Opioid receptors on the surfaces of brain cells are involved in pleasure, pain, addiction, depression, psychosis, and other conditions. The receptors bind to both innate opioids and drugs ranging from hospital anesthetics to opium. Researchers at The Scripps Research Institute, supported by the NIGMS Protein Structure Initiative, determined the first three-dimensional structure of a human opioid receptor, a kappa-opioid receptor. In this illustration, the submicroscopic receptor structure is shown while bound to an agonist (or activator). The structure is superimposed on a poppy flower, the source of opium.