Linking Essential Tremor to the Cerebellum

Linking Essential Tremor to the Cerebellum: Neurochemical Evidence

Juan Marin-Lahoz & Alexandre Gironell

The Cerebellum ISSN 1473-4222 Cerebellum DOI 10.1007/s12311-015-0735-z

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Author's personal copy Cerebellum DOI 10.1007/s12311-015-0735-z

REVIEW

Linking Essential Tremor to the Cerebellum: Neurochemical Evidence Juan Marin-Lahoz 1 & Alexandre Gironell 1

# Springer Science+Business Media New York 2015

Abstract The pathophysiology and the exact anatomy of essential tremor (ET) is not well known. One of the pillars that support the cerebellum as the main anatomical locus in ET is neurochemistry. This review examines the link between neurochemical abnormalities found in ET and cerebellum. The review is based on published data about neurochemical abnormalities described in ET both in human and in animal studies. We try to link those findings with cerebellum. γ-aminobutyric acid (GABA) is the main neurotransmitter involved in the pathophysiology of ET. There are several studies about GABA that clearly points to a main role of the cerebellum. There are few data about other neurochemical abnormalities in ET. These include studies with noradrenaline, glutamate, adenosine, proteins, and T-type calcium channels. One single study reveals high levels of noradrenaline in the cerebellar cortex. Another study about serotonin neurotransmitter results negative for cerebellum involvement. Finally, studies on Ttype calcium channels yield positive results linking the rhythmicity of ET and cerebellum. Neurochemistry supports the cerebellum as the main anatomical locus in ET. The main neurotransmitter involved is GABA, and the GABA hypothesis remains the most robust pathophysiological theory of ET to date. However, this hypothesis does not rule out other

* Alexandre Gironell [email protected] 1

Movement Disorders Unit, Department of Neurology, Hospital de la Santa Creu i Sant Pau, Autonomous University of Barcelona, Av.Sant Antoni Maria Claret, 167, 08025 Barcelona, Catalonia, Spain

mechanisms and may be seen as the main scaffold to support findings in other systems. We clearly need to perform more studies about neurochemistry in ET to better understand the relations among the diverse systems implied in ET. This is mandatory to develop more effective pharmacological therapies.

Keywords Essential tremor . Cerebellum . GABA . Noradrenaline . Serotonin . T-type calcium channels

Introduction Essential tremor (ET) is a common movement disorder in adults, characterized by rhythmic shaking of the arms and possibly other parts of the body [1, 2]. The etiology, pathophysiology, and exact anatomy of ET are not yet well known. Furthermore, ET is probably not a single entity but a cluster of disorders [3]. There is growing consensus about the involvement of the cerebellum in ET. One of the main pillars that support the cerebellum as the main anatomical locus in ET is neurochemistry. Although knowledge of the neurochemical abnormalities underlying ET remains incomplete, the implication of the GABA system is undeniable. Little is known, however, about other neurochemical phenomena underlying ET. This review is structured in three parts. The first part links the cerebellum and ET, the second part discusses the neurochemical abnormalities found in ET, and the third part combines neurochemical abnormalities and the cerebellum. Our aim is to review the pathophysiology of ET with special focus on neurochemical targets in order to foster the development of more effective pharmacological therapies for this prevalent disease.

