Amino acid sequences for identified prolactin (PRL)-releasing peptides (PrRPs) were conserved in mammals (>90%) or teleost fishes (100%), but there were considerable differences between these classes in the sequence (<65%) as well as in the role of PrRP. In species other than fishes and mammals, we have identified frog PrRP. The cDNA encoding Xenopus laevis prepro-PrRP, which can generate putative PrRPs, was cloned and sequenced. Sequences for the coding region showed higher identity with teleost PrRPs than mammalian homologues, but suggested the occurrence of putative PrRPs of 20 and 31 residues as in mammals. The amino acid sequence of PrRP20 was only one residue different from teleost PrRP20, but shared 70% identity with mammalian PrRP20s. In primary cultures of bullfrog (Rana catesbeiana) pituitary cells, Xenopus PrRPs increased prolactin concentrations in culture medium to 130-160% of the control, but PrRPs was much less potent than thyrotropin-releasing hormone (TRH) causing a three- to four-fold increase in prolactin concentrations. PrRP mRNA levels in the developing Xenopus brain peak in early prometamorphosis, different from prolactin levels. PrRP may not be a major prolactin-releasing factor (PRF), at least in adult frogs, as in mammals.
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