Melanopsin is a photopigment found in some retinal ganglion cells in the eyes of humans and other vertebrates. These cells, known as intrinsically photosensitive retinal ganglion cells, perceive light but are much slower to react to visual changes than the better-known rod and cone cells. They have been shown to affect circadian rhythms, the pupillary light reflex, and several other functions related to ambient light.
In structure, melanopsin is an opsin, a retinylidene protein variety of G-protein-coupled receptor. Melanopsin is most sensitive to blue light. A melanopsin based receptor has been linked to the association between light sensitivity and migraine pain.
Melanopsin differs from other opsin photopigments in vertebrates. In fact, it resembles invertebrate opsins in many respects, including its amino acid sequence and downstream signaling cascade. Like invertebrate opsins, melanopsin appears to be a bistable photopigment, with intrinsic photoisomerase activity, and to signal through a G-protein of the Gq family.
Evidence supports prior theories that melanopsin is the photopigment responsible for the entrainment of the central „body clock“, the suprachiasmatic nuclei (SCN), in mammals. Fluorescent immunocytochemistry was used to visualize melanopsin distribution throughout the rat retina and showed that melanopsin was found in approximately 2.5% of the total rat retinal ganglion cells (RGCs) and that these cells were indeed ipRGCs. Using β-galactosidase as a marker for the melanopsin gene, X-gal labeling of these ipRGCs showed that their axons directly target the SCN, providing further evidence that melanopsin is important in entrainment through the retinohypothalamic tract (RHT).
More about melanopsin can be read here.