Channelrhodopsins are photoreceptive proteins that are employed as optogenetic actuators. Many optogenetic actuators are found naturally in single cell prokaryotes, algae, or other microbes, and contribute to processes such as photosynthesis.
Optogenetics is a technique used to deliver light to living tissue via a fibre optic cable in order to selectively activate individual or groups of cells. The specific functions of these cells can then be investigated by controlling and observing an animal’s corresponding movements, behaviour, or biological processes.
The brain is often the focus of optogenetic research. The function of specific regions and neural networks within the brain is revealed by selectively targeting neurons. By studying brains with genetic/neurological disorders, the groups of neurons that contribute to those disorders can be identified and effective treatments discovered.
An “optogenetic actuator” is a photoreceptive protein that, when attached to the membrane of a cell, functions as a light-gated ion channel. In the case of nerve cells, light of a particular wavelength causes the channel to open by converting it into an electrical signal i.e. a flow of positive ions. It is this in-flux of ions that cause neurons to fire.
Delivery of the optogenetic actuator can be targeted to specific populations of neurons only in the presence of a “promotor gene”.
The first step in optogenetic experiments and treatments is to establish an optogenetic actuator in the region of interest. The gene that encodes the actuator protein must be extracted from its naturally occurring source and artificially combined with a “promoter” gene. The resulting genetically modified molecule is inserted into a virus and injected into the region of interest. Only neurons that naturally encode the promoter gene are infected with the virus, enabling specific neurons or neural circuits to be targeted.
Individual or groups of neurons can be selectively fired by illuminating the infected region with pulses of blue light after infecting the brain with the genetic code for channelrhodopsin.
Light from an LED or laser source is delivered to the infected region via a fibre optic cannula.
Neurons can also be deactivated, or “silenced”, by infecting them with a virus containing a different optogenetic actuator and delivering the appropriate wavelength of light. This concept can be used as a potential treatment by switching off the cells that cause a neurological disorder.
Individual or groups of neurons can be selectively silenced by illuminating the infected region with pulses of yellow light after infecting the brain with the genetic code for halorhodopsin.