Reduced pallidal output causes dystonia

Rethinking the external globus pallidus and information flow in cortico-basal ganglia-thalamic circuits

For decades, the external globus pallidus (GPe) has been viewed as a passive way-station in the indirect pathway of the cortico-basal ganglia-thalamic (CBGT) circuit, sandwiched between striatal inputs and basal ganglia outputs. According to this model, one-way descending striatal signals in the indirect pathway amplify the suppression of downstream thalamic nuclei by inhibiting GPe activity. Here, we revisit this assumption, in light of new and emerging work on the cellular complexity, connectivity and functional role of the GPe in behaviour. We show how, according to this new circuit-level logic, the GPe is ideally positioned for relaying ascending and descending control signals within the basal ganglia. Focusing on the problem of inhibitory control, we illustrate how this bidirectional flow of information allows for the integration of reactive and proactive control mechanisms during action selection. Taken together, this new evidence points to the GPe as being a central hub in the CBGT circuit, participating in bidirectional information flow and linking multifaceted control signals to regulate behaviour.

[Dynamic activity model of movement disorders: a unified view to understand their pathophysiology]

Malfunction of the basal ganglia leads to movement disorders such as Parkinson's disease, dystonia, Huntington's disease, dyskinesia, and hemiballism, but their underlying pathophysiology is still subject to debate. To understand their pathophysiology in a unified manner, we propose the "dynamic activity model", on the basis of alterations of cortically induced responses in individual nuclei of the basal ganglia. In the normal state, electric stimulation in the motor cortex, mimicking cortical activity during initiation of voluntary movements, evokes a triphasic response consisting of early excitation, inhibition, and late excitation in the output stations of the basal ganglia of monkeys, rodents, and humans. Among three components, cortically induced inhibition, which is mediated by the direct pathway, releases an appropriate movement at an appropriate time by disinhibiting thalamic and cortical activity, whereas early and late excitation, which is mediated by the hyperdirect and indirect pathways, resets on-going cortical activity and stops movements, respectively. Cortically induced triphasic response patterns are systematically altered in various movement disorder models and could well explain the pathophysiology of their motor symptoms. In monkey and mouse models of Parkinson's disease, cortically induced inhibition is reduced and prevents the release of movements, resulting in akinesia/bradykinesia. On the other hand, in a mouse model of dystonia, cortically induced inhibition is enhanced and releases unintended movements, inducing involuntary muscle contractions. Moreover, after blocking the subthalamic nucleus activity in a monkey model of Parkinson's disease, cortically induced inhibition is recovered and enables voluntary movements, explaining the underlying mechanism of stereotactic surgery to ameliorate parkinsonian motor signs. The "dynamic activity model" gives us a more comprehensive view of the pathophysiology underlying motor symptoms of movement disorders and clues for their novel therapies.

External segment of the globus pallidus in health and disease: Its interactions with the striatum and subthalamic nucleus

The external segment of the globus pallidus (GPe) has long been considered a homogeneous structure that receives inputs from the striatum and sends processed information to the subthalamic nucleus, composing a relay nucleus of the indirect pathway that contributes to movement suppression. Recent methodological revolution in rodents led to the identification of two distinct cell types in the GPe with different fiber connections. The GPe may be regarded as a dynamic, complex and influential center within the basal ganglia circuitry, rather than a simple relay nucleus. On the other hand, many studies have so far been performed in monkeys to clarify the functions of the basal ganglia in the healthy and diseased states, but have not paid much attention to such classification and functional differences of GPe neurons. In this minireview, we consider the knowledge on the rodent GPe and discuss its impact on the understanding of the basal ganglia circuitry in monkeys.

Rethinking the external globus pallidus and information flow in cortico-basal ganglia-thalamic circuits

For decades the external globus pallidus (GPe) has been viewed as a passive way-station in the indirect pathway of the cortico-basal ganglia-thalamic (CBGT) circuit, sandwiched between striatal inputs and basal ganglia outputs. According to this model, one-way descending striatal signals in the indirect pathway amplify the suppression of downstream thalamic nuclei by inhibiting GPe activity. Here we revisit this assumption, in light of new and emerging work on the cellular complexity, connectivity, and functional role of the GPe in behavior. We show how, according to this new circuit-level logic, the GPe is ideally positioned for relaying ascending and descending control signals within the basal ganglia. Focusing on the problem of inhibitory control, we illustrate how this bidirectional flow of information allows for the integration of reactive and proactive control mechanisms during action selection. Taken together, this new evidence points to the GPe as being a central hub in the CBGT circuit, participating in bidirectional information flow and linking multifaceted control signals to regulate behavior.

