Solvent ototoxicity in the rat and guinea pig

Recommedation for an occupational exposure limit for toluene

The Lower Olefins and Aromatics (LOA) REACH Consortium, which includes toluene registrants in the EU, established a Working Group (WG) to conduct a review of the occupational exposure limit (OEL) for toluene. The review focussed on CNS and neuro-behavioural toxicity, ototoxicity, effects on colour vision, reproductive and developmental effects, as safety signals for these effects were identified. The WG also examined the need for a skin notation and/or a short-term exposure limit (STEL). The WG critically reviewed and discussed the strengths and weaknesses of the available published information describing the effects of toluene in animals and humans, to assess its adequacy as a potential point of departure for the establishment of an OEL for toluene and to derive an OEL. As a result, the WG recommendation for a toluene OEL is 20 ppm 8 h TWA, with a 15 min STEL of 100 ppm and a skin notation.

Styrene targets sensory and neural cochlear function through the crossroad between oxidative stress and inflammation

BACKGROUND

Although styrene is an established ototoxic agent at occupational exposure levels, the mechanisms of styrene toxicity in the auditory system are still unclear.

OBJECTIVES

The aim of this study was to identify the consequences of styrene chronic exposure in cochlear structures, looking for the mechanisms of ototoxicity of this organic compound and focusing on cell targets and oxidative stress/inflammatory processes.

METHODS

Male adult Wistar rats were exposed to styrene (400 mg/kg by gavage for 5 days/week, 3 consecutive weeks). Hearing loss was evaluated by measuring auditory brainstem responses (ABR), morphological analysis were performed to evaluate hair cell and spiral ganglion neuron survival, as well as synaptic damage. Analysis of apoptotic (p53) and inflammatory (NF-κB, TNF-α, IL-1β and IL-10) mediators were performed by immunofluorescence analysis and western blot.

RESULTS

Styrene ototoxic effects induced a hearing loss of about 35-40 dB. Immunofluorescence and western blotting analyses demonstrated that styrene administration induced redox imbalance and activated inflammatory processes, targeting sensory hair cell and neural dysfunction by a cross-talk between oxidative and inflammatory mediators.

DISCUSSION

Major findings connect styrene ototoxicity to an interplay between redox imbalance and inflammation, leading to the intriguing assumption of a mixed sensory and neural styrene-induced ototoxicity. Thus, in a clinical perspective, data reported here have important implications for styrene risk assessment in humans.

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.

Distortion product otoacoustic emission sensitivity to different solvents in a population of industrial painters

Objective: To evaluate the ototoxic effect of the exposure to different organic solvents and noise using distortion product otoacoustic emissions (DPOAEs).Design: The exposure to different solvents was evaluated by measuring, before and at the end of the work-shift, the urinary concentrations of solvent metabolites used as dose biomarkers. The urinary concentrations of DNA and RNA oxidation products were also measured as biomarkers of oxidative damage. The simultaneous exposure to noise was also evaluated. DPOAEs and pure tone audiometry (PTA) were used as outcome variables, and were correlated to the exposure variables using mixed effect linear regression models.Study sample: Seventeen industrial painters exposed to a solvent mixture in a naval industry. A sample size of 15 was estimated from previous studies as sufficient for discriminating small hearing level and DPOAE level differences (5 dB and 2 dB, respectively) at a 95% confidence level.Results: Statistically significant associations were found between the DPOAE level and the urinary dose biomarkers and the oxidative damage biomarkers. DPOAE level and the logarithm of the metabolite concentration showed a significant negative correlation.Conclusions: DPOAE are sensitive biomarkers of exposure to ototoxic substances and can be effectively used for the early detection of hearing dysfunction.

