G protein-coupled receptors (GPCRs) are the largest family of membrane receptors that activate intracellular signaling cascades and undergo endocytosis, recycling, or degradation upon stimulation. The (mu) m-, delta (d) and kappa (k) opioid receptors are GPCRs of the nervous system, which control pain, stress, and addictive behaviors. The mu opioid receptor (MOR-1) is a high-affinity receptor for enkephalins and beta-endorphins with low affinity for dynorphins. The opiate alkaloids morphine and codeine bind to MOR-1.