Morphology of the dorsal cochlear nucleus in C57BL/6J and CBA/J mice across the life span

Age-related changes of auditory sensitivity across the life span of CBA/CaJ mice

The inbred mouse strain CBA/CaJ is a frequently used animal model of age-related hearing loss in humans. These mice display significant hearing loss at a relatively advanced age, similar to most humans, with progressive loss of hearing as the mouse continues to age. While important descriptions of hearing loss in this mouse strain at multiple ages have previously been published, shortcomings persist in the data for hearing over the lifespan of the mouse. Therefore, we analyzed auditory brainstem response threshold data from records maintained by our research group to yield an extensive database of thresholds over nearly the entire life span of the CBA/CaJ mouse (from 79 to 1085 days). Data was collected from in-house bred mice of CBA/CaJ stock, initially from The Jackson Laboratory. Data was collected using BiosigRZ software and TDT System III hardware. Thresholds were routinely measured in conjunction with behavioral and electrophysiological experiments; only responses from baseline or experimentally naïve animals were analyzed. The resulting data set comprised 376 female mice and 441 males. At the lowest and highest frequencies (8 & 32 kHz), initial thresholds were just under 30 dB SPL and increased slowly until they were significantly different at 16-18 months compared to 1-3 months age, with the difference increasing over subsequent ages. At the middle frequencies (12 & 16 kHz), initial thresholds were just under 20 dB SPL and increased until they became different from initial at 16-18 months. At 24 kHz, initial thresholds were just above 20 dB and became different from initial at 13-16 months of age. The rate of change of thresholds with age were similar for all frequencies until about 30 months of age, when 32 kHz threshold changes lagged behind other frequencies. Generally, CBA/CaJ mice in our colony display relatively low thresholds until approximately 16 months of age, depending on frequency. After 16-18 months, thresholds become significantly worse. After approximately 20-22 months thresholds increase linearly with age.

A class of anti-inflammatory lipids decrease with aging in the central nervous system

Lipids contribute to the structure, development, and function of healthy brains. Dysregulated lipid metabolism is linked to aging and diseased brains. However, our understanding of lipid metabolism in aging brains remains limited. Here we examined the brain lipidome of mice across their lifespan using untargeted lipidomics. Co-expression network analysis highlighted a progressive decrease in 3-sulfogalactosyl diacylglycerols (SGDGs) and SGDG pathway members, including the potential degradation products lyso-SGDGs. SGDGs show an age-related decline specifically in the central nervous system and are associated with myelination. We also found that an SGDG dramatically suppresses LPS-induced gene expression and release of pro-inflammatory cytokines from macrophages and microglia by acting on the NF-κB pathway. The detection of SGDGs in human and macaque brains establishes their evolutionary conservation. This work enhances interest in SGDGs regarding their roles in aging and inflammatory diseases and highlights the complexity of the brain lipidome and potential biological functions in aging.

Impact of Aging on the Auditory System and Related Cognitive Functions: A Narrative Review

Age-related hearing loss (ARHL), presbycusis, is a chronic health condition that affects approximately one-third of the world's population. The peripheral and central hearing alterations associated with age-related hearing loss have a profound impact on perception of verbal and non-verbal auditory stimuli. The high prevalence of hearing loss in the older adults corresponds to the increased frequency of dementia in this population. Therefore, researchers have focused their attention on age-related central effects that occur independent of the peripheral hearing loss as well as central effects of peripheral hearing loss and its association with cognitive decline and dementia. Here we review the current evidence for the age-related changes of the peripheral and central auditory system and the relationship between hearing loss and pathological cognitive decline and dementia. Furthermore, there is a paucity of evidence on the relationship between ARHL and established biomarkers of Alzheimer's disease, as the most common cause of dementia. Such studies are critical to be able to consider any causal relationship between dementia and ARHL. While this narrative review will examine the pathophysiological alterations in both the peripheral and central auditory system and its clinical implications, the question remains unanswered whether hearing loss causes cognitive impairment or vice versa.

Strain-specific differences in the development of neuronal excitability in the mouse ventral nucleus of the trapezoid body

This investigation compared the development of neuronal excitability in the ventral nucleus of the trapezoid body (VNTB) between two strains of mice with differing progression rates for age-related hearing loss. In contrast to CBA/Ca (CBA) mice, the C57BL/6J (C57) strain are subject to hearing loss from a younger age and are more prone to damage from sound over-exposure. Higher firing rates in the medial olivocochlear system (MOC) are associated with protection from loud sounds and these cells are located in the VNTB. We postulated that reduced neuronal firing of the MOC in C57 mice could contribute to hearing loss in this strain by reducing efferent protection. Whole cell patch clamp was used to compare the electrical properties of VNTB neurons from the two strains initially in two age groups: before and after hearing onset at ∼ P9 and ∼P16, respectively. Prior to hearing onset VNTB neurons electrophysiological properties were identical in both strains, but started to diverge after hearing onset. One week after hearing onset VNTB neurons of C57 mice had larger amplitude action potentials but in contrast to CBA mice, their waveform failed to accelerate with increasing age, consistent with the faster inactivation of voltage-gated potassium currents in C57 VNTB neurons. The lower frequency action potential firing of C57 VNTB neurons at P16 was maintained to P28, indicating that this change was not a developmental delay. We conclude that C57 VNTB neurons fire at lower frequencies than in the CBA strain, supporting the hypothesis that reduced MOC firing could contribute to the greater hearing loss of the C57 strain.

Volumes of cochlear nucleus regions in rodents

The cochlear nucleus receives all the coded information about sound from the cochlea and is the source of auditory information for the rest of the central auditory system. As such, it is a critical auditory nucleus. The sizes of the cochlear nucleus as a whole and its three major subdivisions - anteroventral cochlear nucleus (AVCN), posteroventral cochlear nucleus (PVCN), and dorsal cochlear nucleus (DCN) - have been measured in a large number of mammals, but measurements of its subregions at a more detailed level for a variety of species have not previously been made. Size measurements are reported here for the summed granular regions, DCN layers, AVCN, PVCN, and interstitial nucleus in 15 different rodent species, as well as a lagomorph, carnivore, and small primate. This further refinement of measurements is important because the granular regions and superficial layers of the DCN appear to have some different functions than the other cochlear nucleus regions. Except for DCN layers in the mountain beaver, all regions were clearly identifiable in all the animals studied. Relative regional size differences among most of the rodents, and even the 3 non-rodents, were not large and did not show a consistent relation to their wide range of lifestyles and hearing parameters. However, the mountain beaver, and to a lesser extent the pocket gopher, two rodents that live in tunnel systems, had relative sizes of summed granular regions and DCN molecular layer distinctly larger than those of the other mammals. Among all the mammals studied, there was a high correlation between the size per body weight of summed granular regions and that of the DCN molecular layer, consistent with other evidence for a close relationship between granule cells and superficial DCN neurons.

