Thalamic GABA may modulate cognitive control in restless legs syndrome

Unraveling the pathophysiology of restless legs syndrome from multimodal MRI techniques: A systematic review

BACKGROUND

Restless Legs Syndrome (RLS) is a common neurological disorder currently diagnosed based on clinical features only, and characterized by a compulsive urge to move the legs triggered by rest or diminished arousal. This systematic review aimed at integrating all current brain magnetic resonance imaging (MRI) modalities for a convergent pathophysiological understanding of RLS phenomenology.

METHODS

We performed a MEDLINE (PubMed)-based systematic review for research articles in patients with primary RLS published in English from 2010 till November 2023. Studies meeting the inclusion criteria according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses criteria were systematically assessed for quality using modality-specific checklists, bias using AXIS tool and a narrative synthesis of the results was conducted.

RESULTS

A total of 49 studies (22 structural, 12 DTI, 7 iron-imaging, 4 spectroscopy with 10 datasets combining multiple approaches) involving 1273 patients (414 males) and 1333 healthy controls (478 males) met the eligibility criteria. Despite participant, technical/device-related and statistical heterogeneity, most agree that patients with primary RLS have structural and metabolite alterations, changes in multiple white matter tract architectures, and disrupted functional connectivity within multiple brain areas. Most of the studies (n = 43, 88 %) have a low-risk of bias on the AXIS scale. Scores on the modality-specific checklist ranged from 46 to 92 %, 70-93 % and 54-92 % for structural MRI, DTI and MRS Datasets, respectively.

CONCLUSIONS

Notwithstanding the large heterogeneity in the methods employed, global connectivity alterations suggest the utility of casting RLS within a system-level perspective rather than viewing it as related to the dysfunction of a single or particular brain region. A holistic approach and its integration within the framework of molecular vulnerability and neurotransmitter alterations are warranted to disentangle the complex pathophysiology of RLS and to identify new therapeutic targets.

Sensory aspects of restless legs syndrome: Clinical, neurophysiological and neuroimaging prospectives

Restless Legs Syndrome (RLS) is a complex sensorimotor disorder, classified among the sleep-related movement disorders. Although sensory symptoms appear as key features of the disorder, they are still poorly characterized from a clinical perspective and conceptualized from a pathophysiological point of view. In this review, we aim to describe the clinical and functional substrates of RLS, focusing mainly on its sensory symptoms and on their neurophysiological and anatomical correlates. Knowledge of both subjective sensory symptoms and objective sensory signs are still controversial. Current data also indicate that the sensory component of RLS seems to be subserved by anomalies of sensorimotor integration and by mechanism of central sensitization. Overall, electrophysiological findings highlight the involvement of multiple generators in the pathogenesis of RLS, eventually resulting in an increased nervous system excitability and/or alterations in inhibition within the somatosensory and nociceptive pathways. Structural and functional neuroimaging data show the involvement of several crucial areas and circuits, among which the thalamus appears to play a pivotal role. A holistic approach looking at brain connectivity, structural or functional abnormalities, and their interplay with molecular vulnerability and neurotransmitter alterations is warranted to disentangle the complex framework of RLS.

A Historical Overview of the Role of Benzodiazepines including Clonazepam in the Treatment of Adult Restless Legs Syndrome and Periodic Limb Movements in Sleep

In a recent survey of 16,694 people receiving treatment for Restless Legs Syndrome (RLS), approximately 25% were treated with benzodiazepines either singly or in combination with other RLS treatments. Because of the large number of people receiving benzodiazepines for treatment of RLS, we conducted a historical overview of the therapeutic role of benzodiazepines in RLS and its associated condition Periodic Limb Movements in Sleep (PLMS). We found 17 articles on the use of clonazepam in RLS, PLMS, or both, 3 on triazolam and PLMS, 1 on alprazolam and RLS, 1 on temazepam and PLMS, and 1 on nitrazepam and PLMS. The order of benefit of benzodiazepines from the summarized literature is Sleep>RLS>PLMS and arousals > PLMS. Most of the studies on clonazepam employed dosages of 0.5-2.0 mg. Dosages of 3 or 4 mg caused lethargy, somnolence and confusion. An epidemiological study on the therapy of RLS suggests that treatment of RLS with most types of RLS medications including benzodiazepines in combination with other RLS therapies lowers the future cardiovascular risk associated with RLS. The major effect of benzodiazepines is through potentiation of the effect of GABA on the GABA A receptor. Neuroimaging studies suggest that GABA is altered either positively or negatively in various brain regions in RLS and genetic studies suggest that there are alterations in the GABA receptor in RLS. These results suggest that medications with different GABAergic mechanisms such as tiagabine (Gabitril) or others should be investigated in RLS for their possible therapeutic benefit.

Highlights

Benzodiazepines are frequently used as therapy in Restless Legs Syndrome (RLS) and Periodic Limb Movements in Sleep. The order of benefit is Sleep>RLS>PLMS and arousals > PLMS. For clonazepam dosages of 0.5 mg-2.0 mg/day are most frequently employed. Benzodiazepines exert their therapeutic effect through GABA-ergic mechanisms.

