Melanopsin (gene symbol:Opn4) is the G protein-coupled photopigment that confers photosensitivity upon intrinsically photosensitive retinal ganglion cells (ipRGCs). ipRGCs are the third class of retinal photoreceptor in mammals, complementing the two previously identified classes, the rods and cones. This novel class, however, differs from rods and cones in many significant ways. First, ipRGCs are more similar morphologically to other retinal ganglion cell classes than to other retinal photoreceptors, i.e., rods and cones. Instead of having photopigment concentrated in a specialized light-absorbing cellular domain such as the outer segment, ipRGCs have photopigment distributed throughout the plasma membrane of the cell. Second, the phototransduction cascade of ipRGCs more closely resembles that of the rhabdomeric photoreceptors that are typically found in the invertebrates rather than that of ciliary photoreceptors typical of vertebrate visual systems. Accordingly, like the rhabdomeric photoreceptors of invertebrates, ipRGCs depolarize in response to illumination while rods and cones hyperpolarize. Third, in addition to their inherent light sensitivity, ipRGCs also function as a conduit for information that originates in the rods and cones and is conveyed to the brain for the purposes of generating non-visual light responses.WIREs Membr Transp Signal 2012, 1:228–237. doi: 10.1002/wmts.29
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