Potassium channel blockers inhibit the triggers of attacks in the calcium channel mouse mutant tottering

Inborn Errors of Metabolism with Ataxia: Current and Future Treatment Options

A number of hereditary ataxias are caused by inborn errors of metabolism (IEM), most of which are highly heterogeneous in their clinical presentation. Prompt diagnosis is important because disease-specific therapies may be available. In this review, we offer a comprehensive overview of metabolic ataxias summarized by disease, highlighting novel clinical trials and emerging therapies with a particular emphasis on first-in-human gene therapies. We present disease-specific treatments if they exist and review the current evidence for symptomatic treatments of these highly heterogeneous diseases (where cerebellar ataxia is part of their phenotype) that aim to improve the disease burden and enhance quality of life. In general, a multimodal and holistic approach to the treatment of cerebellar ataxia, irrespective of etiology, is necessary to offer the best medical care. Physical therapy and speech and occupational therapy are obligatory. Genetic counseling is essential for making informed decisions about family planning.

Physiology of Dystonia: Animal Studies

Dystonia is currently ranked as the third most prevalent motor disorder. It is typically characterized by involuntary muscle over- or co-contractions that can cause painful abnormal postures and jerky movements. Dystonia is a heterogenous disorder-across patients, dystonic symptoms vary in their severity, body distribution, temporal pattern, onset, and progression. There are also a growing number of genes that are associated with hereditary dystonia. In addition, multiple brain regions are associated with dystonic symptoms in both genetic and sporadic forms of the disease. The heterogeneity of dystonia has made it difficult to fully understand its underlying pathophysiology. However, the use of animal models has been used to uncover the complex circuit mechanisms that lead to dystonic behaviors. Here, we summarize findings from animal models harboring mutations in dystonia-associated genes and phenotypic animal models with overt dystonic motor signs resulting from spontaneous mutations, neural circuit perturbations, or pharmacological manipulations. Taken together, an emerging picture depicts dystonia as a result of brain-wide network dysfunction driven by basal ganglia and cerebellar dysfunction. In the basal ganglia, changes in dopaminergic, serotonergic, noradrenergic, and cholinergic signaling are found across different animal models. In the cerebellum, abnormal burst firing activity is observed in multiple dystonia models. We are now beginning to unveil the extent to which these structures mechanistically interact with each other. Such mechanisms inspire the use of pre-clinical animal models that will be used to design new therapies including drug treatments and brain stimulation.

Cerebellar neuronal dysfunction accompanies early motor symptoms in spinocerebellar ataxia type 3

Spinocerebellar ataxia type 3 (SCA3) is an adult-onset, progressive ataxia. SCA3 presents with ataxia before any gross neuropathology. A feature of many cerebellar ataxias is aberrant cerebellar output that contributes to motor dysfunction. We examined whether abnormal cerebellar output was present in the CMVMJD135 SCA3 mouse model and, if so, whether it correlated with the disease onset and progression. In vivo recordings showed that the activity of deep cerebellar nuclei neurons, the main output of the cerebellum, was altered. The aberrant activity correlated with the onset of ataxia. However, although the severity of ataxia increased with age, the severity of the aberrant cerebellar output was not progressive. The abnormal cerebellar output, however, was accompanied by non-progressive abnormal activity of their upstream synaptic inputs, the Purkinje cells. In vitro recordings indicated that alterations in intrinsic Purkinje cell pacemaking and in their synaptic inputs contributed to abnormal Purkinje cell activity. These findings implicate abnormal cerebellar physiology as an early, consistent contributor to pathophysiology in SCA3, and suggest that the aberrant cerebellar output could be an appropriate therapeutic target in SCA3.

Summary: In a mouse model of spinocerebellar ataxia type 3 (SCA3), aberrant cerebellar physiology is apparent early in disease, prior to cerebellar neuronal pathology. Aberrant cerebellar output could be a therapeutic target in SCA3.

Mechanism of stress-induced attacks in an episodic neurologic disorder

Stress is the most common trigger among episodic neurologic disorders. In episodic ataxia type 2 (EA2), physical or emotional stress causes episodes of severe motor dysfunction that manifest as ataxia and dystonia. We used the tottering (tg/tg) mouse, a faithful animal model of EA2, to dissect the mechanisms underlying stress-induced motor attacks. We find that in response to acute stress, activation of α1-adrenergic receptors (α1-Rs) on Purkinje cells by norepinephrine leads to their erratic firing and consequently motor attacks. We show that norepinephrine induces erratic firing of Purkinje cells by disrupting their spontaneous intrinsic pacemaking via a casein kinase 2 (CK2)-dependent signaling pathway, which likely reduces the activity of calcium-dependent potassium channels. Moreover, we report that disruption of this signaling cascade at a number of nodes prevents stress-induced attacks in the tottering mouse. Together, our results suggest that norepinephrine and CK2 are required for the initiation of stress-induced attacks in EA2 and provide previously unidentified targets for therapeutic intervention.

Effects of antipsychotic drugs and potassium channel modulators on spectral properties of local field potentials in mouse hippocampus and pre-frontal cortex

Local field potentials (LFPs) recorded intracranially display a range of location-specific oscillatory spectra which have been related to cognitive processes. Although the mechanisms producing LFPs are not completely understood, it is likely that voltage-gated ion channels which produce action potentials and patterned discharges play a significant role. It is also known that antipsychotic drugs (APDs) affect LFP spectra and a direct inhibitory effect on voltage-gated potassium channels has been reported. Additionally, voltage-gated potassium channels have been implicated in the pathophysiology of schizophrenia, a disorder for which APDs are primary therapies. In this study we sought to: i) better characterise the effects of two APDs on LFPs spectra and connectivity measures and ii) examine the effects of potassium channel modulators on LFPs and potential overlap of effects with APDs. Intracranial electrodes were implanted in hippocampus (HIP) and pre-frontal cortex (PFC) of C57BL/6J mice; power spectra, coherence and phase-amplitude cross-frequency coupling were measured. Drugs tested were APDs haloperidol and clozapine as well as voltage-gated potassium channel modulators (KVMs) 4-aminopyridine (4-AP), tetraethylammonium, retigabine and E-4031. Both APDs and KVMs significantly reduced gamma power except 4-AP, which conversely increased gamma power. Clozapine and retigabine additionally reduced gamma coherence between HIP and PFC, while 4-AP demonstrated the opposite effect. Phase-amplitude coupling between theta and gamma oscillations in HIP was significantly reduced by the administration of haloperidol and retigabine. These results provide previously undescribed effects of APDs on LFP properties and demonstrate novel modulation of LFP characteristics by KVMs that intriguingly overlap with the APD effects.

