Maturation of cochlear glutathione-S-transferases correlates with the end of the sensitive period for ototoxicity

Ventilation with hyperoxia promotes cochlear bleeding in rabbits with congenital diaphragmatic hernia

OBJECTIVE

The authors hypothesized that ventilation and hyperoxia may harm the cochlea vasculature in an experimental model of Congenital Diaphragmatic Hernia (CDH) performed in rabbits.

METHODS

New Zealand rabbits underwent CDH creation at 25 days of gestation (term = 30 days). CDH was created in fetuses (n = 15) and compared with Controls (n = 15). Six groups were studied: Control, Control Ventilated 21% FiO2 (Control 21%), Control Ventilated 100% FiO2 (Control 100%), CDH, CDH Ventilated 21% FiO2 (CDH 21%) and CDH Ventilated 100% FiO2 (CDH 100%). Dynamic Compliance (CRS), dynamic Elastance (ERS), and dynamic Resistance (RRS) were measured. The cochleae were then removed, and the apical, middle, and basal slopes of the cochleae were evaluated. Samples were graded using a scoring system for the severity of bleeding: 0 (absent), 1 (mild), 2 (moderate), and 3 (severe). Statistical analysis was performed by contingence and ANOVA.

RESULTS

There was no difference in the severity of cochlear bleeding between Controls and CDH without ventilation. Control 21% and 100% had average scores of 2 and 1.2 respectively; CDH 21% and 100% had average scores of 0.4 and 3.8 respectively; RR [CDH 100% / Control 100%] (95% CI) = 3.16 (p < 0.005).

CONCLUSIONS

The severity of bleeding was 3.16 times worse with 100% oxygenation in CDH. This information may be helpful for future therapeutic strategies for decreasing SNHL in CDH patients.

Glutathione S-transferase pi isoform (GSTP1) expression in murine retina increases with developmental maturity

BACKGROUND AND AIMS

Glutathione S-transferase pi isoform (GSTP1) is an intracellular detoxification enzyme that catalyzes reduction of chemically reactive electrophiles and is a zeaxanthin-binding protein in the human macula. We have previously demonstrated that GSTP1 levels are decreased in human age-related macular degeneration (AMD) retina compared to normal controls (Joshi et al., Invest Ophthalmol Vis Sci, e-abstract, 2009). We also showed that GSTP1 levels parallel survival of human retinal pigment epithelial (RPE) cells exposed to ultraviolet (UV) light, and GSTP1 over-expression protects them against UV light damage (Joshi et al., Invest Ophthalmol Vis Sci, e-abstract, 2010). In the present work, we determined the developmental time course of GSTP1 expression in murine retina and in response to light challenge.

METHODS

Eyes from BALB/c mice at postnatal day 20, 1 month, and 2 months of age were prepared for retinal protein extraction and cryo sectioning, and GSTP1 levels in the retina were analyzed by Western blot and immunohistochemistry (IHC). Another group of BALB/c mice with the same age ranges was exposed to 1000 lx of white fluorescent light for 24 h, and their retinas were analyzed for GSTP1 expression by Western blot and IHC in a similar manner.

RESULTS

GSTP1 levels in the murine retina increased in ascending order from postnatal day 20, 1 month, and 2 months of age. Moreover, GSTP1 expression in murine retina at postnatal day 20, 1 month, and 2 months of age increased in response to brief light exposure compared to age-matched controls under normal condition.

CONCLUSIONS

GSTP1 expression in retina increases with developmental age in mice and accompanies murine retinal maturation. Brief exposure to light induces GSTP1 expression in the murine retina across various developmental ages. GSTP1 induction may be a protective response to light-induced oxidative damage in the murine retina.

