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 encodingprolactin (Prl1) in the brainand its receptors (PrlRa and PrlRb) in the brain, kidneys, gills,and intestine of the three-spined stickleback (Gasterosteusaculeatus L.) were studied in the context of freshwateradaptation. Males and females of the marine morph were adapted tofreshwater for 72 h, and Prl1, PrlRa and PrlRb expressionlevels were assessed using RT-qPCR. In seawater, no sex differencesin Prl1 expression in the brainwere detected, but they appeared after freshwater adaptation dueto significantly increased Prl1 expressionin females. In seawater, PrlRa expressionin the brain of females was significantly lower compared to males,but increased significantly after freshwater adaptation, becomingcomparable in both sexes. PrlRb expressionin the brain increased significantly in females following freshwateradaptation. In the kidneys, PrlRa expressionwas indistinguishable between females and males in either seawateror freshwater, but significantly decreased after freshwater adaptationin both sexes. PrlRb expressionin the kidneys was sex-independent in seawater, but freshwater adaptationled to its significant increase in females and a decrease in males.In the gills, PrlRa and PrlRb expression was independentof both sex and salinity. In the intestine, PrlRa expressiondid not depend on either sex or salinity, whereas PrlRb gene expression also showedno sex differences, while decreasing in both sexes after transitioningto freshwater. It is concluded that the dependence of the prolactinosmoregulatory function on the sex of sticklebacks is manifestedin sex differences in prolactin gene expression in the brain, aswell as in the sensitization of the brain and osmoregulatory organsto this hormone during freshwater adaptation.