Controlling Individual Neurons
In April 2007 Karl Deisseroth, Assistant Professor of Bioengineering at Stanford University reported that he had created genetically modified C. elegans with individual neurons that could be switched on and off as he desired. It has long been known that electric current can simulate neurons. This is how most neuroscientists activate neurons. However there are two problems with using electrodes; even the smallest electrode activates thousands of neurons and although electrodes are able to activate neurons they cant deactivate them. Deisseroths method overcomes both these drawbacks. His worms have been genetically modified so that they have genes for two exotic proteins in specifically targeted neurons. A protein imported from photosynthetic algae turns on neurons, shine a blue light on the protein and it pumps neuron activating sodium into the cell. A different protein found in bacteria that live in salt flats is used to switch off the neural activity, irradiation with a yellow light turns this protein on and causes it to pump chloride ions into the cell. The negative charge on the chloride ion negates the positive charge on the sodium ion and thereby deactivates the neural activity. The light induced on-off switch also works in higher organisms. The expression of the chloride pump was observed using enhanced YFP and the sodium pump was monitored with mCherry. We were able to introduce both of these genes, the excitatory and inhibitory signals, into the brains of mice. We were able to take out a slice of brain tissue - a little like taking a circuit board out of a computer - and we were able to drive and control neural circuitry in this intact mammalian tissue, says Deisseroth. Ultimately this technique should help researchers understand neurodegenerative diseases such as Parkinsons disease.
The yellow circle is shown to indicate illumination with yellow light (off-switch). The genetically modified C. elegans worm responds within 1 ms. (Courtesy: K. Deisseroth)