Loss Of Spiral Ganglion Cells Research Articles

Combined brain-derived neurotrophic factor and neurotrophin-3 treatment is preferred over either one separately in the preservation of the auditory nerve in deafened guinea pigs.

Severe hearing loss or deafness is often caused by cochlear hair cell loss and can be mitigated by a cochlear implant (CI). CIs target the auditory nerve, consisting of spiral ganglion cells (SGCs), which degenerate gradually, following hair cell loss. In animal models, it has been established that treatment with the neurotrophins brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) reduce SGC degeneration. In this study, we aimed to investigate whether treatment with both BDNF and NT-3 (Cocktail) is superior to treatment with each neurotrophin separately regarding cell preservation and neural responsiveness to electrical stimulation. To this end, deafened guinea pigs received neurotrophic treatment in their right ear via a gelatin sponge on the perforated round window membrane, followed by cochlear implantation 4 weeks later in the same ear for electrophysiological recordings to various stimulation paradigms. Normal-hearing and deafened untreated guinea pigs were included as positive and negative controls, respectively. Substantial SGC loss occurred in all deafened animals. Each of the neurotrophic treatments led to enhanced SGC survival mainly in the basal turn of the cochlea, gradually decreasing toward the apex. The Cocktail treatment resulted in the highest SGC survival in the treated ear, followed by BDNF, with the least protection of SGCs following NT-3 treatment. Survival of the SGC’s peripheral processes (PPs) followed the same trend in response to the treatment. However, survival of SGCs and PPs in the contralateral untreated ears was also highest in the Cocktail group. Consequently, analysis of the ratio between the treated and untreated ears showed that the BDNF group, which showed low SGC survival in the untreated ear, had the highest relative SGC survival of the three neurotrophin-treated groups. Neurotrophic treatment had positive effects in part of the electrically evoked compound action-potential recording paradigms. These effects were only observed for the BDNF or Cocktail treatment. We conclude that treatment with either BDNF or a cocktail of BDNF and NT-3 is preferred to NT-3 alone. Furthermore, since the Cocktail treatment resulted in better electrophysiological responsiveness and overall higher SGC survival than BDNF alone, we are inclined to recommend the Cocktail treatment rather than BDNF alone.

Changes in the Electrically Evoked Compound Action Potential over time After Implantation and Subsequent Deafening in Guinea Pigs

The electrically evoked compound action potential (eCAP) is a direct measure of the responsiveness of the auditory nerve to electrical stimulation from a cochlear implant (CI). CIs offer a unique opportunity to study the auditory nerve’s electrophysiological behavior in individual human subjects over time. In order to understand exactly how the eCAP relates to the condition of the auditory nerve, it is crucial to compare changes in the eCAP over time in a controlled model of deafness-induced auditory nerve degeneration. In the present study, 10 normal-hearing young adult guinea pigs were implanted and deafened 4 weeks later, so that the effect of deafening could be monitored within-subject over time. Following implantation, but before deafening, most examined eCAP characteristics significantly changed, suggesting increasing excitation efficacy (e.g., higher maximum amplitude, lower threshold, shorter latency). Conversely, inter-phase gap (IPG) effects on these measures – within-subject difference measures that have been shown to correlate well with auditory nerve survival – did not vary for most eCAP characteristics. After deafening, we observed an initial increase in excitability (steeper slope of the eCAP amplitude growth function (AGF), lower threshold, shorter latency and peak width) which typically returned to normal-hearing levels within a week, after which a slower process, probably reflecting spiral ganglion cell loss, took place over the remaining 6 weeks (e.g., decrease in maximum amplitude, AGF slope, peak area, and IPG effect for AGF slope; increase in IPG effect for latency). Our results suggest that gradual changes in peak width and latency reflect the rate of neural degeneration, while peak area, maximum amplitude, and AGF slope reflect neural population size, which may be valuable for clinical diagnostics.

Temporal Bone Pathology Secondary to Head Trauma-A Human Temporal Bone Study.

