The primate subthalamic nucleus. I. Functional properties in intact animals

Impact of High-Frequency Stimulation Parameters on the Pattern of Discharge of Subthalamic Neurons

In clinical conditions, high-frequency stimulation (HFS) of subthalamic (STN) neurons in Parkinson's disease is empirically applied at ≥100 Hz (130–185 Hz), with pulses of short duration (60–100 μs) and 1- to 3-mA amplitude. Other parameter values produce no effect or aggravate the symptoms. To gain a better understanding of the mechanisms that underlie the therapeutic action of HFS, we have compared the effects of different combinations of parameter values delivered by clinical stimulators on the activity of STN neurons recorded in whole cell patch-clamp configuration in slices. We showed that none of tested combinations of parameters silenced the neurons. Non-therapeutic combinations i.e., low-frequency pulses (10–50 Hz), even at large amplitude or width, further excited the STN neurons with respect to their spontaneous activity. In contrast, combinations in the therapeutic range (80–185 Hz, 90–200 μs, 500–800 μA) replaced the preexisting activity by spikes, time-locked to the stimuli and thus presenting a striking regularity. When increasing pulse width or amplitude in this high-frequency range, the dual effect was still present but the activity generated became more irregular. We propose that during HFS at clinically relevant parameters, STN neurons behave as stable oscillators entirely driven by the stimulation, giving an average stable STN output that overrides spontaneous activity and introduces high-frequency regular spiking in the basal ganglia network.

Reward-related neuronal activity in the subthalamic nucleus of the monkey

The subthalamic nucleus is a key structure for motor information processing in the basal ganglia. Little is known about its involvement in other aspects of behavior such as motivation. We investigated neuronal activity in the subthalamic nucleus while a monkey performed arm-reaching movements to obtain a liquid reward. Most neurons were modulated both during the movement and reward phases of the task. The changes in activity occurring after or just before the delivery of reward consisted of either increases or decreases in firing and were not directly related to mouth movements. These findings indicate that STN neurons are involved in the detection and expectation of reward, consistent with a role for these neurons in the processing of motivational information.

Ballism after stroke responds to standard physical therapeutic interventions

OBJECTIVE

To report the effects of noninvasive standard physical therapy (PT) interventions on an involuntary movement disorder after stroke.

DESIGN

Single case with clinical follow-up over 2 years.

SETTING

Inpatient stroke and rehabilitation services and outpatient clinic.

PARTICIPANT

A man with acute bilateral ballism after unilateral subthalamic infarction.

INTERVENTIONS

Rhythmic coordinated bilateral limb movements and firm tactile stimulation to the hand.

MAIN OUTCOME MEASURES

We had not anticipated that dyskinesia itself would specifically improve during treatment. Consequently, we used qualitative clinical observations, including review of videotaped performance, and self-reported limb control. Hypotheses concerning treatment effects were developed after data collection.

RESULTS

Involuntary movements recurrently improved within treatment sessions.

CONCLUSIONS

PT may improve dyskinesia after stroke. The benefit may be adjunctive or alternative to current invasive treatments of movement disorders after brain injury and merits confirmation. The improvements are consistent with current research indicating that (1) intact cortical, subcortical, cerebellar, and spinal areas interact to generate bilateral rhythmic limb movements that can overcome dyskinesia and (2) tactile stimulation can improve motor deficits associated with basal ganglia disorders. Finally, because functional activities assessments improved our evaluation of ballism, these should be routinely used along with conventional neurologic examination to assess involuntary movement disorders.

Somatotopy in the basal ganglia: experimental and clinical evidence for segregated sensorimotor channels

Growing experimental and clinical evidence supports the notion that the cortico-basal ganglia-thalamo-cortical loops proceed along parallel circuits linking cortical and subcortical regions subserving the processing of sensorimotor, associative and affective tasks. In particular, there is evidence that a strict topographic segregation is maintained during the processing of sensorimotor information flowing from cortical motor areas to the sensorimotor areas of the basal ganglia. The output from the basal ganglia to the motor thalamus, which projects back to neocortical motor areas, is also organized into topographically segregated channels. This high degree of topographic segregation is demonstrated by the presence of a well-defined somatotopic organization in the sensorimotor areas of the basal ganglia. The presence of body maps in the basal ganglia has become clinically relevant with the increasing use of surgical procedures, such as lesioning or deep brain stimulation, which are selectively aimed at restricted subcortical targets in the sensorimotor loop such as the subthalamic nucleus (STN) or the globus pallidus pars interna (GPi). The ability to ameliorate the motor control dysfunction without producing side effects related to interference with non-motor circuits subserving associative or affective processing requires the ability to target subcortical areas particularly involved in sensorimotor processing (currently achieved only by careful intraoperative microelectrode mapping). The goal of this article is to review current knowledge about the somatotopic segregation of basal ganglia sensorimotor areas and outline in detail what is known about their body maps.

Modulation of spontaneous firing in rat subthalamic neurons by 5-HT receptor subtypes

The subthalamic nucleus (STN) is considered to be one of the driving forces in the basal ganglia circuit. The STN is innervated by serotonergic afferents from the raphe nucleus and expresses a variety of 5-HT receptor subtypes. We investigated the effects of 5-HT and 5-HT receptor subtype agonists and antagonists on the firing properties of STN neurons in rat brain slices. We used cell-attached, perforated-patch, and whole cell recording techniques to detect changes in firing frequency and pattern and electrical membrane properties. Due to the depolarization of membrane potential caused by reduced potassium conductance, 5-HT (10 microM) increased the firing frequency of STN neurons without changing their firing pattern. Cadmium failed to occlude the effect of 5-HT on firing frequency. 5-HT had no effect on afterhyperpolarization current. These results indicated that the 5-HT action was not mediated by high-voltage-activated calcium channel currents and calcium-dependent potassium currents. 5-HT had no effect on hyperpolarization-activated cation current (I(H)) amplitude and voltage-dependence of I(H) activation, suggesting that I(H) was not involved in 5-HT-induced excitation. The increased firing by 5-HT was mimicked by 5-HT(2/4) receptor agonist alpha-methyl-5-HT and was partially mimicked by 5-HT2 receptor agonist DOI or 5-HT4 receptor agonist cisapride. The 5-HT action was partially reversed by 5-HT4 receptor antagonist SB 23597-190, 5-HT2 receptor antagonist ketanserin, and 5-HT2C receptor antagonist RS 102221. Our data indicate that 5-HT has significant ability to modulate membrane excitability in STN neurons; modulation is accomplished by decreasing potassium conductance by activating 5-HT4 and 5-HT2C receptors.

Motor control in basal ganglia circuits using fMRI and brain atlas approaches

In this study, we examined how the motor, premotor and associative basal ganglia territories process movement parameters such as the complexity and the frequency of movement. Twelve right-handed volunteers were studied using EPI BOLD contrast (3 T) while performing audio-paced finger tapping tasks designed to differentiate basal ganglia territories. Tasks varied movement complexity (repetitive index tapping, simple sequence of finger movements and complex sequence of 10 moves) and frequency (from 0.5 to 3 Hz). Activation maps were coregistered onto a 3-D brain atlas derived from post-mortem brains. Three main patterns of activation were observed. In the posterior putamen and the sensorimotor cortex, signal increased with movement frequency but not with movement complexity. In premotor areas, the anterior putamen and the ventral posterolateral thalamus, signal increased regularly with increasing movement frequency and complexity. In rostral frontal areas, the caudate nucleus, the subthalamic nucleus and the ventral anterior/ventrolateral thalamus, signal increased mainly during the complex task and the high frequency task (3 Hz). These data show the different roles of motor, premotor and associative basal ganglia circuits in the processing of motor-related operations and suggest that activation can be precisely located within the entire circuitry of the basal ganglia.

