cone visual cycle
The visual processing of humans is primarily reliant upon the sensitivity of cone photoreceptors to light during daylight conditions. This underscores the importance of understanding how cone photoreceptors maintain the ability to detect light.
The vertebrate retina consists of a combination of both rod and cone photoreceptors. Subsequent to light exposure, both rod and cone photoreceptors are dependent upon the recycling of vitamin A to regenerate photopigments, the proteins responsible for detecting light. Metabolic processing of vitamin A in support of rod photopigment renewal, the so-called "rod visual cycle", is well established.
The activities of Isomerase I and II have been investigated but the nature of the Isomerase II protein remains unknown. The activity of the all-trans retinyl ester synthase remains to be investigated since it is not a known enzyme such as LRAT, nor all-trans ARAT. The RDH in the photoreceptor and in the Müller cell have not been fully investigated at this time.
However, the metabolic processing of vitamin A in support of cone photopigment renewal remains a challenge for characterization in the recently discovered "cone visual cycle".
The specificity and importance of CRALBP in the retinol isomerase reaction has been underscored. The contribution of retinoid-binding proteins such as CRALBP and IRBP to throughput (flux?) in specific steps of the cone visual cycle remains a topic for future research. The role of CRALBP in supporting the activity of Isomerase II in the retina may prove to be similar to the pivotal role of isomerase I in the RPE.
It has been hypothesized that a separate cone visual cycle was first developed in duplex retinas to allow cones to avoid competition with rods for 11-cis retinal supplied by the RPE.
Thus, it is important to examine whether a neuro-retinal cone visual cycle co-exists with the RPE rod cycle in duplex retinas, from which modern cone-rich retinas were derived, and if so, the degree to which the two visual cycles interact under bright and dim-light conditions.
Rod-dominated human retinas contain a cone-only area centralis and a cone-rich fovea/macula. It is not known if a cone cycle, similar to that described herein for cone-dominated retinas, exists in these locations to support cone pigment regeneration.
References
Muniz A, Villazana-Espinoza ET, Hatch AL, Trevino SG, Allen DM, Tsin AT. A novel cone visual cycle in the cone-dominated retina. Exp Eye Res. 2007 Aug;85(2):175-84. PMID: 17618621. Free in PMC