We hypothesize that following head trauma there is a difference in temporal bone (TB) pathology in cases with and without skull fracture. Although conductive, sensorineural, mixed hearing loss, and TB pathology following head trauma have been reported, to our knowledge, there are no studies that have compared the pathology of the TB in cases with and without skull fracture. We analyzed 34 TBs from donors who had a history of head trauma (20 with skull fracture and 14 without fracture), and 25 age-matched controls without clinical or histological evidence of otologic disorders. We documented the presence and location of TB fracture, ossicular injury, and cochlear hemorrhage and evaluated the loss of spiral ganglion cells and sensory hair cells, damage to the stria vascularis, and the presence of endolymphatic hydrops. We found a significant loss of outer hair cells in the upper basal, lower, and upper middle turns of the cochlea (p = 0.009, =0.019, =0.040, respectively), a significant loss of spiral ganglion cells (p = 0.023), and cochlear hemorrhage predominantly in the basal turns secondary to head trauma. Interestingly, these findings were significantly observed in TBs from donors with a history of head trauma without skull fracture. The greatest damage was to the cochlear basal turn. Our findings suggest that head trauma may result in tonotopic high frequency sensorineural hearing loss. TBs from donors with skull fracture have less pathologic changes than those without.

PIN1 Protects Hair Cells and Auditory HEI-OC1 Cells against Senescence by Inhibiting the PI3K/Akt/mTOR Pathway.

A growing amount of evidence has confirmed the crucial role of the prolyl isomerase PIN1 in aging and age-related diseases. However, the mechanism of PIN1 in age-related hearing loss (ARHL) remains unclear. Pathologically, ARHL is primarily due to the loss and dysfunction of hair cells (HCs) and spiral ganglion cells (SGCs) in the cochlea. Therefore, in this study, we aimed to investigate the role of PIN1 in protecting hair cells and auditory HEI-OC1 cells from senescence. Enzyme-linked immunosorbent assays, immunohistochemistry, and immunofluorescence were used to detect the PIN1 protein level in the serum of ARHL patients and C57BL/6 mice in different groups, and in the SGCs and HCs of young and aged C57BL/6 mice. In addition, a model of HEI-OC1 cell senescence induced by H2O2 was used. Adult C57BL/6 mice were treated with juglone, or juglone and NAC, for 4 weeks. Interestingly, we found that the PIN1 protein expression decreased in the serum of patients with ARHL, in senescent HEI-OC1 cells, and in the cochlea of aged mice. Moreover, under H2O2 and juglone treatment, a large amount of ROS was produced, and phosphorylation of p53 was induced. Importantly, PIN1 expression was significantly increased by treatment with the p53 inhibitor pifithrin-α. Overexpression of PIN1 reversed the increased level of p-p53 and rescued HEI-OC1 cells from senescence. Furthermore, PIN1 mediated cellular senescence by the PI3K/Akt/mTOR signaling pathway. In vivo data from C57BL/6 mice showed that treatment with juglone led to hearing loss. Taken together, these findings demonstrated that PIN1 may act as a vital modulator in hair cell and HEI-OC1 cell senescence.

Effects of estrogen on apoptosis of spiral ganglion cells in cochlea of aged C57BL/6J mice

Objective: To observe the effects of estrogen on cochlear spiral ganglia cell apoptosis in aged C57BL/6J mice, and to explore the possible mechanism of estrogen's protective effects on senile deafness. Methods: Forty C57BL/6J mice were divided into the following four groups (10 mice/group): 3 m group (3 months old group), 12 m group (12 months old sham operation group); In the 12 m OVX group (ovariectomized at 12 months), bilateral oophorectomy was performed at the age of 9 months and normal feeding was performed until the age of 12 months.The 12m OVX+E2 group (estrogen intervention group) underwent bilateral oophorectomy at 9 months of age. After the one-month washout period, mice in the other groups were treated with estrogen at the dose of 100 μg/(kg·d) by subcutaneous injection, lasting 2 months to 12 months old. Mice in the other groups were fed normally.Blood samples were collected from the tail vein at the end of the treatment in 12 m OVX+E2 group. Enzyme-linked immunosorbent assays (ELISAs) was used to determine the serum estrogen levels. Auditory brainstem response (ABR) was used to detect the changes of hearing threshold in each group.Mice were anesthetized with 2% pentobarbital sodium. Bilateral cochlea was extracted after neck amputation and paraffin-embedded sections were performed.Hematoxylin eosin (HE) staining was used to observe the morphological changes in the cochlea spiral ganglion neurons (SGN), and TUNEL staining was used to observe the apoptosis of SGN. The expression levels of Caspase-3, Bax and Bcl-2 mRNA of the apoptotic proteins in cochlear spiral ganglion were measured by real-time fluorescence quantitative PCR (QRT-PCR). Results: Compared with the 3 m group, the hearing threshold of the 12 m group was improved, the loss of spiral ganglion cells was aggravated, and the apoptosis of the cells was increased(P＜0.01). After removal of the ovaries, the hearing threshold of the mice in the 12 m OVX group was higher than that in the 12 m control group (P＜0.01), and this increased threshold was accompanied by an increased loss of spiral ganglion cells, and increased apoptosis (P＜0.01). Meanwhile, the mRNA levels of apoptotic protein Caspase-3 and Bax were increased (P＜0.01), while the mRNA level of anti-apoptotic protein Bcl-2 was decreased (P＜0.01). After exogenous estrogen was given to the 12 m OVX+E2 group, the hearing threshold was lower than that in 12 m OVX group(P＜0.01). At the same time, the apoptosis of helical ganglion cells was reduced, the mRNA levels of Caspase-3 and Bax were decreased (P＜0.01), and the Bcl-2 mRNA level was increased (P＜0.01). Conclusion: Estrogen inhibited apoptosis of cochlear spiral ganglion cells in aged C57BL/6J mice ,thus achieving a protective effect on presbycusis.

