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categories: ["cybernetics", "genetics"]categories: ["cybernetics", "genetics"]tags: ["neuroscience", "opsins", "gene delivery", "circuit mapping", "vision restoration", "research tools"]tags: ["neuroscience", "opsins", "gene delivery", "circuit mapping", "vision restoration", "research tools"]summary: "A technique that makes selected neurons light-sensitive by expressing microbial opsin genes in them, allowing their activity to be switched on or off on a millisecond timescale."summary: "A technique that makes selected neurons light-sensitive by expressing microbial opsin genes in them, allowing their activity to be switched on or off on a millisecond timescale."updated: "2026-07-27"updated: "2026-10-29"humanEvidence: "One published case report describes partial visual recovery in a patient with retinitis pigmentosa after opsin delivery to retinal ganglion cells; every use outside the eye, from engram control to cochlear and cardiac work, is in animals."humanEvidence: "One published case report describes partial visual recovery in a patient with retinitis pigmentosa after opsin delivery to retinal ganglion cells; every use outside the eye, from engram control to cochlear and cardiac work, is in animals."access: "Nothing to obtain clinically: no optogenetic therapy is approved anywhere and the only human use is inside ophthalmology trials, while opsin constructs are standard laboratory reagents."access: "Nothing to obtain clinically: no optogenetic therapy is approved anywhere and the only human use is inside ophthalmology trials, while opsin constructs are standard laboratory reagents."reversibility: "irreversible"reversibility: "irreversible"lines 32–178 → 32–17517 unchanged lines not shown
`````` **Optogenetics** is the use of light-sensitive proteins, introduced into cells by gene transfer, to**Optogenetics** is the use of light-sensitive proteins, introduced into cells by gene transfer, tocontrol the electrical activity of those cells with light. In neuroscience it made a specific andcontrol the electrical activity of those cells with light. In neuroscience it made possible apreviously unavailable experiment possible: activating or silencing one genetically definedspecific experiment that had not been available before: activating or silencing one geneticallypopulation of neurons, in a behaving animal, on the timescale of a single action potential, whiledefined population of neurons, in a behaving animal, on the timescale of a single action potential,leaving neighbouring cells untouched. That combination of cell-type specificity and millisecondwhile leaving neighbouring cells untouched. Electrical methods such as [[deep-brain-stimulation]]timing is what electrical methods such as [[deep-brain-stimulation]] cannotcannot provide that combination of cell-type specificity and millisecond timing.[^boyden2005]provide.[^boyden2005] ## How it works## How it works The active ingredient is a microbial opsin, a membrane protein from algae, archaea, or bacteria thatThe active ingredient is a microbial opsin, a membrane protein from algae, archaea or bacteria thatabsorbs light and moves ions across the membrane. Channelrhodopsin-2, from the green algaabsorbs light and moves ions across the membrane. Channelrhodopsin-2, from the green alga*Chlamydomonas reinhardtii*, opens a cation channel when illuminated with blue light, depolarizing*Chlamydomonas reinhardtii*, opens a cation channel when illuminated with blue light, depolarizingthe cell. Halorhodopsin, a chloride pump from a haloarchaeon, and archaerhodopsin, a proton pump,the cell. Halorhodopsin, a chloride pump from a haloarchaeon, and archaerhodopsin, a proton pump,hyperpolarize the cell under yellow or green light and therefore silence it. All require thehyperpolarize the cell under yellow or green light and so silence it. All of them require thecofactor all-*trans* retinal, which vertebrate brain tissue supplies in sufficient quantity — acofactor all-*trans* retinal. Vertebrate brain tissue happens to supply it in sufficient quantity,biological accident without which the technique would not work in mammals.and without that biological accident the technique would not work in mammals. An experiment has three parts. The opsin gene is delivered to the target cells, usually byAn experiment has three parts. First the opsin gene is delivered to the target cells, usually by[[aav-vectors]] carrying a cell-type-specific promoter, or by crossing a transgenic animal line[[aav-vectors]] carrying a cell-type-specific promoter, or by crossing a transgenic animal lineexpressing Cre recombinase in a defined population with a Cre-dependent opsin construct. Light isexpressing Cre recombinase in a defined population with a Cre-dependent opsin