Inhaled toluene can modulate the effects of anesthetics on the middle-ear acoustic reflex

Three-dimensional cultured ampullae from rats as a screening tool for vestibulotoxicity: Proof of concept using styrene

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.

Isoflurane anesthesia suppresses distortion product otoacoustic emissions in rats

A commonly used anesthetic, isoflurane, can impair auditory function in a dose-dependent manner. However, in rats, isoflurane-induced auditory impairments have only been assessed with auditory brainstem responses; a measure which is unable to distinguish if changes originate from the central or peripheral auditory system. Studies performed in other species, such as mice and guinea-pigs, suggests auditory impairment stems from disrupted OHC amplification. Despite the wide use of the rat in auditory research, these observations have yet to be replicated in the rat animal model. This study used distortion product otoacoustic emissions to assess outer hair cell function in rats that were anesthetized with either isoflurane or a ketamine/xylazine cocktail for approximately 45 min. Results indicate that isoflurane can significantly reduce DPOAE amplitudes compared to ketamine/xylazine, and that responses were more variable with isoflurane than ketamine/xylazine over the 45-min test period. Based on these observations, isoflurane should be used with caution when assessing peripheral auditory function to avoid potentially confounding effects.

Effects of thiocolchicoside, a commonly used myorelaxant, on the acoustic reflex

OBJECTIVE

To determine whether thiocolchicoside, a commonly used myorelaxant, may impair the acoustic reflex.

METHODS

Forty-two patients scheduled to receive thiocolchicoside treatment for different reasons were enrolled in the study. Acoustic reflex thresholds at 500, 1000, 2000 and 4000 Hz were determined and analysed statistically pre-treatment and on the 5th day of treatment.

RESULTS

Increases were observed in the mean acoustic reflex thresholds on the 5th day of treatment compared to pre-treatment, at all frequencies, except right contralateral thresholds at 500 and 2000 Hz. These increases were statistically significant for right ipsilateral thresholds at 2000 and 4000 Hz, left ipsilateral thresholds at 500, 1000, 2000 and 4000 Hz, and left contralateral thresholds at 2000 and 4000 Hz (p ≤ 0.05), but not at other frequencies (p > 0.05).

CONCLUSION

Muscle relaxant drugs, especially those affecting the central nervous system, may weaken the stapedial muscle so that the ability of noise to cause acoustic trauma may become evident. For this reason, physicians should advise their patients to avoid loud noises when muscle relaxant therapy is prescribed.

Membrane fluidity does not explain how solvents act on the middle-ear reflex

Some volatile aromatic solvents have similar or opposite effects to anesthetics in the central nervous system. Like for anesthetics, the mechanisms of action involved are currently the subject of debate. This paper presents an in vivo study to determine whether direct binding or effects on membrane fluidity best explain how solvents counterbalance anesthesia's depression of the middle-ear reflex (MER). Rats were anesthetized with a mixture of ketamine and xylazine while also exposed to solvent vapors (toluene, ethylbenzene, or one of the three xylene isomers) and the amplitude of their MER was monitored. The depth of anesthesia was standardized based on the magnitude of the contraction of the muscles involved in the MER, determined by measuring cubic distortion product oto-acoustic emissions (DPOAEs) while triggering the bilateral reflex with contralateral acoustic stimulation. The effects of the aromatic solvents were quantified based on variations in the amplitude of the DPOAEs. The amplitude of the alteration to the MER measured in anesthetized rats did not correlate with solvent lipophilocity (as indicated by logKow values). Results obtained with the three xylene isomers indicated that the positions of two methyl groups around the benzene ring played a determinant role in solvent/neuronal cell interaction. Additionally, Solid-state Nuclear Magnetic Resonance (NMR) spectra for brain microsomes confirmed that brain lipid fluidity was unaffected by solvent exposure, even after three days (6h/day) at an extremely high concentration (3000ppm). Therefore, aromatic solvents appear to act directly on the neuroreceptors involved in the acoustic reflex circuit, rather than on membrane fluidity. The affinity of this interaction is determined by stereospecific parameters rather than lipophilocity.