INTRODUCTION: The ability to control reproductive outcomes is key to successful Atlantic salmon aquaculture production. Steroidogenic enzymes shape circulating steroid hormone profiles and metabolites needed to induce changes in fish physiology that ultimately lead to spawning. This study reports on the steroidogenic capabilities of ovary, interrenal plasma and ovarian fluid in female Atlantic salmon at the time of spawning.
METHODS AND RESULTS: Ovary, interrenal, plasma, ovarian fluid and skin samples were collected from female (n = 15) Atlantic salmon at the time of spawning. These tissues were analyzed for steroidogenic enzyme transcript abundance and enzyme activity as well as steroid hormone concentrations. The abundance of mRNA for steroid hormone enzymes CYP11A, STAR, HSD3B1, CYP19A1, CYP19A2, HSD11B, CYP11, CYP17, CYP21, SRD5A1 and SRD5A2 were quantified by real-time quantitative PCR. Enzyme activity was analyzed for 3β-HSD, 5α-DHP, and the Cytochrome P450 enzyme group from isolated microsomes. Hormone concentrations from the enzyme activity assays and tissue concentrations were determined using LC-MS/MS to measure 26 hormones from all steroid classes. Transcripts of HSD11B and both isoforms of SDR5A were detected in salmon skin. CYP17 was only measurable in testis and ovary tissues and CYP19A1 was only measurable in ovary. CYP19A2 was detected in all tissues except skin. Hormones of all classes were detectable in interrenal, ovarian tissues and ovarian fluid. Glucocorticoids were primarily measured in the interrenal tissue and pregnanes were mainly measured in ovarian tissues. Interestingly, high concentration of 5α reduced testosterone (5αDHT) was measured in the ovarian tissue and slightly less was measured in the interrenal tissue. Additionally, high levels of 17 hydroxyallopregnanolone, an identified pheromone, was measured in the interrenal tissue.
DISCUSSION: This study was the first to measure steroidogenic enzymes in the skin of Atlantic salmon suggesting that skin is a site of steroid metabolism in reproductively mature Atlantic salmon. This study was also the first to identify tissue-specific synthesis of steroid hormone enzyme (via mRNA expression) coupled with steroid hormone concentrations to indicate where key hormones are produced in salmon. This information can potentially be used to better understand spawning in Atlantic salmon.