What makes dopamine such a powerful chemical?
Nora Volkow: Dopamine is a chemical substance that serves to send messages between two cells in the brain and that's what we call neurotransmitters. There are many neurotransmitters and dopamine plays a key role in modulating the areas of the brain that enable a wide variety of functions, best known is movement. When your dopamine cells die for example, you cannot initiate movement and that's Parkinson's disease.
The effects of dopamine go away beyond movement and one of actions very relevant for drug abuse and addiction is it modulates the areas of the brain involved with our ability to perceive reward or reinforcement and to be motivated by that reward to do actions.
For example, it's so important that if you can generate through transgenetic, genetic technologies, you can generate in a mice those that not produce synthesize dopamine. These animals will die of starvation because they don't have the motivation to engage in the behaviors to go and eat the food. You can rescue this animal by injecting dopamine into the areas of the brain that control this. If you don't do that, these animals will die of starvation. That really epitomizes how extraordinary important dopamine is. It gives you that energy that drives to do things.
Drugs of abuse, we know that for the past 50 years, all of them, whether it's legal or illegal, they don't make any distinctions over there in terms of the pharmacology. They increase dopamine in specific areas of the brain, of the limbic brain and this ability to increase dopamine when a person takes a drug is associated directly with what we call rewarding or reinforcing effects. If you can actually manipulate it in such a way that to interfere with the ability of a drug to increase dopamine then that drug is no longer pleasurable. That's why dopamine is so important and understanding why certain drugs can produce addiction and others not.
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 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.
These mechanisms are not developed in our brains to take drugs. These mechanisms of dopamine signal in reinforcement and once you have experienced it, getting conditioned to it is extraordinary important way for nature to ensure that humans, as well as animals, will perform behaviors that are indispensable for survival.
Therefore, it shouldn't surprise us that behaviors such as eating or sexual behavior are linked with increases in dopamine and in the same areas that drugs do it. There are differences though because natural reinforcers increase dopamine as a function very much of the context. What do I mean by that? If you're hungry, for example and you get exposed to food, that will increase dopamine much more than if you just finished eating. So, as you eat, the ability of food to increase dopamine goes down and eventually disappears and because it disappears, you are no longer motivated to eat the food.
How is dopamine related to drug addiction?
Nora Volkow: What happens with drug though on the other hand is they do not decline the ability to increase dopamine. 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 and this is believed that these differences between the normal responses of the dopamine stimuli as it was developed through evolution to serve physiological functions versus the ways that drugs do it, much more potently, longer duration of action and it does not decline with repeated administration. It's believed to trigger the adaptations, the plastic changes in our brain that eventually will lead in those individuals that are vulnerable to the process of addiction which is a condition whereupon the person with repeated administration of drugs no longer can control his or her ability to decide when they take or they don't take the drug.
This is fundamentally a stage where that individual has lost control and has intense drive to compulsively take the drug and that's what we call addiction. Not everybody that takes drugs actually becomes addicted. We've come to recognize, for example, that approximately 50% of the vulnerability of a person to become addicted is genetically determined. That vulnerability has a very strong genetic component. It also has other processes that determine vulnerabilities.
For example, if you get exposed to drugs when you're very young, very early adolescence, you're much more likely to become addicted than if you get exposed to the same drugs when you're an adult. This has to do with the fact that the adolescent brain is much more neuroplastic than the adult brain and as a result of that, a drug which triggers this adaptation process is likely to produce these changes faster in an adolescent and it's also the duration of those changes is likely to be much longer lasting in an adolescent than it is an adult. These are two processes as it relates to addiction.
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