Repeated drug exposure also changes brain function.
Positron emission tomography (PET) images show similar changes in brain dopamine receptors resulting from addiction to different substances. Dopamine D2 receptors are one of five types of receptors that bind dopamine in the brain. The brain images on the left are those of controls, while those 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 (in the left column), 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 repeated over-stimulation of the dopamine receptors. Brain adaptations such as this contribute to the compulsion to abuse drugs.
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National Institute on Drug Abuse (July 1 2008). Repeated drug exposure also changes brain function.. In Addiction Science: From Molecules to Managed Care. Retrieved from http://www.drugabuse.gov/publications/addiction-science/drug-use-changes-brain/repeated-drug-exposure-also-changes-brain-function