Visual phototransduction is thesensory transduction process of thevisual system by whichlight is detected byphotoreceptor cells (rods andcones) in the vertebrateretina. Aphoton is absorbed by aretinalchromophore (each bound to anopsin), which initiates a signal cascade through several intermediate cells, then through theretinal ganglion cells (RGCs) comprising the optic nerve.

Light enters the eye, passes through the optical media, then the inner neural layers of the retina before finally reaching thephotoreceptor cells in the outer layer of the retina. The light may be absorbed by achromophore bound to anopsin, whichphotoisomerizes the chromophore, initiating both thevisual cycle, which "resets" the chromophore, and the phototransduction cascade, which transmits the visual signal to the brain. The cascade begins with graded polarization (ananalog signal) of the excited photoreceptor cell, as itsmembrane potential increases from aresting potential of −70 mV, proportional to the light intensity. At rest, the photoreceptor cells are continually releasing glutamate at thesynaptic terminal to maintain the potential.^[1] The transmitter release rate is lowered (hyperpolarization) as light intensity increases. Each synaptic terminal makes up to 500 contacts withhorizontal cells andbipolar cells.^[1] These intermediate cells (along withamacrine cells) perform comparisons of photoreceptor signals within areceptive field, but their precise functionalities are not well understood. The signal remains as a graded polarization in all cells until it reaches theRGCs, where it is converted to anaction potential and transmitted to the brain.^[1]

Thephotoreceptor cells involved in vertebrate vision are therods, thecones, and thephotosensitive ganglion cells (ipRGCs). These cells contain achromophore (11-cis-retinal, thealdehyde ofvitamin A1 and light-absorbing portion) that is bound to a cell membrane protein,opsin. Rods are responsible for vision under low light intensity and contrast detections. Because they all have the same response across frequencies, no colour information can be deduced from the rods only, as in low light conditions for example. Cones, on the other hand, are of different kinds with different frequency response, such that colour can be perceived through comparison of the outputs of different kinds of cones. Each cone type responds best to certainwavelengths, or colours, of light because each type has a slightly different opsin. The three types of cones are L-cones, M-cones and S-cones that respond optimally to long wavelengths (reddish colour), medium wavelengths (greenish colour), and short wavelengths (bluish colour) respectively. Humans havetrichromaticphotopic vision consisting of threeopponent process channels that enablecolour vision.^[2] Rod photoreceptors are the most common cell type in the retina and develop quite late. Most cells become postmitotic before birth, but differentiation occurs after birth. In the first week after birth, cells mature and the eye becomes fully functional at the time of opening. The visual pigment rhodopsin (rho) is the first known sign of differentiation in rods.^[3]

To understand the photoreceptor's behaviour to light intensities, it is necessary to understand the roles of different currents.

There is an ongoing outwardpotassium current through nongated K⁺-selective channels. This outward current tends to hyperpolarize the photoreceptor at around −70 mV (the equilibrium potential for K⁺).

There is also an inward sodium current carried bycGMP-gatedsodium channels. This "dark current" depolarizes the cell to around −40 mV. This is significantly more depolarized than most other neurons.

A high density ofNa⁺-K⁺ pumps enables the photoreceptor to maintain a steady intracellular concentration of Na⁺ and K⁺.

When light intensity increases, the potential of the membrane decreases (hyperpolarization). Because as the intensity increases, the release of the stimulating neurotransmitter glutamate of the photoreceptors is reduced. When light intensity decreases, that is, in the dark environment, glutamate release by photoreceptors increases. This increases the membrane potential and produces membrane depolarization.^[1]

Photoreceptor cells are unusual cells in that they depolarize in response to absence of stimuli or scotopic conditions (darkness). In photopic conditions (light), photoreceptors hyperpolarize to a potential of −60 mV.

In the dark, cGMP levels are high and keep cGMP-gated sodium channels open allowing a steady inward current, called the dark current. This dark current keeps the cell depolarized at about −40 mV, leading toglutamate release which inhibits excitation of neurons.

The depolarization of the cell membrane inscotopic conditions opens voltage-gated calcium channels. An increased intracellular concentration ofCa²⁺ causesvesicles containing glutamate, aneurotransmitter, to merge with the cell membrane, therefore releasing glutamate into thesynaptic cleft, an area between the end of one cell and the beginning of anotherneuron. Glutamate, though usually excitatory, functions here as an inhibitory neurotransmitter.

