While both rods and cones sense light in the vertebrate retina, they have distinct functional differences, including sensitivity to light and temporal resolution and differences in their underlying cell biology. Both types of photoreceptors turn light into an electrical signal through a series of enzyme activities known as the phototransduction cascade. The isolated and purified cells of the carp retina were used to biochemically compare each reaction in the phototransduction cascades of rods and cones. This allowed researchers to gain insight into the mechanisms underlying the functional distinctions between rods and cones. Although the proteins in the cascade are the same or very similar in function between rods and cones, their activity and expression levels are very different. Cones are slightly less successful than rods in reactions that generate a response, but each reaction for terminating and recovering from a response is significantly more efficient in cones. These results account for why cones have less light sensitivity and shorter light responses than rods. Furthermore, these findings imply that the Ca 2+– binding protein S-modulin or recovery has a role in structuring light responses that have not been previously appreciated. After comparing the expression levels of proteins and/or mRNAs using isolated cells, several proteins were determined to be exclusively or primarily expressed in cones. Future research aiming to characterize the distinctions between rods and cones may find these proteins of interest.
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