Different functional loops between cerebral cortex and the subthalmic area in Parkinson's disease

We investigate the extent to which functional circuits coupling cortical and subthalamic activity are multiple and segregated by frequency in untreated Parkinson's disease (PD). To this end, we recorded EEG and local field potentials (LFPs) from macroelectrodes inserted into the subthalamic nucleus area (SA) in nine awake patients following functional neurosurgery for PD. Patients were studied after overnight withdrawal of medication. Coherence between EEG and SA LFPs was apparent in the theta (3-7 Hz), alpha (8-13 Hz), lower beta (14-20 Hz) and upper beta (21-32 Hz) bands, although activity in the alpha and upper beta bands dominated. Theta coherence predominantly involved mesial and lateral areas, alpha and lower beta coherence the mesial and ipsilateral motor areas, and upper beta coherence the midline cortex. SA LFPs led EEG in the theta band. In contrast, EEG led the depth LFP in the lower and upper beta bands. SA LFP activity in the alpha band could either lead or lag EEG. Thus there are several functional sub-loops between the subthalamic area and cerebral cortical motor regions, distinguished by their frequency, cortical topography and temporal relationships. Tuning to distinct frequencies may provide a means of marking and segregating related processing, over and above any anatomical segregation of processing streams.

Motor and non motor effects during intraoperative subthalamic stimulation for Parkinson's disease

Spatial distribution of the clinical effects induced by deep brain stimulation during the intraoperative investigation of the subthalamic nucleus (STN) for Parkinson's disease (PD) was analysed in 17 patients under local anesthesia. The stimulation parameters were 130 hertz, 100 micros, and voltage ranged from 0.05 to 5 volts. Optimal motor response was assessed as the total and lasting disappearance of wrist rigidity on the side opposite to stimulation. Among the adverse effects induced by stimulation, special attention was given to frequently observed autonomic effects (AE). Full motor response was achieved in 49.2% of the 301 points evaluated,with a mean voltage (MV) of 0.94 volts; paresthesiae occurred in 6.6% (MV: 2 volts), dystonia in 10.6% (MV: 3.4 volts), autonomic effects in 19.6% (MV: 3.1 volts) and oculomotor effects in 31.6% (MV: 3 volts). The motor target was located in the posterodorsal part of the nucleus and the optimal point for motor response was close to the superior limit of the nucleus. Whereas other adverse effects occurred relatively far from the motor target, AE occurred with statistic significance near this point. Their neural substrates, such as limbic system and their relationship with postoperative behavioral disorders, are discussed.

Timing and direction selectivity of subthalamic and pallidal neurons in patients with Parkinson disease

Current models of basal ganglia function suggest that some manifestations of Parkinson disease (PD) arise from abnormal activity and decreased selectivity of neurons in the subthalamic nucleus (STN) and globus pallidus internus (Gpi). Our goal was to examine the timing and direction selectivity of neuronal activity relative to visually guided movements in the STN and Gpi of patients with PD. Recordings were made from 152 neurons in the STN and 33 neurons in the Gpi of awake subjects undergoing surgery for PD. Corresponding EMG data were obtained for half the cells. We employed a structured behavioral task in which the subjects used a joystick to guide a cursor to one of four targets displayed on a monitor. Each direction was tested over multiple trials. Movement-related modulation of STN activity began on average 264+/-10 ms before movement initiation and 92+/-13 ms before initial EMG activity, while modulation of Gpi activity began 204+/-21 ms before overt movement initiation. In the STN, 40% of cells demonstrated perimovement activity, and of these 64% were directionally selective. In Gpi, 45% of cells showed perimovement activity of which 80% were selective. In both nuclei, directionally selective cells had significantly lower baseline firing rates than nonselective cells (41+/-5 vs 59+/-4 spikes/s in STN, and 50+/-9 vs 74+/-15 spikes/s in Gpi). These results suggest that STN activity occurs earlier than previously reported, and that higher neuronal firing rates maybe associated with decreased direction selectivity in PD patients.

The basal ganglia: anatomy, physiology, and pharmacology

The basal ganglia are perceived as important nodes in cortico-subcortical networks involved in the transfer, convergence, and processing of information in motor, cognitive, and limbic domains. How this integration might occur remains a matter of some debate, particularly given the consistent finding in anatomic and physiologic studies of functional segregation in cortico-subcortical loops. More recent theories, however, have raised the notion that modality-specific information might be integrated not spatially, but rather temporally, by coincident processing in discrete neuronal populations. Basal ganglia neurotransmitters, given their diverse roles in motor performance, learning, working memory, and reward-related activity are also likely to play an important role in the integration of cerebral activity. Further work will elucidate this to a greater extent, but for now, it is clear that the basal ganglia form an important nexus in the binding of cognitive, limbic, and motor information into thought and action.

Glutamatergic-receptors blockade does not regularize the slow wave sleep bursty pattern of subthalamic neurons

The subthalamic nucleus (STN) has been implicated in movement disorders observed in Parkinson's disease because of its pathological mixed burst firing mode and hyperactivity. In physiological conditions, STN bursty pattern has been shown to be dependent on slow wave cortical activity. Indeed, cortical ablation abolished STN bursting activity in urethane-anaesthetized intact or dopamine depleted rats. Thus, glutamate afferents might be involved in STN bursting activity during slow wave sleep (SWS) when thalamic and cortical cells oscillate in a low-frequency range. The present work was aimed to test, on non-anaesthetized rats, if it was possible to regularize the SWS STN bursty pattern by microiontophoresis of kynurenate, a broad-spectrum glutamate ionotropic receptors antagonist. As glutamatergic effects might be masked by GABAergic inputs arriving tonically and during the entire sleep-wake cycle on STN neurons, kynurenate was also co-iontophoresed with bicuculline, a GABA(A) receptors antagonist. Kynurenate iontophoretic applications had a weak inhibitory effect on the discharge rate of STN neurons whatever the vigilance state, and did not regularize the SWS STN bursty pattern. But, the robust bursty bicuculline-induced pattern was impaired by kynurenate, which elicited the emergence of single spikes between remaining bursts. These data indicate that the bursty pattern exhibited by STN neurons specifically in SWS, does not seem to exclusively depend on glutamatergic inputs to STN cells. Furthermore, GABA(A) receptors may play a critical role in regulating the influence of excitatory inputs on STN cells.

Conductance-Based Model of the Voltage-Dependent Generation of a Plateau Potential in Subthalamic Neurons

Because the subthalamic nucleus (STN) acts as a driving force of the basal ganglia, it is important to know how the activities of STN neurons are regulated. Previously, we have reported that a subset of STN neurons generates a plateau potential in a voltage-dependent manner. These plateau potentials can be evoked only when the cell is hyperpolarized. Here, to examine the mechanism of the voltage-dependent generation of the plateau potential in STN neurons, we constructed a conductance-based model of the plateau-generating STN neuron based on experimental observations and compared simulation results with recordings in slices. The model consists of a single compartment containing a Na+ current, a delayed-rectifier K+ current, an A-type K+ current, an L-like long-lasting Ca2+ current, a T-type Ca2+ current, a Ca2+-dependent K+ current, and a leak current. Our simulation results showed that a plateau potential in the model could be induced in a voltage-dependent manner that depended on the inactivation properties of L-like long-lasting Ca2+ current. The model could also reproduce the generation of a plateau potential as a rebound potential after termination of hyperpolarizing current injection. In addition, we tested the stability of simulated plateau potentials against inhibitory perturbation and found that the model showed similar properties observed for the plateau potentials of STN neurons in slices. The effects of K+ channel blockade by TEA and intracellular Ca2+ ion chelation by BAPTA on the plateau duration were also tested in the model and were found to match experimental observations. Thus our STN neuron model could qualitatively reproduce a number of experimental observations on plateau potentials. Our results suggest that the inactivation of L-type Ca2+ channels plays an important role in the voltage-dependent generation of the plateau potential.