Aminopyridines and Acetyl-DL-leucine: New Therapies in Cerebellar Disorders

Cerebellar ataxia is a frequent and often disabling syndrome severely impairing motor functioning and quality of life. Patients suffer from reduced mobility, and restricted autonomy, experiencing an even lower quality of life than, e.g., stroke survivors. Aminopyridines have been demonstrated viable for the symptomatic treatment of certain forms of cerebellar ataxia. This article will give an outline of the present pharmacotherapy of different cerebellar disorders. As a current key-therapy for the treatment of downbeat nystagmus 4-aminopyridine (4-AP) is suggested for the treatment of downbeat nystagmus (5-10 mg Twice a day [TID]), a frequent type of persisting nystagmus, due to a compromise of the vestibulo-cerebellum. Studies with animals have demonstrated, that a nonselective blockage of voltage-gated potassium channels (mainly Kv1.5) increases Purkinje- cell (PC) excitability. In episodic ataxia type 2 (EA2), which is frequently caused by mutations of the PQ-calcium channel, the efficacy of 4-AP (5-10 mg TID) has been shown in a randomized controlled trial (RCT). 4-AP was well tolerated in the recommended dosages. 4-AP was also effective in elevating symptoms in cerebellar gait ataxia of different etiologies (2 case series). A new treatment option for cerebellar disease is the amino-acid acetyl-DL-leucine, which has significantly improved cerebellar symptoms in three case series. There are on-going randomized controlled trials for cerebellar ataxia (acetyl-DL-leucine vs placebo; ALCAT), cerebellar gait disorders (SR-form of 4-AP vs placebo; FACEG) and EA2 (sustained-release/SR-form of 4-AP vs acetazolamide vs placebo; EAT2TREAT), which will provide new insights into the pharmacological treatment of cerebellar disorders.

Aberrant cerebellar Purkinje cell activity as the cause of motor attacks in a mouse model of episodic ataxia type 2

Many cerebellar-induced neurological disorders, such as ataxias and cerebellar-induced dystonias, are associated with abnormal Purkinje cell activity. In tottering mice, a well-established mouse model of episodic ataxia type 2 (EA2), cerebellar Purkinje cells are required for the initiation of motor attacks. How Purkinje cells contribute to the initiation of attacks is not known, and to date there are no reports on the activity of Purkinje cells during motor attacks in the tottering mice. Here, we show that tottering Purkinje cells exhibit high-frequency burst firing during attacks, reminiscent of other mouse models of cerebellar-induced motor dysfunction. We recorded the activity of Purkinje cells in awake head-restrained tottering mice at baseline, or during caffeine-induced attacks. During motor attacks, firing of Purkinje cells transformed to high-frequency burst firing. Interestingly, the extent to which the activity of Purkinje cells was erratic was correlated with the severity of the motor dysfunction. In support of a causal role for erratic activity in generating motor dysfunction, we found that direct infusion of the small conductance calcium-activated potassium (SK) channel activator NS309 into the cerebellum of tottering mice in the midst of an attack normalized the firing of Purkinje cells and aborted attacks. Conversely, we found that inducing high-frequency burst firing of Purkinje cells in wild-type animals is sufficient to produce severe motor signs. We report that erratic activity of wild-type Purkinje cells results in ataxia and dystonic postures. Moreover, this aberrant activity is the cause of motor attacks in the tottering mice.

Summary: Here, we report that in the well-established mouse model of episodic ataxia type 2, tottering, the severe episodic motor signs are caused by highly erratic activity of Purkinje cells.

Episodic ataxias

Primary episodic ataxias (EAs) are a group of dominantly inherited disorders characterized by transient recurrent incoordination and truncal instability, often triggered by physical exertion and emotional stress, variably associated with progressive baseline ataxia. There are now eight designated subtypes based largely on genetic loci. Mutations have been identified in multiple individuals and families with EA1, EA2, and EA6, mostly with onset before adulthood. EA1 and EA2 are prototypical neurologic channelopathies. EA1 is caused by heterozygous mutations in KCNA1, which encodes the α1 subunit of a neuronal voltage-gated potassium channel, Kv1.1. EA2, the most common and best characterized, is caused by heterozygous mutations in CACNA1A, which encodes the α1A subunit of a neuronal voltage-gated calcium channel, Cav2.1. EA6 is caused by heterozygous mutations in SLC1A3, which encodes a subunit of a glial excitatory amino acid transporter, EAAT1. The other EA subtypes were defined in single families awaiting gene identification and further confirmation. This chapter focuses on the best-characterized EA syndromes, the clinical assessment and genetic diagnosis of EA, and the management of EA, as well as newly recognized allelic disorders that have greatly expanded the clinical spectrum of EA2. Illustrative cases are discussed, with a focus on sporadic patients with congenital features without episodic ataxia who present diagnostic and therapeutic challenges.

Current Opinions and Areas of Consensus on the Role of the Cerebellum in Dystonia

A role for the cerebellum in causing ataxia, a disorder characterized by uncoordinated movement, is widely accepted. Recent work has suggested that alterations in activity, connectivity, and structure of the cerebellum are also associated with dystonia, a neurological disorder characterized by abnormal and sustained muscle contractions often leading to abnormal maintained postures. In this manuscript, the authors discuss their views on how the cerebellum may play a role in dystonia. The following topics are discussed: The relationships between neuronal/network dysfunctions and motor abnormalities in rodent models of dystonia. Data about brain structure, cerebellar metabolism, cerebellar connections, and noninvasive cerebellar stimulation that support (or not) a role for the cerebellum in human dystonia. Connections between the cerebellum and motor cortical and sub-cortical structures that could support a role for the cerebellum in dystonia. Overall points of consensus include: Neuronal dysfunction originating in the cerebellum can drive dystonic movements in rodent model systems. Imaging and neurophysiological studies in humans suggest that the cerebellum plays a role in the pathophysiology of dystonia, but do not provide conclusive evidence that the cerebellum is the primary or sole neuroanatomical site of origin.

Power Spectral Density Analysis of Purkinje Cell Tonic and Burst Firing Patterns From a Rat Model of Ataxia and Riluzole Treated

Introduction:

Purkinje Cell (PC) output displays a complex firing pattern consisting of high frequency sodium spikes and low frequency calcium spikes, and disruption in this firing behavior may contribute to cerebellar ataxia. Riluzole, neuroprotective agent, has been demonstrated to have neuroprotective effects in cerebellar ataxia. Here, the spectral analysis of PCs firing in control, 3-acetylpyridine (3-AP), neurotoxin agent, treated alone and riluzole plus 3-AP treated were investigated to determine changes in the firing properties. Difference in the power spectra of tonic and burst firing was assessed. Furthermore, the role of calcium-activated potassium channels in the power spectra was evaluated.

Methods:

Analysis was performed using Matlab. Power spectral density (PSD) of PCs output were obtained. Peak frequencies were extracted from the spectrum and statistical comparisons were done. In addition, a multi-compartment computational model of a Purkinje cell was used. This computational stimulation allowed us to study the changes in the power spectral density of the PC output as a result of alteration in ion channels.

Results:

Spectral analysis showed that in the spectrum of tonic and burst firing pattern only high sodium frequency and low calcium frequency was seen, respectively. In addition, there was a significant difference between the frequency components of PCs firing obtained from normal, ataxia and riluzole treated rats. Results indicated that sodium firing frequency of normal, ataxic and treated PCs occurred in approximate frequency of 22.53±5.49, 6.46±0.23, and 31.34±4.07 Hz, respectively; and calcium frequency occurred in frequency of 4.22±2.02, 1.52±1.19, and 3.88±1.37 Hz, respectively. The simulation results demonstrated that blockade of calcium-activated potassium channels in the PC model changed the PSD of the PC model firing activity. This change was similar to PSD changes in ataxia condition.

