Effect of salicylate on electrically evoked otoacoustic emissions elicited in the first and third turns of the guinea pig cochlea.

OBJECTIVE

Electrically evoked otoacoustic emissions (EEOAEs) are sounds emitted in the ear canal when an alternating current is delivered to the cochlea. Salicylate, which causes reversible hearing loss, decreases both the electromotility of the outer hair cells (OHCs) and the EEOAE output. The frequency response of EEOAEs is known to depend on the location of the stimulating electrode. Using electrodes placed in the first or third turn of the cochlea, we measured EEOAEs before and after salicylate administration in order to clarify the frequency range of EEOAEs from which exact information can be acquired.

MATERIAL AND

METHODS

A sinusoidal electrical signal (160 microA root-mean-square) at frequencies ranging from 500 Hz to 16 kHz was delivered to the first or third turn of the scala tympani of the guinea pig cochlea in order to determine EEOAEs. The EEOAE outputs were measured before and every 5 min after administration of salicylate (500 mg/kg) for 60 min. The threshold of the compound action potential (CAP) was measured before and 60 min after salicylate administration.

RESULTS

CAP thresholds were reduced at frequencies ranging from 2 to 12 kHz after salicylate administration. The maximum EEOAE output recorded with the first-turn electrode was obtained at 12 kHz with a gentle slope of low cutoff, whereas the frequency response of the EEOAE recorded with the third-turn electrode demonstrated a steep dip at 2 kHz.

CONCLUSION

EEOAE reduction after salicylate administration was restricted to frequencies close to that of the characteristic frequency (CF) of the electrode position. This was probably related to the fact that the EEOAE is the sum of waves from the CF of the electrode position and the stimulus frequency. In terms of studying the electromotility of OHCs, useful information obtained from EEOAEs is restricted to the narrow frequency band close to the CF of the electrode location.