|Opsin 4 (melanopsin)|
Melanopsin is a photopigment found in specialized photosensitive ganglion cells of the retina that are involved in the regulation of circadian rhythms, pupillary light reflex, and other non-visual responses to light. In structure, melanopsin is an opsin, a retinylidene protein variety of G-protein-coupled receptor.
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.
Melanopsin was originally discovered in 1998 in specialized light-sensitive cells of frog skin by Dr. Ignacio Provencio and his colleagues. In 1999 Dr. Russell Foster showed that a third class of photoreceptor existed in mammalian eyes. In 2000, Provencio showed that mammals, including humans, also produce melanopsin and that it is found only in a rare subtype of retinal ganglion cells, the output cells of the retina.
They also showed that these responses persisted when pharmacological agents blocked synaptic communication in the retina, and when single melanopsin-containing ganglion cells were physically isolated from other retinal cells. These findings showed that melanopsin-containing ganglion cells are intrinsically photosensitive, thus named intrinsically photosensitive Retinal Ganglion Cells (ipRGC). They constitute a third class of photoreceptors in the mammalian retina, beside the already known rod and cone photoreceptors.
Further studies from Berson's lab have concluded that melanopsin-containing ganglion cells exhibit both light and dark adaptation, that is, that they adjust their sensitivity according to the recent history of light exposure. In this respect, they are similar to rods and cones. Whereas rods and cones are responsible for the analysis of images, patterns, motion and color, a number of studies have shown that melanopsin-containing ganglion cells contribute to various reflexive responses of the brain and body to the presence of (day)light.
Melyan et al. in England in 2005 reported rendering a mouse paraneuronal cell line (Neuro-2a), which normally is not photosensitive, photoreceptive by the addition of human melanopsin. Under such conditions, melanopsin acts as a sensory photopigment, performing physiological light detection. The melanopsin photoresponse is selectively sensitive to short-wavelength light (peak absorption ~480 nm), while it also has an intrinsic photoisomerase regeneration function that is chromatically shifted to longer wavelengths.
These targets include the olivary pretectal nucleus (a center responsible for controlling the pupil of the eye) and, through the retinohypothalamic tract (RHT), the suprachiasmatic nucleus of the hypothalamus (the master pacemaker of circadian rhythms).
Melanopsin-containing ganglion cells are thought to influence these targets by releasing from their axon terminals the neurotransmitters glutamate and pituitary adenylate cyclase activating polypeptide (PACAP).
Melanopsin-containing ganglion cells also receive input from rods and cones that modifies or adds to the input to these pathways.
Experiments have shown that entrainment to light, by which periods of behavioral activity or inactivity (sleep) are synchronized with the light-dark cycle, is not as effective in melanopsin knockout mice, but mice lacking rods and cones still exhibit circadian entrainment. The pupillary reflex is also retained in mice lacking rods and cones but has severely reduced sensitivity, identifying a crucial input from the rods and cones.
Blind people who entrain to the 24-hour light/dark cycle have eyes with functioning retinas including the operative non-visual light-sensitive cells which convey their signals to the "circadian clock" via the retinohypothalamic tract.
Melanopsin has a very similar pattern of tissue distribution among all mammals studied so far, including rodents, monkeys, and humans. Specifically, melanopsin is expressed only in the retina, and only in 1-2% of the ganglion cells.
In non-mammalian vertebrates, however, such as birds, fish and amphibians, melanopsin is found in certain other retinal cells, and also outside the retina in structures known or presumed to be directly photosensitive, such as the iris muscle of the eye, deep brain regions, the pineal gland, and the skin.