Cochlear pathology induced by styrene

Effects of coexposure to noise and mixture of toluene, ethylbenzene, xylene, and styrene (TEXS) on hearing loss in petrochemical workers of southern China

Many harmful factors existing simultaneously with noise are reported to induce hearing impairment, such as organic solvents. However, the existing hearing safety limits and current risk assessment for hearing loss rely on single noise exposure. It is urgent to clarify the combined effect of noise and other harmful factors on hearing loss. Petrochemical workers are always exposed to noise and organic solvents, mainly benzene, toluene, ethylbenzene, xylene, and styrene (BTEXS), while the combined effect of their coexposure on hearing remains unclear. Herein we conducted a cross-sectional survey, measuring pure-tone audiometry of 1496 petrochemical workers in southern China. Participants exposed to BTEXS were 569, 524, 156, 452, and 177 respectively. Individual cumulative noise exposure (CNE) levels and BTEXS exposure were assessed. The average CNE was 93.27 ± 4.92 dB(A)·years, and the concentrations of BTEXS were far below the occupational exposure limits of China. Logistic regression analyses showed that CNE was consistently positively associated with hearing loss (HL) and high-frequency hearing loss (HFHL) but not related to speech-frequency hearing loss (SFHL). Compared with participants in the lowest quartile of CNE, those in the highest quartile showed an OR of 5.229 (95% CI: 3.179, 8.598) for HFHL. Two-pollutant model analysis indicated that TEXS exposure was positively associated with HL (OR 1.679, 95%CI 1.086, 2.597), SFHL (OR 2.440, 95%CI 1.255, 4.744), and HFHL (OR 1.475, 95%CI 1.077, 2.020). However, no interactions were observed between CNE and TEXS coexposure on hearing loss. In our study, covariates including smoking and drinking status, body mass index (BMI), ear protection and personal protective equipment, and use of earphone/headphone were adjusted. In conclusion, coexposure to noise and low-level TEXS could induce more severe damage on hearing function than exposure to each alone, especially SFHL. Therefore, petrochemical workers simultaneously exposed to noise and TEXS, even at low-level, should be included in hearing protection programs.

Styrene alters potassium endolymphatic concentration in a model of cultured utricle explants

Despite well-documented neurotoxic and ototoxic properties, styrene remains commonly used in industry. Its effects on the cochlea have been extensively studied in animals, and epidemiological and animal evidence indicates an impact on balance. However, its influence on the peripheral vestibular receptor has yet to be investigated. Here, we assessed the vestibulotoxicity of styrene using an in vitro model, consisting of three-dimensional cultured newborn rat utricles filled with a high‑potassium (K⁺) endolymph-like fluid, called "cysts". K⁺ entry in the cyst ("influx") and its exit ("efflux") are controlled by secretory cells and hair cells, respectively. The vestibular epithelium's functionality is thus linked to K⁺ concentration, measured using a microelectrode. Known inhibitors of K⁺ efflux and influx validated the model. Cysts were subsequently exposed to styrene (0.25; 0.5; 0.75 and 1 mM) for 2 h or 72 h. The decrease in K⁺ concentration measured after both exposure durations was dose-dependent, and significant from 0.75 mM styrene. Vacuoles were visible in the cytoplasm of epithelial cells from 0.5 mM after 2 h and from 0.25 mM after 72 h. The results presented here are the first evidence that styrene may deregulate K⁺ homeostasis in the endolymphatic space, thereby altering the functionality of the vestibular receptor.

Evaluation of potential health effects associated with occupational and environmental exposure to styrene - an update

The potential chronic health risks of occupational and environmental exposure to styrene were evaluated to update health hazard and exposure information developed since the Harvard Center for Risk Analysis risk assessment for styrene was performed in 2002. The updated hazard assessment of styrene's health effects indicates human cancers and ototoxicity remain potential concerns. However, mechanistic research on mouse lung tumors demonstrates these tumors are mouse-specific and of low relevance to human cancer risk. The updated toxicity database supports toxicity reference levels of 20 ppm (equates to 400 mg urinary metabolites mandelic acid + phenylglyoxylic acid/g creatinine) for worker inhalation exposure and 3.7 ppm and 2.5 mg/kg bw/day, respectively, for general population inhalation and oral exposure. No cancer risk value estimates are proposed given the established lack of relevance of mouse lung tumors and inconsistent epidemiology evidence. The updated exposure assessment supports inhalation and ingestion routes as important. The updated risk assessment found estimated risks within acceptable ranges for all age groups of the general population and workers with occupational exposures in non-fiber-reinforced polymer composites industries and fiber-reinforced polymer composites (FRP) workers using closed-mold operations or open-mold operations with respiratory protection. Only FRP workers using open-mold operations not using respiratory protection have risk exceedances for styrene and should be considered for risk management measures. In addition, given the reported interaction of styrene exposure with noise, noise reduction to sustain levels below 85 dB(A) needs be in place.