The transcription regulator Lmo3 is required for the development of medial ganglionic eminence derived neurons in the external globus pallidus

The external globus pallidus (GPe) is an essential component of the basal ganglia, a group of subcortical nuclei that are involved in control of action. Changes in the firing of GPe neurons are associated with both passive and active body movements. Aberrant activity of GPe neurons has been linked to motor symptoms of a variety of movement disorders, such as Parkinson's Disease, Huntington's disease and dystonia. Recent studies have helped delineate functionally distinct subtypes of GABAergic GPe projection neurons. However, not much is known about specific molecular mechanisms underlying the development of GPe neuronal subtypes. We show that the transcriptional regulator Lmo3 is required for the development of medial ganglionic eminence derived Nkx2.1+ and PV+ GPe neurons, but not lateral ganglionic eminence derived FoxP2+ neurons. As a consequence of the reduction in PV+ neurons, Lmo3-null mice have a reduced GPe input to the subthalamic nucleus.

Cell and circuit complexity of the external globus pallidus

The external globus pallidus (GPe) of the basal ganglia has been underappreciated owing to poor understanding of its cells and circuits. It was assumed that the GPe consisted of a homogeneous neuron population primarily serving as a 'relay station' for information flowing through the indirect basal ganglia pathway. However, the advent of advanced tools in rodent models has sparked a resurgence in interest in the GPe. Here, we review recent data that have unveiled the cell and circuit complexity of the GPe. These discoveries have revealed that the GPe does not conform to traditional views of the basal ganglia. In particular, recent evidence confirms that the afferent and efferent connections of the GPe span both the direct and the indirect pathways. Furthermore, the GPe displays broad interconnectivity beyond the basal ganglia, consistent with its emerging multifaceted roles in both motor and non-motor functions. In summary, recent data prompt new proposals for computational rules of the basal ganglia.

DYT1 dystonia: Neurophysiological properties of the pallidal activity

OBJECTIVES

The aim of this paper is to find the differences in the physiology of the pallidal neurons in DYT1 and non-DYT1 dystonia.

METHODS

We performed microelectrode recording of the single unit activity in both segments of the globus pallidus during stereotactic implantation of electrodes for deep brain stimulation (DBS).

RESULTS

We found a reduced firing rate, reduced burst rate, and increased pause index in both pallidal segments in DYT1. Also, in DYT1 the activity in both pallidal segments was similar, but not so in non-DYT1.

CONCLUSION

The results suggest a common pathological focus for both pallidal segments, located in the striatum. We also speculate that strong striatal influence on GPi and GPe overrides other input sources to the pallidal nuclei causing similarity in neuronal activity.

SIGNIFICANCE

We found significant differences in neuronal activity between DYT1 and non-DYT1 neurons. Our findings shed light on the pathophysiology of DYT-1 dystonia which can be very different from non-DYT1 dystonia and have other efficient treatment tactics.

Novel targets in deep brain stimulation for movement disorders

The neurosurgical treatment of movement disorders, primarily via deep brain stimulation (DBS), is a rapidly expanding and evolving field. Although conventional targets including the subthalamic nucleus (STN) and internal segment of the globus pallidus (GPi) for Parkinson's disease and ventral intermediate nucleus of the thalams (VIM) for tremor provide substantial benefit in terms of both motor symptoms and quality of life, other targets for DBS have been explored in an effort to maximize clinical benefit and also avoid undesired adverse effects associated with stimulation. These novel targets primarily include the rostral zona incerta (rZI), caudal zona incerta (cZI)/posterior subthalamic area (PSA), prelemniscal radiation (Raprl), pedunculopontine nucleus (PPN), substantia nigra pars reticulata (SNr), centromedian/parafascicular (CM/PF) nucleus of the thalamus, nucleus basalis of Meynert (NBM), dentato-rubro-thalamic tract (DRTT), dentate nucleus of the cerebellum, external segment of the globus pallidus (GPe), and ventral oralis (VO) complex of the thalamus. However, reports of outcomes utilizing these targets are scattered and disparate. In order to provide a comprehensive resource for researchers and clinicians alike, we have summarized the existing literature surrounding these novel targets, including rationale for their use, neurosurgical techniques where relevant, outcomes and adverse effects of stimulation, and future directions for research.

