Testing for cognitive function in animals in a regulatory context

Developmental Neurotoxicity and Behavioral Screening in Larval Zebrafish with a Comparison to Other Published Results

With the abundance of chemicals in the environment that could potentially cause neurodevelopmental deficits, there is a need for rapid testing and chemical screening assays. This study evaluated the developmental toxicity and behavioral effects of 61 chemicals in zebrafish (Danio rerio) larvae using a behavioral Light/Dark assay. Larvae (n = 16-24 per concentration) were exposed to each chemical (0.0001-120 μM) during development and locomotor activity was assessed. Approximately half of the chemicals (n = 30) did not show any gross developmental toxicity (i.e., mortality, dysmorphology or non-hatching) at the highest concentration tested. Twelve of the 31 chemicals that did elicit developmental toxicity were toxic at the highest concentration only, and thirteen chemicals were developmentally toxic at concentrations of 10 µM or lower. Eleven chemicals caused behavioral effects; four chemicals (6-aminonicotinamide, cyclophosphamide, paraquat, phenobarbital) altered behavior in the absence of developmental toxicity. In addition to screening a library of chemicals for developmental neurotoxicity, we also compared our findings with previously published results for those chemicals. Our comparison revealed a general lack of standardized reporting of experimental details, and it also helped identify some chemicals that appear to be consistent positives and negatives across multiple laboratories.

Issues in the design, analysis, and application of rodent developmental neurotoxicology studies

Developmental neurotoxicity (DNT) studies could benefit from revisions to study design, data analysis, and some behavioral test methods to enhance reproducibility. The Environmental Protection Agency (EPA) reviewed 69 studies submitted to the Office of Pesticide Programs. Two of the behavioral tests identified the lowest observable adverse effect level (LOAEL) 20 and 13 times, respectively, while the other two tests identified the LOAEL only 3 and 4 times, respectively. The EPA review showed that the functional observational battery (FOB) was least effective at detecting the LOAEL, whereas tests of learning and memory (L&M) had methodological shortcomings. Human neurodevelopmental toxicity studies over the past 30 years show that most of the adverse effects are on higher cognitive functions such as L&M. The results of human studies together with structure-function relationships from neuroscience, suggest that tests of working memory, spatial navigation/memory, and egocentric navigation/memory should be added to guideline studies. Collectively, the above suggest that EPA and EU DNT studies would better reflect human findings and be more relevant to children by aligning L&M tests to the same domains that are affected in children, removing less useful methods (FOB), and using newer statistical models to better account for random factors of litter and litter × sex. Common issues in study design and data analyses are discussed: sample size, random group assignment, blinding, elimination of subjective rating methods, avoiding confirmation bias, more complete reporting of species, housing, test protocols, age, test order, and litter effects. Litter in DNT studies should at least be included as a random factor in ANOVA models and may benefit from inclusion of litter × sex as random factors.

Developmental neurotoxicity test guidelines: problems and perspectives

Epidemiologic evidence has demonstrated associations between early life exposure to industrial chemicals and the occurrence of disease states, including cognitive and behavioral abnormalities, in children. The developing brain in the fetal and infantile periods is extremely vulnerable to chemicals because the blood-brain barrier is not completely formed during these periods. The Organisation for Economic Co-operation and Development (OECD) developmental neurotoxicity (DNT) test guideline, TG426, updated in 2007, comprises in vivo behavioral observational tests and other tests intended to assess DNT induced by exposure to industrial chemicals. These chemicals may enter the market without having been subjected to DNT testing, as DNT test data is not mandated by law at the time of chemical registration. In addition, proprietary rights have led to problems concerning the non-disclosure of industrial chemical toxicity test data, including DNT test data. To overcome the disadvantages of high-cost and low time efficiency of in vivo DNT tests, in vitro or in silico tests are the proposed alternatives, but it is unlikely that the results of such tests would reflect changes in higher brain functions. Accordingly, the current DNT test guidelines need to be revised to avoid overlooking or neglecting the occurrence of DNT induced by exposure to low doses of chemicals. This review also proposes the introduction of novel in vivo DNT testing methods in light of a cost-performance analysis.

