Developing an exposure-dose-response model for the acute neurotoxicity of organic solvents: overview and progress on in vitro models and dosimetry

We are developing an exposure–dose–response (EDR) model for volatile organic compounds (VOCs) to predict acute effects of VOCs on nervous system function from exposure data (concentration and duration of inhalation). This model contains both toxicokinetic and toxicodynamic components. One advantage of the EDR model will be its ability to relate in vitro effects of solvents on cellular ion channels (putative targets) to in vivo effects, using a combination of physiologically-based toxicokinetic (PBTK) modeling (to estimate VOC concentrations in the blood and brain) and in vitro studies to clarify the mode of action of the VOCs. Recent work in vitro has focused on quantifying the inhibitory effects of toluene, trichloroethylene (TCE) and perchloroethylene (PERC) on ion channel currents. All three VOCs inhibit current through voltage-sensitive calcium channels (VSCCs) in pheochromocytoma cells; PERC blocked calcium currents and altered the current–voltage relationship at lower concentrations than did toluene or TCE. Recombinant nicotinic acetylcholine receptors (nAChRs), expressed in Xenopus oocytes, were also inhibited by PERC and toluene in a concentration-dependent manner. PERC inhibited 7 receptors more than 4β2 receptors in recombinant human and rat nAChRs. However, human and rat 7 receptors were equally sensitive to PERC and TOL. These in vitro studies will be used to identify an appropriate neuronal receptor system to serve as an index of acute effects of VOCs in vivo. The PBTK model incorporates physiological input parameters derived from radiotelemetered heart rate data from rats performing operant tests of cognitive and motor functions. These studies should improve predictions of target organ concentrations of inhaled VOCs in subjects actively performing behavioral tests over a range of physical activity levels.     

A meta-analytical approach to neurobehavioural effects of occupational toluene exposure

Twenty-two studies investigating neurobehavioural effects of toluene were reviewed. Repeatedly applied neuropsychological performance tests and appropriately documented results allowed to include 10 of the studies into a meta-analysis based on effect sizes. Mean exposure level of the studies was 57 ppm (range 20–117 ppm) toluene.

Five of the six analyses of neuropsychological tests obtained effect sizes suggesting a negative impact of toluene, but for none of the analyses a significant effect size was estimated at averaged exposure levels between 33 and 89 ppm. The tests represented the psychological domains of attention and constructional performance. Additionally, the relation between exposure conditions (level of exposure, length of exposure), potential confounders (age, verbal intelligence), and effect sizes was analysed. Only pre-exposure intellectual capacity showed a consistent relation to effect sizes.

The study suggests that homogenisation of study groups with respect to intelligence, cultural background and practice trials is important when investigating low-level exposure. Efforts to homogenize studies should be completed by a thorough documentation of feasible influences.     

接触苯、甲苯、二甲苯对工人脂质过氧化作用的影响

目的探讨苯、甲苯、二甲苯对人体的脂质过氧化作用及抗氧化作用的影响。方法选择50名长期接触苯、甲苯、二甲苯作业的工人和55名不接触任何毒物的工人为研究对象,进行现场职业卫生学监测,对工人血清中丙二醛(MDA)含量和超氧化物歧化酶(SOD)、谷胱甘肽过氧化物酶(GSH Px)活力进行测定。结果接触组MDA含量[(3.55±1.58)]nmol/L及SOD活力[(95.45±11.05)×103单位/L]高于对照组,而GSH Px活力[(70.54±38.24)×104单位/L]低于对照组,差异均有显著性(P<0.05)。结论长期接触低浓度苯、甲苯、二甲苯,将导致MDA含量和SOD活力增加及GSH Px活力下降。     

