Stanford Report Online



Stanford Report, March 21, 2001
Research shows dopamine plays crucial role in sleep regulation

BY KRISTA CONGER

The search for a better understanding of a drug used to treat narcolepsy has revealed a previously unsuspected pathway in sleep regulation, say researchers at Stanford University Medical Center.

The scientists found that the drug, modafinil, changes the way dopamine affects nerve cell signaling. A lack of dopamine causes the jerky movements and fixed facial expressions of people with Parkinson's disease, clearly implicating the substance in motor control. But until now it wasn't thought to be of much importance in sleep regulation.

"The role of dopamine in sleep has not been seriously examined for a couple of decades," said Jonathan Wisor, PhD, a postdoctoral researcher in the laboratory of Dale Edgar, PhD, associate professor of psychiatry and behavioral science. Wisor is lead author of a paper about the research that appeared in the March 1 issue of the Journal of Neuroscience.

"We found that the dopamine transporter is blocked by modafinil," Wisor said. Modafinil is a promising new compound that works to promote wakefulness without causing the jittery nerves and subsequent "crash" that accompany other stimulants.

Dopamine is just one of many neurotransmitters used for communications between brain cells known as neurons. Neurotransmitters convey a nervous impulse across the gap that separates one neuron from another. After the signal has been passed along, the neurotransmitters either diffuse away, are actively dismantled by enzymes or are reabsorbed by protein complexes called transporters on the surface of the originating neuron.

Previous sleep research has shown that neurons using certain neurotransmitters, such as serotonin and norepinephrine, have levels of activity that wax and wane in clear correlation with waking and sleeping ­ alert animals have more of these neurons firing than do sleeping animals. In contrast, the activity of dopamine-dependent neurons varies only slightly over a 24-hour period.

Even so, sleep researchers had a few clues that dopamine might be important in regulating sleep. Parkinson's patients frequently wake inappropriately at night, and methamphetamine ­ a potent stimulant ­ blocks the transporter complexes for dopamine as well as serotonin and norepinephrine. And previous work in the laboratory of Emmanuel Mignot, MD, PhD, associate professor of psychiatry and behavioral sciences at Stanford, had shown that modafinil binds to the dopamine transporter. But the actual mechanism by which the two compounds promote wakefulness was unclear.

"No one was absolutely sure how they worked," said Mignot, director of the Stanford Center for Narcolepsy and a co-author of the paper. "Amphetamines act nonspecifically on several neurotransmitters, so it was difficult to tell which one was most important."

To clear up the confusion, the researchers studied mice engineered to be unable to express the dopamine transporter. Because these mice lack a transporter to mop up the leftover dopamine, the transmitter hangs around in the synapse much longer after signaling than it does in wild-type mice.

By monitoring the brain activity of the mice, the researchers found that although the mutant mice had activity levels similar to those of their wild-type counterparts, they slept less deeply and were awake more often and for longer periods of time.

The researchers then gave both strains of mice similar doses of modafinil and watched them for five hours. As expected, the wild-type mice were awake for an additional hour compared to their sleep levels prior to taking the drug. In contrast, the mutant mice spent no more time awake than before. The researchers saw similar results when they tested the effect of methamphetamine on the mice.

The experiments show both drugs require the dopamine transporter to promote wakefulness.

"It's very clear; the drugs were completely ineffective in these mice," said Mignot. "We've demonstrated that these drugs taken to stay awake work because they act on the dopamine transporter."

Wisor is even more interested in the results of a subsequent experiment involving caffeine. In contrast to their reaction to modafinil and methamphetamine, the mutant mice retained their ability to respond to caffeine. In fact, they spent even more time awake after a simulated jolt of java than did the wild-type mice.

"Caffeine is the most widely used stimulant in the world," said Wisor. "And it's thought to work through another neurochemical, adenosine."

Sleep researchers think adenosine accumulates steadily in the brain during the waking hours. When it reaches a certain threshold level, adenosine receptors are triggered and initiate a slowing of brain activity that leads to sleep. Caffeine works by blocking the receptors for adenosine and preventing the sleep signal from being transmitted.

Wisor and Edgar hypothesize that in a wild-type brain, dopamine signaling works to keep the animal awake until the rising levels of adenosine overwhelm its effect. Caffeine tips the balance in dopamine's favor and keeps the animal awake by blocking the action of adenosine. The dopamine lingering in the synapse of the mutant mice heightens caffeine's effect.

"It's known that the adenosine receptor and the dopamine receptors are present in some of the same areas of the brain, and sometimes even on the same cell. This fits in perfectly with our idea that dopamine fights against the tendency of adenosine to promote sleep," Wisor said.

Other co-authors of the study include Edgar and senior research scientist Seiji Nishino, MD, PhD.