Responses of substantia nigra pars reticulata and globus pallidus complex to high frequency stimulation of the subthalamic nucleus in rats: electrophysiological data

Effect of subthalamic nucleus stimulation patterns on motor performance in Parkinson's disease

We studied different patterns of deep brain stimulation (DBS), but same average rate, in seven Parkinson disease patients performing a wrist flexion/extension task. Movement times were shorter with regular (continuous) stimulation than cycled 'on' and 'off' for 0.1s (mean difference 0.129 s; 95% confidence interval [CI], 0.228-0.029 s; P < 0.007) and tended to be shorter than cycling at 0.5s (mean difference 0.076 s, 95% CI, 0.171 to -0.020 s; P < 0.6). Movement times under stimulation cycling at 0.5 s tended to be shorter than cycling at 0.1 s (mean difference, 0.083 s, 95% CI, 0.188 to -0.022 s, P < 0.12). Any therapeutic mechanisms of action of DBS must account for patterns of stimulation.

Subthalamic stimulation activates internal pallidus: evidence from cGMP microdialysis in PD patients

Parkinson's disease patients benefit from deep brain stimulation (DBS) in subthalamic nucleus (STN), but the basis for this effect is still disputed. In this intraoperative microdialysis study, we found elevated cGMP extracellular concentrations in the internal segment of the globus pallidus, despite negligible changes in glutamate levels, during a clinically effective STN-DBS. This supports the view that a clinically beneficial effect of STN-DBS is paralleled by an augmentation (and not an inactivation) of the STN output onto the GPi.

Therapeutic electrical stimulation of the central nervous system

The electrical effects on the nervous system have been known for long. The excitatory effect has been used for diagnostic purposes or even for therapeutic applications, like in pain using low-frequency stimulation of the spinal cord or of the thalamus. The discovery that High-Frequency Stimulation (HFS) mimics the effect of lesioning has opened a new field of therapeutic application of electrical stimulation in all places where lesion of neuronal structures, such as nuclei of the basal ganglia, had proven some therapeutic efficiency. This was first applied to the thalamus to mimic thalamotomy for the treatment of tremor, then to the subthalamic nucleus and the pallidum to treat some advanced forms of Parkinson's disease and control not only the tremor but also akinesia, rigidity and dyskinesias. The field of application is increasingly growing, currently encompassing dystonias, epilepsy, obsessive compulsive disease, cluster headaches, and experimental approaches are being made in the field of obesity and food intake control. Although the effects of stimulation are clear-cut and the therapeutic benefit is clearly recognized, the mechanism of action of HFS is not yet understood. The similarity between HFS and the effect of lesions in several places of the brain suggests that this might induce an inhibition-like process, which is difficult to explain with the classical concept of physiology where electrical stimulation means excitation of neural elements. The current data coming from either clinical or experimental observations are providing elements to shape a beginning of an understanding. Intra-cerebral recordings in human patients with artefact suppression tend to show the arrest of electrical firing in the recorded places. Animal experiments, either in vitro or in vivo, show complex patterns mixing inhibitory effects and frequency stimulation induced bursting activity, which would suggest that the mechanism is based upon the jamming of the neuronal message, which is by this way functionally suppressed. More recent data from in vitro biological studies show that HFS profoundly affects the cellular functioning and particularly the protein synthesis, suggesting that it could alter the synaptic transmission by reducing the production of neurotransmitters. It is now clear that this method has a larger field of application than currently known and that its therapeutical applications will benefit to several diseases of the nervous system. The understanding of the mechanism has opened a new field of research, which will call for reappraisal of the basic effects of electricity on the living tissues.

Monopolar versus bipolar high frequency stimulation in the rat subthalamic nucleus: differences in histological damage

The aim of the present study was to determine the effects of monopolar and bipolar high frequency stimulation (HFS) on histological damage and current flow using a commonly applied stimulus amplitude (300 microA). Bipolar HFS resulted in a large amount of histological damage whereas with monopolar HFS no damage was observed except for the electrode trajectory. Oscilloscopic readings confirmed that this was due to the application of twice as much current to the target with bipolar HFS. Our results demonstrate that there are differences in tissue damage dependent of polarity. In order to create a better comparison to the clinical condition, we suggest that the present rodent models for studying the effect of chronic HFS require further adjustment. This can be achieved by decreasing the present current densities to a level comparable to the human situation.

Complex locking rather than complete cessation of neuronal activity in the globus pallidus of a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated primate in response to pallidal microstimulation

High-frequency stimulation of the globus pallidus (GP) has emerged as a successful tool for treating Parkinson's disease and other motor disorders. However, the mechanism governing its therapeutic effect is still under debate. To shed light on the basic mechanism of deep brain stimulation (DBS), we performed microstimulation in the GP of a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated monkey while recording with other microelectrodes in the same nucleus. We used robust methods to reduce the stimulus artifact, and 600-3000 repetitions of a single stimulus and of high-frequency short trains (10-40 stimuli), enabling high temporal resolution analysis of neural responses. Low-frequency stimulation yielded a typical three-stage response: short-term (2-3 msec duration) activity, followed by mid-term (15-25 msec) inhibition, and occasionally longer-term (30-40 msec) excitation. Trains of high-frequency stimuli elicited complex locking of the response to the stimuli in most neurons. The locking displayed a stereotypic temporal structure consisting of three short-duration (1-2 msec) phases: an initial (mean latency = 2.9 msec) excitation followed by an inhibition (4.6 msec) and a second excitation (6.3 msec). The change in the mean firing rate was mixed; the majority of the neurons displayed partial inhibition during the stimulus train. Slow inhibitory and excitatory multiphase changes in the firing rate were observed after the stimulus trains. The activity of neurons recorded simultaneously displayed rate correlations but no spike-to-spike correlations. Our results suggest that the effect of DBS on the GP is not complete inhibition but rather a complex reshaping of the temporal structure of the neuronal activity within that nucleus.

Neurotransmitter release from high-frequency stimulation of the subthalamic nucleus

Object. High-frequency stimulation (HFS) delivered through implanted electrodes in the subthalamic nucleus (STN) has become an established treatment for Parkinson disease (PD). The precise mechanism of action of deep brain stimulation (DBS) in the STN is unknown, however. In the present study, the authors tested the hypothesis that HFS within the STN changes neuronal action potential firing rates during the stimulation period by modifying neurotransmitter release.

Methods. Intracellular electrophysiological recordings were obtained using sharp electrodes in rat STN neurons in an in vitro slice preparation. A concentric bipolar stimulating electrode was placed in the STN slice, and electrical stimulation (pulse width 50–100 µsec, duration 100–2000 µsec, amplitude 10–500 µA, and frequency 10–200 Hz) was delivered while simultaneously obtaining intracellular recordings from an STN neuron.

High-frequency stimulation of the STN either generated excitatory postsynaptic potentials (EPSPs) and increased the action potential frequency or it generated inhibitory postsynaptic potentials and decreased the action potential frequency of neurons within the STN. These effects were blocked after antagonists to glutamate and γ-aminobutyric acid were applied to the tissue slice, indicating that HFS resulted in the release of neurotransmitters. Intracellular recordings from substantia nigra pars compacta (SNc) dopaminergic neurons during HFS of the STN revealed increased generation of EPSPs and increased frequency of action potentials in SNc neurons.

Conclusions. During HFS of STN neurons the mechanism of DBS may involve the release of neurotransmitters rather than the primary electrogenic inhibition of neurons.

