By Daniel Lende
In your brain you have a system that comes up from some of the oldest evolved parts of your brain to some of the most recently evolved parts. Reptile parts to ape parts. In brain research on addiction, it’s generally called the mesolimbic dopamine pathway or system. All the main addictive drugs affect this system, making the mesolimbic pathway a core component in addictive behavior. Addictive experiences—gambling, shopping, eating and sex—also impact the mesolimbic dopamine system.
In both scientific research and the popular press, the dopamine system is often cast in the role of “bad boy,” a hard-wired brain circuit that has gotten out of control, self-indulging in an orgy of pleasure. That neat story tells us a lot about how we cast our own morals onto the brain, selectively picking out research to provide a nice scientific sheen. Hard-wired for hedonism, we have to work even harder at self-control.
It strikes me as the same sort of story that addicts know how to spin so well. So let’s be blunt. Denial!
Blaming others, whether people or circumstances, is nearly as rampant in the study of addiction as it is in addiction itself. So here’s the first step—blaming addiction on dopamine. This chemical becomes the prime cause. It’s a simple story: our excesses are caused by a chemical in the brain.
“[T]he brain’s reward system [is] powered largely by the neurotransmitter dopamine… The hope is that if you can dampen the effect of the brain chemical that carries the pleasurable signal, you can loosen the drug’s hold.” -From “How We Get Addicted” by Michael Lemonick, Time, July 5, 2007
When challenged by people who know neurotransmitters do not work in a cause-and-effect fashion, along comes the next step. We have a hard-wired system in our brain, and something goes wrong with it.
“We believe that these [addictive] disorders are linked by a common biological substrate, a ‘hard-wired’ system in the brain (consisting of cells and signaling molecules) that provides pleasure in the process of rewarding certain behavior… Although each substance of abuse appears to act on different parts of this circuit, the end result is the same: Dopamine is released in the nucleus accumbens and the hippocampus (Koob and Bloom 1988). Dopamine appears to be the primary neurotransmitter of reward at these reinforcement sites.” -From “Reward Deficiency Syndrome” by Kenneth Blum, John G. Cull, Eric R. Braverman and David E. Comings, American Scientist, March-April 1996
But our brains are not some fifth-grade science project on electricity, circuits clinked onto a preset board. It has complex patterns of neurons firing together, and those patterns of activation are linked to both body and environment.
In other words, the hard-wired-pleasure story denies basic facts about the brain and basic facts about addiction. So let us take a closer look at the mesolimbic dopamine system and its relationship to addictive behavior.
Carlton Erickson, in his book The Science of Addiction, presents a wonderful way to grasp where the mesolimbic dopamine system is located. Make the peace sign (or the victory sign) and stick those two fingers in the middle of your forehead. Using your other hand, point with one finger directly above your ear. Imagine where your fingers would intersect. That’s where the mesolimbic dopamine system is, right in the middle of your brain (yes, the two fingers matter—we have two brain hemispheres). It runs from the ventral tegmental area in your midbrain to the nucleus accumbens in your limbic system, with projections into the frontal cortices.
Being in the middle of the brain matters. It means that the mesolimbic dopamine system mediates between more basic functions, like arousal, and higher-level functions, like abstract thought. So, what does it do?
Here is where things get tricky. Our normal approach is to imagine our brain like a car engine or a computer, with specific functions located in specific pieces. Forget that image, it’s wrong.
A better way to think about the dopamine system is as a chess piece, say, the knight. The knight has a limited repertoire of moves, different shaped L’s. So what really matters is what pieces are left on the board and what the other person is doing. The brain works that way—the role of each piece depends on what other pieces are doing, and how that works vis-à-vis a changing environment.
So, here are some basic results from research on the mesolimbic dopamine system: dopamine is involved in seeking and wanting, dopamine is involved in both pain and pleasure, dopamine function can be viewed as a learning signal, and dopamine can modulate emotions, attention and memories.
What?! How can one system do all that? Here is where using the chess piece metaphor is important—one piece can accomplish many things but still have some basic patterns of movement. What it can accomplish depends on the overall layout of pieces on the board (the environment) and the strategy of the player (the overall behavior of the person).
In discussing brain function and plasticity, my friend Greg Downey also makes an important comment: “Our cultural categories of perception — folk concepts, phenomenological traits, or even physical categories from audiology such as timbre, pitch, and location — may not be relevant categories for the neural division of labor.” In other words, expecting the dopamine system to have one function—such as pleasure or reward—is based on our own expectations and categories, not on actual brain function.
Actually, the structure of the system lends itself to a varied view of function. As I said before, it goes from the midbrain through the limbic system and into the frontal cortices. There are pathways that come down from the frontal cortices to shape mesolimbic dopamine signaling, as well as other pathways from association cortices.
Put differently, it’s a big system with broad and diffuse projections and with feedback systems from memory and judgment/control circuits. Of course it’s not going to accomplish one simple thing. Put differently, the mesolimbic dopamine system is a major modulator of your brain. But would you expect anything different from a system that plays a role in sex, eating and most other sorts of “get up and go” activities?
(Author’s note: This is a draft for one section in the opening chapter of a book I am writing about my research on addiction. The book is aimed for a popular audience. If anyone has comments on the writing, as well as on my characterization of the mesolimbic dopamine system, I would appreciate hearing them. Feel free to email me at firstname.lastname@example.org or to post your comment below.)