Visual phototransduction

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Homo sapiens
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Visual phototransduction is the process by which photon absorption by visual pigment molecules in photoreceptor cells is converted to an electrical cellular response. The events in this process are photochemical, biochemical and electrophysiological and are highly conserved across many species. This process occurs in two types of photoreceptors in the retina, rods and cones. Each type consists of two parts, the outer segment which detects a photon signal and the inner segment which contains the necessary machinery for cell metabolism. Each type of cell functions differently. Rods are very light sensitive but their flash response is slow so they work best in twilight conditions but are not good at detecting objects moving quickly. Cones are less light-sensitive and have a fast flash response so they work best in daylight conditions and are better at detecting fast moving objects than rods.

The visual pigment consists of a chromophore (11-cis-retinal, 11cRAL, A1) covalently attached to a GPCR opsin family member. The linkage is via a Schiff base forming retinylidene protein. Upon photon absorption, 11cRAL isomerises to all-trans retinal (atRAL), changing the conformation of opsin to an activated form which can activate the regulatory G protein transducin (Gt). The alpha subunit of Gt activates phosphodiesterase which hydrolyses cGMP to 5'-GMP. As high level of cGMP keep cGMP-gated sodium channels open, the lowering of cGMP levels closes these channels which causes hyperpolarization of the cell and subsequently, closure of voltage-gated calcium channels. As calcium levels drop, the level of the neurotransmitter glutamate also drops causing depolarization of the cell. This effectively relays the light signal to postsynaptic neurons as electrical signal (Burns & Pugh 2010, Korenbrot 2012, Pugh & Lamb 1993).

11cRAL cannot be synthesised in vertebrates. Vitamin A from many dietary sources is the precursor for 11cRAL. It is taken from food in the form of esters such as retinyl acetate or palmitate or one of four caretenoids (alpha-carotene, beta-carotene, gamma-carotene and beta-cryptoxanthin). Retinoids are transported from the gut to be stored in liver, until required by target organs such as the eye (Harrison & Hussain 2001, Harrison 2005). In the eye, in the form 11cRAL, it is used in the retinoid (visual) cycle to initiate phototransduction and for visual pigment regeneration to ready the photoreceptor for the next phototransduction event (von Lintig 2012, Blomhoff & Blomhoff 2006, von Lintig et al. 2010, D'Ambrosio et al. 2011, Wang & Kefalov 2011, Kefalov 2012, Wolf 2004).

Literature References
PubMed ID Title Journal Year
22658984 Speed, sensitivity, and stability of the light response in rod and cone photoreceptors: facts and models

Korenbrot, JI

Prog Retin Eye Res 2012
8382952 Amplification and kinetics of the activation steps in phototransduction

Pugh, EN, Lamb, TD

Biochim. Biophys. Acta 1993
22074928 Rod and cone visual pigments and phototransduction through pharmacological, genetic, and physiological approaches

Kefalov, VJ

J. Biol. Chem. 2012
21111842 The cone-specific visual cycle

Wang, JS, Kefalov, VJ

Prog Retin Eye Res 2011
20430952 Lessons from photoreceptors: turning off G-protein signaling in living cells

Burns, ME, Pugh, EN

Physiology (Bethesda) 2010
11340090 Mechanisms involved in the intestinal digestion and absorption of dietary vitamin A

Harrison, EH, Hussain, MM

J Nutr 2001
15384919 The visual cycle of the cone photoreceptors of the retina

Wolf, G

Nutr. Rev. 2004
21350678 Vitamin A metabolism: an update

D'Ambrosio, DN, Clugston, RD, Blaner, WS

Nutrients 2011
20188572 The biochemical and structural basis for trans-to-cis isomerization of retinoids in the chemistry of vision

von Lintig, J, Kiser, PD, Golczak, M, Palczewski, K

Trends Biochem. Sci. 2010
16688755 Overview of retinoid metabolism and function

Blomhoff, R, Blomhoff, HK

J. Neurobiol. 2006
22074927 Metabolism of carotenoids and retinoids related to vision

von Lintig, J

J. Biol. Chem. 2012
16011460 Mechanisms of digestion and absorption of dietary vitamin A

Harrison, EH

Annu Rev Nutr 2005
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