The rat animal model for noise-induced hearing loss

Rats make excellent models for the study of medical, biological, genetic, and behavioral phenomena given their adaptability, robustness, survivability, and intelligence. The rat's general anatomy and physiology of the auditory system is similar to that observed in humans, and this has led to their use for investigating the effect of noise overexposure on the mammalian auditory system. The current paper provides a review of the rat model for studying noise-induced hearing loss and highlights advancements that have been made using the rat, particularly as these pertain to noise dose and the hazardous effects of different experimental noise types. In addition to the traditional loss of auditory function following acoustic trauma, recent findings have indicated the rat as a useful model in observing alterations in neuronal processing within the central nervous system following noise injury. Furthermore, the rat provides a second animal model when investigating noise-induced cochlear synaptopathy, as studies examining this in the rat model resemble the general patterns observed in mice. Together, these findings demonstrate the relevance of this animal model for furthering the authors' understanding of the effects of noise on structural, anatomical, physiological, and perceptual aspects of hearing.

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.

Occupational Styrene Exposure on Auditory Function Among Adults: A Systematic Review of Selected Workers

A review study was conducted to examine the adverse effects of styrene, styrene mixtures, or styrene and/or styrene mixtures and noise on the auditory system in humans employed in occupational settings. The search included peer-reviewed articles published in English language involving human volunteers spanning a 25-year period (1990–2015). Studies included peer review journals, case–control studies, and case reports. Animal studies were excluded. An initial search identified 40 studies. After screening for inclusion, 13 studies were retrieved for full journal detail examination and review. As a whole, the results range from no to mild associations between styrene exposure and auditory dysfunction, noting relatively small sample sizes. However, four studies investigating styrene with other organic solvent mixtures and noise suggested combined exposures to both styrene organic solvent mixtures may be more ototoxic than exposure to noise alone. There is little literature examining the effect of styrene on auditory functioning in humans. Nonetheless, findings suggest public health professionals and policy makers should be made aware of the future research needs pertaining to hearing impairment and ototoxicity from styrene. It is recommended that chronic styrene-exposed individuals be routinely evaluated with a comprehensive audiological test battery to detect early signs of auditory dysfunction.

Styrene enhances the noise induced oxidative stress in the cochlea and affects differently mechanosensory and supporting cells

Experimental and human investigations have raised the level of concern about the potential ototoxicity of organic solvents and their interaction with noise. The main objective of this study was to characterize the effects of the combined noise and styrene exposure on hearing focusing on the mechanism of damage on the sensorineural cells and supporting cells of the organ of Corti and neurons of the ganglion of Corti. The impact of single and combined exposures on hearing was evaluated by auditory functional testing and histological analyses of cochlear specimens. The mechanism of damage was studied by analyzing superoxide anion and lipid peroxidation expression and by computational analyses of immunofluorescence data to evaluate and compare the oxidative stress pattern in outer hair cells versus the supporting epithelial cells of the organ of Corti. The oxidative stress hypothesis was further analyzed by evaluating the protective effect of a Coenzyme Q₁₀ analogue, the water soluble Q_ter, molecule known to have protective antioxidant properties against noise induced hearing loss and by the analysis of the expression of the endogenous defense enzymes. This study provides evidence of a reciprocal noise-styrene synergism based on a redox imbalance mechanism affecting, although with a different intensity of damage, the outer hair cell (OHC) sensory epithelium. Moreover, these two damaging agents address preferentially different cochlear targets: noise mainly the sensory epithelium, styrene the supporting epithelial cells. Namely, the increase pattern of lipid peroxidation in the organ of Corti matched the cell damage distribution, involving predominantly OHC layer in noise exposed cochleae and both OHC and Deiters' cell layers in the styrene or combined exposed cochleae. The antioxidant treatment reduced the lipid peroxidation increase, potentiated the endogenous antioxidant defense system at OHC level in both exposures but it failed to ameliorate the oxidative imbalance and cell death of Deiters' cells in the styrene and combined exposures. Current antioxidant therapeutic approaches to preventing sensory loss focus on hair cells alone. It remains to be seen whether targeting supporting cells, in addition to hair cells, might be an effective approach to protecting exposed subjects.

Oxidative stress biomarkers and otoacoustic emissions in humans exposed to styrene and noise

OBJECTIVE

Evaluating the correlation between otoacoustic emission levels, styrene exposure, and oxidative stress biomarkers concentration in styrene-exposed subjects, to investigate the role of oxidative stress in outer hair cell damage.

