#CAN2014 Brain, illuminated: new optogenetic tools for brain control
Notes from the 2014 Canadian Association for Neuroscience annual meeting in Montreal, Cananda.
The brain's a tough nut to crack. The spongy computer is densely packed with over 86 billions neurons, each of which springing thousands of connections organized into dynamic neural circuits that process information on millisecond scales. Add in to that a complex web of crosstalk between neurons and another 84.6 billions of non-neural brain cells known as glia, and "solving the brain"seems practically impossible.
But across multiple computation levels neuroscientist are making leeway in decoding the brain. Molecular biologists are beginning to understand how life experiences orchestrate gene transcription and protein modifications, which work together to change the firing properties of single synapses and neurons. Electrophysiologists have eavesdropped on the chatter between neurons from diverse brain areas, providing clues to information processing within brain subregions. Viral vectors have helped scientists map out the anatomy of neural circuits one by one. By integrating data from individual circuits, scientists are beginning to puzzle out how the pieces fit together.
Yet one link is conspicuously missing from the whole picture: what are the rules that transform the firing of an ensemble of neurons into our thoughts, emotions and behaviour? Traditionally, scientists have relied on inferring network activity by recording changes in neuronal activity or blood flow (in the case of fMRI), but neither technique actually traces the firing pathways that control behaviour. To establish causality, we need tools to directly tweak the activity of a particular neural pathway and observe the behavioural outcome - the more precise, the better.
A flash of light
One such tool is optogenetics. The core of this technology is light-sensitive proteins called opsins, which are naturally found in unassuming sources like algae and bacteria. Large and complex, opsins are made up of columns of protein subunits that cluster into a channel. In response to blue, green or yellow light the channel opens to allow ions to pass through. Since the basis of neural activity relies on the balance and movement of ions through the membrane, these electricity-generating proteins are the perfect controllers of brain activity.
By genetically introducing opsins into the mouse brain, scientists can use optic wires - tethered to a laser and implanted into the brain - to stimulate or silence these genetically modified neurons. Scientists can also target opsins to a particular brain area or to neurons that release certain chemicals (such as "dopamine neurons"). Pick a circuit, load some opsins, and we can finally parse out which cells are doing what in controlling complex behaviour. Take comfort eating: which cells are driving us to eat in the absence of hunger, which ones are inhibiting this urge, and how do the circuits interact to lead us to binge or abstain in the end? Optogenetics gives us answers, and these answers in turn hands us the keys to controlling behaviour.
With a flash of light, scientists can now drive well-satiated animals to overeat, "implant" false memory associations, block established habits and switch on and off repetitive behaviours related to OCD. Perhaps one day, light-induced inhibition may help us quiet down overactive circuits in epilepsy and other diseases. Optogenetics is only taking off - imagine being able to monitor whole brain activity as an animals learns, then simultaneously manipulating multiple circuits across the brain to speed up learning and strengthen memory. Even crazier, imagine using the recorded neural activity to precisely activate the same circuits in a different animal - will it "learn" vicariously?
Such fanciful goals are still a long way off. First, scientists need to develop optogenetic (and recording) tools with better physical and chemical properties. Current generation opsins are far from perfect. To start off, light itself is damaging to neurons; to minimize injury, scientists are engineering opsins with exquisite light-sensitivity so they can function at low irradiance. Opsins also need extremely fast on-and-off kinetics to mimic the lightning speed of neural transmission- something unfortunately often at odds with light-sensitiviity - and be able to produce large changes in neural activity. Colour matters too: current opsins only respond to either blue, green or yellow light, depending on their chemical makeup. Scientists are now mining a palette of opsins from the natural world that respond to light colours across the spectrum. Red is particularity attractive, as it penetrates deeper into the brain - roughly a millimetre, or about a quarter in depth of a mouse brain. Lying at the far end of the spectrum, red is also less likely to activate opsins that respond to blue light. This allows scientists to use two types of opsins and tinker with two circuits at the same time.
The hunt so far has been successful. At the Canadian Association for Neuroscience 2014 meeting in Montreal, Ed Boyden, a professor at MIT and pioneer of optogenetics, introduced several new tools ready for use in neuroscience research: Jaws, a red-responsive opsin that silences neural activity evoked by environmental stimuli; Crimson, a complimentary red-activated opsin that stimulates neural activity; Chronos, a rare find that combines the two opposing properties of fast kinetics and extreme light-sensitivity in one; Chrome, a mutant designed to manipulate intracellular organelles; Lumitoxin, an adaptor that transforms our very own proteins into light-responsive electricity generators.
LED helmets and recording robots
The improvements don't stop at manufacturing better opsins. Unsatisfied with the current state of optogenetic hardware, Boyden devised a strategy to "cut the cord". In the past, optic fibres needed to be implanted into the brain for light delivery, a process that takes skill, time and causes mechanical injury to the tissue. Boyden's solution is a wireless optogenetic helmet with 16 LED lights to control opsin-modified neurons in rodents. Plop it on, plug it in, and voila, you're ready for some brain control.
What about the tedious - if not impossible - task of recording activity from a mass of neurons one-by-one in animals? Boyden's answer is to develop an automated robot that expertly picks up single-cell activity (whole-cell electrophysiology) in animals and does so better than most human experimenters. If you're picturing tiny robots sticking electrodes into neurons across the brain, that's not exactly accurate. In this setup, "robot" simply refers to a plain-looking computer circuit integrated with standard electrophysiology equipment; the algorithms programmed into the circuit are doing the actual work. However, as far fetched as it sounds, advances in nanotechnology and material sciences may one day allow us to develop actual nanorobots that cruise the brain.
After all, with the birth of optogenetics and various brain-mapping tools, science fiction is already becoming science reality.
For those interested in learning more, check out this video of Ed Boyden talking about optogenetics, via the BRAIN initiative.