Low-level toluene disrupts auditory function in guinea pigs

Toluene appears to have adverse effects on the human auditory system, but it is difficult to estimate its potency since it is commonly present in the workplace in combination with noise exposure; workplace noise exposures are often highly variable. Studies designed to assess toluene ototoxicity specifically have been limited to high-dose studies in a single laboratory animal model, the rat. Here permanent hearing loss has been observed at concentrations of 1000 ppm toluene and greater after inhalation exposure for 5 days, 6 h/day. The OSHA threshold limit value for toluene is only 100 ppm. The current study focuses on the onset of toluene ototoxicity acutely in the guinea pig and in adducing a mechanism of effect. In this study, evidence is presented for the impairment of auditory function by toluene in the guinea pig, at a concentration substantially lower than that used for studying permanent impairment in the rat. The impaired function was correlated with reduced energy metabolism in outer hair cells. Assessment of auditory function was made using distortion product otoacoustic emissions (DPOAE) with subsequent measurement of succinate dehydrogenase (SDH) staining density in hair cells using surface preparations. Temporary disruption of auditory function in guinea pigs is seen in subjects exposed to 250, 500, and 1000 ppm toluene for 8 h/day, 5 day/week for 1 and 4 weeks. Concentrations as low as 250 ppm toluene were able to disrupt auditory function acutely in the guinea pig, and 500 and 1000 ppm toluene produced greater acute dysfunction. SDH staining suggests that reduced enzyme activity in the midfrequency region of the cochlea occurs acutely following toluene exposure. Although the auditory dysfunction progressed between 1 and 4 weeks of exposure, a permanent loss did not develop for these subjects and hair cell death was not seen. The current study identifies early evidence of auditory system impairment in the guinea pig at low toluene concentration and evidence for impairment of energy production in hair cells. While even a transient auditory impairment has implications for workplace safety, additional study on the transition from such acute effects to permanent impairment is essential. (C) 2000 Academic Press.