Glyphosate and neurological outcomes: A systematic literature review of animal studies.

Studies of nervous system effects of glyphosate, a widely used herbicide, have not been critically examined. The aim of this paper was to systematically review glyphosate-induced neurotoxicity literature to determine its usefulness in regulatory decision-making. The review was restricted to mammalian studies of behavior, neuropathology, and neuropharmacology; in vitro and other biochemical studies were considered supplementary information. Glyphosate formulation studies were also considered, despite uncertainties regarding toxicities of the formulated products; no studies used a formulation vehicle as the control. Inclusion criteria were developed a priori to ensure consistent evaluation of studies, and in vivo investigations were also ranked using ToxRTool software to determine reliability. There were 27 in vivo studies (open literature and available regulatory reports), but 11 studies were considered unreliable (mostly due to critical methodological deficiencies). There were only seven acceptable investigations on glyphosate alone. Studies differed in terms of dosing scenarios, experimental designs, test species, and commercial product. Limitations included using only one dose and/or one test time, small sample sizes, limited data presentation, and/or overtly toxic doses. While motor activity was the most consistently affected endpoint (10 of 12 studies), there were considerable differences in outcomes. In six investigations, there were no marked neuropathological changes in the central or peripheral nervous system. Other neurological effects were less consistent, and some outcomes were less convincing due to influences including high variability and small effect sizes. Taken together, these studies do not demonstrate a consistent impact of glyphosate on the structure or function of the mammalian nervous system.

Genotoxicity as a toxicologically relevant endpoint to inform risk assessment: A case study with ethylene oxide.

The purpose of the present investigation is to analyze the in vivo genotoxicity dose-response data of ethylene oxide (EO) and the applicability of the derived point-of-departure (PoD) values when estimating permitted daily exposure (PDE) values. A total of 40 data sets were identified from the literature, and benchmark dose analyses were conducted using PROAST software to identify a PoD value. Studies employing the inhalation route of exposure and assessing gene or chromosomal mutations and chromosomal damage in various tissues were considered the most relevant for assessing risk from EO, since these effects are likely to contribute to adverse health consequences in exposed individuals. The PoD estimates were screened for precision and the values were divided by data-derived adjustment factors. For gene mutations, the lowest PDE was 285 parts per trillion (ppt) based on the induction of lacI mutations in the testes of mice following 48 weeks of exposure to EO. The corresponding lowest PDE value for chromosomal mutations was 1,175 ppt for heritable translocations in mice following 8.5 weeks of EO exposure. The lowest PDE for chromosomal aberrations was 238 ppt in the mouse peripheral blood lymphocytes following 48 weeks of inhalation exposure. The diverse dose-response data for EO-induced genotoxicity enabled the derivation of PoDs for various endpoints, tissues, and species and identified 238 ppt as the lowest PDE in this retrospective analysis.

This is your teen brain on drugs: In search of biological factors unique to dependence toxicity in adolescence.

Response variability across the lifespan is an important consideration in toxicology and risk assessment, and the toxic effects of drugs and chemicals during adolescence need more research. This paper summarizes a workshop presented in March 2019, at the Society of Toxicology Annual Meeting in Baltimore, Maryland, that brought together experts in research on drug dependence and toxicity related to nicotine, cannabis, cocaine, and other illicit drugs during adolescence. The goal of the workshop was to address the following issues: (1) Do the effects of adolescent exposure differ from the same exposure in adults? (2) Are there unique biological markers of adolescent brain development? If so, what are they and how reliable are they? (3) Since multiple factors influence substance use disorder, can we disentangle risk factors for abuse and/or toxicity? What are the underlying biological susceptibilities that lead to dependence and neurotoxicity? What are the social, psychosocial and environmental factors that contribute to abuse susceptibilities? This paper reviews drug policy and national trends in adolescent substance use; the public health consequences of e-cigarettes; rat models of adolescent-onset nicotine self-administration and persisting effects of gestational nicotine; sex-dependent effects of delta-9-tetrahydrocannabinol on adolescent brain-behavior relationships; and translational approaches for identifying adolescent risk factors for transition to drug dependence. There is strong evidence that drug exposure prior to adulthood has longer lasting effects on behavior and the underlying neural circuitry. These effects, which are sex-dependent and influenced by stress, may be candidates as predictors of adolescent vulnerability. A major challenge to determining if adolescents have a unique susceptibility to dependence is whether and to what extent the human data allow distinction between the increased risk due to biological immaturity, an underlying biological susceptibility to dependence, or psychosocial and environmental factors for substance dependence. Factors important to consider for development of animal models include the timing and pattern of exposure as it relates to adolescence; age of assessment, and direct comparison with similar effects following exposures to adults to demonstrate that these effects are unique to adolescence. Here we provide a roadmap for further research into what makes adolescent brain development unique.