Conclusion:

These alterations in the spectrum of PC output may be a basis for developing possible new treatment strategies to improve cerebellar ataxia.

Aminopyridines for the treatment of neurologic disorders

PURPOSE OF REVIEW

To identify the different indications for the treatment of neurologic disorders with the potassium channel blockers 4-aminopyridine (4-AP) and 3,4-diaminopyridine (3,4-DAP).

RECENT FINDINGS

4-AP is an effective symptomatic treatment for downbeat nystagmus (DBN), episodic ataxia type 2 (EA2) (5-10 mg TID), and impaired gait in multiple sclerosis (MS) (10 mg BID). 3,4-DAP (5 mg/d-20 mg TID) improves symptoms in Lambert-Eaton myasthenic syndrome (LEMS) (randomized placebo-controlled trials for all 4 entities). 4-AP may also be effective in cerebellar gait ataxia of different etiologies (2 case series), upbeat nystagmus, and limb ataxia in MS (single cases). In the recommended dosages, they are well tolerated. The assumed mode of action is a blockade of mainly Kv_1.5: in DBN, this increases the excitability of Purkinje cells (PC), and in EA2, restores the precision of resting discharge of PC. In MS, 4-AP improves the conduction of action potentials in demyelinated axons, and in LEMS, 3,4-DAP facilitates the transmission at the neuromuscular endplate by prolonging the action potential duration.

SUMMARY

There is sufficient evidence that APs are indicated for the symptomatic treatment of DBN, EA2, gait ataxia due to MS and cerebellar disorders, and LEMS with a reasonable risk-benefit profile.

Pharmacotherapy of vestibular and cerebellar disorders and downbeat nystagmus: translational and back-translational research

There are currently eight groups of drugs for the pharmacotherapy of vertigo, nystagmus, and cerebellar disorders: antiemetics; anti-inflammatories, antimenieres, and antimigraineous medications; antidepressants, anticonvulsants, aminopyridines, and acetyl-DL-leucine ("the eight A's"). In acute unilateral vestibulopathy, corticosteroids improve the recovery of peripheral vestibular function, but there is not sufficient current evidence for a general recommendation. There is also insufficient evidence that 48 or 144 mg/day betahistine has an effect in Ménière's disease. Therefore, higher dosages are currently recommended; in animal studies, it was shown that betahistine increases cochlear blood flow. In vestibular paroxysmia, oxcarbazepine was effective (one yet not randomized controlled trial (RCT)). Aminopyridines are recommended for the treatment of downbeat nystagmus (two RCTs) and episodic ataxia type 2 (EA2, one RCT). There are so far no RCTs on vestibular migraine, so currently no treatment can be recommended. Acetyl-dl-leucine improves cerebellar ataxia (three observational studies); it also accelerates central compensation in an animal model of acute unilateral lesion, but RCTs were negative. There are ongoing RCTs on vestibular paroxysmia with carbamazepine (VESPA), acute unilateral vestibulopathy with betahistine (BETAVEST), vestibular migraine with metoprolol (PROVEMIG), benign paroxysmal positional vertigo with vitamin D (VitD@BPPV), EA2 with 4-aminopyridine versus acetazolamide (EAT-2-TREAT), and cerebellar ataxias with acetyl-DL-leucine (ALCAT).

Consensus paper: management of degenerative cerebellar disorders

Treatment of motor symptoms of degenerative cerebellar ataxia remains difficult. Yet there are recent developments that are likely to lead to significant improvements in the future. Most desirable would be a causative treatment of the underlying cerebellar disease. This is currently available only for a very small subset of cerebellar ataxias with known metabolic dysfunction. However, increasing knowledge of the pathophysiology of hereditary ataxia should lead to an increasing number of medically sensible drug trials. In this paper, data from recent drug trials in patients with recessive and dominant cerebellar ataxias will be summarized. There is consensus that up to date, no medication has been proven effective. Aminopyridines and acetazolamide are the only exception, which are beneficial in patients with episodic ataxia type 2. Aminopyridines are also effective in a subset of patients presenting with downbeat nystagmus. As such, all authors agreed that the mainstays of treatment of degenerative cerebellar ataxia are currently physiotherapy, occupational therapy, and speech therapy. For many years, well-controlled rehabilitation studies in patients with cerebellar ataxia were lacking. Data of recently published studies show that coordinative training improves motor function in both adult and juvenile patients with cerebellar degeneration. Given the well-known contribution of the cerebellum to motor learning, possible mechanisms underlying improvement will be outlined. There is consensus that evidence-based guidelines for the physiotherapy of degenerative cerebellar ataxia need to be developed. Future developments in physiotherapeutical interventions will be discussed including application of non-invasive brain stimulation.

Flocculus Purkinje cell signals in mouse Cacna1a calcium channel mutants of escalating severity: an investigation of the role of firing irregularity in ataxia

Mutation of the Cacna1a gene for the P/Q (CaV2.1) calcium channel invariably leads to cerebellar dysfunction. The dysfunction has been attributed to disrupted rhythmicity of cerebellar Purkinje cells, but the hypothesis remains unproven. If irregular firing rates cause cerebellar dysfunction, then the irregularity and behavioral deficits should covary in a series of mutant strains of escalating severity. We compared firing irregularity in floccular and anterior vermis Purkinje cells in the mildly affected rocker and moderately affected tottering Cacna1a mutants and normal C57BL/6 mice. We also measured the amplitude and timing of modulations of floccular Purkinje cell firing rate during the horizontal vestibuloocular reflex (VOR, 0.25-1 Hz) and the horizontal and vertical optokinetic reflex (OKR, 0.125-1 Hz). We recorded Purkinje cells selective for rotational stimulation about the vertical axis (VAPCs) and a horizontal axis (HAPCs). Irregularity scaled with behavioral deficit severity in the flocculus but failed to do so in the vermis, challenging the irregularity hypothesis. Mutant VAPCs exhibited unusually strong modulation during VOR and OKR, the response augmentation scaling with phenotypic severity. HAPCs exhibited increased OKR modulation but in tottering only. The data contradict prior claims that modulation amplitude is unaffected in tottering but support the idea that attenuated compensatory eye movements in Cacna1a mutants arise from defective transfer of Purkinje cell signals to downstream circuitry, rather than attenuated synaptic transmission within the cerebellar cortex. Shifts in the relative sizes of the VAPC and HAPC populations raise the possibility that Cacna1a mutations influence the development of floccular zone architecture.

The first knockin mouse model of episodic ataxia type 2

Episodic ataxia type 2 (EA2) is an autosomal dominant disorder associated with attacks of ataxia that are typically precipitated by stress, ethanol, caffeine or exercise. EA2 is caused by loss-of-function mutations in the CACNA1A gene, which encodes the α1A subunit of the CaV2.1 voltage-gated Ca(2+) channel. To better understand the pathomechanisms of this disorder in vivo, we created the first genetic animal model of EA2 by engineering a mouse line carrying the EA2-causing c.4486T>G (p.F1406C) missense mutation in the orthologous mouse Cacna1a gene. Mice homozygous for the mutated allele exhibit a ~70% reduction in CaV2.1 current density in Purkinje cells, though surprisingly do not exhibit an overt motor phenotype. Mice hemizygous for the knockin allele (EA2/- mice) did exhibit motor dysfunction measurable by rotarod and pole test. Studies using Cre-flox conditional genetics explored the role of cerebellar Purkinje cells or cerebellar granule cells in the poor motor performance of EA2/- mice and demonstrate that manipulation of either cell type alone did not cause poor motor performance. Thus, it is possible that subtle dysfunction arising from multiple cell types is necessary for the expression of certain ataxia syndromes.

