Age- and strain-related differences in dehydrogenase activity and glycogen levels in CBA and C57 mouse cochleas

Altered Retinal Dopamine Levels in a Melatonin-proficient Mouse Model of Form-deprivation Myopia

Reduced levels of retinal dopamine, a key regulator of eye development, are associated with experimental myopia in various species, but are not seen in the myopic eyes of C57BL/6 mice, which are deficient in melatonin, a neurohormone having extensive interactions with dopamine. Here, we examined the relationship between form-deprivation myopia (FDM) and retinal dopamine levels in melatonin-proficient CBA/CaJ mice. We found that these mice exhibited a myopic refractive shift in form-deprived eyes, which was accompanied by altered retinal dopamine levels. When melatonin receptors were pharmacologically blocked, FDM could still be induced, but its magnitude was reduced, and retinal dopamine levels were no longer altered in FDM animals, indicating that melatonin-related changes in retinal dopamine levels contribute to FDM. Thus, FDM is mediated by both dopamine level-independent and melatonin-related dopamine level-dependent mechanisms in CBA/CaJ mice. The previously reported unaltered retinal dopamine levels in myopic C57BL/6 mice may be attributed to melatonin deficiency.

Hidden Age-Related Hearing Loss and Hearing Disorders: Current Knowledge and Future Directions

Age-related hearing loss, which affects roughly 35% of those over the age of 70, is the second most common disorder among the elderly. The severity of age related hearing loss may actually be worse if assessments are made under more realistic conditions, such as communicating in noise. Emerging data from humans and animal models suggest that damage to the inner hair cells and/or type I neurons, that relay sound information to the brain may contribute to hearing deficits in a noisy background. Data obtained from carboplatin-treated chinchillas suggest that tone-in-noise thresholds are a sensitive and frequency dependent method of detecting damage to the IHC/type I system. Therefore, tone detection thresholds measured in broadband noise may provide an efficient method of detecting the deficits in specific frequency regions. Preliminary data obtained in elderly subject with normal thresholds in quiet compared to young subjects illustrate the importance of repeating these measurements in broadband noise because thresholds in noise were worse for our elderly subjects than young subjects, even though both groups had similar hearing thresholds in quiet. N-acetyl cysteine supplementation which protects against inner hair cell loss in animal models, may represent a viable therapy for protecting the inner hair cell/type I neurons.

Standardization of experimental animals temporal bone sections

Preparation of the temporal bone for light microscopy is an important step in histological studies of the inner ear. Due to the complexity of structures of the inner ear, it is difficult to measure or compare structures of interest without a commonly accepted standardized measure of temporal bone sections. Therefore, standardization of temporal bone sections is very important for histological assessment of sensory hair cells and peripheral ganglion neurons in the cochlear and vestibular systems. The standardized temporal bone sectioning is oriented to a plane parallel to the outer and internal auditory canals. Sections are collected from the epitympanum to the hypotympanum to reveal layers in the order of the crista ampullaris of the superior and lateral semicircular canals, macula utriculi and macula sacculi, superior vestibular ganglion neurons, macula of saccule and inferior vestibular ganglion neurons, cochlear modiolus, endolymphatic duct and endolymphatic sac, and finally the crista ampullaris of the posterior semicircular canal. Moreover, technical details of preparing for temporal bone sectioning including fixation, decalcification, whole temporal bone staining, embedding penetration, and embedding orientation are also discussed.

Ouabain-induced cochlear degeneration in rat

Ouabain, a potent inhibitor of the Na+/K+-ATPase pump, selectively destroys spiral ganglion neurons (SGNs) in gerbils and mice, whereas in guinea pigs it preferentially damages cochlear hair cells. To elucidate the effects of ouabain on the rat inner ear, a species widely used in research, 5 μl of 1 or 10 mM ouabain was applied to the round window membrane. Distortion product otoacoustic emissions (DPOAE) and auditory brainstem responses (ABR) were used to identify functional deficits in hair cells and neurons, respectively, and histological techniques were used to characterize cochlear pathologies. High-frequency ABR thresholds were elevated after treatment with 1 mM ouabain, whereas DPOAEs remained normal. In contrast, 10 mM ouabain increased ABR thresholds and reduced DPOAE amplitudes. Consistent with the physiological changes, 1 mM ouabain only damaged the SGNs and auditory nerve fibers in the basal turn of the cochlea whereas 10 mM ouabain destroyed both SGNs and cochlear hair cells; damage was greatest near the base and decreased toward the apex. The nuclei of degenerating SGNs and hair cells were condensed and fragmented and many cells were TUNEL-positive, morphological features of apoptotic cell death. Thus, ouabain-induced cochlear degeneration in rats is apoptotic and concentration dependent; low concentrations preferentially damage SGNs in the base of the cochlea, producing an animal model of partial auditory neuropathy, whereas high concentrations damage both hair cells and SGNs with damage decreasing from the base toward the apex.

