Why do we work? Or why do addicts put themselves in harms way by abusing a substance? Researchers are finding clues in the brain chemistry of motivation and depression.
In a review published in the book, Well-being: Foundations of Hedonic Psychology (Russell Sage Foundation, 1999), Princeton behavioral neuroscientist Bartley Hoebel and fellow researchers examine how the chemical dopamine reinforces connections between cognitive inputs and behavioral output, so that successful behaviors are repeated. Their study has implications for social systems of reward and punishment.
Turning to “extreme cases” to illuminate the more mundane, the researchers focus on binge eating, addiction, and depression. They hypothesize that there is a common neural reinforcement system shared between such diverse activities as eating, addiction, mating – even working. The key player may be the dopamine pathway in the brain.
Studies have shown that most abused drugs increase dopamine in the accumbens area of the brain. Other studies show that rats will repeatedly bar-press to self-inject dopamine-releasing drugs such as amphetamine and cocaine directly into their brains. Conversely, when dopamine receptors are blocked, rats show an abnormal lack of motivation. For example, they treat sugar as less sweet.
“But when the dopamine system is functioning properly, the rewards in life have more effect,” observes Hoebel.
In fact, the researchers conclude that mild addiction is somewhat natural in that even very sweet foods can lead to dependency. Sugar triggers the production of the opioids, morphine-like neurotransmitters produced in the brain and body. Studies show that opioids and dopamine work together in promoting feeding. Dopamine tends to initiate food seeking, while opioids can prolong the meal.
“We think that is a key to the addiction process,” Hoebel says. “The brain is getting addicted to its own opioids as it would to morphine or heroin. Drugs give a bigger effect, but it is essentially the same process.”