To investigate the auditory impairment induced by the administration of aminoglycosides in the newborn, the shuttle box method was employed to measure the auditory threshold of rats. Five groups of newborn rats were administered kanamycin sulfate, 250 and 500 mg/kg, streptomycin sulfate, 250 and 500 mg/kg, or 1 ml/kg saline, subcutaneously, from the 10th to the 15th day of birth. The auditory threshold of the control group could be measured by the shuttle box method at the age of 100 days. The auditory threshold of the control group was 52.1 ± 1.0 dB (N = 14). The auditory thresholds of the animals treated with kanamycin 250 mg/kg and streptomycin 250 mg/kg groups were measured in only 1 (61.0 dB) and 4 (64.8±4.6 dB), respectively, since the auditory toxicity of these drugs in newborn rats was stronger than adult rats. Auditory threshold of the 250 mg/kg streptomycin group was significantly higher than that of the control group. The animals which could not be measured for the auditory threshold had the ability to acquire conditioned avoidance response when both conditioned stimuli (tone and light) were presented. However, after differentiation of the stimuli, the percent avoidance to tone in these animals was significantly decreased and did not recover by the following trainings, while the percent avoidance to light was similar to that before the differentiation. From these results, it is suggested that the auditory threshold of rats whose ears are slightly impaired can be measured by our method and the auditory impairment of rats whose ears are seriously impaired such as deaf animals can be detected from acquisition curve of conditioned avoidance response.
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