Age-related loss of spiral ganglion neurons

Anti-apoptotic treatment in mouse models of age-related hearing loss

Age-related hearing loss (AHL), or presbycusis, is the most common neurodegenerative disorder and top communication deficit of the aged population. Genetic predisposition is one of the major factors in the development of AHL. Generally, AHL is associated with an age-dependent loss of sensory hair cells, spiral ganglion neurons and stria vascularis cells in the inner ear. Although the mechanisms leading to genetic hearing loss are not completely understood, caspase-family proteases function as important signals in the inner ear pathology. It is now accepted that mouse models are the best tools to study the mechanism of genetic hearing loss or AHL. Here, we provide a brief review of recent studies on hearing improvement in mouse models of AHL by anti-apoptotic treatment.

Differential effects of Cdh23(753A) on auditory and vestibular functional aging in C57BL/6J mice

The C57BL/6J (B6) mouse strain carries a cadherin 23 mutation (Cdh23(753A), also known as Ahl), which affects inner ear structures and results in age-related hearing loss. The B6.CAST strain harbors the wild type Cdh23 gene, and hence, the influence of Ahl is absent. The purpose of the present study was to characterize the effect of age and gender on gravity receptor function in B6 and B6.CAST strains and to compare functional aging between auditory and vestibular modalities. Auditory sensitivity declined at significantly faster rates than gravity receptor sensitivity for both strains. Indeed, vestibular functional aging was minimal for both strains. The comparatively smaller loss of macular versus cochlear sensitivity in both the B6 and B6.CAST strains suggests that the contribution of Ahl to the aging of the vestibular system is minimal, and thus very different than its influence on aging of the auditory system. Alternatively, there exist unidentified genes or gene modifiers that serve to slow the degeneration of gravity receptor structures and maintain gravity receptor sensitivity into advanced age.

Factors Associated With Age-related Hearing Impairment: A Retrospective Cohort Study

Age-related hearing impairment (ARHI) is a complex degenerative disease in the elderly. As multiple factors interact during the development of ARHI, it is important to elucidate the major influencing factors to understand and prevent ARHI. We aimed to identify risk factors associated with the development of ARHI with a retrospective cohort from 2001 to 2010. The records of the adult subjects over 40 years of age who consecutively underwent a comprehensive health checkup including pure-tone audiometry at the Health Promotion Center were reviewed. During this period, 1560 subjects who underwent pure-tone audiometry more than twice, had no other otologic diseases, and were followed-up more than 2 years were included. A pure-tone average (PTA: 0.5, 1, 2, 4 kHz) was calculated. Development of ARHI was defined as a PTA at follow-up more than 10 dB greater than the baseline PTA. Times to the first development of ARHI were investigated. Overall, 12.7% of subjects developed ARHI within the first 4 years. High blood ionized calcium (hazard ratio [HR] 0.084), albumin (HR 0.239), systolic blood pressure (HR 0.577), thyroid hormone (T3) (HR 0.593), and alpha fetoprotein levels (HR 0.883) were associated with decreased hazard for the development of ARHI. In contrast, high blood high-density lipoprotein (HR 2.105), uric acid (HR 1.684), total protein (HR 1.423), and total bilirubin levels (HR 1.220) were potential risk factors for the development of ARHI. Development of ARHI is common among the aged population, and a variety of factors may interact during this process. The results of this study can be used for counseling of adults at high-risk of developing ARHI with regard to regular audiological check-up.

