Induction of spontaneous otoacoustic emissions in chinchillas from carboplatin-induced inner hair cell loss

The characteristic of otoacoustic emissions in full-term neonates according to ABO blood groups

Introduction

Previous research has suggested that individuals with different blood groups show varied incidences of noise-induced hearing loss. The reduced otoacoustic emissions amplitudes indicate the higher possibilities of outer hair cell damage for noise exposure.

Objective

The objective is to analyze the characteristics of otoacoustic emissions, including the occurrence of spontaneous otoacoustic emission and the amplitudes of distortion product otoacoustic emission at certain frequencies in full term neonates with different ABO blood groups.

Methods

A total of 80 selected full-term female neonates who passed the initial newborn hearing screen were enrolled into the study, with equal number of participants in four ABO blood groups (Blood Group A, Blood Group B, Blood Group AB, Blood Group O). Measurements of spontaneous otoacoustic emission and distortion product otoacoustic emission were performed in both ears for all participants.

Results

(1) The blood group O participants showed significantly fewer spontaneous otoacoustic emission occurrences than the other three blood groups (A = 70%, B = 80%, AB = 67%, O = 25%, p <  0.05). (2) The blood group O participants showed lower DPOAE amplitudes at 1257 Hz (M = 4.55 dB, SD = 8.36), 1587 Hz (M = 11.60 dB, SD = 6.57), 3174 Hz (M = 7.25 dB, SD = 5.99), 5042 Hz (M = 13.60, SD = 6.70) than participants with the other three blood groups in left ears (p < 0.05). In right ears, the blood group O participants showed reduced amplitudes at 1257 Hz (M = 6.55 dB, SD = 8.36), 1587 Hz (M = 13.60 dB, SD = 6.57), 3174 Hz (M = 7.65 dB, SD = 6.43), 5042 Hz (M = 13.65 dB, SD = 6.50) than participants from non-O blood groups (p < 0.05).

Conclusion

Female individuals with blood group O have lower otoacoustic emissions values than individuals with the other three blood groups. We need to further investigate the possible relationships between ABO blood group and cochlear function, including the potential influences of noise damage on cochlear outer hair cells.

N-acetyl-cysteine prevents age-related hearing loss and the progressive loss of inner hair cells in γ-glutamyl transferase 1 deficient mice

Genetic factors combined with oxidative stress are major determinants of age-related hearing loss (ARHL), one of the most prevalent disorders of the elderly. Dwarf grey mice, Ggt1^dwg/dwg, are homozygous for a loss of function mutation of the γ-glutamyl transferase 1 gene, which encodes an important antioxidant enzyme critical for the resynthesis of glutathione (GSH). Since GSH reduces oxidative damage, we hypothesized that Ggt1^dwg/dwg mice would be susceptible to ARHL. Surprisingly, otoacoustic emissions and cochlear microphonic potentials, which reflect cochlear outer hair cell (OHC) function, were largely unaffected in mutant mice, whereas auditory brainstem responses and the compound action potential were grossly abnormal. These functional deficits were associated with an unusual and selective loss of inner hair cells (IHC), but retention of OHC and auditory nerve fibers. Remarkably, hearing deficits and IHC loss were completely prevented by N-acetyl-L-cysteine, which induces de novo synthesis of GSH; however, hearing deficits and IHC loss reappeared when treatment was discontinued. Ggt1^dwg/dwgmice represent an important new model for investigating ARHL, therapeutic interventions, and understanding the perceptual and electrophysiological consequences of sensory deprivation caused by the loss of sensory input exclusively from IHC.

Central gain control in tinnitus and hyperacusis

Sensorineural hearing loss induced by noise or ototoxic drug exposure reduces the neural activity transmitted from the cochlea to the central auditory system. Despite a reduced cochlear output, neural activity from more central auditory structures is paradoxically enhanced at suprathreshold intensities. This compensatory increase in the central auditory activity in response to the loss of sensory input is referred to as central gain enhancement. Enhanced central gain is hypothesized to be a potential mechanism that gives rise to hyperacusis and tinnitus, two debilitating auditory perceptual disorders that afflict millions of individuals. This review will examine the evidence for gain enhancement in the central auditory system in response to cochlear damage. Further, it will address the potential cellular and molecular mechanisms underlying this enhancement and discuss the contribution of central gain enhancement to tinnitus and hyperacusis. Current evidence suggests that multiple mechanisms with distinct temporal and spectral profiles are likely to contribute to central gain enhancement. Dissecting the contributions of these different mechanisms at different levels of the central auditory system is essential for elucidating the role of central gain enhancement in tinnitus and hyperacusis and, most importantly, the development of novel treatments for these disorders.

