Effects of 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin (TCDD) on development of anuran amphibians

Abstract

We exposed anuran eggs and tadpoles to vehicle control (0.7% acetone) or waterborne [3H]2,3,7,8‐tetrachlorodibenzo‐p‐dioxin (TCDD) for 24 h (American toad, 0.003‐30 μg/L; leopard frog, 3 μg/L; green frog, 0.3‐100 μg/L) and subsequently raised them in clean water. Neither American toads nor green frogs exhibited TCDD‐related mortality, but leopard frogs showed significantly increased (10%) mortality over controls. Eggs and tadpoles eliminated TCDD relatively quickly compared with published data for other vertebrates, with t1/2 of 1 to 5 d (American toad), 2 to 7 d (leopard frog), and 4 to 6 d (green frog). Elimination rates were slowest for tadpoles fed nothing, fastest for those fed a low‐fat diet, and intermediate for those fed a high‐fat diet. Although not significant, American toads exposed to ⩾0.03 μg TCDD/L appeared to metamorphose earlier, and those exposed to higher TCDD treatments appeared to metamorphose at a larger body mass than controls. Comparisons of these results with studies of fish early life stages suggest that anuran eggs and tadpoles eliminate TCDD more rapidly and are 100‐ to 1,000‐fold less sensitive to its deleterious effects during development. These differences may be related to differences in metabolic rate, patterns of lipid storage and utilization, and aryl hydrocarbon receptor binding and signal transduction.

An Aryl Hydrocarbon Receptor from the Salamander Ambystoma mexicanum Exhibits Low Sensitivity to 2,3,7,8-Tetrachlorodibenzo-p-dioxin

“…Frogs and toads (order Anura) are substantially insensitive to the toxic effects of TCDD 22, 23 . Although rapid elimination contributes to TCDD insensitivity following the onset of feeding in tadpoles 23, 66 , the low TCDD affinity of Xenopus laevis AHRs is well established and likely plays a substantial mechanistic role in all life stages 25 .…”

“…Although rapid elimination contributes to TCDD insensitivity following the onset of feeding in tadpoles 23, 66 , the low TCDD affinity of Xenopus laevis AHRs is well established and likely plays a substantial mechanistic role in all life stages 25 . Low TCDD affinity is attributed to three specific amino acid residues bearing side chains that are modeled to protrude into the ligand binding pocket 26 .…”

“…Low dioxin affinity results from a unique combination of specific residues within the LBD at the same positions shown to govern variations in TCDD affinity in mammal and bird AHRs 26 . It is hypothesized that low TCDD affinity is a property shared with other frog and toad AHRs 23 , but the AHR binding properties and TCDD sensitivities of other amphibian groups are not well understood.…”

Responsiveness of a Xenopus laevis cell line to the aryl hydrocarbon receptor ligands 6-formylindolo[3,2-b]carbazole (FICZ) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

“…As a group, frogs are remarkably insensitive to the toxic effects of dioxin-like compounds [10]. Recent studies in the African clawed frog, Xenopus laevis , demonstrate that the AHRs bind TCDD with low affinity, at least 25-fold lower than the high-affinity mouse AHR b-1 [11], a property that likely underlies the lack of TCDD toxicity.…”

Cytochrome P450 1 Genes in Birds: Evolutionary Relationships and Transcription Profiles in Chicken and Japanese Quail Embryos

“…Features of the AHR may explain differences in sensitivity to dioxin-like compounds in certain mouse strains [46], and in birds, where for instance turkey and quail are much less sensitive than chicken [19], [47]. Frogs show a low sensitivity to dioxin toxicity and have an AHR with low dioxin affinity [48], [49]. The differing sensitivities to dioxin of mouse strains were shown to be due to differences in specific amino acid residues in the AHR ligand binding domain [46].…”