(Excerpts from an old Harvard Mental Health Letter) dated July 2011

Addiction exerts a long and powerful influence on the brain that manifests in three distinct ways: craving for the object of addiction, loss of control over its use, and continuing involvement with it despite adverse consequences.

In the 1930s, when researchers first began to investigate what caused addictive behavior, they believed that people who developed addictions were somehow morally flawed or lacking in willpower. Today we recognize addiction as a chronic disease that changes both brain structure and function. Addiction hijacks the brain.

Nobody starts out intending to develop an addiction, but many people get caught in its snare. Genetic vulnerability contributes to the risk of developing an addiction. Twin and adoption studies show that about 40% to 60% of susceptibility to addiction is hereditary. But behavior plays a key role, especially when it comes to reinforcing a habit.

Pleasure Principle

The brain registers all pleasures in the same way, whether they originate with a psychoactive drug, a monetary reward, a sexual encounter, or a satisfying meal. In the brain, pleasure has a distinct signature: the release of the neurotransmitter dopamine in the nucleus accumbens, a cluster of nerve cells lying underneath the cerebral cortex. Dopamine release in the nucleus accumbens is so consistently tied with pleasure that neuroscientists refer to the region as the brain's pleasure center. All drugs of abuse, like Nicotine, cause a particularly powerful surge of dopamine in the nucleus accumbens. The likelihood that the use of Nicotine will lead to addiction is directly linked to the speed with which it promotes dopamine release, the intensity of that release, and the reliability of that release. Smoking, as opposed to swallowing a pill, for example, generally produces a faster, stronger dopamine signal and is more likely to lead to drug misuse.

Learning Process

Scientists once believed that the experience of pleasure alone was enough to prompt people to continue seeking an addictive substance. But newer research suggests that the situation is more complicated. Dopamine not only contributes to the pleasure experience, but also plays a role in learning and memory — two key elements in the transition from liking something to becoming addicted to it.

Dopamine interacts with another neurotransmitter, glutamate, to take over the brain's system of reward-related learning. This system has an important role in sustaining life because it links activities needed for human survival (such as eating and sex) with pleasure and reward. The reward circuit in the brain includes areas involved with motivation and memory as well as with pleasure. Addictive substances stimulate the same circuit — and then overload it. Repeated exposure to an addictive substance (Nicotine) causes nerve cells in the nucleus accumbens and the prefrontal cortex (the area of the brain involved in planning and executing tasks) to communicate in a way that couples “liking” something with wanting it, in turn driving us to go after it. That is, this process motivates us to take action to seek out the pleasure source.

Tolerance and Compulsion

Addictive drugs, like Nicotine, provide a shortcut, flooding the brain with dopamine and other neurotransmitters. Our brains do not have an easy way to withstand the onslaught. Nicotine can release two to 10 times the amount of dopamine that natural rewards do, and they do it more quickly and reliably. In a person who becomes addicted, brain receptors become overwhelmed. The brain responds by producing less dopamine or eliminating dopamine receptors — an adaptation similar to turning the volume down on a loudspeaker when noise becomes too loud. As a result, dopamine has less impact on the brain's reward center. People who develop an addiction typically find that, in time, the desired substance no longer gives them as much pleasure. They have to take more of it to obtain the same "high" because their brains have adapted — an effect known as tolerance.

At this point, compulsion takes over. The pleasure associated with an addictive drug subsides — and yet the memory of the desired effect and the need to recreate it (the wanting) persists. The learning process comes into play. The hippocampus and the amygdala store information about environmental cues associated with, so that it can be located again. These memories help create a conditioned response — intense craving. Cravings contribute not only to addiction, but to relapse. A person addicted to Nicotine may be in danger of relapse when he sees someone smoking. Total abstinence is the ONLY way to stay quit.