Molecular cloning of bullfrog prolactin receptor cDNA: Changes in prolactin receptor mRNA level during metamorphosis

In amphibian larvae, prolactin (PRL) is known to possess growth-promoting and anti-metamorphic activities. For further understanding of the role of PRL in larvae, bullfrog PRL receptor (bfPRLR) cDNA was obtained from the tail fin of premetamorphic tadpoles by use of the reverse transcription-polymerase chain reaction (RT-PCR) coupled with 5^′- and 3^′- rapid amplification of cDNA ends (RACE). The predicted bfPRLR was composed of 617 amino acids, contained a single transmembrane domain, and showed 33-57% sequence homologies with known sequences of vertebrate PRLRs. When bfPRLR was transiently expressed, specific binding of ¹²⁵I-labeled bullfrog PRL (bfPRL) was observed. By Northern blot analysis, a 3-kb transcript was detected in the tail fin. By RT-PCR bfPRLR mRNA expression was detected mainly in the brain, kidney, skin, and tail throughout prometamorphic and middle climactic periods. The results of an RNase protection assay revealed that the bfPRLR mRNA level in the tail fin increased around the onset of climax (stage XX) and was maintained at a relatively high value at least until mid-climax (stage XXII). It also revealed that bfPRLR mRNA level in the kidney of larvae gradually rose as metamorphosis progressed. The results support the view that PRL in larval period acts not only on the larval organs but also on the organs that are necessary for the adult life to maintain or develop their structures and functions.