Hidden cochlear impairments

Pure tone audiometry is a routine clinical examination used to identify hearing loss. A normal pure tone audiogram is usually taken as evidence of normal hearing. Auditory deficits detected in subjects with normal audiograms, such as poor sound discrimination and auditory perceptual disorders, are generally attributed to central problems. Does the pure tone audiogram truly reflect cochlear status? Recent evidence suggests that individuals with normal audiogram may still have reduced peripheral auditory responses but normal central responses, indicating that the pure tone audiometry may not detect some types of cochlear injuries. In the cochlea, the outer hair cells (OHCs), inner hair cells (IHCs), and the spiral ganglion neurons that synapse with IHCs are the 3 key cochlear components in transducing acoustical vibrations into the neural signals. This report reviews three types of cochlear damage identified in laboratory animals that may not lead to overt hearing loss. The first type of cochlear impairment, such as missing a certain proportion of IHCs without damage to OHCs, may reduce the cochlear output and elevate response threshold; however, the reduced peripheral auditory sensitivity may be restored along the auditory pathway via central gain enhancement. The second type of cochlear impairment, such as selective damage to the synapses of the high-threshold thin auditory nerve fibers (ANFs), reduces cochlear output at high stimulation levels with no effect on response threshold. In this case the reduced cochlear output may be compensated along the auditory pathway as well. The third type of cochlear impairment, such as missing a certain number of OHCs without damage to others, may not even affect cochlear function at all. These “hidden” cochlear impairments do not result in overt hearing loss, but they may increase the vulnerability of the cochlea to traumatic exposure and lead to disrupted central auditory processing.

A weight of evidence approach for the assessment of the ototoxic potential of industrial chemicals

There is accumulating epidemiological evidence that exposure to some solvents, metals, asphyxiants and other substances in humans is associated with an increased risk of acquiring hearing loss. Furthermore, simultaneous and successive exposure to certain chemicals along with noise can increase the susceptibility to noise-induced hearing loss. There are no regulations that require hearing monitoring of workers who are employed at locations in which occupational exposure to potentially ototoxic chemicals occurs in the absence of noise exposure. This project was undertaken to develop a toxicological database allowing the identification of possible ototoxic substances present in the work environment alone or in combination with noise exposure. Critical toxicological data were compiled for chemical substances included in the Quebec occupational health regulation. The data were evaluated only for noise exposure levels that can be encountered in the workplace and for realistic exposure concentrations up to the short-term exposure limit or ceiling value (CV) or 5 times the 8-h time-weighted average occupational exposure limit (TWA OEL) for human data and up to 100 times the 8-h TWA OEL or CV for animal studies. In total, 224 studies (in 150 articles of which 44 evaluated the combined exposure to noise and a chemical) covering 29 substances were evaluated using a weight of evidence approach. For the majority of cases where potential ototoxicity was previously proposed, there is a paucity of toxicological data in the primary literature. Human and animal studies indicate that lead, styrene, toluene and trichloroethylene are ototoxic and ethyl benzene, n-hexane and p-xylene are possibly ototoxic at concentrations that are relevant to the occupational setting. Carbon monoxide appears to exacerbate noise-induced hearing dysfunction. Toluene interacts with noise to induce more severe hearing losses than the noise alone.

Protective effect of N-acetyl-L-cysteine (L-NAC) against styrene-induced cochlear injuries

CONCLUSION

Styrene exposure causes hair cell death through both apoptotic and necrotic pathways and treatment with N-acetyl-L-cysteine (L-NAC) reduces styrene ototoxicity.

OBJECTIVE

Exposure to styrene causes hearing loss and hair cell death in the middle frequency region in the cochlea. The current study was designed to examine the cell death pathways and the protective effect of L-NAC against styrene-induced cochlear injuries.

MATERIALS AND METHODS

Seventeen rats were exposed to styrene by gavage at 400 mg/kg 5 days per week for 3 weeks. Nine of the styrene-treated rats received L-NAC by intraperitoneal injection (325 mg/kg), and the remaining eight rats received saline injections as controls. The styrene-induced hearing loss was assessed by auditory brainstem responses (ABRs). Apoptotic, necrotic, and missing hair cells were quantified using combined methods, including nuclear staining with propidium iodide, F-actin staining with FITC-phalloidin, and the TUNEL assay.

RESULTS

The styrene exposure caused a threshold shift of 15±4.3 dB. Both apoptosis and necrosis were involved in the pathogenesis of the cochlear lesion, but apoptosis appeared to be the major cell death pathway leading to the styrene ototoxicity. Treatment with L-NAC reduced the number of missing and dying outer hair cells (OHCs) and reduced the styrene-induced hearing loss.

