Aromatic solvents disturb the stapedial reflex involved in hearing

The olivocochlear and stapedial reflexes are both involved in hearing physiology: the first one is mainly dedicated to the high frequency discrimination, whereas the second protects the cochlea by contracting the middle-ear muscles. In the present study, the measured effects of the olivocochlear reflex are negligible regarding those of the stapedial reflex (SR). For this reason, we will speak about acoustic reflex (Rumeau, 2011), a more general concept which can be considered as the stapedial reflex in our experimental context.
For over thirty years, it is known that anesthetics inhibit the SR function (Farkas, 1983). Recently, Campo et al (2013) demonstrated that aromatic solvents can counterbalance the inhibitory effects of anesthetics on the reflex. However, the mechanism of the interaction between solvents and anesthetics are still questionable; it would depend on either the structure, or the physico-chemical characteristics of the solvents (Campo, 2007).
In the present study, the mechanisms of the interaction between anesthetics and solvents were studied to conceive a model capable of characterizing the action of the solvent on the SR and thereby to predict a possible synergy of noise and chemical effects on hearing. Given the large number of occupational chemicals, the model would allow to screen the chemicals capable of potentiating the noise effects.
Hearing from Brown Norway rats was tested by measuring the amplitude of oto-acoustic emissions generated by two pure tones: f1 & f2, with f1/f2=1.2. The oto-emissions were measured at the frequency 2f1-f2 and were called distortion product emissions (DPOAEs). As regards the SR, it was triggered for 3 s every 30 s by a contralateral stimulation at a constant intensity of 95 dB SPL. In this experimental context, the SR amplitude was stabilized at approximately 1.5, 1.8 dB SPL by adjusting the anesthesia depth with a ketamine and xylazine mixture administrated by ip with a driven syringe. Once the amplitude of the SR was stabilized, the animal was intratracheally exposed to a solvent (figure 1).
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