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

Saccadometry: a novel clinical tool for quantification of the motor effects of subthalamic nucleus stimulation in Parkinson's disease

INTRODUCTION

High frequency stimulation of the subthalamic nucleus (STN HFS) is the therapy of choice in the surgical management of patients suffering from advanced Parkinson's disease (PD). Worldwide, more than 30,000 patients have undergone this procedure. At present, there is a need for a rapid method of assessing its therapeutic effect that is ideally also target-specific, objective, automated, quantitative, and with high overall reliability. Saccadic latency, that reflects the operation of central decision-making mechanisms, is increasingly being used as a way of obtaining quantitative, objective information about cerebral performance in general.

OBJECTIVE

In this study we investigated the possible application of saccadic eye movements as a tool for assessing the therapeutic motor effects of STN HFS in patients with advanced PD.

METHODS

Patients received bilateral STN HFS and were evaluated in stimulation and medication on and off conditions, together with conventional motor assessments under the same conditions.

RESULTS

We found that saccadometry can provide a sensitive and objective measure of the effects of STN HFS in PD patients that correlates well with conventional, subjective, evaluation of motor impairment. The effect appears to be specific to the site of stimulation, and in terms of the LATER model appears to be due to an increase in the mean rate of rise of the underlying decision signal, combined with a degree of suppression of the mechanism responsible for the early component.

INTERPRETATION

A possible interpretation of our findings is that electrical stimulation of the STN, which is known to have a powerful influence on substantia nigra pars reticulata, enhances both the descending facilitation that passes from the cortex to the colliculus via the basal ganglia, thus increasing the mean rate of rise of the decision signal, and also the tonic background inhibition that normally suppresses unwanted early responses.

Toward feedback controlled deep brain stimulation: dynamics of glutamate release in the subthalamic nucleus in rats

Deep brain stimulation (DBS) is an effective symptomatic treatment in Parkinson's disease. High frequency stimulation (HFS) of the subthalamic nucleus elicits neurotransmitter release in multiple nuclei. Therefore, we tested the hypothesis that neurotransmitter release during HFS may be used to provide feedback control of the intensity and pattern of HFS. We studied the dynamic relationship between extracellular glutamate levels and HFS in and around the STN in anesthetized rats. We used a pseudorandom binary sequence (PRBS) of stimulation in the STN, the independent forcing function, while measuring extracellular glutamate in the same nucleus, the dependent variable. The PRBS consisted of 90 s periods during which stimulation (100 microA, 150Hz, 10% duty cycle) was either off or on. The stimulation and extracellular glutamate levels were fitted using an autoregressive exogenous model (ARX) to determine the transfer function between HFS and the extracellular glutamate concentration in the STN. The ARX model fit the dynamics of extracellular glutamate levels well (correlation coefficients ranged from 0.74 to 0.99; n=11). The transfer function accurately predicted extracellular glutamate levels in the STN even when the pattern of HFS was modified. We used the transfer function to develop a feedback controlled stimulation algorithm. Feedback controlled HFS maintained extracellular glutamate concentrations at any predefined level, but only intermittent HFS was required. We conclude that the transfer function between HFS and neurotransmitter levels in the brain can be used to design DBS protocols that generate specific temporal patterns of glutamate release in the STN.

Monophasic but not biphasic pulses induce brain tissue damage during monopolar high-frequency deep brain stimulation

OBJECTIVE

Electrical high-frequency stimulation (HFS) of deep brain structures has been successfully used as a treatment for patients with movement disorders. The mechanisms of HFS allowing therapeutic clinical effects remain unclear, which justifies experimental studies to address these questions. These experiments require an external stimulator, which may offer the possibility to deliver a current with monophasic or biphasic pulses. The aim of the present study was to quantify the evolution of a potentially deleterious effect of HFS according to the duration and/or intensity in monophasic and biphasic conditions.

