ABSTRACT
Loss of adaptability rather than loss of sensitivity may be one of the initial signs of auditory impairment following exposure to noise. One way to examine the adaptability of hearing in experimental investigation is to measure the magnitude of the suppression, exerted by the medial olivocochlear efferent system, on the ipsilateral otoacoustic emissions in response to contralateral sound stimulation. Thus, in order to test the hypothesis it was decided to measure hearing thresholds (HT), the cubic DPOAE and suppression of cubic DPOAE by contralateral wide band noise in rats exposed to long-term, low level noise (90 days of 90 dBlin 4-20 kHz wide band noise 4 hours/day, 5 days/week). Measurements of HT were performed by assessment of the ABR, elicited by tone-pips from the same probe assembly used in the measurements of DPOAE. The suppression of the cubic distortion product (CDP) was determined in ketamine/xylazine anaesthesia, allowing a stable response for a minimum of 20 min. Of the frequencies tested, the rats exposed to noise had an increase in HT at 12.8 kHz only (6.8 dB, P<0.05), while a reduction on the CDP was evident with f2 going from 9.2 kHz to the upper limit at 17.4 kHz. Further, the rats exposed to noise had little suppression of the CDP at low levels of contralateral noise (CN), but no difference from the control animals was seen as the CN noise level was increased. The measurement of DPOAE suppression did not reveal any effects of the low level noise exposure that was not paralleled also by shifts in hearing thresholds. The most sensitive assessment of the auditory changes in the study was the measurements of DPOAE, and further elaboration on the bandwidth and frequency distribution of the CN is necessary, before auditory changes in the high frequency range can be probably assessed.
ABSTRACT
Hearing loss in workers exposed to organic solvents has been shown to be the effect of interaction between the exposure to solvents and noise. Synergistic interaction has been demonstrated in rats following simultaneous exposure to toluene and noise, but only at high-level toluene exposure. The present study was initiated to investigate the potential interaction of exposure to noise and toluene on the auditory system of the rat, covering a dose-range of toluene exposure (0, 500, 1000, 1500, and 2000 ppm, 6 h/d, 10 d). Exposed to toluene only, the rats exposed at the 1500 and 2000 ppm level developed a mid-frequency ABR threshold shift, whereas rats exposed to 0, 500, and 1000 ppm did not exhibit signs of auditory impairment. Rats exposed to 500 ppm toluene and noise (96 dB SPL, 2h following the daily toluene exposure, 10 d) developed a small, but statistically significant threshold shift, equal to the hearing loss in rats exposed to noise only (0 ppm). Synergistic interaction was evident at the 1000, 1500, and 2000 ppm toluene exposure levels. There was no further hearing loss at the 2000 ppm than at the 1500 ppm level, indicating that a saturation of the auditory impairment had been reached. When acute noise exposure (105 dB SPL, 4 h) followed the toluene exposure by 30 days, interaction was noted at the 1500 ppm toluene exposure level, but not at the 1000 ppm level. However, the latter type of interaction is of indirect nature and should be distinguished from the direct interaction, taking place when toluene is physically present in the cochlea during exposure to noise. Further investigations in animal models should preferentially be carried out as long-term, low-level exposure studies, showing the possible interaction at low exposure levels, where exposure to each factor alone is without any effect.