Genetic background effects on age-related hearing loss associated with Cdh23 variants in mice

Inbred strain variants of the Cdh23 gene have been shown to influence the onset and progression of age-related hearing loss (AHL) in mice. In linkage backcrosses, the recessive Cdh23 allele (ahl) of the C57BL/6J strain, when homozygous, confers increased susceptibility to AHL, while the dominant allele (Ahl+) of the CBA/CaJ strain confers resistance. To determine the isolated effects of these alleles on different strain backgrounds, we produced the reciprocal congenic strains B6.CBACa-Cdh23(Ahl)(+) and CBACa.B6-Cdh23(ahl) and tested 15-30 mice from each for hearing loss progression. ABR thresholds for 8 kHz, 16 kHz, and 32 kHz pure-tone stimuli were measured at 3, 6, 9, 12, 15 and 18 months of age and compared with age-matched mice of the C57BL/6J and CBA/CaJ parental strains. Mice of the C57BL/6N strain, which is the source of embryonic stem cells for the large International Knockout Mouse Consortium, were also tested for comparisons with C57BL/6J mice. Mice of the C57BL/6J and C57BL/6N strains exhibited identical hearing loss profiles: their 32 kHz ABR thresholds were significantly higher than those of CBA/CaJ and congenic strain mice by 6 months of age, and their 16 kHz thresholds were significantly higher by 12 months. Thresholds of the CBA/CaJ, the B6.CBACa-Cdh23(Ahl)(+), and the CBACa.B6-Cdh23(ahl) strain mice differed little from one another and only slightly increased throughout the 18-month test period. Hearing loss, which corresponded well with cochlear hair cell loss, was most profound in the C57BL/6J and C57BL/6NJ strains. These results indicate that the CBA/CaJ-derived Cdh23(Ahl)(+) allele dramatically lessens hearing loss and hair cell death in an otherwise C57BL/6J genetic background, but that the C57BL/6J-derived Cdh23(ahl) allele has little effect on hearing loss in an otherwise CBA/CaJ background. We conclude that although Cdh23(ahl) homozygosity is necessary, it is not by itself sufficient to account for the accelerated hearing loss of C57BL/6J mice.

Overexpression of X-linked inhibitor of apoptosis protein protects against noise-induced hearing loss in mice

Apoptosis is responsible for cochlear cell death induced by noise. Here, we show that transgenic (TG) mice that overexpress X-linked inhibitor of apoptosis protein (XIAP) under control of the ubiquitin promoter display reduced hearing loss and cochlear damage induced by acoustic overstimulation (125 dB sound pressure level, 6 h) compared with wild-type (WT) littermates. Hearing status was evaluated using the auditory brainstem response (ABR), whereas cochlear damage was assessed by counts of surviving hair cells (HCs) and spiral ganglion neurons (SGNs) as well as their fibers to HCs. Significantly smaller threshold shifts were found for TG mice than WT littermates. Correspondingly, the TG mice also showed a reduced loss of HCs, SGNs and their fibers to HCs. HC loss was limited to the basal end of the cochlea that detects high frequency sound. In contrast, the ABRs demonstrated a loss of hearing sensitivity across the entire frequency range tested (2-32 kHz) indicating that the hearing loss could not be fully attributed to HC loss alone. The TG mice displayed superior hearing sensitivity over this whole range, suggesting that XIAP overexpression reduces noise-induced hearing loss not only by protecting HCs but also other components of the cochlea.

Antioxidant enzymes, presbycusis, and ethnic variability

OBJECTIVE

A proposed mechanism for presbycusis is a significant increase in oxidative stress in the cochlea. The enzymes glutathione S-transferase (GST) and N-acetyltransferase (NAT) are two classes of antioxidant enzymes active in the cochlea. In this work, we sought to investigate the association of different polymorphisms of GSTM1, GSTT1, and NAT2 and presbycusis and analyze whether ethnicity has an effect in the genotype-phenotype associations.