The Effect of Aging on the Electrically Evoked Compound Action Potential.

To examine the effect of aging on electrically evoked compound action potential (eCAP) growth functions and their relationship with speech recognition in noise in cochlear implant (CI) users. Aging typically leads to difficulty understanding speech in background noise. Previous research has explored cognitive and central auditory mechanisms contributing to these age-related changes. However, it is likely that the peripheral auditory system may also play a role. One challenge is separating the effects of aging on cochlear structures from the effects of aging on the auditory nerve in humans. CI users provide a unique way to address this issue, as intracochlear electrical stimulation bypasses surviving hair cells and activates the auditory nerve directly. Studies in animal models suggest that age-related loss of spiral ganglion cells could lead to shallower eCAP growth functions and/or increased eCAP thresholds and potentially negatively impact speech recognition. Ten younger and 10 older postlingually deafened, adult CI recipients participated in this study. eCAP amplitude-intensity functions were recorded from a mid-array electrode and fit using linear functions. Speech recognition in noise was assessed using the Quick Speech-in-Noise (QuickSIN) test. Older CI users had significantly shallower eCAP growth functions and higher eCAP thresholds than younger CI users. eCAP growth functions were not correlated with speech recognition in noise. Results of this study suggest that older adults may have poorer neural survival, resulting in higher eCAP thresholds and shallower eCAP growth functions. These findings expand our understanding of mechanisms underlying age-related changes in the peripheral auditory system.

Single-sided deafness after sudden hearıng loss: late effect on cochlear nerve size.

Single-sided deafness (SSD) is a condition where an individual has non-functional hearing in one ear and receives no clinical benefit from amplification in that ear, with the contralateral ear possessing normal audiometric function. Cochlear implant presents a good choice in rehabilitation of SSD. The presence of a deficient cochlear nerve (CN) has been linked to poor performance with cochlear implants. CN can be measured by imaging techniques. The objective of this study was to compare CN diameter in patients who had a history of single-sided deafness because of sudden hearing loss. Retrospective study where radiologist was blind designed. 53 SSD patients who had a history of sudden hearing loss and MRI at least 5years after the sudden hearing loss were included in this retrospective study. High-resolution 3-D constructive interference in steady state (CISS)-sequence magnetic resonance (MRI) images was review by the neurotology-experienced blind radiologist. Vertical, horizontal and area measurements of cochlear nerve between the deaf and the normal ear were made. There was no statistically significant difference between the normal ear and deaf ear of the subjects in terms of cochlear nerve vertical diameter, horizontal diameter and area. (respectively, p = 0.250; p = 0.183; p = 0.874) CONCLUSION: The numbers of remaining cochlear neurons and spiral ganglion cells in the implanted deaf ears are critical and evidence was not found for spiral ganglion cell loss in the sudden hearing loss deaf ear with SSD in MRI. In the light of all these findings, implantation would be a good choice for rehabilitation in postlingual SSD regardless of auditory deprivation duration.

Rate of Spiral Ganglion Cell Loss in Idiopathic Sudden Sensorineural Hearing Loss.