Calcium-dependent subthreshold oscillations determine bursting activity induced by N-methyl-D-aspartate in rat subthalamic neurons in vitro

We used whole-cell patch recordings in current clamp to investigate the ionic dependence of burst firing induced by N-methyl-d-aspartate (NMDA) in neurons of the subthalamic nucleus (STN) in slices of rat brain. NMDA (20 microm) converted single-spike firing to burst firing in 87% of STN neurons tested. NMDA-induced bursting was blocked by AP5 (50 microm), and was not mimicked by the non-NMDA receptor agonist AMPA (0.6 microm). Tetrodotoxin (1 microm) converted bursts to oscillations of membrane potential, which were most robust when oscillations ranged between -50 and -70 mV. The NMDA bursts were blocked by an elevated extracellular concentration of Mg(2+), but superfusate containing no added Mg(2+) either reduced or increased burst firing, depending upon the amount of intracellular current injection. Block of K(+) conductances by apamin and tetraethylammonium prolonged burst duration, but iberiotoxin had no effect. NMDA-induced burst firing and membrane oscillations were completely blocked by superfusate containing no added Ca(2+), and they were significantly reduced when patch pipettes contained BAPTA. Selective antagonists for T-type (mibefradil, 10 microm), L-type (nifedipine, 3 microm), and N-type (omega-conotoxin GVIA, 1 micro m) Ca(2+) channels had no effect on NMDA burst firing. Superfusate containing a low concentration of Na(+) (20 mm) completely abolished NMDA-induced burst firing. Flufenamic acid (10 microm), which blocks current mediated by Ca(2+)-activated nonselective cation channels (I(CAN)), reversibly abolished NMDA-depended bursting. These results are consistent with the hypothesis that NMDA-induced burst firing in STN neurons requires activation of either an I(CAN) or a Na(+)-Ca(2+) exchanger.

How laminar frontal cortex and basal ganglia circuits interact to control planned and reactive saccades

How does the brain learn to balance between reactive and planned behaviors? The basal ganglia (BG) and frontal cortex together allow animals to learn planned behaviors that acquire rewards when prepotent reactive behaviors are insufficient. This paper proposes a new model, called TELOS, to explain how laminar circuitry of the frontal cortex, exemplified by the frontal eye fields, interacts with the BG, thalamus, superior colliculus, and inferotemporal and parietal cortices to learn and perform reactive and planned eye movements. The model is formulated as fourteen computational hypotheses. These specify how strategy priming and action planning (in cortical layers III, Va and VI) are dissociated from movement execution (in layer Vb), how the BG help to choose among and gate competing plans, and how a visual stimulus may serve either as a movement target or as a discriminative cue to move elsewhere. The direct, indirect and hyperdirect pathways through the BG are shown to enable complex gating functions, including deferred execution of selected plans, and switching among alternative sensory-motor mappings. Notably, the model can learn and gate the use of a What-to-Where transformation that enables spatially invariant object representations to selectively excite spatially coded movement plans. Model simulations show how dopaminergic reward and non-reward signals guide monkeys to learn and perform saccadic eye movements in the fixation, single saccade, overlap, gap, and delay (memory-guided) saccade tasks. Model cell activation dynamics quantitatively simulate seventeen established types of dynamics exhibited by corresponding real cells during performance of these tasks.

Event‐related beta desynchronization in human subthalamic nucleus correlates with motor performance

Although the basal ganglia play an important role in self-generated movement, their involvement in externally paced voluntary movement is less clear. We recorded local field potentials (LFPs) from the region of the subthalamic nuclei of eight patients with Parkinson's disease during the performance of a warned reaction time task in which an imperative cue instructed the subject to move or not to move. In 'go' trials, LFP activity in the beta frequency band ( approximately 20 Hz) decreased prior to movement, with an onset latency that strongly correlated with mean reaction time across patients. This was followed by a late post-movement increase in beta power. In contrast, in 'nogo' trials the beta power drop following imperative signals was prematurely terminated compared with go trials and reversed into an early beta power increase. These differences were manifest as power increases when go trials were subtracted from nogo trials. In six patients these relative beta power increases in nogo-go difference trials were of shorter latency than the respective reaction time. The findings suggest that, firstly, the subthalamic nucleus is involved in the preparation of externally paced voluntary movements in humans and, secondly, the degree of synchronization of subthalamic nucleus activity in the beta band may be an important determinant of whether motor programming and movement initiation is favoured or suppressed.

Microelectrode recording revealing a somatotopic body map in the subthalamic nucleus in humans with Parkinson disease

Object. The subthalamic nucleus (STN) is a key structure for motor control through the basal ganglia. The aim of this study was to show that the STN in patients with Parkinson disease (PD) has a somatotopic organization similar to that in nonhuman primates.

Methods. A functional map of the STN was obtained using electrophysiological microrecording during placement of deep brain stimulation (DBS) electrodes in patients with PD. Magnetic resonance imaging was combined with ventriculography and intraoperative x-ray film to assess the position of the electrodes and the STN units, which were activated by limb movements to map the sensorimotor region of the STN. Each activated cell was located relative to the anterior commissure—posterior commissure line. Three-dimensional coordinates of the cells were analyzed statistically to determine whether those cells activated by movements of the arm and leg were segregated spatially.

Three hundred seventy-nine microelectrode tracks were created during placement of 71 DBS electrodes in 44 consecutive patients. Somatosensory driving was found in 288 tracks. The authors identified and localized 1213 movement-related cells and recorded responses from 29 orofacial cells, 480 arm-related cells, 558 leg-related cells, and 146 cells responsive to both arm and leg movements. Leg-related cells were localized in medial (p < 0.0001) and ventral (p < 0.0004) positions and tended to be situated anteriorly (p = 0.063) relative to arm-related cells.

Conclusions. Evidence of somatotopic organization in the STN in patients with PD supports the current theory of highly segregated loops integrating cortex—basal ganglia connections. These loops are preserved in chronic degenerative diseases such as PD, but may subserve a distorted body map. This finding also supports the relevance of microelectrode mapping in the optimal placement of DBS electrodes along the subthalamic homunculus.

Overwhelmingly asynchronous firing of rat subthalamic nucleus neurones in brain slices provides little evidence for intrinsic interconnectivity

In Parkinson's disease the neurones of the subthalamic nucleus show increased synchrony and oscillatory burst discharge, thought to reflect a breakdown of parallel processing in basal ganglia circuitry. To understand better the mechanisms underlying this transition, we sought to mimic this change in firing pattern within sagittal slices of rat midbrain. The firing patterns of up to four simultaneously extracellularly recorded subthalamic nucleus (STN) neurones were analysed using burst and oscillation detection programs, and correlated activity between pairs of neurones assessed. In control conditions all but 11 of 488 (2%) neurones fired in a predominantly tonic pattern (with mean oscillation frequency >3 Hz), with no significantly cross-correlated activity in any of 393 pairs of neurones. The glutamate antagonists DL-2-amino-phosphonopentanoic acid (APV), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 6-methyl-2-(phenylethynyl)pyridine (MPEP) did not change the firing rate or pattern of these cells, providing no evidence for a role of glutamatergic collaterals within the STN under these conditions. The GABA(A) receptor antagonist bicuculline and GABA(B) receptor antagonist (2S)-3-[[(1S)-1-(3,4-dichlorophenyl)ethyl]amino-2-hydroxypropyl]phenylmethyl phosphinic acid (CGP 55845) were also without effect on firing rate or pattern in these cells, suggesting that there was no active input from other GABAergic basal ganglia nuclei in this slice. The dopamine receptor antagonist haloperidol caused no significant change to firing rate or pattern of firing in these cells, suggesting that there was no active dopaminergic input in this slice. Excitations of STN neurones by muscarine, (+)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD), N-methyl-D-aspartic acid (NMDA) or dopamine were all unaccompanied by a change in firing pattern or any significant correlated activity between STN neurone pairs. Burst firing could be induced in STN neurones with either the potassium channel blocker tetraethylammonium (TEA; 10 mM; in 100/138 [72%] of cells) or with a combination of NMDA and the calcium-activated potassium channel blocker apamin (in 101/216 [47%] of cells). Burst firing in TEA was unchanged by CNOX and APV, MPEP, CGP55845, haloperidol, dopamine, and ACPD, although muscarine produced a significant increase in oscillation frequency. Burst firing in NMDA and apamin was unchanged by CNQX and APV, dopamine, muscarine and ACPD, although bicuculline caused a significant increase in oscillation frequency. Such burst firing was not accompanied by synchrony in any condition, either alone, or during application of excitatory agents or glutamate or GABA antagonists. As the bursting seen here was unaccompanied by the synchronous activity that has often been observed (pathologically) in vivo, it probably reflects solely intrinsic STN neuronal properties, rather than network activity. No functional role was found for glutamatergic collaterals within the STN, either when cells are firing tonically or burst firing. The circuitry needed to produce synchrony in the STN is most likely not intrinsic to the STN itself, but requires connections with other basal ganglia nuclei, and/or the cortex, which are not present in this preparation.