Reevaluation of Historical Exposures to Ethylene Oxide Among U.S. Sterilization Workers in the National Institute of Occupational Safety and Health (NIOSH) Study Cohort.

The 2016 U.S. Environmental Protection Agency (EPA) Integrated Risk Information System (IRIS) assessment for ethylene oxide (EO) estimated a 10-6 increased inhalation cancer risk of 0.1 parts per trillion, based on National Institute of Occupational Safety and Health (NIOSH) epidemiology studies of sterilization facility workers exposed to EO between 1938 and 1986. The worker exposure estimates were based on a NIOSH statistical regression (NSR) model "validated" with EO levels measured after 1978. Between 1938 and 1978, when EO data was unavailable, the NSR model predicts exposures lowest in 1938 increasing to peak levels in 1978. That increasing EO concentration trend arose, in part, because engineering/industrial-hygiene (E/IH) factors associated with evolving EO-sterilization equipment and operations before 1978 were not properly considered in the NSR model. To test the NSR model trend prediction, a new E/IH-based model was developed using historical data on EO kill concentrations, EO residue levels in sterilized materials, post-wash EO concentrations in a sterilization chamber, and information on facility characteristics and sterilizer operator practices from operators familiar with pre-1978 industry conditions. The E/IH 90th percentile of 8 h time-weighted average EO exposures (C90) for highly exposed sterilizer operators was calibrated to match 1978 C90 values from the NSR model. E/IH model C90 exposures were estimated to decrease over time from levels 16 and were four-fold greater than NSR-estimated exposures for workers during 1938-1954 and 1955-1964. This E/IH modeled trend is opposite to that of NSR model predictions of exposures before 1978, suggesting that EPA's exclusive reliance on the NIOSH cohort to estimate EO cancer risk should be re-examined.

Practical considerations for developmental thyroid toxicity assessments: What's working, what's not, and how can we do better?

Thyroid hormones (THs; T3 and T4) play a role in development of cardiovascular, reproductive, immune and nervous systems. Thus, interpretation of TH changes from rodent studies (during pregnancy, in fetuses, neonates, and adults) is critical in hazard characterization and risk assessment. A roundtable session at the 2017 Society of Toxicology (SOT) meeting brought together academic, industry and government scientists to share knowledge and different perspectives on technical and data interpretation issues. Data from a limited group of laboratories were compiled for technical discussions on TH measurements, including good practices for reliable serum TH data. Inter-laboratory historical control data, derived from immunoassays or mass spectrometry methods, revealed: 1) assay sensitivities vary within and across methodologies; 2) TH variability is similar across animal ages; 3) laboratories generally achieve sufficiently sensitive TH quantitation levels, although issues remain for lower levels of serum TH and TSH in fetuses and postnatal day 4 pups; thus, assay sensitivity is critical at these life stages. Best practices require detailed validation of rat serum TH measurements across ages to establish assay sensitivity and precision, and identify potential matrix effects. Finally, issues related to data interpretation for biological understanding and risk assessment were discussed, but their resolution remains elusive.

Recommendations for harmonization of data collection and analysis of developmental neurotoxicity endpoints in regulatory guideline studies: Proceedings of workshops presented at Society of Toxicology and joint Teratology Society and Neurobehavioral Teratology Society meetings.