construct. Light isdelivered by an optical fibre, an implanted micro-LED array, or, in transparent preparations, fromthen delivered by an optical fibre, an implanted micro-LED array, or, in transparent preparations,outside. Activity is then read out by behaviour, electrophysiology, or fluorescent calcium imaging.from outside. Activity is read out by behaviour, electrophysiology or fluorescent calcium imaging. The engineered variants matter as much as the original discovery. Trafficking sequences improveThe engineered variants matter as much as the original discovery. Trafficking sequences improvemembrane targeting; faster mutants follow spike trains above 100 Hz; step-function opsins stay openmembrane targeting, and faster mutants follow spike trains above 100 Hz. Step-function opsins stayfor minutes after a brief pulse; red-shifted opsins absorb longer wavelengths that scatter less inopen for minutes after a brief pulse. Red-shifted opsins absorb longer wavelengths that scatter lesstissue; and soma-targeted opsins restrict expression to the cell body so that holographicin tissue. Soma-targeted opsins restrict expression to the cell body, so that holographic two-photontwo-photon illumination can address individual neurons without exciting passing axons.illumination can address individual neurons without exciting passing axons. ## Development history## Development history ```timeline```timeline[[ { "year": "1971–1979", "title": "Precursors", "text": "Bacteriorhodopsin is characterized as a light-driven proton pump. Francis Crick later argues that neuroscience needs a method to control one cell type at a time, and speculates that light might provide it." }, { "year": "1971-1979", "title": "Precursors", "text": "Bacteriorhodopsin is characterized as a light-driven proton pump. Francis Crick later argues that neuroscience needs a method to control one cell type at a time, and speculates that light might provide it." }, { "year": "2002–2003", "title": "Channelrhodopsins identified", "text": "Georg Nagel, Peter Hegemann and colleagues characterize channelrhodopsin-1 and channelrhodopsin-2 from green algae as directly light-gated ion channels." }, { "year": "2002-2003", "title": "Channelrhodopsins identified", "text": "Georg Nagel, Peter Hegemann and colleagues characterize channelrhodopsin-1 and channelrhodopsin-2 from green algae as directly light-gated ion channels." }, { "year": "2002", "title": "First genetically targeted photostimulation", "text": "Gero Miesenböck's group controls neurons using a multi-component invertebrate phototransduction system, establishing the concept before a single-gene tool existed." }, { "year": "2002", "title": "First genetically targeted photostimulation", "text": "Gero Miesenböck's group controls neurons using a multi-component invertebrate phototransduction system, establishing the concept before a single-gene tool existed." }, { "year": "2005", "title": "Optogenetics in neurons", "text": "Boyden, Zhang, Deisseroth and colleagues show that channelrhodopsin-2 alone confers millisecond-precision light control on mammalian neurons." }, { "year": "2005", "title": "Optogenetics in neurons", "text": "Boyden, Zhang, Deisseroth and colleagues show that channelrhodopsin-2 alone confers millisecond-precision light control on mammalian neurons." }, { "year": "2007–2010", "title": "Inhibition and colour range", "text": "Halorhodopsin and archaerhodopsin provide optical silencing; red-shifted and faster variants broaden the toolkit." }, { "year": "2007-2010", "title": "Inhibition and colour range", "text": "Halorhodopsin and archaerhodopsin provide optical silencing; red-shifted and faster variants broaden the toolkit." }, { "year": "2012–2013", "title": "Memory engrams manipulated", "text": "Optogenetic reactivation of hippocampal cells labelled during learning elicits the corresponding memory in mice, and a false memory is created by pairing artificial reactivation with a new experience." }, { "year": "2012-2013", "title": "Memory engrams manipulated", "text": "Optogenetic reactivation of hippocampal cells labelled during learning elicits the corresponding memory in mice, and a false memory is created by pairing artificial reactivation with a new experience." }, { "year": "2013–2021", "title": "Recognition", "text": "The Brain Prize, the Shaw Prize and the Lasker Basic Medical Research Award are given for the development of optogenetics." }, { "year": "2013-2021", "title": "Recognition", "text": "The Brain Prize, the Shaw Prize and the Lasker Basic Medical Research Award are given for the development of optogenetics." }, { "year": "2021", "title": "First human result", "text": "A patient with retinitis pigmentosa recovers partial visual function after retinal ganglion cells are made light-sensitive