Physiologic status of regenerated hair cells in the avian inner ear following aminoglycoside ototoxicity.

Regeneration of avian inner ear hair cells has been demonstrated after administration of aminoglycoside and after acoustic trauma. However, no published study to date has documented functional recovery of these regenerated sensory receptor cells. Newborn chicks were treated with gentamicin sulfate (50 mg/kg/day) for a total of either 5 (n = 10) or 10 (n = 76) days. Evoked potential thresholds were obtained one day after the 5-day treatment, or at intervals between one day and 20 weeks after the 10-day treatment course, and compared to thresholds of age-matched control animals. A significant hearing loss, predominantly in the high frequencies, was present after as few as 5 days of drug administration. The magnitude of hearing loss continued to increase, especially at lower frequencies, as survival increased from 1 day to 5 weeks after gentamicin treatment. Sixteen-to-20 weeks after treatment, partial recovery of thresholds was evident. These findings demonstrate that functional recovery does occur in the avian inner ear following aminoglycoside administration. Recovery occurs at all frequencies, but predominantly at low and middle frequencies, leaving significant residual high-frequency threshold elevation. Recovery lags 14 to 18 weeks behind anatomic evidence of hair cell regeneration, which was demonstrated in one study by 2 weeks after comparable administration of gentamicin.