The abundance and spatial distribution of retinal cone photoreceptors
change during thyroid hormone (TH)-induced and natural development of
rainbow trout (Oncorhynchus mykiss). These changes are thought to
allow the fish to adapt to different photic environments throughout its
life history. To date, the ontogeny of rainbow trout cone photoreceptors
has been examined using physiological and morphological approaches. In
this study, we extended these observations by measuring opsin gene
expression in retinal quadrants during natural and TH-induced development.
Gene expression during natural development was investigated in retinae
from fish at both parr and smolt stages. The role of TH in modulating
opsin gene expression was determined in TH-treated parr and control fish
sampled after two, nine, and 22 days of treatment. Total RNA was isolated
from each retinal quadrant and steady-state opsin mRNA levels were
measured using reverse transcriptase real-time quantitative polymerase
chain reaction (QPCR) analysis. Expression of ultraviolet-sensitive opsin
(SWS1), rod opsin (RH1), middle wavelength-sensitive opsin (RH2), and long
wavelength-sensitive opsin (LWS) transcripts vary spatially in the parr
retina. Smolts, compared to parr, had downregulated SWS1 expression in all
quadrants, lower LWS expression dorsally, higher RH1 expression nasally,
and higher RH2 expression dorsally. In TH-treated parr, SWS1 opsin
expression was downregulated in the nasal quadrants by two days. SWS1
displayed the greatest degree of downregulation in all quadrants after
nine days of treatment, with an increase in short wavelength-sensitive
(SWS2) and RH2 opsin mRNA expression in the temporal quadrants. This study
reveals that opsin genes display spatially significant differences within
rainbow trout retina in their level of mRNA expression, and that
regulation of opsin expression is a dynamic process that is influenced by
TH. This is particularly evident for SWS1 gene expression in parr
following TH-induced and natural development.