To calculate the spiral ganglion neural decay rate among patients diagnosed with idiopathic sudden sensorineural hearing loss (ISSNHL). Retrospective. Temporal bone histopathology bank. Subjects diagnosed during life with unrecovered unilateral ISSNHL. Spiral ganglion cell count in the bilateral reconstructed Rosenthal canal. Decay rate of spiral ganglion cells. Nine patients were enrolled. The average age of ISSNHL diagnosis and death was 52 and 63 years, respectively. The total and segmental SGCC decay shows a temporal dependency. The time lag between the ISSNHL event and death was linearly proportional to the SGCC decay. Subjects who died less than 5 years from diagnosis of ISSNHL had no more than 13% loss of their total SGCC, and no more than 14% of SGCC per Rosenthal canal segment. When a longer period passed from ISSNHL diagnosis to death (19-20 yr), a loss of 16% and 13 to 18% of total and segmental SGCC occurred, respectively. SGCC decline from ISSNHL diagnosis through life.

Ouabain Does Not Induce Selective Spiral Ganglion Cell Degeneration in Guinea Pigs.

Round window membrane (RWM) application of ouabain is known to selectively destroy type I spiral ganglion cells (SGCs) in cochleas of several rodent species, while leaving hair cells intact. This protocol has been used in rats and Mongolian gerbils, but observations in the guinea pig are conflicting. This is why we reinvestigated the effect of ouabain on the guinea pig cochlea. Ouabain solutions of different concentrations were placed, in a piece of gelfoam, upon the RWM of the right cochleas. Auditory function was assessed using acoustically evoked auditory brainstem responses (aABR). Finally, cochleas were fixed and processed for histological examination. Due to variability within treatment groups, histological data was pooled and three categories based upon general histological observations were defined: cochleas without outer hair cell (OHC) and SGC loss (Category 1), cochleas with OHC loss only (Category 2), and cochleas with OHC and SGC loss (Category 3). Animals treated with 1 mM or 10 mM ouabain showed shifts in hearing thresholds, corresponding with varying histological changes in their cochleas. Most cochleas exhibited complete outer hair cell loss in the basal and middle turns, while some had no changes, together with either moderate or near-complete loss of SGCs. Neither loss of inner hair cells nor histological changes of the stria vascularis were observed in any of the animals. Cochleas in Category 1 had normal aABRs and morphology. On average, in Category 2 OHC loss was 46.0±5.7%, SGC loss was below threshold, ABR threshold shift was 44.9±2.7 dB, and ABR wave II amplitude was decreased by 17.1±3.8 dB. In Category 3 OHC loss was 68.3±6.9%, SGC loss was 49.4±4.3%, ABR threshold shift was 39.0±2.4 dB, and ABR amplitude was decreased by 15.8±1.6 dB. Our results show that ouabain does not solely destroy type I SGCs in the guinea pig cochlea.

Is the cause of sensorineural hearing loss in patients with facial schwannomas multifactorial?

Facial nerve schwannomas (FNSs) and vestibular nerve schwannomas (VNSs) may occupy similar locations in the internal auditory canal (IAC). Risk factors for hearing loss in patients with FNS are unknown. Similar to VNSs, we hypothesize that the size and location of FNSs do not correlate with hearing loss. Retrospective cohort study and otopathology case review. A single institution retrospective review of all patients diagnosed with FNS was performed. Demographic data, tumor size and location, severity and type of hearing loss, and facial nerve function were analyzed. Otopathologic review of a case of FNS was also performed. Thirty-three patients with FNS were identified. Mean age was 44 years, and 61% (n = 21) were female. Nine patients had tumors involving the IAC facial nerve, whereas 24 had distal tumors without IAC involvement. Eleven patients (33%) had hearing loss, and 55% of patients with sensorineural hearing loss (SNHL) had tympanic segment tumors without IAC involvement. The presence of SNHL was not associated with IAC tumor location (P = 0.21). Maximal dimensions of FNSs were 3 mm to 42 mm, without correlation between size and SNHL severity (r = -0.017, P = 0.926). Otopathologic review of an untreated 5-mm IAC FNS demonstrated disproportionate spiral ganglion and hair cell loss within the ipsilateral cochlea compared to the contralateral ear. Tympanic segment FNS without IAC involvement may result in SNHL. Tumor size is not associated with the presence or severity of SNHL. Otopathologic review suggests degenerative cochlear changes in FNS. Mechanisms for SNHL in schwannomas are likely multifactorial and may be unrelated to cochlear nerve compression. 4. Laryngoscope, 127:1676-1682, 2017.

Low Iron Diet Increases Susceptibility to Noise-Induced Hearing Loss in Young Rats.

