Abstract
Amphibian metamorphosis is an excellent example of hormone-dependent control of development. Thyroid hormones (THs) regulate almost all aspects of metamorphosis, including brain development and larval neuroendocrine function. Sex steroids are also important for early brain function, although little is known about interactions between the two hormonal systems. In the present study, we established brain developmental profiles for thyroid hormone receptors (tralpha and trbeta), deiodinases (dio1, dio2 and dio3), aromatase (cyp19) mRNA and activity, oestrogen receptors (eralpha and erbeta), androgen receptor (ar) and 5α-reductases (srd5alpha1 and srd5alpha2) mRNA during Silurana (Xenopus) tropicalis metamorphosis. Real-time reverse transcriptase-polymerase chain reaction analyses revealed that all of the genes were expressed in the brain and for most of the genes expression increased during development, with the exception of dio2, srd5alpha1 and srd5alpha2. The ability of premetamorphic tadpoles to respond to exogenous THs was used to investigate the regulation of TH- and sex steroid-related genes in the brain during development. Exposure of premetamorphic tadpoles to triiodothyronine (T3; 0, 0.5, 5 and 50 nm) for 48 h resulted in concentration-dependent increases in trbeta, dio2, dio3, eralpha and erbeta. Expression of srd5alpha2 showed large increases (six- to 7.5-fold) for all three concentrations of T3. No changes were detected in dio1, ar and cyp19 transcript levels; however, cyp19 activity increased significantly at 50 nm T3. The results obtained suggest that expression of TH-related genes and er during development could be regulated by rising levels of THs, as previously documented in Lithobates (Rana) pipiens. The positive regulation of srd5alpha by T3 in the brain suggests that endogenous TH levels help maintain or control the rate at which srd5alpha mRNA levels decrease as metamorphosis progresses. Finally, we have identified sex steroid-related genes that are responsive to T3, providing additional evidence of crosstalk between THs and sex steroids in the tadpole brain.
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