Cortical Effects of Subthalamic Stimulation Correlate with Behavioral Recovery from Dopamine Antagonist Induced Akinesia

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.

Targeting papez circuit for cognitive dysfunction- insights into deep brain stimulation for Alzheimer's disease

Hippocampus is the most widely studied brain area coupled with impairment of memory in a variety of neurological diseases and Alzheimer's disease (AD). The limbic structures within the Papez circuit have been linked to various aspects of cognition. Unfortunately, the brain regions that include this memory circuit are often ignored in terms of understanding cognitive decline in these diseases. To properly comprehend where cognition problems originate, it is crucial to clarify any aberrant contributions from all components of a specific circuit -on both a local and a global level. The pharmacological treatments currently available are not long lasting. Deep Brain Stimulation (DBS) emerged as a new powerful therapeutic approach for alleviation of the cognitive dysfunctions. Metabolic, functional, electrophysiological, and imaging studies helped to find out the crucial nodes that can be accessible for DBS. Targeting these nodes within the memory circuit produced significant improvement in learning and memory by disrupting abnormal circuit activity and restoring the physiological network. Here, we provide an overview of the neuroanatomy of the circuit of Papez along with the mechanisms and various deep brain stimulation targets of the circuit structures which could be significant for improving cognitive dysfunctions in AD.

Neural pathway activation in the subthalamic region depends on stimulation polarity

Deep brain stimulation (DBS) is an effective treatment for Parkinson's disease (PD); however, there is limited understanding of which subthalamic pathways are recruited in response to stimulation. Here, by focusing on the polarity of the stimulus waveform (cathodic vs. anodic), our goal was to elucidate biophysical mechanisms that underlie electrical stimulation in the human brain. In clinical studies, cathodic stimulation more easily triggers behavioral responses, but anodic DBS broadens the therapeutic window. This suggests that neural pathways involved respond preferentially depending on stimulus polarity. To experimentally compare the activation of therapeutically relevant pathways during cathodic and anodic subthalamic nucleus (STN) DBS, pathway activation was quantified by measuring evoked potentials resulting from antidromic or orthodromic activation in 15 PD patients undergoing DBS implantation. Cortical evoked potentials (cEP) were recorded using subdural electrocorticography, DBS local evoked potentials (DLEP) were recorded from non-stimulating contacts and EMG activity was recorded from arm and face muscles. We measured: 1) the amplitude of short-latency cEP, previously demonstrated to reflect activation of the cortico-STN hyperdirect pathway, 2) DLEP amplitude thought to reflect activation of STN-globus pallidus (GP) pathway, and 3) amplitudes of very short-latency cEP and motor evoked potentials (mEP) for activation of cortico-spinal/bulbar tract (CSBT). We constructed recruitment and strength-duration curves for each EP/pathway to compare the excitability for different stimulation polarities. We compared experimental data with the most advanced DBS computational models. Our results provide experimental evidence that subcortical cathodic and anodic stimulation activate the same pathways in the STN region and that cathodic stimulation is in general more efficient. However, relative efficiency varies for different pathways so that anodic stimulation is the least efficient in activating CSBT, more efficient in activating the HDP and as efficient as cathodic in activating STN-GP pathway. Our experiments confirm biophysical model predictions regarding neural activations in the central nervous system and provide evidence that stimulus polarity has differential effects on passing axons, terminal synapses, and local neurons. Comparison of experimental results with clinical DBS studies provides further evidence that the hyperdirect pathway may be involved in the therapeutic mechanisms of DBS.

Systematic review of rodent studies of deep brain stimulation for the treatment of neurological, developmental and neuropsychiatric disorders

Deep brain stimulation (DBS) modulates local and widespread connectivity in dysfunctional networks. Positive results are observed in several patient populations; however, the precise mechanisms underlying treatment remain unknown. Translational DBS studies aim to answer these questions and provide knowledge for advancing the field. Here, we systematically review the literature on DBS studies involving models of neurological, developmental and neuropsychiatric disorders to provide a synthesis of the current scientific landscape surrounding this topic. A systematic analysis of the literature was performed following PRISMA guidelines. 407 original articles were included. Data extraction focused on study characteristics, including stimulation protocol, behavioural outcomes, and mechanisms of action. The number of articles published increased over the years, including 16 rat models and 13 mouse models of transgenic or healthy animals exposed to external factors to induce symptoms. Most studies targeted telencephalic structures with varying stimulation settings. Positive behavioural outcomes were reported in 85.8% of the included studies. In models of psychiatric and neurodevelopmental disorders, DBS-induced effects were associated with changes in monoamines and neuronal activity along the mesocorticolimbic circuit. For movement disorders, DBS improves symptoms via modulation of the striatal dopaminergic system. In dementia and epilepsy models, changes to cellular and molecular aspects of the hippocampus were shown to underlie symptom improvement. Despite limitations in translating findings from preclinical to clinical settings, rodent studies have contributed substantially to our current knowledge of the pathophysiology of disease and DBS mechanisms. Direct inhibition/excitation of neural activity, whereby DBS modulates pathological oscillatory activity within brain networks, is among the major theories of its mechanism. However, there remain fundamental questions on mechanisms, optimal targets and parameters that need to be better understood to improve this therapy and provide more individualized treatment according to the patient's predominant symptoms.

Modulation of DBS-induced cortical responses and movement by the directionality and magnitude of current administered

Subthalamic deep brain stimulation (STN-DBS) is an effective therapy for alleviating motor symptoms in people with Parkinson's disease (PwP), although some may not receive optimal clinical benefits. One potential mechanism of STN-DBS involves antidromic activation of the hyperdirect pathway (HDP), thus suppressing cortical beta synchrony to improve motor function, albeit the precise mechanisms underlying optimal DBS parameters are not well understood. To address this, 18 PwP with STN-DBS completed a 2 Hz monopolar stimulation of the left STN during MEG. MEG data were imaged in the time-frequency domain using minimum norm estimation. Peak vertex time series data were extracted to interrogate the directional specificity and magnitude of DBS current on evoked and induced cortical responses and accelerometer metrics of finger tapping using linear mixed-effects models and mediation analyses. We observed increases in evoked responses (HDP ~ 3-10 ms) and synchronization of beta oscillatory power (14-30 Hz, 10-100 ms) following DBS pulse onset in the primary sensorimotor cortex (SM1), supplementary motor area (SMA) and middle frontal gyrus (MFG) ipsilateral to the site of stimulation. DBS parameters significantly modulated neural and behavioral outcomes, with clinically effective contacts eliciting significant increases in medium-latency evoked responses, reductions in induced SM1 beta power, and better movement profiles compared to suboptimal contacts, often regardless of the magnitude of current applied. Finally, HDP-related improvements in motor function were mediated by the degree of SM1 beta suppression in a setting-dependent manner. Together, these data suggest that DBS-evoked brain-behavior dynamics are influenced by the level of beta power in key hubs of the basal ganglia-cortical loop, and this effect is exacerbated by the clinical efficacy of DBS parameters. Such data provides novel mechanistic and clinical insight, which may prove useful for characterizing DBS programming strategies to optimize motor symptom improvement in the future.

Myogenic and cortical evoked potentials vary as a function of stimulus pulse geometry delivered in the subthalamic nucleus of Parkinson's disease patients

Introduction

The therapeutic efficacy of deep brain stimulation (DBS) of the subthalamic nucleus (STN) for Parkinson's disease (PD) may be limited for some patients by the presence of stimulation-related side effects. Such effects are most often attributed to electrical current spread beyond the target region. Prior computational modeling studies have suggested that changing the degree of asymmetry of the individual phases of the biphasic, stimulus pulse may allow for more selective activation of neural elements in the target region. To the extent that different neural elements contribute to the therapeutic vs. side-effect inducing effects of DBS, such improved selectivity may provide a new parameter for optimizing DBS to increase the therapeutic window.

Methods

We investigated the effect of six different pulse geometries on cortical and myogenic evoked potentials in eight patients with PD whose leads were temporarily externalized following STN DBS implant surgery. DBS-cortical evoked potentials were quantified using peak to peak measurements and wavelets and myogenic potentials were quantified using RMS.

Results

We found that the slope of the recruitment curves differed significantly as a function of pulse geometry for both the cortical- and myogenic responses. Notably, this effect was observed most frequently when stimulation was delivered using a monopolar, as opposed to a bipolar, configuration.

Discussion

Manipulating pulse geometry results in differential physiological effects at both the cortical and neuromuscular level. Exploiting these differences may help to expand DBS' therapeutic window and support the potential for incorporating pulse geometry as an additional parameter for optimizing therapeutic benefit.

