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  Table of Contents    
Year : 2014  |  Volume : 16  |  Issue : 72  |  Page : 257-264
Renexin as a rescue regimen for noise-induced hearing loss

Department of Otorhinolaryngology-Head and Neck Surgery, College of Medicine, The Catholic University of Korea, Seoul, Korea

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Date of Web Publication10-Sep-2014

Renexin, a compound of cilostazol and ginkgo biloba extract, has been reported to produce neuroprotective effects through antioxidant, antiplatelet, and vasodilatory mechanisms. This study was designed to investigate the protective effects of renexin on hearing, the organ of Corti (OC), and medial olivocochlear efferents against noise-induced damage. C57BL/6 mice were exposed to 110 dB SPL white noise for 60 min and then randomly divided into three groups: high- and low-dose renexin-treated groups and noise only group. Renexin were administered for 7 days: 90 mg/kg to the low-dose, and 180 mg/kg to the high-dose groups. All mice, including the controls underwent hearing tests on postnoise day 8 and were killed for cochlear harvest. We compared the hearing thresholds and morphology of the OC and cochlear efferents across the groups. The renexin-treated groups recovered from the immediate threshold shifts in a dose-dependent manner, while the noise group showed a permanent hearing loss. The renexin-treated ears demonstrated less degeneration of the OC. The diameters of the efferent terminals labeled with α-synuclein were preserved in the high-dose renexin-treated group. In the western blot assay of the cochlear homogenates, the treated groups displayed stronger expressions of α-synuclein than the noise and control groups, which may indicate that noise-induced enhanced activity of the cochlear efferent system was protected by renexin. Our results suggest that pharmacologic treatment with renexin is hopeful to reduce or prevent noise-induced hearing loss as a rescue regimen after noise exposure.

Keywords: Antioxidant, cilostazol, ginkgo biloba extract, medial olivocochlear efferent, organ of Corti, α -synuclein

How to cite this article:
Park SY, Back SA, Kim HL, Kim DK, Yeo SW, Park SN. Renexin as a rescue regimen for noise-induced hearing loss. Noise Health 2014;16:257-64

How to cite this URL:
Park SY, Back SA, Kim HL, Kim DK, Yeo SW, Park SN. Renexin as a rescue regimen for noise-induced hearing loss. Noise Health [serial online] 2014 [cited 2023 Nov 28];16:257-64. Available from: https://www.noiseandhealth.org/text.asp?2014/16/72/257/140497

  Introduction Top

Excessive noise exposure widely affects the cellular and mechanical structures of the cochlea, and the eventual sensory hair cell loss leads to permanent threshold shift (PTS). Oxidative stress by overproduction of reactive oxygen species associated with overdriven mitochondria, ischemia/reperfusion, and glutamate excitotoxicity plays an important role in the pathogenic mechanisms of noise-induced hearing loss (NIHL) and cochlear damage. [1] Exposure to hazardous industrial, military, recreational, or accidental noise is frequently unrecognized at that time and thus cannot be prevented in many cases, which makes postexposure management more significant than pretreatment. Recent studies on pharmacological interventions for NIHL have targeted the molecular cascades from oxidative stress to hair cell death. The protective effects of various compounds, especially antioxidants, have shown promise in clinical application to reduce or prevent NIHL. [2],[3] The medial olivocochlear (MOC) efferent system projecting mainly to the outer hair cells (OHCs) is inherently suppressive. It modulates cochlear mechanics to reduce the gain of the cochlear amplifiers through sound-evoked MOC reflex, and exerts protective function from acoustic injury. The MOC system also plays a role in improving the acoustic signal detection in background noise. [4],[5]

