When most people think of dopamine, they think of things that can get you high. Things that feel good. Cocaine. Sex. Food. We imagine floods of dopamine in our brains as the pleasurable feelings take hold. As more and more media outlets cover neuroscience, we get the idea that serotonin means happiness, but dopamine means...pleasure.
And sure, sometimes it does. But dopamine is nothing without its RECEPTORS. And the D2 receptor is one of the big ones. Sure, D2 is a dopamine receptor. But it doesn't mean pleasure, and it's a wonderful example of the ways that the brain uses to put the brakes on such an...addictive system.
This current paper looks at the way we look at D2 receptor function. But it also provides an interesting perspective on possible targeting of those who might be at risk for addiction. Because it turns out, the ability to stop a cocaine binge is only as good as the brakes on your system.
Bello, et al. "Cocaine supersensitivity and enhanced motivation for reward in mice lacking dopamine D2 autoreceptors." Nature Neuroscience. 2011.
In order to understand how the D2 receptors these authors are looking at work, we're going to have to go back to the basics of neurotransmission (for a more detailed explanation, see my SCIENCE 101 post here).
What we're looking at here is a synapse, the space between one neuron and another where a signal has to get passed along. Since the two neurons are not connected, one neuron (the presynaptic neuron on the top) releases chemicals (neurotransmitters) from little vesicles (the bubbles in the pic) into the space between the neurons. The neurotransmitters travel across the space and bump into receptors on the other side (the postsynaptic neuron at the bottom). The response of the receptors will then pass the signal on (stimulating or repressing the signal on the way) to the next neuron, and around and around the brain it goes.
You can also see the little pink flip things on the presynaptic neuron. These are dopamine transporters (DAT). You obviously don't want your one signal to continue forever, so you need to clear the dopamine from the synapse, so the receptors will not continue on being stimulated. So the dopamine transporter helped with that, sucking the dopamine back up into the presynaptic neuron.
But there's something missing in this charming picture, where it's been left out for simplification. You see, on the PREsynaptic neuron, there are also RECEPTORS. These receptors are D2 receptors. You might wonder why on earth you'd want receptors around on a neuron that's already fired. But that's because these neurons are inhibitory, and help the presynaptic neuron stop the signal. When the dopamine in the synapse hits the presynaptic D2 receptors, the presynaptic neuron is inhibited, helping the neuron to stop firing (though not stopping it completely).
Of course, we THINK that's what happens. We can see it when we look at neurons in a dish. In the brain, well that's harder. Because there are D2 receptors both pre- and post- synaptically, and separating them out in a whole brain is difficult to do. So sure, you can SAY that low doses of an agonist prefer the presynaptic and high doses prefer the postsynaptic...but you're not really being specific. And so, figuring out the BEHAVIORAL effects of presynaptic D2 receptors was pretty fuzzy.
But that was then. This is now. And now, the authors on this paper can take a mouse, and knockout the D2 receptors ONLY in the neurons expressing DAT. When a neuron expressed DAT, meaning it has the ability to recycle dopamine, you're pretty sure that neuron FIRES dopamine. SO if you limit your D2 knockout ability only to neurons that are FIRING dopamine (not receiving), you can limit your D2 knockout only to the presynaptic D2 receptors. Very elegant.
This allows you to look at the role of D2 receptors, what they are really doing with behavior and drug responses. And look at that role they did.
This is part of figure 2 from the paper, and shows a technical called voltammetry, where you use a carbon fiber electrode to stimulate dopamine release from a brain slice (you can also do it in a whole brain, but I'm pretty sure this here's a slice). You can see that when you stimulate with the electrode on the left, you get that nice iconic spike in neurotransmitter that we're all used to seeing (though usually we see electrophysiology and a cell firing, this is just neurotransmitter release). On the right, you have your D2 knockout mouse, and you can see the signal is BIGGER. The DA neuron D2 knockout has MORE dopamine and releases MORE dopamine per stimulation. Because it has no D2 brakes to pull it back!
But what does it mean behaviorally?
It means these Dopamine neuron D2 knockout mice like to move it move it!
