Vestibular-oculomotor, opto-oculomotor and visual function in the rat after long-term inhalation exposure to toluene

Combined exposure to carbon disulfide and low-frequency noise reversibly affects vestibular function

Chronic occupational exposure to carbon disulfide (CS2) has debilitating motor and sensory effects in humans, which can increase the risk of falls. Although no mention of vestibulotoxic effects is contained in the literature, epidemiological and experimental data suggest that CS2 could cause low-frequency hearing loss when associated with noise exposure. Low-frequency noise might also perturb the peripheral balance receptor through an as-yet unclear mechanism. Here, we studied how exposure to a low-frequency noise combined with 250-ppm CS2 affected balance in rats. Vestibular function was tested based on post-rotary nystagmus recorded by a video-oculography system. These measurements were completed by behavioral tests and analysis of the cerebellum to measure expression levels for gene expression associated with neurotoxicity. Assays were performed prior to and following a 4-week exposure, and again after a 4-week recovery period. Functional measurements were completed by histological analyses of the peripheral organs.Nystagmus was unaltered by exposure to noise alone, while CS2 alone caused a moderate 19% decrease of the saccade number. In contrast, coexposure to 250-ppm CS2 and low-frequency noise decreased both saccade number and duration by 33% and 34%, respectively. After four weeks, recovery was only partial but measures were not significantly different from pre-exposure values. Real-time quantitative polymerase chain reaction (RT-qPCR) analysis of cerebellar tissue revealed a slight but significant modification in expression levels for two genes linked to neurotoxicity in CS2-exposed animals. However, neither histopathological changes to the peripheral receptor nor behavioral differences were observed. Based on all these results, we propose that the effects of CS2 were due to reversible neurochemical disturbance of the efferent pathways managing post-rotatory nystagmus. Because the nervous structures involving the vestibular function appear particularly sensitive to CS2, post-rotary nystagmus could be used as an early, non-invasive measurement to diagnose CS2 intoxication as part of an occupational conservation program.

The toxic effects of toluene on the optokinetic nystagmus in pigmented rats

The effects of 375 mgm(-3) (100 ppm) toluene in air inhalation were evaluated on pigmented rats during either repeated exposures over five consecutive days 3h a day or during a single 4-h exposure. At the end of the inhalation period, the animals were returned to fresh air to evaluate their ability to recover optokinetic performance. The optokinetic responses were analyzed using a magnetic search coil technique previously described. After repeated toluene exposure, the eye position at rest of all the rats was unsteady. In response to visual stimulation, the eye velocity was slower and more irregular than in the control state. At the end of the stimulation, the environment of the animals became stationary, but the eye did not immediately return to a fixed stable position. A similar effect was observed after a single exposure. An increase of the optokinetic deficit was observed after single or repeated 375 mgm(-3) toluene exposures. No recovery was observed even after a single exposure. In view of the fact that toluene is a widely used solvent, these results show that inhalation of low concentrations, even for short single exposures, must be taken into account, because gaze destabilization could cause vertigo symptoms.

Auditory and vestibular functions after single or combined exposure to toluene: a review

Toluene is a widely used organic solvent, heavily employed in many manufacturing industries. Recently, evidence has begun to accumulate on the deleterious effect of toluene exposure has on the auditory and vestibular systems. Although little published information exists regarding these effects, the reported findings indicate a need for further investigation. The results of such investigations may dramatically affect occupational hearing conservation practices and legislation. Both human and animal studies will be summarized in discussing the effects of toluene alone or in combination with noise or other chemicals. Gaps in scientific knowledge are highlighted to assist future research.

Function of the auditory system, the visual system, and peripheral nerve and long-term combined exposure to toluene and ethanol in rats

Male pigmented rats (n = 36) were exposed to toluene and/or ethanol (1000 p.p.m. toluene in the inhaled air 21 hr/day, and 5.7-8.0% ethanol in the drinking water continuously) during 8 weeks. Electrophysiological recordings were made 1 week after the exposure. Auditory sensitivity (auditory brainstem response) was reduced only after exposures including toluene. At 20 kHz, ethanol antagonized toluene-induced loss of auditory sensitivity (P < 0.05). Flash evoked potentials were not affected in any group. In peripheral nerve, exposures containing ethanol were followed by increased amplitudes of nerve and muscle action potentials. Exposures including toluene were followed by an increase in liquid consumption.

Function of the auditory and visual systems, and of peripheral nerve, in rats after long-term combined exposure to n-hexane and methylated benzene derivatives. I. Toluene

Rats were exposed to n-hexane, toluene, or toluene together with n-hexane, each solvent 1000 p.p.m. (1000 + 1000 p.p.m. in combined exposure), 21 hr/day, 7 days/week during 28 days. Neurophysiological recordings were made 2 days, 3 months, and one year after end of exposure. A reduction in auditory sensitivity, recorded by click evoked auditory brainstem response, was observed 2 days after exposure to toluene alone, or to toluene together with n-hexane, but not after exposure to n-hexane alone. The reduction lasted one year after the exposure. Three months after combined exposure, a synergistic enhancement of loss of auditory sensitivity was observed in the mixed exposure group. One amplitude in the flash evoked potentials was lowered in the n-hexane exposed group 2 days after exposure. No such reduction was seen after simultaneous exposure to toluene. Exposure to n-hexane alone caused a marked decrease in peripheral nerve conduction velocity 2 days and 3 months after exposure, while exposure to n-hexane together with toluene had only a small effect on this velocity.