New Research Identifies Adenosine Role in RLS

JULY 21, 2018

By Kris Schanilec

If you have RLS, you know the relentless leg discomfort that can drive you out of bed at night.

Chances are, you also have two other symptoms: periodic limb movements in sleep (PLMS) – 80 percent of those suffering from RLS display this symptom; and constant wakefulness or “hyperarousal” – people with RLS typically are not sleepy during the day, even if they don’t sleep well at night.

Now, scientists have gained new insight into what causes these symptoms – and new avenues to investigate for treatment.
In 2014 and 2015, Sergi Ferré, MD, PhD, of the National Institute on Drug Abuse (Intramural Research Program) received grants from the RLS Foundation to more closely examine how dopamine and glutamate work in the brain.

Dr. Ferré has found that low levels of receptors of adenosine, a chemical in the brain that regulates neurochemicals, lead to abnormally high levels of glutamate and dopamine. This may be the underlying cause of both PLMS and hyperarousal in RLS. The researchers have also pinpointed a subtype of receptor in the brain – the dopamine D4 receptor – as a new and better target for dopamine drug development.

These findings are the culmination of a four-year study funded by the RLS Foundation, and were recently published in Annals of Neurology¹ and Frontiers in Neuroscience.²

Building on decades of RLS research

Dr. Ferré’s research into RLS builds on decades of science by others on the roles of dopamine and glutamate in the brain for those who suffer from RLS.

It has long been believed that RLS is related to a malfunction in the way the brain uses dopamine. Dopamine is a neurotransmitter (a brain chemical that sends messages) associated with muscle activity and movement. Dopamine medications for RLS act by decreasing the amount of dopamine produced by neurons (brain cells). This has the effect of reducing PLMS. However, use of dopamine medications is limited by the serious side effect of augmentation.

In 2013, scientists at Johns Hopkins identified the involvement of another neurotransmitter – glutamate, which is associated with arousal (wakefulness). Researchers found elevated levels of glutamate in the brains of people who had RLS. The extra glutamate is thought to contribute to PLMS – and to hyperarousal. Many RLS patients take alpha-2-delta medications to control their symptoms by reducing glutamate levels.

In the current study, Dr. Ferré’s laboratory used its first RLS Foundation grant to create an animal model that will help develop and test new RLS treatments. A rodent was fed an iron-deficient diet to mirror the brain iron deficiency found in people with RLS. The researchers invented a method that combines optogenetics and microdialysis. They attached a light-activated protein to specific neurons between the cortex and striatum in the brain of the rodent. When they shone a light on the neurons, this triggered the release of glutamate – which was greater in the animals with iron-deficiency than in animals fed a regular diet.

“The first finding was to show that in the rodent with brain iron deficiency, a well-accepted pathogenetic model of RLS, we were able to show that brain iron deficiency produces increased sensitivity of the cortico-striatal terminals to release glutamate,” says Dr. Ferré. Recently, Dr. Ferré’s team tested this animal model with several RLS drugs: the dopamine agonists pramipexole and ropinirole, and the alpha-2-delta drug gabapentin. The researchers validated that these medications work by counteracting the release of glutamate.

In the process, the team also pinpointed a specific type of protein, called the dopamine D4 receptor, that controls the transmission of glutamate in these cells. This protein may be an effective target for future dopamine medications. “Thanks to this mechanism, we were able to find which subtype of dopamine receptor needs to be targeted,” says Dr. Ferré. “I believe that augmentation is acting on other subtypes. That opens up new hope for therapeutic approaches that probably will have less secondary problems like augmentation and will be more successful.”

Uncovering a new player: Adenosine

If RLS medications work by reversing unusually high dopamine and glutamate levels, how could these high levels be prevented in the first place?

That is the scientific question – and Dr. Ferré feels he may have found an answer in adenosine. Adenosine is a neurotransmitter that regulates the levels of other chemicals; it acts as a brake for the dopamine and glutamate systems in the brain.