Noise exposure modulates cochlear inner hair cell ribbon volumes, correlating with changes in auditory measures in the FVB/nJ mouse

Cochlear neuropathy resulting from unsafe noise exposure is a life altering condition that affects many people. This hearing dysfunction follows a conserved mechanism where inner hair cell synapses are lost, termed cochlear synaptopathy. Here we investigate cochlear synaptopathy in the FVB/nJ mouse strain as a prelude for the investigation of candidate genetic mutations for noise damage susceptibility. We used measurements of auditory brainstem response (ABR) and distortion product otoacoustic emissions (DPOAE) to assess hearing recovery in FVB/nJ mice exposed to two different noise levels. We also utilized confocal fluorescence microscopy in mapped whole mount cochlear tissue, in conjunction with deconvolution and three-dimensional modeling, to analyze numbers, volumes and positions of paired synaptic components. We find evidence for significant synapse reorganization in response to both synaptopathic and sub-synaptopathic noise exposures in FVB/nJ. Specifically, we find that the modulation in volume of very small synaptic ribbons correlates with the presence of reduced ABR peak one amplitudes in both levels of noise exposures. These experiments define the use of FVB/nJ mice for further genetic investigations into the mechanisms of noise damage. They further suggest that in the cochlea, neuronal-inner hair cell connections may dynamically reshape as part of the noise response.

Frequency-specific corticofugal modulation of the dorsal cochlear nucleus in mice

The primary auditory cortex (AI) modulates the sound information processing in the lemniscal subcortical nuclei, including the anteroventral cochlear nucleus (AVCN), in a frequency-specific manner. The dorsal cochlear nucleus (DCN) is a non-lemniscal subcortical nucleus but it is tonotopically organized like the AVCN. However, it remains unclear how the AI modulates the sound information processing in the DCN. This study examined the impact of focal electrical stimulation of AI on the auditory responses of the DCN neurons in mice. We found that the electrical stimulation induced significant changes in the best frequency (BF) of DCN neurons. The changes in the BFs were highly specific to the BF differences between the stimulated AI neurons and the recorded DCN neurons. The DCN BFs shifted higher when the AI BFs were higher than the DCN BFs and the DCN BFs shifted lower when the AI BFs were lower than the DCN BFs. The DCN BFs showed no change when the AI and DCN BFs were similar. Moreover, the BF shifts were linearly correlated to the BF differences. Thus, our data suggest that corticofugal modulation of the DCN is also highly specific to frequency information, similar to the corticofugal modulation of the AVCN. The frequency-specificity of corticofugal modulation does not appear limited to the lemniscal ascending pathway.

FVB/NJ mice demonstrate a youthful sensitivity to noise-induced hearing loss and provide a useful genetic model for the study of neural hearing loss

The hybrid mouse diversity panel (HMDP), a panel of 100 strains, has been employed in genome wide association studies (GWAS) to study complex traits in mice. Hearing is a complex trait and the CBA/CaJ mouse strain is a widely used model for age-related hearing loss (ARHI) and noise induced hearing loss (NIHL). The CBA/CaJ strain's youthful sensitivity to noise and limited age-related loss led us to attempt to identify additional strains segregating a similar phenotype for our panel. FVB/NJ is part of the HMDP and has been previously described as having a similar ARHI phenotype to CBA/CaJ. For these reasons, we have studied the FVB/NJ mouse for ARHI and NIHL phenotypes in hopes of incorporating its phenotype into HMDP studies. We demonstrate that FVB/NJ exhibits ARHI at an earlier age than CBA/CaJ and young FVB/NJ mice are vulnerable to NIHL up until 10 to 12 weeks. This suggests that FVB/NJ may be used as an additional genetic model for neural forms of progressive hearing loss and for the study of youthful sensitivity to noise.

The relationship between age-related hearing loss and synaptic changes in the hippocampus of C57BL/6J mice

To explore the relationship between age-related hearing loss (presbycusis) and synaptic degeneration in the hippocampal CA3 region of C57BL/6J mice, we investigated both cognitive performance and synaptic changes within the hippocampus of C57BL/6J mice from three age groups of 6-8, 24-26, and 42-44 weeks; CBA/CaJ mice served as controls. The auditory brainstem response was used as a measure of hearing threshold, and cognitive behavior was evaluated using the Morris water maze. The ultrastructure of synapses was observed with transmission electron microscopy, and the quantity and distribution of the synaptic markers synaptophysin and PSD-95 were observed with immunohistochemistry. The hearing threshold of C57BL/6J mice was significantly higher at 24-26 weeks than at 6-8 weeks, and hearing loss was profound at 42-44 weeks. This was accompanied by progressive degeneration of synapses within the auditory cortex. In contrast, the hearing threshold of CBA/CaJ mice was relatively unchanged at 24-26 weeks of age, and these mice developed only mild hearing loss at 42-44 weeks of age. Interestingly, C57BL/6J, but not CBA/CaJ mice clearly exhibited both decreased performance in the Morris water maze and degeneration of synapses within the hippocampus. We therefore conclude that age-related hearing loss is accompanied by the degeneration of synapses in the hippocampal CA3 region of C57BL/6J mice.

Auditory sensitivity and the outer hair cell system in the CBA mouse model of age-related hearing loss

Age-related hearing loss is a highly prevalent sensory disorder, from both the clinical and animal model perspectives. Understanding of the neurophysiologic, structural, and molecular biologic bases of age-related hearing loss will facilitate development of biomedical therapeutic interventions to prevent, slow, or reverse its progression. Thus, increased understanding of relationships between aging of the cochlear (auditory portion of the inner ear) hair cell system and decline in overall hearing ability is necessary. The goal of the present investigation was to test the hypothesis that there would be correlations between physiologic measures of outer hair cell function (otoacoustic emission levels) and hearing sensitivity (auditory brainstem response thresholds), starting in middle age. For the CBA mouse, a useful animal model of age-related hearing loss, it was found that correlations between these two hearing measures occurred only for high sound frequencies in middle age. However, in old age, a correlation was observed across the entire mouse range of hearing. These findings have implications for improved early detection of progression of age-related hearing loss in middle-aged mammals, including mice and humans, and distinguishing peripheral etiologies from central auditory system decline.