Renexin (SK Chemicals, Korea) is a combination drug of cilostazol and ginkgo biloba extracts. Cilostazol, a phosphodiesterase III inhibitor, induces vasodilation, and inhibits platelet aggregation by an increase in intracellular cyclic adenosine monophosphate (cAMP) concentration in blood vessels and platelets. [6],[7] Increased cAMP by cilostazol has been reported to suppress superoxide production in endothelial cells. [8] Ginkgo biloba extracts that exert neuroprotective effects by complex mechanisms such as potent platelet-activating factor antagonism, increased blood flow, and free-radical scavenging have been used for cerebrovascular insufficiency, dementia, stroke, tinnitus, and others. [9],[10],[11] The combination of these two drugs has been documented to enhance antiplatelet/antithrombotic, [12] anti-atherogenic, [13] and neuroprotective effects [14] of each drug. The present study was designed to investigate the postexposure protective effects of renexin on hearing, the organ of Corti (OC), and MOC efferents against damage caused by noise.

  Methods Top


Thirty-two male C57BL/6 (B6) mice, 4 weeks old, and weighing 13-17 g, were obtained from experimental animal center (Orient Bio, Korea). Mice were kept in the animal colony of the facilities (two mice/cage, 23°C, 50% relative humidity, 12-h light-dark cycle), and fed with regular rodent chow and distilled water ad libitum. All procedures in this study were performed in compliance with the national ethical guidelines and relevant laws, and the experimental protocol was approved by the Animal Care and Use Committee of the Catholic University of Korea College of Medicine (IACUC-CUMC-2012-0002-02).

Experimental design

All mice were screened for hearing in a sound-attenuated chamber before the experiment. Continuous white noise was generated by the same method as described in our previous report. [15] The control animals remained naive, and the experimental mice were exposed to an 110 dB sound pressure level (SPL) noise for 60 min, awake and unrestrained within a specially designed acrylic box containing a pie-shaped, wire-mesh cage with eight compartments (a single mouse for each compartment), on the top of which a noise speaker was attached. Immediately after noise exposure, 24 experimental animals underwent hearing tests and then were randomly assigned to one of the three experimental groups: The RX180, RX90, and noise groups (n = 8, each group). The renexin treatment was started 24 h after the cessation of noise when the hearing tests were done, and the animals recovered completely from anesthesia. Two different dosages of renexin (suspension in 0.5% carboxymethyl cellulose) provided by SK Chemicals were administered intragastrically using a sonde to the renexin-treated groups for 7 consecutive days: 90 mg/kg to the RX90 and 180 mg/kg to the RX180 groups. All mice survived for 1 week after noise exposure, allowing recovery from the initial threshold shifts with or without renexin treatment [Figure 1].
Figure 1: Experimental design

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Hearing tests

Animals underwent auditory brainstem response (ABR) and distortion product otoacoustic emission (DPOAE) testing under the anesthesia with zolazepam-tiletamine (5 mg/kg) and xylazine (5 mg/kg). ABR was recorded by SmartEP and software version 2.33 is the pair of hard- and software that operate in a single machine. To obtain ABRs, subdermal stainless steel needle electrodes applied to the vertex (active) and below the left pinna (reference) were connected to a preamplifier with filter settings at 0.1 to 3 kHz. Acoustic stimuli - click (0.1 ms duration, 19.3/s rate) and tone bursts of 8, 16, and 32 kHz (1.5 ms duration, 21.1/s rate) - were presented through an insert earphone from 90 dB SPL, and attenuated in 5-10 dB steps to determine the threshold. The evoked potentials were obtained in the first 12 ms after the auditory stimuli and averaged from 256 sweeps. Threshold was defined as the lowest intensity at which the typical ABR pattern could be replicated. DPOAEs were recorded to obtain DP-grams over 6-32 kHz geometric-mean (GM) frequencies using the SmartOAE version 4.26 (Intelligent Hearing Systems, Miami, USA). Etymotic 10B+ probe was inserted into the ear canal in conjunction with two types of transducers: Etymotic ER2 stimulator for frequencies from 6 to 16 kHz, and IHS high-frequency transducer for frequencies from 16 to 32 kHz. The response signals were sampled at a rate of 128 kHz using a 16-bit D/A converter. L 1 amplitude was set to 65 dB SPL; L 2 , 55 dB SPL with f 1 /f 2 ratio of 1.22. Four blocks were acquired in total, each block consisting of 32 sweeps. Statistical analyses were performed using SPSS 18.0 software (IBM Corp., NY, USA) at a two-tailed significance level of 0.05. Prenoise and immediate postnoise ABR thresholds/DPOAE levels were compared between the control and experimental mice using t-test. ABRs on postnoise day 8 were compared across the four groups using one-way ANOVA and Tukey post hoc test; DPOAEs, Kruskal-Wallis test. Descriptive statistical analyses were also carried out for ABR thresholds on day 8. The effect sizes were calculated using the mean difference and standard deviation (SD). The effect size (Cohen's d) of around 0.5 was considered a medium effect; 0.8 to infinity, a large effect by Cohen's criteria. When hearing tests were completed, mice were killed under the anesthesia to harvest the cochleae for morphological evaluation.

