Prolactin—thyroid interaction was analysed in the second metamorphosis of Diemictylus viridescens. Intraperitoneal injections of either 3 μg thyroxine (T4) or 0·2 mg propylthiouracil (PTU) on alternate days failed to induce second metamorphosis in the red eft (land stage); 0·1 mg PTU inhibited prolactin-induced second metamorphosis. The effect of prolactin was facilitated by 0·1 μg T4. However, 0·5 μg T4 inhibited water-drive behaviour after initial stimulation of integumentary changes and water-drive. Larger doses of T4 (1–3 μg) proved fatal to the efts when given together with 0·5–40 μg prolactin.
The results indicate that a certain optimal level of thyroid hormone is needed for the initiation of second metamorphosis. Nevertheless, prolactin—thyroid antagonism, previously reported in tadpoles, also occurs in the newt. Second metamorphosis appears to be brought about by a reversal of the low prolactin—high thyroid level of the eft phase to a high prolactin—low thyroid level.
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