某空气净化杀菌设备的消毒净化效果检测

[目的]了解某牌空气净化杀菌设备的消毒与净化空气效果。[方法]2004年8～10月将污染到空气中甲醛、甲苯、氨及自然菌用某空气净化杀菌设备进行了现场净化杀菌效果试验。[结果]该空气净化杀菌设备在45m3房间,作用30min对自然菌的消毒消亡率达100%;作用60min对污染到空气中的甲醛、甲苯和氨清除率分别为96.1%、99.9%、94.9%;作用60min对60m3的密闭室净化度达万级。[结论]某空气净化杀菌设备对空气中细菌消毒效果好,洁净成度高,对空气中的有害气体甲醛、甲苯、氨有较高的清除率。     

Selective inhibition of gamma-aminobutyric acid type A receptors in human IMR-32 cells by low concentrations of toluene

Effects of the neurotoxic organic solvent toluene on human neuronal nicotinic acetylcholine (nACh) and γ-aminobutyric acid type A (GABA_A) neurotransmitter receptors were investigated in whole-cell voltage-clamped IMR-32 neuroblastoma cells. Ion currents evoked by near maximum effective concentrations of 1 mM acetylcholine (ACh) and 1 mM γ-aminobutyric acid (GABA) are inhibited by toluene in a concentration-dependent way. Concentration–effect curves of toluene yield IC₅₀ values of 276±26 and 39±6 μM and slope factors of 1.4±0.2 and 0.8±0.1 for inhibition of the ACh- and GABA-induced ion currents, respectively. The results demonstrate the selective inhibition of human GABA_A receptors by toluene at concentrations comparable with brain concentrations associated with occupational exposure.     

Diagnosis and prevention of diseases induced by isocyanate

Isocyanates are among the most frequent causes of occupational asthma in industrialized countries. Early diagnosis of diisocyanate asthma followed by prompt termination of chemical exposure can prevent chronic morbidity due to persistent asthma. Chronic exposure to isocyanates also induces hypersensitivity pneumonitis (HP). The accurate diagnosis of diisocynate asthma requires a systematic approach that combines information obtained from the occupational history, immunologic tests and physiologic studies. The prevention of health problems from toluene diisocyanate (TDI), 4,4′-methylenediphenyl diisocyanate (MDI) and 1,6′-hexamethylene diisocyanate (HDI) is essential for all those handling the chemicals. Regulatory exposure limits should be observed. However, wheezing, coughing or even asthmatic attacks may occur after exposure much below the regulatory exposure limits especially in sensitive individuals. Preventing or minimizing exposure is of prime importance and should be supported by the installation of engineering controls, by education of the workforce, by regular monitoring of the workplace exposure and by medical surveillance. To prevent such asthma it is suggested that workers should be tested airway sensitivity and should avoid working in areas that have dust containing specific-IgE. Such tests must be periodically performed after working. Symptoms induced by isocyanate need earlier discover and early isolation of the associated individuals.     

Toluene and temporal discrimination in rats: effects on accuracy, discriminability, and time estimation

Thirty-five rats acquired a temporal discrimination of seven signal durations in an operant chamber with two levers. If a 2-s or an 8-s tone signal was presented, rats were required to press one lever (”short” response) or the other lever (”long” response) to be reinforced, respectively. Neither response was reinforced when five intermediate signals were presented. Percentages of a long response in each of the seven signals were calculated and the psychophysical function between signal durations and long response percentages was obtained. Intraperitoneal injections of 50 mg/kg and 100 mg/kg toluene steepened a gradient of the psychophysical function and elevated correct responses in 2-s and 8-s signals. The function showed lower difference limen and Weber fraction. Accuracy and discriminability of temporal discrimination were enhanced. However, 400 mg/kg and 600 mg/kg toluene resulted in a shallower gradient and reduced correct responses. Higher difference limen and Weber fraction were also obtained. Accuracy and discriminability deteriorated and, in particular, behavioral depression was observed for 600 mg/kg toluene. The point of indifference was changed and dose-related overestimation or underestimation of time were suggested to occur. Blood toluene levels were reported to be 11–18 μg/ml for 50–100 mg/kg toluene and 47–73 μg/ml for 400–600 mg/kg toluene. It is speculated that temporal discrimination was sharpened by 10–20 μg/ml toluene in blood and disrupted by 50–70 μg/ml toluene in blood.     

