Critical periods of vulnerability for the developing nervous system: evidence from humans and animal models

Vulnerable periods during the development of the nervous system are sensitive to environmental insults because they are dependent on the temporal and regional emergence of critical developmental processes (i.e., proliferation, migration, differentiation, synaptogenesis, myelination, and apoptosis). Evidence from numerous sources demonstrates that neural development extends from the embryonic period through adolescence. In general, the sequence of events is comparable among species, although the time scales are considerably different. Developmental exposure of animals or humans to numerous agents (e.g., X-ray irradiation, methylazoxymethanol, ethanol, lead, methyl mercury, or chlorpyrifos) demonstrates that interference with one or more of these developmental processes can lead to developmental neurotoxicity. Different behavioral domains (e.g., sensory, motor, and various cognitive functions) are subserved by different brain areas. Although there are important differences between the rodent and human brain, analogous structures can be identified. Moreover, the ontogeny of specific behaviors can be used to draw inferences regarding the maturation of specific brain structures or neural circuits in rodents and primates, including humans. Furthermore, various clinical disorders in humans (e.g., schizophrenia, dyslexia, epilepsy, and autism) may also be the result of interference with normal ontogeny of developmental processes in the nervous system. Of critical concern is the possibility that developmental exposure to neurotoxicants may result in an acceleration of age-related decline in function. This concern is compounded by the fact that developmental neurotoxicity that results in small effects can have a profound societal impact when amortized across the entire population and across the life span of humans.

Critical periods of vulnerability for the developing nervous system: evidence from humans and animal models

Vulnerable periods during the development of the nervous system are sensitive to environmental insults because they are dependent on the temporal and regional emergence of critical developmental processes (i.e., proliferation, migration, differentiation, synaptogenesis, myelination, and apoptosis). Evidence from numerous sources demonstrates that neural development extends from the embryonic period through adolescence. In general, the sequence of events is comparable among species, although the time scales are considerably different. Developmental exposure of animals or humans to numerous agents (e.g., X-ray irradiation, methylazoxymethanol, ethanol, lead, methyl mercury, or chlorpyrifos) demonstrates that interference with one or more of these developmental processes can lead to developmental neurotoxicity. Different behavioral domains (e.g., sensory, motor, and various cognitive functions) are subserved by different brain areas. Although there are important differences between the rodent and human brain, analogous structures can be identified. Moreover, the ontogeny of specific behaviors can be used to draw inferences regarding the maturation of specific brain structures or neural circuits in rodents and primates, including humans. Furthermore, various clinical disorders in humans (e.g., schizophrenia, dyslexia, epilepsy, and autism) may also be the result of interference with normal ontogeny of developmental processes in the nervous system. Of critical concern is the possibility that developmental exposure to neurotoxicants may result in an acceleration of age-related decline in function. This concern is compounded by the fact that developmental neurotoxicity that results in small effects can have a profound societal impact when amortized across the entire population and across the life span of humans.

Human health effects of trichloroethylene: key findings and scientific issues

BACKGROUND

In support of the Integrated Risk Information System (IRIS), the U.S. Environmental Protection Agency (EPA) completed a toxicological review of trichloroethylene (TCE) in September 2011, which was the result of an effort spanning > 20 years.

OBJECTIVES

We summarized the key findings and scientific issues regarding the human health effects of TCE in the U.S. EPA's toxicological review.

METHODS

In this assessment we synthesized and characterized thousands of epidemiologic, experimental animal, and mechanistic studies, and addressed several key scientific issues through modeling of TCE toxicokinetics, meta-analyses of epidemiologic studies, and analyses of mechanistic data.

DISCUSSION

Toxicokinetic modeling aided in characterizing the toxicological role of the complex metabolism and multiple metabolites of TCE. Meta-analyses of the epidemiologic data strongly supported the conclusions that TCE causes kidney cancer in humans and that TCE may also cause liver cancer and non-Hodgkin lymphoma. Mechanistic analyses support a key role for mutagenicity in TCE-induced kidney carcinogenicity. Recent evidence from studies in both humans and experimental animals point to the involvement of TCE exposure in autoimmune disease and hypersensitivity. Recent avian and in vitro mechanistic studies provided biological plausibility that TCE plays a role in developmental cardiac toxicity, the subject of substantial debate due to mixed results from epidemiologic and rodent studies.

CONCLUSIONS

TCE is carcinogenic to humans by all routes of exposure and poses a potential human health hazard for noncancer toxicity to the central nervous system, kidney, liver, immune system, male reproductive system, and the developing embryo/fetus.

Do Hydrofluorocarbons Destroy Stratospheric Ozone?

Hydrofluorocarbons, many of which contain a CF(3) group, are one of the major substitutes for the chlorofluorocarbons and halons that are being phased out because they contribute to stratospheric ozone depletion. It is critical to ensure that CF(3) groups do not cause significant ozone depletion. The rate coefficients for the key reactions that determine the efficiency of the CF(3) radical as a catalyst for ozone loss in the stratosphere have been measured and used in a model to calculate the possible depletion of ozone. From these results, it is concluded that the ozone depletion potentials related to the presence of the CF(3) group in hydrofluorocarbons are negligibly small.

Neuronal differentiation in PC12 cells is inhibited by chlorpyrifos and its metabolites: is acetylcholinesterase inhibition the site of action?