Deep brain stimulation

During the last decade deep brain stimulation (DBS) has become a routine method for the treatment of advanced Parkinson's disease (PD), leading to striking improvements in motor function and quality of life of PD patients. It is associated with minimal morbidity. The rationale of targeting specific structures within basal ganglia such as the subthalamic nucleus (STN) or the internal segment of the globus pallidus (GPi) is strongly supported by the current knowledge of the basal ganglia pathophysiology, which is derived from extensive experimental work and which provides the theoretical basis for surgical therapy in PD. In particular, the STN has advanced to the worldwide most used target for DBS in the treatment of PD, due to the marked improvement of all cardinal symptoms of the disease. Moreover on-period dyskinesias are reduced in parallel with a marked reduction of the equivalent daily levodopa dose following STN-DBS. The success of the therapy largely depends on the selection of the appropriate candidate patients and on the precise implantation of the stimulation electrode, which necessitates careful imaging-based pre-targeting and extensive electrophysiological exploration of the target area. Despite the clinical success of the therapy, the fundamental mechanisms of high-frequency stimulation are still not fully elucidated. There is a large amount of evidence from experimental and clinical data that stimulation frequency represents a key factor with respect to clinical effect of DBS. Interestingly, high-frequency stimulation mimics the functional effects of ablation in various brain structures. The main hypotheses for the mechanism of high-frequency stimulation are: (1) depolarization blocking of neuronal transmission through inactivation of voltage dependent ion-channels, (2) jamming of information by imposing an efferent stimulation-driven high-frequency pattern, (3) synaptic inhibition by stimulation of inhibitory afferents to the target nucleus, (4) synaptic failure by stimulation-induced neurotransmitter depletion. As the hyperactivity of the STN is considered a functional hallmark of PD and as there is experimental evidence for STN-mediated glutamatergic excitotoxicity on neurons of the substantia nigra pars compacta (SNc), STN-DBS might reduce glutamatergic drive, leading to neuroprotection. Further studies will be needed to elucidate if STN-DBS indeed provides a slow-down of disease progression.

Focal changes of oxygen consumption in cerebral cortex of patients with Parkinson's disease during subthalamic stimulation

Motor symptoms of Parkinson's disease (PD) are substantially improved by bilateral high-frequency electrical stimulation of the subthalamic nucleus (STN). Altered cerebral blood flow (CBF) in a network of frontal cortical and subcortical structures has been reported in numerous studies of patients undergoing subthalamic stimulation. However, CBF is a controversial indicator of brain activation because measures of blood flow bear a variable relation to measures of brain work and energy metabolism. We hypothesized that STN stimulation would alter the rate of oxygen consumption (CMRO(2)) in cerebral cortical areas in proportion to previously reported changes in CBF in patients undergoing stimulation at rest. We used quantitative PET to map CMRO(2) in brain of seven patients with Parkinson's disease, first in a baseline condition with pause of stimulation and medication for a period of 12 h, and again after 4 h of stimulation. Comparison of these two conditions revealed activation of CMRO(2) in the cerebellum, and in specific posterior neocortical regions, most notably in the left lingual gyrus and in the right lateral occipitotemporal gyrus, both of which latter regions are linked to higher-order visual processing. CMRO(2) was unaffected in the frontal cortex. Thus, the present findings do not support the original hypothesis, but suggest that STN stimulation increases energy metabolism in the posterior cerebral cortex, especially in regions involved in perception of movement and the direction of movement to visual cues.

Uncovering the mechanism(s) of action of deep brain stimulation: activation, inhibition, or both

High-frequency deep brain stimulation (DBS) of the thalamus or basal ganglia represents an effective clinical technique for the treatment of several medically refractory movement disorders. However, understanding of the mechanisms responsible for the therapeutic action of DBS remains elusive. The goal of this review is to address our present knowledge of the effects of high-frequency stimulation within the central nervous system and comment on the functional implications of this knowledge for uncovering the mechanism(s) of DBS. Four general hypotheses have been developed to explain the mechanism(s) of DBS: depolarization blockade, synaptic inhibition, synaptic depression, and stimulation-induced modulation of pathological network activity. Using the results from functional imaging, neurochemistry, neural recording, and neural modeling experiments we address the general hypotheses and attempt to reconcile what have been considered conflicting results from these different research modalities. Our analysis suggests stimulation-induced modulation of pathological network activity represents the most likely mechanism of DBS; however, several open questions remain to explicitly link the effects of DBS with therapeutic outcomes.

High frequency stimulation of the subthalamic nucleus eliminates pathological thalamic rhythmicity in a computational model

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) or the internal segment of the globus pallidus (GPi) has recently been recognized as an important form of intervention for alleviating motor symptoms associated with Parkinson's disease, but the mechanism underlying its effectiveness remains unknown. Using a computational model, this paper considers the hypothesis that DBS works by replacing pathologically rhythmic basal ganglia output with tonic, high frequency firing. In our simulations of parkinsonian conditions, rhythmic inhibition from GPi to the thalamus compromises the ability of thalamocortical relay (TC) cells to respond to depolarizing inputs, such as sensorimotor signals. High frequency stimulation of STN regularizes GPi firing, and this restores TC responsiveness, despite the increased frequency and amplitude of GPi inhibition to thalamus that result. We provide a mathematical phase plane analysis of the mechanisms that determine TC relay capabilities in normal, parkinsonian, and DBS states in a reduced model. This analysis highlights the differences in deinactivation of the low-threshold calcium T -current that we observe in TC cells in these different conditions. Alternative scenarios involving convergence of thalamic signals in the cortex are also discussed, and predictions associated with these results, including the occurrence of rhythmic rebound bursts in certain TC cells in parkinsonian states and their drastic reduction by DBS, are stated. These results demonstrate how DBS could work by increasing firing rates of target cells, rather than shutting them down.

Subthalamic high frequency stimulation induced rotations are differentially mediated by D1 and D2 receptors

High frequency stimulation (HFS) of the subthalamic nucleus (STN) has clinically emerged as a promising approach in the treatment of Parkinson's disease, epilepsy, dystonia as well as compulsive and possibly other mood disorders. The underlying mechanisms are incompletely understood, but are definitely related to high frequency and likely to involve the dopamine (DA)-system. To further test this hypothesis the present study investigated the modulation of STN-HFS-induced circling by systemic and intracerebral injection of drugs acting on DA receptors in naive freely moving rats. Within this experimental setup, unilateral STN-HFS alone induced intensity-dependent circling. Systemic injections of selective D1- (SCH-23390) and D2-((-)-sulpiride) antagonists as well as the mixed D1 and D2 agonist apomorphine dose-dependently reduced STN-HFS-induced rotational behavior. Intracerebral microinjections of (-)-sulpiride but not SCH-23390 decreased circling when injected intrastriatally and increased the number of rotations when injected intranigrally (pars reticulata (SNr)). These data reveal that STN-HFS-induced contralateral circling is differentially modulated by D1 and D2 receptors. While D2 receptor-mediated effects involve the dorso-/ventrolateral striatum and the SNr, D1 receptors probably exert their actions via brain areas outside the striatum and SNr. These findings suggest the nigrostriatal DA-system to be specifically involved in the mediation of STN-HFS-induced motor effects.

La stimulation cérébrale profonde dans la maladie de Parkinson

The present renewal of the surgical treatment of Parkinson's disease, almost abandoned for twenty Years, arises from two main reasons. The first is the better understanding of the functional organization of the basal ganglia. It was demonstrated in animal models of Parkinson's disease that the loss of dopaminergic neurons within the substantia nigra, at the origin of the striatal dopaminergic defect, induces an overactivity of the excitatory glutamatergic subthalamo-internal pallidum pathway. The decrease in this hyperactivity might lead to an improvement in the pakinsonian symptoms. The second reason is the improvement in stereotactic neurosurgery in relation with the progress in neuroimaging techniques and with intraoperative electrophysiological microrecordings and stimulations, which help determine the location of the deep brain targets. In the 1970s chronic deep brain stimulation in humans was applied to the sensory nucleus of the thalamus for the treatment of intractable pain. In 1987, Benabid and colleagues suggested high frequency stimulation of the ventral intermediate nucleus of the thalamus in order to treat drug-resistant tremors and to avoid the adverse effects of thalamotomies. How deep brain stimulation works is not well known but it has been hypothetized that it could change the neuronal activities and thus avoid disease-related abnormal neuronal discharges. Potential candidates for deep brain stimulation are selected according to exclusion and inclusion criteria. Surgery can be applied to patients in good general and mental health, neither depressive nor demented and who are severely disabled despite all available drug therapies but still responsive to levodopa. The first session of surgery consists in the location of the target by ventriculography and/or brain MRI. The electrodes are implanted during the second session. The last session consists in the implantation of the neurostimulator. The ventral intermediate nucleus of the thalamus was the first target in which chronic deep brain stimulation electrodes were implanted in order to alleviate tremor. This technique can be applied bilaterally without the adverse effects of bilateral thalamotomies. Like pallidotomy, internal globus pallidum stimulation has a dramatic beneficial effect on levodopa-induced dyskinesia but its effects on the parkinsonian triad are less constant and opposite motor effects are sometimes observed in relation with the stimulated contact. The inconstant results, perhaps related to the complexity of the structure led to the development of subthalamic nucleus stimulation. The alleviation of motor fluctuations and the improvement in all motor symptoms allows a significant decrease in levodopa daily dose and in levodopa-induced dyskinesia. Presently, deep brain stimulation is a fashionable neurosurgical technique to treat Parkinson's disease. Subthalamic nucleus stimulation seems to be the most suitable target to control the parkinsonian triad and the motor fluctuations. Because of the possible adverse effects it must be reserved for disabled parkinsonian patients. No large randomized study comparing different targets and different neurosurgical techniques has been performed yet. Such studies, including cost benefit studies would be useful to assess the respective value of these different techniques.

