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]
^"Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
^"Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
^Cite error: The named reference Hankins_2007 was invoked but never defined (see the help page).
^Provencio I, Warthen DM (2012). "Melanopsin, the photopigment of intrinsically photosensitive retinal ganglion cells". Wiley Interdisciplinary Reviews: Membrane Transport and Signaling. 1 (2): 228–237. doi:10.1002/wmts.29.
^Cite error: The named reference S Hatt was invoked but never defined (see the help page).
^Cite error: The named reference pmid18804284 was invoked but never defined (see the help page).
^Nissilä J, Mänttäri S, Tuominen H, Särkioja T, Takala T, Saarela S, et al. (2012). "P-780 – The abundance and distribution of melanopsin (OPN4) protein in human brain". European Psychiatry. 27: 1–8. doi:10.1016/S0924-9338(12)74947-7. S2CID82045589.