Cochlear neuropathy in the rat exposed for a long period to moderate-intensity noises.

Damaging effects on the cochlea of high-intensity acoustic overexposures have been extensively documented, but only few works have focused on the danger of moderate noise levels. Using scanning and transmission electron microscopy, we explored the noise-induced neuroepithelial changes that occur in the cochlea of rats subjected to moderate intensities, 70 and 85 dB SPL, for an extended period of time (6 hr/day over 3 months). Although the full quota of outer and inner sensory hair cells remained present, we detected discrete abnormalities, likely resulting from metabolic impairment, in both types of hair cell within the basal region of the cochlea. In contrast, important noise-dependent losses of spiral ganglion neurons had occurred. In addition, we found cytoplasmic accumulations of lipofuscin-like aggregates in most of the surviving cochlear neurons. These results strongly suggest that noise levels comparable to those of certain working environments, with sufficient exposure duration, pose a severe risk to the cochlea. Moreover, our data support the notion that long-duration exposure to moderate noise is a causative factor of presbycusis.

Assessment of cochlear effects and extra-auditory disorders in male rats exposed repetitively to low noise.

BACKGROUND: The noise is considered as a factor of environmental stress, causing a wide range of health effects such as acoustic, cardiovascular, nervous and endocrine systems. PURPOSE: The present study was conducted to examine the affects of repeated exposure to noise on the peripheral auditory system, adrenal gland and heart tissue. METHOD: The White strain rats "Wistar" were exposed to chronic and repetitive exposure noise at two different intensity levels of 70 and 85dB (A). The noise level was generated by the Audacity® software to an octave-band noise (8616 kHz). The sound exposure duration was 6 hr/day, 5 days per week for 3 months. Quantitative and qualitative investigations were performed by using electron microscopy. The ganglion neuron counting was examined via light microscopy. RESULTS: The results show that exposure to sound intensities 70 and 85 dB (A) for long periods, lead to changes in the morphological structure of the cochlea (inner ear), adrenal cortex and cardiac tissue which involve cell disruption which over time can lead to pathological effects. CONCLUSION: This study provides morphological evidence that repetitive exposure noise at moderate sound levels to 70 and 85 dB (A) induces changes in the peripheral auditory system, the adrenal cortex and heart tissue.

Morphological changes of adrenal gland and heart tissue after varying duration of noise exposure in adult rat.

Noise was considered an environmental stressor causing a wide range of health effects such as acoustic, cardiovascular, nervous, and endocrine systems. The present study was performed to examine the effects of a repeated noise exposure on adrenal gland and heart tissue. The results showed that exposure to moderate intensity sound (70 dB[A]) causes time-dependent changes in the morphological structure of the adrenal cortex that involve disarrangement of cells and modification in thickness of the different layers of the adrenal gland. The experiment revealed important changes depending on exposure duration in the morphological structure of heart tissue that causes irreversible cell damage leading to cell death or necrosis.

Qualitative and quantitative assessment of noise at moderate intensities on extra-auditory system in adult rats.

Noise has long been realized as an environmental stress causing physiological, psychological and behavioral changes in humans. The aim of the present study was to determinate the effect of chronic noise at moderate intensities on both glandular and cardiac function and oxidative status. Our problem comes from working conditions in call centers where operators are responsible for making simple and repetitive tasks. One wishes to ascertain the effects of moderate sound levels on rats exposed to the same noise levels during similar periods to those experienced by call center operators. Male Wistar rats were exposed to 70 and 85 dB(A) to an octave-band noise (8-16 kHz) 6 h/day for 3 month. Corticosterone levels, oxidative status and functional exploration of adrenal and thyroid glands and cardiac tissue were determined. Exposure to long-term noise for different intensities (70 and 85 dB(A)) resulted in increased corticosterone levels, affected various parameters of the endocrine glands and cardiac function. Markers of oxidative stress (catalase, superoxide dismutase and lipid peroxidation) were increased. These results imply that long-term exposure to noise even at moderate levels may enhance physiological function related to neuroendocrine modulation and oxidative imbalance. In these data, the physiological changes occur during the different sounds suggests the concept of allostatic load or homeostatic response of the body.