Cortical stimulation evokes abnormal responses in the dopamine-depleted rat basal ganglia

A Model with Dopamine Depletion in Basal Ganglia and Cerebellum Predicts Changes in Thalamocortical Beta Oscillations

Parkinsonism is presented as a motor syndrome characterized by rigidity, tremors, and bradykinesia, with Parkinson's disease (PD) being the predominant cause. The discovery that those motor symptoms result from the death of dopaminergic cells in the substantia nigra led to focus most of parkinsonism research on the basal ganglia (BG). However, recent findings point to an active involvement of the cerebellum in this motor syndrome. Here, we have developed a multiscale computational model of the rodent brain's BG-cerebellar network. Simulations showed that a direct effect of dopamine depletion on the cerebellum must be taken into account to reproduce the alterations of neural activity in parkinsonism, particularly the increased beta oscillations widely reported in PD patients. Moreover, dopamine depletion indirectly impacted spike-time-dependent plasticity at the parallel fiber-Purkinje cell synapses, degrading associative motor learning as observed in parkinsonism. Overall, these results suggest a relevant involvement of cerebellum in parkinsonism associative motor symptoms.

Synaptic Changes in Pallidostriatal Circuits Observed in the Parkinsonian Model Triggers Abnormal Beta Synchrony with Accurate Spatio-temporal Properties across the Basal Ganglia

Excessive oscillatory activity across basal ganglia (BG) nuclei in the β frequencies (12-30 Hz) is a hallmark of Parkinson's disease (PD). While the link between oscillations and symptoms remains debated, exaggerated β oscillations constitute an important biomarker for therapeutic effectiveness in PD. The neuronal mechanisms of β-oscillation generation however remain unknown. Many existing models rely on a central role of the subthalamic nucleus (STN) or cortical inputs to BG. Contrarily, neural recordings and optogenetic manipulations in normal and parkinsonian rats recently highlighted the central role of the external pallidum (GPe) in abnormal β oscillations, while showing that the integrity of STN or motor cortex is not required. Here, we evaluate the mechanisms for the generation of abnormal β oscillations in a BG network model where neuronal and synaptic time constants, connectivity, and firing rate distributions are strongly constrained by experimental data. Guided by a mean-field approach, we show in a spiking neural network that several BG sub-circuits can drive oscillations. Strong recurrent STN-GPe connections or collateral intra-GPe connections drive γ oscillations (>40 Hz), whereas strong pallidostriatal loops drive low-β (10-15 Hz) oscillations. We show that pathophysiological strengthening of striatal and pallidal synapses following dopamine depletion leads to the emergence of synchronized oscillatory activity in the mid-β range with spike-phase relationships between BG neuronal populations in-line with experiments. Furthermore, inhibition of GPe, contrary to STN, abolishes oscillations. Our modeling study uncovers the neural mechanisms underlying PD β oscillations and may thereby guide the future development of therapeutic strategies.

Closed-loop modulation of model parkinsonian beta oscillations based on CAR-fuzzy control algorithm

Closed-loop deep brain stimulation (DBS) can apply on-demand stimulation based on the feedback signal (e.g. beta band oscillation), which is deemed to lower side effects of clinically used open-loop DBS. To facilitate the application of model-based closed-loop DBS in clinical, studies must consider state variations, e.g., variation of desired signal with different movement conditions and variation of model parameters with time. This paper proposes to use the controlled autoregressive (CAR)-fuzzy control algorithm to modulate the pathological beta band (13-35 Hz) oscillation of a basal ganglia-cortex-thalamus model. The CAR model is used to identify the relationship between DBS frequency parameter and beta oscillation power. Then the error between the one-step-ahead predicted beta power of CAR model and the desired value is innovatively used as the input of fuzzy controller to calculate the next step stimulation frequency. Compared with 130 Hz open-loop DBS, the proposed closed-loop DBS method could lower the mean stimulation frequency to 74.04 Hz with similar beta oscillation suppression performance. The Mamdani fuzzy controller is selected because which could establish fuzzy controller rules according to human operation experience. Adding prediction module to closed-loop control improves the accuracy of fuzzy control, compared with proportional-integral control and fuzzy control, the proposed CAR-fuzzy control algorithm has higher tracking reliability, response speed and robustness.

Synaptic network structure shapes cortically evoked spatio-temporal responses of STN and GPe neurons in a computational model

Introduction

The basal ganglia (BG) are involved in motor control and play an essential role in movement disorders such as hemiballismus, dystonia, and Parkinson's disease. Neurons in the motor part of the BG respond to passive movement or stimulation of different body parts and to stimulation of corresponding cortical regions. Experimental evidence suggests that the BG are organized somatotopically, i.e., specific areas of the body are associated with specific regions in the BG nuclei. Signals related to the same body part that propagate along different pathways converge onto the same BG neurons, leading to characteristic shapes of cortically evoked responses. This suggests the existence of functional channels that allow for the processing of different motor commands or information related to different body parts in parallel. Neurological disorders such as Parkinson's disease are associated with pathological activity in the BG and impaired synaptic connectivity, together with reorganization of somatotopic maps. One hypothesis is that motor symptoms are, at least partly, caused by an impairment of network structure perturbing the organization of functional channels.

Methods

We developed a computational model of the STN-GPe circuit, a central part of the BG. By removing individual synaptic connections, we analyzed the contribution of signals propagating along different pathways to cortically evoked responses. We studied how evoked responses are affected by systematic changes in the network structure. To quantify the BG's organization in the form of functional channels, we suggested a two-site stimulation protocol.

Results

Our model reproduced the cortically evoked responses of STN and GPe neurons and the contributions of different pathways suggested by experimental studies. Cortical stimulation evokes spatio-temporal response patterns that are linked to the underlying synaptic network structure. Our two-site stimulation protocol yielded an approximate functional channel width.

Discussion/conclusion

The presented results provide insight into the organization of BG synaptic connectivity, which is important for the development of computational models. The synaptic network structure strongly affects the processing of cortical signals and may impact the generation of pathological rhythms. Our work may motivate further experiments to analyze the network structure of BG nuclei and their organization in functional channels.

A neurorobotics approach to behaviour selection based on human activity recognition

Behaviour selection has been an active research topic for robotics, in particular in the field of human-robot interaction. For a robot to interact autonomously and effectively with humans, the coupling between techniques for human activity recognition and robot behaviour selection is of paramount importance. However, most approaches to date consist of deterministic associations between the recognised activities and the robot behaviours, neglecting the uncertainty inherent to sequential predictions in real-time applications. In this paper, we address this gap by presenting an initial neurorobotics model that embeds, in a simulated robot, computational models of parts of the mammalian brain that resembles neurophysiological aspects of the basal ganglia-thalamus-cortex (BG-T-C) circuit, coupled with human activity recognition techniques. A robotics simulation environment was developed for assessing the model, where a mobile robot accomplished tasks by using behaviour selection in accordance with the activity being performed by the inhabitant of an intelligent home. Initial results revealed that the initial neurorobotics model is advantageous, especially considering the coupling between the most accurate activity recognition approaches and the computational models of more complex animals.

