One downside of cocaine and amphetamines: Doing stimulants can cut your motivation to do anything else. Researchers have found that stifled ambition may be an enduring side effect of a stimulant addiction, persisting long after the habit is gone. In addition, an area of the brain involved in curiosity seems to be permanently damaged by the drugs.

The findings, published in the Proceedings of the National Academy of Sciences, explain why stimulant abusers have a hard time kicking the addiction and often relapse. In the brain, stimulants stunt the growth of spiny neurons in the nucleus accumbens and pyramidal cells in the parietal cortex. The former is involved in motivation and reward and the latter helps with sensory and motor function.

In the experiment, a team led by Bryan Kolb, professor of psychology and neuroscience at the University of Lethbridge in Alberta, Canada, exposed one group of laboratory rats to amphetamine and cocaine for twenty days. A similar group was treated with a sodium control solution. After this time, both groups of rats were transferred to new accommodations. Some were housed in normal laboratory cages, while others stayed in more interesting pens, with multiple levels, ramps, bridges, tunnels and even a climbing chain. These rats also got toys, which were rearranged each week to encourage exploration.

After three months, the researchers inspected the condition of the spiny neurons in each rat's brain. Non-drugged rats that were placed in the complex cages had spiny neurons with increased dendritic branching, reflecting new neuronal development. For the stimulant-treated rats, the elaborate settings did nothing to improve or develop these neurons. Their minds were the same as the rats left in the simple cages.

Kolb says these findings help explain the repetitive and compulsive behavior of people addicted to stimulants. Further research may find experiences that can counteract the long-term effects of stimulants on the brain.

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