Perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) acutely affect human α1ß2γ2L GABAA receptor and spontaneous neuronal network function in vitro.

Concerns about the neurotoxic potential of polyfluoroalkyl substances (PFAS) such as perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) increase, although their neurotoxic mechanisms of action remain debated. Considering the importance of the GABAA receptor in neuronal function, we investigated acute effects of PFAS on this receptor and on spontaneous neuronal network activity. PFOS (Lowest Observed Effect Concentration (LOEC) 0.1 µM) and PFOA (LOEC 1 µM) inhibited the GABA-evoked current and acted as non-competitive human GABAA receptor antagonists. Network activity of rat primary cortical cultures increased following exposure to PFOS (LOEC 100 µM). However, exposure of networks of human induced pluripotent stem cell (hiPSC)-derived neurons decreased neuronal activity. The higher sensitivity of the α1ß2γ2L GABAA receptor for PFAS as compared to neuronal networks suggests that PFAS have additional mechanisms of action, or that compensatory mechanisms are at play. Differences between rodent and hiPSC-derived neuronal networks highlight the importance of proper model composition. LOECs for PFAS on GABAA receptor and neuronal activity reported here are within or below the range found in blood levels of occupationally exposed humans. For PFOS, LOECs are even within the range found in human serum and plasma of the general population, suggesting a clear neurotoxic risk.

Neurotoxicity and risk assessment of brominated and alternative flame retardants.

Brominated flame retardants (BFRs) are widely used chemicals that prevent or slow the onset and spreading of fire. Unfortunately, many of these compounds pose serious threats for human health and the environment, indicating an urgent need for safe(r) and less persistent alternative flame retardants (AFRs). As previous research identified the nervous system as a sensitive target organ, the neurotoxicity of past and present flame retardants is reviewed. First, an overview of the neurotoxicity of BFRs in humans and experimental animals is provided, and some common in vitro neurotoxic mechanisms of action are discussed. The combined epidemiological and toxicological studies clearly underline the need for replacing BFRs. Many potentially suitable AFRs are already in use, despite the absence of a full profile of their environmental behavior and toxicological properties. To prioritize the suitability of some selected halogenated and non-halogenated organophosphorous flame retardants and inorganic halogen-free flame retardants, the available neurotoxic data of these AFRs are discussed. The suitability of the AFRs is rank-ordered and combined with human exposure data (serum concentrations, breast milk concentrations and house dust concentrations) and physicochemical properties (useful to predict e.g. bioavailability and persistence in the environment) for a first semi-quantitative risk assessment of the AFRs. As can be concluded from the reviewed data, several BFRs and AFRs share some neurotoxic effects and modes of action. Moreover, the available neurotoxicity data indicate that some AFRs may be suitable substitutes for BFRs. However, proper risk assessment is hampered by an overall scarcity of data, particularly regarding environmental persistence, human exposure levels, and the formation of breakdown products and possible metabolites as well as their toxicity. Until these data gaps in environmental behavioral and toxicological profiles are filled, large scale use of these chemicals should be cautioned.

Effects of neonatal exposure to the flame retardant tetrabromobisphenol-A, aluminum diethylphosphinate or zinc stannate on long-term potentiation and synaptic protein levels in mice.

Brominated flame retardants such as tetrabromobisphenol-A (TBBPA) may exert (developmental) neurotoxic effects. However, data on (neuro)toxicity of halogen-free flame retardants (HFFRs) are scarce. Recent in vitro studies indicated a high neurotoxic potential for some HFFRs, e.g., zinc stannate (ZS), whereas the neurotoxic potential of other HFFRs, such as aluminum diethylphosphinate (Alpi), appears low. However, the in vivo (neuro)toxicity of these compounds is largely unknown. We therefore investigated effects of neonatal exposure to TBBPA, Alpi or ZS on synaptic plasticity in mouse hippocampus. Male C57bl/6 mice received a single oral dose of 211 µmol/kg bw TBBPA, Alpi or ZS on postnatal day (PND) 10. On PND 17-19, effects on hippocampal synaptic plasticity were investigated using ex vivo extracellular field recordings. Additionally, we measured levels of postsynaptic proteins involved in long-term potentiation (LTP) as well as flame retardant concentrations in brain, muscle and liver tissues. All three flame retardants induced minor, but insignificant, effects on LTP. Additionally, TBBPA induced a minor decrease in post-tetanic potentiation. Despite these minor effects, expression of selected synaptic proteins involved in LTP was not affected. The flame retardants could not be measured in significant amounts in the brains, suggesting low bioavailability and/or rapid elimination/metabolism. We therefore conclude that a single neonatal exposure on PND 10 to TBBPA, Alpi or ZS does affect neurodevelopment and synaptic plasticity only to a small extent in mice. Additional data, in particular on persistence, bioaccumulation and (in vivo) toxicity, following prolonged (developmental) exposure are required for further (human) risk assessment.

