research highlights

neuroscience Dawn on the mice

By directly delivering light deep into the activated cation channel that can excite a neu- signaling pathways can be controlled with fine brain, scientists can now study the basis of ron. Halorhodopsin (NpHR), in contrast, is a temporal precision. neurological therapy and animal behavior. chloride pump that can inhibit a neuron. As To test the optoXRs in vivo, Deisseroth The mammalian brain is a secluded and with electrical stimulation, photostimulation and colleagues expressed them in brain neu- enigmatic organ. Despite the progress in offers precise temporal control. rons and used the optrode to stimulate and molecular and cellular biology, we still know Deisseroth is interested in using optogenet- record activity. They found that spike fre- very little about the brain circuitry and how ics to help understand how deep brain stimu- quency in the nucleus accumbens, the reward it controls behavior. One of the challenges of lation works and to improve the procedure. center in the brain, decreases and increases brain studies is accessibility. The brain is pro- Viviana Gradinaru, a graduate student in his upon photoactivation of opto-β2AR and tected by the opaque skull, which makes bio- laboratory, has designed a hybrid instrument opto-α1AR, respectively. They thus set up a chemical and genetic manipulations difficult. called an ‘optrode’, which consists of fiber behavioral paradigm to test whether high One way to access deep inside the brain is optics for photostimulation and an electrode nucleus accumbens activity encodes reward to insert thin electrodes through the skull. for activity recording. By inserting an optrode memory. They allowed a mouse with opto- Deep brain stimulation is one of these mini- precisely into a specific brain region, she can α1AR expression in the nucleus accumbens to mally invasive methods and is used to treat simultaneously photostimulate the area and freely roam for one day. On the second day, Parkinson’s disease and depression. Deep measure electrical activity there. whenever the mouse entered the right side of brain stimulation electrically stimulates the To understand how deep brain stimulation the cage, they sent light via an optrode to the subthalamic nucleus, a complex structure with works, Deisseroth and colleagues expressed nucleus accumbens to induce reward mem- neurons, glia, and efferent and afferent fibers. ChR2 or NpHR in different subthalamic ory. On the third day, they sent no light, but “[Deep brain stimulation] is a fundamentally nucleus cell populations in a Parkinson’s the mouse now preferred to stay on the right mysterious intervention. There is not much disease model (Gradinaru et al., 2009). They side of the cage in expectation of receiving the knowledge, but plenty of hypotheses, in terms stimulated the subthalamic nucleus with an reward stimulation. Therefore, optoXRs can of the circuit element that’s been recruited,” optrode to see whether any cell type–specific control neuronal activity and mouse behavior explains to Karl Deisseroth, a psychiatrist and manipulation reverses the behavioral defect of through biochemical signals. neuroscientist at Stanford University. the model. After systematically testing excit- These reports demonstrate the versatility © All rights reserved. 2009 Inc. Nature America, Deisseroth’s group has pioneered a ver- atory neurons and astroglia without seeing a of optogenetics. A major consideration is the satile approach called optogenetics, which therapeutic effect, they finally found that the availability of the light-absorbing retinoids uses light-gated microbial ion channels afferent fibers originated from the cortex as an needed to form the rhodopsin chromophore. to control neuron membrane potential. important target of deep brain stimulation. “It is possible that retinoid may be used Channelrhodopsin-2 (ChR2) is a light- Meanwhile, another student in Deisseroth’s throughout mammalian tissue for trophic laboratory, Raag Airan, was developing a radi- and signaling roles; most tissues will likely cal approach to broaden the reach of opto- have sufficient retinoids,” says Deissseroth. genetics. In addition to electrical signals, In addition to the nucleus accumbens, biochemical signals are also important in the other brain areas, such as the amygdala, and brain. “This axis [of biochemical signals] is behaviors, such as anxiety, can be studied by not directly accessible to microbial opsins,” optogenetics. Finally, with additional genetic says Deisseroth. Airan engineered genetically engineering, optoXRs for other signaling encoded optical tools called ‘optoXRs’ con- pathways can be made. It has begun to dawn sisting of extracellular and transmembrane on the mice, and on us, too, that the grow- domains of bovine rhodopsin and intracel- ing optogenetics toolbox will be invaluable in lular loops of another G protein–coupled revealing the mysteries of the brain. receptor to control a distinct intracellular Wayne Peng signaling pathway (Airan et al., 2009). They RESEARCH PAPERS used β -adrenergic receptor loops (opto- Airan, R.D. et al. Temporally precise in vivo control Mouse with optrode inserted for targeted nucleus 2 β AR) to elevate cAMP and loops from the of intracellular signalling. Nature advance online accumbens stimulation. Blue light is delivered 2 publication (18 March 2009). α -adrenergic receptor (opto-α AR) to mobi- to the optrode via an optical fiber linking the 1 1 Gradinaru, V. et al. Optical deconstruction of laser diode and the freely moving mouse. Image lize calcium. With this repertoire of photoac- Parkinsonian neural circuitry. Science advance online courtesy of K. Deisseroth. tivated G protein–coupled receptor chimeras, publication (19 March 2009).

nature methods | VOL.6 NO.5 | MAY 2009 | 319