9 April 2007. Scientists would love to map the activity of neural circuits that drive behavior, thought, and emotion. Doing that requires that one can direct neurons to fire or fall silent from afar, a bit like a lecturer might point to a spot on the screen with a laser pointer. In the living brain, that’s largely the stuff of science fiction. Now, Karl Deisseroth and colleagues from Stanford University in California have developed a new tool for accomplishing this task using flashes of colored light that control neuron activity quickly and precisely. Together with collaborators from the Johann Wolfgang-Goethe-University and the Max Planck Institute, both in Frankfurt, Germany, Deisseroth engineered neurons to express a light-sensitive ion channel or a chloride pump, or both. This allows the investigators to turn on neuronal firing with a flash of blue light, or turn it off with yellow. By expressing the channels in the worm C. elegans, the researchers showed they could control swimming behavior in live worms with pulses of light. The work appears in the April 4 Nature.
Artist's Rendering of Optical Remote-control in Action
Blue and yellow lights evoke or inhibit electrical activity in neurons. Image credit: Feng Zhang, Steve Dixon, and Karl Deisseroth
“The system provides researchers with a two-knob remote control for increasing or decreasing the activity of specific neurons using different colors of light,” write Michael Hausser and Spencer Smith at University College London, United Kingdom, in a commentary accompanying the paper. “The overwhelming advantages of this new approach should revolutionize the field,” they write, since having both stimulatory and inhibitory control over neuronal activity will allow researchers to prove when activity is either necessary or sufficient to trigger behavioral effects.
Another paper, out in the March issue of PLoS ONE from Xue Han of Stanford and Edward Boyden of MIT in Cambridge, Massachusetts, shows similar results with the same pump in cultured neurons.
Previously, Deisseroth and colleagues had shown that they can express the algal protein channel rhodopsin-2 (ChR2) in neurons, and that the protein rendered the neurons sensitive to depolarization by treatment with flashes of blue light (Boyden et al., 2005). That channel constituted a remote “on” switch for the genetically modified neurons. In the new work, the scientists introduce the companion “off” switch—a light-driven halorhodopsin chloride pump from the archaebacteria Natronomonas pharaonis (NpHR), which responds to yellow light. Expression of NpHR allowed the rapid and reversible hyperpolarization of cells, shutting down spiking activity. The responses to light were so fast, on the millisecond scale, that it was possible to block single action potentials in a train of spikes.
That the channel and pump register different wavelengths of light was a lucky break. It raised the possibility that cells expressing both proteins could be rapidly toggled between depolarization and hyperpolarization. The researchers showed that, indeed, they could use mixed light pulses on doubly expressing cells to generate predetermined on/off firing patterns either in cultured cells or in tissue slices from mouse brain. What’s more, by tagging the pump and channel with fluorescent proteins, and loading cells with a calcium dye, they could find modified cells, optically activate them, and measure the resulting calcium transients in cortical slices. Using these technologies together allowed the researchers to “identify, observe and control” intact living neural circuitry, the authors write.
To test their system in living animals, Deisseroth and coworkers turned to C. elegans. When they expressed the NpHR in either body wall muscle cells or cholinergic neurons, they were able to paralyze swimming worms with pulses of yellow light. The two proteins worked together, as well: in doubly expressing worms, ChR2 activation caused muscle contraction (the worms appeared shorter), which was reversed by NpHR activation.
Besides opening up a new frontier in circuit analysis, the technique could have clinical use, too. Deep brain stimulation, like that used to treat Parkinson disease, could be accomplished with fiber optic illumination of suitably engineered cells. The authors conjure up the idea of an “optical neuronal prosthetic,” for the treatment of disease.—Pat McCaffrey.
Zhang F, Wang LP, Brauner M, Liewald JF, Kay K, Watzke N, Wood PG, Bamberg E, Nagel G, Gottschalk A, Deisseroth K. Multimodal fast optical interrogation of neural circuitry. Nature. 2007 Apr 5;446(7136):633-9. Abstract
Hausser M, Smith SL. Neuroscience: controlling neural circuits with light. Nature. 2007 Apr 5;446(7136):617-9. No abstract available. Abstract
Han X, Boyden E. Multiple-color optical activation, silencing and desynchronization of neural activity, with single-spike temporal resolution. PLoS One. 2007 March 21; 2(3):e299. Abstract