Phototransduction in Vertebrate Rods and Cones

Abstract

Vision begins in rods and cones with light detection by visual pigments in the membranes of the cells' outer segments, and the conversion of this initial quantum event into a series of highly amplified biochemical reactions leading to closure of channels in the outer segment plasma membrane and electrical hyperpolarization. The first stages are membrane-delimited activation of the G protein, transducin, and the cGMP phosphodiesterase, PDE6. The diffusible messenger cGMP carries the signal to cyclic nucleotide-gated channels in the plasma membrane, which close as [cGMP] decreases, leading to reduction in the dark current and hyperpolarization. Upon return to darkness, every step of activation is terminated or reversed, primarily through additional membrane-delimited reactions, with an important role played by Ca2+-feedback mechanisms, involving (the channels?), a Na+/Ca2+-K+ exchanger, and Ca2+-binding proteins. The outlines of this pathway are well established, but many quantitative and structural details remain to be determined.