Morphometry of the organ of Corti by light microscopy

The mouse cochleae (n = 4, each group) were gently perfused through the round and oval windows with 2% paraformaldehyde and 2% glutaraldehyde in 0.1 M phosphate buffered saline (PBS) at pH 7.4, and preserved in the same fixative overnight at 4°C. Specimens were postfixed in 1% phosphate-buffered osmium tetroxide and decalcified in 0.135 M ethylenediaminetetraacetic acid for 2-4 days. Decalcified cochleae were dehydrated in ethanol and propylene oxide, embedded in Araldite 502 resin to be sectioned at 5 μm, and stained with toluidine blue. We randomly selected six sections from the total midmodiolar sections of a cochlea and examined the basal, middle, and apical turns in light microscopy. The degree of morphological degeneration of the OC was graded on a scale of 1-5 by the rank order grading methods modified from Leake et al.[16] Scores were assigned to indicate the following conditions: 5 = Normal cytoarchitecture of the OC with intact hair cells; 4 = Maintenance of the normal cytoarchitecture of the OC with intact supporting cells; 3 = Partial collapse of the OC with subtypes of supporting cells still recognizable; 2 = Collapsed OC with a low cuboidal cell layer without recognizable supporting cells; 1 = Complete degeneration of the OC to a single flattened cell layer. Partial collapse of the OC was defined as altered, but still noticeable pillars and inner tunnel (e.g. pillar buckling with the height of the tunnel lowered); collapse, merged tunnel. The mean grades of the six sections at each turn were compared across the groups using Kruskal-Wallis test.

Scanning electron microscopy of the organ of Corti

The cochleae (n = 4, each group) were perfused with 2% glutaraldehyde in 0.1 M PBS, preserved in the same fixative as above, and postfixed in 1% osmium tetroxide for 2 h. After the cochleae were microdissected with a drill, the opened cochleae were submerged in PBS and chemically dehydrated in a series of graded acetone solutions from 50% to 100% for 12 h at each concentration. The specimens were transferred to hexamethyldisilazane, gold-coated, and examined under scanning electron microscope (SEM) (JSM-5410LV, JEOL, Japan). A mouse frequency-place map was adopted from the previous study [17] to localize the 8 kHz (18% distance from the apex), 16 kHz (43%), and 32 kHz regions (68%). The number of missing and altered OHC stereocilia - fused or broken - were counted from each region. The degree of ultrastructural changes of the OC was assessed with the mean number of pathology to categorize it as none (0-1), mild (2-3), moderate (4-6), and severe (>7).