DESIGN

Distortion product otoacoustic emissions were measured in the exposed workers and in a control group. Separation between the distortion and reflection otoacoustic components was performed by time-frequency-domain filtering. The urinary concentration of the DNA and RNA oxidation products, namely 8-oxo-7,8-dihydroguanine (oxoGua), 8-oxo-7,8-dihydro-2'-deoxyguanosine (oxodGuo), and 8-oxo-7,8-dihydroguanosine (oxoGuo), were evaluated.

STUDY SAMPLE

Nine subjects exposed to styrene in a fiberglass factory, eight control subjects. The two groups were statistically equivalent in mean age.

RESULTS

Statistically significant differences were found in the distortion component levels between the exposed and the control group. High levels of the oxidative damage biomarkers were found in the workers exposed to high levels of styrene. Significant negative correlation was found between the otoacoustic emission distortion component levels and the concentration of the oxoGuo biomarker.

CONCLUSIONS

Exposure-induced damage of the cochlear amplifier is shown in the mid-frequency range, confirming animal experiments, in which hair cells in the cochlear middle turn were damaged. Hearing damage is consistent with the outer hair cell apoptosis pathway associated with oxidative stress.

The Safety Pharmacology of Auditory Function

Safety pharmacology satisfies a key requirement in the process of drug development. Safety pharmacology studies are required to assess the impact of a new chemical entity (NCE) or biotechnology-derived product for human use on vital systems, such as those subserving auditory function. Safety pharmacology studies accordingly are defined as those studies that investigate the potential undesirable effects of a substance on auditory functions in relation to exposure in and above the therapeutic range. Auditory safety studies should be designed with the primary objective of determining how administration of a compound influences normal hearing. If an effect on hearing is identified, then it is necessary to determine through histopathology the underlying mechanism for the observed hearing loss. Since the auditory system contains a heterogeneous mixture of structural and cellular components that are organized in a very complex and integrated manner, it is necessary to clearly identify the underlying primary mechanism or target of the new chemical entity that produced the hearing loss. This chapter will highlight major components of auditory function with regard to potential opportunities for drug interaction. Aspects of designing ototoxicity studies will be discussed with an emphasis on standards deemed necessary by the US Food and Drug Administration. Additionally, classes of ototoxic compounds and their proposed mechanisms of action are described in depth.

Otoacoustic emission sensitivity to exposure to styrene and noise

The ototoxic effect of the exposure to styrene is evaluated, also in the presence of simultaneous exposure to noise, using otoacoustic emissions as biomarkers of mild cochlear damage. Transient-evoked and distortion product otoacoustic emissions were recorded and analyzed in a sample of workers (15 subjects) exposed to styrene and noise in a fiberglass manufacturing facility and in a control group of 13 non-exposed subjects. Individual exposure monitoring of the airborne styrene concentrations was performed, as well as biological monitoring, based on the urinary concentration of two styrene metabolites, the Mandelic and Phenylglyoxylic acids. Noise exposure was evaluated using wearable phonometers, and hearing loss with pure tone audiometry. Due to their different job tasks, one group of workers was exposed to high noise and low styrene levels, another group to higher styrene levels, close to the limit of 20 ppm, and to low noise levels. A significant negative correlation was found between the otoacoustic emission levels and the concentration of the styrene urinary metabolites. Otoacoustic emissions, and particularly distortion products, were able to discriminate the exposed workers from the controls, providing also a rough estimate of the slope of the dose-response relation between otoacoustic levels and styrene exposure.

Subchronic JP-8 jet fuel exposure enhances vulnerability to noise-induced hearing loss in rats