Selective vulnerability of adult cochlear nucleus neurons to de-afferentation by mechanical compression

It is well established that the cochlear nucleus (CN) of developing species is susceptible to loss of synaptic connections from the auditory periphery. Less information is known about how de-afferentation affects the adult auditory system. We investigated the effects of de-afferentation to the adult CN by mechanical compression. This experimental model is quantifiable and highly reproducible. Five weeks after mechanical compression to the axons of the auditory neurons, the total number of neurons in the CN was evaluated using un-biased stereological methods. A region-specific degeneration of neurons in the dorsal cochlear nucleus (DCN) and posteroventral cochlear nucleus (PVCN) by 50% was found. Degeneration of neurons in the anteroventral cochlear nucleus (AVCN) was not found. An imbalance between excitatory and inhibitory synaptic transmission after de-afferentation may have played a crucial role in the development of neuronal cell demise in the CN. The occurrence of a region-specific loss of adult CN neurons illustrates the importance of evaluating all regions of the CN to investigate the effects of de-afferentation. Thus, this experimental model may be promising to obtain not only the basic knowledge on auditory nerve/CN degeneration but also the information relevant to the application of cochlear or auditory brainstem implants.

Auditory efferent feedback system deficits precede age-related hearing loss: contralateral suppression of otoacoustic emissions in mice

The C57BL/6J mouse has been a useful model of presbycusis, as it displays an accelerated age-related peripheral hearing loss. The medial olivocochlear efferent feedback (MOC) system plays a role in suppressing cochlear outer hair cell (OHC) responses, particularly for background noise. Neurons of the MOC system are located in the superior olivary complex, particularly in the dorsomedial periolivary nucleus (DMPO) and in the ventral nucleus of the trapezoid body (VNTB). We previously discovered that the function of the MOC system declines with age prior to OHC degeneration, as measured by contralateral suppression (CS) of distortion product otoacoustic emissions (DPOAEs) in humans and CBA mice. The present study aimed to determine the time course of age changes in MOC function in C57s. DPOAE amplitudes and CS of DPOAEs were collected for C57s from 6 to 40 weeks of age. MOC responses were observed at 6 weeks but were gone at middle (15-30 kHz) and high (30-45 kHz) frequencies by 8 weeks. Quantitative stereological analyses of Nissl sections revealed smaller neurons in the DMPO and VNTB of young adult C57s compared with CBAs. These findings suggest that reduced neuron size may underlie part of the noteworthy rapid decline of the C57 efferent system. In conclusion, the C57 mouse has MOC function at 6 weeks, but it declines quickly, preceding the progression of peripheral age-related sensitivity deficits and hearing loss in this mouse strain.

Somatic mtDNA mutations cause progressive hearing loss in the mouse

Mitochondrial dysfunction has been implicated in the commonly occurring age-associated hearing loss (presbyacusis). We have previously generated mtDNA mutator mice with increased levels of somatic mtDNA point mutations causing phenotypes consistent with premature ageing. We have now utilized these mice to investigate whether elevated levels of somatic mtDNA mutations affect the auditory system. The mtDNA mutator mice develop a progressive impairment of hearing (ABR thresholds). Quantitative assessment of hair cell loss in the cochlea did not show any significant difference between the mutator and wild-type mice. The mtDNA mutator mice showed progressive apoptotic cell loss in the spiral ganglion and increased pathology with increasing age in the stria vascularis. The neurons in the cochlear nucleus showed an accelerated progressive degeneration with increasing age in the mutator mice compared to the wild-type mice. Both physiological and histological characterization thus reveals a striking resemblance between the auditory system pathology of mtDNA mutator mice and humans with presbyacusis. Somatic mtDNA mutations accumulate during normal ageing and further studies in humans are now warranted to investigate whether presbyacusis can be linked to mitochondrial dysfunction.

Age-related changes in the response properties of cartwheel cells in rat dorsal cochlear nucleus

The fusiform cell and deep layers of the dorsal cochlear nucleus (DCN) show neurotransmitter and functional age-related changes suggestive of a downregulation of inhibitory efficacy onto DCN output neurons. Inhibitory circuits implicated in these changes include vertical and D-multipolar cells. Cartwheel cells comprise a large additional population of DCN inhibitory neurons. Cartwheel cells receive excitatory inputs from granule cell parallel fibers and provide a source of glycinergic inhibitory input onto apical dendrites of DCN fusiform cells. The present study compared the response properties from young and aged units meeting cartwheel-cell criteria in anesthetized rats. Single unit recordings from aged cartwheel cells revealed significantly higher thresholds, increased spontaneous activity and significantly altered rate-level functions characterized by hyperexcitability at higher intensities. Aged cartwheel cells showed a significant reduction in off-set suppression. Collectively, these findings suggest a loss of tonic and perhaps response inhibition onto aged DCN cartwheel neurons. These changes likely reflect a compensatory downregulation of synaptic inhibition in response to a loss of excitatory drive from auditory and non-auditory excitatory inputs via granule cells. The impact of increased excitability of cartwheel cells on DCN output neurons is likely to be complex, influenced by loss of glycinergic release and/or subunit receptor changes which would only partially off-set age-related loss of inhibition onto the somata and basal dendrites of fusiform cells.

Age-related structural and functional changes in the cochlear nucleus

Presbycusis - age-related hearing loss - is a key communication disorder and chronic medical condition of our aged population. The cochlear nucleus is the major site of projections from the auditory portion of the inner ear. Relative to other levels of the peripheral and central auditory systems, relatively few studies have been conducted examining age-related changes in the cochlear nucleus. The neurophysiological investigations suggest declines in glycine-mediated inhibition, reflected in increased firing rates in cochlear nucleus neurons from old animals relative to young adults. Biochemical investigations of glycine inhibition in the cochlear nucleus are consistent with the functional aging declines of this inhibitory neurotransmitter system that affect complex sound processing. Anatomical reductions in neurons of the cochlear nucleus and their output pathways can occur due to aging changes in the brain, as well as due to age-dependent plasticity of the cochlear nucleus in response to the age-related loss of inputs from the cochlea, particularly from the basal, high-frequency regions. Novel preventative and curative biomedical interventions in the future aimed at alleviating the hearing loss that comes with age, will likely emanate from increasing our knowledge and understanding of its neural and molecular bases. To the extent that this sensory deficit resides in the central auditory system, including the cochlear nucleus, future neural therapies will be able to improve hearing in the elderly.

