Melanopsin is a type of
photopigment belonging to a larger family of light-sensitive
retinal proteins called
opsin
Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become Retinylidene proteins, but are usually still called opsins regardless. Most ...
s and encoded by the gene ''Opn4''.
In the mammalian
retina
The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which the ...
, there are two additional categories of opsins, both involved in the formation of visual images:
rhodopsin and
photopsin
Vertebrate visual opsins are a subclass of ciliary opsins and mediate vision in vertebrates. They include the opsins in human rod and cone cells. They are often abbreviated to ''opsin'', as they were the first opsins discovered and are still th ...
(types I, II, and III) in the
rod and
cone photoreceptor cells, respectively.
In humans, melanopsin is found in
intrinsically photosensitive retinal ganglion cells (ipRGCs).
It is also found in the iris of mice and primates.
Melanopsin is also found in rats,
amphioxus, and other chordates.
ipRGCs are photoreceptor cells which are particularly sensitive to the absorption of short-wavelength (blue) visible light and communicate information directly to the area of the brain called the
suprachiasmatic nucleus
The suprachiasmatic nucleus or nuclei (SCN) is a tiny region of the brain in the hypothalamus, situated directly above the optic chiasm. It is responsible for controlling circadian rhythms. The neuronal and hormonal activities it generates regu ...
(SCN), also known as the central "body clock", in mammals.
Melanopsin plays an important non-image-forming role in the
setting
Setting may refer to:
* A location (geography) where something is set
* Set construction in theatrical scenery
* Setting (narrative), the place and time in a work of narrative, especially fiction
* Setting up to fail a manipulative technique to e ...
of circadian rhythms as well as other functions. Mutations in the ''Opn4'' gene can lead to clinical disorders, such as
Seasonal Affective Disorder (SAD).
According to one study, melanopsin has been found in eighteen sites in the human brain (outside the retinohypothalamic tract), intracellularly, in a granular pattern, in the cerebral cortex, the cerebellar cortex and several phylogenetically old regions, primarily in neuronal soma, not in nuclei.
Melanopsin is also expressed in human cones. However, only 0.11% to 0.55% of human cones express melanopsin and are exclusively found in the peripheral regions of the retina. The human peripheral retina senses light at high intensities that is best explained by four different photopigment classes.
Discovery
Melanopsin was discovered by
Ignacio Provencio as a new
opsin
Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become Retinylidene proteins, but are usually still called opsins regardless. Most ...
in the
melanophores, or light-sensitive skin cells, of the
African clawed frog in 1998.
A year later, researchers found that mice without any
rods or
cones, the cells involved in image-forming vision, still
entrained to a light-dark cycle.
This observation led to the conclusion that neither rods nor cones, located in the outer
retina
The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which the ...
, are necessary for circadian entrainment and that a third class of photoreceptor exists in the mammalian eye.
Provencio and colleagues then found in 2000 that melanopsin is also present in mouse retina, specifically in
ganglion cells, and that it mediates non-visual photoreceptive tasks.
Melanopsin is encoded by the ''Opn4'' gene with
orthologs in a variety of organisms.
These retinal ganglion cells were found to be innately photosensitive, since they responded to light even while isolated, and were thus named
intrinsically photosensitive Retinal Ganglion Cells (ipRGCs).
They constitute a third class of
photoreceptor cell
A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiat ...
s in the mammalian retina, besides the already known rods and cones, and were shown to be the principal conduit for light input to circadian
photoentrainment.
In fact, it was later demonstrated by Satchidananda Panda and colleagues that melanopsin pigment may be involved in entrainment of a
circadian oscillator to light cycles in mammals since melanopsin was necessary for blind mice to respond to light.
Species distribution
Mammals have
orthologous melanopsin genes named ''Opn4m'', which are derived from one branch of the ''Opn4'' family, and are approximately 50-55% conserved.
However, non-mammalian vertebrates, including chickens and zebrafish, have another version of the melanopsin gene, ''Opn4x'', which appears to have a distinct lineage that diverged from ''Opn4m ''about 360 million years ago. Mammals lost the gene ''Opn4x'' relatively early in their evolution, leading to a general reduction in photosensory capability. It is thought that this event can be explained by the fact that this occurred during the time in which nocturnal mammals were evolving.
Structure
The human melanopsin gene, ''opn4'', is expressed in
ipRGCs
Intrinsically photosensitive retinal ganglion cells (ipRGCs), also called photosensitive retinal ganglion cells (pRGC), or melanopsin-containing retinal ganglion cells (mRGCs), are a type of neuron in the retina of the mammalian eye. The presence ...
