Expression of the prolactin receptor (tiPRL-R) gene in tilapia Oreochromisniloticus: tissue distribution and cellular localization in osmoregulatoryorgans

The expression of the prolactin receptor (PRL-R) gene has been investigated in various tissues of tilapia (Oreochromis niloticus) reared in fresh or b rackish water. Using a cDNA probe spanning the extracellular domain of the tilapia PRL-R and Northern blot analysis,the presence of tilapia PRL-R mRNA has been confirmed in the osmoregulatory organs and has been detected in o ther tissues, including the skin, the brain, the reproductive organs, and t he two major hematopoietic organs (spleen and head kidney), as well as circ ulating lymphocytes. These findings suggest a conservation of the physiolog ical processes regulated by prolactin throughout the vertebrates, including immunity and central nervous activity. A non-radioactive in situ hybridiza tion procedure has allowed us to detect the expression of the tilapia PRL-R in the branchial chloride cells and the intestinal mucosal layer of fresh water animals, confirming the direct control exerted by prolactin on the wa ter and ionic exchanges in tilapia. In all the tissues examined one unique PRL-R transcript has been detected with a similar size (3.2 kb) whatever th e salinity conditions. Thus, the transcriptional expression of the tilapia PRL-R strongly differs from the complex RNA pattern reported for the higher vertebrates PRL-R and provides an additional argument for the existence of a single PRL-R for both prolactin isoforms in this fish species.