Age-related increases in calcium-binding protein immunoreactivity in the cochlear nucleus of hearing impaired C57BL/6J mice

Aging C57BL/6J (C57) mice (1-30 months old), were used to study calcium-binding protein immunoreactivity (parvalbumin, calbindin and calretinin) in the cochlear nucleus. A quantitative stereological method, the optical fractionator was used to determine the total number of neurons, and the total number of immunostained neurons in the posteroventral- and dorsal cochlear nuclei (PVCN and DCN). A statistically significant age-related decrease of the total number of neurons was found in the PVCN and DCN using Nissl staining. In the DCN, an age-related increase in the total number of parvalbumin-positive neurons was found, while no changes in the total number of calbindin or calretinin positive neurons were demonstrated. In the PVCN, the total number of parvalbumin, calbindin, or calretinin positive neurons remained stable with increasing age. The percentage of parvalbumin, calbindin, and calretinin positive neurons significantly increased in the DCN, and the percentage of parvalbumin and calbindin-positive neurons increased in the PVCN. These findings imply that there is a relative up-regulation of calcium-binding proteins in neurons that had not previously expressed these proteins. This plastic response in the profoundly hearing impaired C57 mouse may be a survival strategy for cochlear nucleus neurons.

Loss of Kv3.1 tonotopicity and alterations in cAMP response element-binding protein signaling in central auditory neurons of hearing impaired mice

The promoter for the kv3.1 potassium channel gene is regulated by a Ca2+-cAMP responsive element, which binds the transcription factor cAMP response element-binding protein (CREB). Kv3.1 is expressed in a tonotopic gradient within the medial nucleus of the trapezoid body (MNTB) of the auditory brainstem, where Kv3.1 levels are highest at the medial end, which corresponds to high auditory frequencies. We have compared the levels of Kv3.1, CREB, and the phosphorylated form of CREB (pCREB) in a mouse strain that maintains good hearing throughout life, CBA/J (CBA), with one that suffers early cochlear hair cell loss, C57BL/6 (BL/6). A gradient of Kv3.1 immunoreactivity in the MNTB was detected in both young (6 week) and older (8 month) CBA mice. Although no gradient of CREB was detected, pCREB-immunopositive cells were grouped together in distinct clusters along the tonotopic axis. The same pattern of Kv3.1, CREB, and pCREB localization was also found in young BL/6 mice at a time (6 weeks) when hearing is normal. In contrast, at 8 months, when hearing is impaired, the gradient of Kv3.1 was abolished. Moreover, in the older BL/6 mice there was a decrease in CREB expression along the tonotopic axis, and the pattern of pCREB labeling appeared random, with no discrete clusters of pCREB-positive cells along the tonotopic axis. Our findings are consistent with the hypothesis that ongoing activity in auditory brainstem neurons is necessary for the maintenance of Kv3.1 tonotopicity through the CREB pathway.

The aging of the middle ear in 129S6/SvEvTac and CBA/CaJ mice: measurements of umbo velocity, hearing function, and the incidence of pathology

Measurements of umbo velocity and auditory brainstem response (ABR) were made on two different strains of mice, 129S6/SvEvTac (129S6) and CBA/CaJ (CB), within three different age ranges. The velocity measurements were made with a laser Doppler vibrometer using a semiclosed sound delivery system; the frequency range of accurate velocity measurements is from 1 to 21 kHz. The visual detection threshold of the ABR was determined at selected frequencies between 2 and 32 kHz. The velocity results suggest a small but significant change in umbo velocity with age in both strains, between the youngest (1.5-3 months) and mid-aged (12-14 months) groups. There is also a clear difference in the umbo velocity in the youngest animals of the two strains, with the 129S6 having more sensitive middle-ear function than the CB. These results support the existence of a small age-related loss in middle-ear sensitivity in the mouse that was first described in the BALB/6J strain. The age-related changes in middle-ear function observed in both CB and 129S6 are much smaller than the age-related decreases in ABR. Our results also describe a statistically significant increase in the incidence of middle-ear pathology with age in the 129S6.

Impaired gap encoding in aged mouse inferior colliculus at moderate but not high stimulus levels

Age-related deterioration of auditory temporal acuity has been identified as a contributing factor in presbycusis. In the present study, the effects of aging and stimulus level on gap encoding and gap recovery functions were investigated by measuring near-field auditory evoked potentials in the inferior colliculus of eight 3 month old and eight 24 month old CBA/CaJ mice, in response to gap stimuli embedded in broadband noise (40, 60, and 80 dB SPL). Gap encoding was assessed by measuring latencies and amplitudes of peak features of the near-field response, and also with a procedure that calculated the root mean square of the response within specific time windows. The chief differences in gap encoding between young and old mice were longer gap thresholds, slower recovery functions, and longer response peak latencies for old mice at 60, but not 80 dB SPL, although the latency of the earliest measured peak remained delayed for this condition in the old compared with the young mice. These results demonstrate that age-related changes in temporal acuity may interact with stimulus level, and suggest that adequate amplification may be critically important for maintaining temporal acuity with advancing age.

Age-related synaptic changes in the anteroventral cochlear nucleus of Fischer-344 rats

Previous studies have demonstrated age-related decreases in the transmitters glycine and glutamate in the cochlear nucleus (CN) of the Fischer-344 (F344) rat, along with declining levels of binding for glycine receptors. The purpose of this study was to evaluate structural correlates to the transmitter and receptor losses that accompany aging in the anteroventral CN (AVCN). Thin sections were obtained from the middle-frequency area of the right AVCNs from five 3-month-, four 19-month-, and five 28-month-old F344 rats. Montages were constructed from electron micrographs taken of several sites in each AVCN section. The presynaptic terminals were classified by vesicle type and postsynaptic target, and their perimeters and synaptic lengths were traced using morphometry software. The calibers of all dendritic profiles were also measured, and cell counts were performed on semi-thin sections. The data were compared among the three age groups using analysis of variance followed by Tukey's Honestly Significant Difference for pairwise comparisons. There were significant age-related decreases in the size of terminals contacting small-caliber (<2 microm) dendrites. Dendrites of this size comprised the largest percentage of dendrites in the AVCN. On these targets, round and pleomorphic-vesicle terminals were reduced in volume by nearly 44% and 24%, respectively, in 28-month olds when compared to the 3-month olds. On the other hand, the densities and numbers of synaptic terminals and dendritic profiles did not differ among age groups, and no neuronal losses were evident in the older animals. Also, there were no detectable changes in synaptic area among groups. The decrease in terminal size may be related to age-associated reductions in neurotransmitter levels previously described in the F344 CN. The observations presented here contrast with those previously described in the inferior colliculus (IC), in which there were significant age-related losses of synaptic terminals and dendrites, but no change in the size of synaptic terminals. The lack of synaptic and dendritic losses suggests that the structural connectivity of the rat AVCN remains relatively intact during aging, which is interesting in light of the synaptic and dendritic changes evident in the IC, a major target of its projections.

