. IHCs are intact at 2 days, but 20-30% are missing at 3 days. We studied the electrophysiological correlates of this progressive morphological damage by recording cochlear microphonics (CM), distortion product otoacoustic emissions (DPOAE), summating potentials (SP), compound action potentials (CAP) and midbrain evoked potentials (IC-EVP) before and 1 h, 12 h, 1 days, 3 days, 5 days, 7 days and 14 days after carboplatin injection (75 mg/kg IP) in four chinchillas. CM and DPOAEs tended to be unchanged or enhanced. CAP and SP showed little change until Day 3, when amplitudes were reduced in all animals and CAP thresholds were elevated by 9 dB; amplitudes declined further between Days 3 and 5 but not thereafter. IC-EVP amplitudes decreased on Days 3 or 5 but thresholds were relatively unchanged. All animals showed some recovery of IC-EVP between Days 7 and 14, including one with 70% enhancement on Day 14. The results indicate that threshold and amplitude measures fail to detect peripheral pathology until some relatively high threshold level of damage has been exceeded. This has important implications for monitoring peripheral damage and interpreting electrophysiological test results in animals and humans.

Electrophysiological findings in the Sprague-Dawley rat induced by moderate-dose carboplatin

Carboplatin is a second generation platinum-containing anti-tumor drug which selectively alters the micromechanical function of the inner hair cells (IHCs) of the organ of Corti in the chinchilla. Data from a recent study [Wake et al., Acta Otolaryngol. 116 (1996) 374-381], using the chinchilla model, have suggested that a moderate dose of carboplatin alters the efferent feedback loop gain of the OHCs. The present study was designed to evaluate the possible 'efferent feedback alteration mechanism' in the Sprague-Dawley rat using distortion product otoacoustic emissions (DPOAEs). A moderate dose of carboplatin (50 mg/kg body weight) was administered by a 30 min i.p. infusion. Pre- and 72-h post-treatment DPOAE and auditory brainstem response (ABR) recordings were acquired from a group of 12 rats. The animals were anesthetized with a ketamine-atropin anesthesia administered in two consecutive phases. The DPOAE responses (cubic distortion products) were recorded with four asymmetrical protocols: P1=60-50, P2=50-40, P3=40-30 and P4=30-20 dB SPL (sound pressure level), in the frequency range from 4.0 to 16 kHz. ABR responses were obtained for bipolar clicks and tone pips at the frequencies 8.0, 10.0, 20.0 and 30 kHz using stimuli in the range from 100 to 30 dB SPL. Significant ABR threshold shifts of 15 dB were observed at 30 kHz, and shifts of 10 dB at 20, 16 and 10 kHz. The comparison of pre- and post-treatment DPOAE responses did not reveal any significant changes for protocols P1, P2 and P4. Data from the P3 protocol indicated a decrease of the DPOAE amplitude. The findings from the rat model suggest that (a) moderate doses of carboplatin do not affect the efferent feedback loop OHC function and (b) the cochlear susceptibility to carboplatin across species is different, even at moderate-dose regimes.

DPOAE level shifts and ABR threshold shifts compared to detailed analysis of histopathological damage from noise

