Interdependence of corticosterone and thyroid hormones in larval toads (Bufo boreas). I. Thyroid hormone‐dependent and independent effects of corticosterone on growth and development

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Abstract
In a previous study (Hayes et al. [1993] J. Exp. Zool., 266: 206–215), we demonstrated that exogenous corticosterone (B) inhibited growth, and had varied effects on development and metamorphosis in the toad(Bufo boreas). The current study determined the relation between the action (length and height), rear leg growth and differentiation, and foreleg emergence (FLE). Thiourea (Thio; a goitrogen) and metyrapone (MTP; a glucocorticoid synthesis inhibitor) were used to determine the role of endogenous hormones in growth and development. These inhibitors were also used in various combinations with the thyroid hormones, thyroxine (T4) and triiodothyronine (T3), to determine the extent to which B's actions depend on the thyroid hormones. B was ineffective at inducing tail reduction (length and height) in the presence of Thio, but B enhanced the effects of both thyroid hormones, suggesting that the action of B on the tail were dependent on thyroid hormones. B inhibited body growth even in the presence of Thio, but did not enhance thyroid hormone's inhibition of growth. B alone stimulated foreleg emergence (FLE) and enhanced thyroid hormone's activity on FLE when B and the thyroid hormones were given in combination, but did not induce FLE in the presence of Thio. B stimulated rear leg development, but not in the presence of Thio, suggesting that this effect was due to interactions with thyroid hormones. Furthermore, MTP antagonized the stimulatory effect of T4 on rear leg development, suggesting that endogenous B also interacted with exogenous thyroid hormones. While the thyroid hormones stimulated rear leg growth, B inhibited rear leg growth and antagonized the effects of the thyroid hormones when the hormones were given together. This study demonstrated that while some of the effects of B (such as the stimulation of tail resorption) depend on endogenous thyroid hormones, other effects (such as the inhibition of rear leg growth) are antagonistic to the thyroid hormones.