Prolactin prevents the autoinduction of thyroid hormone receptor mRNAs during amphibian metamorphosis.

We have recently reported that prolactin (PRL) inhibits both morphogenesis and cell death in thyroid hormone (T3)-induced amphibian metamorphosis (Tata et al., 1991), and that the autoinduction of T3 receptor (TR alpha and beta) mRNA is among the most rapid responses of premetamorphic Xenopus tadpoles to T3 (Kawahara et al., 1991). We now demonstrate that PRL prevents the rapid T3-induced upregulation of TR alpha and beta mRNAs in stages 50-54 Xenopus tadpoles and in organ cultures of tadpole tails. This effect is followed by the inhibition of the de novo activation of 63-kDa keratin gene by T3. We present an experimentally testable model whereby PRL exerts its juvenilizing action by preventing the amplification of TR by its autoinduction by T3.