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by permitting energy mobilization and probably acting at the onset of
puberty. Cortisol may therefore control the metabolic challenge occurring
during both metamorphosis and puberty/reproduction in teleosts. It may
also act in synergy with different other hormones to induce the various
morphological changes observed: with sex steroids during eel silvering,
with THs during larval metamorphosis and with GH and TH during
salmonid smoltifi cation.
7.3.1.4 Gonadotropic axis
As highlighted by several authors, the silvering process is also characterized
by gonadal modifi cations (Lopez and Fontaine, 1990; Fontaine and Dufour,
1991). Field studies clearly demonstrated an increase in gonad weight and
modifi cations of the oocyte structure throughout the silvering process
(Marchelidon et al., 1999; Aroua et al., 2005; Durif et al., 2005). In addition,
measurement of sexual steroids, estrogens (E2) and androgens (T and 11-
KT), in the plasma showed an increase between yellow and silver stage
(
A. australis and A. dieffenbachii
: Lokman et al., 1998;
A. anguilla
: Sbaihi et
al., 2001; Aroua et al., 2005;
A. rostrata
: Cottrill et al., 2001;
A. japonica
: Han
et al., 2003). A fi rst increase in E2 levels at the early stages of the silvering
process (in intermeditae eels), then a further increase of E2, accompanied
by an increase in 11-KT and T in seilver eels (Aroua et al., 2005) (Fig. 4b).
All these results suggest that the gonadotropic axis occupies an important
position during the silvering process.
Recent studies have focused on the two pituitary hormones involved
in the control of reproduction, the gonadotropins (luteinizing hormone, LH
and follicle-stimulating hormone, FSH). In
A. japonica
(Han et al., 2003), as
well as in
A. anguilla
(Aroua et al., 2005), variation on mRNA levels of the
alpha and the beta subunits of the gonadotropins were observed throughout
silvering. In
A. japonica,
authors observed a concomitant increase in mRNA
of the different subunits, LHβ, FSHβ and the glycoprotein alpha (GPα) (Han
et al., 2003). In
A. anguilla
, LH and FSH were shown to be differentially
expressed during the silvering process, with an early increase in FSHβ
expression and a late increase of LHβ expression (Aroua et al., 2005)
(Fig. 4a). These data suggest that FSH could play an early role in the
activation of gonads, while LH may have an important role later in the
silvering process. Indeed, a concomitance exists between the increase in
FSH expression and the start of lipid incorporation in oocytes (also called
“endogenous vitellogenesis”), which suggests that FSH could be responsible
for the initiation of lipidic vitellogenesis. The early increase in FSH may
also be responsible for the fi rst increase in steroid production (E2), observed
in « intermediate » eels. In contrast, the late increase in vitellogenin (Vg)
plasma levels, concomitant with the late increase in LH expression, and the
