The retinas of many vertebrates have cone photoreceptors that express multiple visual pigments. In many of these animals, including humans, the original cones to appear in the retina (which express opsins maximally sensitive to UV or blue light) may change opsin types giving rise to new cone phenotypes. In my dissertation, I used various cellular, molecular and biophysical techniques to determine the spatial and temporal progression of opsin appearance, the distribution of UV and blue cones, and the role of thyroid hormone (TH) in regulating opsin expression during retinal development of salmonid fishes. The sequence of cone opsin appearance in salmonid fishes is SWS1 (UV), followed by LWS (red), RH2 (green) and SWS2 (blue) opsin. All opsins are first expressed in a small patch of cells in the centrotemporal retina, and expression then proceeds towards the dorsal retina. Single cones express a UV opsin at hatching (£fmax of the visual pigment ~ 365 nm), and these cones later transform into blue cones by opsin switch (£fmax of the blue visual pigment ~ 434 nm). Cones undergoing the UV to blue opsin switch exhibit a spectral absorbance profile characterized by blue absorbance at the base and UV absorbance at the tip of the outer segment. Double cones contain green (ƒÜmax ~ 510 nm) or red (ƒÜmax ~ 565 nm) visual pigments (one per member) and, like the rods (ƒÜmax ~ 508 nm), do not exhibit opsin changeover. Treatment of juvenile fish with TH induced a premature switch from UV to blue opsin expression. This appears to be regulated at the transcriptional level by TH-mediated repression of the UV opsin promoter. Cessation of induced hyperthyroidism reverted the opsin switch, suggesting that additional factors are required to maintain it, as occurs during natural development. This thesis provides direct evidence for an opsin switch in the retina of a vertebrate and demonstrates that this switch is mediated by TH. These are fundamental cellular mechanisms that establish the chromatic organization of the retina as evidenced by ongoing findings in other vertebrates including mammals.
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