METHODS

In all rats, HFS was performed with monophasic pulses in deep brain structures of 1 hemisphere and with biphasic pulses symmetrically in the other hemisphere. The effect of HFS was tested, first for various durations of HFS at a constant intensity (100 microA) and, second, for measuring the effect of various current intensities of HFS at constant duration (10 minutes). At the end of each stimulation test, the volume of lesion was determined and analyzed.

RESULTS

In all hemispheres in which stimulation using biphasic pulses was delivered, we never found any relevant lesions. Conversely, monophasic electrical stimulation always created a lesion: at 100 microA, a minimal duration of HFS of 5 minutes induced a tissue damage volume of 0.0055 +/- 0.0015 mm(3). For 10 minutes of HFS, a minimal intensity of 100 microA induced a tissue damage volume of 0.0062 +/- 0.0017 mm(3). Regression analysis showed that the extent of lesion increased linearly with the intensity and duration.

CONCLUSION

In conclusion, this study proved that HFS using monophasic pulses systematically created tissue damage after 5 minutes of stimulation at 100 microA. HFS is safe when biphasic pulses are used for intensities as high as 2 mA and durations as long as 120 minutes. Monophasic pulses can be safely used only during short stimulation and at low intensities.

Characterisation of tremor-associated local field potentials in the subthalamic nucleus in Parkinson's disease

We simultaneously recorded local field potentials (LFPs) in the subthalamic nucleus (STN) and surface electromyographic signals (EMGs) from the extensor and flexor muscles of the contralateral forearm in eight patients with idiopathic tremor-dominant Parkinson's disease (resting tremor) during the bilateral implantation of deep brain stimulation electrodes. Recordings were made at different heights (in 0.5- to 2.0-mm steps beginning outside the STN) using up to five concentrically configured macroelectrodes (2 mm apart). The patients were instructed to relax their contralateral forearm (rest condition). We analysed the coherence between tremor EMGs and STN LFPs, which showed significant tremor-associated coupling at single tremor and double tremor frequencies. Moreover, the EMG-LFP coherences were characterised by differences between antagonistic muscles (flexor, extensor) and by the spatial distribution of LFPs within the STN. Coherence at single and double tremor frequencies occurred significantly more frequently within STN than above STN (in the zona incerta). In this study, we were able to show that, within STN, tremor-associated LFP activity varied with spatial distribution and with the contralateral antagonistic forearm muscles. These findings suggest the existence of distribution- and muscle-specific tremor-associated LFP activity at different tremor frequencies and an organisation of tremor-related subloops within the STN.

Subthalamic nucleus stimulation modulates thalamic neuronal activity

Deep brain stimulation (DBS) in the subthalamic nucleus (STN) is an effective tool for the treatment of advanced Parkinson's disease. The mechanism by which STN DBS elicits its beneficial effect, however, remains unclear. We previously reported STN stimulation increased the rate and produced a more regular and periodic pattern of neuronal activity in the internal segment of the globus pallidus (GPi). Here we extend our observations to neurons in the pallidal [ventralis lateralis pars oralis (VLo) and ventralis anterior (VA)] and cerebellar [ventralis lateralis posterior pars oralis (VPLo)] receiving areas of the motor thalamus during STN DBS. Stimulation parameters that produced improvement in rigidity and bradykinesia resulted in changes in the pattern and power of oscillatory activity of neuronal activity that were similar in both regions of the motor thalamus. Neurons in both VA/VLo and VPLo tended to become more periodic and regular with a shift in oscillatory activity from low to high frequencies. Burst activity was reduced in VA/VLo, but was not significantly changed in VPLo. There was also a significant shift in the population of VA/VLo neurons that were inhibited during STN DBS, whereas VPLo neurons tended to be activated. These data are consistent with the hypothesis that STN DBS increases output from the nucleus and produces a change in the pattern and periodicity of neuronal activity in the basal ganglia thalamic network, and that these changes include cerebellar pathways likely via activation of adjacent cerebello-thalamic fiber bundles.

