Evaluating D-methionine dose to attenuate oxidative stress-mediated hearing loss following overexposure to noise.

Noise exposure causes an excessive reactive oxygen species (ROS) generation as an unwanted byproduct of high metabolic activity. Oxidative stress and antioxidative protective mechanisms have been therefore proposed as the most interesting issues in the development of noise-induced hearing loss. The aim of this study was to examine changes in superoxide dismutase (SOD), catalase (CAT) and the auditory brainstem response (ABR) in the cochlea of C57BL/6 mice 1, 7 and 14 days after exposure to 4 kHz octave band noise at the intensity of 110 dB SPL for 8 h. The evaluation of three D-methionine (D-met) doses (100, 200 and 400 mg/kg) has been performed in order to choose an optimal concentration displaying most effectively its antioxidant and thereby otoprotective functions. Administering D-met at the dose of 400 mg/kg resulted in a significant decrease in threshold shift (TS) independently of the evaluation time after exposure to noise. SOD activity was strongly supported by the same concentration (400 mg/kg) of D-met. This effect was seen not shortly, but 7 and 14 days after exposure to noise. CAT activity was induced only by noise and it reached the peak levels 7 days after exposure. D-Met at the doses of 200 and 400 mg/kg significantly decreased noise-induced changes in CAT activity. The findings of this study indicate that the protective effect depends on the concentration of D-met and can be fully expressed only when the drug is administered in the dose 400 mg/kg.

Candidate gene association study for noise-induced hearing loss in two independent noise-exposed populations.

Millions of people are daily exposed to high levels of noise. Consequently, noise-induced hearing loss (NIHL) is one of the most important occupational health hazards worldwide. In this study, we performed an association study for NIHL based on a candidate gene approach. 644 Single Nucleotide Polymorphisms (SNPs) in 53 candidate genes were analyzed in two independent NIHL sample sets, a Swedish set and part of a Polish set. Eight SNPs with promising results were selected and analysed in the remaining part of the Polish samples. One SNP in PCDH15 (rs7095441), resulted in significant associations in both sample sets while two SNPs in MYH14 (rs667907 and rs588035), resulted in significant associations in the Polish sample set and significant interactions with noise exposure level in the Swedish sample set. Calculation of odds ratios revealed a significant association of rs588035 with NIHL in the Swedish high noise exposure level group. Our studies suggest that PCDH15 and MYH14 may be NIHL susceptibility genes, but further replication in independent sample sets is mandatory.

Noise-induced time-dependent changes in oxidative stress in the mouse cochlea and attenuation by D-methionine.

Oxidative stress in the cochlea is considered to play an important role in noise-induced hearing loss. This study determined changes in superoxide dismutase (SOD), catalase, lipid peroxidation (LPO) and the auditory brainstem response (ABR) in the cochlea of C57BL/6 mice prior to and immediately, 1, 3, 7, 10, 14 and 21 days after noise exposure (4 kHz octave band at the intensity of 110 dB SPL for 4 h). A significant increase in SOD activity immediately and on 1st day after noise exposure, without a concomitant increase in catalase activity suggested a difference in the time dependent changes in the scavenging enzymes, which facilitates the increase in LPO observed on day 7. The ABR indicated significant noise-induced functional deficits which stabilized in 2 weeks with a permanent threshold shift (PTS) of 15 dB at both 4 kHz and 8 kHz. The antioxidant D-methionine (D-Met) reversed the noise-induced changes in LPO levels and enzyme activities. It also significantly reduced the PTS observed on the 14th day from 15 dB to 5 dB for 4 kHz. In summary, the findings indicate that time-dependent alterations in scavenging enzymes facilitate the production of reactive oxygen species and that D-met effectively attenuates noise-induced oxidative stress and the associated functional loss in the mouse cochlea.

Protective effect of melatonin against in vitro iron ions and 7 mT 50 Hz magnetic field-induced DNA damage in rat lymphocytes.

We have previously shown that simultaneous exposure of rat lymphocytes to iron ions and 50Hz magnetic field (MF) caused an increase in the number of cells with DNA strand breaks. Although the mechanism of MF-induced DNA damage is not known, we suppose that it involves free radicals. In the present study, to confirm our hypothesis, we have examined the effect of melatonin, an established free radicals scavenger, on DNA damage in rat peripheral blood lymphocytes exposed in vitro to iron ions and 50Hz MF. The alkaline comet assay was chosen for the assessment of DNA damage. During pre-incubation, part of the cell samples were supplemented with melatonin (0.5 or 1.0mM). The experiments were performed on the cell samples incubated for 3h in Helmholtz coils at 7mT 50Hz MF. During MF exposure, some samples were treated with ferrous chloride (FeCl2, 10microg/ml), while the rest served as controls. A significant increase in the number of cells with DNA damage was found only after simultaneous exposure of lymphocytes to FeCl2 and 7mT 50Hz MF, compared to the control samples or those incubated with FeCl2 alone. However, when the cells were treated with melatonin and then exposed to iron ions and 50Hz MF, the number of damaged cells was significantly reduced, and the effect depended on the concentration of melatonin. The reduction reached about 50% at 0.5mM and about 100% at 1.0mM. Our results indicate that melatonin provides protection against DNA damage in rat lymphocytes exposed in vitro to iron ions and 50Hz MF (7mT). Therefore, it can be suggested that free radicals may be involved in 50Hz magnetic field and iron ions-induced DNA damage in rat blood lymphocytes. The future experimental studies, in vitro and in vivo, should provide an answer to the question concerning the role of melatonin in the free radical processes in the power frequency magnetic field.