Author's personal copy Cerebellum

Cerebellum and Essential Tremor

Neurochemical Abnormalities of Essential Tremor

The most important and most studied function of the cerebellum is motor control. The cerebellum performs the integration of motor afferents from the diencephalon, allowing precise control of movements. Although the exact nature of the computations performed by the cerebellum is unknown, the main model used is proportional integral derivative control. An important characteristic of this and other human-made controllers is their tendency to produce a rhythmic oscillating error over the target when one of their settings fails. When the final output of a system is in position, this oscillation becomes tremor [4]. Beyond this modeling logic, several types of studies locate ET pathology in the cerebellum. Verified cerebellum disorders are known to cause tremor [5]. In some cases, however, ET has ameliorated following a cerebellar stroke [6]. While this may sound contradictory, it is logical if we take into account the structure and functioning of the cerebellum. Cerebellar afferents synapse with Purkinje cells in the cerebellar cortex. Purkinje cells link the cerebellar cortex with the deep nuclei by inhibiting them. These deep nuclei project to the rest of the central nervous system (CNS). Therefore, a lesion in the cerebellar cortex increases deep nuclei output while a direct lesion in the deep nuclei diminishes it. Postmortem studies have shown neuropathological changes in the cerebellum of ET patients when comparing with healthy controls [7]. Specifically, ET patients show fewer and lower linear density of Purkinje cells in the cerebellar cortex. [8–10]. However, some studies did not find Purkinje cell loss and considered pathophysiology of ET as structural changes in synapse, neurotransmitter metabolic pathways dysfunction, or receptor dysfunction [11]. The controversial results may be related to patient selection biases, different methodologies and counting methods, and differences in field sizes and lack of standardization across studies. Neuroimaging studies showed no decrease in cerebellar gray matter [12]. However, MRI spectroscopy has shown a reduced NAA/tCR ratio in the cerebellum, indicating neural damage [13, 14]. Also, some [15–17], but not all [18], functional neuroimaging studies show an increase in the cerebellar tracer uptake. Electrophysiologic evidence also shows a relation between the cerebellum and ET. Repetitive transcranial magnetic stimulation (rTMS) targeting the cerebellum has shown tremorolytic power in ET, both in the short term [19] and in the long term [20]. A possible mechanism would be activation of Purkinje cells through parallel fibers, which would inhibit deep cerebellar nuclei [21]. In summary, there is consistent evidence about the involvement of the cerebellum in the pathophysiology of ET.

Neurotransmitters Several neurotransmitters have been implied in ET; a summary can be found in Table 1. Amino Acids GABA GABA is the main inhibitory neurotransmitter in mammals [50]. Because of its widespread presence and use, GABA is involved in all functions and structures of the CNS, including the cerebellum. In fact, the GABA system is the target of a wide range of drugs that act on the CNS, such as anxiolytics, sedative-hypnotics, general anesthetics, and anticonvulsants. For a schematic view of GABA synapse, refer to Fig. 1. GABA is synthesized by a specific enzyme, l-glutamic acid decarboxylase (GAD) in one step from l-glutamate. Thus, glutamate must be available in certain nerve endings for biosynthesis of GABA. Much of the glutamate and GABA used as neurotransmitter is derived from glial storage pools of glutamine. GABA is released from electrically stimulated inhibitory nerve cells. The application of GABA and structural analogues to cells innervated by GABAergic neurons produces effects on that target cell identical to those produced by stimulating the inhibitory innervation. GABA is removed from the synaptic cleft by GABA transporters and is metabolized. Besides, GABA can be converted into glutamate. GABA acts through two kinds of receptors: GABAA and GABAB. GABAA receptors are linked to chloride (Cl−) channels which lead to fast hyperpolarization when open. The Table 1

Main neurotransmitter findings in ET

Neurotransmitter

Findings

GABA

Reduced CSF levels [22, 23] Reduced postmortem GABA receptors [24] Reduced postmortem GABAergic neurons [25] Increased flumacemil binding in cerebellum [26] Flumacemil binding correlated with tremor severity [27] GABAA receptor knockout mice model [28] Several GABAergic drugs relieve ET [29–39] Increased CSF levels [22] Beta adrenergic antagonist ameliorate ET [31, 36, 40–44] High postmortem cerebellar levels [45] Trazodone ameliorates ET in some trials [46–49] Theophylline ameliorates tremor [44]

Glutamate Noradrenaline

Serotonin Adenosine

Only positive findings are shown ET essential tremor, GABA γ-aminobutyric acid, CSF cerebrospinal fluid