The potential for developmental neurotoxicity (DNT) of environmental chemicals may be evaluated using specific test guidelines from the US Environmental Protection Agency or the Organisation for Economic Cooperation and Development (OECD). These guidelines generate neurobehavioral, neuropathological, and morphometric data that are evaluated by regulatory agencies globally. Data from these DNT guideline studies, or the more recent OECD extended one-generation reproductive toxicity guideline, play a pivotal role in children's health risk assessment in different world areas. Data from the same study may be interpreted differently by regulatory authorities in different countries resulting in inconsistent evaluations that may lead to inconsistencies in risk assessment decisions internationally, resulting in regional differences in public health protection or in commercial trade barriers. These issues of data interpretation and reporting are also relevant to juvenile and pre-postnatal studies conducted more routinely for pharmaceuticals and veterinary medicines. There is a need for development of recommendations geared toward the operational needs of the regulatory scientific reviewers who apply these studies in risk assessments, as well as the scientists who generate DNT data sets. The workshops summarized here draw upon the experience of the authors representing government, industry, contract research organizations, and academia to discuss the scientific issues that have emerged from diverse regulatory evaluations. Although various regulatory bodies have different risk management decisions and labeling requirements that are difficult to harmonize, the workshops provided an opportunity to work toward more harmonized scientific approaches for evaluating DNT data within the context of different regulatory frameworks. Five speakers and their coauthors with neurotoxicology, neuropathology, and regulatory toxicology expertise discussed issues of variability, data reporting and analysis, and expectations in DNT data that are encountered by regulatory authorities. In addition, principles for harmonized evaluation of data were suggested using guideline DNT data as case studies.

A critical review of neonicotinoid insecticides for developmental neurotoxicity.

A comprehensive review of published and previously unpublished studies was performed to evaluate the neonicotinoid insecticides for evidence of developmental neurotoxicity (DNT). These insecticides have favorable safety profiles, due to their preferential affinity for nicotinic receptor (nAChR) subtypes in insects, poor penetration of the mammalian blood-brain barrier, and low application rates. Nevertheless, examination of this issue is warranted, due to their insecticidal mode of action and potential exposure with agricultural and residential uses. This review identified in vitro, in vivo, and epidemiology studies in the literature and studies performed in rats in accordance with GLP standards and EPA guidelines with imidacloprid, acetamiprid, thiacloprid, clothianidin, thiamethoxam, and dinotefuran, which are all the neonicotinoids currently registered in major markets. For the guideline-based studies, treatment was administered via the diet or gavage to primiparous female rats at three dose levels, plus a vehicle control (≥20/dose level), from gestation day 0 or 6 to lactation day 21. F1 males and females were evaluated using measures of motor activity, acoustic startle response, cognition, brain morphometry, and neuropathology. The principal effects in F1 animals were associated with decreased body weight (delayed sexual maturation, decreased brain weight, and morphometric measurements) and acute toxicity (decreased activity during exposure) at high doses, without neuropathology or impaired cognition. No common effects were identified among the neonicotinoids that were consistent with DNT or the neurodevelopmental effects associated with nicotine. Findings at high doses were associated with evidence of systemic toxicity, which indicates that these insecticides do not selectively affect the developing nervous system.

Recommended Methods for Brain Processing and Quantitative Analysis in Rodent Developmental Neurotoxicity Studies.

Neuropathology methods in rodent developmental neurotoxicity (DNT) studies have evolved with experience and changing regulatory guidance. This article emphasizes principles and methods to promote more standardized DNT neuropathology evaluation, particularly procurement of highly homologous brain sections and collection of the most reproducible morphometric measurements. To minimize bias, brains from all animals at all dose levels should be processed from brain weighing through paraffin embedding at one time using a counterbalanced design. Morphometric measurements should be anchored by distinct neuroanatomic landmarks that can be identified reliably on the faced block or in unstained sections and which address the region-specific circuitry of the measured area. Common test article-related qualitative changes in the developing brain include abnormal cell numbers (yielding altered regional size), displaced cells (ectopia and heterotopia), and/or aberrant differentiation (indicated by defective myelination or synaptogenesis), but rarely glial or inflammatory reactions. Inclusion of digital images in the DNT pathology raw data provides confidence that the quantitative analysis was done on anatomically matched (i.e., highly homologous) sections. Interpreting DNT neuropathology data and their presumptive correlation with neurobehavioral data requires an integrative weight-of-evidence approach including consideration of maternal toxicity, body weight, brain weight, and the pattern of findings across brain regions, doses, sexes, and ages.

Pesticide exposure and neurodevelopmental outcomes: review of the epidemiologic and animal studies.

