Preliminary Assessment of the Cane Toad (Bufo marinus) as a Model for a Bioassay for Thyroid Axis Disruption

The amphibian metamorphosis assay (AMA) is a bioassay for assessing chemical agents for their potential to disrupt the hypothalamic-pituitary-thyroid axis, and is founded on the dependence of anuran metamorphosis on this endocrine pathway. The commonly used model animal for the AMA is the African clawed frog, Xenopus laevis, although other anuran species have been used in adaptations and modifications of this bioassay. The very cosmopolitan and invasive cane toad, Bufo marinus, is not among those species whose potential as a model for the AMA has been investigated. In light of this gap, the current study aimed to assess the utility of B. marinus for an alternative bioassay, with the ultimate aim of providing baseline for the development of a cane toad-based, fairly inexpensive adaptation of the AMA. Acclimated Gosner stage 33 and 37 tadpoles were exposed for seven days to normal tapwater or 25, 50, and 100 μg/mL propylthiouracil (PTU) for thyroid hormone (TH) inhibition. Trends in developmental stage, weight, total length, hindlimb length, and forelimb length across treatments after 7 days revealed significantly retarded metamorphosis in PTU-exposed cane toad tadpoles compared to the normal tadpoles, suggesting the potential of B. marinus as an alternative model for the metamorphosis assay. Results also point to 50 μg/mL as a recommended concentration of PTU appropriate for use as an inhibitory control in a B. marinus-based AMA.