The functional age of hearing loss in a mouse model of presbycusis. I. Behavioral assessments

Presbycusis is a common form of hearing loss that progresses from high to low frequencies with advancing age. C57BL/6J mice experience a rapid progression of presbycusis-like hearing deficits and thus provide a convenient animal model for evaluating behavioral, physiological and anatomical correlates of the disorder. Previous studies of C57BL/6J mice have relied on short-term observations of age-matched subject groups to reconstruct a time course for auditory pathologies. Such statistical approaches are weakened by the variability of hearing thresholds in young mice and the inconsistent timing of degenerative effects in older mice. The present study was designed to resolve these ambiguities by tracking the hearing abilities of individual C57BL/6J mice from age 16 weeks until the onset of hearing loss in specific listening conditions. Testing at frequencies of 8 and 16 kHz in quiet confirmed the high-to-low frequency progression that is characteristic of presbycusis. Often the hearing loss developed in two phases, one gradual and the other abrupt. Testing in noise revealed deficits that were first manifested as threshold instability and then an increased susceptibility to masking. These changes occurred before hearing loss in quiet. CBA/CaJ mice did not show significant loss during a similar period of observation. Our findings provide a well-ordered chronology for isolating the functional consequences of multiple cochlear pathologies that arise during the time course of presbycusis. This neurobehavioral assessment is termed the functional age of hearing loss. Neuroanatomical assessments of behaviorally characterized C57BL/6J mice are presented in the companion paper [Hear. Res. 183 (2003) 29-36].     

Hereditary deafness occurring in cd/1 mice.

Different strains of mice provide a valuable research tool for studying both hereditary and acquired forms of deafness. The cd/1 strain has been found to demonstrate hereditary cochlear pathology. The characteristics of hearing loss in cd/1 mice have not previously been reported. In this investigation auditory thresholds were obtained by measuring evoked brain stem responses in subjects of three different ages: 3 weeks, 10 weeks and 6 months. The results were compared with thresholds obtained from CBA/Ca mice (which have normal hearing) and C57BL/6 mice (which are known to have a genetically determined pre-senile progressive cochlear hearing loss). A significant hearing loss was observed which progressed from high to low frequencies, and with age. Extensive degeneration was observed throughout the organ of Corti. cd/1 mice may provide a useful model for studying genetically determined deafness.     

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.     

Age-related hearing loss and the ahl locus in mice

C57BL/6 (B6) mice experience hearing loss and cochlear degeneration beginning about mid-life, whereas CAST/Ei (CAST) mice retain normal hearing until old age. A locus contributing to the hearing loss of B6 mice, named age-related hearing loss (ahl), was mapped to Chromosome 10. A homozygous, congenic strain of mice (B6.CAST-+ahl ), generated by crossing B6 (ahl/ahl) and CAST (+ahl/+ahl) mice has the same genomic material as the B6 mice except in the region of the ahl locus, which is derived from CAST. In this study, we have determined the extent of the CAST-derived region of Chromosome 10 in the congenic strain and have examined mice of all three strains for hearing loss and cochlear morphology between 9 and 25 months of age. Results for B6 mice were similar to those described previously. CAST mice showed no detectable hearing loss even at 24 months of age; however, they had a small amount of ganglion cell degeneration. B6.CAST-+ahl mice were protected from early onset hearing loss and basal turn degeneration, but older animals did show some hearing loss and ganglion cell degeneration. We conclude that loci in addition to ahl contribute to the differences in hearing loss between B6 and CAST mice. These results illustrate the complex inheritance of age-related hearing loss in mice and may have implications for the study of human presbycusis.     