We evaluated the role of iron deficiency (ID) without anemia on hearing function and cochlear pathophysiology of young rats before and after noise exposure. We used rats at developmental stages as an animal model to induce ID without anemia by dietary iron restriction. We have established this dietary restriction model in the rat that should enable us to study the effects of iron deficiency in the absence of severe anemia on hearing and ribbon synapses. Hearing function was measured on Postnatal Day (PND) 21 after induction of ID using auditory brainstem response (ABR). Then, the young rats were exposed to loud noise on PND 21. After noise exposure, hearing function was again measured. We observed the morphology of ribbon synapses, hair cells and spiral ganglion cells (SGCs), and assessed the expression of myosin VIIa, vesicular glutamate transporter 3 and prestin in the cochlea. ID without anemia did not elevate ABR threshold shifts, but reduced ABR wave I peak amplitude of young rats. At 70, 80, and 90 dB SPL, amplitudes of wave I (3.11 ± 0.96 µV, 3.52 ± 1.31 µV, and 4.37 ± 1.08 µV, respectively) in pups from the ID group were decreased compared to the control (5.92 ± 1.67 µV, 6.53 ± 1.70 µV, and 6.90 ± 1.76 µV, respectively) (p < 0.05). Moreover, ID without anemia did not impair the morphology hair cells and SGCs, but decreased the number of ribbon synapses. Before noise exposure, the mean number of ribbon synapses per inner hair cell (IHC) was significantly lower in the ID group (8.44 ± 1.21) compared to that seen in the control (13.08 ± 1.36) (p < 0.05). In addition, the numbers of ribbon synapses per IHC of young rats in the control (ID group) were 6.61 ± 1.59, 3.07 ± 0.83, 5.85 ± 1.63 and 12.25 ± 1.97 (3.75 ± 1.45, 2.03 ± 1.08, 3.81 ± 1.70 and 4.01 ± 1.65) at 1, 4, 7 and 14 days after noise exposure, respectively. Moreover, ABR thresholds at 4 and 8 kHz in young rats from the ID group were significantly elevated at 7 and 14 days after noise exposure compared to control (p < 0.05). The average number of young rat SGCs from the ID group were significantly decreased in the basal turn of the cochlea compared to the control (p < 0.05). Therefore, ID without anemia delayed the recovery from noise-induced hearing loss and ribbon synapses damage, increased SGCs loss, and upregulated prestin after noise exposure. Thus, the cochleae in rat pups with ID without anemia were potentially susceptible to loud noise exposure, and this deficit may be attributed to the reduction of ribbon synapses and SGCs.

E2F1-CDK1 pathway activation in kanamycin-induced spiral ganglion cell apoptosis and the protective effect of CR8

Cochlear hair cell loss results in the secondary loss of spiral ganglion cells (SGCs). The death of these SGCs is due to apoptosis. The E2F1-cyclin dependent kinase 1 (CDK1) pathway is believed to represent an important mechanism of neuronal cell death. However, the role of this pathway in spiral ganglion neuronal apoptosis has not yet been reported. In this study, we deafened guinea pigs with a subcutaneous injection of kanamycin followed by an intravenous infusion of furosemide and then assayed the expression levels of cleaved caspase-3, E2F1, CDK1 and cleaved caspase-9 during the induced SGC apoptosis. Our results revealed that co-administration of kanamycin and furosemide rapidly induced hair cell loss in the guinea pigs and then resulted in a progressive loss of SGCs. Expression levels of E2F1 and CDK1 were obviously up-regulated at 1 and 3 days after deafening. Cleaved caspase-9 also increased robustly 1 or 2 weeks after the deafening procedure. The up-regulation of E2F1, CDK1 and cleaved caspase-9 was significantly attenuated by the systemic injection of CR8 (1mg/kg/day, intraperitoneally) starting at 5min after deafening. These findings indicate that the activation of the E2F1-CDK1 pathway and cell cycle re-entry contributes to the apoptosis of SGCs and that the selective inhibition of this signaling cascade may represent an attractive therapeutic strategy. CR8 has the potential to protect SGCs from apoptosis.

