Lecture 12: Optogenetics or How to Manipulate Neurons with Light

okay uh this is the pictures that Christophe and I have been giving it's almost exactly six months after we started to the day and I'll be speaking today about optogenetics or how to manipulate neurons with light and you'll see how I'm trying to bring the the chorus full circle from the very beginning when I discussed the basics of the visual system and discussed how in particular the retina was put together in order to capture light and turn it into neural signals optogenetics is a very new field it's really seven or eight years old and it's essentially the same thing that the thing that evolution did to give us eyes and turn photons into neural activity neuroscientists have gone into the genome of various species in particular bacteria and taken molecules that are sensitive to light in order to turn light again into neural activity and let me just start at the beginning which is really the first thing I showed at the beginning of this course a non Center neuron in the brain and really just to say how incredibly amazing and convenient a vision is if one is a neuroscientist because one can do the simplest thing possible just take a spot of light and have an animal through the miracle of its optics take that spot of light focus it on the retina and transduce that life into nervous activity and not just do it in a simple where you might imagine that photons are translated into neural activity but in fact the pattern of activation of the receptor neurons of a whole class of neurons affects in a very complex way how downstream neurons even in the retina are excited and inhibited not by life by nervous activity so for multiple reasons really I'll get to throughout this lecture the visual system is an ideal window brain that's not just a metaphor it's an actual literal fact the visual system in particular the eye is a window into the brain and the objects of the eye allow us to see in and allows the animal to see out in the first lecture I didn't I left out one part of visual neuroscience and that's the human eye and the human eye consists of the entire eye itself which is a primarily clear all the way from the cornea the front covering of the eye the anterior chamber which is a fluid filled filled region um the iris which is very much like the aperture and it lands in a photographic system the lens itself of course and then vitreous fluid which is another very large fluid filled space and really the lens is not the only lens in the eye but it's it's the cornea the anterior chamber and the lens itself all of which are optical elements that help to focus photons on to the eye and the eye really has you know throughout philosophy I should have kept in the slide I had from al-kindi an Arab philosopher and physicists and physiologist really or a optical physiologist who first diagram the eye but really from the period when people finally understood how the I worked it was used as an example of perhaps the most beautiful structure that we believe evolution has given us but it was often used to really justify the existence of God how could how could anything but a divine being create such a perfect system as the eye and Darwin says it very nicely to suppose that the eye with all its inimitable contrivance is for adjusting the focus to different distances and the next phrase really reminds you that Darwin in the 1860s you think of Darwin is fairly long ago but really of physics and our understanding of many things was it was pretty advanced back then for admitting different amounts of light and for a correction of spherical and chromatic aberration that's the current state of knowledge of the optics of the eye in the 1850s could have been formed by natural selection seems I freely confess absurd in the highest degree and but he goes on reason tells me that of numerous gradations from simple and imperfect eye to one complex and perfect can be shown to exist - there's a lot of discussion of selection then a very complicated clause then the difficulty of believing that a perfect and complex I could be formed by natural selection though in Super Bowl by our imagination should not be considered as subversive to the theory so it's almost a triple negative that the difficulty should not be considered subversive to the theory Darwin white writes beautifully but sometimes it in a in a complex fashion but essentially Darwin is saying that here's an example of something that is just so beautifully engineered that it's hard to imagine evolution creating it but a evolution has a lot of time and a lot of intermediate steps and what's really amazing is essentially what Darwin said and we really with a fair amount of optical sophistication that the eye is like a microscope objective you know what's it what's really amazing about it the eye not that it focuses light that's that's the basic idea but it's really a pretty good microscope objective here's a picture of a pretty good microscope objective pretty good as it means it has a numerical aperture of 0.3 it doesn't have the highest resolution capability doesn't gather all the light that the best objectors gather but you can imagine taking a pretty good microscope objective and focusing it on cones in the fovea and here's a nice picture from a beautiful paper in 1990 from Kuroshio and Hendrickson of the spacing of cones in the fovea and at its finest they can be down to a little bit less than 2 microns spacing and the eye which has a numerical aperture of 0.2 isn't capable of seeing so much better it is incapable of focusing much better than that but it actually says that it given that it has a numerical aperture or 0.2 a certain like