Confocal microscopy of the whole-mounted cochleae

The cochleae (n = 4, each group) were perfused with 2% paraformaldehyde, maintained in the same fixative, and then decalcified by the same method as described above. After the otic capsule was opened, the lateral wall, Reissner's, and tectorial membranes were removed. To visualize the efferent terminals (ETs) innervating the OHCs, immunofluorescent staining was performed in whole-mount cochleae using monoclonal antibody to α-synuclein, an efferent synaptic vesicular protein (purified mouse anti-α-synuclein, BD Biosciences, CA, USA). Specimens were preincubated in 0.3% Triton X100 and 5% normal goat serum for 1 h, and then incubated with 1:100 anti-α-synuclein antibody at 4°C overnight in a humid chamber. They were rinsed and incubated with 1:4000 fluorescent secondary antibody (Alexa Fluor 555 goat anti-mouse immunoglobin (Ig) G, Life technologies, CA, USA) for 2 h. After the secondary antibodies had been removed, specimens were exposed to DAPI (Vectashield, CA, USA), which allowed visualization of the OHC nuclei. The ETs were examined using confocal laser-scanning microscopy (LSM 510 Meta, Zeiss, Germany). The images of the very middle portion of the apical, middle, and basal turns were taken. Considering the multiple layers of the ETs, we took the Z-stacked pictures for image capture in the consistent settings of depth, exposure time, step, and magnification. The captured images were imported into NIH ImageJ software 1.47v (National Institutes of Health, Bethesda, MD, USA) for quantitative analysis. In the cochlea, the diameter was measured in approximately 80 ETs; the number of ETs per OHC was counted in approximately 30 OHCs from each turn. The mean values were compared across the four groups, and among the three turns within the control group using the Kruskal-Wallis and post hoc Mann-Whitney tests.

Western blot to detect α-synuclein in cochlear homogenates

Cochleae (n = 4, each group) were dissected and homogenized in lysis buffer (T-PER Tissue Protein Extraction Reagent, Thermo science, IL, USA). Homogenates were centrifuged at 6500 rpm at 4°C for 20 s and the extracts were stored at -70°C. The protein concentration in the supernatant was determined by the BCA protein assay. Equal amounts (100 μg) of total protein derived from mixed cochlear homogenates of four cochleae from each group were loaded per lane on 10% gel and separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis. After separation, proteins were transferred onto nitrocellulose membranes. The membranes were blocked for 1 h with 5% bovine serum albumin in TBS-T (tris buffer saline +0.1% Tween 20). Blocked membranes were incubated with α-synuclein antibodies (1:1000) at 4°C overnight, washed in TBS-T, and incubated with peroxidase-conjugated secondary antibody (1:5000 anti-rabbit/mouse Ig G, Sigma) at room temperature for 2 h. Immunoreactive bands were visualized with enhanced chemiluminescence reagents (Las 3000, Fujifilm, Tokyo, Japan) and the densities of the bands were analyzed semi-quantitatively by imaging densitometer (Fujifilm multi gauge 3.0). The density of α-synuclein was normalized with respect to β-actin. Western blot of a-synuclein was duplicated using the same specimen. The ratios of a-synuclein were averaged, and the effect sizes were calculated.

  Results Top


Auditory brainstem response thresholds on pre-/post-noise day 0 for click, 8, 16, and 32 kHz tone burst were 21.2 ± 4.3/42.9 ± 3.3, 21.3 ± 4.7/41.7 ± 3.3, 20.3 ± 3.4/41.7 ± 3.3, and 23.8 ± 3.9/42.1 ± 3.3 (mean ± SD), respectively, showing significant difference [Figure 2]a. On postnoise day 8, the thresholds for click, 8, 16, and 32 kHz tone burst in the control/RX180/RX90/noise groups were 21.9 ± 3.7/23.8 ± 2.3/26.3 ± 5.2/43.1 ± 3.7, 24.4 ± 4.2/23.8 ± 3.5/28.1 ± 3.7/43.8 ± 3.5, 23.1 ± 3.7/25.6 ± 4.2/30.6 ± 5.6/42.5 ± 3.8, and 26.3 ± 3.5/26.9 ± 3.7/31.9 ± 6.5/45.0 ± 3.8, respectively [Figure 2]b. All thresholds of the control and both renexin-treated groups were much lower than those of the noise group. There were no statistically significant differences among the three groups except the 16 kHz threshold of the RX90 group, which was significantly higher than that of the control group. The effect sizes were determined to compare the mean thresholds. Between the control and RX180 groups, Cohen's d was 0.60/0.16/0.63/0.17 for click/8/16/32 kHz tone bursts (medium effects for click and 16 kHz); between the RX180 and RX90 groups, 0.67/1.21/1.02/0.98 (medium effect for click and large effects for all tone bursts). DPOAEs obtained immediately after noise were significantly lower than the prenoise levels at low GM frequencies of 6, 7, and 10 kHz. At high GM frequencies over 17 kHz, DPOAE responses were small in all mice without intergroup differences [Figure 3]a. On postnoise day 8, DPOAEs were not different at all frequencies among the experimental groups; not different at ≥10 kHz across the groups [Figure 3]b. For both ears gave the same ABR thresholds and DPOAEs in a mouse, we used the data on the right ear for statistical analyses.
Figure 2: Auditory brainstem response (ABR) thresholds for click and 8/16/32 kHz tone bursts in B6 mice. (a) Prenoise and postnoise thresholds to all stimuli show significant differences. *P = 0.000 versus prenoise. (b) On postnoise day 8, ABR thresholds of the noise group are higher than those of the other groups. *P = 0.000 versus the other groups; **P = 0.01 versus control group. Error bars indicate standard error of mean

