Evidence of a Common Pathway in Noise-Induced Hearing Loss and Carboplatin Ototoxicity.

In spite of the differences in the nature of the insult, the hearing loss from ototoxic drugs and noise exposure share a number of similarities in cochlear pathology. This paper explores the common factors between noise-induced hearing loss and ototoxicity by experimentally manipulating cochlear glutathione (GSH). In the first experiment, chinchillas were treated with a drop of saline (50 &mgr;l) on the round window of one ear and a drop of buthionine sulfoximine (BSO, 50 &mgr;l of 200 mM) on the other ear. BSO is a drug that blocks GSH synthesis and it was hypothesised that GSH-depressed ears would be more vulnerable to noise. Six hours after treatment, the animals were exposed to a 105 dB 4 kHz octave band noise for 4 hours, then a second dose of BSO was applied 2 hours later. The BSO treated ears showed more temporary threshold shifts and reduced GSH staining at day 4 post exposure, but there was no BSO effect in terms of greater permanent threshold shift (PTS) or hair cell loss. In the second experiment, chinchillas were pretreated with BSO and 3 days later were given either a single dose of carboplatin (25 mg/kg i.p.), a double dose (day 3 and 7) or only BSO. Chinchillas that received BSO and the double dose of carboplatin had significantly greater loss of inner and outer hair cells than the carboplatin chinchillas. In addition, the BSO and carboplatin chinchillas also had larger decreases in evoked response amplitudes suggesting that GSH depletion potentiated the ototoxicity of carboplatin. These results are discussed in terms of the role of reactive oxygen species in creating hearing loss and the potential protective role of glutathione.