Using Light instead of Drugs for Pain Relief

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Researchers used optogenetics to manipulate the reward response and pain response in brains of mice

Opioids, a type of pain-killing drug, interact with opiod receptors on brain cells to reduce the body’s pain response. Neuroscientists at the Washington University School of Medicine in St Louis recently discovered a way to activate opioid receptors with light, and their research was published in the journal, Neuron.

The scientists merged the light-sensing protein, rhodopsin, to key parts of opioid receptors. Then they injected this new compound into the brains of mice. The researchers used light instead of drugs, to activate opioid receptors and successfully stimulated a reward response in brains of mice.

This discovery of using light instead of pain-killing drugs eliminates the side effects of pain-killing drugs, such as addiction. Edward R.Siuda, graduate student and first author, shared that, “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 pain-killing drug with 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 reduce pain sensations. However, patients may develop tolerance and addiction. Opioids could also reduce breathing rate and cause constipation. In theory, receptors activated by light may not present the same danger.

Furthermore, using light instead of drugs allows researchers to understand how receptors work in cells and circuits of the brain and spinal cord. The principal investigator, Michael R. Bruchas, PhD, stated that, “It’s been difficult to determine exactly how opioid receptors work because they have multiple functions in the body. These receptors interact with pain-killing drugs called opiates, but they also are involved in breathing, are found in the gastrointestinal tract and play a role in the reward response.” Hence Bruchas and his team of researchers sought a method to limit opioid receptors to performing a single task at a time, and it turned out to be almost as simple as flipping on a light switch.

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From left, co-first author Bryan A. Copits, PhD, Michael R. Bruchas, PhD, and co-first author Edward R. Siuda used light to activate opiod receptors in the brains of mice

The researchers combined the rhodopsin protein, which senses light in the eye’s retina, with a specific opioid receptor, the Mu opioid receptor, to build a receptor that responds to light in the same way opioid receptors respond to pain-killing drugs. Pain-killing drugs act on opiod receptors to initiate activity in specific chemical pathways in the brain and spinal cord. When the researchers used light to activate receptors that contained rhodopsin, the same cellular pathways were activated.

Bruchas and his team plan to continue using these receptors to investigate ways to control brain cells and circuits that mediate pain and reward with light.

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