Drug addiction is a devastating disease that tends to rip lives apart, and often people find it’s easiest just to blame an addict for their own problems—but a new study out of Cambridge has added to the growing pile of evidence that addiction is far more complicated than someone’s willful lack of self-control. In fact, the researchers discovered a sort of “back door” in the brain that completely skips over self-control in the first place.
The study, which is published in Nature Communications, sought to explain why, in the case of relapses, only four out of ten people who do so felt actual cravings for the drug—meaning that six out of ten people who relapse do so for reasons other than “needing” it.
“Most people who use cocaine do so initially in search of a hedonic ‘high’,” said David Belin, from the Department of Pharmacology at the University of Cambridge, in a statement. “In some individuals, though, frequent use leads to addiction, where use of the drug is no longer voluntary, but ultimately becomes a compulsion. We wanted to understand why this should be the case.”
The researchers looked to the brains of rats addicted to cocaine for answers.
Previously, research by Barry Everitt from the Department of Psychology at Cambridge showed that, in the case of rats being allowed to self-administer cocaine, the rats who sought the drug showed activity tied to dopamine (a neurotransmitter associated with happiness) in a brain region known as the nucleus accumbens—which is associated with “goal-directed” behavior. As in: The goal was to continue pleasurable feelings.
But this brain activity quickly shifted following prolonged exposure to cocaine. Dopamine activity began to occur now in the dorsolateral striatum, a brain area associated with habitual behavior—suggesting that the rats had lost control, and had developed a drug-taking habit.
At the same time, chronic exposure to drugs also alters a region known as the basolateral amygdala, which is associated with linking a stimulus to an emotion. In this way, the basolateral amygdala actually ties cocaine (a stimulus) to pleasurable feelings, and then stores them as memories.
Or, in short, the rats soon developed an automatic habit by way of cocaine use, and the basolateral amygdala strengthened the habit by storing positive associations and memories in connection to the drug.
Decisions, decisions
The prefrontal cortex, or the control center of the brain, was thought to ultimately decide whether or not an addicted person continues their habits. To do so, it recalls the memories stored in the basolateral amygdala, but does not just take into account the good feelings found there when deciding whether or not a person should take a drug.
Instead, it manipulates such information, weighing in other factors to help it come to a choice. A common thought was, in the brains of addicts, the other factors (like cravings) overwhelmed the prefrontal cortex’s ability to decide, leading to further drug abuse.
However, for the six out of ten people who relapsed while feeling no cravings, this model didn’t quite fit—and that’s where the new research undertaken by the Cambridge team entered.
They have discovered that in rats chronically exposed to the drug, the decision-making prefrontal cortex isn’t overwhelmed by information while trying to decide—because it’s actually skipped over entirely, granting it no chance to stop the behavior.
In actuality, the pleasurable-memory storing basolateral amygdala indirectly links to the habit-associated dorsolateral striatum, avoiding the prefrontal cortex. Which means that an addicted individual might not be aware of their desire to relapse.
“We’ve always assumed that addiction occurs through a failure of our self-control, but now we know this is not necessarily the case,” explained Belin. “We’ve found a back door directly to habitual behaviour.
“Drug addiction is mainly viewed as a psychiatric disorder, with treatments such as cognitive behavioural therapy focused on restoring the ability of the prefrontal cortex to control the otherwise maladaptive drug use. But we’ve shown that the prefrontal cortex is not always aware of what is happening, suggesting these treatments may not always be effective.”
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