By Sri Sindhu Bhatta
What is Optogenetics?
In 2005 a group of three scientists, Edward S Boyden, Feng Zhang, and Karl Deisseroth,
collectively contributed to a technique called optogenetics. This technique uses a combination of light and genetic engineering (manipulating the genetic information of a living organism by inserting or deleting information in the genetic code) to control and monitor the activity of the brain cells.
Slowly, optogenetics became very popular in neuroscience and is now being used in brain research laboratories all around the world.
This field shows us new ways to perceive and understand the human brain and also helps us to unravel new things about the brain.
How does the human brain work?
To figure out how the brain works, many researchers had to do thousands of experiments and find ways to examine and test the brain.
But before getting into that, we need to understand the basics.
Let’s start with what is known as the most fundamental unit of the brain - neurons. Neurons are special cells in the brain and nervous system that work together to produce all our thoughts and behaviors. In other words, they're the heart of the underlying brain activity.
To understand how the brain can control behavior, we need to understand how neurons communicate.
Neurons communicate using both electrical and chemical signals. They communicate with each other via electrical events called 'action potentials' and chemical signals called ‘neurotransmitters’. At the junction between two neurons (synapse), an action potential causes the neuron to release a chemical neurotransmitter.
We now learned that neurons communicate through action potentials by causing voltage changes within the neurons and their surroundings.
Rising Era of Optogenetics
The human brain is very complex but luckily shares many similarities with those of other animals.
In 1780 an Italian scientist Luigi Galvani observed that the muscles of the dead frog's legs twitched during a lightning storm.
He hypothesized that the electricity of the storm was activating the nerves in the frog legs.
He tested the hypothesis in his laboratory by allowing electricity to flow through the frog’s nerve using an electrode causing the frog leg to twitch. It was the first time that anyone performed the electrical current simulation in neuroscience.
Galvani concluded that neurons communicate through electrical signals to pass information among them.
Experiment: Effect of electricity on muscular movement
Image Source: https://www.cell.com/trends/neurosciences/comments/S0166-2236(97)01101-6
Now that we know how brain activity works, we can understand all the underlying mechanisms that operate to keep the brain functioning.
We can use the electrical signals to turn on specific neurons and see what happens.
It wasn’t until Dr. Wilder Penfeild, a brain surgeon in the 1930s, used electrical stimulation to map the human brain.
He was working with epilepsy patients. Epilepsy causes abnormal electrical signals in the brain and can be lethal. In the extreme stages, the patient requires brain surgery to stop epilepsy.
Dr. Penfeild wanted to know which brain areas are the most important. So he went ahead and tried to map the brains of his patients. To map the brain, he used electrical stimulation, just as Galvani did. He observed that stimulation of one area of the brain caused a finger twitch, while stimulation of another area caused a foot twitch. It made him realize that specific regions are responsible for controlling specific parts of the body. He created diagrams of his results, giving us the first-ever functional map of the motor areas in the human brain.
Image Source: https://www.vox.com/science-and-health/2018/1/26/16932476/wilder-penfield-brain-surgery-epilepsy-google-doodle
But this method had some limitations. One of the drawbacks is that the inserted electrode might damage the brain - stimulation in general causes the activation of the whole tissue instead of a specific region.
It is like using an excavator when a shovel will do- an excavator is useful, but it is not as precise as the shovel.
Discovery of Optogenetics
In 2005, a new technique was developed to make more precise brain stimulation. Edward S Boyden, Feng Zhang, and Karl Deisseroth came across a unique protein in algae called channelrhodopsin that converts the specific wavelength of light into electricity. The three scientists collectively hypothesized that if somehow these proteins were inserted into the neurons, they could stimulate specific neuronal circuits, giving scientists more precision and control.
How did they do it?
The most famous opsin (light-sensitive protein) in neuroscience is channelrhodopsin-2 (ChR2) which comes from the green algae Chlamydomonas reinhardtii. This protein helps the algae move towards the light. Channelrhodopsin gets activated in the presence of blue light and doesn't respond to other types of light.
Image Source: https://www.sciencedirect.com/science/article/pii/B9781907568671500101
They specifically isolated the gene that produced this protein and inserted it into the genetic sequence of a virus. The virus containing the gene was then injected into the brain of a mouse, and a fiber-optic light implanted into its head. When the blue light was turned on, only neurons expressing channelrhodopsin generated action potentials, even though the light hit millions of neurons.
The six steps to optogenetics — Image source: http://optogenetics.weebly.com/why--how.html
To get the opsin into the neurons of a mouse, the genetic code for the opsin must be carefully inserted into the genetic code for the neurons in the mouse. Since we understand a lot about the mouse’s genetic code, we can choose where to put the opsin. We can insert the code into a specific type of neuron or a particular location in the brain. We can choose which neurons we want to control. It gives scientists precision and control over the time of neuronal activity.
What happens at a molecular level?
ChR2 absorbs blue light, causing a conformational change that allows H+, Na+, K+, and Ca+ ions to diffuse down their concentration gradients passively. When expressed in neurons, the opening of these channels causes a rapid depolarization of the plasma membrane that can cause action potentials. More importantly, the channel also closes very rapidly when the blue light is switched off. Therefore, a brief pulse of blue light can generate single action potentials with no long-lasting residual effects of stimulation. ChR2 is capable of driving neuronal activity at frequencies from 1 to 40 Hz. The great change in membrane potential generates an action potential.
What are the applications of optogenetics in neuroscience?
The extreme precision that optogenetics offers allowed scientists to do some pretty cool experiments on mice. They were able to target specific sets of neurons to create and modify the memory of the mice.
Optogenetics made it possible to determine which cells and connections across the brain were important in defining and assembling the different features of anxiety, including respiratory-rate changes and risk avoidance, into a distinct behavioral state.
Clinically inspired discoveries have also been made, helping to shed light on cellular activities associated with conditions such as epilepsy, Parkinson’s disease, Huntington’s disease, stroke, chronic pain, obsessive-compulsive disorder, drug addiction, depression, social dysfunction, and anxiety.
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