John M. DeSesso

The Potential of Selected Brominated Flame Retardants to Affect Neurological Development

Various brominated flame retardants (BFR), including polybrominated diphenyl ether (PBDE) congeners, hexabromocyclododecane (HBCD), and tetrabromobisphenol A (TBBPA), are commonly used in household items and electronics and have been detected in the environment and/or the bodily fluids of people, including children. Some studies in animals suggest that exposure to PBDE congeners, HBCD, or TBBPA during the perinatal period may affect locomotor activity and/or memory and learning. Epidemiological studies showing similar effects in humans, however, are lacking. To assess whether an association exists between perinatal exposure and development of consistent neurobehavioral alterations, published animal studies investigating perinatal exposure to PBDE congeners, HBCD, or TBBPA with specific neurobehavioral evaluations—particularly, assessments of motor activity—were reviewed for consistency of results. Our analysis shows that although the majority of studies suggest that perinatal exposure affects motor activity, the effects observed were not consistent. This lack of consistency includes the type of motor activity (locomotion, rearing, or total activity) affected, the direction (increase or decrease) and pattern of change associated with exposure, the existence of a dose response, the permanency of findings, and the possibility of gender differences in response. Interestingly, Good Laboratory Practices (GLP)-compliant studies that followed U.S. Environmental Protection Agency (EPA)/Organization for Economic Cooperation and Development (OECD) guidelines for developmental neurotoxicity testing found no adverse effects associated with exposure to PBDE209, HBCD, or TBBPA at doses that were orders of magnitude higher and administered over longer durations than those used in the other studies examined herein. The lack of consistency across studies precludes establishment of a causal relationship between perinatal exposure to these substances and alterations in motor activity.

Estimation of cancer risks and benefits associated with a potential increased consumption of fruits and vegetables.

The current paper provides an analysis of the potential number of cancer cases that might be prevented if half the U.S. population increased its fruit and vegetable consumption by one serving each per day. This number is contrasted with an upper-bound estimate of concomitant cancer cases that might be theoretically attributed to the intake of pesticide residues arising from the same additional fruit and vegetable consumption. The cancer prevention estimates were derived using a published meta-analysis of nutritional epidemiology studies. The cancer risks were estimated using U.S. Environmental Protection Agency (EPA) methods, cancer potency estimates from rodent bioassays, and pesticide residue sampling data from the U.S. Department of Agriculture (USDA). The resulting estimates are that approximately 20,000 cancer cases per year could be prevented by increasing fruit and vegetable consumption, while up to 10 cancer cases per year could be caused by the added pesticide consumption. These estimates have significant uncertainties (e.g., potential residual confounding in the fruit and vegetable epidemiologic studies and reliance on rodent bioassays for cancer risk). However, the overwhelming difference between benefit and risk estimates provides confidence that consumers should not be concerned about cancer risks from consuming conventionally-grown fruits and vegetables.

Contrasting the Gastrointestinal Tracts of Mammals: Factors that Influence Absorption

Publisher Summary The rate and extent of absorption of orally ingested compounds are influenced by properties that are intrinsic to ingested substances themselves as well as factors that are associated with the milieu of the alimentary canal and its absorptive surface. This chapter presents comparative, quantitative data for the previously identified important anatomical and physiological parameters of the gastrointestinal tract for several additional mammalian species. Data for the dog, rabbit, mouse, pig, and monkey, and updated information for rats and humans is presented. It has been found that perfused jejunal segments of normal human subjects absorb fluids at a rate that is 5–10 times greater per unit area of mucosa than that of laboratory rats. It is emphasized that the parameters described relate to the crossing of materials from the gastrointestinal lumen into the bloodstream. The information about absorption of materials from the gastrointestinal tract is the first step in determining potential biological effects.

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 ≥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 BMDL10 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 BMDL10 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.

Relationship Between Bent Long Bones, Bent Scapulae, and Wavy Ribs: Malformations or Variations?

