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P. K. Saarinen, J. Pahlberg, A. Koskelainen, K. Donner; Fractional Sensitivity in Long-Wavelength-Sensitive Cones of Different Species Correlates Inversely With Estimated Thermal Events of Visual Pigment. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3006.
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© ARVO (1962-2015); The Authors (2016-present)
Steady background illumination of sufficient intensity light-adapts photoreceptor cells, decreasing the amplitude of the single-quantum response expressed as a fraction of the saturated response ("fractional sensitivity"). However, even in the absence of real light, cells experience an intrinsic background of light-identical "dark" events due to spontaneous (thermal) activations of visual-pigment molecules. Photoreceptor cells with high rates of such events might potentially be "light-adapted" even in complete darkness. The purpose of our work was to study the impact of thermal pigment activations on photoreceptor sensitivity in an evolutionary perspective.
We collected data on fractional sensitivity and calculated rates of dark events in different L-cones, which possess visual pigments thought (as a class) to be particularly susceptible to thermal activation. For comparison, similar data were collected for different rods, which have low rates of dark events.
In 8 species of fish, amphibian, reptile and mammalian L-cones with estimated dark event rates ranging from ca. 10 to ca. 700 per cell per second (Rh*s-1), we found a significant inverse correlation with fractional sensitivity. By contrast, in 11 species of rods with dark event rates ranging from ca. 0.005 to ca. 0.30 Rh*s-1, there was at the most a hint of a decrease in fractional sensitivity at the upper end of the range.
The results are consistent with the idea that the high rate of thermal activation of L-cone pigment constitutes an evolutionary limitation on the dark-adapted sensitivity of L-cones.
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