Behavioral and electrophysiological effects of exposure to an augmented acoustic environment in adolescent/young adult DBA/2J and C57BL/6J mice
Turner, Jeremy G.
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Partial hearing loss serves to deprive the central auditory system of normal input and has been shown to result in changes in auditory system anatomy, physiology, and behavior. However, little is known about how these changes might be exacerbated, counteracted, or otherwise modified by increasing the level of acoustic stimulation as hearing loss develops (e.g., as might be the case for treatments such as the hearing aid). The present study begins to address this topic by assessing the effects of increased acoustic stimulation during hearing loss. Two strains of mice which differ with respect to hearing loss were used. DBA/2J (DBA, which demonstrate considerable hearing loss by 55 days of age) and C57BL/6J mice (C57, which do not) were exposed to an augmented acoustic environment (AAE; broadband noise bursts, 70 dB SPL, 200 ms duration, 10 ms rise/fall, 2 Hz rate) for 10 consecutive 12-hour nights during one of three age periods: 25-35, 35-45, or 45-55 days. Littermate controls were not exposed to the noise. Prepulse inhibition (PPI), a technique for measuring the behavioral salience of sounds, was measured before and after the 10-day period using 70 dB (SPL) tone pips of 4, 8, 12, 16, and 24 kHz. Auditory brainstem response (ABR) thresholds, an electrophysiological estimate of auditory sensitivity, were obtained for the same five frequencies. \ In DBA mice, acoustic exposure from 25-35 days of age resulted in poorer PPI whereas the same exposure in the older age groups led to better PPI. In addition, AAE exposure resulted in better ABR thresholds for some frequencies in the youngest two age groups and larger startle amplitudes in the oldest age group. No significant effects of acoustic exposure on PPI, startle amplitudes, or ABR thresholds were observed for normal-hearing C57 mice of any age group. These findings suggest that AAE exposure implemented at the appropritate times in a hearing-impaired animal can a) improve auditory sensitivity as measured by ABR thresholds and startle amplitude, and b) improve or impair auditory behavioral/perceptual abilities as measured by PPI.