Age-related changes of auditory brainstem responses in nonhuman primates

Nonhuman primates, compared with humans and rodents, have historically been far less used for studies of age-related hearing loss, primarily because of their long life span and high cost of maintenance. Strong similarities in genetics, anatomy, and neurophysiology of the auditory nervous system between humans and monkeys, however, could provide fruitful opportunities to enhance our understanding of hearing loss. The present study used a common, noninvasive technique for testing hearing sensitivity in humans, the auditory brainstem response (ABR), to assess the hearing of 48 rhesus macaques from 6 to 35 yr of age to clicks and tone stimuli between 0.5 and 16.0 kHz. Old monkeys, particularly those above 21.5 yr of age, had missing ABR waveforms at high frequencies. Regression analyses revealed that ABR threshold increased as a function of age at peaks II and IV simultaneously. In the suprathreshold hearing condition (70 dB peak sound pressure level), ABR-based audiograms similarly varied as a function of age such that old monkeys had smaller peak amplitudes and delayed latencies at low, middle, and high frequencies. Peripheral hearing differences remained a major influence associated with age-related changes in audiometric functions of old monkeys at a comparable sensation level across animals. The present findings suggest that hearing loss occurs in old monkeys across a wide range of frequencies and that these deficits increase in severity with age. Parallel to prior studies in monkeys, we found weak effects of sex on hearing, and future investigations are necessary to clarify its role in age-related hearing loss.

Age-related changes in auditory nerve-inner hair cell connections, hair cell numbers, auditory brain stem response and gap detection in UM-HET4 mice

This study compared the timing of appearance of three components of age-related hearing loss that determine the pattern and severity of presbycusis: the functional and structural pathologies of sensory cells and neurons and changes in gap detection (GD), the latter as an indicator of auditory temporal processing. Using UM-HET4 mice, genetically heterogeneous mice derived from four inbred strains, we studied the integrity of inner and outer hair cells by position along the cochlear spiral, inner hair cell-auditory nerve connections, spiral ganglion neurons (SGN), and determined auditory thresholds, as well as pre-pulse and gap inhibition of the acoustic startle reflex (ASR). Comparisons were made between mice of 5-7, 22-24 and 27-29 months of age. There was individual variability among mice in the onset and extent of age-related auditory pathology. At 22-24 months of age a moderate to large loss of outer hair cells was restricted to the apical third of the cochlea and threshold shifts in the auditory brain stem response were minimal. There was also a large and significant loss of inner hair cell-auditory nerve connections and a significant reduction in GD. The expression of Ntf3 in the cochlea was significantly reduced. At 27-29 months of age there was no further change in the mean number of synaptic connections per inner hair cell or in GD, but a moderate to large loss of outer hair cells was found across all cochlear turns as well as significantly increased ABR threshold shifts at 4, 12, 24 and 48 kHz. A statistical analysis of correlations on an individual animal basis revealed that neither the hair cell loss nor the ABR threshold shifts correlated with loss of GD or with the loss of connections, consistent with independent pathological mechanisms.

The mouse as a model for age-related hearing loss - a mini-review

The most common form of sensory disability is age-related hearing loss (ARHL), also referred to as presbycusis. ARHL is a complex disorder with a mixture of genetic and environmental components, a combination that leads to a progressive decline in hearing function with increased age. In the last 15 years, there has been a vast increase in our knowledge of the genes that underlie congenital deafness and the critical components of hearing. In contrast, knowledge of the pathological processes involved in ARHL remains very limited. The mouse has proved an essential tool in the identification of early-onset deafness genes and in revealing the basic mechanisms of hearing. As focus is now turning toward elucidating the most common form of hearing loss, ARHL, the mouse will again play a fundamental role in this research. Here, we review the need for an animal model and discuss the suitability of the mouse as an ARHL model. Finally, we outline the ways in which hearing researchers are utilising the mouse in the investigation of ARHL and provide perspectives on the need for these data to be integrated with the results of human genetic studies.

