Effects of some monocyclic aromatic solvents and their metabolites on brain dopamine in rabbits

A critical review finds styrene lacks direct endocrine disruptor activity

The European Commission lists styrene (S) as an endocrine disruptor based primarily on reports of increased prolactin (PRL) levels in S-exposed workers. The US Environmental Protection Agency included S in its list of chemicals to be tested for endocrine activity. Therefore, the database of S for potential endocrine activity is assessed. In vitro and in vivo screening studies, as well as non-guideline and guideline investigations in experimental animals indicate that S is not associated with (anti)estrogenic, (anti)androgenic, or thyroid-modulating activity or with an endocrine activity that may be relevant for the environment. Studies in exposed workers have suggested elevated PRL levels that have been further examined in a series of human and animal investigations. While there is only one definitively known physiological function of PRL, namely stimulation of milk production, many normal stress situations may lead to elevations without any chemical exposure. Animal studies on various aspects of dopamine (DA), the PRL-regulating neurotransmitter, in the central nervous system did not give mechanistic explanations on how S may affect PRL levels. Overall, a neuroendocrine disruption of PRL regulation cannot be deduced from a large experimental database. The effects in workers could not consistently be reproduced in experimental animals and the findings in humans represented acute reversible effects clearly below clinical and pathological levels. Therefore, unspecific acute workplace-related stress is proposed as an alternative mode of action for elevated PRL levels in workers.

The styrene metabolite, phenylglyoxylic acid, induces striatal-motor toxicity in the rat: influence of dose escalation/reduction over time

Exposure to the industrial solvent, styrene, induces locomotor and cognitive dysfunction in rats, and parkinsonian-like manifestations in man. The antipsychotic, haloperidol (HP), well known to induce striatal toxicity in man and animals, and styrene share a common metabolic pathway yielding p-fluoro phenylglyoxylic acid and phenylglyoxylic acid (PGA), respectively. Using an exposure period of 30 days and the vacous chewing movement (VCM) model as an expression of striatal-motor toxicity, we found that incremental PGA dosing (220–400 mg/kg) significantly increased VCMs up to day 25, but decreased to control levels shortly after reaching maximum dose. However, a diminishing dose of PGA (400–200 mg/kg) did not evoke an immediate worsening of VCMs but precipitated a significant increase in VCMs following dosage reduction to 200 mg/kg on day 22. PGA exposure, therefore, compromises striatal-motor function that is especially sensitive to changes in exposure dose. Longer alternating dose exposure studies are needed to establish whether motor dysfunction is progressive in severity or longevity. These findings are of significance for the environmental toxicology of styrene in the chemical industry.

Effects of subchronic exposure to styrene on the extracellular and tissue levels of dopamine, serotonin and their metabolites in rat brain

At present, there is controversy over the neurotoxic potential of styrene. Several epidemiological and clinical studies have shown that styrene exposure causes alterations of central nervous system functions in humans. Neurotransmitters have been implicated in the pathogenesis of styrene neurotoxicity in rodents. Several studies carried out on postmortem brain tissue suggest that styrene may alter dopaminergic neurotransmission in rabbit or rat brain. Moreover, in vitro studies suggest that both styrene and styrene oxide inhibit the uptake of dopamine (DA) in purified synaptic vesicles prepared from rat brain striata. To date, biochemical studies on animals have explored global tissue levels of neurotransmitters with sub-acute exposures to styrene. However, extracellular levels of neurotransmitters are more closely related to behaviour than are global tissue levels. The present study determined changes in the extracellular concentrations of DA, serotonin (5-HT) and their acid metabolites, dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindolacetic acid (5-HIAA), in striatal dialysates from freely moving adult male rats after exposure to 750 and 1,000 ppm styrene, 6 h per day, 5 days per week for 4 weeks. We also determined the concentrations of DA, 5-HT and their acid metabolites in striatum, nucleus accumbens and prefrontal cortex obtained postmortem from similarly exposed rats. Exposure to 1,000 ppm of styrene caused a significant decrease in extracellular acid metabolite concentrations. Tissue levels of acid metabolites were also decreased to a lesser extent. The effects were observed 72 h after discontinuing exposure but had vanished 17 days later. There was no change in DA or 5-HT concentrations either in the dialysates or tissues. Exposure to 750 ppm styrene caused no changes in the concentrations of DA, 5-HT and their acid metabolites either in the dialysates or tissues. The possibility that the effect of styrene is mediated by monoamine oxidase (MAO) inhibition is discussed.

