Optical deconstruction of parkinsonian neural circuitry

Electrical stimulation for epilepsy: experimental approaches

Direct electrical stimulation of the brain is an increasingly popular means of treating refractory epilepsy. Although there has been moderate success in human trials, the rate of seizure freedom does not yet compare favorably to resective surgery. It therefore remains critical to advance experimental investigations aimed toward understanding brain stimulation and its utility. This article introduces the concepts necessary for understanding these experimental studies, describing recording and stimulation technology, animal models of epilepsy, and various subcortical targets of stimulation. Bidirectional and closed-loop device technologies are also highlighted, along with the challenges presented by their experimental use.

Optogenetics: potentials for addiction research

Research on the biology of addiction has advanced significantly over the last 50 years expanding our understanding of the brain mechanisms underlying reward, reinforcement and craving. Novel experimental approaches and techniques have provided an ever increasing armory of tools to dissect behavioral processes, neural networks and molecular mechanisms. The ultimate goal is to reintegrate this knowledge into a coherent, mechanistic framework of addiction to help identify new treatment. This can be greatly facilitated by using tools that allow, with great spatial and temporal specificity, to link molecular changes with altered activation of neural circuits and behavior. Such specificity can now be achieved by using optogenetic tools. Our review describes the general principles of optogenetics and its use to understand the links between neural activity and behavior. We also provide an overview of recent studies using optogenetic tools in addiction and consider some outstanding questions of addiction research that are particularly amenable for optogenetic approaches.

High frequency stimulation of the subthalamic nucleus increases c-fos immunoreactivity in the dorsal raphe nucleus and afferent brain regions

High frequency stimulation (HFS) of the subthalamic nucleus (STN) is the neurosurgical therapy of choice for the management of motor deficits in patients with advanced Parkinson's disease, but this treatment can elicit disabling mood changes. Our recent experiments show that in rats, HFS of the STN both inhibits the firing of 5-HT (5-hydroxytryptamine; serotonin) neurons in the dorsal raphe nucleus (DRN) and elicits 5-HT-dependent behavioral effects. The neural circuitry underpinning these effects is unknown. Here we investigated in the dopamine-denervated rat the effect of bilateral HFS of the STN on markers of neuronal activity in the DRN as well as DRN input regions. Controls were sham-stimulated rats. HFS of the STN elicited changes in two 5-HT-sensitive behavioral tests. Specifically, HFS increased immobility in the forced swim test and increased interaction in a social interaction task. HFS of the STN at the same stimulation parameters, increased c-fos immunoreactivity in the DRN, and decreased cytochrome C oxidase activity in this region. The increase in c-fos immunoreactivity occurred in DRN neurons immunopositive for the GABA marker parvalbumin. HFS of the STN also increased the number of c-fos immunoreactive cells in the lateral habenula nucleus, medial prefrontal cortex but not significantly in the substantia nigra. Collectively, these findings support a role for circuitry involving DRN GABA neurons, as well as DRN afferents from the lateral habenula nucleus and medial prefrontal cortex, in the mood effects of HFS of the STN.

In situ characterization of the brain-microdevice interface using device-capture histology

Accurate assessment of brain-implantable microdevice bio-integration remains a formidable challenge. Prevailing histological methods require device extraction prior to tissue processing, often disrupting and removing the tissue of interest which had been surrounding the device. The Device-Capture Histology method, presented here, overcomes many limitations of the conventional Device-Explant Histology method, by collecting the device and surrounding tissue intact for subsequent labeling. With the implant remaining in situ, accurate and precise imaging of the morphologically preserved tissue at the brain/microdevice interface can then be collected and quantified. First, this article presents the Device-Capture Histology method for obtaining and processing the intact, undisturbed microdevice-tissue interface, and imaging using fluorescent labeling and confocal microscopy. Second, this article gives examples of how to quantify features found in the captured peridevice tissue. We also share histological data capturing (1) the impact of microdevice implantation on tissue, (2) the effects of an experimental anti-inflammatory coating, (3) a dense grouping of cell nuclei encapsulating a long-term implant, and (4) atypical oligodendrocyte organization neighboring a long term implant. Data sets collected using the Device-Capture Histology method are presented to demonstrate the significant advantages of processing the intact microdevice-tissue interface, and to underscore the utility of the method in understanding the effects of the brain-implantable microdevices on nearby tissue.

Dissecting local circuits in vivo: integrated optogenetic and electrophysiology approaches for exploring inhibitory regulation of cortical activity

Local cortical circuit activity in vivo comprises a complex and flexible series of interactions between excitatory and inhibitory neurons. Our understanding of the functional interactions between these different neural populations has been limited by the difficulty of identifying and selectively manipulating the diverse and sparsely represented inhibitory interneuron classes in the intact brain. The integration of recently developed optical tools with traditional electrophysiological techniques provides a powerful window into the role of inhibition in regulating the activity of excitatory neurons. In particular, optogenetic targeting of specific cell classes reveals the distinct impacts of local inhibitory populations on other neurons in the surrounding local network. In addition to providing the ability to activate or suppress spiking in target cells, optogenetic activation identifies extracellularly recorded neurons by class, even when naturally occurring spike rates are extremely low. However, there are several important limitations on the use of these tools and the interpretation of resulting data. The purpose of this article is to outline the uses and limitations of optogenetic tools, along with current methods for achieving cell type-specific expression, and to highlight the advantages of an experimental approach combining optogenetics and electrophysiology to explore the role of inhibition in active networks. To illustrate the efficacy of these combined approaches, I present data comparing targeted manipulations of cortical fast-spiking, parvalbumin-expressing and low threshold-spiking, somatostatin-expressing interneurons in vivo.

Optogenetics in neural systems

Both observational and perturbational technologies are essential for advancing the understanding of brain function and dysfunction. But while observational techniques have greatly advanced in the last century, techniques for perturbation that are matched to the speed and heterogeneity of neural systems have lagged behind. The technology of optogenetics represents a step toward addressing this disparity. Reliable and targetable single-component tools (which encompass both light sensation and effector function within a single protein) have enabled versatile new classes of investigation in the study of neural systems. Here we provide a primer on the application of optogenetics in neuroscience, focusing on the single-component tools and highlighting important problems, challenges, and technical considerations.

