= Visual cycle
The \b[visual cycle] is the set of biochemical transformations that regenerates the light-sensitive retinal supply of visual pigments after illumination. In the classical rod pathway, a <photon> absorbed by <rhodopsin> converts its bound <11-cis-retinal> toward <all-trans-retinal>. Subsequent processing involving the <photoreceptor> and the <retinal pigment epithelium> returns retinal to a form that can rebuild light-sensitive pigment. This recycling connects a rapid light-triggered molecular event to slower recovery of sensitivity.<Crystal Structure of Rhodopsin: A G Protein-Coupled Receptor by Krzysztof Palczewski and others>[Page 739: photoisomerization, dissociation of photolyzed retinal and regeneration from 11-cis-retinal supplied by adjacent epithelial cells.]{ref}
A useful experiment interrupted this supply rather than removing the receptor itself. <T. M. Redmond> and collaborators disrupted <RPE65> in mice and measured retinal compounds and visual responses. The animals lacked normal <rhodopsin> and rod <electroretinography> responses even though rod <opsin> remained; all-trans retinyl esters accumulated, while 11-cis retinoids were absent. These results established that <RPE65> is necessary for the normal production of 11-cis vitamin A derivatives in this pathway.<Rpe65 is necessary for production of 11-cis-vitamin A in the retinal visual cycle by T. M. Redmond and others>[Abstract and experimental comparison of Rpe65-deficient and control mice.]{ref}
The contrast supplies an illuminating example of biochemical dependence: a cell may retain a receptor <protein> yet lose the response because it cannot regenerate the small molecule required by that receptor. The <visual cycle> is the chemical recycling pathway; <electroretinography> measures an electrical consequence of its success or failure. The knockout establishes necessity in the tested pathway and conditions, rather than proving that every <photoreceptor> follows an identical regeneration route.
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