There are lots of challenges when it comes to studying the brain, but one of the biggest is that it’s very hard to see. Aside from being, you know, inside your skull, the many electrical and chemical signals which the brain uses are impossible to see with the naked eye.
We have ways to look at neurons and how they convey information. For example, to record the electrical signals from a single neuron, you can piece it with a tiny electrode, to get access inside the membrane (electrophysiology). You can then stimulate the neuron to fire, or record as it fires spontaneously. For techniques like optogenetics, you can insert a gene into the neuron that makes it fire (or not) in response to light. When you shine the light, you can make the neuron fire. So you can make a neuron fire, or see a neuron fire. With things like voltammetry, we can see neurotransmitters, chemicals as they are released from a neuron and sent as signals on to other neurons. Techniques like these have made huge strides in what we understand about neurons and how they work.
But…you can only do this for a few neurons at a time. This becomes a problem, because the brain does not work as one neuron at a time. Instead, neurons organize into networks, A neuron fires, which impinges upon many more neurons, all of which will react in different ways, depending on what input they receive and when. Often many neurons have to fire to get a result, often it’s a single specific pattern of neurons. An ideal technique would be one where we could see neurons fire spontaneously, in real time, and then see where those signals GO, to actually see a network in action. And where we could see it…without taking the brain out first.
It looks like that technique might be here.
Cao et al. “Genetically Targeted Optical Electrophysiology in Intact Neural Circuits” Cell, 2013.
When a neuron “fires”, it undergoes something called an action potential, a wave of changing voltages that goes along the membrane until it reaches the synapse. At the synapse, the changing voltages trigger neurotransmission, where chemical signals are released from one neuron to then simulate receptors on other neurons.
This new technique, called Arclight, is the latest in genetically encoded voltage indicators (GEVIs), where you insert a gene into your organism that is attached to a fluorescence indicator. The gene itself is sensitive to the voltage of a cell. So as the action potential occurs, and the voltage of the neuron changes, the voltage sensitive indicator activates the flourenscent protein attached to it. The voltage of the neuron changes…and the neuron itself GLOWS*.
This means that you can take these GEVIs and insert them into a series of neurons…and then watch as the neurons fire. As a proof of concept, Cao and his group, the Nitabach laboratory at Yale, put these GEVIs into the circadian neurons of a fruit fly.
Above you can see the changes in florescence in the morning (red) and the evening (black), as the fly wakes up and goes to sleep. They also put the GEVIs into the olfactory (smell) system, and exposed the flies to various chemical scents. They were able to watch as the neurons responded specifically to some scents over others.
Now, this isn’t the first florescent sensor. A lot of great work has been done with calcium sensors, which can measure cell activity by measuring the amount of calcium. Calcium sensors can measure tens of thousands of cells at a time, and are a very valuable tool. But calcium is an indirect indicator of cell activity, and calcium signals do not occur for every single change in neuronal potential (for example, you get no calcium for inhibitory signals). Arclight has the advantage of measuring the membrane potential directly.
Of course, other techniques, like electrophysiology, measure membrane potential directly as well, but they cannot do it across the WHOLE CELL. If you watch this video of Arclight in action, you will see the membrane potential change across the entire cell. Of course, Arclight cannot actually determine the membrane potential, but it has the advantage of being non-invasive (you do have to put in a gene, but you don’t have to pierce the cell membrane with an electrode).
The lab hopes to further refine Arclight, making it more sensitive (right now the signal to noise can be difficult). It can also be combined with other techniques, such as optogenetics. “Optogenetics refers to using light to *manipulate* neuronal membrane potential, and ArcLight is a means for using light to *measure* neuronal membrane potential” says Mike Nitabach. “So they are two sides of the same coin, and the prospect of deploying them together in an experiment would allow wholly optical closed-loop control of neuronal circuits.”
The best part? The Arclight technique is already available for other scientists. The cDNA is available from Addgene, and the flies are available as well! The Nitabach lab is glad to share knowledge of the technique and is glad to help any scientists who want in on the technique to obtain the reagents. Currently, it works best in things like fruit flies, who already have relatively transparent brains, but the lab is already working on mouse neurons. And the implications for the technique are already huge. From fly networks, it’s a step to mouse, and from mouse, who knows where we might go?
*Please note that in that video, courtesy of the Nitabach lab, the color map for the florescence has been inverted. Usually a higher voltage (and a neuron firing) would be a decrease in florescence with this technology. This has been pseudocolored so the higher voltage is an increase in florescence. Easier to look at.
**Note: Ed Yong also did great coverage of the new technique.
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