Medial olivocochlear efferent terminals are protected by sound conditioning

Synaptophysin immunoreactivity was used as a marker for the olivocochlear e fferent system that innervates the outer hair cells of the cochlea. An inte nse noise exposure at either 6.3 kHz or 1.0 kHz caused a significant reduct ion in anti-synaptophysin immunoreactivity within the 8-6 mm or 14-11 mm di stance from the round window, respectively. In the region of the main lesio n, the reduction in synaptophysin immunoreactivity for both the 6.3 and 1.0 kHz exposures correlated well with outer hair cell loss. In regions periph eral to the main lesion, some remnants of efferent nerve endings could rema in even when their associated outer hair cells were missing. Pre-treatment with a low level sound conditioner (either at 6.3 tone or 1.0 kHz) effectiv ely reduced the efferent and outer hair cell pathology induced by the 6.3 a nd 1.0 kHz intense noise exposures, respectively. The results demonstrate t he feasibility of using anti-synaptophysin immunoreactivity as an effective means of quantifying pathological alterations to the medial cochlear effer ent terminals throughout the cochlea. Furthermore, the results show that so und conditioning significantly reduces damage to the efferent terminals. (C ) 1999 Elsevier Science B.V. All rights reserved.