Assessment of Learning, Memory, and Attention in Developmental Neurotoxicology Regulatory Testing: Commentary on essentiality of cognitive assessment for protecting child health

This commentary is in response to the remarks of Drs. Christina Sobin, Mari Golub, and David Herr on the Special Issue of this Journal entitled "Assessment of Learning, Memory, and Attention in Developmental Neurotoxicology Regulatory Testing." I endorse the views expressed by Drs. Sobin, Golub, and Herr and add some discussion on a recent Organization for Economic Cooperation and Development (OECD) guideline, the Extended One Generation Reproductive Toxicology Guideline (OECD 446), in which testing for higher cognitive function (learning and memory) has been eliminated. The case against this decision is offered. It is noted that deficits in higher cognitive function are one of the hallmarks of human studies that find neurobehavioral toxicity in children after exposure to environmental agents such as lead, methylmercury, PCB, pesticides, and other environmental agents. It is noted that the OECD decision is at variance with the views of the scientific community in this field, including those of Drs. Sobin, Golub, and Herr. Why OECD took such action without the advice and consent of the field of developmental neurotoxicology is deeply concerning and potentially hazardous to children. I also endorse Dr. Herr's recommendation that in the future the Environmental Protection Agency negotiate study designs in advance with submitters as the Food and Drug Administration does to improve data quality for all neurobehavioral methods, and especially for tests of learning and memory that have not been adequately conducted in many past studies.

Assessment of learning, memory, and attention in developmental neurotoxicity regulatory studies: synthesis, commentary, and recommendations

Cognitive tests of learning and memory (L&M) have been required by U.S. Environmental Protection Agency (EPA) developmental neurotoxicity test (DNT) guidelines for more than two decades. To evaluate the utility of these guidelines, the EPA reviewed 69 pesticide DNT studies. This review found that the DNT provided or could provide the point-of-departure for risk assessment by showing the Lowest Observable Adverse Effect Level (LOAEL) in 28 of these studies in relation to other reported end points. Among the behavioral tests, locomotor activity and auditory/acoustic startle provided the most LOAELs, and tests of cognitive function and the Functional Observational Battery (FOB) the fewest. Two issues arose from the review: (1) what is the relative utility of cognitive tests versus tests of unconditioned behavior, and (2) how might cognitive tests be improved? The EPA sponsored a symposium to address this. Bushnell reviewed studies in which both screening (locomotor activity, FOB, reflex ontogeny, etc.) and complex tests (those requiring training) were used within the same study; he found relatively little evidence that complex tests provided a LOAEL lower than screening tests (with exceptions). Levin reviewed reasons for including cognitive tests in regulatory studies and methods and evidence for the radial arm maze and its place in developmental neurotoxicity assessments. Driscoll and Strupp reviewed the value of serial reaction time operant methods for assessing executive function in developmental neurotoxicity studies. Vorhees and Williams reviewed the value of allocentric (spatial) and egocentric cognitive tests and presented methods for using the Morris water maze for spatial and the Cincinnati water maze for egocentric cognitive assessment. They also reviewed the possible use of water radial mazes. The relatively lower impact of cognitive tests in previous DNT studies in the face of the frequency of human complaints of chemical-induced cognitive dysfunction indicates that animal cognitive tests need improvement. The contributors to this symposium suggest that if the guidelines are updated, they be made more specific by recommending preferred tests and providing greater detail on key characteristics of such tests. Additionally, it is recommended that guidance be developed to address important issues with cognitive tests and to provide the information needed to improve the design, conduct, and interpretation of tests of higher function within a regulatory context. These steps will maximize the value of cognitive tests for use in hazard evaluation and risk assessment.

Assessment of health risks resulting from early-life exposures: Are current chemical toxicity testing protocols and risk assessment methods adequate?

Abstract Over the last couple of decades, the awareness of the potential health impacts associated with early-life exposures has increased. Global regulatory approaches to chemical risk assessment are intended to be protective for the diverse human population including all life stages. However, questions persist as to whether the current testing approaches and risk assessment methodologies are adequately protective for infants and children. Here, we review physiological and developmental differences that may result in differential sensitivity associated with early-life exposures. It is clear that sensitivity to chemical exposures during early-life can be similar, higher, or lower than that of adults, and can change quickly within a short developmental timeframe. Moreover, age-related exposure differences provide an important consideration for overall susceptibility. Differential sensitivity associated with a life stage can reflect the toxicokinetic handling of a xenobiotic exposure, the toxicodynamic response, or both. Each of these is illustrated with chemical-specific examples. The adequacy of current testing protocols, proposed new tools, and risk assessment methods for systemic noncancer endpoints are reviewed in light of the potential for differential risk to infants and young children.