Activation of SC during electrical stimulation of LGN: retinal antidromic stimulation or corticocollicular activation?

We have recently used combined electrostimulation, neurophysiology, microinjection and functional magnetic resonance imaging (fMRI) to study the cortical activity patterns elicited during stimulation of cortical afferents in monkeys. We found that stimulation of a site in lateral geniculate nucleus (LGN) increases the fMRI signal in the regions of primary visual cortex receiving input from that site, but suppresses it in the retinotopically matched regions of extrastriate cortex. Intracortical injection experiments showed that such suppression is due to synaptic inhibition. During these experiments, we have consistently observed activation of superior colliculus (SC) following LGN stimulation. Since LGN does not directly project to SC, the current study investigated the origin of SC activation. By examining experimental manipulations inactivating the primary visual cortex, we present here evidence that the robust SC activation, which follows the stimulation of LGN, is due to the activation of corticocollicular pathway.

Serotonin-dependent depression in Parkinson's disease: a role for the subthalamic nucleus?

Depression is the most common neuropsychiatric co-morbidity in Parkinson's disease (PD). The underlying mechanism of depression in PD is complex and likely involves biological, psychosocial and therapeutic factors. The biological mechanism may involve changes in monoamine systems, in particular the serotonergic (5-hydroxytryptamine, 5-HT) system. It is well established that the 5-HT system is markedly affected in the Parkinsonian brain, with evidence including pathological loss of markers of 5-HT axons as well as cell bodies in the dorsal and median raphe nuclei of the midbrain. However, it remains unresolved whether alterations to the 5-HT system alone are sufficient to confer vulnerability to depression. Here we propose low 5-HT combined with altered network activity within the basal ganglia as critically involved in depression in PD. The latter hypothesis is derived from a number of recent findings that highlight the close interaction between the basal ganglia and the 5-HT system, not only in motor but also limbic functions. These findings include evidence that clinical depression is a side effect of deep brain stimulation (DBS) of the subthalamic nucleus (STN), a treatment option in advanced PD. Further, it has recently been demonstrated that STN DBS in animal models inhibits 5-HT neurotransmission, and that this change may underpin depressive-like side effects. This review provides an overview of 5-HT alterations in PD and a discussion of how these changes might combine with altered basal ganglia network activity to increase depression vulnerability.

Viral-mediated overexpression of mutant huntingtin to model HD in various species

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by an expansion of CAG repeats in the huntingtin (Htt) gene. Despite intensive efforts devoted to investigating the mechanisms of its pathogenesis, effective treatments for this devastating disease remain unavailable. The lack of suitable models recapitulating the entire spectrum of the degenerative process has severely hindered the identification and validation of therapeutic strategies. The discovery that the degeneration in HD is caused by a mutation in a single gene has offered new opportunities to develop experimental models of HD, ranging from in vitro models to transgenic primates. However, recent advances in viral-vector technology provide promising alternatives based on the direct transfer of genes to selected sub-regions of the brain. Rodent studies have shown that overexpression of mutant human Htt in the striatum using adeno-associated virus or lentivirus vectors induces progressive neurodegeneration, which resembles that seen in HD. This article highlights progress made in modeling HD using viral vector gene transfer. We describe data obtained with of this highly flexible approach for the targeted overexpression of a disease-causing gene. The ability to deliver mutant Htt to specific tissues has opened pathological processes to experimental analysis and allowed targeted therapeutic development in rodent and primate pre-clinical models.

Multi-institutional evaluation of deep brain stimulation targeting using probabilistic connectivity-based thalamic segmentation

OBJECT

Due to the lack of internal anatomical detail with traditional MR imaging, preoperative stereotactic planning for the treatment of tremor usually relies on indirect targeting based on atlas-derived coordinates. The object of this study was to preliminarily investigate the role of probabilistic tractography-based thalamic segmentation for deep brain stimulation (DBS) targeting for the treatment of tremor.

METHODS

Six patients undergoing bilateral implantation of DBS electrodes in the thalamus for the treatment of upper-extremity tremor were studied. All patients underwent stereotactic surgical implantation using traditional methods (based on indirect targeting methodologies and intraoperative macrostimulation findings) that were programmed for optimal efficacy, independent of tractography-based segmentations described in this report. Connectivity-based thalamic segmentations were derived by identifying with which of 7 cortical target regions each thalamic voxel had the highest probability of connectivity. The authors retrospectively analyzed the location of the optimal contact for treatment of tremor with connectivity-based thalamic segmentations. Findings from one institution (David Geffen School of Medicine at UCLA) were validated with results from 4 patients at another institution (University of Virginia Health System).

RESULTS

Of 12 electrodes implanted using traditional methodologies, all but one resulted in efficacious tremor control. Connectivity-based thalamic segmentation consistently revealed discrete thalamic regions having unique connectivity patterns with distinct cortical regions. Although the authors initially hypothesized that the most efficacious DBS contact for controlling tremor would colocalize with the thalamic region most highly connected with the primary motor cortex, they instead found it to highly colocalize with those thalamic voxels demonstrating a high probability of connectivity with premotor cortex (center-to-center distance: 0.36 ± 0.55 mm). In contrast to the high degree of colocalization with optimal stimulation site, the precise localization of the premotor cortex-defined thalamic region relative to the anterior and posterior commissures was highly variable. Having defined a connectivity-based target for thalamic stimulation in a cohort of patients at David Geffen School of Medicine at UCLA, the authors validated findings in 4 patients (5 electrodes) who underwent surgery at a different institution (University of Virginia Health System) by a different surgeon.

CONCLUSIONS

This report identifies and provides preliminary external validation of a novel means of targeting a patient-specific therapeutic thalamic target for the treatment of tremor based on individualized analysis of thalamic connectivity patterns. This novel thalamic targeting approach is based on identifying the thalamic region with the highest probability of connectivity with premotor and supplementary motor cortices. This approach may prove to be advantageous over traditional preoperative methods of indirect targeting, providing patient-specific targets that could improve the precision, efficacy, and efficiency of deep brain stimulation surgery. Prospective evaluation and development of methodologies to make these analyses more widely available to neurosurgeons are likely warranted.