Assessment of whether pesticide exposure is associated with neurodevelopmental outcomes in children can best be addressed with a systematic review of both the human and animal peer-reviewed literature. This review analyzed epidemiologic studies testing the hypothesis that exposure to pesticides during pregnancy and/or early childhood is associated with neurodevelopmental outcomes in children. Studies that directly queried pesticide exposure (e.g., via questionnaire or interview) or measured pesticide or metabolite levels in biological specimens from study participants (e.g., blood, urine, etc.) or their immediate environment (e.g., personal air monitoring, home dust samples, etc.) were eligible for inclusion. Consistency, strength of association, and dose response were key elements of the framework utilized for evaluating epidemiologic studies. As a whole, the epidemiologic studies did not strongly implicate any particular pesticide as being causally related to adverse neurodevelopmental outcomes in infants and children. A few associations were unique for a health outcome and specific pesticide, and alternative hypotheses could not be ruled out. Our survey of the in vivo peer-reviewed published mammalian literature focused on effects of the specific active ingredient of pesticides on functional neurodevelopmental endpoints (i.e., behavior, neuropharmacology and neuropathology). In most cases, effects were noted at dose levels within the same order of magnitude or higher compared to the point of departure used for chronic risk assessments in the United States. Thus, although the published animal studies may have characterized potential neurodevelopmental outcomes using endpoints not required by guideline studies, the effects were generally observed at or above effect levels measured in repeated-dose toxicology studies submitted to the U.S. Environmental Protection Agency (EPA). Suggestions for improved exposure assessment in epidemiology studies and more effective and tiered approaches in animal testing are discussed.

Potential effects of chlorpyrifos on fetal growth outcomes: implications for risk assessment.

Chlorpyrifos (CPF) is one of the most widely used organophosphate insecticides in the United States. By December 2000, nearly all residential uses were voluntarily canceled, so that today, CPF is only used to control insect pests on a variety of crops. Periodic review of the potential effects of CPF on all developmental outcomes is necessary in the United States because the Food Quality Protection Act mandates special consideration of risk assessments for infants and children. This article reviews epidemiologic studies examining the association of potential CPF exposure with growth indices, including birth weight, birth length, and head circumference, and animal studies focusing on related somatic developmental endpoints. It differs from earlier reviews by including an additional cohort study and providing in-depth systematic evaluation of the patterns of association across different studies with respect to specificity of biomarkers for CPF, consistency, dose response, strength of association, temporality, and biological plausibility (Hill 1965), as well as consideration of the potential role of effect modification and bias. The review did not identify any strong associations exhibiting consistent exposure-response patterns that were observed in more than one of the four cohort studies evaluated. In addition, the animal data indicate that developmental effects occur at doses that produce substantial maternal toxicity and red blood cell (RBC) acetylcholinesterase (AChE) inhibition. Based on consideration of both the epidemiologic and animal data, maternal RBC AChE inhibition is a more sensitive endpoint for risk assessment than somatic developmental effects reviewed in this article.

Evaluation of epidemiology and animal data for risk assessment: chlorpyrifos developmental neurobehavioral outcomes.

Developmental neurobehavioral outcomes attributed to exposure to chlorpyrifos (CPF) obtained from epidemiologic and animal studies published before June 2010 were reviewed for risk assessment purposes. For epidemiological studies, this review considered (1) overall strength of study design, (2) specificity of CPF exposure biomarkers, (3) potential for bias, and (4) Hill guidelines for causal inference. In the case of animal studies, this review focused on evaluating the consistency of outcomes for developmental neurobehavioral endpoints from in vivo mammalian studies that exposed dams and/or offspring to CPF prior to weaning. Developmental neuropharmacologic and neuropathologic outcomes were also evaluated. Experimental design and methods were examined as part of the weight of evidence. There was insufficient evidence that human developmental exposures to CPF produce adverse neurobehavioral effects in infants and children across different cohort studies that may be relevant to CPF exposure. In animals, few behavioral parameters were affected following gestational exposures to 1 mg/kg-d but were not consistently reported by different laboratories. For postnatal exposures, behavioral effects found in more than one study at 1 mg/kg-d were decreased errors on a radial arm maze in female rats and increased errors in males dosed subcutaneously from postnatal day (PND) 1 to 4. A similar finding was seen in rats exposed orally from PND 1 to 21 with incremental dose levels of 1, 2, and 4 mg/kg-d, but not in rats dosed with constant dose level of 1 mg/kg-d. Neurodevelopmental behavioral, pharmacological, and morphologic effects occurred at doses that produced significant brain or red blood cell acetylcholinesterase inhibition in dams or offspring.

