Retrospectacle: A Neuroscience Blog

Cocaine abuse is a major public health issue, with estimations of as high as 1% of the US population over 12 as abusers. Addiction to cocaine, and most substances for that matter, results in physical modifications in your brain which are persistant, and detrimental to many mental fuctions like learning. During and following cocaine addiction, changes occur in a brain region called the nucleus accumbens (NAc), which is important in natural reward pathways. It is a key player in the reinforcement of drug-taking.

A recent brief communication in Nature Neuroscience (Martin et al. 2006) describes a facinating study which provides a neural explantion as to why learning and memory is abnormally disrupted in drug abusers, due to modifications in activity in the NAc region. (More below the fold...)

What this group found that was so different was this: there exists a difference in NAc activity between rats that can control their drug intake vs. those who receive it passively. The group compared four animal groups: rats which controlled their drug intake by pressing a lever, rats which received drug (via a catheter) passively, rats which received nothing (sham controls), and rats which could control their food intake. Following a 19-day training period where each group received either drug, food, or nothing, synaptic plasticity in each group was recorded in an attempt to measure LTD. (LTD refers to "long term depression" which is a form of neural activity which, like "long term potentiation," is necessary for types of learning and memory. More on that here.

The results were surprising. The group which self-administered cocaine had a significant decrease in LTD in the NAc following a one-day abstinence. The groups which received no drug and the ones that received self-administered food had no change in NAc activity, which could be expected, but the group which received passive cocaine also had no significant change in LTD.

What this translates to is this: it was not the pharmacological properties of cocaine which triggered the change in LTD (or else it would have occured in passive cocaine animals too) but rather the control over cocaine administration made all the difference. This suggests that an important component of addition is the drug-seeking behavior, as well as making a "choice" at some level to administer. The authors speculated that it is this mechanism which contributes to the difficulty of avoiding relapse following drug abstinence, insofar that the drug-seeking behaviors are biologically reinforced at least as much as the physical addition itselt.