Swimming suppresses hepatic vitellogenesis in European female silver eels as shown by expression of the estrogen receptor 1, vitellogenin1 and vitellogenin2 in the liver.

BACKGROUND: When European silver eels (Anguilla anguilla) venture into the Atlantic Ocean for their 6,000 km semelparous spawning run to the Sargasso Sea, they are still in a prepubertal stage. Further sexual development appears to be blocked by dopaminergic inhibition of hypothalamus and pituitary activity. Recently, we found that swimming for several weeks in freshwater stimulated the incorporation of fat droplets in the oocytes. So, it was hypothesized that long term swimming in seawater would release the inhibition further and would also stimulate the production of vitellogenin by the liver. METHODS: For this study a swim-flume was constructed to allow simulated migration of migratory female silver eels for 3 months (1,420 km) in natural seawater at 20 degrees C. Primers were designed for polymerase chain reactions to measure the mRNA expression of estrogen receptor 1 (esr1), vitellogenin1 (vtg1) and vitellogenin2 (vtg2) genes in the liver of European female silver eels. RESULTS: In comparison to resting eels, swimming eels showed a diminished expression of esr1, vtg1 and vtg2 in the liver. They also had lower plasma calcium (Ca; indicative of vitellogenin) levels in their blood. This showed that vitellogenesis is more strongly suppressed in swimming than in resting eels. However, when eels were subsequently stimulated by 3 weekly carp pituitary extract injections, the expression of the same genes and plasma levels of Ca strongly increased in both groups to similar levels, thus equalizing the initial differences between resting and swimming. CONCLUSIONS: It is concluded that vitellogenesis remains suppressed during resting and even more during swimming. The fact that swimming stimulates fat deposition in the oocytes but suppresses vitellogenesis indicates that these events are separated in nature and occur sequentially. Swimming-suppressed vitellogenesis may imply that in nature eels undergo vitellogenesis and final maturation near or at the spawning grounds.

Temporal expression of hepatic estrogen receptor 1, vitellogenin1 and vitellogenin2 in European silver eels.

Because European silver eels have never been caught during or after their 6000-km reproductive migration to the Sargasso Sea, all existing knowledge on their sexual maturation comes from hormonal stimulation. Silver eels that start their oceanic migration are still immature with pre-vitellogenic oocytes. Hence we assumed that vitellogenesis should start with the expression of the estrogen receptor in the liver before the circulating 17beta-estradiol (E2) can have any effect. In this study we followed the hepatic vitellogenesis upon 4 weekly injections with carp pituitary extracts (CPE). New molecular primers for the expression of the estrogen receptor 1 (esr1), vitellogenin1 (vtg1) and vitellogenin2 (vtg2) in the liver were developed. Sequences of vtg2 and esr1 were not previously described in Anguilla anguilla. All eels showed weekly increase of the eye size and pectoral fin length, which are signs of early maturation. The same occurred with the gonadosomatic index, the oocyte stage and diameter, and number of deposited fat droplets. Early vitellogenesis appeared as a 3-step process (1) E2-levels and esr1 expression were significantly increased already after one injection, (2) vtg1 and vtg2 expression were significantly increased after one and two injections, respectively, and (3) vtg1 and vtg2 expression increased further after three and four injections. Then also plasma calcium (corresponds with plasma vitellogenin) increased and yolk globuli appeared in the oocytes. These results show that esr1 is the first of the three genes examined that is expressed during the onset of hepatic vitellogenesis. Furthermore, ovarian vitellogenesis (appearance of yolk globuli in oocytes) occurs 1-2 weeks later than the onset of hepatic vitellogenesis.

Male silver eels mature by swimming.

BACKGROUND: If European silver eels are prevented from reproductive migration, they remain in a prepubertal stage by dopaminergic inhibition of pituitary activity. Because this inhibition is likely a requirement for an extended female growth stage, we tested if it is sex-specific by subjecting both sexes to stimulation by GnRHa (Gonadotropin-Releasing Hormone agonist) - injection or 3-months swimming in seawater. RESULTS: In contrast to females, males showed a two- to three-fold higher LHbeta (luteinising hormone beta subunit) - expression, a three- to five-fold higher GSI (Gonadosomatic index) and induced spermatogenesis when compared with the untreated control group. CONCLUSION: Dopaminergic inhibition is thus not effective in males and swimming results in natural maturation, probably via GnRH-release.