Basal ganglia network dynamics and function: Role of direct, indirect and hyper-direct pathways in action selection

Basal ganglia (BG) are a widely recognized neural basis for action selection, but its decision-making mechanism is still a difficult problem for researchers. Therefore, we constructed a spiking neural network inspired by the BG anatomical data. Simulation experiments were based on the principle of dis-inhibition and our functional hypothesis within the BG: the direct pathway, the indirect pathway, and the hyper-direct pathway of the BG jointly implement the initiation execution and termination of motor programs. Firstly, we studied the dynamic process of action selection with the network, which contained intra-group competition and inter-group competition. Secondly, we focused on the effects of the stimulus intensity and the proportion of excitation and inhibition on the GPi/SNr. The results suggested that inhibition and excitation shape action selection. They also explained why the firing rate of GPi/SNr did not continue to increase in the action-selection experiment. Finally, we discussed the experimental results with the functional hypothesis. Uniquely, this paper summarized the decision-making neural mechanism of action selection based on the direct pathway, the indirect pathway, and the hyper-direct pathway within BG.

Motor cortico-nigral and cortico-entopeduncular information transmission and its modulation by buspirone in control and after dopaminergic denervation

Cortical information is transferred to the substantia nigra pars reticulata (SNr) and the entopeduncular nucleus (EP), the output structures of the basal ganglia (BG), through three different pathways: the hyperdirect trans-subthalamic and the direct and indirect trans-striatal pathways. The nigrostriatal dopamine (DA) and the activation of 5-HT_1A receptors, distributed all along the BG, may modulate cortical information transmission. We aimed to investigate the effect of buspirone (5-HT_1A receptor partial agonist) and WAY-100635 (5-HT_1A receptor antagonist) on cortico-nigral and cortico-entopeduncular transmission in normal and DA loss conditions. Herein, simultaneous electrical stimulation of the motor cortex and single-unit extracellular recordings of SNr or EP neurons were conducted in urethane-anesthetized sham and 6-hydroxydopamine (6-OHDA)-lesioned rats before and after drug administrations. Motor cortex stimulation evoked monophasic, biphasic, or triphasic responses, combination of an early excitation, an inhibition, and a late excitation in both the SNr and EP, while an altered pattern of evoked response was observed in the SNr after 6-OHDA lesion. Systemic buspirone potentiated the direct cortico-SNr and cortico-EP transmission in sham animals since increased duration of the inhibitory response was observed. In DA denervated animals, buspirone administration enhanced early excitation amplitude in the cortico-SNr transmission. In both cases, the observed effects were mediated via a 5-HT_1A-dependent mechanism as WAY-100635 administration blocked buspirone’s effect. These findings suggest that in control condition, buspirone potentiates direct pathway transmission and DA loss modulates responses related to the hyperdirect pathway. Overall, the results may contribute to understanding the role of 5-HT_1A receptors and DA in motor cortico-BG circuitry functionality.

Deep brain stimulation rectifies the noisy cortex and irresponsive subthalamus to improve parkinsonian locomotor activities

The success of deep brain stimulation (DBS) therapy indicates that Parkinson's disease is a brain rhythm disorder. However, the manifestations of the erroneous rhythms corrected by DBS remain to be established. We found that augmentation of α rhythms and α coherence between the motor cortex (MC) and the subthalamic nucleus (STN) is characteristically prokinetic and is decreased in parkinsonian rats. In multi-unit recordings, movement is normally associated with increased changes in spatiotemporal activities rather than overall spike rates in MC. In parkinsonian rats, MC shows higher spike rates at rest but less spatiotemporal activity changes upon movement, and STN burst discharges are more prevalent, longer lasting, and less responsive to MC inputs. DBS at STN rectifies the foregoing pathological MC-STN oscillations and consequently locomotor deficits, yet overstimulation may cause behavioral restlessness. These results indicate that delicate electrophysiological considerations at both cortical and subcortical levels should be exercised for optimal DBS therapy.

Subthalamic nucleus stabilizes movements by reducing neural spike variability in monkey basal ganglia

The subthalamic nucleus projects to the external and internal pallidum, the modulatory and output nuclei of the basal ganglia, respectively, and plays an indispensable role in controlling voluntary movements. However, the precise mechanism by which the subthalamic nucleus controls pallidal activity and movements remains elusive. Here, we utilize chemogenetics to reversibly reduce neural activity of the motor subregion of the subthalamic nucleus in three macaque monkeys (Macaca fuscata, both sexes) during a reaching task. Systemic administration of chemogenetic ligands prolongs movement time and increases spike train variability in the pallidum, but only slightly affects firing rate modulations. Across-trial analyses reveal that the irregular discharges in the pallidum coincides with prolonged movement time. Reduction of subthalamic activity also induces excessive abnormal movements in the contralateral forelimb, which are preceded by subthalamic and pallidal phasic activity changes. Our results suggest that the subthalamic nucleus stabilizes pallidal spike trains and achieves stable movements.

Chemogenetic inactivation of the subthalamic nucleus in monkeys increases spike train variability in the pallidum and prolongs movement time, suggesting its role in stabilizing pallidal spike trains to achieve stable motor control.

The Origin of Abnormal Beta Oscillations in the Parkinsonian Corticobasal Ganglia Circuits

Parkinson's disease (PD) is a neurodegenerative brain disorder associated with motor and nonmotor symptoms. Exaggerated beta band (15–30 Hz) neuronal oscillations are widely observed in corticobasal ganglia (BG) circuits during parkinsonism. Abnormal beta oscillations have been linked to motor symptoms of PD, but their exact relationship is poorly understood. Nevertheless, reduction of beta oscillations can induce therapeutic effects in PD patients. While it is widely believed that the external globus pallidus (GPe) and subthalamic nucleus (STN) are jointly responsible for abnormal rhythmogenesis in the parkinsonian BG, the role of other cortico-BG circuits cannot be ignored. To shed light on the origin of abnormal beta oscillations in PD, here we review changes of neuronal activity observed in experimental PD models and discuss how the cortex and different BG nuclei cooperate to generate and stabilize abnormal beta oscillations during parkinsonism. This may provide further insights into the complex relationship between abnormal beta oscillations and motor dysfunction in PD, which is crucial for potential target-specific therapeutic interventions in PD patients.

Transient Response of Basal Ganglia Network in Healthy and Low-Dopamine State

The basal ganglia (BG) are crucial for a variety of motor and cognitive functions. Changes induced by persistent low-dopamine (e.g., in Parkinson’s disease; PD) result in aberrant changes in steady-state population activity (β band oscillations) and the transient response of the BG. Typically, a brief cortical stimulation results in a triphasic response in the substantia nigra pars reticulata (SNr; an output of the BG). The properties of the triphasic responses are shaped by dopamine levels. While mechanisms underlying aberrant steady state activity are well studied, it is still unclear which BG interactions are crucial for the aberrant transient responses in the BG. Moreover, it is also unclear whether mechanisms underlying the aberrant changes in steady-state activity and transient response are the same. Here, we used numerical simulations of a network model of BG to identify the key factors that determine the shape of the transient responses. We show that an aberrant transient response of the SNr in the low-dopamine state involves changes in the direct pathway and the recurrent interactions within the globus pallidus externa (GPe) and between GPe and subthalamic nucleus (STN). However, the connections from D2-type spiny projection neurons (D2-SPN) to GPe are most crucial in shaping the transient response and by restoring them to their healthy level, we could restore the shape of transient response even in low-dopamine state. Finally, we show that the changes in BG that result in aberrant transient response are also sufficient to generate pathologic oscillatory activity in the steady state.

