An Update on the Neurochemistry of Essential Tremor

Essential tremor - drug treatments present and future

INTRODUCTION

The main treatment options for essential tremor (ET), which is probably one of the most common movement disorders, have been propranolol and primidone, for many years. This review aims to synthesize therapeutic attempts with other drugs.

AREAS COVERED

We have reviewed the current state of the pharmacological treatment of ET, both in patients and in experimental models of this disease, with special emphasis on the data published in the last 5 years. Based on the results in experimental models of ET, proposals have been made for future alternative therapeutic options.

EXPERT OPINION

The use of drugs other than propranolol and primidone has not shown a greater degree of efficacy than these in the treatment of ET, although according to certain evidence-based guidelines topiramate and phenobarbital could be alternative drugs. The results on the effectiveness of other drugs have been variable. For patients with refractory ET, especially those with head tremor, local injections with botulinum toxin A may be useful. According to the results of various experimental models, T calcium channel blockers, modulators of GABAA receptors (GABAARs), GABAB receptors (GABABRs), and glutamatergic neurotransmission, and drugs that decrease the expression of LINGO-1 could be interesting options for the future, among others.

Perampanel: Medical Alternative for Essential Tremor?

OBJECTIVES

The aim of this study was to assess the safety and efficacy of perampanel in patients with refractory essential tremor (ET).

METHODS

We recruited patients from our movement disorders clinic with the diagnosis of severe refractory ET, and perampanel 4 mg at night was initiated.Assessments were conducted at baseline and after 1 month of treatment with perampanel 4 mg/d. Details about tolerance and effectiveness were collected. Clinical evaluation was conducted with the Fahn-Tolosa-Marín scale, and statistical analysis was carried out with Wilcoxon matched pairs signed rank test.

RESULTS

This study included 18 patients with severe ET (11 females, 7 males; mean age: 75.1 ± 12.03 years; mean duration of ET: 17.4 ± 17.03 years). Perampanel significantly improved patients' average score with refractory ET ( P ≤ 0.0001). This improvement has been occasionally quite relevant. However, a proportion of patients did not tolerate perampanel because of several adverse effects including dizziness, ataxia, irritability, and instability.

CONCLUSIONS

Perampanel had a markedly positive antitremor effect in patients with ET and could be an alternative treatment. However, this drug is not devoid of adverse effects.

Locus coeruleus degeneration and cerebellar gray matter changes in essential tremor

BACKGROUND

The pathophysiology of essential tremor (ET) is not fully understood, and studies suggest pathological changes mainly occur in the cerebellum and locus coeruleus (LC).

METHODS

Fifty-three ET patients, including 30 patients with head tremor (h-ET), 23 patients without head tremor (nh-ET), 71 age and education matched healthy controls (HCs) were enrolled. All participants underwent Neuromelanin-sensitive magnetic resonance imaging (NM-MRI) and T1 scans on a 3-Tesla MR system. Next, we assessed the relationship between the contrast-to-noise ratio of LC (CNR_LC) and the score of The Essential Tremor Rating Assessment Scale (TETRAS) and cerebellum gray matter (GM) volume.

RESULTS

Significant difference of CNR_LC was found between ET and HC groups. The CNR_LC of ET groups is lower than the HC group (p = 0.031). Subgroup analysis showed that the CNR_LC in nh-ET was significantly lower than HCs (p = 0.016). Compared to HCs, h-ETs showed marked atrophy in the cerebellum: the vermis IV-V and lobule VI (GRF corrected, p < 0.05). A significant negative correlation was found between CNR_LC and the vermis lobule IV-V in h-ETs (r = - 0.651, p < 0.001). No significant correlation was found between CNR_LC and TETRAS scores.

CONCLUSION

The LC and the cerebellum might both involve in the pathophysiology of ET. LC evaluation using NM-MRI might be an effective tool for us to explore the pathophysiology of ET further.

α6-Containing GABAA Receptors: Functional Roles and Therapeutic Potentials

GABAA receptors containing the α6 subunit are highly expressed in cerebellar granule cells and less abundantly in many other neuronal and peripheral tissues. Here, we for the first time summarize their importance for the functions of the cerebellum and the nervous system. The cerebellum is not only involved in motor control but also in cognitive, emotional, and social behaviors. α6βγ2 GABAA receptors located at cerebellar Golgi cell/granule cell synapses enhance the precision of inputs required for cerebellar timing of motor activity and are thus involved in cognitive processing and adequate responses to our environment. Extrasynaptic α6βδ GABAA receptors regulate the amount of information entering the cerebellum by their tonic inhibition of granule cells, and their optimal functioning enhances input filtering or contrast. The complex roles of the cerebellum in multiple brain functions can be compromised by genetic or neurodevelopmental causes that lead to a hypofunction of cerebellar α6-containing GABAA receptors. Animal models mimicking neuropsychiatric phenotypes suggest that compounds selectively activating or positively modulating cerebellar α6-containing GABAA receptors can alleviate essential tremor and motor disturbances in Angelman and Down syndrome as well as impaired prepulse inhibition in neuropsychiatric disorders and reduce migraine and trigeminal-related pain via α6-containing GABAA receptors in trigeminal ganglia. Genetic studies in humans suggest an association of the human GABAA receptor α6 subunit gene with stress-associated disorders. Animal studies support this conclusion. Neuroimaging and post-mortem studies in humans further support an involvement of α6-containing GABAA receptors in various neuropsychiatric disorders, pointing to a broad therapeutic potential of drugs modulating α6-containing GABAA receptors. SIGNIFICANCE STATEMENT: α6-Containing GABAA receptors are abundantly expressed in cerebellar granule cells, but their pathophysiological roles are widely unknown, and they are thus out of the mainstream of GABAA receptor research. Anatomical and electrophysiological evidence indicates that these receptors have a crucial function in neuronal circuits of the cerebellum and the nervous system, and experimental, genetic, post-mortem, and pharmacological studies indicate that selective modulation of these receptors offers therapeutic prospects for a variety of neuropsychiatric disorders and for stress and its consequences.