gathering capacity and and theoretical resolution it does a very good job of using all that light and all of that focusing ability at a focusing light on two individual cones and we know from physiology and probably better from psychophysics from a perceptual experiments that we really do use individual cones and therefore the focus on two individual cones in perceiving so that means that the the eye is a pretty good microscope and actually like modern microscopes what it does is it it's focused at infinity it takes photons out at great distances and focuses it down onto the fovea to almost the reticle limit of its resolution the microscope does the opposite it takes photons down here at where in front of the front element and enlarges it and really all microscope of Directors nowadays or infinity corrected meaning it's focused at infinity so really a the eye is a camera but perhaps better the eye is a microscope objective and let's go back to the to the retina again slides from in my first lecture and the logical projection the logical progression of how the retina is built is that there are photoreceptors rods and cones that collect I light up here in these outer segments they're called these stacks upon stacks of membranes that are just filled with proteins known as options rhodopsin and the cone options and there's layer upon layer of membrane with these light-sensitive molecules so rather than having to just go through a single layer on the surface of the cells and have one chance of catching a photon it's many you know hundreds of layers of pigment and hundreds of chances of capturing capturing those very scarce photons that we really want to use when we're in a dark room or at nice and you know the magic of the photoreceptors is it turns the photons into biophysical signals a change the voltage across between the inside and the outside of these cells and if we had given really a biophysical introduction to neuroscience rather than a visual centric view of neuroscience you'd know but I'm sure everyone in the room knows that by changing the potential the voltage across the membrane of a neuron that indirectly changes the amount of neurotransmitter that comes out of the axons in the nerve cell those neurotransmitters get send signals to the next level and in this case the next level is bipolar cells that transient transmitted information from photoreceptors to ganglion cells and finally we have the ganglion cells the output of the retina the in the first slide you weren't listening to a ganglion cell but you're listening to the next step in the circuit the real a neuron between ganglion cells and visual cortex so the Gambian cells transmit information from the retina to the rest of the brain and as I said in the first lecture the retina is built upside down light must go has to go through coming from up here photons have to travel all the way through this roughly 50 microns of brain tissue the embryologically the the retina is part of the central nervous system so it's traveling through brain tissue through three layers of cell bodies all the way out to these photoreceptor outer segments and that's why it's called outer segments that's and to capture the light and very importantly and certainly importantly for this lecture brain tissue especially the retina it's fairly transparent that photons can hit the brain but they don't stop dead in their tracks that they can transmit through brain tissue a certain amount why is that important well certainly important if you have a flashing spot of lice out in at infinity or at some distance from an animal that flashing spot of light that you saw in the first movie gets focused on to a small region in the photoreceptors and really doesn't care doesn't isn't really affected by the nervous tissue between the light and the photoreceptors one thing I didn't say in the first lecture is anything about phototransduction and phototransduction is the churning of photons into signal and a photo transduction in the retina is fairly complex and i'm only going to tell you the tiniest bit about this complicated subject the simplest thing is kind of surprising when you first learn it that here this is a simple graph where the x-axis is time the and there are two traces the first race is just luminance you can think of it as luminance of a spot it starts out dark gets bright and then gets dark again and the simple thing you'd expect is that when you excite the retina with light you will be exciting neurons and it's in fact the opposite is true that photoreceptors are hyperpolarized by light hyper so all nervous cells have a negative voltage negative potential compared to the outside and when that potential becomes positive that can lead to an action potential or even a graded potential which causes the neuron to emit neurotransmitters when the voltage gets more negative or hyperpolarized that's inhibiting the neuron so light in fact inhibits photoreceptor cells and decreases the amount of neurotransmitter that gets to bipolar cells so I think I should also say that the transduction isn't simple in any way shape or form that you typically when something happens to an Iran to change its potential typically a gate opens or closes that allows ions to flow in and into or out of a neuron um the rhodopsins in the animal kingdom are in fact or many of them most of them are receptors that in fact start an intracellular cascade and it's one of a very very large class of g-protein coupled receptors and g-proteins are now I'm going to simplify radically G proteins are essentially enzymes that catalyze biochemical reactions so life turns into biochemistry and then the biochemistry in the Cascade opens or