Osmoregulation and Reproduction: Evolutionary Trends in Prolactin Functions from Fish to Mammals

The study of prolactin function evolution provides key insights into the diverse effects of this hormone in mammals, both in health and disease, which is relevant from both theoretical and practical perspectives. This article reviews both original and literature data concerning the role of prolactin and its receptors in regulating the sexual dimorphism of freshwater adaptation in the three-spined stickleback Gasterosteus aculeatus L. The mRNA (messenger RNA) expression of prolactin 1 gene (one of two prolactin paralogs) and of its receptor PRLRA (prolactin receptor A) in the brain increase only in female sticklebacks upon transition to fresh water. The brain and kidney of the stickleback are androgen-dependent organs characterized by sex-dependent expression of Prlra (prolactin receptor A gene) under seawater conditions. It is assumed that sex-dependent osmoregulatory effects of prolactin are realized through the PRLRA receptors in these organs. The PRLRB (prolactin receptor B) receptors are expressed in the kidney and brain of sticklebacks under seawater conditions regardless of sex, but the expression of these receptors is more sensitive to decrease in water salinity. Apparently, the PRLRB receptors participate more actively in the implementation of sex-independent osmoregulatory functions of prolactin. The gills and intestine are osmoregulatory organs expressing the PRLRA and PRLRB receptors regardless of sex in both seawater and freshwater. During freshwater adaptation of stickleback females, the expression of prolactin 1 gene Prl1 in the brain and the expression of Atp1a1 (α1a subunit of Na + /K + -ATPase), Nhe3 (sodium-proton antiport gene NHE3), and Ecac (epithelial calcium channel gene) in their gills increase concurrently. It is assumed that these gill genes are under the positive control of prolactin. Exploring the potential for prolactin osmoregulatory function in mammals brings to the idea that this hormone may manifest itself under conditions of certain pathologies accompanied by increased expression of prolactin receptor isoforms in osmoregulatory organs, such as cholestasis in female rats. The latter is accompanied by expression of Prlr (prolactin receptor gene) isoforms’ increase and by the changes in the activity and ratio of Na + /K + -ATPase subunits in the kidneys. It is assumed that the osmoregulatory function of prolactin in fish is sex-dependent; in mammals, it may manifest itself at disrupted water-salt exchange.