Acute inhalation study of allyl alcohol for derivation of acute exposure guideline levels.

An acute, whole-body inhalation study for allyl alcohol in Sprague-Dawley rats was designed to support derivation of AEGL values, with emphasis on establishing NOAELs for irreversible effects of different exposure concentrations and durations. Groups of 10 rats were exposed for 1 hour (0, 50, 200, or 400 ppm), 4 hours (0, 20, 50, or 100 ppm), or 8 hours (0, 10, 20, or 50 ppm). Clinical evaluations were performed during exposure and in an open field within 22-71 minutes after termination of exposure. Clinical pathology, gross necropsy, and histopathology (nasal tissues, larynx, trachea, lungs/bronchi, liver, and kidneys) were evaluated 14 days after exposure. Mortality was limited to 1 male exposed for 8 hours to 50 ppm. Clinical findings of gasping, rales, increased respiration noted at higher exposure levels were rapidly reversed. No treatment-related findings were observed in the liver and kidneys, or in the lungs of surviving animals. Histopathology in the nasal cavity was noted at all exposure levels following 1, 4, or 8 hours of exposure. Mild nasal inflammation was found at the lowest exposure levels (50-ppm/1-hour, 20-ppm/4-hour, and 10-ppm/8-hour). These effects were considered reversible and were not associated with related clinical signs. Severe, irreversible nasal olfactory epithelial lesions were present in 50 ppm/8-hour males. The NOELs for irreversible effects were 400-ppm/1-hour, 100-ppm/4-hour, and 20-ppm/8-hour. The incidence of severe findings was positively dependent on both concentration and the exposure duration. In contrast, the incidence of mild reversible findings did not appear to be dependent on duration.

Integration of epidemiology and animal neurotoxicity data for risk assessment.

Most human health risk assessments are based on animal studies that can be conducted under conditions where exposure to multiple doses of a single chemical can be controlled. Data from epidemiology studies also provide valuable information about human exposure and response to pesticides. Human studies have the potential of evaluating neurobehavioral and other outcomes that may be more difficult to evaluate in animals. The human data together with animal data can contribute to a weight-of-evidence analysis in the characterization of human health risks. Epidemiology data do, however, pose challenges with respect to characterizing human health risks. Similarly, animal data at high doses or routes of exposure not typical for humans also pose challenges to dose-response evaluations needed for risk assessments. This paper summarizes some of the presentations given at a symposium held at the Xi'an, China, International Neurotoxicology Conference held in June 2011. This symposium brought together scientists from government, industry and academia to discuss approaches to evaluating and conducting animal and human neurotoxicity studies for risk assessment purposes, using the pesticides paraquat and chlorpyrifos as case studies.

Strategies and tools for preventing neurotoxicity: to test, to predict and how to do it.

A change in paradigm is needed in the prevention of toxic effects on the nervous system, moving from its present reliance solely on data from animal testing to a prediction model mostly based on in vitro toxicity testing and in silico modeling. According to the report published by the National Research Council (NRC) of the US National Academies of Science, high-throughput in vitro tests will provide evidence for alterations in "toxicity pathways" as the best possible method of large scale toxicity prediction. The challenges to implement this proposal are enormous, and provide much room for debate. While many efforts address the technical aspects of implementing the vision, many questions around it need also to be addressed. Is the overall strategy the only one to be pursued? How can we move from current to future paradigms? Will we ever be able to reliably model for chronic and developmental neurotoxicity in vitro? This paper summarizes four presentations from a symposium held at the International Neurotoxicology Conference held in Xi'an, China, in June 2011. A. Li reviewed the current guidelines for neurotoxicity and developmental neurotoxicity testing, and discussed the major challenges existing to realize the NCR vision for toxicity testing. J. Llorens reviewed the biology of mammalian toxic avoidance in view of present knowledge on the physiology and molecular biology of the chemical senses, taste and smell. This background information supports the hypothesis that relating in vivo toxicity to chemical epitope descriptors that mimic the chemical encoding performed by the olfactory system may provide a way to the long term future of complete in silico toxicity prediction. S. Ceccatelli reviewed the implementation of rodent and human neural stem cells (NSCs) as models for in vitro toxicity testing that measures parameters such as cell proliferation, differentiation and migration. These appear to be sensitive endpoints that can identify substances with developmental neurotoxic potential. C. Suñol reviewed the use of primary neuronal cultures in testing for neurotoxicity of environmental pollutants, including the study of the effects of persistent exposures and/or in differentiating cells, which allow recording of effects that can be extrapolated to human developmental neurotoxicity.

