Biology Reference
In-Depth Information
index, GSI, HSI and plasma levels of vitellogenin and cortisol. Female silver
eels that swam for 1.5 or 3 months had reduced ERα, VTG1 and VTG2 liver
expression and lower plasma calcium levels in their blood, indicative of
suppressed hepatic vitellogenesis (Palstra et al., 2010).
Males stimulated by three month-swimming showed a similar two-
to three- fold higher LHβ expression level than males injected with GnRH
agonist, compared to controls (Palstra et al., 2008b). Both treatments also
caused a three- to fi ve-fold increase in GSI and an induced spermatogenesis
(>80% presence of spermatogonia late type b); one male in the swimmer
group even showed the formation of spermatocytes (Palstra et al., 2008b).
However, in this study, females were not stimulated by SW-swimming nor
by GnRH agonist, and even showed regression of maturation overt time
as demonstrated by lower LHβ expression, GSI and oocyte diameters in
all groups after 3 months (Palstra et al., 2008b). This unresponsiveness of
females may be due to the fact that, in contrast to males, their pituitaries
are not sensitized and still under dopaminergic control.
Different mechanisms have been suggested by van Thillart and
collaborators. Swimming may activate lipid metabolism and/or
inhibits
vitellogenesis (Palstra et al., 2009a).
A possible role of cortisol in this
phenomenon has been hypothetized as silver eels have higher cortisol
levels (Van Ginneken et al., 2007a) and higher cortisol levels have been
measured in swimming eels of Lake Grevelingen and Lake Balaton (Palstra
et al., 2009b).
In summary, swimming trials performed by van den Thillart's group
did show different effects in males and females. In silver males, swimming
trials continued sexual maturation. In contrast, suppressed gonadotropin
expression and vitellogenesis, associated with stimulated lipid deposition
in oocytes are observed after swimming trials in silver females. These latter
observations in females suggest that in nature, a different sequence of
events may occur compared to artifi cial maturation and eels may undergo
vitellogenesis and fi nal maturation near or at the spawning grounds.
7.4 Environmental Factors Involved in the Control of Eel Silvering
7.4.1 Photoperiod/melatonin
Melatonin is a hormone principally released by the pineal organ during the
night known to act as a transmitter of environmental cues, notably photic
information, and to play a role in synchronising various behaviours and
physiological processes (for review: Zachmann et al., 1992; Pandi-Perumal
et al., 2006).
In the eel, a long-term melatonin treatment increased brain tyrosine
hydroxylase (TH, the rate limiting enzyme of DA synthesis) mRNA