closes channels and in the case of mammalian photoreceptors light inhibits those photoreceptors um I just added this slide because I really love this review and I decided that there's a wanted to have an evolutionary theme in this lecture and this is a lovely review that I highly recommend to anyone to read by Josh Saenz and there is a Persky comparing vertebrate and invertebrate vision and in both vertebrates and in flies in this case there are receptor cells that synapse in the case of vertebrates on to these of bipolar cells and bipolar cells synapse on to retinal ganglion cells in the fly the photoreceptors go to the lamina which is the second stage or processing and then all on to the medulla and these are separate structures but if one goes and compares them side by side and actually an amazing thing is that qahal studied both vision very intensively in mammals but also studied vision intensively in flies and cahal made this this analogy you know hundred years ago and the Persky in Saenz go through the same thing in modern in a modern way and there's a very very close symbology between photoreceptors to bipolar cells to retinal ganglion cells compared to photoreceptors to lamina neurons and trans medullary neurons in in the fly the reason I really wanted to show this is that a the counterintuitive fact that photons hyperpolarize or inhibit receptors in vertebrates the opposite is true in invertebrates the biochemical cascade that hyperpolarizes neurons in the vertebrate a a different biochemical cascade depolarizes neurons in in in the fly so really there's a from the very beginning things are different but it really points out that the transduction of photons into nervous signals is almost arbitrary that if the nervous system can take positive and negative signals and process them and in fact invert them to have on and off cells then it really doesn't matter except in probably in terms of energetics in terms of energy expended what is the convention the nervous system chooses to take and back to a slide that I showed part of in the first lecture we know that um the receptive fields of ganglion cells come in two flavors and probably the more talked about version are on center cells on Center cells when you excite the center of the receptive field as we heard earlier results in more action potentials so light excites this neuron how's that possible you you might think that there's an inhibitory neuron interposed in the this the pathway to take the inhibitory signal them the minus signal of the photoreceptors invert it with with another bit of inhibition to make light excite these neurons and the most logical way of doing that in the way that's that's done throughout the nervous system is to have an inhibitory neuron that uses a neurotransmitter structures gaba but a a very unusual thing happens in there are unusual glutamate receptors where glutamate is the canonical excitatory neurotransmitter they're the glutamate receptors in the retina that actually are inhibitory that inhibit the neuron so that changes sign but the therefore the simpler type of ganglion cell to build is the off-center neuron where the office of a photoreceptor the the rods and cones of the mammalian retina are in fact off cells you think they're light they're light detectors but there are really dark detectors there they they send excitation by the removal of life and therefore the job of the off pathway is quite simple just a glutamatergic excitatory neurotransmission all the way through the retina so more Darwin um here uh was when Darwin lived everyone is used to pictures of Darwin with various lengths of great white beards but this is the very intense Darwin in 1855 when he was writing the origin of the species the quote I showed earlier is a very common quote that it's a very interesting debate you know where eyes came from and how can evolution do it this is a somewhat less common um quote but I I like it anyway how a nerve comes to be sensitive to light hardly concerns us more than how life itself originated it it he doesn't find it it's sort of an unknowable a question of when exactly that happened but I may remark that is some of the lowest organisms in which nerves cannot be detective are capable of perceiving light it does not seem impossible again all these negatives that certain sensitive elements in their SAR code and sour code is really just the cytoplasm of neurons of not neurons this is non neural cells should become aggregated and development to nerves and it endowed with this special Sensibility so for him um the real origin of all this is not the creation of nerves that transmit information about light and even the evolution of the optics but that the first thing is is the creation sometime in evolution of these options these light-sensitive elements that you call them and um here's a nice paper there they're actually a lot of papers on the evolution of opsins this is a particularly nice one that really just is what is that it's 150 years after a Darwin of course um and it really goes through the evolution of all the options in the animal kingdom and all the way from humans through a port Estonia actually through a bilateral invertebrates this is a Horton prep where a great deal of vision research that happened but primarily because the neurons are so big so horseshoe crab has a multi-faceted eye an eye with many omote idea very much like an insect but it was studied by heartland another heart line was in several slides back the person who coined the term receptive field he coined that term from studying invertebrates the horseshoe crab and also from frogs so these are the well known visual animals but in the animal kingdom