An enzyme-linked immunosorbent assay for hippuric acid: its potential application for biological monitoring of toluene exposure

An enzyme-linked immunosorbent assay (ELISA) for hippuric acid (HA) was developed using polyclonal anti-HA antibodies. Anti-HA antibodies were obtained by immunizing rabbits with N-benzoyl-cysteine (B-Cys) or N--benzoyl-lysine (B-Lys). An antibody with highest reactivity to HA was obtained from anti-B-Lys antiserum by affinity chromatography with B-Cys-Sepharose. The ELISA system was composed of solid-phase B-Cys, anti-HA antibody, and horseradish peroxidase-conjugated anti-rabbit immunoglobulin antibody. The detection limit of the ELISA for HA was around 1 g/ml. The urinary HA concentration determined by the ELISA system correlated well with that obtained by high-performance liquid chromatography (HPLC). The ELISA system was considered to be useful in the biological monitoring of toluene exposure, and to be more advantageous than time-consuming HPLC, especially when measuring a large number of samples.     

Detection of specific IgE in isocyanate and phthalic anhydride exposed workers: comparison of RAST RIA, Immuno CAP System FEIA, and Magic Lite SQ

Sera from 94 workers occupationally exposed to isocyanate were tested by RAST RIA, Immuno CAP FEIA, and Magic Lite SQ (ML) against the allergens HDI, MDI, TDI, and phthalic anhydride. Twenty sera showed increased levels of diisocyanate-specific IgE antibodies, and five sera were phthalic anhydride positive. High total IgE titer was not correlated with positive specific IgE titer ( r = 0.47), showing that nonspecific IgE binding was low. The results of ML and CAP correlated well ( r = 0.91), but tended to be slightly higher than the results obtained with the RAST isotope test. CAP and RAST data also correlated significantly ( r = 0.90), but the correlation between ML and RAST data was lower ( r = 0.82). The results of the new in vitro tests, CAP and ML, showed good reproducibility (5% CV for CAP and 8% CV for ML). In summary, the CAP and ML methods were found to be appropriate for routine diagnosis of specific IgE antibodies against the allergens HDI, MDI, TDI, and phthalic anhydride.     

Evidence for neutrophil activation in occupational asthma

In order to evaluate the role of neutrophils in the pathogenesis of occupational asthma (OA), 15 toluene diisocyanate (TDI)-asthma and six grain dust-asthma patients were recruited. Controls were the same number of subjects showing negative bronchoprovocation test (BPT) and six house dust mite-sensitive asthma. Bronchoscopic biopsy specimens were stained with monoclonal antibodies to mast cell (AA1), eosinophil (EG2), pan T cell (CD3) and neutrophil (NE). Serum neutrophil chemotactic activity (NCA) was measured before and 10-420 min after BPT. Sputum interleukin-8 (IL-8) and myeloperoxidase (MPO) were also measured. There was a significant increase of NE+ cells as well as AA1+ and EG2+ cells in grain dust- and TDI-asthma compared with house dust-sensitive asthma (P < 0.05). Neutrophil+ cells and AA1+ cells showed a significant correlation in TDI-asthma (r = 0.73, P = 0.02). Serum NCA was significantly increased at 10 min after BPT and decreased at 60 min in subjects with TDI-asthma. In grain dust-asthma, serum NCA increased at 30 min and decreased at 240 min after BPT (P < 0.05). Sputum IL-8 and MPO were significantly increased after BPT in both TDI- and grain dust-asthma (P < 0.05). These findings suggested that neutrophils in the lungs might contribute to bronchoconstriction induced by either TDI or grain dust. The possible involvement of IL-8 in activation of neutrophils was also suggested.