Comparison of plate reader-based methods with fluorescence microscopy for measurements of intracellular calcium levels for the assessment of in vitro neurotoxicity.

The intracellular calcium concentration ([Ca(2+)]i) is an important readout for in vitro neurotoxicity since calcium is critically involved in many essential neurobiological processes, including neurotransmission, neurodegeneration and neurodevelopment. [Ca(2+)]i is often measured with considerable throughput at the level of cell populations with plate reader-based assays or with lower throughput at the level of individual cells with fluorescence microscopy. However, these methodologies yield different quantitative and qualitative results. In recent years, we demonstrated that the resolution and sensitivity of fluorescence microscopy is superior compared to plate reader-based assays. However, it is currently unclear if the use of plate reader-based assays results in more 'false negatives' or 'false positives' in neurotoxicity screening studies. In the present study, we therefore compared a plate reader-based assay with fluorescence microscopy using a small test set of environmental pollutants consisting of dieldrin, lindane, polychlorinated biphenyl 53 (PCB53) and tetrabromobisphenol-A (TBBPA). Using single-cell fluorescence microscopy, we demonstrate that all test chemicals reduce the depolarization-evoked increase in [Ca(2+)]i, whereas lindane, PCB53 and TBBPA also increase basal [Ca(2+)]i, though via different mechanisms. Importantly, none of these effects were confirmed with the plate reader-based assay. We therefore conclude that standard plate reader-based methods are not sufficiently sensitive and reliable to measure the highly dynamic and transient changes in [Ca(2+)]i that occur during chemical exposure.

A comparison of the in vitro cyto- and neurotoxicity of brominated and halogen-free flame retardants: prioritization in search for safe(r) alternatives.

Brominated flame retardants (BFRs) are abundant persistent organic pollutants with well-studied toxicity. The toxicological and ecological concerns associated with BFRs argue for replacement by safe(r) alternatives. Though previous research identified the nervous system as a sensitive target organ for BFRs, the (neuro) toxic potential of alternative halogen-free flame retardants (HFFRs) is largely unknown. We therefore investigated the in vitro (neuro) toxicity of 13 HFFRs and three BFRs in dopaminergic pheochromocytoma (PC12) and neuroblastoma (B35) cells by assessing several cytotoxic and neurotoxic endpoints. Effects on cell viability and production of reactive oxygen species (ROS) were measured using a combined Alamar Blue and Neutral Red assay and a H2-DCFDA assay, respectively, whereas effects on calcium homeostasis were measured using single-cell fluorescent Ca(2+)-imaging. The majority of the tested flame retardants induced negligible cytotoxicity, except zinc hydroxystannate (ZHS) and zinc stannate (ZS). A considerable fraction of flame retardants affected ROS production (decabromodiphenyl ether (BDE-209), triphenylphosphate (TPP), aluminium trihydroxide (ATH), ammonium polyphosphate (APP), magnesium hydroxide (MHO), ZHS, ZS and melamine polyphosphate (MPP)). Interestingly, ATH, ZHS, ZS and montmorillonite (MMT) increased the basal intracellular calcium concentration ([Ca(2+)]i), whereas tetrabromobisphenol A (TBBPA), resorcinol bis (diphenylphosphate) (RDP), TPP, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), ATH, ZHS, ZS and MMT reduced depolarization-evoked increases in [Ca(2+)]i as a result of inhibition of voltage-gated calcium channels. These combined data on the in vitro (neuro) toxicity of HFFRs in comparison with BFRs are essential for prioritization of safe(r) flame retardants. Though additional data are required for a complete (toxic) risk assessment, our data demonstrate that several HFFRs could be suitable substitutes for BFRs.

Persistence, bioaccumulation, and toxicity of halogen-free flame retardants.

