Your Eyes Not The Only Part Of Your Body That See.
Melanopsin is a type of photopigment belonging to a larger family of light-sensitive retinal proteins called opsins and encoded by the gene Opn4. It 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, the cells involved in image-forming vision, 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. In the mammalian retina, there are two additional 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). It is also found in the iris of mice and primates. 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, also known as the central “body clock,” in mammals. 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. 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. It 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.
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