The following is Part 3 of a three-part series of selected presentations from the 2015 annual meeting of the Society for Neuroscience, held October 17-21, 2015, in Chicago, Illinois, US. Also see Part 1 and Part 2.
Optogenetics is having a moment in neuroscience—except that moments pass, and optogenetics only seems to be gaining ground as a go-to experimental technique. As one attendee at the annual meeting of the Society for Neuroscience put it, if you ask what researchers are using optogenetics to study these days, you might as well ask what they’re studying with electrophysiology—just about everything. Pain is no exception, as evidenced by a multitude of posters and a mini-symposium featuring optogenetics research in pain.
When it comes to translating optogenetic technology from bench to bedside, therapeutic devices are still years away. However, intractable chronic pain might be among the first conditions to be treated with optogenetic technology, because the target neurons are found in the periphery and spinal cord, making them much more accessible than neurons deep in the brain that are implicated in other neurological disorders.
Two main hurdles—and they are large—stand between patients and optogenetic therapies. First, the technology requires that human neurons be genetically modified to express opsins, the light-sensitive ion channels that form the cornerstone of the technique. Second, those neurons must be reachable with light from a light-emitting diode (LED), requiring an implanted device and a power source. Research presented at the meeting has made strides toward developing wireless devices for pain research in rodents.
Robert Gereau and colleagues at Washington University, Saint Louis, US, in collaboration with John Rogers, a materials scientist at University of Illinois at Urbana-Champaign, US, presented a novel, stretchable, ultra-miniaturized implantable LED device for control of neurons in the sciatic nerve in the leg or in the spinal cord in mice. The devices are wirelessly powered by a radio frequency (RF) field. The data were presented in a poster by graduate student Melanie Pullen and published online November 9 in Nature Biotechnology (Park et al., 2015).
The chief limitation of LED devices for optogenetic research in behaving animals has been the requirement for a power source for the light, which has necessitated that animals with implanted LEDs either be tethered to a power source or have a battery mounted to the skull. The device developed by Rogers is made of soft, flexible loops of wire that function as an antenna to capture an RF signal emitted by a remote power source, which blankets the rodent’s entire cage with a field of energy. A major challenge was to develop a functional antenna from pliable material. “We want to put these devices in tissues where a rigid device could damage delicate nerve tissue,” Gereau said. The new implantable device’s tissue-like properties permit it to operate in areas of the body—such as the limbs and spine—that undergo a lot of motion, Gereau said. “That allows huge opportunities in basic research.”
The researchers tested whether the device could evoke pain behaviors in freely moving mice. They used a Cre recombinase-based transgenic approach to express channel rhodopsin 2 (ChR2), a light-sensitive opsin that activates neurons when exposed to blue light, either in all sensory neurons or a subset of nociceptive neurons. The researchers implanted the device in one of two locations in mice: atop the sciatic nerve or in the epidural space of the spinal cord. Following implantation, the mice showed no signs of either immune cell activation or motor impairment. When the blue light was switched on, mice expressing ChR2 in all sensory neurons displayed nocifensive behaviors. Next, the researchers used a Y-arm maze to test place aversion, a measure of ongoing pain. Whereas control mice spent equal time in the maze’s two arms, mice expressing ChR2 in all sensory neurons or nociceptors avoided the arm in which RF power—and therefore the LED—was turned on.
In a separate presentation, Gereau described ongoing experiments using archaerhodopsin, a light-activated proton pump that can effectively quiet neurons. Gereau’s team transfected mouse sensory neurons with archaerhodopsin and used yellow light, delivered to the lumen of the bladder, to inhibit the neurons and quell bladder pain in mice. “You can use light to inhibit bladder pain completely in animals,” Gereau said during his presentation.
Ada Poon, an electrical engineer at Stanford University working in collaboration with Scott Delp and Karl Deisseroth, gave a talk describing a different wireless, implantable, miniaturized LED, which also uses an RF energy field as a power source. Poon and colleagues successfully used their LED device to stimulate ChR2-expressing neurons in the premotor cortex, which elicited circling behaviors in mice. Following viral delivery of ChR2 to nociceptive neurons, LED stimulation of the spinal cord or peripheral nerve endings resulted in neuronal activation. Light activation of peripheral nerve endings also increased reflexive nocifensive behaviors in the animals. The work was published October 12 in Nature Methods (Montgomery et al., 2015).
In one sense, Gereau said, the bar has already been met for testing the interface between stimulating devices and the human spinal cord, because spinal stimulators are already in use for pain relief. But those devices indiscriminately stimulate large populations of neurons, which can include motor neurons, producing side effects. Because the stimulation (or silencing) of neurons can be precisely targeted to subsets of neurons—perhaps one day even subsets that transduce specific forms of chronic pain—Gereau is hopeful that optogenetic therapies can one day be far more selective, leaving normal sensation—and perhaps even acute pain transmission—intact. “The rate-limiting step will be the genetic strategy to get opsin proteins into neurons,” Gereau said.
In that arena, too, researchers are making strides. For example, a strategy to deliver opioid proteins to neurons using the herpes simplex viral (HSV) vector is moving through clinical trials (see Fink et al., 2011). And Gereau’s group has also used HSV to successfully transfect isolated dorsal root ganglion (DRG) neurons from human organ donors with ChR2, which made the cells responsive to blue light, he told PRF.
Stephani Sutherland, PhD, is a neuroscientist, yogi, and freelance writer in Southern California.
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