ABSTRACT
At least two sets of mechanisms lead to and maintain addiction. First, there are the direct effects of the drug itself. When the brain is repeatedly exposed to drugs, tolerance and physical dependence inevitably develop. Based on the difference between heroin addicts and pain patients who are taking strong opiates as medication, it is now reasonable to believe
that additional changes take place in the brain that account for the psychological dependence that distinguishes addiction from tolerance and physical dependence. Neuroscientists have now described alterations in receptors, second messengers, enzymes that synthesize neurotransmitters, and other components of the brain reward system in the brains of human addicts and animals that have been chronically exposed to drugs. There is even evidence suggesting that chronic drug exposure might reduce the number of proteins that help support the structure of the dopamine-containing neurons in the ventral tegmental area, shrinking them and reducing the number of dendrites they contain.