The effects of early bilateral deafening on calretinin expression in the dorsal cochlear nucleus of aged CBA/CaJ mice

The aim of this study was to test the hypothesis that calretinin (CR) levels in the aged mouse auditory brainstem depend upon hearing ability. Old animals with good hearing, and thus higher sound-evoked activity levels, were predicted to have higher levels of CR immunoreactivity than old animals with hearing loss. CR immunoreactivity was analyzed in the deep layer (layer III) of the dorsal cochlear nucleus (DCN) in CBA/CaJ mice that were bilaterally deafened at 3 months of age with kanamycin, and then aged until 24 months. This manipulation partially mimics the lack of sound-evoked auditory activity experienced by old C57BL/6J mice, who are deaf at 24 months of age (but show residual hearing at 15 months) and have lower levels of CR immunoreactivity than old CBA mice with normal hearing [Hear. Res. 158 (2001) 131]. Cell counts revealed that the density of CR+ cells in DCN layer III of the deafened CBA mice was statistically different from old intact CBA mice raised under identical conditions. Old deafened CBAs showed a decline of 47% in the mean density of CR+ cells compared to old hearing CBAs, thus supporting the hypothesis. Interestingly, while there tended to be fewer CR+ cells in the old deaf C57s as compared to young C57s and young and old CBAs with normal hearing, the difference was not statistically significant. It is possible that the residual hearing of C57 mice at 15 months may provide sufficient auditory input to maintain CR at levels higher than CBA mice that are deafened completely at 3 months of age, and are profoundly deaf for a much longer time (21 months).

Cellular regulatory mechanisms influencing the activity of the cochlear nucleus: a review

The cochlear nucleus is the site in the auditory pathway where the primary sensory information carried by the fibres of the acoustic nerve is transmitted to the second-order neurones. According to the generally accepted view this transmission is not a simple relay process but is considered as the first stage where the decoding of the auditory information begins. This notion is based on the diverse neurone composition and highly ordered structure of the nucleus, on the complex electrophysiological properties and activity patterns of the neurones, on the activity of local and descending modulatory mechanisms and on the presence of a highly sophisticated intracellular Ca2+ homeostasis. This review puts emphasis on introducing the experimental findings supporting the above statements and on the questions which should be answered in order to gain a better understanding of the function of the cochlear nucleus.

Auditory peripheral influences on calcium binding protein immunoreactivity in the cochlear nucleus during aging in the C57BL/6J mouse

The C57BL/6J (C57) mouse was selected as a suitable model for early presbyacusis to determine if there were correlations between peripheral pathology (spiral ganglion loss, inner and outer hair cell loss) and calcium binding immunoreactivity in the cochlear nucleus during aging. The quantitative stereological method, the optical fractionator, was used for determining the total number of neurons and calcium binding immunopositive neurons (calbindin, parvalbumin and calretinin) during aging in the posteroventral- and dorsal cochlear nucleus (PVCN and DCN) in C57 mice. Comparing 30-month-old to 1-month-old C57 mice, a percent increase in parvalbumin and calbindin immunoreactivity was evident in both the PVCN and DCN. Correlations were made between peripheral pathology (spiral ganglion and inner and outer hair cell loss) and calcium binding protein expression. Significant correlations between cochlear pathology and the percentage of parvalbumin and calretinin immunoreactive neurons were demonstrated in the DCN. Moreover, significant correlations were found between cochlear pathology and parvalbumin and calbindin in the PVCN. In summary, the findings imply that degenerative changes in the auditory periphery can modulate neuronal homeostasis by increasing calcium binding proteins in the PVCN and DCN during aging. Taken together, these findings suggest a role for calcium binding proteins in protecting against age-induced calcium toxicity.

Morphology, GluR1 and GRIP-C localization differ in octopus cells of C57BL6 and B6Cast mice

The C57BL6 mouse (B6) is homozygous for the gene for age-related hearing loss (ahl/ahl) and shows normal adult-like hearing before subtle changes in hearing begin at about 30 days of age. The B6Cast mouse is congenic to B6, having the wild type allele for normal hearing from Castaneous Ei on a B6 background. It has normal hearing throughout most of its lifespan. This study characterized the morphology of octopus cell (OC) somata in the posterior-ventral cochlear nucleus and of synaptic terminals on the OC somata in 8-week-old B6 and B6Cast mice, and the immunolocalization of antibodies to GluR1 (glutamate receptor subunit 1) and GRIP-C (glutamate receptor interacting protein-C terminus). By 8 weeks of age there are significant changes in the morphology of OCs and synaptic terminals around their somata in B6 mice compared to B6Cast mice. The distribution of immunoreactivity for the proteins GluR1 and GRIP is also significantly different in B6 mice from that in B6Cast mice. The modest degenerative changes reported in some B6 outer hair cells of the basal turn at this age do not seem adequate to explain the major changes observed in most OCs at a time when physiological studies show that many measures of the animals' hearing are still near normal. The findings suggest that changes in the alpha-amino-3-hydroxy-5-methyl-4-isoxazole glutamate receptor subunits and/or their binding proteins are part of the phenotype of ahl, and may reflect a role of the glutamate receptor pathway in the mechanism of ahl.

Plastic changes in the central auditory system after hearing loss, restoration of function, and during learning

Traditionally the auditory system was considered a hard-wired sensory system; this view has been challenged in recent years in light of the plasticity of other sensory systems, particularly the visual and somatosensory systems. Practical experience in clinical audiology together with the use of prosthetic devices, such as cochlear implants, contributed significantly to the present view on the plasticity of the central auditory system, which was originally based on data obtained in animal experiments. The loss of auditory receptors, the hair cells, results in profound changes in the structure and function of the central auditory system, typically demonstrated by a reorganization of the projection maps in the auditory cortex. These plastic changes occur not only as a consequence of mechanical lesions of the cochlea or biochemical lesions of the hair cells by ototoxic drugs, but also as a consequence of the loss of hair cells in connection with aging or noise exposure. In light of the aging world population and the increasing amount of noise in the modern world, understanding the plasticity of the central auditory system has its practical consequences and urgency. In most of these situations, a common denominator of central plastic changes is a deterioration of inhibition in the subcortical auditory nuclei and the auditory cortex. In addition to the processes that are elicited by decreased or lost receptor function, the function of nerve cells in the adult central auditory system may dynamically change in the process of learning. A better understanding of the plastic changes in the central auditory system after sensory deafferentation, sensory stimulation, and learning may contribute significantly to improvement in the rehabilitation of damaged or lost auditory function and consequently to improved speech processing and production.