The Clinical Spectrum of Autosomal-Dominant Episodic Ataxias

Autosomal-dominant episodic ataxias (EAs) represent a clinically and genetically heterogeneous group of disorders characterized by recurrent episodes of cerebellar ataxia (CA). Ataxia episodes are usually of short duration and often triggered by specific stimuli. There are currently seven classified subtypes of EA. EA types 1 and 2 have the highest prevalence and are therefore the clinically most relevant. Between attacks, EA 1 is associated with myokymia. In EA 2, often an interictal downbeat nystagmus with other cerebellar ocular dysfunctions is present; patients with EA 2 may display slowly progessive ataxia and vermian atrophy. EA 1 and 2 are both channelopathies, affecting the potassium channel gene, KCNA1, in EA 1 and the PQ calcium channel-encoding gene, CACNA1A, in EA 2. The types EA 3 to 7 are very rare and have to be further elucidated. Here, we review the historical, clinical, and genetic aspects of autosomal-dominant EAs and their current treatment, focusing on EA 1 and 2.

Introducing treatment strategy for cerebellar ataxia in mutant med mice: combination of acetazolamide and 4-aminopyridine

Purkinje neurons are the sole output neuron of the cerebellar cortex, and they generate high-frequency action potentials. Electrophysiological dysfunction of Purkinje neurons causes cerebellar ataxia. Mutant med mice have the lack of expression of the Scn8a gene. This gene encodes the NaV1.6 protein. In med Purkinje neurons, regular high-frequency firing is slowed, and med mice are ataxic. The aim of this study was to propose the neuroprotective drugs which could be useful for ataxia treatment in med mice, and to investigate the neuroprotective effects of these drugs by simulation. Simulation results showed that Kv4 channel inhibitors and BK channel activators restored the normal electrophysiological properties of the med Purkinje neurons. 4-Aminopyridine (4-AP) and acetazolamide (ACTZ) were proposed as neuroprotective drugs for Kv4 channel inhibitor and BK channel activator, respectively.

4-aminopyridine does not enhance flocculus function in tottering, a mouse model of vestibulocerebellar dysfunction and ataxia

The potassium channel antagonist 4-aminopyridine (4-AP) improves a variety of motor abnormalities associated with disorders of the cerebellum. The most rigorous quantitative data relate to 4-AP's ability to improve eye movement deficits in humans referable to dysfunction of the cerebellar flocculus. Largely based on work in the ataxic mouse mutant tottering (which carries a mutation of the Cacna1a gene of the P/Q voltage-activated calcium channel), 4-AP is hypothesized to function by enhancing excitability or rhythmicity of floccular Purkinje cells. We tested this hypothesis by determining whether systemic or intrafloccular administration of 4-AP would ameliorate the eye movement deficits in tottering that are attributable to flocculus dysfunction, including the reductions in amplitude of the yaw-axis vestibulo-ocular reflex (VOR) and vision-enhanced vestibulo-ocular reflex (VVOR), and the optokinetic reflex (OKR) about yaw and roll axes. Because tottering's deficits increase with age, both young and elderly mutants were tested to detect any age-dependent 4-AP effects. 4-AP failed to improve VOR, VVOR, and OKR gains during sinusoidal stimuli, although it may have reduced the tendency of the mutants' responses to VOR and VVOR to decline over the course of a one-hour recording session. For constant-velocity optokinetic stimuli, 4-AP generated some enhancement of yaw OKR and upward-directed roll OKR, but the effects were also seen in normal C57BL/6 controls, and thus do not represent a specific reversal of the electrophysiological consequences of the tottering mutation. Data support a possible extra-floccular locus for the effects of 4-AP on habituation and roll OKR. Unilateral intrafloccular 4-AP injections did not affect ocular motility, except to generate mild eye elevations, consistent with reduced floccular output. Because 4-AP did not produce the effects expected if it normalized outputs of floccular Purkinje cells, there is a need for further studies to elucidate the drug's mechanism of action on cerebellar motor dysfunction.

Stress, caffeine and ethanol trigger transient neurological dysfunction through shared mechanisms in a mouse calcium channelopathy

Several episodic neurological disorders are caused by ion channel gene mutations. In patients, transient neurological dysfunction is often evoked by stress, caffeine and ethanol, but the mechanisms underlying these triggers are unclear because each has diverse and diffuse effects on the CNS. Attacks of motor dysfunction in the Ca(V)2.1 calcium channel mouse mutant tottering are also triggered by stress, caffeine and ethanol. Therefore, we used the tottering mouse attacks to explore the pathomechanisms of the triggers. Despite the diffuse physiological effects of these triggers, ryanodine receptor blockers prevented attacks induced by all of them. In contrast, compounds that potentiate ryanodine receptors triggered attacks suggesting a convergent biochemical pathway. Tottering mouse attacks were both induced and blocked within the cerebellum suggesting that the triggers act locally to instigate attacks. In fact, stress, caffeine and alcohol precipitated attacks in Ca(V)2.1 mutant mice in which genetic pathology was limited to cerebellar Purkinje cells, suggesting that the triggers initiate dysfunction within a specific brain region. The surprising biochemical and anatomical specificity of the triggers and the discovery that the triggers operate through shared mechanisms suggest that it is possible to develop targeted therapies aimed at blocking the induction of episodic neurological dysfunction, rather than treating the symptoms once provoked.

The ataxic mouse as a model for studying downbeat nystagmus

Downbeat nystagmus (DBN) is a common eye movement complication of cerebellar disease. Use of mice to study pathophysiology of vestibulocerebellar disease is increasing, but it is unclear if mice can be used to study DBN; it has not been reported in this species. We determined whether DBN occurs in the ataxic mutant tottering, which carries a mutation in the Cacna1a gene for P/Q calcium channels. Spontaneous DBN occurred only rarely, and its magnitude did not exhibit the relationship to head tilt seen in human patients. DBN during yaw rotation was more common and shares some properties with the tilt-independent, gaze-independent component of human DBN, but differs in its dependence on vision. Hyperactivity of otolith circuits responding to pitch tilts is hypothesized to contribute to the gaze-independent component of human DBN. Mutants exhibited hyperactivity of the tilt maculo-ocular reflex (tiltMOR) in pitch. The hyperactivity may serve as a surrogate for DBN in mouse studies. TiltMOR hyperactivity correlates with hyperdeviation of the eyes and upward deviation of the head during ambulation; these may be alternative surrogates. Muscimol inactivation of the cerebellar flocculus suggests a floccular role in the tiltMOR hyperactivity and provides insight into the rarity of frank DBN in ataxic mice.