Both laboratory and epidemiological studies published over the past two decades have identified the risk of excess hearing loss when specific chemical contaminants are present along with noise. The objective of this study was to evaluate the potency of JP-8 jet fuel to enhance noise-induced hearing loss (NIHL) using inhalation exposure to fuel and simultaneous exposure to either continuous or intermittent noise exposure over a 4-wk exposure period using both male and female Fischer 344 rats. In the initial study, male (n = 5) and female (n = 5) rats received inhalation exposure to JP-8 fuel for 6 h/d, 5 d/wk for 4 wk at concentrations of 200, 750, or 1500 mg/m³. Parallel groups of rats also received nondamaging noise (constant octave band noise at 85 dB(lin)) in combination with the fuel, noise alone (75, 85, or 95 dB), or no exposure to fuel or noise. Significant concentration-related impairment of auditory function measured by distortion product otoacoustic emissions (DPOAE) and compound action potential (CAP) threshold was seen in rats exposed to combined JP-8 plus noise exposure when JP-8 levels of 1500 mg/m³ were presented with trends toward impairment seen with 750 mg/m³ JP-8 + noise. JP-8 alone exerted no significant effect on auditory function. In addition, noise was able to disrupt the DPOAE and increase auditory thresholds only when noise exposure was at 95 dB. In a subsequent study, male (n = 5 per group) and female (n = 5 per group) rats received 1000 mg/m³ JP-8 for 6 h/d, 5 d/wk for 4 wk with and without exposure to 102 dB octave band noise that was present for 15 min out of each hour (total noise duration 90 min). Comparisons were made to rats receiving only noise, and thosereceiving no experimental treatment. Significant impairment of auditory thresholds especially for high-frequency tones was identified in the male rats receiving combined treatment. This study provides a basis for estimating excessive hearing loss under conditions of subchronic JP-8 jet fuel exposure.

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.

Ototoxic potential of JP-8 and a Fischer-Tropsch synthetic jet fuel following subacute inhalation exposure in rats

This study was undertaken to identify the ototoxic potential of two jet fuels presented alone and in combination with noise. Rats were exposed via a subacute inhalation paradigm to JP-8 jet fuel, a kerosene-based fuel refined from petroleum, and a synthetic fuel produced by the Fischer-Tropsch (FT) process. Although JP-8 contains small ( approximately 5%) concentrations of aromatic hydrocarbons some of which known to be ototoxic, the synthetic fuel does not. The objectives of this study were to identify a lowest observed adverse effect level and a no observed adverse effect level for each jet fuel and to provide some preliminary, but admittedly, indirect evidence concerning the possible role of the aromatic hydrocarbon component of petroleum-based jet fuel on hearing. Rats (n = 5-19) received inhalation exposure to JP-8 or to FT fuel for 4 h/day on five consecutive days at doses of 500, 1000, and 2000 mg/m(3). Additional groups were exposed to various fuel concentrations followed by 1 h of an octave band of noise, noise alone, or no exposure to fuel or noise. Significant dose-related impairment in the distortion product otoacoustic emissions (DPOAE) was seen in subjects exposed to combined JP-8 plus noise exposure when JP-8 levels of at least 1000 mg/m(3) were presented. No noticeable impairment was observed at JP-8 levels of 500 mg/m(3) + noise. In contrast to the effects of JP-8 on noise-induced hearing loss, FT exposure had no effect by itself or in combination with noise exposure even at the highest exposure level tested. Despite an observed loss in DPOAE amplitude seen only when JP-8 and noise were combined, there was no loss in auditory threshold or increase in hair cell loss in any exposure group.

Otoacoustic detection of risk of early hearing loss in ears with normal audiograms: a 3-year follow-up study

INTRODUCTION

Distortion product otoacoustic emissions (DPOAEs) are known to represent the contractile amplifier function of cochlear outer hair cells. It is known that low or absent DPOAEs are associated with hearing loss on audiograms. However, low DPOAEs can also be found associated with normal audiograms. It is unknown whether low DPOAEs in normal hearing ears are risk markers for subsequent early hearing loss when subjects are exposed to noise.

