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How does addiction change the brain? To what extent are drug addicts responsible for their behavior? And how can policy-makers minimize the harm addictive drugs cause to individuals and society?
Researchers discussed the science and ethical aspects of drug addiction in a symposium at the British Neuroscience Association Festival of Neuroscience in Edinburgh earlier this month, sponsored by the European Dana Alliance for the Brain (EDAB) and the International Neuroethics Society.
Addiction is increasingly viewed as a complex disease involving maladaptive learning that exaggerates the rewarding effects of dangerous substances, leading to compulsive behaviors that often have negative consequences. Cognitive neuroscientist Trevor Robbins of the University of Cambridge explained addiction in terms of a transition from voluntary to habit-based behavior; the drug user gradually loses the ability to control their substance use. This compulsivity leads to persistent and inappropriate actions that bear no obvious relationship to their life goals.
Robbins and his colleagues have shown that high impulsivity predicts the shift towards compulsive self-administration of cocaine in rats, and have developed a model that might explain the neural basis of this process. In their model, the shift from voluntary drug use to addiction is associated with a breakdown of frontal cortex executive control functions. As a result, drug-seeking behaviors come under control of the striatum, which includes structures that make up the brain’s reward system.
“Executive control mechanisms normally hold these subcortical behaviors in check.” said Robbins, an EDAB member. “We hypothesize that this executive control is lost with increasing drug use, so these habits become essentially out of control, and by repetition they become the building blocks for compulsive behaviors.”
Indirect evidence for this model comes from a brain scanning study published in 2011, which linked cocaine addiction to reduced grey matter volume in the orbitofrontal cortex and an increase in the volume of the striatum.
“These differences are also present in [users’] siblings who do not take drugs, so the propensity for habit-forming may be hereditary” Robbins added. “It suggests that there are pre-existing structural changes in the brain in areas controlling volitional behavior which are correlated to substance abuse.”
Gunter Schumann, a professor of biological psychiatry at Kings College London, discussed the role of genetics in addiction, particularly alcohol abuse and dependence. In 2011, Schumann and colleagues published a genome-wide association study, involving more than 26,000 participants, in which they identified a gene that influences alcohol consumption.
Alcoholism is a complex behavior, however, with no single cause. Last year, a large longitudinal study of adolescent alcohol abusers showed that around half of all cases of alcohol abuse disorders have a genetic component, but also identified many other factors, including psychological and social ones, that contribute to the behavior.
“There are different ways to become alcohol-dependent,” said Schumann. “Some people become alcohol abusers because of a lack of executive control, others want to reduce stress and anxiety. The important take-home message is that there is a substantial amount of heritability, but there is no single gene that causes one to become alcoholic.”
Schumann noted that psychopathology and alcohol and substance abuse in adolescents is often associated with life events. Stressful events early in life in particular have been linked to a greater frequency of binge drinking during the teenage years, possibly because such events tend to decrease resilience to stress. (see “Stress and the Brain: What Makes Some of Us More Vulnerable Than Others“)
He also presented preliminary data showing that adolescent alcohol abuse affects development of a number of brain areas, including the prefrontal cortex, consistent with the model presented by Robbins.
Neuropsychopharmacologist David Nutt of Imperial College London argued that our society has responded to drugs unethically.
In 2010, Nutt and his colleagues published a large analysis ranking drugs according to the harm they cause. The study showed that alcohol was by far the most harmful substance in the UK, with heroin and crack cocaine in second and third place respectively.
Nutt noted that, while the incidence of many kinds of diseases has gradually declined, that of liver disease has increased dramatically since the early 1970s, and some 80% of this increase is due to alcohol use. Alcohol is now the leading cause of death in men under age 50, and one-third of all homicides are committed by people who are under its influence. Nevertheless, alcohol is legal, and its consumption actively encouraged through advertising campaigns.
“This should be a public health emergency, but we have systematically avoided doing anything about it,” said Nutt, also an EDAB member. “The most concern is about the drugs that cause the least harm. We encourage people to harm themselves by only allowing them access to certain drugs, and I believe all drugs that are less harmful than alcohol should be decriminalized.”
Furthermore, abstinence-based recovery programs are themselves harmful, he argued. Using the experiences of singer Amy Winehouse and actor Phillip Seymour Hoffman as examples, he said that abstinence lowers a user’s tolerance, and that as a result of this, relapsing with a large dose after staying “clean” for weeks or months can kill.
Nutt further argued that U.K. drug laws are outdated, irrational, and not based on evidence of relative harm, and that the distinction between legal and illegal drugs is a political decision rather than a scientific one. He also argued that laws regarding cannabis do more harm than the drug itself; 1 million people in the U.K. have a criminal record due merely to having possession of it, reducing their chances at a steady life, and also that banning certain drugs makes users turn to other, more harmful substances.
For example, “when mephedrone entered the market as a legal high, there was a significant fall in cocaine and amphetamine deaths as users switched to it,” he said. “Mephedrone then got banned due to media hysteria, and now cocaine deaths are on the rise again.”
Finally, Julian Savulescu, director of the Oxford Centre for Neuroethics, discussed the philosophy of addiction. He noted that drugs can facilitate people’s well-being, and argued that people should be free to harm themselves if they wish.
Freedom, he said, is the ability to make choices and live one’s life according to one’s own rules. “We become responsible when we set rules and live according to them,” said Savulescu. “Some people have better impulse control than others, and that has implications for whether they become addicts, end up in prison, or become successful.”
“There are many factors that make it more or less difficult to exercise control over whatever rules we set ourselves. Addiction is an expression of the rough-and-ready nature of the brain’s learning and reward systems, and the fragility of our proneness to environmental insults, genetic susceptibility, and so on.”
Savulescu concluded by reiterating that prohibition never works. “When the human animal wants something, it finds a way of getting it, so harm reduction strategies are necessary and will be more effective,” he said.
“We have enough knowledge to use drugs safely, and the challenge is to use that in a way that enables people to live their lives the way they want. It’s time to ethically readdress the laws that govern our behavior.”