Addicts May Be Predisposed

According to a recent study by Harvard researchers—including the University’s provost—cocaine addiction may be caused by more than simple use of the drug.

Released last Thursday in the journal Neuron, the article indicates that certain people may have a genetic predisposition toward cocaine addiction, raising important public policy issues and ethical questions about the nature of addiction.

According to Hans C. Breiter, co-director of the Motivation and Emotion Neuroscience Collaboration (MENC) and senior author of the study, this key discovery lies in the structural differences within a specific part of the brains of cocaine addicts.

This section of the brain, the amygdala, is responsible for creating the feeling of craving, according to Breiter, who is also an assistant professor of psychiatry at Harvard Medical School.

More importantly, he said, the amygdala allows people to see potentially harmful outcomes of their actions, thus serving as a deterrent for most people from abusing drugs.

“Cocaine addicts have a terrible time seeing negative consequences of their actions—this is why they have to ‘hit bottom,’” said Breiter.

Nikolaos Makris, instructor in neurology and an author of the study who developed the technology to image and measure the size of the amygdala, found that the amygdala of cocaine addicts is significantly smaller than non-addicts.

Breiter said that such reduction in size was “unbelievable.”

“You’re seeing as much shrinkage as people who die of Alzheimer’s,” he said.

In addition, Makris also noticed that the reduction of amygdala size is much more pronounced on the right side, and that the end result is a symmetrical amygdala.

According to Breiter, the amygdala is only symmetrical in children. As we mature, however, the right side of the amygdala enlarges at a faster rate than the left side, establishing asymmetry.

“Asymmetries in body organs and cell lineages are set up by the actions of specific genes,” said Gregory P. Gasic, the other co-director for the MENC and an author of the study. “Therefore, a loss of asymmetry can be due to a loss of function in these genes.”

The study found that people who had been addicted to cocaine for one year had about the same amount of amygdala shrinkage as those who had been addicted for up to 27 years. This confirmed that genes, and not the drug itself, were responsible.

Provost Steven E. Hyman, another author of the study, warned that medical diagnosis of predisposition toward drug abuse raises potential questions in a “nascent field called neuro-ethics.”

“Let’s say it’s 2020, you’re a 15 or 16 year old, and let’s say your genotype is on your credit card,” he said. “We can do a high resolution MRI, and know about your brain structure. This kind of information may be extraordinarily useful for predicting prevention measures, but this type of information could be used in a terrible way.”

However, scientists do not yet know whether genes are solely to blame.

“There are two possible types of predisposition,” said Breiter. “Either the amygdala was going to develop normally, but the drug caused it not to, or the amygdala was never going to develop normally in the first place.”

Regardless, Breiter said that these findings will cause major public policy changes in the future.

“If cocaine causes the amygdala to change in shape, it means that we have a mandate to get it off the street,” said Breiter. “If this is purely genetic, then we should develop better medical diagnosis and prevention skills—we can determine who is at risk.”

The study is part of the greater Phenotype Genotype Project in Addiction and Depression, the largest neuro-imaging and genetics project in the United States.

—Staff writer Risheng Xu may be reached at xu4@fas.harvard.edu.