Toluene and Styrene Intoxication Route in the Rat Cochlea

doi:10.1016/S0892-0362(99)00010-0

Neurotoxicology and Teratology

Volume 21, Issue 4, July–August 1999, Pages 427-434

https://doi.org/10.1016/S0892-0362(99)00010-0 Get rights and content

Abstract

It is well established that organic solvents such as toluene and styrene are ototoxic in the rat; however, the intoxication route used to reach the organ of Corti is still questionable. The distribution of toluene and styrene in various tissues of Long-Evans rats (n = 2 × 8) was studied after inhalation of either 1750 ppm toluene or 1750 ppm styrene for 10 h (6 consecutive h + 4 h the following day). At the end of the solvent exposures, blood, brain, auditory nerves, the organ of Corti, cerebrospinal (CSF), and inner ear fluids (IEF) were sampled or removed to measure the rates of solvent uptake in each tissue by gas chromatography. Results indicate that CSF and IEF were free from detectable solvents, whereas the organ of Corti, the nerves, and the brain were contaminated. Therefore, both toluene- and styrene-induced hearing losses are caused by tissue intoxication rather than by fluid contamination. It is proposed that the outer sulcus is used as an intoxication route to reach the organ of Corti.

Section snippets

Animals

Male Long-Evans rats (450 g) were purchased from a local supplier (Janvier Laboratories). The rats (n = 32) were housed in individual cages (350 × 180 × 184 mm) 1 month before the start of the experiments. Food (UAR Cie France; A04 10) and tap water were available ad libitum. Temperature in the animal quarters was 22 ± 1°C and the humidity ranged from 50% to 55%. Fluorescent lights were on from 0700 to 1900 h. The investigators followed the Guiding Principles in the Use of Animals in

Toluene

The toluene concentrations obtained in the different tissues are presented in Table 1. Toluene was detected in blood, brain (Part A, B), auditory nerve, or cochleae, but not in the fluids of the treated animals. There is no significant difference [1.02 μg/g (−2.94, 4.99), t₁₄ = 0.55, p = 0.59] in toluene between Part A and B. As a result, the comparison between brain and other tissue will be done with the average between A and B [47.61 μg/g (42.3, 52.92)]. The difference of toluene [21.48 μg/g

Discussion

The solvent concentrations found in brain are higher than those in blood, as previously mentioned by Withey (26) and Benignus et al. (3). In the present study, the toluene concentrations are 47.6 μg/g in brain and 26.1 μg/g in blood, which are similar to the values reported by Kishi et al. (13), 45 and 30 μg/g, 1 h after 4-h toluene exposure to 2000 ppm. Due to the discrepancies between the experimental protocols, it is risky to compare our styrene results with the small amount of data reported

Acknowledgements

The authors thank Dr. L. Fechter and T. Morata for helpful critiques of the manuscript, C. Barthélémy (N/PE) and P. Bonnet (N/TIE) for their technical assistance, and extend special thanks for M. Grzebyk, who performed the statistical analyses.