Comparison of the auditory sensitivity of neurons in the cochlear nucleus and inferior colliculus of young and aging C57BL/6J and CBA/J mice

Thresholds of neurons to sounds were compared as a function of central auditory structure [ventral cochlear nucleus (VCN), dorsal cochlear nucleus (DCN), and inferior colliculus (IC)] in young and middle-aged C57BL/6J mice (multiple- and single-unit recordings) and in young and old CBA/J mice (single-unit recordings). Middle-aged C57 mice show progressive loss of sensitivity to high frequencies and noise due to cochlear pathology; CBA mice show little loss of sensitivity through most of their lifespan. Multiple-unit threshold curves (MTCs) for tones indicated that neurons in the C57 VCN suffered a greater degree of age-related loss of sensitivity than neurons in the IC (from an earlier study). Furthermore, whereas the low frequency portions of MTCs in IC neurons in high frequency tonotopic regions typically become 'sensitized' in middle-aged C57 mice (i.e., lower thresholds than young mice), such was not the case for VCN neurons. In contrast to VCN neurons, MTCs of the population of DCN neurons studied were statistically indistinguishable from those of the IC. Measurements of single-unit response areas in C57 mice corroborated the MTCs. In CBA mice, little effect of age was found in comparing single-unit response areas of young and old mice. The findings indicate that sensorineural impairment in middle-aged C57 mice is accompanied by threshold changes that are more severe in the VCN than in the IC or DCN. Because the VCN and DCN are believed to play different roles in hearing, the functions they support should, likewise, be affected to different extents by age-related hearing loss.     

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×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.     

Pattern of degeneration of the spiral ganglion cell and its processes in the C57BL/6J mouse

Although degeneration of spiral ganglion cells has been described as a histopathologic correlate of hearing loss both in animals and humans, the pattern and sequence of this degeneration remain controversial. Degeneration of hair cells and of spiral ganglion cells and their dendritic processes was evaluated in the C57BL/6J mouse, in which there is a genetically determined progressive sensorineural loss starting in the high frequencies that is similar to the pattern commonly seen in the human. Auditory function was evaluated by brainstem evoked responses, and degeneration of hair cells, ganglion cells and their dendrites was evaluated histologically at 3, 8, 12 and 18 months of age. Progressive loss of auditory sensitivity was correlated with the loss of outer and inner hair cells and spiral ganglion cells and their dendritic processes. In addition, dendritic counts were consistently lower at a distal location in the osseous spiral lamina (i.e. near the organ of Corti) than at a proximal location (i.e. near the spiral ganglion), and the difference between the number of distal dendrites and the number of proximal dendrites tended to be greater with advancing age. These observations suggest an age-related progressive retrograde degeneration of spiral ganglion cells. Thus, in degenerating cochleas, some remaining spiral ganglion cells may have no distal dendritic processes near the organ of Corti. This may have implications for successful stimulation of the cochlear neuron in cochlear implantation.     

Low-Frequency Tone Pips Elicit Exaggerated Startle Reflexes in C57BL/6J Mice with Hearing Loss

The strength of the acoustic startle reflex (ASR) as a function of age was studied in adult C57BL/6J and CBA/CaJ mice, because altered ASR levels are a potential behavioral consequence of the neural reorganization that accompanies the early-onset hearing loss of the C57BL, in contrast to the normal-hearing CBA. For C57BL mice at 14–36 weeks of age, compared with 7-week-old mice, high-frequency thresholds measured with the auditory brainstem response (ABR) were less sensitive by about 25–30 dB while the hearing loss at low frequencies was 10–15 dB, but by 60 weeks losses of 45–50 dB were present across the entire spectrum. Their ASR amplitudes for 16 kHz tone pips were highest at 7 weeks and then declined with age, but, for 4 kHz tones the ASR increased in strength at 18 weeks and beyond to levels above that of the younger mice. This hyperreactivity persisted even in 60-week-old mice. The ASR for 16 kHz stimuli was positively correlated with hearing sensitivity, but the ASR for 4 kHz stimuli was positively correlated with hearing loss for mice that were 18–36 weeks of age. Furthermore, ASR amplitudes for 4 kHz stimuli were positively correlated with the 16 kHz ASR in young C57BL mice but negatively correlated in older mice. There were no similar ASR or ABR changes in adult CBA mice through 19 weeks of age. Correlations between ASR and ABR scores were always weakly positive, and correlations between 4 kHz and 16 kHz ASR amplitudes were always strongly positive. The ASR data in older C57BL mice with hearing loss are consistent with reports describing their increased neural representation of low-frequency sounds and reinforce the value of this strain for studying the functional consequences that accompany age-related cochlear degeneration.     