High-frequency stimulation of both zona incerta and subthalamic nucleus induces a similar normalization of basal ganglia metabolic activity in experimental parkinsonism

High-frequency stimulation (HFS) of the subthalamic nucleus (STN) alleviates dramatically motor symptoms in Parkinson's disease, and recently it has been suggested that zona incerta (ZI) stimulation might be as beneficial to patients. We used in situ cytochrome oxidase (CoI) mRNA hybridization to investigate and compare the effects of HFS of the STN and the ZI on metabolic activity of the STN, globus pallidus (GP), and substantia nigra reticulata (SNr) in normal rats as well as in rats with 6-hydroxydopamine (6-OHDA) lesion, an animal model of Parkinson's disease. In normal rats, HFS of the STN, as well as of the ZI, induced a significant decrease in CoI mRNA expression within the STN and SNr but an increase within the GP. In 6-OHDA rats, HFS of the STN reversed dopamine denervation-induced changes in the expression of CoI mRNA in the STN, SNr, and GP. Similar results were obtained with HFS of the ZI except for the STN, which showed only a trend toward normalization. These data suggest that the ZI, as well as the STN, are implicated in the functional mechanism of HFS supporting the involvement of GABA transmission for the reduction of neuronal activity in the basal ganglia output structures.

Cellular effects of deep brain stimulation: model-based analysis of activation and inhibition

Deep brain stimulation (DBS) is an effective therapy for medically refractory movement disorders. However, fundamental questions remain about the effects of DBS on neurons surrounding the electrode. Experimental studies have produced apparently contradictory results showing suppression of activity in the stimulated nucleus, but increased inputs to projection nuclei. We hypothesized that cell body firing does not accurately reflect the efferent output of neurons stimulated with high-frequency extracellular pulses, and that this decoupling of somatic and axonal activity explains the paradoxical experimental results. We studied stimulation using the combination of a finite-element model of the clinical DBS electrode and a multicompartment cable model of a thalamocortical (TC) relay neuron. Both the electric potentials generated by the electrode and a distribution of excitatory and inhibitory trans-synaptic inputs induced by stimulation of presynaptic terminals were applied to the TC relay neuron. The response of the neuron to DBS was primarily dependent on the position and orientation of the axon with respect to the electrode and the stimulation parameters. Stimulation subthreshold for direct activation of TC relay neurons caused suppression of intrinsic firing (tonic or burst) activity during the stimulus train mediated by activation of presynaptic terminals. Suprathreshold stimulation caused suppression of intrinsic firing in the soma, but generated efferent output at the stimulus frequency in the axon. This independence of firing in the cell body and axon resolves the apparently contradictory experimental results on the effects of DBS. In turn, the results of this study support the hypothesis of stimulation-induced modulation of pathological network activity as a therapeutic mechanism of DBS.

Effects of subthalamic nucleus stimulation on the micturation reflex in cats

High frequency stimulation (HFS) of the subthalamic nucleus (STN) has been performed to reverse motor dysfunction in severe parkinsonian patients. Recent studies suggested that neural circuitry in the basal ganglia might regulate micturition function as well. In 15 adult male cats under ketamine anesthesia, in which spontaneous isovolumetric micturition reflex had been generated, we performed electrical stimulation and extracellular single unit recording in the STN. Electrical stimulation applied in the STN elicited inhibition of the micturition reflex. None of the responses was facilitatory. Effective amplitude of the electrical stimulation for evoking inhibitory responses was less than 50 microA, which gradually increased and exceeded 250 microA as the location of the stimulation exceeded an area of the STN. Effective frequency of the electrical stimulation with given stimulus intensity was 50 Hz and higher. Total 10 neurons were recorded in the STN that were related to urinary storage/micturition cycles. All neurons were tonically active throughout storage/micturition cycles with storage phase predominance, with almost constant firing activities during the storage phase. In conclusion, our results showed that HFS-STN inhibited the micturition reflex and there were micturition-related neuronal firings in the STN in cats, suggesting the STN may be involved in neural control of micturition. The results also provide an implication that clinical HFS-STN may alter urinary function in parkinsonian patients.

Neurochemical Mechanisms Induced by High Frequency Stimulation of the Subthalamic Nucleus: Increase of Extracellular Striatal Glutamate and GABA in Normal and Hemiparkinsonian Rats

High frequency stimulation (HFS) (130 Hz) of the subthalamic nucleus (STN) provides beneficial effects in patients suffering from severe parkinsonism, but the mechanisms underlying these clinical results remain to be clarified. To date, very little is known concerning the effects of STN-HFS on neurochemical transmission in the different basal ganglia nuclei and in particular the striatum. This study examines the effects of STN-HFS in intact and hemiparkinsonian rats on extracellular striatal glutamate (Glu) and GABA levels by means of intracerebral microdialysis. Unilateral STN-HFS was found to induce a significant bilateral increase of striatal Glu and GABA both in intact and in dopamine-lesioned animals. In intact rats, these increases were reversed by local administration of the D1 antagonist SCH 23390, but were potentiated by the D2 antagonist sulpiride. Potentiation was also observed after local administration of both D1 and D2 antagonists whose amplitude was similar to that measured in hemiparkinsonian rats. These data furnish the first evidence that STN-HFS influences striatal amino-acid transmission and that this influence is modulated by dopamine. They provide evidence that the effects of STN-HFS are not only restricted to the direct STN targets, but also involve adaptive changes within other structures of the basal ganglia circuitry.

Deep brain stimulation for Parkinson’s disease

Deep brain stimulation at high frequency was first used in 1997 to replace thalamotomy in treating the characteristic tremor of Parkinson's disease, and has subsequently been applied to the pallidum and the subthalamic nucleus. The subthalamic nucleus is a key node in the functional control of motor activity in the basal ganglia. Its inhibition suppresses symptoms in animal models of Parkinson's disease, and high frequency chronic stimulation does the same in human patients. Acute and long-term results after deep brain stimulation show a dramatic and stable improvement of a patient's clinical condition, which mimics the effects of levodopa treatment. The mechanism of action may involve a functional disruption of the abnormal neural messages associated with the disease. Long-term changes, neural plasticity and neural protection might be induced in the network. Similar effects of stimulation and lesioning have led to the extension of this technique for other targets and diseases.

Spontaneous and evoked activity of substantia nigra pars reticulata neurons during high-frequency stimulation of the subthalamic nucleus

The subthalamic nucleus (STN), a major component of the basal ganglia, exerts an excitatory influence on the output structures of this system i.e., the substantia nigra pars reticulata (SNR) and the internal segment of the globus pallidus. High-frequency stimulation of the STN is a method currently used to treat parkinsonian symptoms. The aim of the present study was to analyze the effects of STN high-frequency stimulation on the activity of SNR neurons and to investigate its impact on the transfer of information between the cerebral cortex and the SNR. During STN high-frequency stimulation, the activity of SNR cells was decreased at low-intensity stimulation, whereas it was increased at a higher intensity. The decrease in the discharge of SNR cells likely results from the activation of a GABAergic transmission in the SNR because this effect was blocked by local application of bicuculline. The increased activity likely results from the activation of the glutamatergic subthalamonigral projection because the latency of the evoked excitations was consistent with the conduction time of the subthalamonigral neurons. Finally, during STN high-frequency stimulation, the transmission of cortical information along the direct trans-striatal pathway was preserved, whereas the functionality of the trans-subthalamic pathways was partly preserved or completely blocked depending on the stimulation intensity. The present data indicate that STN high-frequency stimulation influences the activity of SNR cells through activation of their excitatory and inhibitory synaptic afferent pathways as well as antidromic activation of the projection neurons.