Work exposure to urban pollutants and urinary homovanillic acid

The aim of this study was to evaluate whether traffic policemen exposed to urban pollutants could be at risk of alterations on urinary homovanillic acid in 24h HVA(U) excretion levels, an end product of dopamine catabolism, compared with a control group. Traffic policemen were matched by sex, age, and working life with control group after excluding principal confounding factors; 50 traffic policemen (29 men and 21 women) with outdoor activity exposed to urban pollutants and 50 not exposed subjects (29 men and 21 women) with indoor activity were included in the study. The HVA(U) excretion levels were significantly higher in male and female traffic policemen compared to not exposed subjects (respectively P=0.003; P=0.023). The authors hypothesize an effect on the excretion of HVA(U) in traffic policemen exposed to chemical and physical stressors, according to HVA(U) modifications found by other authors in workers exposed in factories.

Relation between colour vision loss and occupational styrene exposure level

AIMS

To investigate the relation between colour vision loss and the exposure level of styrene. Exposure level included the current exposure concentration, past cumulative exposure, and the maximum exposure level in the past.

METHODS

Colour vision was examined by the Lanthony desaturated panel D-15 test for 76 subjects exposed to styrene in a fibreglass reinforced plastics boat plant (as an exposed group) and 102 non-exposed subjects (as a control group). The current exposure level was expressed by the concentration of atmospheric styrene and end shift urinary mandelic acid (MA) and phenylglyoxylic acid (PGA) levels. The individual cumulative exposure index (CEI) was calculated, based on the exposure frequency and urinary MA concentrations measured for the past eight years.

RESULTS

The Colour Confusion Index (CCI) of the exposed group showed a significant difference from the age matched controls. However, only a slight significant relation was found between CCI and the concentration of urinary MA plus PGA. In this study, the exposed group was further divided into two subgroups (as sub-MA+PGA groups) by the median of urinary MA plus PGA of each subject. The dividing line between the subgroups was 0.24 g/g creatinine, which was equivalent to an atmospheric concentration of styrene of about 10 ppm. The CCI values of both the sub-MA+PGA groups were significantly higher than that of the control group. The relation between CCI value and the maximum exposure concentration in the past eight years was examined. It was found that the CCI values of the group with the maximum exposure concentration of styrene over 50 ppm were significantly higher than that of the other groups.

CONCLUSIONS

Exposure to styrene would impair colour vision even if the exposure concentration was lower than 10 ppm. Furthermore, if the maximum concentration of styrene exposure transiently exceeded 50 ppm in the past, the styrene related damage might remain. Thus, the safe limit of exposure to styrene and the relation between exposure to styrene and the degree of damage to ocular structure, retina, optic nerve, and brain need to be re-examined.

Liquid chromatography/electrospray tandem mass spectrometry characterization of styrene metabolism in man and in rat

