Date of Award


Document Type

Open Access Dissertation


Environmental Health Sciences

First Advisor

David C Volz


Human health and ecological risk assessors are currently facing increased demands and public pressure to evaluate the potential risk of hundreds to thousands of chemicals based on limited toxicity data. Therefore, scientifically sound models are needed to predict toxicity without sole reliance on conventional whole-organism (vertebrate) bioassays. To help address this challenge, adverse outcome pathways (AOPs) have recently been proposed as conceptual tools to help link direct, molecular-level initiating events to adverse outcomes at higher levels of biological organization. Additionally, using AOPs as frameworks, high-throughput screening (HTS) and high-content screening (HCS) assays have been recently proposed as key components of a tiered regulatory toxicity testing strategy that enables screening and prioritization of chemicals for further testing using conventional bioassays. Therefore, using zebrafish embryonic development as a model, the overall research aims of this dissertation were to 1) rely on paraoxon - the active metabolite of the organophosphorus (OP) insecticide parathion - as a reference acetylcholinesterase (AChE) inhibitor to examine whether the common molecular initiating event (AChE inhibition) was associated with adverse neurological effects at higher levels of biological organization and 2) develop and optimize a HCS assay to identify chemicals impacting cardiovascular function and potentially serve as a platform for guiding AOP discovery and development. Data presented within Chapter 1 suggests that (1) normal AChE activity is not required for secondary motoneuron development and (2) spontaneous tail contractions at 26 hours postfertilization (hpf) are sensitive to paraoxon exposure, an effect that may be independent of AChE inhibition. Data presented within Chapter 2 suggests that, compared to existing zebrafish-based assays, our HCS assay provides a comprehensive chemical screening and AOP discovery platform with 1) increased sample sizes; 2) broad concentration-response format; and 3) the ability to identify chemicals that target cardiovascular function at non-teratogenic concentrations. Overall, this dissertation highlights the scientific and logistical challenges in developing quantitative AOPs to support chemical screening and prioritization strategies. Nonetheless, qualitative AOPs can play a role in identification of HTS/HCS assays that capture key events along an AOP, and results of HTS/HCS-based chemical screens can be used to facilitate development of current and new AOPs.


© 2013, Krystle Yozzo

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