and driven by image-projecting goggles." } { "year": "2021", "title": "First human result", "text": "A patient with retinitis pigmentosa recovers partial visual function after retinal ganglion cells are made light-sensitive and driven by image-projecting goggles." }]]`````` ## What it is used for## What it is used for Optogenetics is, in practice, a research tool, and its influence on neuroscience comes from turningIn practice optogenetics is a research tool, and its influence on neuroscience comes from turningcorrelational claims into causal ones. Before it, showing that a population of neurons was activecorrelational claims into causal ones. Before it, showing that a population of neurons was activeduring a behaviour was straightforward and showing that the activity caused the behaviour was not.during a behaviour was straightforward, but showing that the activity caused the behaviour was not.Optogenetics supplies the missing manipulation, and it has been used to map circuits for fear,Optogenetics supplies the missing manipulation. It has been used to map circuits for fear, reward,reward, feeding, aggression, thirst, sleep-wake transitions, and locomotion, generally by activatingfeeding, aggression, thirst, sleep-wake transitions and locomotion, generally by activating oror silencing a projection defined by both its cell type and its anatomical target.silencing a projection defined by both its cell type and its anatomical target. The most striking demonstrations concern memory. Cells active during fear learning can be taggedThe most striking demonstrations concern memory. Cells active during fear learning can be taggedwith an opsin using activity-dependent promoters; reactivating those cells later elicits thewith an opsin using activity-dependent promoters, and reactivating those cells later elicits thebehavioural signature of the memory in the absence of the original cue.[^liu2012] Pairing artificialbehavioural signature of the memory without the original cue.[^liu2012] Pairing artificialreactivation of a context representation with an aversive stimulus produces behaviour consistentreactivation of a context representation with an aversive stimulus produces behaviour consistentwith a memory of an event that never occurred. These experiments are the strongest existing evidencewith a memory of an event that never occurred. These experiments are the strongest existing evidencethat a discrete, manipulable physical substrate of a specific memory exists, which bears on thethat a specific memory has a discrete physical substrate that can be manipulated. That bears on thepremises of [[whole-brain-emulation]] and on the encoding assumptions behind apremises of [[whole-brain-emulation]] and on the encoding assumptions behind a[[memory-prosthesis]].[[memory-prosthesis]]. Optogenetics also underpins "all-optical" physiology: an opsin for writing, a genetically encodedOptogenetics also underpins "all-optical" physiology, which combines an opsin for writing, acalcium or voltage indicator for reading, and holographic light shaping to address chosen neurons.genetically encoded calcium or voltage indicator for reading, and holographic light shaping toThis is the closest thing neuroscience has to a bidirectional interface with single-cell resolution,address chosen neurons. This is the closest thing neuroscience has to a bidirectional interface withand it is available only in animals; no [[brain-computer-interface]] in a person approaches thatsingle-cell resolution, and it is available only in animals. No [[brain-computer-interface]] in aprecision on either the read or the write side. Structural mapping methods such asperson approaches that precision on either the read or the write side. Structural mapping methods[[connectomics]] give the wiring; optogenetics tests what the wiring does.such as [[connectomics]] give the wiring, and optogenetics tests what the wiring does. ## Human applications## Human applications Only one clinical route has produced a published human result. In an approach that treats the eye asOnly one clinical route has produced a published human result. It treats the eye as a uniquelya uniquely favourable target — accessible, transparent, small, and relatively immune-privileged — afavourable target, being accessible, transparent, small and relatively immune-privileged. Ared-shifted opsin was delivered by intravitreal AAV to the surviving retinal ganglion cells of ared-shifted opsin was delivered by intravitreal AAV to the surviving retinal ganglion cells of apatient blinded by retinitis pigmentosa, and paired with goggles that convert the visual scene intopatient blinded by retinitis pigmentosa, and paired with goggles that convert the visual scene intoamber light pulses. The patient regained the ability to locate, count, and touch objects, which heamber light pulses. The patient regained the ability to locate, count and touch objects, which hecould not do beforehand.