Boosting brain excitability by transcranial high frequency stimulation in the ripple range

Alleviating the symptoms of neurological diseases by increasing cortical excitability through transcranial stimulation is an ongoing scientific challenge. Here, we tackle this issue by interfering with high frequency oscillations (80–250 Hz) via external application of transcranial alternating current stimulation (tACS) over the human motor cortex (M1). Twenty-one subjects participated in three different experimental studies and they received on separate days tACS at three frequencies (80 Hz, 140 Hz and 250 Hz) and sham stimulation in a randomized order. tACS with 140 Hz frequency increased M1 excitability as measured by transcranial magnetic stimulation-generated motor evoked potentials (MEPs) during and for up to 1 h after stimulation. Control experiments with sham and 80 Hz stimulation were without any effect, and 250 Hz stimulation was less efficient with a delayed excitability induction and reduced duration. After-effects elicited by 140 Hz stimulation were robust against inversion of test MEP amplitudes seen normally under activation. Stimulation at 140 Hz reduced short interval intracortical inhibition, but left intracortical facilitation, long interval cortical inhibition and cortical silent period unchanged. Implicit motor learning was not facilitated by 140 Hz stimulation. High frequency stimulation in the ripple range is a new promising non-invasive brain stimulation protocol to increase human cortical excitability during and after the end of stimulation.

A role of diffusion tensor imaging fiber tracking in deep brain stimulation surgery: DBS of the dentato-rubro-thalamic tract (drt) for the treatment of therapy-refractory tremor

INTRODUCTION

Deep brain stimulation (DBS) can alleviate tremor of various origins. A number of regions are targeted. In recent work our group was able to show the involvement of the dentato-rubro-thalamic tract (drt) in tremor control with fiber tracking techniques. Here we report for the first time the successful use of magnetic resonance tractography in combination with traditional landmark-based targeting techniques to perform the implantation of a bilateral DBS system in a patient with dystonic head tremor.

METHODS

We report on a 37-year-old female with long-standing pure head tremor from myoclonus dystonia. She was identified as a candidate for thalamic DBS. The use of head fixation in a stereotactic frame would blur target symptoms (head tremor) during surgery and was therefore avoided. Her dentate-rubro-thalamic tracts were visualized with preoperative diffusion tensor imaging (DTI) and tractography, and then directly targeted stereotactically with DBS electrodes.

RESULTS

Three months after implantation, tremor control was excellent (>90%). A close evaluation of the active electrode contact positions revealed clear involvement of the drt.

CONCLUSION

This is the first time that direct visualization of fiber tracts has been employed for direct targeting and successful movement disorder tremor surgery. In the reported case, additional knowledge about the position of the drt, which previously has been shown to be a structure for modulation to achieve tremor control, led to a successful implantation of a DBS system, although there was a lack of intra-operatively testable tremor symptoms. In concordance with studies in optogenetic neuromodulation, fiber tracts are the emerging target structures for DBS. The routine integration of DTI tractography into surgical planning might be a leading path into the future of DBS surgery and will add to our understanding of the pathophysiology of movement disorders. Larger study populations will have to prove these concepts in future research.

Alterations in brain connectivity underlying beta oscillations in Parkinsonism

Cortico-basal ganglia-thalamocortical circuits are severely disrupted by the dopamine depletion of Parkinson's disease (PD), leading to pathologically exaggerated beta oscillations. Abnormal rhythms, found in several circuit nodes are correlated with movement impairments but their neural basis remains unclear. Here, we used dynamic causal modelling (DCM) and the 6-hydroxydopamine-lesioned rat model of PD to examine the effective connectivity underlying these spectral abnormalities. We acquired auto-spectral and cross-spectral measures of beta oscillations (10-35 Hz) from local field potential recordings made simultaneously in the frontal cortex, striatum, external globus pallidus (GPe) and subthalamic nucleus (STN), and used these data to optimise neurobiologically plausible models. Chronic dopamine depletion reorganised the cortico-basal ganglia-thalamocortical circuit, with increased effective connectivity in the pathway from cortex to STN and decreased connectivity from STN to GPe. Moreover, a contribution analysis of the Parkinsonian circuit distinguished between pathogenic and compensatory processes and revealed how effective connectivity along the indirect pathway acquired a strategic importance that underpins beta oscillations. In modelling excessive beta synchrony in PD, these findings provide a novel perspective on how altered connectivity in basal ganglia-thalamocortical circuits reflects a balance between pathogenesis and compensation, and predicts potential new therapeutic targets to overcome dysfunctional oscillations.

Optogenetic control of cells and circuits

The absorption of light by bound or diffusible chromophores causes conformational rearrangements in natural and artificial photoreceptor proteins. These rearrangements are coupled to the opening or closing of ion transport pathways, the association or dissociation of binding partners, the enhancement or suppression of catalytic activity, or the transcription or repression of genetic information. Illumination of cells, tissues, or organisms engineered genetically to express photoreceptor proteins can thus be used to perturb biochemical and electrical signaling with exquisite cellular and molecular specificity. First demonstrated in 2002, this principle of optogenetic control has had a profound impact on neuroscience, where it provides a direct and stringent means of probing the organization of neural circuits and of identifying the neural substrates of behavior. The impact of optogenetic control is also beginning to be felt in other areas of cell and organismal biology.