Developmental expression of AChE has been associated with neuronal differentiation (P. G. Layer and E. Willbold, Prog. Histochem. Cytochem. 29, 1-94, 1995). In this study we used pheochromocytoma (PC12) cells, a noncholinergic cell line, rich in acetylcholinesterase (AChE) activity, to examine the effects of cholinesterase-inhibiting pesticides on neural differentiation. The experimental paradigm was focused on whether alterations in cholinesterase (ChE) activity by a pesticide or its metabolites would affect neurite outgrowth, a morphological marker of neuronal differentiation. Results indicated that (1) in controls, both total ChE and AChE activities were significantly increased in NGF-primed PC12 cells compared to NGF-unprimed cells, while the basal expression of butyrylcholinesterase (BuChE) activity was much lower (1.3-7% of total ChE activity) in either the presence or the absence of NGF; (2) an increase in AChE activity was highly correlated (r(2) = 0.99) with the extension of neurite outgrowth, suggesting a link between the expression of AChE activity and the elaboration of neurite outgrowth; (3) NGF increased neurite outgrowth in a time- and concentration-dependent manner; and (4) either chlorpyrifos (CPF) or its metabolites (CPF oxon and TCP) inhibited NGF-induced neurite outgrowth (branches per cell, fragments per cell, total neurite outgrowth per cell) in PC12 cells. These data suggest that the expression of AChE activity is associated with the extension of neurite outgrowth. Both enzyme activity and neurite branching were disrupted by CPF oxon; however, CPF and its other metabolite TCP (1 microgram/ml) caused inhibition of neurite outgrowth in the absence of ChE inhibition, suggesting an alternative mechanism(s) may be involved in pesticide-induced inhibition of differentiation.

Differential patterns of nerve growth factor, brain-derived neurotrophic factor and neurotrophin-3 mRNA and protein levels in developing regions of rat brain

The present studies were undertaken to characterize the regional and temporal patterns of neurotrophin messenger RNA and protein levels for beta-nerve growth factor, brain-derived neurotrophic factor and neurotrophin-3 in the developing CNS. We have examined the levels of these neurotrophin messenger RNAs with ribonuclease protection assays and corresponding protein levels with enzyme-linked immunosorbent assays in the developing Long-Evans rat hippocampus, neocortex and cerebellum on postnatal days 1, 7, 14, 21, and 92. In addition, immunohistochemistry was used to localize the neurotrophins in these developing brain regions. Results indicated that in neocortex and hippocampus, messenger RNA for both nerve growth factor and brain-derived neurotrophic factor increased in an age-dependent manner, reaching a plateau by postnatal day 14. In the neocortex, nerve growth factor and brain-derived neurotrophic factor protein levels both peaked at postnatal day 14. In hippocampus, nerve growth factor protein peaked at postnatal day 7 while brain-derived neurotrophic factor peaked at postnatal day 14. In cerebellum, nerve growth factor messenger RNA levels were flat, while nerve growth factor protein peaked at postnatal day 7. Brain-derived neurotrophic factor messenger RNA increased in an age-dependent manner while the pattern for its protein levels was mixed. Neurotrophin-3 messeger RNA levels increased in an age-dependent manner in hippocampus, peaked at postnatal day14 in cerebellum, and no changes occurred in neocortex. Neurotrophin-3 protein was at its peak at postnatal day 1 and thereafter decreased at other postnatal days in all three brain regions. Results of neurotrophin immunohistochemistry often paralleled and complemented enzyme-linked immunosorbent assay data, demonstrating specific cell groups containing neurotrophin proteins in these regions. Within each region, patterns with regard to messenger RNA and respective protein levels for each neurotrophin were unique. No consistent relationship between patterns of neurotrophin messenger RNAs and their cognate proteins was observed between regions. The different regional patterns for neurotrophin messengerRNA and protein levels in each brain region indicate that messenger RNA studies of neurotrophin messenger RNA must be augmented by protein determination to fully characterize spatial and temporal neurotrophin distribution.

Qualitative and quantitative estimates of apoptosis from birth to senescence in the rat brain

Apoptosis is crucial for proper development of the CNS, wherein a significant percentage of all central neurons produced during early ontogeny die by apoptosis. To characterize the pattern of developmental programmed cell death, we assayed rat brainstem, neocortex, hippocampus, and cerebellum from birth through senescence. Quantitatively, using an ELISA for oligonucleosomal DNA fragments, we demonstrated that PND1 brainstem, neocortex, and hippocampus have the highest levels of fragmented DNA compared to older ages. Cerebellum displayed a large peak at PND10 and a smaller peak at PND21. Low levels were observed throughout adulthood and into senescence, which was corroborated qualitatively by agarose gel and TUNEL data. These data provide a temporal and regional baseline for further studies of the effects of perturbations of cell death during neural development. Quantitative and qualitative changes in these regional profiles of apoptosis due to environmental insults during early ontogeny may alter neuron number and function later in life.