all the way down through sponges have a photo receptor proteins and eye presumably down here is what Darwin was talking about in very very lowly animals that have photo reception but in fact they're even in in bacteria there are options and there appears to be somewhat of a controversy whether there's parallel evolution or they in fact stem from each other but the options in bacteria are are quite different from the options in in the animal kingdom and one of the great things about them is that they're directly connected to channels rather than a biochemical cascade so they're very simple and you can imagine they might be very useful in studying neurons because if you can control the channel that's much easier than having to rely on a complex biochemical cascade in order to control the level of polarization of a neuron so here in 2005 there was really a landmark paper from Boyden Fang bamberg Nagel and dice are off and three of those authors are really the people who are known as the leaders the early leaders and creators of the field of optogenetics in 2003 Nagel discovered channelrhodopsin a cation channel which i'll tell you about later in in certain bacteria and in carl dice Roth's lab ed Boyden and Karl Deisseroth figured out how to put the these channels the channelrhodopsin into individual neurons and begin the use of genetically introduced molecules of that control neural excitation with light so here's the two sort of key figures in Boyden and Al in 2005 this is showing a the expression of a GFP actually in that that was put along with the channelrhodopsin just to be able to visualize which neurons were expressing the the Chandra Dobson molecule but here's the important figure the physiology and it looks very much like a physiology experiment that I've already shown you a visual physiology experiment where you flash a bit of light the light and you know back in the 1950s the convention was you put a little bar over there when the light is is on and then you superimpose that with a neural trace where you get action potentials when the light is shown this is exactly the same thing in a culture of cortical neurons so this is a cortical neuron where you shine light these channelrhodopsin is is sensitive to blue light and when it's receiving blue photons it opens a check depolarizes the neuron there here's the the internal voltage starting negative a little bit less negative and then action potentials fire so it turns a cortical neuron directly into a non neuron and um here's one of multiple reviews of the field but a very nice review from Carl dice Roth's group neurons can be excited with channelrhodopsin or inhibited with halorhodopsin or a number of different inhibitory classes of that of of options that hyperpolarized neurons and their multiple different excitatory options as well and here is a figure that tell what these two molecules do and how they differ they both are excited by light but when Chalmer Dopson is excited by light which is incidentally blue it opens a channel and this channel is fairly selective it only passes positive ions and but it's not that terrifically selective at the type of positive ion it passes both calcium ions and sodium ions and then that effect of both is to depolarize the neuron or excite the neuron halorhodopsin is similar in that it's excited by light and then opens a channel but this channel is highly selective for the negative ion chloride the negative chloride ions and that hyperpolarizes a neuron when it receives a yellow light so this is just two of the growing toolbox of molecules that you can put into neurons and turn them from the things that typically can only be modulated by chemical neurotransmission or a chemical neuromodulation or in some cases the passage of ions with gap junctions to neurons that are directly excited or inhibited by light so I'm just going to give a few examples of the use of these options in cortical neurons probably the neurons that we have spoken about in this course so far and really the novel thing that one is able to do is to turn a circuit which one only could study with either physiology with conventional means such as exciting the visual system through the eye or other behavioral modifications or sometimes pharmacology one is able to go into the circuit and really crack its logic figure out its logic by making individual neurons sensitive and it's not just a the optic part of optogenetics but really it's the genetic part of optogenetics that makes it so extraordinary powerful these are proteins that one can make express in neurons of the brain but now that we have so many tools to selectively target proteins to different classes of neurons we can target these light-sensitive molecules to all of the classes of neurons that we have genetic access to for instance this is a paper I chose it a because I like it very much and B because the first author Shawn Olson will be joining us at the Institute this summer and it's a it's his work from a postdoc in Massimo's con Jionni's lab and they targeted a certain class of layer six cortical neurons and the class of layer six neurons that provide feedback to the thalamus the other reason I chose it is that Christoph's favored pathway is the is the layer six feedback that last week he gave his consciousness lecture and he and others really consider feedback a of sort of an important or perhaps the important a characteristic of cortical networks of a network of networks that allows not just the feed-forward passive a response to how the periphery impinges upon the cortex and it might drive it like a fancy TV camera but in fact allowing higher centers and all the way from centers in the cortex association