Polymers are synthetic organic materials having a high carbon and hydrogen content, which make them readily combustible. Polymers have many indoor uses and their flammability makes them a fire hazard. Therefore, flame retardants (FRs) are incorporated into these materials as a safety measure. Brominated flame retardants (BFRs), which accounted for about 21% of the total world market of FRs, have several unintended negative effects on the environment and human health. Hence, there is growing interest in finding appropriate alternative halogen-free flame retardants (HFFRs). Many of these HFFRs are marketed already, although their environ- mental behavior and toxicological properties are often only known to a limited extent, and their potential impact on the environment cannot yet be properly assessed. Therefore, we undertook this review to make an inventory of the available data that exists (up to September 2011) on the physical-chemical properties, pro- duction volumes, persistence, bioaccumulation, and toxicity (PBT) of a selection of HFFRs that are potential replacements for BFRs in polymers. Large data gaps were identified for the physical-chemical and the PBT properties of the reviewed HFFRs. Because these HFFRs are currently on the market, there is an urgent need to fill these data gaps. Enhanced transparency of methodology and data are needed to reevaluate certain test results that appear contradictory, and, if this does not provide new insights, further research should be performed. TPP has been studied quite extensively and it is clearly persistent, bioaccumulative, and toxic. So far, RDP and BDP have demonstrated low to high ecotoxicity and persistence. The compounds ATH and ZB exerted high toxicity to some species and ALPI appeared to be persistent and has low to moderate reported ecotoxicity. DOPO and MPP may be persistent, but this view is based merely on one or two studies, clearly indicating a lack of information. Many degradation studies have been performed on PER and show low persistence, with a few exceptions. Additionally, there is too l ittle information on the bioaccumulation potential of PER. APP mostly has low PBT properties; however, moderate ecotoxicity was reported in two studies. Mg(OH)2, ZHS, and ZS do not show such remarkably high bioaccumulation or toxicity, but large data gaps exist for these compounds also. Nevertheless, we consider the latter compounds to be the most promising among alternative HFFRs. To assess whether the presently reviewed HFFRs are truly suitable alternatives, each compound should be examined individually by comparing its PBT values with those of the relevant halogenated flame retardant. Until more data are available, it remains impossible to accurately evaluate the risk of each of these compounds, including the ones that are already extensively marketed.

Modulation of human α4ß2 nicotinic acetylcholine receptors by brominated and halogen-free flame retardants as a measure for in vitro neurotoxicity.

Brominated flame retardants (BFRs) are abundant persistent organic pollutants with well-studied toxicity. The toxicological and ecological concern associated with BFRs argues for replacement by safer alternatives. However, the (neuro)toxic potential of alternative halogen-free flame retardants (HFFRs) is unknown. Previous research identified the nervous system as a sensitive target organ for BFRs, with modulation of excitatory nicotinic acetylcholine (nACh) receptors as one of the modes of action. Since it is essential to assess the (neuro)toxic potential of HFFRs before large scale use, we measured the effects of three BFRs and 13 HFFRs on the function of human α(4)ß(2) nACh receptors, expressed in Xenopus oocytes, using the two-electrode voltage-clamp technique. The results demonstrate that some BFRs (TBBPA and to a lesser extent BDE-209) and HFFRs (TPP, Alpi, APP, MMT and to a lesser extent ATH, ATO, MHO, MPP, RDP and ZHS) act as nACh receptor antagonists. Contrary, BPS, BDP, DOPO and ZS were unable to modulate nACh receptors. Despite the lack of toxicological data on HFFRs and the need for additional studies to perform a full (neuro)toxic risk assessment, the current data on antagonistic effects on nACh receptors could be an important step in prioritizing viable HFFRs for substitution of BFRs.

Multiple novel modes of action involved in the in vitro neurotoxic effects of tetrabromobisphenol-A.