Spiral Ligament Pathology: A Major Aspect of Age-Related Cochlear Degeneration in C57BL/6 Mice

Data from systematic, light microscopic examination of cochlear histopathology in an age-graded series of C57BL/6 mice (1.5-15 months) were compared with threshold elevations (measured by auditory brain stem response) to elucidate the functionally important structural changes underlying age-related hearing loss in this inbred strain. In addition to quantifying the degree and extent of hair cell and neuronal loss, all structures of the cochlear duct were qualitatively evaluated and any degenerative changes were quantified. Hair cell and neuronal loss patterns suggested two degenerative processes. In the basal half of the cochlea, inner and outer hair cell loss proceeded from base to apex with increasing age, and loss of cochlear neurons was consistent with degeneration occurring secondary to inner hair cell loss. In the apical half of the cochlea with advancing age, there was selective loss of outer hair cells which increased from the middle to the extreme apex. A similar gradient of ganglion cell loss was noted, characterized by widespread somatic aggregation and demyelination. In addition to these changes in hair cells and their innervation, there was widespread degeneration of fibrocytes in the spiral ligament, especially among the type IV cell class. The cell loss in the ligament preceded the loss of hair cells and/or neurons in both space and time suggesting that fibrocyte pathology may be a primary cause of the hearing loss and ultimate sensory cell degeneration in this mouse strain.     

Comparison of cochlear morphology and apoptosis in mouse models of presbycusis

Objectives

Morphological studies on presbycusis, or age-related hearing loss, have been performed in several different strains of mice that demonstrate hearing loss with auditory pathology. The C57BL/6 (C57) mouse is a known model of early onset presbycusis, while the CBA mouse is characterized by relatively late onset hearing loss. We performed this study to further understand how early onset hearing loss is related with the aging process of the cochlea.

Methods

We compared C57 cochlear pathology and its accompanying apoptotic processes to those in CBA mice. Hearing thresholds and outer hair cell functions have been evaluated by auditory brainstem response (ABR) recordings and distortion product otoacoustic emission (DPOAE).

Results

ABR recordings and DPOAE studies demonstrated high frequency hearing loss in C57 mice at P3mo of age. Cochlear morphologic studies of P1mo C57 and CBA mice did not show differences in the organ of Corti, spiral ganglion, or stria vascularis. However, from P3mo and onwards, a predominant early outer hair cell degeneration at the basal turn of the cochlea in C57 mice without definitive degeneration of spiral ganglion cells and stria vascularis/spiral ligament, compared with CBA mice, was observed. Additionally, apoptotic processes in the C57 mice also demonstrated an earlier progression.

Conclusion

These data suggest that the C57 mouse could be an excellent animal model for early onset ''sensory'' presbycusis in their young age until P6mo. Further studies to investigate the intrinsic or extrinsic etiologic factors that lead to the early degeneration of organ of Corti, especially in the high frequency region, in C57 mice may provide a possible pathological mechanism of early onset hearing loss.     

C57BL/6J 小鼠耳蜗血管纹Na-K-2Cl联合转运子-1的年龄相关性变化

目的观察不同周龄C57BL/6J(C57)小鼠听力及血管纹Na-K-2Cl联合转运子-1(Na-K-2Cl co-transporter-1,NKCC1)表达的情况。方法应用听性脑干反应(auditory brainstemresponse,ABR)分别检测4、8、16、32、48、64周龄组C57小鼠的听力;采用免疫组织化学染色法观察其血管纹NKCC1表达的变化。结果C57小鼠随年龄增大出现听力下降,自16周龄时ABR阈值出现显著性增高(P<0.05);血管纹NKCC1表达也出现年龄相关性减少,其灰度值自16周龄时显著增高(P<0.01)。结论C57小鼠血管纹NKCC1蛋白表达随年龄增长而减少,可能与年龄相关性听力损失具有一定相关性。     

Endocochlear potentials and compound action potential recovery: functions in the C57BL/6J mouse

