A certain group of brain structures, often referred to as the “reward system”, has long been recognized by scientists as having an important role in addiction. The structures of the reward system are related in that they are all part of a major dopamine network, the mesotelencephalic dopamine system. Dopamine neurons that arise from the ventral tegmental area (VTA), a nucleus in the midbrain, and project to the nucleus accumbens (NAc) are thought to play the largest part in addiction. The NAc is part of the striatum, a subcortical region that gets its name from its combination of white and gray matter that give it a striped, or striated, appearance.
David Belin and Barry Everitt of the University of Cambridge have been studying this system in rats. Their goal was to elucidate the mechanism that causes early experiences with drugs to turn into the compulsive drug-seeking behavior characteristic of addiction. Previous studies have suggested that addiction occurs when behavioral control over drug-seeking is transferred from the ventral to the dorsal striatum.
Belin and Everitt taught a group of rats to press a lever to receive cocaine. When their behavior became compulsive, the researchers severed the connection between the two striatal areas. This caused the compulsive behavior to diminish. In a second experiment, the researchers showed rats with the severed connection could still be trained to push a lever for a reward. It seems the disconnection affects only the compulsivity associated with addiction. The severed striatum disrupts a major line of dopamine transmission that Belin and Everitt feel must be essential for addiction to occur.