Pallidal stimulation suppresses pathological dysrhythmia in the parkinsonian motor cortex

Why so slow? Models of parkinsonian bradykinesia

Bradykinesia, or slowness of movement, is a defining feature of Parkinson disease (PD) and a major contributor to the negative effects on quality of life associated with this disorder and related conditions. A dominant pathophysiological model of bradykinesia in PD has existed for approximately 30 years and has been the basis for the development of several therapeutic interventions, but accumulating evidence has made this model increasingly untenable. Although more recent models have been proposed, they also appear to be flawed. In this Perspective, I consider the leading prior models of bradykinesia in PD and argue that a more functionally related model is required, one that considers changes that disrupt the fundamental process of accurate information transmission. In doing so, I review emerging evidence of network level functional connectivity changes, information transfer dysfunction and potential motor code transmission error and present a novel model of bradykinesia in PD that incorporates this evidence. I hope that this model may reconcile inconsistencies in its predecessors and encourage further development of therapeutic interventions.

Dysfunction of motor cortices in Parkinson's disease

The cerebral cortex has long been thought to be involved in the pathophysiology of motor symptoms of Parkinson's disease. The impaired cortical function is believed to be a direct and immediate effect of pathologically patterned basal ganglia output, mediated to the cerebral cortex by way of the ventral motor thalamus. However, recent studies in humans with Parkinson's disease and in animal models of the disease have provided strong evidence suggesting that the involvement of the cerebral cortex is much broader than merely serving as a passive conduit for subcortical disturbances. In the present review, we discuss Parkinson's disease-related changes in frontal cortical motor regions, focusing on neuropathology, plasticity, changes in neurotransmission, and altered network interactions. We will also examine recent studies exploring the cortical circuits as potential targets for neuromodulation to treat Parkinson's disease.

Comparative motor effectiveness of non-invasive brain stimulation techniques in patients with Parkinson's disease: A network meta-analysis

BACKGROUND

Although noninvasive brain stimulation (NIBS) techniques are an effective alternative treatment option, their relative effects in patients with Parkinson's disease (PD) remain undefined. Here, we aimed to compare motor efficacy of the NIBS techniques in PD.

METHODS

We carried out an electronic search in PubMed, Embase, Cochrane Library, CINAHL, PEDro and PsycINFO (accessed via Ovid) for articles published until August 2022. The treatment efficacy of motor function was quantified by the Unified Parkinson's disease rating scale part III.

RESULTS

28 randomized controlled trials with parallel group were included in the analysis, enrolling 1057 patients. In the "on" state, high-frequency repetitive transcranial magnetic stimulation (HFrTMS) conferred better short-term and long-term efficacy compared to transcranial direct current stimulation. Surface under the cumulative ranking curve rank showed that HFrTMS combined with transcranial direct current stimulation and low-frequency TMS ranked first among PD in improving motor function. In the "off" state, there were no significant differences in most of the treatments, but surface under the cumulative ranking curve rank showed that continuous theta burst stimulation and low-frequency TMS had the highest short- and long-term effect in improving motor function.

CONCLUSION

HFrTMS is an effective intervention in improving motor function. Besides, its combination with another NIBS technique produces better therapeutic effects in the "on" state.

Value of functional connectivity in outcome prediction for pallidal stimulation in Parkinson disease

OBJECTIVE

Functional connectivity shows the ability to predict the outcome of subthalamic nucleus deep brain stimulation (DBS) in Parkinson disease (PD). However, evidence supporting its value in predicting the outcome of globus pallidus internus (GPi) DBS remains scarce. In this study the authors investigated patient-specific functional connectivity related to GPi DBS outcome in PD and established connectivity models for outcome prediction.

METHODS

The authors reviewed the outcomes of 21 patients with PD who received bilateral GPi DBS and presurgical functional MRI at the Ruijin Hospital. The connectivity profiles within cortical areas identified as relevant to DBS outcome in the literature were calculated using the intersection of the volume of tissue activated (VTA) and the local structures as the seeds. Combined with the leave-one-out cross-validation strategy, models of the optimal connectivity profile were constructed to predict outcome.

