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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.〔 Two other opsins in the mammalian retina are both involved in the formation of visual images: rhodopsin and photopsin in the rod and cone photoreceptor cells, respectively.
In humans, melanopsins is found in intrinsically photosensitive retinal ganglion cells (ipRGCs), as well as the cerebral cortex of the brain and in epidermal skin. Melanopsin also is found rats, mice, amphioxus, and other chordates. ipRGCs are photoreceptor cells which are particularly sensitive to the absorption of short-wavelength visible light and communicate information directly to the area of the brain called the suprachiasmatic nucleus (SCN), otherwise known as the central "body clock" in mammals.〔 Consequently, 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).〔 According to one study, melanopsin has been found in eighteen sites in the human brain (outside of 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.〔
== Discovery ==
Melanopsin was first discovered by Ignacio Provencio as a novel opsin in the melanophores, or light sensitive skin cells, of the African clawed frog in 1998. A year later, researchers found that mice without any rods or cones still entrained to a light-dark cycle. This observation led to the conclusion that neither rods nor cones, located in the outer retina, are necessary for circadian entrainment and that a third class of photoreceptor exists in the mammalian eye. Provencio and colleagues then found in 2000 that melanopsin is also present in mouse retina, specifically in ganglion cells, and that it mediates non visual photoreceptive tasks.〔 Melanopsin was found to be encoded by Opn4 with orthologs in a variety of organisms.〔
These retinal ganglion cells were found to be innately photosensitive, since they responded to light even while isolated, and were thus named intrinsically photosensitive Retinal Ganglion Cells (ipRGCs). They constitute a third class of photoreceptor cells in the mammalian retina, besides the already known rods and cones, and were shown to be the principal conduit for light input to circadian photoentrainment.〔 In fact, it was later demonstrated by Satchidananda Panda and colleagues that melanopsin pigment may be involved in entrainment of a circadian oscillator to light cycles in mammals since melanopsin was necessary for blind mice to respond to light.
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