Using the animal model, Dr. Ferré’s team found that when there are fewer adenosine receptors (specifically of the A1 subtype), there is increased dopamine and glutamate. This links adenosine to PLMS symptoms.

But there’s more. “Adenosine by the way, regulates the sleepiness associated with prolonged wakefulness,” explains Dr. Ferré. In the cortex, thalamus and other areas of the brain, adenosine modulates homeostatic sleep. Low levels of adenosine receptors could explain the hyperarousal of RLS. “The beauty of the adenosine story is that it connects the hyperarousal symptoms with movement [PLMS],” he says.

“What is really important about our story is that maybe we have found a mechanism – the adenosine system – that links brain iron deficiency to hyperglutamatergic and hyperdopaminergic states.”

This insight can be used to potentially develop RLS treatments that increase the concentration of adenosine to prevent both PLMS and hyperarousal. One such drug is already available clinically: dipyridamole, an antiplatelet drug that protects against stroke and heart attack.

Dr. Ferré’s team validated dipyridamole using the animal model. He also collaborated with Diego García-Borreguero, MD, PhD, director of the Sleep Research Institute in Madrid, to trial the drug in a small group of RLS patients – with promising results. As a next step, Dr. García- Borreguero will coordinate a larger clinical trial in Europe.

Dr. Ferré is also working with several research laboratories to pursue drugs similar to dipyridamole that are more potent and can more easily cross the blood-brain barrier.

“The story now reaches the clinical level,” says Dr. Ferré. “Anything else that could modify the release of glutamate by those terminals could theoretically be good therapeutically for RLS. That’s the case with dipyridamole. We just got results that validate once more our animal model and, in fact, dipyridamole decreases glutamate release by cortico- striatal neuronal terminals.”

Study Results At a Glance

• Using a rodent model of RLS, the researchers showed that people with RLS may have an increased sensitivity in specific brain cells to release dopamine and glutamate.
• They also found that a decrease in the concentration of receptors for adenosine in the brain may cause higher levels of dopamine and glutamate, explaining movement-related RLS symptoms – namely, PLMS.
• Since adenosine also regulates sleep, the low concentration of adenosine may also explain the hyperarousal symptom in RLS.
• Therapies that increase adenosine may help with RLS. Dipyridamole may be one, and others can be developed.
• The researchers also pinpointed a subtype of protein in the brain that can be targeted by dopamine medications, potentially without augmentation as a side effect.

Thanks to the RLS Foundation

Dr. Ferré credits the RLS Foundation with making the last four years of progress in his laboratory possible. “The RLS Foundation research grant has made what we’ve done so far possible. Above all, it has made it possible for a scientist on our team, Gabriel Yepes, to devote his time entirely to this project. And it has been a complete success.”

He also points to the Foundation’s role in cultivating a community of scientists dedicated to solving the mysteries of RLS who support each other’s work. Forty-four researchers have received grants from the RLS Foundation since 1997, and nine have served on the Foundation’s advisory board. Their work spans several continents and many disciplines. “Thanks to the RLS Foundation, I can connect to different specialties quickly and get their input right away,” says Dr. Ferré.

“We are much closer to understanding the mechanisms of RLS. If you understand the mechanisms, that directs your research,” he adds. “We are very close to getting new approaches – not just new molecules that are acting the same way. We are discovering new mechanisms, and that should open a line to new approaches. To hope.”

¹Yepes G, Guitart X, Rea W, Newman AH, Allen RP, Earley CJ, Quiroz C, Ferré S. Dec 2017. “Targeting hypersensitive corticostriatal terminals in restless legs syndrome.” Ann Neurol 82(6): 951–60. doi:10.1002/ana.25104.
²Ferré S, Quiroz C, Guitart X, Rea W, Seyedian A, Moreno E, Casadó-Anguera V, Díaz-Ríos M, Casadó V, Clemens S, Allen RP, Earley CJ, García-Borreguero D. 2018. “Pivotal Role of Adenosine Neurotransmission in Restless Legs Syndrome.” Front Neurosci 11: 722. doi:10.3389/fnins.2017.00722.