Effects of Aromatic Solvents on Acoustic Reflexes Mediated by Central Auditory Pathways

Abstract

From previous in vivo investigations, it has been shown that toluene can mimic the effects of cholinergic receptor antagonists and may thereby modify the response of protective acoustic reflexes. The current study aimed to define the relative effects of aromatic solvents on the middle ear and inner ear acoustic reflexes. Toward this end, the cochlear microphonic (CMP) elicited with a band noise centered at 4 kHz, and the compound action potential (CAP) elicited with 4-kHz tone pips was measured in rats. Both potentials were recorded before, during, and after triggering the protective reflexes by a 110-dB SPL contralateral octave band noise centered at 12.5 kHz (12.5 kHz-OBN). In several rats, the middle ear muscles were severed to identify the relative effects of toluene on the two reflexes. While the reflex elicitor was capable of decreasing both the CMP and CAP amplitudes, an injection of 116.2 mM toluene cancelled this suppressor effect induced by the contralateral sound. In the rats with nonfunctional middle ear muscles, a solvent injection did not modify the electrophysiological responses of the cochlea. Different solvents were tested to study the relationship of the chemical structure of the solvents on the acoustic reflexes. The present study showed that aromatic solvents can inhibit the action of the middle ear reflex by their anticholinergic effect on the efferent motoneurons. An aromatic nucleus and the presence of one side chain of no more than 3 C seem to be required in the solvent structure to inhibit the efferent motoneurons.

Solvent-Induced Hearing Loss: Mechanisms and Prevention Strategy

“…More recently, Bale et al [18] showed with an in vitro experiment that the cholinergic and, more specifically, the nicotinic receptors were sensitive to toluene. In a recent experiment, we confirmed their results with an in vivo experimental approach [19,20]. Besides, we showed that toluene, like most of the aromatic solvents, can mimic the effects of antagonists of the cholinergic receptors (AchRs) and thereby inhibit the action of both protective reflexes: the inner-ear and middle-ear acoustic reflexes.…”

Caracterização do perfil auditivo de frentistas de postos de combustível

“…Besides suggesting the involvement of the auditory nerve/low brainstem (14) , they may demonstrate risk for the development of retrocochlear pathology. It is believed that the aromatic solvents may inhibit the action of the acoustic reflex in the stapedius muscle, due to its anticholinergic effects on the efferent motor neurons (15) . The toluene, solvent present in the gasoline, seems to be able to act upon the efferent auditory system, inhibiting the acoustic reflexes (16) .…”

Chemical exposure and hearing loss

“…76 A dysfunction of this reflex would increase risks to hearing by allowing higher acoustic energy levels to penetrate the inner ear. 75,82 This would make co-exposure more dangerous than exposure to noise or to styrene alone.…”

“…This is important as ototoxicants were recently shown to affect the central nuclei driving the middle-ear acoustic reflex. 76,82 Therefore, from a safety point of view, it would be more efficient to collect and measure performances both in the middle ear and in the inner ear. The acoustic reflex would allow chemically induced retrocochlear disorders to be evaluated.…”