Discharge patterns of pallidal neurons with input from various cortical areas during movement in the monkey

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

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

Mechanisms of Network Interactions for Flexible Cortico-Basal Ganglia-Mediated Action Control

In humans, finely tuned γ synchronization (60-90 Hz) rapidly appears at movement onset in a motor control network involving primary motor cortex, the basal ganglia and motor thalamus. Yet the functional consequences of brief movement-related synchronization are still unclear. Distinct synchronization phenomena have also been linked to different forms of motor inhibition, including relaxing antagonist muscles, rapid movement interruption and stabilizing network dynamics for sustained contractions. Here, I will introduce detailed hypotheses about how intrasite and intersite synchronization could interact with firing rate changes in different parts of the network to enable flexible action control. The here proposed cause-and-effect relationships shine a spotlight on potential key mechanisms of cortico-basal ganglia-thalamo-cortical (CBGTC) communication. Confirming or revising these hypotheses will be critical in understanding the neuronal basis of flexible movement initiation, invigoration and inhibition. Ultimately, the study of more complex cognitive phenomena will also become more tractable once we understand the neuronal mechanisms underlying behavioral readouts.

Parvalbumin+ and Npas1+ Pallidal Neurons Have Distinct Circuit Topology and Function

The external globus pallidus (GPe) is a critical node within the basal ganglia circuit. Phasic changes in the activity of GPe neurons during movement and their alterations in Parkinson's disease (PD) argue that the GPe is important in motor control. Parvalbumin-positive (PV+) neurons and Npas1+ neurons are the two principal neuron classes in the GPe. The distinct electrophysiological properties and axonal projection patterns argue that these two neuron classes serve different roles in regulating motor output. However, the causal relationship between GPe neuron classes and movement remains to be established. Here, by using optogenetic approaches in mice (both males and females), we showed that PV+ neurons and Npas1+ neurons promoted and suppressed locomotion, respectively. Moreover, PV+ neurons and Npas1+ neurons are under different synaptic influences from the subthalamic nucleus (STN). Additionally, we found a selective weakening of STN inputs to PV+ neurons in the chronic 6-hydroxydopamine lesion model of PD. This finding reinforces the idea that the reciprocally connected GPe-STN network plays a key role in disease symptomatology and thus provides the basis for future circuit-based therapies.SIGNIFICANCE STATEMENT The external pallidum is a key, yet an understudied component of the basal ganglia. Neural activity in the pallidum goes awry in neurologic diseases, such as Parkinson's disease. While this strongly argues that the pallidum plays a critical role in motor control, it has been difficult to establish the causal relationship between pallidal activity and motor function/dysfunction. This was in part because of the cellular complexity of the pallidum. Here, we showed that the two principal neuron types in the pallidum have opposing roles in motor control. In addition, we described the differences in their synaptic influence. Importantly, our research provides new insights into the cellular and circuit mechanisms that explain the hypokinetic features of Parkinson's disease.

Weakly correlated activity of pallidal neurons in behaving monkeys

The basal ganglia play a crucial role in the control of voluntary movements. Neurons in both the external and internal segments of the globus pallidus, the connecting and output nuclei of the basal ganglia, respectively, change their firing rates in relation to movements. Firing rate changes of movement-related neurons seem to convey signals for motor control. On the other hand, coincident spikes among neurons, that is, correlated activity, may also contribute to motor control. To address this issue, we first identified multiple pallidal neurons receiving inputs from the forelimb regions of the primary motor cortex and supplementary motor area, recorded neuronal activity of these neurons simultaneously, and analyzed their spike correlations while monkeys performed a hand-reaching task. Most (79%) pallidal neurons exhibited task-related firing rate changes, whereas only a small fraction (20%) showed significant but small and short correlated activity during the task performance. These results suggest that motor control signals are conveyed primarily by firing rate changes in the external and internal segments of the globus pallidus and that the contribution of correlated activity may play only a minor role in the healthy state.

Contribution of the Entopeduncular Nucleus and the Globus Pallidus to the Control of Locomotion and Visually Guided Gait Modifications in the Cat

We tested the hypothesis that the entopeduncular (EP) nucleus (feline equivalent of the primate GPi) and the globus pallidus (GPe) contribute to both the planning and execution of locomotion and voluntary gait modifications in the cat. We recorded from 414 cells distributed throughout these two nuclei (referred to together as the pallidum) while cats walked on a treadmill and stepped over an obstacle that advanced towards them. Neuronal activity in many cells in both structures was modulated on a step-by-step basis during unobstructed locomotion and was modified in the step over the obstacle. On a population basis, the most frequently observed change, in both the EP and the GPe, was an increase in activity prior to and/or during the swing phase of the step over the obstacle by the contralateral forelimb, when it was the first limb to pass over the obstacle. Our results support a contribution of the pallidum, in concert with cortical structures, to the control of both the planning and the execution of the gait modifications. We discuss the results in the context of current models of pallidal action on thalamic activity, including the possibility that cells in the EP with increased activity may sculpt thalamo-cortical activity.

Integrating Brain and Biomechanical Models-A New Paradigm for Understanding Neuro-muscular Control

To date, realistic models of how the central nervous system governs behavior have been restricted in scope to the brain, brainstem or spinal column, as if these existed as disembodied organs. Further, the model is often exercised in relation to an in vivo physiological experiment with input comprising an impulse, a periodic signal or constant activation, and output as a pattern of neural activity in one or more neural populations. Any link to behavior is inferred only indirectly via these activity patterns. We argue that to discover the principles of operation of neural systems, it is necessary to express their behavior in terms of physical movements of a realistic motor system, and to supply inputs that mimic sensory experience. To do this with confidence, we must connect our brain models to neuro-muscular models and provide relevant visual and proprioceptive feedback signals, thereby closing the loop of the simulation. This paper describes an effort to develop just such an integrated brain and biomechanical system using a number of pre-existing models. It describes a model of the saccadic oculomotor system incorporating a neuromuscular model of the eye and its six extraocular muscles. The position of the eye determines how illumination of a retinotopic input population projects information about the location of a saccade target into the system. A pre-existing saccadic burst generator model was incorporated into the system, which generated motoneuron activity patterns suitable for driving the biomechanical eye. The model was demonstrated to make accurate saccades to a target luminance under a set of environmental constraints. Challenges encountered in the development of this model showed the importance of this integrated modeling approach. Thus, we exposed shortcomings in individual model components which were only apparent when these were supplied with the more plausible inputs available in a closed loop design. Consequently we were able to suggest missing functionality which the system would require to reproduce more realistic behavior. The construction of such closed-loop animal models constitutes a new paradigm of computational neurobehavior and promises a more thoroughgoing approach to our understanding of the brain's function as a controller for movement and behavior.

