Your Brain on Drugs: Dopamine and Addiction

What is happening to the neurochemistry of an addict's brain that makes that person so unable to do without cocaine, heroin, or methamphetamines?

"You can turn your back on a person, but never turn your back on a drug, especially when it's waving a razor sharp hunting knife in your eye," wrote gonzo journalist Hunter S. Thompson, no stranger himself to the compelling nature of addiction.


As has been demonstrated many, many, many times over, drug addiction is a powerful force that can take control of the lives of users. In the past, addiction was thought to be a weakness of character, but in recent decades research has increasingly found that addiction to drugs like cocaine, heroin and methamphetamine is a matter of brain chemistry.

Dr. Nora Volkow, the director of the National Institute on Drug Abuse, says that the way a brain becomes addicted to a drug is related to how a drug increases levels of the naturally-occurring neurotransmitter dopamine, which modulates the brain's ability to perceive reward reinforcement. The pleasure sensation that the brain gets when dopamine levels are elevated creates the motivation for us to proactively perform actions that are indispensable to our survival (like eating or procreation). Dopamine is what conditions us to do the things we need to do.

Using addictive drugs floods the limbic brain with dopamine—taking it up to as much as five or 10 times the normal level. With these levels elevated, the user's brain begins to associate the drug with an outsize neurochemical reward. Over time, by artificially raising the amount of dopamine our brains think is "normal," the drugs create a need that only they can meet.

"If a drug produces increases in dopamine in these limbic areas of the brain, then your brain is going to understand that signal as something that is very reinforcing, and will learn it very rapidly," says Volkow. "And so that the next time you get exposed to that stimuli, your brain already has learned that that's reinforcing, and you immediately—what we call a type of memory that's conditioning—will desire that particular drug." Over time, the consistently high levels of dopamine create plastic changes to the brain, desensitizing neurons so that they are less affected by it, and decreasing the number of receptors. That leads to the process of addiction, wherein a person loses control and is left with an intense drive to compulsively take the drug.

According to Volkow, the reason that dopamine-producing drugs are so addictive is that they have the ability to constantly fill a need for more dopamine. "So a person may take a hit of cocaine, snort it, it increases dopamine, takes a second, it increases dopamine, third, fourth, fifth, sixth. So there's never that decrease that ultimately leads to the satiety," she says.

Adam Kepecs, a neuroscientist at Cold Spring Harbor Laboratory says that addiction has to do with the brain's expectations. An emerging idea, he says, is that drugs basically "hijack" the brain's normal computational enjoyment and reward mechanisms.

"Let’s say you’re happy about a great chocolate ice cream," says Kepecs, as an example."Over time you learn to expect that the chocolate ice cream is really great and you have no more dopamine released in expectation of that when you receive it. Whereas, if you take an addictive drug, you can never learn to expect it because the drug itself will release an extra kick of dopamine. And when that happens, the value of that drug keeps increasing because now you’re learning that 'Wow my expectations were violated, therefore this must be much more valuable than what I thought before.' So basically what ends up happening: the dopamine system gets hijacked by these drugs."

Volkow notes that there are other components to addiction—like genetics and age of exposure—which is why not everyone who takes drugs becomes an addict. She says approximately 50% of the vulnerability of a person to become addicted is genetically determined, and research indicates that if a person is exposed to drugs in early adolescence they are much more likely to become addicted than if they were exposed to the same drugs as an adult.

Takeaway

One of the key functions of the neurotransmitter dopamine is to create feelings of pleasure that our brains associate with necessary physiological actions like eating and procreating. We are driven to perform these vital functions because our brains are conditioned to expect the dopamine rush that accompanies them.

Addictive drugs flood the brain with dopamine and condition us to expect artificially high levels of the neurotransmitter. Over time, the user's brain requires more dopamine than it can naturally produce, and it becomes dependent on the drug, which never actually satisfies the need it has created.

More Resources

— "What Addicts Need," 2008 article in Newsweek about how scientists are using insights about neurochemistry to treat addiction.

— "Addictive Research," 2007 article in TheScientist.com describing the latest

— University of Texas Addiction Science Research and Education Center description of the role dopamine plays in addiction.

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Homo sapiens have been on earth for 200,000 years — give or take a few ten-thousand-year stretches. Much of that time is shrouded in the fog of prehistory. What we do know has been pieced together by deciphering the fossil record through the principles of evolutionary theory. Yet new discoveries contain the potential to refashion that knowledge and lead scientists to new, previously unconsidered conclusions.