Surgery of the subthalamic nucleus: use of movement-related neuronal activity for surgical navigation

THE BASAL GANGLIA have important roles in somatic motor, oculomotor, limbic, and associative functions. These functions are represented in anatomically distinct territories in each basal ganglion nucleus. During surgery of the subthalamic nucleus for Parkinson's disease, the primary goal is to influence the physiology of the motor territory without affecting nonmotor areas. This article describes the use of movement-related cellular activity during single-unit microelectrode mapping to identify and to navigate within the motor territory of the subthalamic nucleus.

Macro-architecture of basal ganglia loops with the cerebral cortex: use of rabies virus to reveal multisynaptic circuits

We have used retrograde transneuronal transport of rabies virus to examine basal ganglia connections with the cerebral cortex. We injected rabies into the primary motor cortex (M1) or into Area 46 of cebus monkeys. A 4-day survival time was long enough to allow transport of rabies from the injection site to 'third-order' neurons in the basal ganglia. After either M1 or Area 46 injections, third-order neurons were found in the external segment of the globus pallidus (GPe), striatum and subthalamic nucleus (STN). In each of these nuclei, the third-order neurons that innervate M1 were spatially separated from those that innervate Area 46. Thus, distinct basal ganglia-thalamocortical circuits innervate M1 and Area 46. Next, we injected a conventional tracer into M1 to define its terminations in the putamen and STN. We found that the regions of the putamen and STN that receive input from M1 are the same as those that contain third-order neurons after M1 injections of virus. On the other hand, virus injections into M1 also labeled a relatively dense group of third-order neurons in a region of the ventral putamen that is not innervated by M1. This region of the putamen is the target of efferents from the amygdala. Thus, the ventral putamen may provide a route for the limbic system to influence motor output. Overall, our results indicate that basal ganglia circuits with the cerebral cortex can be characterized by both open- and closed-loop macro-architectures.

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.

A model of reverse spike frequency adaptation and repetitive firing of subthalamic nucleus neurons

Subthalamic nucleus neurons exhibit reverse spike-frequency adaptation. This occurs only at firing rates of 20-50 spikes/s and higher. Over this same frequency range, there is an increase in the steady-state frequency-intensity (F-I) curve's slope (the secondary range). Specific blockade of high-voltage activated calcium currents reduced the F-I curve slope and reverse adaptation. Blockade of calcium-dependent potassium current enhanced secondary range firing. A simple model that exhibited these properties used spike-triggered conductances similar to those in subthalamic neurons. It showed: 1) Nonaccumulating spike afterhyperpolarizations produce positively accelerating F-I curves and spike-frequency adaptation that is complete after the second spike. 2) Combinations of accumulating aftercurrents result in a linear F-I curve, whose slope depends on the relative contributions of inward and outward currents. Spike-frequency adaptation can be gradual. 3) With both accumulating and nonaccumulating aftercurrents, primary and secondary ranges will be present in the F-I curve. The slope of the primary range is determined by the nonaccumulating conductance; the accumulating conductances govern the secondary range. The transition is determined by the relative strengths of accumulating and nonaccumulating currents. 4) Spike-threshold accommodation contributes to the secondary range, reducing its slope at high firing rates. Threshold accommodation can stabilize firing when inward aftercurrents exceed outward ones. 5) Steady-state reverse adaptation results when accumulated inward aftercurrents exceed outward ones. This requires spike-threshold accommodation. Transient speedup arises when inward currents are smaller than outward ones at steady state, but accumulate more rapidly. 6) The same mechanisms alter firing in response to irregular patterns of synaptic conductances, as cell excitability fluctuates with changes in firing rate.

Noradrenaline increases the firing rate of a subpopulation of rat subthalamic neurones through the activation of α1-adrenoceptors

In the rat subthalamic nucleus, which plays a critical role in the control of motor behaviour, specific binding of [3H]-prazosin was detected by radioligand binding to homogenates and by autoradiography in slices. [3H]-Prazosin binding to homogenates (Bmax 71 +/- 5 fmol/mg protein; Kd 0.27 +/- 0.05 nM) was competed for by alpha1-antagonists. In subthalamic nucleus slices and in the presence of 10 mM LiCl, noradrenaline (100 microM) produced a modest, but consistent, stimulation of [3H]-inositol phosphate accumulation (146 +/- 6% of basal), reversed by the alpha1-antagonist prazosin (1 microM). Extracellular single-unit recordings in slices showed that in a subpopulation (61 out of 94 cells) of rat subthalamic neurones with regular, single-spike firing pattern, noradrenaline induced a concentration-dependent increase in the firing rate (EC50 2.5 +/- 0.2 microM, maximum effect 272 +/- 33% of basal). The action of noradrenaline was mimicked by the selective alpha1-agonist phenylephrine but not by selective alpha2- or beta-agonists, and was blocked by the alpha1-antagonist prazosin but not by alpha2- or beta-antagonists. The excitatory effect of noradrenaline was not prevented by perfusion with low Ca2+/high Mg2+ solution. In four out of 11 neurones perfusion with 3 microM noradrenaline resulted in a shift from bursting to regular firing. Taken together, our results indicate that rat subthalamic neurones express alpha1-adrenoceptors responsible for noradrenaline-induced stimulation of the firing rate of a subpopulation of neurones. By modulating the spontaneous activity of STN neurones, noradrenergic pathways might have a significant role in regulating basal ganglia function and thus motor activity.

Information processing, dimensionality reduction and reinforcement learning in the basal ganglia

Modeling of the basal ganglia has played a major role in our understanding of this elusive group of nuclei. Models of the basal ganglia have undergone evolutionary and revolutionary changes over the last 20 years, as new research in the fields of anatomy, physiology and biochemistry of these nuclei has yielded new information. Early models dealt with a single pathway through the nuclei and focused on the nature of the processing performed within it, convergence of information versus parallel processing of information. Later, the Albin-DeLong "box-and-arrow" model characterized the inter-nuclei interaction as multiple pathways while maintaining a simplistic scalar representation of the nuclei themselves. This model made a breakthrough by providing key insights into the behavior of these nuclei in hypo- and hyper-kinetic movement disorders. The next generation of models elaborated the intra-nuclei interactions and focused on the role of the basal ganglia in action selection and sequence generation which form the most current consensus regarding basal ganglia function in both normal and pathological conditions. However, new findings challenge these models and point to a different neural network approach to information processing in the basal ganglia. Here, we take an in-depth look at the reinforcement driven dimensionality reduction (RDDR) model which postulates that the basal ganglia compress cortical information according to a reinforcement signal using optimal extraction methods. The model provides new insights and experimental predictions on the computational capacity of the basal ganglia and their role in health and disease.

Lateralized effects of subthalamic nucleus stimulation on different aspects of speech in Parkinson's disease

In this report, we have analyzed the effects of left and right STN stimulation separately on different aspects of speech. Significant differences were found between left and right stimulation. It appears that selective left-sided stimulation has a profoundly negative effect on prosody, articulation and hence intelligibility. Right-sided stimulation does not display this side-effect. There is no significant difference in speech characteristics between bilateral stimulation on and off. We suggest that a balanced tuning of bilateral basal ganglia networks is necessary for speech, and that the left circuit is probably dominant.