In the cone pathway, glutamate:

• Hyperpolarizes on-centrebipolar cells. Glutamate that is released from the photoreceptors in the dark binds to metabotropic glutamate receptors (mGluR6), which, through a G-protein coupling mechanism, causes non-specific cation channels in the cells to close, thus hyperpolarizing the bipolar cell.
• Depolarizes off-centre bipolar cells. Binding of glutamate to ionotropic glutamate receptors results in an inward cation current that depolarizes the bipolar cell.

In summary:Light closes cGMP-gated sodium channels, reducing the influx of both Na⁺ and Ca²⁺ ions. Stopping the influx of Na⁺ ions effectively switchesoff the dark current. Reducing this dark current causes the photoreceptor tohyperpolarise, which reduces glutamate release which thus reduces theinhibition of retinal nerves, leading toexcitation of these nerves. This reduced Ca²⁺ influx during phototransduction enables deactivation and recovery from phototransduction, as discussed below in§ Deactivation of the phototransduction cascade.

1. A photon interacts with aretinal molecule in anopsin complex in aphotoreceptor cell. The retinal undergoesisomerization, changing from the 11-cis-retinal to the all-trans-retinal configuration.
2. Opsin therefore undergoes a conformational change to metarhodopsin II.
3. Metarhodopsin II activates aG protein known astransducin. This causes transducin to dissociate from its boundGDP, and bindGTP; then the alpha subunit of transducin dissociates from the beta and gamma subunits, with the GTP still bound to the alpha subunit.
4. The alpha subunit-GTP complex activatesphosphodiesterase, also known as PDE6. It binds to one of two regulatory subunits of PDE (which itself is a tetramer) and stimulates its activity.
5. PDE hydrolysescGMP, formingGMP. This lowers the intracellular concentration of cGMP and therefore the sodium channels close.^[5]
6. Closure of the sodium channels causes hyperpolarization of the cell due to the ongoing efflux of potassium ions.
7. Hyperpolarization of the cell causes voltage-gated calcium channels to close.
8. As the calcium level in the photoreceptor cell drops, the amount of the neurotransmitter glutamate that is released by the cell also drops. This is because calcium is required for the glutamate-containing vesicles to fuse with cell membrane and release their contents (seeSNARE proteins).
9. A decrease in the amount of glutamate released by the photoreceptors causes depolarization of on-centre bipolar cells (rod and cone On bipolar cells) and hyperpolarization of cone off-centre bipolar cells.

In light, low cGMP levels close Na⁺ and Ca²⁺ channels, reducing intracellular Na⁺ and Ca²⁺. During recovery (dark adaptation), the low Ca²⁺ levels induce recovery (termination of the phototransduction cascade), as follows:

1. Low intracellular Ca²⁺ causes Ca²⁺ to dissociate fromguanylate cyclase activating protein (GCAP). The liberated GCAP ultimately restores depleted cGMP levels, which re-opens the cGMP-gated cation channels (restoring dark current).
2. Low intracellular Ca²⁺ causes Ca²⁺ to dissociate fromGTPase-activating protein (GAP), also known asregulator of G protein signalling. The liberated GAP deactivates transducin, terminating the phototransduction cascade (restoring dark current).
3. Low intracellular Ca²⁺ makes intracellular Ca-recoverin-RK dissociate into Ca²⁺ andrecoverin andrhodopsin kinase (RK). The liberated RK then phosphorylates the Metarhodopsin II, reducing its binding affinity for transducin.Arrestin then completely deactivates the phosphorylated-metarhodopsin II, terminating the phototransduction cascade (restoring dark current).
4. Low intracellular Ca²⁺ make the Ca²⁺/calmodulin complex within the cGMP-gated cation channels more sensitive to low cGMP levels (thereby, keeping the cGMP-gated cation channel open even at low cGMP levels, restoring dark current)^[6]

In more detail:

GTPase Accelerating Protein (GAP) of RGS (regulators of G protein signalling) interacts with the alpha subunit of transducin, and causes it to hydrolyse its bound GTP to GDP, and thus halts the action of phosphodiesterase, stopping the transformation of cGMP to GMP. This deactivation step of the phototransduction cascade (the deactivation of the G protein transducer) was found to be the rate limiting step in the deactivation of the phototransduction cascade.^[7]

In other words: Guanylate Cyclase Activating Protein (GCAP) is a calcium binding protein, and as the calcium levels in the cell have decreased, GCAP dissociates from its bound calcium ions, and interacts with Guanylate Cyclase, activating it. Guanylate Cyclase then proceeds to transform GTP to cGMP, replenishing the cell's cGMP levels and thus reopening the sodium channels that were closed during phototransduction.

