Involvement of retinoic acid-induced peroxiredoxin 6 expression in recovery of noise-induced temporary hearing threshold shifts.

All-trans retinoic acid (ATRA) is reported to reduce hair cell loss and hearing deterioration caused by noise-induced hearing loss (NIHL). The present study investigates the involvement of peroxiredoxin 6 (Prdx 6) in ATRA-mediated protection of temporary threshold shift of hearing. Mice fed with ATRA before or after exposure to white noise showed a faster recovery than untreated controls within 1 week, with a concomitant increase of cochlear Prdx 6 expression. Treatment of mouse auditory cells with ATRA induced Prdx 6 expression. A putative retinoic acid (RA)-response element (RARE) was identified in a murine Prdx 6 promoter region. Prdx 6 promoter activities were elevated in wild-type reporter plasmid-transfected cells, whereas no significant change in activity was in those with RARE-disrupted mutant reporter. RA receptor α (RARα) functions as a transactivator of Prdx 6 gene expression. These findings suggest that ATRA-induced Prdx 6 expression may be associated with rapid recovery from temporary NIHL.

Retinoic acid applied after noise exposure can recover the noise-induced hearing loss in mice.

CONCLUSION: The early post-exposure treatment with All-trans retinoic acid (ATRA) can reduce hair cell loss and hearing deterioration in mice in which permanent threshold shift has been induced. OBJECTIVE: One of the mechanism by which intense noise induces apoptosis of cochlea hair cells is the C-Jun NH(2)-terminal kinase (JNK) pathway. ATRA is a potent inhibitor of activator protein 1, a transcription factor of the JNK pathway. In this study we evaluated that the effect of post-exposure treatment of ATRA on noise-induced hearing loss and aimed to determine a time window for effective post-exposure treatment of ATRA. METHODS: All mice were exposed to white noise for 3 h per day for three consecutive days and induced permanent threshold shift. The treatment groups fed with ATRA from 1 h, one day, two days, and three days after noise exposure for five days were compared with mice fed with same dosage of sesame oil. We measured the threshold shifts of hearing and survival rates of hair cells on the cytocochleogram. RESULTS: Mice fed with ATRA beginning within two days after noise had less threshold shifts and more hair cell survivals than mice fed with sesame oil.

Circadian changes in serum corticosterone levels affect hearing in mice exposed to noise.

We assessed the relationship between changes in corticosterone concentrations and hearing in mice exposed to noise during the light (inactive) and dark (active) phases. Serum corticosterone concentrations and hearing levels were measured before, and 1, 3, 5, 7, and 10 days after, noise exposure between 8:00-11:00 h and 15:00-18:00 h. Serum corticosterone concentrations were significantly lower at 8:00-11:00 h than at 15:00-18:00 h and were significantly lower before than after noise exposure. In addition, serum corticosterone concentrations were significantly lower at 11:00 h after noise exposure than at 18:00 h before noise exposure. Mice exposed to noise at 8:00-11:00 h showed significantly elevated threshold shifts after noise exposure than did mice exposed to noise at 15:00-18:00 h. Endogenous serum corticosterone concentration has a significant effect on hearing after noise exposure. Noise exposure during the inactive phase of the hypothalamic-pituitary-adrenal axis may be more harmful to the auditory system than noise exposure during the active phase of the hypothalamic-pituitary-adrenal axis.

Lipoic acid rescues DBA mice from early-onset age-related hearing impairment.

We fed DBA mice with alpha-lipoic acid (100 mg/kg body weight/day), beginning 2, 4, or 8 weeks after birth. Hearing thresholds were measured weekly. At 12 weeks after birth, control mice not fed with alpha-lipoic acid showed significant hearing decreases at all frequencies. In contrast, mice fed with alpha-lipoic acid beginning at 2 weeks after birth showed significantly better hearing at all frequencies. Mice fed with alpha-lipoic acid beginning at 4 and 8 weeks after birth also showed significantly better hearing than control mice after they were fed with alpha-lipoic acid. The stria vascularis of mice fed with alpha-lipoic acid showed reduced 8-oxoguanine residues in DNA and cytoplasm compared with that of control mice. Western blotting showed that the level of hypoxia-inducible factor-1alpha was lower in mice fed with alpha-lipoic acid than in control mice. From these results, we suggest that alpha-lipoic acid prevented early-onset hearing impairment in DBA mice.