The neurotoxicant trimethyltin induces apoptosis via caspase activation, p38 protein kinase, and oxidative stress in PC12 cells

Acute exposure to trimethyltin (TMT) causes neuronal degeneration in the hippocampus, amygdala, pyriform cortex, and neocortex [Am. J. Pathol. 97 (1979) 59]. Despite extensive efforts elucidating neuropathological changes and behavioral deficits following TMT exposure, only a limited amount of work has examined the molecular signaling mechanisms that lead to these changes. The present paper demonstrates that TMT impairs neurite outgrowth and cell viability in an in vitro model of neuronal development. The decrease in cell viability is paralleled by a decrease in cell body size, an increase in DNA fragmentation, activation of caspase-9, and cleavage of the caspase substrate poly-ADP ribose polymerase (PARP). These results suggest that TMT induces apoptosis. Pharmacological inhibition of caspase activity, p38 stress-responsive protein kinase activity, or oxidative stress prevented TMT-induced cell death. This work provides the first evidence for a TMT-initiated apoptotic pathway requiring oxidative stress, caspase activation, and p38 protein kinase activity.

The Next Generation of Risk Assessment Multi-Year Study-Highlights of Findings, Applications to Risk Assessment, and Future Directions

BACKGROUND

The Next Generation (NexGen) of Risk Assessment effort is a multi-year collaboration among several organizations evaluating new, potentially more efficient molecular, computational, and systems biology approaches to risk assessment. This article summarizes our findings, suggests applications to risk assessment, and identifies strategic research directions.

OBJECTIVE

Our specific objectives were to test whether advanced biological data and methods could better inform our understanding of public health risks posed by environmental exposures.

METHODS

New data and methods were applied and evaluated for use in hazard identification and dose-response assessment. Biomarkers of exposure and effect, and risk characterization were also examined. Consideration was given to various decision contexts with increasing regulatory and public health impacts. Data types included transcriptomics, genomics, and proteomics. Methods included molecular epidemiology and clinical studies, bioinformatic knowledge mining, pathway and network analyses, short-duration in vivo and in vitro bioassays, and quantitative structure activity relationship modeling.

DISCUSSION

NexGen has advanced our ability to apply new science by more rapidly identifying chemicals and exposures of potential concern, helping characterize mechanisms of action that influence conclusions about causality, exposure-response relationships, susceptibility and cumulative risk, and by elucidating new biomarkers of exposure and effects. Additionally, NexGen has fostered extensive discussion among risk scientists and managers and improved confidence in interpreting and applying new data streams.

CONCLUSIONS

While considerable uncertainties remain, thoughtful application of new knowledge to risk assessment appears reasonable for augmenting major scope assessments, forming the basis for or augmenting limited scope assessments, and for prioritization and screening of very data limited chemicals. Citation: Cote I, Andersen ME, Ankley GT, Barone S, Birnbaum LS, Boekelheide K, Bois FY, Burgoon LD, Chiu WA, Crawford-Brown D, Crofton KM, DeVito M, Devlin RB, Edwards SW, Guyton KZ, Hattis D, Judson RS, Knight D, Krewski D, Lambert J, Maull EA, Mendrick D, Paoli GM, Patel CJ, Perkins EJ, Poje G, Portier CJ, Rusyn I, Schulte PA, Simeonov A, Smith MT, Thayer KA, Thomas RS, Thomas R, Tice RR, Vandenberg JJ, Villeneuve DL, Wesselkamper S, Whelan M, Whittaker C, White R, Xia M, Yauk C, Zeise L, Zhao J, DeWoskin RS. 2016. The Next Generation of Risk Assessment multiyear study-highlights of findings, applications to risk assessment, and future directions. Environ Health Perspect 124:1671-1682; http://dx.doi.org/10.1289/EHP233.

Gestational exposure to chlorpyrifos: apparent protection of the fetus?

Previous studies have shown that, in general, young, postnatal animals are more sensitive than adults to the toxic effects of anticholinesterase (antiChE) pesticides. Paradoxically, often fetal brain cholinesterase (ChE) is less inhibited than maternal brain after gestational exposure to an antiChE, presumably due to placental and fetal detoxification of the antiChE. The present investigation was designed to study selected toxicokinetic and toxicodynamic factors surrounding the toxicity of chlorpyrifos (CPF; [O,O'-diethyl O-3,5,6-trichloro-2-pyridyl] phosphorothionate) in pregnant rats dosed repeatedly or singly during late gestation. Dams were dosed daily (po) with CPF in corn oil (0 or 7 mg/kg) on gestational days (GD) 14 to 18. Animals were euthanized at 2 to 120 h after the last dose and tissues were collected for enzyme analysis. Using this dosing regimen, we found that (1) the time of maximal ChE inhibition was the same (i.e., 5-10 h after dosing) for both maternal and fetal brain, (2) the degree of fetal brain ChE inhibition was 4.7 times less than maternal brain inhibition, and (3) the detoxification potential (i.e., carboxylesterase and chlorpyrifos-oxonase) of the fetal tissues was very low compared to the maternal tissues. A separate group of experiments showed that if pregnant dams received only one oral dose of 7 or 10 mg/kg CPF on GD18, the degree of ChE inhibition in the fetal brain was comparable to the maternal brain ChE inhibition. Taking into consideration the net increase (more than fourfold) in fetal brain ChE activity from GD14 to 18 in control animals, and the fact that maternal brain ChE was inhibited more than fetal brain ChE only in a repeated-dosing regimen, we conclude that the fetus is not genuinely protected from the toxic effects of a given dose of CPF. We propose that fetal brain ChE is simply able to recover more fully between each dose as compared to maternal brain ChE, giving the illusion that the fetal compartment is less affected than the maternal compartment.

