Miércoles 10 de Agosto de 2005, Ip nº 121

Cocaine use prevents adaptive behaviour
Por Gaia Vince

Cocaine may keep users from adapting to new situations by disrupting connections between key brain regions, suggests a new study in rats. The finding may shed light on the impulsive behaviour seen in cocaine addicts, researchers say.

A team looked at the connections between two regions of the brain: one involved with learning, memory and processing information - the prefrontal cortex and hippocampus - and one involved with pleasure seeking, emotion and reward behaviour - the nucleus accumbens in the limbic system.

Normally, these two regions are held in balance with connections between the two sending information in both directions and allowing “plasticity” of thought, so that behaviour can be modified and adapted to different situations.

Neuroscientists Yuriori Goto and Anthony Grace at the University of Pittsburgh, US, believe that cocaine disrupts these connections and causes the limbic system to become over stimulated.

“It may explain why cocaine addicts are oriented towards pleasure rather than other goals, and have an impaired ability to make decisions. It could be why addicts go back to taking more of the drug and ex-addicts often become addicted again faster than those who have never taken it,” says Grace.

Changing the rules
The team electrically stimulated the two different regions of the brain in a group of rats to examine the flow of information between the areas. They then injected them with cocaine and measured this “plasticity”. They found that, after taking the drug, the normal connections between the two regions were interrupted and that neurons in the limbic area were over-firing. They then carried out a behavioural analysis.

Half of the rats were given cocaine regularly for several days, while the rest were drug free. The rodents were placed in a T-shaped maze where, based on a coloured signal, they had to make a choice over which arm of the maze to run down in order to receive a food-based reward.

At first, the drugged rats learned the rules fastest and so got their rewards quicker. But when the rules were changed, it was the drug-free rats that were able to learn the new rules most quickly and get their treats – the cocaine appeared to dull the drugged rats’ adaptation abilities. “The cocaine was producing a deficit in their learning response strategy,” Grace suggests.

The researchers hope that understanding how connections operate between these brain regions may lead to new ways of treating drug addiction and a greater understanding of disorders such as schizophrenia, which is linked to the brain regions studied.

  20/07/2005. New Scientist Magazine.