Pituitary gonadotropin and ovarian gonadotropin receptor transcript levels: Seasonal and photoperiod-induced changes in the reproductive physiology of female Atlantic salmon (Salmo salar)

In female Atlantic salmon kept at normal light conditions, pituitary follicle-stimulating hormone beta (fshb) transcript levels were transiently elevated one year before spawning, re-increased in February, and remained high during spawning in November and in post-ovulatory fish in December. The first increase in plasma 17b-estradiol (E2), testosterone (T) and gonadosomatic index (GSI) was recorded in January; E2 rose up to one month prior to ovulation, while T and GSI kept increasing until ovulation. Pituitary luteinizing hormone beta (lhb) transcript levels peaked at the time of ovulation. Except for transient changes before and after ovulation, ovarian follicle stimulating hormone receptor (fshr) transcript amounts were relatively stable at a high level. By contrast, luteinizing hormone receptor (lhcgr) transcript levels started out low and increased in parallel to GSI and plasma E2 levels. Exposure to continuous light (LL) induced a bimodal response where maturation was accelerated or arrested. The LL-arrested females showed previtellogenic oil droplet stage follicles or primary yolk follicles only, and fshb and E2 plasma levels collapsed while fshr increased. The LL-accelerated females showed elevated lhb transcript levels and slightly elevated E2 levels during early vitellogenesis, and significantly elevated lhcgr E2 and GSI levels in late vitellogenesis. We conclude that Fsh-dependent signaling stimulates recruitment into and the sustained development through vitellogenesis. Up-regulation of lhcgr gene expression during vitellogenesis may reflect an estrogenic effect, while elevated fshr gene expression following ovulation or during LL-induced arrestment may be associated with ovarian tissue remodeling processes.