Optogenetic Activation of Striatopallidal Neurons Reveals Altered HCN Gating in DYT1 Dystonia

Firing activity of external globus pallidus (GPe) is crucial for motor control and is severely perturbed in dystonia, a movement disorder characterized by involuntary, repetitive muscle contractions. Here, we show that GPe projection neurons exhibit a reduction of firing frequency and an irregular pattern in a DYT1 dystonia model. Optogenetic activation of the striatopallidal pathway fails to reset pacemaking activity of GPe neurons in mutant mice. Abnormal firing is paralleled by alterations in motor learning. We find that loss of dopamine D2 receptor-dependent inhibition causes increased GABA input at striatopallidal synapses, with subsequent downregulation of hyperpolarization-activated, cyclic nucleotide-gated cation (HCN) channels. Accordingly, enhancing in vivo HCN channel activity or blocking GABA release restores both the ability of striatopallidal inputs to pause ongoing GPe activity and motor coordination deficits. Our findings demonstrate an impaired striatopallidal connectivity, supporting the central role of GPe in motor control and, more importantly, identifying potential pharmacological targets for dystonia.

Neurons under genetic control: What are the next steps towards the treatment of movement disorders?

Since the implementation of deep-brain stimulation as a therapy for movement disorders, there has been little progress in the clinical application of novel alternative treatments. Movement disorders are a group of neurological conditions, which are characterised with impairment of voluntary movement and share similar anatomical loci across the basal ganglia. The focus of the current review is on Parkinson's disease and Huntington's disease as they are the most investigated hypokinetic and hyperkinetic movement disorders, respectively. The last decade has seen enormous advances in the development of laboratory techniques that control neuronal activity. The two major ways to genetically control the neuronal function are: 1) expression of light-sensitive proteins that allow for the optogenetic control of the neuronal spiking and 2) expression or suppression of genes that control the transcription and translation of proteins. However, the translation of these methodologies from the laboratories into the clinics still faces significant challenges. The article summarizes the latest developments in optogenetics and gene therapy. Here, I compare the physiological mechanisms of established electrical deep brain stimulation to the experimental optogenetical deep brain stimulation. I compare also the advantages of DNA- and RNA-based techniques for gene therapy of familial movement disorders. I highlight the benefits and the major issues of each technique and I discuss the translational potential and clinical feasibility of optogenetic stimulation and gene expression control. The review emphasises recent technical breakthroughs that could initiate a notable leap in the treatment of movement disorders.

Can Pallidal Deep Brain Stimulation Rescue Borderline Dystonia? Possible Coexistence of Functional (Psychogenic) and Organic Components

The diagnosis and treatment of functional movement disorders are challenging for clinicians who manage patients with movement disorders. The borderline between functional and organic dystonia is often ambiguous. Patients with functional dystonia are poor responders to pallidal deep brain stimulation (DBS) and are not good candidates for DBS surgery. Thus, if patients with medically refractory dystonia have functional features, they are usually left untreated with DBS surgery. In order to investigate the outcome of functional dystonia in response to pallidal DBS surgery, we retrospectively included five patients with this condition. Their dystonia was diagnosed as organic by dystonia specialists and also as functional according to the Fahn and Williams criteria or the Gupta and Lang Proposed Revisions. Microelectrode recordings in the globus pallidus internus of all patients showed a cell-firing pattern of bursting with interburst intervals, which is considered typical of organic dystonia. Although their clinical course after DBS surgery was incongruent to organic dystonia, the outcome was good. Our results question the possibility to clearly differentiate functional dystonia from organic dystonia. We hypothesized that functional dystonia can coexist with organic dystonia, and that medically intractable dystonia with combined functional and organic features can be successfully treated by DBS surgery.

Miswiring of Frontostriatal Projections in Schizophrenia

We investigated brain wiring in chronic schizophrenia and healthy controls in frontostriatal circuits using diffusion magnetic resonance imaging tractography in a novel way. We extracted diffusion streamlines in 27 chronic schizophrenia and 26 healthy controls connecting 4 frontal subregions to the striatum. We labeled the projection zone striatal surface voxels into 2 subtypes: dominant-input from a single cortical subregion, and, functionally integrative, with mixed-input from diverse cortical subregions. We showed: 1) a group difference for total striatal surface voxel number (P = .045) driven by fewer mixed-input voxels in the left (P  = .007), but not right, hemisphere; 2) a group by hemisphere interaction for the ratio quotient between voxel subtypes (P  = .04) with a left (P  = .006), but not right, hemisphere increase in schizophrenia, also reflecting fewer mixed-input voxels; and 3) fewer mixed-input voxel counts in schizophrenia (P  = .045) driven by differences in left hemisphere limbic (P  = .007) and associative (P  = .01), but not sensorimotor, striatum. These results demonstrate a less integrative pattern of frontostriatal structural connectivity in chronic schizophrenia. A diminished integrative pattern yields a less complex input pattern to the striatum from the cortex with less circuit integration at the level of the striatum. Further, as brain wiring occurs during early development, aberrant brain wiring could serve as a developmental biomarker for schizophrenia.

