Opioids, a type of pain-killing drug, interact with opioid 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.