Age effects on tinnitus and hearing loss in CBA/CaJ mice following sound exposure

Tinnitus is a maladaptive neuropathic condition that develops in humans and laboratory animals following auditory insult. In our previous study we demonstrated that sound exposure leads to development of behavioral evidence of tinnitus in a sample of exposed mice. However, this tinnitus mouse model did not account for long-term maladaptive plasticity or aging, factors that are commonly linked to the human tinnitus population. Therefore the same group of mice was monitored for tinnitus for 360 days post exposure. Tinnitus was assessed behaviorally by measuring gap-induced pre-pulse suppression of the acoustic startle (GPIAS). Cochlear histology was performed on both control (unexposed) and experimental mice to determine whether sound exposure caused any evident cochlear damage. We found that 360 days after exposure the vast majority of exposed mice exhibited similar gap detection deficits as detected at 84 days post exposure. These mice did not demonstrate significant loss of inner/outer hair cells or spiral ganglion neurons compared to the control sample. Lastly, we demonstrated that GPIAS deficits observed in exposed animals were unlikely exclusively caused by cochlear damage, but could be a result of central auditory maladaptive plasticity. We conclude that CBA/CaJ mice can be considered a good animal model to study the possible contribution of age effects on tinnitus development following auditory insult.

Recognition and control of the progression of age-related hearing loss

Recent breakthroughs have provided notable insights into both the pathogenesis and therapeutic strategies for age-related hearing loss (ARHL). Simultaneously, these breakthroughs enhance our knowledge about this neurodegenerative disease and raise the question of whether the disorder is preventable or even treatable. Discoveries relating to ARHL have revealed a unique link between ARHL and the underlying pathologies. Therefore, we need to better understand the pathogenesis or the mechanism of ARHL and learn how to take full advantage of various therapeutic strategies to prevent the progression of ARHL.

Role of fibroblast growth factor 8 in neurite outgrowth from spiral ganglion neurons in vitro

Many neurons degenerate after injuries resulting from overstimulation, drugs, genetic mutations, and aging. Although several growth factors and neurotrophins delay degeneration and promote regrowth of neural processes, the role of fibroblast growth factor 8 (FGF8) in mammalian spiral ganglion neurons (SGN) neurite outgrowth has not been examined. This study develops and uses SGN cell cultures suitable for experimental analysis, it investigates whether FGF8a and FGF8b isoforms affect the neurite outgrowth from SGN cultured in vitro. We found that both FGF8a and FGF8b promoted the outgrowth of neurites from cultured SGN. This response is mediated by FGF receptors and involves the activation of IκBα-mediated NFκB signaling pathway. These findings suggest that, besides its morphogenetic role during development, FGF8 may have trophic functions in the adult which are relevant to regeneration.

Metabolic changes in the auditory cortex in presbycusis demonstrated by MR spectroscopy

In humans, aging is accompanied by the deterioration of the hearing function--presbycusis. The major etiology for presbycusis is the loss of hair cells in the inner ear; less well known are changes in the central auditory system. Therefore, we used 1H magnetic resonance spectroscopy at 3T tomograph to examine metabolite levels in the auditory cortex of three groups of subjects: young healthy subjects less than 30 years old and subjects older than 65 years either with mild presbycusis corresponding to their age or with expressed presbycusis. Hearing function in all subjects was examined by pure tone audiometry (125-16,000 Hz). Significant differences were found in the concentrations of glutamate and N-acetylaspartate, with lower levels in aged subjects. Lactate was particularly increased in subjects with expressed presbycusis. Significant differences were not found in other metabolites, including GABA, between young and elderly subjects. The results demonstrate that the age-related changes of the inner ear are accompanied by a decrease in the excitatory neurotransmitter glutamate as well as a lactate increase in the auditory cortex that is more expressed in elderly subjects with large hearing threshold shifts.

Age-related hearing loss in rhesus monkeys is correlated with cochlear histopathologies

Audiometric hearing deficits are a common symptom of age-related hearing loss (ARHL), as are specific histopathological changes in the cochlea; however, very little data have been collected in non-human primates. To examine this relationship further, we collected auditory brainstem responses (ABRs) from rhesus monkeys spanning in age from 10 to 35 years old, and examined four different morphological features of their cochleae. We found significant correlations between ABR thresholds and the loss of outer hair cells and spiral ganglion cells, but not with the loss of inner hair cells or a reduced thickness of the stria vascularis. The strongest correlation with ABR thresholds was the number of different pathologies present. These findings show that while aged rhesus monkeys experience audiometric hearing deficits similar to that seen in humans, they are not correlated with a single peripheral deficit, but instead with a number of different underlying cochlear histopathologies, indicating that similar histopathologies may exist in geriatric humans as well.