Development of stereotaxic recording system for awake marmosets (Callithrix jacchus)

The common marmoset has been proposed as a potential alternative to macaque monkey as a primate model for neuroscience and medical research. Here, we have newly developed a stereotaxic neuronal recording system for awake marmosets under the head-fixed condition by modifying that for macaque monkeys. Using this system, we recorded neuronal activity in the cerebral cortex of awake marmosets and successfully identified the primary motor cortex by intracortical microstimulation. Neuronal activities of deep brain structures, such as the basal ganglia, thalamus, and cerebellum, in awake marmosets were also successfully recorded referring to magnetic resonance images. Our system is suitable for functional mapping of the brain, since the large recording chamber allows access to arbitrary regions over almost the entire brain, and the recording electrode can be easily moved stereotaxically from one site to another. In addition, our system is desirable for neuronal recording during task performance to assess motor skills and cognitive function, as the marmoset sits in the marmoset chair and can freely use its hands. Moreover, our system can be used in combination with cutting-edge techniques, such as two-photon imaging and optogenetic manipulation. This recording system will contribute to boosting neuroscience and medical research using marmosets.

Information processing from the motor cortices to the subthalamic nucleus and globus pallidus and their somatotopic organizations revealed electrophysiologically in monkeys

To understand how the information derived from different motor cortical areas representing different body parts is organized in the basal ganglia, we examined the neuronal responses in the subthalamic nucleus (STN), and the external (GPe) and internal (GPi) segments of the globus pallidus (input, relay and output nuclei, respectively) to stimulation of the orofacial, forelimb and hindlimb regions of the primary motor cortex (MI) and supplementary motor area (SMA) in macaque monkeys under the awake state. Most STN and GPe/GPi neurons responded exclusively to stimulation of either the MI or SMA, and one‐fourth to one‐third of neurons responded to both. STN neurons responding to the hindlimb, forelimb and orofacial regions of the MI were located along the medial–lateral axis in the posterolateral STN, while neurons responding to the orofacial region of the SMA were located more medially than the others in the anteromedial STN. GPe/GPi neurons responding to the hindlimb, forelimb and orofacial regions of the MI were found along the dorsal–ventral axis in the posterolateral GPe/GPi, and neurons responding to the corresponding regions of the SMA were similarly but less clearly distributed in more anteromedial regions. Moreover, neurons responding to the distal and proximal forelimb MI regions were found along the lateral–medial axis in the STN and the ventral–dorsal axis in the GPe/GPi. Most STN and GPe/GPi neurons showed kinaesthetic responses with similar somatotopic maps. These observations suggest that the somatotopically organized inputs from the MI and SMA are well preserved in the STN and GPe/GPi with partial convergence.

Parkinson's Disease and the Basal Ganglia: Lessons from the Laboratory and from Neurosurgery

During the last decade, a clearer understanding of the circuitry of the basal ganglia and their mode of operation has emerged. The basal ganglia are now viewed as parts of larger, segregated circuits that involve the thalamus and cerebral cortex. A pathophysiological model has been elaborated and tested in which Parkinsonian signs are viewed as resulting from increased activity of neurons in the "motor" portion of the internal pallidum, the major output nucleus of the basal ganglia, leading to increased inhibition of thalamocortical projection neurons and decreased activation of the precentral motor fields. Increased internal pallidal activity is thought to result from striatal dopamine loss, leading to decreased inhibition of the internal pallidum via a monosynaptic ("direct") striatopallidal pathway and to excessive excitatory glutamatergic drive via a polysynaptic ("indirect") striatopallidal pathway. Because current medical therapies for Parkinson's disease, aimed at systemically replacing dopamine, often lose their effectiveness after several years, with patients suffering from motor fluctuations and drug-induced dyski nesias, several new therapeutic strategies have been developed. In addition to the transplantation of dopaminergic tissue, other strategies attempt to reduce increased basal ganglia outflow directly by the placement of stereotactic lesions into the sensorimotor portion of the internal pallidum (pallidotomy) or by the chronic electric stimulation of the subthalamic nucleus. Preliminary results suggest that these new techniques may lead to significant improvement in Parkinsonian signs, motor fluctuations, and drug- induced dyskinesias. The Neuroscientist 1:236-244, 1995

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.

Origins of basal ganglia output signals in singing juvenile birds

Across species, complex circuits inside the basal ganglia (BG) converge on pallidal output neurons that exhibit movement-locked firing patterns. Yet the origins of these firing patterns remain poorly understood. In songbirds during vocal babbling, BG output neurons homologous to those found in the primate internal pallidal segment are uniformly activated in the tens of milliseconds prior to syllable onsets. To test the origins of this remarkably homogenous BG output signal, we recorded from diverse upstream BG cell types during babbling. Prior to syllable onsets, at the same time that internal pallidal segment-like neurons were activated, putative medium spiny neurons, fast spiking and tonically active interneurons also exhibited transient rate increases. In contrast, pallidal neurons homologous to those found in primate external pallidal segment exhibited transient rate decreases. To test origins of these signals, we performed recordings following lesion of corticostriatal inputs from premotor nucleus HVC. HVC lesions largely abolished these syllable-locked signals. Altogether, these findings indicate a striking homogeneity of syllable timing signals in the songbird BG during babbling and are consistent with a role for the indirect and hyperdirect pathways in transforming cortical inputs into BG outputs during an exploratory behavior.

