The transient increase in auditory evoked response amplitude was observed before the complete hearing loss after daily administration of Kanamycin (KM). Such transient phenomena in the auditory system can be seen occasionally in the basic experiments such as noise exposure experiments. The mechanism of this transient amplitude change, however, has not been well established.
By examinig the course of recovery time of the depressed amplitude of the auditory evoked responses immediately after intense noise exposure, which is well known as temporary threshold shift (TTS), we obtained following results:
(1) From our preliminary experimental results, the 2KHz pure tone at 115dBSPL with 10min duration was used as the exposure sound.
(2) In 10 normal hearing guinea pigs without KM administration, the recovery function of auditory brain stem responses (ABR) which was evoked by 2KHz short tone burst at 95dBSPL with repetition rate of 20/sec, was examined after the completion of the sound exposure. The recovery time to the pre-exposure level (100%) was about 5min.
(3) The cochlear microphonics (CM) recorded chronically from the round window after the sound exposure showed no suppression contrary to that of ABR.
(4) Five of 14 animals which received 400mg/kg KM daily injection exhibited overshoots (20-100%) of the ABR amplitude after several to 10 days treatments of KM and showed sudden disappearance of the ABR about 14 days after the first administration.
(5) During the period of the transient amplitude increase, the input-output curves showed a recruitment-like overshoot by high intensity sound stimuli at above 80dBSPL and a slight threshold elevation by low intensity stimuli.
(6) The recovery time to the pre-exposure level after the sound exposure was about 10min in KM-treated animals at the period of the transient amplitude increase.