A detailed comparison of 2f(1)-f(2) distortion product otoacoustic emission (DPOAE) level shifts (LS) and auditory brainstem response (ABR) threshold shifts with noise-induced histopathology was conducted in chinchillas. DPOAE levels (i.e., L(1) and L(2)) at f(1) and f(2), respectively, ranged from 55-75 dB sound pressure level (SPL), with f(2)/f(1)=1.23, 6 points/octave, f(2)=0.41-20 kHz, and ABR thresholds at 0.5-20 kHz, 2 points/octave, were determined pre-exposure. The exposure was a 108 dB SPL octave band of noise centered at 4 kHz (1-1.75 h, n=6) or 80-86 dB SPL (24 h, n=5). DPOAE LSs (magnitude pre- minus post-exposure) and ABR threshold shifts (TS) were determined at 0 days and up to 28 days post-exposure. The cochleae were fixed, embedded in plastic and dissected into flat preparations. The length of the organ of Corti (OC) was measured; missing inner (IHC) and outer (OHC) hair cells counted; stereocilia damage rated; and regions of OC and nerve-fiber loss determined. Cytocochleograms were made showing functional loss and structural damage with the LS and TS overlaid. Some unexpected results were obtained. First, the best correlation of LS with histopathology required plotting the DPOAE data at f(1) with respect to the chinchilla-place map. The best correlation of TS was with IHC and nerve-fiber loss. Second, wide regions of up to 10% scattered OHC loss in the apical half of the OC showed little or no LS. Third, with the 108 dB SPL noise, there was 20-40 dB of recovery for DPOAEs at mid-high frequencies (3-10 kHz) in eight of 12 cochleae where there was 70-100% OHC loss in the basal half of the OC. The largest recovery at mid-high frequencies occurred in regions where the OC was entirely missing. Fourth, with the 80-86 dB SPL noise, there was no LS at small focal lesions (100% loss of OHCs over 0.4 mm) when the frequency place of either f(1) or f(2) was within the lesion but not both. There was no correlation of LS with OHC stereocilia loss, fusion or disarray. These results suggest that, after noise exposure, DPOAEs at mid-high frequencies can originate from or be augmented by generators located at someplace other than the frequency place of f(2), possibly the basal 20% of the OC when this region is intact. Also, noise-induced DPOAE LSs seemed to reflect differing mechanisms for temporary and permanent hearing loss.

Effect of inner and outer hair cell lesions on electrically evoked otoacoustic emissions

When the cochlea is stimulated by a sinusoidal current, the inner ear emits an acoustic signal at the stimulus frequency, termed the electrically evoked otoacoustic emission (EEOAE). Recent studies have found EEOAEs in birds lacking outer hair cells (OHCs), raising the possibility that other types of hair cells, including inner hair cells (IHCs), may generate EEOAEs. To determine the relative contribution of IHCs and OHCs to the generation of the EEOAE, we measured the amplitude of EEOAEs, distortion product otoacoustic emissions (DPOAEs), the cochlear microphonic (CM) and the compound action potential (CAP) in normal chinchillas and chinchillas with IHC lesions or IHC plus OHC lesions induced by carboplatin. Selective IHC loss had little or no effect on CM amplitude and caused a slight reduction in mean DPOAE amplitude. However, IHC loss resulted in a massive reduction in CAP amplitude. Importantly, selective IHC lesions did not reduce EEOAE amplitude, but instead, EEOAE amplitude increased at high frequencies. When both IHCs and OHCs were destroyed, the amplitude of the CM, DPOAE and EEOAE all decreased. The increase in EEOAE amplitude seen with IHC loss may be due to (1) loss of tonic efferent activity to the OHCs, (2) change in the mechanical properties of the cochlea or (3) elimination of EEOAEs produced by IHCs in phase opposition to those from OHCs.

Cochlear microphonics and otoacoustic emissions in chronically de-efferented chinchilla

The effects of eliminating the olivocochlear bundle (OCB) on cochlear electromechanical properties were examined by measuring cochlear microphonics (CM) and distortion product otoacoustic emissions (DPOAEs) in chronically de-efferented chinchillas. The OCB fibers to the right ears were successfully sectioned in six out of 15 adult chinchillas via a posterior paraflocular fossa approach. At the end of the experiment, these ears were histologically verified as being deprived of both lateral and medial OCB fibers. The opposite (left) ears from the animals served as controls. Following de-efferentation, changes of the inter-modulation distortion components (2f(1)-f(2), f(2)-f(1), 3f(1)-2f(2), 3f(2)-2f(1)) varied, depending on the frequencies and levels of the stimuli. DPOAE amplitudes to low-level stimuli were within the 95% confidence intervals around mean DPOAE amplitudes of the control ears at all the frequencies (1-8 kHz). At high stimulus levels, DPOAE amplitudes increased by 5-20 dB at 1 and 2 kHz while remaining in the normal range at 4 and 8 kHz. In contrast, the CM input/output functions to stimuli from 1 to 8 kHz were significantly reduced by approximately 40-50% at all input levels. The results suggest that the OCB may play a role in modulating electrical properties of the outer hair cells and in reducing the magnitude of cochlear distortion to high-level stimuli.