Multi-scale and cross-dimensional TMS mapping: A proof of principle in patients with Parkinson's disease and deep brain stimulation

Introduction

Transcranial magnetic stimulation (TMS) mapping has become a critical tool for exploratory studies of the human corticomotor (M1) organization. Here, we propose to gather existing cutting-edge TMS-EMG and TMS-EEG approaches into a combined multi-dimensional TMS mapping that considers local and whole-brain excitability changes as well as state and time-specific changes in cortical activity. We applied this multi-dimensional TMS mapping approach to patients with Parkinson's disease (PD) with Deep brain stimulation (DBS) of the sub-thalamic nucleus (STN) ON and OFF. Our goal was to identifying one or several TMS mapping-derived markers that could provide unprecedent new insights onto the mechanisms of DBS in movement disorders.

Methods

Six PD patients (1 female, mean age: 62.5 yo [59-65]) implanted with DBS-STN for 1 year, underwent a robotized sulcus-shaped TMS motor mapping to measure changes in muscle-specific corticomotor representations and a movement initiation task to probe state-dependent modulations of corticospinal excitability in the ON (using clinically relevant DBS parameters) and OFF DBS states. Cortical excitability and evoked dynamics of three cortical areas involved in the neural control of voluntary movements (M1, pre-supplementary motor area - preSMA and inferior frontal gyrus - IFG) were then mapped using TMS-EEG coupling in the ON and OFF state. Lastly, we investigated the timing and nature of the STN-to-M1 inputs using a paired pulse DBS-TMS-EEG protocol.

Results

In our sample of patients, DBS appeared to induce fast within-area somatotopic re-arrangements of motor finger representations in M1, as revealed by mediolateral shifts of corticomuscle representations. STN-DBS improved reaction times while up-regulating corticospinal excitability, especially during endogenous motor preparation. Evoked dynamics revealed marked increases in inhibitory circuits in the IFG and M1 with DBS ON. Finally, inhibitory conditioning effects of STN single pulses on corticomotor activity were found at timings relevant for the activation of inhibitory GABAergic receptors (4 and 20 ms).

Conclusion

Taken together, these results suggest a predominant role of some markers in explaining beneficial DBS effects, such as a context-dependent modulation of corticospinal excitability and the recruitment of distinct inhibitory circuits, involving long-range projections from higher level motor centers and local GABAergic neuronal populations. These combined measures might help to identify discriminative features of DBS mechanisms towards deep clinical phenotyping of DBS effects in Parkinson's Disease and in other pathological conditions.

DBS-evoked cortical responses index optimal contact orientations and motor outcomes in Parkinson's disease

Although subthalamic deep brain stimulation (DBS) is a highly-effective treatment for alleviating motor dysfunction in patients with Parkinson's disease (PD), clinicians currently lack reliable neurophysiological correlates of clinical outcomes for optimizing DBS parameter settings, which may contribute to treatment inefficacies. One parameter that could aid DBS efficacy is the orientation of current administered, albeit the precise mechanisms underlying optimal contact orientations and associated clinical benefits are not well understood. Herein, 24 PD patients received monopolar stimulation of the left STN during magnetoencephalography and standardized movement protocols to interrogate the directional specificity of STN-DBS current administration on accelerometer metrics of fine hand movements. Our findings demonstrate that optimal contact orientations elicit larger DBS-evoked cortical responses in the ipsilateral sensorimotor cortex, and importantly, are differentially predictive of smoother movement profiles in a contact-dependent manner. Moreover, we summarize traditional evaluations of clinical efficacy (e.g., therapeutic windows, side effects) for a comprehensive review of optimal/non-optimal STN-DBS contact settings. Together, these data suggest that DBS-evoked cortical responses and quantitative movement outcomes may provide clinical insight for characterizing the optimal DBS parameters necessary for alleviating motor symptoms in patients with PD in the future.

Patterned Stimulation of the Chrimson Opsin in Glutamatergic Motor Thalamus Neurons Improves Forelimb Akinesia in Parkinsonian Rats

Parkinson's disease (PD) is a motor disorder charactertised by altered neural activity throughout the basal ganglia-thalamocortical circuit. Electrical deep brain stimulation (DBS) is efficacious in alleviating motor symptoms, but has several notable side-effects, most likely reflecting the non-specific nature of electrical stimulation and/or the brain regions targeted. We determined whether specific optogenetic activation of glutamatergic motor thalamus (Mthal) neurons alleviated forelimb akinesia in a chronic rat model of PD. Parkinsonian rats (unilateral 6-hydroxydopamine injection) were injected with an adeno-associated viral vector (AAV5-CaMKII-Chrimson-GFP) to transduce glutamatergic Mthal neurons with the red-shifted Chrimson opsin. Optogenetic stimulation with orange light at 15 Hz tonic and a physiological pattern, previously recorded from a Mthal neuron in a control rat, significantly increased forelimb use in the reaching test (p < 0.01). Orange light theta burst stimulation, 15 Hz and control reaching patterns significantly reduced akinesia (p < 0.0001) assessed by the step test. In contrast, forelimb use in the cylinder test was unaffected by orange light stimulation with any pattern. Blue light (control) stimulation failed to alter behaviours. Activation of Chrimson using complex patterns in the Mthal may be an alternative treatment to recover movement in PD. These vector and opsin changes are important steps towards translating optogenetic stimulation to humans.

The impact of pulse timing on cortical and subthalamic nucleus deep brain stimulation evoked potentials

The impact of pulse timing is an important factor in our understanding of how to effectively modulate the basal ganglia thalamocortical (BGTC) circuit. Single pulse low-frequency DBS-evoked potentials generated through electrical stimulation of the subthalamic nucleus (STN) provide insight into circuit activation, but how the long-latency components change as a function of pulse timing is not well-understood. We investigated how timing between stimulation pulses delivered in the STN region influence the neural activity in the STN and cortex. DBS leads implanted in the STN of five patients with Parkinson's disease were temporarily externalized, allowing for the delivery of paired pulses with inter-pulse intervals (IPIs) ranging from 0.2 to 10 ms. Neural activation was measured through local field potential (LFP) recordings from the DBS lead and scalp EEG. DBS-evoked potentials were computed using contacts positioned in dorsolateral STN as determined through co-registered post-operative imaging. We quantified the degree to which distinct IPIs influenced the amplitude of evoked responses across frequencies and time using the wavelet transform and power spectral density curves. The beta frequency content of the DBS evoked responses in the STN and scalp EEG increased as a function of pulse-interval timing. Pulse intervals <1.0 ms apart were associated with minimal to no change in the evoked response. IPIs from 1.5 to 3.0 ms yielded a significant increase in the evoked response, while those >4 ms produced modest, but non-significant growth. Beta frequency activity in the scalp EEG and STN LFP response was maximal when IPIs were between 1.5 and 4.0 ms. These results demonstrate that long-latency components of DBS-evoked responses are pre-dominantly in the beta frequency range and that pulse interval timing impacts the level of BGTC circuit activation.

The cortical evoked potential corresponds with deep brain stimulation efficacy in rats

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) antidromically activates the motor cortex (M1), and this cortical activation appears to play a role in the treatment of hypokinetic motor behaviors (Gradinaru V, Mogri M, Thompson KR, Henderson JM, Deisseroth K. Science 324: 354-359, 2009; Yu C, Cassar IR, Sambangi J, Grill WM. J Neurosci 40: 4323-4334, 2020). The synchronous antidromic activation takes the form of a short-latency cortical evoked potential (cEP) in electrocorticography (ECoG) recordings of M1. We assessed the utility of the cEP as a biomarker for STN DBS in unilateral 6-hydroxydopamine-lesioned female Sprague Dawley rats, with stimulating electrodes implanted in the STN and the ECoG recorded above M1. We quantified the correlations of the cEP magnitude and latency with changes in motor behavior from DBS and compared them to the correlation between motor behaviors and several commonly used spectral-based biomarkers. The cEP features correlated strongly with motor behaviors and were highly consistent across animals, whereas the spectral biomarkers correlated weakly with motor behaviors and were highly variable across animals. The cEP may thus be a useful biomarker for assessing the therapeutic efficacy of DBS parameters, as its features strongly correlate with motor behavior, it is consistent across time and subjects, it can be recorded under anesthesia, and it is simple to quantify with a large signal-to-noise ratio, enabling rapid, real-time evaluation. Additionally, our work provides further evidence that antidromic cortical activation mediates changes in motor behavior from STN DBS and that the dependence of DBS efficacy on stimulation frequency may be related to antidromic spike failure.NEW & NOTEWORTHY We characterize a new potential biomarker for deep brain stimulation (DBS), the cortical evoked potential (cEP), and demonstrate that it exhibits a robust correlation with motor behaviors as a function of stimulation frequency. The cEP may thus be a useful clinical biomarker for changes in motor behavior. This work also provides insight into the cortical mechanisms of DBS, suggesting that motor behaviors are strongly affected by the rate of antidromic spike failure during DBS.

