Why can't people just quit using drugs? Are addicts somehow more drawn to the "highs"? Are their bodies physically dependent on drugs, or do they just lack willpower to quit? The truth is that although some drugs do cause physical dependence and therefore severe symptoms of withdrawal if one stops using them abruptly, there are also real changes that happen in the brain of drug users.

So, even though we can medically treat physical symptoms like the shakes, hallucinations, and changes in heart rate and blood pressure, we also need to pay attention to what is happening in the brain's command centre that makes people crave the drugs, and ultimately come back for more.

In the short term, drugs such as cocaine, heroin, marijuana, alcohol, and ecstasy affect thinking, decision-making, inhibition, and mood, which make the user feel relaxed or happy…resulting in the feeling of being "high". But, with long-term, repeated drug exposure there are lasting structural and functional changes in the brain that persist long after a person has stopped using.

Many factors influence the reality of addiction. Genetics may predispose some people to be more susceptible to drug addiction. Additionally, people with other medical problems such as depression, attention deficit disorder, chronic anxiety and chronic pain are also more susceptible. Environmental conditions are a third factor that may be involved in some people's addictions. In fact, studies have shown that rats raised in isolation are more likely to self-administer cocaine, for example.

Thinking of addiction as a brain disease does not mean that a person cannot be held responsible for his or her own actions, but it can mean that quitting will require more than strength of will alone.

Neurons, the cells of our brain, communicate by sending each other electrical and/or chemical signals via different levels of substances called neurotransmitters. These transmitters are sent across a space called a synapse, and are then picked up by receptors on neighbouring neurons (see Figure 1). This communication between brain cells has the ultimate effect of causing us to move faster, remember a piece of music, feel happiness and even fall in love.

Dopamine is a neurotransmitter that is closely associated with pleasure. It is an important messenger in the limbic system, a primitive area of our brain responsible for emotions and instincts, and more specifically in the nucleus accumbens, our brain's "reward centre". There are naturally increased levels of dopamine in these areas when we experience something we like, including chocolate and possibly even sugar.

Dopamine regulates not just feelings of pleasure, but also regulates cravings in response to cues, such as places and people, that can trigger addicts to use again. Rats that learn to turn on electrodes implanted into their limbic reward system, which stimulates that sensation of "reward", will do so thousands of times an hour…to the exclusion of food and water. These rats will even go so far as either starving to death or crossing painful electrical grids just to self-stimulate!

Drugs and dopamine levels are intimately related since drugs of abuse often cause rises in dopamine levels, and thus an increased sense of euphoria. It has been shown in animals that cocaine prevents dopamine from being mopped up by the neurons that released it in the first place. As a result, dopamine hangs around in the synapses for a longer period of time, allowing for prolonged activation of the nucleus accumbens, which then results in the "rush" people get from cocaine and methamphetamine use.

Similarly, people who smoke cigarettes have been shown to have less of one enzyme that breaks down dopamine, and thus may have more of this "pleasure chemical" hanging around, rewarding and reinforcing the activity of smoking.

Dopamine signaling is not the only communication pathway that is affected by drugs. In fact, opiate signaling can also be targeted. Opiates such as heroin and morphine bind to specialized opiate receptors. These are naturally occurring receptors for opiates that are normally released by the brain. Binding of opiates, either natural or drug-derived, to these receptors dramatically increases the amount of dopamine released to cause intense pleasure.

Alcohol works a bit differently. The alcohol molecules bind to receptors for a chemical called GABA, and increase the effect of GABA on neurons, which is to dampen their electrical activity. The end result is that many functions controlled by the brain are slowed down, such as perception, movement, decision making, and consequentially, response time.

Prolonged use of drugs and alcohol are known to cause long term changes in the synapses between neurons: The number and function of receptors, and amount of enzymes that break down the neurotransmitters change so that more drug is needed to have the same effect…These fundamental changes in the brain's physiology are what leads to tolerance.

But tolerance is not just about "needing more" to attain the same high. Adaptation of the brain to long-term drug use also means that the brain becomes wired to need the drug to function normally. After the person stops using, symptoms such as nausea, tremors, irritability, anxiety, and seizures develop. Persistent brain abnormalities then lead to discomfort and cravings for more drugs.

Detoxification strategies that wean people off drugs are usually aimed at a gradual reduction of dose to minimizing withdrawal symptoms and to allow the brain to adapt to the changes. Other times, one drug is replaced with another, less life-threatening drug. For example, methadone is given to heroin addicts. Methadone is a long-acting opioid that does not give the same intense rush that heroin, but does act on the same opiate receptors to stabilize the person over a longer period of time while they also pursue other methods, such as counseling, towards a more permanent rehabilitation solution.

The brain is an extremely complex, but fascinating organ that is influenced by genetics and environment. It has taken years for researchers to understand the science behind addiction…and we still don't know the whole story. But, we now know that to quit any addictive substances, from chocolate to cocaine, both psychological and biological issues must be addressed. It can be difficult process, but not impossible. Most important, however, is to identify and recognize if there is an addiction problem so that it can be treated successfully.

Article first published January 20, 2006

Dr. Hasini Reddy is a Rhodes Scholar whose PhD research focused on brain imaging techniques (University of Oxford, England). She completed her medical degree at Memorial University (St. John’s, ND), and is now a medical resident at the University of Western Ontario specializing in neurology and pathology. She enjoys reading, traveling and horseback riding.

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