Melanopsin

Melanopsin is a type of photopigment belonging to a larger family of light-sensitive retinal proteins called opsins and encoded by the gene Opn4.^[5] In the mammalian retina, there are two additional categories of opsins, both involved in the formation of visual images: rhodopsin and photopsin (types I, II, and III) in the rod and cone photoreceptor cells, respectively.

In humans, melanopsin is found in intrinsically photosensitive retinal ganglion cells (ipRGCs).^[6] It is also found in the iris of mice and primates.^[7] Melanopsin is also found in rats, amphioxus, and other chordates.^[8] ipRGCs are photoreceptor cells which are particularly sensitive to the absorption of short-wavelength (blue) visible light and communicate information directly to the area of the brain called the suprachiasmatic nucleus (SCN), also known as the central "body clock", in mammals.^[9] Melanopsin plays an important non-image-forming role in the setting of circadian rhythms as well as other functions. Mutations in the Opn4 gene can lead to clinical disorders, such as Seasonal Affective Disorder (SAD).^[10] According to one study, melanopsin has been found in eighteen sites in the human brain (outside the retinohypothalamic tract), intracellularly, in a granular pattern, in the cerebral cortex, the cerebellar cortex and several phylogenetically old regions, primarily in neuronal soma, not in nuclei.^[11] Melanopsin is also expressed in human cones. However, only 0.11% to 0.55% of human cones express melanopsin and are exclusively found in the peripheral regions of the retina.^[12] The human peripheral retina senses light at high intensities that is best explained by four different photopigment classes.^[13]