Acquired resistance to acoustic trauma by sound conditioning is primarily mediated by changes restricted to the cochlea, not by systemic responses

Abstract

Hearing loss caused by intense sound exposure can be significantly reduced by pre-exposing subjects to moderate-level acoustic stimuli. This phenomenon occurs in a variety of mammals. We investigated whether sound conditioning provides acquired resistance to acoustic trauma through local mechanisms selectively in the conditioned ears or if systemic mechanisms are involved that would yield contralateral protection in unconditioned ears. Guinea pigs (group I) in which one external ear canal was occluded were exposed to conditioning sound (2–20 kHz, 85 dB SPL, 5 h/day, 10 days). After removing the occlusion, the animals were then subjected bilaterally to intense noise (2–20 kHz, 110 dB SPL, 5 h) 5 days after the last conditioning exposure. Animals without ear canal occlusion were also exposed to the intense sound without conditioning (group II) or following the same conditioning exposure (group III). Intense sound exposure caused significantly greater permanent ABR threshold shifts at all frequencies tested (4, 8, 12, 16 and 20 kHz) in group II than in group III. In group I, the occluded ears showed significantly greater threshold shifts at all frequencies compared to the unoccluded ears. The threshold shifts in the occluded ears in group I were identical to those observed in group II; and the shifts in unoccluded ears in group I were identical to those in group III. Protective effects provided by sound conditioning were almost the same in group III and in the unoccluded ears in group I. The extent of hair cell damage supported the physiological findings. These results indicate that acquired resistance to acoustic trauma provided by sound conditioning is restricted to the cochlea exposed to conditioning sound, suggesting that conditioning protection is mediated primarily by the changes that occur locally within the conditioned cochlea. This animal model, with unilateral external ear canal occlusion during sound conditioning, is useful for studies of the mechanisms of conditioning protection.