Marijuana acts as neurotransmitter, attaching to dopamine receptors and causing the release of dopamine in the brain.

Addiction as a 'Brain Disease'

We become addicted to a substance or activity for the same reason that we initially try it: Because we like the way it makes us feel. And although some people may try a drug, take a drink or eat a donut and never become hooked, almost all of us have the capability to become addicted. Users cross a threshold and undergo a transition to addiction.

Research has shone light on the changes that take place in the brain after this transition, developing the "brain disease" model of addiction. It's currently the most widely held view of addiction among the scientific community.

The way we learn to survive is based on a reward system. When we do something that aids in our survival, like eating or exercising, our brain's limbic system rewards us for this behavior by releasing dopamine, a chemical that makes us feel good. Since we like the way we feel, we learn to repeat the behavior.

Different substances approach the limbic system -- the reward center -- in our brains in different ways, but all substances of abuse cause the brain to release high levels of dopamine. This release can be two to 10 times the amount our brain releases normally, giving the user a sense of a "rush" or "high."

Because of this release and its impact on the brain's reward center, users learn very quickly to use a substance or engage in an activity. They learn this in the same way they learn to eat or exercise, but even faster and with more intensity, since the release of dopamine is so much larger. Since the amount of dopamine released is abnormal, the brain struggles to regain its normal chemical balance after a substance wears off. This produces a hangover, or withdrawal, from a substance, which can manifest in physical pain, depression and even dangerous behavior.

Over time, prolonged use of a substance can lead the brain to stop producing as much dopamine as it naturally does. This creates further withdrawal, leading to a physical dependency -- the addict needs to use more of the substance just to feel normal, creating a vicious cycle that can be difficult to break.

Because of this learning process and eventual physical dependence on a substance, the substance user becomes a substance abuser. As a result, the abuser loses control over the act of taking a substance or engaging in an activity. This has led to the idea that in order to cure an addiction, abstinence -- total discontinuation of substance use or behavior -- is necessary.

Under the disease model of addiction, the brain's motivational center becomes reorganized. The priorities are shuffled so that finding and using the substance (or another substance that will produce similar effects) becomes top priority as far as the brain is concerned. In this sense, the drug has essentially taken over the brain, and the addict is no longer in control of his behavior. An alcoholic won't, for example, have trouble deciding whether or not to get in his car and drive to the store to get more alcohol -- the urge will be irresistible.

But simply going to the store to buy alcohol is not a definitive sign of alcoholism. So how can you tell the difference between using a substance and being addicted to it? In the next section, we'll learn about the symptoms of addiction.