Model-based analysis and control of a network of basal ganglia spiking neurons in the normal and parkinsonian states

Controlling the spatiotemporal firing pattern of an intricately connected network of neurons through microstimulation is highly desirable in many applications. We investigated in this paper the feasibility of using a model-based approach to the analysis and control of a basal ganglia (BG) network model of Hodgkin-Huxley (HH) spiking neurons through microstimulation. Detailed analysis of this network model suggests that it can reproduce the experimentally observed characteristics of BG neurons under a normal and a pathological Parkinsonian state. A simplified neuronal firing rate model, identified from the detailed HH network model, is shown to capture the essential network dynamics. Mathematical analysis of the simplified model reveals the presence of a systematic relationship between the network's structure and its dynamic response to spatiotemporally patterned microstimulation. We show that both the network synaptic organization and the local mechanism of microstimulation can impose tight constraints on the possible spatiotemporal firing patterns that can be generated by the microstimulated network, which may hinder the effectiveness of microstimulation to achieve a desired objective under certain conditions. Finally, we demonstrate that the feedback control design aided by the mathematical analysis of the simplified model is indeed effective in driving the BG network in the normal and Parskinsonian states to follow a prescribed spatiotemporal firing pattern. We further show that the rhythmic/oscillatory patterns that characterize a dopamine-depleted BG network can be suppressed as a direct consequence of controlling the spatiotemporal pattern of a subpopulation of the output Globus Pallidus internalis (GPi) neurons in the network. This work may provide plausible explanations for the mechanisms underlying the therapeutic effects of deep brain stimulation (DBS) in Parkinson's disease and pave the way towards a model-based, network level analysis and closed-loop control and optimization of DBS parameters, among many other applications.

The corticostriatal and corticosubthalamic pathways: two entries, one target. So what?

The basal ganglia receive cortical inputs through two main stations - the striatum and the subthalamic nucleus (STN). The information flowing along the corticostriatal system is transmitted to the basal ganglia circuitry via the "direct and indirect" striatofugal pathways, while information that flows through the STN is transmitted along the so-called "hyperdirect" pathway. The functional significance of this dual entry system is not clear. Although the corticostriatal system has been thoroughly characterized anatomically and electrophysiologically, such is not the case for the corticosubthalamic system. In order to provide further insights into the intricacy of this complex anatomical organization, this review examines and compares the anatomical and functional organization of the corticostriatal and corticosubthalamic systems, and highlights some key issues that must be addressed to better understand the mechanisms by which these two neural systems may interact to regulate basal ganglia functions and dysfunctions.

Avoiding mouse traps in schizophrenia genetics: lessons and promises from current and emerging mouse models

Schizophrenia is one of the most common psychiatric disorders, but despite progress in identifying the genetic factors implicated in its development, the mechanisms underlying its etiology and pathogenesis remain poorly understood. Development of mouse models is critical for expanding our understanding of the causes of schizophrenia. However, translation of disease pathology into mouse models has proven to be challenging, primarily due to the complex genetic architecture of schizophrenia and the difficulties in the re-creation of susceptibility alleles in the mouse genome. In this review we highlight current research on models of major susceptibility loci and the information accrued from their analysis. We describe and compare the different approaches that are necessitated by diverse susceptibility alleles, and discuss their advantages and drawbacks. Finally, we discuss emerging mouse models, such as second-generation pathophysiology models based on innovative approaches that are facilitated by the information gathered from the current genetic mouse models.

Emerging, reemerging, and forgotten brain areas of the reward circuit: Notes from the 2010 Motivational Neural Networks conference

On April 24-27, 2010, the Motivational Neuronal Networks meeting took place in Wrightsville Beach, North Carolina. The conference was devoted to "Emerging, re-emerging, and forgotten brain areas" of the reward circuit. A central feature of the conference was four scholarly discussions of cutting-edge topics related to the conference's theme. These discussions form the basis of the present review, which summarizes areas of consensus and controversy, and serves as a roadmap for the next several years of research.

Striatal gating through up states and oscillations in the basal ganglia: Implications for Parkinson's disease

Up states are a hallmark of striatal physiology. Spontaneous activity in the thalamo-cortical network drives robust plateau depolarizations in the medium spiny projection neurons of the striatum. Medium spiny neuron firing is only possible during up states and is very tightly regulated by dopamine and NMDA receptors. In a rat model of Parkinson's disease the medium spiny neurons projecting to the globus pallidus (indirect pathway) show more depolarized up states and increased firing. This is translated into abnormal patterns of synchronization between the globus pallidus and frontal cortex, which are believed to underlie the symptoms of Parkinson's disease. Here we review our work in the field and propose a mechanism through which the lack of D2 receptor stimulation in the striatum allows the establishment of fixed routes of information flow in the cortico-striato-pallidal network.

Illusions of visual motion elicited by electrical stimulation of human MT complex

Human cortical area MT(+) (hMT(+)) is known to respond to visual motion stimuli, but its causal role in the conscious experience of motion remains largely unexplored. Studies in non-human primates demonstrate that altering activity in area MT can influence motion perception judgments, but animal studies are inherently limited in assessing subjective conscious experience. In the current study, we use functional magnetic resonance imaging (fMRI), intracranial electrocorticography (ECoG), and electrical brain stimulation (EBS) in three patients implanted with intracranial electrodes to address the role of area hMT(+) in conscious visual motion perception. We show that in conscious human subjects, reproducible illusory motion can be elicited by electrical stimulation of hMT(+). These visual motion percepts only occurred when the site of stimulation overlapped directly with the region of the brain that had increased fMRI and electrophysiological activity during moving compared to static visual stimuli in the same individual subjects. Electrical stimulation in neighboring regions failed to produce illusory motion. Our study provides evidence for the sufficient causal link between the hMT(+) network and the human conscious experience of visual motion. It also suggests a clear spatial relationship between fMRI signal and ECoG activity in the human brain.

Deep-Brain Stimulation for Basal Ganglia Disorders

The realization that medications used to treat movement disorders and psychiatric conditions of basal ganglia origin have significant shortcomings, as well as advances in the understanding of the functional organization of the brain, has led to a renaissance in functional neurosurgery, and particularly the use of deep brain stimulation (DBS). Movement disorders are now routinely being treated with DBS of 'motor' portions of the basal ganglia output nuclei, specifically the subthalamic nucleus and the internal pallidal segment. These procedures are highly effective and generally safe. Use of DBS is also being explored in the treatment of neuropsychiatric disorders, with targeting of the 'limbic' basal ganglia-thalamocortical circuitry. The results of these procedures are also encouraging, but many unanswered questions remain in this emerging field. This review summarizes the scientific rationale and practical aspects of using DBS for neurologic and neuropsychiatric disorders.