Liquid chromatography with electrospray tandem mass spectrometry was used to characterize the metabolism of styrene in man and in rat. To improve identification and characterization of minor styrene metabolites, rats were co-exposed to styrene and styrene-d(8). In addition to the main styrene metabolites, mandelic acid and phenylglyoxylic acid, and specific mercapturic acids, phenylhydroxyethylmercapturic acids (PHEMAs), other minor metabolites, including phenylglycine, N-acetyl-S-(phenacyl)cysteine, 4-vinylphenol and styreneglycol conjugates (glucuronides and sulfates) were identified and determined both in human and rat urine. Phenylglycine and N-acetyl-S-(phenacyl)cysteine have been hypothesized to occur, but never detected in human or rat urine after styrene exposure. 4-Vinylphenol and styrene glycol had already been recognized as styrene metabolites, but never determined as intact glucuronide and sulfate conjugates. Failure to identify 1- and 2-phenylethanol conjugates suggests that phenylethanol might be an intermediate metabolite, but it is not a conjugated catabolite. A method for the simultaneous determination of mandelic acid, phenylglyoxylic acid, phenyglycine and the four PHEMA diastereoisomers has been developed and validated. For those glucuronide and sulfate conjugates whose standards are not commercially available, a method for semiquantitative analysis, based on the use of structurally similar compounds as standards, has been developed. This approach was found to be valid for the determination of 4-vinylphenol glucuronide and 4-vinylphenol sulfate.

The use of surrogate endpoints to assess potential toxicity in humans

Data on toxic effects in humans may come from epidemiology studies, accidental poisonings, surveillance schemes or following intentional exposures. In many cases, a surrogate endpoint related to the adverse effect is investigated. Effects produced following intentional exposures are usually restricted to readily reversible, mild surrogate endpoints of the adverse effect of concern. Not all initial interactions within the target organ are related to the toxic effect, and many measurements are biomarkers of exposure not response. Biomarkers of response represent surrogate endpoints of response only if they are critical to the mode of action. The use of biomarkers and the possible problems with using surrogate endpoints are illustrated with data on aniline, cadmium, carbon monoxide, erythrosine, paracetamol (acetaminophen) and styrene. In vivo surrogate endpoints are normally used in risk assessment directly, whereas in vitro surrogate endpoints can be incorporated by the development of a biologically based dose-response model, or used to replace a default uncertainty factor by a chemical-specific adjustment factor.

A review of the developmental and reproductive toxicity of styrene

The reproductive and developmental toxicity of styrene has been studied in animals and humans. The animal studies on styrene have diverse study designs and conclusions. Developmental or reproductive toxicity studies have been conducted in rats, mice, rabbits, and hamsters. In most cases, high doses are required to elicit effects, and the effects are not unique to reproduction or development. In a number of the reports, either the experimental designs are limited or the descriptions of the designs and the endpoints measured are insufficient to draw conclusions about the toxicity of styrene. The more complete and better-reported studies show that styrene does not cause developmental toxicity at dose levels that are not maternally toxic. Some neurochemical or neurobehavioral effects have been reported at high exposures. Styrene does not affect fertility or reproductive function. Considerable animal toxicity data on styrene support the conclusion that styrene is neither an endocrine-active substance nor an endocrine disrupter. Human studies often suffer from either inadequate exposure data or exposure to a wide variety of materials, so that attribution of effects to styrene exposure is impossible. Furthermore, investigators often have failed to account for other exposures in the workplace or for other potentially confounding factors in their studies. Menstrual cycle irregularities and congenital abnormalities were initially reported; however, the better and more recent reports do not show that styrene causes developmental or reproductive effects in humans. Human studies also support the conclusion that styrene is not an endocrine disrupter. Although some study authors have concluded that styrene is either a human or an animal reproductive or developmental toxicant, careful review demonstrates that such conclusions are not justified.

Altered regulation of dopaminergic activity and impairment in motor function in rats after subchronic exposure to styrene

