Estrogen receptor mRNA expression patterns in the liver and ovary of female rainbow trout over a complete reproductive cycle

Estrogens are critical hormones involved in reproduction and need to bind to estrogen receptors in target organs for biological activity. Fishes have two distinct estrogen receptor subtypes, alpha (α) and beta (β), with variable combinations of additional isoforms of each subtype dependent on the history of genome duplication within a taxon. The comparative expression patterns of estrogen receptor isoforms during the female reproductive cycle will provide important insights into the unique function and importance of each. The purpose of this study was to measure the mRNAs for the four estrogen receptor isoforms (erα1, erα2, erβ1, erβ2) in the liver and ovary of adult, female rainbow trout over the course of an annual reproductive cycle. The expression of estrogen receptor mRNA isoforms was measured by quantitative real-time RT-PCR. Several reproductive indices (gonadosomatic index, maximum oocyte diameter, plasma estradiol-17β, plasma vitellogenin, and ovulation) were also quantified for comparison and used in a correlation analysis to examine any inter-relationships. Of the four isoforms, the expression of erα1 was highest in the liver, and had a significant positive correlation with liver erβ1 expression. Liver expression of erα2 mRNA was the lowest, but showed a significant positive correlation with maximum oocyte diameter in the ovary. The pattern of the erβ isoforms in liver was one of initially elevated mRNA expression followed by a gradual decrease as reproductive development proceeded. In the ovary the erβ1 isoform had the highest mRNA expression of all estrogen receptor isoforms, at the beginning of the reproductive cycle, but then decreased afterward. Both ovarian erβ isoforms had a significant positive correlation with one another. In contrast, erα2 mRNA expression showed a high maximum level in the ovary near the end of the cycle along with a significant positive correlation with plasma estradiol-17β levels; the highest gonadosomatic indices, maximum oocyte diameter, and vitellogenin levels occurred then too.