Author's personal copy Cerebellum Fig. 1 GABA synapsis schematic. Please note that GABAA and GABAB receptors are found in different neuron populations. Adapted from Splette [51]

pharmacology of this receptor is quite interesting. While, for most other neurotransmitters, drugs acting in their receptor bind them at the same place than the original neurotransmitter does (e.g., opioids, dopamine agonists) where they may stimulate the receptor in a similar way or impede the neurotransmitter to bind and therefore inhibit its effects (in rare cases, they are inverse agonists). However, most of the drugs acting on the GABAergic system modulate the GABAA receptor, making it more likely to get activated by the original ligand. Although ethanol has multiple CNS effects, its main effect is considered to be a positive allosteric modulator of GABAA receptors. Importantly, this could be considered the only effect at low doses. [52–54]. Gabapentin does not bind to the GABAA receptor but increases GABA synthesis [50, 52]. Primidone is metabolized into phenobarbital which also acts as an allosteric modulator enhancing GABA transmission and at higher doses as a GABA agonist. GABAB is a metabotropic receptor coupled to G proteins. It mediates inhibitory responses mainly via potassium channels. They also reduce the activity of adenylyl cyclase and neuronal conductance to Ca++ [55, 56]. Whereas GABAA receptors mediate fast responses, GABAB receptors mediate slow responses. In addition to GABA, main known agonists of GABAB receptors are baclofen and gammahydroxybutyrate (GHB). Baclofen has been studied in the harmaline model of tremor but not in ET. It ameliorates the tremor in the rat model [57] but not in the mouse model [58]. More interestingly, GHB has shown some benefit in ET in two open-label clinical trials [59, 60]. The GABA involvement in ET is in several investigations that show GABA system abnormalities in ET. In 1996, Málly et al. found reduced levels of GABA in the cerebrospinal fluid in a series of ET patients [22, 23]. Some neuropathological studies of ET have targeted GABA. In 2012 Paris-Robidas et al. reported a postmortem decrease in GABAA (35 %

reduction) and GABAB (22–31 % reduction) receptors in the dentate nucleus of the cerebellum in individuals with ET (n= 10), when compared to controls (n=16) or individuals with Parkinson’s disease (n=10), as assessed by receptor-binding autoradiography [24]. Furthermore, concentrations of GABAB receptors in the dentate nucleus were inversely correlated with the duration of ET symptoms, suggesting that the loss of GABAB receptors follows the progression of the disease. The authors proposed that a decrease in GABA receptors in the dentate nucleus disinhibits cerebellar pacemaker output activity, propagating along the cerebello-thalamo-cortical pathways to generate tremors. In the same year, Shill et al. found reduced levels of parvalbumin (a marker of GABAergic neurons) in the pons and locus coeruleus but not in the cerebellum [25]. Basic electrophysiology has also provided valuable support for the GABA implication. In 2008, Uusiaari and Knopfel demonstrated that all dentate nucleus cells are sensitive to GABAergic input from Purkinje cells [61]. Using electrophysiological recordings of deep cerebellar nuclei neurons, they showed that both GABAA and GABAB receptors mediate the inhibitory input coming from Purkinje cells. [11C]Flumazenil is a tracer that specifically binds to the central benzodiazepine receptor sites of the GABAA receptor complex. To date, there are two main controlled PET studies using this tracer in ET. The first article was published in 2010 [26]. It was a comparative study of [11C]flumazenil PET in eight patients with bilateral ET, with 11 healthy controls. Parametric distribution volume images were calculated for focally altered [11C]flumazenil binding at the sites of tremor genesis, in particular at the level of the cerebellum and interconnected thalamocortical pathways. The authors found significant increases in binding of [11C]flumazenil at the benzodiazepine receptor site of the GABA A receptor in the


