In search of alternatives to dopaminergic ligands for the treatment of restless legs syndrome: iron, glutamate, and adenosine

Self-administered non-invasive vagus nerve stimulation therapy for severe pharmacoresistant restless legs syndrome: outcomes at 6 months

Severe pharmacoresistant restless legs syndrome (RLS) is difficult to manage and a source of suffering to patients. We studied the effectiveness at 6 months of an innovative treatment: transauricular vagus nerve stimulation (taVNS) in the left cymba concha in a case series of 15 patients, 53% male, mean (SD) age 62.7 (12.3) years with severe pharmacoresistant RLS (mean [SD] International Restless Legs Rating Scale [IRLS] score of 31.9 [2.9]) at baseline. Following an 8-week non-randomised hospital-based study with eight 1-h sessions of taVNS, patients were trained to administer taVNS at home and were followed up for 6 months. The primary outcome measure was the IRLS score, secondary outcome measures were quality of life, mood disorders using the Hospital Anxiety and Depression scale (HAD) subscales for depression (HADD) and anxiety (HADA). At the 6-month follow-up 13/15 patients continued to use weekly taVNS. Symptom severity decreased (mean [SD] IRLS score 22.2 [9.32] at 6 months, p = 0.0005). Four of the 15 patients had an IRLS score of <20 at 6 months and two an IRLS score of 5. Quality of life significantly improved compared to baseline (mean [SD] score at baseline 49.3 [18.1] versus 65.66 [22.58] at 6 months, p = 0.0005) as did anxiety and depression symptoms (mean [SD] HADA score at baseline 8.9 [5.4] versus 7.53 [4.42] at 6 months, p = 0.029; and HADD score at baseline 5.2 [4.5] versus 4.73 [4.44] at 6 months, p = 0.03). Treatment was well tolerated, and no adverse events were reported. Our case series shows a potential role for self-administered taVNS in patients with severe pharmacoresistant RLS. Randomised controlled trials are needed to confirm the utility of taVNS.

Vitamin D and Restless Legs Syndrome: A Review of Current Literature

This review presents a detailed summary of the current literature regarding RLS and vitamin D deficiency. To our knowledge it is the first review of its kind. We review the prevalence of vitamin D deficiency in RLS as well as the evidence for the use of vitamin D supplementation in RLS management. We further examine the literature for proteomic and genetic evidence of a role for vitamin D in the pathogenesis of RLS. An alteration in vitamin D binding protein in RLS is one of the most consistent findings in the proteomic studies. Furthermore, we examine the interaction of vitamin D with calcium, phosphorus, and parathyroid hormone and the possible role of these connections in RLS. We also explore the possible nexus between RLS and vitamin D in renal disease, cardiovascular and cerebrovascular disease as well as inflammation. In addition, we review the potential interaction between vitamin D and RLS with iron, dopamine and other neurotransmitter systems including the endogenous opiate, serotoninergic, glutamatergic and adenosinergic systems. We also explore the role of vitamin D in RLS Augmentation (i.e., the paradoxical worsening of RLS symptoms when dopaminergic agents are used as a therapy for RLS). Although the literature is not entirely consistent in affirming vitamin D deficiency in RLS or the amelioration of RLS symptoms with vitamin D therapy, the collective studies overall indicate that vitamin D deficiency is common enough in RLS patients to suggest that RLS patients should have their vitamin D levels checked and any deficiency corrected as a standard of care.

Highlights 

Patients with Restless Legs Syndrome (RLS) may be deficient in vitamin D and therapy with vitamin D may ameliorate RLS. We present the first review dedicated solely to evaluating the relationship between RLS and vitamin D and present a case for the role of vitamin D in RLS pathogenesis.

Noninvasive Vagus Nerve Stimulation: A New Therapeutic Approach for Pharmacoresistant Restless Legs Syndrome

AIMS

This work aimed to study the effect of noninvasive vagus nerve stimulation on severe restless legs syndrome (RLS) resistant to pharmacotherapy.

