Addiction & the Reward Pathway

From the lab to the legislature, the push is on to recognize and treat addiction as a mental illness. Getting hooked on alcohol or drugs is not the result of bad behavior or a weak character, researchers say; rather, it’s caused by genes and by biochemical disruptions in the brain that distort mood, clear thinking and compulsion control. It’s all rooted in the ‘reward pathway,’ a looped circuit deep in the human brain.

How does addiction work in the brain?

A HEALTHY PATHWAY: The ‘reward pathway ‘ produces feelings of pleasure in response to naturally enjoyable stimuli, such as food and sex. Connected to other brain regions, including memory storage, the pathway motivates us to repeat activities that perpetuate the species.

AN ADDICTED PATHWAY: Drinking or doing drugs hijacks the reward pathway. But in genetically vulnerable people, this altered state leads to an addiction that they are, on their own, powerless to overcome. Someone with an addiction can’t talk herself out of the compulsion any more than someone can talk herself out of depression.

Electrical and chemical signals pass between neurons in the reward pathway that trigger the release of dopamine. Dopamine is, among other things, the pleasure chemical.

• REWARD Activating the reward pathway is a gradual, step-by-step process that first engages the five senses, slowly triggering a dopamine release and making us feel good. For example, in a hungry person the release would start with the anticipation of food and decline as desire is sated.
• IN CONTRAST Psychoactive substances such as alcohol, methamphetamines and tobacco bypass the senses to work directly on brain circuitry, launching the pathway to a sudden high.
• THE HIGH The result provokes an exaggerated release of dopamine, leading to an over-accumulation of the pleasure chemical in the brain. This produces the feelings of euphoria, increased energy, confidence and relaxation.
• THE LOWS The brain adjusts for the overabundance of pleasure chemicals by reducing the number of receptors in an effort to moderate dopamine levels.
• REPEAT USE The cravings motivate a user to seek drugs to activate the reward pathway again, as memories connecting to past highs feed and reinforce the urge. Research has found that, even decades after a user has been clean, the mere image of a drug can stimulate the pathway.
• DAMAGE Due to the shrinking numbers of dopamine receptors, however, users require greater amounts of a drug to achieve the same high. In turn, this again prompts the brain to limit dopamine receptors, creating a vicious circle.
• ADDICTION The motivation to continue using drugs becomes an addiction, driven more by fear of the negative emotional and physical feelings associated with withdrawal than the desire to be high. “More and more experts agree … addictions are themselves a mental illness. ” - Remi Quirion, professor of psychiatry at McGill University and scientific director of the Institute of Neurosciences for the Canadian Institutes of Health Research

RECEPTOR Receptors are like keyholes on the surface of brain cells. They regulate dopamine levels in the brain.
DOPAMINE This neurotransmitter is linked to motivation.

Sources: GLOBE REPORTING, MCGILL UNIVERSITY, THE UNIVERSITY OF UTAH GENETIC SCIENCE LEARNING CENTER

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RESEARCH FINDINGS

THE GENE FACTOR Scientists have identified gene types that lead to addiction. Genes that make people metabolize drugs quickly, for example, make them more likely to become addicted quickly. Some genes produce a euphoric response to alcohol similar to a heroin high. Other genes govern dopamine levels in the brain. Overall, scientists estimate that genetic factors account for 40 to 60 per cent of a person’s vulnerability to addiction. For example, research shows that 33 out of 100 will become addicted to cigarettes after the first few tries, 16 out of 100 to cocaine.

THE OVERLAP Research in the U.S. suggests that more than half of people with a mental illness also suffer from a substance dependence, compared with 6 per cent of the general population.

DRINKING AND DEPRESSION A recent national survey backed by the Canadian Institutes of Health Research, based on interviews with 14,000 men and women, has found strong links between depression and alcohol. The findings suggest it is not how often you drink but how much. People who binge drink are more likely to be depressed than people who frequently consume moderate amounts of alcohol. The pattern appears to be especially pronounced in women, said lead investigator Katherine Graham, a researcher with the Centre for Addiction and Mental Health.

MARIJUANA AND THE YOUNG MIND Researchers have found that repeated cannabis use in adolescence can increase the risk of developing schizophrenia - particularly in young people predisposed to the disease. A 2005 report in the British Medical Journal concluded that teenagers who used marijuana three times before age 15 faced a 10-per-cent chance of developing schizophrenia by age 26, compared to a 3-per-cent risk in the general population.

HOOKED FOR GOOD Research shows that addictions are more difficult to break if they are learned at a young age. “You don’t forget how to ride a bicycle and you don’t forget how to be an addict,” said Charles O’Brien, psychiatry professor at the University of Pennsylvania. “It’s a physical response in the brain….even when the drug is out of the body for a long time, there are changes in the brain that do not go back to normal - there’s always the risk of relapse.”

THE POLICY PUSH The Diagnostic and Statistical Manual, the dictionary of mental conditions for clinicians in North America, lists addictions as Dependence Disorders. But a move is afoot to call addiction a mental illness in its own right in the next edition of the DSM, due in 2012. Charles O’Brien, who chairs the committee reviewing the addiction category, says the name change could help combat stigma and help addicts get the treatment they need. “We don’t blame someone with schizophrenia for the disease, but we do blame the addict… when addiction is a chronic disease that becomes involuntary.” In March, the U.S. House of Representatives passed a bill requiring group health insurance plans to provide the same coverage for substance-use disorders and mental illness as for medical and surgical services. In Canada, treatments for addiction remain a patchwork of public, private and not-for-profit programs.