Alteration of Leopard Frog (Rana pipiens) Metamorphosis by the Herbicide Acetochlor

Abstract.

Based on the geographic correlation between the use of the pre-emergent herbicide acetochlor [2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methylphenyl) acetamide] and the natural range of Northern leopard frogs (Rana pipiens), we investigated the effects of acetochlor (ACETO) on frog metamorphosis. We specifically examined the interaction of ACETO with thyroid hormone (T₃) and corticosterone (CORT), hormones that regulate natural metamorphosis. ACETO, T₃, and CORT were administered via immersion. Growth, developmental stage, and onset of metamorphic climax (forelimb emergence, FLE) were measured. We examined three hypotheses: (1) ACETO may alter metamorphosis. Premetamorphic tadpoles with low endogenous T₃ were exposed to ACETO ± 10⁻⁹ M T₃ for 7 days. 67% of tadpoles exposed to ACETO + T₃ attained FLE, while 0% of T₃ treated animals did. (2) ACETO mimics T₃ action at the thyroid receptor (TR). Tadpoles were pretreated with T₃ for 3 days to induce TR expression, then treated for 7 days with vehicle (DMSO), T₃, or ACETO ± T₃. ACETO treatment after T₃ priming did not accelerate FLE, suggesting that ACETO does not interact directly with the TR. Cotreatment with ACETO + T₃ after T₃ priming accelerated FLE relative to tadpoles primed with T₃, then treated with T₃. Because the ACETO + T₃ acceleration of FLE appeared similar to the effect of CORT, we examined a third hypothesis: (3) ACETO may interact with CORT to accelerate FLE. Premetamorphic tadpoles were exposed to various doses of ACETO ± T₃ in the presence or absence of 10⁻⁷ M CORT. CORT inhibited growth and hindlimb development and delayed FLE. ACETO never inhibited growth or hindlimb development, but ACETO did counteract the effects of CORT when T₃ was present. ACETO consistently accelerated T₃-induced metamorphosis, apparently interacting with T₃ via a non-TR–mediated mechanism.