Effects of prenatal exposure to styrene on neurochemical levels in rat brain

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.

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.

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.

Effects of pregnancy, age and sex in the metabolism of styrene in rat liver in relation to the regulation of cytochrome P450 enzymes

To elucidate the effect of maternal styrene exposure, which is due to various postnatal changes in the development and behavior of offspring, we investigated pregnancy-induced changes in the metabolism of styrene in rat liver in relation to the regulation of cytochrome P450 enzymes. We also examined age and sex-induced changes in the metabolism of styrene. Pregnancy appeared to exert a negative effect on cytochrome P450 content at the late stage, whereas microsomal protein content showed little change during pregnancy. Pregnancy significantly decreased the rate of formation of styrene glycol at the late stage. The percentage of remaining activity in microsomes exposed to anti-CYP2E1 was lower than that exposed to anti-CYP2C11/6 in pregnant and non-pregnant female rats and immature male rats, indicating that CYP2E1 contributes to the metabolism of styrene more than CYP2C11/6 in these rats. Although pregnancy seemed to decrease styrene metabolism, the contribution of CYP2E1 seemed to be slightly increasing. In conclusion, pregnancy clearly influences the metabolism of styrene as well as other characteristic factors such as age and sex. It is very important to elucidate the changes in specific P450 isozyme composition related to their characteristic modification and in their affinity for chemicals.

Developmental neurotoxicity study of styrene by inhalation in Crl-CD rats

This study was conducted to assess potential adverse functional and/or morphological effects of styrene on the neurological system in the F2 offspring following F0 and F1 generation whole-body inhalation exposures. Four groups of male and female Crl:CD (SD)IGS BR rats (25/sex/group) were exposed to 0, 50, 150, and 500 ppm styrene for 6 hr daily for at least 70 consecutive days prior to mating for the F0 and F1 generations. Inhalation exposure continued for the F0 and F1 females throughout mating and through gestation day 20. On lactation days 1 through 4, the F0 and F1 females received styrene in virgin olive oil via oral gavage at dose levels of 66, 117, and 300 mg/kg/day (divided into three equal doses, approximately 2 hr apart). Inhalation exposure of the F0 and F1 females was re-initiated on lactation day 5 and continued through weaning of the F1 or F2 pups on postnatal day (PND) 21. Developmental landmarks were assessed in F1 and F2 offspring. The neurological development of randomly selected pups from the F2 generation was assessed by functional observational battery, locomotor activity, acoustic startle response, learning and memory evaluations, brain weights and dimension measurements, and brain morphometric and histologic evaluation. Styrene exposure did not affect survival or the clinical condition of the animals. As expected from previous studies, slight body weight and histopathologic effects on the nasal olfactory epithelium were found in F0 and F1 rats exposed to 500 ppm and, to a lesser extent, 150 ppm. There were no indications of adverse effects on reproductive performance in either the F0 or F1 generation. There were exposure-related reductions in mean body weights of the F1 and F2 offspring from the mid and high-exposure groups and an overall pattern of slightly delayed development evident in the F2 offspring only from the 500-ppm group. This developmental delay included reduced body weight (which continued through day 70) and slightly delayed acquisition of some physical landmarks of development. Styrene exposure of the F0 and F1 animals had no effect on survival, the clinical condition or necropsy findings of the F2 animals. Functional observational battery evaluations conducted for all F1 dams during the gestation and lactation periods and for the F2 offspring were unaffected by styrene exposure. Swimming ability as determined by straight channel escape times measured on PND 24 were increased, and reduced grip strength values were evident for both sexes on PND 45 and 60 in the 500-ppm group compared to controls. There were no other parental exposure-related findings in the F2 pre-weaning and post-weaning functional observational battery assessments, the PND 20 and PND 60 auditory startle habituation parameters, in endpoints of learning and memory performance (escape times and errors) in the Biel water maze task at either testing age, or in activity levels measured on PND 61 in the 500-ppm group. Taken together, the exposure-related developmental and neuromotor changes identified in F2 pups from dams exposed to 500 ppm occurred in endpoints known to be both age- and weight-sensitive parameters, and were observed in the absence of any other remarkable indicators of neurobehavioral toxicity. Based on the results of this study, an exposure level of 50 ppm was considered to be the NOAEL for growth of F2 offspring; an exposure level of 500 ppm was considered to be the NOAEL for F2 developmental neurotoxicity.