Computational insights into the neuroprotective action of riluzole on 3-acetylpyridine-induced ataxia in rats

OBJECTIVE

Intra-peritoneal administration of riluzole has been shown to preserve the membrane properties and firing characteristics of Purkinje neurons in a rat model of cerebellar ataxia induced by 3-acetylpyridine (3-AP). However, the exact mechanism(s) by which riluzole restores the normal electrophysiological properties of Purkinje neurons is not completely understood. Changes in the conductance of several ion channels, including the BK channels, have been proposed as a neuro protective target of riluzole. In this study, the possible cellular effects of riluzole on Purkinje cells from 3-AP-induced ataxic rats that could be responsible for its neuro protective action have been investigated by computer simulations.

MATERIALS AND METHODS

This is a computational stimulation study. The simulation environment enabled a change in the properties of the specific ion channels as the possible mechanism of action of riluzole. This allowed us to study the resulted changes in the firing activity of Purkinje cells without concerns about its other effects and interfering parameters in the experiments. Simulations were performed in the NEURON environment (Version 7.1) in a time step of 25 μs; analyses were conducted using MATLAB r2010a (The Mathworks). Data were given as mean ± SEM. Statistical analyses were performed by the student's t test, and differences were considered significant if p< 0.05.

RESULTS

The computational findings demonstrated that modulation of an individual ion channel current, as suggested by previous experimental studies, should not be considered as the only possible target for the neuro protective effects of riluzole to restore the normal firing activity of Purkinje cells from ataxic rats.

CONCLUSION

Changes in the conductance of several potassium channels, including voltage- gated potassium (Kv1, Kv4) and big Ca(2+)-activated K(+) (BK) channels may be responsible for the neuro protective effect of riluzole against 3-AP induced alterations in the firing properties of Purkinje cells in a rat model of ataxia.

Purkinje cell-specific ablation of Cav2.1 channels is sufficient to cause cerebellar ataxia in mice

The Cacna1a gene encodes the α(1A) subunit of voltage-gated Ca(V)2.1 Ca(2+) channels that are involved in neurotransmission at central synapses. Ca(V)2.1-α(1)-knockout (α1KO) mice, which lack Ca(V)2.1 channels in all neurons, have a very severe phenotype of cerebellar ataxia and dystonia, and usually die around postnatal day 20. This early lethality, combined with the wide expression of Ca(V)2.1 channels throughout the cerebellar cortex and nuclei, prohibited determination of the contribution of particular cerebellar cell types to the development of the severe neurobiological phenotype in Cacna1a mutant mice. Here, we crossed conditional Cacna1a mice with transgenic mice expressing Cre recombinase, driven by the Purkinje cell-specific Pcp2 promoter, to specifically ablate the Ca(V)2.1-α(1A) subunit and thereby Ca(V)2.1 channels in Purkinje cells. Purkinje cell Ca(V)2.1-α(1A)-knockout (PCα1KO) mice aged without difficulties, rescuing the lethal phenotype seen in α1KO mice. PCα1KO mice exhibited cerebellar ataxia starting around P12, much earlier than the first signs of progressive Purkinje cell loss, which appears in these mice between P30 and P45. Secondary cell loss was observed in the granular and molecular layers of the cerebellum and the volume of all individual cerebellar nuclei was reduced. In this mouse model with a cell type-specific ablation of Ca(V)2.1 channels, we show that ablation of Ca(V)2.1 channels restricted to Purkinje cells is sufficient to cause cerebellar ataxia. We demonstrate that spatial ablation of Ca(V)2.1 channels may help in unraveling mechanisms of human disease.

The role of regularity and synchrony of cerebellar Purkinje cells for pathological nystagmus

Previous theories assumed that the beneficial effect of the potassium channel blocker 4-aminopyridine (4-AP) for patients suffering from downbeat nystagmus (DBN) or episodic ataxia type 2 (EA2) is due to an increase of excitability of cerebellar Purkinje cells (PC). Recent experimental results using therapeutic doses of 4-AP with a mouse model of EA2 challenged the theory showing that 4-AP does not change the firing rate of PC but their regularity. Based on a mathematical model of the ocular motor and cerebellar circuitry, we show that changes in regularity have no effect without synchrony in PC firing. Together with synchronous firing, an increase in regularity may lead to a decrease in overall inhibition and may invert the inhibitory to an excitatory response due to imprinting, a novel effect of synchronized neural inhibition. Both effects are unlikely to be the causative mechanism for the success of 4-AP in treating cerebellar disorders.

Aminopyridines correct early dysfunction and delay neurodegeneration in a mouse model of spinocerebellar ataxia type 1

The contribution of neuronal dysfunction to neurodegeneration is studied in a mouse model of spinocerebellar ataxia type 1 (SCA1) displaying impaired motor performance ahead of loss or atrophy of cerebellar Purkinje cells. Presymptomatic SCA1 mice show a reduction in the firing rate of Purkinje cells (both in vivo and in slices) associated with a reduction in the efficiency of the main glutamatergic synapse onto Purkinje cells and with increased A-type potassium current. The A-type potassium channel Kv4.3 appears to be internalized in response to glutamatergic stimulation in Purkinje cells and accumulates in presymptomatic SCA1 mice. SCA1 mice are treated with aminopyridines, acting as potassium channel blockers to test whether the treatment could improve neuronal dysfunction, motor behavior, and neurodegeneration. In acutely treated young SCA1 mice, aminopyridines normalize the firing rate of Purkinje cells and the motor behavior of the animals. In chronically treated old SCA1 mice, 3,4-diaminopyridine improves the firing rate of Purkinje cells, the motor behavior of the animals, and partially protects against cell atrophy. Chronic treatment with 3,4-diaminopyridine is associated with increased cerebellar levels of BDNF, suggesting that partial protection against atrophy of Purkinje cells is possibly provided by an increased production of growth factors secondary to the reincrease in electrical activity. Our data suggest that aminopyridines might have symptomatic and/or neuroprotective beneficial effects in SCA1, that reduction in the firing rate of Purkinje cells can cause cerebellar ataxia, and that treatment of early neuronal dysfunction is relevant in neurodegenerative disorders such as SCA1.

Large Genomic Deletions in CACNA1A Cause Episodic Ataxia Type 2

Episodic ataxia (EA) syndromes are heritable diseases characterized by dramatic episodes of imbalance and incoordination. EA type 2 (EA2), the most common and the best characterized subtype, is caused by mostly nonsense, splice site, small indel, and sometimes missense mutations in CACNA1A. Direct sequencing of CACNA1A fails to identify mutations in some patients with EA2-like features, possibly due to incomplete interrogation of CACNA1A or defects in other EA genes not yet defined. Previous reports described genomic deletions between 4 and 40 kb in EA2. In 47 subjects with EA (26 with EA2-like features) who tested negative for mutations in the known EA genes, we used multiplex ligation-dependent probe amplification to analyze CACNA1A for exonic copy number variations. Breakpoints were further defined by long-range PCR. We identified distinct multi-exonic deletions in three probands with classic EA2-like features: episodes of prolonged vertigo and ataxia triggered by stress and fatigue, interictal nystagmus, with onset during infancy or early childhood. The breakpoints in all three probands are located in Alu sequences, indicating errors in homologous recombination of Alu sequences as the underlying mechanism. The smallest deletion spanned exons 39 and 40, while the largest deletion spanned 200 kb, missing all but the first three exons. One deletion involving exons 39 through 47 arose spontaneously. The search for mutations in CACNA1A appears most fruitful in EA patients with interictal nystagmus and onset early in life. The finding of large heterozygous deletions suggests haploinsufficiency as a possible pathomechanism of EA2.