Animal and human studies suggest a dopamine-mediated effect of styrene neurotoxicity. However, the results reported to date are incomplete and not consistent. As such, the mechanism of its neurotoxicity is still unclear. The present study has, therefore, reexamined the central dopaminergic system in relation to some neurobehavioral effects in rats following subchronic exposure to styrene. Groups of adult male Sprague-Dawley rats received 0, 0.25, or 0.5 g styrene per kg b.wt. by gavage for 13 consecutive weeks. Twenty-four hours after cessation of such treatment with the higher dose (0.5 g/kg), the contents of dopamine (DA) and its metabolites were significantly reduced in the corpus striatum, hypothalamus, and lateral olfactory tract regions. In vitro styrene showed a significant increase in DA release from rat striatal synaptosomes similar to that of tyramine. Significant loss of motor function was observed on days 56, 70, and 84 during the styrene treatment with the higher dose, and lasted over a month after such treatment. However, the treated animals recovered their motor function within 45-60 days after cessation of such treatment, along with the recovery of normal levels of dopamine and its metabolites. Furthermore, styrene-induced initial impairments in measures of dopaminergic activity cannot be attributed to altered regulation of tyrosine hydroxylase activity. Specific [3H]-spiroperidol binding was also unaltered 7 or 15 days after subchronic treatment with styrene. These data imply that despite the dopaminergic neuronal loss due to styrene, dopaminergic transmission was not reduced to a level that would result in an overall development of dopamine receptor supersensitivity in the striatum. Collectively, these studies indicate that the subchronic neurotoxic action of styrene may be primarily presynaptic in nature and may involve impaired regulation of DA content and stimulation of DA release.

Styrene and styrene oxide affect the transport of dopamine in purified rat striatal synaptic vesicles

Animal and human studies suggest a dopamine-mediated effect of styrene neurotoxicity. To date, mechanisms of cerebral membrane transport of neurotransmitter amines in the presence of styrene in relation to its neurotoxicity have not been addressed properly. So, the present study has examined to test the hypothesis that dopaminergic malfunction in vesicular transport is a critical component in styrene-induced neurotoxicity in rats. Both styrene and its intermediate reactive metabolite, styrene oxide antagonized the in vitro striatal binding of [3H] tyramine, a putative marker of the vesicular transporter for dopamine. Both styrene and styrene oxide potently inhibited the uptake of [3H] dopamine in purified synaptic vesicles prepared from rat brain striata, in a dose-related manner, with inhibitory constants (Ki) 2.5 and 2.2 microM respectively. However, neither styrene nor styrene oxide significantly increased the basal efflux of [3H] dopamine that has been preloaded into striatal vesicles in vitro. On the other hand, both styrene and styrene oxide have failed to significantly inhibit the uptake of either [3H] norepinephrine, or [3H] serotonin into striatal synaptic vesicles. It is concluded that both styrene and styrene oxide are capable of producing impairments in dopaminergic transport in purified striatal synaptic vesicles, an effect which may be a critical component in styrene-induced neurotoxicity.

Platelet monoamine oxidase B activity in workers exposed to styrene

A cross-sectional study was conducted to evaluate monoamine oxidase type B (MAO-B) activity in platelets as a biomarker of effect of styrene and perchloroethylene exposures. MAO-B is an enzyme system involved in dopamine catabolism, the impairment of which has been postulated as a mechanism of styrene-induced neurotoxicity. We previously observed an inverse association between blood styrene and MAO-B among reinforced plastics manufacturing workers. The present study included 59 male boat plant workers exposed to styrene (exposure range < 1-144 ppm, 8-h TWA). Two comparison groups comprised six male dry cleaning workers exposed to perchloroethylene (PCE; exposure range < 2-37 ppm) and 14 male laundry workers not exposed to either agent. Respiratory protection was not used by any of the styrene- or PCE-exposed workers; thus, air concentrations were regarded as valid exposure indicators. MAO-B activity (pmol/10(8) cells/h) was measured in peripheral blood platelets, using phenylethylamine as substrate. Only small overall mean differences in MAO-B were observed among the three groups; mean values were 4.21, 4.51, and 4.12 for the styrene-exposed, PCE-exposed, and laundry workers, respectively. Despite the absence of gross differences among the groups, styrene exposure was inversely related to MAO-B. Mean values for four increasing exposure group quartiles were: 5.60, 4.13, 3.69, and 3.44. The Spearman rank correlation coefficient for styrene with MAO-B was -0.41. Adjustment for age, medication use, smoking, and alcohol consumption had only a minimal effect on this trend.(ABSTRACT TRUNCATED AT 250 WORDS)