RESULTS

Connectivity between the pallidal areas and primary motor area, supplementary motor area (SMA), and premotor cortex was identified through the literature as related to GPi DBS outcome. The similarity between the connectivity profile within the primary motor area, SMA, pre-SMA, and premotor cortex seeding from the VTA-GPi intersection from an out-of-sample patient and the constructed in-sample optimal connectivity profile predicts GPi DBS outcome (R = 0.58, p = 0.006). The predictions on average deviated by 13.1% ± 11.3% from actual improvements. On the contrary, connectivity profiles seeding from the GPi (R = -0.12, p = 0.603), the VTA (R = 0.23, p = 0.308), the VTA outside the GPi (R = 0.12, p = 0.617), or other local structures were found not to be predictive.

CONCLUSIONS

The results showed that patient-specific functional connectivity seeding from the VTA-GPi intersection could help in GPi DBS outcome prediction. Reproducibility remains to be determined across centers in larger cohorts stratified by PD motor subtype.

Revisiting the "Paradox of Stereotaxic Surgery": Insights Into Basal Ganglia-Thalamic Interactions

Basal ganglia dysfunction is implicated in movement disorders including Parkinson Disease, dystonia, and choreiform disorders. Contradicting standard "rate models" of basal ganglia-thalamic interactions, internal pallidotomy improves both hypo- and hyper-kinetic movement disorders. This "paradox of stereotaxic surgery" was recognized shortly after rate models were developed, and is underscored by the outcomes of deep brain stimulation (DBS) for movement disorders. Despite strong evidence that DBS activates local axons, the clinical effects of lesions and DBS are nearly identical. These observations argue against standard models in which GABAergic basal ganglia output gates thalamic activity, and raise the question of how lesions and stimulation can have similar effects. These paradoxes may be resolved by considering thalamocortical loops as primary drivers of motor output. Rather than suppressing or releasing cortex via motor thalamus, the basal ganglia may modulate the timing of thalamic perturbations to cortical activity. Motor cortex exhibits rotational dynamics during movement, allowing the same thalamocortical perturbation to affect motor output differently depending on its timing with respect to the rotational cycle. We review classic and recent studies of basal ganglia, thalamic, and cortical physiology to propose a revised model of basal ganglia-thalamocortical function with implications for basic physiology and neuromodulation.

Effects of stereotactic radiofrequency thermocoagulation in the globus pallidus internus on refractory tic disorders

OBJECTIVE

To investigate the effect of stereotactic radiofrequency thermocoagulation in the globus pallidus internus on refractory tic disorders.

MATERIALS AND METHODS

Forty patients with refractory tic disorders were enrolled between January 2015 and July 2017 to experience stereotactic radiofrequency thermocoagulation in the globus pallidus internus. All clinical data, Yale Global Tic Severity Scale (YGTSS) scores, serum dopamine (SDA), and 5-hydroxytryptamine (5-HT) were analyzed.

RESULTS

Radiofrequency thermocoagulation was successfully performed in all patients. Periprocedural complications occurred in two patients (5.0%), one with fever (2.5%) and one with a urination disorder (2.5%); both returned to normal after treatment. After 12 months of follow-ups, excellent improvement was exhibited in 18 patients (45.0%), marked improvement in 10 (25.0%), good improvement in 9 (22.5%), and invalid in 3 (7.5%), with a total efficacy rate of 92.5% (37/40). Twenty-eight patients (70%) showed excellent or marked improvement without additional treatment after surgery. YGTSS scores were significantly (p < 0.05) decreased after compared with before thermocoagulation. SDA was significantly (p < 0.05) decreased 6 months (80.78 ± 18.82 ng/ml) and 12 months (75.65 ± 15.23 ng/ml) after compared with before (125.63 ± 35.26 ng/ml) surgery, whereas 5-HT was significantly (p < 0.05) increased 6 months (58.93 ± 16.88 ng/ml) and 12 months (62.63 ± 15.21 ng/ml) after compared with before (35.62 ± 3.41 ng/ml) surgery.

CONCLUSION

Stereotactic radiofrequency thermocoagulation can be safely applied in the globus pallidus internus to treat refractory tic disorders, resulting in significant tic symptom relief and a decrease in SDA but increase in 5-HT.