Synaptic plasticity in rat subthalamic nucleus induced by high-frequency stimulation

The technique of deep brain stimulation (DBS) has become a preferred surgical choice for the treatment of advanced Parkinson's disease. The subthalamic nucleus (STN) is presently the most promising target for such DBS. In this study, whole-cell patch-clamp recordings were made from 46 STN neurons in rat brain slices to examine the effect of high-frequency stimulation (HFS) of the STN on glutamatergic synaptic transmission in STN neurons. HFS, consisting of trains of stimuli at a frequency of 100 Hz for 1 min, produced three types of synaptic plasticity in 17 STN neurons. First, HFS of the STN induced short-term potentiation (STP) of evoked postsynaptic current (EPSC) amplitude in four neurons. STP was associated with a reduction in the EPSC paired-pulse ratio, suggesting a presynaptic site of action. Second, HFS of the STN generated long-term potentiation (LTP) of EPSC amplitude in eight neurons. Although the EPSC paired-pulse ratio was reduced transiently in the first 2 min following HFS, ratios measured 6-20 min after HFS were unchanged from control. This suggests that LTP is maintained by a postsynaptic mechanism. Third, HFS produced long-term depression (LTD) of EPSC amplitude in five STN neurons. LTD was associated with a significant increase in EPSC paired-pulse ratios, indicating a presynaptic site of action. These results suggest that HFS can produce long-term changes in the efficacy of synaptic transmission in the STN. HFS-induced synaptic plasticity might be one mechanism underlying the effectiveness of DBS in the STN as a treatment of advanced Parkinson's disease.

Electrophysiological and metabolic evidence that high‐frequency stimulation of the subthalamic nucleus bridles neuronal activity in the subthalamic nucleus and the substantia nigra reticulata

High-frequency stimulation (HFS) of the subthalamic nucleus (STN) has been shown to produce a dramatic alleviation of motor symptoms in patients with advanced Parkinson's disease. Its functional mechanism, however, remains obscure. We used extracellular recording and in situ cytochrome oxidase (CoI) mRNA hybridization to investigate the effects of HFS of the STN on neuronal activity of the STN and the substantia nigra reticulata (SNr) in normal rats and rats with 6-hydroxydopamine (6-OHDA) lesion of the substantia nigra compacta (SNc). To allow detection of spikes and analysis of firing activity, artifacts recorded during stimulation were scaled down using a template subtraction method. In both normal and lesioned rats, the activity of a majority of STN neurons was inhibited during stimulation. In the SNr, HFS also induced an inhibition of the activity of a majority of neurons in normal and lesioned rats. In situ hybridization histochemistry confirmed these results in that it showed a significant decrease in levels of CoI mRNA expression in the STN and SNr in both normal and lesioned rats during stimulation. These data afford an interesting insight into the functional mechanism of deep brain stimulation and support the hypothesis that HFS exerts an inhibitory influence on STN neuronal firing.

High frequency stimulation of the subthalamic nucleus improves treadmill locomotion in unilateral 6-hydroxydopamine lesioned rats

This study investigated the influence of electrical stimulation of the subthalamic nucleus (STN) on motor impairment induced by unilateral 6-hydroxydopamine (6-OHDA) lesions in the medial forebrain bundle. Rats were trained to walk on a treadmill and then implanted with microelectrode arrays in and near the STN. The neurotoxin 6-OHDA was injected into the medial forebrain bundle (MFB) unilaterally to produce a targeted lesion of the dopaminergic system. Successful lesions produced impaired treadmill walking behavior. High frequency stimulation (HFS) of the STN improved treadmill walking immediately and restored normal walking patterns. The same HFS failed to evoke visible side effects such as stepping, turning, raising of the head or facial muscle contraction in the absence of treadmill movement, or to change rotational behaviors elicited by the dopamine (DA) agonist apomorphine in unilateral lesioned rats. This suggests that the stimulation did not cause movement by an activation of brainstem locomotor regions or an increase attention leading to movement. Apomorphine-induced rotation may represent an imbalance of dopaminergic activation which remains during HFS. This work may provide a rodent model for deep brain stimulation (DBS) in patients with Parkinson's disease, and be suitable for further investigation of the neural mechanisms underlying the therapeutic effects of DBS.

High frequency stimulation of the subthalamic nucleus has beneficial antiparkinsonian effects on motor functions in rats, but less efficiency in a choice reaction time task

Chronic subthalamic nucleus high frequency stimulation (STN HFS) improves motor function in Parkinson's disease. However, its efficacy on cognitive function and the mechanisms involved are less known. The aim of this study was to assess the effects of STN HFS in hemiparkinsonian awake rats performing different specific motor tests and a cognitive operant task. Unilateral STN HFS applied in unilaterally DA-depleted rats decreased the apomorphine-induced circling behaviour and reduced catalepsy induced by the neuroleptic haloperidol. DA-depleted rats exhibited severe deficits in the operant task, among which the inability to perform the task was not alleviated by STN HFS. However, in a few animals showing less impairment, STN HFS significantly reduced the contralateral neglect induced by the lesion. These results are the first to demonstrate a beneficial effect of STN HFS applied in awake rats on basic motor functions. However, STN HFS appears to be less effective on impaired cognitive functions.

Subthalamic deep brain stimulation does not induce striatal dopamine release in Parkinson's disease

Deep brain stimulation of the subthalamic nucleus (STN) is an increasingly prevalent treatment for advanced Parkinson's disease (PD). Its main mechanism of action is thought to be a reduction in the inhibitory outflow from basal ganglia to cerebral cortex. However, recent animal experiments have led to the suggestion that high frequency stimulation of the STN also acts by promoting dopamine release. We tested this hypothesis by performing [11C]raclopride PET on and off stimulation in six patients with PD and implanted STN stimulators. There was no difference in tracer binding in the striatum between the two testing conditions. We conclude that high frequency stimulation of the STN does not act by increasing dopamine release.

Presynaptic inhibition of synaptic transmission by adenosine in rat subthalamic nucleus in vitro

Whole-cell patch clamp recordings were made from the subthalamic nucleus in rat brain slice preparations to examine the effect of adenosine on inhibitory and excitatory synaptic transmission. Adenosine reversibly inhibited both GABA-mediated inhibitory and glutamate-mediated excitatory postsynaptic currents. Adenosine at 100 microM reduced the amplitude of inhibitory and excitatory postsynaptic currents by 42+/-5% and 34+/-6%, respectively. Reductions in the amplitude of both inhibitory and excitatory postsynaptic currents were accompanied by increases in paired-pulse ratios. In addition, adenosine decreased the frequency of spontaneous miniature excitatory postsynaptic currents but had no effect on their amplitude. These results are consistent with a presynaptic site of action. The adenosine A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine completely reversed the adenosine-induced attenuation of inhibitory and excitatory postsynaptic currents, but 8-cyclopentyl-1,3-dipropylxanthine alone had no effect on synaptic currents evoked at 0.1 Hz. However, 8-cyclopentyl-1,3-dipropylxanthine inhibited a time-dependent depression of excitatory postsynaptic currents that was normally observed in response to a 5 Hz train of stimuli, suggesting that endogenous adenosine could be released during higher frequencies of stimulation. These results suggest that adenosine inhibits synaptic release of GABA and glutamate by stimulation of presynaptic A(1) receptors in the subthalamic nucleus.

Long-term electrical capsular stimulation in patients with obsessive-compulsive disorder

OBJECTIVE

Because of the irreversibility of lesioning procedures and their possible side effects, we studied the efficacy of replacing bilateral anterior capsulotomy with chronic electrical capsular stimulation in patients with severe, long-standing, treatment-resistant obsessive-compulsive disorder.

