Biology Reference
In-Depth Information
7.3.1
Hormones
7.3.1.1 Thyrotropic axis
Callamand and Fontaine (1942) observed a hyperactivity of the thyroid
gland before and during the downstream migration. It was then thought
that the thyroid activation was responsible for the important morphological
modifications observed during silvering. Especially, concerning eye
changes, some cases of exophthalmy can be observed in humans having
hyperthyroidian pathology (for reviews: Bradley, 2001; Wiersinga and
Bartalena, 2002), supporting the former hypothesis of a role of thyroid
hormones in eel eye changes. Moreover, Fontaine (1953) suggested that the
thyroid activation during eel silvering at the moment of the downstream
migration could be responsible for the increased locomotor activity
of the
animals, as is observed during smoltifi cation. In subadult American eels,
elevated T4 plasma levels are correlated with increased locomotor activity
under natural conditions (Castonguay et al., 1990).
In contrast with the previous hypothesis, the study of the expression
profi les of TSH showed a non-signifi cant or a weak increase in TSHβ mRNA
between yellow and silver eels (
Anguilla anguilla
: Aroua et al., 2005;
Anguilla
japonica
: Han et al., 2004) (Fig. 3a). Moreover, measurement of plasma
levels of thyroid hormones in yellow and silver eels showed a moderate
increase in thyroxine (T4) and no signifi cant variations in triiodothyronine
(T3) during silvering (
Anguilla anguilla
: Marchelidon et al., 1999; Aroua et
al., 2005;
Anguilla japonica
: Han et al., 2004) (Fig. 3b). These recent results
suggest that the thyrotropic axis is poorly implicated in the neuroendocrine
control of the silvering process. The weak variations observed on TSHβ
mRNA and T4 plasma level could be involved in the increased activity of
eels related to their migratory behavior. This role in motility may not be
specifi c to the silver stage as thyroid hormones can also induce an increase
in locomotor activity
in yellow (Castonguay et al., 1990)
and glass eels
(Edeline et al., 2005).
Future studies should aim at investigating the brain control of pituitary
TSH. Interestingly, preliminary data
on eel pituitary cells
suggest a role for
CRH in the control of TSH, rather than TRH (personal communication).
7.3.1.2 Somatotropic axis
A recent study showed no signifi cant differences in mRNA levels between
yellow and silver stage, and a signifi cant decrease of GH pituitary content
(Aroua et al., 2005). These data suggest that, unlike in salmonid smoltifi cation
where GH has a strong role as a factor controlling osmoregulation and
seawater adaptability (Björnsson, 1997), in eel silvering the GH role is less