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Figure 3: Average DP-grams for 65 dB SPL primary tone showing distortion product otoacoustic emission (DPOAE) levels as a function of geometric-mean frequencies from 6 to 32 kHz. (a) Postnoise DPOAE levels decreased significantly at 6, 7, and 10 kHz compared with prenoise baseline levels. *P = 0.000 versus postnoise day 0. (b) On postnoise day 8, experimental groups show lower DPOAE levels than the control group at 6 and 7 kHz. **P = 0.014/0.048 for 6/7 kHz. A noise floor averaged for all groups is indicated at the bottom of each plot

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The organ of Corti

Light microscopic examination of the OC revealed less hair cell loss in the RX180 group than in the RX90 and noise groups in the basal turn. The height of the OC was relatively preserved in the RX90 group compared to the noise group that showed partially collapsed OC due to buckled pillars [Figure 4]a. The apical-turn grading score for OC degeneration of the control/RX180/RX90/noise group was 4.4 ± 0.1/4.5 ± 0.2/4.4 ± 0.2/3.9 ± 0.2; the middle-turn grading score, 4.6 ± 0.1/4.6 ± 0.2/4.4 ± 0.1/3.9 ± 0.2. In the apical and middle turns, the grading scores for OC degeneration of the control, RX180, and RX90 groups were significantly better than those of the noise group. However, the basal-turn grading score of the RX90 group (2.8 ± 0.5) was similar with that of the noise group (2.7 ± 0.5) and significantly lower than that of the control/RX180 group (4.2 ± 0.1/4.4 ± 0.2). The control and RX180 groups showed significantly higher scores than the noise group in all turns [Figure 4]b. The SEM findings of the OC showed ultrastructural changes in the experimental groups. The RX180 group exhibited mild disarray of stereocilia of both inner hair cells (IHCs) and OHCs in the 16 and 32 kHz regions. The RX90 group displayed moderately disturbed stereocilia with mild to moderate stereociliary loss in the 16 and 32 kHz regions similar to the noise group. The noise group showed severe stereociliary damage in the 32 kHz region; mild stereociliary changes and losses even in the 8 kHz region. The 8 kHz region was relatively spared in both renexin-treated groups [Figure 5].
Figure 4: (a) Representative photomigrographs showing radial section of the organ of Corti (OC) from basal turn. Compared to noise and RX90 groups, RX180 group exhibits relatively preserved outer hair cells (OHCs). Note the decreased height (from reticular lamina to basilar membrane) of the OC due to pillar buckling (arrowheads) in the noise group. (b) Grading scores for the degeneration of OC are compared across groups. I = Inner hair cell; O1, O2, O3 = Three OHC rows; IT = Inner tunnel of Corti

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Figure 5: Representative scanning electron micrographs of the organ of Corti (OC) from 32 kHz region. RX180 group shows subtle disarray of outer hair cell stereocilia. Disturbed and missing stereocilia (asterisks) are found in the RX90 and noise groups. Furthermore, note the fused, disarrayed inner hair cell stereocilia of the experimental groups. The degree of ultrastructural alterations at three frequency regions is demonstrated in the figure table. −: none (0-1); +: mild (2-3); ++: moderate (4-6); +++: severe (>7)

