Dopamine is the primary neurotransmitter involved in the brain’s Limbic “reward” system. When dopamine spikes, it excites the neurons in the reward system, resulting in the feeling of pleasure. Its role as a molecular signal of reward isn’t limited to humans, however. Such circuits have been traced back hundreds of millions of years in animals that existed long before the human brain evolved rational brain structures.1 Thus, this system is pre-rational.
Dopamine concentrations in the brain normally fluctuate between 20-30%.2 How much dopamine any individual produces depends on one’s genetic make-up.3 (For more click on Dopamine D2 gene and Dopamine D1 & 3 Genes.)
Psychoactive drugs trigger spikes of dopamine that are much higher than the brain is accustomed to — as much as 10 times that of natural rewards.4 Thus, drugs significantly overstimulating the reward system.
Scientists used to think dopamine’s role was limited to triggering reward, which was more than enough to implicate it in addiction. Abundant data showed that a variety of drugs unleashed dopamine surges5 and that during withdrawal, dopamine (and another neurotransmitter, serotonin) dropped precipitously.6
As far back as 1975, researchers at the University of Toronto reported that addicted rats given doses of dopamine-blocking chemicals stopped pushing levers they were trained to press to administer doses of methamphetamine. Later research showed that destruction of dopamine-producing cells in rats’ brains stopped cocaine-seeking behavior.7
However, with the advent of better technology, data emerged that didn’t fit the prevailing theory of dopamine’s function. For example, dopamine was also found to surge in anticipation of rewards, which surprised researchers. A buzzer associated with food-delivery increased dopamine in rats’ brains even when no food was given. Dopamine increased 44% when rats anticipated sex, not just on copulation. Similarly, a light associated with food increased dopamine in monkeys, whereas the food itself did not.8
Scientists subsequently discovered that dopamine spikes are triggered by the unexpectedness of a reward. Dr. Wolfram Schultz of Cambridge University, tracked dopamine production in the brains of monkeys when given small squirts of apple juice. He found that when the monkeys got unpredictable squirts or larger ones than they were anticipating, dopamine production surged. In contrast, when they got juice they were expecting, dopamine-producing neurons remained inactive, and when the monkeys expected juice but didn’t get any, dopamine production fell.9
Similarly, MRIs of cocaine abusers who were given the drug registered activity in the nucleus accumbens, a key part of the reward system. This supported the prevailing theory of dopamine as an agent of reward. However, when the nucleus accumbens was also activated in anticipation of another dose, what scientists call the “craving stage,” it challenged the prevailing wisdom.10
One research team studied individual monkey neurons and found that “dopamine cells respond to reward only when it occurs unpredictably.” They said the dopamine response revealed the difference between what the brain predicts and what actually occurs, a feedback system that “looks like the perfect teaching signal.”11
Scientists concluded that dopamine plays a much broader role than originally conceived. It draws the brain’s attention to significant or surprising events, highlighting them to enable the brain to replicate the experience. Dopamine does play a role in reward, but its primary function is to signal the brain to “pay attention” and remember. “Think of dopamine,” suggests P. Read Montague of Baylor College of Medicine, “as the proverbial carrot, a reward the brain doles out to networks of neurons for making survival-enhancing choices.”12
Researchers now think there are at least two systems for rewards involving the neurotransmitter. One, described as the “liking” system, is stimulated on receiving a reward, incorporating dopamine’s originally conceived role. The other, the “wanting” system, motivates behavior in pursuit of a reward, which explains why dopamine rises in anticipation. Neuroscientists say the balance between these systems can be altered by drug abuse to the point that the wanting system dominates the liking one. In effect, it hijacks the brain, “thus offering an explanation for a core paradox of addiction,”13 that the motivation to take drugs outlasts the pleasure they produce.
So dopamine isn’t only about inducing pleasure. Its role in reward is a stepping-stone to a more important function: learning. The brain learns to repeat gratifying experiences and to avoid negative ones. Drugs magnify reward and that distorts the learning process. As learning is key to memory formation, drugs also take a toll on the memories based on that distorted learning. As a result researchers conclude that addiction is a form of toxic learning that impairs the motivation for normal pleasures while strengthening the motivation to take drugs to boost dopamine (and get high). Addiction treatment, they say, is a process of unlearning those lasting malignant memories.
Though dopamine is the primary neurotransmitter implicated in addiction, other neurotransmitters also play a significant role. Alcohol, for example, activates another neurotransmitter system, the opioid system in addition to dopamine. Other neurotransmitters, including glutamates, serotonin and corticotropin-releasing factor (CRF) have also been linked to addiction. The involvement of multiple neurotransmitters illustrates the complexity of the brain’s reactions to addictive drugs and how addiction is a multi-faceted disease. For more on this subject, click on Other (non-dopamine) Neurotransmitters.
1. Psychoactive Drug Use in Evolutionary Perspective, Science Magazine, October 3, 1997.
2. Getting the Brain’s Attention, Science, Oct. 3, 1997.
3. Drug Abuse: Hedonic Homeostatic Dysregulation, Science, Oct. 3, 1997; Brain Experts Now Follow the Money, New York Times, June 17, 2003.
4. Getting the Brain’s Attention, Science, Oct. 3, 1997.
5. Brain Experts Now Follow the Money, New York Times, June 17, 2003.
7. Hijacking the Brain Circuits With A Nickel Slot Machine, New York Times, February 19, 2002.
8. Getting the Brain’s Attention, Science, Oct. 3, 1997.
9. Getting the Brain’s Attention, Science, Oct. 3, 1997.
10. Addiction, Time Magazine, May 5, 1997.
11. Getting the Brain’s Attention, Science, Oct. 3, 1997.
12. Psychoactive Drug Use in Evolutionary Perspective, Science, Oct. 3, 1997.
13. Psychoactive Drug Use in Evolutionary Perspective, Science, Oct. 3, 1997.