Neuroendocrine Mechanisms Involved in the Control of Ovarian Development and Vitellogenesis in the Rainbow Trout

Abstract

This thesis provides a detailed study of the neuroendocrine mechanisms that control vitellogenesis and ovarian development in the female rainbow trout. Fluorometric, electrophoretic and chromatographic techniques and radioimmunoassay were used to determine serum levels of oestrone, oestradiol-17B, testosterone, vitellogenin and calcium and these correlated with histological data both during the reproductive cycle and after treatment with hormones. Single and double injections of oestradiol-17B respectively produced primary and much more rapid secondary increases in levels of vitellogenin, total calcium and phosphoprotein phosphorus. The dose of oestradiol-17B was correlated with the levels of vitellogenin and with hepatosomatic and gonadosomatic indices. Although the liver was at no time completely refractory to oestradiol-17B, there was a marked seasonal variation in sensitivity, being highest in October and lowest in January. During the reproductive cycles of three strains of female trout, initial increases in oestrone and oestradiol-17B were followed by increases in vitellogenin, calcium and testosterone. Levels ‘of both oestrogens were correlated with vitellogenin during vitellogenesis, which supports a role for both hormones in this process. All three strains showed similar sequences of changes in both endocrinological and histological development. Similar, although much reduced, changes were observed in immature fish of one strain at a time of year suggestive of a practice-run, one year before the first spawning, as a rehearsal for full reproductive development. Exposing fish to long and then short days, or a condensed 6 month seasonal photoperiod, advanced spawning by 4-5 months. These results indicate that the different spawning times shown by the three strains may be the result of a requirement for a different number of stimuli by a specific length of daily photoperiod. The application and value of techniques of hormonal and photoperiodic manipulation, for the improvement of egg quality and alteration of spawning time, in the farming of salmonid fish are discussed.