Determination of auditory threshold was carried out under a response duration schedule of tone and/or light- dipper presentation. This schedule enabled an exact assessment of auditory sensitivity, since the direction and distance from rat to sound source were constant and the rat could thus acquire the conditioned behavior in a short time. The rat has to press the lever and hold it until a CS(tone and/or light) was presented. The animal could get water-reinforcement with release of the lever during the period of CS. The intensity of 3 KHz pure tone was reduced, from 110 dB (starting point), by 2 dB steps in each trial, until the subject made 3 non-response trials(i.e., no dipper approach within 0.7 sec after the tone onset) among 5 trials. The highest tone intensity among non-response trials was taken as the threshold. The auditory sensitivity of the rats with cotton-stuffed ears and the pierced eardrums decreased about 10 and 20 dB, respectively. The auditory sensitivity in rats was decreased by administration of streptomycin sulfate(SM), 300 mg/kg/day, i.m., in combination with ethacrynic acid(EA), 50 mg/kg/day, p.o., for forty days and in combination with EA, 100 mg/ kg/day, p.o. for ten days. The auditory sensitivity was not influenced by administration of SM, 300 mg/kg/ day, i.m., for forty days.
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