Dopamine and the Brain's Reward System

Dopamine is an important neurotransmitter (a brain chemical involved in the communication between nerve cells) that influences movement, emotion, learning, and behavior.  Most notably, it is the brain chemical that controls feelings of pleasure.  Frequently associated with the brain’s reward system, dopamine is discharged through the areas of the brain that are activated whenever a person experiences a positive outcome in response to a particular behavior (in psychological terms, a reward).  In everyday life, neurons located in your mid-brain release dopamine in conjunction with the activities that make you happy, like eating, drinking, having sex, being with the people you love, and appreciating a good joke.   All of these behaviors are beneficial for your survival, and the brain’s release of dopamine makes it likely that you will engage in these activities again and again.   Ironically, however, the very same reward system that naturally facilitates the learning of beneficial behaviors also responds to the artificial rewards of taking drugs or engaging in unhealthy behaviors.

How do drugs affect the way nerve cells typically communicate with each other?                      

Although narcotics affect the entire body, these powerful chemicals primarily target the brain, and their capacity to alter the normal functioning of dopamine and the brain’s reward system proves to be particularly devastating.  In general, neurons communicate by discharging neurotransmitters into the tiny spaces called synapses between the cells in the brain.  Each different type of neurotransmitter is then captured by unique receptors on other neurons.   Once it binds with the receptor, the neurotransmitter instigates a particular action in the receiving cell and is then carried away from the synaptic space by transporters that recycle the chemical.  Drugs enter the picture by exerting effects on a variety of different kinds of neurotransmitters.  Drugs can influence every step of this intricate communication system, and scientists are only beginning to understand the precise mechanisms by which they do so.

How do drugs affect the normal functioning of dopamine and the brain’s reward system?         

Scientists have found that drugs tend to concentrate within the brain’s reward pathways, deluging them with dopamine.  Some drugs, including marijuana and heroin, very closely resemble dopamine, causing the brain’s dopamine receptor cells to behave abnormally.  Other drugs—like cocaine, amphetamine, and methamphetamine—work by triggering the release of enormous amounts of dopamine and inhibiting the natural uptake of this neurotransmitter from the synapses between nerve cells.  In either case, this massive rush of dopamine—between two and ten times the amount of dopamine released in response to natural rewards—is responsible for the euphoric highs experienced by drug users, as well as the addictive nature of recreational drugs. 

Aren’t massive amounts of pleasure a good thing?

After a while, the brain adapts to being flooded by dopamine by producing less and less dopamine and reducing the number of available dopamine receptors.  In other words, it develops a tolerance for that drug.  Chronic drug users must continue to take larger and larger amounts of a drug just to feel normal, and eventually they become numb to the naturally rewarding activities, like eating and spending time with loved ones, that once made them happy.