Improved alpha-beta power reduction via combined electrical and ultrasonic stimulation in a parkinsonian cortex-basal ganglia-thalamus computational model

Objective. To investigate computationally the interaction of combined electrical and ultrasonic modulation of isolated neurons and of the parkinsonian cortex-basal ganglia-thalamus loop.Approach. Continuous-wave or pulsed electrical and ultrasonic neuromodulation is applied to isolated Otsuka plateau-potential generating subthalamic nucleus (STN) and Pospischil regular, fast and low-threshold spiking cortical cells in a temporally alternating or simultaneous manner. Similar combinations of electrical/ultrasonic waveforms are applied to a parkinsonian biophysical cortex-basal ganglia-thalamus neuronal network. Ultrasound-neuron interaction is modelled respectively for isolated neurons and the neuronal network with the NICE and SONIC implementations of the bilayer sonophore underlying mechanism. Reduction inα-βspectral energy is used as a proxy to express improvement in Parkinson's disease by insonication and electrostimulation.Main results. Simultaneous electro-acoustic stimulation achieves a given level of neuronal activity at lower intensities compared to the separate stimulation modalities. Conversely, temporally alternating stimulation with50 Hzelectrical and ultrasound pulses is capable of eliciting100 HzSTN firing rates. Furthermore, combination of ultrasound with hyperpolarizing currents can alter cortical cell relative spiking regimes. In the parkinsonian neuronal network, continuous-wave and pulsed ultrasound reduce pathological oscillations by different mechanisms. High-frequency pulsed separated electrical and ultrasonic deep brain stimulation (DBS) reduce pathologicalα-βpower by entraining STN-neurons. In contrast, continuous-wave ultrasound reduces pathological oscillations by silencing the STN. Compared to the separated stimulation modalities, temporally simultaneous or alternating electro-acoustic stimulation can achieve higher reductions inα-βpower for the same safety contraints on electrical/ultrasonic intensity.Significance. Focused ultrasound has the potential of becoming a non-invasive alternative of conventional DBS for the treatment of Parkinson's disease. Here, we elaborate on proposed benefits of combined electro-acoustic stimulation in terms of improved dynamic range, efficiency, spatial resolution, and neuronal selectivity.

Subthalamic burst firing: A pathophysiological target in Parkinson's disease

Understanding the pathophysiological mechanism of Parkinson's disease (PD) in the subthalamic nucleus (STN) has become a critical issue since deep brain stimulation (DBS) in this region has been proven as an effective treatment for this disease. The STN possesses a special ability to switch from the spike to the burst firing mode in response to dopamine deficiency in parkinsonism, and this STN burst is considered an electrophysiological signature of the cortico-basal ganglia circuit in the brains of PD patients. This review focuses on the role of STN burst firing in the pathophysiology of PD and during DBS. Here, we review existing literature on how STN bursts originate and the specific factors affecting their formation; how STN burst firing causes motor symptoms in PD and how interventions can rescue these symptoms. Finally, the similarities and differences between the two electrophysiological hallmarks of PD, STN burst firing and beta-oscillation, are discussed. STN burst firing should be considered as a pathophysiological target in PD during treatment with DBS.

Adaptive Parameter Modulation of Deep Brain Stimulation Based on Improved Supervisory Algorithm

Clinically deployed deep brain stimulation (DBS) for the treatment of Parkinson's disease operates in an open loop with fixed stimulation parameters, and this may result in high energy consumption and suboptimal therapy. The objective of this manuscript is to establish, through simulation in a computational model, a closed-loop control system that can automatically adjust the stimulation parameters to recover normal activity in model neurons. Exaggerated beta band activity is recognized as a hallmark of Parkinson's disease and beta band activity in model neurons of the globus pallidus internus (GPi) was used as the feedback signal to control DBS of the GPi. Traditional proportional controller and proportional-integral controller were not effective in eliminating the error between the target level of beta power and the beta power under Parkinsonian conditions. To overcome the difficulties in tuning the controller parameters and improve tracking performance in the case of changes in the plant, a supervisory control algorithm was implemented by introducing a Radial Basis Function (RBF) network to build the inverse model of the plant. Simulation results show the successful tracking of target beta power in the presence of changes in Parkinsonian state as well as during dynamic changes in the target level of beta power. Our computational study suggests the feasibility of the RBF network-driven supervisory control algorithm for real-time modulation of DBS parameters for the treatment of Parkinson's disease.

Elimination of the Cortico-Subthalamic Hyperdirect Pathway Induces Motor Hyperactivity in Mice

The substantia nigra pars reticulata (SNr) is the output station of the basal ganglia and receives cortical inputs by way of the following three basal ganglia pathways: the cortico-subthalamo (STN)-SNr hyperdirect, the cortico-striato-SNr direct, and the cortico-striato-external pallido-STN-SNr indirect pathways. Compared with the classical direct and indirect pathways via the striatum, the functions of the hyperdirect pathway remain to be fully elucidated. Here we used a photodynamic technique to selectively eliminate the cortico-STN projection in male mice and observed neuronal activity and motor behaviors in awake conditions. After cortico-STN elimination, cortically evoked early excitation in the SNr was diminished, while the cortically evoked inhibition and late excitation, which are delivered through the direct and indirect pathways, respectively, were unchanged. In addition, locomotor activity was significantly increased after bilateral cortico-STN elimination, and apomorphine-induced ipsilateral rotations were observed after unilateral cortico-STN elimination, suggesting that cortical activity was increased. These results are compatible with the notion that the cortico-STN-SNr hyperdirect pathway quickly conveys cortical excitation to the output station of the basal ganglia, resets or suppresses the cortical activity related to ongoing movements, and prepares for the forthcoming movement.SIGNIFICANCE STATEMENT The basal ganglia play a pivotal role in the control of voluntary movements, and their malfunctions lead to movement disorders, such as Parkinson's disease and dystonia. Understanding their functions is important to find better treatments for such diseases. Here we used a photodynamic technique to selectively eliminate the projection from the motor cortex to the subthalamic nucleus, the input station of the basal ganglia, and found greatly reduced early excitatory signals from the cortex to the output station of the basal ganglia and motor hyperactivity. These results suggest that the neuronal signals through the cortico-subthalamic hyperdirect pathway reset or suppress ongoing movements and that blockade of this pathway may be beneficial for Parkinson's disease, which is characterized by oversuppression of movements.

Cell Type-Specific Decrease of the Intrinsic Excitability of Motor Cortical Pyramidal Neurons in Parkinsonism

The hypokinetic motor symptoms of Parkinson's disease (PD) are closely linked with a decreased motor cortical output as a consequence of elevated basal ganglia inhibition. However, whether and how the loss of dopamine (DA) alters the cellular properties of motor cortical neurons in PD remains undefined. We induced parkinsonism in adult C57BL/6 mice of both sexes by injecting neurotoxin, 6-hydroxydopamine (6-OHDA), into the medial forebrain bundle. By using ex vivo patch-clamp recording and retrograde tracing approach, we found that the intrinsic excitability of pyramidal tract neurons (PTNs) in the primary motor cortical (M1) layer (L)5b was greatly decreased in parkinsonism; but the intratelencephalic neurons (ITNs) were not affected. The cell type-specific intrinsic adaptations were associated with a depolarized threshold and broadened width of action potentials (APs) in PTNs. Moreover, the loss of midbrain dopaminergic neurons impaired the capability of M1 PTNs to sustain high-frequency firing, which could underlie their abnormal pattern of activity in the parkinsonian state. We also showed that the decreased excitability in parkinsonism was caused by an impaired function of both persistent sodium channels and the large conductance, Ca2+-activated K+ channels. Acute activation of dopaminergic receptors failed to rescue the impaired intrinsic excitability of M1 PTNs in parkinsonian mice. Altogether, our data demonstrated a cell type-specific decrease of the excitability of M1 pyramidal neurons in parkinsonism. Thus, intrinsic adaptations in the motor cortex provide novel insight in our understanding of the pathophysiology of motor deficits in PD.SIGNIFICANCE STATEMENT The degeneration of midbrain dopaminergic neurons in Parkinson's disease (PD) remodels the connectivity and function of cortico-basal ganglia-thalamocortical network. However, whether and how dopaminergic degeneration and the associated basal ganglia dysfunction alter motor cortical circuitry remain undefined. We found that pyramidal neurons in the layer (L)5b of the primary motor cortex (M1) exhibit distinct adaptations in response to the loss of midbrain dopaminergic neurons, depending on their long-range projections. Besides the decreased thalamocortical synaptic excitation as proposed by the classical model of Parkinson's pathophysiology, these results, for the first time, show novel cellular and molecular mechanisms underlying the abnormal motor cortical output in parkinsonism.

