Fig. 7

Proposed model for the photic cone visual cycle in Müller glial cells and cone photoreceptors Absorption of a photon (hv) by a cone-opsin pigment isomerizes the 11cRAL to atRAL, activating visual transduction. Shortly afterward, atRAL dissociates from the opsin, is reduced to atROL by RDH12, and is released into the extracellular space for recycling. The atROL is taken up by the Müller cell where it is re-oxidized by RDH10 to atRAL, which binds covalently to apo-RGR opsin. Absorption of a photon by RGR opsin isomerizes atRAL to 11cRAL. 30 The 11cRAL is reduced, again by RDH10, to 11cROL, which is released by Müller cells into the extracellular space. 15 Finally, the 11cROL is taken up by the cone photoreceptor where it is oxidized by RDH12 to 11cRAL, which combines with cone apo-opsin to form a new cone visual pigment. Because only cones can utilize 11cROL to regenerate their visual pigments, the visual cycle in Müller cells helps cones avoid competition with rods for chromophore under bright-light conditions.