Alteration of leopard frog (Rana pipiens) metamorphosis by the herbicide acetochlor

Based on the geographic correlation between the use of the pre-emergent her bicide acetochlor [2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methylphenyl) ace tamide] and the natural range of Northern leopard frogs (Rana pipiens), we investigated the effects of acetochlor (ACETO) on frog metamorphosis. We sp ecifically examined the interaction of ACETO with thyroid hormone (T-3) and corticosterone (CORT), hormones that regulate natural metamorphosis. ACETO , T-3, and CORT were administered via immersion. Growth, developmental stag e, and onset of metamorphic climax (forelimb emergence, FLE) were measured. We examined three hypotheses: (1) ACETO may alter metamorphosis. Premetamo rphic tadpoles with low endogenous T-3 were exposed to ACETO +/- 10(-9) M T -3 for 7 days. 67% of tadpoles exposed to ACETO + T-3 attained FLE, while 0 % of T-3 treated animals did. (2) ACETO mimics T-3 action at the thyroid re ceptor (TR). Tadpoles were pretreated with T-3 for 3 days to induce TR expr ession, then treated for 7 days with vehicle (DMSO), T-3, Or ACETO + T-3. A CETO treatment after T-3 priming did not accelerate FLE, suggesting that AC ETO does not interact directly with the TR. Cotreatment with ACETO + T-3 af ter T-3 priming accelerated FLE relative to tadpoles primed with T-3, then treated with T-3. Because the ACETO + T-3 acceleration of FLE appeared simi lar to the effect of CORT, we examined a third hypothesis: (3) ACETO may in teract with CORT to accelerate FLE. Premetamorphic tadpoles were exposed to various doses of ACETO + T-3 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 eff ects of CORT when Tg was present. ACETO consistently accelerated T-3-induce d metamorphosis, apparently interacting with T-3 via a non-TR-mediated mech anism.