Intrinsic Disruption of the M1 Cortical Network in a Mouse Model of Parkinson's Disease

BACKGROUND

Parkinson's disease (PD) disrupts motor performance by affecting the basal ganglia system. Yet, despite the critical position of the primary motor cortex in linking basal ganglia computations with motor performance, its contribution to motor disability in PD is largely unknown. The objective of this study was to characterize the role of the primary motor cortex in PD-related motor disability.

METHODS

Two-photon calcium imaging and optogenetic stimulation of primary motor cortex neurons was done during performance of a dexterous reach-to-grasp motor task in control and 6-hydroxydopamine-induced PD mice.

RESULTS

Experimental PD disrupted performance of the reach-to-grasp motor task and especially initiation of the task, which was partially restored by optogenetic activation of the primary motor cortex. Two-photon calcium imaging during task performance revealed experimental-PD affected the primary motor cortex in a cell-type-specific manner. It suppressed activation of output layer 5 pyramidal tract neurons, with greater effects on freeze versus nonfreeze trials. In contrast, it did not attenuate the initial movement-related activation response of layer 2/3 pyramidal neurons while diminishing the late inhibitory phase of their response. At the network level, experimental PD disrupted movement-related population dynamics of the layer 5 pyramidal tract network while almost not affecting the dynamics of the layer 2/3 neuronal population. It also disrupted short- and long-term robustness and stability of the pyramidal tract subnetwork, with reduced intertrial temporal accuracy and diminished reproducibility of motor parameter encoding and temporal recruitment of the output pyramidal tract neurons over repeated daily sessions.

CONCLUSIONS

Experimental PD disrupts both external driving and intrinsic properties of the primary motor cortex. Motor disability in experimental PD results primarily from the inability to generate robust and stable output motor sequences in the parkinsonian primary motor cortex output layer 5 pyramidal tract subnetwork. © 2021 International Parkinson and Movement Disorder Society.

Frequency-Specific Optogenetic Deep Brain Stimulation of Subthalamic Nucleus Improves Parkinsonian Motor Behaviors

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) is an effective therapy for the motor symptoms of Parkinson's disease (PD). However, the neural elements mediating symptom relief are unclear. A previous study concluded that direct optogenetic activation of STN neurons was neither necessary nor sufficient for relief of parkinsonian symptoms. However, the kinetics of the channelrhodopsin-2 (ChR2) used for cell-specific activation are too slow to follow the high rates required for effective DBS, and thus the contribution of activation of STN neurons to the therapeutic effects of DBS remains unclear. We quantified the behavioral and neuronal effects of optogenetic STN DBS in female rats following unilateral 6-hydroxydopamine (6-OHDA) lesion using an ultrafast opsin (Chronos). Optogenetic STN DBS at 130 pulses per second (pps) reduced pathologic circling and ameliorated deficits in forelimb stepping similarly to electrical DBS, while optogenetic STN DBS with ChR2 did not produce behavioral effects. As with electrical DBS, optogenetic STN DBS exhibited a strong dependence on stimulation rate; high rates produced symptom relief while low rates were ineffective. High-rate optogenetic DBS generated both increases and decreases in firing rates of single neurons in STN, globus pallidus externa (GPe), and substantia nigra pars reticular (SNr), and disrupted β band oscillatory activity in STN and SNr. High-rate optogenetic STN DBS can indeed ameliorate parkinsonian motor symptoms through reduction of abnormal oscillatory activity in the STN-associated neural circuit, and these results highlight that the kinetic properties of opsins have a strong influence on the effects of optogenetic stimulation.SIGNIFICANCE STATEMENT Whether STN local cells contribute to the therapeutic effects of subthalamic nucleus (STN) deep brain stimulation (DBS) in Parkinson's disease (PD) remains unclear. We re-examined the role of STN local cells in mediating the symptom-relieving effects of STN DBS using cell type-specific optogenetic stimulation with a much faster opsin, Chronos. Direct optogenetic stimulation of STN neurons was effective in treating the symptoms of parkinsonism in the 6-hydroxydopamine (6-OHDA) lesion rat. These results highlight that the kinetic properties of opsins can have a strong influence on the effects of optogenetic activation/inhibition and must be considered when employing optogenetic to study high-rate neural stimulation.

