Low Rank Monkeys More Prone to Cocaine Addiction

Posted by Richard Linney on Jan 21, 2002 at 10:22
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Monkeys at the bottom of the social dominance hierarchy are more prone to cocaine addiction, say researchers. But they caution against making direct comparisons with humans.

"The positive spin on our findings is that enriching the environment can produce large and robust changes in the brain," says Michael Nader at Wake Forest University. "These lower the propensity for using drugs."

But he rejects the simplistic interpretation that the changes he has identified might underlie any link between cocaine use and social exclusion in humans.

The researchers set up a social group of 20 macaques and after three months the dominance hierarchy in the group was established. PET scans of the monkeys' brains revealed that dominant monkeys had a 20 per cent increase in D2 dopamine receptors compared with subordinates.

Dopamine is a chemical messenger that is released at junctions between nerves. The pathway transmits pleasure and pain sensations and is directly affected by drugs such as cocaine and ecstasy which boost dopamine levels.

The monkeys were trained to self-administer cocaine. While both dominants and subordinates used the drug, only the subordinate monkeys became addicted.

Widespread use

Nader thinks that cocaine may be more addictive for subordinates because the drug, via its effect on dopamine, is stimulating fewer receptors. As a result, the dopamine boost hits a greater proportion of those receptors.

But he warns that "there is a lot more going on than just dopamine and D2 receptors." Indeed, mice genetically engineered to lack D2 still become addicted to cocaine so the dopamine pathway cannot be all of the story.

Other experiments in humans have shown that people with low numbers of D2 receptors found ritalin more pleasant. This drug has a similar effect to cocaine on the dopamine pathway. But no-one has yet investigated a link between social dominance in humans and high D2 activity.

"Cocaine use is widely distributed amongst every strata of UK society," says John Marsden an expert on drug addiction at Kings College London. Its demographics do not reflect a picture of abuse by disenfranchised individuals on the margins of society, he says.

Figures from the 2000 British Crime Survey support his view. These point to higher drug use in affluent urban areas than poor areas. In addition, the richest and poorest people use drugs more heavily that those on a moderate income. Marsden agrees that the monkey experiments are intriguing, but says social interactions are much more complicated in humans.

Journal reference: Nature Neuroscience (DOI: 10.1038/nn798)

Article downloaded from NewScientist.com



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