A new approach for estimation of spiketrain patterns in basal ganglia

The pathophysiological model of dystonia proposed that in addition to reduced firing rate in the internal pallidum, changes in the pattern may also play a role in disease manifestation. While common methods for patterns separation depends on arbitrary spiketrain parameters, we considered the new method for neural patterns based on spike density histograms and hierarchical clustering of real datasets. We used the single unit activity recordings from the globus pallidus external (GPe) and the globus pallidus internal (GPi) from 10 cervical dystonia (CD), 7 segmental dystonia (SD) and 8 generalized dystonia (GD) patients undergoing deep brain stimulation surgery. Using novel method, we separated three patterns of activity: burst, burst-like and tonic. Using this separation, we revealed the differences both in firing rate and pattern distribution between dystonia patients. We have shown the suitability of the proposed method for pattern clusterization on real data and assume that further application of this method would facilitate more detailed study of the neural activity in the basal ganglia and the search for neurophysiological biomarkers of movement disorders.

A review of basal ganglia circuits and physiology: Application to deep brain stimulation

INTRODUCTION

Drawing on the seminal work of DeLong, Albin, and Young, we have now entered an era of basal ganglia neuromodulation. Understanding, re-evaluating, and leveraging the lessons learned from neuromodulation will be crucial to facilitate an increased and improved application of neuromodulation in human disease.

METHODS

We will focus on deep brain stimulation (DBS) - the most common form of basal ganglia neuromodulation - however, similar principles can apply to other neuromodulation modalities. We start with a brief review of DBS for Parkinson's disease, essential tremor, dystonia, and Tourette syndrome. We then review hallmark studies on basal ganglia circuits and electrophysiology resulting from decades of experience in neuromodulation. The organization and content of this paper follow Dr. Okun's Lecture from the 2018 Parkinsonism and Related Disorders World Congress.

RESULTS

Information gained from neuromodulation has led to an expansion of the basal ganglia rate model, an enhanced understanding of nuclei dynamics, an emerging focus on pathological oscillations, a revision of the tripartite division of the basal ganglia, and a redirected focus toward individualized symptom-specific stimulation. Though there have been many limitations of the basal ganglia "box model," the construct provided the necessary foundation to advance the field. We now understand that information in the basal ganglia is encoded through complex neural responses that can be reliably measured and used to infer disease states for clinical translation.

CONCLUSIONS

Our deepened understanding of basal ganglia physiology will drive new neuromodulation strategies such as adaptive DBS or cell-specific neuromodulation through the use of optogenetics.

Corticopallidal Connectome of the Globus Pallidus Externus in Humans: An Exploratory Study of Structural Connectivity Using Probabilistic Diffusion Tractography

BACKGROUND AND PURPOSE

Electrophysiologic abnormalities of the globus pallidus externus have been shown in several disease processes including Parkinson disease, dystonia, and Huntington disease. However, the connectivity, nuclear structure, and function of the globus pallidus externus are still not well-understood. Increasing evidence for the existence of direct corticopallidal connections challenges traditional understanding of the connectivity of the globus pallidus externus; nevertheless, these corticopallidal connections have yet to be fully characterized in humans. The objective of this study was to assess the corticopallidal connections of the globus pallidus externus by means of probabilistic diffusion-weighted MR imaging tractography using high-resolution, multishell data.

MATERIALS AND METHODS

Imaging data from the open-access Human Connectome Project data base were used to perform probabilistic tractography between the globus pallidus externus and the cerebral cortex using 34 distinct cortical regions. Group averages were calculated for normalized percentages of tracts reaching each of the cortical targets, and side-to-side comparison was made.

RESULTS

Cortical connectivity was demonstrated between the globus pallidus externus and multiple cortical regions, including direct connection to putative sensorimotor, associative, and limbic areas. Connectivity patterns were not significantly different between the right and left hemispheres with the exception of the frontal pole, which showed a greater number of connections on the right (P = .004).

CONCLUSIONS

Our in vivo study of the human globus pallidus externus using probabilistic tractography supports the existence of extensive corticopallidal connections and a tripartite functional division, as found in animal studies. A better understanding of the connectivity of the globus pallidus externus may help to understand its function and elucidate the effects of programming the higher contacts in pallidal deep brain stimulation.

