The cGMP-Gated Channels of Rod and Cone Photoreceptors
Light generates a hyperpolarizing response in retinal photoreceptors by stopping a steady inward current (the dark current) that is present at their outer segments in darkness.(1–3) The mechanism underlying this phototransduction process has been a subject of intense interest and controversy for many years, and is only now becoming fairly clear. The present picture for rods(4–6) is that upon absorbing a photon a visual pigment molecule isomerizes and catalytically activates a GTP-binding protein (also called G-protein) that is bound peripherally to disk membranes within the outer segment. The activated G-protein in turn stimulates a phosphodiesterase that hydrolyzes cGMP, a cyclic nucleotide known to be present at a high concentration in the outer segment. In darkness, this high cGMP level maintains a plasma membrane cationic conductance (the light-regulated conductance) in the open state, thus sustaining the dark current. In the light, the fall in the cGMP level leads to the closure of this conductance, and therefore the cessation of the dark current. This picture for phototransduction in rods now seems to apply to cones as well,(7) despite their different surface membrane geometries.
KeywordsDivalent Cation Dark Current Outer Segment Tiger Salamander cGMP Concentration
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