Pathophysiologic responses of the inner ear to intense sound

Interaction of aminooxyacetic acid and ethacrynic acid with intense sound at the level of the cochlea

Results of previous investigation of the interaction of intense sound and drugs have, in general, failed to show a protective effect mediated by pre-administration with a drug having transient ototoxic effects. The present investigation was designed to further evaluate a protective effect found previously at the anatomical level and explained with an electrochemical theory of noise damage. The alternating current (a.c) potential and compound eighth nerve action potential (CAP) amplitude were monitored in aminooxyacetic acid (AOAA)- or ethacrynic acid (EA)-treated guinea pigs exposed to either moderate or high levels of intense sound and compared to changes observed in the same potentials in animals exposed to the intense sounds alone. Results showed protective effects only in the moderate--intense sound-exposure groups, with changes in sensitivity and voltage on the linear part of the input--output curve of the a.c cochlear potential found to be the only conditions where differences occurred. These results were difficult to interpret in terms of a protective effect and point to the need for obtaining additional data before an electrochemical mechanism is shown to play a role in the effect of intense sound on the cochlea.

Effects of noise on cochlear potentials and endolymph potassium concentration recorded with potassium-selective electrodes

Guinea pig cochleas were exposed to either broad-band noise at intensities between 95 and 115 dBA or octave-band noise centered at 380 Hz or 4.2 kHz at intensities between 115 and 125 dB SPL. Cochlear microphonics (CM), summating potentials (SP) and action potentials (AP) were recorded from differential electrodes in the perilymphatic scalae between successive 20-min periods of noise exposure. The endocochlear potential (EP) and endolymph potassium concentration [Kendo+] were recorded continuously from scala media using double-barreled potassium-sensitive electrodes. It was found that the initial exposure to noise at 115 dBA produced considerable suppression of the CM and AP, while the EP and [Kendo+] were elevated above their normal values. When animals previously treated with kanamycin were subjected to the same level of noise exposure no systematic increase in either EP ro [Kendo+] was observed. After prolonged exposure to 380 Hz octave-band noise at 125 dB SPL, a slow decline of EP and [Kendo+] was observed. The relationships between the changes in EP, [Kendo+] and CM are discussed.