Neurotoxicological data on the widely used brominated flame retardant tetrabromobisphenol-A (TBBPA) is limited. Since recent studies indicated that inhibitory GABA(A) and excitatory α(4)ß(2) nicotinic acetylcholine (nACh) receptors are sensitive targets for persistent organic pollutants, we investigated the effects of TBBPA on these receptors, expressed in Xenopus oocytes, using the two-electrode voltage-clamp technique. Our results demonstrate that TBBPA acts as full (≥ 10 µM) and partial (≥ 0.1 µM) agonist on human GABA(A) receptors, whereas it acts as antagonist (≥ 10 µM) on human α(4)ß(2) nACh receptors. Next, neuronal B35 cells were used to further study the effects of TBBPA on calcium-permeable nACh receptors using single-cell fluorescent calcium imaging. These results demonstrate that TBBPA (≥ 1 µM) inhibits acetylcholine (ACh) receptors as evidenced by a reduction in the ACh-evoked increases in the intracellular calcium concentration ([Ca(2+)](i)). Additionally, TBBPA (> 1 µM) induced a strong and concentration-dependent increase in basal [Ca(2+)](i) in B35 cells. Similarly, TBBPA (> 1 µM) increases basal [Ca(2+)](i) in dopaminergic PC12 cells. This increase is also evident under calcium-free conditions, indicating it originates from intracellular calcium stores. Moreover, depolarization-evoked increases in [Ca(2+)](i) are strongly reduced by TBBPA (≥ 1 µM), indicating TBBPA-induced inhibition of voltage-gated calcium channels. Our in vitro studies thus demonstrate that TBBPA exerts several adverse effects on functional neurotransmission endpoints with effect concentrations that are only two orders of magnitude below the highest cord serum concentrations. Although epidemiological proof for adverse TBBPA effects is lacking, our data justify the quest for flame retardants with reduced neurotoxic potential.

Activation and potentiation of human GABAA receptors by non-dioxin-like PCBs depends on chlorination pattern.

The neurotoxic potential of non-dioxin-like polychlorinated biphenyls (NDL-PCBs) is characterized by disruption of presynaptic processes, including calcium homeostasis and neurotransmitter transport. Recently, using a limited set of congeners, we demonstrated that PCB28 and PCB52 can potentiate postsynaptic GABA(A) receptors. In the present study, effects of 20 NDL-PCBs and 2 dioxin-like PCBs, selected based on their chemical variation and abundance in the environment, on human GABA(A) receptors were investigated. GABA(A) receptors were expressed in Xenopus oocytes, and NDL-PCB effects were determined using the two-electrode voltage-clamp technique. Results demonstrate that lower chlorinated PCB19, PCB28, PCB47, PCB51, PCB52, PCB95, and PCB100 act as a partial agonists (at low receptor occupancy), i.e., potentiating the receptor response during coapplication with GABA (at EC(20)). Importantly, PCB19, PCB47, PCB51, and PCB100 can also act as full agonist, i.e., activate the GABA(A) receptor in the absence of GABA. Potentiation and activation of the GABA(A) receptor is concentration dependent and limited to NDL-PCBs that have 3-5 chlorine atoms, 1-3 ortho-substitutions, an equal number (0-1) of meta-substitutions on both phenyl rings, and do not have an adjacent para- and meta-substitution on the same phenyl ring. Activation and potentiation of the GABA(A) receptor by PCB47, the most potent congener (lowest observed effect concentration of 10nM), is attenuated when coapplied with PCB19, PCB28, PCB153, or PCB180, indicative for competitive binding. Considering the importance of GABA-ergic signaling for brain development, motor coordination, learning, and memory, this mode of action can contribute to the previously observed NDL-PCB-induced neurobehavioral and neurodevelopmental effects and should be included in human risk assessment.

PCB-47, PBDE-47, and 6-OH-PBDE-47 differentially modulate human GABAA and alpha4beta2 nicotinic acetylcholine receptors.

