Triiodothyronine (T3) action on aquatic locomotor behavior during metamorphosis of the bullfrog Rana catesbeiana
Author
Fernández-Mongil, Marisabel
Venza, Celia J.
Rivera, Amelia
Lasalde-Dominicci, José A.
Burggren, Warren
Rojas, Legier V.
Type
ArticleDate
2009Metadata
Show full item recordAbstract
Thyroid hormones - particularly triiodothyronine, T3 - play a critical role in the morphological
transformations comprising metamorphosis in larval bullfrogs (Rana catesbeiana). Traditional
staging criteria for anuran larvae incompletely distinguish physiological and behavioral changes
during growth. We therefore first developed a new parameter to describe larval growth, the
developmental index (DI), which is simply the ratio between the tail length of the larva and its head
diameter. Using the DI we were able to identify two distinct populations classifying the larvae during
growth along a continuous linear scale with a cutoff value of DI at 2.8. Classification based on the
DI, used in this study, proved an effective complement to existing classifications based on
developmental staging into pre- or pro-metamorphic stages. Exposure to T3 in the water induced a
rapid (beginning within 5 min) and significant decrease (~20–40%) in locomotor activity, measured
as total distance traversed and velocity. The largest decrease occurred in more developed larvae (DI<
2.8). To determine correlated changes in the neuromuscular junctions during metamorphosis and
apoptotic tail loss, miniature endplate currents from tail muscle were recorded during acute exposure
to a hypertonic solution, which simulates an apoptotic volume decrease. Our results support a role
for T3 in regulating larval locomotor activity during development, and suggest an enhanced response
to volume depletion at the neuromuscular junction of older larvae (DI<2.8) compared to younger
animals (DI≥2.8). We discuss the significance of the possible role of an apoptotic volume decrease
at the level of the neuromuscular junction.