Finally, Metarhodopsin II is deactivated. Recoverin, another calcium binding protein, is normally bound to Rhodopsin Kinase when calcium is present. When the calcium levels fall during phototransduction, the calcium dissociates from recoverin, and rhodopsin kinase is released and phosphorylatesmetarhodopsin II, which decreases its affinity for transducin. Finally, arrestin, another protein, binds the phosphorylated metarhodopsin II, completely deactivating it. Thus, finally, phototransduction is deactivated, and the dark current and glutamate release is restored. It is this pathway, where Metarhodopsin II is phosphorylated and bound to arrestin and thus deactivated, which is thought to be responsible for the S2 component of dark adaptation. The S2 component represents a linear section of the dark adaptation function present at the beginning of dark adaptation for all bleaching intensities.

The visual cycle occurs viaG-protein coupled receptors calledretinylidene proteins which consists of a visualopsin and achromophore11-cis-retinal. The 11-cis-retinal is covalently linked to theopsin receptor viaSchiff base. When it absorbs aphoton, 11-cis-retinal undergoesphotoisomerization toall-trans-retinal, which changes the conformation of the opsinGPCR leading tosignal transduction cascades which causes closure of cyclic GMP-gated cation channel, and hyperpolarization of the photoreceptor cell. Following photoisomerization, all-trans-retinal is released from the opsin protein and reduced to all-trans-retinol, which travels to theretinal pigment epithelium to be "recharged". It is firstesterified bylecithin retinol acyltransferase (LRAT) and then converted to 11-cis-retinol by the isomerohydrolaseRPE65. The isomerase activity of RPE65 has been shown; it is uncertain whether it also acts as the hydrolase.^[8] Finally, it is oxidized to 11-cis-retinal before travelling back to thephotoreceptor cell outer segment where it is again conjugated to anopsin to form new, functional visual pigment (retinylidene protein), namelyphotopsin orrhodopsin.

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Visual phototransduction ininvertebrates like thefruit fly differs from that of vertebrates, described up to now. The primary basis of invertebrate phototransduction is thePI(4,5)P₂ cycle. Here, light induces the conformational change intorhodopsin and converts it into meta-rhodopsin. This helps in dissociation of G-protein complex. Alpha sub-unit of this complex activates thePLC enzyme (PLC-beta) which hydrolyse thePIP2 intoDAG. This hydrolysis leads to opening ofTRP channels and influx of calcium.^{[citation needed]}

Invertebratephotoreceptor cells differ morphologically and physiologically from their vertebrate counterparts. Visual stimulation in vertebrates causes a hyperpolarization (weakening) of the photoreceptor membrane potential, whereas invertebrates experience a depolarization with light intensity. Single-photon events produced under identical conditions in invertebrates differ from vertebrates in time course and size. Likewise, multi-photon events are longer than single-photon responses in invertebrates. However, in vertebrates, the multi-photon response is similar to the single-photon response. Both phyla have light adaptation and single-photon events are smaller and faster. Calcium plays an important role in this adaptation. Light adaptation in vertebrates is primarily attributable to calcium feedback, but in invertebrates cyclic AMP is another control on dark adaptation.^{[9][verification needed]}

The genome ofDrosophila encodes seven opsins,^[11] five of those are expressed in the omatidia of the eye. The photoreceptor cells R1-R6 express the opsin Rh1,^[12] which absorbs maximally blue light (around 480 nm),^[13][14][15] however the R1-R6 cells cover a broader range of the spectrum than an opsin would allow due to a sensitising pigment^[16][17] that adds two sensitivity maxima in the UV-range (355 and 370 nm).^[15] The R7 cells come in two types with yellow and pale rhabdomeres (R7y and R7p).^[18][19] The pale R7p cells express the opsin Rh3,^[20][21] which maximally absorbsUV-light (345 nm).^[22] The R7p cells are strictly paired with the R8p cells that express Rh5,^[21] which maximally absorbs violet light (437 nm).^[15] The other, the yellow R7y cells express a blue-absorbing screening pigment^[18] and the opsin Rh4,^[23] which maximally absorbs UV-light (375 nm).^[22] The R7y cells are strictly paired with R8y cells that express Rh6,^[24] which maximally absorbs UV-light (508 nm).^[15] In a subset of omatidia both R7 and R8 cells express the opsin Rh3.^[21]

