A paper that’s about to be published in the journal Science has found at least part of the difference between the brains of addicted individuals and those that use recreationally.
The question as to why only some people get addicted to drugs has been a difficult one to answer. Still, there’s no doubt that only a relatively small fraction of those exposed to drugs develop the compulsive, often destructive pattern of use we associate with addiction. The pattern holds in animal research too – even though all the animals in an experiment get the same amount of drugs, delivered in the same way, only some of them develop addictive drug taking. It seems there’s something different about addicts’ brains, but what is it?
What’s different about addicts’ brains?
We’ve found quite a few things that differentiate addicts’ brains from those of normal research participants. These include lower density of a certain type of dopamine receptor (D2), reduced activity in specific brain parts like the OFC (orbitofrontal cortex) that are important in decision making and behavioral control. Still, if we start with what is supposed to be a pretty similar group of rats and give them all the same drug, for the same time, in the same amounts, why do only some get addicted?
This recent study found that a specific neuronal process called LTD (Long Term Depression), that is important in learning (or what we call plasticity) is suppressed in addicted animals for far longer than in animals that end up not not displaying addictive behavior. Even though all animals displayed this sort of deficiency in LTD right after learning to take drugs, only the addicted animals showed it when tested two months later.
Since the difference was seen in an area of addicts’ brains called the Nucleus Accumbens, a very important area for learning about rewards, it seems likely that it plays an important role in addicts’ inability to change their behavior after they’ve started using drugs. Past research has already identified this as a problem with something we call “reversal learning” but it seems we may have just found at least part of the mechanism.
Now we have to figure out why some animals show this sort of pattern and others don’t. Genetic variability seems like a good place to start here.
F. Kasanetz, V. Deroche-Gamonet, N. Berson, E. Balado, M. Lafourcade, O. Manzoni, P. V. Piazza, Transition to addiction is associated with a persistent impairment in synaptic plasticity. Science 328, 1709–1712 (2010).