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Numerous ototoxic drugs, such as some antibiotics and chemotherapeutics, are both cochleotoxic and vestibulotoxic (causing hearing loss and vestibular disorders). However, the impact of some industrial cochleotoxic compounds on the vestibular receptor, if any, remains unknown. As in vivo studies are long and expensive, there is considerable need for predictive and cost-effective in vitro models to test ototoxicity. Here, we present an organotypic model of cultured ampullae harvested from rat neonates. When cultured in a gelatinous matrix, ampulla explants form an enclosed compartment that progressively fills with a high-potassium (K⁺) endolymph-like fluid. Morphological analyses confirmed the presence of a number of cell types, sensory epithelium, secretory cells, and canalar cells. Treatments with inhibitors of potassium transporters demonstrated that the potassium homeostasis mechanisms were functional. To assess the potential of this model to reveal the toxic effects of chemicals, explants were exposed for either 2 or 72 h to styrene at a range of concentrations (0.5–1 mM). In the 2-h exposure condition, K⁺ concentration was significantly reduced, but ATP levels remained stable, and no histological damage was visible. After 72 h exposure, variations in K⁺ concentration were associated with histological damage and decreased ATP levels. This in vitro 3D neonatal rat ampulla model therefore represents a reliable and rapid means to assess the toxic properties of industrial compounds on this vestibular tissue, and can be used to investigate the specific underlying mechanisms.
Neurotoxicity of organic solvents: An update on mechanisms and effects
2022, Advances in Neurotoxicology
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The ototoxic solvents were active by either inhalation or oral routes of exposure, although it is hard to equate tissue concentrations across exposure routes. Delivery to the cochlea appears to be blood-borne rather the via the cerebrospinal fluid or the inner ear fluid (Campo et al., 1999). Gagnaire and Langlais (2005) observed that the ototoxic aromatic organic solvents differed widely in potency, and their activity (ototoxic or not) and potency of the ototoxic substances was unrelated to LogP.
Organic solvents are a diverse set of chemicals and chemical mixtures that are among the most widely used class of chemicals in industry and commerce. Many solvents are volatile leading to a risk of inhalation exposure. Typically, solvents are also highly lipid soluble and readily partition into lipid-rich brain tissue. In the nervous system, solvents act acutely to alter the function of multiple ligand-gated and voltage-gated nerve membrane ion channels to cause acute neurological dysfunctions, behavioral impairments, and eventually generalized CNS depression. The momentary solvent concentration in the brain determines the strength of these acute effects. Upon repeated or chronic exposures, organic solvents can also reduce neuronal plasticity, generate reactive oxygen species, disrupt nerve membrane integrity, increase intracellular calcium, induce neuroinflammation and potentially cause long-term impairments. Chronic occupational exposures to organic solvents have been linked to impairments of cognitive function, mood, memory, vision, and hearing. Hearing deficits are augmented by co-exposures to noise. Peripheral neuropathy and developmental neurotoxicity are also concerns. In this chapter, types of organic solvents, their uses, mode of actions, and the evidence for acute and chronic exposures to produce neurotoxic effects in humans and laboratory animals are discussed.
An in vitro model to assess the peripheral vestibulotoxicity of aromatic solvents
2021, NeuroToxicology
Epidemiological and experimental studies indicate that a number of aromatic solvents widely used in the industry can affect hearing and balance following chronic exposure. Animal studies demonstrated that long-term exposure to aromatic solvents directly damages the auditory receptor within the inner ear: the cochlea. However, no information is available on their effect on the vestibular receptor, which shares many structural features with the cochlea and is also localized in inner ear. The aim of this study was to use an in vitro approach to assess and compare the vestibular toxicity of different aromatic solvents (toluene, ethylbenzene, styrene and ortho-, meta-, para-xylene), all of which have well known cochleotoxic properties. We used a three-dimensional culture model of rat utricles (“cysts”) with preserved functional sensory and secretory epithelia, and containing a potassium-rich (K⁺) endolymph-like fluid for this study. Variations in K⁺ concentrations in this model were considered as biomarkers of toxicity of the substances tested. After 72 h exposure, o-xylene, ethylbenzene and styrene decreased the K⁺ concentration by 78 %, 37 % and 28 %, respectively. O- xylene and styrene both caused histopathological alterations in secretory and sensory epithelial areas after 72 h exposure, whereas no anomalies were observed in ethylbenzene-exposed samples.