[^sahel2021] This is a single case reported from an ongoing trial, and thecould not do beforehand.[^sahel2021] This is a single case reported from an ongoing trial, and therecovered function is far below normal vision, comparable in magnitude to what arecovered function is far below normal vision, comparable in magnitude to what a [[retinal-implant]][[retinal-implant]] delivers. Other opsin-based programmes target bipolar cells rather than gangliondelivers. Other opsin-based programmes target bipolar cells rather than ganglion cells and remain incells and remain in trials; the eye is still the only tissue in which the technique has been tried intrials. The eye is still the only tissue in which the technique has been tried in people at all.people at all. The advantage over an electronic implant is that no hardware goes inside the eye and the number ofCompared with an electronic implant, the approach puts no hardware inside the eye, and the number ofaddressable cells is limited by expression rather than by electrode count. The disadvantages areaddressable cells is limited by expression rather than by electrode count. On the other side, itthat it requires a functioning ganglion cell layer, that the opsin's light sensitivity is far belowrequires a functioning ganglion cell layer, and the opsin's light sensitivity is so far below thatthat of natural photoreceptors so external amplification is mandatory, and that expressing aof natural photoreceptors that external amplification is mandatory. Expressing a microbial proteinmicrobial protein in a human tissue for life carries an immunological question that no long-termin a human tissue for life also raises an immunological question that no long-term data yet answers.data yet answers. Beyond the eye, proposals are earlier. An optical [[cochlear-implant]] using opsin-expressing spiralOutside the eye, proposals are at an earlier stage. An optical [[cochlear-implant]] usingganglion neurons would exploit light's poor spread in tissue to deliver more independent channelsopsin-expressing spiral ganglion neurons would exploit light's poor spread in tissue to deliver morethan electrical stimulation allows; the concept has been demonstrated in rodents. Cardiacindependent channels than electrical stimulation allows, and the concept has been demonstrated inoptogenetics for pacing and defibrillation works in animal hearts. Neither has a human study.rodents. Cardiac optogenetics for pacing and defibrillation works in animal hearts. Neither has ahuman study. > [!caution] A research tool, not yet a therapy> [!caution] A research tool, not yet a therapy> Optogenetics transformed animal neuroscience within a decade. Its clinical footprint after twenty> Optogenetics transformed animal neuroscience within a decade, yet after twenty years its clinical> years is one published case report in a rare form of blindness. The gap is not scientific> footprint is one published case report in a rare form of blindness. The mechanism is not in doubt.> uncertainty about the mechanism; it is that every therapeutic use requires solving gene delivery> The obstacle is that every therapeutic use requires solving gene delivery and light delivery at> and light delivery simultaneously, in a specific tissue, with acceptable immunology.> the same time, in a specific tissue, with acceptable immunology. ## Delivery and physical limits## Delivery and physical limits Gene delivery is the same problem faced by [[somatic-gene-therapy]] generally: viral tropism,Gene delivery is the same problem faced by [[somatic-gene-therapy]] generally: viral tropism,pre-existing immunity to common AAV serotypes, limited packaging capacity, and the difficulty ofpre-existing immunity to common AAV serotypes, limited packaging capacity, and the difficulty ofrestricting expression to one cell type without a Cre driver line that humans do not have. Editingrestricting expression to one cell type without the Cre driver lines that humans do not have.tools such as [[crispr-cas9]] do not solve this, because the problem is delivery and cell-typeEditing tools such as [[crispr-cas9]] do not solve this, because the problem is delivery andtargeting rather than the sequence change itself. Non-viralcell-type targeting rather than the sequence change itself. Non-viral carriers such ascarriers such as [[lipid-nanoparticles]] do not efficiently reach neurons after systemic[[lipid-nanoparticles]] do not efficiently reach neurons after systemic administration.administration. Light delivery is the constraint with no biological workaround. Blue light is scattered and absorbedLight delivery is the constraint with no