Resting-State Pallidal-Cortical Oscillatory Couplings in Patients With Predominant Phasic and Tonic Dystonia

Pallidal deep brain stimulation (DBS) improves the symptoms of dystonia. The improvement processes of dystonic movements (phasic symptoms) and tonic symptoms differ. Phasic symptoms improve rapidly after starting DBS treatment, but tonic symptoms improve gradually. This difference implies distinct neuronal mechanisms for phasic and tonic symptoms in the underlying cortico-basal ganglia neuronal network. Phasic symptoms are related to the pallido-thalamo-cortical pathway. The pathway related to tonic symptoms has been assumed to be different from that for phasic symptoms. In the present study, local field potentials of the globus pallidus internus (GPi) and globus pallidus externus (GPe) and electroencephalograms from the motor cortex (MCx) were recorded in 19 dystonia patients to analyze the differences between the two types of symptoms. The 19 patients were divided into two groups, 10 with predominant phasic symptoms (phasic patients) and 9 with predominant tonic symptoms (tonic patients). To investigate the distinct features of oscillations and functional couplings across the GPi, GPe, and MCx by clinical phenotype, power and coherence were calculated over the delta (2-4 Hz), theta (5-7 Hz), alpha (8-13 Hz), and beta (14-35 Hz) frequencies. In phasic patients, the alpha spectral peaks emerged in the GPi oscillatory activities, and alpha GPi coherence with the GPe and MCx was higher than in tonic patients. On the other hand, delta GPi oscillatory activities were prominent, and delta GPi-GPe coherence was significantly higher in tonic than in phasic patients. However, there was no significant delta coherence between the GPi/GPe and MCx in tonic patients. These results suggest that different pathophysiological cortico-pallidal oscillations are related to tonic and phasic symptoms.

The external globus pallidus: progress and perspectives

The external globus pallidus (GPe) of the basal ganglia is in a unique and powerful position to influence processing of motor information by virtue of its widespread projections to all basal ganglia nuclei. Despite the clinical importance of the GPe in common motor disorders such as Parkinson's disease, there is only limited information about its cellular composition and organizational principles. In this review, recent advances in the understanding of the diversity in the molecular profile, anatomy, physiology and corresponding behaviour during movement of GPe neurons are described. Importantly, this study attempts to build consensus and highlight commonalities of the cellular classification based on existing but contentious literature. Additionally, an analysis of the literature concerning the intricate reciprocal loops formed between the GPe and major synaptic partners, including both the striatum and the subthalamic nucleus, is provided. In conclusion, the GPe has emerged as a crucial node in the basal ganglia macrocircuit. While subtleties in the cellular makeup and synaptic connection of the GPe create new challenges, modern research tools have shown promise in untangling such complexity, and will provide better understanding of the roles of the GPe in encoding movements and their associated pathologies.

Preserved dichotomy but highly irregular and burst discharge in the basal ganglia in alert dystonic rats at rest

Despite its prevalence, the underlying pathophysiology of dystonia remains poorly understood. Using our novel tri-component classification algorithm, extracellular neuronal activity in the globus pallidus (GP), STN, and the entopeduncular nucleus (EP) was characterized in 34 normal and 25 jaundiced dystonic Gunn rats with their heads restrained while at rest. In normal rats, neurons in each nucleus were similarly characterized by two physiologically distinct types: regular tonic with moderate discharge frequencies (mean rates in GP, STN and EP ranging from 35-41 spikes/s) or irregular at slower frequencies (17-20 spikes/s), with a paucity of burst activity. In dystonic rats, these nuclei were also characterized by two distinct principal neuronal patterns. However, in marked difference, in the dystonic rats, neurons were primarily slow and highly irregular (12-15 spikes/s) or burst predominant (14-17 spikes/s), with maintained modest differences between nuclei. In GP and EP, with increasing severity of dystonia, burstiness was moderately further increased, irregularity mildly further increased, and discharge rates mildly further reduced. In contrast, these features did not appreciably change in STN with worsening dystonia. Findings of a lack of bursting in GP, STN and EP in normal rats in an alert resting state and prominent bursting in dystonic Gunn rats suggest that cortical or other external drive is normally required for bursting in these nuclei and that spontaneous bursting, as seen in dystonia and Parkinson's disease, is reflective of an underlying pathophysiological state. Moreover, the extent of burstiness appears to most closely correlate with the severity of the dystonia.