Apoptotic pattern of cochlear outer hair cells and frequency-specific hearing threshold shift in noise-exposed BALB/c mice.

OBJECTIVES: Apoptosis of outer hair cell (OHC) can be identified through nuclear staining by specific nuclear changes. The change of filamentous actin (F-actin) is also involved in early cell death process. The study was designed to investigate OHC death along the whole length of the organ of Corti. METHODS: BALB/c hybrid mice were used in this study. The noise group was exposed to white noise of 120 dB SPL for 3 hr per day for 3 consecutive days. The tone burst auditory brainstem response (ABR) test was conducted and cochleas from each group were obtained for the immunostaining of FITC phalloidin for F-actin and propidium iodide (PI) for nuclei. RESULTS: ABR threshold of the noise group significantly increased after noise exposure (P<0.001). No threshold shift was found in the control group. Threshold shift of the noise group constantly increased from 4 to 16 kHz, but threshold shifts at 16 kHz and 32 kHz were similar. Patterns of OHC staining were subclassified as FITC+PI- cells, FITC+ PI+ cells, FITC-PI+ cells and missing cells. Proportion of normal live OHCs (FITC+PI-) rapidly decreased from the apex to the base. In the basal turn, FITC-PI+ cells and vacancy OHC (missing cells) were observed easily. Apoptotic and missing cells were most abundant at 60% of the whole length of the Corti organ. CONCLUSION: We could subclassify morphologic changes in OHC death after noise exposure. Quantitative changes in OHCs along the whole Corti organ showed a plateau pattern similar to that of a frequency-specific threshold shift.

Hypoxic changes in the central nervous system of noise-exposed mice.

CONCLUSION: After a noise-induced transient threshold shift, hypoxia occurred in the central nervous system, especially in the auditory cortex, the hippocampus, and the inferior colliculus. OBJECTIVES: Noise-induced inner ear hypoxia was shown by measurement of an increase in hypoxia-inducible factor-1 alpha, which is expressed? in the nucleus under hypoxic conditions. This study uses pimonidazole to localize site-specific hypoxic changes occurring in the mouse central auditory pathway during noise-induced auditory threshold shift. METHOD: BALB/c hybrid mice with normal hearing were exposed to 122 dB SPL white noise for 3 h. Immediately after exposure to the noise, and 7 d after noise exposure, the brains of mice were collected. Brains were cryosectioned into slices 15 microm thick and examined by immunofluorescence after staining with pimonidazole HCl. RESULTS: After 3 h of exposure to 120 dB SPL noise, the hearing thresholds of mice decreased to 51.1+/-8.6 dB SPL (n =14), but hearing recovered in 7 d. After noise exposure, pimonidazole signal increased in the auditory cortex, the hippocampus, and the inferior colliculus. The pimonidazole signal remained elevated after 7 d. In control mice, pimonidazole did not stain any brain region.

The effect of isoflurane, halothane and pentobarbital on noise-induced hearing loss in mice.