Pedunculopontine Nucleus Stimulation Improves Gait Freezing in Parkinson Disease

BACKGROUND

Pedunculopontine nucleus (PPN) stimulation is a novel therapy for Parkinson disease. However, controversies remain regarding the clinical application of this new therapy, including patient selection, electrode positioning, and how best to assess outcomes.

OBJECTIVE

To clarify the clinical application of PPN stimulation in Parkinson disease.

METHODS

Five consecutive patients with Parkinson disease complicated by severe gait freezing, postural instability, and frequent falls (all persisting even while the patient was on medication) received bilateral stimulation of the mid-lower PPN without costimulation of other brain targets. Outcomes were assessed prospectively over 2 years with gait-specific questionnaires and the Unified Parkinson Disease Rating Scale (part III).

RESULTS

The primary outcome, the Gait and Falls Questionnaire score, improved significantly with stimulation. Benefits were maintained over 2 years. Unified Parkinson Disease Rating Scale (part III) items assessing gait and posture were relatively insensitive to these treatment effects. Beneficial effects often appeared to outlast stimulation for hours or longer. Thus, single-session on- vs off-stimulation assessments may be susceptible to “delayed washout effects.” Stimulation of the PPN did not change akinesia scores or dopaminergic medication requirements.

CONCLUSION

Bilateral stimulation of the mid-lower PPN (more caudal than previous reports) without costimulation of other brain targets may be beneficial for the subgroup of patients with Parkinson disease who experience severe gait freezing and postural instability with frequent falls, which persist even while on medication. Choosing appropriate outcome measures and accounting for the possibility of prolonged stimulation washout effects appear to be important for detecting the clinical benefits.

Neuromodulation in Epilepsy

Neuromodulation strategies have been proposed to treat a variety of neurological disorders, including medication-resistant epilepsy. Electrical stimulation of both central and peripheral nervous systems has emerged as a possible alternative for patients who are not deemed to be good candidates for resective procedures. In addition to well-established treatments such as vagus nerve stimulation, epilepsy centers around the world are investigating the safety and efficacy of neurostimulation at different brain targets, including the hippocampus, thalamus, and subthalamic nucleus. Also promising are the preliminary results of responsive neuromodulation studies, which involve the delivery of stimulation to the brain in response to detected epileptiform or preepileptiform activity. In addition to electrical stimulation, novel therapeutic methods that may open new horizons in the management of epilepsy include transcranial magnetic stimulation, focal drug delivery, cellular transplantation, and gene therapy. We review the current strategies and future applications of neuromodulation in epilepsy.

Return of bradykinesia after subthalamic stimulation ceases: relationship to electrode location

In 20 subjects we quantified the rate at which subthalamic nucleus deep brain stimulation effects on Parkinson's bradykinesia "washed-out" after stimulation ceased. We found that wash-out was a two-step process, consisting of an initial fast decrease in stimulation's therapeutic effect, followed by a further, slow decline. Moreover, the relative contribution of the fast and slow components differed between patients. Finally, we found that lateral stimulation caused more of the fast-decaying component, while medial stimulation caused more of the slow-decaying component. This implies the existence of at least two separate mechanisms by which subthalamic nucleus deep brain stimulation improves bradykinesia, associated with activation of spatially separate zones in the vicinity of the subthalamic nucleus.

Nanoscale materials for tackling brain cancer: recent progress and outlook

This article reports on recent progress in the development of advanced nanoscale photoreactive, magnetic and multifunctional materials applicable to brain cancer diagnostics, imaging, and therapy, with an emphasis on the latest contributions and the novelty of the approach, along with the most promising emergent trends.

Subthalamic nucleus versus pedunculopontine nucleus stimulation in Parkinson disease: synergy or antagonism?

Stimulation of the subthalamic nucleus (STN) improves the cardinal features of Parkinson disease (PD). However, its efficacy on gait disorders is less satisfying in the long term. In recent years, the pedunculopontine (PPN) nucleus has emerged as a possible promising deep brain stimulation target for gait disorders in PD. In this review, we examine whether STN and PPN act synergistically or antagonistically. Results suggest that the combination of STN and PPN stimulations leads to a significant further improvement in gait as compared with STN stimulation alone, but additive effects on the classical motor triad are questionable. Thus, they highlight the specificity of STN stimulation over PPN's for the PD cardinal features and the specificity of PPN stimulation over STN for gait disorders. In addition, low-frequency stimulation of the PPN may improve alertness. The additive rather than potentiating effects of STN and PPN stimulations suggest that they may be mediated by distinct pathways. Nevertheless, considering the inconsistencies in published results regarding the influence of PPN stimulation on gait disorders, work is still needed before one can know whether it will convert into a standard surgical treatment and to decipher its place beside STN stimulation.

Chaotic desynchronization as the therapeutic mechanism of deep brain stimulation

High frequency deep-brain stimulation of the subthalamic nucleus (deep brain stimulation, DBS) relieves many of the symptoms of Parkinson's disease in humans and animal models. Although the treatment has seen widespread use, its therapeutic mechanism remains paradoxical. The subthalamic nucleus is excitatory, so its stimulation at rates higher than its normal firing rate should worsen the disease by increasing subthalamic excitation of the globus pallidus. The therapeutic effectiveness of DBS is also frequency and intensity sensitive, and the stimulation must be periodic; aperiodic stimulation at the same mean rate is ineffective. These requirements are not adequately explained by existing models, whether based on firing rate changes or on reduced bursting. Here we report modeling studies suggesting that high frequency periodic excitation of the subthalamic nucleus may act by desynchronizing the firing of neurons in the globus pallidus, rather than by changing the firing rate or pattern of individual cells. Globus pallidus neurons are normally desynchronized, but their activity becomes correlated in Parkinson's disease. Periodic stimulation may induce chaotic desynchronization by interacting with the intrinsic oscillatory mechanism of globus pallidus neurons. Our modeling results suggest a mechanism of action of DBS and a pathophysiology of Parkinsonism in which synchrony, rather than firing rate, is the critical pathological feature.

