Neuroscientists have attached the light-sensing protein rhodopsin to opioid receptor parts to activate the receptor pathways using light from a laser fiber-optic device. They also influenced the behavior of mice using light, rather than drugs, to activate the reward response. When an opioid receptor is exposed to a pain-killing drug, it initiates activity in specific chemical pathways in the brain and spinal cord. And when the researchers shone light on the receptors that contained rhodopsin, the same cellular pathways were activated. Neurons in that part of the brain release chemicals such as dopamine that create feelings of euphoria.