MATERIALS AND METHODS

Patients with severe pharmacoresistant RLS were recruited from a tertiary care sleep center. Intervention was one-hour weekly sessions of transauricular vagus nerve stimulation (tVNS) in the left cymba concha, for eight weeks. The primary outcome measure was the score on the International Restless Legs Rating Scale (IRLS); secondary outcome measures were quality of life (Restless Legs Syndrome Quality of Life scale [RLSQOL]), mood disorders using the Hospital Anxiety and Depression scale subscale for depression (HADD) and Hospital Anxiety and Depression scale subscale for anxiety (HADA), and objective sleep latency, sleep duration, efficiency, and leg movement time measured by actigraphy.

RESULTS

Fifteen patients, 53% male, aged mean 62.7 ± 12.3 years with severe RLS, reduced quality of life, and symptoms of anxiety and depression, were included. The IRLS improved from baseline to session eight: IRLS 31.9 ± 2.9 vs 24.6 ± 5.9 p = 0.0003. Of these participants, 27% (4/15) had a total response with a decrease below an IRLS score of 20; 40% (6/15) a partial response with an improvement in the IRLS > 5 but an IRLS above 20; and 33% (5/15) were nonresponders. After tVNS, quality of life improved (RLSQOL 49.3 ± 18.1 vs 80.0 ± 19.6 p = 0.0005), as did anxiety (HADA 8.9 ± 5.4 vs 6.2 ± 5.0 p = 0.001) and depression (HADD 5.2 ± 4.5 vs 4.0 ± 4.0 p = 0.01). No significant change was found in actigraphic outcome measures.

CONCLUSIONS

In this pilot study, tVNS improved the symptoms of RLS in 66% of participants (10/15) with severe pharmacoresistant RLS, with concomitant improvements in quality of life and mood. Randomized controlled trials evaluating therapeutic efficacy of tVNS in RLS are needed to confirm these promising findings.

Single‐Vesicle Electrochemistry Following Repetitive Stimulation Reveals a Mechanism for Plasticity Changes with Iron Deficiency

Deficiency of iron, the most abundant transition metal in the brain and important for neuronal activity, is known to affect synaptic plasticity, causing learning and memory deficits. How iron deficiency impacts plasticity by altering neurotransmission at the cellular level is not fully understood. We used electrochemical methods to study the effect of iron deficiency on plasticity with repetitive stimulation. We show that during iron deficiency, repetitive stimulation causes significant decrease in exocytotic release without changing vesicular content. This results in a lower fraction of release, opposite to the control group, upon repetitive stimulation. These changes were partially reversible by iron repletion. This finding suggests that iron deficiency has a negative effect on plasticity by decreasing the fraction of vesicular release in response to repetitive stimulation. This provides a putative mechanism for how iron deficiency modulates plasticity.

Emerging Treatments for Insomnia, Sleep Apnea, and Restless Leg Syndrome Among Dialysis Patients

Sleep disturbances are highly prevalent in patients with predialysis chronic kidney disease, end-stage kidney disease, and after a kidney transplant. They contribute to impairment in daily function and are associated with a high burden of physical and psychiatric symptoms, decreased quality of life, and increased morbidity and mortality. Sleep disturbances also may precipitate and accelerate kidney disease progression. They often evolve across the spectrum of kidney dysfunction and may persist or re-emerge in kidney transplant recipients. Investigation into the multifaceted and dynamic relationships between sleep disturbance and chronic kidney disease requires consideration of myriad contributors including the progression of kidney disease itself, the role of treatment via dialysis and kidney transplant, psychosocial factors, and underlying sleep disorders. Despite sleep disturbance being identified as a priority to address by patients and caregivers, sleep disorders including insomnia, sleep apnea, and restless leg syndrome remain under-recognized and undertreated, and innovation in their management remains modest. In this article, we review the relationships between sleep disturbance and kidney disease, the impact of sleep disturbance and sleep disorders on symptom burden and mental health, and treatment opportunities that may address overlapping symptoms across the spectrum of kidney disease and that could improve patient-related and clinical outcomes.

[Restless legs syndrome]

Restless legs syndrome (RLS) is seen widely in clinical practice. RLS commonly occurs at night time and presents with unpleasant or uncomfortable sensations in the legs that causes an urge to move them. This article describes the epidemiology, risk factors and pathophysiology of RLS. There is a detailed description of clinical presentations, diagnostic criteria and also management of RLS.

