There's another reason to say no to drugs. The results of a rat study published online this week by the Proceedings of the National Academy of Sciences indicate that past use of amphetamines and cocaine can impair the brains ability to learn from new experiences.

Our brains typically respond to novel situations by forming additional connections between neurons. This rewiring is thought to underlie learning and memory, as well as other cognitive and behavioral functions. Because drug use has also been shown to cause changes in the brain, Bryan Kolb of the University of Lethbridge in Alberta and his colleagues devised a study to investigate the interactive effect of fresh experiences and pyschostimulant drugs (namely amphetamine and cocaine) on the brain. The researchers first gave rats one of the drugs or a saline solution for 20 days. At the end of this period, half of the animals were moved to complex new cages containing bridges, ramps, tunnels and other toys, while the rest remained in ordinary laboratory enclosures. After three and a half months, the scientists examined the rats' brains and counted the number and density of branches of neurons known as dendrites. The team found that rats that had received saline solutions and had lived in a challenging environment had a greater number of neuronal connections than control animals did. Rats given either amphetamines or cocaine, in contrast, did not experience a similar increase in brain connections after living in the stimulating surroundings.

"The findings from this study indicate that at least some of the cognitive and behavioral advantages that accrue with experience may be diminished by prior exposure to psychostimulant drugs," Kolb says. "This impairment of the ability of specific brain circuits to change in response to experiences may help explain some of the behavioral and cognitive deficits seen in people who are addicted to drugs." The researchers also note, however , that the relationship might work both ways. That is, certain experiences might be able to influence later effects of drugs.