Who’d have thought that a protein isolated from pond scum would transform the way researchers investigate the brain? The protein, called channelrhodopsin (ChR), is found in algae and other microbes, and is related to the molecule in human photoreceptors that captures light particles. Both versions control the electrical currents that constantly flow in and out of cells; one regulates the algae’s movements in response to light, the other generates the nervous impulses sent along the optic nerve to the brain. Unlike its human equivalent, the algal ChR controls the currents directly because it forms a pore that spans the cell membrane. When expressed in neurons, it renders the cells sensitive to light, and they can be switched on or off very precisely using lasers.
This discovery led to the emergence of a new field called optogenetics. Early studies showed that the technique can be used to control the behaviour of small organisms such as nematode worms and fruit flies. Last year, Karl Deisseroth‘s group at Stanford University demonstrated, for the first time, that it can also be used to control reward and motivation behaviours in mice. Josh Johansen of the Center for Neural Science at New York University and his colleagues have now taken this one step further. Working in collaboration with Deisseroth, they show that optogenetics can also be used to induce a simple form of associative learning called fear conditioning.