Neuronal Circuits Involved in the Middle-Ear Acoustic Reflex

Abstract

Human and animal studies have shown that certain aromatic solvents such as toluene can cause hearing loss and can exacerbate the effects of noise. The latter effects might be due to a modification of responses of motoneurons controlling the middle-ear acoustic reflex. In the present investigation, the audition of Long-Evans rats was evaluated by measuring cubic (2f1 - f2) distortion otoacoustic emissions (f1 = 8000 Hz; f2 = 9600 Hz; f1/f2 = 1.2) prior to, during, and after activation of the middle-ear acoustic reflex. A noise suppressor was used to modify the amplitude of the 2f1 - f2 distortion otoacoustic emissions. It was delivered either contralaterally (band noise centered at 4 kHz), or ipsilaterally (3.5 kHz sine wave) to test the role played by the central auditory nuclei. This audiometric approach was used to study the physiological efficiency of the middle-ear acoustic reflex during an injection of a bolus of Intralipid (as a vehicle) containing 58.4, 87.4, or 116.2mM toluene via the carotid artery. The results showed that toluene could either increase or decrease middle-ear acoustic reflex efficiency, depending on the toluene concentration and the ear receiving noise suppressor. A new neuronal circuit of the middle-ear acoustic reflex has been proposed to explain findings obtained in this investigation. Finally, the depressing action of toluene on the central auditory nuclei driving the middle-ear acoustic reflex might explain the synergistic effects of a co-exposure to noise and aromatic solvents.

Chemical exposure and hearing loss

“…This reflex is driven by a cholinergic efferent system. 76 This type of effect suggests that the hearing loss observed in solvent-exposed subjects might be due to dysfunctions beyond the peripheral auditory system, which could at least partially explain the synergistic effects of co-exposure to noise and aromatic solvents.…”

“…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.…”

Use of the kurtosis statistic in an evaluation of the effects of noise and solvent exposures on the hearing thresholds of workers: An exploratory study

“…In today's complex industrial environments, noise and chemicals, such as solvents, are found in many workplaces, and are part of the daily life of numerous workers. Studies conducted with experimental animals provide robust evidence that solvents, such as toluene, styrene, xylene, and ethyl benzene, can affect the auditory function through their toxic action on the organ of Corti, auditory pathways, as well as on the middle-ear reflex (Venet et al, 2011;Wathier et al, 2016). In addition, aromatic solvents can interact synergistically with noise.…”

“…The mechanism behind this specific interaction involves an effect of solvents on acoustic reflexes. The middle-ear acoustic reflex (MER) is driven by a cholinergic efferent system and in vivo studies have shown that it can be affected by solvents (Maguin et al, 2009;Venet et al, 2011). A dysfunction of this reflex would increase the risk for hearing loss by allowing higher acoustic energy levels (at least 5 dB) to penetrate the inner ear (Campo et al, 2014;Carreres Pons et al, 2017).…”