Summary
Highlights
Opsins are broadly categorized into Type One (microbial group, in one-celled organisms, sometimes acting as ion pumps) and Type Two (in eukaryotes and higher animals, mostly for vision and circadian rhythms). Type Two opsins branched into ciliary and rhabdomeric groups. Ciliary opsins are in vertebrate rods and cones, mediating vision. Melanopsin is a rhabdomeric opsin. The discovery of melanopsin in the human eye and ciliary opsins in invertebrates was surprising, suggesting a shared evolutionary origin for retinal structures.
The retina functions with three layers of nerve cells: photoreceptors, bipolar cells, and ganglion cells. Amacrine and horizontal cells also provide connections. Ganglion cells, along with amacrine and horizontal cells, appear to be derived from the rhabdomeric line, explaining the presence of melanopsin in ganglion cells. Rods, cones, and bipolar cells seem to originate from the ciliary line. This new understanding helps piece together the evolutionary wiring of the retina.
Only a small percentage of ganglion cells contain melanopsin. Experiments with mice lacking rods and cones showed that melanopsin-containing cells still mediated pupil response and regulated circadian rhythms. However, mice lacking both rods, cones, and melanopsin lost all pupil response and the ability to set circadian clocks. This indicates that melanopsin is crucial for non-visual light sensing, such as pupil constriction and synchronizing the body's internal clock, with potential applications for human adaptation to light and dark cycles.
Melanopsin is a visual pigment that doesn't contribute to vision but provides important information about the eye. It is distinct from melatonin, though both are involved in circadian rhythms. Melanopsin is a protein found in the retina, while melatonin is a tryptophan derivative found in the pineal gland. While humans have four other opsins for vision (three in cones for color and one in rods for low light), melanopsin is a fifth, unique opsin.
Unlike other opsins, melanopsin is not located in rods or cones and does not contribute to vision. Instead, it is found in light-sensing ganglion cells in the retina. Photoreceptors, including rods and cones, consist of retinal (derived from vitamin A) which absorbs light, and an opsin protein that determines the light's color sensitivity. Melanopsin's presence in ganglion cells highlights a surprising aspect of retinal structure.
There are two main types of photoreceptor cells in the animal kingdom: rhabdomeric, found mostly in invertebrates (e.g., squids, insects), and ciliary, generally associated with vertebrate eyes (rods and cones). Rhabdomeric cells have villi to multiply opsin molecules and recycle retinal in place using a second photon, utilizing R opsins, including melanopsin. Ciliary cells use membrane folds or plates for opsin exposure and require a complex recycling process for retinal, utilizing C opsins.