Developmental origins of neuroendocrine dysfunction associated with an embryonic atrazine exposure
Abstract
During embryonic development, organisms exhibit a high level of developmental plasticity allowing for alterations of the genetic landscape in response to the surrounding environment which can ultimately contribute to disease onset during adulthood. This concept has been defined as the ‘developmental origins of health and adult disease’ (DOHaD) hypothesis. Studies have begun to identify a link between in utero or early developmental chemical exposure to genetic and phenotypic alterations. A major class of chemicals is the endocrine disrupting chemicals (EDCs). EDCs are found in a wide array of products making human exposure a likely event, therefore, increasing the need for research investigating their mechanistic effects. Atrazine is a pre-emergent herbicide used extensively throughout the Midwestern United States as well as other parts of the globe on a variety of agricultural crops. Atrazine frequently contaminates potable water supplies and is a suspected EDC. Studies have investigated the adverse health effects of atrazine exposure and their associated mechanisms during development and adulthood; however, few studies have aimed to address the adverse effects of low environmentally relevant atrazine concentrations in addition to the later-in-life outcomes. The studies in this dissertation were designed to investigate the later-in-life effects from exposure to environmentally relevant concentrations of atrazine using the zebrafish vertebrate model. In addition, an in vitro study was conducted in order to provide novel mechanistic data following an atrazine exposure. Zebrafish embryos were exposed to 0, 0.3, 3, or 30 parts per billion (ppb; µg/L) atrazine from 1-72 hours post fertilization (hpf). Following the dosing period, larvae were rinsed and allowed to mature in clean aquaria water until adulthood. Neurotransmitter analysis revealed a decrease in 5-hydroxyindoleacetic acid (5-HIAA) and serotonin turnover in adult female zebrafish; while no neurotransmitter alterations were observed in adult male zebrafish or in zebrafish larvae. Transcriptomic profiling of adult female brain tissue showed alterations in gene expression associated with nervous system development and function with specific genes targeting the serotonergic system (Chapter 2). As atrazine primarily targets the reproductive system through the hypothalamus-pituitary-gonadal (HPG) axis, reproductive function was assessed. A significant decrease in spawning events was observed in the 30 ppb treatment group. In addition, female zebrafish in the 30 ppb treatment group displayed an increase in follicular atresia and in ovarian progesterone. Furthermore, offspring of the exposed generation (F2) displayed alterations in head-to-body ratios (Chapter 3). Adult male zebrafish did not show any significant outcomes associated with various morphological and histological endpoints on the reproductive system. However, transcriptomic analysis of gonad and brain tissue revealed numerous alterations in gene expression associated with neuroendocrine dysfunction (Chapter 4). As the cellular mechanisms of action elicited by atrazine are still under investigation, genotoxicity through the generation of copy number alterations (CNAs) in a zebrafish fibroblast cell line was assessed. Results demonstrated that atrazine elicits genotoxicity through the generation of CNAs. Furthermore, these chromosomal amplifications and deletions can be linked to previously identified mRNA alterations in adult female and male zebrafish exposed to atrazine during embryogenesis (Chapter 5). Overall, data obtained from these studies show support of neuroendocrine dysfunction later-in-life caused by an embryonic low dose atrazine exposure.
Degree
Ph.D.
Advisors
Freeman, Purdue University.
Subject Area
Toxicology|Surgery
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.