Effects of Erlong Zuoci decoction on the age-related hearing loss in C57BL/6J mice

Ethnopharmacological relevanceErlong Zuoci decoction (ELZCD), a typical traditional Chinese medicine (TCM) prescription, has long been clinically used in treatment of deafness and tinnitus with the syndrome of “kidney yin deficiency”. However, there are few studies to investigate its pharmacological mechanisms. Until now, there is not report about its effects on the age-related hearing loss (ARHL). Aim of studyThe present study was conducted to observe the effects of ELZCD on the ARHL in C57BL/6J mice and explore the mechanisms. Materials and methodsELZCD was fed to C57BL/6J mice from 3 months to 6 months in ELZCD group as a dose of 6g/kg/d. And the same volume of saline was fed to mice in ARHL group. 3-months-old C57BL/6J mice were used as control group. High performance liquid chromatography (HPLC) was used for the quality control of ELZCD. Auditory brainstem response (ABR) was used to assess the hearing function of mice. The morphologic changes were observed by hematoxylin eosin (HE) staining. Apoptosis was tested by terminal dexynucleotidyl transferase (TdT)-mediated dUTP nick end labeling (TUNEL) method. Mitochondrial damage was detected by transmission electron microscopy (TEM). Quantitative RT-PCR (qRT-PCR) was used to observe the mRNA expression of p53 and Bak. Fluorescence immunohistochemical technique was used to test the protein expression of p53 and Bak. ResultsThe hearing threshold of ARHL group was higher than that of control group (P<0.001) and ELZCD decreased the rise of hearing threshold levels of ARHL mice (P<0.001), which suggested ELZCD inhibited the hearing loss of ARHL mice. HE staining showed that ELZCD decreased the spiral ganglion (SG) cell damage and loss in ARHL. TUNEL test showed that the apoptotic SG cells increased in ARHL group compared to control group and decreased in ELZCD group compared to ARHL group. TEM observation showed that mitochondrial damage was obvious in SG cells of ARHL group and ELZCD inhibited the mitochondrial damage. The qRT-PCR results showed that the mRNA expression of p53 and Bak in ARHL group increased compared to that of control group (P<0.05), and ELZCD reduced the elevated mRNA expression levels of p53 and Bak (P<0.01, P<0.05). In addition, ELZCD inhibited the increased proteins expression (green fluorescence) of p53 and Bak. ConclusionThe results demonstrated that ELZCD prevented ARHL in C57BL/6J mice and p53/Bak-mediated mitochondrial apoptosis of SG cells might be involved in the mechanisms.

Hair Cell Loss, Spiral Ganglion Degeneration, and Progressive Sensorineural Hearing Loss in Mice with Targeted Deletion of Slc44a2/Ctl2.

SLC44A2 (solute carrier 44a2), also known as CTL2 (choline transporter-like protein 2), is expressed in many supporting cell types in the cochlea and is implicated in hair cell survival and antibody-induced hearing loss. In mice with the mixed C57BL/6-129 background, homozygous deletion of Slc44a2 exons 3–10 (Slc44a2Δ/Δ) resulted in high-frequency hearing loss and hair cell death. To reduce effects associated with age-related hearing loss (ARHL) in these strains, mice carrying the Slc44a2Δ allele were backcrossed to the ARHL-resistant FVB/NJ strain and evaluated after backcross seven (N7) (99 % FVB). Slc44a2Δ/Δ mice produced abnormally spliced Slc44a2 transcripts that contain a frameshift and premature stop codons. Neither full-length SLC44A2 nor a putative truncated protein could be detected in Slc44a2Δ/Δ mice, suggesting a likely null allele. Auditory brain stem responses (ABRs) of mice carrying the Slc44a2Δ allele on an FVB/NJ genetic background were tested longitudinally between the ages of 2 and 10 months. By 6 months of age, Slc44a2Δ/Δ mice exhibited hearing loss at 32 kHz, but at 12 and 24 kHz had sound thresholds similar to those of wild-type Slc44a2+/+ and heterozygous +/Slc44a2Δ mice. After 6 months of age, Slc44a2Δ/Δ mutants exhibited progressive hearing loss at all frequencies and +/Slc44a2Δ mice exhibited moderate threshold elevations at high frequency. Histologic evaluation of Slc44a2Δ/Δ mice revealed extensive hair cell and spiral ganglion cell loss, especially in the basal turn of the cochlea. We conclude that Slc44a2 function is required for long-term hair cell survival and maintenance of hearing.

Soybean β-Conglycinin Prevents Age-Related Hearing Impairment.

