The neurotransmitter dopamine is crucial for learning adaptive behaviors, but addictive drugs exploit this normal mechanism by flooding the brain with dopamine.

Question: How does dopamine help us learn adaptive behaviors?

Adam Kepecs: So dopamine is a very interesting neurotransmitter and what is really unique about it, it’s a handful of neuromodulators which are special in the sense that they’re neurons.  The source of the signal is localized to specific brain areas and from these brain areas they project all across the brain.  There is a handful of other ones like serotonin, acetylcholine and these are very interesting because you can think of them as broadcast systems.  Some small area functionally computing something very simple perhaps can send this information across the brain and we understand a lot about dopamine.  It turned out to be incredibly interesting and there is a very simple summary of what we understand about dopamine, which is that these neurons are active when your expectations are violated.  They suddenly send a burst of dopamine when you’re expecting something and you’re not getting it and if you think about it this is exactly the right kind of signal for learning.  If you’re expecting something and you’re getting there is nothing to change about your behavior, whereas if you’re expecting something and you’re not getting it and you have a burst of dopamine now it is a great signal for learning.

Question: What role does dopamine play in addition?

Adam Kepecs: So addiction is really interesting.  It turned out that essentially all of the addictive drugs that we know of end up activating the dopamine circuitry.  They end up releasing dopamine. So an emerging idea is that what happens during addiction is that you have a normal computational process whereby dopamine sends signals out precisely what is the degree to which you violated your expectations and that allows you to precisely learn in proportion to what you need to.

And, on top of this an addictive drug ends up releasing extra dopamine, so what happens is that when you get those drugs you might feel happy.  Let’s say you’re happy about a great chocolate ice cream.  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, so it’s a normal process of learning that gets hijacked by addictive drugs.

Recorded August 20, 2010

Interviewed by Max Miller