WINSTON-SALEM, N.C. - Social rank, whether an individual is dominant or subordinate, has a significant influence on susceptibility to cocaine abuse in monkeys, according to research conducted at Wake Forest University School of Medicine and published by the journal Nature Neuroscience.
After being socially housed for three months, monkeys were introduced to cocaine and allowed to self-administer the drug in controlled sessions. "During these sessions, subordinate monkeys had significantly higher intakes than dominant monkeys," says an article by Michael A. Nader, Ph.D., and nine colleagues.
"The present study demonstrates that social context can have profound effects on brain dopaminergic function in adult, non-human primates," says the article, prepared for the journal''s February print editions and online publication Jan. 22.
Earlier research by scientists at Wake Forest and elsewhere has revealed that cocaine increases levels of the neurotransmitter dopamine in the synapses, or spaces between nerve cells. The increased dopamine correlates to the "high" that cocaine users feel. Dopamine is released during normal nerve impulse transmission in the brain, but is then reabsorbed into dopamine "transporters." Cocaine blocks the reabsorption of the dopamine, creating higher levels. In the Wake Forest study, a group of 20 male cynomolgus macaque monkeys were first housed individually, and extensive measurements were taken of their hormonal and behavioral activity. They also underwent positron emission tomography (PET) scans to measure chemical activity in the brain. The monkeys were then housed in social groups of four, and the groups stratified according to levels of dominance or submission. Dominant monkeys were far more aggressive, and as a result, had access to more resources compared to the subordinate monkeys. After the monkeys were socially housed for three months, PET imaging revealed a significant change in dopamine function in the brains of the dominant monkeys, causing them to be much less likely to abuse cocaine. The findings indicate that this change in brain activity was a result of becoming a dominant monkey, rather than a condition that existed before they were socially housed. The findings suggest that environment can increase or decrease the likelihood of drug abuse. Nader, associate professor of physiology and pharmacology and of radiology at Wake Forest and the principal investigator on the study, said the application of brain-imaging techniques in this research holds much promise for investigation of drug abuse and other diseases. "The findings show that environment can produce changes in the brain and that those changes have consequences, and one of those is in the susceptibility to abuse drugs," Nader said. He said there is a strong possibility for applying the findings of the animal research to human populations. The article notes that between 1994 and 1998, the annual number of new cocaine users rose from 500,000 to more than 900,000.
The new research may provide another piece in the puzzle of why some of those new users go on to become abusers and addicts and some do not. The research was supported by a grant from the National Institute on Drug Abuse.
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