Obesity-related complications are associated with the development of age-related hearing impairment. β-Conglycinin (β-CG), one of the main storage proteins in soy, offers multiple health benefits, including anti-obesity and anti-atherosclerotic effects. Here, to elucidate the potential therapeutic application of β-CG, we investigated the effect of β-CG on age-related hearing impairment. Male wild-type mice (age 6 months) were randomly divided into β-CG-fed and control groups. Six months later, the body weight was significantly lower in β-CG-fed mice than in the controls. Consumption of β-CG rescued the hearing impairment observed in control mice. Cochlear blood flow also increased in β-CG-fed mice, as did the expression of eNOS in the stria vascularis (SV), which protects vasculature. β-CG consumption also ameliorated oxidative status as assessed by 4-HNE staining. In the SV, lipofuscin granules of marginal cells and vacuolar degeneration of microvascular pericytes were decreased in β-CG-fed mice, as shown by transmission electron microscopy. β-CG consumption prevented loss of spiral ganglion cells and reduced the frequencies of lipofuscin granules, nuclear invaginations, and myelin vacuolation. Our observations indicate that β-CG ameliorates age-related hearing impairment by preserving cochlear blood flow and suppressing oxidative stress.

Neuronal erythropoietin overexpression protects mice against age-related hearing loss (presbycusis)

So far, typical causes of presbycusis such as degeneration of hair cells and/or primary auditory (spiral ganglion) neurons cannot be treated. Because erythropoietin's (Epo) neuroprotective potential has been shown previously, we determined hearing thresholds of juvenile and aged mice overexpressing Epo in neuronal tissues. Behavioral audiometry revealed in contrast to 5 months of age, that 11-month-old Epo-transgenic mice had up to 35 dB lower hearing thresholds between 1.4 and 32 kHz, and at the highest frequencies (50–80 kHz), thresholds could be obtained in aged Epo-transgenic only but not anymore in old C57BL6 control mice. Click-evoked auditory brainstem response showed similar results. Numbers of spiral ganglion neurons in aged C57BL6 but not Epo-transgenic mice were dramatically reduced mainly in the basal turn, the location of high frequencies. In addition, there was a tendency to better preservation of inner and outer hair cells in Epo-transgenic mice. Hence, Epo's known neuroprotective action effectively suppresses the loss of spiral ganglion cells and probably also hair cells and, thus, development of presbycusis in mice.

Immediate and delayed cochlear neuropathy after noise exposure in pubescent mice.

Moderate acoustic overexposure in adult rodents is known to cause acute loss of synapses on sensory inner hair cells (IHCs) and delayed degeneration of the auditory nerve, despite the completely reversible temporary threshold shift (TTS) and morphologically intact hair cells. Our objective was to determine whether a cochlear synaptopathy followed by neuropathy occurs after noise exposure in pubescence, and to define neuropathic versus non-neuropathic noise levels for pubescent mice. While exposing 6 week old CBA/CaJ mice to 8-16 kHz bandpass noise for 2 hrs, we defined 97 dB sound pressure level (SPL) as the threshold for this particular type of neuropathic exposure associated with TTS, and 94 dB SPL as the highest non-neuropathic noise level associated with TTS. Exposure to 100 dB SPL caused permanent threshold shift although exposure of 16 week old mice to the same noise is reported to cause only TTS. Amplitude of wave I of the auditory brainstem response, which reflects the summed activity of the cochlear nerve, was complemented by synaptic ribbon counts in IHCs using confocal microscopy, and by stereological counts of peripheral axons and cell bodies of the cochlear nerve from 24 hours to 16 months post exposure. Mice exposed to neuropathic noise demonstrated immediate cochlear synaptopathy by 24 hours post exposure, and delayed neurodegeneration characterized by axonal retraction at 8 months, and spiral ganglion cell loss at 8-16 months post exposure. Although the damage was initially limited to the cochlear base, it progressed to also involve the cochlear apex by 8 months post exposure. Our data demonstrate a fine line between neuropathic and non-neuropathic noise levels associated with TTS in the pubescent cochlea.

A high-fat diet delays age-related hearing loss progression in C57BL/6J mice.