METHODS

We stereotactically implanted quadripolar electrodes in both anterior limbs of the internal capsules into six patients with severe obsessive-compulsive disorder. Psychiatrists and psychologists performed a double-blind clinical assessment. A blinded random crossover design was used to assess four of those patients, who underwent continuous stimulation thereafter.

RESULTS

The psychiatrist-rated Yale-Brown Obsessive Compulsive Scale score was lower in the stimulation-on condition (mean, 19.8 +/- 8.0) than in the postoperative stimulator-off condition (mean, 32.3 +/- 3.9), and this stimulation-induced effect was maintained for at least 21 months after surgery. The Clinical Global Severity score decreased from 5 (severe; standard deviation, 0) in the stimulation-off condition to 3.3 (moderate to moderate-severe; standard deviation, 0.96) in the stimulation-on condition. The Clinical Global Improvement scores were unchanged in one patient and much improved in the other three during stimulation. During the stimulation-off period, symptom severity approached baseline levels in the four patients. Bilateral stimulation led to increased signal on functional magnetic resonance imaging studies, especially in the pons. Digital subtraction analysis of preoperative [(18)F]2-fluoro-2-deoxy-d-glucose positron emission tomographic scans and positron emission tomographic scans obtained after 3 months of stimulation showed decreased frontal metabolism during stimulation.

CONCLUSION

These observations indicate that capsular stimulation reduces core symptoms 21 months after surgery in patients with severe, long-standing, treatment-refractory obsessive-compulsive disorder. The stimulation elicited changes in regional brain activity as measured by functional magnetic resonance imaging and positron emission tomography.

Stimulation of the subthalamic nucleus changes the firing pattern of pallidal neurons

To clarify the mechanism underlying improvement of parkinsonian signs by high-frequency electrical stimulation (HFS) of the subthalamic nucleus (STN), we investigated the effects of STN HFS on neuronal activity of the internal and external segment of the globus pallidus (GPi and GPe, respectively) in two rhesus monkeys rendered parkinsonian by administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. A scaled-down version of the chronic stimulating electrode used in humans, consisting of four metal contacts 0.50 mm in length each separated by 0.50 mm, was implanted through a cephalic chamber targeting the STN. Histological reconstruction revealed that the cathode was located in the STN in both monkeys. Extracellular recordings from a total of 110 pallidal neurons during STN stimulation were performed. Poststimulus time histograms of single neurons triggered by 2 Hz STN stimulation pulses at 2.4-3.0 V revealed short-latency excitations at 2.5-4.5 and 5.5-7.0 msec after stimulation onset and inhibitions at 1.0-2.5, 4.5-5.5, and 7.0-9.0 msec for both GPe and GPi neurons. These short-latency responses were present with 136 Hz stimulation, at voltages effective for alleviation of parkinsonian signs, resulting in a significant increase in mean discharge rate and a stimulus-synchronized regular firing pattern. These results indicate that activation of the STN efferent fibers and resultant changes in the temporal firing pattern of neurons in GPe and GPi underlie the beneficial effect of HFS in the STN in Parkinson's disease and further support the role of temporal firing patterns in the basal ganglia in the development of Parkinson's disease and other movement disorders.

Local suppression of epileptiform activity by electrical stimulation in rat hippocampus in vitro

High frequency electrical stimulation of deep brain structures (DBS) has been effective at controlling abnormal neuronal activity in Parkinson's patients and is now being applied for the treatment of pharmacologically intractable epilepsy. The mechanisms underlying the therapeutic effects of DBS are unknown. In particular, the effect of the electrical stimulation on neuronal firing remains poorly understood. Previous reports have showed that uniform electric fields with both AC (continuous sinusoidal) or DC waveforms could suppress epileptiform activity in vitro. In the present study, we tested the effects of monopolar electrode stimulation and low-duty cycle AC stimulation protocols, which more closely approximate those used clinically, on three in vitro epilepsy models. Continuous sinusoidal stimulation, 50 % duty-cycle sinusoidal stimulation, and low (1.68 %) duty-cycle pulsed stimulation (120 micros, 140 Hz) could completely suppress spontaneous low-Ca2+ epileptiform activity with average thresholds of 71.11 +/- 26.16 microA, 93.33 +/- 12.58 microA and 300 +/- 100 microA, respectively. Continuous sinusoidal stimulation could also completely suppress picrotoxin- and high-K+-induced epileptiform activity with either uniform or localized fields. The suppression generated by the monopolar electrode was localized to a region surrounding the stimulation electrode. Potassium concentration and transmembrane potential recordings showed that AC stimulation was associated with an increase in extracellular potassium concentration and neuronal depolarization block; AC stimulation efficacy was not orientation-selective. In contrast, DC stimulation blocked activity by membrane hyperpolarization and was orientation-selective, but had a lower threshold for suppression.

Effects of high-frequency stimulation in the internal globus pallidus on the activity of thalamic neurons in the awake monkey

The reduction in symptoms of Parkinson's disease produced by high-frequency stimulation (HFS) in the internal globus pallidus (GPi) has been proposed to be due to stimulus-induced inactivation of pallidal neurons and resulting disinhibition of thalamic neurons. We tested this in awake Macaca fascicularis by stimulating between pairs of electrodes inserted into GPi under electrophysiological control and recording the responses evoked in thalamic neurons. HFS produced a reduction, not an increase, in discharge frequency during the stimulus train in 77% of the responsive thalamic neurons. Only 16% of the responsive cells showed an increase in discharge during stimulation and, for some of these, stimulation at a similar intensity produced contralateral muscle contraction, a probable sign of current spread to the internal capsule. The few thalamic neurons studied during bursting had a reduction in burst frequency and duration during HFS. We conclude that high-frequency stimulation within GPi does not necessarily facilitate thalamic discharge, and it may act, instead, to interrupt abnormal patterns of thalamic discharge associated with parkinsonian symptoms.

The effect of electrical stimulation of the subthalamic nucleus on seizures is frequency dependent

PURPOSE

Animal studies and anecdotal human case reports have indicated that the subthalamic nucleus (STN) may be a site of anticonvulsant action.

METHODS

We tested the hypothesis that continuous electrical stimulation of the STN inhibits seizures acutely. We determined the effects of three stimulation frequencies, 130 Hz, 260 Hz, and 800 Hz, on generalized clonic and tonic-clonic flurothyl seizures. Adult male rats were implanted with concentric bipolar stimulating electrodes in the STN bilaterally. After recovery, rats underwent flurothyl seizures to compare the effects of each stimulation frequency on seizure threshold. Rats were tested 4 times, twice in the stimulated condition, and twice in the unstimulated condition. The order of trials was random, except that stimulation trials alternated with control trials. Flurothyl seizure thresholds under each stimulation condition were compared with control values from the same animal.

RESULTS

Bilateral stimulation of the STN at 130 Hz produced a significant increase in the seizure threshold for clonic flurothyl seizures, whereas stimulation at 260 Hz did not appear to have any effect on seizures. STN stimulation at 800 Hz significantly lowered seizure threshold for tonic-clonic seizures.

CONCLUSIONS

We conclude that electrical stimulation of the STN can be anticonvulsant, but the effects appear to depend on the stimulation frequency and the type of seizure.

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.

Effect of chronic pallidal deep brain stimulation on off period dystonia and sensory symptoms in advanced Parkinson's disease

OBJECTIVE

To investigate the efficacy of chronic pallidal deep brain stimulation (DBS) on off period dystonia, cramps, and sensory symptoms in advanced Parkinson's disease (PD).

METHODS

16 patients (6 women, 10 men; mean age at surgery 65 years) suffering from advanced PD were followed up prospectively for one year after implantation of a monopolar electrode in the posteroventral lateral globus pallidus internus. Unilateral DBS was performed in 9 patients. 10 patients had bilateral procedures (contemporaneous bilateral surgery in 7 and staged bilateral surgery in 3 instances). The decision whether to perform unilateral or bilateral surgery depended on the clinical presentation of the patient. Patients were formally assessed preoperatively, at 3-5 days, 3 months, and 12 months after surgery.