BACKGROUND Shortened and bent long bones and bent scapulae are sometimes reported in fetuses with wavy ribs (Carney and Kimmel, ). Wavy ribs are typically seen in the presence of maternal and developmental toxicity, are transient and reversible postnatally, and are considered to be variations rather than malformations. METHODS We further assessed the literature cited in Kimmel and Carney () as well as papers published since then to determine under what conditions bent long bones in the absence of gross limb defects and bent scapulae were reported and whether information was available on the transient or permanent nature of these effects. RESULTS Long bone and/or scapular changes almost always occurred at a lower incidence than wavy ribs. In every case, maternal and fetal toxicity occurred at the same dose levels. In a few studies, pups were followed sequentially after birth and bent long bones and scapulae were transient in nature and appeared normal by the time of weaning. Rabbits were much less likely to show wavy ribs or long bone and scapular changes at birth, even in the presence of severe maternal and fetal toxicity. This species difference may be due in part to the great increase in bone mass and remodeling that occurs during the first few postnatal weeks in rodents, but which takes place during the longer fetal period in rabbits. CONCLUSION Our conclusion from this review is that bent long bones and scapulae, like wavy ribs, appear to be secondary to maternal and developmental toxicity, are transient, and like wavy ribs should be considered variations rather than malformations.

In utero arsenic exposure in mice and early life susceptibility to cancer

In its review of the U.S. Environmental Protection Agencys toxicological review of inorganic arsenic (iAs), the National Academy of Sciences identified carcinogenic endpoints among the highest priority health effects of concern and stated the need to consider evidence that early life exposures may increase the risk of adverse health effects. Recent studies in mice suggest that in utero exposure to arsenic increases susceptibility to cancer later in life. These data are striking in light of the general lack of evidence for carcinogenicity in rodents exposed to iAs. To evaluate the transplacental carcinogenic potential of iAs, a detailed analysis of the toxicology literature evaluating the role of in utero arsenic exposure in carcinogenesis was conducted. Bladder, lung, and skin tumors, which are the tumor types most consistently reported in humans exposed to high arsenic levels, were not consistently increased in mouse studies. There was also a lack of concordance across studies for other tumor types not typically reported in humans. Therefore, we considered methodological and other critical issues that may have contributed to variable results and we suggest additional studies to address these issues. It was concluded that the available data do not provide evidence of a causal link between in utero arsenic exposure and cancer or indicate early life-stage susceptibility to arsenic-induced cancer, particularly at environmentally relevant doses.

Gestational/perinatal chlorpyrifos exposure is not associated with autistic-like behaviors in rodents.

Abstract Although animal models cannot exactly replicate human psychiatric disorders, they may be useful to investigate whether the behaviors associated with certain exposures in animals parallel those observed in people. According to the most current version of the Diagnostic and Statistical Manual of Mental Disorders, autism is diagnosed based on (1) persistent deficits in social communication and social interaction; and (2) the presence of restricted, repetitive patterns of behavior, interests and activities. To address whether developmental chlorpyrifos (CPF) exposure was associated with the development of autistic behaviors, a literature search was conducted to identify studies in rats and mice involving gestational or early postnatal exposure to CPF or CPF oxon (CPO, the active metabolite of CPF) and subsequent behavioral testing to assess behaviors related to autism. A total of 13 studies conducted in six different laboratories were identified. Analysis of these studies found that perinatal CPF exposure was generally associated with (1) no effect or increased social communications; (2) no effect or increased social encounters; (3) no effect, reduced stereotypies, or conflicting findings on stereotypic behaviors; and (4) no effect or increased preference for novelty and reduced anxiety in novel environments. These behavioral findings are generally inconsistent with the types of behaviors that would be expected in children with clinical autism. Based on the results of this analysis of rodent model studies involving CPF/CPO exposure, it cannot be concluded that gestational and/or perinatal CPF exposure is likely to be associated with the development of autism-like behaviors in humans.

Adaptation of the ToxRTool to Assess the Reliability of Toxicology Studies Conducted with Genetically Modified Crops and Implications for Future Safety Testing

To determine the reliability of food safety studies carried out in rodents with genetically modified (GM) crops, a Food Safety Study Reliability Tool (FSSRTool) was adapted from the European Centre for the Validation of Alternative Methods’ (ECVAM) ToxRTool. Reliability was defined as the inherent quality of the study with regard to use of standardized testing methodology, full documentation of experimental procedures and results, and the plausibility of the findings. Codex guidelines for GM crop safety evaluations indicate toxicology studies are not needed when comparability of the GM crop to its conventional counterpart has been demonstrated. This guidance notwithstanding, animal feeding studies have routinely been conducted with GM crops, but their conclusions on safety are not always consistent. To accurately evaluate potential risks from GM crops, risk assessors need clearly interpretable results from reliable studies. The development of the FSSRTool, which provides the user with a means of assessing the reliability of a toxicology study to inform risk assessment, is discussed. Its application to the body of literature on GM crop food safety studies demonstrates that reliable studies report no toxicologically relevant differences between rodents fed GM crops or their non-GM comparators.