Electrophysiological characterization of the hyperdirect pathway and its functional relevance for subthalamic deep brain stimulation

The subthalamic nucleus (STN) receives input from various cortical areas via hyperdirect pathway (HDP) which bypasses the basal-ganglia loop. Recently, the HDP has gained increasing interest, because of its relevance for STN deep brain stimulation (DBS). To understand the HDP's role cortical responses evoked by STN-DBS have been investigated. These responses have short (<2 ms), medium (2-15 ms), and long (20-70 ms) latencies. Medium-latency responses are supposed to represent antidromic cortical activations via HDP. Together with long-latency responses the medium responses can potentially be used as biomarker of DBS efficacy as well as side effects. We here propose that the activation sequence of the cortical evoked responses can be conceptualized as high frequency oscillations (HFO) for signal analysis. HFO might therefore serve as marker for antidromic activation. Using existing knowledge on HFO recordings, this approach allows data analyses and physiological modeling to advance the pathophysiological understanding of cortical DBS-evoked high-frequency activity.

Image-based biophysical modeling predicts cortical potentials evoked with subthalamic deep brain stimulation

BACKGROUND

Subthalamic deep brain stimulation (DBS) is an effective surgical treatment for Parkinson's disease and continues to advance technologically with an enormous parameter space. As such, in-silico DBS modeling systems have become common tools for research and development, but their underlying methods have yet to be standardized and validated.

OBJECTIVE

Evaluate the accuracy of patient-specific estimates of neural pathway activations in the subthalamic region against intracranial, cortical evoked potential (EP) recordings.

METHODS

Pathway activations were modeled in eleven patients using the latest advances in connectomic modeling of subthalamic DBS, focusing on the hyperdirect pathway (HDP) and corticospinal/bulbar tract (CSBT) for their relevance in human research studies. Correlations between pathway activations and respective EP amplitudes were quantified.

RESULTS

Good model performance required accurate lead localization and image fusions, as well as appropriate selection of fiber diameter in the biophysical model. While optimal model parameters varied across patients, good performance could be achieved using a global set of parameters that explained 60% and 73% of electrophysiologic activations of CSBT and HDP, respectively. Moreover, restricted models fit to only EP amplitudes of eight standard (monopolar and bipolar) electrode configurations were able to extrapolate variation in EP amplitudes across other directional electrode configurations and stimulation parameters, with no significant reduction in model performance across the cohort.

CONCLUSIONS

Our findings demonstrate that connectomic models of DBS with sufficient anatomical and electrical details can predict recruitment dynamics of white matter. These results will help to define connectomic modeling standards for preoperative surgical targeting and postoperative patient programming applications.

An Introspective Approach: A Lifetime of Parkinson's Disease Research and Not Much to Show for it Yet?

I feel part of a massive effort to understand what is wrong with motor systems in the brain relating to Parkinson’s disease. Today, the symptoms of the disease can be modified slightly, but dopamine neurons still die; the disease progression continues inexorably. Maybe the next research phase will bring the power of modern genetics to bear on halting, or better, preventing cell death. The arrival of accessible human neuron assemblies in organoids perhaps will provide a better access to the processes underlying neuronal demise.

Primary motor cortex in Parkinson's disease: Functional changes and opportunities for neurostimulation

Movement abnormalities of Parkinson's disease (PD) arise from disordered neural activity in multiple interconnected brain structures. The planning and execution of movement requires recruitment of a heterogeneous collection of pyramidal projection neurons in the primary motor cortex (M1). The neural representations of movement in M1 single-cell and field potential recordings are directly and indirectly influenced by the midbrain dopaminergic neurons that degenerate in PD. This review examines M1 functional alterations in PD as uncovered by electrophysiological recordings and neurostimulation studies in patients and experimental animal models. Dysfunction of the parkinsonian M1 depends on the severity and/or duration of dopamine-depletion and the species examined, and is expressed as alterations in movement-related firing dynamics; functional reorganisation of local circuits; and changes in field potential beta oscillations. Neurostimulation methods that modulate M1 activity directly (e.g., transcranial magnetic stimulation) or indirectly (subthalamic nucleus deep brain stimulation) improve motor function in PD patients, showing that targeted neuromodulation of M1 is a realistic therapy. We argue that the therapeutic profile of M1 neurostimulation is likely to be greatly enhanced with alternative technologies that permit cell-type specific control and incorporate feedback from electrophysiological biomarkers measured locally.

Frequency-dependent spike-pattern changes in motor cortex during thalamic deep brain stimulation

The cerebellar-receiving area of the motor thalamus is the primary anatomical target for treating essential tremor with deep brain stimulation (DBS). Although neuroimaging studies have shown that higher stimulation frequencies in this target correlate with increased cortical metabolic activity, less is known about the cellular-level functional changes that occur in the primary motor cortex (M1) with thalamic stimulation and how these changes depend on the frequency of DBS. In this study, we used a preclinical animal model of DBS to collect single-unit spike recordings in M1 before, during, and after DBS targeting the cerebellar-receiving area of the motor thalamus (VPLo, nucleus ventralis posterior lateralis pars oralis). The effects of VPLo-DBS on M1 spike rates, interspike interval entropy, and peristimulus phase-locking were compared across stimulus pulse train frequencies ranging from 10 to 130 Hz. Although VPLo-DBS modulated the spike rates of 20-50% of individual M1 cells in a frequency-dependent manner, the population-level average spike rate only weakly depended on stimulation frequency. In contrast, the population-level entropy measure showed a pronounced decrease with high-frequency stimulation, caused by a subpopulation of cells that exhibited strong phase-locking and general spike-pattern regularization. Contrarily, low-frequency stimulation induced an entropy increase (spike-pattern disordering) in a relatively large portion of the recorded population, which diminished with higher stimulation frequencies. These results also suggest that changes in phase-locking and spike-pattern entropy are not necessarily equivalent pattern phenomena, but rather that they should both be weighed when quantifying stimulation-induced spike-pattern changes.NEW & NOTEWORTHY The network mechanisms of thalamic deep brain stimulation (DBS) are not well understood at the cellular level. This study investigated the neuronal firing rate and pattern changes in the motor cortex resulting from stimulation of the cerebellar-receiving area of the motor thalamus. We showed that there is a nonintuitive relationship between general entropy-based spike-pattern measures and phase-locked regularization to DBS.

A Custom Made Electrode Construct and Reliable Implantation Method That Allows for Long-Term Bilateral Deep Brain Stimulation in Mice

Objectives

The underlying mechanisms behind the therapeutic and side effects of deep brain stimulation (DBS) need further investigation. The utilization of transgenic mouse lines is a suitable approach to better understand the cellular and network effects of DBS. However, not many bilateral DBS studies have been conducted in mice. This might be due to a lack of commercially available bilateral DBS constructs.

Materials and Methods

We developed an approach to perform repetitive long‐term DBS in freely moving mice. In this study, we implanted an in‐house custom‐made DBS construct containing two bipolar concentric electrodes to target the subthalamic nucleus (STN) bilaterally. Subsequently, we stimulated half of the animals with clinically relevant parameters three to five times a week with a duration of 20 min for ten weeks. Several behavioral tests were conducted of which the open field test (OFT) is shown to validate the reliability of this electrode construct and implantation method. Furthermore, we performed fiber photometry measurements to show the acute effect of STN‐DBS on serotonin network activity in the dorsal raphe nucleus.

Results

Repetitive DBS and long‐term behavioral testing were performed without complications. STN‐DBS resulted in an increase of the distance traveled in the OFT and a reduction of calcium levels in serotonergic neurons of the dorsal raphe nucleus. None of the mice had lost their electrodes and postmortem evaluation of the tissue showed accurate targeting of the STN without excessive gliosis.

Conclusion

The DBS electrode construct and implantation method described can be used for long‐term DBS studies to further investigate the mechanisms underlying DBS.

Direct Activation of Primary Motor Cortex during Subthalamic But Not Pallidal Deep Brain Stimulation

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) and globus pallidus internus (GPi) is an effective treatment for parkinsonian motor signs. Though its therapeutic mechanisms remain unclear, it has been suggested that antidromic activation of the primary motor cortex (M1) plays a significant role in mediating its therapeutic effects. This study tested the hypothesis that antidromic activation of M1 is a prominent feature underlying the therapeutic effect of STN and GPi DBS. Single-unit activity in M1 was recorded using high-density microelectrode arrays in two parkinsonian nonhuman primates each implanted with DBS leads targeting the STN and GPi. Stimulation in each DBS target had similar therapeutic effects, however, antidromic activation of M1 was only observed during STN DBS. Although both animals undergoing STN DBS had similar beneficial effects, the proportion of antidromic-classified cells in each differed, 30 versus 6%. Over 4 h of continuous STN DBS, antidromic activation became less robust, whereas therapeutic benefits were maintained. Although antidromic activation waned over time, synchronization of spontaneous spiking in M1 was significantly reduced throughout the 4 h. Although we cannot discount the potential therapeutic role of antidromic M1 activation at least in the acute phase of STN DBS, the difference in observed antidromic activation between animals, and target sites, raise questions about its hypothesized role as the primary mechanism underlying the therapeutic effect of DBS. These results lend further support that reductions in synchronization at the level of M1 are an important factor in the therapeutic effects of DBS.SIGNIFICANCE STATEMENT Recently there has been great interest and debate regarding the potential role of motor cortical activation in the therapeutic mechanisms of deep brain stimulation (DBS) for Parkinson's disease. In this study we used chronically implanted high density microelectrode arrays in primary motor cortex (M1) to record neuronal population responses in parkinsonian nonhuman primates during subthalamic nucleus (STN) DBS and globus pallidus internus (GPi) DBS. Our results suggest a contribution of antidromic activation of M1 during STN DBS in disrupting synchronization in cortical neuronal populations; however, diminishing antidromic activity over time, and differences in observed antidromic activation between animals and target sites with antidromic activation not observed during GPi DBS, raise questions about its role as the primary mechanism underlying the therapeutic effect of DBS.