Impact of noise or styrene exposure on the kinetics of presbycusis

Presbycusis, or age-related hearing loss is a growing problem as the general population ages. In this longitudinal study, the influence of noise or styrene exposure on presbycusis was investigated in Brown Norway rats. Animals were exposed at 6 months of age, either to a band noise centered at 8 kHz at a Lex,8h = 85 dB (86.2 dB SPL for 6 h), or to 300 ppm of styrene for 6 h per day, five days per week, for four weeks. Cubic distortion product otoacoustic emissions (2f1-f2 DPOAEs) were used to test the capacity of the auditory receptor over the lifespan of the animals. 2f1-f2DPOAE measurements are easy to implement and efficiently track the age-related deterioration of mid- and high-frequencies. They are good indicators of temporary auditory threshold shift, especially with a level of primaries close to 60 dB SPL. Post-exposure hearing defects are best identified using moderate, rather than high, levels of primaries. Like many aging humans, aging rats lose sensitivity to high-frequencies faster than to medium-frequencies. Although the results obtained with the styrene exposure were not entirely conclusive, histopathological data showed the presbycusis process to be enhanced. Noise-exposed rats exhibit a loss of spiral ganglion cells from 12 months and a 7 dB drop in 2f1-f2DPOAEs at 24 months, indicating that even moderate-intensity noise can accelerate the presbycusis process. Even though the results obtained with the styrene exposure are less conclusive, the histopathological data show an enhancement of the presbycusis process.

Expression of genes implicated in oxidative stress in the cochlea of newborn rats

Oxidative stress is an important mechanism inducing ototoxicity-, age- and noise-induced hearing loss. To better understand this phenomenon, we examined cochlear tissues for the expression of following genes involved directly or indirectly in the oxidative stress response: glyceraldehyde-3-phosphate dehydrogenase (Gapdh); solute carrier family-2 (facilitated glucose transporter), member-1 (Slc2a1); heme oxygenase-1 (Hmox1); heme oxygenase-2 (Hmox2); inducible nitric oxide synthase-2 (Nos2); transferrin (Tf); transferrin receptor (Tfrc); glutathione S-transferase A3 (Gsta3) and metallothionein-1a (Mt1a). Cochlear tissues were dissected from the p3-p5 Wistar rats, divided into the organ of Corti (OC), modiolus (MOD) and stria vascularis together with spiral ligament (SV + SL) and processed immediately or cultured under normoxic conditions or a short-term, mild hypoxia followed by re-oxygenation. After 24 h, explants were collected and total RNA isolated, transcribed and amplified in the real time RT-PCR. We found all genes listed above expressed in the freshly isolated cochlear tissues. In the OC and MOD, Slc2a1, Tf, and Mt1a were expressed on a lower level than in the SV + SL. In the OC, Hmox1 was expressed on a lower level than in the MOD and SV + SL. Hypoxic and normoxic cultures increased the transcript number of Gapdh, Slc2a1 and Hmox1 in all cochlear tissues. The expression of Nos2, Tf, Gsta3 and Mt1a increased in a tissue-specific manner. In the SV + SL, Mt1a expression decreased after normoxic and hypoxic conditions. Taken together, using real time RT-PCR, our results imply that oxidative stress may be an important component of cochlear injury during the developing period. In spite of the immaturity of the tissue, a differential response of antioxidant enzymes/proteins with respect to the pathway, the expression levels and regions was observed.

Divergence of noise vulnerability in cochleae of young CBA/J and CBA/CaJ mice

CBA/CaJ and CBA/J inbred mouse strains appear relatively resistant to age- and noise-related cochlear pathology, and constitute the predominant 'good hearing' control strains in mouse studies of hearing and deafness. These strains have often been treated as nearly equivalent in their hearing characteristics, and have even been mixed in some studies. Nevertheless, we recently showed that their trajectories with regard to age-associated cochlear pathology diverge after one year of age (Ohlemiller et al., 2010a). We also recently reported that they show quite different susceptibility to cochlear noise injury during the 'sensitive period' of heightened vulnerability to noise common to many models, CBA/J being far more vulnerable than CBA/CaJ (Fernandez et al., 2010 J. Assoc. Res. Otolaryngol. 11:235-244). Here we explore this relation in a side-by-side comparison of the effect of varying noise exposure duration in young (6 week) and older (6 month) CBA/J and CBA/CaJ mice, and in F1 hybrids formed from these. Both the extent of permanent noise-induced threshold shifts (NIPTS) and the probability of a defined NIPTS were determined as exposure to intense broadband noise (4-45 kHz, 110 dB SPL) increased by factors of two from 7 s to 4 h. At 6 months of age the two strains appeared very similar by both measures. At 6 weeks of age, however, both the extent and probability of NIPTS grew much more rapidly with noise duration in CBA/J than in CBA/CaJ. The 'threshold' exposure duration for NIPTS was <1.0 min in CBA/J versus >4.0 min in CBA/CaJ. F1 hybrid mice showed both NIPTS and hair cell loss similar to that in CBA/J. This suggests that dominant-acting alleles at unknown loci distinguish CBA/J from CBA/CaJ. These loci have novel effects on hearing phenotype, as they come into play only during the sensitive period, and may encode factors that demarcate this period. Since the cochlear cells whose fragility defines the early window appear to be hair cells, these loci may principally impact the mechanical or metabolic resiliency of hair cells or the organ of Corti.