Apamin-sensitive small conductance calcium-activated potassium channels, through their selective coupling to voltage-gated calcium channels, are critical determinants of the precision, pace, and pattern of action potential generation in rat subthalamic nucleus neurons in vitro

Distinct activity patterns in subthalamic nucleus (STN) neurons are observed during normal voluntary movement and abnormal movement in Parkinson's disease (PD). To determine how such patterns of activity are regulated by small conductance potassium (SK)/calcium-activated potassium (KCa) channels and voltage-gated calcium (Cav) channels, STN neurons were recorded in the perforated patch configuration in slices, [which were prepared from postnatal day 16 (P16)-P30 rats and held at 37 degrees C] and then treated with the SK KCa channel antagonist apamin or the SK KCa channel agonist 1-ethyl-2-benzimidazolinone or the Cav channel antagonists w-omega-conotoxin GVIA (Cav2.2-selective) or nifedipine (Cav1.2-1.3-selective) [corrected]. In other experiments, fura-2 was introduced as an indicator of intracellular calcium dynamics. A component of the current underlying single-spike afterhyperpolarization was sensitive to apamin, phase-locked to calcium entry via Cav2.2 channels, and necessary for precise, autonomous, single-spike oscillation. SK KCa/Cav2.2 channel coupling did not underlie spike-frequency adaptation but limited activity in response to current injection by encoding the accumulation of intracellular calcium, maintained the characteristic sigmoidal frequency-intensity relationship and generated a post-train afterhyperpolarization. In addition, SK KCa channels terminated rebound burst activity more effectively in neurons with short-duration bursts (<100 msec) than neurons with long-duration bursts (>100 msec), presumably through their activation by Cav3 channels. Cav1.2-1.3 channels were not strongly coupled to SK KCa channels and therefore supported secondary range and long-duration rebound burst firing. In summary, SK KCa channels play a fundamental role in autonomous, driven, and rebound activity and oppose the transition from autonomous, rhythmic, single-spike activity to burst firing in STN neurons.

Locations of movement-related cells in the human subthalamic nucleus in Parkinson's disease

The subthalamic nucleus (STN) is an emerging target for deep brain stimulator (DBS) implantation for the treatment of advanced Parkinson's disease (PD). Understanding the somatotopic organization of the STN is important for surgical navigation within the nucleus. We analyzed intraoperative data obtained during 54 procedures for the implantation of STN stimulators to assess the locations of movement-related cells. Cells were considered movement-related if they exhibited modulation of the cell discharge during passive movement of the contralateral upper or lower extremity. Microelectrode track reconstructions were plotted on a human brain atlas, using the location of the DBS electrode from postoperative magnetic resonance images as a registration mark in reconstructing microelectrode track locations. Movement-related cells were predominantly located in the dorsal part of the nucleus. The majority of the cells were related to proximal joint manipulation. Arm-related cells were located laterally and at the rostral and caudal poles, whereas leg-related cells were located medially and centrally. The finding of three or more leg-related cells on a given microelectrode track was predictive of a medial localization within the motor area. Our findings are consistent with the small number of published studies on STN somatopy in the human and the nonhuman primate.

Anatomic and neurophysiological methods for the targeting and lesioning of the subthalamic nucleus: Cuban experience and review

OBJECTIVE

To develop a method to place a lesion precisely in the subthalamic nucleus (STN) and evaluate its effectiveness.

METHODS

A retrospective study of targeting data collected during stereotactic planning to lesion the STN in 31 patients with Parkinson's disease and of results in more than 50 procedures was performed. The targeting method was based on computed tomographic imaging together with semimicroelectrode recording digital processing and electrical stimulation. Two statistical methods were used to correlate initial with final target coordinates and assess the efficacy of the targeting procedure.

RESULTS

The anatomic target based on computed tomographic imaging data showed electrical activity in the subthalamus in the first pass in 82% of the procedures. In the remaining 18%, the STN was an average of 1.93 mm away from the nearest trajectory that recorded the STN (range, 1.41-2.24 mm). The average number of trajectories per procedure was 7.2; the location of the first trajectory relative to the center of the nucleus determined by electrical and physiological means (P < 0.01, analysis of variance, Student's t test) was as follows: in the lateral direction, 1.25 +/- 1.15 mm; in the anteroposterior direction, 1.53 +/- 1.31 mm; and in the vertical direction, 0.67 +/- 0.51 mm. The average number of tracts necessary to lesion the STN was two.

CONCLUSION

The combination of computed tomographic imaging, semimicroelectrode recording, and microstimulation provides an effective method to identify the STN lesion in parkinsonian patients. The method used for anatomic localization and electrophysiological mapping of the subthalamus was found to be effective in reaching the sensorimotor region of the nucleus. We carried out an accurate determination of the subthalamus location and its volume in the lesioning.

Parkinson's Disease: Surgical Options

Surgical therapy for Parkinson's disease (PD) has been a treatment option for over 100 years. Advances in the knowledge of basal ganglia physiology and in techniques of stereotactic neurosurgery and neuroimaging have allowed more accurate placement of lesions or "brain pacemakers" in the sensorimotor regions of target nuclei. This, in turn, has led to improved efficacy with fewer complications than in the past. Currently, bilateral deep brain stimulation (DBS) of the subthalamic nucleus (STN) or the internal segment of the globus pallidus (GPi) is the preferred option (and is approved by the US Food and Drug Administration) for the surgical treatment of PD. The most important predictors for outcome for DBS for PD are patient selection and electrode location. Patients should have a documented preoperative improvement from dopaminergic medication of at least 30% in the patient's Unified Parkinson's Disease Rating Scale motor disability scores. A levodopa challenge may be needed to document the best "on" state. Dementia or active cognitive decline must be excluded. Active psychiatric disease should be treated preoperatively. Patients should be motivated, with good support systems, and committed to the postoperative management of DBS therapy. Deep brain stimulation should be considered when the patient begins to experience dyskinesia and on-off fluctuations despite optimal medical therapy. Deep brain stimulation is not a good option at the final stages of the disease because of the increased incidence of dementia and severe comorbidity. The DBS electrode should be placed in the sensorimotor region of the GPi or STN. Subthalamic nucleus and GPi DBS can improve all motor aspects of PD, as well as predictable "on" time, without dyskinesia or fluctuations. On average, STN DBS results in a greater reduction of dopaminergic medication compared with GPi DBS. Because of the smaller size of the target region, the pulse generator battery life is longer with STN then with GPi DBS. Deep brain stimulation programming is a skill that is readily learned and may be required of all neurologists in the future. Emerging surgical therapies are restorative, and they aim to replace or regenerate degenerating dopaminergic neurons. These include embryonic mesencephalic tissue transplantation, human embryonic stem cell transplantation, and gene-derived methods of intracerebral implantation of growth factors and dopamine- producing cell lines. It will be important to determine whether DBS, if performed before the onset of motor response complications to medical therapy, may prevent this stage of disease altogether or delay it for a significant period of time. The same question applies to the future with restorative therapy.

Movement-related neurons of the subthalamic nucleus in patients with Parkinson disease

OBJECT

The subthalamic nucleus (STN) is a target in the surgical treatment of Parkinson disease (PD). Little is known about the neurons within the human STN that modulate movement. The authors' goal was to examine the distribution of movement-related neurons within the STN of humans by using microelectrode recording to identify neuronal receptive fields.

METHODS

Data were retrospectively collected from microelectrode recordings that had been obtained in 38 patients with PD during surgery for placement of STN deep brain stimulation electrodes. The recordings had been obtained in awake, nonsedated patients. Antiparkinsonian medications were withheld the night before surgery. Neuronal discharges were amplified, filtered, and displayed on an oscilloscope and fed to an audio monitor. The receptive fields were identified by the presence of reproducible, audible changes in the firing rate that were time-locked to the movement of specific joint(s). The median number of electrode tracks per patient was six (range two-nine). The receptive fields were identified in 278 (55%) of 510 STN neurons studied. One hundred one tracks yielded receptive field data. Fourteen percent of 64 cells tested positive for face receptive fields, 32% of 687 cells tested positive for upper-extremity receptive fields, and 21% of 242 cells tested positive for lower-extremity receptive fields. Sixty-eight cells (24%) demonstrated multiple-joint receptive fields. Ninety-three cells (65%) with movement-related receptive fields were located in the dorsal half of the STN, and 96.8% of these were located in the rostral two thirds of the STN. Analysis of receptive field locations from pooled data and along individual electrode tracks failed to reveal a consistent somatotopic organization.