areas that might make decisions or really think and the theory is that without feedback the the top-down influence the cognitive influence disappears and Francis Crick and Kristoff had a theory that you might be able to create a zombie animal a zombie Mouse by cutting out the feedback so here's an example of exciting these feedback neurons and let me show you what happens in the cortex that this is a very interesting paper that has a lot of other a lot of results but I really just want to show you the simple phenomenology the simple result of what happens when you excite these layer six neurons with photons um so the experiment is very similar to the experiments that I've been speaking about for the last six months that you have a mouse looking at a computer monitor and one is recording in the brain to see brain activity and in this case it's not recording optically but recording with a multi electrode with a probe you can stick into the brain that has in this case more than ten different sites each one of these sites has an independent electrical recording of the local neurons so you can record neurons in layer six five four and then layer two three and over here we have the physiological activity of these neurons of neurons on these electrodes when a visual stimulus is is presented and on the x-axis as always is time on the y-axis is different traces of different streams of action potentials and these at corresponding to different trials and the electrodes were in were lumped into four different ranges of depths in the cortex from less than 350 microns at the surface of the cortex down deeper and deeper all the way down to greater than 650 microns and more importantly on the right-hand side that that corresponds to layer 2 3 2 the input layer layer 4 and then layers 5 and 6 so what happens when light is trying not only in the into the eye but also into the back of the brain what happens well of course the deep layer neurons layer 6 are excited by this light so they're they're excited here by vision but when the the blue light is turned on they're excited even more there are even more action potentials the these little tick marks get denser but something potentially quite surprising happens in the superficial layers that the neurons in layer 5 4 and layer 2 3 are in fact turned off and this is almost certainly or certainly because not that the layer 6 neurons are inhibitory but the the strongest thing that they do when they project to these various layers is as excite inhibitory neurons and those inhibitory neurons inhibit all of the neurons in the in the local neighborhood so that's just one example of many where we are now able to dissect neural circuits at the level of individual cell types I I'm leaving out a a field that's really just beginning to emerge the field of not only optogenetics that excites that might excite or inhibit all the individual neurons of a given class but I feel that a lot of people like to call random access optogenetics wouldn't it be great to have multiple cells in layer 6 expressing these genetically encoded molecules and not just exciting all of them but exciting perhaps this cell here but not that cell there and of course that would that'll allow us to really treat the tumor people like to use random access to sort of a technical term that one can randomly excite different neurons Raphael you say and I like to both like to use the word playing the piano that in the limit I think fairly soon one is going to be able to to use advanced optical techniques to first image many cells in the cortex or anywhere else in the surface of the brain but really the cortex and excite individual neurons using advanced microscopy and really trying to reverse engineer the the circuit if this neuron is excited might it excite that neuron there might excite a specific neuron and layer for um and that's just optics but as I said at the beginning nature through evolution has given us the easiest optical system to excite neurons in the brain and that's the eye so I'm going to finish up with a couple of experiments that are both elegant but really go beyond just the intellectual exercise of trying to figure out how circuits work to try to cure major and serious disease or a condition blindness there are certainly millions of individuals who are legally blind or hundreds of thousands who are really almost completely blind due primarily to afflictions that affect the retina and a large class of those afflictions is a retinitis pigmentosa that that attacks the retina from the outside in from the photoreceptors starting at the outer segments where the the photo pigment is and eventually that through mechanisms both known and unknown results in a degeneration of the entire retina but there's a long period at which in which just the outer segments of the of the retinal cells are damaged of the photoreceptors are damaged but certainly the inner retina and really often the cell bodies of these of photoreceptors are spared so the question is and you have a very important clinical question is can some forms of retinal blindness be cured without the genetics and the simplest way that one can imagine doing this is starting with the the cells that signal to the brain that light is present at this position in in the eye and therefore give some amount of form vision you know being making the the eye photosensitive but not giving you any form vision is in terrifically encouraging but if one could label many or perhaps all of the ganglion cells or not labeled but create or cause the expression of these genetically encoded options in the ganglion cells one can get signals from the eye to the brain and in and use the optics