Cultured pituitary cell GtH response to GnRH at different stages of rainbow trout oogenesis and influence of steroid hormones

Abstract

In rainbow trout, a variable in vivo pituitary sensitivity to GnRH has been previously observed, depending on the stage of oogenesis. The purpose of the present work was to study, in vitro, the role of oestradiol (E2) and 17α-hydroxy,20β-dihydroprogesterone (17α20βP), respectively, involved in vitellogenesis and in oocyte maturation, upon this variability. The study was performed using primary cultures of whole pituitary cells from animals at different stages of oogenesis and subjected to increasing doses of salmon GnRH (sGnRH) after a 3-day pretreatment with control medium or medium supplemented with the steroid at levels corresponding to those circulating at the time of particular events of the sexual cycle (maturation and vitellogenesis). In control cultures, pituitary GtH responsiveness to sGnRH was maximal at ovulation, since at this time the gonadotrophs were able to respond to 10 −9 M sGnRH, whereas during vitellogenesis and preovulatory stages, the minimal effective dose of sGnRH ranged between 10 −6 and −8 M. We have demonstrated that 17α20βP has a positive or negative effect by acting directly on pituitary cell responsiveness to sGnRH, depending on the stages at which it is applied; its effect is positive during early vitellogenic and preovulatory stages whereas it is negative at the time of ovulation. E2 also increased pituitary responsiveness to sGnRH when applied during early vitellogenesis at low doses, corresponding to circulating levels at the time of ovulation; higher levels of E2, corresponding to circulating levels found during the last stages of vitellogenesis, did not modify pituitary responsiveness but increased cell GtH content. Results obtained are discussed in relation to hormonal changes (gonadotropin and steroid hormones) observed by different authors during the sexual cycle.