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Medial olivocochlear efferents

We used α-synuclein antibody to label the ETs, and DAPI to visualize the hair cell nucleus [Figure 6]. The mean ET diameters of the control and RX180 groups were larger than those of the RX90 and noise groups in the apex and middle. The mean number of ETs per OHC showed significant differences between the control and RX90/noise groups in the middle turn. Even in the control cochleae, the ETs were smaller and fewer in the base than in the apex and middle [Figure 7]. In western blot assay, the ratios of a-synuclein were 24.1 ± 1.7%, 29.7 ± 2.1%, 26.1 ± 2.6%, and 20.1 ± 1.2 % in the control, RX180, RX90, and noise groups, respectively [Figure 8]. The effect size of the difference between the control and RX180/the control and RX90/the control and noise/the RX180 and RX90/the RX180 and noise/the RX90 and noise groups was 2.9/0.9/2.8/1.5/5.8/3.2, showing large effects.
Figure 6: Representative confocal Z-stacks of whole-mounted cochleae from middle turn labeled with α-synuclein and stained with DAPI for the efferent terminals (ETs) and outer hair cell nuclei. The ETs appear smaller and less distinct in the RX90 and noise groups than in the normal and RX180 groups

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Figure 7: Quantitative analyses of the diameter of the efferent terminals (ETs) and number of the ETs per outer hair cell

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Figure 8: Western blot assay of α-synuclein in cochlear homogenates. Renexin-treated groups show increased ratios of α-synuclein compared with the control and noise groups

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  Discussion Top

We have performed this study to investigate the postnoise protective effects of renexin on hearing, the OC, and MOC efferents against noise-induced damage. Renexin-treated mice recovered from noise-induced threshold shifts. Acoustic stimulation with a 60-min white noise at a level of 110 dB SPL induced significant ABR threshold shifts of 19-21 dB in all mice immediately after the cessation of noise. After 8 days, the RX180 group showed almost complete recovery from the threshold shifts; the RX90 group, partial recovery; the noise group, PTS at all frequencies. Considering the effect sizes, the experimental groups displayed graded threshold levels that were inversely proportional to the renexin dosage. These findings suggest that the high-dose renexin (180 mg/kg) is more effective in protection from noise-induced threshold shifts than low-dose one (90 mg/kg). Similar results have been reported in a recent study on protective effects of renexin against cisplatin-induced ototoxicity by free radical scavenging. [18] A common damaging factor of oxidative stress in NIHL and ototoxicity can explain the similar patterns of hearing loss and cochlear pathology [19] as well as the similar efficacy of the same regimen between them. The protective effects of renexin on DPOAE were not as remarkable as on ABR. The experimental groups eventually showed no recovery from the reduced DPOAE levels by the time of cochlear harvest. We postulated the possible mechanisms in two ways. First, it can be attributed to the genetic susceptibility of this mouse strain. B6 mice has been known to have early-onset age-related hearing loss with a progressive basal to apical hair cell loss [20] due to the recessive Ahl gene, which is also implicated in the pronounced vulnerability to NIHL. [21],[22] Baseline DPOAE of B6 mouse is weak at middle to high frequencies. This finding has already been shown in our previous studies on B6 mice, a mouse model for presbycusis [23] and NIHL. [15] In the present study, the experimental groups showed reduced DPOAEs after noise, especially in the low GM frequencies. The low baseline DPOAEs in the middle to high GM frequencies were not significantly different across all groups including the controls. Second, as seen in SEM findings, stereocilia of both IHCs and OHCs were damaged in a dose-dependent manner in the experimental groups: mild in the RX180, moderate in the RX90, and severe in the noise group. It is probable that the altered stereocilia affected mechanoelectrical transmission associated with OHC motility and the reduced DPOAEs mirrored the affected OHC function. However, the relationship between the genetic and stereociliary susceptibility remain unknown. Further study may be needed with the use of the other mouse strains.

