Sex steroids differentially regulate , and expression in Atlantic cod ().

Depending on the stage of gonad maturation, as well as other factors, gonadal steroids can exert either a positive or negative feedback at the brain and pituitary level. While this has been demonstrated in many teleost species, little is known about the nature of steroid feedback in Gadiform fish. Using an optimized model system of the Atlantic cod pituitary, the present study investigated the potential effects of two physiologically relevant doses of estradiol, testosterone (TS) or dihydrotestosterone (DHTS) on cell viability and gene expression of gonadotropin subunits (/) and two suggested reproduction-relevant gonadotropin-releasing hormone receptors (/) during three stages of sexual maturity. In general, all steroids stimulated cell viability in terms of metabolic activity and membrane integrity. Furthermore, all steroids affected expression, with the effect depending on both the specific steroid, dose and maturity status. Conversely, only DHTS exposure affected levels, and this occurred only during the spawning season. Using single-cell qPCR, co-transcription of and was confirmed to both and expressing gonadotropes, with being the most prominently expressed isoform. While steroid exposure had no effect on expression, all steroids affected transcript levels in at least one maturity stage. These and previous results from our group point to Gnrhr2a as the main modulator of gonadotropin regulation in cod and that regulation of its gene expression level might function as a direct mechanism for steroid feedback at the pituitary level.