Pharmacologic Treatment of Restless Legs Syndrome

Restless legs syndrome (RLS)/Willis-Ekbom disease is a neurologic disorder characterized by a strong desire to move when at rest (usually in the evening) and paraesthesia in their lower legs. The most widely used therapies for first-line treatment of RLS are dopaminergic drugs; however, their long-term use can lead to augmentation. α2δ Ligands, opioids, iron, glutamatergic drugs, adenosine, and sleep aids have been investigated as alternatives. The pathogenesis of RLS is not well understood. Despite the efficacy of dopaminergic drugs in the treatment of this disorder, unlike in Parkinson’s disease dopaminergic cell loss in the substantia nigra has not been observed in RLS. The etiology of RLS is likely complex, involving multiple neural pathways. RLS-related genes identified in genome-wide association studies can provide insight into the mechanistic basis and pathophysiology of RLS. Here we review the current treatments and knowledge of the mechanisms underlying RLS.

Restless Legs Syndrome and Other Common Sleep-Related Movement Disorders

PURPOSE OF REVIEW

In this article, the different sleep-related movement disorders are discussed with special attention given to restless legs syndrome (RLS).

RECENT FINDINGS

The differential diagnosis of sleep-related movement disorders can often be challenging; therefore, it is essential to have accurate information to make a correct diagnosis. This article focuses on RLS, highlighting the change in the paradigm of initial treatment, the role played by iron (pathophysiologic and therapeutic), and how to approach possible complications occurring with long-term treatment.

SUMMARY

RLS is one of the most common neurologic conditions, and it is common in clinical practice to find patients experiencing symptoms suggestive of RLS. Neurologists must be careful and thorough in the diagnosis, excluding RLS mimics. The decisions regarding which specific sleep-related movement disorder is present and how it should be treated are important because in certain cases, especially in RLS, adverse effects and long-term complications are frequently reported with the use of certain drugs.

Sleep-Wake Disorders in Childhood

PURPOSE OF REVIEW

The presentation of sleep issues in childhood differs from the presentation in adulthood and may be more subtle. Sleep issues may affect children differently than adults, and distinct treatment approaches are often used in children.

RECENT FINDINGS

Sodium oxybate was approved by the US Food and Drug Administration (FDA) in October 2018 for an expanded indication of treatment of sleepiness or cataplexy in patients with narcolepsy type 1 or narcolepsy type 2 aged 7 years or older, with side effect and safety profiles similar to those seen in adults. Restless sleep disorder is a recently proposed entity in which restless sleep, daytime sleepiness, and often iron deficiency are observed, but children do not meet the criteria for restless legs syndrome or periodic limb movement disorder.

SUMMARY

Children's sleep is discussed in this article, including normal sleep patterns and effects of insufficient sleep. Sleep disorders of childhood are reviewed, including insomnia, obstructive sleep apnea, restless legs syndrome, parasomnias, narcolepsy, and Kleine-Levin syndrome. Children with neurologic issues or neurodevelopmental disorders frequently have sleep disorders arising from an interaction of heterogeneous factors. Further attention to sleep may often be warranted through a polysomnogram or referral to a pediatric sleep specialist. Sleep disorders may cause indelible effects on children's cognitive functioning, general health, and well-being, and awareness of sleep disorders is imperative for neurologists who treat children.

Emerging Concepts of the Pathophysiology and Adverse Outcomes of Restless Legs Syndrome

Restless legs syndrome (RLS), also known as Willis-Ekbom disease (WED), is a common neurological disorder affecting up to 5% to 10% of the population, but it remains an underdiagnosed condition. RLS/WED is characterized by uncomfortable sensations, mainly in the legs, which appear during inactivity and worsen in the evening or at night. The prevalence of RLS/WED and periodic leg movements (PLMs) is increased in patients with sleep-disordered breathing, particularly in those with OSA, the most common sleep disorder encountered in sleep centers. New advances in the pathophysiology of RLS/WED have shown important implications for various genetic markers, neurotransmitter dysfunction, and iron deficiency. A practical approach to RLS/WED management includes an accurate diagnosis, the identification of reversible contributing factors, and the use of nonpharmacological therapies, including iron substitution (oral or IV) therapy. Many pharmacological agents are effective for the treatment of RLS/WED. Until recently, the first-line treatment of RLS/WED consisted of low-dose dopamine agonists (DA). However, given the fact that DAs cause high rates of augmentation of symptoms, international guidelines recommend that whenever possible the initial treatment of choice should be an α2δ ligand, and avoidance of dopaminergic agents unless absolutely necessary. If necessary, the lowest effective dose should be used for only the shortest possible time. The symptoms of RLS/WED can disrupt the quality of sleep as well as the quality of life. IV iron therapy may be considered in patients with refractory RLS. A better understanding of RLS/WED pathophysiology will allow patients to receive tailored therapy, resulting in an improved quality of life.