Structure-activity comparison of organotin species: dibutyltin is a developmental neurotoxicant in vitro and in vivo

Human exposure to the organotins can occur due to their use as polyvinyl chloride heat stabilizers and as marine biocides. The consequences of this exposure for human health are unknown. We initially compared the toxicity of monomethyltin, dimethyltin, and dibutyltin to the known neurotoxicant trimethyltin using an in vitro model of neuronal development in PC12 cells. Dibutyltin, a compound traditionally thought to target the immune system, was the most potent neurotoxicant. Dibutyltin significantly inhibited neurite outgrowth and caused cell death at concentrations approximately 40-fold lower than the lowest toxic concentrations of trimethyltin. Dimethyltin was less potent than trimethyltin and monomethyltin was not toxic at any concentration examined. These results suggested the importance of prioritizing in vivo neurotoxicity testing with dibutyltin. To accomplish this, pregnant rats were dosed orally with low levels of dibutyltin from gestational day 6 through weaning. In response to developmental dibutyltin exposure, the incidence of apoptotic cell death, measured by DNA fragmentation and TUNEL staining, was increased in the neocortex and hippocampus of postnatal day 38 offspring. No effect was observed at other ages examined.

Workshop to identify critical windows of exposure for children's health: neurobehavioral work group summary

This paper summarizes the deliberations of a work group charged with addressing specific questions relevant to risk estimation in developmental neurotoxicology. We focused on eight questions. a) Does it make sense to think about discrete windows of vulnerability in the development of the nervous system? If it does, which time periods are of greatest importance? b) Are there cascades of developmental disorders in the nervous system? For example, are there critical points that determine the course of development that can lead to differences in vulnerabilities at later times? c) Can information on critical windows suggest the most susceptible subgroups of children (i.e., age groups, socioeconomic status, geographic areas, race, etc.)? d) What are the gaps in existing data for the nervous system or end points of exposure to it? e) What are the best ways to examine exposure-response relationships and estimate exposures in vulnerable life stages? f) What other exposures that affect development at certain ages may interact with exposures of concern? g) How well do laboratory animal data predict human response? h) How can all of this information be used to improve risk assessment and public health (risk management)? In addressing these questions, we provide a brief overview of brain development from conception through adolescence and emphasize vulnerability to toxic insult throughout this period. Methodological issues focus on major variables that influence exposure or its detection through disruptions of behavior, neuroanatomy, or neurochemical end points. Supportive evidence from studies of major neurotoxicants is provided.

The effects of perinatal tebuconazole exposure on adult neurological, immunological, and reproductive function in rats

Studies are under way to address concerns of potential persistent immunotoxic, reproductive, and neurotoxic effects of perinatal exposure to several pesticides. Tebuconazole, a triazole fungicide, was evaluated as part of this project. Sprague-Dawley dams were administered tebuconazole (0, 6, 20, or 60 mg/kg) by oral gavage daily from gestational day 14 to postnatal day (PND)7; the pups were then dosed daily at the same levels from PND7-42. Separate groups of rats were used for testing of immunological parameters, neurobehavioral testing using a screening battery of functional tests, and cognitive evaluations. Other groups of rats were evaluated for reproductive development and function, while yet others were sacrificed at the end of the dosing period for histological analyses of major organs systems, including neuropathological assessments. Pup viability and body weight were decreased in the highest dose group. There were no differences in the fertility indices in the exposed rats mated as adults. In the sheep RBC-immunized high-dose rats, spleen weights and cellularity were increased, and the ratio of cell types was altered compared to controls. There were, however, no biologically significant changes in the immune function of these rats. At necropsy on PND46 or 152, kidney, liver, and spleen weights were altered by tebuconazole treatment, but a dose-response relationship was not clear for most organs; only decreased kidney and increased liver weights were consistent in both sexes. Histological analyses were generally unremarkable outside of the brain. One month after the end of dosing, acquisition of learning the platform location in a water tank (i.e., Morris water maze) was impaired in the high-dose group; there were no differences in neuromuscular ability, motor activity, or swim speed to account for this finding. Furthermore, there was no effect on recall of the position during a free-swim trial. Neuropathological evaluations revealed pyknotic cells across hippocampal cell fields in animals of all tebuconazole treatment groups, with the highest incidence in the 20 and 60 mg/kg/day dose groups, coincident with cell loss within pyramidal cell layer of CA3-4 cell fields of the hippocampus and layer V of the neocortex. Thus, perinatal exposure to tebuconazole produced neurobehavioral deficits and neuropathology in rats, but did not alter immunological or reproductive function.

