Light Repair Work
Researchers literally shed light on the brain in a new form of treatment.
Deep brain stimulation, in which implanted electrodes inactivate specific regions of the brain, can have a near-miraculous effect when used to treat depression. Yet many types of neurons are at play in the disease, and electrodes, unable to discriminate among them, create a halo effect, inducing unpredictable changes.
Enter a tool whose precision could leave deep brain stimulation in the dust: light. An “optical” technique, being pioneered in animal models by Ed Boyden at the Media Lab and the McGovern Institute for Brain Research at the Massachusetts Institute of Technology, involves engineering neurons to carry photosensitive proteins that act as on-off switches. Ultrathin fiber-optic cables implanted in the brain flash millisecond-length pulses of light to control the firing of the neurons bearing those proteins. A pulse of blue light instantly switches on a protein called channelrhodopsin, causing the neurons to fire, whereas yellow light activates halorhodopsin, which turns off the neurons.
Optical control can target selected neurons without affecting other types of neighboring cells. What’s more, unlike electrodes that can only stimulate neurons, not turn them off (the inactivation of brain regions in deep brain stimulation is actually accomplished by activating inhibitory circuits), the optical method can directly inhibit neurons for more nuanced control of brain function.
Boyden and others are testing optical technology to treat such conditions as stroke, epilepsy and schizophrenia using animals specially bred to carry the light-sensitive proteins in certain classes of neurons.
People can’t be genetically modified before birth, however, so human studies will require gene therapy, Boyden says, which means the optical method might be a long time coming. A harmless virus, carrying genes that code for the manufacture of the light-sensitive proteins, would “infect” targeted neurons. Boyden expects the light source to come from light-emitting diodes under the scalp and, as with deep brain stimulation, to be driven by an implanted pacemaker-type device.