OTOTOXIC EFFECTS OF CARBOPLATIN IN ORGANOTYPIC CULTURES IN CHINCHILLAS AND RATS

Carboplatin, a second-generation platinum chemotherapeutic drug, is considerably less ototoxic than cisplatin. While common laboratory species such as mice, guinea pigs and rats are highly resistant to carboplatin ototoxicity, the chinchilla stands out as highly susceptible. Moreover, carboplatin causes an unusual gradient of cell death in chinchillas. Moderate doses selectively damage type I spiral ganglion neurons (SGN) and inner hair cells (IHC) and the lesion tends to be relatively uniform along the length of the cochlea. Higher doses eventually damage outer hair cells (OHC), but the lesion follows the traditional gradient in which damage is more severe in the base than the apex. While carboplatin ototoxicity has been well documented in adult animals in vivo, little is known about its in vitro toxicity. To elucidate the ototoxic effects of carboplatin in vitro, we prepared cochlear and vestibular organotypic cultures from postnatal day 3 rats and adult chinchillas. Chinchilla cochlear and vestibular cultures were treated with carboplatin concentrations ranging from 50 µM to 10 mM for 48 h. Consistent with in vivo data, carboplatin selectively damaged IHC at low concentrations (50-100 µM). Surprisingly, IHC loss decreased at higher doses and IHC were intact at doses exceeding 500 µM. The mechanisms underlying this nonlinear response are unclear but could be related to a decrease in carboplatin uptake via active transport mechanisms (e.g., copper). Unlike the cochlea, the carboplatin dose-response function increased with dose with the highest dose destroying all chinchilla vestibular hair cells. Cochlear hair cells and auditory nerve fibers in rat cochlear organotypic cultures were unaffected by carboplatin concentrations <10 µM; however, the damage in OHC were more severe than IHC once the dose reached 100 µM. A dose at 500 µM destroyed all the cochlear hair cells, but hair cell loss decreased at high concentrations and nearly all the cochlear hair cells were present at the highest dose, 5 mM. Unlike the nonlinear dose-response seen with cochlear hair cells, rat auditory nerve fiber and spiral ganglion losses increased with doses above 50 µM with the highest dose destroying virtually all SGN. The remarkable species differences seen in vitro suggest that chinchilla IHC and type I SGN posse some unique biological mechanism that makes them especially vulnerable to carboplatin toxicity.

Frequency distribution of synchronized spontaneous otoacoustic emissions showing sex-dependent differences and asymmetry between ears in 2- to 4-day-old neonates

OBJECTIVE

The mature pattern of frequency distribution of synchronized spontaneous otoacoustic emissions (SSOAEs) has been reported to be bimodal in adults and children between 5 and 11 years of age; however, little is known about the distribution in neonates between 2 and 4 days after birth. Furthermore, overall differences in frequency distribution resulting from difference in sex and asymmetry between ears have not been carefully examined. The aim of this study is to determine the frequency distribution of SSOAEs in neonates at 2 to 4 days of age, evaluate the maturity of the pattern of distribution in this age group, and to evaluate the effects of differences in sex and asymmetry between left and right ears on the frequency distribution.

METHODS

We evaluated 224 ears in 112 newborns (59 girls, 53 boys) whose ages ranged from 2 to 4 days. The SSOAEs were measured using ILO96.

RESULTS

Most of the SSOAEs (86.5%) appeared at frequencies between 1.01 and 4.50 kHz. The overall frequency distribution of the SSOAEs showed a 'peak-valley-peak' pattern when plotted. Two peaks with maxima at 1.41-1.60 and 3.01-3.20 kHz were separated by a valley with a minimum at 2.41-2.60 kHz. Both girls and boys had approximate monomodal patterns in the distribution of SSOAEs. Significant sex-dependent differences were noted with more SSOAEs at the lower frequencies (<or=2 kHz) in boys (46.1%) than in girls (32.0%) (P<0.05) and more SSOAEs at the higher frequencies (2.51- 4.50 kHz) in girls (50.9%) than in boys (37.5%) (P<0.05). Both the right and left ears showed the 'peak-valley-peak' pattern that was similar to the overall distribution pattern. But, compared with the peaks measured in the left ears at 1.01-1.50 and 3.01-3.50 kHz, the peaks of the right ears at 1.51-2.00 and 2.51-3.00 kHz were much closer to the valley.

