melanopsine

Melanopsin, also called melanopsine in some languages, is a photopigment protein encoded by the OPN4 gene. It is expressed in a subset of retinal ganglion cells known as intrinsically photosensitive retinal ganglion cells (ipRGCs). Unlike classical photoreceptors, ipRGCs can be directly activated by light, enabling non-image-forming vision in addition to daylight perception. Melanopsin-containing ipRGCs project to brain regions such as the suprachiasmatic nucleus (SCN), the body’s central circadian clock, and to the olivary pretectal nucleus, which mediates the pupil light reflex.

Biochemically, melanopsin is a blue-light–sensitive G protein-coupled receptor. It binds 11-cis retinal as a chromophore and, upon absorption of photons, signals through Gq/11 pathways to activate phospholipase C, increasing intracellular calcium and producing a sustained depolarizing response in ipRGCs. The photic response is relatively slow and long-lasting compared with rods and cones, enabling the encoding of cumulative light exposure rather than fast image formation. The action spectrum of melanopsin peaks near 480 nanometers (blue light).

Functions attributed to melanopsin include entrainment of circadian rhythms to the light-dark cycle, suppression of melatonin during daytime, and the pupil light reflex. It also contributes to various non-image-forming responses to light, including alertness and mood modulation in some contexts. Rod and cone signaling interacts with melanopsin pathways to shape the overall retinal response to ambient light.

Melanopsin is highly conserved in vertebrates and is a key component of the non-image-forming visual system. OPN4 gene expression in ipRGCs provides a complementary photoreception mechanism alongside rods and cones, supporting physiological and behavioral adaptations to environmental light.