Free The Addicted Brain by Michael Kuhar Page B

http://faculty.washington.edu/chudler/slice.html .)

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    PET Scanning
    Positron Emission Tomography (PET) is an imaging technique that produces a three-dimensional picture of the distribution of a radioactive substance in the body. If the substance is preferentially bound to some receptor, for example, then the distributionof radioactivity shows the distribution and quantity of the receptors. If the radioactivity reflects metabolism, then the distribution of radioactivity shows areas that are highly metabolic or functional. PET is one of the most important research tools available today. It allows us to look inside the body for important molecules and processes without invading the tissues of the body. PET can also be combined with other powerful imaging techniques such as CT and MRI to provide even more information.

    This schematic shows how PET scanning works. If a radioactive substance that emits positrons and binds preferentially to D2 dopamine receptors, for example, is injected into a subject, then the substance will settle onto D2 receptors in the brain. As the positrons are emitted during radioactive decay, they encounter electrons, and, being antiparticles, they annihilate each other and produce gamma radiation (see lower left) that is detected by a ring of detectors arranged around the head. The information about the annihilations is then processed and sent to a computer where the spatial distribution of the radioactivity (and the receptors) is reconstructed.
    Image adapted from “Positron Emission Tomography,” in http://en.wikipedia.org/wiki/Positron_emission_tomography , accessed November 18, 2010.)
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    Studies using brain-imaging techniques have shown that continued use of drugs causes long-lasting changes in brain chemistry and function. For example, dopamine receptors, specifically the D2 type of receptor, are decreased in the brains of drug abusers who take cocaine, methamphetamine, alcohol, or heroin. When an established addict stops taking cocaine or some other drugs, the D2 dopamine receptor levels do not immediately increase to normal (see Figure 7-2 ). In fact, they remain suppressed for months and months, and this has proven to be the case in several studies. The low levels of the receptors have suggested that the dopamine system is dysfunctional or under-functioning in these people. In other studies, low D2 levels were also found in obese subjects, echoing the importance of dopamine in “natural” rewards, and that drugs insert themselves in circuits for natural rewards such as feeding. Thus, low levels of D2 dopamine receptors are a suggestive marker for increased vulnerability to drug use, and perhaps other addictive behaviors as well.
    Figure 7-2. Levels of D2 dopamine receptors in a normal brain (top), a brain from a cocaine user after one month of withdrawal (middle), and a brain from a cocaine user after four months of withdrawal (bottom). Each row shows two different slices of brain from the same subject and comparisons are made by examining the images in each column. The bright areas in the image show the places where D2 dopamine receptors are the highest—the larger the brighter area, the greater the number of receptors. For example, consider the left column that shows the same brain levels from three individuals, one with no drug history and two users. The top image from an individual with no drug history has the most receptors, the middle image from a user abstinent for one month has many fewer receptors, and the third or lowest level shows perhaps a slightly higher level compared to the middle image. But it is clear that even after four months of abstinence, D2 dopamine receptor levels are not back to normal. The images are from PET scans of D2 dopamine receptors, which were first carried out by a team of which the author was a member. (Adapted from “Figure 2” from Volkow et al. “Decreased Dopamine D2 Receptor Availability Is Associated with Reduced Frontal Metabolism in