Induced metamorphosis in Ambystoma mexicanum: Lack of effect of triiodothyronine on tissue or blood levels of thyrotropin-releasing hormone (TRH)

In order to test the hypothesis that thyroid hormone affects thyrotropin-releasing hormone (TRH) during metamorphosis, the latter was induced in the facultative neotenic amphibian, Ambystoma mexicanum, by injection of triiodothyronine (T 3). Plasma levels of T 3 were readily detectable up to 2 weeks after injection. Immunoreactive TRH was found in large quantities in the hypothalamus and brain, in much smaller amounts in the pituitary, pineal, and skin, and was only occasionally detectable in blood. No changes in tissue or blood levels of TRH were found during metamorphosis after treatment with T 3, suggesting that any hypothalamic action of thyroid hormone during metamorphosis does not involve TRH in this species.