Toluene-induced hearing loss in rats evidenced by the brainstem auditory-evoked response

Behavioral results showing that toluene causes hearing loss in rats precipitated an electrophysiologic study of the auditory thresholds of these rats using the brainstem auditory-evoked response (BAER). Twenty-three-day-old male Fischer-344 rats had been exposed to 1400 or 1200 ppm toluene 14 hrs/day, for 4 or 5 weeks while a control group was exposed only to air. The rats were tested 2.5 mo after termination of the exposures. BAERs, recorded with 25-gauge needle electrodes placed over the nose and posterior skull, were evoked by 100-microseconds-duration clicks and 1-msec-duration tone pips at eight intensities. Thresholds for the appearance of BAERs in the toluene-exposed rats were elevated by 13 to 27 dB, and latency-intensity functions were consistent with the occurrence of sensorineural hearing loss. The amplitudes of the third and fifth components of the BAER were attenuated at high stimulus intensities in the toluene-exposed rats. These behavioral and electrophysiologic results are apparently the first to indicate the ototoxicity of toluene in experimental animals.