Abstract
Abstract: :
Purpose: There are two visual pigment chromophores that can be employed in vertebrate retinas; 11-cis retinal and 11-cis 3,4-dehydroretinal. Altering the visual pigment chromophore results in a measurable shift in spectral absorbance of individual photoreceptors that can be used to calculate the ratio of the two chromophores. Temperature, day length, light intensity and hormones have all been found to invoke shifts in chromophore ratio. Coho salmon follow a seasonal pattern in which the visual pigment composition varies gradually with season. However, some researchers have reported measurable shifts in as little as 3 days. The aim of this work was to determine the rate of chromophore interchange in coho salmon that have been treated with thyroid hormone, and to determine if both rods and cones shift simultaneously. Methods: Coho salmon were reared under 12 hr light: 12 hr dark at 12 °C in two tanks one receiving a control diet and the other the same diet plus hormone treatment. Fish reared on the control diet (commercial salmon food sprayed with alcohol and then evaporated and frozen) received the same food throughout the experiment. The hormone treated fish received food sprayed with triiodothyronine (12ppm) and thyroxine (120ppm) dissolved in alcohol, for the first four weeks. After four weeks the treatment fish were switched to the control diet. The spectral absorbance of rods and cones were measured at weekly intervals using CCD-microspectrophotometry. Results: Control fish maintained a constant mean λmax absorbance in their rods of @ 510 nm throughout the experiment. Thyroid hormone treated fish increased their mean absorbance of their rods from @ 510 nm at week 0 to @ 520 nm at the end of week 4 before being removed from the treatment diet. Within two weeks of stoping the hormone treatment, the mean λmax of rods in the treatment fish had returned to the same level as the control fish. The green/red double cones also differed in mean λmax between control and hormone treated fish: green cones from @ 511nm to @ 522 nm, and red cones from @ 568 nm to @ 576 nm for control and treated respectively. Conclusions: Coho salmon treated with thyroid hormone, doubled the concentration of 11-cis 3,4-dehydroretinal in their visual pigments in approximately four weeks and were capable of returning to their pretreatment ratio in only two weeks. The direction of shift in chromophores was from retinal to dehydroretinal regardless of the season and the natural direction of shift. The shift in chromophores resulted in shifts in absorbance of both rod and cones which would result in considerable alterations of both photopic and scotopic spectral sensitivity.
Keywords: photoreceptors • opsins • retinoids/retinoid binding proteins