Biology Reference
In-Depth Information
and Dufour, 2004). Our
in vitro
study in the European eel demonstrates
that the brain inhibitory control of GH is exerted by somatostatin (SRIH)
and that insulin-like growth factor 1 (IGF-1) exerts a negative feedback
on GH. Thus, during silvering, the major regulation of the somatotroph
cells, which are responsible of GH synthesis, would be an increase of their
inhibitory control.
It is important to note that seasonal data suggest that a peak in GH and
body growth during summer may occur (Durif et al., 2005). This suggests
that while GH may not be involved in the control of the silvering process
itself, the somatotropic axis may participate earlier in the initiation of the
silvering. A similar growth surge is observed at puberty in mammals.
7.3.1.3 Corticotropic axis
Only a few studies have focused on the corticotropic axis during the
transition from yellow to silver stage. It is probably because of the diffi culty
of sampling blood in order to measure plasma cortisol levels without
stressing animals. Recently, Van Ginneken et al. (2007a) demonstrated
elevated cortisol levels in silver eels prior to migration. This is in agreement
with the fact that during the downstream migration, eels are fasting and
it is well know that the production of cortisol is induced in response to
starvation. A role of cortisol may be to permit the mobilization of energy
stores needed by the fi sh at this critical period.
Forrest et al. (1973) found that Na+, K+-ATPase activity rose very
slowly after transfer of yellow american eels (
A. rostrata
) to SW. If the eels
were treated with cortisol before transfer, the enzyme activity increased to
levels found in SW-acclimated eels and did not increase further following
transfer to SW. Injection of yellow American eels with cortisol, in addition
to increasing Na+, K+-ATPase of gill and intestine, caused their ventral
surface to turn silver (Epstein et al., 1971).
In addition to the effect of cortisol on energy mobilization and seawater
adaptation, it was previously demonstrated in the European eel that cortisol
had also a strong positive effect on LH production
in vivo
as well as
in vitro
(Huang et al., 1999). This stimulation is stronger when eels are treated by
a combination of cortisol and androgens, indicating synergistic action of
these hormones on LH (Huang, 1998; Sbaihi,
2001). It is interesting to note
that while in amphibians, cortisol has a synergistic effect with thyroid
hormones on metamorphosis, a synergy between cortisol and sex steroids
is observed in the control of eel silvering. In the eel, cortisol has also an
inhibitory action on digestive tract somatic index and can mobilize mineral
stores from vertebral skeleton (Sbaihi et al., 2009).
The various effects of cortisol demonstrated in the eel indicate that
the corticotropic axis may play an important role throughout silvering