The total number of neurons and calcium binding protein positive neurons during aging in the cochlear nucleus of CBA/CaJ mice: a quantitative study

The quantitative stereological method, the optical fractionator, was used for determining the total number of neurons and the total number of neurons immunostained with parvalbumin, calbindin-D28k (calbindin), and calretinin in the dorsal and posteroventral cochlear nucleus (DCN and PVCN) in CBA/CaJ (CBA) mice during aging (1-39 months old). CBA mice have only a modest sensorineural pathology late in life. An age-related decrease of the total number of neurons was demonstrated in the DCN (r=-0.54, P<0.03), while the total number of neurons in the PVCN did not show any significant age-related differences (r=0.16, P=0.57). In the DCN 5.5% of neurons were parvalbumin positive in the very old (30-39 months) mice, vs. 2.2% in the 1 month old mice. In the DCN 3% of the neurons were calbindin immunopositive in the 30-39 months mice compared to 1.9% in the 1 month old group. In the PVCN, 20% of the neurons in the very old mice were parvalbumin immunopositive, compared to 12% in the young mice. Calbindin did not show any significant age-related differences in the PVCN. The total number of calretinin immunopositive neurons both in the DCN and PVCN did not show any significant change with increasing age. In conclusion, the total neuronal number in the DCN and PVCN was age-related and region-specific. While the neuronal number in the DCN and PVCN was decreased or unchanged, respectively, the calcium binding protein positive neuronal number showed a graded increase during aging in a region-specific and protein-specific manner.

Age-related loss of distortion product otoacoustic emissions in four mouse strains

Changes in cochlear function in four inbred strains of mice, CBA/CaJ (CBA), C57BL/6J (C57), BALB/cByJ (BALB), and WB/ReJ (WB), previously used to study age-related hearing loss, were evaluated serially as a function of age with 2f(1)-f(2) distortion-product otoacoustic emissions (DPOAEs). DPOAE levels in response to equilevel primary tones for geometric-mean (GM) frequencies from 5.6 to 48.5 kHz were recorded systematically as DP-grams and response/growth or input/output (I/O) functions at monthly intervals from about 2 to 15 months of age. Over the approximate 13-month measurement period, CBAs showed robust and unchanged DPOAEs for all tested frequencies, while BALBs, C57s, and WBs showed strain-specific, age-related decreases in DPOAEs that progressed systematically from the high to low frequencies. Specifically, for the youngest WBs at 2 months of age, no DPOAEs were recordable for GM frequencies > or = 32 kHz, while C57s and BALBs reached the identical stage of cochlear dysfunction by 5 and 8 months, respectively. The differential decline in DPOAE activity shown for WB, C57, and BALB mice supports the notion that they represent unique animal models of age-related changes in cochlear function. In contrast, the unchanging DPOAEs for CBAs over the same time period indicate that this strain makes an effective control for normal cochlear function in the mouse, at least, up to 15 months of age.

Preservation of synapses on principal cells of the central nucleus of the inferior colliculus with aging in the CBA mouse

The light and electron microscopic features of principal neurons of the central nucleus of the inferior colliculus were quantitated in the CBA mouse. Three age groups of mice were examined, including young (3 months), middle-aged (8 months) and old (25 months). No changes were noted in the size of the principal neurons over the age range examined. At the ultrastructural level, synapses on the somata of the principal neurons showed no change in the number or type of synapses, the length of synaptic apposition nor the size of synaptic terminal area. These results are in contrast with the moderately severe synapse loss which we previously reported in the C57BL/6 mouse strain, a strain which has a genetic deficit producing progressive sensorineural hearing loss starting in young adulthood (Kazee et al., 1995). In contrast, hearing is quite well-preserved across the lifespan in the CBA mouse strain, making this a useful animal to study the intrinsic effects of aging in the auditory system versus the effects of sensorineural hearing loss. The preservation of synapses on principal neurons in this strain suggests that synaptic loss is not an inevitable event in aging, but may be related to the preservation of peripheral auditory function and input to the neurons.

Inputs to a physiologically characterized region of the inferior colliculus of the young adult CBA mouse

Presbycusis is a sensory perceptual disorder involving loss of high-pitch hearing and reduced ability to process biologically relevant acoustic signals in noisy environments. The present investigation is part of an ongoing series of studies aimed at discerning the neural bases of presbycusis. The purpose of the present experiment was to delineate the inputs to a functionally characterized region of the dorsomedial inferior colliculus (IC, auditory midbrain) in young, adult CBA mice. Focal, iontophoretic injections of horseradish peroxidase were made in the 18-24 kHz region of dorsomedial IC of the CBA strain following physiological mapping experiments. Serial sections were reacted with diaminobenzidine or tetramethylbenzidine, counterstained and examined for retrogradely labeled cell bodies. Input projections were observed contralaterally from: all three divisions of cochlear nucleus; intermediate and dorsal nuclei of the lateral lemniscus (LL); and the central nucleus, external nucleus and dorsal cortex of the IC. Input projections were observed ipsilaterally from: the medial and lateral superior olivary nuclei; the superior paraolivary nucleus; the dorsolateral and anterolateral periolivary nuclei; the dorsal and ventral divisions of the ventral nucleus of LL; the dorsal and intermediate nuclei of LL; the central nucleus, external nucleus and dorsal cortex of the IC outside the injection site; and small projections from central gray and the medial geniculate body. These findings in young, adult mice with normal hearing can now serve as a baseline for similar experiments being conducted in mice of older ages and with varying degrees of hearing loss to discover neural changes that may cause age-related hearing disorders.

Age-related changes in the cochlea and cochlear nuclei of dogs

To study the underlying morphological changes of presbycusis, cochlea and cochlear nuclei from twenty three dogs, ranging in age from 3 days to 17 years, were examined histologically. Dogs used in this study were house dogs kept in an environment similar to that of humans. Four types of histological changes reported in human presbycusis, that is, loss of spiral ganglion cells, atrophy of the organ of Corti, atrophy of the stria vascularis, and thickening of the basilar membrane were observed in dogs. The changes were prominent at the base of the cochlea. Less intense changes were also observed in the apex of the cochlea. The degree of these changes appeared to progress as a function of age. All four types of changes with varied intensity were found in all dogs over 12 years old. In addition to the changes in the cochlea, cochlear nuclei changes including nerve cell loss, astrogliosis and ubiquitin deposition were found in dogs over 10 years old. Hearing dysfunction was accompanied by the morphological changes, though the degree of the hearing dysfunction did not always parallel to that of morphological changes. The morphological changes seen in the cochlea and cochlear nuclei of dogs were qualitatively and quantitatively similar to those reported in aged humans, indicating that otopathologic changes in the inner ear may be due to aging plus exposure to certain environmental ototoxic factors.