, which comprises only 1-2% of
RGCs in the inner mammalian retina, as studied by
Samer Hattar and colleagues.
The gene spans approximately 11.8 kb and is mapped to the long arm of
chromosome 10. The gene includes nine
introns and ten
exons
An exon is any part of a gene that will form a part of the final mature RNA produced by that gene after introns have been removed by RNA splicing. The term ''exon'' refers to both the DNA sequence within a gene and to the corresponding seque ...
compared to the four to seven exons typically found in other human opsins.
In non-mammalian vertebrates, melanopsin is found in a wider subset of retinal cells, as well as in photosensitive structures outside the retina, such as the
iris muscle of the eye, deep brain regions, the
pineal gland
The pineal gland, conarium, or epiphysis cerebri, is a small endocrine gland in the brain of most vertebrates. The pineal gland produces melatonin, a serotonin-derived hormone which modulates sleep patterns in both circadian and seasonal cy ...
, and the skin.
[ ] Paralogs of ''Opn4'' include
OPN1LW,
OPN1MW
Green-sensitive opsin is a protein that in humans is encoded by the ''OPN1MW'' gene.
OPN1MW2 is a similar opsin.
See also
* Opsin
Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, ...
,
rhodopsin and
encephalopsin.
Melanopsin, like all other animal
opsins
Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become Retinylidene proteins, but are usually still called opsins regardless. Most pro ...
(e.g.
rhodopsin), is a
G-protein-coupled receptor (GPCR). The melanopsin protein has an extarcellular
N-terminal domain
The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the ami ...
, an intracellular
C-terminal domain
The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain ( protein or polypeptide), terminated by a free carboxyl group (-COOH). When the protein is ...
, and seven
alpha helices spanning through the plasma membrane.
The seventh helix has a
lysine that corresponds to Lys296
7.43 in
cattle
Cattle (''Bos taurus'') are large, domesticated, cloven-hooved, herbivores. They are a prominent modern member of the subfamily Bovinae and the most widespread species of the genus '' Bos''. Adult females are referred to as cows and adult ...
rhodopsin and that is conserved in almost all opsins.
This lysine binds covalently
retinal via a
Schiff-base
In organic chemistry, a Schiff base (named after Hugo Schiff) is a compound with the general structure ( = alkyl or aryl, but not hydrogen). They can be considered a sub-class of imines, being either secondary ketimines or secondary aldimi ...
, which makes melanopsin light sensitive. In fact this is abolished if the lysine is replaced by an
alanine.
Melanopsin is more closely related to
invertebrate
Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chorda ...
visual opsins, which are rhabdomeric opsin, than to
vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
visual opsins, which are cliary opsins.
This is also reflected by the downstream
signaling cascade
A biochemical cascade, also known as a signaling cascade or signaling pathway, is a series of chemical reactions that occur within a biological cell when initiated by a stimulus. This stimulus, known as a first messenger, acts on a receptor tha ...
, melanopsin couples in ipRGCs to the
G-proteins
G(q),
G(11), and
G(14), which are all of the G(q)-type. In fact, they can functionally replace each other, as a
knocking out
A knockout (abbreviated to KO or K.O.) is a fight-ending, winning criterion in several full-contact combat sports, such as boxing, kickboxing, muay thai, mixed martial arts, karate, some forms of taekwondo and other sports involving striking ...
only two of them has no
phenotypical effect. The G-proteins activate the
phospholipase C
Phospholipase C (PLC) is a class of membrane-associated enzymes that cleave phospholipids just before the phosphate group (see figure). It is most commonly taken to be synonymous with the human forms of this enzyme, which play an important role ...
PLCB4
1-Phosphatidylinositol-4,5-bisphosphate phosphodiesterase beta-4 is an enzyme that in humans is encoded by the ''PLCB4'' gene.
Function
The protein encoded by this gene catalyzes the formation of inositol 1,4,5-trisphosphate and diacylglycerol ...
,
which causes the
TRP-channels TRPC6 and
TRPC7
Transient receptor potential cation channel, subfamily C, member 7, also known as TRPC7, is a human gene encoding a protein of the same name.
See also
* TRPC
TRPC is a family of transient receptor potential cation channels in animals.
TRPC ...
mediate to open so that the cell
depolarizes.
This is like in the photoreceptor cells of the
Drosophila
''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many speci ...
eye, and in contrast to the vertebrate
rod and
cone cells, where
phototransduction Visual phototransduction is the sensory transduction process of the visual system by which light is detected to yield nerve impulses in the rod cells and cone cells in the retina of the eye in humans and other vertebrates. It relies on the visu ...
eventually makes the cells
hyperpolarize. Like other rhabdomeric opsins, Melanopsin has intrinsic
photoisomerase
Retinal isomerase () is an enzyme that catalyzes the isomerisation of all-trans-retinal in the eye into 11-cis-retinal which is the form that most opsins bind.