Progressive changes in striatal dopaminergic markers, nigral volume, and rotational behavior following iron infusion into the rat substantia nigra

Excess iron (Fe) within the substantia nigra zona compacta (SNc) has been implicated in the pathogenesis of Parkinson's disease (PD). We recently reported that intranigral Fe infusion into the rat substantia nigra (SN) induces dose-dependent SN neurodegeneration and associated reductions in striatal dopaminergic (DA) markers. The objective of the present study was to determine whether infused Fe is capable of inducing persistent/progressive neurodegenerative changes relevant to PD. Following unilateral infusions of vehicle, 1.25 or 2.10 nmol Fe into the rat SN, SNc neuronal loss, SN volume, striatal neurochemical markers, and apomorphine-induced rotational behavior were assessed at 2, 4, and 6 months. Semiquantitative analysis of thionine-stained SNc neurons demonstrated an initial modest neuronal loss which remained stable through 6 months postinfusion. Fe-induced SN atrophy was dose-dependent and progressive through 6 months. Striatal DA and homovanillic acid levels were progressively decreased at least through 4 months following 1.25 nmol Fe infusion; both doses of Fe induced significant reductions of both DA markers at 4 months with no recovery evident through 6 months. Apomorphine-induced rotational behavior progressively increased for both Fe infusion groups through the 6 months of testing. These data indicate that a single exposure of the SN to a modest amount of Fe can induce persistent/progressive changes occurring through a number of months postinfusion and further establishes intranigral Fe infusion as an animal model for PD.

Methylmercury decreases NGF-induced TrkA autophosphorylation and neurite outgrowth in PC12 cells

Neurotrophin signaling through Trk receptors is important for differentiation and survival in the developing nervous system. The present study examined the effects of CH(3)Hg on (125)I-nerve growth factor (NGF) binding to the TrkA receptor, NGF-induced activation of the TrkA receptor, and neurite outgrowth in an in vitro model of differentiation using PC12 cells. Whole-cell binding assays using (125)I-NGF revealed a single binding site with a K(d) of approximately 1 nM. Methylmercury (CH(3)Hg) at 30 nM (EC(50) for neurite outgrowth inhibition) did not affect NGF binding to TrkA. TrkA autophosphorylation was measured by immunoblotting with a phospho-specific antibody. TrkA autophosphorylation peaked between 2.5 and 5 min of exposure and then decreased but was still detectable at 60 min. Concurrent exposure to CH(3)Hg and NGF for 2.5 min resulted in a concentration-dependent decrease in TrkA autophosphorylation, which was significant at 100 nM CH(3)Hg. To determine whether the observed inhibition of TrkA was sufficient to alter cell differentiation, NGF-stimulated neurite outgrowth was examined in PC12 cells after exposure to 30 nM CH(3)Hg, a concentration that inhibited TrkA autophosphorylation by approximately 50%. For comparison, a separate group of PC12 cells were exposed to a concentration of the selective Trk inhibitor K252a (30 nM), which had been shown to produce significant inhibition of TrkA autophosphorylation. Twenty-four hour exposure to either CH(3)Hg or K252a reduced neurite outgrowth to a similar degree. Our results suggest that CH(3)Hg may inhibit differentiation of PC12 cells by interfering with NGF-stimulated TrkA autophosphorylation.

Repeated exposure of adult rats to Aroclor 1254 causes brain region-specific changes in intracellular Ca2+ buffering and protein kinase C activity in the absence of changes in tyrosine hydroxylase

Polychlorinated biphenyls (PCBs) are ubiquitous environmental contaminants, some of which may be neurotoxic. In vitro studies from this laboratory indicated that noncoplanar PCBs perturbed intracellular signal transduction mechanisms including Ca2+ homeostasis, receptor-mediated inositol phosphate production, and translocation of protein kinase C (PKC). In the present study, we examined the effects of PCBs in vivo by dosing adult male Long-Evans rats orally with Aroclor 1254 (0, 10, or 30 mg/kg/day; 5 days/week for 4 weeks) in corn oil. At 24 h after the last dose, rats were tested for motor activity in a photocell device for 30 min. Immediately, the rats were euthanized, blood was collected for thyroid hormone analysis, and brains were removed, dissected into regions (cerebellum, frontal cortex, and striatum), and subcellular fractions were obtained for neurochemical analysis. Following Aroclor 1254 treatment, body weight gain in the high-dose group was significantly lower than the control and low-dose groups. Horizontal motor activity was significantly lower in rats dosed with 30 mg/kg Aroclor 1254. Ca2+ buffering by microsomes was significantly lower in all three brain regions from the 30 mg/kg group. In the same dose group, mitochondrial Ca2+ buffering was affected in cerebellum but not in cortex or striatum. Similarly, total cerebellar PKC activity was decreased significantly while membrane-bound PKC activity was significantly elevated at 10 and 30 mg/kg. PKC activity was not altered either in cortex or the striatum. Neurotransmitter levels in striatum or cortex were slightly altered in PCB-exposed rats compared to controls. Furthermore, repeated oral administration of Aroclor 1254 to rats did not significantly alter forebrain tyrosine hydroxylase immunoreactivity or enzymatic activity. Circulating T4 (total and free) concentrations were severely depressed at both doses in Aroclor 1254-exposed rats compared to control rats, suggesting a severe hypothyroid state. These results indicate that (1) in vivo exposure to a PCB mixture can produce changes in second messenger systems that are similar to those observed after in vitro exposure of neuronal cell cultures; (2) second messenger systems seem to be more sensitive than alterations in neurotransmitter levels or tyrosine hydroxylase involved in dopamine synthesis during repeated exposure to PCBs; and (3) the observed motor activity changes were independent of changes in striatal dopamine levels.