Peripheral markers of neurochemical function among workers exposed to styrene

A cross sectional study of biological markers of neurochemical function in peripheral blood cells, and self reported nervous system symptoms, was conducted among 60 workers exposed to styrene in three reinforced plastics plants and 18 reference workers not exposed to styrene or other solvents. Concentrations of styrene in the air at the plants ranged from less than 1 to 160 ppm. Biomarkers of neurochemical function measured were: sigma receptor binding in lymphocytes, monoamine oxidase type B (MAO-B) activity in platelets, and serotonin uptake by platelets. Blood styrene concentration was used as the exposure index to take account of the use of protective equipment and dermal uptake. Four blood styrene exposure groups were defined as: non-exposed (reference) and exposed to less than 0.05, 0.05-0.19, and greater than or equal to 0.20 micrograms/ml. The prevalences of headache, dizziness, light headedness, fatigue, irritability, memory loss, and feeling "drunk" at work increased with increasing blood styrene concentration. No effect on sigma receptor binding was seen. A slight positive correlation was found for uptake of serotonin, which has been used as an exposure related effect indicator in previous studies of workers exposed to solvents. The MAO-B activity decreased with increasing blood styrene concentration; the mean (SE) MAO-B values for the four groups were 34.2 (3.0), 28.1 (5.3), 20.1 (4.8), and 16.9 (7.7) pmol/10(7) cells/min. The MAO-B activity also correlated negatively with the number of reported nervous system symptoms, whereas no associations were seen between prevalence of symptoms and either serotonin uptake or sigma receptor binding. The findings for MAO-B activity are consistent with previously reported experimental data, and suggest that MAO-B may be a useful marker of styrene neurotoxicity.

Long term neurotoxicity of styrene. A quantitative study of glial fibrillary acidic protein (GFA) and S-100

Little information exists about the possible neurotoxicity of styrene. The present study was designed to explore whether long term inhalation exposure (three months) to styrene (90 and 320 ppm) could induce long lasting astroglial alterations in Sprague Dawley rats, traceable four months after exposure ceased. Styrene exposure at 320 ppm induced such alterations as shown by raised concentrations of the glial cell marker, glial fibrillary acidic protein (GFA) in the sensory motor cortex and in the hippocampus. GFA is the structural protein of the astroglial filaments and formation of these filaments has been shown after damage to the central nervous system from any cause. It is concluded that exposure to styrene at moderate exposure levels induces regional, long lasting astroglial reactions that serve as an indicator of solvent induced brain damage.

Brain dopamine as a target for solvent toxicity: effects of some monocyclic aromatic hydrocarbons

Adult male rabbits were exposed to toluene, xylene, styrene, ethylbenzene, vinyltoluene or were dosed with hippuric, methylhippuric, mandelic, phenylglyoxylic, and 7-methyl-mandelic acids. Styrene, vinyltoluene and ethylbenzene caused a marked depletion of striatal and tubero-infundibular dopamine. Such an effect was also caused by treatment with mandelic and phenylglyoxylic acids. These results indicate that dopamine is a target for some solvents of their metabolites, the presence of a lateral vinyl- or ethyl-chain which may be biotransformed into alpha-keto acids being crucial for the effect. Experiments in vitro suggest that dopamine condenses non-enzymatically with reactive carbonylic groups of such and other alpha-keto acids, thus becoming ineffective as neurotransmitter. This mechanism might account for the neurobehavioral and neuroendocrine changes which have been reported in workers occupationally exposed to styrene and to some solvent mixtures.

Neurobehavioral investigation as a tool for revealing preclinical disorders

This paper calls attention to the methodologies designed to investigate the higher cortical functions in order to elicit signis of encephalopathy in apparently normal conditions. This can be done by testing the blobal hemispheric funcionts or the interhemispheric functional balance. This shows up the clinical sequels that may precede or be the outcome both of transient pathological disorders, such as transient global anemia, migraine, TIAs and subarachnoid hemorrhage without apparent clinical consequences and of nontransient pathological conditions, such as epilepsy, occupational diseases, arterial hypertension and cerebral revascularization.