Biology Reference
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6.3.3.1 Hormonal changes
6.3.3.1.1 Cortisol
The cells of interrenal tissue undergo hypertrophy during smoltifi cation
(Specker, 1982). This change coincides with the elevation of plasma cortisol
in coho salmon (Specker, 1982); between January and April, plasma
corticosteroid levels decrease and by the end of May in coho salmon smolts
retained in fresh water, levels increase (Specker and Schreck, 1982). Young et
al. (1989) showed, in coho salmon, increase in plasma cortisol from February
to late March, a plateau from mid-April to July and a decline in September
and October. Plasma cortisol levels increased during spring in both wild
and hatchery-reared juvenile coho salmon (Shrimpton et al., 1994) and
plasma clearance rate of cortisol is also elevated during the spring in this
species (Patino et al., 1985). Using in vitro system (head kidney fragments),
Young (1986) examined changes in interrenal sensitivity to ACTH during
smoltifi cation of coho salmon. He showed maximal in vitro responsiveness
of interrenal tissue to ACTH in April, correlated with peak plasma T4 and
enhanced hypoosmoregulatory ability, but not with plasma cortisol, which
occurred in May.
In Atlantic salmon, plasma cortisol was constant from March to early
May, and then started to increase, peaking in late June (Virtanen and Soivio,
1985). The rise of plasma cortisol concentration took place at the same
time as the darkening of dorsal, pectoral and caudal fi ns (Virtanen and
Soivio, 1985). Serum cortisol levels became elevated during smoltifi cation,
but after SW transfer, cortisol titers fell sharply (Langhorne and Simpson,
1986); these authors suggested that high serum cortisol levels represent a
secondary response caused by the development of hypoosmoregulatory
ability while still resident in FW. Comparing Atlantic salmon parrs separated
by size into upper and lower mode, Shrimpton and McCormick (1998b)
demonstrated that plasma cortisol levels increased signifi cantly in upper
mode fi sh in May, concurrently with increased smolt characteristics, but
not in lower mode fi sh.
Recently, Kiilerich et al. (2007) reported a 2-fold increase in gill
glucocorticoid receptor (GR) transcript levels during smoltification
of Atlantic salmon. These data are consistent with the increased GR
concentration and expression during smolting (coho salmon: Shrimpton et
al., 1994; steelhead trout: McLeese et al., 1994; Atlantic salmon: Mazurais
et al., 1998; Shrimpton and McCormick, 1998b; masu salmon: Mizuno et
al., 2001b). This increase occurred before the increase in plasma cortisol,
possibly explaining the increased responsiveness of gill tissue to cortisol
observed in early spring (coho and Atlantic salmon: McCormick et al., 1991b)
and demonstrated in vitro (rainbow trout: Shrimpton and McCormick,
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