Immunoreactivities for glutathione S-transferases and glutathione peroxidase in the lateral wall of pigmented and albino guinea pig cochlea

Dark-skinned people are known to be more tolerant of ototraumatic noise than are light-skinned people, and pigmented animals are more tolerant of ototraumatic noise and aminoglycoside ototoxicity than are albino animals. Such tolerance may be dependent on the local ability of detoxification and antioxidant enzymes, including glutathione S-transferase (GST) and glutathione peroxidase (GSPx). In the present study, we examined the difference in GST/GSPx expression in the lateral wall of the cochlea between pigmented and albino guinea pigs. Eight-week-old male pigmented and albino guinea pigs were killed by transcardiac perfusion with 2% paraformaldehyde. The cochlear ducts were isolated, further fixed with 4% paraformaldehyde, decalcified, and then embedded in paraffin. Sections prepared at 5-microm thickness were incubated with anti-GST-alpha,-mu,-pi, or anti-GSPx antibody, reacted with Alexa Fluorconjugated secondary antibody, and examined under a Carl Zeiss Axioskop 2 plus fluorescence microscope. The cochlea ducts were also subjected to immunoelectron microscopy for GST-pi by the postembedment method. The stria vascularis of pigmented guinea pigs was strongly immunoreactive for GST-alpha,-mu,-pi, and GSPx, whereas no or only weak immunoreactivities were seen in the stria vascularis of albino guinea pigs. The spiral ligament showed positive but different immunoreactivities for these enzymes between the strains. Double-stained immunofluorescence micrographs for GST-pi and GSPx showed a close resemblance of localization between the two enzymes in both pigmented and albino guinea pigs. At the ultrastructural level, immunoreactivity for GST-pi was localized preferentially in the melanin cells of pigmented guinea pigs. These results suggest that correlation between pigmentation and inner ear susceptibility is, at least partially, attributed to the different distribution of GST/GSPx in the stria vascularis.

Association between long-term neuro-toxicities in testicular cancer survivors and polymorphisms in glutathione-s-transferase-P1 and -M1, a retrospective cross sectional study

Background

To assess the impact of polymorphisms in Glutathione S-transferase (GST) -P1, -M1, and -T1 on self-reported chemotherapy-induced long-term toxicities in testicular cancer survivors (TCSs).

Methods

A total of 238 TCSs, who had received cisplatin-based chemotherapy at median twelve years earlier, had participated in a long-term follow-up survey which assessed the prevalence of self-reported paresthesias in fingers/toes, Raynaud-like phenomena in fingers/toes, tinnitus, and hearing impairment. From all TCSs lymphocyte-derived DNA was analyzed for the functional A→G polymorphism at bp 304 in GSTP1, and deletions in GST-M1 and GST-T1. Evaluation of associations between GST polymorphisms and self-reported toxicities included adjustment for prior treatment.

Results

All six evaluated toxicities were significantly associated with the cumulative dose of cisplatin and/or bleomycin. Compared to TCSs with either GSTP1-AG or GSTP1-AA, the 37 TCSs with the genotype GSTP1-GG, were significantly less bothered by paresthesias in fingers and toes (p = 0.039, OR 0.46 [0.22–0.96] and p = 0.023, OR 0.42 [0.20–0.88], respectively), and tinnitus (p = 0.008, OR 0.33 [0.14–0.74]). Furthermore, absence of functional GSTM1 protected against hearing impairment (p = 0.025, OR 1.81 [1.08–3.03]).

Conclusion

In TCSs long-term self-reported chemotherapy-induced toxicities are associated with functional polymorphisms in GSTP1 and GSTM1. Hypothetically, absence of GST-M1 leaves more glutathione as substrate for the co-expressed GST-P1. Also intracellular inactivation of pro-apoptotic mediators represents a possible explanation of our findings. Genotyping of these GSTs might be a welcomed step towards a more individualized treatment of patients with metastatic testicular cancer.

