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.