Understanding Parkinson's disease and deep brain stimulation: Role of monkey models

Parkinson's disease (PD) is a progressive neurodegenerative movement disorder affecting over 10 million people worldwide. In the 1930s and 1940s there was little understanding regarding what caused PD or how to treat it. In a desperate attempt to improve patients' lives different regions of the neuraxis were ablated. Morbidity and mortality were common, but some patients' motor signs improved with lesions involving the basal ganglia or thalamus. With the discovery of l-dopa the advent of medical therapy began and surgical approaches became less frequent. It soon became apparent, however, that medical therapy was associated with side effects in the form of drug-induced dyskinesia and motor fluctuations and surgical therapies reemerged. Fortunately, during this time studies in monkeys had begun to lay the groundwork to understand the functional organization of the basal ganglia, and with the discovery of the neurotoxin MPTP a monkey model of PD had been developed. Using this model scientists were characterizing the physiological changes that occurred in the basal ganglia in PD and models of basal ganglia function and dysfunction were proposed. This work provided the rationale for the return of pallidotomy, and subsequently deep brain stimulation procedures. In this paper we describe the evolution of these monkey studies, how they provided a greater understanding of the pathophysiology underlying the development of PD and provided the rationale for surgical procedures, the search to understand mechanisms of DBS, and how these studies have been instrumental in understanding PD and advancing the development of surgical therapies for its treatment.

Differential Effects of Open- and Closed-Loop Intracortical Microstimulation on Firing Patterns of Neurons in Distant Cortical Areas

Intracortical microstimulation can be used successfully to modulate neuronal activity. Activity-dependent stimulation (ADS), in which action potentials recorded extracellularly from a single neuron are used to trigger stimulation at another cortical location (closed-loop), is an effective treatment for behavioral recovery after brain lesion, but the related neurophysiological changes are still not clear. Here, we investigated the ability of ADS and random stimulation (RS) to alter firing patterns of distant cortical locations. We recorded 591 neuronal units from 23 Long-Evan healthy anesthetized rats. Stimulation was delivered to either forelimb or barrel field somatosensory cortex, using either RS or ADS triggered from spikes recorded in the rostral forelimb area (RFA). Both RS and ADS stimulation protocols rapidly altered spike firing within RFA compared with no stimulation. We observed increase in firing rates and change of spike patterns. ADS was more effective than RS in increasing evoked spikes during the stimulation periods, by producing a reliable, progressive increase in stimulus-related activity over time and an increased coupling of the trigger channel with the network. These results are critical for understanding the efficacy of closed-loop electrical microstimulation protocols in altering activity patterns in interconnected brain networks, thus modulating cortical state and functional connectivity.

Circuit Mechanisms of Parkinson's Disease

Parkinson's disease (PD) is a complex, multi-system neurodegenerative disorder. The second most common neurodegenerative disorder after Alzheimer's disease, it affects approximately 1% of adults over age 60. Diagnosis follows the development of one or more of the core motor features of the disease, including tremor, slowing of movement (bradykinesia), and rigidity. However, there are numerous other motor and nonmotor disease manifestations. Many PD symptoms result directly from neurodegeneration; others are driven by aberrant activity patterns in surviving neurons. This latter phenomenon, PD circuit dysfunction, is an area of intense study, as it likely underlies our ability to treat many disease symptoms in the face of (currently) irreversible neurodegeneration. This Review will discuss key clinical features of PD and their basis in neural circuit dysfunction. We will first review important disease symptoms and some of the responsible neuropathology. We will then describe the basal ganglia-thalamocortical circuit, the major locus of PD-related circuit dysfunction, and some of the models that have influenced its study. We will review PD-related changes in network activity, subdividing findings into those that touch on the rate, rhythm, or synchronization of neurons. Finally, we suggest some critical remaining questions for the field and areas for new developments.

