Abstract
Animal studies have shown that toluene can cause hearing loss and can exacerbate the effects of noise by inhibiting the middle ear acoustic reflex. In this investigation, carried out in Long-Evans rats, the tensor tympani tendon was cutoff and the stapedius muscle was electrocoagulated in one or both middle ears. Rat hearing was evaluated by measuring cubic distortion otoacoustic emissions (2f1-f2; f1 = 8000 Hz; f2 = 9600 Hz; f1/f2 = 1.2) prior to, during, and after activation of the olivocochlear (OC) reflex. A band noise centered at 4 kHz was used as suppressor noise. It was delivered contralaterally to decrease 2f1-f2 amplitude. The strength of the inner ear acoustic reflex was tested by increasing contralateral noise intensity, and toluene injected into the carotid artery was used to study physiological efficacy. Results showed that the protective effect of the OC reflex is intensity dependent. In addition, the OC reflex was found to be less sensitive to toluene than the middle ear acoustic reflex. This may be because the efferent neurons involved in inner ear and middle ear reflexes are located differently. In conclusion, the synergistic effects on hearing of co-exposure to noise and aromatic solvents are because of solvents depressing the central nuclei, which mainly drive the middle ear acoustic reflex.
Highlights
Noise is clearly the predominant occupational hazard to hearing, research on hearing conservation proves that noise is often present in occupational settings where chemical exposure occurs (EU-OSHA, 2009; Johnson and Morata, 2010)
We obtained a significant (MW 1⁄4 0; p 1⁄4 0.002) decrease in 2f1-f2 DPOAE amplitude because of the noise suppressor (NS) delivered to the contralateral side (Fig. 3A)
In a previous paper (Venet et al, 2011), we showed that toluene could either increase or decrease the efficiency of the middle ear acoustic reflex depending on the toluene concentration applied and the ear receiving NS stimulation
Summary
Noise is clearly the predominant occupational hazard to hearing, research on hearing conservation proves that noise is often present in occupational settings where chemical exposure occurs (EU-OSHA, 2009; Johnson and Morata, 2010). Our research team showed that toluene can affect the auditory nervous system involved in ear-protective reflexes, in particular the middle ear reflex. This reflex arc is elicited by sound-evoked efferent feedback and provokes contraction of the stapedius and tensor tympani muscles, reducing the acoustic energy penetrating the cochleae. The protective effects induced by the inner ear reflex were too weak compared with those induced by the middle ear reflex (2 vs 16 dB) to be accurately observed. OC efferent neurons generate a sound-evoked reflex pathway to the inner ear, and activity in this pathway can suppress cochlear responses (Liberman et al, 1996 ; Puria et al, 1996)
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