Current and future directions of deep brain stimulation for neurological and psychiatric disorders

Deep brain stimulation (DBS) has evolved considerably over the past 4 decades. Although it has primarily been used to treat movement disorders such as Parkinson's disease, essential tremor, and dystonia, recently it has been approved to treat obsessive-compulsive disorder and epilepsy. Novel potential indications in both neurological and psychiatric disorders are undergoing active study. There have been significant advances in DBS technology, including preoperative and intraoperative imaging, surgical approaches and techniques, and device improvements. In addition to providing significant clinical benefits and improving quality of life, DBS has also increased the understanding of human electrophysiology and network interactions. Despite the value of DBS, future developments should be aimed at developing less invasive techniques and attaining not just symptom improvement but curative disease modification.

Cellular, molecular, and clinical mechanisms of action of deep brain stimulation-a systematic review on established indications and outlook on future developments

Deep brain stimulation (DBS) has been successfully used to treat movement disorders, such as Parkinson's disease, for more than 25 years and heralded the advent of electrical neuromodulation to treat diseases with dysregulated neuronal circuits. DBS is now superseding ablative techniques, such as stereotactic radiofrequency lesions. While serendipity has played a role in developing DBS as a therapy, research during the past two decades has shown that electrical neuromodulation is far more than a functional lesion that can be switched on and off. This understanding broadens the field to enable new types of stimulation, clinical indications, and research. This review highlights the complex effects of DBS from the single cell to the neuronal network. Specifically, we examine the electrical, cellular, molecular, and neurochemical mechanisms of DBS as applied to Parkinson's disease and other emerging applications.

Cortical Activation Elicited by Subthalamic Deep Brain Stimulation Predicts Postoperative Motor Side Effects

OBJECTIVE

Although deep brain stimulation (DBS) is an effective treatment for movement disorders, improvement varies substantially in individuals, across clinical trials, and over time. Noninvasive biomarkers that predict the individual response to DBS could be used to optimize outcomes and drive technological innovation in neuromodulation. We sought to evaluate whether noninvasive event related potentials elicited by subthalamic DBS during surgical targeting predict the tolerability of a given stimulation site in patients with advanced Parkinson's disease.

METHODS

Using electroencephalography, we measured event related potentials elicited by 20 Hz DBS over a range of stimulus intensities across the spatial extent of the implanted electrode array in 11 patients. We correlated event related potential timing and morphology with the stimulus amplitude thresholds for motor side effects during postoperative programming at ≥130 Hz.

RESULTS

During surgical targeting, DBS at 20 Hz elicits large amplitude, high frequency activity (evoked HFA) with mean onset latency of 9.0 ± 0.3 msec and a mean frequency of 175.8 ± 7.8 Hz. The lowest DBS amplitude that elicits the HFA predicts thresholds for motor side effects during postoperative stimulation at ≥130 Hz (p < 0.001, ANOVA).

CONCLUSION

Event related potentials elicited by DBS can predict clinically relevant corticospinal activation by stimulation after surgery. Noninvasive scalp physiology requires no patient interaction and could serve as a biomarker to guide targeting, postoperative programming, and emerging technologies such as directional and closed-loop stimulation.

Optogenetic Stimulation of the M2 Cortex Reverts Motor Dysfunction in a Mouse Model of Parkinson's Disease

Neuromodulation of deep brain structures (deep brain stimulation) is the current surgical procedure for treatment of Parkinson's disease (PD). Less studied is the stimulation of cortical motor areas to treat PD symptoms, although also known to alleviate motor disturbances in PD. We were able to show that optogenetic activation of secondary (M2) motor cortex improves motor functions in dopamine-depleted male mice. The stimulated M2 cortex harbors glutamatergic pyramidal neurons that project to subcortical structures, critically involved in motor control, and makes synaptic contacts with dopaminergic neurons. Strikingly, optogenetic activation of M2 neurons or axons into the dorsomedial striatum increases striatal levels of dopamine and evokes locomotor activity. We found that dopamine neurotransmission sensitizes the locomotor behavior elicited by activation of M2 neurons. Furthermore, combination of intranigral infusion of glutamatergic antagonists and circuit specific optogenetic stimulation revealed that behavioral response depended on the activity of M2 neurons projecting to SNc. Interestingly, repeated M2 stimulation combined with l-DOPA treatment produced an unanticipated improvement in working memory performance, which was absent in control mice under l-DOPA treatment only. Therefore, the M2-basal ganglia circuit is critical for the assembly of the motor and cognitive function, and this study demonstrates a therapeutic mechanism for cortical stimulation in PD that involves recruitment of long-range glutamatergic projection neurons.SIGNIFICANCE STATEMENT Some patients with Parkinson's disease are offered treatment through surgery, which consists of delivering electrical current to regions deep within the brain. This study shows that stimulation of an area located on the brain surface, known as the secondary motor cortex, can also reverse movement disorders in mice. Authors have used a brain stimulation technique called optogenetics, which allowed targeting a specific type of surface neuron that communicates with the deep part of the brain involved in movement control. The study also shows that a combination of this stimulation with drug treatment might be useful to treat memory impairment, a kind of cognitive problem in Parkinson's disease.

Distinct cortical responses evoked by electrical stimulation of the thalamic ventral intermediate nucleus and of the subthalamic nucleus

Objective

To investigate the spatial and temporal pattern of cortical responses evoked by deep brain stimulation (DBS) of the subthalamic nucleus (STN) and ventral intermediate nucleus of the thalamus (VIM).

Methods

We investigated 7 patients suffering from Essential tremor (ET) and 7 patients with Parkinson's Disease (PD) following the implantation of DBS electrodes (VIM for ET patients, STN for PD patients). Magnetoencephalography (MEG) was used to record cortical responses evoked by electric stimuli that were applied via the DBS electrode in trains of 5 Hz. Dipole fitting was applied to reconstruct the origin of evoked responses.

Results

Both VIM and STN DBS led to short latency cortical responses at about 1 ms. The pattern of medium and long latency cortical responses following VIM DBS consisted of peaks at 13, 40, 77, and 116 ms. The associated equivalent dipoles were localized within the central sulcus, 3 patients showed an additional response in the cerebellum at 56 ms. STN DBS evoked cortical responses peaking at 4 ms, 11 ms, and 27 ms, respectively. While most dipoles were localized in the pre- or postcentral gyrus, the distribution was less homogenous compared to VIM stimulation and partially included prefrontal brain areas.

Conclusion

MEG enables localization of cortical responses evoked by DBS of the VIM and the STN, especially in the sensorimotor cortex. Short latency responses of 1 ms suggest cortical modulation which bypasses synaptic transmission, i.e. antidromic activation of corticofugal fiber pathways.

•

Cortical responses evoked by VIM or STN DBS can be precisely described using MEG.

•

Both STN and VIM DBS primarily evoke cortical responses within the sensorimotor region.

•

Short latency responses of 1 ms both observed in VIM and STN DBS suggest antidromic activation of corticofugal fibers.