Estimation of electrode location in a rat motor cortex by laminar analysis of electrophysiology and intracortical electrical stimulation

While the development of microelectrode arrays has enabled access to disparate regions of a cortex for neurorehabilitation, neuroprosthetic and basic neuroscience research, accurate interpretation of the signals and manipulation of the cortical neurons depend upon the anatomical placement of the electrode arrays in a layered cortex. Toward this end, this report compares two in vivo methods for identifying the placement of electrodes in a linear array spaced 100 µm apart based on in situ laminar analysis of (1) ketamine-xylazine-induced field potential oscillations in a rat motor cortex and (2) an intracortical electrical stimulation-induced movement threshold. The first method is based on finding the polarity reversal in laminar oscillations which is reported to appear at the transition between layers IV and V in laminar 'high voltage spindles' of the rat cortical column. Analysis of histological images in our dataset indicates that polarity reversal is detected 150.1 ± 104.2 µm below the start of layer V. The second method compares the intracortical microstimulation currents that elicit a physical movement for anodic versus cathodic stimulation. It is based on the hypothesis that neural elements perpendicular to the electrode surface are preferentially excited by anodic stimulation while cathodic stimulation excites those with a direction component parallel to its surface. With this method, we expect to see a change in the stimulation currents that elicits a movement at the beginning of layer V when comparing anodic versus cathodic stimulation as the upper cortical layers contain neuronal structures that are primarily parallel to the cortical surface and lower layers contain structures that are primarily perpendicular. Using this method, there was a 78.7 ± 68 µm offset in the estimate of the depth of the start of layer V. The polarity reversal method estimates the beginning of layer V within ±90 µm with 95% confidence and the intracortical stimulation method estimates it within ±69.3 µm. We propose that these methods can be used to estimate the in situ location of laminar electrodes implanted in the rat motor cortex.

Subthalamic nucleus high-frequency stimulation generates a concomitant synaptic excitation-inhibition in substantia nigra pars reticulata

Deep brain stimulation is an efficient treatment for various neurological pathologies and a promising tool for neuropsychiatric disorders. This is particularly exemplified by high-frequency stimulation of the subthalamic nucleus (STN-HFS), which has emerged as an efficient symptomatic treatment for Parkinson's disease. How STN-HFS works is still not fully elucidated. With dual patch-clamp recordings in rat brain slices, we analysed the cellular responses of STN stimulation on SNr neurons by simultaneously recording synaptic currents and firing activity. We showed that STN-HFS caused an increase of the spontaneous spiking activity in half of SNr neurons while the remaining ones displayed a decrease. At the synaptic level, STN stimulation triggered inward current in 58% of whole-cell recorded neurons and outward current in the remaining ones. Using a pharmacological approach, we showed that STN-HFS-evoked responses were mediated in all neurons by a balance between AMPA/NMDA receptors and GABA(A) receptors, whose ratio promotes either a net excitation or a net inhibition. Interestingly, we observed a higher excitation occurrence in 6-hydroxydopamine (6-OHDA)-treated rats. In vivo injections of phaseolus revealed that GABAergic pallido-nigral fibres travel through the STN whereas striato-nigral fibres travel below it. Therefore, electrical stimulation of the STN does not only recruit glutamatergic axons from the STN, but also GABAergic passing fibres probably from the globus pallidus. For the first time, we showed that STN-HFS induces concomitant excitatory-inhibitory synaptic currents in SNr neurons by recruitment of efferences and passing fibres allowing a tight control on basal ganglia outflow.

Functional imaging of subthalamic nucleus deep brain stimulation in Parkinson's disease

Deep brain stimulation of the subthalamic nucleus is an accepted treatment for the motor complications of Parkinson's disease. The therapeutic mechanism of action remains incompletely understood. Although the results of deep brain stimulation are similar to the results that can be obtained by lesional surgery, accumulating evidence from functional imaging and clinical neurophysiology suggests that the effects of subthalamic nucleus-deep brain stimulation are not simply the result of inhibition of subthalamic nucleus activity. Positron emission tomography/single-photon emission computed tomography has consistently demonstrated changes in cortical activation in response to subthalamic nucleus-deep brain stimulation. However, the technique has limited spatial and temporal resolution, and therefore the changes in activity of subcortical projection sites of the subthalamic nucleus (such as the globus pallidus, substantia nigra, and thalamus) are not as clear. Clarifying whether clinically relevant effects from subthalamic nucleus-deep brain stimulation in humans are mediated through inhibition or excitation of orthodromic or antidromic pathways (or both) would contribute to our understanding of the precise mechanism of action of deep brain stimulation and may allow improvements in safety and efficacy of the technique. In this review we discuss the published evidence from functional imaging studies of patients with subthalamic nucleus-deep brain stimulation to date, together with how these data inform the mechanism of action of deep brain stimulation.

High frequency stimulation of the subthalamic nucleus acutely rescues motor deficits and neocortical movement representations following 6-hydroxydopamine administration in rats

Loss of frontal neocortical activation is one of the main neurophysiological abnormalities of Parkinson's disease (PD) and can be observed in rodent models of nigrostriatal degeneration. High-frequency deep brain stimulation (DBS) of the subthalamic nucleus improves motor deficits in PD. However, it is unknown whether this general therapeutic effect is associated with a restoration of frontal output function. To address this question, chronic stimulating electrodes were implanted bilaterally into the subthalamic nuclei of adult rats that received either bilateral intrastriatal 6-hydroxydopamine (6-OHDA) or vehicle infusion to induce nigrostriatal degeneration. Forelimb use and locomotor activity were assessed based on the cylinder and open field tests in intact, post-lesion+sham DBS, and post-lesion+DBS conditions. Intracortical microstimulation was then used to probe frontal output function of forelimb motor areas. DBS was found to improve motor deficits arising from 6-OHDA lesions, increase forelimb map area, and decrease movement thresholds relative to baseline. These effects were significantly greater in 6-OHDA lesion rats compared to vehicle controls. Results indicate that changes in motor map expression can take place during subthalamic DBS following dopamine depletion in a rodent model of PD.

