Neuroscientists have figured out a way to use light to activate opioid receptors—which interact with painkillers such as OxyContin and Vicodin.
“It’s conceivable that with much more research we could develop ways to use light to relieve pain without a patient needing to take a painkilling drug with side effects,” says Edward R. Siuda, a graduate student at Washington University School of Medicine in St. Louis.
But before that’s possible, the researchers are attempting to learn the most effective ways to activate and deactivate the opioid receptor’s pathways in brain cells.
Michael R. Bruchas, an assistant professor of anesthesiology and of neurobiology and the study’s principal investigator, says that working with light rather than painkilling drugs makes it much easier to understand how the receptors function within the complex array of cells and circuits in the brain and spinal cord.
“It’s been difficult to determine exactly how opioid receptors work because they have multiple functions in the body,” Bruchas explains. “These receptors interact with painkilling drugs called opiates, but they also are involved in breathing, are found in the gastrointestinal tract, and play a role in the reward response.”
So the researchers sought a way to limit opioid receptors to performing a single task at a time, and it turned out to be almost as easy as flipping on a light switch, according to Bruchas and colleagues.
By combining the rhodopsin protein, which senses light in the eye’s retina, with a specific type of opioid receptor called a Mu opioid receptor, the researchers were able to build a receptor that responds to light in exactly the same way that standard opioid receptors respond to painkilling drugs.
How painkillers work
When an opioid receptor is exposed to a painkilling drug, it initiates activity in specific chemical pathways in the brain and spinal cord. Shining light on the receptors that contained rhodopsin had the same effect.
In a test tube and in cells, Siuda exposed the receptors to light and then watched as they released the same chemicals that standard opioid receptors release.
Then, in mice, the researchers implanted a light-emitting diode (LED) device the size of a human hair into a brain region linked to the reward response. They injected the light-sensing receptors they had genetically manufactured into the same brain region. Neurons in that part of the brain release chemicals such as dopamine that create feelings of euphoria.
In decades of past opioid studies, researchers have observed mice and rats press a lever to receive a dose of morphine, for example. The morphine would activate opioid receptors and the release of dopamine, and the animals would enjoy the response and press the lever again to continue feeling that reward sensation.
This is one of the reasons opiates are so often abused in patients being treated for pain—people like the way the drugs make them feel as much as the pain relief they provide—and rates of abuse have skyrocketed over the past 10 years.
Working to deliver a similar reward sensation using light, the researchers put the mice into an enclosed chamber. In one part of the chamber, the lighted laser fiber-optic device stimulated the release of dopamine in the brain.
When the animals left that part of the chamber, the light in the brain turned off. Soon after, the mice returned to the part of the chamber that activated the fiber-optic device so that the brain could receive more light stimulation.
“By activating the receptors with light, we are presumably causing the brain to release more dopamine,” Bruchas explains. “Rather than a drug such as morphine activating an opioid receptor, the light provides the reward.”
The researchers were able to vary the animals’ response depending on the amount and type of light emitted by the LED. Different colors of light, longer and shorter exposure to light, and whether the light pulsed or was constant all produced slightly different effects.
Fewer side effects?
When a person takes an opioid drug such as Vicodin or OxyContin to relieve pain, such drugs interact with receptors in the brain to blunt pain sensations. But over time, patients develop tolerance and sometimes addiction. Opioids also can dramatically slow a person’s breathing, too, and typically cause constipation.
In theory, receptors tuned to light may not present the same danger. Siuda says it someday may be possible to activate, or deactivate, nerve cells without affecting any of the other receptors that painkilling drugs trigger, although achieving that goal will be difficult.
Bruchas’ team is planning future studies that will use these receptors to test ways to control the brain cells that mediate pain and reward behavior with light rather than drugs.
The National Institutes of Health, the Howard Hughes Medical Institute, and a W.M. Keck Fellowship in Molecular Medicine funded the research, which was published in the journal Neuron.