Voxel-based analysis in neuroferritinopathy expands the phenotype and determines radiological correlates of disease severity

Neuroferritinopathy is an autosomal dominant adult-onset movement disorder which occurs due to mutations in the ferritin light chain gene (FTL). Extensive iron deposition and cavitation are observed post-mortem in the basal ganglia, but whether more widespread pathological changes occur, and whether they correlate with disease severity is unknown. 3D-T1w and quantitative T2 whole brain MRI scans were performed in 10 clinically symptomatic patients with the 460InsA FTL mutation and 10 age-matched controls. Voxel-based morphometry (VBM) and voxel-based relaxometry (VBR) were subsequently performed. Clinical assessment using the Unified Dystonia Rating Scale (UDRS) and Unified Huntington's Disease Rating Scale (UHDRS) was undertaken in all patients. VBM detected significant tissue changes within the substantia nigra, midbrain and dentate together with significant cerebellar atrophy in patients (FWE, p < 0.05). Iron deposition in the caudate head and cavitation in the lateral globus pallidus correlated with UDRS score (p < 0.001). There were no differences between groups with VBR. Our data show that progressive iron accumulation in the caudate nucleus, and cavitation of the globus pallidus correlate with disease severity in neuroferritinopathy. We also confirm sub-clinical cerebellar atrophy as a feature of the disease. We suggest that VBM is an effective technique to detect regions of iron deposition and cavitation, with potential wider utility to determine radiological markers of disease severity for all NBIA disorders.

Biochemical mechanisms of pallidal deep brain stimulation in X-linked dystonia parkinsonism

OBJECTIVE

Invasive techniques such as in-vivo microdialysis provide the opportunity to directly assess neurotransmitter levels in subcortical brain areas.

METHODS

Five male Filipino patients (mean age 42.4, range 34-52 years) with severe X-linked dystonia-parkinsonism underwent bilateral implantation of deep brain leads into the internal part of the globus pallidus (GPi). Intraoperative microdialysis and measurement of gamma aminobutyric acid and glutamate was performed in the GPi in three patients and globus pallidus externus (GPe) in two patients at baseline for 25/30 min and during 25/30 min of high-frequency GPi stimulation.

RESULTS

While the gamma-aminobutyric acid concentration increased in the GPi during high frequency stimulation (231 ± 102% in comparison to baseline values), a decrease was observed in the GPe (22 ± 10%). Extracellular glutamate levels largely remained unchanged.

CONCLUSIONS

Pallidal microdialysis is a promising intraoperative monitoring tool to better understand pathophysiological implications in movement disorders and therapeutic mechanisms of high frequency stimulation. The increased inhibitory tone of GPi neurons and the subsequent thalamic inhibition could be one of the key mechanisms of GPi deep brain stimulation in dystonia. Such a mechanism may explain how competing (dystonic) movements can be suppressed in GPi/thalamic circuits in favour of desired motor programs.

Mechanism of Deep Brain Stimulation: Inhibition, Excitation, or Disruption?

Deep brain stimulation (DBS), applying high-frequency electrical stimulation to deep brain structures, has now provided an effective therapeutic option for treatment of various neurological and psychiatric disorders. DBS targeting the internal segment of the globus pallidus, subthalamic nucleus, and thalamus is used to treat symptoms of movement disorders, such as Parkinson’s disease, dystonia, and tremor. However, the mechanism underlying the beneficial effects of DBS remains poorly understood and is still under debate: Does DBS inhibit or excite local neuronal elements? In this short review, we would like to introduce our recent work on the physiological mechanism of DBS and propose an alternative explanation: DBS dissociates input and output signals, resulting in the disruption of abnormal information flow through the stimulation site.

Changes of cerebral white matter in patients suffering from Pantothenate Kinase-Associated Neurodegeneration (PKAN): A diffusion tensor imaging (DTI) study

BACKGROUND

To look for microstructural white matter alterations in patients with dystonia due to Pantothenate Kinase-Associated Neurodegeneration.

MATERIAL AND METHODS

We examined 21 genetically confirmed patients and an age-matched group of 21 healthy controls by diffusion tensor imaging. Evaluation of data was performed by tract-based spatial statistics analysis and a voxel-wise comparison of calculated maps of fractional anisotropy. Findings were compared between groups and correlated to the dystonia score of the Burke-Fahn-Marsden Scale (p ≤ 0.05).

RESULTS

Patients showed reductions of fractional anisotropy mainly in the periventricular substance surrounding the third ventricle, in the medial part of both putamina and in the frontal white matter including the anterior limbs of the internal capsules and the corpus callosum. Infratentorially, the cerebellar white matter and dorsal parts of the pons and medulla were affected.

CONCLUSION

In addition to cortical grey matter changes, we now have a second structural finding pointing to a more widespread affection of cerebral tissue in PKAN dystonia than just the lesion and iron accumulation in the globus pallidus.

