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S.E. Temple, W.T. Allison, T.J. Haimberger, C.W. Hawryshyn; Visual Pigment Shifting in Zebrafish (Danio rerio) . Invest. Ophthalmol. Vis. Sci. 2003;44(13):5114.
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Purpose: Numerous freshwater fishes have the ability to shift between 11-cis retinal (for vitamin A1 based visual pigments) and 11-cis 3,4-dehydroretinal (for vitamin A2 based visual pigments) as their chromophore. The ratio of the two chromophores in the retinas of most fish is dynamic and varies with environmental factors such as temperature and day length. This shift from A1- to A2-based visual pigment results in a bathochromic shift in spectral absorbance of the photoreceptors. Therefore, measurements and conclusions about the spectral sensitivity of a fish must consider the ratio of the two visual pigments. Numerous reports indicate that zebrafish use only A1-based visual pigments. However, cyprinids are known to posses a labile system, and recent reports have invoked the hypothesis that zebrafish have A2-based visual pigments to explain variance in spectral sensitivity. The goal of this work was to determine if zebrafish have a chromophore interchange system. Methods: Zebrafish were reared in three temperature treatments covering the full range of that found in their natural environment; 15, 20, and 28 °C. The fourth treatment was a thyroid hormone bath at 28 °C. We also performed a chromophore replacement experiment in which we bleached zebrafish retinas and reconstituted the chromophores using lipid vesicles charged with 11-cis retinal. For both experiments, photoreceptor absorbance was measured using CCD-microspectrophotometry. Results: We found no significant effect of temperature with all fish having predominantly vitamin A1-based visual pigments after 5-6 weeks. In rods, the thyroid hormone treatment resulted in a shift from A1- to A2-based visual pigments within two weeks. Utilization of A2-based visual pigments was measured in both rod and cone photoreceptors. Bleaching and reconstitution resulted in all receptor types measured possessing vitamin A1-based visual pigments. Conclusions: Zebrafish have a chromophore interchange system, that enables them to shift between A1- and A2-based visual pigments in both rods and cones. Visual pigment composition was not correlated to temperature as has been found in several other fishes. Thyroid hormone treatment did affect a shift to A2-based visual pigment dominance, which has been demonstrated in other fishes, including cyprinids. The existence of a chromophore interchange system has important implications for studies examining spectral sensitivity in zebrafish, particularly in the case of genetic, hormonal or regenerative manipulations. Supported by a Fellowship from the Alzheimer Society of Canada and the CIHR Institute of Aging (WTA), operating grants from NSERC (CWH).
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