MATERIALS AND METHODS

A 3-year follow-up study was carried out on a population of pilots aged 20-40 years (n=521). Data collection consisted of tonal audiograms, DPOAEs measurements with a calculation of an index of abnormality (the IaDPOAE). Of the 521 pilots enrolled, 350 (67%) had follow-up data 3 years later. In pilots with normal audiograms (n=219, all frequencies=10dB HL), we observed the occurrence of hearing threshold shifts after 3 years depending on whether the IaDPOAE was initially high (group 1) or low (group 2). We used this index to test the hypothesis that reduced DPOAEs levels are potential ear vulnerability biomarkers in apparent normal hearing ears. After a 3-year follow-up, the initial IaDPOAE in normal hearing subjects was correlated with final noise-induced hearing threshold shifts at high frequencies (p<0.01). The occurrence of abnormal audiograms was significantly higher in group 1 compared to group 2 (p=0.003). In group 1, 13% of audiograms were found with at least one frequency 25dB HL compared to 3% of audiograms in group 2. In both groups, impairments occurred at high frequencies and hearing in the 4kHz frequency range was significantly more impaired in group 1 (p=0.035). Group 1 was associated with a relative risk of 2.29 (95% CI 1.26-4.16, p=0.005) of sustaining early hearing loss. There was no significant differences between groups for age and noise exposure.

DISCUSSION

In adults with a normal audiogram, ear vulnerability to noise could be elicited by the use of objective DPOAE measurements. A high IaDPOAE that corresponded to reduced DPOAE levels constitutes a risk for early hearing loss. This study emphasised the interest of DPOAE measurements in public health and occupational noise prevention policies. The IaDPOAE calculation may also be interesting for clinicians because no DPOAE index of abnormality is currently available.

Methyl mercapturate synthesis: an efficient, convenient and simple method

A safe and simple method for methyl S-arylmercapturate synthesis is described. Thirteen such compounds, to be used afterwards in metabolism studies, have been obtained with yields ranging from 71 to 99.6%. These compounds were obtained using a sulfa-Michael addition and synthesized by adding the corresponding thiophenols to a mixture composed of methyl 2-acetamidoacrylate (MAA), potassium carbonate and a phase transfer catalyst, Aliquat 336. MAA, the initial synthon, was itself isolated in quasi quantitative yield following a fully described synthesis.

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.

Ototoxicity of organic solvents - from scientific evidence to health policy

The scientific workshop, organized under the 6th European Framework Programme, the Marie Curie Host Fellowship for the Transfer of Knowledge "NoiseHear" Project, by the Nofer Institute of Occupational Medicine (Łódź, Poland, 15-16 November 2006), gathered world specialists in noise, chemicals, and ototoxicity, including hearing researchers, toxicologists, otolaryngologists, audiologists and occupational health physicians.The workshop examined the evidence and the links between isolated exposure to organic solvents, combined exposure to noise and solvents, and effects on the auditory system. Its main purpose was to review the key scientific evidence to gather the necessary knowledge for developing adequate occupational health policies. This paper summarizes the workshop sessions and subsequent discussions.

Streptomycin action to the mammalian inner ear vestibular organs: comparison between pigmented guinea pigs and rats

Streptomycin is the antibiotic of choice to treat tuberculosis and other infectious diseases but it causes vestibular malfunction and hipoacusia. Rodents are usually employed as models of drug action to the inner ear and results are extrapolated to what happens in humans. In rats, streptomycin destroys macular sensory cells and does not affect cochlear ones, whereas in guinea pigs the contrary is true. Action on the vestibular cristae cells involved in vestibulo-ocular reflex integrity is less clear. Thus, we compared this response in both pigmented guinea pigs (Cavia cobaya) and rats (Rattus norvegicus) after parallel streptomycin chronic treatment. In guinea pigs, the reflex was obliterated along treatment time; in rats this behavior was not observed, suggesting that the end organ target was diverse. In recent studies, streptidine, a streptomycin derivative found in the blood of humans and rats treated with streptomycin, was the actual ototoxic agent. The putative streptomycin vestibular organ target observed in humans corresponds with the guinea pig observations. Results observed in rats are controversial: streptidine did not cause any damage either to vestibular cristae nor auditory cells. We hypothesize differential drug metabolism and distribution and conclude that results in laboratory animals may not always be applicable in the human situation.