Current treatment of vestibular, ocular motor disorders and nystagmus

Vertigo and dizziness are among the most common complaints with a lifetime prevalence of about 30%. The various forms of vestibular disorders can be treated with pharmacological therapy, physical therapy, psychotherapeutic measures or, rarely, surgery. In this review, the current pharmacological treatment options for peripheral and central vestibular, cerebellar and ocular motor disorders will be described. They are as follows for peripheral vestibular disorders. In vestibular neuritis recovery of the peripheral vestibular function can be improved by treatment with oral corticosteroids. In Menière's disease a recent study showed long-term high-dose treatment with betahistine has a significant effect on the frequency of the attacks. The use of aminopyridines introduced a new therapeutic principle in the treatment of downbeat and upbeat nystagmus and episodic ataxia type 2 (EA 2). These potassium channel blockers presumably increase the activity and excitability of cerebellar Purkinje cells, thereby augmenting the inhibitory influence of these cells on vestibular and cerebellar nuclei. A few studies showed that baclofen improves periodic alternating nystagmus, and gabapentin and memantine, pendular nystagmus. However, many other eye movement disorders such as ocular flutter opsoclonus, central positioning, or see-saw nystagmus are still difficult to treat. Although progress has been made in the treatment of vestibular neuritis, downbeat and upbeat nystagmus, as well as EA 2, state-of-the-art trials must still be performed on many vestibular and ocular motor disorders, namely Menière's disease, bilateral vestibular failure, vestibular paroxysmia, vestibular migraine, and many forms of central eye movement disorders.

A randomized trial of 4-aminopyridine in EA2 and related familial episodic ataxias

OBJECTIVE

The therapeutic effects of 4-aminopyridine (4AP) were investigated in a randomized, double-blind, crossover trial in 10 subjects with familial episodic ataxia with nystagmus.

METHODS

After randomization, placebo or 4AP (5 mg 3 times daily) was administered for 2 3-month-long treatment periods separated by a 1-month-long washout period. The primary outcome measure was the number of ataxia attacks per month; the secondary outcome measures were the attack duration and patient-reported quality of life (Vestibular Disorders Activities of Daily Living Scale [VDADL]). Nonparametric tests and a random-effects model were used for statistical analysis.

RESULTS

The diagnosis of episodic ataxia type 2 (EA2) was genetically confirmed in 7 subjects. Patients receiving placebo had a median monthly attack frequency of 6.50, whereas patients taking 4AP had a frequency of 1.65 (p = 0.03). Median monthly attack duration decreased from 13.65 hours with placebo to 4.45 hours with 4AP (p = 0.08). The VDADL score decreased from 6.00 to 1.50 (p = 0.02). 4AP was well-tolerated.

CONCLUSIONS

This controlled trial on EA2 and familial episodic ataxia with nystagmus demonstrated that 4AP decreases attack frequency and improves quality of life.

LEVEL OF EVIDENCE

This crossover study provides Class II evidence that 4AP decreases attack frequency and improves the patient-reported quality of life in patients with episodic ataxia and related familial ataxias.

Pharmacotherapy of vestibular and ocular motor disorders, including nystagmus

We review current pharmacological treatments for peripheral and central vestibular disorders, and ocular motor disorders that impair vision, especially pathological nystagmus. The prerequisites for successful pharmacotherapy of vertigo, dizziness, and abnormal eye movements are the "4 D's": correct diagnosis, correct drug, appropriate dosage, and sufficient duration. There are seven groups of drugs (the "7 A's") that can be used: antiemetics; anti-inflammatory, anti-Ménière's, and anti-migrainous medications; anti-depressants, anti-convulsants, and aminopyridines. A recovery from acute vestibular neuritis can be promoted by treatment with oral corticosteroids. Betahistine may reduce the frequency of attacks of Ménière's disease. The aminopyridines constitute a novel treatment approach for downbeat and upbeat nystagmus, as well as episodic ataxia type 2 (EA 2); these drugs may restore normal "pacemaker" activity to the Purkinje cells that govern vestibular and cerebellar nuclei. A limited number of trials indicate that baclofen improves periodic alternating nystagmus, and that gabapentin and memantine improve acquired pendular and infantile (congenital) nystagmus. Preliminary reports suggest suppression of square-wave saccadic intrusions by memantine, and ocular flutter by beta-blockers. Thus, although progress has been made in the treatment of vestibular neuritis, some forms of pathological nystagmus, and EA 2, controlled, masked trials are still needed to evaluate treatments for many vestibular and ocular motor disorders, including betahistine for Ménière's disease, oxcarbazepine for vestibular paroxysmia, or metoprolol for vestibular migraine.

KCa channels as therapeutic targets in episodic ataxia type-2

Episodic ataxia type-2 (EA2) is an inherited movement disorder caused by mutations in the gene encoding the Ca(v)2.1alpha1 subunit of the P/Q-type voltage-gated calcium channel that result in an overall reduction in the P/Q-type calcium current. A consequence of these mutations is loss of precision of pacemaking in cerebellar Purkinje cells. This diminished precision reduces the information encoded by Purkinje cells and is thought to contribute to symptoms associated with this disorder. The loss of the precision of pacemaking in EA2 is the consequence of reduced activation of calcium-dependent potassium channels (K(Ca)) by the smaller calcium current and in vitro can be pharmacologically restored by K(Ca) activators. We used a well established mouse model of EA2, the tottering (tg/tg) mouse, to examine the potential therapeutic utility of one such Food and Drug Administration (FDA)-approved compound, chlorzoxazone (CHZ). Compared with wild-type Purkinje cells, we found the firing rate of tg/tg Purkinje cells in acutely prepared cerebellar slices to be very irregular. Bath application of CHZ successfully restored the precision of pacemaking in a dose-dependent manner. Oral administration of CHZ to tg/tg mice improved their baseline motor performance and reduced the severity, frequency, and duration of episodes of dyskinesia without producing any adverse effects. We propose the use of CHZ, which is currently FDA approved as a muscle relaxant, as a safe and novel treatment of EA2.

The therapeutic mode of action of 4-aminopyridine in cerebellar ataxia

Episodic ataxia type 2 (EA2) is a hereditary cerebellar ataxia associated with mutations in the P/Q-type voltage-gated calcium (Ca(2+)) channels. Therapeutic approaches for treatment of EA2 are very limited. Presently, the potassium (K(+)) channel blocker 4-aminopyridine (4-AP) constitutes the most promising treatment, although its mechanism of action is not understood. Here we show that, in contrast to what is commonly believed, therapeutic concentrations of 4-AP do not increase the inhibitory drive of cerebellar Purkinje cells. Instead, 4-AP restores the severely diminished precision of pacemaking in Purkinje cells of EA2 mutant mice by prolonging the action potential and increasing the action potential afterhyperpolarization. Consistent with this mode of action, the therapeutic efficacy of 4-AP was comparable, and not additive, to chlorzoxazone, an activator of Ca(2+)-dependent K(+) channels that also restores the precision of Purkinje cell pacemaking. The likely target of 4-AP at the concentrations used are the K(v)1 family of K(+) channels, possibly the K(v)1.5 subtype. Because at higher concentrations 4-AP blocks a large array of K(+) channels and is a proconvulsant, use of selective K(v)1 channel blockers is likely to be a safer substitute for treatment of cerebellar ataxia.

