Study of psychiatric disorders is difficult in man and mouse

One of the challenges with treating psychiatric disorders is finding a way to study them outside of the human brain. When there is no fundamental understanding of how a disease works, it becomes that much harder to find comparable symptoms in an animal or cell. And when you’re working with diseases such as depression that have symptoms that are hard to objectively quantify, there’s an extra layer of complexity.

Psychiatric disorders have many potential interacting causes: Environment, stress, genetics and more. Investigating specific mutations in genes associated with disorders like attention-deficit/hyperactivity disorder helps scientists understand how genetic changes might interact with other factors to produce symptoms of the disorder. A new study highlights one of the ways that scientists can study genetic contributions to human psychiatric disorders in animal models, by taking a mutated gene from a human and putting it into a mouse. The work shows how a single change in a gene’s code  could contribute to behavioral changes in humans can be studied in detail in animals, and how knowledge from such studies can help us understand psychiatric diseases such as ADHD.

It all begins with dopamine. Dopamine is a chemical we often associate with pleasure, movement, reward and attention. It is a chemical messenger that is released when we initiate movement or when we encounter things we need to pay attention to, things that give us pleasure, or other signals indicating that something nice might be on the way.

But like other chemicals released into the spaces between neurons, dopamine has to be removed. A protein called the dopamine transporter vacuums up dopamine from outside the cell and brings it inside to be broken down or recycled. This transporter is the target of many drugs — both medical and illegal. Stimulants including cocaine, amphetamine (commercially known as Adderall) and methylphenidate (commercially known as Ritalin) all act on the dopamine transporter. Cocaine and methylphenidate stop up the transporter, preventing it from sucking dopamine back into the neuron. With more dopamine left in the space between neurons, more of it binds to receptors on other neurons, perpetuating a signal that should have ended. These extraneous piles of dopamine are what produces cocaine’s high and the increased focus after methylphenidate.

Amphetamine is a bit different. It also causes dopamine to persist in the spaces between brain cells. But it goes the extra mile, altering the transporter’s interactions with other proteins so it pumps dopamine out of the cell instead of in. The result is that dopamine isn’t cleared from the extracellular space, and additional dopamine is dumped in. This extra dopamine increases focus in people ADHD when it is released in small doses as a pill. It also causes a high if taken in high doses, or snorted to produce speedy dopamine spikes.

Neuropharmacologist Randy Blakely and his lab at Vanderbilt University in Nashville have identified a naturally occurring genetic variant of the dopamine transporter that he says “does exactly what amphetamine does.” The mutation alters a single amino acid on the edge of the transporter, outside the area where dopamine is moved through into the cell but in an area where the dopamine transporter interacts with other proteins required for its function. When the lab studied the gene variant, called Ala559Val, using cells, it produced a leaky transporter: It took up dopamine, but it also released dopamine back out.

Now the researchers have taken this human gene variant and bred it into a mouse. The leaky transporter gave the mice much higher levels of dopamine in the spaces between their neurons than normal. The scientists soon realized that the variant produces “a transporter that’s ‘on amphetamine’ all the time,” Blakely says. The new work was published October 20 in the Proceedings of the National Academy of Sciences.

Unlike a mouse that’s actually on amphetamine, the mice with the gene variant don’t have increased motor activity. Instead, Blakely says, they exhibit an odd “darting” behavior when they are approached. The scientists will be pursuing the behavior in future studies, trying to determine if this rare gene variant produces mice that model the symptoms of ADHD in a human.

While it seems somewhat paradoxical, mice with high levels of extracellular dopamine are sometimes viewed as having symptoms associated with human ADHD. The authors compare their newly created mouse to one known as DAT-KO, which has a genetically knocked out dopamine transporter. Without any dopamine transporters, the chemical has no way to get back into the neuron from the extracellular space. With all this extra dopamine around, the mice are excessively hyperactive. But when dosed with amphetamine, they are less hyperactive. So when scientists create other mice with higher levels of dopamine between neurons, they often seek to determine if they, too, might provide a way to study ADHD. 

This variant used in the new study is vanishingly rare. So far there are only a few instances of humans with the change. One is a woman with bipolar disorder; another is a pair of brothers with autism. So it’s unlikely that this single variant would be the underlying cause for most cases of ADHD, bipolar disorder or autism. But tweaks like this to the dopamine transporter gene provide scientists the opportunity to take something associated with psychiatric symptoms in a human and examine its impact in detail in a mouse.

The study authors hope to determine how exactly this tiny change might affect the way the transporter interacts with other proteins. “Everything in the dopamine system is so poised, so tightly balanced,” Blakely says. “This gene variant starts a Rube Goldberg machine. One little change ripples through the whole protein and all its interactions.”

And this gene variant is just one of many that occur in the gene for the transporter. Some produce transporters that don’t work. But others produce transporters with similarly altered function, that pump dopamine out as well as in. “All these reports make a convincing case for the critical role of this protein in ADHD,” says Raul Gainetdinov, a neuropharmacologist at Saint Petersburg State University in Russia. “But being a developmental disorder, ADHD probably could result also from deficits in proteins involved in development. I would not be surprised if mutations in related processes and functions could have an important role in this disorder as well.”

Lei Shi, a biophysicist at Cornell University, says that studies like these help researchers learn how natural human mutations might change how a particular protein functions. This, in turn, will help scientists learn more about what happens when those protein functions go wrong. Eventually, Shi hopes that studies like this will “help us to develop specific treatment by targeting either the dopamine transporter or its associated proteins.”

With enough gene variants, and enough mice, scientists may one day understand how the dopamine transporter functions and the many ways in which the delicate balance of dopamine might be upset. These functional changes could provide an important piece in the genetic, environmental and developmental influences that underlie psychiatric disorders.

Editor’s Note: This article was updated November 3, 2014 to clarify the studies on ADHD and autism.