Motor thalamus integration of cortical, cerebellar and basal ganglia information: implications for normal and parkinsonian conditions

Motor thalamus (Mthal) is implicated in the control of movement because it is strategically located between motor areas of the cerebral cortex and motor-related subcortical structures, such as the cerebellum and basal ganglia (BG). The role of BG and cerebellum in motor control has been extensively studied but how Mthal processes inputs from these two networks is unclear. Specifically, there is considerable debate about the role of BG inputs on Mthal activity. This review summarizes anatomical and physiological knowledge of the Mthal and its afferents and reviews current theories of Mthal function by discussing the impact of cortical, BG and cerebellar inputs on Mthal activity. One view is that Mthal activity in BG and cerebellar-receiving territories is primarily "driven" by glutamatergic inputs from the cortex or cerebellum, respectively, whereas BG inputs are modulatory and do not strongly determine Mthal activity. This theory is steeped in the assumption that the Mthal processes information in the same way as sensory thalamus, through interactions of modulatory inputs with a single driver input. Another view, from BG models, is that BG exert primary control on the BG-receiving Mthal so it effectively relays information from BG to cortex. We propose a new "super-integrator" theory where each Mthal territory processes multiple driver or driver-like inputs (cortex and BG, cortex and cerebellum), which are the result of considerable integrative processing. Thus, BG and cerebellar Mthal territories assimilate motivational and proprioceptive motor information previously integrated in cortico-BG and cortico-cerebellar networks, respectively, to develop sophisticated motor signals that are transmitted in parallel pathways to cortical areas for optimal generation of motor programmes. Finally, we briefly review the pathophysiological changes that occur in the BG in parkinsonism and generate testable hypotheses about how these may affect processing of inputs in the Mthal.

Basal ganglia output to the thalamus: still a paradox

The basal ganglia (BG)-recipient thalamus controls motor output but it remains unclear how its activity is regulated. Several studies report that thalamic activation occurs via disinhibition during pauses in the firing of inhibitory pallidal inputs from the BG. Other studies indicate that thalamic spiking is triggered by pallidal inputs via post-inhibitory 'rebound' calcium spikes. Finally excitatory cortical inputs can drive thalamic activity, which becomes entrained, or time-locked, to pallidal spikes. We present a unifying framework where these seemingly distinct results arise from a continuum of thalamic firing 'modes' controlled by excitatory inputs. We provide a mechanistic explanation for paradoxical pallidothalamic coactivations observed during behavior that raises new questions about what information is integrated in the thalamus to control behavior.

The representation of egocentric space in the posterior parietal cortex

The posterior parietal cortex (PPC) is the most likely site where egocentric spatial relationships are represented in the brain. PPC cells receive visual, auditory, somaesthetic, and vestibular sensory inputs; oculomotor, head, limb, and body motor signals; and strong motivational projections from the limbic system. Their discharge increases not only when an animal moves towards a sensory target, but also when it directs its attention to it. PPC lesions have the opposite effect: sensory inattention and neglect. The PPC does not seem to contain a "map" of the location of objects in space but a distributed neural network for transforming one set of sensory vectors into other sensory reference frames or into various motor coordinate systems. Which set of transformation rules is used probably depends on attention, which selectively enhances the synapses needed for making a particular sensory comparison or aiming a particular movement.

High-frequency pallidal stimulation disrupts information flow through the pallidum by GABAergic inhibition

To elucidate the mechanism of deep brain stimulation (DBS) targeting the internal segment of the globus pallidus (GPi), neuronal activity of the GPi and the external segment of the globus pallidus (GPe) was examined during local electrical microstimulation in normal awake monkeys. Single-pulse stimulation of the GPi evoked brief inhibition in neighboring GPi neurons, which was mediated by GABA(A) receptors. High-frequency stimulation of the GPi completely inhibited spontaneous firings of GPi neurons by activation of GABA(A) and GABA(B) receptors. Local single-pulse stimulation directly excited some GPi neurons. Such directly evoked responses were also inhibited by high-frequency stimulation through GABA(A) receptors. In contrast to the GPi, single-pulse and high-frequency stimulation of the GPe induced complex responses composed of GABAergic inhibition and glutamatergic excitation in neighboring GPe neurons. Cortically evoked triphasic responses of GPi neurons were completely inhibited during high-frequency GPi stimulation. These findings suggest that GPi-DBS dissociates inputs and outputs in the GPi by intense GABAergic inhibition and disrupts information flow through the GPi.

Changes in basal ganglia processing of cortical input following magnetic stimulation in Parkinsonism

Parkinsonism is associated with major changes in neuronal activity throughout the cortico-basal ganglia loop. Current measures quantify changes in baseline neuronal and network activity but do not capture alterations in information propagation throughout the system. Here, we applied a novel non-invasive magnetic stimulation approach using a custom-made mini-coil that enabled us to study transmission of neuronal activity throughout the cortico-basal ganglia loop in both normal and parkinsonian primates. By magnetically perturbing cortical activity while simultaneously recording neuronal responses along the cortico-basal ganglia loop, we were able to directly investigate modifications in descending cortical activity transmission. We found that in both the normal and parkinsonian states, cortical neurons displayed similar multi-phase firing rate modulations in response to magnetic stimulation. However, in the basal ganglia, large synaptically driven stereotypic neuronal modulation was present in the parkinsonian state that was mostly absent in the normal state. The stimulation-induced neuronal activity pattern highlights the change in information propagation along the cortico-basal ganglia loop. Our findings thus point to the role of abnormal dynamic activity transmission rather than changes in baseline activity as a major component in parkinsonian pathophysiology. Moreover, our results hint that the application of transcranial magnetic stimulation (TMS) in human patients of different disorders may result in different neuronal effects than the one induced in normal subjects.