cerebellum, the ventrolateral thalamus, and the lateral premotor cortex in the ET group. The second paper appeared in 2012 [27]. The authors performed correlated clinical scale scores and parametric binding potential images of [11C]flumazenil PET in 10 ET patients at different stages of clinical severity. The severity of tremor statistically correlated with the abnormalities found in GABA receptor binding in the cerebellar vermis, bilateral posterior lobes, and right anterior lobe. In 2005, Kralic et al. performed an interesting investigation using GABAA receptor alpha1 subunit knockout mice [28]. Although the features of this and other models of tremor are a matter of debate in terms of translation to human ET [62], this mouse model exhibited postural and kinetic tremor and motor incoordination characteristic of ET. The mice were tested using current ET drug therapies such as primidone, propranolol, and gabapentin, and the amplitude of the pathologic tremor decreased. Moreover, one of the major premises of the GABA hypothesis in ET was the antitremoric effects observed in drugs that enhance GABA activity including muscimol, theophylline, gabapentin, topiramate, pregabalin, primidone, benzodiazepines, and alcohol [29–39]. Many genetic studies have tried to identify variants related to the presence of ET. Studies used different approaches. Some have looked for variants of candidate genes such as alpha 1 subunit of the GABAA receptor. However, no relation was found between genetics of this subunit and the phenotype [63]. Other studies have targeted single nucleotide polymorphisms (SNPs) in GABA-related genes without success. In a population of German and Danish origin, 15 polymorphisms of the GABAA receptor and four GABA transporter genes were analyzed using SNPs, but no evidence of association between those genes and ET was found [64]. Similar studies have been performed in Spanish population. There were no significant differences in SNPs between patients and controls. SNPs were also unrelated to ET clinical characteristics such as age of onset of alcohol response [65, 66]. Recently, a review focused on the limitations of genetic studies for ET that could explain why genetic studies yield no positive result. The main limitations considered were the following: lack of stringent diagnostic criteria, a high phenocopy rate, small samples, non-Mendelian inheritance, and high locus heterogeneity [67]. Other evidence against the involvement of GABA is the absence of pathological changes in some postmortem studies in ET [68, 69]. Finally, also of concern is the therapeutic failure of several GABAergic drugs in controlled studies. These include progabide, gabapentin, topiramate, tiagabine, and pregabalin [70–76]. Glutamate and Other Amino Acids The concentration of amino acids in cerebrospinal fluid (CSF) and blood has

also been studied. Málly et al. found increased levels of excitatory amino acids—aspartate and glutamate—in 19 ET patients compared to 10 controls. However, this elevation was only significant for aspartate. They also found reduced levels of glutamine, glycine, isoleucine, leucine, threonine, and asparagine [22]. In another study by the same group, the concentration of the inhibitory amino acid glycine was low in both CSF and serum, and GABA was also low in CSF [23]. However, the results concerning excitatory amino acids differed from those of the previous study: glutamate was higher in CSF in ET patients but aspartate was lower. The serum concentration of both neurotransmitters was higher in patients. Taken together, these studies suggest an increase in excitatory transmitters and a decrease in inhibitory transmitters. The inconsistencies found may be due to the small sample size. Another group that studied amino acids in ET compared these with amino acids in Parkinson’s disease but found no differences [77]. Monoamines Noradrenaline Most evidence of catecholamine involvement in ET is based on the effectiveness of beta adrenergic antagonists in ameliorating ET [31, 36, 40–44]. However, both peripheral and central mechanisms have been proposed [78, 79] and there are few basic data on catecholaminergic system in ET. One postmortem study revealed high levels of noradrenaline in the locus coeruleus, dentate nucleus, and cerebellar cortex [45]. Other studies found no changes in striatal tyrosine hydroxylase in ET or dopamine beta hydroxylase in the locus coeruleus [25]. Serotonin Trazodone achieved clinical benefit in some patients with ET [46] which was replicated in a small clinical trial [47]. Based on response to trazodone, serotonin activity has also been investigated in ET [48] and in the harmaline tremor model [49]. However, bigger therapeutical trial in ET patients with trazodone [80] and a molecular neuroimaging study of [27] have been negative. Others Adenosine Adenosine activity imbalance has been considered as a potential mechanism underlying ET mainly because theophylline, an adenosine antagonist, was found to be as effective as propranolol in a small blinded clinical trial [44]. Its effect is considered to be related to a reduction in the release of excitatory neurotransmitters during chronic use—an upregulation of GABA receptors has also been postulated [29]—but there are no reports of primary changes in adenosine function that could cause ET. Adenosine has a very short half-life