CONCLUSIONS

The overall distribution of frequency of SSOAEs in 2- to 4-day-old neonates had the similar mature 'peak-valley-peak' distribution pattern seen in adults. Significant sex-dependent differences of the SSOAEs frequency distributions have been found. However, only slight ear asymmetries of the SSOAEs frequency distributions can observed in this age group.

Changes to spontaneous otoacoustic emissions (SOAEs) due to cisplatin administration

We investigated the influence of cisplatin on spontaneous otoacoustic emissions (SOAEs) by measuring SOAEs, before and after cisplatin administration, in 18 ears of nine patients (one female and eight males) who had received chemotherapy with cisplatin for a brain tumor. No hearing loss was observed after cisplatin administration in eight ears. Before cisplatin administration SOAE was present in four out of these eight ears, and only mild frequency fluctuation was observed even after administration. In 10 ears, sensory neural hearing loss was observed after cisplatin administration. Before cisplatin administration SOAE was present in four out of these 10 ears, and SOAE decreased or disappeared in three ears after administration. In two ears, SOAE was not present before cisplatin administration, but newly appeared after administration. It was indicated that SOAE principally disappeared at the frequencies where the region of the outer hair cells responsible for the same frequencies was injured, but new SOAEs appeared at the frequencies where the region of the outer hair cells was not injured after cisplatin administration.

Carboplatin-induced oxidative injury in rat inferior colliculus

Carboplatin is currently being used as an anticancer drug against human cancers. However, high dose of carboplatin chemotherapy resulted in ototoxicity in cancer patients. Carboplatin-induced ototoxicity was related to oxidative stress to the cochlea and inner hair cell loss in animals. It is likely that initial oxidative injury spreads throughout the neuroaxis of the auditory system later. The study aim was to evaluate carboplatin-induced hearing loss and oxidative injury to the central auditory system (inferior colliculus) of the rat. Male Wistar rats were divided into two groups of seven animals each and treated as follows: (1) control (normal saline, intraperitoneal [i.p.]) and (2) carboplatin (256 mg/kg, i.p.). Auditory brain-evoked responses (ABRs) were recorded before and 4 days after treatments. The animals were sacrificed on the 4th day and inferior colliculus from brain stem and cerebellum were isolated and analyzed. Carboplatin significantly elevated the hearing threshold shifts at clicks, 2-, 4-, 8-, 16-, and 32-kHz tone burst stimuli. Carboplatin significantly increased nitric oxide and lipid peroxidation, xanthine oxidase, and manganese superoxide dismutase activities in the inferior colliculus, but not in the cerebellum, indicating an enhanced flux of free radicals in the central auditory system. Carboplatin significantly depressed the reduced to oxidized glutathione ratio, antioxidant enzyme activities, such as copper-zinc superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase, and glutathione S-transferase, and enzyme protein expressions in the inferior colliculus, but not in the cerebellum, 4 days after treatment. The data suggest that carboplatin induced oxidative injury specifically in the inferior colliculus of the rat leading to hearing loss.

Time response of carboplatin-induced hearing loss in rat

Carboplatin is currently being used as an anticancer drug against human cancers. However, high dose of carboplatin chemotherapy resulted in hearing loss in cancer patients. We have shown that carboplatin-induced hearing loss was related to dose-dependent oxidative injury to the cochlea in rat model. However, the time response of ototoxic dose of carboplatin on hearing loss and oxidative injury to cochlea has not been explored. The aim of the study was to evaluate the time response of carboplatin-induced hearing loss and oxidative injury to the cochlea of the rat. Male Wistar rats were divided into two groups of 30 animals each and treated as follows: (1) control (normal saline, i.p.) and (2) carboplatin (256 mg/kg, a single i.p. bolus injection). Auditory brain-evoked responses (ABRs) were recorded before and 1-5 days after treatments. The animals (n = 6) from each group were sacrificed on day 1, 2, 3, 4, and 5 and cochleae were isolated and analyzed. Carboplatin significantly elevated the hearing thresholds to clicks and to 2, 4, 8, 16, and 32 kHz tone burst stimuli only 3-5 days post-treatment. Carboplatin significantly increased nitric oxide (NO), malondialdehyde (MDA) levels and manganese superoxide dismutase (Mn-SOD) activity in the cochlea 4-5 and 3-5 days post-treatment, respectively, indicating enhanced influx of free radicals and oxidative injury to the cochlea. Carboplatin significantly depressed the reduced to oxidized glutathione (GSH/GSSG) ratio, antioxidant enzyme activities such as copper/zinc-superoxide dismutase (CuZn-SOD), catalase (CAT), and glutathione peroxidase (GSH-Px) as well as enzyme protein expressions in the cochlea 3-5 days after treatment. The data suggest that carboplatin-induced hearing loss involves oxidative injury to the cochlea of the rat in a time-dependent manner.

