Measuring the middle-ear reflex: A quantitative method to assess effects of industrial solvents on central auditory pathways

Volatile organic solvents are frequently present in industrial atmospheres. Their lipophilic properties mean they quickly reach the brain following inhalation. Acute exposure to some solvents perturbs the middle ear reflex, which could jeopardize cochlear protection against loud noises. As the physiological mechanisms involved in this protective reflex are highly complex, in vivo rodent models are required to allow rapid and reliable identification of any adverse effects of solvents on the middle ear reflex (MER). In this study, MER amplitude was measured in anesthetized Brown-Norway rats by monitoring the decrease in distortion product otoacoustic emissions (DPOAEs) caused by a contralateral stimulation. Our screening test consisted in measuring the impact of inhalation of solvent vapors at 3000 ppm for 15 min on the MER amplitude. We had previously studied a selection of aromatic solvents with this model; here, we extended the analysis to volatile compounds from other chemical families. The results obtained shed light on the mechanisms involved in the interactions between solvents and their neuronal targets. Thus, benzene and chlorobenzene had the greatest effect on MER (>= + 1.8 dB), followed by a group composed of toluene, styrene, p-xylene, m-xylene, tetrachloroethylene and cyclohexane, which had a moderate effect on the MER (between + 0.3 and + 0.7 dB). Finally, trichloroethylene, n-hexane, methyl-ethyl-ketone, acetone, o-xylene, and ethylbenzene had no effect on the MER. Thus, the effect of solvents on the MER is not simply linked to their lipophilicity, rather it depends on specific interactions with neuronal targets. These interactions appear to be governed by the compound's chemical structure, e.g. the presence of an aromatic ring and its steric hindrance. In addition, perturbation of the MER by a solvent is independent of its toxic effects on cochlear cells. As the MER plays a protective role against exposure to high-intensity noises, these findings could have a significant impact in terms of prevention for subjects exposed to both noise and solvents.