Biology Reference
In-Depth Information
hypothesis concerning opsin shift, proposed by Temple and collaborators
(see paragraph 1.2).
In rainbow trout, Hawryshyn and collaborators have done many studies
on ultraviolet photosensitivity and the role of thyroid hormones. They
showed that T4 could induce a precocious loss of UV photosensitivity and an
associated change in the retinal photoreceptor cell mosaic in small rainbow
trout (Browman and Hawryshyn, 1992, 1994; Deutschlander et al., 2001). Six
weeks after termination of hormone treatment, the same individuals once
again possessed UV sensitivity due to UVS cone reappearance (Browman
and Hawryshyn, 1994). T4 treatment to larger trout, which had lost their UV
photoreceptor mechanism during normal development, were once again UV
photosensitive and UVS cones were found in their retinae (Browman and
Hawryshyn, 1994). These data suggest that thyroid hormones are involved
in both the loss and reappearance of UV photosensitivity. Deutschlander
and collaborators (2001) demonstrated that the reduction of UV sensitivity
in T4-treated juvenile
O. mykiss
and anadromous steelhead
O. mykiss
smolts
occurred primarily in the ventral retina. The remodelling of the smolt cone
mosaic in response to T4 treatment is marked by localized proliferation
or neuroregeneration of UVS cones in the ventral and nasal retinal areas
(Hawryshyn et al., 2003). The optical quality of trout ocular lens was found
to be decreased after T4 treatment; however, T3 treatment had no effect on
trout lenses in culture (van Doorn et al., 2005).
In vitro
, T3 is able to directly act on isolated coho salmon retinal pigment
epithelial cells in order to alter the production of didehydro-derivatives of
retinoids (Alexander et al., 2001).
6.3.1.2.4 Olfaction
An early study showed that artifi cially elevating thyroid hormone induced
parr to imprint to artifi cial odorants, while parr with unaltered hormone
levels did not. Moreover, pre-smolt coho salmon injected with TSH in doses
suffi cient to increase the T3 and T4 concentrations to smolt levels 5 months
before smolt transformation would normally occurs, and simultaneously
exposed to synthetic chemicals, became imprinted to the chemicals, whereas
control fi sh receiving either ACTH or solvent did not (Hasler and Scholz,
1983). More recently, using intraperitoneal implants of T3 for 16-20 days
to mimic smolting, Lema and Nevitt (2004) demonstrated that T3 could
induce olfactory cellular proliferation in juvenile coho salmon. Scholz et
al. (1985) demonstrated ontogenetic change in receptor binding kinetics
during smolt transformation with all the putative target tissues for TH
containing consistently more radioactivity in the TSH-induced smolts than
in saline treated presmolts. In accordance with this result, it was shown
that the olfactory epithelium of masu salmon (
O. masou
) became enriched