Comment on “Glyphosate impairs male offspring reproductive development by disrupting gonadotropin expression” by Romano et al. 2012

Romano et al. (2012) recently published the results of a study in which pregnant rats were treated with a glyphosate-based herbicide formulation from gestational day 18 to postnatal day 5. Various parameters related to sexual behavior and reproductive development were assessed in the resulting male offspring. Based on their results, the study authors claim that glyphosate exposure in the perinatal period causes changes in males that are associated with hypersecretion of androgens. There are a number of issues with this study, however, that preclude such conclusions. First, as a premise for this study, the authors incorrectly assert that glyphosate is a ‘‘potential endocrine chemical disruptor.’’ As evidence, they cite three in vitro studies of altered aromatase activity conducted in a single laboratory (Benachour et al. 2007; Gasnier et al. 2009; Richard et al. 2005), and their own earlier work suggesting reduced serum testosterone concentrations in response to treatment (Romano et al. 2010). These studies, however, were conducted using glyphosate-based herbicide formulations rather than the active ingredient alone. Exposures were at high concentrations; for instance, in the study by Benachour et al. (2007), the concentration of glyphosate in the tissue culture medium was reported to be 1.27 mM (approximately 200,000 lg/L), which is not relevant to concentrations in target tissue concentrations or environmental media. Further, although sporadic positive responses were reported, aromatase activity was generally unaffected in the few experiments that tested glyphosate by itself. It should be additionally noted that the EPA OPPTS guideline for conducting the aromatase assay (U.S. Environmental Protection Agency 2009a) clearly warns that all glassware and apparatus used need to be free of detergent residue; this is because microsomes are extremely sensitive to the surfactants found in detergents. Surfactants are a major component of the various herbicide formulations tested in these studies and are the likely cause for the observed aromatase inhibition. Therefore, the claim that glyphosate is an endocrine disruptor is not substantiated. Second, it is not clear to what agent(s) the animals were exposed. The study authors indicate that Roundup Transorb (480 g/L glyphosate, 648 g/L isopropylamine salt, and 594 g/L inert ingredients) was the product administered. However, they report no additional details regarding the composition of the herbicide formulation used such as the identity and amount of surfactant(s) in the formulation. This is important because in other studies, Levine et al. (2007) have shown that steroidogenesis in Leydig cell cultures is inhibited by the presence of surfactants, which perturb membrane function of mitochondria. An appropriate control group that was exposed to the surfactant(s) only was not included in the study nor was there a group exposed to glyphosate only. As such, the study results are confounded. Third, the authors did not describe any methods to control for potential litter effects, and it is not clear whether this was done or not. Because the dams were the unit of exposure in these experiments, the exposed litter (the dam) is the appropriate unit of analysis. This is particularly important when assessing these experiments because dosing of the dams occurred from gestational day 18 through postnatal day 5; however, glyphosate does not cross well

Functional Anatomy of the Brain

The central nervous system comprises the brain and spinal cord which provide sensation, control of movement, emotion, aesthetics, reason and self-awareness. The tissue that makes up the central nervous system is highly differentiated and exceedingly ordered, yet plastic. The central nervous system is well protected throughout by a fluid-filled, tri-layered, connective tissue covering (the meninges) and various osseous claddings. The cranium provides a rigid armor for the brain whereas the vertebral column constitutes the flexible protection of the spinal cord. Because the focus of this volume is the study of various disease states that affect the functions of the brain, it is important to understand the normal relationship of the brain to its surrounding structures including its bony case and its connective tissue coverings, its blood supply, and its internal organization, as well as how the perturbation of the relationships among these structures can impact brain functions. It is the purpose of this chapter to present an overview of this information. More detailed anatomical information is readily available in textbooks of gross anatomy and neuroscience.