of the eye that the microscope in our eye to excite different ganglion cells at different positions in the same way that the optics of the eye excites photoreceptors at different positions so this is an animal model of retinitis pigmentosa Rd one it's a retinal degeneration a mouse that has been study for a long time because it's a good model of retinitis pigmentosa and histologically when one looks at the retina in these animals compared to the wild type there is a loss of the outer retina so that what's left is in this case cell bodies of various types but in particular the the retinal ganglion cells and here Sheila Nuremberg and and Pandora a colleague they created this mouse where the ganglion cells were photosensitive and made the animals go from a state where they didn't there was no sign that the eyes were collecting light and processing that life to a state where they did process the light and so but something interesting happened I don't have a slide of it is that when they did the simple thing when they took just simple images and showed those images a very bright version of those images to the mouse what they were trying to do is to assess the processing of visual information as measured by tracking the of animals including mice we'll track a visual stimulus as its moved along and they found that just throwing up a visual stimulus wasn't terrifically good at making these previously blind animals track but they did something that she learner Berg who was really a it has a lot of has done a lot of work in the processing and really the computation performed by the retina when she rather than putting photons on the retina that were just simple images of objects she projected the images that the on ganglion cells would transmit to to the to the brain that the the on Gambian cells because they have these complicated center-surround organization they don't the array of them don't just project a picture to the brain they project a highly processed picture to the brain and when that highly processed picture is actually put into the eye the animals at least behaviorally were close to normal but that really bring points out something that naive experiments that one might want to do to cure the blind eye may not always be the right thing as as you go deeper and deeper into the visual system for instance if you wanted to excite the visual cortex to cure blindness you'd have to do all sorts of different types of transformations to have the properly encoded information into the brain if however what's the one place in the visual system where there is a only slightly processed version of the outside world that's in the photoreceptors and in this really lovely paper um that I'm about to show that actually preceded the Nuremberg paper by two years of a paper that came out in 2010 and vote on rosca had the great idea that if photoreceptors are in fact hyperpolarized by light let's put one of these inhibitory options into the photoreceptors that are spared in a retinal degeneration and see how that works so here we have a very similar set of figures we have on the bottom we have a wild-type mouse a totally normal Mouse and what the rasca team did was target proteins to the photoreceptors and in a normal Mouse these proteins you'd see them both where the cell body so here here we have the the retina in a montage both that shows the cell bodies and the green protein that's being put into the eye here are the cell bodies of the photoreceptors exist and here we have the outer segments the the parts of the retina or of the photoreceptors that receive light and in the wild-type animal these outer segments are perfectly healthy and in this case they they express these proteins that are put in genetically but here we have a diseased retina this is again a retinal degeneration animal frd is a fast retinal degeneration um and you see there's really there's nothing out here where the outer segments the photoreceptors or the photo pigment would normally exist but in fact sort of surprisingly when people knew this you know naively surprisingly that if you target photoreceptors to express exogenous proteins even in a highly degenerated retina you have a certain amount of expression there are there are a significant number of spared photoreceptors that really only exists out here where the cell body is and where the synapses are but in this case they put in a marker protein but more importantly the halorhodopsin the chloride channel which hyperpolarizes these neurons and sends exactly the signal that you'd want from photoreceptor to the bipolar cells and all of the the rest of the retina so what they did was put the the mice in a very bright or presented very bright stimuli to the mice that were completely ineffective in the disease Mouse without this rescuing optogenetic protein um and then they did a retinal physiology and they found really exactly what Koffler discovered in in the 1950s that they could record seemingly quite normal on center cells and off center cells that when light is is introduced classes of ganglion cells responded when the light was introduced by the sign inverting synapse presumably and off cells were similarly responsive when the light is turned on there are no responses but when the light is turned off there's there's a response of these neurons so in in the the first experiment that I showed you all of the Gambian cells have the same on type responses and that that presumably is not the greatest thing for the rest of the brain to process but in this strategy this is really a perfect way of restoring all of vision and using all of the circuitry of the eye and another beautiful part of this paper is that they got