Review Rod and cone photoreceptors: Molecular basis of the difference in their physiology Satoru Kawamura ⁎, Shuji Tachibanaki Graduate School of Frontier Biosciences and Department of Biology, Graduate School of Science, Osaka University, Yamada-Oka 1-3, Suita, Osaka 565-0871, Japan abstract article info Article history: Received 5 March 2008 Received in revised form 15 April 2008 Accepted 15 April 2008 Available online 26 April 2008 Keywords: Rods Cones Phototransduction Twilight vision Daylight vision Sensitivity Vertebrate retinal photoreceptors consist of two types of cells, the rods and cones. Rods are highly light- sensitive but their flash response time course is slow, so that they can detect a single photon in the dark but are not good at detecting an object moving quickly. Cones are less light-sensitive and their flash response time course is fast, so that cones mediate daylight vision and are more suitable to detect a moving object than rods. The phototransduction mechanism was virtually known by the mid 80s, and detailed mechanisms of the generation of a light response are now understood in a highly quantitative manner at the molecular level. However, most of these studies were performed in rods, but not in cones. Therefore, the mechanisms of low light-sensitivity or fast flash response time course in cones have not been known. The major reason for this slow progress in the study of cone phototransduction was due to the inability of getting a large quantity of purified cones to study them biochemically. We succeeded in its purification using carp retina, and have shown that each step responsible for generation of a light response is less effective in cones and that the reactions responsible for termination of a light response are faster in cones. Based on these findings, we speculated a possible mechanism of evolution of rods that diverged from cones. © 2008 Elsevier Inc. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369 2. Molecular basis underlying the light response in rods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370 2.1. Phototransduction in rods: a general overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370 2.2. Recovery of a light response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371 2.3. Light-adaptation: control of light-sensitivity by a Ca 2+ -feedback mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371 3. Phototransduction in cones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372 3.1. Isolation of cone photoreceptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372 3.2. Activation of cone phototransduction cascade . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372 3.3. Inactivation of cone phototransduction cascade . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373 4. Comparison of in vivo and in vitro studies on the difference of phototransduction efficiencies in rods and cones . . . . . . . . . . . . . . 373 4.1. Amplification in tiger salamander cones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373 4.2. Amplification in carp rods and cones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374 5. A hypothetical model of evolution of rods from cones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375 6. Future perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376 1. Introduction In the vertebrate retina, there are two types of photoreceptors, the rods and cones (Fig. 1A). The rods and cones consist of two parts, outer segment and inner segment. The outer segment is the specialized site to detect a photon signal, and the inner segment contains the nucleus and other cellular organelles necessary for cell metabolism. Rods and cones are distinctive in their morphology of the outer segment. In rods, stacks of disk membranes are surrounded by a plasma membrane, and in cones, the plasma membrane invaginates repeat- edly to form a tightly stacked lamellae structure. Rods and cones are also different in their characteristics in responding to light: the Comparative Biochemistry and Physiology, Part A 150 (2008) 369–377 ⁎ Corresponding author. Tel.: +816 6879 4610; fax: +81 6 6879 4614. E-mail address: kawamura@fbs.osaka-u.ac.jp (S. Kawamura). 1095-6433/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.cbpa.2008.04.600 Contents lists available at ScienceDirect Comparative Biochemistry and Physiology, Part A journal homepage: www.elsevier.com/locate/cbpa