:all-trans-retinal \rightleftharpoons 11-cis-retinal
Hence, this enzyme has one subst ...
activity.
Function
Melanopsin-containing ganglion cells,
like rods and cones, exhibit both light and dark
adaptation
In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the po ...
; they adjust their sensitivity according to the recent history of light exposure.
However, while rods and cones are responsible for the reception of images, patterns, motion, and color, melanopsin-containing
ipRGCs
Intrinsically photosensitive retinal ganglion cells (ipRGCs), also called photosensitive retinal ganglion cells (pRGC), or melanopsin-containing retinal ganglion cells (mRGCs), are a type of neuron in the retina of the mammalian eye. The presence ...
contribute to various reflexive responses of the brain and body to the presence of light.
Evidence for melanopsin's physiological light detection has been tested in mice. A mouse cell line that is not normally photosensitive,
Neuro-2a, is rendered light-sensitive by the addition of human melanopsin. The photoresponse is selectively sensitive to short-wavelength light (peak absorption ~479 nm),
and has an intrinsic
photoisomerase
Retinal isomerase () is an enzyme that catalyzes the isomerisation of all-trans-retinal in the eye into 11-cis-retinal which is the form that most opsins bind.
:all-trans-retinal \rightleftharpoons 11-cis-retinal
Hence, this enzyme has one subst ...
regeneration function that is chromatically shifted to longer wavelengths.
Melanopsin photoreceptors are sensitive to a range of wavelengths and reach peak light absorption at blue light wavelengths around 480 nanometers.
Other wavelengths of light activate the melanopsin signaling system with decreasing efficiency as they move away from the optimum 480 nm. For example, shorter wavelengths around 445 nm (closer to violet in the
visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called '' visible light'' or simply light. A typical human eye will respond to ...
) are half as effective for melanopsin photoreceptor stimulation as light at 480 nm.
Melanopsin in the iris of some, primarily nocturnal, mammals closes the iris when it is exposed to light. This local pupil light reflex (PLR) is absent from primates, even though their irises express melanopsin.
Mechanism
When light with an appropriate frequency enters the eye, it activates the melanopsin contained in
intrinsically photosensitive retinal ganglion cells (ipRGCs), triggering an
action potential
An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells ...
. These neuronal electrical signals travel through neuronal
axons to specific brain targets, such as the center of pupillary control called the
olivary pretectal nucleus (OPN) of the midbrain. Consequently, stimulation of melanopsin in ipRGCs mediates behavioral and physiological responses to light, such as pupil constriction and inhibition of
melatonin
Melatonin is a natural product found in plants and animals. It is primarily known in animals as a hormone released by the pineal gland in the brain at night, and has long been associated with control of the sleep–wake cycle.
In vertebrat ...
release from the
pineal gland
The pineal gland, conarium, or epiphysis cerebri, is a small endocrine gland in the brain of most vertebrates. The pineal gland produces melatonin, a serotonin-derived hormone which modulates sleep patterns in both circadian and seasonal cy ...
. The ipRGCs in the mammalian retina are one terminus of the
retinohypothalamic tract
In neuroanatomy, the retinohypothalamic tract (RHT) is a photic neural input pathway involved in the circadian rhythms of mammals. The origin of the retinohypothalamic tract is the intrinsically photosensitive retinal ganglion cells (ipRGC), wh ...
that projects to the
suprachiasmatic nucleus
The suprachiasmatic nucleus or nuclei (SCN) is a tiny region of the brain in the hypothalamus, situated directly above the optic chiasm. It is responsible for controlling circadian rhythms. The neuronal and hormonal activities it generates regu ...
(SCN) of the
hypothalamus
The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamus ...
. The suprachiasmatic nucleus is sometimes described as the brain's "master clock", as it maintains the
circadian rhythm
A circadian rhythm (), or circadian cycle, is a natural, internal process that regulates the sleep–wake cycle and repeats roughly every 24 hours. It can refer to any process that originates within an organism (i.e., endogenous) and responds to ...
, and nerve signals from ipRGCs to the SCN entrain the internal circadian rhythm to the rising and setting of the sun.
The SCN also receives input from rods and cones through the retinohypothalamic tract, so information from all three photosensitive cell types (rods, cones, and ipRGCs) in the mammalian retina are transmitted to the (SCN) SCN.
