Methylmercury (MeHg) has been an environmental concern to public health and regulatory agencies for over 50 years because of its toxicity to the human nervous system. Its association with nervous system toxicity in adults and infants near Minamata Bay, Japan, in the 1950s initiated environmental health research inquiries that continue to this day. Observations of greater neurotoxicity with gestational compared with adult exposure suggest a unique susceptibility of the developing nervous system to MeHg. Despite extensive research conducted over the last half century, determination of definitive molecular mechanisms underlying the observed neurotoxic effects of MeHg have not been identified. This paper summarizes results of a series of experiments conducted to examine the effects of MeHg on neuroepithelial cell proliferation, a hypothesized mode of action for its selective effects on neurogenesis. Observed effects of MeHg on cell cycle entry and progression were associated with alterations in a variety of cell cycle regulatory molecules, including p21 signaling pathways. We place these studies in the context of other cellular responses involved in signal transduction, including oxidative stress, altered protein phosphorylation, and altered intracellular calcium homeostasis. Although existing information suggests that no single mechanism underlies the diverse array of effects associated with MeHg-induced developmental neurotoxicity, we demonstrate characteristic effects of MeHg on cell signaling that contribute to observed effects on cell proliferation. Experimentally derived cell cycle kinetic and cytotoxicity data allowed development of a biologically based dose-response model of MeHg-induced alterations in neurodevelopment, which can form the basis for information synthesis and hypothesis testing and for use in assessing risks from environmental exposures.

For more than three decades chronic studies in rodents have been the benchmark for assessing the potential long-term toxicity, and particularly the carcinogenicity, of chemicals. With doses typically administered for about 2 years (18 months to lifetime), the rodent bioassay has been an integral component of testing protocols for food additives, pesticides, pharmaceuticals, industrial chemicals, and all manner of byproducts and environmental contaminants. Over time, the data from these studies have been used to address an increasing diversity of questions related to the assessment of human health risks, adding complexity to study design and interpretation. An earlier ILSI RSI working group developed a set of principles for the selection of doses for chronic rodent studies (ILSI, 1997). The present report builds on that work, examining some of the issues that arise and offering new perspectives and approaches for putting the principles into practice. Dose selection is considered both from the prospective viewpoint of the choosing of dose levels for a study and from the retrospective interpretation of study results in light of the doses used. A main theme of this report is that the purposes and objectives of chronic rodent studies vary and should be clearly defined in advance. Dose placement, then, should be optimized to achieve study objectives. For practical reasons, most chronic studies today must be designed to address multiple objectives, often requiring trade-offs and innovative approaches in study design. A systematic approach to dose selection should begin with recognition that the design of chronic studies occurs in the context of a careful assessment of the accumulated scientific information on the test substance, the relevant risk management questions, priorities and mandates, and the practical limitations and constraints on available resources. A stepwise process is described. The aim is to increase insofar as possible the utility of an expensive and time-consuming experiment. The kinds of data that are most commonly needed for dose selection and for understanding the dose-related results of chronic rodent studies, particularly carcinogenicity studies, are discussed as "design/interpretation factors." They comprise both the inherent characteristics of the test substance and indicators of biological damage, perturbation or stress among the experimental animals. They may be primary toxicity endpoints, predictors or indicators of appropriate dose selection, or indicators of conditions to be avoided in dose selection. The application and interpretation of design/interpretation factors is conditioned by the study objectives-what is considered desirable will depend on the strategy for choice of doses that is being followed. The challenge is to select doses that accommodate all of the issues raised by the relevant design/interpretation factors. Three case studies are presented here that illustrate the interplay between study objectives and the design and selection of doses for chronic rodent studies. These exa...

5'-bromodeoxyuridine (BrdU) labeling was employed to explore the effects of methylmercury (MeHg) on cell cycle kinetics in the developing rat midbrain during gestational days (GDs) 11 to 14. Contrary to what has been previously reported in mice, no effects of MeHg on cell cycle kinetics were observed up to embryonic brain concentrations of 3-4 microg/g. The absence of an effect was confirmed using stereology and counts of midbrain cell number. Treatment with colchicine, the positive control, resulted in significant effects on cell cycle kinetics in the developing rat midbrain. The parallelogram method, borrowed from genetic toxicology, was subsequently used to place the data obtained in the present study in the context of previously collected in vitroand in vivo data on MeHg developmental neurotoxicity. This required developing a common dose metric (microg Hg/g cellular material) to allow in vitro and in vivo study comparisons. Evaluation suggested that MeHg's effects on neuronal cell proliferation show a reasonable degree of concordance across mice, rats, and humans, spanning approximately an order of magnitude. Comparisons among the in vivo data suggest that humans are at least or more sensitive than the rodent and that mice may be a slightly better model for MeHg human developmental neurotoxicity than the rat. Such comparisons can provide both a quantitative and a qualitative framework for utilizing both in vivo and in vitro data in human health risk assessment.

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