Striatal Direct Pathway Targets Npas1+ Pallidal Neurons

The classic basal ganglia circuit model asserts a complete segregation of the two striatal output pathways. Empirical data argue that, in addition to indirect-pathway striatal projection neurons (iSPNs), direct-pathway striatal projection neurons (dSPNs) innervate the external globus pallidus (GPe). However, the functions of the latter were not known. In this study, we interrogated the organization principles of striatopallidal projections and their roles in full-body movement in mice (both males and females). In contrast to the canonical motor-promoting response of dSPNs in the dorsomedial striatum (DMSdSPNs), optogenetic stimulation of dSPNs in the dorsolateral striatum (DLSdSPNs) suppressed locomotion. Circuit analyses revealed that dSPNs selectively target Npas1+ neurons in the GPe. In a chronic 6-hydroxydopamine lesion model of Parkinson's disease, the dSPN-Npas1+ projection was dramatically strengthened. As DLSdSPN-Npas1+ projection suppresses movement, the enhancement of this projection represents a circuit mechanism for the hypokinetic symptoms of Parkinson's disease that has not been previously considered. In sum, our results suggest that dSPN input to the GPe is a critical circuit component that is involved in the regulation of movement in both healthy and parkinsonian states.SIGNIFICANCE STATEMENT In the classic basal ganglia model, the striatum is described as a divergent structure: it controls motor and adaptive functions through two segregated, opposing output streams. However, the experimental results that show the projection from direct-pathway neurons to the external pallidum have been largely ignored. Here, we showed that this striatopallidal subpathway targets a select subset of neurons in the external pallidum and is motor-suppressing. We found that this subpathway undergoes changes in a Parkinson's disease model. In particular, our results suggest that the increase in strength of this subpathway contributes to the slowness or reduced movements observed in Parkinson's disease.

Abnormal Cortico-Basal Ganglia Neurotransmission in a Mouse Model of l-DOPA-Induced Dyskinesia

l-3,4-dihydroxyphenylalanine (l-DOPA) is an effective treatment for Parkinson's disease (PD); however, long-term treatment induces l-DOPA-induced dyskinesia (LID). To elucidate its pathophysiology, we developed a mouse model of LID by daily administration of l-DOPA to PD male ICR mice treated with 6-hydroxydopamine (6-OHDA), and recorded the spontaneous and cortically evoked neuronal activity in the external segment of the globus pallidus (GPe) and substantia nigra pars reticulata (SNr), the connecting and output nuclei of the basal ganglia, respectively, in awake conditions. Spontaneous firing rates of GPe neurons were decreased in the dyskinesia-off state (≥24 h after l-DOPA injection) and increased in the dyskinesia-on state (20-100 min after l-DOPA injection while showing dyskinesia), while those of SNr neurons showed no significant changes. GPe and SNr neurons showed bursting activity and low-frequency oscillation in the PD, dyskinesia-off, and dyskinesia-on states. In the GPe, cortically evoked late excitation was increased in the PD and dyskinesia-off states but decreased in the dyskinesia-on state. In the SNr, cortically evoked inhibition was largely suppressed, and monophasic excitation became dominant in the PD state. Chronic l-DOPA treatment partially recovered inhibition and suppressed late excitation in the dyskinesia-off state. In the dyskinesia-on state, inhibition was further enhanced, and late excitation was largely suppressed. Cortically evoked inhibition and late excitation in the SNr are mediated by the cortico-striato-SNr direct and cortico-striato-GPe-subthalamo-SNr indirect pathways, respectively. Thus, in the dyskinesia-on state, signals through the direct pathway that release movements are enhanced, while signals through the indirect pathway that stop movements are suppressed, underlying LID.SIGNIFICANCE STATEMENT Parkinson's disease (PD) is caused by progressive loss of midbrain dopaminergic neurons, characterized by tremor, rigidity, and akinesia, and estimated to affect around six million people world-wide. Dopamine replacement therapy is the gold standard for PD treatment; however, control of symptoms using l-3,4-dihydroxyphenylalanine (l-DOPA) becomes difficult over time because of abnormal involuntary movements (AIMs) known as l-DOPA-induced dyskinesia (LID), one of the major issues for advanced PD. Our electrophysiological data suggest that dynamic changes in the basal ganglia circuitry underlie LID; signals through the direct pathway that release movements are enhanced, while signals through the indirect pathway that stop movements are suppressed. These results will provide the rationale for the development of more effective treatments for LID.

Evaluation of Frequency-Dependent Effects of Deep Brain Stimulation in a Cortex-Basal Ganglia-Thalamus Network Model of Parkinson's Disease

Parkinson's disease (PD) is a chronic neurodegenerative disease whose motor symptoms are accompanied by an exaggerated power in the alpha-beta (7-35Hz) band and an increased synchronization of neurons encompassing the cortex-basal ganglia-thalamus network. Currently, deep brain stimulation (DBS) is used as an effective therapy for reducing the excessive power and synchrony observed in brain circuits, thereby ameliorating the PD symptoms. In the present study, we used a biologically plausible computational model of cortex-basal ganglia-thalamus network, which represents both healthy and PD conditions, to systematically investigate the effects of DBS frequency on the model outputs. DBS was applied to the subthalamic nucleus (STN) at different stimulation frequencies (40Hz to 300Hz). Spike train variability and spectral power in the 7-35Hz band were measured from the several nuclei represented in the model. In addition, the magnitude squared coherence between the nuclei was assessed. An increased DBS frequency tended to produce interspike intervals (ISIs) with higher variability as compared to PD condition. Also, DBS significantly reduced the alpha-beta power for almost all brain nuclei. The median of the magnitude-squared coherence matrix (which is a metric of global network synchronization) decreased significantly with the increase of DBS frequency.

Dysregulation of external globus pallidus-subthalamic nucleus network dynamics in parkinsonian mice during cortical slow-wave activity and activation

KEY POINTS

Reciprocally connected GABAergic external globus pallidus (GPe) and glutamatergic subthalamic nucleus (STN) neurons form a key network within the basal ganglia. In Parkinson's disease and its models, abnormal rates and patterns of GPe-STN network activity are linked to motor dysfunction. Using cell class-specific optogenetic identification and inhibition during cortical slow-wave activity and activation, we report that, in dopamine-depleted mice, (1) D2 dopamine receptor expressing striatal projection neurons (D2-SPNs) discharge at higher rates, especially during cortical activation, (2) prototypic parvalbumin-expressing GPe neurons are excessively patterned by D2-SPNs even though their autonomous activity is upregulated, (3) despite being disinhibited, STN neurons are not hyperactive, and (4) STN activity opposes striatopallidal patterning. These data argue that in parkinsonian mice abnormal, temporally offset prototypic GPe and STN neuron firing results in part from increased striatopallidal transmission and that compensatory plasticity limits STN hyperactivity and cortical entrainment.

