Monkeys of a lower social standing are more likely to choose cocaine over food in stressful situations than their higher status counterparts, a new study from the U.S. suggests.

The study, presented Sunday at the Experimental Biology conference in San Diego, also found that improving the monkeys' quality of life diminished cocaine use, a result with implications for the treatment of drug abuse in humans.

"We believe this type of research can be used to identify better treatment strategies, including providing environmental enrichment, that may affect the likelihood of abusing drugs," said lead researcher Michael Nader in a news release.

"We think that reducing drug use in people is going to require changes in their environment — things that are enriching, such as alternative activities," he elaborated in an e-mail to CBC News on Monday.

Such enrichment could include a job, said Nader, a professor in the school's department of physiology and pharmacology.

"The point is that abstinence or incarceration won't be effective to most people in the absence of environmental changes," he said.

Subordinate monkeys used more cocaine, ate less

For the study, researchers from the Wake Forest University School of Medicine in North Carolina set up the monkeys, male cynomolgus macaques, with the ability to self-administer cocaine injections and exposed them to both stressful and enriched situations.

Cynomolgus macaques ascribe to a strict social hierarchy, and divide themselves accordingly, from the dominant male down to the most subordinate.

Earlier research found that low-status monkeys were more likely to use cocaine if it was offered to them.

To see how the monkeys would react under stress, the researchers introduced the monkeys to another social group by putting them in a cage beside the strangers' pen for 15 minutes. The monkeys were physically safe, but could hear the new monkeys.

After spending time in the cage, the monkeys were given the choice between two levers, one which they knew delivered cocaine, the other known to deliver a food reward.

The researchers found that the subordinate monkeys used more cocaine and ate less while the higher-status monkeys tended to choose the food pellets.

"We found that stressful events increased cocaine use especially in subordinate monkeys (who are already more stressed than the dominant animals," Nader said.

The human applications

For the second part of the study, the monkeys were given three days in an "enriched environment," that is, a larger pen where they had the chance to explore and learn new ways to get treats. Spending time in this pen led to less cocaine use in all of the monkeys, the researchers found.

Calling the results "very significant," Nader said that the findings could be used to treat cocaine addiction in humans.

"These findings suggest that changes in the environment only can have profound effects on the likelihood to use cocaine. Enrichment can decrease use," he said.

The environmental changes reduced cocaine use but didn't eliminate it in the monkeys, he added.

"We think a combination of drug treatments, whatever those may be, plus environmental changes will have a better likelihood of eliminating drug use than either manipulation alone."

Nader said that in addition to the positive effects in changing the environment, the researchers also saw individual differences in the results depending on the animal's social rank.

"That suggests that the same manipulation won't necessarily have the same effect in everyone — a finding that is certainly true with humans," he said.

Other research conducted at the school and presented Sunday included a study on the changes in the brain associated with stressful situations. Researchers found that when stressed, the subordinate monkeys showed decreased activity in the areas of the brain related to stress, anxiety, reward and emotion, while dominant monkeys showed increases in reward-related areas.

The researchers said this difference may explain why the subordinate monkeys got a greater reward from cocaine than food when compared with the higher-status monkeys.

Addiction could be treated with 'replacement' drugs

Additional research, conducted by assistant professor of physiology and pharmacology Paul Czoty, found that it would be possible to treat cocaine addiction with a "replacement" drug that mimics cocaine with less potential for abuse, similar to the way nicotine addiction is treated.

The researchers found that treating monkeys with amphetamine reduced the self-administration of cocaine for up to a month.

Czoty said that while it is unlikely that amphetamines will turn out to be the best treatment for cocaine addictions, "these drugs allow us to prove the concept of using a replacement drug to combat cocaine addiction."

He said the study may also help identify the best dose and schedule for administering replacement drugs.