which limits its study, but no changes in adenosine receptors have been found. Proteins Proteins in CSF have also been studied in patients with tremor. A study compared protein levels in different tremoric diseases: Parkinson’s disease, chronic alcoholism, and ET. Abnormal proteins were found in 17 % of patients with Parkinson’s disease, 80 % with chronic alcoholism, and 94 % of those with ET. However, electrophoresis was inconclusive [81]. Somatostatin has been directly targeted in the study of tremoric diseases. Parkinson’s disease patients showed low levels, but patients with ET were similar to controls [82].

Since the discovery of T-type calcium currents, a lot of research has focused on them—mainly because of their role in epilepsy—and selective antagonist have appeared [90]. Several compounds antagonizing T-type calcium channels have succeeded to suppress harmaline tremor. Furthermore, the same drugs also suppress tremor in GABAA alpha1 knockout models [91]. These findings suggest that ET is not due to an only neurochemical imbalance but probably to a series of related neurochemical changes in the same way parkinsonian syndrome appears after insults which increase GPi activity (e. g., SNpc dopaminergic neurons loss, calcium blockade, dopamine blockade, or subthalamic lesion) [92, 93].

Neurochemical Links to the Cerebellum Other Neurochemical Phenomena T-Type Calcium Channels T-type calcium channels are responsible for the postinhibitory rebound phenomenon in thalamocortical neurons. They lead to a depolarization following a hyperpolarization [83] which causes a higher firing rate. In the appropriate conditions (i.e., when inhibitory interneurons or a more complex circuit inhibits the neuron presenting postinhibitory rebound), an oscillatory phenomenon arises [77]. Such conditions occur in the inferior olive. In turn, all the ascending fibers modulating Purkinje cells correspond to inferior olive neurons and therefore the rhythmic activity is transmitted to the cerebellum [84]. These oscillations are avoided if there is enough external inhibition and should not happen when external inhibition is intact. However, reciprocally innervated circuits may present oscillations if the excitability of their neurons is increased, even with a normal inhibitory system. This could be due to an increase in hyperpolarization-activated cation currents (Ih) or low-threshold calcium current (IT). Using a HodgkinHuxley-type model, Shaikh et al. [85] showed how modifying those currents leads to oscillations reproducing ET features. Limitations to this model are that the authors do not propose an only location in the nervous system where that phenomenon may happen and that the characteristics of the simulated currents are based on recordings from neurons of several brain locations and therefore do not represent a known circuit. Harmaline hyperpolarizes olivary neurons and induces action potentials which extend to the cerebellar cortex and manifest as tremor [86, 87]. Harmaline tremor reminds of ET in several characteristics: it is an action tremor and has a similar drug response. This makes harmaline useful to develop animal models of ET. Interestingly, octanol, a calcium antagonist, suppresses harmaline tremor [88]. However, octanol is not selective for T-type calcium channels but also acts through GABA potentiation [89].

Here, we have provided evidence linking cerebellum and ET based on studies about neurochemical phenomena in ET. However, direct evidence of neurochemical disturbances in the cerebellum is scarce. Table 2 summarizes findings to date. The main neurotransmitter linking cerebellum and ET is GABA (the GABA hypothesis), although other neurochemical links are also involved (Fig. 2). Cerebellum and the GABA System The cerebellum and the thalamus are the main regions involved in the genesis of tremor in humans. Clinical case studies have shown that lesions of these structures can alleviate tremor [6, 94]. The ventral intermediate nucleus is the most effective target for stereotactic interventions to treat tremor [95]. Two studies using repetitive transcranial magnetic stimulation applied over the cerebellum found a tremorlytic action in ET patients [19, 20]. Furthermore, PET activation studies show that regional cerebral blood flow in the cerebellum is abnormally increased [15–17]. MRI studies including spectroscopy, voxel-based morphometry, and functional MRI also support cerebellar involvement in ET [12–14]. [7] In 2007, Louis and coworkers studied 33 ET brains and found that about 75 % of cases exhibited cerebellar pathology with a loss of Purkinje cells and increased numbers of axonal torpedoes [7]. Furthermore, microscopic cerebellar pathology has been identified in these patients, associated with cerebellar Purkinje cell loss with torpedoes and Bergmann gliosis [7]. Cerebellar Purkinje cells are large neurons that have many branching extensions. They are found in the cortex of the cerebellum and play a fundamental role in controlling motor movement [5]. These cells are characterized by cell bodies that are flash-like in shape, the result of numerous branching dendrites, and by a single long axon. Most Purkinje cells release the neurotransmitter GABA, which exerts inhibitory