Medical treatment of vestibular disorders

BACKGROUND

The lifelong prevalence of rotatory vertigo is 30%. Despite this high figure, patients with vertigo generally receive either inappropriate or inadequate treatment. However, the majority of vestibular disorders have a benign cause, take a favorable natural course, and respond positively to therapy.

OBJECTIVE

This review puts special emphasis on the medical rather than the physical, operative, or psychotherapeutic treatments available.

METHODS

A selected review of recent reports and studies on the medical treatment of peripheral and central vestibular disorders.

RESULTS/CONCLUSIONS

In vestibular neuritis, recovery of the peripheral vestibular function can be improved by oral corticosteroids; in Menière's disease, there is first evidence that high-dose, long-term administration of betahistine reduces attack frequency; carbamazepine or oxcarbamazepine is the treatment of first choice in vestibular paroxysmia, a disorder mainly caused by neurovascular cross-compression; the potassium channel blocker aminopyridine provides a new therapeutic principle for treatment of downbeat nystagmus, upbeat nystagmus, and episodic ataxia type 2.

Low-frequency oscillations in the cerebellar cortex of the tottering mouse

The tottering mouse is an autosomal recessive disorder involving a missense mutation in the gene encoding P/Q-type voltage-gated Ca2+ channels. The tottering mouse has a characteristic phenotype consisting of transient attacks of dystonia triggered by stress, caffeine, or ethanol. The neural events underlying these episodes of dystonia are unknown. Flavoprotein autofluorescence optical imaging revealed transient, low-frequency oscillations in the cerebellar cortex of anesthetized and awake tottering mice but not in wild-type mice. Analysis of the frequencies, spatial extent, and power were used to characterize the oscillations. In anesthetized mice, the dominant frequencies of the oscillations are between 0.039 and 0.078 Hz. The spontaneous oscillations in the tottering mouse organize into high power domains that propagate to neighboring cerebellar cortical regions. In the tottering mouse, the spontaneous firing of 83% (73/88) of cerebellar cortical neurons exhibit oscillations at the same low frequencies. The oscillations are reduced by removing extracellular Ca2+ and blocking L-type Ca2+ channels. The oscillations are likely generated intrinsically in the cerebellar cortex because they are not affected by blocking AMPA receptors or by electrical stimulation of the parallel fiber-Purkinje cell circuit. Furthermore, local application of an L-type Ca2+ agonist in the tottering mouse generates oscillations with similar properties. The beam-like response evoked by parallel fiber stimulation is reduced in the tottering mouse. In the awake tottering mouse, transcranial flavoprotein imaging revealed low-frequency oscillations that are accentuated during caffeine-induced attacks of dystonia. During dystonia, oscillations are also present in the face and hindlimb electromyographic (EMG) activity that become significantly coherent with the oscillations in the cerebellar cortex. These low-frequency oscillations and associated cerebellar cortical dysfunction demonstrate a novel abnormality in the tottering mouse. These oscillations are hypothesized to be involved in the episodic movement disorder in this mouse model of episodic ataxia type 2.

Paroxysmal dyskinesias in mice

Animal models of human disease are important tools for revealing the underlying mechanisms of pathophysiology and developing therapeutic strategies. Several unique mouse calcium channel mutants have been identified with nonepileptic, episodic dyskinetic movements that are phenotypically similar to human paroxysmal dyskinesias. In this report, video demonstrations of these motor attacks are provided for two previously described mouse mutants, tottering and lethargic, as well as a new one, rocker. Semiquantitative comparisons using two different rating scales reveal differences in attack morphology, severity, and duration among the strains. These mice provide three independent models of paroxysmal dyskinesia and support for prior proposals that channelopathies may underlie the human disorders.

Altered functional expression of Purkinje cell calcium channels precedes motor dysfunction in tottering mice

In tottering mice, a point mutation in the gene encoding P-type (Ca(v)2.1) voltage-gated calcium channels results in ataxia, absence epilepsy, and motor dystonia that appear 3-4 weeks postnatally. The aberrant motor behaviors have been linked to cerebellar dysfunction, and adult Purkinje cells (PCs) of tottering mice exhibit calcium-dependent changes in gene transcription suggestive of altered calcium homeostasis. In an attempt to identify early postnatal events important for the development of the behavioral phenotype, we examined calcium channel expression in cerebellar PCs from postnatal days 6-15 (P6-15). Whole cell recording was combined with selective calcium channel antagonists to allow discrimination of the various voltage-activated calcium channels types; early age-dependent differences between tottering and wild-type PCs were found. Wild-type PCs experienced a steady increase in P current density over this period, resulting in a twofold change by P15. In tottering, by contrast, P current density remained unchanged from P6-8 and was only 25% of the wild-type level by P8. A developmental delay in functional expression was implicated in this early deficit, since ensuing gains over the subsequent week brought tottering P current density close to the wild-type level by P15. At this age, tottering PCs also exhibited a 2.2-fold higher L-type calcium current density than that expressed by wild-type PCs. Increases in N current were apparent at some ages, most strikingly within a subset of tottering PCs at P15. Functional R- and T-type calcium current densities were equivalent to wild-type levels at all ages. We conclude that the tottering mutation brings about selective changes in functional calcium channel expression 1 to 2 weeks prior to the appearance of the behavioral deficits, raising the possibility that they represent an early, primary event along the path to motor dysfunction in tottering.

Fampridine-SR for multiple sclerosis and spinal cord injury

Fampridine-SR is a sustained-release tablet form of the K(+) channel-blocking compound 4-aminopyridine that has been shown to restore conduction in focally demyelinated axons, to enhance synaptic transmission in many types of neurons and to potentiate muscle contraction. The present review describes the mechanism of action and chemistry of Fampridine-SR, its pharmacokinetics and safety, and the outcomes of clinical trials of its safety and efficacy for enhancing neuromuscular function in patients with multiple sclerosis or spinal cord injury. Randomized clinical trials completed to date indicate that this form of K(+) channel blockade may be useful for the improvement of walking ability in patients with multiple sclerosis.