Changes in total cell numbers of the basal ganglia in patients with multiple system atrophy - A stereological study

Total numbers of neurons, oligodendrocytes, astrocytes, and microglia in the basal ganglia and red nucleus were estimated in brains from 11 patients with multiple system atrophy (MSA) and 11 age- and gender-matched control subjects with unbiased stereological methods. Compared to the control subjects, the MSA patients had a substantially lower number of neurons in the substantia nigra (p=0.001), putamen (p=0.001), and globus pallidus (p<0.001), and, to a lesser extent in the caudate nucleus (p=0.03). A significantly lower number of oligodendrocytes were only observed in the putamen (p=0.04) and globus pallidus (p=0.01). In the MSA brains the total number of astrocytes was significantly higher in the putamen (p=0.04) and caudate nucleus (p=0.01). In all examined regions a higher number of microglia were found in the MSA brains with the greatest difference observed in the otherwise unaffected red nucleus (p=0.001). The results from the stereological study were supported by cell marker expression analyses showing increased markers for activated microglia. Our results suggest that microgliosis is a consistent and severe neuropathological feature of MSA, whereas no widespread and substantial loss of oligodendrocytes was observed. We have demonstrated significant neuronal loss in the substantia nigra, striatum, and globus pallidus of patients with MSA, while neurons in other basal ganglia nuclei were spared, supporting the region-specific patterns of neuropathological changes in MSA.

Grey matter alterations in patients with Pantothenate Kinase-Associated Neurodegeneration (PKAN)

BACKGROUND

Pantothenate Kinase-Associated Neurodegeneration (PKAN) is a rare heritable disease marked by dystonia and loss of movement control. In contrast to the well-known "Eye-of-the-Tiger" sign affecting the globus pallidus, little is known about other deviations of brain morphology, especially about grey matter changes.

METHODS

We investigated 29 patients with PKAN and 29 age-matched healthy controls using Magnet Resonance Imaging and Voxel-Based Morphometry.

RESULTS

As compared to controls, children with PKAN showed increased grey matter density in the putamen and nucleus caudatus and adults with PKAN showed increased grey matter density in the ventral part of the anterior cingulate cortex. A multiple regression analysis with dystonia score as predictor showed grey matter reduction in the cerebellum, posterior cingulate cortex, superior parietal lobule, pars triangularis and small frontal and temporal areas and an analysis with age as predictor showed grey matter decreases in the putamen, nucleus caudatus, supplementary motor area and anterior cingulate cortex.

CONCLUSIONS

The grey matter increases may be regarded as a secondary phenomenon compensating the increased activity of the motor system due to a reduced inhibitory output of the globus pallidus. With increasing age, the grey matter reduction of cortical midline structures however might contribute to the progression of dystonic symptoms due to loss of this compensatory control.

Deep brain stimulation: a mechanistic and clinical update

Deep brain stimulation (DBS), the practice of placing electrodes deep into the brain to stimulate subcortical structures with electrical current, has been increasing as a neurosurgical procedure over the past 15 years. Originally a treatment for essential tremor, DBS is now used and under investigation across a wide spectrum of neurological and psychiatric disorders. In addition to applying electrical stimulation for clinical symptomatic relief, the electrodes implanted can also be used to record local electrical activity in the brain, making DBS a useful research tool. Human single-neuron recordings and local field potentials are now often recorded intraoperatively as electrodes are implanted. Thus, the increasing scope of DBS clinical applications is being matched by an increase in investigational use, leading to a rapidly evolving understanding of cortical and subcortical neurocircuitry. In this review, the authors discuss recent innovations in the clinical use of DBS, both in approved indications as well as in indications under investigation. Deep brain stimulation as an investigational tool is also reviewed, paying special attention to evolving models of basal ganglia and cortical function in health and disease. Finally, the authors look to the future across several indications, highlighting gaps in knowledge and possible future directions of DBS treatment.

IP3R1 deficiency in the cerebellum/brainstem causes basal ganglia-independent dystonia by triggering tonic Purkinje cell firings in mice

The type 1 inositol 1,4,5- trisphosphate receptor (IP₃R1) is a Ca²⁺ channel on the endoplasmic reticulum and is a predominant isoform in the brain among the three types of IP₃Rs. Mice lacking IP₃R1 show seizure-like behavior; however the cellular and neural circuit mechanism by which IP₃R1 deletion causes the abnormal movements is unknown. Here, we found that the conditional knockout mice lacking IP₃R1 specifically in the cerebellum and brainstem experience dystonia and show that cerebellar Purkinje cell (PC) firing patterns were coupled to specific dystonic movements. Recordings in freely behaving mice revealed epochs of low and high frequency PC complex spikes linked to body extension and rigidity, respectively. Remarkably, dystonic symptoms were independent of the basal ganglia, and could be rescued by inactivation of the cerebellum, inferior olive or in the absence of PCs. These findings implicate IP₃R1-dependent PC firing patterns in cerebellum in motor coordination and the expression of dystonia through the olivo-cerebellar pathway.

