Repeated drug exposure also changes brain function. In this case, brain imaging technologies are used to demonstrate these changes in human drug abusers. Positron emission tomography (PET) images are illustrated showing similar brain changes in dopamine receptors resulting from addiction to different substances. Dopamine D2 receptors are one of five receptors that bind dopamine in the brain. In this slide, the brains on the left are those of controls, while the brains on the right are from individuals addicted to cocaine, methamphetamine, alcohol, or heroin. The striatum (which contains the reward and motor circuitry) shows up as bright red and yellow in the controls, indicating numerous D2 receptors. Conversely, the brains of addicted individuals (in the right column) show a less intense signal, indicating lower levels of D2 receptors. This reduction likely stems from a chronic over-stimulation of the second (post-synaptic) neuron (schematically illustrated in the far right hand column), a drug-induced alteration that contributes to the addict’s compulsion to abuse drugs.