Difference in the Gain in the Phototransduction Cascade Between Rods and Cones in Carp

In the vertebrate retina, there are two types of photoreceptors, rods and cones. Rods are highly light-sensitive and cones are less light-sensitive. One of the possible mechanisms accounting for the lower light-sensitivity in cones would be lower signal amplification, i.e., lower gain in the phototransduction cascade in cones. In this study, we compared the difference in the gain between rods and cones electrophysiologically in carp. The initial rising phases of the light responses were analyzed to determine an index of the gain, G, a parameter that can be used to compare the gain among cells of varying outer segment volumes. G (in fL.sec(-2)) was 91.2 +/- 14.8 (n = 5) in carp rods and 25.3 +/- 3.2 (n = 4) in carp red cones, so that the gain in carp red cones is similar to 1/4 of that in carp rods. G was also determined in bullfrog rods and was 81.0 +/- 17.2 (n = 3) which was very similar to that in carp rods. The difference in the gain between rods and cones in carp determined in this study (similar to 1/4 in cones compared with rods) is consistent with that we recently determined biochemically (similar to 1/5 in cones compared with rods). Together with the result obtained in bullfrog rods in this study and the results obtained by others, we concluded that the gain in the cascade is several-fold lower in cones than in rods in carp and probably in other animal species also.