ABSTRACT

Reciprocally connected GABAergic external globus pallidus (GPe) and glutamatergic subthalamic nucleus (STN) neurons form a key, centrally positioned network within the basal ganglia. In Parkinson's disease and its models, abnormal rates and patterns of GPe-STN network activity are linked to motor dysfunction. Following the loss of dopamine, the activities of GPe and STN neurons become more temporally offset and strongly correlated with cortical oscillations below 40 Hz. Previous studies utilized cortical slow-wave activity and/or cortical activation (ACT) under anaesthesia to probe the mechanisms underlying the normal and pathological patterning of basal ganglia activity. Here, we combined this approach with in vivo optogenetic inhibition to identify and interrupt the activity of D2 dopamine receptor-expressing striatal projection neurons (D2-SPNs), parvalbumin-expressing prototypic GPe (PV GPe) neurons, and STN neurons. We found that, in dopamine-depleted mice, (1) the firing rate of D2-SPNs was elevated, especially during cortical ACT, (2) abnormal phasic suppression of PV GPe neuron activity was ameliorated by optogenetic inhibition of coincident D2-SPN activity, (3) autonomous PV GPe neuron firing ex vivo was upregulated, presumably through homeostatic mechanisms, (4) STN neurons were not hyperactive, despite being disinhibited, (5) optogenetic inhibition of the STN exacerbated abnormal GPe activity, and (6) exaggerated beta band activity was not present in the cortex or GPe-STN network. Together with recent studies, these data suggest that in dopamine-depleted mice abnormally correlated and temporally offset PV GPe and STN neuron activity is generated in part by elevated striatopallidal transmission, while compensatory plasticity prevents STN hyperactivity and limits cortical entrainment.

Synaptic and cellular plasticity in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disease, which causes a tremendous socioeconomic burden. PD patients are suffering from debilitating motor and nonmotor symptoms. Cardinal motor symptoms of PD, including akinesia, bradykinesia, resting tremor, and rigidity, are caused by the degeneration of dopaminergic neurons in the substantia nigra pars compacta. In addition, decreased amounts of dopamine (DA) level in the basal ganglia induces numerous adaptive changes at the cellular and synaptic levels in the basal ganglia circuits. These cellular and synaptic adaptations are believed to underlie the emergence and propagation of correlated, rhythmic pattern of activity throughout the interconnected cortico-basal ganglia-thalamocortical network. The widespread pathological pattern of brain activity is closely linked to the devastating motor symptoms of PD. Accumulating evidence suggests that both dopaminergic degeneration and the associated abnormal cellular and circuit activity in the basal ganglia drive the motor symptoms of PD. In this short review I summarize the recent advances in our understanding of synaptic and cellular alterations in two basal ganglia nuclei (i.e. the striatum and the subthalamic nucleus) following a complete loss of DA, and in our conceptual understanding of the cellular and circuit bases for the pathological pattern of brain activity in parkinsonian state.

Beta-Band Resonance and Intrinsic Oscillations in a Biophysically Detailed Model of the Subthalamic Nucleus-Globus Pallidus Network

Increased beta-band oscillatory activity in the basal ganglia network is associated with Parkinsonian motor symptoms and is suppressed with medication and deep brain stimulation (DBS). The origins of the beta-band oscillations, however, remains unclear with both intrinsic oscillations arising within the subthalamic nucleus (STN)-external globus pallidus (GPe) network and exogenous beta-activity, originating outside the network, proposed as potential sources of the pathological activity. The aim of this study was to explore the relative contribution of autonomous oscillations and exogenous oscillatory inputs in the generation of pathological oscillatory activity in a biophysically detailed model of the parkinsonian STN-GPe network. The network model accounts for the integration of synaptic currents and their interaction with intrinsic membrane currents in dendritic structures within the STN and GPe. The model was used to investigate the development of beta-band synchrony and bursting within the STN-GPe network by changing the balance of excitation and inhibition in both nuclei, and by adding exogenous oscillatory inputs with varying phase relationships through the hyperdirect cortico-subthalamic and indirect striato-pallidal pathways. The model showed an intrinsic susceptibility to beta-band oscillations that was manifest in weak autonomously generated oscillations within the STN-GPe network and in selective amplification of exogenous beta-band synaptic inputs near the network's endogenous oscillation frequency. The frequency at which this resonance peak occurred was determined by the net level of excitatory drive to the network. Intrinsic or endogenously generated oscillations were too weak to support a pacemaker role for the STN-GPe network, however, they were considerably amplified by sparse cortical beta inputs and were further amplified by striatal beta inputs that promoted anti-phase firing of the cortex and GPe, resulting in maximum transient inhibition of STN neurons. The model elucidates a mechanism of cortical patterning of the STN-GPe network through feedback inhibition whereby intrinsic susceptibility to beta-band oscillations can lead to phase locked spiking under parkinsonian conditions. These results point to resonance of endogenous oscillations with exogenous patterning of the STN-GPe network as a mechanism of pathological synchronization, and a role for the pallido-striatal feedback loop in amplifying beta oscillations.

Verifying Feighner's Hypothesis; Anorexia Nervosa Is Not a Psychiatric Disorder

Mental causation takes explanatory priority over evolutionary biology in most accounts of eating disorders. The evolutionary threat of starvation has produced a brain that assists us in the search for food and mental change emerges as a consequence. The major mental causation hypothesis: anxiety causes eating disorders, has been extensively tested and falsified. The subsidiary hypothesis: anxiety and eating disorders are caused by the same genotype, generates inconsistent results because the phenotypes are not traits, but vary along dimensions. Challenging the mental causation hypothesis in Feighner et al. (1972) noted that anorexic patients are physically hyperactive, hoarding for food, and they are rewarded for maintaining a low body weight. In 1996, Feighner's hypothesis was formalized, relating the patients' behavioral phenotype to the brain mechanisms of reward and attention (Bergh and Södersten, 1996), and in 2002, the hypothesis was clinically verified by training patients how to eat normally, thus improving outcomes (Bergh et al., 2002). Seventeen years later we provide evidence supporting Feighner's hypothesis by demonstrating that in 2012, 20 out of 37 patients who were referred by a psychiatrist, had a psychiatric diagnosis that differed from the diagnosis indicated by the SCID-I. Out of the 174 patients who were admitted in 2012, most through self-referral, there was significant disagreement between the outcomes of the SCID-I interview and the patient's subjective experience of a psychiatric problem in 110 of the cases. In addition, 358 anorexic patients treated to remission scored high on the Comprehensive Psychopathological Rating Scale, but an item response analysis indicated one (unknown) underlying dimension, rather than the three dimensions the scale can dissociate in patients with psychiatric disorders. These results indicate that psychiatric diagnoses, which are reliable and valid in patients with psychiatric disorders, are less well suited for patients with anorexia. The results are in accord with the hypothesis of the present Research Topic, that eating disorders are not always caused by disturbed psychological processes, and support the alternative, clinically relevant hypothesis that the behavioral phenotype of the patients should be addressed directly.