Deep brain stimulation of the ventroanterior and ventrolateral thalamus improves motor function in a rat model of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disease with affected individuals exhibiting motor symptoms of bradykinesia, muscle rigidity, tremor, postural instability and gait dysfunction. The current gold standard treatment is pharmacotherapy with levodopa, but long-term use is associated with motor response fluctuations and can cause abnormal movements called dyskinesias. An alternative treatment option is deep brain stimulation (DBS) with the two FDA-approved brain targets for PD situated in the basal ganglia; specifically, in the subthalamic nucleus (STN) and globus pallidus pars interna (GPi). Both improve quality of life and motor scores by ~50-70% in well-selected patients but can also elicit adverse effects on cognition and other non-motor symptoms. Therefore, identifying a novel DBS target that is efficacious for patients not optimally responsive to current DBS targets with fewer side-effects has clear clinical merit. Here, we investigate whether the ventroanterior (VA) and ventrolateral (VL) motor nuclei of the thalamus can serve as novel and effective DBS targets for PD. In the limb-use asymmetry test (LAT), hemiparkinsonian rats showcased left forelimb akinesia and touched only 6.5 ± 1.3% with that paw. However, these animals touched equally with both forepaws with DBS at 10 Hz, 100 μsec pulse width and 100 uA cathodic stimulation in the VA (n = 7), VL (n = 8) or at the interface between the two thalamic nuclei which we refer to as the VA|VL (n = 12). With whole-cell patch-clamp recordings, we noted that VA|VL stimulation in vitro increased the number of induced action potentials in proximal neurons in both areas albeit VL neurons transitioned from bursting to non-bursting action potentials (APs) with large excitatory postsynaptic potentials time-locked to stimulation. In contrast, VA neurons were excited with VA|VL electrical stimulation but with little change in spiking phenotype. Overall, our findings show that DBS in the VA, VL or VA|VL improved motor function in a rat model of PD; plausibly via increased excitation of residing neurons.

Added value of money on motor performance feedback: Increased left central beta-band power for rewards and fronto-central theta-band power for punishments

Monetary rewards and punishments have been shown to respectively enhance retention of motor memories and short-term motor performance, but their underlying neural bases in the context of motor control tasks remain unclear. Using electroencephalography (EEG), the present study tested the hypothesis that monetary rewards and punishments are respectively reflected in post-feedback beta-band (20-30 Hz) and theta-band (3-8 Hz) oscillatory power. While participants performed upper limb reaching movements toward visual targets using their right hand, the delivery of monetary rewards and punishments was manipulated as well as their probability (i.e., by changing target size). Compared to unrewarded and unpunished trials, monetary rewards and the successful avoidance of punishments both entailed greater beta-band power at left central electrodes overlaying contralateral motor areas. In contrast, monetary punishments and reward omissions both entailed increased theta-band power at fronto-central scalp sites. Additional analyses revealed that beta-band power was further increased when rewards were lowly probable. In light of previous work demonstrating similar beta-band modulations in basal ganglia during reward processing, the present results may reflect functional communication of reward-related information between the basal ganglia and motor cortical regions. In turn, the increase in fronto-central theta-band power after monetary punishments may reflect an emphasized cognitive need for behavioral adjustments. Globally, the present work identifies possible neural substrates for the growing behavioral evidence showing beneficial effects of monetary feedback on motor learning and performance.

Intermittent Stimulation of the Nucleus Basalis of Meynert Improves Working Memory in Adult Monkeys

Acetylcholine in the neocortex is critical for executive function [1-3]. Degeneration of cholinergic neurons in aging and Alzheimer's dementia is commonly treated with cholinesterase inhibitors [4-7]; however, these are modestly effective and are associated with side effects that preclude effective dosing in many patients [8]. Electrical activation of the nucleus basalis (NB) of Meynert, the source of neocortical acetylcholine [9, 10], provides a potential method of improving cholinergic activation [11, 12]. Here we tested whether NB stimulation would improve performance of a working memory task in a nonhuman primate model. Unexpectedly, intermittent stimulation proved to be most beneficial (60 pulses per second, for 20 s every minute), whereas continuous stimulation often impaired performance. Pharmacological experiments confirmed that the effects depended on cholinergic activation. Donepezil, a cholinesterase inhibitor, restored performance in animals impaired by continuous stimulation but did not improve performance further during intermittent stimulation. Intermittent stimulation was rendered ineffective by either nicotinic or muscarinic receptor antagonists. In the months after stimulation began, performance also improved in sessions without stimulation. Our results reveal that intermittent NB stimulation can improve working memory, a finding that has implications for restoring cognitive function in aging and Alzheimer's dementia.

