Scandinavian audiology. Supplementum
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Review
Genetic influences on susceptibility of the auditory system to aging and environmental factors.
Auditory susceptibility to aging and environmental factors (noise and toluene) was investigated in two genotypes of mice, CBA/Ca (showing a moderate hearing loss with onset late in life) and C57BL/6J (undergoing spontaneous auditory degeneration with onset during young adulthood). Both strains of mice were often used together as a model of human age-related hearing loss, with a primary decline in auditory high-frequency sensitivity. Auditory function was analysed by recording auditory brainstem response (ABR). ⋯ First an additivity and then, at total threshold shifts of 30-50 dB, a blocking-like interaction were observed. The rate of progression in post-noise hearing loss did not exceed the spontaneous rate of age-related degeneration. The difference between exposed and non-exposed groups decreased with advancing age for both strains of mice.(ABSTRACT TRUNCATED AT 400 WORDS)
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The aim of the present investigation was to determine: 1) the relationship between changes in auditory sensitivity and alterations in stereocilia micromechanics and tectorial membrane morphology after acoustic overstimulation; 2) the rate of growth of a threshold shift in stereocilia following in vitro overstimulation; and 3) if the damaging effects of noise trauma can be reduced by pre-exposure to a low level acoustic stimulus. After exposure to a 1.0 kHz pure tone signal at 105 dB SPL for 72 hours, the threshold of the auditory brainstem response was broadly elevated by approximately 40-50 dB; the inner hair cell stereocilia became less stiff; and morphological alterations were observed in the middle zone of the tectorial membrane. The location of both the stereocilia and tectorial membrane alterations corresponded to the region of the cochlea demonstrating a threshold shift. ⋯ After 6 minutes of overstimulation, the threshold shift exhibited a plateau whose magnitude was dependent upon the exposure intensity. Stereocilia micromechanics were shown to be dependent on the metabolism of the hair cell. The pre-treatment to a low level acoustic stimulus (81 dB SPL) prior to exposure to a stimulus known to yield a permanent threshold shift resulted in a 20 dB reduction in the threshold shift relative to the group not pre-exposed as well as complete recovery from the threshold shift after 2 months.