Calbindin D-28k immunoreactivity in the medial nucleus of the trapezoid body declines with age in C57BL/6, but not CBA/CaJ, mice

This study compared calbindin D-28k immunoreactivity in the medial nucleus of the trapezoid body (MNTB) in young (3-4 month old) and old (24-26 month old) CBA/CaJ mice, and young (3-4 month old), middle-aged (6.5-8.5 month old), and old (24-29 month old) C57BL/6 mice. C57BL/6 mice exhibit progressively more severe peripheral (sensorineural) hearing loss between 4 and 12 months of age, whereas CBA/CaJ mice show little change in peripheral sensitivity until very late in life. We obtained auditory brainstem response audiograms on all subject mice. Old CBA mice were selected for study whose audiograms matched those of young CBA and C57 controls. Middle-aged C57 mice showed elevated thresholds indicative of peripheral degeneration. Brain sections were reacted with anti-calbindin D-28k (CB). Staining patterns in Nissl and anti-CB material were characterized and cells were counted. We found no significant change in the number of CB+ cells or the total number of cells in the MNTB of old CBA mice compared to young controls. However, the mean number of CB+ cells decreased by 11% in middle-aged, and by 14.8% in old C57 mice. Since the decline in C57 mice was significant by 6.5-8.5 months of age, the decrease could be the consequence of a loss of input from the cochlear nucleus where cell numbers are known to decline by this age in this strain. The total number of neurons in MNTB assessed from Nissl material showed a more modest 7.1% decline with age in C57 mice, implying that the greater loss of CB immunoreactive cells with age cannot be completely attributed to a reduction in the total number of cells.

Age-related decline of auditory function in the chinchilla (Chinchilla laniger)

The aim of this study was to examine the functional consequences of aging in the chinchilla, a rodent with a relatively long life span and a range of hearing similar to that of humans. Subjects were 21 chinchillas aged 10-15 years, and 23 young controls. Thresholds were determined from auditory evoked potentials (EVPs), and outer hair cell (OHC) functioning was assessed by measuring 2f1-f2 distortion product otoacoustic emissions (DPOAEs). Six cochleas from 11-12-year-old animals were examined for hair cell loss and gross strial pathology. The results show that the chinchilla exhibits a small but significant decline of auditory sensitivity and OHC functioning between 3 and 15 years of age, with high-frequency losses exceeding and growing more rapidly than low-frequency losses. Compared to rodents with shorter life spans, the chinchilla has a rate of loss that is more similar to that of humans, which could make it a valuable model for understanding the etiology of human presbycusis.

Efferent projections of a physiologically characterized region of the inferior colliculus of the young adult CBA mouse

The present investigation is part of an ongoing series of studies aimed at discerning the neural bases of presbycusis. Presbycusis is a sensory perceptual disorder involving loss of high-pitch hearing and reduced ability to process biologically relevant acoustic signals in noisy environments. The purpose of the present experiment was to delineate the efferent projections of a functionally characterized region of the dorsomedial inferior colliculus (IC, auditory midbrain) in young, adult CBA mice. The CBA strain's progressive loss of hearing over its lifespan approximates many aspects of the mild-to-moderate hearing loss experienced by a significant number of humans suffering from presbycusis. Focal, iontophoretic injections of HRP were made in the 18-24 kHz region of dorsomedial IC of the CBA strain following physiological mapping experiments. Serial sections were reacted with a chromagen, counterstained and examined for anterogradely labeled fibers and boutons. Efferent projections were observed ipsilaterally in: medial and ventral divisions of the medial geniculate body (MGB); middle layers of the superior colliculus; central gray; and external nucleus (E), dorsal cortex (DC) and central nucleus of IC. Contralaterally, labeled fibers and boutons were seen in the IC at a location homologous to the injection site, as well as in E and DC. A small projection was noted in contralateral MGB. These findings in young, adult mice with normal hearing can now serve as a baseline for similar experiments being conducted in mice and animals of other species of older ages and with varying degrees of hearing loss.

Morphological changes in the anteroventral cochlear nucleus that accompany sensorineural hearing loss in DBA/2J and C57BL/6J mice

Morphological measurements were made on histological sections of the anteroventral cochlear nucleus (AVCN) in mice of the DBA/2J and C57BL/6J strains to determine the effects of sensorineural cochlear pathology on the number, packing density, and size of neurons and on AVCN volume. Both strains possess alleles that cause progressive cochlear pathology initially affecting the organ of Corti: in DBA mice, hearing loss is evident at 4 weeks of age and progresses rapidly; in C57 mice, hearing loss begins after 2 months of age and progresses more slowly. In both strains AVCN volume decreased, some loss of neurons occurred, and these changes paralleled the progression of peripheral hearing loss. Central changes were rapid in DBA mice, but the ultimate magnitude of the changes in 1-year-old mice did not differ between strains. Both strains differed from well-hearing CBA/J mice which exhibited no changes in the AVCN measures. The findings indicate that pathology of the organ of Corti in adult mice results in degenerative changes in the cochlear nucleus. The data also support earlier findings indicating that, if cochlear pathology does not begin prior to young adulthood, the age of onset and duration of sensorineural impairment have little effect on the ultimate magnitude of central effects.

Synaptic loss in the central nucleus of the inferior colliculus correlates with sensorineural hearing loss in the C57BL/6 mouse model of presbycusis

Between 3 and 25 months of age, light and electron microscopic features of principal neurons in the central nucleus of the inferior colliculus of the C57BL/6 mouse were quantitated. This mouse strain has a genetic defect producing progressive sensorineural hearing loss which starts during young adulthood (2 months of age) with high-frequency sounds. During the second year of life, hearing is severely impaired, progressively involving all frequencies. The hearing loss was documented in the present study by auditory brainstem recordings of the mice at various ages. The cochleas from many of the same animals showed massive loss of both inner and outer hair cells beginning at the base (high-frequency region) and progressing with age along the entire length to the apex (low-frequency region). In the inferior colliculi, there was a significant decrease in the size of principal neurons in the central nucleus. There was a dramatic decrease in the number of synapses of all morphologic types on principal neuronal somas. The percentage of somatic membrane covered by synapses decreased by 67%. A ventral (high frequency) to dorsal (low frequency) gradient of synaptic loss could not be identified within the central nucleus. These synaptic changes may be related to the equally dramatic physiologic changes which have been noted in the central nucleus of the inferior colliculus, in which response properties of neurons normally sensitive to high-frequency sounds become more sensitive to low-frequency sounds. The synaptic loss noted in this study may be due to more than the loss of primary afferent pathways. It may represent alterations of the complex synaptic circuitry related to the central deficits of presbycusis.

