Developmental regulation and estrogenic endocrine disruption of metamorphosis in the Northern leopard frog, Rana pipens

Amphibian metamorphosis is a dramatic larvae-to-adult transformation regulated by temporal changes in thyroid hormones (TH) as well as interactions with other hormone systems. This developmental process is extremely responsive to changes in environmental conditions and susceptible to aquatic contaminants that interfere with hormone-dependent processes. While many of these endocrine disrupting compounds (EDCs) have been studied for their estrogenic effects on reproduction, the plasticity of amphibian metamorphosis provides a unique model for examining the thyroid-mediated effects of developmental exposure to estrogenic EDCs.;The first objective of this thesis was to assess the toxicological response and developmental effects of estrogenic EDCs during the larval life-stages of amphibian development. After establishing baseline toxicity data for the test species, Rana pipiens, a developmental exposure was conducted to determine the estrogen-sensitivity of distinct periods during metamorphosis. Stage-specific exposures altered tadpole growth parameters and resulted in lasting effects such as delayed metamorphosis and female-biased sex ratios. Therefore, the second objective was to identify potential mechanisms by which xenoestrogens alter TH-dependent metamorphosis using a targeted gene expression approach. Development of multiplex and simplex real-time PCR assays and subsequent analysis established that thyroid- and estrogen-responsive transcripts in the tadpole brain are differentially regulated by exogenous TH and upon exposure to an estrogenic EDC. A short-term "challenge" assay confirmed that estrogenic exposure suppresses and in some cases blocks the ability of TH to induce the expression of target genes involved in mediating tissue-specific TH sensitivity during metamorphosis. The results of this research (1) provide evidence of cross-regulation between thyroid and estrogen-regulated genes during amphibian development and (2) indicate that developmental exposure to estrogenic EDCs may affect normal TH-responsiveness necessary for proper timing of metamorphosis.