Presynaptic inhibition in restless legs syndrome

Introduction: Restless legs syndrome (RLS) is a condition that particularly urges at night in resting and causes the need to move the legs. Although the pathophysiology has not yet been clarified, dopamine and iron metabolism and spinal cord pathologies are blamed for causing the condition. There are few studies on spinal reflex mechanisms on RLS. In the present study, we aimed to investigate the role of presynaptic inhibition (PreI) in the spinal cord in RLS.Methods: Fourteen patients with RLS and 14 controls with similar demographic characteristics were included in the study. Soleus muscle H-reflex (Ht) investigation was performed for subjects whose electrophysiologic investigation was normal. The Ht response was conditioned to the stimulation of the common peroneal nerve (CPN) (Hc). The test and conditioned stimulation intervals were kept between 10 ms, 20 ms, 30 ms, 40 ms, 50 ms, 75 ms, 100 ms, 150 ms and 200 ms. In each inter-stimulus interval, nonparametric repeat measurement evaluations were conducted with the percentage value of Hc/Ht. The Hc/Ht values of the study and control groups in the same intervals were compared separately.Results: A significant decrease was detected in Hc values in the control group in the repeat measurement values at 20 ms and 100 ms inter-stimulus intervals; however, there was not decrease in any intervals in the patient's group.Conclusion: The absence of any decrease in Hc reflexes for 20-100 ms intervals revealed that discernible PreI was vanished in RLS patients.

Restless legs syndrome: Clinical changes in nervous system excitability at the spinal cord level

Restless legs syndrome (RLS) is a complex multifactorial disorder whose aetiology has yet to be fully elucidated. Some of the features of RLS, such as processing of sensations and activation of movement, may result from a dysfunction in spinal processing giving rise to a state of spinal hyperexcitability. In the current article we review studies investigating spinal excitability in RLS patients looking specifically at electrophysiological studies of spinal activity, sensory evaluations, and spinal reflex studies. Increased spinal excitability has been shown in RLS patients based on the combined data from electrophysiological studies. Results from studies assessing sensory evaluations in RLS patients show enhanced spinal processing of nociceptive inputs possibly due to central sensitisation. However, not all sensory modalities demonstrate an increase in sensitivity. An increase in nervous system excitability would result in an increase in reflex responses in RLS patients however the data from reflex analyses in RLS patients has failed to consistently show this expected result. Overall changes to RLS spinal excitability have been demonstrated though these changes might be heterogeneous as not all afferent input appears to be affected in the same manner. There may be phase-dependent and modality-dependent alterations in spinal excitability suggesting that the theory of absolute spinal hyperexcitability in RLS patients' needs to be reconsidered.

Non-dopaminergic vs. dopaminergic treatment options in restless legs syndrome

Two types of drugs have been extensively investigated for the treatment of restless legs syndrome (RLS)/Willis-Ekbom disease (WED): dopamine agonists and α2δ ligands to the α2δ subunit of calcium channels. Comparative studies show that both classes of drugs are similarly effective in treating RLS symptoms over the short- and long-term. While dopamine agonists are more effective in treating periodic limb movements (PLMs), α2δ ligands are more effective in consolidating sleep. However, given the fact that dopamine agonists cause high rates of augmentation of symptoms, recent international guidelines recommend that whenever possible the initial treatment of choice should be an α2δ ligand. In fact, the most effective preventive strategy involves not using dopaminergic agents unless absolutely necessary. Indeed, should dopaminergic treatment be needed to handle the symptoms effectively, then it is recommended that the dopaminergic load be reduced by using the lowest effective dose for the shortest possible period of time. However, it must be taken into account that the only α2δ ligand approved for RLS/WED is gabapentin enacarbil, which is not yet available in Europe. Furthermore, recent studies have also reported on the efficacy of opioids as a second-line treatment of RLS/WED, following treatment failure with dopamine agonists. Recent guidelines have taken these new data into account and highlight that a low dose of an opioid (prolonged-release oxycodone or methadone) may be considered in patients with very severe augmentation of symptoms. Alternative non-dopaminergic treatment concepts based on glutamatergic and adenosinergic mechanisms are currently in development, and are likely to provide encouraging therapeutic alternatives.

