Paul Kenny of the Scripps Center in Jupiter, Florida first turned rats fat using the same treats we bulk up on, and found that the numbers of a dopamine receptor called D2 fell, just as they did in cocaine addicts. Knocking down those receptors with a virus also resulted in addictive behavior.
Biochemically the addiction process is the same in both cases. Overuse reduces the number of D2 receptors, so more overuse is needed to deliver the same result. The cycle feeds on itself.
The money quote from Kenny is "drug addiction and obesity are based on the same underlying neurobiological mechanisms."
The link between dopamine receptors and addiction is not new. PET images of addicts were showing the effect early in the last decade. Studies of food addiction have also looked at dopamine receptors for years. Kenny has just proven the link, and that the link is the same one found in stimulant abuse.
One obvious result, proven in 2005, was that blocking the D2 receptor could eliminate a craving for cocaine. The antipsychotics used for this, risperidone and olanzapine, have nasty side effects, but the Australian study proving their effectiveness against cocaine showed they only had to be given for a short time in order to work.
Not all addictions work in this way. Studies of opioid addiction have not found a link between dopamine and addiction. The link, British scientists wrote in 2008, appears limited to stimulants.
Still, the idea of using brain phamacology against morbid obesity is likely to get a test soon.