Melanopsin-containing ganglion cells are thought to influence these targets by releasing the
neurotransmitters glutamate and
pituitary adenylate cyclase activating polypeptide (PACAP) from their axon terminals. Melanopsin-containing ganglion cells also receive input from rods and cones that can add to the input to these pathways.
Effects on circadian rhythm
Melanopsin serves an important role in the
photoentrainment of circadian rhythms in mammals. An organism that is
photoentrained has aligned its activity to an approximately 24-hour cycle, the
solar cycle
The solar cycle, also known as the solar magnetic activity cycle, sunspot cycle, or Schwabe cycle, is a nearly periodic 11-year change in the Sun's activity measured in terms of variations in the number of observed sunspots on the Sun's surf ...
on Earth. In mammals, melanopsin expressing axons target the
suprachiasmatic nucleus (SCN) through the
retinohypothalamic tract
In neuroanatomy, the retinohypothalamic tract (RHT) is a photic neural input pathway involved in the circadian rhythms of mammals. The origin of the retinohypothalamic tract is the intrinsically photosensitive retinal ganglion cells (ipRGC), wh ...
(RHT).
In mammals, the eye is the main photosensitive organ for the transmission of light signals to the brain. However, blind humans are still able to entrain to the environmental light-dark cycle, despite having no conscious perception of the light. One study exposed subjects to bright light for a prolonged duration of time and measured their
melatonin
Melatonin is a natural product found in plants and animals. It is primarily known in animals as a hormone released by the pineal gland in the brain at night, and has long been associated with control of the sleep–wake cycle.
In vertebrat ...
concentrations. Melatonin was not only suppressed in visually unimpaired humans, but also in blind participants, suggesting that the photic pathway used by the circadian system is functionally intact despite blindness. Therefore, physicians no longer practice
enucleation of blind patients, or removal of the eyes at birth, since the eyes play a critical role in the photoentrainment of the circadian pacemaker.
In mutant breeds of mice that lacked only rods, only cones, or both rods and cones, all breeds of mice still entrained to changing light stimuli in the environment, but with a limited response, suggesting that
rods and
cones are not necessary for circadian photoentrainment and that the mammalian eye must have another photopigment required for the regulation of the
circadian
A circadian rhythm (), or circadian cycle, is a natural, internal process that regulates the sleep–wake cycle and repeats roughly every 24 hours. It can refer to any process that originates within an organism (i.e., Endogeny (biology), endogeno ...
clock.
Melanopsin-
knockout mice display reduced photoentrainment. In comparison to wild-type mice that expressed melanopsin normally, deficits in light-induced phase shifts in locomotion activity were noted in melanopsin-null mice (''Opn4 -/-'').
These melanopsin-deficient mice did not completely lose their circadian rhythms, as they were still able to entrain to changing environmental stimuli, albeit more slowly than normal. This indicated that, although melanopsin is sufficient for entrainment, it must work in conjunction with other photopigments for normal photoentrainment activity. Triple-mutant mice that were rod-less, cone-less, and melanopsin-less display a complete loss in the circadian rhythms, so all three photopigments in these photoreceptors,
rhodopsin,
photopsin
Vertebrate visual opsins are a subclass of ciliary opsins and mediate vision in vertebrates. They include the opsins in human rod and cone cells. They are often abbreviated to ''opsin'', as they were the first opsins discovered and are still th ...
and melanopsin, are necessary for photoentrainment. Therefore, there is a functional redundancy between the three photopigments in the photoentrainment pathway of mammals. Deletion of only one photopigment does not eliminate the organism's ability to entrain to environmental light-dark cycles, but it does reduce the intensity of the response.
Regulation
Melanopsin undergoes
phosphorylation
In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, wh ...
on its intracellular
carboxy tail as a way to deactivate its function. Compared to other opsins, melanopsin has an unusually long carboxy tail that contains 37
serine and
threonine amino acid sites that could undergo phosphorylation. However, a cluster of seven amino acids are sufficient to deactivate zebrafish melanopsin. These sites are dephosphorylated when melanopsin is exposed to light and are unique from those that regulate rhodopsin. They are important for proper response to calcium ions in ipRGCs; lack of functional phosphorylation sites, particularly at serine-381 and serine-398, reduce the cell's response to light-induced calcium ion influx when voltage-gated calcium ion channels open.
In terms of the gene Opn4,
Dopamine
Dopamine (DA, a contraction of 3,4-dihydroxyphenethylamine) is a neuromodulatory molecule that plays several important roles in cells. It is an organic chemical of the catecholamine and phenethylamine families. Dopamine constitutes about 80% o ...
