The Brain’s Inherent Susceptibility To Addiction

Psychoactive drugs produce euphoria because they overstimulate the neurons in the brain’s Limbic system, the site responsible for feeling pleasure. Some drugs, like heroin and marijuana, have molecular structures so similar to the brain’s natural neurotransmitters that they mimic the neurotransmitters themselves, dock with receptors on the neurons in the Limbic system and overstimulate the reward circuits.

Other drugs use a different mechanism to reach the same result. Rather than mimicking neurotransmitters, they induce an increase in the output of neurotransmitters which stimulate reward. Amphetamines, for example increase production of dopamine, the primary reward-center neurotransmitter.

Cocaine uses a third mechanism: it prevents the brain from mopping-up excess dopamine. No matter what mechanism, neurons experience dopamine spikes, resulting in a high.

A dopamine high is essentially the same no matter what drug is taken: “If you give an animal a general injection of an opiate like heroin, it is likely to slump to the floor and not get up. But if you inject heroin directly into the part of the brain where it is rewarding, it makes the animal run around like it was given amphetamine,” Dr. Roy Wise, of the National Institute of Drug Abuse says. That’s “the common denominator of addiction.”Why do opiates make you sleepy, while amphetamines keep you awake? They’re side-effects from the differences in the ways the drugs act in brain structures outside the Limbic System, not involving dopamine.1

Because psychoactive drugs have the ability to manipulated naturally-occurring neurotransmitters that overstimulate the brain’s reward center, scientists say the human brain is innately vulnerable to addiction. Having evolved over millennia, the brain is “not designed to cope with ready access to” modern drugs of relatively high purity that are commonly available today: “Pure psychoactive drugs and direct routes of administration are evolutionarily novel features of our environment. They are inherently pathogenic [ie., disease causing] because they bypass adaptive information processing systems and act directly on ancient brain mechanisms that control emotion and behavior.”2

“Any organism with a chemically mediated incentive system and technological capabilities is intrinsically vulnerable to addiction, but these special design features of vertebrate reward systems magnify the risks …”3 (For more, click on Why Drugs Are Gratifying.) 

Given the way the brain is engineered, occasional drug use to feel good is anything but aberrational, scientists say. It’s normal, and most people who use drugs don’t get addicted. The National Academy of Sciences reported, for example, that only 32% of those who tried tobacco became dependent. For heroin, it was 23%, cocaine 17%, alcohol 15%, and marijuana 9%.4

Drug abuse, however, causes physical and structural changes to the neurons in the reward system, which can lead to tolerance and addiction. And, since a dopamine high is essentially the same regardless of the drug that causes it, drugs are fungible. If an addict’s drug of choice isn’t available, rather than stop, he or she will substitute other drugs that are available, which will result in the same high.

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1. Alcohol Abuse Is Hereditary,, July 5, 2007.
2. Psychoactive Drug Use in Evolutionary Perspective, Science, Oct. 3, 1997.
3. Psychoactive Drug Use in Evolutionary Perspective, Science,
4. Addiction: A Brain Ailment, not a Moral Lapse; New York Times, September 30, 2003.

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