Toxicity testing in the 21st century: a vision and a strategy.

With the release of the landmark report Toxicity Testing in the 21st Century: A Vision and a Strategy, the U.S. National Academy of Sciences, in 2007, precipitated a major change in the way toxicity testing is conducted. It envisions increased efficiency in toxicity testing and decreased animal usage by transitioning from current expensive and lengthy in vivo testing with qualitative endpoints to in vitro toxicity pathway assays on human cells or cell lines using robotic high-throughput screening with mechanistic quantitative parameters. Risk assessment in the exposed human population would focus on avoiding significant perturbations in these toxicity pathways. Computational systems biology models would be implemented to determine the dose-response models of perturbations of pathway function. Extrapolation of in vitro results to in vivo human blood and tissue concentrations would be based on pharmacokinetic models for the given exposure condition. This practice would enhance human relevance of test results, and would cover several test agents, compared to traditional toxicological testing strategies. As all the tools that are necessary to implement the vision are currently available or in an advanced stage of development, the key prerequisites to achieving this paradigm shift are a commitment to change in the scientific community, which could be facilitated by a broad discussion of the vision, and obtaining necessary resources to enhance current knowledge of pathway perturbations and pathway assays in humans and to implement computational systems biology models. Implementation of these strategies would result in a new toxicity testing paradigm firmly based on human biology.

Oral gavage subchronic neurotoxicity and inhalation subchronic immunotoxicity studies of ethylbenzene in the rat.

The potential for neurotoxicological and immunotoxicological effects of ethylbenzene was studied in young adult Crl:CD(SD) rats following 90-day oral (neurotoxicity) or 28-day inhalation (immunotoxicity) exposures. In the neurotoxicity study, ethylbenzene was administered orally via gavage twice daily at 0, 25, 125, or 250 mg/kg per dose (total daily dosages of 0, 50, 250, or 500 mg/kg bwt/day [mg/kg bwt/day]) for 13 weeks and the functional observational battery (FOB), automated tests for motor activity and neuropathological examination were conducted. In the immunotoxicity study, animals were exposed by inhalation to 0, 25, 100, or 500 ppm ethylbenzene (approximately 26, 90, or 342 mg/kg bwt/day as calculated from physiologically based pharmacokinetic modeling). Immunotoxicity was evaluated in female rats using the splenic antibody-forming cell plaque-forming assay in sheep red blood cell sensitized animals. The no-observed-effect level for the oral gavage study was 50mg/kg bwt/day based on increased relative weights of the liver and kidneys in the male rats. The no-observed-adverse-effect level (NOAEL) for adult neurotoxicity was the highest dose tested 500 mg/kg bwt/day. The NOAEL for the immunotoxicity evaluation was the highest tested exposure concentration, 500 ppm (342 mg/kg bwt/day).

Analysis and integration of developmental neurotoxicity and ancillary data into risk assessment: a case study of dimethoate.

Dimethoate is an organophosphate (OP) pesticide used to control a wide variety of insects on agricultural crops and ornamentals. To ensure that dimethoate is used safely, it is important to determine exposure levels that protect against adverse effects at all life stages, including the developing fetus, infant, and child. Based on an analysis of a developmental neurotoxicity (DNT) study, a cholinesterase (ChE) sensitivity study, a cross-fostering study, and several single- and multigenerational reproductive toxicity studies, two potential critical endpoints for dimethoate were identified: brain ChE inhibition (ChEI) in adult females, and pup mortality. An initial evaluation concluded that pup mortality was a preferable endpoint, based on an increased number of pup deaths born to dams dosed with > or =3 mg/kg dimethoate via oral gavage. Closer examination, however, revealed that the pup deaths were clustered in a small number of litters in which the dams providing postnatal care exhibited maternal care deficits. When the data were analyzed using the dam as the unit of statistical significance, a significant increase in the average litter proportion of pup deaths was observed only when the dams were dosed postnatally with 6 mg/kg dimethoate while they were raising the pups. Gestational exposure (i.e., during pregnancy only) to 6 mg/kg dimethoate exerted no effect on pup survival. This leads to the conclusion that it is postnatal exposure of the nursing dams that is associated with pup mortality. Furthermore, a previous benchmark dose (BMD) meta-analysis approach revealed that BMDL(10) for adult females (the lower 95% bound of the dose resulting in a 10% reduction in the parameter of interest) for ChEI was > 3-fold lower than the BMDL(10) for pup mortality (0.19 and 0.68 mg/kg, respectively). Overall, this study underscores the importance of using the dam as the unit of statistical significance when assessing data collected in the perinatal period, and it is concluded that adult brain ChEI is the correct critical endpoint for assessing risk of dimethoate toxicity.