However, these absorption maxima of the opsins where measured in white eyed flies without screening pigments (Rh3-Rh6),^[22][15] or from the isolated opsin directly (Rh1).^[13] Those pigments reduce the light that reaches the opsins depending on the wavelength. Thus in fully pigmented flies, the effective absorption maxima of opsins differs and thus also the sensitivity of their photoreceptor cells. With screening pigment, the opsin Rh3 is short wave shifted from 345 nm^[a] to 330 nm and Rh4 from 375 nm to 355 nm. Whether screening pigment is present does not make a practical difference for the opsin Rh5 (435 nm and 437 nm), while the opsin R6 is long wave shifted by 92 nm from 508 nm to 600 nm.^[10]

Additionally of the opsins of the eye,Drosophila has two more opsins: The ocelli express the opsin Rh2,^[25][26] which maximally absorbs violet light (~420 nm).^[26] And the opsin Rh7, which maximally absorbs UV-light (350 nm) with an unusually long wavelength tail up to 500 nm. The long tail disappears if a lysine at position 90 is replaced byglutamic acid. This mutant then absorbs maximally violet light (450 nm).^[27] The opsin Rh7 entrains withcryptochrome thecircadian rhythm ofDrosophila to the day-night-cycle in the central pacemakerneurons.^[28]

EachDrosophila opsin binds thecarotenoid chromophore 11-cis-3-hydroxyretinal via a lysine.^[29][30] Thislysine is conserved in almost all opsins, only a few opsins have lost it duringevolution.^[31] Opsins without it are not light sensitive.^[32][33][34] In particular, the Drosophila opsins Rh1, Rh4, and Rh7 function not only asphotoreceptors, but also aschemoreceptors foraristolochic acid. These opsins still have the lysine like other opsins. However, if it is replaced by anarginine in Rh1, then Rh1 loses light sensitivity but still responds to aristolochic acid. Thus, the lysine is not needed for Rh1 to function as chemoreceptor.^[33]

As invertebrate vision, visual transduction ininvertebrates occurs via a G protein-coupled pathway. However, invertebrates, theG protein istransducin, while the G protein in invertebrates is Gq (dgq inDrosophila). When rhodopsin (Rh) absorbs aphoton of light its chromophore, 11-cis-3-hydroxyretinal, isisomerized to all-trans-3-hydroxyretinal. Rh undergoes a conformational change into its active form, metarhodopsin. Metarhodopsin activates Gq, which in turn activates aphospholipase Cβ (PLCβ) known as NorpA.^[35]

PLCβ hydrolysesphosphatidylinositol (4,5)-bisphosphate (PIP₂), aphospholipid found in thecell membrane, into solubleinositol triphosphate (IP₃) anddiacylglycerol (DAG), which stays in the cell membrane. DAG, a derivative of DAG, or PIP₂ depletion cause acalcium-selectiveion channel known astransient receptor potential (TRP) to open and calcium andsodium flows into the cell.^[36] IP₃ is thought to bind toIP₃ receptors in the subrhabdomeric cisternae, an extension of theendoplasmic reticulum, and cause release of calcium, but this process does not seem to be essential for normal vision.^[35]

Calcium binds to proteins such ascalmodulin (CaM) and an eye-specificprotein kinase C (PKC) known as InaC. These proteins interact with other proteins and have been shown to be necessary for shut off of the light response. In addition, proteins calledarrestins bind metarhodopsin and prevent it from activating more Gq. Asodium-calcium exchanger known as CalX pumps the calcium out of the cell. It uses the inwardsodium gradient to export calcium at astoichiometry of 3 Na⁺/ 1 Ca⁺⁺.^[37]

TRP, InaC, and PLC form a signalling complex by binding ascaffolding protein called InaD. InaD contains five binding domains calledPDZ domain proteins, which specifically bind the C termini of target proteins. Disruption of the complex by mutations in either the PDZ domains or the target proteins reduces the efficiency of signalling. For example, disruption of the interaction between InaC, the protein kinase C, and InaD results in a delay in inactivation of thelight response.

Unlike vertebrate metarhodopsin, invertebrate metarhodopsin can be converted back into rhodopsin by absorbing a photon of orange light (580 nm).

• Visual pigments and visual transduction at med.utah.edu
• Transduction of Light Prezi
• A General Overview on Visual Perception at brynmawr.eduArchived 2011-07-09 at theWayback Machine
• Phototransduction at the U.S. National Library of MedicineMedical Subject Headings (MeSH)

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• Nervous system
• Sensory receptors
• Metabolism

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