Relation between outer hair cell loss and hearing loss in rats exposed to styrene

The relationship between outer hair cell (OHC) loss and cochlear sensitivity is still unclear, because in many animal models there exist surviving but dysfunctional OHCs and also injured/dead inner hair cells (IHC). Styrene is an ototoxic agent, which targets and destroys OHCs starting from the third row to the second and first rows depending on the exposure level. The remaining cells may be less affected. In this experiment, rats were exposed to styrene by gavage at different doses (200-800 mg/kg/day) for varying periods (5 days/week for 3-12 weeks). An interesting finding was that the cochlear sensitivity was not affected in a few rats with all OHCs in the third row being destroyed by styrene. A further loss of OHCs was usually accompanied with a linear input/output (I/O) function of cochlear compound action potentials (CAP), indicating the loss of cochlear amplification. However, normal CAP amplitudes at the highest stimulation level of 90 dB SPL were often observed when all OHCs were destroyed, indicating normal function of the remaining IHCs. The OHC-loss/hearing-loss relation appeared to be a sigmoid-type function. Initially, styrene-induced OHC losses (<33%) did not result in a significant threshold shift. Then CAP threshold shift increased dramatically with OHC loss from 33% to 66%. Then, CAP threshold changed less with OHC loss. The data suggest a tri-modal relationship between OHC loss and cochlear amplification. That is, under the condition that all surviving OHCs are ideally functioning, the cochlear amplifier is not affected until 33% of OHCs are absent, then the gain of the amplifier decreases proportionally with the OHC loss, and at last the amplifier may fail completely when more than 67% of OHCs are lost.

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

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.

Ototoxicity of the three xylene isomers in the rat

Numerous experiments have shown that the aromatic solvents can affect the auditory system in the rat, the cochlea being targeted first. Solvents differ in cochleotoxic potency: for example, styrene is more ototoxic than toluene or xylenes. The goal of this study was to determine the relative ototoxicity of the three isomers of xylene (o-, m- or p-xylene). Moreover, by dosing with the two urinary metabolites of xylene, methylhippuric (MHAs) and mercapturic acids (MBAs), this study points toward a causal relationship between the cochleotoxic effects and potential reactive intermediates arising from the biotransformation of the parent molecules. Separate groups of rats were exposed by inhalation to one isomer following this schedule: 1800 ppm, 6 h/d, 5 d/wk for 3 wk. Auditory thresholds were determined with brainstem-auditory evoked potentials. Morphological analysis of the organ of Corti was performed by counting both sensory and spiral ganglion cells. Among the three isomers, only p-xylene was cochleotoxic. A 39-dB permanent threshold shift was obtained over the tested frequencies range from 8 to 20 kHz. Whereas outer hair cells were largely injured, no significant morphological change was observed within spiral ganglia. The concentrations of urinary p-, o- or m-MHA were greater (p-MHA: 33.2 g/g; o-MHA: 7.8 g/g; m-MHA: 20.4 g/g) than those obtained for MBAs (p-MBA: 0.04 g/g; o-MBA: 6.2 g/g; m-MBA: 0.03 g/g). Besides, there is a large difference between o-MBA (6.2 g/g) and p-MBA (0.04 g/g). As a result, since the cysteine conjugates are not determinant in the ototoxic process of xylenes, the location of the methyl groups around the benzene nucleus could play a key role.

