Using pulses of light to control the activity of specific types of neurons, researchers can now show precisely how these neurons function in networks in the brain. These “optogenetics” methods, which use light-sensitive proteins to select and influence a neuron, are driving a wave of discoveries about long-misunderstood neural phenomena.
“With optogenetics we can achieve high-temporal-resolution control over activity patterns in targeted cell types [even] in freely moving mammals,” says Karl Deisseroth, a psychiatrist and bioengineer whose lab at Stanford University’s School of Medicine has pioneered the use of the technology.
“Just about everybody I know is going to start using these methods,” says Gary Aston-Jones, a neuroscientist at the Medical University of South Carolina.
Optogenetics techniques, which combine optics and genetics, are based on a class of light-sensitive electrochemical switching proteins called opsins. These are found in the retinal cells of animals but also in a variety of other cells, and in plants and bacteria. Four years ago, Deisseroth and his students reported using a viral vector to deliver an opsin derived from algae, Channelrhodopsin-2 (ChR2), into specific neurons in mice.
Embedded in the membrane of each neuron, the ChR2 protein opens an ion channel when exposed to blue light, exciting the neuron and causing it to fire. With fast pulses of this light, Deisseroth’s team caused ChR2-containing neurons to spike with precise timing. In 2007, Deisseroth’s team reported using a new, microbe-derived opsin, NpHR, to precisely inhibit the activity of neurons with light pulses, allowing them to turn the neurons on or off.
Since then, the lab has published a flurry of optogenetics-based papers in high-profile journals, demonstrating how to use the technique to answer important questions of neuroscience or neurology.
In the April 23 issue of Nature, for example, Deisseroth’s team reported combining opsins with proteins found in a broad class of neuronal receptors—called G-protein-coupled receptors—so that the receptor activity itself can be switched on and off with light pulses. The team implanted a gene sequence into the nucleus accumbens, a key reward-circuit region, in the brains of live mice. They then used light pulses via an optical fiber to create the receptor switching that leads to a sense of reward in the mice in a standard place-preference test. This technique, says Deisseroth, takes optogenetics “beyond control of electricity to control of biochemistry in neurons and other cell types.”
Related reports from the Deisseroth lab have addressed classic questions in neuroscience. A paper in 2007, for example, described the use of opsins to manipulate the activity of hypocretin neurons in the hypothalamus in live mice, increasing their probability of waking and showing how these neurons play a role in pushing the brain from sleep to a waking state.
A study that appeared online April 26 in Nature describes the use of opsin proteins ChR2 and NpHR to investigate a population of neurons in the cortex known as fast-spiking parvalbumin interneurons. This class of brain cells had long been suspected as the source of “gamma wave” oscillations in the cortex. These coherent neuronal spikings with frequencies of 30 to 80 cycles per second have been associated with attention and improved cognitive functioning and are notably impaired in conditions such as schizophrenia and autism.
Neuroscientists didn’t have the tools to demonstrate the interneurons’ role in gamma oscillations—a “fundamental question of relevance to both basic neuroscience and understanding of human disease,” says Deisseroth. “The question of the relevant cell type, and important millisecond-scale activity patterns, had been fairly tantalizing and mysterious for quite some time.”
Deisseroth’s team showed that by inhibiting or exciting these cells with light pulses, they could turn gamma oscillations off or on repeatedly. Moreover, they demonstrated that by turning these gamma-generating neurons on, they effectively improved the signal-to-noise properties of the cortical circuits into which they were wired, indicating how well the signal was transmitted.
The gamma-oscillation question drew the interest of another research team, based in the laboratory of Christopher Moore at the Massachusetts Institute of Technology. Also reporting April 26 in Nature, Moore and colleagues, with assistance from Feng Zhang and Deisseroth from Stanford, showed similar results in the brains of live mice.
To highlight the potential medical applications of the technique, Deisseroth’s team performed a study that clarifies the effect of deep brain stimulation (DBS), an increasingly common treatment for Parkinson’s disease and in experimental use for other disorders. DBS involves electrode stimulation of brain tissue in and around the subthalamic nucleus, but how that alleviates Parkinson’s symptoms hasn’t been clear. By systematically exciting or inhibiting individual circuits in this region using optogenetics techniques in live mice, Deisseroth’s group identified incoming sensory-cortex nerves as the ones whose stimulation led to the kind of improved hypothalamic activity seen in human patients. They reported the results in Science on April 17.
Aston-Jones notes that optogenetics techniques have limitations, especially that they are invasive, which means research in humans is far less likely. “There also are limitations for the penetration of light in tissue, so if you’re trying to reach for example a large subcortical area, it might be difficult,” he says.
Even so, his lab is preparing use the techniques for new research. Aston-Jones also praises the Deisseroth lab for having made information about the techniques, including the necessary equipment, freely available on its lab Web site. “That Web site and their collaborative nature will continue, I’m sure, to facilitate the use of this method by lots of labs,” Aston-Jones says.
The number of studies using optogenetics techniques may rise in coming years. “There are more tools on the way with new properties, and we are beginning to target this arsenal of optical tools to other basic-neuroscience and disease-model questions,” says Deisseroth. In particular, he adds, “in my clinical work, I see patients with depression and autism-spectrum disease, and we are beginning to explore questions relating to these devastating diseases as well.”