CONCLUSIONS

Data from this study demonstrate a regional compartmentalization of neurons with movement-related receptive fields within the STN, supporting the existence of specific motor territories within the STN in patients suffering from PD.

The switch of subthalamic neurons from an irregular to a bursting pattern does not solely depend on their GABAergic inputs in the anesthetic-free rat

The subthalamic nucleus (STN) powerfully controls basal ganglia outputs and has been implicated in movement disorders observed in Parkinson's disease because of its pathological mixed burst firing mode and hyperactivity. A recent study suggested that reciprocally connected glutamatergic STN and GABAergic globus pallidus (GP) neurons act in vitro as a generator of bursting activity in basal ganglia. In vivo, we reported that GP neurons increased their firing rate in wakefulness (W) compared with slow-wave sleep (SWS) without any change in their random pattern. In contrast, STN neurons exhibited similar firing rates in W and SWS, with an irregular pattern in W and a bursty one in SWS. Thus, the pallidal GABAergic tone might control the STN pattern. This hypothesis was tested by mimicking such variations with microiontophoresis of GABA receptor ligands. GABA agonists specifically decreased the STN firing rate but did not affect its firing pattern. GABA(A) (but not GABA(B)) antagonists strongly enhanced the STN mean discharge rate during all vigilance states up to three to five times its basal activity. However, such applications did not change the typical W random pattern. When applied during SWS, GABA(A) antagonists strongly reinforced the spontaneous bursty pattern into a particularly marked one with instantaneous frequencies reaching 500-600 Hz. SWS-W transitions occurring during ongoing antagonist iontophoresis invariably disrupted the bursty pattern into a random one. Thus GABA(A) receptors play a critical, but not exclusive, role in regulating the excitatory STN influence on basal ganglia outputs.

Implantation of deep brain stimulators into the subthalamic nucleus: technical approach and magnetic resonance imaging-verified lead locations

OBJECT

Chronic deep brain stimulation (DBS) of the subthalamic nucleus (STN) is a procedure that is rapidly gaining acceptance for the treatment of symptoms in patients with Parkinson disease (PD), but there are few detailed descriptions of the surgical procedure itself. The authors present the technical approach used to implant 76 stimulators into the STNs of patients with PD and the lead locations, which were verified on postoperative magnetic resonance (MR) images.

METHODS

Implantation procedures were performed with the aid of stereotactic MR imaging, microelectrode recording (MER) in the region of the stereotactic target to define the motor area of the STN, and intraoperative test stimulation to assess the thresholds for stimulation-induced adverse effects. All patients underwent postoperative MR imaging, which was performed using volumetric gradient-echo and T2-weighted fast-spin echo techniques, computational reformatting of the MR image into standard anatomical planes, and quantitative measurements of lead location with respect to the midcommissural point and the red nucleus. Lead locations were statistically correlated with physiological data obtained during MER and intraoperative test stimulation.

CONCLUSIONS

The authors' approach to implantation of DBS leads into the STN was associated with consistent lead placement in the dorsolateral STN, a low rate of morbidity, efficient use of operating room time, and robust improvement in motor function. The mean coordinates of the middle of the electrode array, measured on postoperative MR images, were 11.6 mm lateral, 2.9 mm posterior, and 4.7 mm inferior to the midcommissural point, and 6.5 mm lateral and 3.5 mm anterior to the center of the red nucleus. Voltage thresholds for several types of stimulation-induced adverse effects were predictive of lead location. Technical nuances of the surgery are described in detail.

Short-term plasticity shapes the response to simulated normal and parkinsonian input patterns in the globus pallidus

Basal ganglia structures show strong activity modulation during movement and synchronous bursting in Parkinson's disease. Recent work has shown that short-term synaptic plasticity (STP) can play an important role in the effect of temporal activity patterns on postsynaptic targets. To determine the role of STP in the subthalamic nucleus (STN) to globus pallidus (GP) connection, which has been suggested to underlie rhythmical bursting in Parkinson's disease, we first measured STP using trains of electrical input stimulation in vitro. We found that STN inputs to GP typically show both facilitation and depression with input frequencies of 10-100 Hz and that facilitation is dominant for the first few inputs in a train but that depression takes over subsequently. We quantified the strength and time course of facilitation and depression using a computational model of STP. Using the STP model, we constructed synaptic conductance patterns of normal and Parkinsonian STN activity and applied these conductances to GP neurons in vitro using the technique of dynamic clamping. We show that STP controls the slope and shape of the function describing the steady-state level of GP neuron firing in response to different levels of STN input. In addition, we show that STP modulates responses of GP neurons to bursts and pauses in the input pattern. These findings indicate that STP plays an important role in modulating both spike rates and temporal patterns of GP activity in the normal state, as well as in Parkinson's disease.

Dependence of subthalamic nucleus oscillations on movement and dopamine in Parkinson's disease

Local field potentials and pairs of neurones in the subthalamic nucleus (STN) of patients with Parkinson's disease show high-frequency oscillations (HFOs) at 15-30 Hz. This study explores how these HFOs are modulated by voluntary movements and by dopaminergic medication. We examined 15 patients undergoing implantation of bilateral deep brain stimulating electrodes using microelectrode recordings of pairs of STN neurones (eight patients) and macroelectrode recordings of local field potentials from the STN (14 patients). Synchronized HFOs between STN neurones were observed in 28 out of 37 pairs in five patients who had tremor in the operating room and none of 45 pairs in three patients who did not. In two of the three non-tremulous patients, HFOs in the frequency spectra of local field potentials were detected but were weaker than in those patients with tremor. Active movement suppressed synchronized HFOs in three out of five pairs of neurones, independent of changes in firing rate. HFOs observed in the local field potentials in nine out of 14 patients were reduced with voluntary movement in six of the eight patients tested. Dopaminergic medication decreased the incidence of synchronized HFOs in STN neurone pairs, reduced HFO synchrony in a pair of tremor cells concurrent with a reduction in firing rate and limb tremor, and decreased HFOs of local field potentials in the STN. These results demonstrate that HFO synchronization in the STN is reduced by voluntary movements and by exogenous dopaminergic medication. A mechanism for neuronal oscillatory synchronization in basal ganglia is proposed. It is suggested that the firing of STN neurones can be synchronized by 15-30 Hz cortical beta oscillatory activity, particularly when dopamine deficiency results in a higher background firing rate of STN neurones, and that this synchronization contributes to parkinsonian pathophysiology.

Movement-related changes in synchronization in the human basal ganglia

There is a wealth of data suggesting that behavioural events are reflected in the basal ganglia through phasic changes in the discharge of individual neurones. Here we investigate whether events are also reflected in momentary changes in the degree of synchronization between neuronal elements. We simultaneously recorded local potentials (LPs) from the subthalamic nucleus (STN) and/or ipsilateral globus pallidus interna (GPi) or scalp EEG during voluntary movements of a hand-held joystick in six awake patients following neurosurgery for Parkinson's disease. Without medication the power within the STN and the coherence between the STN and the GPi were dominated by activity with a frequency of <30 Hz. This coupling was attenuated by movement. In the presence of exogenous dopaminergic stimulation, power within the STN and coherence between the STN and the GPi was dominated by activity at 70-85 Hz, which increased with movement. The movement-related changes in coherence between the STN and EEG showed a similar pattern of pharmacological dependence, as seen subcortically. Movement-related frequency-specific changes in synchronization occur in the basal ganglia and extend to involve subcortico-cortical motor loops. The dynamic organization of activities in the frequency domain might provide a means for temporal co-ordination within and across different processing streams in the basal ganglia. This organization is critically dependent on the level of dopaminergic activity.

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.