Cortical Potentials Evoked by Subthalamic Stimulation Demonstrate a Short Latency Hyperdirect Pathway in Humans

A monosynaptic projection from the cortex to the subthalamic nucleus is thought to have an important role in basal ganglia function and in the mechanism of therapeutic subthalamic deep-brain stimulation, but in humans the evidence for its existence is limited. We sought physiological confirmation of the cortico-subthalamic hyperdirect pathway using invasive recording techniques in patients with Parkinson's disease (9 men, 1 woman). We measured sensorimotor cortical evoked potentials using a temporary subdural strip electrode in response to low-frequency deep-brain stimulation in patients undergoing awake subthalamic or pallidal lead implantations. Evoked potentials were grouped into very short latency (<2 ms), short latency (2-10 ms), and long latency (10-100 ms) from the onset of the stimulus pulse. Subthalamic and pallidal stimulation resulted in very short-latency evoked potentials at 1.5 ms in the primary motor cortex accompanied by EMG-evoked potentials consistent with corticospinal tract activation. Subthalamic, but not pallidal stimulation, resulted in three short-latency evoked potentials at 2.8, 5.8, and 7.7 ms in a widespread cortical distribution, consistent with antidromic activation of the hyperdirect pathway. Long-latency potentials were evoked by both targets, with subthalamic responses lagging pallidal responses by 10-20 ms, consistent with orthodromic activation of the thalamocortical pathway. The amplitude of the first short-latency evoked potential was predictive of the chronic therapeutic stimulation contact.SIGNIFICANCE STATEMENT This is the first physiological demonstration of the corticosubthalamic hyperdirect pathway and its topography at high spatial resolution in humans. We studied cortical potentials evoked by deep-brain stimulation in patients with Parkinson's disease undergoing awake lead implantation surgery. Subthalamic stimulation resulted in multiple short-latency responses consistent with activation of hyperdirect pathway, whereas no such response was present during pallidal stimulation. We contrast these findings with very short latency, direct corticospinal tract activations, and long-latency responses evoked through polysynaptic orthodromic projections. These findings underscore the importance of incorporating the hyperdirect pathway into models of human basal ganglia function.

Frequency-dependent antidromic activation in thalamocortical relay neurons: effects of synaptic inputs

OBJECTIVE

Deep brain stimulation (DBS) generates action potentials (APs) in presynaptic axons and fibers of passage. The APs may be antidromically propagated to invade the cell body and/or orthodromically transmitted to downstream structures, thereby affecting widespread targets distant from the electrode. Activation of presynaptic terminals also causes trans-synaptic effects, which in turn alter the excitability of the post-synaptic neurons. Our aim was to determine how synaptic inputs affect the antidromic invasion of the cell body.

APPROACH

We used a biophysically-based multi-compartment model to simulate antidromic APs in thalamocortical relay (TC) neurons. We applied distributed synaptic inputs to the model and quantified how excitatory and inhibitory inputs contributed to the fidelity of antidromic activation over a range of antidromic frequencies.

MAIN RESULTS

Antidromic activation exhibited strong frequency dependence, which arose from the hyperpolarizing afterpotentials in the cell body and its respective recovery cycle. Low-frequency axonal spikes faithfully invaded the soma, whereas frequent failures of antidromic activation occurred at high frequencies. The frequency-dependent pattern of the antidromic activation masked burst-driver inputs to TC neurons from the cerebellum in a frequency-dependent manner. Antidromic activation also depended on the excitability of the cell body. Excitatory synaptic inputs improved the fidelity of antidromic activation by increasing the excitability, and inhibitory inputs suppressed antidromic activation by reducing soma excitability. Stimulus-induced depolarization of neuronal segments also facilitated antidromic propagation and activation.

SIGNIFICANCE

The results reveal that synaptic inputs, stimulus frequency, and electrode position regulate antidromic activation of the cell body during extracellular stimulation. These findings provide a biophysical basis for interpreting the widespread inhibition/activation of target nuclei during DBS.

Action potential initiation, propagation, and cortical invasion in the hyperdirect pathway during subthalamic deep brain stimulation

BACKGROUND

High frequency (∼130 Hz) deep brain stimulation (DBS) of the subthalamic region is an established clinical therapy for the treatment of late stage Parkinson's disease (PD). Direct modulation of the hyperdirect pathway, defined as cortical layer V pyramidal neurons that send an axon collateral to the subthalamic nucleus (STN), has emerged as a possible component of the therapeutic mechanisms. However, numerous questions remain to be addressed on the basic biophysics of hyperdirect pathway stimulation.

OBJECTIVE

Quantify action potential (AP) initiation, propagation, and cortical invasion in hyperdirect neurons during subthalamic stimulation.

METHODS

We developed an anatomically and electrically detailed computational model of hyperdirect neuron stimulation with explicit representation of the stimulating electric field, axonal response, AP propagation, and synaptic transmission.

RESULTS

We found robust AP propagation throughout the complex axonal arbor of the hyperdirect neuron. Even at therapeutic DBS frequencies, stimulation induced APs could reach all of the intracortical axon terminals with ∼100% fidelity. The functional result of this high frequency axonal driving of the thousands of synaptic connections made by each directly stimulated hyperdirect neuron is a profound synaptic suppression that would effectively disconnect the neuron from the cortical circuitry.

CONCLUSIONS

The synaptic suppression hypothesis integrates the fundamental biophysics of electrical stimulation, axonal transmission, and synaptic physiology to explain a generic mechanism of DBS.

Model-based deconstruction of cortical evoked potentials generated by subthalamic nucleus deep brain stimulation

Parkinson's disease is associated with altered neural activity in the motor cortex. Chronic high-frequency deep brain stimulation (DBS) of the subthalamic nucleus (STN) is effective in suppressing parkinsonian motor symptoms and modulates cortical activity. However, the anatomical pathways responsible for STN DBS-mediated cortical modulation remain unclear. Cortical evoked potentials (cEP) generated by STN DBS reflect the response of cortex to subcortical stimulation, and the goal of this study was to determine the neural origin of STN DBS-generated cEP using a two-step approach. First, we recorded cEP over ipsilateral primary motor cortex during different frequencies of STN DBS in awake healthy and unilateral 6-OHDA-lesioned parkinsonian rats. Second, we used a detailed, biophysically based model of the thalamocortical network to deconstruct the neural origin of the recorded cEP. The in vivo cEP included short (R1)-, intermediate (R2)-, and long-latency (R3) responses. Model-based cortical responses to simulated STN DBS matched remarkably well the in vivo responses. The short-latency response was generated by antidromic activation of layer 5 pyramidal neurons, whereas recurrent activation of layer 5 pyramidal neurons via excitatory axon collaterals reproduced the intermediate-latency response. The long-latency response was generated by polysynaptic activation of layer 2/3 pyramidal neurons via the cortico-thalamic-cortical pathway. Antidromic activation of the hyperdirect pathway and subsequent intracortical and cortico-thalamo-cortical synaptic interactions were sufficient to generate cortical potential evoked by STN DBS, and orthodromic activation through basal ganglia-thalamus-cortex pathways was not required. These results demonstrate the utility of cEP to determine the neural elements activated by STN DBS that might modulate cortical activity and contribute to the suppression of parkinsonian symptoms. NEW & NOTEWORTHY Subthalamic nucleus (STN) deep brain stimulation (DBS) is increasingly used to treat Parkinson's disease (PD). Cortical potentials evoked by STN DBS in patients with PD exhibit consistent short-latency (1-3 ms), intermediate-latency (5-15 ms), and long-latency (18-25 ms) responses. The short-latency response occurs as a result of antidromic activation of the hyperdirect pathway comprising corticosubthalamic axons. However, the neural origins of intermediate- and long-latency responses remain elusive, and the dominant view is that these are produced through the orthodromic pathway (basal ganglia-thalamus-cortex). By combining in vivo electrophysiology with computational modeling, we demonstrate that antidromic activation of the cortico-thalamic-cortical pathway is sufficient to generate the intermediate- and long-latency cortical responses to STN DBS.

Changes in neuronal activity of cortico-basal ganglia-thalamic networks induced by acute dopaminergic manipulations in rats

The basal ganglia are thought to be particularly sensitive to changes in dopaminergic tone, and the realization that reduced dopaminergic signaling causes pronounced motor dysfunction is the rationale behind dopamine replacement therapy in Parkinson's disease. It has, however, proven difficult to identify which neurophysiological changes that ultimately lead to motor dysfunctions. To clarify this, we have here recorded neuronal activity throughout the cortico-basal ganglia-thalamic circuits in freely behaving rats during periods of immobility following acute dopaminergic manipulations, involving both vesicular dopamine depletion and antagonism of D1 and D2 type dopamine receptors. Synchronized and rhythmic activities were detected in the form of betaband oscillations in local field potentials and as cortical entrainment of action potentials in several basal ganglia structures. Analyses of the temporal development of synchronized oscillations revealed a spread from cortex to gradually also include deeper structures. In addition, firing rate changes involving neurons in all parts of the network were observed. These changes were typically relatively balanced within each structure, resulting in negligible net rate changes. Animals treated with D1 receptor antagonist showed a rapid onset of hypokinesia that preceded most of the neurophysiological changes, with the exception of these balanced rate changes. Parallel rate changes in functionally coupled ensembles of neurons in different structures may therefore be the first step in a cascade of neurophysiological changes underlying motor symptoms in the parkinsonian state. We suggest that balanced rate changes in distributed networks are possible mechanism of disease that should be further investigated in conditions involving dopaminergic dysfunction.

Theta-phase closed-loop stimulation induces motor paradoxical responses in the rat model of Parkinson disease

BACKGROUND

High-frequency deep brain stimulation (DBS) has become a widespread therapy used in the treatment of Parkinson's Disease (PD) and other diseases. Although it has proved beneficial, much recent attention has been centered around the potential of new closed-loop DBS implementations.

