Researchers have found that like other inherited traits – addiction could be a legacy from parents. A child of drug-addicted parents is eight times more likely to become an addict than a child growing up in a drug-free home. However in families whose very brains seem primed for addiction, some children still go on to lead productive lives free of drugs, according to new research.
Behavioral neuroscientist Karen Ersche of the University of Cambridge in the United Kingdom and colleagues examined whether drug abusers begin life with miswired brain circuitry or merely end up that way. Imaging studies of addicts show dramatic differences in brain areas involved in motivation, reward, and self-control, to name just a few. But it's less clear whether these differences are the cause or the effect of drug abuse. Because both addiction and brain structure are likely to be inherited traits, many researchers suspect that drug abusers have faulty brain circuitry based in their genes.
Dr Karen Ersche said, “It has long been known that not everyone who takes drugs becomes addicted…It shows that drug addiction is not a choice of lifestyle, it is a disorder of the brain and we need to recognize this.”
To explain the connection Ersche and colleagues worked with 50 pairs of biological siblings; one in each pair was addicted to cocaine or amphetamines while the other had no history of drug abuse. Also included were 50 healthy, drug-free, unrelated volunteers.
The researchers first tested the subjects' self-control. Participants pressed a left- or right-arrow key when seeing a similar arrow on a computer screen—unless they heard a tone, in which case they were to do nothing. People with poor self-control, including most drug addicts, find it difficult to refrain from pressing the key; studies show that impaired performance on the test is linked to irregularities in the brain areas involved in addiction.
They found that siblings who didn't use drugs performed as poorly on the test as the ones who did. All of the sibling pairs did worse than the healthy controls, the team reports in the 3 February issue of Science.
Additionally they performed brain scans and found that both members of the sibling pairs had abnormal interconnections between parts of the brain that exert control and those involved with drive and reward. Some individual brain structures were abnormal as well; the putamen, which plays a key role in habit formation, was larger in the siblings than in control subjects, as was the medial temporal lobe, which is involved in learning and memory.
Ersche believes the finding provides a measurable, biological basis for vulnerability to addiction. But she adds that even among siblings who share so many risk factors for addiction—genes, family environment, brain circuitry, and behavioral test results—“some people just don't go down that road. There must be other factors that convey resilience.”
Imaging specialist Nora Volkow of the National Institute on Drug Abuse in Bethesda, Maryland, adds that key differences between the brains of the siblings might account for some of the resilience. For example, in the addicted sibling, an area called the orbitofrontal cortex, which provides for flexibility during changing circumstances, was smaller, possibly making it difficult to break out of compulsive patterns such as drug addiction. Increased activity in this region, on the other hand, is linked to positive emotions, which are thought to protect against addiction.
“The findings are good news,” Volkow says. She explains that even in children as young as 4 to 12, traits such as self-control and flexibility can be improved by targeted interventions, including exercise training, martial arts, yoga, and computer games designed to enhance working memory (the ability to hold complex information in mind). These approaches, she says, could be used to help prevent addiction in those at risk for drug abuse.
Dr Paul Keedwell, a consultant psychiatrist at Cardiff University, said, “Addiction, like most psychiatric disorders, is the product of nature and nurture. We need to follow up people over time to quantify the relative risk of nature versus nurture.” “If we could get a handle on what makes unaffected relatives of addicts so resilient we might be able to prevent a lot of addiction from taking hold,” said Dr Keedwell.
The chief pharmacist for Derbyshire Mental Health Trust, David Branford, said the study, “implies that addiction does not produce noticeable changes to brain structure and function which means that there may be provision for looking at new treatment techniques for addiction”.
Prof Les Iversen, from the department of pharmacology at the University of Oxford, said, “These new findings reinforce the view that the propensity to addiction is dependent on inherited differences in brain circuitry, and offer the possibility of new ways of treating high-risk individuals to develop better 'self control'.”
Derek Hill, a professor of Medical Imaging Science at University College London, said it was a “clearly designed” piece of research which showed that this sort of brain scanning might be used to find so-called biomarkers to help develop new treatments for other self-control-related conditions such as over-eating. “Unfortunately, it takes years to develop an imaging method like this to the level of maturity needed to help develop new treatments, so practical benefits are some way in the future,” he said.
A study in the Lancet medical journal in January said that as many as 200 million people use illicit drugs worldwide each year, with use highest in wealthy countries. Unhealthy addictions can also range from narcotics and prescription medicines to legal substances like cigarettes and alcohol and lifestyle factors such as over-eating or gambling.
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