Is essential tremor a disorder of primary GABA dysfunction? Yes

Dysfunction in gamma-aminobutyric acid (GABA) neurotransmission has emerged as a prime suspect for the underlying neurochemical dysfunction in essential tremor (ET). This dysfunction has been termed the GABA hypothesis. We review findings to date supporting the 4 steps in this hypothesis in studies of cerebrospinal fluid, pathology, genetics, animal models, imaging, computational models, and human drugs, while not overlooking the evidence of negative studies and controversies. It remains to be elucidated whether reduced GABAergic tone is a primary contributing factor to ET pathophysiology, a consequence of altered Purkinje cell function, or even a result of Purkinje cell death. More studies are clearly needed to confirm both the neurodegenerative nature of ET and the reduction in GABA activity in the cerebellum. Also necessary is to test further therapies to enhance GABA transmission specifically focused on the cerebellar area.

Primidone Intolerance in Essential tremor: Is it More than Just Age?

Background:

There are few medications for the treatment of essential tremor (ET). One of these, primidone, which is one of only two front-line agents, is associated with considerable adverse drug reactions (ADRs). It is unclear why some primidone-treated ET patients develop ADRs whereas others do not, and why these ADRs seem to be more prevalent in ET patients than primidone-treated patients with epilepsy.

Objective:

To review several possible explanations underlying the above-referenced differences.

Methods:

A literature search was conducted in PubMed in October 2021. Studies reporting the ADRs of primidone in different neurological conditions were comprehensively reviewed.

Discussion:

Although there were no head-to-head data, a review of the previous studies on ET and epilepsy patients indicates that the former is relatively more intolerant to primidone. Moreover, not all ET patients develop ADR of similar nature or severity. We discuss several potential mechanisms for this variability in the intolerance to primidone. These include: (i) older age (ET vs. epilepsy patients), (ii) cross-tolerance to primidone in patients with epilepsy, (iii) neurobiological (GABA-related) abnormalities associated with ET.

Conclusion:

We speculate that there are several possible explanations for primidone intolerance in ET. These possibilities should be tested in future studies, and we propose the roadmap for designing these studies. It is of value to obtain detailed insight into these complex issues because primidone remains one of the few frontline anti-tremor medications in ET. Answers to issues we have raised in this article could facilitate more customized formulation of primidone in ET patients.

Genomic Markers for Essential Tremor

There are many reports suggesting an important role of genetic factors in the etiopathogenesis of essential tremor (ET), encouraging continuing the research for possible genetic markers. Linkage studies in families with ET have identified 4 genes/loci for familial ET, although the responsible gene(s) have not been identified. Genome-wide association studies (GWAS) described several variants in LINGO1, SLC1A2, STK32B, PPARGC1A, and CTNNA3, related with ET, but none of them have been confirmed in replication studies. In addition, the case-control association studies performed for candidate variants have not convincingly linked any gene with the risk for ET. Exome studies described the association of several genes with familial ET (FUS, HTRA2, TENM4, SORT1, SCN11A, NOTCH2NLC, NOS3, KCNS2, HAPLN4, USP46, CACNA1G, SLIT3, CCDC183, MMP10, and GPR151), but they were found only in singular families and, again, not found in other families or other populations, suggesting that some can be private polymorphisms. The search for responsible genes for ET is still ongoing.

Sleep Disorders in Patients with Essential Tremor

PURPOSE OF THE REVIEW

Patients diagnosed with essential tremor (ET) report frequent sleep complaints. This review focuses on the main findings of studies addressing sleep features in patients diagnosed with ET, updating previously reported information. Bad quality of sleep and excessive daytime somnolence are very frequent in patients with ET, although the effects of the drugs used for the therapy of ET could contribute to these complaints. REM sleep behavior disorder, restless legs, insomnia, and nocturia are frequent complaints as well. There is a lack of studies addressing polysomnographic features of ET.

Current and Future Neuropharmacological Options for the Treatment of Essential Tremor

BACKGROUND

Essential Tremor (ET) is likely the most frequent movement disorder. In this review, we have summarized the current pharmacological options for the treatment of this disorder and discussed several future options derived from drugs tested in experimental models of ET or from neuropathological data.

METHODS

A literature search was performed on the pharmacology of essential tremors using PubMed Database from 1966 to July 31, 2019.

RESULTS

To date, the beta-blocker propranolol and the antiepileptic drug primidone are the drugs that have shown higher efficacy in the treatment of ET. Other drugs tested in ET patients have shown different degrees of efficacy or have not been useful.

CONCLUSION

Injections of botulinum toxin A could be useful in the treatment of some patients with ET refractory to pharmacotherapy. According to recent neurochemical data, drugs acting on the extrasynaptic GABAA receptors, the glutamatergic system or LINGO-1 could be interesting therapeutic options in the future.