The retinoid cycle in cones (daylight vision)

The retinoid cycle in cones (daylight vision) Rods and cones share the same mechanism for the phototransduction process but perform functionally different roles. Although cone photoreceptors make up around 5% of all photoreceptor cells and are outnumbered 20 to 1 by rod photoreceptors, they mediate daylight vision in the human eye whereas rods mediate twilight vision. Also, cones are around 100-times less light-sensitive than rods thereby depriving us of colour vision in dark conditions in which cones cannot function. Rod function saturates in even moderate amounts of light whereas cones can adjust to even very bright light conditions, a process called light adaptation. In bright conditions, rods can take up to one hour to regain their sensitivity whereas cones can recover in a few minutes, a process called dark adaptation and which allows us to retain visual perception in changing light conditions.

Cone cells express three types of opsin which allow colour discrimination. Long Wavelength Sensitive Opsin (OPN1LW) detects red , Short Wavelength Sensitive Opsin (OPN1SW) detects blue, and Medium Wavelength Sensitive Opsin (OPN1MW) detects green regions of the light spectrum.

In the canonical retinoid (visual) cycle, the visual chromophore is regenerated in reactions involving the rod outer segments (ROS) and the retinal pigment epithelium (RPE). For cones, chromophore recycling is independent of the RPE and instead involves Muller cells in the retina which supply the chromophore selectively to cones. The molecular steps of the cone retinoid (visual) cycle are outlined in this section. The ability of cones to react to bright and differing light conditions means it has to regenerate the chromophore much quicker than rods. All-trans-retinol (atROL) released from cone outer segments is taken up by Muller cells where it is directly isomerized back to 11-cis-retinol (11cROL) then esterified by LRAT. When required, these 11-cis-retinyl esters can be hydrolysed by 11-cis-RE hydrolases back to 11cROL then oxidised in the cone photoreceptor cell to regenerate 11-cis-retinal (11cRAL), the visual chromophore (see reviews von Lintig 2012, Wang & Kefalov 2011, Kefalov 2012, Wolf 2004).