Abstract
Purpose: :
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.
Methods: :
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.
Results: :
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.
Conclusions: :
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.
Keywords: photoreceptors • retina