A set of 8-million-year-old teeth may have done just that. Researchers recently inspected the upper and lower jaw of an ancient European ape. Their conclusions suggest that humanity's forebearers may have arisen in Europe before migrating to Africa, potentially upending a scientific consensus that has stood since Darwin's day.

Rethinking humanity's origin story

The frontispiece of Thomas Huxley's Evidence as to Man's Place in Nature (1863) sketched by natural history artist Benjamin Waterhouse Hawkins. (Photo: Wikimedia Commons)

As reported in New Scientist, the 8- to 9-million-year-old hominin jaw bones were found at Nikiti, northern Greece, in the '90s. Scientists originally pegged the chompers as belonging to a member of Ouranopithecus, an genus of extinct Eurasian ape.

David Begun, an anthropologist at the University of Toronto, and his team recently reexamined the jaw bones. They argue that the original identification was incorrect. Based on the fossil's hominin-like canines and premolar roots, they identify that the ape belongs to a previously unknown proto-hominin.

The researchers hypothesize that these proto-hominins were the evolutionary ancestors of another European great ape Graecopithecus, which the same team tentatively identified as an early hominin in 2017. Graecopithecus lived in south-east Europe 7.2 million years ago. If the premise is correct, these hominins would have migrated to Africa 7 million years ago, after undergoing much of their evolutionary development in Europe.

Begun points out that south-east Europe was once occupied by the ancestors of animals like the giraffe and rhino, too. "It's widely agreed that this was the found fauna of most of what we see in Africa today," he told New Scientists. "If the antelopes and giraffes could get into Africa 7 million years ago, why not the apes?"

He recently outlined this idea at a conference of the American Association of Physical Anthropologists.

It's worth noting that Begun has made similar hypotheses before. Writing for the Journal of Human Evolution in 2002, Begun and Elmar Heizmann of the Natural history Museum of Stuttgart discussed a great ape fossil found in Germany that they argued could be the ancestor (broadly speaking) of all living great apes and humans.

"Found in Germany 20 years ago, this specimen is about 16.5 million years old, some 1.5 million years older than similar species from East Africa," Begun said in a statement then. "It suggests that the great ape and human lineage first appeared in Eurasia and not Africa."

Migrating out of Africa

In the Descent of Man, Charles Darwin proposed that hominins descended out of Africa. Considering the relatively few fossils available at the time, it is a testament to Darwin's astuteness that his hypothesis remains the leading theory.

Since Darwin's time, we have unearthed many more fossils and discovered new evidence in genetics. As such, our African-origin story has undergone many updates and revisions since 1871. Today, it has splintered into two theories: the "out of Africa" theory and the "multi-regional" theory.

The out of Africa theory suggests that the cradle of all humanity was Africa. Homo sapiens evolved exclusively and recently on that continent. At some point in prehistory, our ancestors migrated from Africa to Eurasia and replaced other subspecies of the genus Homo, such as Neanderthals. This is the dominant theory among scientists, and current evidence seems to support it best — though, say that in some circles and be prepared for a late-night debate that goes well past last call.

The multi-regional theory suggests that humans evolved in parallel across various regions. According to this model, the hominins Homo erectus left Africa to settle across Eurasia and (maybe) Australia. These disparate populations eventually evolved into modern humans thanks to a helping dollop of gene flow.

Of course, there are the broad strokes of very nuanced models, and we're leaving a lot of discussion out. There is, for example, a debate as to whether African Homo erectus fossils should be considered alongside Asian ones or should be labeled as a different subspecies, Homo ergaster.

Proponents of the out-of-Africa model aren't sure whether non-African humans descended from a single migration out of Africa or at least two major waves of migration followed by a lot of interbreeding.

Did we head east or south of Eden?

Not all anthropologists agree with Begun and his team's conclusions. As noted by New Scientist, it is possible that the Nikiti ape is not related to hominins at all. It may have evolved similar features independently, developing teeth to eat similar foods or chew in a similar manner as early hominins.

Ultimately, Nikiti ape alone doesn't offer enough evidence to upend the out of Africa model, which is supported by a more robust fossil record and DNA evidence. But additional evidence may be uncovered to lend further credence to Begun's hypothesis or lead us to yet unconsidered ideas about humanity's evolution.