Intracerebral neural stem cell transplantation improved the auditory of mice with presbycusis

Stem cell-based regenerative therapy is a potential cellular therapeutic strategy for patients with incurable brain diseases. Embryonic neural stem cells (NSCs) represent an attractive cell source in regenerative medicine strategies in the treatment of diseased brains. Here, we assess the capability of intracerebral embryonic NSCs transplantation for C57BL/6J mice with presbycusis in vivo. Morphology analyses revealed that the neuronal rate of apoptosis was lower in the aged group (10 months of age) but not in the young group (2 months of age) after NSCs transplantation, while the electrophysiological data suggest that the Auditory Brain Stem Response (ABR) threshold was significantly decreased in the aged group at 2 weeks and 3 weeks after transplantation. By contrast, there was no difference in the aged group at 4 weeks post-transplantation or in the young group at any time post-transplantation. Furthermore, immunofluorescence experiments showed that NSCs differentiated into neurons that engrafted and migrated to the brain, even to sites of lesions. Together, our results demonstrate that NSCs transplantation improve the auditory of C57BL/6J mice with presbycusis.

Silent damage of noise on cochlear afferent innervation in guinea pigs and the impact on temporal processing

Noise-exposure at levels low enough to avoid a permanent threshold shift has been found to cause a massive, delayed degeneration of spiral ganglion neurons (SGNs) in mouse cochleae. Damage to the afferent innervation was initiated by a loss of synaptic ribbons, which is largely irreversible in mice. A similar delayed loss of SGNs has been found in guinea pig cochleae, but at a reduced level, suggesting a cross-species difference in SGN sensitivity to noise. Ribbon synapse damage occurs “silently” in that it does not affect hearing thresholds as conventionally measured, and the functional consequence of this damage is not clear. In the present study, we further explored the effect of noise on cochlear afferent innervation in guinea pigs by focusing on the dynamic changes in ribbon counts over time, and resultant changes in temporal processing. It was found that (1) contrary to reports in mice, the initial loss of ribbons largely recovered within a month after the noise exposure, although a significant amount of residual damage existed; (2) while the response threshold fully recovered in a month, the temporal processing continued to be deteriorated during this period.

Intranuclear localization of apoptosis-inducing factor and endonuclease G involves in peroxynitrite-induced apoptosis of spiral ganglion neurons

OBJECTIVES

The present study was designed to determine whether or not the caspase-independent apoptotic pathway participated in the cellular death of spiral ganglion neurons (SGNs) after exposure to peroxynitrite (ONOO(-)), with particular attention given to the intranuclear translocation of mitochondrial apoptosis-inducing factor (AIF) and endonuclease G (Endo G) in this process.

METHODS

The rat SGNs were isolated and primary cultured in vitro and were exposed to ONOO(-) with pre-treatment of pan-caspase inhibitor. Morphological changes of SGNs were observed by acridine orange cytochemistry staining, and apoptosis was examined by flow cytometry. The translocation of mitochondrial AIF and Endo G was detected by immunocytochemistry and Western blot. The protein expressions of Bcl-2 family in SGNs exposed to ONOO(-) were determined by Western blot.

RESULTS

Treatment of SGNs with ONOO(-) resulted in the occurrence of caspase-independent apoptosis as evidenced by acridine orange staining and flow cytometry analysis. The immunocytochemical analysis showed that AIF and Endo G labeling were marked in neuronal nuclei, while the Western blot demonstrated the intranuclear localization of AIF and Endo G in SGNs treated with ONOO(-). Western blot analysis demonstrated that ONOO(-) increased the Bax expression while reducing Bcl-2 expression, which was not prevented by pre-treatment with caspase inhibitor.

CONCLUSION

These data indicate that ONOO(-) can trigger caspase-independent apoptosis in SGNs associated with mitochondrial AIF and Endo G intranuclear localization.