RESULTS

In patients who underwent unilateral surgery, pain was present in 7 (78%), off dystonia in 5 (56%), cramps in 6 (67%), and dysaesthesia in 4 (44%). In patients who underwent bilateral surgery, pain was present in 7 (70%), off dystonia in 6 (60%), cramps in 7 (70%), and dysaesthesia in 4 (40%). With unilateral DBS, contralateral off period dystonia was improved by 100% at 1 year postoperatively, pain by 74%, cramps by 88%, and dysaesthesia by 100%. There was less pronounced amelioration of ipsilateral off period dystonia and sensory symptoms. With bilateral DBS, total scores for dystonia were improved by 86%, for pain by 90%, for cramps by 90%, and for dysaesthesia by 88%. The benefit appeared early at the first evaluation 3-5 days after surgery and was stable throughout the follow up period.

CONCLUSIONS

Pallidal DBS yields major improvement of off period dystonia, cramps, and sensory symptoms in patients with advanced PD.

Extracellular stimulation of central neurons: influence of stimulus waveform and frequency on neuronal output

The objective of this project was to examine the influence of stimulus waveform and frequency on extracellular stimulation of neurons with their cell bodies near the electrode (local cells) and fibers of passage in the CNS. Detailed computer-based models of CNS cells and axons were developed that accurately reproduced the dynamic firing properties of mammalian motoneurons including afterpotential shape, spike-frequency adaptation, and firing frequency as a function of stimulus amplitude. The neuron models were coupled to a three-dimensional finite element model of the spinal cord that solved for the potentials generated in the tissue medium by an extracellular electrode. Extracellular stimulation of the CNS with symmetrical charge balanced biphasic stimuli resulted in activation of fibers of passage, axon terminals, and local cells around the electrode at similar thresholds. While high stimulus frequencies enhanced activation of fibers of passage, a much more robust technique to achieve selective activation of targeted neuronal populations was via alterations in the stimulus waveform. Asymmetrical charge-balanced biphasic stimuli, consisting of a long-duration low-amplitude cathodic prepulse phase followed by a short-duration high-amplitude anodic stimulus phase, enabled selective activation of local cells. Conversely, an anodic prepulse phase followed by a cathodic stimulus phase enabled selective activation of fibers of passage. The threshold for activation of axon terminals in the vicinity of the electrode was lower than the threshold for direct activation of local cells, independent of the stimulus waveform. As a result, stimulation induced trans-synaptic influences (indirect depolarization/hyperpolarization) on local cells altered their neural output, and this indirect effect was dependent on stimulus frequency. If the indirect activation of local cells was inhibitory, there was little effect on the stimulation induced neural output of the local cells. However, if the indirect activation of the local cells was excitatory, attempts to activate selectively fibers of passage over local cells was limited. These outcomes provide a biophysical basis for understanding frequency-dependent outputs during CNS stimulation and provide useful tools for selective stimulation of the CNS.

Subthalamic nucleus deep brain stimulus evoked potentials: physiological and therapeutic implications

The effect of subthalamic nucleus (STN) stimulation on cortical electroencephalographic activity was examined in 10 patients with Parkinson's disease and 4 patients with epilepsy. Evoked potentials were created by time-locking electroencephalography to the onset of electrical stimulation delivered through the lead implanted in the STN of patients who had previously undergone deep brain stimulation (DBS) surgery. The effect of different patterns of stimulation on the evoked response, including single- and paired-pulse as well as burst stimulation, was explored. Cortical evoked potentials to single pulses were observed with latencies as short as 1 to 2 msec after a single pulse of stimulation, with activity continuing, in some cases, for up to 400 msec. Paired-pulse experiments revealed refractory periods on the order of 0.5 msec, suggesting that stimulation of axons contributed to the generation of at least some portion of the evoked potential waveform. Evoked potentials were also present in response to 100-msec bursts of stimulation, with some evidence that the potential was initiated within the burst artifact. The potential implications of the types of responses observed as well as potential applications are discussed.

EEG and evoked potential recording from the subthalamic nucleus for deep brain stimulation of intractable epilepsy

OBJECTIVES

The substantia nigra in the animal model has been implicated in the control of epilepsy. The substantia nigra pars reticulata (SNpr) receives afferents from the subthalamic nucleus (STN), which thus may have an effect on the control of epilepsy. There is evidence in the animal model of a direct connection from the cortex to the STN. High-frequency STN stimulation is being used in experimental trial for the management of intractable epilepsy. Our primary objective in this study was to determine if there was epileptiform activity recorded from the STN in association with scalp recorded epileptiform activity to support the presence of a pathway from the cortex to the STN in humans as described in animals that may be important for the management of epilepsy. This article describes the interictal and ictal electroencephalographic (EEG) findings as well as evoked potential recordings from the STN in these patients with intractable epilepsy.

METHODS

Four patients (3 males) ranging from 19 to 45 years with intractable focal epilepsy refractory to anti-epileptic drugs were studied. Two patients failed vagal nerve stimulation and one patient had previous epilepsy surgery. Depth electrodes were implanted stereotactically in the STN bilaterally. A comparative analysis of the interictal and ictal activities recorded from the scalp and STN electrodes was performed. Median nerve somatosensory evoked potentials (SEPs) and auditory evoked potentials (AEPs) were also recorded.

RESULTS

Interictal sharp waves recorded in the scalp EEG were always negative in polarity. These sharp waves were always associated with sharp waves recorded at the ipsilateral STN electrode contacts that were always positive in polarity. In addition repetitive spikes were recorded independently at the left or right STN electrode contacts, with no reflection at the scalp. These spikes were extremely stereotyped, of high amplitude and short duration, and were positive or negative in polarity. Focal scalp EEG seizures were also recorded at the ipsilateral STN electrodes. In 3 patients SEPs were recorded from the contralateral STN electrodes corresponding to the P14/N18 far-field complex. In two patients AEPs were recorded, and wave V (near-field) and wave VII (far-field) from the contralateral STN electrodes.

CONCLUSIONS

This study demonstrates that scalp recorded epileptiform activity is reflected at the ipsilateral STN either following or preceding the scalp sharp waves. The STN sharp waves are most probably an expression of the direct cortico-STN glutamatergic pathways that have been demonstrated previously in animals. This pathway in man may be important with regard to a possible mechanism for the treatment of epilepsy with STN stimulation.

Deep brain stimulation for Parkinson's disease: disrupting the disruption

Many people are disabled by Parkinson's disease (PD) despite the drug treatments that are currently available. For these patients, neurosurgery has the potential to help restore their function. The most effective neurosurgical procedures to date use electrical stimulation--deep brain stimulation (DBS)--of small targets in the brain by use of a pacemaker-like device to deliver constant stimulation. Although these operations can produce striking results, the mechanism by which delivery of electrical stimulation to targets deep in the brain can restore function in the motor system is not clear. This type of surgery probably works by interfering with and shutting down abnormal brain activity in areas where the current is delivered, such as the thalamus, globus pallidus, or the subthalamic nucleus. With this abnormal neuronal activity neutralised, motor areas of the brain can resume their function and normal movements are reinstated. Current research is aimed at elucidating how DBS works and using this information to develop better treatments for patients with PD and other neurological disorders.

High-frequency stimulation of the subthalamic nucleus selectively reverses dopamine denervation-induced cellular defects in the output structures of the basal ganglia in the rat

High-frequency stimulation (HFS) of the subthalamic nucleus (STN) is now recognized as an effective treatment for advanced Parkinson's disease, but the molecular basis of its effects remains unknown. This study examined the effects of unilateral STN HFS (2 hr of continuous stimulation) in intact and hemiparkinsonian awake rats on STN neuron metabolic activity and on neurotransmitter-related gene expression in the basal ganglia, by means of in situ hybridization histochemistry and immunocytochemistry. In both intact and hemiparkinsonian rats, this stimulation was found to induce c-fos protein expression but to decrease cytochrome oxidase subunit I mRNA levels in STN neurons. STN HFS did not affect the dopamine lesion-mediated overexpression of enkephalin mRNA or the decrease in substance P in the ipsilateral striatum. The lesion-induced increases in intraneuronal glutamate decarboxylase 67 kDa isoform (GAD67) mRNA levels on the lesion side were reversed by STN HFS in the substantia nigra, partially antagonized in the entopeduncular nucleus but unaffected in the globus pallidus. The stimulation did not affect neuropeptide or GAD67 mRNA levels in the side contralateral to the dopamine lesion or in intact animals. These data furnish the first evidence that STN HFS decreases the metabolic activity of STN neurons and antagonizes dopamine lesion-mediated cellular defects in the basal ganglia output structures. They provide molecular substrate to the therapeutic effects of this stimulation consistent with the current hypothesis that HFS blocks STN neuron activity. However, the differential impact of STN HFS on the effects of dopamine lesion among structures receiving direct STN inputs suggests that this stimulation may not cause simply interruption of STN outflow.