Pathophysiology of nonparkinsonian tremors

Patients with nonparkinsonian tremors are the second largest group treated with functional neurosurgery. We summarize the present pathophysiological knowledge of these conditions. Essential tremor (ET) may be due to oscillations within the olivocerebellar circuit. There is experimental evidence from animal models for such a mechanism, and clinical data indicate an abnormal function of the cerebellum in ET. Cerebellar tremor may be closely related to the tremor seen in advanced ET. The malfunction of the cerebellum causes a pathological feed-forward control. Additionally an oscillator within the cerebellum or its input/output pathways may cause cerebellar tremor. Almost nothing is known about the pathophysiology of dystonic tremor. Holmes tremor is based on a nigral and a cerebellar malfunction and presents clinically as the combination of tremor in Parkinson's disease and cerebellar tremor. Neuropathic tremor can be extremely disabling and is thought to be due to an abnormal interaction of the disturbances within the periphery and abnormal cerebellar feedback. Unlike the case of Parkinson's disease, functional neurosurgery of nonparkinsonian tremors is not yet based on a solid pathophysiological background.

From single extracellular unit recording in experimental and human Parkinsonism to the development of a functional concept of the role played by the basal ganglia in motor control

Parkinson's disease (PD) is a progressive neurodegenerative disorder that affects the whole basal ganglia (BG). Various techniques have been used to study BG physiology and pathophysiology. Among these, extracellular single unit recording remains of particular importance. An impressive number of studies of BG electrophysiological activity have been carried out, both in non-human and in human primates, but the data collected show many omissions and disparities. BG activity has been well defined in the physiological situation, but remains far from clear in the Parkinsonian and virtually unexplored in the dopamine (DA)-replacement situation. This paper provides a brief synopsis of (i) recording techniques and (ii) BG electrophysiological activity in normal, Parkinsonian, and dopamine-replacement situations. We have restricted the data used to those obtained in BG structures of human and non-human primates. Only single unit recordings have been reported and four electrophysiological characteristics retained: mean firing frequency, firing pattern, periodic oscillation, and response to both passive and active movement. We have attempted to summarize (i) the commonly accepted characteristics of each BG structure in the three situations, (ii) discrepancies that exist, and (iii) missing elements. Then, the main successive theories aimed to explain the role played by BG in motor control are presented and discussed in the light of the most recently obtained results using the latest technological advances.

Regulation of the timing and pattern of action potential generation in rat subthalamic neurons in vitro by GABA-A IPSPs

The regulation of activity in the subthalamic nucleus (STN) by GABAergic inhibition from the reciprocally connected globus pallidus (GP) plays an important role in normal movement and disorders of movement. To determine the precise manner in which GABAergic synaptic input, acting at A-type receptors, influences the firing of STN neurons, we recorded the response of STN neurons to GABA-A inhibitory postsynaptic potentials (IPSPs) that were evoked by supramaximal electrical stimulation of the internal capsule using the perforated-patch technique in slices at 37 degrees C. The mean equilibrium potential of the GABA-A IPSP (EGABA-A IPSP) was -79.4 +/- 7.0 mV. Single IPSPs disrupted the spontaneous oscillation that underlies rhythmic single-spike firing in STN neurons. As the magnitude of IPSPs increased, the effectiveness of prolonging the interspike interval was related more strongly to the phase of the oscillation at which the IPSP was evoked. Thus the largest IPSPs tended to reset the oscillatory cycle, whereas the smallest IPSPs tended to produce relatively phase-independent delays in firing. Multiple IPSPs were evoked at various frequencies and over different periods and their impact was studied on STN neurons held at different levels of polarization. Multiple IPSPs reduced and/or prevented action potential generation and/or produced sufficient hyperpolarization to activate a rebound depolarization, which generated a single spike or restored rhythmic spiking and/or generated a burst of activity. The pattern of IPSPs and the level of polarization of STN neurons were critical in determining the nature of the response. The duration of bursts varied from 20 ms to several hundred milliseconds, depending on the intrinsic rebound properties of the postsynaptic neuron. These data demonstrate that inhibitory input from the GP can produce a range of firing patterns in STN neurons, depending on the number and frequencies of IPSPs and the membrane properties and voltage of the postsynaptic neuron.

Enhanced food-related motivation after bilateral lesions of the subthalamic nucleus

Although inactivation of the subthalamic nucleus (STN) has beneficial effects on motor symptoms of parkinsonism, little is known of possible actions on nonmotor symptoms of cognition or mood. Here, we used several forms of converging evidence to show that STN lesions can enhance behavioral motivation. Thus, bilateral fiber-sparing lesions of the STN in rats reduced the time required to eat a standard number of food reward pellets, without affecting food intake, and altered performance on a number of behavioral measures consistent with enhanced motivation for food. Thus, STN-lesioned rats showed greater levels of locomotor activity conditioned to food presentation, enhanced control over responding by food-related conditioned reinforcers, and a higher breaking point associated with elevated rate of lever press under a progressive ratio schedule of reinforcement. These results reveal a new functional role schedule for STN, possibly because of its involvement in ventral, as well as dorsal, striatal circuitry and are relevant to the therapeutic effects of STN stimulation in Parkinson's disease.

Effects of transient focal inactivation of the basal ganglia in parkinsonian primates

Ablative and chronic stimulation procedures targeting the internal pallidum (GPi) and the subthalamic nucleus (STN) have led to major advancements in the treatment of Parkinson's disease and other movement disorders. Although these procedures have evolved to primarily target the posterior ventrolateral sensorimotor portion of GPi and to less selectively target STN, centrally, the ideal targets within these structures remain to be fully established. In this study, we sought to identify the optimal targeting sites in GPi and STN for reversal of parkinsonian signs through a series of reversible injections of the GABA(A) agonist muscimol in these nuclei in parkinsonian primates. Akinesia and bradykinesia were strongly ameliorated by discrete inactivation within the centromedial extent of the sensorimotor territory in GPi and the lateral portion of the sensorimotor territory in STN. This suggests that akinesia and bradykinesia might, in fact, originate from abnormalities in the same, or at least overlapping, motor circuits in the parkinsonian state. Inactivation of areas outside of the motor territories did not improve parkinsonism but induced circling and behavioral abnormalities. The segregation of basal ganglia-thalamocortical circuits appears to be therefore maintained, at least to a large extent, in the parkinsonian state. These results underscore that inactivation of discrete regions in the central territory of GPi and the lateral portion of STN are sufficient to ameliorate parkinsonian motor signs and that extension of lesions into nonmotor territories may be deleterious. Surgical outcomes might therefore be optimized by placing more discrete lesions and by restricting the extent of chronic stimulation.

Neurochemical organization of the human basal ganglia: anatomofunctional territories defined by the distributions of calcium-binding proteins and SMI-32

The distribution of the calcium-binding proteins calbindin-D28K (CB), parvalbumin (PV) and calretinin (CR), and of the nonphosphorylated neurofilament protein (with SMI-32) was investigated in the human basal ganglia to identify anatomofunctional territories. In the striatum, gradients of neuropil immunostaining define four major territories: The first (T1) includes all but the rostroventral half of the putamen and is characterized by enhanced matriceal PV and SMI-32 immunoreactivity (-ir). The second territory (T2) encompasses most part of the caudate nucleus (Cd) and rostral putamen (PuT), which show enhanced matriceal CB-ir. The third and fourth territories (T3 and T4) comprise rostroventral parts of Cd and PuT characterized by complementary patch/matrix distributions of CB- and CR-ir, and the accumbens nucleus (Acb), respectively. The latter is separated into lateral (prominently enhanced in CB-ir) and medial (prominently enhanced in CR-ir) subdivisions. In the pallidum, parallel gradients also delimit four territories, T1 in the caudal half of external (GPe) and internal (GPi) divisions, characterized by enhanced PV- and SMI-32-ir; T2 in their rostral half, characterized by enhanced CB-ir; and T3 and T4 in their rostroventral pole and in the subpallidal area, respectively, both expressing CB- and CR-ir but with different intensities. The subthalamic nucleus (STh) shows contrasting patterns of dense PV-ir (sparing only the most medial part) and low CB-ir. Expression of CR-ir is relatively low, except in the medial, low PV-ir, part of the nucleus, whereas SMI-32-ir is moderate across the whole nucleus. The substantia nigra is characterized by complementary patterns of high neuropil CB- and SMI-32-ir in pars reticulata (SNr) and high CR-ir in pars compacta (SNc) and in the ventral tegmental area (VTA). The compartmentalization of calcium-binding proteins and SMI-32 in the human basal ganglia, in particular in the striatum and pallidum, delimits anatomofunctional territories that are of significance for functional imaging studies and target selection in stereotactic neurosurgery.