OBJECTIVE

Here we present a new closed-loop DBS scheme based on the phase of the theta activity recorded from the motor cortex. By testing the implementation on freely moving 6-OHDA lesioned and control rats, we assessed the behavioral and neurophysiologic effects of this implementation and compared it against the classical high-frequency DBS.

RESULTS

Results show that both stimulation modalities produce significant and opposite changes on the movement and neurophysiological activity. Close-loop stimulation, far from improving the animals' behavior, exert contrary effects to those of high-frequency DBS which reverts the parkinsonian symptoms. Motor improvement during open-loop, high-frequency DBS was accompanied by a reduction in the amount of cortical beta oscillations while akinetic and disturbed behavior during close-loop stimulation coincided with an increase in the amplitude of beta activity.

CONCLUSION

Cortical-phase-dependent close-loop stimulation of the STN exerts significant behavioral and oscillatory changes in the rat model of PD. Open-loop and close-loop stimulation outcomes differed dramatically, thus suggesting that the scheme of stimulation determines the output of the modulation even if the target structure is maintained. The current framework could be extended in future studies to identify the correct parameters that would provide a suitable control signal to the system. It may well be that with other stimulation parameters, this sort of DBS could be beneficial.

Ultrasoft microwire neural electrodes improve chronic tissue integration

Chronically implanted neural multi-electrode arrays (MEA) are an essential technology for recording electrical signals from neurons and/or modulating neural activity through stimulation. However, current MEAs, regardless of the type, elicit an inflammatory response that ultimately leads to device failure. Traditionally, rigid materials like tungsten and silicon have been employed to interface with the relatively soft neural tissue. The large stiffness mismatch is thought to exacerbate the inflammatory response. In order to minimize the disparity between the device and the brain, we fabricated novel ultrasoft electrodes consisting of elastomers and conducting polymers with mechanical properties much more similar to those of brain tissue than previous neural implants. In this study, these ultrasoft microelectrodes were inserted and released using a stainless steel shuttle with polyethyleneglycol (PEG) glue. The implanted microwires showed functionality in acute neural stimulation. When implanted for 1 or 8weeks, the novel soft implants demonstrated significantly reduced inflammatory tissue response at week 8 compared to tungsten wires of similar dimension and surface chemistry. Furthermore, a higher degree of cell body distortion was found next to the tungsten implants compared to the polymer implants. Our results support the use of these novel ultrasoft electrodes for long term neural implants.

STATEMENT OF SIGNIFICANCE

One critical challenge to the translation of neural recording/stimulation electrode technology to clinically viable devices for brain computer interface (BCI) or deep brain stimulation (DBS) applications is the chronic degradation of device performance due to the inflammatory tissue reaction. While many hypothesize that soft and flexible devices elicit reduced inflammatory tissue responses, there has yet to be a rigorous comparison between soft and stiff implants. We have developed an ultra-soft microelectrode with Young's modulus lower than 1MPa, closely mimicking the brain tissue modulus. Here, we present a rigorous histological comparison of this novel ultrasoft electrode and conventional stiff electrode with the same size, shape and surface chemistry, implanted in rat brains for 1-week and 8-weeks. Significant improvement was observed for ultrasoft electrodes, including inflammatory tissue reaction, electrode-tissue integration as well as mechanical disturbance to nearby neurons. A full spectrum of new techniques were developed in this study, from insertion shuttle to in situ sectioning of the microelectrode to automated cell shape analysis, all of which should contribute new methods to the field. Finally, we showed the electrical functionality of the ultrasoft electrode, demonstrating the potential of flexible neural implant devices for future research and clinical use.

Motor learning in animal models of Parkinson's disease: Aberrant synaptic plasticity in the motor cortex

In Parkinson's disease (PD), dopamine depletion causes major changes in the brain, resulting in the typical cardinal motor features of the disease. PD neuropathology has been restricted to postmortem examinations, which are limited to only a single time of PD progression. Models of PD in which dopamine tone in the brain is chemically or physically disrupted are valuable tools in understanding the mechanisms of the disease. The basal ganglia have been well studied in the context of PD, and circuit changes in response to dopamine loss have been linked to the motor dysfunctions in PD. However, the etiology of the cognitive dysfunctions that are comorbid in PD patients has remained unclear until now. In this article, we review recent studies exploring how dopamine depletion affects the motor cortex at the synaptic level. In particular, we highlight our recent findings on abnormal spine dynamics in the motor cortex of PD mouse models through in vivo time-lapse imaging and motor skill behavior assays. In combination with previous studies, a role of the motor cortex in skill learning and the impairment of this ability with the loss of dopamine are becoming more apparent. Taken together, we conclude with a discussion on the potential role for the motor cortex in PD, with the possibility of targeting the motor cortex for future PD therapeutics. © 2017 International Parkinson and Movement Disorder Society.

Subthalamic nucleus deep brain stimulation on motor-symptoms of Parkinson's disease: Focus on neurochemistry

Deep brain stimulation (DBS) has become a standard therapy for Parkinson's disease (PD) and it is also currently under investigation for other neurological and psychiatric disorders. Although many scientific, clinical and ethical issues are still unresolved, DBS delivered into the subthalamic nucleus (STN) has improved the quality of life of several thousands of patients. The mechanisms underlying STN-DBS have been debated extensively in several reviews; less investigated are the biochemical consequences, which are still under scrutiny. Crucial and only partially understood, for instance, are the complex interplays occurring between STN-DBS and levodopa (LD)-centred therapy in the post-surgery follow-up. The main goal of this review is to address the question of whether an improved motor control, based on STN-DBS therapy, is also achieved through the additional modulation of other neurotransmitters, such as noradrenaline (NA) and serotonin (5-HT). A critical issue is to understand not only acute DBS-mediated effects, but also chronic changes, such as those involving cyclic nucleotides, capable of modulating circuit plasticity. The present article will discuss the neurochemical changes promoted by STN-DBS and will document the main results obtained in microdialysis studies. Furthermore, we will also examine the preliminary achievements of voltammetry applied to humans, and discuss new hypothetical investigational routes, taking into account novel players such as glia, or subcortical regions such as the pedunculopontine (PPN) area. Our further understanding of specific changes in brain chemistry promoted by STN-DBS would further disseminate its utilisation, at any stage of disease, avoiding an irreversible lesioning approach.

State-of-the-Art Analysis of High-Frequency (Gamma Range) Electroencephalography in Humans

Gamma oscillations (>30 Hz) in the brain are involved in attention, perception and memory. They are altered in various pathological states, as well as by neuropharmaceuticals, so that they are of interest in drug and clinical investigations. However, when the human electroencephalogram is recorded on the scalp, this neural high-frequency signal is buried under a range of other electrical signals such that, without careful handling, recordings of the high-frequency electroencephalogram cannot be considered reliable. The artefacts of concern originate from: power line noise, saccade-associated contraction of the extra-ocular muscles, activity of muscles in the scalp, face and neck, screen refresh artefacts and activity of the muscles associated with blinking. Recent progress in dealing with these artefacts is described, including either noise cancellation or phased noise template subtraction for power line noise, regression or independent component analysis for correcting extra-ocular muscle activity and mathematical modelling for reducing scalp, face and neck muscle activity. If the artefacts are properly addressed, the neural gamma signal can be uncovered.

Are the Symptoms of Parkinsonism Cortical in Origin?

We present three reasons to suspect that the major deleterious consequence of dopamine loss from the striatum is a cortical malfunction. We suggest that it is cortex, rather than striatum, that should be considered as the source of the debilitating symptoms of Parkinson's disease (PD) since:1.

Cortical synapses onto striatal dendritic spines are lost in PD.

2.

All known treatments of the symptoms of PD disrupt beta oscillations. Oscillations that are also disrupted following antidromic activation of cortical neurons.

3.

The final output of basal ganglia directly modulates thalamic connections to layer I of frontal cortical areas, regions intimately associated with motor behaviour.

These three reasons combined with evidence that the current summary diagram of the basal ganglia involvement in PD is imprecise at best, suggest that a re-orientation of the treatment strategies towards cortical, rather than striatal malfunction, is overdue.

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Suggested experimental contributions support the proposal of a cortical participation in PD.

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DBS produces antidromic activation of motor cortex and desynchronizes beta oscillations.

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Loss of dopamine decreases dendritic spines in the striatal D2 projection neurons.

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Motor thalamus distributes terminals into frontal cortex layer I.

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Thalamocortical-layer I activity increases with locomotion.

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.