Does the nervous system depend on kinesthetic information to control natural limb movements?

This target article draws together two groups of experimental studies on the control of human movement through peripheral feedback and centrally generated signals of motor commands. First, during natural movement, feedback from muscle, joint, and cutaneous afferents changes; in human subjects these changes have reflex and kinesthetic consequences. Recent psychophysical and microneurographic evidence suggests that joint and even cutaneous afferents may have a proprioceptive role. Second, the role of centrally generated motor commands in the control of normal movements and movements following acute and chronic deafferentation is reviewed. There is increasing evidence that subjects can perceive their motor commands under various conditions, but that this is inadequate for normal movement; deficits in motor performance arise when the reliance on proprioceptive feedback is abolished either experimentally or because of pathology. During natural movement, the CNS appears to have access to functionally useful input from a range of peripheral receptors as well as from internally generated command signals. The unanswered questions that remain suggest a number of avenues for further research.

Implications of neural networks for how we think about brain function

Engineers use neural networks to control systems too complex for conventional engineering solutions. To examine the behavior of individual hidden units would defeat the purpose of this approach because it would be largely uninterpretable. Yet neurophysiologists spend their careers doing just that! Hidden units contain bits and scraps of signals that yield only arcane hints about network function and no information about how its individual units process signals. Most literature on single-unit recordings attests to this grim fact. On the other hand, knowing a system's function and describing it with elegant mathematics tell one very little about what to expect of interneuronal behavior. Examples of simple networks based on neurophysiology are taken from the oculomotor literature to suggest how single-unit interpretability might decrease with increasing task complexity. It is argued that trying to explain how any real neural network works on a cell-by-cell, reductionist basis is futile and we may have to be content with trying to understand the brain at higher levels of organization.

Electrophysiological evidences of organization of cortical motor information in the Basal Ganglia

During the last two decades, the many developments in the treatment of movement disorders such as Parkinson disease and dystonia have enhanced our understanding on organization of the basal ganglia, and this knowledge has led to other advances in the field. According to many electrophysiological and anatomical findings, it is considered that motor information from different cortical areas is processed through several cortico-basal ganglia loops principally in a parallel fashion and somatotopy from each cortical area is also well preserved in each loop. Moreover, recent studies suggest that not only the parallel processing but also some convergence of information occur through the basal ganglia. Information from cortical areas whose functions are close to each other tends to converge in the basal ganglia. The cortico-basal ganglia loops should be comprehended more as a network rather than as separated subdivisions. However, the functions of this convergence still remain unknown. It is important even for clinical doctors to be well informed about this kind of current knowledge because some symptoms of movement disorders may be explained by disorganization of the information network in the basal ganglia.

Somatotopic organization of the primate Basal Ganglia

Somatotopic organization is a fundamental and key concept to understand how the cortico-basal ganglia loop works. It is also indispensable knowledge to perform stereotaxic surgery for movement disorders. Here I would like to describe the somatotopic organization of the basal ganglia, which consist of the striatum, subthalamic nucleus, globus pallidus, and substantia nigra. Projections from motor cortical regions representing different body parts terminate in different regions of these nuclei. Basal ganglia neurons respond not only to the stimulation of the corresponding regions of the motor cortices, but also to active and passive movements of the corresponding body parts. On the basis of these anatomical and physiological findings, somatotopic organization can be identified in the motor territories of these nuclei in the basal ganglia. In addition, projections from functionally interrelated cortical areas partially converge through the cortico-basal ganglia loop, but nevertheless the somatotopy is still preserved. Disorganized somatotopy may explain, at least in part, the pathophysiology of movement disorders, such as Parkinson's disease and dystonia.

Substantia Nigra Output to Prefrontal Cortex Via Thalamus in Monkeys. II. Activity of Thalamic Relay Neurons in Delayed Conditional Go/No-Go Discrimination Task

The present report investigated the involvement of primate nigro-thalamo-cortical projections in discrimination of visual signals with behavioral meaning. We tested the extracellular unit activity of mediodorsal (MD) and ventral anterior (VA) thalamic neurons monosynaptically receiving inhibitory input from the substantia nigra pars reticulata (SNr) and projecting to the frontal cortex in Japanese monkeys performing a delayed conditional go/no-go discrimination task. In the task two colored stimuli (S1, S2) intervened by delay period required the monkeys lifting a lever (go) or not (no-go); the same and different colored pairs of S1 and S2 meant go and no-go signals, respectively. Prominent task-relevant responses were sustained activity with color preference to S1 during delay period and S2-related activity with different firing rates between go and no-go trials. In particular, a high proportion of such go/no-go differential S2-related activity was found in thalamic relay neurons, receiving input from the caudolateral SNr and projecting to the prefrontal area (PSv) ventral to the principal sulcus, in the rostrolateral MD. The findings suggest that the caudolateral SNr–rostrolateral MD–PSv pathways may be possible conduits of signals coding the behavioral meaning of the visual stimuli and thus may be responsible for generating similar neuronal activity in the PSv.

