How to modulate the network in the brain? Part I.

One of the most important part in every neuroscience study besides the anatomical information is the optogenetical part. Namely, which method was used to achieve the results. There are plenty of different methods on the market. Companies offer various solutions from low budget to high-end range. Sometimes the scientists build their own solutions for the certain experiment but if we count the manpower as well, it will not be cheaper. Another important question whether is it compatible with fluorescence imaging or only with electrophysiology? How precise is it regarding spatial and temporal resolution? What other components are necessary for the experiments? And so on. These are not easy questions.

There are different available solutions to modulate the network from one single cell to thousands of neurons. They have many advantages and disadvantages as well. We can categorize them in different aspects like spatial and temporal resolution, but an application point of view is also a great way. I would choose spatial dependent comparison. In this case there are three main groups.

The first is the full field LED stimulation. In my opinion the main advantage of this technique is price. That does not mean this is a cheap method, but it is quite affordable compare to other opto-methods. Other good things are the simplicity, combinability with other techniques (e.g. electrophysiology) and precise timing. If the scientific question is behavior related, LED stimulation could be enough. It is also true that LED can be added later to the rig (in most cases) and various colors are available on the market. It is important to note there are two crucial things can pop up when you are considering upgrading your rig with LED. First is the timing. How precise is the control of the LED? What is the minimum time for a single stimulation? How long is the latency period of the LED? These questions must be answered before any experiment, otherwise data can be corrupted. Second crucial thing is the problem of gating. If you are planning to use fluorescent imaging during the stimulation the protection of the PMT is a very important thing. The most obvious option is to cut off the stimulation light (e. g. 472 nm blue light for excite ChR2) from the detection path. It is also true other stimulation solutions which are operate with visible light. The best way to achieve it using electrical PMT shutter. This solution is extremely fast (µs range) but can also increase the final price tag.

Very similar technique is the built-in fiber optic stimulation. In this case a fiber implanted in the brain next to target area, so the illumination volume is decreased. In addition, it can be used with freely moving animal experiments and even brain oscillations (e.g. epileptic paradigm) can be recorded in parallel. When the experiment requires fine details and the main question is about single cell responses or even dendritic level then Galvo-CW might be the right answer. In this configuration the CW laser path goes through a Galvo-galvo scanner. This allows the user to select various patterns (e.g. point, line, tornado, zig-zag, circle or even mixed) and set precise stimulation time for every pattern. Most of the providers offer a possibility to combinate the stimulation with electrophysiology or/and 2P fast resonant scanning. It is also possible to add extra laser sources to the light path which can open new doors for network modulation. This kind of stimulation only possible in two dimensions but the whole field of view can be used during the experiment. Last but not least the price tag is quite good compare to SLM holography or acousto based techniques. Since it is only one-photon excitation it does not really useful for in vivo mice or thick acute brain slices because of the profile of 1P excitation and a degree of the shorter wavelength photon scattering. Nevertheless, using CW laser for optogenetics applications is still a very good idea.

These were the simpler methods for optogenetics applications. Next time the veil from high-end optogenetics tools will be removed.