The power of dopamine
The dopamine system evolved as a way to help ensure the survival of the species. Surges of dopamine accompany the consumption of food, the sexual act and the drive to be part of a group. Yet dopamine is more than a chemical of pleasure and reward. It is a messenger of salience.
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“Something that is pleasurable is salient. Something that’s aversive is salient. Something that is unexpected is salient,” says Nora Volkow, M.D., director of the National Institute on Drug Abuse. A salient stimulus will “engage your motivational circuits and drive all of your behavior to attend to (it).”
The approach of a hungry lion, for example, will trigger the “flight or fight” response. The flow of dopamine even may underlie heroic acts “beyond the call of nature” to save the life of another, she says.
Salience also may help explain Ritalin’s effectiveness as a treatment for attention deficit hyperactivity disorder (ADHD).
Like cocaine, Ritalin is a stimulant that increases the supply of dopamine in the synapse, or gap, between nerve cells. It does this by blocking the dopamine transporter, a kind of molecular “vacuum cleaner” that normally sweeps excess dopamine back into the nerve cell.
Ritalin may not be as addictive as cocaine, in part because—when taken in pill form—the drug is slow to occupy the dopamine transporters and slow to be washed out of the brain. In comparison, cocaine’s “high”—and its addictive potential—relates to the speed with which it gets into the brain, particularly when it is injected intravenously.
There is some evidence that some children with ADHD may have too many dopamine transporters. While they may produce normal amounts of dopamine, the neurotransmitter is vacuumed up too quickly for it to do its job.
In 2001, Volkow and her colleagues showed for the first time that Ritalin, when given orally to healthy volunteers in doses used to treat ADHD, significantly increased dopamine concentration in the striatum, the part of the brain involved in motivation and attention.
By increasing dopamine levels, Volkow believes, Ritalin may increase the saliency of an academic task and therefore the motivation to pay attention. In the same way, adults drink coffee, which also activates the dopamine system, to make a boring afternoon meeting more interesting.
Drugs of abuse can take control of this circuitry by triggering rapid, intense surges of dopamine. Not only do they affect the brain’s pleasure centers, they also have reinforcing effects on the brain’s motivational pathways.
Other, “normal” stimuli no longer bring pleasure. “The main drive becomes the drug,” Volkow says. This may explain why addicts are compelled to continue using drugs long after the “high” has been replaced by the “lows” of deteriorating health and social status.
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