Deep brain stimulation of the globus pallidus internus in the parkinsonian primate: local entrainment and suppression of low-frequency oscillations

Competing theories seek to account for the therapeutic effects of high-frequency deep brain stimulation (DBS) of the internal globus pallidus (GPi) for medically intractable Parkinson's disease. To investigate this question, we studied the spontaneous activity of 102 pallidal neurons during GPiDBS in two macaques rendered parkinsonian by administration of MPTP. Stimulation through macroelectrodes in the GPi (> or =200 microA at 150 Hz for 30 s) reduced rigidity in one animal and increased spontaneous movement in both. Novel artifact subtraction methods allowed uninterrupted single-unit recording during stimulation. GPiDBS induced phasic (78% of cells) or sustained (22%) peristimulus changes in firing in both pallidal segments. A subset of cells responded at short latency (<2 ms) in a manner consistent with antidromic driving. Later phasic increases clustered at 3- to 5-ms latency. Stimulation-induced decreases were either phasic, clustered at 1-3 ms, or sustained, showing no peristimulus modulation. Response latency and magnitude often evolved over 30 s of stimulation, but responses were relatively stable by the end of that time. GPiDBS reduced mean firing rates modestly and only in GPi (-6.9 spikes/s). Surprisingly, GPiDBS had no net effect on the prevalence or structure of burst firing. GPiDBS did reduce the prevalence of synchronized low-frequency oscillations. Some cell pairs became synchronized instead at the frequency of stimulation. Suppression of low-frequency oscillations did not require high-frequency synchronization, however, or even the presence of a significant peristimulus response. In summary, the therapeutic effects of GPiDBS may be mediated by an abolition of low-frequency synchronized oscillations as a result of phasic driving.

Effect of quinolinic acid-induced lesions of the subthalamic nucleus on performance on a progressive-ratio schedule of reinforcement: a quantitative analysis

The subthalamic nucleus (STN), a major relay in the indirect striatofugal pathway, plays an important role in extrapyramidal motor control. Recent evidence indicates that it may also be involved in regulating the incentive value of food reinforcers.

OBJECTIVE

To examine the effect of lesions of the STN on performance on a progressive-ratio schedule using a quantitative model that dissociates effects of interventions on motor and motivational processes [Killeen PR. Mathematical principles of reinforcement. Behav Brain Sci 1994;17:105-72]. Rats with bilateral quinolinic acid-induced lesions of the STN (n=14) or sham lesions (n=14) were trained to press a lever for food-pellet reinforcers under a progressive-ratio schedule. In Phase 1 (90 sessions) the reinforcer was one pellet; in Phase 2 (30 sessions) it was two pellets; in Phase 3 (30 sessions) it was again one pellet.

RESULTS

The performance of both groups conformed to the model of progressive-ratio schedule performance. The motor parameter, delta, was significantly higher in the STN-lesioned than the sham-lesioned group, reflecting lower overall response rates in the lesioned group. The motivational parameter, a, was significantly higher in the STN-lesioned group than in the sham-lesioned group, consistent with enhanced reinforcer value in the STN-lesioned group compared to the sham-lesioned group. In both groups, a was sensitive to changes in reinforcer size, being significantly greater under the two-pellet condition (Phase 2) than under the one-pellet condition (Phases 1 and 3). The results suggest that destruction of the STN impairs response capacity and enhances the incentive value of food reinforcers.