Recent advances in the study of age-related hearing loss: a mini-review

Hearing loss is a common age-associated affliction that can result from the loss of hair cells and spiral ganglion neurons (SGNs) in the cochlea. Although hair cells and SGNs are typically lost in the same cochlea, recent analysis suggests that they can occur independently, via unique mechanisms. Research has identified both environmental and genetic factors that contribute to degeneration of cochlear cells. Additionally, molecular analysis has identified multiple cell-signaling mechanisms that likely contribute to pathological changes that result in hearing deficiencies. These analyses should serve as useful primers for future work, including genomic and proteomic analysis, to elucidate the mechanisms driving cell loss in the aging cochlea. Significant progress in this field has occurred in the past decade. As our understanding of aging-induced cochlear changes continues to improve, our ability to offer medical intervention will surely benefit the growing elderly population.

Influence of cAMP and protein kinase A on neurite length from spiral ganglion neurons

Regrowth of peripheral spiral ganglion neuron (SGN) fibers is a primary objective in efforts to improve cochlear implant outcomes and to potentially reinnervate regenerated hair cells. Cyclic adenosine monophosphate (cAMP) regulates neurite growth and guidance via activation of protein kinase A (PKA) and Exchange Protein directly Activated by Cylic AMP (Epac). Here we explored the effects of cAMP signaling on SGN neurite length in vitro. We find that the cAMP analog, cpt-cAMP, exerts a biphasic effect on neurite length; increasing length at lower concentrations and reducing length at higher concentrations. This biphasic response occurs in cultures plated on laminin, fibronectin, or tenascin C suggesting that it is not substrate dependent. cpt-cAMP also reduces SGN neurite branching. The Epac-specific agonist, 8-pCPT-2'-O-Me-cAMP, does not alter SGN neurite length. Constitutively active PKA isoforms strongly inhibit SGN neurite length similar to higher levels of cAMP. Chronic membrane depolarization activates PKA in SGNs and also inhibits SGN neurite length. However, inhibition of PKA fails to rescue neurite length in depolarized cultures implying that activation of PKA is not necessary for the inhibition of SGN neurite length by chronic depolarization. Expression of constitutively active phosphatidylinositol 3-kinase, but not c-Jun N-terminal kinase, isoforms partially rescues SGN neurite length in the presence of activated PKA. Taken together, these results suggest that activation of cAMP/PKA represents a potential strategy to enhance SGN fiber elongation following deafness; however such therapies will likely require careful titration so as to promote rather than inhibit nerve fiber regeneration.

Anti-epileptic drugs delay age-related loss of spiral ganglion neurons via T-type calcium channel

Loss of spiral ganglion neurons is a major cause of age-related hearing loss (presbycusis). Despite being the third most prevalent condition afflicting elderly persons, there are no known medications to prevent presbycusis. Because calcium signaling has long been implicated in age-related neuronal death, we investigated T-type calcium channels. This family is comprised of three members (Ca(v)3.1, Ca(v)3.2, and Ca(v)3.3), based on their respective main pore-forming alpha subunits: α1G, α1H, and α1I. In the present study, we report a significant delay of age-related loss of cochlear function and preservation of spiral ganglion neurons in α1H null and heterozygous mice, clearly demonstrating an important role for Ca(v)3.2 in age-related neuronal loss. Furthermore, we show that anticonvulsant drugs from a family of T-type calcium channel blockers can significantly preserve spiral ganglion neurons during aging. To our knowledge, this is the first report of drugs capable of diminishing age-related loss of spiral ganglion neurons.

Why do hair cells and spiral ganglion neurons in the cochlea die during aging?