STUDY DESIGN

Case-control study of 134 DNA samples.

SETTING

University-based tertiary care center.

SUBJECTS AND METHODS

Clinical, audiometric, and DNA testing of 55 adults with presbycusis and 79 control patients with normal hearing.

RESULTS

The GSTM1 null genotype was present in 77 percent of white Hispanics and 51 percent of white non-Hispanics (Fisher's exact test, 2-tail, P = 0.0262). The GSTT1 null genotype was present in 34 percent of control patients and in 60 percent of white presbycusis subjects (P = 0.0067, odds ratio [OR] = 2.843, 95% confidence interval [95% CI] = 1.379-5.860). The GSTM1 null genotype was more frequent in presbycusis subjects, i.e., 48 percent of control patients and 69 percent of white subjects carried this deletion (P = 0.0198, OR = 2.43, 95% CI = 1.163-5.067). The NAT2*6A mutant genotype was more frequent among subjects with presbycusis (60%) than in control patients (34%; P = 0.0086, OR = 2.88, 95% CI = 1.355-6.141).

CONCLUSION

We showed an increased risk of presbycusis among white subjects carrying the GSTM1 and the GSTT1 null genotype and the NAT*6A mutant allele. Subjects with the GSTT1 null genotypes are almost three times more likely to develop presbycusis than those with the wild type. The GSTM1 null genotype was more prevalent in white Hispanics than in white non-Hispanics, but the GSTT1 and NAT2 polymorphisms were equally represented in the two groups.

Over-expression of X-linked inhibitor of apoptosis protein slows presbycusis in C57BL/6J mice

Apoptosis of cochlear cells plays a significant role in age-related hearing loss or presbycusis. In this study, we evaluated whether over-expression of the anti-apoptotic protein known as X-linked Inhibitor of Apoptosis Protein (XIAP) slows the development of presbycusis. We compared the age-related hearing loss between transgenic (TG) mice that over-express human XIAP tagged with 6-Myc (Myc-XIAP) on a pure C57BL/6J genetic background with wild-type (WT) littermates by measuring auditory brainstem responses. The result showed that TG mice developed hearing loss considerably more slowly than WT littermates, primarily within the high-frequency range. The average total hair cell loss was significantly less in TG mice than WT littermates. Although levels of Myc-XIAP in the ear remained constant at 2 and 14 months, there was a marked increase in the amount of endogenous XIAP from 2 to 14 months in the cochlea, but not in the brain, in both genotypes. These results suggest that XIAP over-expression reduces age-related hearing loss and hair cell death in the cochlea.

Glucose transporter 5 is undetectable in outer hair cells and does not contribute to cochlear amplification

Glucose transporter 5 (Glut5) is a high-affinity fructose transporter. It was proposed to be a motor protein or part of the motor complex required for cochlear amplification in outer hair cells (OHCs). Here we show that, in contrast to previous reports, Glut5 is undetectable, and possibly absent, in OHCs harvested from wildtype mice. Further, Glut5-deficient mice display normal OHC morphology and motor function (i.e., nonlinear capacitance and electromotility) and normal cochlear sensitivity and frequency selectivity. We conclude that Glut5 is not required for OHC motility or cochlear amplification.

Effects of exposing C57BL/6J mice to high- and low-frequency augmented acoustic environments: auditory brainstem response thresholds, cytocochleograms, anterior cochlear nucleus morphology and the role of gonadal hormones

Gonadectomized and intact adult C57BL/6J (B6) mice of both sexes were exposed for 12h nightly to an augmented acoustic environment (AAE): repetitive bursts of a 70dB SPL noise band. The high-frequency AAE (HAAE) was a half-octave band centered at 20kHz; the low-frequency AAE (LAAE) was a 2-8kHz band. The effects of sex, gonadectomy, and AAE treatment on genetic progressive hearing loss (a trait of B6 mice) were evaluated by obtaining auditory brainstem response (ABR) thresholds at ages 3-, 6-, and 9-months. At 9-months of age, hair cell counts (cytocochleograms) were obtained, and morphometric measures of the anteroventral cochlear nucleus (AVCN) were obtained. LAAE treatment caused elevation in ABR thresholds (8-24kHz), with the highest thresholds occurring in intact females. LAAE treatment caused some loss of outer hair cells in the basal half of the cochlea (in addition to losses normally occurring in B6 mice), with intact females losing more cells than intact males. The loss of AVCN neurons and shrinkage of tissue volume that typically occur in 9-month-old B6 mice was lessened by LAAE treatment in intact (but not gonadectomized) male mice, whereas the degenerative changes were exacerbated in intact (but not gonadectomized) females. These LAAE effects were prominent in, but not restricted to, the tonotopic low-frequency (ventral) AVCN. HAAE treatment resulted in some loss of neurons in the high-frequency (dorsal) AVCN. In general, LAAE treatment plus male gonadal hormones (intact males) had an ameliorative effect whereas HAAE or LAAE treatment plus ovarian hormones (intact females) had a negative effect on age-related changes in the B6 auditory system.

