Investigations into the roles of thyroid hormone and retinoic acid on opsin expression in juvenile rainbow trout

Thyroid hormone (TH) and retinoic acid (RA) are powerful modulators of photoreceptor differentiation during vertebrate retinal development. In the embryos and young juveniles of salmonid fishes and rodents, TH induces switches in opsin expression within individual cones, a phenomenon that also occurs in adult rodents following prolonged (12 week) hypothyroidism. The ability of TH to modulate opsin expression in the differentiated retina of fish, and the role of RA in inducing opsin switches, if any, is unknown. Here I investigate the action of TH and RA on single cone opsin expression and the absorbance of visual pigments in juvenile rainbow trout. Prolonged TH exposure increased the wavelength of maximum absorbance (λmax) of the rod, and the medium (M, green) and long (L, red) wavelength visual pigments, and affected single cone opsin expression in the alevin. RA did not induce any opsin switches nor change the visual pigment absorbance of photoreceptors.