Mitochondrial role in hair cell survival after injury.

The role of mitochondrial biogenesis in hair cell survival after injury was evaluated by inhibiting mitochondrial protein synthesis with chloramphenicol and then studying the effects on hair cell survival after exposure to two different types of ototoxins, gentamicin and acoustic trauma. Seven- to 10-day-old chicks were treated with either a single injection of gentamicin (250 mg/kg) or noise (1500 Hz at 120 dB sound pressure level for 14 hours). A subset of the gentamicin- and noise-treated animals also received chloramphenicol (1200 mg/kg during a 24-hour period) through a subcutaneous osmotic pump. A control group received chloramphenicol alone (1200 mg/kg during a 24-hour period). All animals were sacrificed after 5 days, and their basilar papillae (cochleas) were prepared for scanning electron microscopy. Hair cell loss was quantified with stereologic techniques. Animals treated with chloramphenicol alone did not have any evidence of hair cell loss. Gentamicin-treated animals had characteristic hair cell loss beginning at the basal tip and tapering out along the inferior edge more distally. The addition of chloramphenicol to gentamicin treatment significantly increased hair cell loss by 30%, extending the area of hair cell loss into the superior hair cell region at the distal boundary of the lesion. Pure-tone noise exposure characteristically produced hair cell loss along the inferior edge and occasionally included hair cells along the most superior edge. Addition of chloramphenicol to noise exposure significantly increased hair cell loss by 80%, with extension of the lesion across the full width of the sensory epithelium and basally. These results demonstrate that mitochondrial biogenesis is involved in cellular responses to injury. They suggest that mitochondrial function may regulate the probability of survival after metabolic challenges to hair cell integrity.