Cortical Plasticity Induction by Pairing Subthalamic Nucleus Deep-Brain Stimulation and Primary Motor Cortical Transcranial Magnetic Stimulation in Parkinson's Disease

Noninvasive brain stimulation studies have shown abnormal motor cortical plasticity in Parkinson's disease (PD). These studies used peripheral nerve stimulation paired with transcranial magnetic stimulation (TMS) to primary motor cortex (M1) at specific intervals to induce plasticity. Induction of cortical plasticity through stimulation of the basal ganglia (BG)-M1 connections has not been studied. In the present study, we used a novel technique of plasticity induction by repeated pairing of deep-brain stimulation (DBS) of the BG with M1 stimulation using TMS. We hypothesize that repeated pairing of subthalamic nucleus (STN)-DBS and M1-TMS at specific time intervals will lead to plasticity in the M1. Ten PD human patients with STN-DBS were studied in the on-medication state with DBS set to 3 Hz. The interstimulus intervals (ISIs) between STN-DBS and TMS that produced cortical facilitation were determined individually for each patient. Three plasticity induction conditions with repeated pairings (180 times) at specific ISIs (∼ 3 and ∼ 23 ms) that produced cortical facilitation and a control ISI of 167 ms were tested in random order. Repeated pairing of STN-DBS and M1-TMS at short (∼ 3 ms) and medium (∼ 23 ms) latencies increased M1 excitability that lasted for at least 45 min, whereas the control condition (fixed ISI of 167 ms) had no effect. There were no specific changes in motor thresholds, intracortical circuits, or recruitment curves. Our results indicate that paired-associative cortical plasticity can be induced by repeated STN and M1 stimulation at specific intervals. These results show that STN-DBS can modulate cortical plasticity.

SIGNIFICANCE STATEMENT

We introduced a new experimental paradigm to test the hypothesis that pairing subthalamic nucleus deep-brain stimulation (STN-DBS) with motor cortical transcranial magnetic stimulation (M1-TMS) at specific times can induce cortical plasticity in patients with Parkinson's disease (PD). We found that repeated pairing of STN-DBS with TMS at short (∼ 3 ms) and medium (∼ 23 ms) intervals increased cortical excitability that lasted for up to 45 min, whereas the control condition (fixed latency of 167 ms) had no effects on cortical excitability. This is the first demonstration of associative plasticity in the STN-M1 circuits in PD patients using this novel technique. The potential therapeutic effects of combining DBS and noninvasive cortical stimulation should be investigated further.

A new biomarker for subthalamic deep brain stimulation for patients with advanced Parkinson's disease--a pilot study

OBJECTIVE

Deep brain stimulation (DBS) has become the standard treatment for advanced stages of Parkinson's disease (PD) and other motor disorders. Although the surgical procedure has improved in accuracy over the years thanks to imaging and microelectrode recordings, the underlying principles that render DBS effective are still debated today. The aim of this paper is to present initial findings around a new biomarker that is capable of assessing the efficacy of DBS treatment for PD which could be used both as a research tool, as well as in the context of a closed-loop stimulator.

APPROACH

We have used a novel multi-channel stimulator and recording device capable of measuring the response of nervous tissue to stimulation very close to the stimulus site with minimal latency, rejecting most of the stimulus artefact usually found with commercial devices. We have recorded and analyzed the responses obtained intraoperatively in two patients undergoing DBS surgery in the subthalamic nucleus (STN) for advanced PD.

MAIN RESULTS

We have identified a biomarker in the responses of the STN to DBS. The responses can be analyzed in two parts, an initial evoked compound action potential arising directly after the stimulus onset, and late responses (LRs), taking the form of positive peaks, that follow the initial response. We have observed a morphological change in the LRs coinciding with a decrease in the rigidity of the patients.

SIGNIFICANCE

These initial results could lead to a better characterization of the DBS therapy, and the design of adaptive DBS algorithms that could significantly improve existing therapies and help us gain insights into the functioning of the basal ganglia and DBS.

Practice changes beta power at rest and its modulation during movement in healthy subjects but not in patients with Parkinson's disease

BACKGROUND

PD (Parkinson's disease) is characterized by impairments in cortical plasticity, in beta frequency at rest and in beta power modulation during movement (i.e., event-related ERS [synchronization] and ERD [desynchronization]). Recent results with experimental protocols inducing long-term potentiation in healthy subjects suggest that cortical plasticity phenomena might be reflected by changes of beta power recorded with EEG during rest. Here, we determined whether motor practice produces changes in beta power at rest and during movements in both healthy subjects and patients with PD. We hypothesized that such changes would be reduced in PD.

METHODS

We thus recorded EEG in patients with PD and age-matched controls before, during and after a 40-minute reaching task. We determined posttask changes of beta power at rest and assessed the progressive changes of beta ERD and ERS during the task over frontal and sensorimotor regions.

RESULTS

We found that beta ERS and ERD changed significantly with practice in controls but not in PD. In PD compared to controls, beta power at rest was greater over frontal sensors but posttask changes, like those during movements, were far less evident. In both groups, kinematic characteristics improved with practice; however, there was no correlation between such improvements and the changes in beta power.

CONCLUSIONS

We conclude that prolonged practice in a motor task produces use-dependent modifications that are reflected in changes of beta power at rest and during movement. In PD, such changes are significantly reduced; such a reduction might represent, at least partially, impairment of cortical plasticity.

Investigating irregularly patterned deep brain stimulation signal design using biophysical models

Parkinson's disease (PD) is a neurodegenerative disorder which follows from cell loss of dopaminergic neurons in the substantia nigra pars compacta (SNc), a nucleus in the basal ganglia (BG). Deep brain stimulation (DBS) is an electrical therapy that modulates the pathological activity to treat the motor symptoms of PD. Although this therapy is currently used in clinical practice, the sufficient conditions for therapeutic efficacy are unknown. In this work we develop a model of critical motor circuit structures in the brain using biophysical cell models as the base components and then evaluate performance of different DBS signals in this model to perform comparative studies of their efficacy. Biological models are an important tool for gaining insights into neural function and, in this case, serve as effective tools for investigating innovative new DBS paradigms. Experiments were performed using the hemi-parkinsonian rodent model to test the same set of signals, verifying the obedience of the model to physiological trends. We show that antidromic spiking from DBS of the subthalamic nucleus (STN) has a significant impact on cortical neural activity, which is frequency dependent and additionally modulated by the regularity of the stimulus pulse train used. Irregular spacing between stimulus pulses, where the amount of variability added is bounded, is shown to increase diversification of response of basal ganglia neurons and reduce entropic noise in cortical neurons, which may be fundamentally important to restoration of information flow in the motor circuit.

Motor and Nonmotor Circuitry Activation Induced by Subthalamic Nucleus Deep Brain Stimulation in Patients With Parkinson Disease: Intraoperative Functional Magnetic Resonance Imaging for Deep Brain Stimulation

OBJECTIVE

To test the hypothesis suggested by previous studies that subthalamic nucleus (STN) deep brain stimulation (DBS) in patients with Parkinson disease would affect the activity of motor and nonmotor networks, we applied intraoperative functional magnetic resonance imaging (fMRI) to patients receiving DBS.

PATIENTS AND METHODS

Ten patients receiving STN DBS for Parkinson disease underwent intraoperative 1.5-T fMRI during high-frequency stimulation delivered via an external pulse generator. The study was conducted between January 1, 2013, and September 30, 2014.

RESULTS

We observed blood oxygen level-dependent (BOLD) signal changes (false discovery rate <0.001) in the motor circuitry (including the primary motor, premotor, and supplementary motor cortices; thalamus; pedunculopontine nucleus; and cerebellum) and in the limbic circuitry (including the cingulate and insular cortices). Activation of the motor network was observed also after applying a Bonferroni correction (P<.001) to the data set, suggesting that across patients, BOLD changes in the motor circuitry are more consistent compared with those occurring in the nonmotor network.

CONCLUSION

These findings support the modulatory role of STN DBS on the activity of motor and nonmotor networks and suggest complex mechanisms as the basis of the efficacy of this treatment modality. Furthermore, these results suggest that across patients, BOLD changes in the motor circuitry are more consistent than those in the nonmotor network. With further studies combining the use of real-time intraoperative fMRI with clinical outcomes in patients treated with DBS, functional imaging techniques have the potential not only to elucidate the mechanisms of DBS functioning but also to guide and assist in the surgical treatment of patients affected by movement and neuropsychiatric disorders.

TRIAL REGISTRATION

clinicaltrials.gov Identifier: NCT01809613.

Therapeutic deep brain stimulation reduces cortical phase-amplitude coupling in Parkinson's disease

Deep brain stimulation (DBS) is increasingly applied for the treatment of brain disorders, but its mechanism of action remains unknown. Here we evaluate the effect of basal ganglia DBS on cortical function using invasive cortical recordings in Parkinson's disease (PD) patients undergoing DBS implantation surgery. In the primary motor cortex of PD patients, neuronal population spiking is excessively synchronized to the phase of network oscillations. This manifests in brain surface recordings as exaggerated coupling between the phase of the beta rhythm and the amplitude of broadband activity. We show that acute therapeutic DBS reversibly reduces phase-amplitude interactions over a similar time course as that of the reduction in parkinsonian motor signs. We propose that DBS of the basal ganglia improves cortical function by alleviating excessive beta phase locking of motor cortex neurons.

