There is increasing evidence that a rise in the prevalence of mental disorders cannot solely be attributed to increased awareness, changing diagnostic criteria, or improvements in technology. Weight of evidence from experimental animal studies and epidemiological human studies suggest that developmental exposure to environmental contaminants is a risk factor for neurodevelopmental disorders. In spite of an increasing need for toxicological data, the low throughput nature of animal experimentation, coupled with the complexity and diversity of developmental neurotoxicity endpoints, has been limiting. The integration of higher-throughput tiered testing paradigm, with prioritization of chemicals for testing followed by identification of the causal relationships between molecular events and adverse developmental outcomes, has been proposed as a solution. However, standardization of endpoints and identification of molecular targets and pathways is still ongoing. The ryanodine receptor (RyR), a sarco/endoplasmic calcium channel, has been implicated as a converging target for environmental contaminants, in particular persistent organic pollutants (POPs) implicated in developmental neurotoxicity, such as the insulators polychlorinated bipheyls (PCBs) or the flame retardants polybrominated diphenyl ethers (PBDEs). This dissertation details the use of higher-throughput in vitro methodologies in quantifying neuronal connectivity through biochemical, morphometric, and functional endpoints and demonstrates that PBDEs can inhibit axon length via a mechanism that requires the RyR.
The development of higher throughput approaches for assessing neuronal connectivity will expedite toxicological screening of neurotoxic compounds. As many neurodevelopmental disorders are thought to reflect altered patterns of synaptic connectivity, including imbalances between excitatory and inhibitory synapses, ontogenetic patterns of in vitro synapse formation in primary hippocampal and cortical cultures were assessed with a combination of high-content imaging and multi-electrode array analysis. Results demonstrated that despite sharing similar synaptophysin expression levels during maturation, hippocampal cultures possessed higher numbers of excitatory and inhibitory synapses but lower firing rates, potentially due to cortical neurons possessing decreased synaptic tone.
PBDEs are widely used flame retardants that persist in the environment and have been implicated in developmental neurotoxicity. Like non-dioxin-like PCBs (NDL-PCBs), PBDEs can sensitize the RyR and disrupt calcium homeostasis. However, results demonstrated that unlike NDL-PCBs, which aberrantly increase dendritic arborization, PBDEs selectivity reduces axon length and delays neuronal polarization through a mechanism that is dependent on RyR. In addition, triiodothyronine or antioxidant supplementation reverses these reductions, suggesting a potential model wherein PBDEs disrupt multiple pathways with RyR as a key intermediary. The studies presented in this dissertation advance our understanding of methodologies useful for measuring neuronal connectivity and provide further mechanistic understanding into the role of RyR in POP developmental neurotoxicity.
|Advisor:||Lein, Pamela J.|
|Commitee:||Pessah, Isaac N., Zito, Karen|
|School:||University of California, Davis|
|Department:||Pharmacology and Toxicology|
|School Location:||United States -- California|
|Source:||DAI-B 78/10(E), Dissertation Abstracts International|
|Subjects:||Neurosciences, Toxicology, Surgery, Environmental science|
|Keywords:||Developmental neurotoxicology, Persistent organic pollutants, Polybrominated diphenyl ethers, Ryanodine receptor|
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