DNA damage in rat lymphocytes treated in vitro with iron cations and exposed to 7 mT magnetic fields (static or 50 Hz).

The present study was undertaken to verify a hypothesis that exposure of the cells to static or 50 Hz magnetic fields (MF) and simultaneous treatment with a known oxidant, ferrous chloride, may affect the oxidative deterioration of DNA molecules. The comet assay was chosen for the assessment of DNA damage. The experiments were performed on isolated rat lymphocytes incubated for 3h in Helmholtz coils at 7 mT static or 50 Hz MF. During MF exposure, part of the cell samples were incubated with 0.01 microM H(2)O(2) and another one with 10 microg/ml FeCl(2,) the rest serving as controls. Lymphocyte exposure to MF at 7 mT did not increase the number of cells with DNA damage in the comet assay. Incubation of lymphocytes with 10 microg/ml FeCl(2) did not produce a detectable damage of DNA either. However, when the FeCl(2)-incubated lymphocytes were simultaneously exposed to 7 mT MF, the number of damaged cells was significantly increased and reached about 20% for static MF and 15% for power frequency MF. In the control samples about 97% of the cells did not have any DNA damage. It is not possible at present to offer a reasonable explanation for the findings of this investigation - the high increase in the number of lymphocytes showing symptoms of DNA damage in the comet assay, following simultaneous exposure to the combination of two non-cytotoxic factors -10 microg/ml FeCl(2) and 7 mT MF. In view of the obtained results we can only hypothesise that under the influence of simultaneous exposure to FeCl(2) and static or 50 Hz MF, the number of reactive oxygen species generated by iron cations may increase substantially. Further studies will be necessary to confirm this hypothesis and define the biological significance of the observed effect.

Hair cell regeneration in the chick basilar papilla after exposure to wide-band noise: evidence for ganglion cell involvement.

It has been demonstrated that the auditory epithelium in the chick basilar papilla may regenerate after acoustic or ototoxic damage. Both types of damage may elicit the appearance of new cells that may develop in to the sensory cells. Factors inducing this process and the role of ganglion cells, the first neuron cells in the auditory pathway, are still unknown. The pattern of auditory damage and regeneration, after octave-band and pure-tone noise exposure, has been well established in research studies on chicks, but there are scarce data on wide-band noise effects. The aim of this study was to investigate the effect of wide-band noise, with different exposure levels applied, on the chick basilar papilla and supporting cells. Further, it was also aimed to determine whether the proliferation of ganglion cells, after wide-band noise exposure, occurs. The morphological changes were assessed with fluorescent, light, and transmission electron microscopy. Cell proliferation was studied based on immunoreactivity assays of proliferating cell nuclear antigen (PCNA). The exposure to wide-band noise at 120 dB SPL for 72 h produced stripe-like lesion of tall hair cells along the neural edge of the basilar papilla, mainly in the middle and, at the lesser extend, in its proximal part. There was no patch-like damage to the region of short hair cells, commonly observed after the exposure to the octave-band or pure-tone noise. The lesion extend depended on the level of exposure. The lower equivalent levels of noise (120 dB SPL for 40 h intermittent exposure) produced proportionally less damage. No morphological changes at light and fluorescent microscopy (apart from tectorial membrane exfoliation) were observed at 110 dB SPL in case of 20 h intermittent exposure. The elimination of dying hair cells took place either by pulling a damaged cell down to the basilar membrane or by extruding the cell to the subtectorial space. New hair cells reappeared at the sensory epithelium on the fifth day after the end of exposure. Cell proliferation started prior to hair cell loss. PCNA-like immunoreactivity was observed after the exposure at all levels in both the damaged and intact areas. PCNA appeared not only in the supporting cells, as indicated in previous studies, but also in the ganglion cells, suggesting ganglion cell involvement in the process of regeneration.

[Hair cell regeneration in a chicken's inner ear after damage due to exposure to industrial noise]. / Ocena regeneracji komórek rzesatych ucha wewnetrznego kurczecia uszkodzonych przez halas przemyslowy.

In mammals, the damage to the hair cells of the inner ear due to exposure to noise or other ototoxic agents is irreversible. In fish, reptiles and birds, however, the hair cells may regenerate, probably from the supporting cells. This regeneration process in the inner ear is being intensively examined in animals in the hope of curing the sensorineural hearing loss in human subjects in future. The aim of the study was to assess hair cell regeneration in the inner ear of chicks after exposure to industrial noise, depending on the level of exposure. The birds were exposed either to the noise at the level of 110 dB (A), 4 hours a day, for 5 consecutive days, or at the level of 125 dB (A), 8 hours a day, for 5 consecutive days. The results confirm that the regeneration starts immediately during the period of recovery from acoustic trauma, and the supporting cells are the main source for new, developing hair cells. Moreover, we found that the complete damage to hair cells is not necessary to the proliferation of supporting cells and that the intensity of proliferation of these cells depends on the level and time of exposure.