Relative ototoxicity of 21 aromatic solvents

Some aromatic solvents (e.g. toluene, p-xylene, styrene, and ethylbenzene) show, in the rat, striking ototoxicity characterized by an irreversible hearing loss, as measured by behavioural or electrophysiological methods, associated with damage to outer hair cells in the cochlea of the exposed animals. To broaden the range of aromatic solvents studied concerning their potential ototoxicity and to compare their ototoxicity quantitatively, 21 aromatic solvents were administered orally by gastric intubation to Sprague-Dawley rats for 5 days/week for a 2-week period. The dose used was 8.47 mmol kg(-1) body weight day(-1). The possible ototoxicity of the aromatic solvents was evaluated by morphological investigation of the cochlea. Whole-mount surface preparations of the organ of Corti were made to quantify the number of missing hair cells (cytocochleogram). Among the 21 solvents studied, eight (toluene, p-xylene, ethylbenzene, n-propylbenzene, styrene, alpha-methylstyrene, trans-beta-methylstyrene, and allylbenzene) caused histological lesions of the organ of Corti. They differed widely in their potency. The least ototoxic solvents caused outer hair cell (OHC) loss in the middle turn of the organ of Corti. The OHC loss was slight in the first row, and greater in the second and third rows. The most ototoxic solvents caused high losses in the three rows of the outer hair cells along the entire length of the basilar membrane. There were also occasional inner hair cell (ICH) losses in the most affected animals. Although no measurements were made of the chemical concentrations reached in the blood or the brain, tentative ranking of an increasing ototoxicity of the eight aromatic solvents could be proposed on the basis of the histological losses observed-alpha-methylstyrene<trans-beta-methylstyrene=toluene< or =p-xylene<n-propylbenzene<styrene=ethylbenzene<allylbenzene. There was no relationship between the degree of ototoxicity and the lipophilic properties of the ototoxic agents as expressed by the octanol/water partition coefficients. However, it seemed that some structural constraint was essential to induce ototoxicity. It seems there must be a single side-chain on the aromatic ring for ototoxicity, except with p-xylene. The other aromatic solvents with two side-chains were not ototoxic. When the saturated side-chain was branched (isopropylbenzene, isobutylbenzene, sec-butylbenzene, tert-butylbenzene), no ototoxicity was observed. The ototoxic potency increased when the length of the saturated side-chain extended from one carbon atom to two carbon atoms. Beyond that point, the ototoxic effect decreased with n-propylbenzene and disappeared with n-butylbenzene. Moreover, unsaturation of the side-chain of allylbenzene increased the ototoxicity of n-propylbenzene substantially. Branching of the unsaturated chain (alpha-methylstyrene and trans-beta-methylstyrene) decreased the ototoxicity of styrene.

Consequences of noise- or styrene-induced cochlear damages on glutamate decarboxylase levels in the rat inferior colliculus

Both noise and styrene can injure the cochlea, resulting in a reduction of incoming inputs from the cochlea to the central nervous system. In addition, styrene is known to have neurotoxic properties at high doses. The loss of inputs caused by noise has been shown to be compensated by a new equilibrium between excitatory and inhibitory influences within the inferior colliculus (IC). The main goal of this study was to determine whether styrene-induced hearing loss could also be counterbalanced by a GABAergic adjustment in the IC. For this purpose, rats were exposed to noise (97 dB SPL octave band noise centered at 8 kHz), or to a non-neurotoxic dose of styrene for 4 weeks (700 ppm, 6 h/day, 5 days/week). Auditory sensitivity was tested by evoked potentials, and cochlear damage was assessed by hair cell counts. Glutamate decarboxylase (GAD) was dosed in the IC by indirect competitive enzyme-linked immunosorbent assay. Both noise and styrene caused PTSs that reached 27.0 and 14.6 dB respectively. Outer hair cell (OHC) loss caused by noise did not exceed 9% in the first row, on the other hand OHC loss induced by styrene reached 63% in the third row. Only the noise caused a decrease of GAD of 37% compared to that measured in the controls. No significant modification of GAD concentration has been shown after styrene exposure. Thus, central compensation for cochlear damage may depend on the nature of the ototoxic agent. Unless styrene directly affects IC function, it is reasonable to assume that noise causes a modification of inhibitory neurotransmission within the structure because of impairment of afferent supply to the auditory brainstem. The present findings suggest that central compensation for cochlear damage can preferably occur when afferent fibers are altered.

Use of DPOAEs for assessing hearing loss caused by styrene in the rat

The study was carried out to test whether or not cubic distortion otoacoustic emissions were more sensitive than auditory-evoked potentials for assessing styrene-induced hearing losses in the Long-Evans rat. For the purposes of comparison, changes in cubic distortion product otoacoustic emissions (DeltaDPOAE), evoked potential permanent threshold shifts (PTS) and outer hair cell losses were measured in a population of styrene-treated rats. Each rat was exposed to either 650 or 750 ppm of styrene for 4 weeks, 5 days per week, 6 h per day. Only the 750 ppm exposure caused significant hearing losses. For this concentration, DPOAEs appeared as sensitive to styrene as the audiometry performed with evoked potentials, but not more. A high coefficient of correlation [0.84< or =r< or =0.91] between DeltaDPOAE and PTS was obtained across the styrene-induced effects for frequencies ranging from 5 to 12 kHz. This experiment demonstrates that DPOAEs can be used to monitor the ototoxicity induced by styrene even though they cannot be considered as a more sensitive index of cochlear pathology than the evoked potentials, at least under our experimental conditions. Likewise evoked potentials, normal DPOAEs may not guarantee a normal cochlear status and therefore results of DPOAE measurements should be interpreted cautiously. The use of both techniques and the determination of the ratio DeltaDPOAE/PTS may be useful in determining the cause of hearing loss: mechanical or chemical process. Moreover, because of its non-invasive and objective characteristics, the use of DPOAEs could play a greater role in a prevention policy.