Episodic ataxia type 2

Episodic ataxia type 2 (EA 2) is a rare neurological disorder of autosomal dominant inheritance resulting from dysfunction of a voltage-gated calcium channel. It manifests with recurrent disabling attacks of imbalance, vertigo, and ataxia, and can be provoked by physical exertion or emotional stress. In the spell-free interval, patients present with central ocular motor dysfunction, mainly downbeat nystagmus. A slow progression of cerebellar signs accompanied by a slight atrophy of midline cerebellar structures is commonly observed during the course of the disease. EA 2 is caused most often by the loss of function mutations of the calcium channel gene CACNA1A, which encodes the Ca(v)2.1 subunit of the P/Q-type calcium channel and is primarily expressed in Purkinje cells. To date, more than 30 mutations have been described. Two effective treatment options have been established for EA 2: acetazolamide (ACTZ), which probably changes the intracellular pH and thereby the transmembraneous potential, and 4-aminopyridine (4-AP), a potassium channel blocker. Approximately 70% of all patients respond to treatment with ACTZ, but the effect is often only transient. In an open trial, 4-AP prevented attacks in five of six patients with EA 2, most likely by increasing the resting activity and excitability of the Purkinje cells. These findings were confirmed by experiments in animal models of EA 2. Many aspects of the pathophysiology (e.g., induction of the attacks) and treatment of EA 2 (e.g., mode of action of ACTZ and 4-AP) still remain unclear and need to be addressed in further animal and clinical studies.

Molecular pathogenesis of spinocerebellar ataxia type 6

Spinocerebellar ataxia type 6 (SCA6) is a neurodegenerative disorder caused by abnormal expansions of a trinucleotide CAG repeat in exon 47 of the CACNA1A gene, which encodes the alpha1A subunit of the P/Q-type voltage-gated calcium channel. The CAG repeat expansion is translated into an elongated polyglutamine tract in the carboxyl terminus of the alpha1A subunit. The alpha1A subunit is the main pore-forming subunit of the P/Q-type calcium channel. Patients with SCA6 suffer from a severe form of progressive ataxia and cerebellar dysfunction. Design of treatments for this disorder will depend on better definition of the mechanism of disease. As a disease arising from a mutation in an ion channel gene, SCA6 may behave as an ion channelopathy, and may respond to attempts to modulate or correct ion channel function. Alternatively, as a disease in which the mutant protein contains an expanded polyglutamine tract, SCA6 may respond to the targets of drug therapies developed for Huntington's disease and other polyglutamine disorders. In this review we will compare SCA6 to other polyglutamine diseases and channelopathies, and we will highlight recent advances in our understanding of alpha1A subunits and SCA6 pathology. We also propose a mechanism for how two seemingly divergent hypotheses can be combined into a cohesive model for disease progression.

Ion channel pharmacology

Because ion channels are involved in many cellular processes, drugs acting on ion channels have long been used for the treatment of many diseases, especially those affecting electrically excitable tissues. The present review discusses the pharmacology of voltage-gated and neurotransmitter-gated ion channels involved in neurologic diseases, with emphasis on neurologic channelopathies. With the discovery of ion channelopathies, the therapeutic value of many basic drugs targeting ion channels has been confirmed. The understanding of the genotype-phenotype relationship has highlighted possible action mechanisms of other empirically used drugs. Moreover, other ion channels have been pinpointed as potential new drug targets. With regards to therapy of channelopathies, experimental investigations of the intimate drug-channel interactions have demonstrated that channel mutations can either increase or decrease affinity for the drug, modifying its potential therapeutic effect. Together with the discovery of channel gene polymorphisms that may affect drug pharmacodynamics, these findings highlight the need for pharmacogenetic research to allow identification of drugs with more specific effects on channel isoforms or mutants, to increase efficacy and reduce side effects. With a greater understanding of channel genetics, structure, and function, together with the identification of novel primary and secondary channelopathies, the number of ion channel drugs for neurologic channelopathies will increase substantially.

Pharmacological advances in the treatment of neuro-otological and eye movement disorders

PURPOSE OF REVIEW

First, to describe the current pharmacological treatment options for peripheral and central vestibular, cerebellar, and ocular motor disorders. Second, to identify vestibular and ocular motor disorders in which treatment trials are warranted.

RECENT FINDINGS

Peripheral vestibular disorders: In vestibular neuritis recovery of the peripheral vestibular function can be improved by treatment with oral corticosteroids. In Ménière's disease treatment strategies range from low-salt diet, diuretics, and betahistine, to intratympanic injection of corticosteroids or gentamicin. Unfortunately most of the trials on Ménière's disease do not have an up-to-date design. In bilateral vestibulopathy steroids do not seem to improve vestibular function.Central vestibular, cerebellar, and ocular motor disorders: The use of aminopyridines introduced a new therapeutic principle in the treatment of downbeat and upbeat nystagmus and episodic ataxia type 2 (EA2). These potassium channel blockers presumably increase the activity and excitability of cerebellar Purkinje cells, thereby augmenting the inhibitory influence of these cells on vestibular and cerebellar nuclei. A few studies showed that baclofen improves periodic alternating nystagmus, and gabapentin and memantine, pendular nystagmus. Many other eye movement disorders, however, such as ocular flutter, opsoclonus, central positioning, or see-saw nystagmus are still difficult to treat.

SUMMARY

Although progress has been made in the treatment of vestibular neuritis, downbeat and upbeat nystagmus, as well as EA2, state-of-the-art trials must still be performed on many vestibular and ocular motor disorders, namely Ménière's disease, bilateral vestibulopathy, vestibular paroxysmia, vestibular migraine, and many forms of central eye movement disorders.

Decreases in the precision of Purkinje cell pacemaking cause cerebellar dysfunction and ataxia

Episodic ataxia type-2 (EA2) is caused by mutations in P/Q-type voltage-gated calcium channels that are expressed at high densities in cerebellar Purkinje cells. Because P/Q channels support neurotransmitter release at many synapses, it is believed that ataxia is caused by impaired synaptic transmission. Here we show that in ataxic P/Q channel mutant mice, the precision of Purkinje cell pacemaking is lost such that there is a significant degradation of the synaptic information encoded in their activity. The irregular pacemaking is caused by reduced activation of calcium-activated potassium (K(Ca)) channels and was reversed by pharmacologically increasing their activity with 1-ethyl-2-benzimidazolinone (EBIO). Moreover, chronic in vivo perfusion of EBIO into the cerebellum of ataxic mice significantly improved motor performance. Our data support the hypothesis that the precision of intrinsic pacemaking in Purkinje cells is essential for motor coordination and suggest that K(Ca) channels may constitute a potential therapeutic target in EA2.

Behavioral seizure correlates in animal models of epilepsy: a road map for assay selection, data interpretation, and the search for causal mechanisms

A broad spectrum of learning/memory, social interaction, and affective behavioral measures serve as functional correlates for neurobiological changes in seizure-prone animals as well as in epileptic clinical populations. The utility of such measures is demonstrated by their ability to distinguish anomalous characteristics in developing organisms predisposed to seizure onset, as well as to discriminate prior seizure history in organisms with established pathology. For instance, typical findings that generalize across species suggest that seizure-experienced organisms exhibit a variety of deficits in cognitive function as well as inappropriate social neglect and aggression. Behavioral testing batteries have also proven useful in assessing neural mechanisms for seizure induction, subcortical neural circuits, and neuropeptide modulators, for example, as well as in identifying neural pathology resulting from prior seizure activity. However, the wanton application of behavioral tests can also produce false positives in the identification of seizure-related disorders unless alternative performance and motivational hypotheses are discounted effectively. Accordingly, the present review attempts to provide the reader interested in behavioral phenotyping and characterization of seizure-prone rats and mice with a roadmap for rational selection, implementation, and interpretation of data from behavior assays while highlighting potential successes and pitfalls inherent in employing functional correlates of brain activity using animal models of epilepsy.