Biology Reference
In-Depth Information
expression in a region dependent way (Sébert et al., 2008). Melatonin
stimulated the dopaminergic system of the preoptic area, which is involved
in the inhibitory control of gonadotrophin (LH and FSH) synthesis and
release. Moreover, the increased TH expression was consistent with
melatonin binding site distribution as shown by 2[
125
I]-melatonin labelling
studies. On the other hand, melatonin had no effects on the two eel native
forms of GnRH (mGnRH and cGnRH-II) mRNA expression. Concerning the
pituitary-gonad axis, melatonin treatment decreased both gonadotrophin
β-subunit (LHβ, FSHβ) mRNA expression and reduced sexual steroid (11-
ketotestosterone, oestradiol) plasma levels. This indicates that melatonin
treatment had a negative effect on eel reproductive function. By this
mechanism melatonin could represent one pathway by which environmental
factors could modulate reproductive function in the eel.
7.4.2 Hydrostatic and atmospheric pressure
Professional fi shermen and researchers have observed that the migration
of silver eel often occurs during storms (i.e., atmospheri depression).
Eels become more active (active swimming and escapement behaviour)
during thunderstorms (Lowe, 1952). Hvidsten (1985) also indicated that
atmospheric pressure is statistically correlated with migration peaks.
Preliminary experiments were performed using female silver eels
kept in a cage 450 m in depth (about 46 atm) during 3 months (Dufour
and Fontaine, 1985; Fontaine et al., 1985). Signifi cantly higher ovarian
development was observed (x1.39) and the pituitary LH content was 27
times higher in immersed eels. Serum LH concentrations, which were
undetectable (≤0.2 ng/ml) in control eels, were signifi cantly increased
(x1.4) in immersed eels. In a more recent study, female silver eels were
submitted to 101 ATA, in a hyperbaric chamber equipped with a freshwater
recirculation system, for 3 weeks (Sébert et al., 2007). Signifi cant increase
in oocyte diameter (x1.11) and in plasma levels of 11-KT (x2.39), E2 (x1.99)
and Vg (x12.47) were observed in females submitted to high hydrostatic
pressure (HP). At the pituitary level, LHβ expression tended to increase,
while FSHβ expression decreased, leading to an increase in LHβ/ FSHβ ratio.
However, even if HP plays a specifi c and positive role in eel reproduction,
additional environmental and internal factors are necessary to ensure
complete sexual maturation.
7.5 Anthropic Factors Impairing Eel Silvering
The effects of contaminants in European eel was recently reviewed
(Geeraerts and Belpaire, 2010).