Neurostimulatory therapeutics in management of treatment-resistant depression with focus on deep brain stimulation

Treatment-resistant depression continues to pose a major medical challenge, as up to one-third of patients with major depressive disorder fail to have an adequate response to standard pharmacotherapies. An improved understanding of the complex circuitry underlying depressive disorders has fostered an explosion in the development of new, nonpharmacological approaches. Each of these treatments seeks to restore normal brain activity via electrical or magnetic stimulation. In this article, the authors discuss the ongoing evolution of neurostimulatory treatments for treatment-resistant depression, reviewing the methods, efficacy, and current research on electroconvulsive therapy, repetitive transcranial magnetic stimulation, magnetic seizure therapy, focal electrically administered stimulated seizure therapy, transcranial direct current stimulation, chronic epidural cortical stimulation, and vagus nerve stimulation. Special attention is given to deep brain stimulation, the most focally targeted approach. The history, purported mechanisms of action, and current research are outlined in detail. Although deep brain stimulation is the most invasive of the neurostimulatory treatments developed to date, it may hold significant promise in alleviating symptoms and improving the quality of life for patients with the most severe and disabling mood disorders.

The influence of subthalamic nucleus lesions on sign-tracking to stimuli paired with food and drug rewards: facilitation of incentive salience attribution?

It is well known that the subthalamic nucleus (STN) plays an important role in regulating motor function, but recent studies suggest the STN is also involved in regulating motivated behavior. For example, bilateral lesions of the STN increase motivation for both food and cocaine as assessed by 'breakpoint' on a progressive ratio schedule. However, the psychological mechanism(s) by which STN lesions increase motivation for rewards is unknown. We hypothesized that STN lesions might influence one specific component of motivation, the attribution of incentive value (incentive salience) to reward-related cues. We tested this hypothesis by quantifying the ability of a discrete cue that had been paired with the non-contingent delivery of either food or cocaine to elicit approach towards it (ie, to produce a 'sign-tracking' conditioned response, CR). STN lesions made prior to training increased asymptotic levels of sign-tracking behavior directed towards a cue paired with either food or cocaine. In addition, when STN lesions were made after animals had already undergone Pavlovian training, and animals were tested under extinction conditions, the STN lesion still facilitated a sign-tracking CR. Finally, reintroduction of the US (food) following extinction immediately restored robust sign-tracking behavior in animals with STN lesions. We speculate, therefore, that the STN is part of a neural system that moderates the amount of incentive salience attributed to reward-related stimuli. Activity in this neural system may help mitigate the development of compulsive behavioral disorders, such as addiction, which are characterized by pathological control over behavior by reward-associated cues.

Pax7 is requisite for maintenance of a subpopulation of superior collicular neurons and shows a diverging expression pattern to Pax3 during superior collicular development

Background

Pax7 encodes a transcription factor well-established as an important determinant of mesencephalic identity and superior collicular development. Pax7 mutant mice, however, present with no obvious morphological impairments to the superior colliculus. This finding is paradoxical and has been attributed to functional redundancy afforded by its paralogue Pax3. Here we utilise Pax7 mutant mice to investigate the precise role of this important developmental regulator during superior collicular development and neuronal specification/differentiation. We also assess its spatiotemporal relationship with Pax3 during embryonic development.

Results

Analysis of the superior colliculus of Pax7 mutant and wildtype mice at a variety of developmental timepoints revealed that whilst correct initial specification is maintained, a subpopulation of dorsal mesencephalic neurons is lost at early postnatal stages. Moreover, a comparative analysis of embryonic Pax3 and Pax7 expression profiles indicate that Pax3 expression overlaps extensively with that of Pax7 initially, but their expression domains increasingly diverge as development progresses, coinciding spatiotemporally with neuronal differentiation and maturation of the tissue. Furthermore, Pax3 expression is perturbed within the CNS of embryonic Pax7 mutant mice.

Conclusion

In summary, these results demonstrate that during superior collicular development, Pax7 is required to maintain a subpopulation of dorsal, mesencephalic neurons and partially regulates, spatiotemporally, Pax3 expression within the CNS. The differential nature of Pax7 and Pax3 with respect to neuronal differentiation may have implications for future stem cell therapies aimed at exploiting their developmental capabilities.