Effects of methylmercury and mercuric chloride on differentiation and cell viability in PC12 cells

The effects of methylmercury (CH3Hg) or mercuric chloride (HgCl(2)) on neurite outgrowth and cell viability were quantified using undifferentiated (unprimed) and differentiated (primed) pheochromocytoma (PC12) cells. In unprimed cells, following 24-h exposure, CH3Hg significantly decreased NGF-stimulated neurite outgrowth at concentrations of 0.3-3 microM. However, HgCl(2) significantly increased both neurite outgrowth and the number of branch points, a component of neurite outgrowth. In primed PC12 cells, following 24-h exposure, both CH3Hg and HgCl(2) inhibited NGF-stimulated neurite outgrowth with an EC(50) of approximately 0.03 microM; however, there was a difference between CH3Hg and HgCl(2) effects on the subcomponents of total neurite outgrowth. CH3Hg significantly decreased both the number of branch points (0.3 microM) and fragment length (0.01 microM), while HgCl(2) only decreased fragment length (0.03 microM). Cell viability was assessed in the same cultures by trypan-blue exclusion. In unprimed cells, the EC(50) for cytotoxicity of CH3Hg in the presence and absence of NGF was 0.21 +/- 0.04 and 0.87 +/- 0.12 microM, respectively, and for HgCl(2) in the presence and absence of NGF was 8.18 +/- 1.52 and 5.02 +/- 0.74 microM, respectively. In primed cells, the EC(50) for cytotoxicity of CH3Hg in the presence or absence of NGF was 1.17 +/- 0.38 and 0.73 +/- 0.14 microM, respectively, and for HgCl(2) in the presence or absence of NGF was 3.96 +/- 0.82 and 3.81 +/- 0.91 microM, respectively. In the primed PC12 model, cytotoxicity occurred at concentrations that were at least 30-fold higher than the EC(50) for neurite outgrowth, suggesting that the mercurial compounds can act selectively on the process of differentiation.

Computational design and engineering of polymeric orthodontic aligners

Transparent and removable aligners represent an effective solution to correct various orthodontic malocclusions through minimally invasive procedures. An aligner-based treatment requires patients to sequentially wear dentition-mating shells obtained by thermoforming polymeric disks on reference dental models. An aligner is shaped introducing a geometrical mismatch with respect to the actual tooth positions to induce a loading system, which moves the target teeth toward the correct positions. The common practice is based on selecting the aligner features (material, thickness, and auxiliary elements) by only considering clinician's subjective assessments. In this article, a computational design and engineering methodology has been developed to reconstruct anatomical tissues, to model parametric aligner shapes, to simulate orthodontic movements, and to enhance the aligner design. The proposed approach integrates computer-aided technologies, from tomographic imaging to optical scanning, from parametric modeling to finite element analyses, within a 3-dimensional digital framework. The anatomical modeling provides anatomies, including teeth (roots and crowns), jaw bones, and periodontal ligaments, which are the references for the down streaming parametric aligner shaping. The biomechanical interactions between anatomical models and aligner geometries are virtually reproduced using a finite element analysis software. The methodology allows numerical simulations of patient-specific conditions and the comparative analyses of different aligner configurations. In this article, the digital framework has been used to study the influence of various auxiliary elements on the loading system delivered to a maxillary and a mandibular central incisor during an orthodontic tipping movement. Numerical simulations have shown a high dependency of the orthodontic tooth movement on the auxiliary element configuration, which should then be accurately selected to maximize the aligner's effectiveness.

Lead exposure in pheochromocytoma (PC12) cells alters neural differentiation and Sp1 DNA-binding

Previous studies have revealed that lead modulates the DNA-binding profile of the transcription factor Sp1 both in vivo and in vitro (Dev Brain Res 1998;107:291). Sp1 is a zinc finger protein, that is selectively up-regulated in certain developing cell types and plays a regulatory role during development and differentiation (Mol Cell Biol 1991;11:2189). In NGF-stimulated PC12 cells, Sp1 DNA-binding activity was induced within 48 h of exposure of NGF naïve cells. Exposure of undifferentiated PC12 cells to lead alone (0.1 microM) also produced a similar increase in Sp1 DNA-binding. Since lead altered the DNA-binding profile of Sp1 in newly differentiating cells, neurite outgrowth was assessed as a morphological marker of differentiation to determine whether or not the effects of lead on differentiation were restricted to the initiation phase (unprimed) or the elaboration phase of this process (NGF-primed). NGF-primed and unprimed PC12 cells were prepared for bioassay following exposure to various concentrations of NGF and/or lead. Neurite outgrowth was measured at 48 and 72 h during early stages of NGF-induced differentiation and at 14 h in NGF primed/replated cells. In the absence of NGF, exposure to lead alone (0.025, 0.05, 0.1 microM) promoted measurable neurite outgrowth in unprimed PC12 cells at 48 and 72 h. A similar phenomenon was also observed in primed/replated PC12 cells at 14 h. However, this effect was two to five times greater than unprimed control cells. In the presence of NGF, a similar trend was apparent at lower concentrations, although the magnitude and temporal nature was different from lead alone. In most cases, the administration of higher lead concentrations (1 and 10 microM), in both the absence or presence of NGF, was less effective than the lower concentrations in potentiating neurite outgrowth. These results suggest that lead alone at low doses may initiate premature stimulation of morphological differentiation that may be related to lead-induced alterations in Sp1 binding to DNA.

