Post-ovulatory secretion of pituitary gonadotropins GtH I and GtH II in the rainbow trout (Oncorhynchus mykiss): regulation by steroids and possible role of non-steroidal gonadal factors

In order to determine the factors of ovarian origin which can modulate the post-ovulatory secretion of the FSH-like gonadotropin (GtH I) and the LH-Li ke gonadotropin (GtH II), freshly ovulated female rainbow trout (Oncorhynch us mykiss) were divided into two groups. In the first group the fish were s tripped in order to eliminate eggs and ovarian fluid from the body cavity, while in the second,group the eggs were kept in the body cavity. Subsequent ly, fish from both groups were implanted with testosterone (10 mg/kg), 17 b eta-estradiol (10 mg/kg) or 17,20 beta-dihydroxy-4-pregnen-3-one (17,20 bet a P) (1 mg/kg) or injected every 2 days with desteroidized ovarian fluid (1 .5 ml/kg). The secretion of GtH I dramatically increased in stripped fish, reaching it s maximum levels 2 weeks after ovulation. The preservation of eggs in the b ody cavity led to the suppression of this increase. The profiles of GtH II secretion were opposite to those encountered for GtH I because the increase of GtH II was observed only in unstripped fish. The administration of steroids showed that testosterone is able to inhibit GtH I release and stimulate that of GtH II in stripped fish, having no effe ct on the release of these gonadotropins in non-stripped animals. 17 beta-E stradiol failed to modify GtH I secretion; however, it decreased the releas e of GtH II in fish containing retained eggs in the body cavity. 17,20 beta P had a delayed stimulating influence on GtH I release in unstripped fish. Finally, multiple injections of desteroidized ovarian fluid into stripped fish led to a significant decrease of GtH I release and to an increase of G tH II secretion. This study demonstrates that factors which are present in ovarian fluid mod ulate the post-ovulatory secretion of both gonadotropins - their net action is negative on GtH I and positive on GtH II. Among the steroids, testoster one is of major importance, being able to inhibit GtH I release and to stim ulate that of GtH II. We also show that non-steroidal factors present in th e ovarian fluid can influence the release of both gonadotropins, which indi rectly supports previous findings about the existence of inhibin/activin-li ke factors in fish.