Age-related decline of cochlear function is mainly due to the loss of hair cells and spiral ganglion neurons (SGNs). Recent findings clearly indicate that survival of these two cell types during aging depends on genetic and environmental interactions, and this relationship is seen at the systemic, tissue, cellular, and molecular levels. At cellular and molecular levels, age-related loss of hair cells and SGNs can occur independently, suggesting distinct mechanisms for the death of each during aging. This mechanistic independence is also observed in the loss of medial olivocochlear efferent innervation and outer hair cells during aging, pointing to a universal independent cellular mechanism for age-related neuronal death in the peripheral auditory system. While several molecular signaling pathways are implicated in the age-related loss of hair cells and SGNs, studies with the ability to locally modify gene expression in these cell types are needed to address whether these signaling pathways have direct effects on hair cells and SGNs during aging. Finally, the issue of whether age-related loss of these cells occurs via typical apoptotic pathways requires further examination. As new studies in the field of aging reshape the framework for exploring these underpinnings, understanding of the loss of hair cells and SGNs associated with age and the interventions that can treat and prevent these changes will result in dramatic benefits for an aging population.

Blocking caspase-3-dependent pathway preserves hair cells from salicylate-induced apoptosis in the guinea pig cochlea

In the present study, we aim to explore whether the caspase-3-dependent pathway is involved in the apoptotic cell death that occurs in the hair cells (HCs) of guinea pig cochlea following a salicylate treatment. Guinea pigs received sodium salicylate (Na-SA), at a dose of 200 mg·kg(-1)·d(-1) i.p., as a vehicle for 5 consecutive days. In some experiments, N-benzyloxycarbonyl-Asp-Glu-Val-Asp-fluoromethylketone (zDEVD-FMK), a specific apoptosis inhibitor, was directly applied into the cochlea via the round window niche (RWN) prior to salicylate treatment for determination of caspase-3 activation. Alterations in auditory function were evaluated with auditory brainstem responses (ABR) thresholds. Caspase-3 activity was determined by measuring the proteolytic cleavage product of caspase-3 (N-terminated peptide substrate). DNA fragmentation within the nuclei was examined with a terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling (TUNEL) method. Ultrastructure variation in the target cell was assessed by electron microscopy (EM). Salicylate treatment initiated an obvious elevation in ABR thresholds with a maximum average shift of 60 dB sound pressure level (SPL), and caused significant apoptosis in both inner (IHCs) and outer (OHCs) hair cells resulted from an evident increasing in immunoreactivity to caspase-3 protease. Transmission electron microscopy (TEM) displayed chromatin condensation and nucleus margination accompanied by cell body shrinkage in the OHCs, but not in the IHCs. Scanning electron microscopy (SEM) showed breakdown, fusion, and loss in the stereociliary bundles at the apex of OHCs rather than IHCs. zDEVD-FMK pretreatment prior to salicylate injection substantially attenuated an expression of the apoptotic protease and protected HCs against apoptotic death, followed by a moderate relief in the thresholds of ABR, an alleviation in the submicroscopic structure was also identified. In particular, disorientation and insertion in the hair bundles at the apex of OHCs was exhibited though no classic apoptotic change found. The above changes were either prevented or significantly attenuated by zDEVD-FMK. These findings indicate that salicylate could damage cochlear hair cells via inducing apoptosis associated with caspase-3 activation.

The border between the central and the peripheral nervous system in the cat cochlear nerve: a light and scanning electron microscopical study

The transition between the central (CNS) and peripheral nervous system (PNS) in cranial and spinal nerve roots, referred to here as the CNS-PNS border, is of relevance to nerve root disorders and factors that affect peripheral-central regeneration. Here, this border is described in the cat cochlear nerve using light microscopical sections, and scanning electron microscopy of the CNS-PNS interfaces exposed by fracture of the nerve either prior to or following critical point drying. The CNS-PNS border represents an abrupt change in type of myelin, supporting elements, and vascularization. Because central myelin is formed by oligodendrocytes and peripheral myelin by Schwann cells, the myelinated fibers are as a rule equipped with a node of Ranvier at the border passage. The border is shallower and smoother in cat cochlear nerve than expected from other nerves, and the borderline nodes are largely in register. The loose endoneurial connective tissue of the PNS compartment is closed at the border by a compact glial membrane, the mantle zone, of the CNS compartment. The mantle zone is penetrated by the nerve fibers, but is otherwise composed of astrocytes and their interwoven processes like the external limiting membrane of the brain surface with which it is continuous. The distal surface of the mantle zone is covered by a fenestrated basal lamina. Only occasional vessels traverse the border. From an anatomical point of view, the border might be expected to be a weak point along the cochlear nerve and thus vulnerable to trauma. In mature animals, the CNS-PNS border presents a barrier to regrowth of regenerating nerve fibers and to invasion of the CNS by Schwann cells. An understanding of this region in the cochlear nerve is therefore relevant to head injuries that lead to hearing loss, to surgery on acoustic Schwannomas, and to the possibility of cochlear nerve regeneration.

