Opioid receptors and their endogenous peptide ligands play important roles in the reward and reinforcement of drugs such as heroin, cocaine, and alcohol. The ?-opioid receptor is a type of opioid receptor which binds the peptide opioid dynorphin as the primary endogenous ligand. ?-opioid receptors are widely distributed in the brain, spinal cord, and in pain neurons. They are associated with the risk for alcohol dependence.