Model-Based Evaluation of Closed-Loop Deep Brain Stimulation Controller to Adapt to Dynamic Changes in Reference Signal

High-frequency deep brain stimulation (DBS) of the subthalamic nucleus (STN) is effective in suppressing the motor symptoms of Parkinson's disease (PD). Current clinically-deployed DBS technology operates in an open-loop fashion, i.e., fixed parameter high-frequency stimulation is delivered continuously, invariant to the needs or status of the patient. This poses two major challenges: (1) depletion of the stimulator battery due to the energy demands of continuous high-frequency stimulation, (2) high-frequency stimulation-induced side-effects. Closed-loop deep brain stimulation (CL DBS) may be effective in suppressing parkinsonian symptoms with stimulation parameters that require less energy and evoke fewer side effects than open loop DBS. However, the design of CL DBS comes with several challenges including the selection of an appropriate biomarker reflecting the symptoms of PD, setting a suitable reference signal, and implementing a controller to adapt to dynamic changes in the reference signal. Dynamic changes in beta oscillatory activity occur during the course of voluntary movement, and thus there may be a performance advantage to tracking such dynamic activity. We addressed these challenges by studying the performance of a closed-loop controller using a biophysically-based network model of the basal ganglia. The model-based evaluation consisted of two parts: (1) we implemented a Proportional-Integral (PI) controller to compute optimal DBS frequencies based on the magnitude of a dynamic reference signal, the oscillatory power in the beta band (13-35 Hz) recorded from model globus pallidus internus (GPi) neurons. (2) We coupled a linear auto-regressive model based mapping function with the Routh-Hurwitz stability analysis method to compute the parameters of the PI controller to track dynamic changes in the reference signal. The simulation results demonstrated successful tracking of both constant and dynamic beta oscillatory activity by the PI controller, and the PI controller followed dynamic changes in the reference signal, something that cannot be accomplished by constant open-loop DBS.

Single-axon tracing of the corticosubthalamic hyperdirect pathway in primates

Individual axons that form the hyperdirect pathway in Macaca fascicularis were visualized following microiontophoretic injections of biotinylated dextran amine in layer V of the primary motor cortex (M1). Twenty-eight singly labeled axons were reconstructed in 3D from serial sections. The M1 innervation of the subthalamic nucleus (STN) arises essentially from collaterals of long-ranged corticofugal axons en route to lower brainstem regions. Typically, after leaving M1, these large caliber axons (2-3 µm) enter the internal capsule and travel between caudate nucleus and putamen without providing any collateral to the striatum. More ventrally, they emit a thin collateral (0.5-1.5 µm) that runs lateromedially within the dorsal region of the STN, providing boutons en passant in the sensorimotor territory of the nucleus. In some cases, the medial tip of the collateral enters the lenticular fasciculus dorsally and yields a few beaded axonal branches in the zona incerta. In other cases, the collateral runs caudally and innervates the ventrolateral region of the red nucleus where large axon varicosities (up to 1.7 µm in diameter) are observed, many displaying perisomatic arrangements. Our ultrastructural analysis reveals a high synaptic incidence (141%) of cortical VGluT1-immunoreactive axon varicosities on distal dendrites of STN neurons, and on various afferent axons. Our single-axon reconstructions demonstrate that the so-called hyperdirect pathway derives essentially from collaterals of long-ranged corticofugal axons that are rarely exclusively devoted to the STN, as they also innervate the red nucleus and/or the zona incerta.

Cyclic AMP-producing chemogenetic activation of indirect pathway striatal projection neurons and the downstream effects on the globus pallidus and subthalamic nucleus in freely moving mice

The indirect pathway striatal medium spiny projection neurons (iMSNs) are critical to motor and cognitive brain functions. These neurons express a high level of cAMP-increasing adenosine A2a receptors. However, the potential effects of cAMP production on iMSN spiking activity have not been established, and recording identified iMSNs in freely moving animals is challenging. Here, we show that in the transgenic mice expressing cAMP-producing G protein G_s -coupled designer receptor exclusively activated by designer drug (Gs-DREADD) in iMSNs, the baseline spike firing in MSNs is normal, indicating DREADD expression does not affect the normal physiology of these neurons. Intraperitoneal injection of the DREADD agonist clozapine-N-oxide (CNO; 2.5 mg/kg) increased the spike firing in 50% of the recorded MSNs. However, CNO did not affect MSN firing in Gs-DREADD-negative mice. We also found that CNO injection inhibited the spike firing of globus pallidus external segment (GPe) neurons in Gs-DREADD-positive mice, further indicating CNO excitation of iMSNs. Temporally coincident with these effects on spiking firing in the indirect pathway, CNO injection selectively inhibited locomotion in D2 Gs-DREADD mice. Taken together, our results strongly suggest that cAMP production in iMSNs can increase iMSN spiking activity and cause motor inhibition, thus addressing a long-standing question about the cellular functions of the cAMP-producing adenosine A2a receptors in iMSNs. Cover Image for this issue: doi: 10.1111/jnc.14181.

Exercise-Induced Neuroprotection of the Nigrostriatal Dopamine System in Parkinson's Disease

Epidemiological studies indicate that physical activity and exercise may reduce the risk of developing Parkinson's disease (PD), and clinical observations suggest that physical exercise can reduce the motor symptoms in PD patients. In experimental animals, a profound observation is that exercise of appropriate timing, duration, and intensity can reduce toxin-induced lesion of the nigrostriatal dopamine (DA) system in animal PD models, although negative results have also been reported, potentially due to inappropriate timing and intensity of the exercise regimen. Exercise may also minimize DA denervation-induced medium spiny neuron (MSN) dendritic atrophy and other abnormalities such as enlarged corticostriatal synapse and abnormal MSN excitability and spiking activity. Taken together, epidemiological studies, clinical observations, and animal research indicate that appropriately dosed physical activity and exercise may not only reduce the risk of developing PD in vulnerable populations but also benefit PD patients by potentially protecting the residual DA neurons or directly restoring the dysfunctional cortico-basal ganglia motor control circuit, and these benefits may be mediated by exercise-triggered production of endogenous neuroprotective molecules such as neurotrophic factors. Thus, exercise is a universally available, side effect-free medicine that should be prescribed to vulnerable populations as a preventive measure and to PD patients as a component of treatment. Future research needs to establish standardized exercise protocols that can reliably induce DA neuron protection, enabling the delineation of the underlying cellular and molecular mechanisms that in turn can maximize exercise-induced neuroprotection and neurorestoration in animal PD models and eventually in PD patients.

Treadmill Exercise Improves Motor Dysfunction and Hyperactivity of the Corticostriatal Glutamatergic Pathway in Rats with 6-OHDA-Induced Parkinson's Disease

Hyperactivity in the corticostriatal glutamatergic pathway (CGP) induces basal ganglia dysfunction, contributing to parkinsonian syndrome (PS). Physical exercise can improve PS. However, the effect of exercise on the CGP, and whether this pathway is involved in the improvement of PS, remains unclear. Parkinson's disease (PD) was induced in rats by 6-hydroxydopamine injection into the right medial forebrain bundle. Motor function was assessed using the cylinder test. Striatal neuron (SN) spontaneous and evoked firing activity was recorded, and the expression levels of Cav1.3 and CaMKII in the striatum were measured after 4 weeks of treadmill exercise. The motor function in PD rats was improved by treadmill exercise. SN showed significantly enhanced excitability, and treadmill exercise reduced SN excitability in PD rats. In addition, firing activity was evoked in SNs by stimulation of the primary motor cortex, and SNs exhibited significantly decreased stimulus threshold, increased firing rates, and reduced latency. The expression of Cav1.3 and p-CaMKII (Thr286) in the striatum were enhanced in PD rats. However, these effects were reversed by treadmill exercise. These findings suggest that treadmill exercise inhibits CGP hyperactivity in PD rats, which may be related to improvement of PS.