Inner hair cell loss leads to enhanced response amplitudes in auditory cortex of unanesthetized chinchillas: evidence for increased system gain

Carboplatin preferentially destroys inner hair cells (IHCs) in the chinchilla inner ear, while retaining a near-normal outer hair cell (OHC) population. The present study investigated the functional consequences of IHC loss on the compound action potential (CAP), inferior colliculus potential (ICP) and auditory cortex potential (ACP) recorded from chronically implanted electrodes. IHC loss led to a reduction in CAP amplitude that was roughly proportional to IHC loss. The ICP amplitude was typically reduced by IHC loss, but the magnitude of this reduction was generally less than that observed for the CAP. In contrast to the CAP and ICP, ACP amplitudes were generally not reduced following IHC loss. In some animals, the ACP amplitude remained at pre-carboplatin values despite substantial IHC loss. However, in other animals, IHC loss led to an increase ('enhancement') of ACP amplitude. ACP enhancement was greatest at 1-2 weeks post-carboplatin, returning towards baseline amplitudes at 5 weeks post-carboplatin. In other animals, the ACP remained enhanced up to 5 weeks post-carboplatin. We interpret the transient and sustained enhancement of ACP amplitude following partial IHC loss as evidence of functional reorganization occurring at or below the level of the auditory cortex. These results suggest that the gain of the central auditory pathway increases following IHC loss to compensate for the reduced input from the cochlea.

Noise-induced hearing loss in the noise-toughened auditory system

The auditory system, toughened by an interrupted noise exposure, has been shown in several reports to be less affected by (or protected from) a subsequent high-level noise exposure. Exposure to 115 dB peak SPL, 1 kHz narrow band (400 Hz) transients presented l/s, 6 h/day, to four groups of chinchillas produced a 10-28 dB toughening effect across the 0.5-8.0 kHz test frequency range. Following either a 30 day or an 18 h recovery period the animals were exposed to the same impulses but presented at 121 or 127 dB peak SPL for five uninterrupted days, thus producing an asymptotic threshold shift (ATS) condition. Comparisons between toughened and untoughened control subjects showed: (1) During the 121 dB exposure there was a statistically significant reduction of 10-25 dB in ATS across the entire test frequency range. Thirty days following the 121 dB exposure there were no significant differences in the postexposure permanent effects on thresholds and sensory cell loss. (2) During the 127 dB exposure only the group with the 30 day interval between the toughening and traumatic exposures showed a small (approximately 10 dB), statistically significant, frequency-specific (8 kHz), reduction in ATS. Thirty days following the 127 dB exposure a statistically significant protective effect on threshold was measured only at 16.0 kHz. However, both toughened groups showed less inner hair cell loss at and above 1.0 kHz, while only the group with the 18 h interval between the toughening and traumatic exposures showed less outer hair cell loss at and above 1.0 kHz. There were no systematic differences in the response of the toughened animals that could be attributed to the 30 day or 18 h post-toughening interval.

Intracochlear infusion of buthionine sulfoximine potentiates carboplatin ototoxicity in the chinchilla

The aim of this experiment was to determine if buthionine sulfoximine (BSO), an inhibitor of glutathione (GSH) synthesis, enhances the ototoxicity of carboplatin. Osmotic pumps were used to infuse BSO into the right cochleas of 12 adult chinchillas for 14 days. The left cochleas served as controls. Animals were assigned to three groups: a drug control group that did not receive carboplatin, a group that received a single dose of carboplatin (25 mg/kg i.p.), and a group that received a double dose of carboplatin (25 mg/kg i.p. x 2), with 4 days between injections. Carboplatin was administered after three days of BSO pre-treatment. Ototoxicity was assessed with evoked potentials recorded from electrodes implanted in the inferior colliculi (ICPs), distortion product otoacoustic emissions (DPOAEs), and cochleograms. BSO infusion itself caused no long-term functional or morphological changes. One of four animals treated with it single dose of carboplatin showed a significant loss of inner hair cells (IHCs), with greater loss in the BSO-treated ear. All animals in the double-dose carboplatin group showed marked differences between BSO-treated and control ears. Average IHC losses were 59% in BSO-treated ears vs. 18% in control ears. Moreover, BSO-treated ears sustained significantly greater outer hair cell (OHC) losses than control ears (37% vs. 2%, respectively). ICP and DPOAE response amplitudes were reduced slightly in BSO-treated ears relative to control ears, consistent with their greater hair cell loss. The results clearly show that BSO can enhance carboplatin ototoxicity in the chinchilla, supporting a role of GSH and reactive oxygen species in platinum ototoxicity.

