Neuroendocrine Impacts of Endocrine-Disrupting Chemicals in Birds: Life Stage and Species Sensitivities

Abstract

Assessing potential risk associated with exposure to endocrine-disrupting chemicals (EDC) has been difficult due to species specific variation in vulnerability and to both short- and long-term effects produced by EDC. In precocial birds, embryonic exposure to EDC impacts sexual differentiation of neuroendocrine systems and behavior. Often, detectable nonlethal effects of EDC diminish as the organism matures such that the chronic impact of EDC may appear relatively innocuous by the time an individual is sexually mature. In addition, studies have not addressed lifetime effects of EDC exposure on birds. Consequently, it is difficult to assess chronic effects of nonlethal exposure on the fitness of an individual and whether there is a potential risk to a wild population. Assessing behavioral and neuroendocrine consequences of exposure is complicated by individual and species variation in sensitivity as well as exposure to complex mixtures. Our studies are designed to examine effects of individual EDC administered to the embryo as well as in a multigenerational dietary study in which birds received low doses of the pesticide methoxychlor (MXC). The influence of dietary MXC exposure was also compared between Japanese quail and northern bobwhite quail. The effects of dietary exposures to 0.5, 5, or 10 ppm that are relatively environmentally low were determined. The selection of these doses was to mimic levels that might be encountered in the field and higher doses that might potentially reveal effects of exposure at relatively low exposures. These doses were also based on the regulations by the U.S. Environmental Protection Agency that mandate a limit 0.04 ppm MXC in drinking water, with a limit of no more than 0.05ppm in water that children drink. Further there is a limit of 1–100 ppm for crops and other food for human and livestock consumption; bottled water has a 0.1 ppm limit for MXC content. Our data are discussed in the context of applicability of toxicological yardsticks, including the toxic equivalent (TEQ) and neurotoxic equivalent (NEQ) as predictive indices for short- and long-term outcomes to nonlethal concentrations of EDC. Other approaches have been developed to address inconsistencies in effects and incorporate diverse data into potency estimates. Perhaps it is time to develop a more inclusive estimation method for endocrine and neuroendocrine effects. An endocrine disruption index (EDI) would (1) complement other indices, (2) focus on endocrine disruption, and (3) include effects beyond those mediated by the aryl hydrocarbon receptor (AhR) for a comparative assessment of nonlethal EDC effects.