Auditory Brainstem Response (ABR) Measurements

Guinea pigs were subjected to ABR measurements (Otometrics, Taastrup, Denmark) before, 1 h, 1 day, 3 days, and 5 days after the noise stimulation. Subsequently, they were anesthetized by i.p injection of 4% pentobarbital (0.3 mL/100 g). After anesthesia, they were moved into a soundproof room and placed on an electric blanket. Cotton swabs were used to clear cerumen from both ears. The recording electrode was inserted subcutaneously into the middle of the vertical line between the ears on the head, the reference electrode was inserted subcutaneously behind the test ear, and the grounding electrode was inserted into the root of the right hind limb. The sound stimulus was composed of a 15-ms tone burst, with a rise-fall time of 1 ms at frequencies of 2, 4, and 8 kHz (Su et al., 2019). Brainstem auditory evoked responses, which were accumulated 600 times, were stimulated by density alternating short click sounds produced by the potentiometer. The signal was monitored by an oscilloscope and then input into a computer through an amplifier. The lowest stimulation intensity that allowed the ABR III wave to be distinguished was considered as the auditory threshold.