Author's personal copy Cerebellum Table 2 Direct evidence from neurochemical studies (both ET and animal models are included)

No. of ET studies

Biochemical resultsa

Cerebellar linkb

+

−

+

−

Not done

Neurotransmiters GABA

20

12

8

9

3

8

Glutamate Noradrenaline

3 2

2 1

1 1

0 1

0 0

3 1

Serotonine

2

0

2

0

1

1

Adenosine Other

1

1

0

0

0

0

2 5

1 5

1 0

0 2

0 0

2 3

Proteins Calcium currents

Drug clinical trials are not considered a

Results concerning the relation between tremor and neurochemical dysfunction

b

Results showing a link between the neurochemical dysfunction and cerebellum

actions on the dentate nucleus and certain deep cerebellar output neurons, and thereby reducing the transmission of nerve impulses. These inhibitory functions enable Purkinje cells to regulate and coordinate motor movements. The major pathophysiological hypothesis of ET during the last two decades has been the GABA hypothesis [96, 97]. This hypothesis implies a disturbance of the GABA system in ET, specially involving the cerebellum. The hypothesis consists of four main steps (see Fig. 1). Steps 1 to 3 take place in the cerebellum. These include cerebellar degeneration with Purkinje cell loss, activity of the GABA system decreased in deep cerebellar neurons, and disinhibition of pacemaker activity of deep cerebellar neurons. The GABA hypothesis has some evidences that agree with the hypothesis [97]. Fig. 2 GABA hypothesis as a scaffold for neurochemical dysfunction in ET: dark boxes indicate GABA hypothesis, light boxes indicate other known possible dysfunctions

Step 1

Step 1: Cerebellar neurodegeneration with cerebellar Purkinje cell loss. – –

Pathological postmortem studies agree with the GABA hypothesis, revealing Purkinje cell loss in ET [7]. The neurodegenerative nature of ET is supported by the progressive course of the disease [98, 99].

Step 2: Decrease in GABA system activity in deep cerebellar neurons – –

GABA metabolism dysfunction

CSF studies demonstrating a decrease of GABA levels [22, 23]. Pathological studies that found a decrease in GABA receptors in dentate nucleus [24].

Neurodegeneration (Purkinje cell loss)

GABA receptor dysfunction

Cerebellar GABA system

Neural inhibition abnormalities

Cerebellar inhibitory activity

T type Calcium currents

Step 3

Deep cerebellar neurons with pacemaker activity

Inferior olivary nucleus rebound

Step 4

Thallamo-cortical rhythmic activity

Excitatory neurotransmitters

Step 2

Tremor


–

– –

In vivo imaging studies, with an increase of binding of flumazenil in GABA receptors in the cerebellar area, thus suggesting a deficit in GABA in these areas [26, 27]. Animal model (knockout mice) of GABA dysfunction exhibits symptoms characteristic of ET [28]. Antitremoric efficacy of drugs that enhance GABA activity including muscimol, theophylline, gabapentin, topiramate, tiagabine, pregabalin, primidone, benzodiazepines, and alcohol [29–39, 43, 72, 100].