The above graph shows locomotor activity of these animals, just running around in a cage. You can see that the DA neuron D2 knockouts (woo, typing that is gonna get OLD...) move around a lot more than their normal counterparts. This is because dopamine controls a lot of locomotor activity, and if an animal has MORE dopamine, they're going to run more.
And that's all well and good, but what about their responses to drugs?
Here you can see some traces of the dopamine release when a bath of cocaine was applied to the neurons. You can see that, on the right, the DA neuron D2 knockout mice showed about an...equal cocaine response. Since cocaine acts by hitting dopamine transporters (and not D2 neurons), and thus increasing the amount of dopamine in the synapse, this isn't a huge surprise. What we're interested in is not the amount of dopamine building up, but the EFFECT it's having.
There we go.
In another graph (not shown here) the DA neurons D2 knockout mice MOVED AROUND more to the same amount of cocaine, showing they were more behaviorally sensitive. But what we're concerned about is what that has to do with drug REWARD. So the graph up here is a measure of something called conditioned place preference. You can do this using mice, or rats, or heck you could probably use humans. You take a room and divide it into two chambers. Inject the mouse with cocaine, and put it in one side. The next time, inject the mouse with saline and put it in the other side. Repeat this for a few days. Then, on test day, put the mouse in the middle, and see where it likes to spend the most of its time. If the mouse tends to find the cocaine injection rewarding (and they usually do), they will spend more time on the side of the room that you paired with cocaine. It's a pretty good measure of drug reward in an animal model where the mouse can't tell you how it's SO HIGH RIGHT NOW.
But the problem is, normal mice tend to like cocaine plenty. So in order to find out if our DA neuron D2 knockout mice are more sensitive, we have to use a LOW dose of cocaine, one that normal mice probably wouldn't really feel. But the D2 knockout mice certainly felt it. You can see above that they showed place preference for the cocaine dose while the normal mice did not. Knocking out presynaptic D2 receptors makes the mice MORE SENSITIVE to cocaine.
But cocaine is one type of reward, and a very specific type at that. What about OTHERS? What about food? After all, recently studies have been coming out showing that not only are D2 receptors lower in people who do a lot of cocaine, D2 receptors are ALSO lower in people who have problems with more 'natural' rewards like binge eating. So the authors put these mice in a chamber where they could press a lever for food pellets. And then they asked how much they liked it.
And boy did they like it. The best graph here is the one on the right, what's called a progressive ratio. Basically, you start out telling the mouse to press the lever once, and it gets a pellet. But the next time, it has to press twice. Then four times. Then 16 times. Then 32 times, then 64...the numbers keep going up. At some point the mouse will decide it wasn't hungry anyway and say screw it, and this is called the breakpoint. If a mouse finds the food to be more reinforcing, it will keep banging on the lever.
And the DA neuron D2 knockout mice did love to bang on that lever. They kept going long after the normal mice had stopped, showing that not only do they prefer cocaine more than normal mice, they prefer food more too. And it looks like it's not just because they are lever happy, they really do prefer the food more.
I'm really interested in these studies. They saw no differences in mouse weight or in anxiety behaviors, etc, etc, but I'm wondering if these mice will get fatter on a high fat diet. What about other natural rewards like running?
And of course, I'm interested in the opposite side of the picture too. In this paper they knocked OUT the D2 receptors on the DA neurons. What if they knocked them IN? What if they overexpressed them? Could you get mice that thought the cocaine was too much? Could you get mice that didn't binge anymore?
As you can tell, this model, figuring out the way D2 receptors work in the brain, has some really interesting implications, both for seeing who might be more vulnerable to addiction (people with lower D2 receptor levels tend to find drugs "feel" better), and also, way in the future, maybe working on a way to help drug addicts stop. In the mean time, it's massively cool to see such a specific knockout, and to be able to see that all of the things we were PRETTY sure were happening with presynaptic D2 receptors...are really actually happening!! It's nice to know we're right sometimes.
Bello EP, Mateo Y, Gelman DM, Noaín D, Shin JH, Low MJ, Alvarez VA, Lovinger DM, & Rubinstein M (2011). Cocaine supersensitivity and enhanced motivation for reward in mice lacking dopamine D(2) autoreceptors. Nature neuroscience PMID: 21743470