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

Hearing loss caused by intense sound exposure can be significantly reduced by pre-exposing subjects to moderate-level acoustic stimuli. This phenomeno n occurs in a variety of mammals. We investigated whether sound conditionin g 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 pig s (group I) in which one external ear canal was occluded were exposed to co nditioning sound (2-20 kHz, 85 dB SPL, 5 h/day, 10 days). After removing th e 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. Ani mals without ear canal occlusion were also exposed to the intense sound wit hout conditioning (group IT) or following the same conditioning exposure (g roup III). Intense sound exposure caused significantly greater permanent AB R threshold shifts at all frequencies tested (4, 8, 12, 16 and 20 kHz) in g roup II than in group III. Tn group I, the occluded ears showed significant ly greater threshold shifts at all frequencies compared to the unoccluded e ars. 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 w ere identical to those in group III. Protective effects provided by sound c onditioning were almost the same in group III and in the unoccluded ears in group I. The extent of hair cell damage supported the physiological findin gs. These results indicate that acquired resistance to acoustic trauma prov ided by sound conditioning is restricted to the cochlea exposed to conditio ning sound, suggesting that conditioning protection is mediated primarily b y the changes that occur locally within the conditioned cochlea. This anima l model, with unilateral external ear canal occlusion during sound conditio ning, is useful for studies of the mechanisms of conditioning protection. ( C) 1999 Elsevier Science B.V. All rights reserved.