Addiction Changes the Brain

Most people know that the use of some street drugs damages the brain. A popular commercial several years ago showed a whole egg as 'your brain' and a frying egg as 'your brain on drugs'. The idea that drugs change our brains is not new.

What is new is the knowledge of how drugs change the brain to cause it to become addicted. Research in this field has uncovered some new and interesting discoveries.

As I discussed in an earlier column, addicted individuals exhibit certain qualities. For example, an addicted person experiences a compulsion to take the substance; an increased tolerability to the substance over time; persistent use even though it is harmful; progressive neglect of other enjoyable activities; and withdrawal symptoms. But what brain mechanisms are involved in these reactions?

New research conducted at the National Institute on Drug Abuse in the US has discovered that the brain dopamine system is central in addiction.

Imaging studies using MRI and PET scans show that almost every drug of abuse (including nicotine, alcohol, cannabis, cocaine, heroin and methamphetamine) elevates the level of the chemical dopamine in the brain. Dopamine is a neurotransmitter involved in attention, memory and pleasure-reinforcing behaviour.

Levels of dopamine increase in response to natural rewards such as food and sex and also in response to stress. It is part of what is known as a reward circuit in the brain.

Addictive drugs stimulate the dopamine system in extreme amounts and usually cause a release of five to 10 times the amount of dopamine as natural rewards.

Repeated excessive stimulation of these 'reward circuits' is what causes addiction. Eventually, the individual's brain becomes altered to the point that natural rewards are no longer sufficient. Judgment and decision-making circuits become damaged and the individual is reduced to an overwhelming need to seek and take drugs.

Not only is the dopamine release increased with initial drug use, but after chronic drug abuse and during withdrawal, the number of dopamine receptors in the brain decreases. This involves dysfunction in the prefrontal regions of the brain and results in decreased sensitivity to things that cause natural reward stimulation. In fact, the numbers of dopamine receptors remain low even after months of abstinence from drugs.

Decreased sensitivity and receptor levels are probably a consequence of conditioned learning in the brain and the resetting of reward thresholds in order to adapt to the abnormally high levels of stimulation induced by the abused drugs.

Genetics also play an important role in drug addiction. It has always been clear that some people are more susceptible to addiction than others. Although everyone experiences a pleasurable sensation from intoxication, not everyone becomes addicted to the intoxicating substance.

High levels of dopamine receptors are protective against addiction. Individuals born with higher numbers of these receptors may be less likely to become addicted. This is probably because, according to research, the subjective response to drugs of abuse is more pleasant with lower numbers of dopamine receptors. Having higher levels of receptors means that the drugs are not as pleasant and do not stimulate as much of a reward reaction in the brain.

While genetics is one factor involved in addiction, some environmental situations can also bring about an increased likelihood that abuse will lead to addiction. Studies in animals show animals that are dominant in their social groups increase the levels of dopamine receptors in their brains.

More research into humans is necessary, but this finding may explain how poverty, abuse and neglect are related to an increased likelihood of addiction at the neurochemical level.

As research in this field continues, more will be discovered about the mechanisms in the brain that are affected by drug abuse. Once the disease of addiction is better understood, effective treatment will be easier to administer and new strategies will be developed.

In the meantime, if you or someone you love experiences a substance abuse problem, seek help. Your doctor can recommend treatment programs and support groups. Although addiction is a disease, it is not impossible to overcome.


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