Building a scientific framework for studying hormonal effects on behavior and on the development of the sexually dimorphic nervous system.

There has been increasing concern that low-dose exposure to hormonally active chemicals disrupts sexual differentiation of the brain and peripheral nervous system. There also has been active drug development research on the therapeutic potential of hormone therapy on behaviors. These different research goals have in common the need to develop reliable animal models to study the effect of hormones on brain function and behaviors that are predictive of effects in humans. This paper summarizes presentations given at the June 2007 11th International Neurotoxicology Association (INA-11) meeting, which addressed these issues. Using a few examples from the bisphenol A neurobehavioral literature for illustrative purposes, Dr. Abby Li discussed some of the methodological issues that should be considered in designing developmental neurobehavioral animal studies so they can be useful for human health risk assessment. Dr. Earl Gray provided an overview of research on the role of androgens and estrogens in the development of the brain and peripheral nervous system and behavior. Based on this scientific foundation, Dr. Gray proposed a rational framework for the study of the effects of developmental exposures to chemicals on the organization of the sexually dimorphic nervous system, including specific recommendations for experimental design and statistical analyses that can increase the utility of the research for regulatory decision-making. Dr. Michael Baum and by Dr. Feng Liu presented basic research on the hormonal mechanisms underlying sexual preference and estrogenic effects of cognition, respectively. These behaviors are among those studied in adult animals following in utero exposure to hormonally active chemicals, to evaluate their potential effects on sexual differentiation of the brain. Understanding of the hormonal mechanisms of these behaviors, and of relevance to humans, is needed to develop biologically plausible hypotheses regarding the potential effects of hormonally active chemicals in humans.

A tiered approach to life stages testing for agricultural chemical safety assessment.

A proposal has been developed by the Agricultural Chemical Safety Assessment (ACSA) Technical Committee of the ILSI Health and Environmental Sciences Institute (HESI) for an improved approach to assessing the safety of crop protection chemicals. The goal is to ensure that studies are scientifically appropriate and necessary without being redundant, and that tests emphasize toxicological endpoints and exposure durations that are relevant for risk assessment. The ACSA Life Stages Task Force proposes a tiered approach to toxicity testing that assesses a compound's potential to cause adverse effects on reproduction, and that assesses the nature and severity of effects during development and adolescence, with consideration of the sensitivity of the elderly. While incorporating many features from current guideline studies, the proposed approach includes a novel rat reproduction and developmental study with enhanced endpoints and a rabbit development study. All available data, including toxicokinetics, ADME data, and systemic toxicity information, are considered in the design and interpretation of studies. Compared to existing testing strategies, the proposed approach uses fewer animals, provides information on the young animal, and includes an estimation of human exposure potential for making decisions about the extent of testing required.

Evaluation of epidemiologic and animal data associating pesticides with Parkinson's disease.

Exposure to pesticides may be a risk factor for developing Parkinson's disease (PD). To evaluate the evidence regarding this association in the scientific literature, we examined both analytic epidemiologic studies of PD cases in which exposure to pesticides was queried directly and whole-animal studies for PD-like effects after systemic pesticide exposure. Epidemiologic studies were considered according to study quality parameters, and results were found to be mixed and without consistent exposure-response or pesticide-specific patterns. These epidemiologic studies were limited by a lack of detailed and validated pesticide exposure assessment. In animal studies, no pesticide has yet demonstrated the selective set of clinical and pathologic signs that characterize human PD, particularly at levels relevant to human populations. We conclude that the animal and epidemiologic data reviewed do not provide sufficient evidence to support a causal association between pesticide exposure and PD.