The heterotetrameric structure of the adenosine A1-dopamine D1 receptor complex: Pharmacological implication for restless legs syndrome

Dopaminergic and purinergic signaling play a pivotal role in neurological diseases associated with motor symptoms, including Parkinson's disease (PD), multiple sclerosis, amyotrophic lateral sclerosis, Huntington disease, Restless Legs Syndrome (RLS), spinal cord injury (SCI), and ataxias. Extracellular dopamine and adenosine exert their functions interacting with specific dopamine (DR) or adenosine (AR) receptors, respectively, expressed on the surface of target cells. These receptors are members of the family A of G protein-coupled receptors (GPCRs), which is the largest protein superfamily in mammalian genomes. GPCRs are target of about 40% of all current marketed drugs, highlighting their importance in clinical medicine. The striatum receives the densest dopamine innervations and contains the highest density of dopamine receptors. The modulatory role of adenosine on dopaminergic transmission depends largely on the existence of antagonistic interactions mediated by specific subtypes of DRs and ARs, the so-called A_2AR-D₂R and A₁R-D₁R interactions. Due to the dopamine/adenosine antagonism in the CNS, it was proposed that ARs and DRs could form heteromers in the neuronal cell surface. Therefore, adenosine can affect dopaminergic signaling through receptor-receptor interactions and by modulations in their shared intracellular pathways in the striatum and spinal cord. In this work we describe the allosteric modulations between GPCR protomers, focusing in those of adenosine and dopamine within the A₁R-D₁R heteromeric complex, which is involved in RLS. We also propose that the knowledge about the intricate allosteric interactions within the A₁R-D₁R heterotetramer, may facilitate the treatment of motor alterations, not only when the dopamine pathway is hyperactivated (RLS, chorea, etc.) but also when motor function is decreased (SCI, aging, PD, etc.).

Comorbidities, treatment, and pathophysiology in restless legs syndrome

Restless legs syndrome, also known as Willis-Ekbom disease, is a common neurological condition whose manifestation is affected by complex environmental and genetic interactions. Restless legs syndrome can occur on its own, mostly at a young age, or with comorbidities such as cardiovascular disease, diabetes, and arterial hypertension, making it a difficult condition to properly diagnose. However, the concept of restless legs syndrome as being two entities, primary or secondary to another condition, has been challenged with genetic data providing further insight into the pathophysiology of the condition. Although dopaminergic treatment was formerly the first-line therapy, prolonged use can result in a serious worsening of symptoms known as augmentation. Clinical studies on pregabalin, gabapentin enacarbil, oxycodone-naloxone, and iron preparations have provided new treatment options, but most patients still report inadequate long-term management of symptoms. Studies of the hypoxic pathway activation and iron deficiency have provided valuable information about the pathophysiology of restless legs syndrome that should now be translated into new, more effective treatments for restless legs syndrome.

Differential Dopamine D1 and D3 Receptor Modulation and Expression in the Spinal Cord of Two Mouse Models of Restless Legs Syndrome