Mechanisms and targets of deep brain stimulation in movement disorders

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

High frequency stimulation and temporary inactivation of the subthalamic nucleus reduce quinpirole-induced compulsive checking behavior in rats

Obsessive-compulsive disorder (OCD) represents a highly prevalent and impairing psychiatric disorder. Functional and structural imaging studies implicate the involvement of basal ganglia-thalamo-cortical circuits in the pathophysiology of this disorder. In patients remaining resistant to pharmaco- and behavioral therapy, modulation of these circuits may consequently reverse clinical symptoms. High frequency stimulation (HFS) of the subthalamic nucleus (STN), an important station of the basal ganglia-thalamo-cortical circuits, has been reported to reduce obsessive-compulsive symptoms in a few Parkinson's disease patients with comorbid OCD. The present study tested the effects of bilateral HFS of the STN and of bilateral pharmacological inactivation of the STN (via intracranial administration of the GABA agonist muscimol) on checking behavior in the quinpirole rat model of OCD. We demonstrate that both HFS and pharmacological inactivation of the STN reduce quinpirole-induced compulsive checking behavior. We conclude that functional inhibition of the STN can alleviate compulsive checking, and suggest the STN as a potential target structure for HFS in the treatment of OCD.

High frequency stimulation of the subthalamic nucleus modulates neurotransmission in limbic brain regions of the rat

Despite the benefit high frequency stimulation (HFS) of the subthalamic nucleus (STN) has on motor symptoms of Parkinson's Disease (PD), accumulating data also suggest effects of STN-HFS on non-motor behavior. This may be related to the involvement of the STN in the limbic basal ganglia-thalamocortical loops. In the present study we investigated the effect of acute STN-HFS on neurotransmission in associated structures of these pathways, i.e. the nucleus accumbens (NAc) core and shell as well as the ventral tegmental area (VTA) using in vivo microdialysis. Experiments were performed in anaesthetized naive rats and rats selectively lesioned in the substantia nigra pars compacta (SNc) or VTA. We demonstrate that: 1. STN-HFS leads to an increase in DA in the NAc, 2., these effects are more pronounced in the NAc shell than in the NAc core, 3. STN-HFS leads to a decrease in GABA in the VTA, 4. preceding lesion of the SNc does not seem to affect the effect of STN-HFS on accumbal DA transmission whereas 5. preceding lesion of the VTA seems to prohibit further detection of DA in the NAc. We conclude that STN-HFS significantly affects neurotransmission in the limbic system, which might contribute to explain the non-motor effects of STN-HFS.

Studies of the neural mechanisms of deep brain stimulation in rodent models of Parkinson's disease

Several rodent models of deep brain stimulation (DBS) have been developed in recent years. Electrophysiological and neurochemical studies have been performed to examine the mechanisms underlying the effects of DBS. In vitro studies have provided deep insights into the role of ion channels in response to brain stimulation. In vivo studies reveal neural responses in the context of intact neural circuits. Most importantly, recording of neural responses to behaviorally effective DBS in freely moving animals provides a direct means for examining how DBS modulates the basal ganglia thalamocortical circuits and thereby improves motor function. DBS can modulate firing rate, normalize irregular burst firing patterns and reduce low frequency oscillations associated with the Parkinsonian state. Our current efforts are focused on elucidating the mechanisms by which DBS effects on neural circuitry improve motor performance. New behavioral models and improved recording techniques will aide researchers conducting future DBS studies in a variety of behavioral modalities and enable new treatment strategies to be explored, such as closed-loop stimulations based on real time computation of ensemble neural activity.

Sensory (nociceptive) stimulation evokes Fos expression in the subthalamus of hemiparkinsonian rats

OBJECTIVE

In an effort to understand cell activity patterns and sensorimotor integration in Parkinson's disease, we have explored the expression of the Fos protein in the subthalamus after sensory (nociceptive) stimulation of hemiparkinsonian Sprague-Dawley rats [6-hydroxydopamine [6OHDA]-lesioned]. Fos is a marker for neuronal activity in most areas of the brain and the subthalamus is a major driving force of the basal ganglia and target for surgical intervention in parkinsonian patients.

