Effects of lidocaine on basilar membrane vibration in the guinea pig

The effects of lidocaine on basilar membrane (BM) vibration and compound ac tion potential (CAP) were studied in guinea pigs in order to elucidate the site of lidocaine action in the cochlea. BM vibration was measured with a l aser Doppler vibrometer through an opening made in the lateral bony wall of the scala tympani at the basal turn. Ten min after local administration of lidocaine (250 mug) into the scala tympani, the velocity of BM vibration a nd the CAP amplitude decreased significantly at around the characteristic f requency of the stimulus sound (p < 0.05). The maximum decreases were 4 dB in the velocity of the BM vibration and 40 dB in the CAP amplitude. In cont rast, such changes were not observed after i.v. injection of lidocaine (1.5 mg/kg). These results suggest that when lidocaine is administered locally in the cochlea it acts not only on the cochlear nerve but also on the outer hair cells.