Photostimulation is the use of light to artificially activate biological compounds, cells, or even whole organisms. Photostimulation can be used to noninvasively probe the causal relationships between different biological processes, using only light. In the long run, photostimulation may be useful as a therapy, using light to adjust the biological state of human patients.

Photostimulation methods fall into two general categories: one set of methods uses light to uncage a compound that then becomes biochemically active, binding to a downstream effector. For example, uncaging glutamate is useful for finding excitatory connections between neurons, since the uncaged glutamate mimics the natural synaptic activity of one neuron impinging upon another. The other major photostimulation method is the use of light to activate a light-sensitive protein such as rhodopsin, which can then excite the cell expressing the opsin.

It has been shown that channelrhodopsin-2, a monolithic protein containing a light sensor and a cation channel, provides electrical stimulation of appropriate speed and magnitude to activate neuronal spike firing. Recently, photoinhibition, the inhibition of neural activity with light, has become feasible with the application of molecules such as the light-activated chloride pump halorhodopsin to neural control. Together, blue-light activated channelrhodopsin-2 and the yellow light-activated chloride pump halorhodopsin enable multiple-color, optical activation and silencing of neural activity. (See also Photobiomodulation)

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