Neurons in both pallidal segments change their firing properties similarly prior to closure of the eyes

Current anatomical models of the cortico-basal ganglia (BG) network predict reciprocal discharge patterns between the external and internal segments of the globus pallidus (GPe and GPi, respectively), as well as cortical driving of BG activity. However, physiological studies revealing similarity in the transient responses of GPe and GPi neurons cast doubts on these predictions. Here, we studied the discharge properties of GPe, GPi, and primary motor cortex neurons of two monkeys in two distinct states: when eyes are open versus when they are closed. Both pallidal populations exhibited decreased discharge rates in the "eye closed" state accompanied by elevated values of the coefficient of variation (CV) of their interspike interval (ISI) distributions. The pallidal modulations in discharge patterns were partially attributable to larger fractions of longer ISIs in the "eye closed" state. In addition, the pallidal discharge modulations were gradual, starting prior to closing of the eyes. Cortical neurons, as opposed to pallidal neurons, increased their discharge rates steeply on closure of the eyes. Surprisingly, the cortical rate modulations occurred after pallidal modulations. However, as in the pallidum, the CV values of cortical ISI distributions increased in the "eye closed" state, indicating a more bursty discharge pattern in that state. Thus changes in GPe and GPi discharge properties were positively correlated, suggesting that the subthalamic nucleus and/or the striatum constitute the main common driving force for both pallidal segments. Furthermore, the early, unexpected changes in the pallidum are better explained by a subcortical rather than a cortical loop through the BG.

Globus pallidus internus firing rate modification after motor-imagination in three Parkinson's disease patients

Motor imagery is thought to involve the same processes of movement preparation as actual movement. Imagination of a simple repetitive movement significantly decreased the firing rate of extracellular micro recording at sensorimotor neurons of globus pallidus internus in three patients with Parkinson's disease, who underwent microelectrode-guided posteroventral pallidotomy. These findings suggest, in agreement with previous clinical and functional neuroimaging studies that the motor corticostriatal circuit could be engaged in mental simulation.

Cortically evoked long-lasting inhibition of pallidal neurons in a transgenic mouse model of dystonia

Dystonia is a neurological disorder characterized by sustained or repetitive involuntary muscle contractions and abnormal postures. To understand the pathophysiology of dystonia, neurophysiological analyses were performed on hyperkinetic transgenic mice generated as a model of DYT1 dystonia. Abnormal muscle activity, such as coactivation of agonist and antagonist muscles and sustained muscle activation, was frequently observed in these mice. Recording of neuronal activity in the awake state revealed reduced spontaneous activity with bursts and pauses in both the external and internal segments of the globus pallidus. Motor cortical stimulation evoked responses composed of excitation and subsequent long-lasting inhibition in both pallidal segments, which were never observed in the normal mice. In addition, the somatotopic arrangements in both pallidal segments were disorganized. Long-lasting inhibition induced by cortical inputs in the internal pallidal segment may disinhibit thalamic and cortical activity, resulting in the motor hyperactivity observed in the transgenic mice.

Seven problems on the basal ganglia

Our knowledge on the functions of the basal ganglia has increased enormously during the last two decades. However, we still do not completely understand the primary function of the basal ganglia. In this article, I review fundamental problems on the basal ganglia that have emerged from recent findings, and propose their solutions in the following seven topics: first, organization of the cortico-basal ganglia loop, second, limitations of the 'direct and indirect pathways model', third, feedforward inhibition in the striatum, fourth, contribution of the basal ganglia to cortical activity through the thalamus, fifth, focused selection of movements and learning, sixth, firing rate model versus firing pattern model for the pathophysiology of movement disorders, and lastly mechanisms of stereotaxic surgery.

Mechanisms and targets of deep brain stimulation in movement disorders

Chronic electrical stimulation of the brain, known as deep brain stimulation (DBS), has become a preferred surgical treatment for medication-refractory movement disorders. Despite its remarkable clinical success, the therapeutic mechanisms of DBS are still not completely understood, limiting opportunities to improve treatment efficacy and simplify selection of stimulation parameters. This review addresses three questions essential to understanding the mechanisms of DBS. 1) How does DBS affect neuronal tissue in the vicinity of the active electrode or electrodes? 2) How do these changes translate into therapeutic benefit on motor symptoms? 3) How do these effects depend on the particular site of stimulation? Early hypotheses proposed that stimulation inhibited neuronal activity at the site of stimulation, mimicking the outcome of ablative surgeries. Recent studies have challenged that view, suggesting that although somatic activity near the DBS electrode may exhibit substantial inhibition or complex modulation patterns, the output from the stimulated nucleus follows the DBS pulse train by direct axonal excitation. The intrinsic activity is thus replaced by high-frequency activity that is time-locked to the stimulus and more regular in pattern. These changes in firing pattern are thought to prevent transmission of pathologic bursting and oscillatory activity, resulting in the reduction of disease symptoms through compensatory processing of sensorimotor information. Although promising, this theory does not entirely explain why DBS improves motor symptoms at different latencies. Understanding these processes on a physiological level will be critically important if we are to reach the full potential of this powerful tool.

Lack of Spike-Count and Spike-Time Correlations in the Substantia Nigra Reticulata Despite Overlap of Neural Responses

Previous studies of single neurons in the substantia nigra reticulata (SNr) have shown that many of them respond to similar events. These results, as well as anatomical studies, suggest that SNr neurons share inputs and thus may have correlated activity. Different types of correlation can exist between pairs of neurons. These are traditionally classified as either spike-count (“signal” and “noise”) or spike-timing (spike-to-spike and joint peristimulus time histograms) correlations. These measures of neuronal correlation are partially independent and have different implications. Our purpose was to probe the computational characteristics of the basal ganglia output nuclei through an analysis of these different types of correlation in the SNr. We carried out simultaneous multiple-electrode single-unit recordings in the SNr of two monkeys performing a probabilistic delayed visuomotor response task. A total of 113 neurons (yielding 355 simultaneously recorded pairs) were studied. Most SNr neurons responded to one or more task-related events, with instruction cue (69%) and reward (63%) predominating. Response-match analysis, comparing peristimulus time histograms, revealed a significant overlap between response vectors. However, no measure of average correlation differed significantly from zero. The lack of significant SNr spike-count population correlations appears to be an exceptional phenomenon in the brain, perhaps indicating unique event-related processing by basal ganglia output neurons to achieve better information transfer. The lack of spike-timing correlations suggests that the basal high-frequency discharge of SNr neurons is not driven by the common inputs and is probably intrinsic.