biological workaround. Blue light is scattered and absorbedwithin roughly a millimetre of brain tissue, so deep targets require an implanted fibre or LED —within roughly a millimetre of brain tissue, so deep targets need an implanted fibre or LED. Thatreintroducing exactly the invasiveness optogenetics was supposed to avoid, plus tissue heating and abrings back the invasiveness optogenetics was supposed to avoid, along with tissue heating and aforeign-body response indistinguishable from that around a recording electrode. Red-shifted opsinsforeign-body response indistinguishable from the one around a recording electrode. Red-shiftedbuy a modest depth increase. Upconversion nanoparticles that convert tissue-penetrating near-infraredopsins gain a modest increase in depth. Upconversion nanoparticles, which convert tissue-penetratinglight into visible emission at the target have been demonstrated in mice, adding a second deliverynear-infrared light into visible emission at the target, have been demonstrated in mice, but theyproblem to the first.[^chen2018]add a second delivery problem to the first.[^chen2018] Related approaches attempt to remove the light source altogether. Chemogenetic receptors activatedRelated approaches try to remove the light source altogether. Chemogenetic receptors activated by anby an inert drug provide cell-type specificity with minutes-to-hours resolution instead ofinert drug provide cell-type specificity with a resolution of minutes to hours rather thanmilliseconds. Sonogenetics uses mechanosensitive channels driven by ultrasound, which penetratesmilliseconds. Sonogenetics uses mechanosensitive channels driven by ultrasound, which penetratestissue far better than light. Magnetogenetic schemes, which claimed magnetic control of engineeredtissue far better than light. Magnetogenetic schemes, which claimed magnetic control of engineeredchannels, have been challenged on the grounds that the reported effects exceed what the physics ofchannels, have been challenged on the grounds that the reported effects exceed what the physics ofmagnetic energy transfer to a protein permits, and remain unresolved — a useful reminder thatmagnetic energy transfer to a protein permits, and the dispute remains unresolved. Tools in thisplausible-sounding tools in this area do not always survive scrutiny.area that sound plausible do not always survive scrutiny. ## Outlook## Outlook The research trajectory is toward larger-scale all-optical control: reading and writing hundreds ofResearch is moving toward all-optical control at larger scale: reading and writing hundreds ofindividually specified neurons in a behaving animal, which would let investigators test whetherindividually specified neurons in a behaving animal, which would let investigators test whetherspecific activity patterns are sufficient for a percept or a decision. That work bears directly onspecific activity patterns are sufficient for a percept or a decision. That work bears directly onthe questions raised under [[neural-correlates-of-consciousness]], since it is the only method thatthe questions raised under [[neural-correlates-of-consciousness]], since it is the only method thatcan impose a precise pattern rather than merely observe one.can impose a precise pattern rather than merely observe one. Clinically, the near-term expectation is narrow: retinal indications where the anatomy cooperates,Clinically, near-term expectations are narrow: retinal indications where the anatomy cooperates, andand possibly the inner ear. A general-purpose optogenetic alternative to implanted electrodes wouldpossibly the inner ear. A general-purpose optogenetic alternative to implanted electrodes would needrequire gene delivery to defined deep populations and a light source that does not need surgery,gene delivery to defined deep populations and a light source that does not need surgery, and noand no current approach supplies both. Whether the technique's extraordinary value in animalcurrent approach supplies both. Twenty years in, it is still an open question whether theresearch ever converts into a broad clinical technology is, twenty years in, still an opentechnique's great value in animal research will ever turn into a broad clinical technology. The morequestion — and the more likely path may be that optogenetics remains the instrument that tells otherlikely path may be that optogenetics remains the instrument that tells other technologies, fromtechnologies, from [[neuroprosthetics]] to [[neural-decoding]], what to aim at.[[neuroprosthetics]] to [[neural-decoding]], what to aim at. ## See also## See also removed, struck through added, underlinedLine numbers count the serialised markdown of each revision, frontmatter included.
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