Pathogenesis of presbycusis in animal models: a review

Presbycusis is the most common cause of hearing loss in aged subjects, reducing individual's communicative skills. Age related hearing loss can be defined as a progressive, bilateral, symmetrical hearing loss due to age related degeneration and it can be considered a multifactorial complex disorder, with both environmental and genetic factors contributing to the aetiology of the disease. The decline in hearing sensitivity caused by ageing is related to the damage at different levels of the auditory system (central and peripheral). Histologically, the aged cochlea shows degeneration of the stria vascularis, the sensorineural epithelium, and neurons of the central auditory pathways. The mechanisms responsible for age-associated hearing loss are still incompletely characterized. This work aims to give a broad overview of the scientific findings related to presbycusis, focusing mainly on experimental studies in animal models.

Age-related synaptic loss of the medial olivocochlear efferent innervation

Age-related functional decline of the nervous system is consistently observed, though cellular and molecular events responsible for this decline remain largely unknown. One of the most prevalent age-related functional declines is age-related hearing loss (presbycusis), a major cause of which is the loss of outer hair cells (OHCs) and spiral ganglion neurons. Previous studies have also identified an age-related functional decline in the medial olivocochlear (MOC) efferent system prior to age-related loss of OHCs. The present study evaluated the hypothesis that this functional decline of the MOC efferent system is due to age-related synaptic loss of the efferent innervation of the OHCs. To this end, we used a recently-identified transgenic mouse line in which the expression of yellow fluorescent protein (YFP), under the control of neuron-specific elements from the thy1 gene, permits the visualization of the synaptic connections between MOC efferent fibers and OHCs. In this model, there was a dramatic synaptic loss between the MOC efferent fibers and the OHCs in older mice. However, age-related loss of efferent synapses was independent of OHC status. These data demonstrate for the first time that age-related loss of efferent synapses may contribute to the functional decline of the MOC efferent system and that this synaptic loss is not necessary for age-related loss of OHCs.

No dramatic age-related loss of hair cells and spiral ganglion neurons in Bcl-2 over-expression mice or Bax null mice

Age-related decline of neuronal function is associated with age-related structural changes. In the central nervous system, age-related decline of cognitive performance is thought to be caused by synaptic loss instead of neuronal loss. However, in the cochlea, age-related loss of hair cells and spiral ganglion neurons (SGNs) is consistently observed in a variety of species, including humans. Since age-related loss of these cells is a major contributing factor to presbycusis, it is important to study possible molecular mechanisms underlying this age-related cell death. Previous studies suggested that apoptotic pathways were involved in age-related loss of hair cells and SGNs. In the present study, we examined the role of Bcl-2 gene in age-related hearing loss. In one transgenic mouse line over-expressing human Bcl-2, there were no significant differences between transgenic mice and wild type littermate controls in their hearing thresholds during aging. Histological analysis of the hair cells and SGNs showed no significant conservation of these cells in transgenic animals compared to the wild type controls during aging. These data suggest that Bcl-2 overexpression has no significant effect on age-related loss of hair cells and SGNs. We also found no delay of age-related hearing loss in mice lacking Bax gene. These findings suggest that age-related hearing loss is not through an apoptotic pathway involving key members of Bcl-2 family.