Modulation of Neuronal Activity in the Motor Thalamus during GPi-DBS in the MPTP Nonhuman Primate Model of Parkinson's Disease

BACKGROUND

The motor thalamus is a key nodal point in the pallidothalamocortical "motor" circuit, which has been implicated in the pathogenesis of Parkinson's disease (PD) and other movement disorders. Although a critical structure in the motor circuit, the role of the motor thalamus in mediating the therapeutic effects of deep brain stimulation (DBS) of the internal segment of the globus pallidus (GPi) is not fully understood.

OBJECTIVE

To characterize the changes in neuronal activity in the pallidal (ventralis lateralis pars oralis (VLo) and ventralis anterior (VA)) and cerebellar (ventralis posterior lateralis pars oralis (VPLo)) receiving areas of the motor thalamus during therapeutic GPi DBS.

METHODS

Neuronal activity from the VA/VLo (n = 134) and VPLo (n = 129) was recorded from two non-human primates made parkinsonian using the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. For each isolated unit, one minute of data was recorded before, during and after DBS; a pulse width of 90 µs and a frequency of 135 Hz were used for DBS to replicate commonly used clinical settings. Stimulation amplitude was determined based on the parameters required to improve motor signs. Severity of motor signs was assessed using the UPDRS modified for nonhuman primates. Discharge rate, presence and characteristics of bursts, and oscillatory activity were computed and compared across conditions (pre-, during, and post-stimulation).

RESULTS

Neurons in both the pallidal and cerebellar receiving areas demonstrated significant changes in their pattern of activity during therapeutic GPi DBS. A majority of the neurons in each nucleus were inhibited during DBS (VA/VLo: 47% and VPLo: 49%), while a smaller subset was excited (VA/VLo: 21% and VPLo: 17%). Bursts changed in structure, becoming longer in duration and both intra-burst and inter-spike intervals and variability were increased in both subnuclei. High frequency oscillatory activity was significantly increased during stimulation with 33% of VA/VLo (likelihood ratio: p < 0.0001) and 34% of VPLo (p < 0.0001) neurons entrained to the stimulation pulse train.

CONCLUSIONS

Therapeutic GPi DBS produced a significant change in neuronal activity in both pallidal and cerebellar receiving areas of the motor thalamus. DBS suppressed activity in the majority of neurons, changed the structure of bursting activity and locked the neuronal response of one-third of cells to the stimulation pulse, leading to an increase in the power of gamma oscillations. These data support the hypothesis that stimulation activates output from the stimulated structure and that GPi DBS produces network-wide changes in neuronal activity that includes both the pallidal and cerebellar thalamo-cortical circuits.

Effects of high-frequency stimulation of the internal pallidal segment on neuronal activity in the thalamus in parkinsonian monkeys

Deep brain stimulation of the internal globus pallidus (GPi) is a major treatment for advanced Parkinson's disease. The effects of this intervention on electrical activity patterns in targets of GPi output, specifically in the thalamus, are poorly understood. The experiments described here examined these effects using electrophysiological recordings in two Rhesus monkeys rendered moderately parkinsonian through treatment with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), after sampling control data in the same animals. Analysis of spontaneous spiking activity of neurons in the basal ganglia-receiving areas of the ventral thalamus showed that MPTP-induced parkinsonism is associated with a reduction of firing rates of segments of the data that contained neither bursts nor decelerations, and with increased burst firing. Spectral analyses revealed an increase of power in the 3- to 13-Hz band and a reduction in the γ-range in the spiking activity of these neurons. Electrical stimulation of the ventrolateral motor territory of GPi with macroelectrodes, mimicking deep brain stimulation in parkinsonian patients (bipolar electrodes, 0.5 mm intercontact distance, biphasic stimuli, 120 Hz, 100 μs/phase, 200 μA), had antiparkinsonian effects. The stimulation markedly reduced oscillations in thalamic firing in the 13- to 30-Hz range and uncoupled the spiking activity of recorded neurons from simultaneously recorded local field potential (LFP) activity. These results confirm that oscillatory and nonoscillatory characteristics of spontaneous activity in the basal ganglia receiving ventral thalamus are altered in MPTP-induced parkinsonism. Electrical stimulation of GPi did not entrain thalamic activity but changed oscillatory activity in the ventral thalamus and altered the relationship between spikes and simultaneously recorded LFPs.