Cisplatin-induced long-term hearing impairment is associated with specific glutathione s-transferase genotypes in testicular cancer survivors

PURPOSE

Cisplatin, a cornerstone of combination chemotherapy in the treatment of testicular cancer, induces hearing impairment with considerable interindividual variations. These differences might be a result of functional polymorphisms in cisplatin-detoxifying enzymes like glutathione S-transferases (GSTs).

PATIENTS AND METHODS

We identified 173 cisplatin-treated testicular cancer survivors (TCSs) who had participated in a long-term survey that included audiometric testing and lymphocyte sampling. The hearing decibel thresholds at 4,000 Hz were categorized into leveled scales by normative decibel percentiles. Known functional polymorphisms (positive or negative) in GSTT1 and GSTM1 and codon 105 A/G (Ile/Val) in GSTP1 were analyzed by multiplex polymerase chain reaction, followed by restriction enzyme cutting, and separated by gel electrophoresis.

RESULTS

The risk of having an inferior audiometric result was more than four times higher in TCSs with 105Ile/105Ile-GSTP1 or 105Val/105Ile-GSTP1 compared with 105Val/105Val-GSTP1 (odds ratio [OR] = 4.21; 95% CI, 1.99 to 8.88; P < .001 when modeled by ordinal logistic regression [OLR]). GSTM1 positivity was detrimental for hearing ability. Two combined genotypes were associated with hearing ability. The presence of pattern 1 (GSTT1 positive, GSTM1 positive, and 105Ile/105Ile-GSTP1) was associated with hearing impairment (OR = 2.76; 95% CI, 1.35 to 5.64; P = .005, OLR). TCSs with pattern 2 (GSTT1 positive, GSTM1 positive, and 105Val/105Val-GSTP1) had better hearing ability than TCSs without this pattern (OR = 5.35; 95% CI, 2.25 to 12.76; P < .001, OLR).

CONCLUSION

The presence of both alleles of 105Val-GSTP1 offered protection against cisplatin-induced hearing impairment. Two genotype patterns with good and poor protection against cisplatin-induced ototoxicity were identified.

Audiologic testing and molecular analysis of 12S rRNA in patients receiving aminoglycosides

BACKGROUND

Pathogenic mutations in the mitochondrial genome are associated with a wide variety of maternally inherited human diseases including sensorineural hearing loss (HL). A specific mutation, m.1555A>G in the mitochondrial 12S rRNA gene, is associated with predisposition to aminoglycoside ototoxicity and HL. Mutation screening in this gene has been recommended before use of aminoglycosides as a preventative strategy to reduce the risk of ototoxicity.

OBJECTIVE

To study the incidence of mutations in the 12S rRNA gene in patients being treated with aminoglycosides and its correlation with ototoxicity.

METHODS

Patients undergoing treatment with aminoglycosides were prospectively enrolled in this study (n = 27). Total dosage administered and therapeutic levels of the antibiotic were noted. All patients underwent high-frequency pure-tone audiometry pre- and posttherapy and sequencing of the 12S rRNA gene. In addition, 12S rRNA gene was also sequenced in 50 controls to characterize population specific polymorphisms.

RESULTS

Five of 27 patients suffered from HL involving the high frequencies: four mild and one moderate. Only one of the five patients with ototoxicity harbored two sequence alterations in 12S rRNA of uncertain pathogenicity. The m.1555A>G and m.961delTInsCn mutations were not detected.

CONCLUSIONS

High-frequency pure-tone audiometry is critical for detection of aminoglycoside-induced HL. In the Swiss population, screening for mutations in the 12S rRNA gene, before the initiation of aminoglycoside therapy, is not supported by this limited study. A larger multicenter and multicultural study is warranted to more definitively address this critical clinical issue.