Microscopic quantitative analysis of the OC degeneration showed that the grading scores of the apex and middle were preserved at the baseline level in both renexin-treated groups; the score of the base, only in the high-dose renexin-treated group. Therefore, high-dose renexin protected all turns; low-dose, the apical and middle turns. It is well-known that there is a spatial gradient in the susceptibility to noise along the cochlear duct from base to apex and the OHCs of the base are the most vulnerable. [1] The ultrastructural alterations of the OC gave the correlative results. Although there was a tendency to degeneration of hair cell stereocilia in the high-frequency region in all experimental groups, the high-dose renexin-treated cochleae exhibited minimal changes. Stereocilia, a hair cell's mechanical transduction apparatus, have repair mechanisms in order to survive constant sound stimulation in life. Dynamic renewal of stereocilia [24] and regeneration of broken tip links [25] have been reported. It is postulated that renexin may help to meet the heavy metabolic demand for stereociliary regeneration after excessive noise exposure, although the exact mechanisms remain to be elucidated.

Alpha-synuclein, a presynaptic vesicular protein, is one of the three (α-, β-, and γ-) synuclein family widely expressed in the central nervous system. In the cochlea, it is localized predominantly to the efferent synapse at the OHC base, playing a potential regulatory role in the transmitter release and synaptic plasticity of the efferent auditory system. [26] Because α-synuclein-labeled ET visualizes the area containing the presynaptic reserve vesicles, it may not represent the actual ET size, especially in the case of swollen or damaged ET. In this regard, the α-synuclein method may be useful for the functional estimate of the ETs. We found significant differences in the ET diameters between the high-dose and low-dose renexin-treated groups in the middle turn, which suggests the protective effect of high-dose renexin on ETs. The effects of renexin on the ETs were not remarkable in the apex and base. The baseline number of ETs was smallest in the base. Furthermore, the base showed small ETs in all cochleae, which may be related to the innate vulnerability of the basal OHCs in this mouse strain. In western blot assay, the order of a-synuclein density was the RX180, RX90, control, and noise groups from the densest. These morphological and biochemical results with respect to a-synuclein in ETs suggest together that the noise-induced enhanced activity of the cochlear efferent system was preserved by high-dose renexin treatment. Earlier studies reported that the activity in the olivocochlear system protects the ear from permanent acoustic injury, while cochlear de-efferentation results in substantially greater PTS and larger OHC lesions. [27],[28] Our results also demonstrated the coherent changes to the OHCs and efferents, although the cause and effect are not clearly known. Hazardous noise affects the efferent system as well as the OC by the previously described mechanisms such as free radical formation and ischemia/reperfusion. In addition, the function of the presynaptic terminal can be influenced by the integrity of the postsynaptic partner. As a result, the ETs can be subjected to a dual stress by noise: directly from oxidative stress, and indirectly from damaged OHCs. Therefore, the protection of both OHCs and ETs by renexin can benefit the cochlear efferent function.

  Conclusion Top

Our results indicate that renexin can protect hearing, the OC, and MOC efferents against noise-induced functional and morphological damage in a dose-dependent manner. We suggest that postnoise pharmacologic treatment with renexin as a rescue regimen is hopeful to reduce or prevent NIHL.

  Acknowledgments Top

This work was supported in part by a grant from Basic Science Research Program of the National Research Foundation of Korea funded by the Ministry of Education, Science, and Technology (2010-0004744 and 2013-011870). The support of SK Chemicals is also gratefully acknowledged.

  References Top

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Correspondence Address:
Dr. Shi Nae Park
Department of Otorhinolaryngology -Head and Neck Surgery, College of Medicine, The Catholic University of Korea, 222 Banpo-daero, Seocho-gu, Seoul, 137-701
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Source of Support: The National Research Foundation of Korea (2010-0004744 and 2013-011870) and SK Chemicals., Conflict of Interest: None

DOI: 10.4103/1463-1741.140497

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