Relationships between serum levels of gonadotropin, oestradiol-17β, and vitellogenin in the control of ovarian development in the rainbow trout

International audience; The present study investigates the effects of seasonally changing photoperiod regimes on the levels of serum gonadotropin, oestradiol-17/3, and vitellogenin (as indicated by phosphoprotein phosphorus and calcium measurements) on the time of spawning of female rainbow trout (Salmo gairdneri). This included a control regime and also seasonal cycles in which the increasing and decreasing components of the photoperiod were maintained but compressed into 6- and 9-month periods. Spawning in the control fish occurred in mid- January at the same time as fish in outside ponds under ambient conditions, whereas under the 6- and 9-month cycles it was advanced by approximately 12 and 6 weeks, respectively. Under both the control and experimental regimes, the primary change was an increase in serum levels of gonadotropin during the early stages of ovarian development, after which levels were reduced. This was followed by increased oestradiol-17β levels which became particularly pronounced during the period of exogenous vitellogenesis. These changes in oestradioL17β appeared to stimulate vitellogenin production and release by the liver as evidenced by increases in serum calcium and phosphoprotein phosphorus levels. Subsequently, vitelogenin levels remained high until after spawning was complete. During the later stages of ovarian development oestradiol levels fell, thus signalhng, probably by a negative feedback mechanism, a further and larger increase in gonadotropin levels presumably required for the control of fmal oocyte maturation and ovulation. This sequence of endocrine changes was similar under the shortened photoperiod cycles, although modifications were apparent in both their timing and duration, resulting in earlier spawning times.