Polychlorinated biphenyls (PCBs) and the structurally related polybrominated diphenyl ethers (PBDEs) are abundant persistent organic pollutants that exert several comparable neurotoxic effects. Importantly, hydroxylated metabolites of PCBs and PBDEs have an increased neurotoxic potency. Recently, we demonstrated that PCBs can act as (partial) agonist on GABA(A) neurotransmitter receptors, with PCB-47 being the most potent congener. It is, however, unknown whether PBDE-47 and its metabolite 6-OH-PBDE-47 exert similar effects and if these effects are limited to GABA(A) receptors only. We therefore investigated effects of PCB-47, PBDE-47, and 6-OH-PBDE-47 on the inhibitory GABA(A) and excitatory α(4)ß(2) nicotinic acetylcholine (nACh) receptor expressed in Xenopus oocytes using the two-electrode voltage-clamp technique. Since human exposure is generally not limited to individual compounds, experiments with binary mixtures were also performed. The results demonstrate that PCB-47 and 6-OH-PBDE-47 act as full and partial agonist on the GABA(A) receptor. However, both congeners act as antagonist on the nACh receptor. PBDE-47 does not affect either type of receptor. Binary mixtures of PCB-47 and 6-OH-PBDE-47 induced an additive activation as well as potentiation of GABA(A) receptors, whereas this mixture resulted in an additive inhibition of nACh receptors. Binary mixtures of PBDE-47 and 6-OH-PBDE-47 yielded similar effects as 6-OH-PBDE-47 alone. These findings demonstrate that GABA(A) and nACh receptors are affected differently by PCB-47 and 6-OH-PBDE-47, with inhibitory GABA(A)-mediated signaling being potentiated and excitatory α(4)ß(2) nACh-mediated signaling being inhibited. Considering these opposite actions and the additive interaction of the congeners, these effects are likely to be augmented in vivo.

Maternal and paternal risk factors for anorectal malformations: a Dutch case-control study.

BACKGROUND: Anorectal malformations (ARM) are major congenital malformations that usually require a multitude of surgical procedures at a very early age and have a large impact on the lives of patients and their parents. The causes of ARM are still largely unknown, but they are assumed to have a multifactorial etiology. A few studies focused on environmental risk factors, but evidence is still scarce. METHODS: In this Dutch case-control study (1996-2008), we investigated the role of maternal and paternal risk factors in the etiology of ARM. Parents of 85 ARM cases and 650 controls filled in a questionnaire. Controls were children treated with ear ventilation tubes. RESULTS: A higher occurrence of fever during the first trimester of pregnancy was found for case mothers compared to control mothers (odds ratio [OR], 5.1; 95% Confidence Interval [CI], 0.9, 28.1). Maternal occupational exposure to industrial cleaning agents and solvents increased the risk of ARM three times (OR, 2.9; 95% CI, 0.9, 9.3). Overweight (Body Mass Index [BMI] > or = 25 kg/m(2)) before pregnancy also seemed to be associated with ARM (OR, 1.8; 95% CI, 1.1, 2.8), as well as maternal multivitamin use during pregnancy (OR, 1.6; 95% CI, 1.0, 2.7), paternal smoking (OR, 1.8; 95% CI, 1.1, 2.9), and paternal occupational exposure to exhaust fumes (OR, 1.9; 95% CI, 1.0, 3.6). Reported ARM in at least one first- or second-degree family member greatly increased the risk of having a child with an ARM (OR, 40.3; 95% CI, 4.8, 342.8). CONCLUSIONS: This study revealed potential risk factors for ARM, including fever during pregnancy, maternal overweight, use of multivitamins, paternal smoking, and occupational exposures, but a familial component seems important as well.

Potentiation of the human GABA(A) receptor as a novel mode of action of lower-chlorinated non-dioxin-like PCBs.

PCBs are still ubiquitous pollutants despite the ban on their industrial and commercial use. To date, risk characterization and assessment of non-dioxin-like PCBs (NDL-PCBs), especially with respect to neurotoxicity, is hampered by a lack of data. Therefore, the effects of six common NDL congeners (PCB28, 52, 101, 138, 153 and 180) on human GABA(A) receptors, expressed in Xenopus oocytes, were investigated using the two-electrode voltage-clamp technique. When coapplied with GABA (at EC(20)), PCB28 and PCB52 concentration-dependently potentiate the GABA(A) receptor-mediated ion current. Though the LOEC for both PCB28 and PCB52 is 0.3 microM, PCB28 is more potent than PCB52 (maximum potentiation at 10 muM amounting to 98.3 +/- 12.5% and 25.5 +/- 1.4%, respectively). Importantly, coapplication of PCB28 (0.3 microM) and PCB52 (10 microM) resulted in an apparently additive potentiation of the GABA(A) response, whereas coapplication of PCB28 (0.3 microM) and PCB153 (10 microM) attenuated the PCB28-induced potentiation. The present results suggest that the potentiation of human GABA(A) receptor function is specific for lower-chlorinated NDL-PCBs and that higher molecular weight PCBs may attenuate this potentiation as a result of competitive binding to human GABA(A) receptors. Nonetheless, this novel mode of action could (partly) underlie the previously recognized NDL-PCB-induced neurobehavioral alterations.