Globus pallidus internal segment

The internal segment of the globus pallidus (GP(i)) gathers many bits of information including movement-related activity from the striatum, external segment of the globus pallidus (GP(e)), and subthalamic nucleus (STN), and integrates them. The GP(i) receives rich GABAergic inputs from the striatum and GP(e), and gamma-aminobutyric acid (GABA) receptors are distributed in the GP(i) in a specific manner. Thus, inputs from the striatum and GP(e) may control GP(i) activity in a different way. The GP(i) finally conveys processed information outside the basal ganglia. Changes in GABAergic neurotransmission have been reported in movement disorders and suggested to play an important role in the pathophysiology of the symptoms.

Context-dependent modulation of movement-related discharge in the primate globus pallidus

A selective contribution of the basal ganglia (BG) to memory-contingent motor control has long been hypothesized. The importance of memory context remains an open question, however, for the BG skeletomotor circuit. To investigate this question, we studied the perimovement discharge of a carefully selected group of 74 "arm-related" pallidal cells in two rhesus monkeys. The animals performed three tasks designed to dissociate multiple independent aspects of memory-contingent reaching while controlling movement kinematics. The activity of most neurons (88%) was influenced strongly by the memory demands of a task (remembering "where" or "when" to move), but the population as a whole showed no systematic preference for memory- or sensory-contingent conditions. The effects of memory context were primarily additive with those of movement kinematics (particularly movement direction). Considered separately, decreases and increases in firing had very different context preferences: decreases were nearly always larger for sensory-triggered movements, whereas increases were enhanced most often under memory-contingent conditions (i.e., self-initiated or self-guided movements). A similar pattern of preferences was found for both pallidal segments. The distinct context-specific enhancements of decreases and increases could not be explained as simple sensory responses or as interactions with preparatory or anticipatory processes present before movement initiation. Rather, they appear related to movement execution under specific contexts. Our results lead to the conclusion that movement facilitatory decreases in internal pallidal (GPi) activity are primarily greater under sensory-triggered conditions. GPi increases and their suppressive effects, perhaps on competing activity in pallidal-recipient centers, have increased prevalence under memory-contingent conditions.

A new approach to understand the pathophysiology of Parkinson's disease

Here I introduce a dynamic model of the basal ganglia functions for the control of voluntary movement: information through major pathways in the cortico-basal ganglia loop, i. e., the cortico-STN-GPi/SNr "hyperdirect", cortico-striato-GPi/SNr "direct" and cortico-striato-GPe-STN-GPi/SNr "indirect" pathways, dynamically controls the activity of the thalamus and cortex and releases only the selected motor program at the selected timing. Based on the dynamic model, the pathophysiology of Parkinson's disease and the mechanism for the effectiveness of stereotaxic surgery can better be explained by an increase or decrease of disinhibition and inhibition in the thalamus and cortex in the temporal and spatial domains.

Dynamic and spatial features of the inhibitory pallidal GABAergic synapses

The globus pallidus, one of the basal ganglia nuclei, plays a major role in both basal ganglia physiology and pathophysiology. The globus pallidus is innervated mainly by striatal spiny neurons and globus pallidus collaterals. These GABAergic synapses constitute 90% of the input to globus pallidus cells. Despite the dominance of this inhibitory GABAergic input, globus pallidus cells are spontaneously active and most of them increase their firing rate in a task related manner. To explain this apparent inconsistency, we studied the dynamic and spatial effects of GABAergic inputs to globus pallidus neurons. To this end, we used intra-cellular recording from globus pallidus neurons in rat brain slices, investigating the effect of bath and local GABA application, as well as the responses to electrical stimulation of the striatum. We showed that the properties of the responses to either local or global GABA applications are similar to the responses of globus pallidus cells to GABA release from nerve terminals. Since the stimulus-evoked responses have been shown to be inhibitory in nature, we concluded that GABAergic inputs to globus pallidus both at soma and dendrite level are inhibitory. Furthermore, we showed that GABA can promote globus pallidus synchronization by affecting the timing of globus pallidus spiking, and that the globus pallidus GABAergic synapse undergoes rapid frequency-dependent depression. This prominent synaptic depression can account for the ability of globus pallidus neurons to fire in the presence of a majority of inhibitory inputs and might indicate that globus pallidus neurons are tuned to detect frequency changes. Furthermore, globus pallidus synaptic depression rules out the possibility of activation of GABAeregic afferents as the main mechanisms of high-frequency deep brain stimulation, used for treatment of severe parkinsonian patients.

Molecular, chemical, and anatomical characterization of globus pallidus dopamine D2 receptor mRNA-containing neurons

Essential for normal movement, the globus pallidus (GP) is a prominent nucleus whose neurons project to all other basal ganglia nuclei. The GP is composed of at least two distinct neuron populations. GP neurons of the rodent contain either the calcium-binding protein parvalbumin (PV) or preproenkephalin (PPE) mRNA, differentially innervate several basal ganglia structures, and have distinct immediate early gene responses to dopamine agonists or antagonists. Recent research has revealed that dopamine directly influences GP neurons, with D2 receptors contributing to both pre- and postsynaptic effects of dopaminergic agents. The existence of D2 mRNA-expressing (D2+) GP neurons has been established, but little is known concerning their numbers, regional distribution, or relationship to pallidal subpopulations identified on the basis of PV immunocytochemistry, PPE mRNA, or axonal targets. Detection of pallidal D2 mRNA with a 35S-cRNA probe revealed that D2+ neurons are found throughout the GP, comprising approximately one-half of pallidal neurons, but they are most dense within a dorsoventral band in lateral GP. While a substantial proportion (42-51%) of all chemically and anatomically labeled pallidal neuron subpopulations expressed D2 transcript, the D2+ neurons exhibited both population-based and regional heterogeneities. Overall, the pallidostriatal cells had a greater density of D2 mRNA than did pallidosubthalamic cells. Also, compared to other pallidal regions, the ventromedial GP contained fewer D2+ cells, and the PPE mRNA-expressing cells in this region had lower densities of D2 mRNA per neuron. These results reveal heterogeneous chemical and anatomical properties of the extensive population of D2+ GP neurons, a potential cellular substrate for dopamine's effects in pallidum.