Determination of hair cell metabolic state in isolated cochlear preparations by two-photon microscopy

Currently there is no accepted method to measure the metabolic status of the organ of Corti. Since metabolism and mitochondrial dysfunction are expected to play a role in many different hearing disorders, here for the first time we employ two-photon metabolic imaging to assess the metabolic status of the cochlea. When excited with ultrashort pulses of 740-nm light, both inner and outer hair cells in isolated murine cochlear preparations exhibited intrinsic fluorescence. This fluorescence is characterized and shown to be consistent with a mixture of oxidized flavoproteins (Fp) and reduced nicotinamide adenine dinucleotide (NADH). The location of the fluorescence within hair cells is also consistent with the different mitochondrial distributions in these cell types. Treatments with cyanide and mitochondrial uncouplers show that hair cells are metabolically active. Both NADH and Fp in inner hair cells gradually become completely oxidized within 50 min from the time of death of the animal. Outer hair cells show similar trends but are found to have greater variability. We show that it is possible to use two-photon metabolic imaging to assess metabolism in the mouse organ of Corti.

Response properties of single auditory nerve fibers in the mouse

The availability of transgenic and mutant lines makes the mouse a valuable model for study of the inner ear, and a powerful window into cochlear function can be obtained by recordings from single auditory nerve (AN) fibers. This study provides the first systematic description of spontaneous and sound-evoked discharge properties of AN fibers in mouse, specifically in CBA/CaJ and C57BL/6 strains, both commonly used in auditory research. Response properties of 196 AN fibers from CBA/CaJ and 58 from C57BL/6 were analyzed, including spontaneous rates (SR), tuning curves, rate versus level functions, dynamic range, response adaptation, phase-locking, and the relation between SR and these response properties. The only significant interstrain difference was the elevation of high-frequency thresholds in C57BL/6. In general, mouse AN fibers showed similar responses to other mammals: sharpness of tuning increased with characteristic frequency, which ranged from 2.5 to 70 kHz; SRs ranged from 0 to 120 sp/s, and fibers with low SR (<1 sp/s) had higher thresholds, and wider dynamic ranges than fibers with high SR. Dynamic ranges for mouse high-SR fibers were smaller (<20 dB) than those seen in other mammals. Phase-locking was seen for tone frequencies <4 kHz. Maximum synchronization indices were lower than those in cat but similar to those found in guinea pig.

Hearing threshold elevation precedes hair-cell loss in prestin knockout mice

Our previous characterization of prestin knockout (-/-) mice demonstrated that prestin is required for the eletromotility of outer hair cells (OHCs) and for the cochlear amplifier. Because hair-cell loss was observed in the basal 25% of cochleae in adult prestin-/- mice, it remained unclear how hair-cell loss progressed, whether hearing thresholds were elevated, and whether OHCs had normal ultra-structure in young prestin-/- mice. We report here that in prestin-/- mice, no significant hair-cell loss occurred before postnatal day 28 (P28); apoptosis of hair cells began at P28; and the loss of inner hair cells lagged behind that of OHCs. The prestin-/- mice had hearing thresholds that were significantly elevated (by approximately 25 dB) as early as P14; their thresholds at high frequencies were significantly elevated (by approximately 50 dB) at P21. The prestin heterozygous (+/-) mice displayed a significant threshold elevation (approximately 3.5 dB) at P21. In addition, transmission electronic microscopy shown that no obvious abnormality occurs in the sterocilla, lateral wall, tight junction and synapses of the outer hair cells. Our results demonstrate that the absence of prestin, not hair-cell loss, is the primary cause of high-frequency hearing threshold elevation in prestin-/- and +/- mice.