METHODS

The medial forebrain bundle (major tract carrying dopaminergic nigrostriatal axons) was injected with either 6OHDA or saline (controls). A week later, some rats were subjected to mechanical stimulation (pinching; activating nociceptive pathways) of the hindpaw for 2 hours, while others received no stimulation. Thereafter, brains were processed using routine tyrosine hydroxylase (TH; marker for dopaminergic cells) or Fos immunocytochemistry.

RESULTS

In the cases that had saline injections combined with mechanical stimulation or with no stimulation, as well as those that had 6OHDA lesions combined with no stimulation, there were no Fos+ cells in the subthalamus. However, in the cases that had 6OHDA-lesions combined with mechanical stimulation, there were many Fos+ cells within the subthalamus of both sides, particularly on the ipsilateral side.

DISCUSSION

Our results indicate that after an increase in sensory (nociceptive) activity, via mechanical stimulation, there is an induction of Fos expression in the subthalamus of 6OHDA-lesioned cases. We suggest that activating nociceptive pathways exacerbates the abnormal cell activity in the basal ganglia generated by the hemiparkinsonian condition.

Deep brain stimulation and medication for parkinsonian tremor during secondary tasks

This study examined the efficacy of subthalamic nucleus (STN), deep brain stimulation (DBS), and medication for resting tremor during performance of secondary tasks. Hand tremor was recorded using accelerometry and electromyography (EMG) from 10 patients with Parkinson's disease (PD) and ten matched control subjects. The PD subjects were examined off treatment, on STN DBS, on medication, and on STN DBS plus medication. In the first experiment, tremor was recorded in a quiet condition and during a cognitive task designed to enhance tremor. In the second experiment, tremor was recorded in a quiet condition and during isometric finger flexion (motor task) with the contralateral limb at 5% of the maximal voluntary contraction (MVC) that was designed to suppress tremor. Results showed that: (1) STN DBS and medication reduced tremor during a cognitive task that exacerbated tremor, (2) STN DBS normalized tremor frequency in both the quiet and cognitive task conditions, whereas tremor amplitude was only normalized in the quiet condition, (3) a secondary motor task reduced tremor in a similar manner to STN DBS. These findings demonstrate that STN DBS still suppresses tremor in the presence of a cognitive task. Furthermore, a secondary motor task of the opposite limb suppresses tremor to levels comparable to STN DBS.

High-frequency stimulation of the subthalamic nucleus prolongs the increase in striatal dopamine induced by acute l-3,4-dihydroxyphenylalanine in dopaminergic denervated rats

High-frequency stimulation of the subthalamic nucleus (STN-HFS) is a powerful approach for treating the motor symptoms of Parkinson's disease (PD). It results in clinical improvement in patients with PD, further reducing the l-3,4-dihydroxyphenylalanine (L-DOPA) requirement and thus L-DOPA-induced dyskinesia. However, it remains unclear how STN-HFS modifies the response to L-DOPA. We investigated the effect of STN-HFS on striatal extracellular concentrations of dopamine and its metabolites following acute L-DOPA administration in intact or partially dopaminergic denervated (DA-PL) rats. L-DOPA treatment significantly increased striatal dopamine levels in intact and DA-PL animals, with the maximal effect observed 1 h after L-DOPA injection. This increase was more pronounced in DA-PL rats (ipsilateral to the lesion) than in intact animals. It remained fairly stable 1 h after the maximal effect of L-DOPA and then decreased towards basal values. STN-HFS in intact rats had no effect on the maximal L-DOPA-induced increase in striatal extracellular dopamine concentration or the return to basal values, the profiles observed being similar to those for non-stimulated intact animals. Conversely, STN-HFS amplified the L-DOPA-induced increase in striatal dopamine levels during the stimulation period (1 h) in DA-PL rats and this increase was sustained throughout the post-stimulation period (2.5 h), without the return to basal levels observed in stimulated intact and non-stimulated rats. These new neurochemical data suggest that STN-HFS interferes with L-DOPA effects, probably synergically, by stabilizing dopamine levels in the striatum and shed light on the mechanisms of STN-HFS in PD.