Loss of Hyperdirect Pathway Cortico-Subthalamic Inputs Following Degeneration of Midbrain Dopamine Neurons

The motor symptoms of Parkinson's disease (PD) are linked to abnormally correlated and coherent activity in the cortex and subthalamic nucleus (STN). However, in parkinsonian mice we found that cortico-STN transmission strength had diminished by 50%-75% through loss of axo-dendritic and axo-spinous synapses, was incapable of long-term potentiation, and less effectively patterned STN activity. Optogenetic, chemogenetic, genetic, and pharmacological interrogation suggested that downregulation of cortico-STN transmission in PD mice was triggered by increased striato-pallidal transmission, leading to disinhibition of the STN and increased activation of STN NMDA receptors. Knockdown of STN NMDA receptors, which also suppresses proliferation of GABAergic pallido-STN inputs in PD mice, reduced loss of cortico-STN transmission and patterning and improved motor function. Together, the data suggest that loss of dopamine triggers a maladaptive shift in the balance of synaptic excitation and inhibition in the STN, which contributes to parkinsonian activity and motor dysfunction.

Enhanced GABA Transmission Drives Bradykinesia Following Loss of Dopamine D2 Receptor Signaling

Bradykinesia is a prominent phenotype of Parkinson's disease, depression, and other neurological conditions. Disruption of dopamine (DA) transmission plays an important role, but progress in understanding the exact mechanisms driving slowness of movement has been impeded due to the heterogeneity of DA receptor distribution on multiple cell types within the striatum. Here we show that selective deletion of DA D2 receptors (D2Rs) from indirect-pathway medium spiny neurons (iMSNs) is sufficient to impair locomotor activity, phenocopying DA depletion models of Parkinson's disease, despite this mouse model having intact DA transmission. There was a robust enhancement of GABAergic transmission and a reduction of in vivo firing in striatal and pallidal neurons. Mimicking D2R signaling in iMSNs with Gi-DREADDs restored the level of tonic GABAergic transmission and rescued the motor deficit. These findings indicate that DA, through D2R activation in iMSNs, regulates motor output by constraining the strength of GABAergic transmission.

Unified neural field theory of brain dynamics underlying oscillations in Parkinson's disease and generalized epilepsies

The mechanisms underlying pathologically synchronized neural oscillations in Parkinson's disease (PD) and generalized epilepsies are explored in parallel via a physiologically-based neural field model of the corticothalamic-basal ganglia (CTBG) system. The basal ganglia (BG) are approximated as a single effective population and their roles in the modulation of oscillatory dynamics of the corticothalamic (CT) system and vice versa are analyzed. In addition to normal EEG rhythms, enhanced activity around 4 Hz and 20 Hz exists in the model, consistent with the characteristic frequencies observed in PD. These rhythms result from resonances in loops formed between the BG and CT populations, analogous to those that underlie epileptic oscillations in a previous CT model, and which are still present in the combined CTBG system. Dopamine depletion is argued to weaken the dampening of these loop resonances in PD, and network connections then explain the significant coherence observed between BG, thalamic, and cortical population activity around 4-8 Hz and 20 Hz. Parallels between the afferent and efferent connection sites of the thalamic reticular nucleus (TRN) and BG predict low dopamine to correspond to a reduced likelihood of tonic-clonic (grand mal) seizures, which agrees with experimental findings. Furthermore, the model predicts an increased likelihood of absence (petit mal) seizure resulting from pathologically low dopamine levels in accordance with experimental observations. Suppression of absence seizure activity is demonstrated when afferent and efferent BG connections to the CT system are strengthened, which is consistent with other CTBG modeling studies. The BG are demonstrated to have a suppressive effect on activity of the CTBG system near tonic-clonic seizure states, which provides insight into the reported efficacy of current treatments in BG circuits. Sleep states of the TRN are also found to suppress pathological PD activity in accordance with observations. Overall, the findings demonstrate strong parallels between coherent oscillations in generalized epilepsies and PD, and provide insights into possible comorbidities.

Cortico-subthalamic inputs from the motor, limbic, and associative areas in normal and dopamine-depleted rats are not fully segregated

The subthalamic nucleus (STN) receives monosynaptic glutamatergic afferents from different areas of the cortex, known as the "hyperdirect" pathway. The STN has been divided into three distinct subdivisions, motor, limbic, and associative parts in line with the concept of parallel information processing. The extent to which the parallel information processing coming from distinct cortical areas overlaps in the different territories of the STN is still a matter of debate and the proposed role of dopaminergic neurons in maintaining the coherence of responses to cortical inputs in each territory is not documented. Using extracellular electrophysiological approaches, we investigated to what degree the motor and non-motor regions in the STN are segregated in control and dopamine (DA) depleted rats. We performed electrical stimulation of different cortical areas and recorded STN neuronal responses. We showed that motor and non-motor cortico-subthalamic pathways are not fully segregated, but partially integrated in the rat. This integration was mostly present through the indirect pathway. The spatial distribution and response latencies were the same in sham and 6-hydroxydopamine lesioned animals. The inhibitory phase was, however, less apparent in the lesioned animals. In conclusion, this study provides the first evidence that motor and non-motor cortico-subthalamic pathways in the rat are not fully segregated, but partially integrated. This integration was mostly present through the indirect pathway. We also show that the inhibitory phase induced by GABAergic inputs from the external segment of the globus pallidus is reduced in the DA-depleted animals.

The external globus pallidus: progress and perspectives

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

A biophysical model of the cortex-basal ganglia-thalamus network in the 6-OHDA lesioned rat model of Parkinson's disease

Electrical stimulation of sub-cortical brain regions (the basal ganglia), known as deep brain stimulation (DBS), is an effective treatment for Parkinson's disease (PD). Chronic high frequency (HF) DBS in the subthalamic nucleus (STN) or globus pallidus interna (GPi) reduces motor symptoms including bradykinesia and tremor in patients with PD, but the therapeutic mechanisms of DBS are not fully understood. We developed a biophysical network model comprising of the closed loop cortical-basal ganglia-thalamus circuit representing the healthy and parkinsonian rat brain. The network properties of the model were validated by comparing responses evoked in basal ganglia (BG) nuclei by cortical (CTX) stimulation to published experimental results. A key emergent property of the model was generation of low-frequency network oscillations. Consistent with their putative pathological role, low-frequency oscillations in model BG neurons were exaggerated in the parkinsonian state compared to the healthy condition. We used the model to quantify the effectiveness of STN DBS at different frequencies in suppressing low-frequency oscillatory activity in GPi. Frequencies less than 40 Hz were ineffective, low-frequency oscillatory power decreased gradually for frequencies between 50 Hz and 130 Hz, and saturated at frequencies higher than 150 Hz. HF STN DBS suppressed pathological oscillations in GPe/GPi both by exciting and inhibiting the firing in GPe/GPi neurons, and the number of GPe/GPi neurons influenced was greater for HF stimulation than low-frequency stimulation. Similar to the frequency dependent suppression of pathological oscillations, STN DBS also normalized the abnormal GPi spiking activity evoked by CTX stimulation in a frequency dependent fashion with HF being the most effective. Therefore, therapeutic HF STN DBS effectively suppresses pathological activity by influencing the activity of a greater proportion of neurons in the output nucleus of the BG.

Dopamine D1 Receptor-Mediated Transmission Maintains Information Flow Through the Cortico-Striato-Entopeduncular Direct Pathway to Release Movements

In the basal ganglia (BG), dopamine plays a pivotal role in motor control, and dopamine deficiency results in severe motor dysfunctions as seen in Parkinson's disease. According to the well-accepted model of the BG, dopamine activates striatal direct pathway neurons that directly project to the output nuclei of the BG through D1 receptors (D1Rs), whereas dopamine inhibits striatal indirect pathway neurons that project to the external pallidum (GPe) through D2 receptors. To clarify the exact role of dopaminergic transmission via D1Rs in vivo, we developed novel D1R knockdown mice in which D1Rs can be conditionally and reversibly regulated. Suppression of D1R expression by doxycycline treatment decreased spontaneous motor activity and impaired motor ability in the mice. Neuronal activity in the entopeduncular nucleus (EPN), one of the output nuclei of the rodent BG, was recorded in awake conditions to examine the mechanism of motor deficits. Cortically evoked inhibition in the EPN mediated by the cortico-striato-EPN direct pathway was mostly lost during suppression of D1R expression, whereas spontaneous firing rates and patterns remained unchanged. On the other hand, GPe activity changed little. These results suggest that D1R-mediated dopaminergic transmission maintains the information flow through the direct pathway to appropriately release motor actions.

