Research Needs Rodent Models of Restless Legs Syndrome, But Assessing RLS in Mice Is Tricky

The International Restless Legs Syndrome Study Group publishes a new guideline on construct validity of rodent models of RLS.

By Jane Kollmer

How do you know if a mouse has restless legs syndrome? It’s a seemingly strange question, but one that is essential to restless legs syndrome (RLS) research because before studies can be conducted in humans, they first need to be validated in rodents.

Now, researchers are buckling down on consistent guidelines that can identify rodents that have true RLS. This year, scientists from the International Restless Legs Syndrome Study Group (IRLSSG) worked as a task force to pin down objective diagnostic biomarkers of RLS in rodents, information that is pivotal for the development of valid animal models.

In August, the task force released a new guideline on the construct validity of rodent models of RLS, detailing 20 biological mechanisms considered to have sufficient support in the literature.1

The scientists grouped the items by sex and genetic factors, iron-related mechanisms, electrophysiological mechanisms, dopaminergic mechanisms, exposure to medications active in the central nervous system, and others.1 The new guideline, which expands on the task force’s previous rodent-model paper in 2021, is a detailed look at how RLS might manifest.2

Not surprisingly, many of the signs of RLS highlighted by the new guideline are seen in the brain. Low iron levels in the brain, increased dopamine synthesis, hyperexcitability of the motor cortex, and increased neuronal activity in the thalamus and cerebellum are all mentioned as potential signs of the disorder.

Other iron-related mechanisms are also mentioned, including peripheral iron deficiency, low levels of cerebrospinal fluid (CSF) ferritin, high levels of CSF transferrin and a reduced CSF ferritin-to-plasma ferritin ratio.2

Overall, the researchers hope that the guidelines to construct the validity of RLS animal models can be used to evaluate previously published research and to generate novel RLS rodent models. Ultimately, they say, this work can move the needle forward on caring for patients by developing better therapeutics.

“Better animal models should lead to better treatments,” says corresponding author Sergi Ferré, MD, PhD, chief of the Integrative Neurobiology Section of the Intramural Research Program at the National Institute on Drug Abuse at the National Institutes of Health.

“Using best-validated models of RLS should promote new discoveries at the preclinical level, which should not only allow a better understanding of the causes and mechanisms involved but also provide new therapeutic strategies. Those new strategies are very necessary in view of the non-wanted secondary effects of the available treatments, such as the ‘augmentation’ phenomenon, which is an increase in the symptomatology after long-term treatment with dopaminergic agents.

Importantly, preclinical and clinical research should validate each other in a continuous dynamic process. The more precise the animal model, the better its predictive value. The rodent model can show changes in brain biochemistry or physiology that are subsequently observed in the human condition, and it can lead to a new successful treatment. And the other way around, newly discovered risk and triggering factors and pathophysiological mechanisms in RLS patients should also be observed in the animal model or lead to better rodent models.

“While the first paper set out to find established parameters that guide us toward animal models, the second paper dives deeper into the experimental approach to probe for the biological construct of restless legs syndrome,” says second guideline author Stefan Clemens, PhD, associate professor of physiology at East Carolina University, Brody School of Medicine and a member of IRLSSG.

He says, “The crutch is now, how can we understand what’s going on in the mouse model and what could be the implications for RLS patients? If we treat these animals with a certain drug or some intervention, could that potentially have a beneficial effect on humans down the line? Possibly. But before—between those experiments and the clinical trial—it will be a couple of other steps to follow through.”

The starting point for the latest guidelines was to determine the most widely accepted risk and triggering factors, as well as the pathophysiological mechanisms of RLS.

At first, the investigators listed a total of 51 options that could be considered RLS risk factors, triggering factors, and pathophysiological mechanisms. Upon a closer examination of the evidence, they whittled it down.1

Selected factors and biological mechanisms were then evaluated for their possible translation in rodent models of RLS. Finally, they proposed guidelines on how to both measure and induce or reproduce these factors in rodent models.1

The resulting paper, along with the 2021 rodent-model paper,2 provide a complete guide for the proper use of rodent models of RLS.

Subsequently, this research may lead to the development of novel and more effective treatments that could improve the quality of life for the millions of people with the disorder.

“It’s a very, very complicated disease, not only to treat, but also because the severity is affected by many different factors,” says Brian B. Koo, MD, associate professor of medicine and director at the sleep medicine laboratory at the Connecticut Veterans Affairs Healthcare System and director of the Yale Center for Restless Legs Syndrome, who was not affiliated with the task force. “Trying to figure out what causes it is very complex, and I think that this is probably one of the reasons why it has eluded scientists for so long.”

The guidelines are based on the present understanding of RLS pathophysiology and should be revised in the future, when new data and approaches become available, the paper authors state.1

“With every new study and with every new finding, the needle moves a little bit,” Koo says. “We have a long way to go because we still don’t know why people have these funny feelings, but I think we are making progress.”

Jane Kollmer is a freelance writer based in Chicago.

References

1. Salminen AV, Clemens S, García-Borreguero D, et al. Consensus guidelines on the construct validity of rodent models of restless legs syndrome. Dis Model Mech. 2022;15(8):dmm049615.

2. Salminen AV, Silvani A, Allen RP, et al. Consensus guidelines on rodent models of restless legs syndrome. Move Discord. 2021 Mar;36(3):558-69.

Illustration 137394117 © Paul Carlsen | Dreamstime.com

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