Males lose hearing earlier in mouse models of late-onset age-related hearing loss; females lose hearing earlier in mouse models of early-onset hearing loss

Gender-related differences in human hearing have been attributed to genetic, environmental, and/or genetic x environmental interactive factors. These differences tend to increase with age, with males showing greater high frequency threshold elevations. An appropriate animal model could aid in prediction, treatment, and prevention of some of these losses. This paper examines inbred strains of mice that are widely used as models of late- (CBA/J and CBA/CaJ) and early- (C57BL/6J) onset age-related hearing loss. In the former two genotypes, the thresholds to high frequency stimuli of the auditory brainstem response (ABR) are higher in the male than in the female. This gender difference was less pronounced in thresholds to the cochlear nerve envelope response of the CBA/CaJ, although this response was more sensitive to the influence of age than was the ABR. In contrast, the male C57BL/6J had more sensitive thresholds than the female, with both measures showing massive loss of sensitivity with increasing age. The data are discussed in terms of the applicability of these animals as tools for examining factors that degrade cochlear function.

Oxidative stress in aging in the C57B16/J mouse cochlea

Presbycusis is a complex of high frequency hearing loss and disproportionate loss of speech discrimination that is seen concomitantly with physical signs of aging. Among the most extensively characterized strains of mice that show an early hearing loss is the C57B16/J strain, a strain that shows early onset of high frequency hearing loss at age 6 months and complete hearing loss by 1 year of age. The histopathology of this strain consists of loss of hair cells and spiral ganglion neurons in the basal turn, with a progression of loss of hair cells and ganglion neurons towards the apical portion of the cochlea as the animal ages. The process of aging has been extensively studied and although details differ in various organisms the consensus today is that oxidative stress, i.e. free radical-mediated tissue damage, is one of the core mechanisms of aging. Aerobic metabolism results in the creation of hydrogen peroxide and reactive oxygen species. These are normally detoxified by a variety of enzymes and free radical scavengers, including superoxide dismutase (SOD), catalase and glutathione. To determine whether oxidative stress plays a role in the pathophysiology of hearing loss in this mouse model of presbycusis we determined the relative change in mRNA production for selected free radical detoxifying enzymes in the C57B16/J mouse cochlea. Using semi-quantitative RT-PCR with tubulin mRNA as a control, relative levels of antioxidant enzyme mRNAs were determined. There was an overall increase in SOD1 mRNA levels when comparing 1 and 9 month time points, and a transient increase in the expression level of catalase mRNA. B6.CAST+ Ahl mice, which carry the C57B16/J genome but receive their Ahl gene from CAST mice, do not show these alteractions in antioxidant enzyme production. Our results suggest that at an age of 9 months, at which point significant hearing loss has developed, the C57B16/J mouse cochlea is exposed to increased levels of free radicals and that the Ahl gene of the C57B16/J mouse mediates this decrease in protective enzymes and therefore increase in levels of oxidative stress.

Modulation of presbycusis: current status and future directions

Literature and ideas are reviewed concerning the modulation of presbycusis - the influence of variables that can alter the severity and/or time course of presbycusis or counteract its negative aspects. Eleven topics are identified: variables related to biological aging; genetics; noise-induced hearing loss; moderately augmented acoustic environment; neural plasticity and the central auditory system; neural plasticity and hearing aids; socioeconomic and cultural barriers to hearing aid use; lifestyle (diet, exercise, etc.); medical variables; pharmaceutical interventions for presbycusis, and cognitive variables. It is concluded that the field of otogerontology will best be served by a comprehensive, integrative interaction among basic researchers and clinical scientists who will continue to learn how the auditory problems associated with presbycusis can be intentionally modulated in beneficial ways.

Succinic dehydrogenase histochemistry as an early marker for hair cell pathology

Density measurements of succinic dehydrogenase (SDH) activity were obtained from the inner and outer hair cells on surface preparations obtained from the guinea pig cochlea. Guinea pigs were exposed to noise (3.85 kHz, 120 dB SPL, 22.5 min) and sacrificed 0, 4 or 24 h after the exposure. By 4 h after exposure, the first- and second-row outer hair cells already demonstrated an altered SDH activity. By 24 h after exposure, a significant decrease in SDH staining in both the inner and outer hair cells at a distance of 10-12 mm from the cochlear apex was demonstrated. After a 1-month recovery period, scanning electron microscopy confirmed the main lesion site to be at a distance of 10-12 mm. In addition, Hensen's cells (supporting cells) at a distance of 10-12 mm from the apex were intensely stained by SDH after noise exposure, indicating an increase in oxidative metabolism. SDH staining in the Hensen's cells from the unexposed cochleae was not found. In conclusion, our findings suggest that the early use of SDH histochemistry can predict later permanent damage to the organ of Corti.