Deep Brain Stimulation for Movement Disorders of Basal Ganglia Origin: Restoring Function or Functionality?

Deep brain stimulation (DBS) is highly effective for both hypo- and hyperkinetic movement disorders of basal ganglia origin. The clinical use of DBS is, in part, empiric, based on the experience with prior surgical ablative therapies for these disorders, and, in part, driven by scientific discoveries made decades ago. In this review, we consider anatomical and functional concepts of the basal ganglia relevant to our understanding of DBS mechanisms, as well as our current understanding of the pathophysiology of two of the most commonly DBS-treated conditions, Parkinson's disease and dystonia. Finally, we discuss the proposed mechanism(s) of action of DBS in restoring function in patients with movement disorders. The signs and symptoms of the various disorders appear to result from signature disordered activity in the basal ganglia output, which disrupts the activity in thalamocortical and brainstem networks. The available evidence suggests that the effects of DBS are strongly dependent on targeting sensorimotor portions of specific nodes of the basal ganglia-thalamocortical motor circuit, that is, the subthalamic nucleus and the internal segment of the globus pallidus. There is little evidence to suggest that DBS in patients with movement disorders restores normal basal ganglia functions (e.g., their role in movement or reinforcement learning). Instead, it appears that high-frequency DBS replaces the abnormal basal ganglia output with a more tolerable pattern, which helps to restore the functionality of downstream networks.

Mechanisms of deep brain stimulation

Deep brain stimulation (DBS) is widely used for the treatment of movement disorders including Parkinson's disease, essential tremor, and dystonia and, to a lesser extent, certain treatment-resistant neuropsychiatric disorders including obsessive-compulsive disorder. Rather than a single unifying mechanism, DBS likely acts via several, nonexclusive mechanisms including local and network-wide electrical and neurochemical effects of stimulation, modulation of oscillatory activity, synaptic plasticity, and, potentially, neuroprotection and neurogenesis. These different mechanisms vary in importance depending on the condition being treated and the target being stimulated. Here we review each of these in turn and illustrate how an understanding of these mechanisms is inspiring next-generation approaches to DBS.

Common therapeutic mechanisms of pallidal deep brain stimulation for hypo- and hyperkinetic movement disorders

Abnormalities in cortico-basal ganglia (CBG) networks can cause a variety of movement disorders ranging from hypokinetic disorders, such as Parkinson's disease (PD), to hyperkinetic conditions, such as Tourette syndrome (TS). Each condition is characterized by distinct patterns of abnormal neural discharge (dysrhythmia) at both the local single-neuron level and the global network level. Despite divergent etiologies, behavioral phenotypes, and neurophysiological profiles, high-frequency deep brain stimulation (HF-DBS) in the basal ganglia has been shown to be effective for both hypo- and hyperkinetic disorders. The aim of this review is to compare and contrast the electrophysiological hallmarks of PD and TS phenotypes in nonhuman primates and discuss why the same treatment (HF-DBS targeted to the globus pallidus internus, GPi-DBS) is capable of ameliorating both symptom profiles. Recent studies have shown that therapeutic GPi-DBS entrains the spiking of neurons located in the vicinity of the stimulating electrode, resulting in strong stimulus-locked modulations in firing probability with minimal changes in the population-scale firing rate. This stimulus effect normalizes/suppresses the pathological firing patterns and dysrhythmia that underlie specific phenotypes in both the PD and TS models. We propose that the elimination of pathological states via stimulus-driven entrainment and suppression, while maintaining thalamocortical network excitability within a normal physiological range, provides a common therapeutic mechanism through which HF-DBS permits information transfer for purposive motor behavior through the CBG while ameliorating conditions with widely different symptom profiles.