Functional and morphological effects of styrene on the auditory system of the rat

The effect of inhaled styrene on the structure and function of the auditory organ of the male Wistar rat was studied. The animals were exposed either to 600, 300 or 100 ppm styrene (12 h/day, 5 days/week, for 4 weeks). Auditory sensitivity was tested prior to and after the exposure by auditory brain stem audiometry (ABR) at frequencies of 1.0, 2.0, 4.0 and 8.0 kHz. Inner ear morphological changes were studied by light- and electron-microscopy. Exposure to 600 ppm styrene caused a 3 dB hearing loss only at the highest test frequency (8 kHz). Quantitative morphological analysis of cochlear hair cells (cytocochleograms) showed that 600 ppm styrene caused a severe outer hair cell (OHC) loss particularly in the third OHC row of the upper basal and lower middle coil. The inner hair cells were usually intact. Exposure to lower styrene concentrations (100 and 300 ppm) caused no unequivocal functional deficit or hair cell damage. We conclude that there appears to be a concentration threshold for styrene ototoxicity in rats (between 300 and 600 ppm).