Evaluation of the effect of acetochlor on thyroid hormone receptor gene expression in the brain and behavior of Rana catesbeiana tadpoles

• Published in: Aquatic toxicology Vol. 80; no. 1; pp. 42 - 51
• Main Authors: Helbing, Caren C., Ovaska, Kristiina, Ji, Lan
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 25.10.2006; Elsevier Science
• Subjects: Acetochlor; Amphibia. Reptilia; Amphibian metamorphosis; animal behavior; animal development; Animal, plant and microbial ecology; Animals; Applied ecology; Behavior; Behavior, Animal - drug effects; Biological and medical sciences; Brain; Brain - drug effects; Brain - metabolism; DNA Primers - chemistry; Ecotoxicology, biological effects of pollution; Freshwater; frogs; Fundamental and applied biological sciences. Psychology; gene expression; Gene Expression - drug effects; General aspects; Herbicides - toxicity; hormone receptors; Larva - drug effects; polymerase chain reaction; Quantitative real-time polymerase chain reaction; Rana catesbeiana; Rana catesbeiana - physiology; Random Allocation; Receptors, Thyroid Hormone - biosynthesis; Receptors, Thyroid Hormone - drug effects; Receptors, Thyroid Hormone - genetics; Reverse Transcriptase Polymerase Chain Reaction - veterinary; tadpoles; thyroid hormones; Tissue biopsy; Toluidines - toxicity; Triiodothyronine - pharmacology; Vertebrates: general zoology, morphology, phylogeny, systematics, cytogenetics, geographical distribution; Acetochlor; Brain; Tissue biopsy; Behavior; Quantitative real-time polymerase chain reaction; Amphibian metamorphosis; Ecotoxicology; Pesticides; Amphibia; Salientia; Gene expression; Herbicide; Polymerase chain reaction; Aquatic environment; Vertebrata; Larva; Development; Thyroid hormone; Organic amide; Ranidae; Developmental stage; Metamorphosis; Rana catesbeiana
• ISSN: 0166-445X; 1879-1514
• DOI: 10.1016/j.aquatox.2006.07.011

The thyroid hormones (THs) including 3,5,3′-triiodothyronine (T 3), are important regulators of growth and development of the brain in vertebrates. Previous studies showed that acetochlor, a widely used herbicide, accelerates T 3-induced frog tadpole metamorphosis and elevates the T 3-dependent accumulation of the mRNAs encoding the TH receptors, TRα and β, in the tail. Here we show that acetochlor affects the expression of these TR isoforms in the brain of Rana catesbeiana tadpoles. Premetamorphic tadpoles exposed to 10 nM acetochlor with and without 100 nM T 3 for 4 days showed substantial increases in TRα and TRβ transcript levels and significant decreases in the TRα/TRβ ratios in their brains. This change in TR ratios is recapitulated with 10 nM acetochlor in R. catesbeiana tadpole brains during prometamorphosis, a period in which THs are endogenously produced. Tail fin biopsies revealed an elevation in TRα and β mRNA levels compared to control animals when exposed to 1 and 10 nM acetochlor for 6 days. When subsequently reared in clean water for 59 days, no alterations in metamorphic hallmarks (forelimb emergence, mouth development, tail regression) were noted compared to the controls. Since alterations in TR ratios/levels may impact brain development, we tested the escape behavior in premetamorphic tadpoles exposed to 10 nM acetochlor for 4 days. We did not detect any statistically significant differences that would indicate that acetochlor affects escape behavior. However, since the gene expression data suggest that brain function may be affected, additional studies examining different behaviors upon acetochlor exposure at environmentally-relevant concentrations are warranted.