A new dynamic model of the cortico-basal ganglia loop

An important issue in the neural control of posture and movement is how motor-related areas of the cerebral cortex modulate the activity of the output nuclei of the basal ganglia. In this chapter, the functional significance of the 'hyperdirect' cortico-subthalamo-pallidal pathway is emphasized, and further a new dynamic model of basal ganglia function is presented. When a voluntary movement is about to be initiated by cortical mechanisms, a corollary signal is conveyed through the 'hyperdirect' pathway to first inhibit large areas of the thalamus and cerebral cortex that are related to both the selected motor program and other competing programs. Next, another corollary signal is sent through the 'direct' cortico-striato-pallidal pathway to disinhibit this second pathway's targets, and ensure activation of only the selected motor program. Finally, a third corollary signal is sent through the 'indirect' cortico-striato-external pallido-subthalamo-internal pallidal pathway to strongly inhibit this third pathway's targets. This sequential information processing ensures that only the selected motor program is initiated, executed and terminated at the appropriate times, whereas other competing programs are canceled.

Group III metabotropic glutamate receptor-mediated modulation of the striatopallidal synapse

The globus pallidus (GP) is a key GABAergic nucleus in the basal ganglia (BG). The predominant input to the GP is an inhibitory striatal projection that forms the first synapse in the indirect pathway. The GP GABAergic neurons project to the subthalamic nucleus, providing an inhibitory control of these glutamatergic cells. Given its place within the BG circuit, it is not surprising that alterations in GP firing pattern are postulated to play a role in both normal and pathological motor behavior. Because the inhibitory striatal input to the GP may play an important role in shaping these firing patterns, we set out to determine the role that the group III metabotropic glutamate receptors (GluRs) play in modulating transmission at the striatopallidal synapse. In rat midbrain slices, electrical stimulation of the striatum evoked GABA(A)-mediated IPSCs recorded in all three types of GP neurons. The group III mGluR-selective agonist L-(+)-2-amino-4-phosphonobutyric acid (L-AP4) inhibited these IPSCs through a presynaptic mechanism of action. L-AP4 exhibited high potency and a pharmacological profile consistent with mediation by mGluR4. Furthermore, the effect of L-AP4 on striatopallidal transmission was absent in mGluR4 knock-out mice, providing convincing evidence that mGluR4 mediates this effect. The finding that mGluR4 may selectively modulate striatopallidal transmission raises the interesting possibility that activation of mGluR4 could decrease the excessive inhibition of the GP that has been postulated to occur in Parkinson's disease. Consistent with this, we find that intracerebroventricular injections of L-AP4 produce therapeutic benefit in both acute and chronic rodent models of Parkinson's disease.

Basal ganglia network mediates the control of movement amplitude

In the present study we address the hypothesis that the basal ganglia are specifically involved in the planning of movement amplitude (or related covariates). This prediction has often been put forward based on the observation that Parkinson's disease (PD) patients exhibit hypokinesia. A close examination of the literature shows, however, that this commonly reported clinical symptom is not consistently echoed by experimental observations. When required to point to visual targets in the absence of vision of the moving limb, PD subjects exhibit various patterns of inaccuracy, including hypometria, hypermetria, systematic direction bias, or direction-dependent errors. They have even been shown to be as accurate as healthy, age-matched subjects. The main aim of the current study is to address the origin of these inconsistencies. To this end, we required nine patients presenting with advanced PD and 15 age-matched control subjects to perform planar reaching movements to visual targets. Eight targets were presented in equally spaced directions around a circle centered on the hand's starting location. Based on a previously validated parsing procedure, end-point errors were segmented into localization and planning errors. Localization errors refer to the existence of systematic biases in the estimation of the initial hand location. These biases can potentially transform a simple pattern of pure amplitude errors into a complex pattern involving both amplitude and direction errors. Results indicated that localization errors were different in the PD patients and the control subjects. This is not surprising knowing both that proprioception is altered in PD patients and that the ability to locate the hand at rest relies mainly on the proprioceptive sense, even when vision is available. Unlike normal subjects, localization errors in PD were idiosyncratic, lacking a consistent pattern across subjects. When the confounding effect of initial hand localization errors was canceled, we found that end-point errors were only due to the implementation of an underscaled movement gain (15%), without direction bias. Interestingly, the level of undershoot was found to increase with the severity of the disease (inferred from the Unified Parkinson's Disease Rating Scale, UPDRS, motor score). We also observed that movement variability was amplified (32%), but only along the main movement axis (extent variability). Direction variability was not significantly different in the patient population and the control group. When considered together, these results support the idea that the basal ganglia are specifically involved in the control of movement amplitude (or of some covariates). We propose that this structure participates in extent planning by modulating cortical activity and/or the tuning of the spinal interneuronal circuits.

Rotational behavior and electrophysiological effects induced by GABA(B) receptor activation in rat globus pallidus

GABA is the major neurotransmitter used in the globus pallidus and there is evidence that GABA(B) receptors exist in this nucleus. Here we show that unilateral microinjection of baclofen, a GABA(B) receptor agonist, induced ipsilateral turning in Sprague-Dawley rats. This effect was prevented by preinjection of the GABA(B) receptor antagonist CGP55845A, which itself did not cause rotation. Thus, activation of GABA(B) receptor may suppress the activity of globus pallidus neurons, which is in line with the finding that the glutamate receptor antagonists (+/-)-2-amino-5-phosphonopentanoic acid and 6-cyano-7-nitroquinoxaline-2,3-dione also caused similar ipsilateral turning when injected into globus pallidus. Furthermore, in the presence of these glutamate receptor antagonists, injection of baclofen resulted in fewer rotations. To test the possibility that baclofen reduced glutamate release onto globus pallidus neurons, the effects of baclofen on miniature excitatory postsynaptic currents were studied in rat brain slices. Patch-clamp recordings showed that baclofen at 30 microM significantly reduced the frequency of the miniature excitatory postsynaptic currents. However, baclofen induced a weak outward current only in a minority of globus pallidus neurons. These pre- and postsynaptic effects of baclofen were reversed or prevented by CGP55845A. These results suggest that GABA(B) receptor in globus pallidus plays an important role in the regulation of movement by modulating glutamatergic inputs at a presynaptic site.