Cubic distortion product otoacoustic emissions in young and aged chinchillas exposed to low-frequency noise

The aim of this study was to examine susceptibility to noise-induced hearing loss in animals with and without age-related hearing loss (AHL), using cubic distortion product otoacoustic emissions (CDPs) to assess the functional status of the outer hair cell (OHC) system. Subjects were young (< or = 3-yr-old) and aged (10- to 15-yr-old) chinchillas. CDP thresholds and input/output (I/O) functions were measured before and after exposure to 95 dB or 106 dB SPL low-frequency noise. The results indicate that (a) aging in the chinchilla is associated with significant elevations of CDP thresholds and depression of CDP I/O functions, (b) noise exposures cause equivalent CDP threshold elevations and amplitude reductions in young animals with normal hearing and older animals with AHL, and (c) CDP threshold and amplitude measures provide information that complements evoked potentials measured from the auditory midbrain.

Effects of inner hair cell loss on inferior colliculus evoked potential thresholds, amplitudes and forward masking functions in chinchillas

The effects of outer hair cell (OHC) loss on evoked potential (EVP) thresholds, amplitudes and forward masking (FWM) functions have been fairly well characterized. In contrast, the effects of inner hair cell (IHC) losses are largely unknown, primarily due to the difficulty of producing selective IHC lesions. Recent studies have shown that IHCs of the chinchilla are preferentially damaged by the anticancer drug, carboplatin. In this study, we administered a single 100 mg/kg dose of carboplatin to four chinchillas, to examine the effects of IHC lesions on EVPs measured from the inferior colliculus (IC-EVPs). Thresholds and amplitude functions were measured for 0.25-16 kHz tone bursts, and FWM functions were measured at 1, 2 and 4 kHz, using masker probe intervals of 2, 5, 10, 20, 40 and 80 ms, before and 1-2 months after carboplatin treatment. Histology revealed IHC lesions ranging from approximately 15 to 90%, with virtually no loss of OHCs. Surprisingly, even massive IHC lesions were not associated with elevations of IC-EVP thresholds. IC-EVP amplitudes at suprathreshold levels were sometimes depressed, sometimes enhanced, and in some cases unchanged. IHC lesions increased susceptibility to FWM, particularly at intermediate (10-20 ms) masker-probe intervals, without significantly changing the overall time course of FWM. The results provide new perspectives on the contribution of IHCs to FWM, and on the ability of the central auditory system to adapt to a significant reduction of neural input from the cochlea.

Noise-induced threshold shift dynamics measured with distortion-product otoacoustic emissions and auditory evoked potentials in chinchillas with inner hair cell deficient cochleas

Chinchillas (n = 6) were treated with carboplatin and, following a 30-day recovery period, were exposed to a 115 dB peak SPL impact noise presented at a rate of l/s for 6 h/day for 10 days. A second group (n = 6) received only the noise treatment. Cubic distortion product otoacoustic emissions (2f1-f2) and auditory evoked potential (AEP) detection thresholds in response to tone bursts were measured before and 30 days after drug treatment and following the first and 10th day of the noise exposure. Thirty days after the final exposure day, permanent changes in AEP detection thresholds and emissions were measured and cochleograms constructed. The drug treatment eliminated over 80% of the inner hair cells (IHC) in the cochlea, leaving the outer hair cell (OHC) population essentially intact prior to the interrupted noise exposure. The drug treatment alone had very little or no effect on AEP detection thresholds and emission metrics. Following the noise exposure, the IHC-deficient animals showed clear 'toughening' effects in the AEP and emission measures which were the same as measured in the group receiving only the noise. After a 30-day post-exposure recovery period. AEP thresholds were elevated about 10 dB at the low frequencies in the drug-noise group whereas emissions returned to near normal despite the massive IHC losses. These results are consistent with the idea that an intact OHC population is required for toughening. However, sound-evoked efferent pathways activated by the few remaining IHCs (approximately 20%) which, in this preparation, are distributed throughout the cochlea, may still contribute significantly to the toughening phenomena.

