AbstractsBiology & Animal Science

Many physiological responses to dioxin-like compounds (DLCs), including polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are mediated by the aryl-hydrocarbon receptor (AhR). In birds, activation of the AhR stimulates the transcription of cytochrome P4501A (CYP1A) genes, including CYP1A4 and CYP1A5, and ultimately leads to expression of biotransformation enzymes, including ethoxyresorufin-O-deethylase (EROD). It is well established that potencies of different DLCs range over several orders of magnitude. There is also a wide variation among birds in their responsiveness to DLCs both in efficacy and threshold for effects. A molecular basis for this differential sensitivity has been suggested. Specifically, a comparison of the AhR ligand-binding domain (LBD) indicated that key amino acid residues are predictive of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) sensitivity. Based on sequencing of the AhR LBD from numerous avian species a sensitive classification scheme has been proposed (in order of decreasing sensitivity, chicken (type I; sensitive) > Common pheasant (type II; moderately sensitive) > Japanese quail (type III; insensitive)). A series of egg injection studies with White-leghorn chicken (Gallus gallus domesticus), Common pheasant (Phasianus colchicus) and Japanese quail (Coturnix japonica) were performed to determine whether molecular and biochemical markers of exposure to DLCs are predictive of the proposed classification scheme. In addition, I was interested in determining whether this classification scheme applies to other DLCs, specifically dibenzofurans. Determining which species are "chicken- like", "pheasant-like" and "quail-like" in their responses to DLCs should allow more refined risk assessments to be conducted as there would be less uncertainty about the potential effects of DLCs in those species for which population-level studies do not exist. Several concentrations of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,4,7,8-pentachlorodibenzofuran (PeCDF), or 2,3,7,8-tetrachlorodibenzofuran (TCDF) (triolein vehicle) were injected into the air cells of Japanese quail, Common pheasant and chicken eggs. Liver from 14 d post-hatch chicks was harvested for analysis of CYP1A4 and CYP1A5 mRNA abundance by quantitative real-time PCR (Q-PCR), and EROD activity. Lowest observed effective concentration (LOEC) and relative potency (ReP) values for CYP1A mRNA abundance and EROD activity were determined and used to make comparisons of sensitivity between each species and DLC potency within each species. The TCDD is widely considered to be the most potent DLC and this is supported by the rank order of LOEC values for CYP1A5 mRNA abundance in White-leghorn chicken (TCDD > PeCDF > TCDF). CYP1A4 mRNA abundance and EROD activity in White-leghorn chicken were significantly increased in the lowest dose exposure groups of each of the three DLCs, so the potency of these compounds based on these endpoints was not established. Interestingly, TCDD was not the most potent DLC in…