Melanopsin

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 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]