The tonotopicity of styrene-induced hearing loss depends on the spectrum of the noise associated with.

Neuropharmacological and cochleotoxic effects of styrene can worsen the noise impact on the peripheral auditory receptor (Campo et al., 2014). The mechanisms by which co-exposures to noise and styrene impair hearing are complex: there is a long cochleotoxic process which can be hidden during a short period by the fast pharmacological effect on the central nervous system. For this reason, the current investigation was designed in order to delineate the auditory frequency range sensitive to noise, to styrene, and to noise and styrene. Such information might be helpful to distinguish the noise-induced hearing loss from the styrene-induced hearing loss, and thereby to point out the factor mainly responsible for a deafness measured in a complex occupational environment. Male Brown Norway rats were exposed either to 600-ppm styrene, to an octave band noise centered at 8 kHz, or to both noise and styrene. The noise exposure had two different natures: impulsive with a LEX,8h of 80 dB and continuous with a LEX,8h of 85 dB SPL. Hearing was tested with a non invasive technique based on distortion product otoacoustic emissions. Hearing data were completed with a histological analysis of cochleae. The results showed that the single styrene exposure provoked outer hair cell losses located in the apical cochlear region or, in other terms, in the region discriminating the low-frequencies. In contrast, the noise-induced hearing loss was located at half an octave above the central frequency of the spectrum, around 10-12 kHz. The impulse noise damages were largely exacerbated by the styrene and the spectrum of the noise had set the location of the cochlear trauma. Following the combined exposure, the cell losses exceeded the summed losses caused by the impulse noise and styrene alone. The fact that the tonotopicity of the styrene-induced damages depends on the spectrum of the noise associated with it makes the diagnosis difficult with a tone-frequency audiometric approach. In conclusion, there is not really a frequency specificity of impairments due to styrene.