Toluene toxicokinetics and metabolism parameters in the rat and guinea pig

Cochlear disruptions induced by toluene were shown in the rat but not in the guinea pig. To better understand the differences between species, three investigations were carried out to study (1) the blood affinity and the pulmonary uptake of the solvent, (2) its clearance and (3) its urinary elimination in both species. The blood affinity of toluene was +44% higher in the rat than in the guinea pig (14.4μg/g versus 10μg/g). Similarly, the pulmonary uptake of toluene was approximately 46.5% more efficient in the rat than in the guinea pig (75.4μg/g versus 40.3μg/g) after 3h inhalation of 1500ppm toluene. Therefore, the physicochemical composition of the blood could explain the difference in the uptake performances between rats and guinea pigs. The clearance of the toluene showed that 10min after an intravenous administration of 400μL of vehicle containing 28μL (43mgkg(-1)) of toluene, the solvent concentration was approximately threefold higher in the rat than in the guinea pig blood. The last experiment was carried out to compare the concentrations of the urinary metabolites. The concentrations of o-cresol, hippuric and benzyl mercapturic acids measured in the urines were different before and after the toluene injection. These data give evidence for large differences of toluene uptake and metabolism between rat and guinea pig. Therefore, it seems reasonable to claim that guinea pigs cochleas are not susceptible to toluene as the blood burden of solvent does not reach the concentration required to induce permanent damages.

Clinical, socio-demographic, neurophysiological and neuropsychiatric evaluation of children with volatile substance addiction

BACKGROUND

Abuse of organic volatile substances in children has become a social health problem that is increasing in the recent years. Among these substances, toluene is highly preferred by abusers because of its euphoric effect, cheapness and easy availability. There is no published research on the clinical and neurophysiological evaluation of children with short-term volatile substance addiction.

METHODS

In this study, socio-demographic characteristics were questioned in 12 children with a mean age of 15 years and a duration of toluene abuse for a mean of 2.3 years, and the clinical characteristics of central and peripheral nervous system damage caused by volatile substances, particularly by toluene were analysed, and probable neurological disorders were investigated by means of neurophysiological and neuropsychological tests. All tests were compared with a control group.

RESULTS

Fifty-eight percent of the children included in the study had pathological findings in the neurological examination. There was pyramidal involvement in 25% and peripheral nerve involvement in 33.3% of the cases. Evaluation of the cognitive functions revealed 33.3% pathology in the 'Short Test of Mental Status' which assesses functions of orientation, attention, learning, arithmetic calculation, abstraction, information, construction and recall. Sensorial polyneuropathy was found in 33.3% of the cases in nerve conduction studies. Somatosensory-evoked potentials revealed pathology in 16.7% of the cases and brainstem-evoked potentials in 50% of the cases. No pathology was observed in electroencephalography and visual-evoked potentials.

CONCLUSION

In our study, neurophysiological and neuropsychiatric tests revealed that toluene causes slow progressive, clinical and subclinical central and peripheral nerve damage. In Turkey, because of cheapness, easy availability and legal use of volatile substances, the clinical extent of systemic and neurological toxicity of volatile substance abuse is increasing. Abuse of volatile substances, a currently increasing social issue, may create important physical problems which can be permanent.

Toxicokinetic parameters of toluene in the rat and guinea pig: a comparative study

Toluene is the most widely used industrial solvent. It has been shown to cause cochlear disruptions in rats but markedly less ototoxic effects in guinea pigs. Susceptibility to the ototoxic properties of toluene is, therefore, species specific. In recent publications, an important difference in the solvent concentration in blood has been identified when rats and guinea pigs were exposed in strictly identical experimental conditions. Solvent concentrations in blood were greater in rats than in guinea pigs. The present studies were designed to compare blood affinity and toxicokinetic parameters of toluene in an attempt to understand the susceptibility differences in both species. The in vitro experiment, in which the headspace concentration of toluene was measured within a sealed vial containing blood, highlighted the greater toluene partition coefficient in rat than in guinea pig blood. The in vivo experiment showed that 10min after a single intravenous administration of 28μL of toluene, the solvent concentration is approximately two-fold lower in guinea pig than in rat blood. Based on the toxicokinetic parameters of toluene and on the relative partition coefficient of toluene in blood, it seems plausible that guinea pigs are not susceptible to organic solvents because the solvent concentration in blood does not reach the concentration required to induce permanent damage. Attempts to explain differences of vulnerability between the rat and guinea pig are addressed in the present paper.