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

Abstract

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:Ϸ1:Ϸ1. 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.

Regulation of Mammalian Cone Phototransduction by Recoverin and Rhodopsin Kinase

“…The role of pigment phosphorylation in shaping the cone light response is much less understood. Studies with zebrafish (14) and carp (43) cones have shown that cone pigments can be phosphorylated in a light-dependent manner. Fish rods and cones use different isoforms of rhodopsin kinase, GRK1 and GRK7, respectively, to phosphorylate their visual pigments.…”

Dephosphorylation during Bleach and Regeneration of Visual Pigment in Carp Rod and Cone Membranes

“…Visual pigments in these membranes were quantified spectrophotometrically. Our purified cone membranes contain all of the cone pigment types but mainly red-sensitive pigment (molar ratio of the pigments, red:green:blue:UV ϭ 3:1:1:negligible) (18). The supernatant was combined with that obtained in the previous centrifugation, and the resultant cytoplasmic proteins were concentrated using Vivaspin 20 (M r 10,000 cutoff, GE Healthcare).…”

Synaptic Ca2+in Darkness Is Lower in Rods than Cones, Causing Slower Tonic Release of Vesicles

“…Rods respond to dim light, generating a slow, prolonged signal, whereas cones require brighter light, generating a rapid, more transient signal (Dowling, 1987). The distinct electrophysiological responses of rods and cones are attributable to differences in the biochemical machinery of phototransduction in outer segments (Tachibanaki et al, 2001;Kefalov et al, 2003Kefalov et al, , 2005Rebrik and Korenbrot, 2004). However, it is unclear whether there are also intrinsic differences in rod and cone synapses that contribute to the distinct messages transmitted by the two cell types.…”