These in vitro results suggest that some widely used aromatic solvents might have vestibulotoxic properties (o-xylene, styrene and ethylbenzene), whereas others may not (p-xylene, m-xylene, toluene). Our results also indicate that variations in endolymphatic K⁺ concentration may be a more sensitive marker of vestibular toxicity than histopathological events. Finally, this study suggests that cochleotoxic solvents might not be necessarily vestibulotoxic, and vice versa.
Styrene alters potassium endolymphatic concentration in a model of cultured utricle explants
2020, Toxicology in Vitro
Citation Excerpt :
Because of its high lipophilicity styrene is assumed to reach the OHCs through the membranes of the cells constituting the outer sulcus within the organ of Corti, and not through the liquids contained in the inner ear. Campo et al. (1999) suggested that styrene reaches the cochlea through the stria vascularis, from where it can access the organ of Corti and thus the OHCs. Several authors have shown that Deiters' cells may be damaged by solvents at doses that do not induce OHC loss (Campo et al., 2001; Chen et al., 2007; Fetoni et al., 2016).
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.
Auditory Toxicology
2018, Comprehensive Toxicology: Third Edition
The sensitivity of the peripheral auditory system to both transient and permanent impairment has been well recognized for decades. It has long been appreciated that aminoglycoside antibiotics, anti-malarial drugs, loop diuretics and platinum-containing chemotherapeutic agents as well as industrial chemicals such as solvents carry a risk of producing functional impairment and injury.
While toxicity is mainly mediated by damage to the hair cells, further investigation is constantly ongoing to elucidate details on the impact to the auditory and vestibular systems. This article describes the structure and normal function of the cochlea, explores potential mechanisms/modes of action responsible for ototoxicity, and surveys a variety of such chemical ototoxicants.
The tonotopicity of styrene-induced hearing loss depends on the associated noise spectrum
2015, Neurotoxicology and Teratology
Citation Excerpt :
Firstly, outer hair cells (OHC) are much more sensitive to styrene than inner hair cells (Loquet et al., 1999). Secondly, the outer sulcus is one of the routes through which the aromatic solvent reaches the organ of Corti (Campo et al., 1999). However, the tonotopicity of the solvent-induced trauma is still questionable, especially when the solvent exposure is polluted with a background noise rich in low-frequencies.
The neuropharmacological and cochleotoxic effects of styrene can exacerbate the impact of noise on the peripheral auditory receptor. The mechanisms through which co-exposure to noise and styrene impairs hearing are complex as the slowly developing cochleotoxic process can be masked in the short-term by the rapid pharmacological effect on the central nervous system. The current investigation was therefore designed to delineate the auditory frequency range sensitive to noise, to styrene, and to noise and styrene combined. In case of different frequency ranges targeted by noise and styrene, it would be possible to point out the main factor responsible for cases of deafness by looking at the location of the audiometric deficits. Male Brown-Norway rats were exposed to 600-ppm styrene, to an octave band noise centered at 8 kHz, or to both noise and styrene. The noise exposure was of two different types: impulse noise with a LEX,8 h (equivalent continuous noise level averaged over 8 h) of 80 dB and continuous noise with a LEX,8 h of 85 dB SPL. Hearing was tested using a non-invasive technique based on distortion product otoacoustic emissions. Hearing data were completed with histological analysis of cochleae. The results showed that exposure to styrene alone caused outer hair cell losses in the apical cochlear region, which discriminates low frequencies. In contrast, noise-induced hearing loss was located at half an octave above the central frequency of the spectrum, around 10–12 kHz. Damage due to impulse noise was significantly exacerbated by styrene, and the noise spectrum defined the location of the cochlear trauma. Combined exposure caused greater cell losses than the sum of losses measured with the impulse noise and styrene alone. The fact that the tonotopicity of the styrene-induced damage depends on the associated noise spectrum complicates the diagnosis of styrene-related hearing loss with a tone-frequency audiometric approach. In conclusion, there is not really a frequency specificity of impairments due to styrene.

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ArticlesToluene and Styrene Intoxication Route in the Rat Cochlea

Abstract

Section snippets

Animals

Toluene

Discussion

Acknowledgements

Otolaryngol. Head Neck Surg.

Toxicol. Appl. Pharmacol.

Environ. Res.

Hear. Res.

Fund. Appl. Toxicol.

Environ. Res.

Environ. Res.

Toxicology

Brain Res.

Toxicol. Appl. Pharmacol.

Hear. Res.

Toxicol. Appl. Pharmacol.

Neurosci. Lett.

Articles
Toluene and Styrene Intoxication Route in the Rat Cochlea