ObjectiveAge-related hearing loss (AHL), or presbycusis, is the most common sensory disorder among the elderly. We used C57BL/6J mice as an AHL model to determine a possible association between AHL and a high-fat diet (HFD).MethodsForty C57BL/6J mice were randomly assigned to a control or HFD group. Each group was divided into the following subgroups: 1-, 3-, 5- and 12-month groups (HFD, n = 5/subgroup; control, n = 5/subgroup). Nine CBA/N-slc mice were also used as a 12-month control (n = 5) or 12-month HFD (n = 4) group. The mice were fed a HFD or normal (control) diet throughout this study. Hearing function was evaluated at 1, 3, 5 and 12 months using auditory evoked brainstem responses (ABRs). Spiral ganglion cells (SGCs) were also counted.ResultsThe elevation of ABR thresholds (at 4 and 32 kHz) at 3 and 5 months was significantly suppressed in the HFD group compared with the control groups for C57BL/6J mice. After 12 months, the elevation of ABR thresholds was significantly suppressed in the HFD group at all frequencies for C57BL/6J mice. In contrast, CBA/N-slc mice displayed opposite outcomes, as ABR thresholds at all frequencies at 12 months were significantly elevated in the HFD group compared with the control group. For the C57BL/6J mice at 12 months, SGC numbers significantly decreased in all parts of the cochleae in the control group compared with the HFD groups. In contrast, for the CBA/N-slc mice, SGC numbers significantly decreased, particularly in the upper parts of the cochleae in the HFD group compared with the control groups.ConclusionsThe elevation in ABR thresholds and SGC loss associated with aging in the HFD-fed C57BL/6J mice were significantly suppressed compared with those in the normal diet-fed mice. These results suggest that HFD delays AHL progression in the C57B/6J mice.

Recovery characteristics of the electrically stimulated auditory nerve in deafened guinea pigs: Relation to neuronal status

Successful cochlear implant performance requires adequate responsiveness of the auditory nerve to prolonged pulsatile electrical stimulation. Degeneration of the auditory nerve as a result of severe hair cell loss could considerably compromise this ability. The main objective of this study was to characterize the recovery of the electrically stimulated auditory nerve, as well as to evaluate possible changes caused by deafness-induced degeneration. To this end we studied temporal responsiveness of the auditory nerve in a guinea pig model of sensorineural hearing loss. Using masker-probe and pulse train paradigms we compared electrically evoked compound action potentials (eCAPs) in normal-hearing animals with those in animals with moderate (two weeks after ototoxic treatment) and severe (six weeks after ototoxic treatment) loss of spiral ganglion cells (SGCs). Masker-probe interval and pulse train inter-pulse interval was varied from 0.3 to 16 ms. Whereas recovery assessed with masker-probe was roughly similar for normal-hearing and both groups of deafened animals, it was considerably faster for six weeks deaf animals (τ ≈ 1.2 ms) than for two weeks deaf or normal-hearing animals (τ ≈ 3–4 ms) when 100-ms pulse trains were applied. Latency increased with decreasing inter-pulse intervals, and this was more pronounced with pulse trains than with masker-probe stimulation. With high frequency pulse train stimulation eCAP amplitudes were modulated for deafened animals, meaning that amplitudes for odd pulse numbers were larger than for even pulses. The relative refractory period (τ) and the modulation depth of the eCAP amplitude for pulse trains, as well as the latency increase for both paradigms significantly correlated with quantified measures of auditory nerve degeneration (size and packing density of SGCs). In addition to these findings, separate masker-probe recovery functions for the eCAP N1 and N2 peaks displayed a robust non-monotonic or shoulder-shaped course in all animals. The time interval between the N1 and N2 correlated with neuronal refractoriness, suggesting that the N2 peak reflects a second firing of part of the SGC population. We conclude that – compared to the commonly used masker-probe recovery functions – recovery functions obtained with pulse train stimulation may provide a means to augment differences and, by doing so, to more potently discriminate between auditory nerve conditions.

Long-term administration of vasopressin can cause Ménière’s disease in mice

Conclusion: A new murine model of Ménière’s disease has been developed, based on long-term administration of vasopressin. Induction of vestibular dysfunction in the present animal model can cause additional stress, by reducing inner ear blood flow. Latanoprost, a selective agonist for the FP prostanoid receptor, may become a new remedy for Ménière’s disease. Objective: The purpose of this study was to develop a more suitable animal model, with a closer resemblance to the pathophysiological process in Ménière’s disease. Methods: Adult CBA/J or ICR mice were treated by subcutaneous injection of vasopressin for 5 days up to 8 weeks. Morphological analyses were performed of the cochlea, vestibular end organs and endolymphatic sac. The effect of latanoprost on the development of endolymphatic hydrops was also examined. Results: All experimental animals showed mild to moderate endolymphatic hydrops, increasing in severity as the vasopressin treatment was prolonged. Animals treated with vasopressin for 8 weeks showed severe endolymphatic hydrops with partial loss of outer hair cells and spiral ganglion cells. These animals also had a reversible vestibular dysfunction following intratympanic injection of epinephrine. Latanoprost inhibited the development of endolymphatic hydrops caused by vasopressin.