Comparative analysis of the expression and localization of prolactin 1A and 1B in the pituitary gland of the red-bellied newt Cynops pyrrhogaster.

The authentic prolactin (PRL) designated PRL1 is a multifunctional hormone secreted by the anterior pituitary gland of vertebrates. The PRL1 gene exists in two forms: fish-type PRL1 gene (PRL1B) and tetrapod-type PRL1 gene (PRL1A). It has been proposed that PRL1B gene underwent duplication during the early amphibian lineage. Subsequently, the ancestral gene (PRL1B) was lost in amniotes, while the newly duplicated gene (PRL1A) was retained. Notably, only amphibians possess both PRL1B and PRL1A genes. In the anuran amphibian bullfrog, PRL1A gene expression is low during the pre- and prometamorphic stages, gradually increasing toward the end of the metamorphic climax stage. In contrast, PRL1B gene expression is relatively high during the early metamorphic stages but significantly declines during the metamorphic climax stage and becomes undetectable after completion of metamorphosis. These differential expression patterns suggest a functional transition of PRL1 paralogs associated with the shift from aquatic to terrestrial life. However, data on PRL1B expression in urodele amphibians remain limited. In this study, we cloned the PRL1B cDNA from the urodele amphibian, the red-bellied newt (Cynops pyrrhogaster) and analyzed its expression in the pituitary gland. Unlike in anuran bullfrog, PRL1B is expressed as the transcript and protein in the adult pituitary of this urodele species, suggesting that the regulatory mechanism controlling PRL1B expression in urodeles differs from that in anurans. This is the first report demonstrating the existence of PRL1B in urodeles in the form of protein as well as mRNA.