Step 3: Disinhibition in output deep cerebellar neurons with pacemaker activity. –

– –

Electrophysiological studies indicating that dentate nucleus cells are sensitive to GABAergic input from Purkinje cells and that deep cerebellar nucleus neurons have pacemaker-like activity [55, 61, 101–104]. Evidence of this step is the increase in output cerebellar metabolism that appears in metabolic PET studies [15, 16, 18]. The antitremoric effect of inhibitory rTMS over the cerebellum also supports an increase in hypermetabolism and activity in this area [19, 20].

Step 4: Increase of rhythmic activity of thalamus and thalamo-cortical circuit. –

Thalamic rhythmic or tremoric neurons, specifically in the VIM nucleus, have been clearly seen in electrophysiological recordings in deep brain stimulation surgery [105, 106].

Other Neurochemical Links with the Cerebellum GABA hypothesis cannot account for all the current knowledge about ET. First, abnormalities in cerebellum are not found in all the pathological studies; second, some studies have found GABA dysfunction in other structures but not in the cerebellum which could account for CSF abnormalities; third, none of the genetic studies targeting GABAergic pathways is positive; fourth, several GABAergic drugs have failed in controlled studies (including progabide, gabapentin, topiramate, tiagabine, and pregabalin) [70–76]; fifth, drugs targeting other receptors are effective; and sixth, some animal models of ET show both similar phenotype and pharmacological response to GABAergic drugs in spite of having an intact GABA system. Therefore, a more complex model may be more useful to understand this disorder.

ET is characterized by its appearance in action and its stable frequency and amplitude, suggesting the presence of an oscillating circuit. This circuit, composed of cerebellum, thalamus, and motor cortex, is complex and reciprocally innervated, meeting the conditions to provoke and convey oscillatory activity. ET can be abolished or decreased by thalamic inhibition using DBS, deep cerebellar nucleus inhibition using rTMS, and reducing motor cortex activity (simply by rest). Computational models have been developed to join both neurotransmitter imbalances and membrane leaking changes (i.e., T-type calcium currents). ET drugs are mainly inhibitory. Some are direct inhibitors such as GABA enhancers while others diminish excitatory signals (propranolol, topiramate). Therefore, ET may be due to an excessive neuronal excitability. On the one hand, a failure of inhibitory systems would potentiate oscillating activity in this circuit. Main cause of this failure probably is a reduction of GABAergic activity diminishing the inhibition in deep cerebellar nuclei. This reduction may be caused by (a) loss of Purkinje cell, (b) diminished conversion of glutamate into GABA, (c) increased conversion of GABA into glutamate, (d) diminished release of GABA in synapses, or (e) diminished binding to GABA receptors. Then an abnormality in the effectory mechanisms of GABA receptors (e. g., chloride channels) would also have similar effects. On the other hand, an excess of excitatory activity in the circuit would make a normal inhibitory system insufficient to defuse oscillatory activity. This increase in excitatory activity can be caused by abnormalities of calcium currents both primarily or secondary to an excess of excitatory neurotransmitters. Any of those alterations would lead to a similar tremor and may respond to several pharmacologic strategies leading to an increase of inhibition, a decrease of postinhibitory rebound, or a decrease of excitatory activity. This is to say that, in a particular patient, one of those mechanisms may be the cause of the tremor and that there is no a single cause of ET. The heterogenicity of the underlying mechanisms may explain the heterogenicity in efficacy of drugs in different patients.

Final Remarks To sum up and conclude, neurochemistry supports the cerebellum as the main anatomical locus in ET. The main neurotransmitter involved is GABA, and the GABA hypothesis remains the most robust pathophysiological theory of ET to date. However, this hypothesis does not rule out other mechanisms and may be seen as the main scaffold to support findings in other systems. We clearly need to perform more studies


about neurochemistry in ET to better understand the relations among the diverse systems implied in ET. This is mandatory to develop more efficient pharmacological therapies.

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Compliance with Ethical Standards 18. Conflict of Interest The authors declare that they have no conflict of interest. This article does not contain any studies with human participants or animals performed by any of the authors.

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