pathological tissue they got human retinas from eye surgeons and were able to show that they could transparent issue so that the photoreceptors expressed this inhibitory opsin so in my final slide really summarizing you know why it's such a great time to be doing this kind of work it's really the same picture I showed you at the end of the first lecture why is it such a great time to be studying the brain well we have our old friend vision that has been used for the past 70 80 years to really allow a very rich entry into complex nervous system is to look at information processing but now we're not limited by single electrical recordings of the brain but in fact we have all sorts of techniques to record from tens or hundreds of neurons electrically or hundreds or thousands of cells optically here's an example of our really earliest optical recording from 100 cells in visual cortex but not only are we able to observe the brain in action we're able to perturb the brain and to really exquisitely go into the circuit and isolate a class of neurons and then really in the limit in the coming years isolate individual neurons and treat those individual neurons as photoreceptors so we can simultaneously when we're saying the physiology of 100 neurons we can take one two or ten neurons excite them or inhibit them and ask how that changes the activity of the circuit really it's it's the the necessary perturbation that one needs to do in order to reverse-engineer a neural circuit but it's not just for us you know these techniques are again great for the intellectual exercise of understanding the brain but optogenetics in particular is one of the great opportunities to use not pharmacology which is often a very very blunt tool blunt instrument for changing brain activity and trying to ameliorate or even cure nervous system disorders with psychiatric disorders or neurological disorders but without the genetics one can go in and either excite or inhibit individual cell types and in the case of the really the multi lab and international effort to cure certain forms of blindness to use the optics of the eye to take opposite ends of the evolutionary tree the you know to take the human eye which is the example of almost unimaginable evolutionary trail that takes the the animal kingdom options and transform them into options that excite neurons options that are in clumps of neurons that gather are hybrid light sensors all the way through optical systems that that allow the outside world to be imaged taking all of that I evolution and throwing us way back in evolution with these bacterial options and using those to effectively cure one of the major afflictions so I think I'll stop there and take questions thanks you yeah no that's the question is I can um if you put options into the eye photoreceptors or ganglion cells or anything else is it possible to use natural light or do you have to use some sort of enhanced light um perhaps in the brightest a sunlit day one can get some response but I think most of these these approaches have the idea of using cameras and very bright screens in front of the eyes but um it's still it's a pretty good a way to do things one if one puts um one tries to cure blindness by exciting cortical neurons you might be able to do that with bright lights but then you're you're missing one very important thing I movements that they're they're movements of the head and then there are movements of the eyes uh if you have a camera system that's yoked to the head and and glasses that are yoked to the head they'll all move rigidly and and and act as a creation of a screen and then the eyes can look at different parts of that screen and you have exactly you know what you want and really you know that's um going back to philosophy and really the the earliest version of how how do we know the outside world Plato's cave that how how do we know that there really is an outside world as opposed to us being creatures reacting to something that looks like the outside world but it's just projected onto the wall of the cave this is really an example of Plato's cave in Tec I wear chat there's nothing so terrifically different between having a good camera and a good bright projector and and looking at the outside world and the users wouldn't really know the difference it's a great question how many photoreceptors synapse onto or with bipolar cells and how many bipolar cells have synapses with ganglion cells and the answer is it's a huge range um in primates there's something known as the system in our phobias and sort of beyond the fovea and in the system that grow up to both bipolar cells and Gambian cells there is a one photoreceptor form synapses action into two bipolar cells went on and went off center bipolar cells and each of those bipolar cells forms synapse with one ganglion cell partner so it really is just a direct labelled line and that's why we're able to use acuity that is essentially sampled at the scale of these two micron cones but out in the periphery uh many cones you know hundreds of cones can synapse onto a given bipolar cell or in our periphery or in all of the mouse retina there hundreds of cells and and for the the rod photoreceptors which are much smaller than the cones it's thousands the convergence is thousands okay well thanks you

Channel: Allen Institute

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Keywords: full-length lecture, vision & coding, coding & vision, lecture series, undergraduate lecture, optogenetics, neural coding, visual system, Clay Reid, R. Clay Reid, Allen Institute, retina, receptive fields, opsin, imaging

Id: LoXye6KBeCI

Length: 59min 6sec (3546 seconds)

Published: Mon Jan 21 2013