Hold your pauses: external globus pallidus neurons respond to behavioural events by decreasing pause activity

Awareness of its rich structural pathways has earned the external segment of the globus pallidus (GPe) recognition as a central figure within the basal ganglia circuitry. Interestingly, GPe neurons are uniquely identified by the presence of prominent pauses interspersed among a high-frequency discharge rate of 50-80 spikes/s. These pauses have an average pause duration of 620 ms with a frequency of 13/min, yielding an average pause activity (probability of a GPe neuron being in a pause) of (620 × 13)/(60 × 1000) = 0.13. Spontaneous pause activity has been found to be inversely related to arousal state. The relationship of pause activity with behavioural events remains to be elucidated. In the present study, we analysed the electrophysiological activity of 200 well-isolated GPe pauser cells recorded from four non-human primates (Macaque fascicularis) while they were engaged in similar classical conditioning tasks. The isolation quality of the recorded activity and the pauses were determined with objective automatic methods. The results showed that the pause probability decreased by 9.09 and 10.0%, and the discharge rate increased by 2.96 and 1.95%, around cue and outcome presentation, respectively. Analysis of the linear relationship between the changes in pause activity and discharge rate showed r(2)  = 0.46 and r(2)  = 0.66 upon cue onset and outcome presentation, respectively. Thus, pause activity is a pertinent element in short-term encoding of relevant behavioural events, and has a significant, but not exclusive, role in the modulation of GPe discharge rate around these events.

Quantitative activation-induced manganese-enhanced MRI reveals severity of Parkinson's disease in mice

We demonstrate that activation-induced manganese-enhanced magnetic resonance imaging with quantitative determination of the longitudinal relaxation time (qAIM-MRI) reveals the severity of Parkinson's disease (PD) in mice. We first show that manganese ion-accumulation depends on neuronal activity. A highly active region was then observed by qAIM-MRI in the caudate-putamen in PD-model mice that was significantly correlated to the severity of PD, suggesting its involvement in the expression of PD symptoms.

Motor cortical oscillations are abnormally suppressed during repetitive movement in patients with Parkinson's disease

OBJECTIVE

Impaired repetitive movement in persons with Parkinson's disease (PD) is associated with reduced amplitude, paradoxical hastening and hesitations or arrest at higher movement rates. This study examined the effects of movement rate and medication on movement-related cortical oscillations in persons with PD.

METHODS

Nine participants with PD were studied off and on medication and compared to nine control participants. Participants performed index finger movements cued by tones from 1 to 3 Hz. Movement-related oscillations were derived from electroencephalographic recordings over the region of the contralateral sensorimotor cortex (S1/M1) during rest, listening, or synchronized movement.

RESULTS

At rest, spectral power recorded over the region of the contralateral S1/M1 was increased in the alpha band and decreased in the beta band in participants with PD relative to controls. During movement, the level of alpha and beta band power relative to baseline was significantly reduced in the PD group, off and on medication, compared to controls. Reduced movement amplitude and hastening at movement rates near 2 Hz was associated with abnormally suppressed and persistent desynchronization of oscillations in alpha and beta bands.

CONCLUSION

Motor cortical oscillations in the alpha and beta bands are abnormally suppressed in PD, particularly during higher rate movements.

SIGNIFICANCE

These findings contribute to the understanding of mechanisms underlying impaired repetitive movement in PD.

Serotonin regulation of subthalamic neurons

The subthalamic nucleus (STN) is a key component of the basal ganglia. As the only basal ganglia nucleus comprised of mostly glutamatergic neurons, STN neurons provide a key driving force to their target neurons. Thus, regulation of STN neuron activity is important. One STN regulator is the serotonin (5-HT) system. The STN receives a dense 5-HT innervation. 5-HT1A, 5-HT1B, 5-HT2C, and 5-HT4 receptors are expressed in the STN. 5-HT may regulate the STN via several mechanisms. First, 5-HT may affect STN neuron excitability directly by either inhibiting a subpopulation of STN neurons via activation of 5-HT1A receptors or exciting STN neurons through activation of 5-HT2C and 5-HT4 receptors. Second, 5-HT may affect synaptic inputs to the STN. Via activation of 5-HT1B receptors on the afferent terminals, 5-HT inhibits glutamatergic input to the STN, but the inhibitory effect on GABAergic input is smaller. Third, 5-HT may regulate the STN glutamatergic output by activating presynaptic 5-HT1B receptors, thus reducing burst firing in target neurons. Last, 5-HT may affect glutamate release at the intra-STN axon collaterals and regulate the recurrent excitation. These mechanisms may work in concert to fine-tune the intensity and pattern of STN activity and reduce STN output bursts.

Posttetanic enhancement of striato-pallidal synaptic transmission

The striato (Str)-globus pallidus external segment (GPe) projection plays major roles in the control of neuronal activity in the basal ganglia under both normal and pathological conditions. The present study used rat brain slice preparations to characterize the enhancement of Str-GPe synapses observed after repetitive conditioning stimuli (CS) of Str with the whole cell patch-clamp recording technique. The results show that 1) the Str-GPe synapses have a posttetanic enhancement (PTE) mechanism, which is considered to be a combination of an augmentation and a posttetanic potentiation; 2) the degree of PTE observed in GPe neurons had a wide range and was positively correlated with a wide range of paired-pulse ratios assessed before application of CS; 3) a wide range of CS, from frequencies as low as 2 Hz with as few as 5 pulses to as high as 100 Hz with 100 pulses, could induce PTE; 4) the decay time constant of PTE was dependent on the strength of CS and was prolonged greatly, up to 120 s, when strong CS were applied; and 5) the level of postsynaptic Cl(-) became a limiting factor for the degree of PTE when strong CS were applied. These results imply that Str-GPe synapses transmit inhibitions in a nonlinear activity-weighted manner, which may be suited for scaling timing and force of repeated or sequential body movements. Other possible factors controlling the induction of PTE and functional implications are also discussed.

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

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

Alterations in neuronal activity in basal ganglia-thalamocortical circuits in the parkinsonian state

In patients with Parkinson’s disease and in animal models of this disorder, neurons in the basal ganglia and related regions in thalamus and cortex show changes that can be recorded by using electrophysiologic single-cell recording techniques, including altered firing rates and patterns, pathologic oscillatory activity and increased inter-neuronal synchronization. In addition, changes in synaptic potentials or in the joint spiking activities of populations of neurons can be monitored as alterations in local field potentials (LFPs), electroencephalograms (EEGs) or electrocorticograms (ECoGs). Most of the mentioned electrophysiologic changes are probably related to the degeneration of diencephalic dopaminergic neurons, leading to dopamine loss in the striatum and other basal ganglia nuclei, although degeneration of non-dopaminergic cell groups may also have a role. The altered electrical activity of the basal ganglia and associated nuclei may contribute to some of the motor signs of the disease. We here review the current knowledge of the electrophysiologic changes at the single cell level, the level of local populations of neural elements, and the level of the entire basal ganglia-thalamocortical network in parkinsonism, and discuss the possible use of this information to optimize treatment approaches to Parkinson’s disease, such as deep brain stimulation (DBS) therapy.