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.

Activation of group I metabotropic glutamate receptors produces a direct excitation and disinhibition of GABAergic projection neurons in the substantia nigra pars reticulata

A pathological increase in excitatory glutamatergic input to substantia nigra pars reticulata (SNr) from the subthalamic nucleus (STN) is believed to play a key role in the pathophysiology of Parkinson's disease. We present an analysis of the physiological roles that group I metabotropic glutamate receptors (mGluRs) play in regulating SNr functions. Immunocytochemical analysis at the light and electron microscopic levels reveal that both mGuR1a and mGluR5 are localized postsynaptically in the SNr. Consistent with this, activation of group I mGluRs depolarizes SNr GABAergic neurons. Interestingly, although both group I mGluRs (mGluR1 and mGluR5) are expressed in these neurons, the effect is mediated solely by mGluR1. Light presynaptic staining for mGluR1a and mGluR5 was also observed in some terminals forming symmetric synapses and in small unmyelinated axons. Consistent with this, activation of presynaptic mGluR1a and mGluR5 decreases inhibitory transmission in the SNr. The combination of direct excitatory effects and disinhibition induced by activation of group I mGluRs could lead to a large excitation of SNr projection neurons. This suggests that group I mGluRs are likely to play an important role in the powerful excitatory control that the STN exerts on basal ganglia output neurons.

Excitatory postsynaptic potentials trigger a plateau potential in rat subthalamic neurons at hyperpolarized states

The subthalamic nucleus (STN) directly innervates the output structures of the basal ganglia, playing a key role in basal ganglia function. It is therefore important to understand the regulatory mechanisms for the activity of STN neurons. In the present study, we aimed to investigate how the intrinsic membrane properties of STN neurons interact with their synaptic inputs, focusing on their generation and the properties of the long-lasting, plateau potential. Whole cell recordings were obtained from STN neurons in slices prepared from postnatal day 14 (P14) to P20 rats. We found that activation of glutamate receptor-mediated excitatory synaptic potentials (EPSPs) evoked a plateau potential in a subpopulation of STN neurons (n = 13/22), in a voltage-dependent manner. Plateau potentials could be induced only when the cell was hyperpolarized to more negative than about -75 mV. Plateau potentials, evoked with a depolarizing current pulse, again only from a hyperpolarized state, were observed in about half of STN neurons tested (n = 162/327). Only in neurons in which a plateau potential could be evoked by current injection did EPSPs evoke plateau potentials. L-type Ca(2+) channels, Ca(2+)-dependent K(+) channels, and TEA-sensitive K(+) channels were found to be involved in the generation of the potential. The stability of the plateau potential, tested by the injection of a negative pulse current during the plateau phase, was found to be robust at the early phase of the potential, but decreased toward the end. As a result the early part of the plateau potential was resistant to membrane potential perturbations and would be able to support a train of action potentials. We conclude that excitatory postsynaptic potentials, evoked in a subpopulation of STN neurons at a hyperpolarized state, activate L-type Ca(2+) and other channels, leading to the generation of a plateau potential. Thus about half of STN neurons can transform short-lasting synaptic excitation into a long train of output spikes by voltage-dependent generation of a plateau potential.

Anatomy, physiology, and pharmacology of the basal ganglia

The basal ganglia consist of five interconnected nuclei in the basal forebrain that influence cortical control of voluntary movement. Synaptic information travels through the basal ganglia using distinct pathways from the input structure, the striatum, to the output nuclei, the substantia nigra pars reticulata and the globus pallidus internal segment. The activity of the striatal output pathways is influenced by glutamatergic input from the cerebral cortex, dopaminergic input from the substantia nigra pars compacta, and cholinergic interneurons. Since the basal ganglia output nuclei tonically inhibit the motor nuclei of the thalamus, the basal ganglia facilitate motor activity by disinhibiting the thalamus.

A pulsed neural network model of bursting in the basal ganglia

We present new techniques for extending the functionality of spiking neurons which allow the incorporation of several aspects of neuron function previously confined to the domain of low level ion-channel based models. These aspects include spontaneous (or endogenous) firing, the complex interaction of multiple ion-species and the spatial distribution of synaptic contacts over the cell membrane. These ideas are applied to a neural circuit consisting of the cortex and a subset of the nuclei in the basal ganglia-the subthalamic nucleus (STN) and the external segment of the globus pallidus (GPe). This circuit has been studied extensively in vitro by Plenz and Kitai [Plenz, D., & Kitai, S. T. (1999). A basal ganglia pacemaker formed by the subthalamic nucleus and external globus pallidus. Nature, 400 677-682] whose data we use to constrain our model. With respect to this circuit, we have obtained three main results. First, that its characteristic burst firing is due to a Ca2+ current mediated mechanism. Second, that noise can assist in the generation of bursting and, paradoxically, stabilise the network behaviour under synaptic weight variations. Third, that a variety of dendritic processing is necessary in order to obtain the full range of bursting behaviour.

Regulation of limbic information outflow by the subthalamic nucleus: excitatory amino acid projections to the ventral pallidum

The subthalamic nucleus (STN), a component of the basal ganglia motor system, sends an excitatory amino acid (EAA)-containing projection to the ventral pallidum (VP), a major limbic system output region. The VP contains both NMDA and AMPA subtypes of EAA receptors. To characterize the physiology of the subthalamic pathway to the VP, and to determine the influence of EAA receptor subtypes, in vivo intracellular recordings, and in vivo extracellular recordings combined with microiontophoresis, were made from VP neurons in anesthetized rats. Of the intracellularly recorded neurons, 86% responded to STN stimulation, and these displayed EPSPs with an onset of 8.7 msec, consistent with a monosynaptic input. The EPSPs evoked in spontaneously firing neurons were nearly twice the amplitude of those in nonfiring cells (13.1 vs 6.8 mV, respectively). As neurons were depolarized by current injection, the latency for spiking decreased from 24.2 to 14.2 msec, although EPSP latency was unaffected. Eighty-seven percent of the extracellularly recorded VP neurons responded to STN stimulation with a rapid and robust enhancement of spiking; the response onset, like the EPSP onset, equaled 8.7 msec. Firing rate was enhanced by NMDA in 94% of the STN-excited cells, and AMPA increased firing in 94% as well. The NMDA-selective antagonist AP-5 attenuated 67% of the STN-evoked excitatory responses, and the AMPA-selective antagonist CNQX attenuated 52%. Both antagonists attenuated 33% of responses, and 78% were attenuated by at least one. This evidence suggests that a great majority of VP neurons are directly influenced by STN activation and that both NMDA and non-NMDA receptors are involved. Moreover, the VP response to STN stimulation appears to be strongly dependent on the depolarization state of the neuron.

Open interconnected model of basal ganglia-thalamocortical circuitry and its relevance to the clinical syndrome of Huntington's disease

The early stages of Huntington's disease (HD) present with motor, cognitive, and emotional symptoms. Correspondingly, current models implicate dysfunction of the motor, associative, and limbic basal ganglia-thalamocortical circuits. Available data, however, indicate that in the early stages of the disease, striatal damage is mainly restricted to the associative striatum. Based on an open interconnected model of basal ganglia-thalamocortical organization, we provide a detailed account of the mechanisms by which associative striatal pathology may lead to the complex pattern of motor, cognitive, and emotional symptoms of early HD. According to this account, the degeneration of a direct and several indirect pathways arising from the associative striatum leads to impaired functioning of: (1) the motor circuit, resulting in chorea and bradykinesia, (2) the associative circuit, resulting in abnormal eye movements, "frontal-like" cognitive deficits and "cognitive disinhibition," and (3) the limbic circuit, resulting in affective and psychiatric symptoms. When relevant, this analysis is aided by comparing the symptomatology of HD patients to that of patients with mild to moderate Parkinson's disease, since in the latter there is similar dysfunction of direct pathways but opposite dysfunction of indirect pathways. Finally, we suggest a potential novel treatment of HD and provide supportive evidence from a rat model of the disease.