Introduction to Photostimulation and Optogenetics

Neuronal activity has been studied historically using pharmacological stimulation through the application of drugs or electrical stimulation such as applying voltage or current via a micro electrode. Photostimulation appeared more recently and involves the activation of biological compounds using light. There are two main types of electrical stimulation: laser uncaging and photoactivation.

Laser uncaging involves injecting or loading the sample with a compound in a caged or inactive form. Light is then applied to the loaded cells to break the covalent bonds of the cage, releasing the active compound. The effect can then be measured using electrophysiology. This can be carried out using caged neurotransmitters such as glutamate, serotonin, GABA etc; and with molecules such as calcium or using ATP.

Photoactivation utilises opsins. These are ion channel proteins from green algae that open in response to light, letting ions into the cell which causes the algae to move towards the light. Using genetic methods, researchers can modify model animals so that their neurons express these light-sensitive channel proteins. When light hits these ion channels, they open and ions enter the cells changing the membrane polarity. The two first opsins to be used for photostimulation were channelrhodopsin-2 and Halorhodopsin. Both are activated by a different wavelength, therefore can be used in the same animal, but as well lead to two opposite effects. channelrhodopsin-2, when stimulated, activates the neurons that express it, while Halorhodopsin inhibits them. Used in combination, these two opsins can be used to decipher the maps of interactions in rather large areas of the brain.

The use of laser or LED light sources coupled with a microscope objective allows precise spatio-temporal photostimulation, giving insight into the role of specific neurons or molecules within neurons, as well as into the role of molecules within a single cell. The approach limits the impact of electrode insertion, gives access to a larger zone than classic approaches and is in principle easier to set up.

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