Potentiation of cochlear hair cell loss by acoustic stimulus and gentamicin in the guinea pig

We explored the possibility of synergism between a pure-tone stimulus and gentamicin in causing cochlear injury by analyzing hair cell loss. Guinea pigs receiving daily injections of gentamicin (200 mg/kg body wt.) for 1 week were exposed to a 2 kHz tone (95 dB SPL, 2 hours daily). Surface preparations of the spiral organ were studied by phase contrast microscopy, and the extent of hair cell loss in the entire organ of Corti was recorded in cytocochleograms. Gentamicin by itself was slightly ototoxic, damaging the innermost row of outer hair cells, whereas exposure to sound alone caused no hair cell loss. Combined antibiotic and acoustic exposure produced extensive cochlear damage. A few animals showed massive hair cell degeneration and collapse of the organ of Corti, except in the apical turn. The site of damage was possibly determined by the frequency of the sound stimulus. Thus, an intermittent tonal stimulus such as that used in the present experiment can be harmless by itself, but causes injury to cochlear hair cells in guinea pigs when administered in combination with gentamicin.