Speed May Slow Learning

Nothing to sniff at. Cocaine and amphetamine prevent neurons from growing more dendrite branches and spines (inset) when an animal explores a new environment.

Despite what the TV told you, a frying egg is at best a loose metaphor for your brain on drugs. However, new research suggests that some recreational stimulants can cause a kind of congealing. Even modest exposure to amphetamine and cocaine prevents the sprouting of new branches on rat neurons that normally occurs when the animals explore an exciting new environment.

Researchers have known for decades that moving rodents from boring cages into an enclosure rich with tunnels, swings, and other playthings changes the animals' brains. In particular, dendrites--the treelike extensions on neurons that receive inputs from other neurons--grow additional branches and build more spines, the tiny protrusions where synapses are found. These changes are thought to be a sign of learning.

To see if drugs interfere with this brainy buildup, behavioral neuroscientist Bryan Kolb of Lethbridge University in Alberta, Canada, and colleagues gave young adult rats a moderate dose of either cocaine or amphetamine once a day for 20 days. They then transferred half the caged animals into a rodent playground. After 3.5 months, the researchers killed the rats and examined their brains, counting dendrite branches and spines. By nearly all measures, both drugs greatly inhibited dendritic growth caused by the enriched environment, the team reports online this week in the Proceedings of the National Academy of Sciences. "It may be that the drug exposure has altered [the rats'] ability to learn from their experience," says Kolb. And it's easy to see a parallel in humans: "The thing that's striking about addicts is they seem not to learn from their experiences," he says.

"It's a very important finding," says Paul Vezina, a neuroscientist at the University of Chicago. Most researchers had suspected that the behavioral and cognitive deficits seen in addicts came about because the drugs created brain lesions, he says. "What they've shown here is that things could be more nefarious." However, he and others say more work is needed to determine whether stopping dendritic remodeling does indeed have an impact on behavior.

Related sites
Vezina's site
Basic information about cocaine, amphetamine, and how they work

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