INTERDEPENDENCE OF CORTICOSTERONE-HORMONES AND THYROID-HORMONES IN TOAD LARVAE (BUFO-BOREAS) .2. REGULATION OF CORTICOSTERONE-HORMONES AND THYROID-HORMONES

Typically, the role of corticosterone(B) in metamorphosis is considered secondary to that of thyroid hormone, with B having only enhancing effects. In the current study, we demonstrate that the relationship between the thyroid hormones and B is much more complex and that thyroxine (T-4) may depend on B for some of its functions. Tadpoles of the western toad (Bufo boreas) were treated with various combinations of corticosterone (B), thyroxine (T-4), triiodothyronine (T-3), a goitrogen (thiourea; Thio), and a corticoid synthesis inhibitor (metyrapone; MTP). Hormones were extracted from individual tadpoles and whole-body hormone levels determined by radioimmunoassay. B-treatment decreased the ratio of T-4 to T-3, suggesting that B increased the conversion of endogenous T-4 to T-3. In addition, B-treatment in combination with T-4 resulted in high whole body-levels of T-3. B also caused a decrease in whole body-thyroid hormone levels (T-4 and T-3), suggesting negative feedback on the hypothalamo-pituitary-thyroid axis and T-3 had a similar effect, decreasing whole body-T-4 levels. T-4-treatment, but not T-3, increased whole body-B levels and MTP-treatment in combination with T-4 prevented the stimulatory effect of T-4 on B production. MTP-treatment alone blocked all steroid metabolism of [H-3]progesterone by the interrenal in vitro, and lowered whole body-B levels three-fold in vivo. Thio-treatment reduced thyroid hormone levels and also resulted in decreased B. Finally, we suggest that these results demonstrate a system in which T-4 may regulate its own potency: increasing T-4 stimulates B production, which increases the conversion of T-4 to its more active form T-3. Rising T-3 levels then cause a decrease in T-4, removing the stimulatory effect of T-4 on B. These interactions between the thyroid hormones and B help clarify the mechanisms underlying the morphological effects of these hormones described in the previous study. (C) 1995 Wiley-Liss, Inc.