Professor Trevor Robbins explains that drug addiction involves chemical and neural processes with dopamine and the nucleus accumbens particularly important.

Well the whole process of addiction is a very complicated one. Addiction to drugs probably occurs at several levels - the cellular/biochemical level, and the neural or brain systems level. We know now how most of the drugs of abuse or addiction actually impact on their initial molecular targets in the brain. We know what the targets are and we know where they are. The important thing, however, to understand is what happens as the drug is given chronically, that is to say repeatedly, and how the brain adapts molecularly to this chronic process. Some people think that as a consequence of chronic drug-taking, certain genes are expressed which changes the functions of the nerve cells in that domain, in that area, perhaps permanently and leads to behavioral patterns that are very rigid hard to change. At another level, the brain systems level, we know that many of these drugs of abuse work ultimately, probably through the dopamine system, as it projects to a structure in the base of the forebrain called the nucleus accumbens. However, although the nucleus accumbens may be the initial site of which drugs of abuse exert their effects, perhaps producing highs and euphoria, as the drug is given chronically, over a long period, the effects of the drug spread to other areas of the brain which we think enhance automatic behaviors including drug addiction, drug-seeking behaviors.

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