The effects of click level, click rate, and level of background masking noise on the inferior colliculus potential (ICP) in the normal and carboplatin-treated chinchilla

Carboplatin produces a selective loss of inner hair cells in chinchilla, substantially reducing the amplitude of the compound action potential. A key question that arises from these experiments is: What effect does a reduction in IHC-eighth-nerve fiber input have on the central auditory nervous system? This investigation evaluated the inferior colliculus potential (ICP) in chinchillas treated with carboplatin. The left ear was surgically destroyed and a recording electrode was placed in the left inferior colliculus. Following thirteen days of recovery time, the ICP was recorded in the awake animal. Click level was varied from 10-20 to 80 dB pSPL. Click rate was varied from 10 to 1000 Hz using both conventional averaging and a cross-correlation procedure. Broadband masking noise was varied from 30 to 70 dB SPL with click level held constant at 80 dB pSPL. The dependent variables were the positive peak latency and peak-to-following trough amplitude of the evoked potential. Following baseline studies, the animals were administered carboplatin (50 mg/kg IP) and retested two weeks later. Prior to carboplatin administration, there was an increase in ICP latency and a decrease in ICP amplitude with decreasing stimulus level, increasing rate and increasing noise level. Mean ICP threshold was 30 dB pSPL. Following carboplatin administration, there was little change in threshold or peak latencies. In contrast, the amplitude of the ICP was reduced on average by one-third, although this effect varied considerably across animals. The magnitude of this amplitude decrement was not strongly dependent on click level, click rate, or the level of background noise.

Magnitude and pattern of inner and outer hair cell loss in chinchilla as a function of carboplatin dose

We examined the relationship between carboplatin dose and pattern of IHC and OHC loss in five groups of chinchillas treated with carboplatin: (I) single dose, 38 mg/kg, (II) double dose, 38 mg/kg, (III) double dose, 63 mg/kg, (IV) double dose, 75 mg/kg, and (V) double dose, 100 mg/kg. The pattern of IHC loss was relatively uniform along the length of the cochlea with all doses. Average IHC loss increased from approximately 20 per cent in group I to approximately 100 per cent in groups III, IV and V. Average OHC loss was small or negligible in groups I, II, III and IV; only group V consistently showed large (> 40 per cent) OHC losses. OHC loss progressed along a base-to-apex gradient. The dose of carboplatin which reliably destroyed OHCs was four to five times greater than that needed to damage IHCs.

Quantitative relationship of carboplatin dose to magnitude of inner and outer hair cell loss and the reduction in distortion product otoacoustic emission amplitude in chinchillas

The outer hair cells (OHCs) are thought to be the dominant source of distortion product otoacoustic emissions (DPOAEs) in the mammalian cochlea; however, little is known about the quantitative relationship between reduction in DPOAE amplitude and the degree of inner hair cell (IHC) and OHC loss. To examine this relationship, we measured the DPOAE input/output functions in the chinchilla before and after destroying the IHCs and/or OHCs with carboplatin. Low-to-moderate doses (38-150 mg/kg, i.p.) of carboplatin selectively destroyed some or all of the IHCs along the entire length of the cochlea while sparing the OHCs. Selective loss of all the IHCs had little effect on DPOAE amplitude as long as the OHCs were present. With high doses of carboplatin (200 mg/kg, i.p.), there was complete destruction of IHCs plus massive OHC loss that decreased from the base towards the apex of the cochlea. OHC loss resulted in a large decrease in DPOAE amplitude. DPOAE amplitude at 9.6 kHz decreased at the rate of 4.1 dB for every 10% loss of OHCs. At 7.2 and 4.8 kHz, DPOAE amplitude decreased 3.1 dB and 2.4 dB per 10% OHC loss, respectively. These results indicate that OHCs are the dominant source of DPOAEs.