Restless Legs Syndrome (RLS) is often and successfully treated with dopamine receptor agonists that target the inhibitory D3 receptor subtype, however there is no clinical evidence of a D3 receptor dysfunction in RLS patients. In contrast, genome-wide association studies in RLS patients have established that a mutation of the MEIS1 gene is associated with an increased risk in developing RLS, but the effect of MEIS1 dysfunction on sensorimotor function remain unknown. Mouse models for a dysfunctional D3 receptor (D3KO) and Meis1 (Meis1KO) were developed independently, and each animal expresses some features associated with RLS in the clinic, but they have not been compared in their responsiveness to treatment options used in the clinic. We here confirm that D3KO and Meis1KO animals show increased locomotor activities, but that only D3KO show an increased sensory excitability to thermal stimuli. Next we compared the effects of dopaminergics and opioids in both animal models, and we assessed D1 and D3 dopamine receptor expression in the spinal cord, the gateway for sensorimotor processing. We found that Meis1KO share most of the tested behavioral properties with their wild type (WT) controls, including the modulation of the thermal pain withdrawal reflex by morphine, L-DOPA and D3 receptor (D3R) agonists and antagonists. However, Meis1KO and D3KO were behaviorally more similar to each other than to WT when tested with D1 receptor (D1R) agonists and antagonists. Subsequent Western blot analyses of D1R and D3R protein expression in the spinal cord revealed a significant increase in D1R but not D3R expression in Meis1KO and D3KO over WT controls. As the D3R is mostly present in the dorsal spinal cord where it has been shown to modulate sensory pathways, while activation of the D1Rs can activate motoneurons in the ventral spinal cord, we speculate that D3KO and Meis1KO represent two complementary animal models for RLS, in which the mechanisms of sensory (D3R-mediated) and motor (D1R-mediated) dysfunctions can be differentially explored.

An update on the treatment of Restless Legs Syndrome/Willis-Ekbom Disease: prospects and challenges

INTRODUCTION

Restless Legs Syndrome/Willis-Ekbom Disease (RLS/WED) is a sleep disorder characterized by an urge to move the legs, frequently associated or triggered by unpleasant sensations in the lower limbs that affects approximately 2.5% of adults. Therapy and management of RLS/WED require long-term interventions, since the typical manifestation of this disorder is chronic. Areas covered: In this review, we provide an update regarding the treatment of RLS/WED with particular attention to future challenges for its management. We reviewed a large variety of treatments studied in clinical trials and supported by the most updated guidelines. Alongside with first-line interventions other pharmacological options including opioids, benzodiazepines, iron therapy, and newly studied drugs are discussed. Furthermore, due to the occurrence of augmentation and worsening of symptoms we also reviewed the development of non-pharmacologic alternatives. Expert commentary: The management of RLS/WED is a challenge because of different long-term issues. Several complications, such as loss of the therapeutic effect of dopaminergic or non-dopaminergic agents and augmentation, are still unsolved concerns. However, the development of new drugs acting on adenosinergic and glutamatergic neurotransmission seems promising. Randomized controlled trials are needed in order to recognize effectiveness of new drugs or non-pharmacological treatment strategies.

Purinergic Receptors in Neurological Diseases With Motor Symptoms: Targets for Therapy

Since proving adenosine triphosphate (ATP) functions as a neurotransmitter in neuron/glia interactions, the purinergic system has been more intensely studied within the scope of the central nervous system. In neurological disorders with associated motor symptoms, including Parkinson's disease (PD), motor neuron diseases (MND), multiple sclerosis (MS), amyotrophic lateral sclerosis (ALS), Huntington's Disease (HD), restless leg syndrome (RLS), and ataxias, alterations in purinergic receptor expression and activity have been noted, indicating a potential role for this system in disease etiology and progression. In neurodegenerative conditions, neural cell death provokes extensive ATP release and alters calcium signaling through purinergic receptor modulation. Consequently, neuroinflammatory responses, excitotoxicity and apoptosis are directly or indirectly induced. This review analyzes currently available data, which suggests involvement of the purinergic system in neuro-associated motor dysfunctions and underlying mechanisms. Possible targets for pharmacological interventions are also discussed.