Relative ototoxicity of 21 aromatic solvents

Some aromatic solvents (e.g. toluene, p-xylene, styrene, and ethylbenzene) show, in the rat, striking ototoxicity characterized by an irreversible hearing loss, as measured by behavioural or electrophysiological methods, associated with damage to outer hair cells in the cochlea of the exposed animals. To broaden the range of aromatic solvents studied concerning their potential ototoxicity and to compare their ototoxicity quantitatively, 21 aromatic solvents were administered orally by gastric intubation to Sprague-Dawley rats for 5 days/week for a 2-week period. The dose used was 8.47 mmol kg(-1) body weight day(-1). The possible ototoxicity of the aromatic solvents was evaluated by morphological investigation of the cochlea. Whole-mount surface preparations of the organ of Corti were made to quantify the number of missing hair cells (cytocochleogram). Among the 21 solvents studied, eight (toluene, p-xylene, ethylbenzene, n-propylbenzene, styrene, alpha-methylstyrene, trans-beta-methylstyrene, and allylbenzene) caused histological lesions of the organ of Corti. They differed widely in their potency. The least ototoxic solvents caused outer hair cell (OHC) loss in the middle turn of the organ of Corti. The OHC loss was slight in the first row, and greater in the second and third rows. The most ototoxic solvents caused high losses in the three rows of the outer hair cells along the entire length of the basilar membrane. There were also occasional inner hair cell (ICH) losses in the most affected animals. Although no measurements were made of the chemical concentrations reached in the blood or the brain, tentative ranking of an increasing ototoxicity of the eight aromatic solvents could be proposed on the basis of the histological losses observed-alpha-methylstyrene<trans-beta-methylstyrene=toluene< or =p-xylene<n-propylbenzene<styrene=ethylbenzene<allylbenzene. There was no relationship between the degree of ototoxicity and the lipophilic properties of the ototoxic agents as expressed by the octanol/water partition coefficients. However, it seemed that some structural constraint was essential to induce ototoxicity. It seems there must be a single side-chain on the aromatic ring for ototoxicity, except with p-xylene. The other aromatic solvents with two side-chains were not ototoxic. When the saturated side-chain was branched (isopropylbenzene, isobutylbenzene, sec-butylbenzene, tert-butylbenzene), no ototoxicity was observed. The ototoxic potency increased when the length of the saturated side-chain extended from one carbon atom to two carbon atoms. Beyond that point, the ototoxic effect decreased with n-propylbenzene and disappeared with n-butylbenzene. Moreover, unsaturation of the side-chain of allylbenzene increased the ototoxicity of n-propylbenzene substantially. Branching of the unsaturated chain (alpha-methylstyrene and trans-beta-methylstyrene) decreased the ototoxicity of styrene.

Antioxidant status and hearing function in noise-exposed workers

The cellular antioxidant system appears to protect cochlear hair cells from oxidative stress due to noise and aging. The role of individual metabolic variables remains poorly understood, however. We examined the role of a number of metabolic factors on human cochlear function in noise-exposed individuals. In 58 factory workers we measured audiometry and distortion product otoacoustic emissions prior to a workshift. Simultaneously we measured levels of vitamin E, vitamin C, and polymorphism status for two metabolic genes related to glutathione S-transferase function (GSTM1 and GSTT1). Age and total noise exposure were predictive of hearing status. Vitamin E levels were negatively correlated with hearing function, and this effect was partly explained by an increase in vitamin E levels with age. No effect was found for vitamin C. Individuals possessing the GSTM1 gene had significantly better high frequency otoacoustic emissions compared to GSTM1 null individuals. The protective effect of GSTM1 was present even after adjusting for age, race, sex, and years of noise exposure. GSTT1 did not exhibit a similarly protective effect. While the cross-sectional nature of the study precludes drawing conclusions about causation, these data suggest that GSTM1, an antioxidant enzyme which is found in the mammalian cochlea, may play a protective role in humans against hair cell damage due to noise or aging.

Aminoglycoside ototoxicity in adult CBA, C57BL and BALB mice and the Sprague-Dawley rat

The availability of genetic information, transgenic and knock-out animals make the mouse a primary model in biomedical research. Aminoglycoside ototoxicity, however, has rarely been studied in mature mice because they are considered highly resistant to the drugs. This study presents models for kanamycin ototoxicity in adult CBA/J, C57BL/6 and BALB/c mouse strains and a comparison to Sprague-Dawley rats. Five-week-old mice were injected subcutaneously twice daily with 400-900 mg kanamycin base/kg body weight for 15 days. Kanamycin induced dose-dependent auditory threshold shifts of up to 70 dB at 24 kHz as measured by auditory brain stem-evoked responses. Vestibular function was also affected in all strains. The functional deficits were accompanied by hair cell loss in both cochlear and vestibular neurosensory epithelia. Concomitant administration of the antioxidant 2,3-dihydroxybenzoate significantly attenuated the kanamycin-induced threshold shifts. In adult male Sprague-Dawley rats, doses of 1 x 500 mg or 2 x 300 mg kanamycin base/kg body weight/day x 14 days induced threshold shifts of approximately 50 dB at 20 kHz. These were accompanied by loss of outer hair cells. The order of susceptibility, BALB>CBA>C57, was not due to differences in the pharmacokinetics of kanamycin. It also did not correlate with the presence of Ahl/Ahl2 genes which predispose C57 and BALB strains, respectively, to accelerated age-related hearing loss. Pigmentation, however, paralleled this rank order suggesting an influence of melanin on cochlear antioxidant status.

