Temporal changes in biological responses and uncertainty in assessing risks of endocrine-disrupting chemicals: insights from intensive time-course studies with fish

Most toxicological testing focuses on defining concentration/dose-response relationships, with little consideration to temporal aspects of the expression of chemical effects. Yet, both dose and time are critical to evaluating potential risks. Our lab has generated an extensive amount of linked dose-response and time-course data for eight model endocrine-disrupting chemicals (EDCs) using adult fathead minnows (Pimephales promelas). Herein we conduct a meta-analysis of this novel dataset, with an emphasis on defining interactions between dose and time on several molecular and biochemical endpoints indicative of endocrine function, both during chemical exposure and recovery. Direct effects of the EDCs were very rapid, often occurring within 24 h or less. Recovery after removal of the chemical stressors was similarly rapid. All experiments provided evidence of system compensation. For example, measures of endocrine perturbation early in an experiment, especially in low-dose treatments, often recovered to control values while a chemical exposure was ongoing. In experiments with several of the EDCs, shortly after cessation of the chemical exposure there was an "overshoot" behavior, in which different measures of endocrine function exceeded control values. Overall, when an endpoint was measured was as an important determinant in identifying a chemical as endocrine-active, as the dose to which the animal was exposed. The compensatory responses during exposure and early recovery after removal of the chemical stressor also produced examples of nonmonotonic dose-response relationships. The types of interactions between time and dose observed in these studies have a number of important implications for screening, testing, and monitoring programs for EDCs.