Functional significance of the cortico-subthalamo-pallidal 'hyperdirect' pathway

How the motor-related cortical areas modulate the activity of the output nuclei of the basal ganglia is an important issue for understanding the mechanisms of motor control by the basal ganglia. The cortico-subthalamo-pallidal 'hyperdirect' pathway conveys powerful excitatory effects from the motor-related cortical areas to the globus pallidus, bypassing the striatum, with shorter conduction time than effects conveyed through the striatum. We emphasize the functional significance of the 'hyperdirect' pathway and propose a dynamic 'center-surround model' of basal ganglia function in the control of voluntary limb movements. When a voluntary movement is about to be initiated by cortical mechanisms, a corollary signal conveyed through the cortico-subthalamo-pallidal 'hyperdirect' pathway first inhibits large areas of the thalamus and cerebral cortex that are related to both the selected motor program and other competing programs. Then, another corollary signal through the cortico-striato-pallidal 'direct' pathway disinhibits their targets and releases only the selected motor program. Finally, the third corollary signal possibly through the cortico-striato-external pallido-subthalamo-internal pallidal 'indirect' pathway inhibits their targets extensively. Through this sequential information processing, only the selected motor program is initiated, executed and terminated at the selected timing, whereas other competing programs are canceled.

Differential role for the striatum and cerebellum in response to novel movements using a motor learning paradigm

The aim of this pilot study was to examine the role of the striatum and cerebellum in the adaptation to a novel movement within a sequence of practiced movements using a motor learning paradigm. The performance of patients in the early or advanced stages of Parkinson's disease (PD) and of patients with damage to the cerebellum (CE) was compared, respectively to a group of aged and young matched controls on an adapted version of the Mirror-Tracing Test. In this task, subjects were required to trace a series of complex figures in two conditions: (1) a Practiced condition, in which the figures were composed of the juxtaposition of three simple designs that were extensively practiced before; and (2) a Mixed condition in which triads were created by replacing the last simple figure of the triads in the Practiced condition by a new simple figure that had never been traced individually before. Results showed that all clinical groups were slower than controls at tracing the Practiced triads. Most interestingly, however, only patients in the advanced stages of PD showed increased completion time to trace the triads in the Mixed condition. This suggests that a bilateral striatal dysfunction affects the ability to adapt to a novel motion within a sequence of practiced movements. Although exploratory, these results support a functional dissociation between the striatum and cerebellum in acquiring visuomotor skilled behaviors.

Neuronal activity in primate striatum and pallidum related to bimanual motor actions

To assess whether striatal and pallidal neurones may contribute to bimanual co-ordination, two macaque monkeys were trained to perform a delayed conditional sequence of co-ordinated pull and grasp movements, executed either bimanually or unimanually. Most of the 58 task-related neurones, recorded from the caudate nucleus, putamen, external and internal divisions of the globus pallidus, exhibited an activity related to the execution of the movements. Only a quarter of neurones displayed preparatory activity. The majority of units exhibited a significant modulation of activity in unimanual trials irrespective of the hand used to perform the task. In bimanual trials, one-third of units exhibited discharge patterns reflecting a bimanual synergy, suggesting a possible role for basal ganglia in inter-limb co-operation.

Dopamine regulates the impact of the cerebral cortex on the subthalamic nucleus-globus pallidus network

The subthalamic nucleus-globus pallidus network plays a central role in basal ganglia function and dysfunction. To determine whether the relationship between activity in this network and the principal afferent of the basal ganglia, the cortex, is altered in a model of Parkinson's disease, we recorded unit activity in the subthalamic nucleus-globus pallidus network together with cortical electroencephalogram in control and 6-hydroxydopamine-lesioned rats under urethane anaesthesia. Subthalamic nucleus neurones in control and 6-hydroxydopamine-lesioned animals exhibited low-frequency oscillatory activity, which was tightly correlated with cortical slow-wave activity (approximately 1 Hz). The principal effect of dopamine depletion was that subthalamic nucleus neurones discharged more intensely (233% of control) and globus pallidus neurones developed low-frequency oscillatory firing patterns, without changes in mean firing rate. Ipsilateral cortical ablation largely abolished low-frequency oscillatory activity in the subthalamic nucleus and globus pallidus. These data suggest that abnormal low-frequency oscillatory activity in the subthalamic nucleus-globus pallidus network in the dopamine-depleted state is generated by the inappropriate processing of rhythmic cortical input. A component (15-20%) of the network still oscillated following cortical ablation in 6-hydroxydopamine-lesioned animals, implying that intrinsic properties may also pattern activity when dopamine levels are reduced. The response of the network to global activation was altered by 6-hydroxydopamine lesions. Subthalamic nucleus neurones were excited to a greater extent than in control animals and the majority of globus pallidus neurones were inhibited, in contrast to the excitation elicited in control animals. Inhibitory responses of globus pallidus neurones were abolished by cortical ablation, suggesting that the indirect pathway is augmented abnormally during activation of the dopamine-depleted brain. Taken together, these results demonstrate that both the rate and pattern of activity of subthalamic nucleus and globus pallidus neurones are altered profoundly by chronic dopamine depletion. Furthermore, the relative contribution of rate and pattern to aberrant information coding is intimately related to the state of activation of the cerebral cortex.