2005 Progress Report: Development of a BBPK Model for the Thyroid Axis in the Pregnant Rat and Fetus for the Dose Response Analysis of Developmental NeurotoxicityEPA Grant Number: R832134
Title: Development of a BBPK Model for the Thyroid Axis in the Pregnant Rat and Fetus for the Dose Response Analysis of Developmental Neurotoxicity
Investigators: Fisher, Jeffrey W. , Ferguson, Duncan , Wagner, John
Institution: University of Georgia
EPA Project Officer: Hahn, Intaek
Project Period: December 1, 2004 through November 30, 2008 (Extended to November 30, 2009)
Project Period Covered by this Report: December 1, 2004 through November 30, 2005
Project Amount: $749,127
RFA: Development and Characterization of Biological Systems for Studying Low Dose Effects of Endocrine Disrupting Chemicals (2004) RFA Text | Recipients Lists
Research Category: Endocrine Disruptors , Health Effects , Economics and Decision Sciences , Health , Safer Chemicals
The objective of this research project is to develop biologically based pharmacokinetic (BBPK) maternal and fetal/neonatal models of the hypothamalic-pituitary-thyroid (HPT) axis in the rat by conducting experimental and computational research. These BBPK models of the HPT will describe the highly nonlinear relationship(s) between administered dose of thyroid active chemical, mode-of-action (MOA), specific perturbations in the HPT axis, and developmental neurotoxicity.
Rats were treated with 0, 3, and 10 ppm propylthiouracil (PTU) in the drinking water from gestation day (GD) 2 through postnatal days (PND) 21-30 when male rat pups were sacrificed, serum collected, and hippocampal slices collected to study the extracellular field excitatory postsynaptic potential (fEPSP) responses from the stratum radiatum layer of the CA1 region of the hippocampus as a measure of synaptic function. Also, tissue samples from hippocampus, cerebral cortex, liver, and thyroid were taken. Male offspring (n = 2) were then sacrificed during each of two time frames, PND 21-30 or PND 90-110. In hippocampal slices obtained from PND 21-31 pups, PTU depressed the stimulus-response curve at treatment doses of 3 and 10 ppm. These doses of PTU induced expected changes in serum thyroid hormones are consistent with previously published reports. Whereas serum total and free T4 concentrations fell and thyroid stimulating hormone (TSH) rose dramatically in response to both doses of PTU, serum T3 concentrations in the serum and notably in the brain remained normal except at the highest dose. The stimulation of Type II 5’-deiodinase enzyme (D2) activity appeared to explain the ability of the central nervous system to maintain tissue T3 concentrations in the face of even a 75 percent decline in serum T4 concentration. In the dams and the PND 25 rat pups, there was a similar decline in serum total and free T4, but, in the dams, serum T3 and cortical tissue T3 were not diminished at the 3 ppm dose, whereas a dose-dependent decline in serum and cortical tissue T3 concentrations was seen in the PND 25 pups. The Type II 5’-deiodinase was already at its maximum at the 3 ppm dose, demonstrating the increased sensitivity of young animals to this dose. These data allowed the development of mathematical relationships between serum T4, cerebrocortical T3, and Type II 5’-deiodinase activity. The tightest correlation with the PND 25 electrophysiological changes was seen with D2 activity, followed by serum T4 concentration, and then cerebrocortical T3 concentration. A tight positive correlation was observed between the cerebrocortical D2 activity with the synaptic response in the hippocampal slices. An increase in cortical D2 activity is widely believed to reflect even mild relative thyroid hormone deficiency. Animals weaned at PND 30 and allowed to mature until GD 90-110 had normal serum thyroid hormone and TSH concentrations, suggesting that they had returned to a euthyroid state, but in the hippocampal slices from these animals, prior PTU exposure did not decrease the stimulus-response relationship but did tend to inhibit long-term potentiation of the fEPSP response. The significance of this finding remains to be determined.
Equations describing the HPT axis were created with a focus on the negative feedback loop. This effort is just getting underway. Chemical specific MOA equations are under development for describing the induction of the hepatic nuclear receptor (Aryl hydrocarbon receptor, AhR) and subsequent phase II metabolism of thyroxine, inhibition of uptake of thyroidal dietary iodide and reduction in thyroid hormone production. These first HPT axis models will be named ‘first generation’ HPT models for developmental neurotoxicity.
Computational efforts to develop HPT axis model(s) for different reproductive states will be undertaken. Dose-response data for HPT perturbation and developmental neurotoxicity will be collected with the prototypical chemical PTU for later use as a standardized probe to be compared with environmental toxicants such as perchlorate and polychlorinated biphenyls.