Biology Reference
In-Depth Information
6.3 Endocrine/internal Factors Involved in the Control of
Smoltifi cation
The involvement of pituitary in the smoltification process has been
suggested by the fact that hypophysectomized fi sh are not able to smoltify
(Nishioka et al., 1987) and that adenohypophysis cytology is clearly
stimulated during this period (Olivereau, 1954; Komourdjian et al., 1976a,
b; Nishioka et al., 1982).
6.3.1
Thyrotropic axis
The thyrotropic axis is mainly involved in the control of development
and metabolism in vertebrates. Thyroid hormones (thyroxine, T4 and
triiodothyronine, T3) are produced and secreted by the thyroid gland under
the control of pituitary thyrotropin (TSH). The hypothalamic control of
TSH is variable during evolution and according to developmental stages.
For example, in amphibians, TSH is under the stimulatory control of
Thyrotropin-Relasing Hormone (TRH) at adult stage, but of Corticotropin-
Relasing Hormone (CRH) at larval stage. In teleosts, whatever the stage is,
CRH seems to be the hypothalamic releasing factor for TSH.
6.3.1.1 Hormonal changes
6.3.1.1.1 Thyroid hormones
Thyroid involvement in smoltifi cation was originally suggested by Hoar
(1939) who observed histological activation of thyroid tissue of the Atlantic
salmon (
Salmo salar
). The availability of RIA procedures has enabled
Dickhoff et al. (1978) to demonstrate a T4 surge in coho salmon during the
smoltifi cation process. Different studies then determined that both plasma
thyroid hormone levels and gill Na+, K+-ATPase activity were possible
indicators of smolt stage (Folmar and Dickhoff, 1981; Langdon and Thorpe,
1984; Boeuf and Prunet, 1985).
Many authors have demonstrated a dramatic
surge of T4 at the end of smolting in freshwater, a few weeks before the
highest level of gill ATPase activity is reached in both Pacifi c and Atlantic
salmon (reviews: Boeuf, 1987; Hoar, 1988). In Atlantic salmon, sometimes T3
also peaks before and after the T4 surge (Boeuf and Prunet, 1985; Virtanen
and Soivio, 1985; Boeuf et al., 1989; Prunet et al., 1989;
Young et al., 1989).
The T4 surge occurs much earlier in Atlantic salmon in long rivers than
in those originating in short streams (Boeuf and Le Bail, 1990). This fact
supports the concept of the role of TH in triggering migratory behaviour
and memorizing the environment (i.e., imprinting).
