Objective method for differentiating between drug-induced vestibulotoxicity and cochleotoxicity

An in vitro model to assess the peripheral vestibulotoxicity of aromatic solvents

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.

Functional and Anatomic Alterations in the Gentamicin-Damaged Vestibular System in the Guinea Pig

HYPOTHESIS

The purpose of this study was to investigate the expected functional and morphologic effect of gentamicin on the vestibular system simultaneously by measurement of vestibular evoked potentials and electron microscopic evaluation.

BACKGROUND

Vestibular short-latency evoked potentials to linear acceleration have been shown to be a useful parameter of vestibular function. In gentamicin-treated animals, the morphologic damage has been well documented, although this has seldom been quantified.

METHODS

Fifteen guinea pigs were divided into three equal groups. Two groups received different dosages of intramuscular gentamicin for 3 weeks; the third group was the control group. Vestibular short-latency evoked potentials to linear acceleration pulses were measured. After the last gentamicin dose, the utricles were prepared for scanning and transmission electron microscopy. On scanning electron microscopy photographs, the surface area damage ratio of the utricles, a simple method of quantifying gross morphologic damage, was calculated.

RESULTS

The vestibular short-latency evoked potential of gentamicin-treated guinea pigs showed a slow-developing, damaging, dose-response effect on the function of the vestibular system (p = 0.01). Scanning electron microscopy and transmission electron microscopy showed severe morphologic damage in the sensory hair cells of the utricle. The surface area damage ratio showed a dose-response relationship (p = 0.01).

CONCLUSION

Functional and anatomic alterations in the gentamicin-damaged vestibular system in the guinea pig are related.