Striatal parvalbuminergic neurons are lost in Huntington's disease: implications for dystonia

Although dystonia represents a major source of motor disability in Huntington's disease (HD), its pathophysiology remains unknown. Because recent animal studies indicate that loss of parvalbuminergic (PARV+) striatal interneurons can cause dystonia, we investigated if loss of PARV+ striatal interneurons occurs during human HD progression, and thus might contribute to dystonia in HD. We used immunolabeling to detect PARV+ interneurons in fixed sections, and corrected for disease-related striatal atrophy by expressing PARV+ interneuron counts in ratio to interneurons co-containing somatostatin and neuropeptide Y (whose numbers are unaffected in HD). At all symptomatic HD grades, PARV+ interneurons were reduced to less than 26% of normal abundance in rostral caudate. In putamen rostral to the level of globus pallidus, loss of PARV+ interneurons was more gradual, not dropping off to less than 20% of control until grade 2. Loss of PARV+ interneurons was even more gradual in motor putamen at globus pallidus levels, with no loss at grade 1, and steady grade-wise decline thereafter. A large decrease in striatal PARV+ interneurons, thus, occurs in HD with advancing disease grade, with regional variation in the loss per grade. Given the findings of animal studies and the grade-wise loss of PARV+ striatal interneurons in motor striatum in parallel with the grade-wise appearance and worsening of dystonia, our results raise the possibility that loss of PARV+ striatal interneurons is a contributor to dystonia in HD.

Supersensitive presynaptic dopamine D2 receptor inhibition of the striatopallidal projection in nigrostriatal dopamine-deficient mice

The dopamine (DA) D2 receptor (D2R)-expressing medium spiny neurons (D2-MSNs) in the striatum project to and inhibit the GABAergic neurons in the globus pallidus (GP), forming an important link in the indirect pathway of the basal ganglia movement control circuit. These striatopallidal axon terminals express presynaptic D2Rs that inhibit GABA release and thus regulate basal ganglion function. Here we show that in transcription factor Pitx3 gene mutant mice with a severe DA loss in the dorsal striatum mimicking the DA denervation in Parkinson's disease (PD), the striatopallidal GABAergic synaptic transmission displayed a heightened sensitivity to presynaptic D2R-mediated inhibition with the dose-response curve shifted to the left, although the maximal inhibition was not changed. Functionally, low concentrations of DA were able to more efficaciously reduce the striatopallidal inhibition-induced pauses of GP neuron activity in DA-deficient Pitx3 mutant mice than in wild-type mice. These results demonstrate that presynaptic D2R inhibition of the striatopallidal synapse becomes supersensitized after DA loss. These supersensitive D2Rs may compensate for the lost DA in PD and also induce a strong disinhibition of GP neuron activity that may contribute to the motor-stimulating effects of dopaminergic treatments in PD.

Frequency-dependent effects of electrical stimulation in the globus pallidus of dystonia patients

Deep brain stimulation (DBS) in the globus pallidus internus (GPi) has been shown to improve dystonia, a movement disorder of repetitive twisting movements and postures. DBS at frequencies above 60 Hz improves dystonia, but the mechanisms underlying this frequency dependence are unclear. In patients undergoing dual-microelectrode mapping of the GPi, microstimulation has been shown to reduce neuronal firing, presumably due to synaptic GABA release. This study examined the effects of different microstimulation frequencies (1–100 Hz) and train length (0.5–20 s), with and without prior high-frequency stimulation (HFS) on neuronal firing and evoked field potentials (fEPs) in 13 dystonia patients. Pre-HFS, the average firing decreased as stimulation frequency increased and was silenced above 50 Hz. The average fEP amplitudes increased up to frequencies of 20–30 Hz but then declined and at 50 Hz, were only at 75% of baseline. In some cases, short latency fiber volleys and antidromic-like spikes were observed and followed high frequencies. Post-HFS, overall firing was reduced compared with pre-HFS, and the fEP amplitudes were enhanced at low frequencies, providing evidence of inhibitory synaptic plasticity in the GPi. In a patient with DBS electrodes already implanted in the GPi, recordings from four neurons in the subthalamic nucleus showed almost complete inhibition of firing with clinically effective but not clinically ineffective stimulation parameters. These data provide additional support for the hypothesis of stimulation-evoked GABA release from afferent synaptic terminals and reduction of neuronal firing during DBS and additionally, implicate excitation of GPi axon fibers and neurons and enhancement of inhibitory synaptic transmission by high-frequency GPi DBS as additional putative mechanisms underlying the clinical benefits of DBS in dystonia.