Biology Reference
In-Depth Information
1999). Changes in gill corticosteroid receptor (CR) concentration (
B
max
) and
dissociation constant (kd) were observed when fi sh smolt during the spring
(
O. kisutch
: Shrimpton, 1996;
S. salar
: Shrimpton and McCormick, 2003).
Elevated GR expression suggests a need for increased cortisol signalling
during smolting when SW-tolerance develops, and seems to occur prior
to/or concurrent with elevated plasma cortisol levels (Shrimpton and
McCormick, 2003).
6.3.3.1.2 ACTH
Measurement of the cell surface and nuclear area in the ACTH cells in
Atlantic salmon showed an activation during smoltification prior to
catadromous migration (Olivereau, 1975). Similarly, in coho salmon, McLeay
(1975) reported a positive correlation between interenal nuclear diameters,
interregnal cell size and ACTH cell nuclear diameters during smoltifi cation.
However, in the same species, Nishioka et al., (1982) demonstrated
that ACTH-producing cells showed only moderate cytological changes
indicative of increased synthetic cytological activity in the smolt.
6.3.3.1.3 CRH
CRH-like activity appears to increase temporally during the smoltifi cation
period in discrete brain regions (cited in Clements and Schreck, 2004).
6.3.3.2 Experimental induction
6.3.3.2.1 Body silvering
The darkening of dorsal, pectoral and caudal fi ns can be induced by ACTH
injection, but neither ACTH nor cortisol induced body silvering (Langdon
et al., 1984).
6.3.3.2.2 Metabolism
Specker (1982) suggested that increased cortisol secretion may be responsible
for the decrease in body lipid content characteristic of smoltifi cation.
Sheridan (1986) demonstrated that coho salmons with cortisol implant
reduced total lipid concentration and triacylglycerol content of the liver
and dark muscle, but increased mesenteric fat lipase activity. Cortisol
replacement to hypophysectomized smolts restored liver lipase activity to
approximately the same levels as those of controls (Sheridan, 1986).