Superior colliculus firing changes after lesion or electrical stimulation of the subthalamic nucleus in the rat

Recent data have suggested a critical role for the basal ganglia in the remote control of epileptic seizures. In particular, it has been shown that inhibition of either substantia nigra pars reticulata or subthalamic nucleus as well as activation of the superior colliculus suppresses generalized seizures in several animal models. It was previously shown that high frequency stimulation of the subthalamic nucleus, thought to act as functional inhibition, stopped ongoing non-convulsive generalized seizures in rats. In order to determine whether high frequency stimulation of the subthalamic nucleus involved an activation of superior colliculus neurons, we examined the effects of subthalamic nucleus manipulation, by either high frequency stimulation or chemical lesion, on the spontaneous electrical activity of superior colliculus neurons. Acute high frequency stimulation of the subthalamic nucleus (frequency 130 Hz) induced an immediate increase of unitary activity in 70% of responding cells, mainly located within the deep layers, whereas a reduction was observed in the remaining 30%. The latter responses are dependent on the intensity and frequency of the stimulation. Unilateral excitotoxic lesion of the subthalamic nucleus induced a delayed and transient decrease of superior colliculus activity. Our data suggest that high frequency stimulation of the subthalamic nucleus suppresses generalised epileptic seizures through superior colliculus activation.

Antiepileptic effect of high-frequency stimulation of the subthalamic nucleus (corpus luysi) in a case of medically intractable epilepsy caused by focal dysplasia: a 30-month follow-up: technical case report

OBJECTIVE AND IMPORTANCE

Currently, some forms of epilepsy are resistant to both pharmacological and surgical interventions. As a result, there is a need for new therapeutic strategies. Because the nigral system modulates neuronal excitability in animal models of epilepsy, we considered therapeutic high-frequency stimulation of the subthalamic nucleus (STN). We were encouraged by the known relationship between the STN and the nigral system, as well as by our experience with high-frequency stimulation of the STN in Parkinsonian patients.

CLINICAL PRESENTATION

A 5-year-old girl with pharmacologically resistant, inoperable epilepsy caused by focal centroparietal dysplasia underwent implantation with a permanent electrode in the left STN and was chronically stimulated. To date, we have followed up this patient for 30 months postoperatively.

TECHNIQUE

High-frequency stimulation of the STN induced a significant voltage-dependent reduction (by 80%) in the number and severity of seizures. In addition, consistent improvement in both motor and cognitive functions was noted as a result of reduced postictal states. The effect was more prominent for seizures occurring in clusters (89% reduction) and during the day (88% reduction) than for those that occurred during sleep (53% reduction).

CONCLUSION

This is the first report of epilepsy control using chronic high-frequency stimulation of the STN. Preliminary observations in three other operated patients (at 2, 12, and 18 mo) confirm these data. We think that high-frequency stimulation of the STN may hold significant future potential as a treatment for epilepsy, similar to its established role in the treatment of Parkinson's disease. This finding opens completely new experimental and therapeutic avenues for the treatment of surgically and medically intractable epilepsy.

Mechanisms of deep brain stimulation

High frequency electrical stimulation by means of electrodes implanted into the brain (deep brain stimulation; DBS) recently has become an accepted technique for the treatment of several movement disorders and in particular for Parkinson's disease. Because the effects produced by DBS are similar to those produced by making a lesion in the same region, it has been proposed that the overall effect of DBS is to inhibit the neural activity in the region stimulated. However, whether this is actually the case is presently not known, but various mechanisms have been proposed in an attempt to explain how DBS could mimic the effects of a lesion. We describe the various mechanisms that have been proposed to account for the inhibition or disruption of the pathologic outflow by high-frequency DBS, ranging from depolarisation block to stimulation-evoked release of gamma-aminobutyric acid and describes preliminary findings that show that stimulation within these structures can result in inhibition.

Intraoperative microrecordings of the subthalamic nucleus in Parkinson's disease

Microelectrode recordings of single unit neuronal activity were used during stereotactic surgery to define the subthalamic nucleus for chronic deep brain stimulation in the treatment of Parkinson's disease. By using five parallel trajectories, often two to three microelectrodes allow us to recognize subthalamic nucleus (STN) neuronal activity. STN neurons were easily distinguished from cells of the overlying zona incerta and the underlying substantia nigra. During a typical exploratory track, we can observe a very low background noise in the zona incerta and almost complete absence of single cell recording. Penetration of the electrode tip into the STN is characterized by a sudden increase in background activity and single cell activity of spontaneously active neurons. The exit of electrode tip out of the STN corresponds to a decrease in background noise and a loss of single cell activity. Spontaneous neuronal activity increases again when the electrode tips enters the substantia nigra pars reticulata (SNr); however, the activity is less rich than in the STN, indicating a more cell-sparse nucleus. STN neurons are characterized by a mean firing rate of 42.30 +/- 22.00 spikes/sec (mean +/- SD). The STN cells exhibited irregular or bursty discharge pattern. The pattern of single cell activity in the SNr is a more regular tonic activity that can easily be distinguished from the bursting pattern in the STN. The most useful criteria to select a trajectory are (1) the length of an individual trajectory displaying typical STN activity, (2) the bursting pattern of activity, and (3) motor responses typical of the sensorimotor part of the nucleus. In conclusion, microelectrode recording of the subthalamic area improves the accuracy of targeting the STN.

Mechanisms of deep brain stimulation

The mechanism of action of high frequency deep brain stimulation is still unknown. However, in all circumstances and in all target nuclei so far stimulated, the effects mimic those of lesions previously made during thalamotomies, pallidotomies or even subthalamotomies, suggesting an inhibition of at least the neuronal network containing the target, if not of the target itself. On the contrary, fiber bundles are consistently activated at low or high frequencies. The hypothetical mechanisms envisioned should therefore be compatible and even produce these observed effects, to be acceptable as hypotheses. The mechanism could be either one or a combination of several causes: jamming of a feedback loop, activation of inhibitory structures included in a more complex network, blockade of membrane ion channels, deplorisation blockade, synaptic exhaustion, induction of early genes, changes in local blood flow, neuroplasticity, etc. It is probable that some are more involved in the acute effects and others in the long term changes, close to neuroplasticity. It is clear that the understanding of this strange and powerful phenomenon will profit from both clinical observation and well designed animal experiments.

Mechanisms of deep brain stimulation: excitation or inhibition

There is little debate that deep brain stimulation (DBS) has been an effective tool in the treatment of Parkinson's disease as well as other movement disorders. There remains however, considerable debate concerning the mechanism(s) underlying its beneficial effect. The comparable effect of stimulation to ablation in the thalamus on tremor, and in the subthalamic nucleus (STN) and internal segment of the globus pallidus (GPi) on the motor signs associated with PD, have led many investigators to conclude that DBS acts to suppress neuronal activity, decreasing output from the stimulated site. There are, however, data that do not support this argument. Microdialysis studies in GPi showed increased levels of glutamate during STN stimulation, suggesting activation of glutamatergic output from the STN to the GPi. Studies in parkinsonian primates have demonstrated increased mean discharge rates of neurons in GPi during chronic stimulation in STN, and GPi stimulation in humans has been associated with a suppression of neuronal activity in the thalamus. Contrary to what one would expect if stimulation inhibits output from the stimulated structure, stimulation in GPe has been demonstrated to improve bradykinesia. Although arguments for increased output from the stimulated structure seem to conflict with the hypothesis that stimulation acts to inhibit neuronal activity, it is possible to explain these observations through a common mechanism, e.g. activation of fiber pathways. Based on this mechanism, the effect of stimulation on cellular activity in the stimulated site would be increased or decreased dependent on the neurotransmitter of the afferent fibers projecting to that site. However, in addition to activation of afferent fibers, projection axons from neurons in the stimulated structure, also readily excitable by electrical stimulation, would also be tonically activated and discharge independently of the soma, thereby increasing output from the structure during extracellular stimulation. Thus, although high frequency stimulation may inhibit neurons via activation of inhibitory afferents, the output from that structure may be increased as the result of activation of axonal elements leaving the target structure. This hypothesis would explain the present experimental results, is consistent with excitability profiles of neuronal elements based on their biophysical properties, and fits with more recent models emphasizing the role of altered patterns of neuronal activity in the development of hypokinetic and hyperkinetic movement disorders.

