Molecular Mechanisms of determination of the photoresponse characteristics of cones

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Our visual system consists of two components, rods and cones. These photoreceptors differ in their light-sensitivity so that rods mediate twilight vision and cones mediate daylight vision. Rods and cones are not only distinguished in the light-sensitivity but also in other response characteristics. The photoresponse time course is much briefer in cones, which improves the time resolution of our daylight vision greatly. Rods are saturated with bright background light and do not respond to more intense light. In contrast, cones are not saturated and respond to very bright light. By using purified rods and cones, we examined the molecular mechanisms underlying these differences between rods and cones.We previously showed that, the activation efficiency of transducin by visual pigment is much lower in cones. In the present study, we examined the activation of cone and rod transducin by rod visual pigment. The result showed that rod visual pigment, when it is activated by light, activated cone transducin at a similar efficiency as rod transducin, The result, therefore suggested that the low activation efficiency of cone transducin by cone visual pigment is due to low potency of cone transducin.The recovery of photoresponse after a brief light flash is much faster in cones. This suggests that the cGMP synthetic activity of guanylate cyclase is higher in cones. To prove it, we measured directly the guanylate cyclase activity. The result showed that the activity is approximately 30 times higher in cones. The cyclase activity is enhanced by GCAP proteins. We determined the subtype expression patterns of GCAPs in rods and cones, and determined the amount of expression of these GCAPs. From these determinations, we now understand the cGMP synthesis in rods and cones in a quantitative way.