BACKGROUND: Ear surgery using mastoid drills can lead to noise-induced hearing loss (NIHL). We investigated whether inhaled anesthetics or pentobarbital could have protective effects on NIHL in mice. METHODS: Mice were exposed to broad band white noise for 3 h per day for 3 consecutive days, with or without anesthesia, using halothane, isoflurane, or pentobarbital. The hearing level of each mouse was analyzed before exposure, and 1 day, 1, 2, and 3 Wk, and 1 mo after noise exposure by measuring auditory brainstem response thresholds. At 1 Wk after noise exposure, the organ of Corti was stained with a fluorescent isothiocyanate-conjugated phalloidin probe and a TUNEL kit. RESULTS: In the unanesthetized control group, the hearing threshold increased to 77.5 +/- 8.0 dB hearing level (HL) after noise stimulation. In the pentobarbital, isoflurane, and halothane groups, hearing threshold increased to 62.5 +/- 6.3 dB HL, 45.5 +/- 9.8 dB HL, and 39.3 +/- 6.2 dB HL, respectively, with all anesthetized groups of mice showing significantly preserved hearing compared with the control group (P < 0.05). But, in mice anesthetized with pentobarbital, hearing loss was more severe than in those treated with the inhaled anesthetics (P < 0.05). Hair cell survival was reduced in unanesthetized control mice and somewhat reduced in pentobarbital-treated mice, but largely unaffected in mice treated with inhaled anesthetics. CONCLUSIONS: These findings indicate that, while halothane, isoflurane and pentobarbital could protect mice against NIHL and hair cell damage, inhaled anesthetics were more effective.

Protective effect of isoflurane anesthesia on noise-induced hearing loss in mice.

HYPOTHESIS/OBJECTIVES: To examine the protective effect of general anesthesia with isoflurane against noise-induced hearing loss in mice. STUDY DESIGN: Animal study using noise stimulation and measurement of hearing in BALB/c mice. METHODS: Mice were exposed to 122 dB peak equivalent sound pressure level click noise for 3 hours per day for 3 consecutive days with or without anesthesia using isoflurane. Hearing levels were measured and hair cell survival ratio was observed. RESULTS: In mice without anesthesia, hearing threshold increased after noise stimulation (73.7 dB hearing level [HL]) and persisted for at least 1 month. However, in mice exposed to noise under anesthesia, hearing loss was less severe (44.1 dB HL) and had recovered more (26.5 dB HL) by one month. Histological examination showed hair cell survival was higher in anesthetized compared to non-anesthetized mice. CONCLUSION: These data indicate isoflurane general anesthesia protects against noise-induced hearing loss and tissue damage in mice.

Anti-apoptotic role of retinoic acid in the inner ear of noise-exposed mice.

Exposure to loud noise can induce temporary or permanent hearing loss, and acoustic trauma is the major cause of hearing impairment in industrial nations. However, the mechanisms underlying the death of hair cells after acoustic trauma remain unclear. In addition to its involvement in cellular stress and apoptosis, the c-Jun N-terminal kinase (JNK), a member of the mitogen-activated protein kinase family, is involved in cell survival, transformation, embryonic morphogenesis, and differentiation. JNK is primarily activated by various environmental stresses including noise, and the phenotypic result appears be to cell death. All-trans retinoic acid (ATRA) is an active metabolite of vitamin A that regulates a wide range of biological processes, including cell proliferation, differentiation, and morphogenesis. We evaluated the role of ATRA in preserving hearing in mice exposed to noise that can induce permanent hearing loss. Mice fed with ATRA before and during 3 consecutive days of noise exposure had a more preserved hearing threshold than mice fed sesame oil or saline. Histological and TUNEL staining of the cochlea showed significantly enhanced preservation of the organ of Corti, including outer hair cells and relatively low apoptotic nuclei, in mice-fed ATRA than in mice-fed sesame oil or saline. Phospho-JNK immunohistochemistry showed that ATRA inhibited the activation of JNK. These results suggest that ATRA has an anti-apoptotic effect on cochleae exposed to noise.

Accumulation of hypoxia-inducible factor-1alpha in mouse inner ear by noise stimulation.

Exposure of mice to a 120 dB SPL broad band click sound for 3 h per day induced expression of hypoxia-inducible factor-1alpha (HIF-1alpha) in the organ of Corti, lateral wall tissue and spiral ganglion cells. In the organ of Corti, HIF-1alpha was expressed in inner and outer hair cells, pillar cells and Deiters' cells, but not in Hensen's cells or the tectorial, Reissner's or basement membrane. HIF-1alpha expression was definitely increased in mice with a permanent threshold shift, where the hearing level did not recover, even after 4 weeks. These results suggest that noise may induce tissue hypoxia in the inner ear and that the further study on the role of HIF-1alpha will be needed.