Low amplification and fast visual pigment phosphorylation as mechanisms characterizing cone photoresponses

Nov 15, 2001Proceedings of the National Academy of Sciences of the United States of America

Low signal strength and quick light pigment changes in cone cell responses to light

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Abstract

Purified cone photoreceptors showed lower signal amplification and faster visual pigment phosphorylation compared to rod photoreceptors.

Cone photoreceptors exhibit lower light sensitivity than rod photoreceptors.
The purified cone fraction contained red-, green-, and blue-sensitive cones in a ratio of 3:1:1.
Signal amplification in cone membranes is lower, which is associated with reduced light sensitivity.
Visual pigment phosphorylation occurs much faster in cone membranes, contributing to both lower light sensitivity and shorter photoresponses.

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Vertebrate cone photoreceptors are known to show lower light sensitivity and briefer photoresponses than rod photoreceptors. To understand the molecular mechanisms characterizing cone photoresponses, we compared some of the reactions in the phototransduction cascade between rods and cones. For this purpose, rods and cones were obtained in quantities large enough to do biochemical studies. The cells were purified from the retina of carp (Cyprinus carpio) with a stepwise Percoll gradient. The purified rod fraction contained almost no other kinds of cells besides rods, and the purified cone fraction contained a mixture of red-, green-, and blue-sensitive cones in the ratio 3: approximately 1: approximately 1. We prepared membrane preparations from the rod and the cone fraction, and in these membranes, we measured activation efficiencies of the reactions in the phototransduction cascade. The results showed that the signal amplification is lower in the cone membranes, which accounts for the lower light sensitivity in cones. Furthermore, we measured the time courses of visual pigment phosphorylation. The result showed that the phosphorylation is much faster in the cone membranes, which also explains the lower light sensitivity and, in addition, the briefer photoresponse in cones.

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Low amplification and fast visual pigment phosphorylation as mechanisms characterizing cone photoresponses

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