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The Transduction Channels of Rod and Cone Photoreceptors

Dimitri Tränkner

CONTENTS

THE TRANSDUCTION CHANNELS OF ROD AND CONE PHOTORECEPTORS THE ROLE OF CNG CHANNELS IN PHOTORECEPTOR PHYSIOLOGY

THE MOLECULAR COMPOSITION OF CNG CHANNELS THE BASIC ACTIVATION PROPERTIES OF CNG CHANNELS

TRANSMEMBRANE TOPOLOGY AND FUNCTIONAL DOMAINS

CNG CHANNELS ARE COMPONENTS OF LARGER PROTEIN COMPLEXES MODULATION BY PHOSPHORYLATION AND ALL-TRANS RETINAL SYNTHESIS, MATURATION, AND TARGETING OF CNG CHANNELS VISUAL DYSFUNCTION CAUSED BY MUTANT CNG CHANNEL GENES REFERENCES

THE TRANSDUCTION CHANNELS OF ROD

AND CONE PHOTORECEPTORS

In contrast to most sensory receptor cells that depolarize in response to a stimulus, vertebrate photoreceptors respond to light with brief hyperpolarization. In darkness, photoreceptors maintain elevated levels of the intracellular messenger guanosine 3,5-cyclic monophosphate (cGMP) that directly activate their transduction channels, the so-called cyclic nucleotide-gated (CNG) ion channels. In the dark, active CNG channels conduct a sustained depolarizing inward current carried by sodium (Na+) and calcium (Ca2+) ions. Light triggers a cascade of enzymatic reactions that culminates in the degradation of cGMP, closure of the CNG channels, and photoreceptor hyperpolarization. Besides functioning as cGMP sensors, CNG channels serve a critical role in the light adaptation of photoreceptors. Light adaptation is Ca2+ dependent and therefore responsive to changes in the Ca2+ flux through CNG channels. The pivotal role of CNG channels in photoreceptor physiology is reflected in the fact that the two photoreceptor types, rods and cones, utilize differently tuned CNG channels to meet specific tasks. The following sections provide an overview of the functional aspects and cell biology of CNG channels in photoreceptors in the normal and diseased state.

From: Ophthalmology Research: Visual Transduction and Non-Visual Light Perception

Edited by: J. Tombran-Tink and C. J. Barnstable © Humana Press, Totowa, NJ

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