The C57BL/6J mouse suffers from cochlear degeneration beginning at an early age and has been used as a model of age-related hearing loss (presbyacusis). Here, the endocochlear potential (EP) and compound action potential (CAP) responses were determined in one-, four-, 12- and 24-month-old C57BL/6J mice. CAP measures included thresholds to tone pips, input/output (I/O) functions, and recovery functions to conditioning tones. EP values among the four age groups did not differ significantly (P>0.05) in either the basal or apical turns. CAP thresholds were increased significantly by 10 to 30 dB in the four-month group compared to the one-month controls at 11.3, 16, 20, and 22.6 kHz. CAP I/O functions were shallower in the four-month group compared to controls at all frequencies. In the 12- and 24-month-old mice, CAP responses were absent, despite normal EP values in these animals. Recovery functions after conditioning tones were obtained at 8, 16, 20 and 22.6 kHz; the functions had fast and slow components at all frequencies tested in both the one- and four-month-old groups. The corresponding recovery curves were identical for both age groups, even with significant threshold shifts in the older group. The two component recovery curves provide the first physiological evidence that different spontaneous rate (SR) classes of auditory neurons exist in the C57BL/6J mouse. Moreover, the unchanged recovery functions in the older group suggest that there was no loss of activity of the low-SR fiber population with age under conditions where the EP remains stable, in contrast to the gerbil model of presbyacusis where there is a loss of low-SR fiber activity and EP does decline with age.     

Degeneration of stria vascularis in age-related hearing loss; a corrosion cast study in a mouse model

Conclusion With age, in a mouse model, degenerative changes to the capillaries of the stria vascularis are observed. These range from a narrowing of vessel lumen to complete degeneration of strial vessels. Other vascular beds in the cochlea are relatively unchanged with age. Strial capillaries at the cochlear base are significantly more affected than those in mid-apical turns.

Objectives Previous work suggests that age-related hearing loss is associated with degenerative changes to cochlear vasculature; the term strial presbyacusis is often cited. This study reports on vascular changes observed in a murine model of presbyacusis, analyzed using corrosion cast techniques.

Methods A novel corrosion cast technique was developed to compare cochlear vasculature in control mice (non-presbycusic, CD1) and old (>?6 months) C57BL/6 animals. ABR measures indicated a significant age-related threshold elevation in the C57BL/6 mice. Cochlear vascular casts were imaged using scanning electron microscopy, and vessel degeneration was quantified by measuring capillary diameters.

Results Corrosion casts of cochlear vasculature in 6–12 month old C57BL/6 mice reveal significant degeneration of stria vascularis. Other capillary beds (spiral ligament and the spiral limbus) appear unchanged. Comparison of strial capillary diameters reveals significantly more damage in basal/lower-turn regions compared with the cochlear mid-turn.     

Effects of exposing gonadectomized and intact C57BL/6J mice to a high-frequency augmented acoustic environment: Auditory brainstem response thresholds and cytocochleograms

Gonadectomized and surgically intact adult C57BL/6J (B6) mice of both sexes were exposed for 12h nightly to a high-frequency augmented acoustic environment (AAE): repetitive bursts of a half-octave noise band centered at 20 kHz, 70 dB SPL. The effects of sex, gonadectomy, and AAE treatment on genetic progressive hearing loss (exhibited by B6 mice) were evaluated by obtaining auditory brainstem response thresholds at ages 3-, 6-, and 9-months; hair cell counts (cytocochleograms) were obtained at 9 months. A sex difference in the rate of genetic progressive hearing loss in B6 mice (observed by earlier studies) was confirmed, with females exhibiting a faster rate of threshold elevations and more severe loss of hair cells at age 9 months. Gonadectomy had no consistent effects on the rate or severity of hearing loss in non-exposed mice of either sex. An unexpected finding was that the high-frequency AAE treatment caused additional ABR threshold elevations and hair cell loss. In an earlier study, the same high-frequency AAE treatment on DBA/2J mice ameliorated hearing loss. The most severe AAE-induced losses occurred in surgically intact females, suggesting a potentiating effect of ovarian hormone(s).     