(DA) is a factor in the regulation of melanopsin
mRNA
In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of synthesizing a protein.
mRNA is created during the ...
in ipRGCs.
Clinical significance
The discovery of the role of melanopsin in non-image forming vision has led to a growth in
optogenetics. This field has shown promise in clinical applications, including the treatment of human eye diseases such as
retinitis pigmentosa and
diabetes
Diabetes, also known as diabetes mellitus, is a group of metabolic disorders characterized by a high blood sugar level ( hyperglycemia) over a prolonged period of time. Symptoms often include frequent urination, increased thirst and increased ...
.
A
missense mutation in Opn4, P10L, has been implicated in 5% of patients with
Seasonal Affective Disorder (SAD).
This is a condition in which people experience depressive thoughts in the winter due to decreased available light. Additionally, a melanopsin based receptor has been linked to
migraine pain.
Restoration of vision
There has been recent research on the role of melanopsin in
optogenetic
Optogenetics is a biological technique to control the activity of neurons or other cell types with light. This is achieved by expression of light-sensitive ion channels, pumps or enzymes specifically in the target cells. On the level of individ ...
therapy for patients with the degenerative eye disease
retinitis pigmentosa (RP).
Reintroducing functional melanopsin into the eyes of mice with retinal degeneration restores the
pupillary light reflex (PLR). These same mice could also distinguish light stimuli from dark stimuli and showed increased sensitivity to room light. The higher sensitivity demonstrated by these mice shows promise for vision restoration that may be applicable to humans and human eye diseases.
Control of sleep/wake patterns
Melanopsin may aid in controlling sleep cycles and wakefulness. Tsunematsu and colleagues created
transgenic mice that expressed melanopsin in
hypothalamic orexin neurons. With a short 4-second pulse of blue light (guided by
optical fibers
An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass ( silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a mea ...
), the transgenic mice could successfully transition from
slow-wave sleep (SWS), which is commonly known as "deep sleep," to long-lasting wakefulness. After switching off the blue light, the hypothalamic
orexin neurons showed activity for several tens of seconds.
It has been shown that rods and cones play no role in the onset of sleep by light, distinguishing them from ipRGCs and melanopsin. This provides strong evidence that there is a link between ipRGCs in humans and alertness, particularly with high frequency light (e.g. blue light). Therefore, melanopsin can be used as a therapeutic target for controlling the sleep-wake cycle.
Regulation of blood glucose levels
In a paper published by Ye and colleagues in 2011, melanopsin was utilized to create an optogenetic synthetic transcription device that was tested in a therapeutic setting to produce
Fc-glucagon-like peptide 1 (Fc-GLP-1), a fusion protein that helps control blood glucose levels in mammals with
Type II Diabetes
Type 2 diabetes, formerly known as adult-onset diabetes, is a form of diabetes mellitus that is characterized by high blood sugar, insulin resistance, and relative lack of insulin. Common symptoms include increased thirst, frequent urinatio ...
. The researchers subcutaneously implanted mice with microencapsulated transgenic
HEK 293 cells that were cotransfected with two vectors including the melanopsin gene and the gene of interest under an NFAT (
nuclear factor of activated T cells) promoter, respectively. It is through this engineered pathway that they successfully controlled the expression of Fc-GLP-1 in doubly recessive diabetic mice and reduced
hyperglycemia
Hyperglycemia is a condition in which an excessive amount of glucose circulates in the blood plasma. This is generally a blood sugar level higher than 11.1 mmol/L (200 mg/dL), but symptoms may not start to become noticeable until even ...
, or high blood glucose levels, in these mice. This shows promise for the use of melanopsin as an optogenetic tool for the treatment of Type II diabetes.
See also
*
Light effects on circadian rhythm
Light effects on circadian rhythm are the effects that light has on circadian rhythm.
Most animals and other organisms have "built-in clocks" in their brains that regulate the timing of biological processes and daily behavior. These "clocks" are ...
*
Opsins
Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become Retinylidene proteins, but are usually still called opsins regardless. Most pro ...
*
Intrinsically Photosensitive Retinal Ganglion Cells (ipRGCs)
*
Suprachiasmatic nucleus (SCN)
*
Retinohypothalamic tract
In neuroanatomy, the retinohypothalamic tract (RHT) is a photic neural input pathway involved in the circadian rhythms of mammals. The origin of the retinohypothalamic tract is the intrinsically photosensitive retinal ganglion cells (ipRGC), wh ...
References
Further reading
*
{{G protein-coupled receptors
G protein-coupled receptors
Circadian rhythm
Human proteins