Age-related reduction of [3H]strychnine binding sites in the cochlear nucleus of the Fischer 344 rat

The present study used quantitative receptor autoradiography to examine the effects of aging on the binding profile of the strychnine-sensitive glycine receptor in the Fischer 344 rat. Glycine receptor binding sites were localized using [3H]strychnine in two principal subdivisions of the cochlear nucleus; the dorsal and anteroventral cochlear nucleus. These central auditory brainstem structures are known to receive extensive glycinergic inputs. In young rats, single concentrations of [3H]strychnine showed significantly higher binding levels in the dorsal cochlear nucleus than the anteroventral cochlear nucleus (+38%, P < 0.001). Little binding was detected in regions of the posteroventral cochlear nucleus, and no specific binding was apparent in the cerebellum. Saturation analysis in the dorsal cochlear nucleus revealed an affinity constant (Kd) of 16.9 nM and a maximum number of binding sites of 850 fmol/mg protein. A significant age-related decrease in [3H]strychnine (8 nM) binding was observed in the anteroventral cochlear nucleus (-37%, P = 0.003) and dorsal cochlear nucleus (-23%, P = 0.034) of 26-month-old rats compared with three-month-old rats. Saturation analysis indicated that the observed decrease in binding was due to a decrease in the total number of binding sites with no significant change in affinity. In the dorsal cochlear nucleus, the number of binding sites was reduced (-26%) in 26-month-old rats compared with three-month-old adults (P = 0.011). Kd was decreased (-22%) in 26-month-old rats when compared with young adults, but this decrease was not statistically significant (P = 0.377).(ABSTRACT TRUNCATED AT 250 WORDS)

Genetics of age-related hearing loss in mice. II. Strain differences and effects of caloric restriction on cochlear pathology and evoked response thresholds

The effects of genotype and diet on age-related hearing loss were evaluated using auditory brainstem response (ABR) thresholds and post-mortem cochlear histopathology in 5 inbred mouse strains, CBA/H-T6J (CH), DBA/2J (D2), C57BL/6J (B6), BALB/cByJ (BY) and WB/ReJ (WB), and their 10 F1 hybrid strains. The mice had been maintained since weaning on either a high-energy (HE) control diet or low-energy (LE) calorically restricted diet. ABR thresholds were obtained when the mice were 23 months old; the mice were allowed to age until they died from natural causes prior to obtaining the histological material. The severity of post-mortem cochlear pathology in mice maintained with the HE diet supports our earlier genetic model which postulated that B6, BY, and WB strains each possessed a different recessive allele causing age-related hearing loss, D2 mice possessed all 3 genes, and CH mice possessed none. The histopathology indicates that the genes act at the cochlear level. Dietary restriction resulted in increased longevity in a number of strains, but age-related changes in cochlear pathology were not ameliorated in any of these; indeed, in some strains long-lived LE mice exhibited severe cochlear degeneration. In strains for which longevity was not extended by caloric restriction, only B6 mice exhibited an ameliorative effect of the LE diet on cochlear pathology. ABRs in 23-month-olds indicated a slowing of age-related hearing loss in LE mice of 3 F1 hybrid strains.

Distribution of glial fibrillary acidic protein (GFAP) in the cochlear nucleus of adult and aged rats

The age-related change in glial fibrillary acidic protein (GFAP) immunoreactivity was analyzed in young (3 months) and old (24 months) adult rat cochlear nuclei (CN). Quantitative analyses show a significant increase with age, in the number of GFAP positive astrocytes and processes in the old adult when compared with the young adult rat. There was also a differential distribution of GFAP immunoreactivity in the young adult CN where it predominates in the granular cell region, whereas in old rats, the GFAP immunoreactivity distribution was homogeneous in all parts of the nucleus. There was no change in the total number of neurons between these two stages in any part of the nucleus except for the antero-ventral CN, where a decrease in neuronal number was observed in the aged rats. The increase in GFAP immunoreactivity was related to an increase of both GFAP positive astrocyte number and processes. The increase of GFAP positive astrocytes may be due either to an alteration of auditory nerve fibers, changing the trophic interactions with post-synaptic cells, or to intrinsic alterations of CN neurons and local circuits reflecting aging of the CN.

Morphology of the cochlear nucleus in CBA/J mice with chronic, severe sensorineural cochlear pathology induced during adulthood

The effects of chronic cochlear impairment on morphological features of the adult cochlear nucleus (CN) were assessed in CBA/J mice in which severe sensorineural damage had been induced by exposure to intense noise. Sections from various CN subdivisions, stained for Nissl substance and fibers, were quantitatively evaluated in four groups of noise-exposed mice that differed with regard to the age at noise exposure (2, 6, or 11 months), age at the time the CN was evaluated (6, 11, or 24 months), and the duration (chronicity) of sensorineural impairment (4, 5, 13, or 18 months). Like-aged, non-exposed CBA mice were used as controls, so the effects of peripheral damage and aging could be compared. Cochlear damage produced significant changes in CN subdivisions thought to receive the heaviest input from cochlear afferents (anteroventral CN, octopus cell area, dorsal CN layer III). These changes included a reduction of neuropil volume, reductions in neuron size, and increases in neuronal packing density that were complementary to reduced volume in these subdivisions. Effects on neuron number were minimal in all subdivisions. Central changes in noise-exposed mice were absent or diminished in DCN layers I and II, which receive relatively less input from primary fibers. The age at onset and chronicity of damage had little to do with the severity of central effects of cochlear damage. The effects of cochlear damage were not additive with age-related changes seen in the old controls.

Morphology of the inferior colliculus in C57BL/6J and CBA/J mice across the life span

Basic anatomical features were evaluated in the inferior colliculus (IC) of C57BL/6J and CBA/J mice across the adult life span (1.5 to 30 months of age). C57BL/6J mice exhibit progressive age-related cochlear pathology and become severely hearing-impaired during the second year of life; CBA/J mice exhibit little hearing loss as they age. Age had little effect on the size of the IC, the size of IC neurons, or the packing density of IC neurons and there was no evidence of age-related neuron loss. However, old CBA/J mice developed numerous spongiform lesions throughout the brainstem. The absence of morphological changes in the IC of hearing-impaired C57BL/6J mice supports the hypothesis that features such as the size of neurons, survival of neurons, and volume of the neuropil are not affected by chronic sensorineural pathology in central auditory nuclei (e.g., as the IC) that do not receive direct input from primary afferent fibers. The data from both strains taken together indicate that certain basic anatomical properties of the mouse IC persist in the face of aging.