Toward sophisticated basal ganglia neuromodulation: Review on basal ganglia deep brain stimulation

This review presents state-of-the-art knowledge about the roles of the basal ganglia (BG) in action-selection, cognition, and motivation, and how this knowledge has been used to improve deep brain stimulation (DBS) treatment of neurological and psychiatric disorders. Such pathological conditions include Parkinson's disease, Huntington's disease, Tourette syndrome, depression, and obsessive-compulsive disorder. The first section presents evidence supporting current hypotheses of how the cortico-BG circuitry works to select motor and emotional actions, and how defects in this circuitry can cause symptoms of the BG diseases. Emphasis is given to the role of striatal dopamine on motor performance, motivated behaviors and learning of procedural memories. Next, the use of cutting-edge electrochemical techniques in animal and human studies of BG functioning under normal and disease conditions is discussed. Finally, functional neuroimaging studies are reviewed; these works have shown the relationship between cortico-BG structures activated during DBS and improvement of disease symptoms.

Modulation of motor cortex neuronal activity and motor behavior during subthalamic nucleus stimulation in the normal primate

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) is a well-established surgical therapy for advanced Parkinson's disease (PD). An emerging hypothesis is that the therapeutic benefit of DBS is derived from direct modulation of primary motor cortex (M1), yet little is known about the influence of STN DBS on individual neurons in M1. We investigated the effect of STN DBS, delivered at discrete interval intensities (20, 40, 60, 80, and 100%) of corticospinal tract threshold (CSTT), on motor performance and M1 neuronal activity in a naive nonhuman primate. Motor performance during a food reach and retrieval task improved during low-intensity stimulation (20% CSTT) but worsened as intensity approached the threshold for activation of corticospinal fibers (80% and 100% CSTT). To assess cortical effects of STN DBS, spontaneous, extracellular neuronal activity was collected from M1 neurons before, during, and after DBS at the same CSTT stimulus intensities. STN DBS significantly modulated the firing of a majority of M1 neurons; however, the direction of effect varied with stimulus intensity such that, at 20% CSTT, most neurons were suppressed, whereas at the highest stimulus intensities the majority of neurons were activated. At a population level, firing rates increased as stimulus intensity increased. These results show that STN DBS influences both motor performance and M1 neuronal activity systematically according to stimulus intensity. In addition, the unanticipated reduction in reach times suggests that STN DBS, at stimulus intensities lower than typically used for treatment of PD motor signs, can enhance normal motor performance.

Quantitative analysis of axonal fiber activation evoked by deep brain stimulation via activation density heat maps

Background: Cortical modulation is likely to be involved in the various therapeutic effects of deep brain stimulation (DBS). However, it is currently difficult to predict the changes of cortical modulation during clinical adjustment of DBS. Therefore, we present a novel quantitative approach to estimate anatomical regions of DBS-evoked cortical modulation.

Methods: Four different models of the subthalamic nucleus (STN) DBS were created to represent variable electrode placements (model I: dorsal border of the posterolateral STN; model II: central posterolateral STN; model III: central anteromedial STN; model IV: dorsal border of the anteromedial STN). Axonal fibers of passage near each electrode location were reconstructed using probabilistic tractography and modeled using multi-compartment cable models. Stimulation-evoked activation of local axon fibers and corresponding cortical projections were modeled and quantified.

Results: Stimulation at the border of the STN (models I and IV) led to a higher degree of fiber activation and associated cortical modulation than stimulation deeply inside the STN (models II and III). A posterolateral target (models I and II) was highly connected to cortical areas representing motor function. Additionally, model I was also associated with strong activation of fibers projecting to the cerebellum. Finally, models III and IV showed a dorsoventral difference of preferentially targeted prefrontal areas (models III: middle frontal gyrus; model IV: inferior frontal gyrus).

Discussion: The method described herein allows characterization of cortical modulation across different electrode placements and stimulation parameters. Furthermore, knowledge of anatomical distribution of stimulation-evoked activation targeting cortical regions may help predict efficacy and potential side effects, and therefore can be used to improve the therapeutic effectiveness of individual adjustments in DBS patients.

Pallidal stimulation suppresses pathological dysrhythmia in the parkinsonian motor cortex

Although there is general consensus that deep brain stimulation (DBS) yields substantial clinical benefit in patients with Parkinson's disease (PD), the therapeutic mechanism of DBS remains a matter of debate. Recent studies demonstrate that DBS targeting the globus pallidus internus (GPi-DBS) suppresses pathological oscillations in firing rate and between-cell spike synchrony in the vicinity of the electrode but has negligible effects on population-level firing rate or the prevalence of burst firing. The present investigation examines the downstream consequences of GPi-DBS at the level of the primary motor cortex (M1). Multielectrode, single cell recordings were conducted in the M1 of two parkinsonian nonhuman primates (Macaca fasicularis). GPi-DBS that induced significant reductions in muscular rigidity also reduced the prevalence of both beta (12-30 Hz) oscillations in single unit firing rates and of coherent spiking between pairs of M1 neurons. In individual neurons, GPi-DBS-induced increases in mean firing rate were three times more common than decreases; however, averaged across the population of M1 neurons, GPi-DBS induced no net change in mean firing rate. The population-level prevalence of burst firing was also not affected by GPi-DBS. The results are consistent with the hypothesis that suppression of both pathological, beta oscillations and synchronous activity throughout the cortico-basal ganglia network is a major therapeutic mechanism of GPi-DBS.

Patterned, but not tonic, optogenetic stimulation in motor thalamus improves reaching in acute drug-induced Parkinsonian rats

High-frequency deep brain stimulation (DBS) in motor thalamus (Mthal) ameliorates tremor but not akinesia in Parkinson's disease. The aim of this study was to investigate whether there are effective methods of Mthal stimulation to treat akinesia. Glutamatergic Mthal neurons, transduced with channelrhodopsin-2 by injection of lentiviral vector (Lenti.CaMKII.hChR2(H134R).mCherry), were selectively stimulated with blue light (473 nm) via a chronically implanted fiber-optic probe. Rats performed a reach-to-grasp task in either acute drug-induced parkinsonian akinesia (0.03-0.07 mg/kg haloperidol, s.c.) or control (vehicle injection) conditions, and the number of reaches was recorded for 5 min before, during, and after stimulation. We compared the effect of DBS using complex physiological patterns previously recorded in the Mthal of a control rat during reaching or exploring behavior, with tonic DBS delivering the same number of stimuli per second (rate-control 6.2 or 1.8 Hz, respectively) and with stimulation patterns commonly used in other brain regions to treat neurological conditions (tonic 130 Hz, theta burst (TBS), and tonic 15 Hz rate-control for TBS). Control rats typically executed >150 reaches per 5 min, which was unaffected by any of the stimulation patterns. Acute parkinsonian rats executed <20 reaches, displaying marked akinesia, which was significantly improved by stimulating with the physiological reaching pattern or TBS (both p < 0.05), whereas the exploring and all tonic patterns failed to improve reaching. Data indicate that the Mthal may be an effective site to treat akinesia, but the pattern of stimulation is critical for improving reaching in parkinsonian rats.

Neuronal activity of the prefrontal cortex is reduced in rats selectively bred for deficient sensorimotor gating

Rats selectively bred for deficient prepulse inhibition (PPI), an operant measure of sensorimotor gating in which a weak prepulse stimulus attenuates the response to a subsequent startling stimulus, may be used to study certain pathophysiological mechanisms and therapeutic strategies for neuropsychiatric disorders with abnormalities in information processing, such as schizophrenia and Tourette's syndrome (TS). Little is known about neuronal activity in the medial prefrontal cortex (mPFC) and the nucleus accumbens (NAC), which are involved in the modulation of PPI. Here, we examined neuronal activity in these structures, and also in the entopeduncular nucleus (EPN), since lesions of this region alleviate the PPI deficit. Male rats with breeding-induced high and low expression of PPI (n=7, each) were anesthetized with urethane (1.4 mg/kg). Single-unit activity and local field potentials were recorded in the mPFC, the NAC and in the EPN. In the mPFC discharge rate, measures of irregularity and burst activity were significantly reduced in PPI low compared to PPI high rats (P<0.05), while analysis in the NAC showed approximately inverse behavior. In the EPN no difference between groups was found. Additionally, the oscillatory theta band activity (4-8 Hz) was enhanced and the beta band (13-30 Hz) and gamma band (30-100 Hz) activity was reduced in the NAC in PPI low rats. Reduced neuronal activity in the mPFC and enhanced activity in the NAC of PPI low rats, together with altered oscillatory behavior are clearly associated with reduced PPI. PPI low rats may thus be used to study the pathophysiology and therapeutic strategies for neuropsychiatric disorders accompanied by deficient sensorimotor gating.