Gestational exposure to methylmercury alters the developmental pattern of trk-like immunoreactivity in the rat brain and results in cortical dysmorphology

Nerve growth factor signal transduction mediated through the trk receptor has been implicated in neuronal growth, differentiation, and survival. In this study, we examined the effects of gestational exposure to the developmental neurotoxicant methylmercury (CH3Hg) on the ontogeny of trk-immunoreactivity (IR). Long-Evans dams were dosed on gestational days 6-15 (p.o.) with 0, 1, or 2 mg/kg CH3Hg dissolved in saline. Pups were sacrificed and perfused with buffered paraformaldehyde on postnatal days (PND) 1, 4, 10, 21 and 85. The brains were sectioned sagitally, Nissl-stained or stained immunohistochemically for trk receptors or glial fibrillary acidic protein (GFAP), and examined throughout the medial to lateral extent of the brain. The greatest density of IR in neural cell bodies was seen in the olfactory bulb, hippocampus, cerebral, and cerebellar cortex, striatum, septum, nucleus basalis, inferior colliculus, pons, and brain stem nuclei. trk IR was not limited to nerve cell bodies, with prominent axonal and dendritic staining in the brainstem, neocortex, hippocampus, cerebellum, and olfactory tract. The regional pattern of trk IR varied in an age-dependent manner. In controls, trk-like IR appeared to peak in most regions between PND4-10 and decreased dramatically after PND21. This age-related difference in trk IR was supported by western blot analysis of PND10 and adult neocortex. This reduced and more adult-like pattern of trk IR was apparent on PND21 with some persistent trk-like IR in the olfactory bulb, hippocampus, neocortex, cerebellum and basal forebrain. In contrast to the normal regional patterns of trk IR, CH3Hg produced a dose-related decrease in trk-like IR in the absence of overt maternal toxicity or neonatal toxicity. CH3Hg-induced decreases in trk-like IR were especially apparent during the early postnatal period when trk IR was the greatest. The effects of CH3Hg exposure were restricted regionally, with the largest decrease in trk-like IR apparent in cortical regions, basal forebrain nuclei, and brain stem nuclei. Subsequent to the effects of CH3Hg on cortical trk-like IR were alterations in the development of cortical laminae on PND10 and 21 of neocortex. These alterations were characterized by quantifiable decreases in cell density, cell size and the widths of the layers of posterior neocortex. Not all of the CH3Hg-induced effects were characterized by decreased trk-like IR. Robust increases in trk IR in glial cells in the corpus callosum and brain stem were observed coincident with increased GFAP IR in cells of similar morphology. The present results localize the cellular and regional ontogeny of trk and suggest that developmental exposure to CH3Hg alters the normal ontogeny of this trophic factor receptor which may be associated with the developmental neurotoxicity of this chemical.

Human health effects of tetrachloroethylene: key findings and scientific issues

BACKGROUND

The U.S. Environmental Protection Agency (EPA) completed a toxicological review of tetrachloroethylene (perchloroethylene, PCE) in February 2012 in support of the Integrated Risk Information System (IRIS).

OBJECTIVES

We reviewed key findings and scientific issues regarding the human health effects of PCE described in the U.S. EPA's Toxicological Review of Tetrachloroethylene (Perchloroethylene).

METHODS

The updated assessment of PCE synthesized and characterized a substantial database of epidemiological, experimental animal, and mechanistic studies. Key scientific issues were addressed through modeling of PCE toxicokinetics, synthesis of evidence from neurological studies, and analyses of toxicokinetic, mechanistic, and other factors (tumor latency, severity, and background rate) in interpreting experimental animal cancer findings. Considerations in evaluating epidemiological studies included the quality (e.g., specificity) of the exposure assessment methods and other essential design features, and the potential for alternative explanations for observed associations (e.g., bias or confounding).

DISCUSSION

Toxicokinetic modeling aided in characterizing the complex metabolism and multiple metabolites that contribute to PCE toxicity. The exposure assessment approach-a key evaluation factor for epidemiological studies of bladder cancer, non-Hodgkin lymphoma, and multiple myeloma-provided suggestive evidence of carcinogenicity. Bioassay data provided conclusive evidence of carcinogenicity in experimental animals. Neurotoxicity was identified as a sensitive noncancer health effect, occurring at low exposures: a conclusion supported by multiple studies. Evidence was integrated from human, experimental animal, and mechanistic data sets in assessing adverse health effects of PCE.

CONCLUSIONS

PCE is likely to be carcinogenic to humans. Neurotoxicity is a sensitive adverse health effect of PCE.

Accumulation of methylmercury or polychlorinated biphenyls in in vitro models of rat neuronal tissue

