HAIR CELL REGENERATION AFTER LOCAL APPLICATION OF GENTAMICIN AT THE ROUND WINDOW OF THE COCHLEA IN THE PIGEON

Hair cells in the basilar papilla of birds have the capacity to regene rate after injury. Methods commonly used to induce cochlear damage are systemic application of ototoxic substances such as aminoglycoside an tibiotics or loud sound. Both methods have disadvantages. The systemic application of antibiotics results in damage restricted to the basal 50% of the papilla and has severe side effects on the kidneys. Loud so und damages only small parts of the papilla and is restricted to the s hort hair cells. The present study was undertaken to determine the eff ect of local aminoglycoside application on the physiology and morpholo gy of the avian basilar papilla. Collagen sponges loaded with gentamic in were placed at the round window of the cochlea in adult pigeons. Th e time course of hearing thresholds was determined from auditory brain stem responses elicited with pure tone bursts within a frequency rang e of 0.35-5.565 kHz. The condition of the basilar papilla was determin ed from scanning electron micrographs. Five days after application of the collagen sponges loaded with gentamicin severe hearing loss, excep t for the lowest frequency tested, was observed. Only at the apical 20 % of the basilar papilla hair cells were left intact, all other hair c ells were missing or damaged. At all frequencies there was little func tional recovery until day 13 after implantation. At frequencies above 1 kHz functional recovery occurred at a rate of up to 4 dB/day until d ay 21, beyond that day recovery continued at a rate below 1 dB/day unt il day 48 at the 5.6 kHz. Below 1 kHz recovery occurred up to day 22, the recovery rate was below 2 dBiday. A residual hearing loss of about 15-25 dB remained at all frequencies, except for the lowest frequency tested. At day 20 new hair cells were seen on the basilar papilla. At day 48 the hair cells appeared to have recovered fully, except for th e orientation of the hair cell bundles. The advantage of the local app lication of the aminoglycoside drug over systemic application is that it damages almost all hair cells in the basilar papilla and it has no toxic side effects. The damage is more extensive than with systemic ap plication. (C) 1998 Elsevier Science B.V. All rights reserved.