Susceptibility to noise exposure during postnatal development in rats

Susceptibility to noise-induced hearing loss was studied during maturation in 20 female pigmented rats (strain Long-Evans). Young rats, 3, 4, 5 and 6-7 weeks old, were exposed for 1 h to a broad-band noise with an intensity of 120 dB SPL. The thresholds and amplitudes of middle latency responses (MLR) recorded from electrodes implanted on the surface of the auditory cortex were analyzed before and after noise exposure. The results were compared with data from our previous study, in which the effects of broad-band noise exposure on MLR were investigated in adult rats [Syka, J. and Rybalko, N. (2000) Hear. Res. 139, 59-68]. The hearing thresholds of 3-7 week old rats before noise exposure were within the normal adult range. Noise exposure in young rats produced an adult-like pattern with an elevation of hearing thresholds. One-two weeks post-exposure a recovery of MLR thresholds was observed, though full recovery only occurred in the low frequency range. Recovery of hearing thresholds in the high frequency range depended on the age of the animal at the time of exposure. In all animals aged less that 6-7 weeks, exposure resulted in a permanent threshold shift in the range of 4-32 kHz. The mean values of permanent threshold shifts at 16 kHz (the frequency of maximal hearing loss) were 53.0+/-4.5, 47.6+/-9.6, 37.5+/-7.5 and 27+/-10 dB for rats exposed at 3, 4, 5 and 6-7 weeks of age, respectively. Similar to adult rats, young rats exposed to noise exhibited an enhancement of MLR amplitudes. This amplitude enhancement was more pronounced in the high frequency range. In several rats exposed at 3-5 weeks of age, the recovery period to normal amplitudes was substantially prolonged and lasted 4-8 weeks in comparison with 1-2 weeks in adult rats. These results demonstrate a greater susceptibility to noise exposure in rats during the first 5 postnatal weeks.

Cryoembedding and sectioning of cochleas for immunocytochemistry and in situ hybridization

Current emphasis on biochemical and molecular aspects of cochlear anatomy underscores the necessity for high quality cryostat sections of the inner ear. The large volume of fluid space within the cochlea makes cryoembedding and sectioning of the organ more problematic than that of other, more homogeneous tissues. Our method for cryoembedding of cochleas for immunocytochemistry and in situ hybridization uses slow infiltration with increasing concentrations of sucrose followed by degassed embedding medium before final orientation and freezing. This method permits high quality cryosections to be cut which preserve overall structure and cellular resolution.

Vulnerability to noise-induced hearing loss in 'middle-aged' and young adult mice: a dose-response approach in CBA, C57BL, and BALB inbred strains

Vulnerability of the cochlea to noise-induced permanent threshold shifts (NIPTS) was examined in young adult (1-2 months) and 'middle-aged' (5-7 months) CBA/CaJ, C57BL/6J, and BALB/cJ inbred mice. For each age and strain, a dose-response paradigm was applied, whereby groups of up to 12 animals were exposed to intense broadband noise (110 dB SPL) for varying durations. Exposure durations reliably associated with <10% and >90% probability of a criterion amount of NIPTS (determined 2 weeks post-exposure) were identified, and the minimum NIPTS exposure and the slope of the dose-response relation were then derived by numerical modeling. For all three strains, young adult mice were more susceptible to NIPTS than older adults; That is, a shorter exposure was able to cause NIPTS in the younger mice. Strain comparisons revealed that C57 mice were more susceptible than CBAs in the older age group only. At both ages examined, however, BALB mice were most susceptible to NIPTS. When animals with a similar amount of NIPTS were compared, outer hair cell loss in the cochlear base was more widespread in the younger animals. BALB mice appear particularly susceptible to noise-induced outer hair cell loss throughout life. Our data suggest that the mechanism or site of noise injury differs between young adults and older adults, and may depend on genetic background. The finding that both BALB and C57 mice, which show pronounced age-related hearing loss, are also especially vulnerable to noise supports the notion that genes associated with age-related hearing loss often act by rendering the cochlea susceptible to insults.