The Subthalamic Nucleus becomes a Generator of Bursts in the Dopamine-Depleted State. Its High Frequency Stimulation Dramatically Weakens Transmission to the Globus Pallidus

Excessive burst firing in the dopamine-depleted basal ganglia correlates with severe motor symptoms of Parkinson's disease that are attenuated by high frequency electrical stimulation of the subthalamic nucleus (STN). Here we test the hypothesis that pathological bursts in dopamine-deprived basal ganglia are generated within the STN and transmitted to globus pallidus neurons. To answer this question we recorded excitatory synaptic currents and potentials from subthalamic and pallidal neurons in the basal ganglia slice (BGS) from dopamine-depleted mice while continuously blocking GABA_A receptors. In control mice, a single electrical stimulus delivered to the internal capsule or the rostral pole of the STN evoked a short duration, small amplitude, monosynaptic EPSC in subthalamic neurons. In contrast, in the dopamine-depleted BGS, this monosynaptic EPSC was amplified and followed by a burst of polysynaptic EPSCs that eventually reverberated three to seven times, providing a long lasting response that gave rise to bursts of EPSCs and spikes in GP neurons. Repetitive (10–120 Hz) stimulation delivered to the STN in the dopamine-depleted BGS attenuated STN-evoked bursts of EPSCs in pallidal neurons after several minutes of stimulation but only high frequency (90–120 Hz) stimulation replaced them with small amplitude EPSCs at 20 Hz. We propose that the polysynaptic pathway within the STN amplifies subthalamic responses to incoming excitation in the dopamine-depleted basal ganglia, thereby transforming the STN into a burst generator and entraining pallidal neurons in pathogenic bursting activities. High frequency stimulation of the STN prevents the transmission of this pathological activity to globus pallidus and imposes a new glutamatergic synaptic noise on pallidal neurons.

Long-term results of electroconvulsive therapy in severe Gilles de la Tourette syndrome

To avoid chronic distress and increasing social isolation, patients with severe, medication-resistant Gilles de la Tourette syndrome (GTS) require treatment alternatives. Electroconvulsive therapy (ECT) is such an alternative treatment, which, however, is rarely mentioned in the literature: a Pubmed search revealed only 7 reports on GTS and ECT, and there were no long-term data on continuously applied maintenance ECT in GTS. This report is the first to document a 5-year-long, full remission from severe GTS after long-term ECT.

Optogenetic-guided cortical plasticity after nerve injury

Peripheral nerve injury causes sensory dysfunctions that are thought to be attributable to changes in neuronal activity occurring in somatosensory cortices both contralateral and ipsilateral to the injury. Recent studies suggest that distorted functional response observed in deprived primary somatosensory cortex (S1) may be the result of an increase in inhibitory interneuron activity and is mediated by the transcallosal pathway. The goal of this study was to develop a strategy to manipulate and control the transcallosal activity to facilitate appropriate plasticity by guiding the cortical reorganization in a rat model of sensory deprivation. Since transcallosal fibers originate mainly from excitatory pyramidal neurons somata situated in laminae III and V, the excitatory neurons in rat S1 were engineered to express halorhodopsin, a light-sensitive chloride pump that triggers neuronal hyperpolarization. Results from electrophysiology, optical imaging, and functional MRI measurements are concordant with that within the deprived S1, activity in response to intact forepaw electrical stimulation was significantly increased by concurrent illumination of halorhodopsin over the healthy S1. Optogenetic manipulations effectively decreased the adverse inhibition of deprived cortex and revealed the major contribution of the transcallosal projections, showing interhemispheric neuroplasticity and thus, setting a foundation to develop improved rehabilitation strategies to restore cortical functions.

Cortical potentials evoked by deep brain stimulation in the subthalamic area

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) has been used since the mid-1990s as a treatment for patients with Parkinson's disease, and more recently also in other conditions, such as dystonia or obsessive compulsive disorder. Non-invasive studies of cortical evoked potentials (EPs) that follow individual STN-DBS stimuli has provided us with insights about the conduction of the DBS pulses to the cortex. Such EPs have multiple components of different latencies, making it possible to distinguish short-latency and long-latency responses (3-8 ms and 18-25 ms latency, respectively). The available evidence indicates that these short- and long-latency EPs correspond to conduction from the STN stimulation site to the cortical recording location via anti- and orthodromic pathways, respectively. In this review we survey the literature from recording studies in human patients treated with STN-DBS for Parkinson's disease and other conditions, as well as recent animal studies (including our own) that have begun to elucidate details of the pathways, frequency dependencies, and other features of EPs. In addition, we comment on the possible clinical utility of this knowledge.

The clinical efficacy of L-DOPA and STN-DBS share a common marker: reduced GABA content in the motor thalamus

At odd with traditional views, effective sub-thalamic nucleus (STN) deep brain stimulation (DBS), in Parkinson's disease (PD) patients, may increase the discharge rate of the substantia nigra pars reticulata and the internal globus pallidus (GPi), in combination with increased cyclic guanosine monophosphate (cGMP) levels. How these changes affect the basal ganglia (BG) output to the motor thalamus, the crucial structure conveying motor information to cortex, is critical. Here, we determined the extracellular GABA concentration in the ventral anterior nucleus (VA) during the first delivery of STN-DBS (n=10) or following levodopa (LD) (n=8). Both DBS and subdyskinetic LD reversibly reduced (−30%) VA GABA levels. A significant correlation occurred between clinical score and GABA concentration. By contrast, only STN-DBS increased GPi cGMP levels. Hence, STN-ON and MED-ON involve partially different action mechanisms but share a common target in the VA. These findings suggest that the standard BG circuitry, in PD, needs revision as relief from akinesia may take place, during DBS, even in absence of reduced GPi excitability. However, clinical amelioration requires fast change of thalamic GABA, confirming, in line with the old model, that VA is the core player in determining thalamo-cortical transmission.