Concurrent excitatory and inhibitory effects of high frequency stimulation: an oculomotor study

OBJECTIVE

To describe a reversible neurological condition resembling a crossed midbrain syndrome resulting from high frequency stimulation (HFS) in the midbrain.

METHODS

Postoperative evaluation of quadripolar electrodes implanted in the area of the subthalamic nucleus of 25 patients with Parkinson's disease (PD) successfully treated by HFS.

RESULTS

Four of the 25 patients experienced reversible acute diplopia, with dystonic posture and tremor in the contralateral upper limb when the white matter between the red nucleus and the substantia nigra was stimulated. The motor signs resembled those caused by lesions of the red nucleus. The ipsilateral resting eye position was "in and down" (three patients) or "in" (one patient). Enophthalmos was seen. Abduction was impaired and vertical eye movements were limited, but adduction was spared. The movements of the controlateral eye were normal. The ocular signs could be best explained by sustained hyperactivity of the extrinsic oculomotor nerve. Simultaneous tonic contraction of the superior rectus, the inferior rectus, and inferior oblique may cause the enophthalmos and partial limitation of upward and downward eye movements. Antagonist tonic contraction of the ipsilateral medial rectus severely impairs abduction.

CONCLUSION

This crossed midbrain syndrome, possibly resulting from simultaneous activation of oculomotor nerve and lesion-like inhibition of the red nucleus suggests that high frequency stimulation has opposite effects on grey and white matter.

A template subtraction method for stimulus artifact removal in high-frequency deep brain stimulation

Deep brain stimulation (DBS) is a neurosurgical technique that has been widely applied for the treatment of tremor or motor symptoms associated with advanced Parkinson's disease. Large stimulus artifacts, however, have hampered investigations of physiological mechanisms underlying DBS effects using extracellular recording techniques. We have developed an off-line procedure for removing stimulus artifacts from recorded neuronal signals (monopolar) and applied this method of artifact subtraction to DBS studies using extracellular recording techniques in a nonhuman primate. The procedure consists of developing a template of the artifact by averaging the artifact signals triggered by its onset. The template is then subtracted from the individual triggered signals. The experimental results indicate that this method is highly effective in removing the majority of the stimulus artifact, while leaving recorded neuronal activity intact. In fact, removal of stimulation artifact using this technique has revealed a short-latency neuronal response to stimulation that was previously obscured by the stimulus artifact. Thus, this technique may not only improve the quality of electrophysiological studies employing DBS techniques, but may also help to elucidate neuronal mechanisms underlying the effect of DBS.

An analysis of the respective risks of hematoma formation in 361 consecutive morphological and functional stereotactic procedures

OBJECTIVE

The risk of hematoma formation in stereotactic procedures is generally considered to range between 1 and 4%, and it has been speculated that morphological procedures may have a higher risk of bleeding than functional procedures.

METHODS

Between 1989 and 1999, all patients who underwent a stereotactic procedure performed by the same surgeon were enrolled sequentially onto the study. All patients had normal preoperative prothrombin time, partial thromboplastin time, and platelet count. High-resolution computed tomography or magnetic resonance imaging with a 1.5-T machine were used for the target definition. None of the patients had an angiogram before surgery.

RESULTS

A total of 361 procedures was performed comprising 175 morphological procedures (139 biopsies, 18 lesion evacuations [cysts, abscesses, and hematomas], and 18 drain implantations) and 186 functional procedures (137 lesions [thalamotomy or pallidotomy], 47 deep brain electrode implantations, and two physiological explorations without lesions or implantations). There were no infections or seizures in either group. Three hematomas (1.7%) occurred in the morphological group, two of them in inflammatory lesions in immunocompromised patients (one death) and one in a pineal tumor. Three hematomas (1.6%) occurred in the functional group (no mortality). There was no statistically significant difference (P > 0.05; Fisher's exact test) in the risk of hematoma formation between morphological and functional stereotactic procedures. The morbidity and mortality related to bleeding also were not statistically different (P > 0.05; Fisher's exact test) between these two groups.

CONCLUSION

In this series, the risk of bleeding was not higher for morphological procedures than for functional procedures. This suggests that the risk of bleeding for stereotactic procedures is related more to the patient than to the type of procedure performed. Our study confirms an overall risk of bleeding of 1.7% for any type of stereotactic procedure, resulting in a mortality of 0.3% and a morbidity of 1.4%.

Unilateral lesion of the nigrostriatal pathway induces an increase of neuronal activity of the pedunculopontine nucleus, which is reversed by the lesion of the subthalamic nucleus in the rat

The role of the pedunculopontine nucleus (PPN) in the pathophysiology of Parkinson's disease is still unclear. Using microrecordings, we investigated the changes occurring in PPN neurons after lesions of the substantia nigra compacta (SNc) and the role of the subthalamic nucleus (STN) in these changes. In normal rats the firing rate of PPN neurons was 10.6 +/- 1.4 spikes/s, the majority of neurons (91%) having a regular firing pattern, 6% irregular and 3% in bursts. In rats with 6-hydroxydopamine lesions of the SNc, the firing rate increased significantly to 18.3 +/- 3.0 spikes/s compared with normal rats. In addition, the firing pattern changed significantly: 70% of the neurons discharged regularly, 27% irregularly and 3% in bursts. In rats with ibotenic acid lesions of the STN, the firing rate decreased significantly to 7.2 +/- 0.9 spikes/s and the firing pattern changed significantly: 50% of the neurons discharged regularly, 43% irregularly and 7% in bursts. The rats with combined SNc and STN lesions showed no change in the firing rate (8.5 +/- 1.0 spikes/s) compared to normal rats. The firing pattern changed significantly: 69% of the cells discharged regularly, 26% irregularly and 5% in bursts. These findings demonstrate that PPN neurons are overactive and more irregular in the 6-hydroxydopamine-lesioned rats, suggesting the implication of this nucleus in the pathophysiology of parkinsonism. Moreover, the fact that STN lesions induced a reduction in the firing rate of the PPN in normal rats and a normalization of the firing rate in rats with 6-hydroxydopamine lesions suggests that this nucleus is under major control of the STN.

Bilateral pallidotomy for treatment of Parkinson's disease induced corticobulbar syndrome and psychic akinesia avoidable by globus pallidus lesion combined with contralateral stimulation

OBJECTIVE

Posteroventral pallidotomy (PVP) has proved to be an effective method for the treatment of Parkinson's disease. However, data on bilateral procedures are still limited. To assess the effects of bilateral globus pallidus (GPi) lesion and to compare it with a combination of unilateral GPi lesion plus contralateral GPi stimulation (PVP+PVS), an open blind randomised trial was designed.

METHODS

A prospective series of patients with severe Parkinson's disease refractory to medical treatment, and severe drug induced dyskinesias, were randomised either to simultaneous bilateral PVP or simultaneous PVP+PVS. All patients were assessed with the core assessment programme for intracerebral transplantation (CAPIT), and a comprehensive neuropsychological and neuropsychiatric battery both before surgery and 3 months later.

RESULTS

The severe adverse effects found in the first three patients subjected to bilateral PVP led to discontinuation of the protocol. All three patients developed depression and apathy. Speech, salivation, and swallowing, as well as freezing, walking, and falling, dramatically worsened. By contrast, all three patients undergoing PVP+PVS had a significant motor improvement.

CONCLUSION

Bilateral simultaneous lesions within the GPi may produce severe motor and psychiatric complications. On the other hand, a combination of PVP+ PVS significantly improves parkinsonian symptoms not associated with the side effects elicited by bilateral lesions.