Review of the role of the endogenous opioid and melanocortin systems in the restless legs syndrome

Restless legs syndrome (RLS) is responsive to opioid, dopaminergic and iron-based treatments. Receptor blocker studies in RLS patients suggest that the therapeutic efficacy of opioids is specific to the opioid receptor and mediated indirectly through the dopaminergic system. An RLS autopsy study reveals decreases in endogenous opioids, β-endorphin and perhaps Met-enkephalin in the thalamus of RLS patients. A total opioid receptor knock-out (mu, delta and kappa) and a mu-opioid receptor knock-out mouse model of RLS show circadian motor changes akin to RLS and, although both models show sensory changes, the mu-opioid receptor knock mouse shows circadian sensory changes closest to those seen in idiopathic RLS. Both models show changes in striatal dopamine, anaemia and low serum iron. However, only in the total receptor knock-out mouse do we see the decreases in serum ferritin that are normally found in RLS. There are also decreases in serum iron when wild-type mice are administered a mu-opioid receptor blocker. In addition, the mu-opioid receptor knock-out mouse also shows increases in striatal zinc paralleling similar changes in RLS. Adrenocorticotropic hormone and α-melanocyte stimulating hormone are derived from pro-opiomelanocortin as is β-endorphin. However, they cause RLS-like symptoms and periodic limb movements when injected intraventricularly into rats. These results collectively suggest that an endogenous opioid deficiency is pathogenetic to RLS and that an altered melanocortin system may be causal to RLS as well.

Exploration of restless legs syndrome under the new concept: A review

Restless leg syndrome (Restless legs syndrome, RLS) is a common neurological disorder. The pathogenesis of RLS remains unknown, and recent pathophysiological developments have shown the contribution of various genetic markers, neurotransmitter dysfunction, and iron deficiency to the disease, as well as other unidentified contributing mechanisms, particularly chronic renal dysfunction. RLS enhancement syndrome is frequently observed in patients with RLS who have received long-term dopamine agonist therapy, manifesting as a worsening of RLS symptoms, usually associated with an increase in the dose of dopamine agonist. Some patients with RLS can adequately control their symptoms with non-pharmacological measures such as massage and warm baths. First-line treatment options include iron supplementation for those with evidence of reduced iron stores, or gabapentin or pregabalin, as well as dopamine agonists, such as pramipexole. Second-line therapies include opioids such as tramadol. RLS seriously affects the quality of life of patients, and because its pathogenesis is unclear, more biological evidence and treatment methods need to be explored.

Sleep and homeostatic control of plasticity

Sleep homeostasis is a complex neurobiologic phenomenon involving a number of molecular pathways, neurotransmitter release, synaptic activity, and factors modulating neural networks. Sleep plasticity allows for homeostatic optimization of neural networks and the replay-based consolidation of specific circuits, especially important for cognition, behavior, and information processing. Furthermore, research is currently moving from an essentially brain-focused to a more comprehensive view involving other systems, such as the immune system, hormonal status, and metabolic pathways. When dysfunctional, these systems contribute to sleep loss and fragmentation as well as to sleep need. In this chapter, the implications of neural plasticity and sleep homeostasis for the diagnosis and treatment of some major sleep disorders, such as insomnia and sleep deprivation, obstructive sleep apnea syndrome, restless legs syndrome, REM sleep behavior disorder, and narcolepsy are discussed in detail with their therapeutical implications. This chapter highlights that sleep is necessary for the maintenance of an optimal brain function and is sensitive to both genetic background and environmental enrichment. Even in pathologic conditions, sleep acts as a resilient plastic state that consolidates prior information and prioritizes network activity for efficient brain functioning.

Association of Peripheral Plasma Neurotransmitters with Cognitive Performance in Chronic High-altitude Exposure

Long-term living at high altitude causes significant impairment of cognitive function. Central neurotransmitters are potential mediators of cognitive performance. We aimed to determine whether there were significant associations between select peripheral plasma neurotransmitters and cognitive performance in humans with chronic high-altitude (HA) exposure and to determine the association between peripheral plasma neurotransmitters and brain neurotransmitters in rats after chronic hypobaric hypoxia (HH) exposure. We demonstrated that 3,4-dihydroxy-L-phenylalanine (DOPA), dopamine, serotonin, 5-hydroxyindole-3-acetic acid (5-HIAA) and GABA in the peripheral plasma were associated with cognitive performance in humans with HA exposure. Consistent with this result, peripheral plasma DOPA, dopamine, serotonin, 5-HIAA and glutamate were associated with brain neurotransmitter levels after chronic HH exposure in rats. These results provide experimental data indicating that neurotransmitter levels and cognitive performance are modified in chronic high-altitude exposure, with a possible causal effect.

Cognitive profile in Restless Legs Syndrome: A signal-to-noise ratio account

Restless legs syndrome (RLS) is a common neurological disorder characterized by a sensorimotor condition, where patients feel an uncontrollable urge to move the lower limbs in the evening and/or during the night. RLS does not only have a profound impact on quality of life due to the disturbed night-time sleep, but there is growing evidence that untreated or insufficiently managed RLS might also cause cognitive changes in patients affected by this syndrome. It has been proposed that RLS is caused by alterations in the signal-to-noise ratio (SNR) and in dopamine (DA) neurotransmission in the nervous system. Based on this evidence, we propose the “SNR-DA hypothesis” as an explanation of how RLS could affect cognitive performance. According to this hypothesis, variations/reductions in the SNR underlie RLS-associated cognitive deficits, which follow an inverted U-shaped function: In unmedicated patients, low dopamine levels worsen the SNR, which eventually impairs cognition. Pharmacological treatment enhances DA levels in medicated patients, which likely improves/normalizes the SNR in case of optimal doses, thus restoring cognition to a normal level. However, overmedication might push patients past the optimal point on the inverted U-shaped curve, where an exaggerated SNR potentially impairs cognitive performance relying on cortical noise such as cognitive flexibility. Based on these assumptions of SNR alterations, we propose to directly measure neural noise via “1/f noise” and related metrics to use transcranial random noise stimulation (tRNS), a noninvasive brain stimulation method which manipulates the SNR, as a research tool and potential treatment option for RLS.

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Restless legs syndrome (RLS) is a common neurological disorder.

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RLS is caused by alterations in the SNR ratio and in DA neurotransmission.

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The SNR- DA hypothesis how RLS affects cognitive performance is presented.