Strategies for prevention of toxicity caused by platinum-based chemotherapy: review and summary of the annual meeting of the Blood-Brain Barrier Disruption Program, Gleneden Beach, Oregon, March 10, 2001

OBJECTIVES

To summarize the findings relevant to otolaryngology from the annual meeting of the Blood-Brain Barrier Disruption Consortium in Gleneden Beach, Oregon, March 10, 2001.

STUDY DESIGN

Summaries are provided by the speakers, as well as related data from the published literature. Findings in otology and oncology regarding ototoxicity that were discussed at the meeting are included.

RESULTS

Data considered included physiological research, animal studies, and clinical trials that relate to platinum-based chemotherapy and prevention of toxicity.

CONCLUSIONS

The dose-limiting side effects of platinum-based chemotherapy are preventable, but questions about the effect of the protective agents on oncological efficacy remain. Strategies for prevention of chemotherapy-induced toxicity include temporal or anatomical separation of cisplatin or carboplatin from sodium thiosulfate, D-methionine, or N-acetyl-cysteine. Clinical application of these methods has begun. The mechanisms presumably involve free radicals or drug conjugation, or both. Understanding the role of free radicals in medicine is likely to become important in the future.

Carboplatin-induced oxidative stress in rat cochlea

Carboplatin is currently being used in the clinic against a variety of human cancers. However, high dose carboplatin chemotherapy resulted in ototoxicity in cancer patients. This is the first study to show carboplatin-induced oxidative stress response in the cochlea of rat. Male Wistar rats were divided into two groups of six animals each and treated as follows: (1) control (normal saline, i.p.) and (2) carboplatin (256 mg/kg, i.p.). Animals in both groups were sedated with ketamine/xylazine and auditory brainstem-evoked responses were recorded before and 4 days after treatments. The animals were sacrificed on the fourth day and cochleae were harvested and analyzed. A significant elevation of the hearing threshold shifts was noted at clicks, 8, 16, and 32 kHz tone burst stimuli following carboplatin administration. Carboplatin significantly increased nitric oxide and malondialdehyde levels, xanthine oxidase and manganese-superoxide dismutase activities in the cochlea indicating enhanced flux of free radicals. Cochlear glutathione levels, antioxidant enzyme activities such as copper zinc-superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase and glutathione S-transferase and enzyme protein levels were significantly depleted 4 days after carboplatin treatment. The data suggest that carboplatin induced free radical generation and antioxidant depletion, and caused oxidative injury in the cochleae of rats.

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.

Effects of AC and DC stimulation on chinchilla SOAE amplitude and frequency

The effects of AC and DC current on spontaneous otoacoustic emissions (SOAEs) were studied in normal chinchillas and chinchillas with selective inner hair cell (IHC) loss. Electrical stimulation was delivered through an electrode on the round window or through an electrode in scala media. SOAE frequencies ranged from 4 to 11 kHz and amplitudes ranged from 13 to 51 dB SPL. AC simulation suppressed SOAE amplitude. The suppression contours had a narrowly tuned, low-threshold tip located above the frequency of the SOAE. AC suppression contours were similar to acoustic suppression contours except that the AC suppression contours lacked a high-threshold, low frequency tail. The lowest threshold of the AC suppression contour was 3.9 microA rms whereas the lowest acoustic suppression threshold was 19 dB SPL. AC stimulation, which induced an electrically evoked otoacoustic emission, interacted with the SOAE to generate distortion product otoacoustic emissions (DPOAEs) of up to 26 dB SPL at 2f(S)-f(AC) (f(S)=SOAE). DPOAE amplitude increased with AC current, but saturated at high levels. DC current steps affected both SOAE frequency and amplitude. Positive current at the round window decreased SOAE amplitude and frequency whereas negative current increased SOAE frequency, but had little effect on amplitude. The effects of AC and DC current on SOAEs in animals with IHC loss were similar to those in normal chinchillas.