Monday January 21 2:37 PM ET
Lower-Status Monkeys More Likely to Take Cocaine
By Faith Reidenbach
NEW YORK (Reuters Health) - The alpha male in a group of monkeys gets the best banana, doesn't have to fight--and is less likely than subordinate monkeys to use cocaine, scientists have observed.
Dr. Michael Nader of Wake Forest University in Winston-Salem, North Carolina and colleagues found that animals who became dominant after moving from solitary housing to social housing showed changes in brain chemistry that made them less likely to use drugs. But monkeys who were subordinate after the move showed no brain chemistry changes.
The findings suggest that environmental changes can affect the brain and, perhaps, the success or failure of treatment for drug addiction, the researchers conclude.
In the study, 20 monkeys that had been living alone in separate cages for 18 months were moved into social housing, with four monkeys to a cage.
One monkey in each group became dominant. In some groups, another monkey eventually claimed the No. 1 rank, but there was always a dominant monkey. During 3 months of living in small groups, the animals were occasionally given access to cocaine.
At the beginning of the study and again during social housing, Nader and his colleagues scanned the monkeys' brains and measured the volume of dopamine D2 receptors.
These receptors interact with the ``feel-good'' brain chemical dopamine, which is part of the brain's ``reward system'' and plays what is thought to be a key role in mood and motivation.
Previous research has suggested that people who are vulnerable to addiction may have fewer-than-normal brain receptors for dopamine. The theory is that this pushes them to make up the difference by using substances--including alcohol and other drugs--that elevate dopamine levels in the body.
In monkeys that were dominant in their groups at the time of the brain scan, the volume of dopamine D2 receptors increased 22% during the course of the study.
Conversely, in subordinate monkeys, the volume of receptors did not change appreciably, the research team reports in the January 22nd online edition of Nature Neuroscience.
Furthermore, subordinate monkeys were more likely to give themselves cocaine than to choose water. Dominant monkeys didn't avoid cocaine altogether, but their intake was significantly lower than that of their subordinates.
``So there was something about the social environment and being the dominant monkey that resulted in this large change in D2 receptors,'' Nader said. Previous studies have shown that ''environmental enrichment''--like moving from solitary to social housing--can increase dopamine receptor numbers, he added.
``When you enhance the environment of the individual, you decrease the propensity to abuse drugs,'' Nader explained.
``What we're thinking is that being the dominant male in that environment is very enriching,'' he added. ``They have access to all the food, they get groomed the most, no other monkey aggresses toward the most dominant monkey--so they live a pretty nice life.''
The study results also suggest that drug abuse ``is not a life sentence,'' Nader said. It may be that when an individual seeks treatment, ``changes in the environment can impact the brain and that can ultimately impact their success rate.''
SOURCE: Nature Neuroscience 2002;10.1038.