 Visual cycle

The 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.^[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.^[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.