Prolactin-releasing peptide, a possible modulator of prolactin in the euryhaline silver sea bream (Sparus sarba): A molecular study

PRL and PrRP cDNAs have been isolated from euryhaline silver sea bream (Sparus sarba). The PRL cDNA consists of 1360 bp encoding 212 amino acids whereas the PrRP cDNA contains 631 bp encoding preproPrRP with 122 amino acids. The mature PrRP sequence within the preprohormone is identical to the PrRPs isolated from other fish species. PRL mRNA was uniquely expressed in sea bream pituitary but PrRP mRNA was expressed in a variety of organs and tissues including the intestines, olfactory rosette and various brain regions such as hypothalamus and pituitary. Expression levels of PRL and PrRP mRNA have been examined in sea bream adapted to different salinities (0, 6, 12, 33 and 50 ppt). In the pituitary, both PRL and PrRP mRNA were significantly higher in fish adapted to low salinities (0 and 6 ppt) and the expression profiles of both hormones closely paralleled each other. However, expression of hypothalamic PrRP was significantly higher in fish adapted to iso-osmotic salinity (12 ppt) when pituitary PRL expression was low. The present study demonstrates, for the first time, a synchronized mRNA expression pattern between PRL and PrRP in fish pituitary but a disparity of mRNA expression levels between hypothalamic PrRP and pituitary PRL during salinity adaptation. These data suggest that PrRP may possibly act as a local modulator in pituitary rather than a hypothalamic factor for regulation of pituitary PRL expression in silver sea bream.