CHANGES IN COCHLEAR ANTIOXIDANT ENZYME-ACTIVITY AFTER SOUND CONDITIONING AND NOISE EXPOSURE IN THE CHINCHILLA

Exposure to low level noise Frier to a high level exposure reduces noi se-induced hearing loss in mammals. This phenomenon is known as sound conditioning or 'toughening'. Reactive oxygen intermediates have been implicated in noise-induced cochlear damage. To evaluate if in situ an tioxidant processes may play a role in the toughening phenomenon initi ated by low level noise exposure we analyzed glutathione reductase, ga mma-glutamyl cysteine synthetase, and catalase in stria vascularis and organ of Corti fractions from cochleae of chinchillas exposed to a so und conditioning paradigm. Chinchillas were either (A) kept in quiet c ages (control), (B) exposed to conditioning noise of a 0.5 kHz octave band (90 dB for 6 h/day for 10 days), (C) exposed to high level noise (105 dB for 4 h) or (D) exposed to conditioning noise (B) followed by exposure to the higher level noise (C). Each of the noise exposure con ditions (B, C, D) induced changes in the levels of these three antioxi dant enzymes. The enzyme-specific activity data for the four subject g roups support the following two hypotheses. (1) Changes in glutathione reductase, gamma-glutamyl cysteine synthetase, and catalase play a ro le in attenuating hearing loss associated with sound conditioning foll owed by high level noise. (2) Hair cells in the organ of Corti an prot ected from noise-induced damage by increasing stria vascularis levels of catalase, a hydrogen peroxide scavenging enzyme, and of enzymes inv olved in maintaining glutathione in the reduced state. The model formu lated by these hypotheses suggests that agents that protect or augment the glutathione system in the cochlea may be protective against noise -induced hearing loss. (C) 1998 Elsevier Science B.V.