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Bail, 1990; Iwata, 1995; Munakata et al., 2000, 2001). Plasma T4 levels were
shown to be higher in migrating fi sh than in non-migrating conspecifi cs
(Biwa salmon, Oncorhynchus rhodurus : Fujioka et al., 1990; chum salmon:
Ojima and Iwata, 2007). However, using exogenous treatments with T4
and/or thiourea, Birks et al. (1985) showed that TH were antagonistic to
mechanism underlying seaward migration of steelhead trout.
6.3.2 Somatotropic axis
The somatotropic axis is classicaly involved in the control of growth, with
the pituitary growth hormone, which can act by itself directly on target/
peripheral tissue or indirectly via the production and release of insulin-
like growth factor 1 (IGF1) by the liver. The stimulatory hypothalamic
control of GH production and release is multifactorial and varied during
evolution, while the inhibitory action of somatostatin is conserved among
vertebrates.
6.3.2.1 Hormonal changes
6.3.2.1.1 GH
Hypertrophy and hyperplasia of somatotropic cells have been observed
during smoltifi cation of Pacifi c salmon (Clarke and Nagahama, 1977; for
review: Donaldson et al., 1979). A rise in plasma GH during smoltifi cation of
coho salmon has been reported (Sweeting et al., 1985). In freshwater, during
Atlantic salmon smoltifi cation, plasma GH levels rose sharply concomitant
with the T3 peak, 2 weeks before the peak of gill ATPase activity (Boeuf et
al., 1989). After transfer to seawater, GH increased signifi cantly remaining
high for 7-10 days, and returning to basal levels after 14 days (Boeuf et
al., 1989). Two peaks in plasma GH levels were observed in 1986, one in
mid-April which coincided with a peak of plasma T3 and the second one
in mid-May which lasted for 1 month, preceded T4 peak by 1-2 weeks
and coincided with maximal gill Na+, K+ -ATPase activity (Prunet et al.,
1989). More recently, in Atlantic salmon, Agustsson et al. (2003) showed
that pituitary GH mRNA expression increased from early-April to reach a
peak in mid-May, in agreement with increased GH secretion and plasma
GH levels during this period (Agustsson et al., 2001).
Due to tetraploidy, two GH (GH1 and GH2) genes were evidenced in
rainbow trout (Agellon et al., 1988; Mori et al., 2001). In Atlantic salmon,
we measured, by quantitative real-time PCR, GH1 and GH2 mRNA levels
during parr-to-smolt transformation and observed a peak in both hormones
concomitant to that of TSH (Fig. 3).
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