Tracking stem cell differentiation in the setting of automated optogenetic stimulation

Membrane depolarization has been shown to play an important role in the neural differentiation of stem cells and in the survival and function of mature neurons. Here, we introduce a microbial opsin into ESCs and develop optogenetic technology for stem cell engineering applications, with an automated system for noninvasive modulation of ESC differentiation employing fast optogenetic control of ion flux. Mouse ESCs were stably transduced with channelrhodopsin-2 (ChR2)-yellow fluorescent protein and purified by fluorescence activated cell sorting (FACS). Illumination of resulting ChR2-ESCs with pulses of blue light triggered inward currents. These labeled ESCs retained the capability to differentiate into functional mature neurons, assessed by the presence of voltage-gated sodium currents, action potentials, fast excitatory synaptic transmission, and expression of mature neuronal proteins and neuronal morphology. We designed and tested an apparatus for optically stimulating ChR2-ESCs during chronic neuronal differentiation, with high-speed optical switching on a custom robotic stage with environmental chamber for automated stimulation and imaging over days, with tracking for increased expression of neural and neuronal markers. These data point to potential uses of ChR2 technology for chronic and temporally precise noninvasive optical control of ESCs both in vitro and in vivo, ranging from noninvasive control of stem cell differentiation to causal assessment of the specific contribution of transplanted cells to tissue and network function.

Balancing the brain: resting state networks and deep brain stimulation

Over the last three decades, large numbers of patients with otherwise treatment-resistant disorders have been helped by deep brain stimulation (DBS), yet a full scientific understanding of the underlying neural mechanisms is still missing. We have previously proposed that efficacious DBS works by restoring the balance of the brain's resting state networks. Here, we extend this proposal by reviewing how detailed investigations of the highly coherent functional and structural brain networks in health and disease (such as Parkinson's) have the potential not only to increase our understanding of fundamental brain function but of how best to modulate the balance. In particular, some of the newly identified hubs and connectors within and between resting state networks could become important new targets for DBS, including potentially in neuropsychiatric disorders. At the same time, it is of essence to consider the ethical implications of this perspective.

Basal ganglia dysfunction in OCD: subthalamic neuronal activity correlates with symptoms severity and predicts high-frequency stimulation efficacy

Functional and connectivity changes in corticostriatal systems have been reported in the brains of patients with obsessive-compulsive disorder (OCD); however, the relationship between basal ganglia activity and OCD severity has never been adequately established. We recently showed that deep brain stimulation of the subthalamic nucleus (STN), a central basal ganglia nucleus, improves OCD. Here, single-unit subthalamic neuronal activity was analysed in 12 OCD patients, in relation to the severity of obsessions and compulsions and response to STN stimulation, and compared with that obtained in 12 patients with Parkinson's disease (PD). STN neurons in OCD patients had lower discharge frequency than those in PD patients, with a similar proportion of burst-type activity (69 vs 67%). Oscillatory activity was present in 46 and 68% of neurons in OCD and PD patients, respectively, predominantly in the low-frequency band (1-8 Hz). In OCD patients, the bursty and oscillatory subthalamic neuronal activity was mainly located in the associative-limbic part. Both OCD severity and clinical improvement following STN stimulation were related to the STN neuronal activity. In patients with the most severe OCD, STN neurons exhibited bursts with shorter duration and interburst interval, but higher intraburst frequency, and more oscillations in the low-frequency bands. In patients with best clinical outcome with STN stimulation, STN neurons displayed higher mean discharge, burst and intraburst frequencies, and lower interburst interval. These findings are consistent with the hypothesis of a dysfunction in the associative-limbic subdivision of the basal ganglia circuitry in OCD's pathophysiology.

Deep brain stimulation of the subthalamic or entopeduncular nucleus attenuates vacuous chewing movements in a rodent model of tardive dyskinesia

Deep brain stimulation (DBS) has recently emerged as a potential intervention for treatment-resistant tardive dyskinesia (TD). Despite promising case reports, no consensus exists as yet regarding optimal stimulation parameters or neuroanatomical target for DBS in TD. Here we report the use of DBS in an animal model of TD. We applied DBS (100 μA) acutely to the entopeduncular nucleus (EPN) or subthalamic nucleus (STN) in rats with well established vacuous chewing movements (VCMs) induced by 12 weeks of haloperidol (HAL) treatment. Stimulation of the STN or EPN resulted in significant reductions in VCM counts at frequencies of 30, 60 or 130 Hz. In the STN DBS groups, effects were significantly more pronounced at 130 Hz than at lower frequencies, whereas at the EPN the three frequencies were equipotent. Unilateral stimulation at 130 Hz was also effective when applied to either nucleus. These results suggest that stimulation of either the EPN or STN significantly alleviates oral dyskinesias induced by chronic HAL. The chronic HAL VCM model preparation may be useful to explore mechanisms underlying DBS effects in drug-induced dyskinesias.

Deep brain stimulation can suppress pathological synchronisation in parkinsonian patients

BACKGROUND

Although deep brain stimulation (DBS) of the subthalamic nucleus (STN) is a highly effective therapeutic intervention in severe Parkinson's disease, its mechanism of action remains unclear. One possibility is that DBS suppresses local pathologically synchronised oscillatory activity.

METHODS

To explore this, the authors recorded from DBS electrodes implanted in the STN of 16 patients with Parkinson's disease during simultaneous stimulation (pulse width 60 μs; frequency 130 Hz) of the same target using a specially designed amplifier. The authors analysed data from 25 sides.

RESULTS

The authors found that DBS progressively suppressed peaks in local field potential activity at frequencies between 11 and 30 Hz as voltage was increased beyond a stimulation threshold of 1.5 V. Median peak power had fallen to 54% of baseline values by a stimulation intensity of 3.0 V.

CONCLUSION

The findings suggest that DBS can suppress pathological 11-30 Hz activity in the vicinity of stimulation in patients with Parkinson's disease. This suppression occurs at stimulation voltages that are clinically effective.