Pivotal Role of Adenosine Neurotransmission in Restless Legs Syndrome

The symptomatology of Restless Legs Syndrome (RLS) includes periodic leg movements during sleep (PLMS), dysesthesias, and hyperarousal. Alterations in the dopaminergic system, a presynaptic hyperdopaminergic state, seem to be involved in PLMS, while alterations in glutamatergic neurotransmission, a presynaptic hyperglutamatergic state, seem to be involved in hyperarousal and also PLMS. Brain iron deficiency (BID) is well-recognized as a main initial pathophysiological mechanism of RLS. BID in rodents have provided a pathogenetic model of RLS that recapitulates the biochemical alterations of the dopaminergic system of RLS, although without PLMS-like motor abnormalities. On the other hand, BID in rodents reproduces the circadian sleep architecture of RLS, indicating the model could provide clues for the hyperglutamatergic state in RLS. We recently showed that BID in rodents is associated with changes in adenosinergic transmission, with downregulation of adenosine A₁ receptors (A1R) as the most sensitive biochemical finding. It was hypothesized that A1R downregulation leads to hypersensitive striatal glutamatergic terminals and facilitation of striatal dopamine release. Hypersensitivity of striatal glutamatergic terminals was demonstrated by an optogenetic-microdialysis approach in the rodent with BID, indicating that it could represent a main pathogenetic factor that leads to PLMS in RLS. In fact, the dopaminergic agonists pramipexole and ropinirole and the α₂δ ligand gabapentin, used in the initial symptomatic treatment of RLS, completely counteracted optogenetically-induced glutamate release from both normal and BID-induced hypersensitive corticostriatal glutamatergic terminals. It is a main tenet of this essay that, in RLS, a single alteration in the adenosinergic system, downregulation of A1R, disrupts the adenosine-dopamine-glutamate balance uniquely controlled by adenosine and dopamine receptor heteromers in the striatum and also the A1R-mediated inhibitory control of glutamatergic neurotransmission in the cortex and other non-striatal brain areas, which altogether determine both PLMS and hyperarousal. Since A1R agonists would be associated with severe cardiovascular effects, it was hypothesized that inhibitors of nucleoside equilibrative transporters, such as dipyridamole, by increasing the tonic A1R activation mediated by endogenous adenosine, could represent a new alternative therapeutic strategy for RLS. In fact, preliminary clinical data indicate that dipyridamole can significantly improve the symptomatology of RLS.

Targeting hypersensitive corticostriatal terminals in restless legs syndrome

OBJECTIVE

The first aim was to demonstrate a previously hypothesized increased sensitivity of corticostriatal glutamatergic terminals in the rodent with brain iron deficiency (BID), a pathogenetic model of restless legs syndrome (RLS). The second aim was to determine whether these putative hypersensitive terminals could constitute a significant target for drugs effective in RLS, including dopamine agonists (pramipexole and ropinirole) and α₂ δ ligands (gabapentin).

METHODS

A recently introduced in vivo optogenetic-microdialysis approach was used, which allows the measurement of the extracellular concentration of glutamate upon local light-induced stimulation of corticostriatal glutamatergic terminals. The method also allows analysis of the effect of local perfusion of compounds within the same area being sampled for glutamate.

RESULTS

BID rats showed hypersensitivity of corticostriatal glutamatergic terminals (lower frequency of optogenetic stimulation to induce glutamate release). Both hypersensitive and control glutamatergic terminals were significant targets for locally perfused pramipexole, ropinirole, and gabapentin, which significantly counteracted optogenetically induced glutamate release. The use of selective antagonists demonstrated the involvement of dopamine D₄ and D₂ receptor subtypes in the effects of pramipexole.

INTERPRETATION

Hypersensitivity of corticostriatal glutamatergic terminals can constitute a main pathogenetic mechanism of RLS symptoms. Selective D₄ receptor agonists, by specifically targeting these terminals, should provide a new efficient treatment with fewer secondary effects. Ann Neurol 2017;82:951-960.

Rotigotine transdermal patch and sleep in Parkinson's disease: where are we now?

A wide range of sleep dysfunction complicates Parkinson’s disease during its course from prodromal to palliative stage. It is now increasingly acknowledged that sleep disturbances are thus integral to the disease and pose a significant burden impacting on quality of life of patients. Sleep fragmentation, restless legs syndrome, nocturia, and nocturnal pain are regarded as one of the main components of night-time sleep dysfunction with possible secondary impact on cognition and well-being. The role of dopaminergic therapies, particularly using a continuous drug delivery strategy in managing some of these sleep issues, have been reported but the overall concept remains unclear. This review provides an overview of several aspects of night-time sleep dysfunction in Parkinson’s disease and describes all available published open-label and blinded studies that investigated the use of rotigotine transdermal patch targeting sleep. Blinded studies have suggested beneficial effects of rotigotine transdermal patch on maintenance insomnia and restless legs syndrome in Parkinson’s disease patients. Open-label studies support these observations and also suggest beneficial effects on nocturia and nocturnal pain.