Expression of Prolactin Axis Genes in the Three-Spined Stickleback G. aculeatus L.: Sex Dependence and Freshwater Adaptation

Sex-related differences in the expression of genes encoding prolactin ( Prl1 ) in the brain and its receptors ( PrlRa and PrlRb ) in the brain, kidneys, gills, and intestine of the three-spined stickleback ( Gasterosteus aculeatus L.) were studied in the context of freshwater adaptation. Males and females of the marine morph were adapted to freshwater for 72 h, and Prl1 , PrlRa and PrlRb expression levels were assessed using RT-qPCR. In seawater, no sex differences in Prl1 expression in the brain were detected, but they appeared after freshwater adaptation due to significantly increased Prl1 expression in females. In seawater, PrlRa expression in the brain of females was significantly lower compared to males, but increased significantly after freshwater adaptation, becoming comparable in both sexes. PrlRb expression in the brain increased significantly in females following freshwater adaptation. In the kidneys, PrlRa expression was indistinguishable between females and males in either seawater or freshwater, but significantly decreased after freshwater adaptation in both sexes. PrlRb expression in the kidneys was sex-independent in seawater, but freshwater adaptation led to its significant increase in females and a decrease in males. In the gills, PrlRa and PrlRb expression was independent of both sex and salinity. In the intestine, PrlRa expression did not depend on either sex or salinity, whereas PrlRb gene expression also showed no sex differences, while decreasing in both sexes after transitioning to freshwater. It is concluded that the dependence of the prolactin osmoregulatory function on the sex of sticklebacks is manifested in sex differences in prolactin gene expression in the brain, as well as in the sensitization of the brain and osmoregulatory organs to this hormone during freshwater adaptation.