In vivo exposure levels for neurotoxicants are often reported in parts per million (ppm) concentration in tissue, whereas exposure levels in experiments utilizing in vitro models are most commonly reported in micromolar (muM) concentration in the exposure solution. The present experiments sought to determine whether or not in vitro solution concentration was an appropriate dose-metric for comparison to in vivo tissue levels for lipophilic compounds. To do so, the accumulation of the polychlorinated biphenyl (PCB) mixture Aroclor 1254 (A1254) or methylmercury (MeHg) was examined in three commonly utilized in vitro neuronal tissue models: nerve growth factor differentiated pheochromocytoma (PC12) cells, primary cultures of rat neocortical cells, and adult rat hippocampal slices. Tissues were exposed to A1254 (0.65 ppm) or to MeHg (0.0033-0.33 ppm) in serum-free media for 1 or 24 h. Total PCB or mercury accumulation was measured by dual column gas chromatography with electron capture detection or by cold vapor atomic absorption, respectively. PC12 cells accumulated 66.7 and 103.8 ppm PCBs after 1 and 24 h exposure to A1254. Neocortical neurons also accumulated significant concentrations of PCBs, but less so than PC12 cells. After 1 h exposure to 0.65 ppm A1254, slices contained 3.46 and 0.81 ppm PCBs when exposed in a static and perfused system, respectively. After 1 h exposure to 0.0033, 0.033, and 0.33 ppm MeHg, PC12 cells contained 0.3, 2.2, and 17.7 ppm mercury, respectively; after 24 h, PC12 cells contained 0.4, 2.8, and 21.9 ppm. Hippocampal slices accumulated 1.7 and 4.8 ppm mercury after 1 and 3 h exposure to 0.33 ppm MeHg. For comparison, mercury accumulation in rat fetal and pup brain tissue after maternal exposure [0, 0.1, 1.0, or 2.0 mg/kg/day MeHg from gestational day (GD) 6-15] ranged from 0.05 to 7.89 ppm in 0.1 mg/kg dose animals on postnatal day 10 and 2.0 mg/kg dose animals on GD16, respectively. These results demonstrate that accumulation of PCBs and MeHg in vitro is tissue-, time-, and concentration-dependent and indicates that tissue levels rather than exposure concentrations are a more appropriate metric for comparison of in vitro to in vivo effects.

The ototoxicity of 3,3'-iminodipropionitrile: functional and morphological evidence of cochlear damage

Previous reports have suggested that IDPN may be ototoxic (Wolff et al., 1977; Crofton and Knight, 1991). The purpose of this research was to investigate the ototoxicity of IDPN using behavioral, physiological and morphological approaches. Three groups of adult rats were exposed to IDPN (0-400 mg/kg/day) for three consecutive days. In the first group, at 9-10 weeks post-exposure, thresholds for hearing of 5.3- and 38-kHz filtered clicks were measured electrophysiologically and brainstem auditory evoked responses (BAERs) were also recorded to a suprathreshold broadband click stimulus. A second set of animals was tested at 9 weeks for behavioral hearing thresholds (0.5- to 40-kHz tones) and at 11-12 weeks post-exposure for BAER thresholds (5- to 80-kHz filtered clicks). A third group of animals was exposed (as above), and killed at 12-14 weeks post-exposure for histological assessment. Kanamycin sulfate was used as a positive control for high-frequency selective hearing loss. Surface preparations of the organ of Corti were prepared in order to assess hair cells, and mid-modiolar sections of the cochlea were used to examine Rosenthal's canal and the stria vascularis. Functional data demonstrate a broad-spectrum hearing loss ranging from 0.5 kHz (30 dB deficit) to 80 kHz (40 dB deficit), as compared to a hearing deficit in kanamycin-exposed animals that was only apparent at frequencies greater than 5 kHz. Surface preparations revealed IDPN-induced hair cell loss in all turns of the organ of Corti, with a basal-to-apical gradient (more damage in the basal turns) at the lower dosages. At higher dosages there was complete destruction of the organ of Corti. There was also a dosage-related loss of spiral ganglion cells in all turns of the cochlea, again with a basal-to-apical gradient at the lower dosages. These data demonstrate that IDPN exposure in the rat results in extensive hearing loss and loss of neural structures in the cochlea.

Developmental neurotoxicity: evaluation of testing procedures with methylazoxymethanol and methylmercury

Testing procedures for identification of potential developmental neurotoxicants were evaluated using two prototypical developmental neurotoxicants, methylazoxymethanol (MAM) and methylmercury (MeHg). Evaluation of offspring of Long-Evans rats incorporated assessments of developmental toxicity, neurochemistry, histology, and behavior, with most testing being completed near weaning. A number of endpoints in the testing strategy were sensitive to the effects of prenatal exposure to MAM [30 mg/kg on Gestation Day (GD) 15]: (1) MAM caused reduced neonatal body weights but did not effect viability or postnatal survivorship; (2) measurement of total and regional brain weight and histological analysis showed that a number of regions, the cortex and hippocampus in particular, were affected by MAM exposure; (3) an assay for glial fibrillary acidic protein (GFAP) showed that the concentration of this protein was significantly increased in the cortex and hippocampus of treated offspring; (4) a T-maze delayed-alternation procedure indicated that MAM-treated pups were slower in the acquisition phase of the task relative to control pups; (5) motor activity testing revealed hyperactivity in treated offspring that persisted into adulthood; and (6) acoustic startle procedures revealed reduced startle amplitudes in preweanlings. Few endpoints were significantly affected by prenatal MeHg exposure (1, 2, or 4 mg/kg on GD 6-15). High fetal and neonatal mortality and lower neonatal body weights were detected at the highest dose of MeHg. Although minimal effects of MeHg may reflect a relative insensitivity of the test species and/or the test methods, the combined results from both chemicals suggest that some procedures not currently required in the developmental neurotoxicity guideline may be useful in hazard identification, and further evaluation with other chemicals, species, strains, and/or exposure paradigms may be warranted.