Anatomical, metabolic and genetic aspects of age-related hearing loss in mice.

Because of their short lifespan and genetic homogeneity, mice can provide valuable insights into the biological basis of age-related hearing loss. In C57BL/6 mice, hair cell loss begins around 1-2 months of age and progresses rapidly along a base-to-apex gradient, whereas CBA mice show relatively little hair cell loss until late in life. This anatomical difference is reflected in dehydrogenase histochemistry, an indirect measure of aerobic energy metabolism. A small, but significant, decrease in hair cell dehydrogenase staining occurred in CBA mice between 1.5 and 18 months of age. Significantly, dehydrogenase levels in 1.5-month C57 mice were substantially lower than in CBA mice of any age. Thus, deficits in aerobic energy metabolism presage degeneration of the hair cells. The superoxide radical, O2*-, a normal byproduct of cellular metabolism, is potentially toxic and can cause cellular damage if it is not inactivated by superoxide dismutases. Cytosolic copper/zinc superoxide dismutase (SOD1) is highly expressed in the cochlea. Knockout mutant mice with a single (HET) or double deletion (KO) of the Sod1 gene coding for SOD1 showed greater age-related losses than wild-type (WT) mice. KO mice had the worst hearing, WT the best, and HETs were intermediate. KOs exhibited considerably greater hair cell loss than WT mice; however, losses in KOs were only slightly greater than in HETs. KO mice showed significantly greater loss of spiral ganglion cells and nerve fibers than WT mice. These results indicate that SOD1 and O2*- play important roles in age-related hearing loss. Intervention strategies targeting O2*- may reduce age-related hearing loss.     

不同种属实验动物耳蜗毛细胞年龄相关性缺失的不同模式

目的 展示自然衰老和耳聋相关基因遗传缺陷之间耳蜗毛细胞缺失的不同模式。方法 用不同龄的长尾猴、南美栗鼠、豚鼠、Sprague-Dawley 大鼠、CBA/CaJ 小鼠、C57BL/6J 小鼠、A/J小鼠、DBA/2J 小鼠和侏儒灰色突变纯合子 (dwg/dwg) 小鼠作为受试对象。所有测试动物的耳蜗基底膜都被制作成平坦的耳蜗基底膜铺片。沿着耳蜗基底膜的全长,基底膜上所有的内外毛细胞都被完整计数,毛细胞的计数结果被输入到耳蜗图软件并自动生成每组实验条件的平均耳蜗图。结果 在天然衰老的动物中,耳蜗毛细胞的缺失总是发生在老年阶段。与此不同的是,在耳聋相关基因缺陷的动物中,耳蜗毛细胞的缺失却是发生在青年阶段甚至幼年阶段。发生在天然老化动物的耳蜗毛细胞缺失总是呈均匀分布或从耳蜗的顶回向底回扩展。 但是,发生在具有耳聋相关基因遗传缺陷动物的耳蜗毛细胞缺失却通常表现为从耳蜗的底回向顶回扩展。结论 本实验观察结果表明,发生在天然衰老的不具备耳聋相关基因缺陷动物身上的年龄相关性耳蜗毛细胞缺失反映的是真正由衰老引起的耳蜗退化性病变,而发生在伴有耳聋相关基因遗传缺陷的年幼动物身上的年龄相关性耳蜗毛细胞缺失可能与耳聋相关基因的遗传缺陷有关。     

Presbycusis: neural degeneration and aging on the auditory receptor of C57/BL6J mice

Presbycusis is a progressive hearing impairment associated with aging, characterized by hearing loss and a degeneration of cochlear structures. In this paper we analyze the effects of aging on the auditory system of C57/BL6J mice, with electrophysiological and morphological studies. With this aim the auditory potentials of mice aging 1, 3, 6, 9, 12, 15, 18, 21 and 24 months were recorded, and then the morphology of the cochleal were analyzed. Auditory potentials revealed an increase in wave latencies, as well as a decrease in their amplitudes during aging. Morphological results showed a total Corti's organ degeneration, being replaced by a flat epithelial layer, and a total absence of hair cells.     

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.     

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.