An immediate application for optogenetics in combination with microscopy is research on layers of cultivated excitable cells. Monitoring the membrane potential of the activated cells, or cells that have direct or indirect contact with the activated cells, reveals insights into the general biology of action potential generation, axonal signal transmission and synaptic activities. Connectivity of neurons and processing of signals in neuronal circuitries is another target for such experiments. Monitoring of the responses is possible with electrical electrodes, usually patch-clamp techniques, or fluorescent sensors – increasingly fluorescent biosensors based on FRET concepts.

To understand wiring and signal processing in real circuits, brain slices or tissue slices in general are frequently used. By introducing optogenetically excitable cells, it becomes possible to study these circuits in the natural context. Shining light onto a region of a brain slice will allow the response to be recorded in other regions, even if they are anatomically remote. Also, a mechanical response, e.g. muscle contraction, is a parameter that can be used to measure the effect of light- induced activation of various target cells. The possibility to transfect only very specifically dedicated cell types sets optogenetic techniques apart from traditional experiments with unspecific electrical or chemical stimulation.

The final step in applying optogenetic techniques to investigations by microscopy is of course the use of living whole animals and studying their response to light-induced alterations of membrane potential or activation of cell signaling pathways. Small animals, like fruit flies, nematodes or maggots, might be illuminated as a whole to trigger the optogenetic responses. The central nervous systems of mammals, very often rat or mouse brain, are studied through cranial windows in living animals. Here, two-photon microscopy is the method of choice, as one is interested in deep imaging – and chemical or physical clearing is not possible in vital specimens. In continuation of the intrinsic optical sectioning feature of two-photon imaging, two-photon illumination is the tool to very specifically activate only single cells in a complex 3D arrangement of neurons like the brain. Also, fast scanning concepts in combination with two-photon activation enable fast alterations in the stimulation of various cells to shed light on their wiring.

At the moment, ideas, concepts, suggestions and results that mention optogenetics are exploding and moving biological sciences forward at an unprecedentedly dizzy speed. Try to keep track [9]!

On the other hand, the method also has its critics. Will humans be remote-controlled in future? Is the experience of joy something that might be induced artificially in order to keep human beings efficient (figure in the introductory paragraph)? And does this mean that there’s no need to treat our fellow men with respect in order to live together in peace [10], as we can repair the mental damage technically?

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