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.

D-Methionine attenuates inner hair cell loss in carboplatin-treated chinchillas

Carboplatin, a second-generation platinum-based antineoplastic drug, preferentially destroys inner hair cells (IHCs) in the chinchilla while sparing outer hair cells (OHCs). D-Methionine (D-Met), a sulfur-containing amino acid, has been shown to protect hair cells from cisplatin damage in rats, but its ability to protect IHCs from carboplatin damage has not yet been evaluated in the chinchilla. We tested whether D-Met would protect the hair cells in the chinchilla from carboplatin. Animals were divided into two groups: a control group that only received carboplatin (100 mg/kg, i.p.) and an experimental group that received 300 mg/kg D-Met (i.p.) 30 min before carboplatin treatment. Ototoxicity was assessed by measuring the amount of IHC and OHC loss. Average IHC loss in the group treated with D-Met was 62% compared with 84% in the untreated control group. Thus, D-Met causes a statistically significant reduction in IHC loss induced by carboplatin.

Auditory plasticity and hyperactivity following cochlear damage

This paper will review some of the functional changes that occur in the central auditory pathway after the cochlea is damaged by acoustic overstimulation or by carboplatin, an ototoxic drug that selectively destroys inner hair cells (IHCs) in the chinchilla. Acoustic trauma typically impairs the sensitivity and tuning of auditory nerve fibers and reduces the neural output of the cochlea. Surprisingly, our results show that restricted cochlear damage enhances neural activity in the central auditory pathway. Despite a reduction in the auditory-nerve compound action potential (CAP), the local field potential from the inferior colliculus (IC) increases at a faster than normal rate and its maximum amplitude is enhanced at frequencies below the region of hearing loss. To determine if this enhancement was due to loss of sideband inhibition, we recorded from single neurons in the IC and dorsal cochlear nucleus before and after presenting a traumatizing above the unit's characteristic frequency (CF). Following the exposure, some neurons showed substantial broadening of tuning below CF, less inhibition, and a significant increase in discharge rate, consistent with a model involving loss of sideband inhibition. The central auditory system of the chinchilla can be deprived of some of its cochlear inputs by selectively destroying IHCs with carboplatin. Selective IHC loss reduces the amplitude of the CAP without affecting the threshold and tuning of the remaining auditory nerve fibers. Although the output of the cochlea is reduced in proportion to the amount of IHC loss, the IC response shows only a modest amplitude reduction, and remarkably, the response of the auditory cortex is enhanced. These results suggest that the gain of the central auditory pathway can be up- or down regulated to compensate for the amount of neural activity from the cochlea.

Selective loss of inner hair cells and type-I ganglion neurons in carboplatin-treated chinchillas. Mechanisms of damage and protection

Carboplatin preferentially destroys inner hair cells (IHCs) and type-I spiral ganglion neurons while sparing outer hair cells (OHCs). Loss of IHCs and type-I ganglion cells is associated with a significant reduction of the compound action potential (CAP). However, the cochlear microphonic (CM) potential and distortion product otoacoustic emissions (DPOAEs) remain normal, indicating that the OHCs are functionally intact. In the vestibular system, carboplatin selectively destroys type-I hair cells and their afferent neurons. Damage of type-I vestibular hair cells and their afferent terminals is associated with significant depression of nystagmus induced by cold, caloric stimulation. Histochemical studies revealed a rapid decrease in succinate dehydrogenase (SDH) staining in IHCs soon after carboplatin treatment, and staining intensity remained depressed in surviving IHCs for at least 1 month after carboplatin treatment. These results suggest that carboplatin depresses the metabolic function in surviving IHCs. Several lines of evidence suggest that free radicals may contribute to carboplatin-induced sensory cell damage. Intracochlear infusion of L-buthionine-[S,R]-sulfoximine (BSO), which depletes intracellular glutathione (GSH), increases IHC and OHC loss. Previous in vitro studies have shown that neurotrophin 4/5 (NT-4/5) promotes the survival of spiral ganglion neurons from cisplatin ototoxicity. In vivo perfusion of NT-4/5 promoted the survival of spiral ganglion neurons, but did not protect the hair cells.