Effects of STN DBS on memory guided force control in Parkinson's disease (June 2007)

This study examined the control of elbow force in nine patients with Parkinson's disease when visual feedback was available and when visual feedback was removed to determine how medication (Meds) and unilateral deep brain stimulation (DBS) of the subthalamic nucleus (STN) affect memory guided force control. Patients were examined in each of four treatment conditions: 1) off treatment; 2) Meds; 3) STN DBS; and 4) Meds plus STN DBS. With visual feedback available, there was no difference in force output across treatment conditions. When visual feedback was removed force output drifted under the target in both the off-treatment and the Meds conditions. However, when on STN DBS or Meds plus STN DBS force output drifted above the target. As such, only STN DBS had a significant effect on force output in the vision removed condition. Increased force output when on STN DBS may have occurred due to disruptions in the basal ganglia-thalamo-cortical circuitry. We suggest that modulation of output of the internal segment of the globus pallidus by STN DBS may drive the effect of STN DBS on memory guided force control.

High-frequency stimulation of the subthalamic nucleus increases glutamate in the subthalamic nucleus of rats as demonstrated by in vivo enzyme-linked glutamate sensor

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) is an effective therapy for Parkinson's disease; however, the mechanism whereby DBS ameliorates the symptoms of Parkinson's disease remains an area of intense research. In the present study, we investigated the hypothesis that the neurotransmitter glutamate is released within the STN during high-frequency stimulation (HFS) of the STN. Direct measurements of extracellular glutamate concentration in the STN were made using a dual enzyme-based electrochemical sensor. The studies were carried out in ketamine/xylazine anesthetized rats placed in a Kopf stereotaxic head frame. Various electrical stimulations (100-micros cathodic pulses; 100-3000 microA; 10- to 1000-Hz frequency; 5-s to 60-min stimulus durations) using bipolar stimulating electrodes were delivered to the STN. Stimulation of the STN elevated the concentration of glutamate in the STN. The concentration of glutamate rose quickly during HFS, remained elevated for the duration of stimulation, and descended slowly towards baseline upon cessation of stimulation. Elevation of the extracellular concentration of glutamate in the STN may be an important mechanism whereby DBS in the STN improves the symptoms of Parkinson's disease. Furthermore, our data argue against the hypothesis that DBS works primarily by electrotonic inhibition of the stimulated structure.

Antidromic propagation of action potentials in branched axons: implications for the mechanisms of action of deep brain stimulation

Electrical stimulation of the central nervous system creates both orthodromically propagating action potentials, by stimulation of local cells and passing axons, and antidromically propagating action potentials, by stimulation of presynaptic axons and terminals. Our aim was to understand how antidromic action potentials navigate through complex arborizations, such as those of thalamic and basal ganglia afferents-sites of electrical activation during deep brain stimulation. We developed computational models to study the propagation of antidromic action potentials past the bifurcation in branched axons. In both unmyelinated and myelinated branched axons, when the diameters of each axon branch remained under a specific threshold (set by the antidromic geometric ratio), antidromic propagation occurred robustly; action potentials traveled both antidromically into the primary segment as well as "re-orthodromically" into the terminal secondary segment. Propagation occurred across a broad range of stimulation frequencies, axon segment geometries, and concentrations of extracellular potassium, but was strongly dependent on the geometry of the node of Ranvier at the axonal bifurcation. Thus, antidromic activation of axon terminals can, through axon collaterals, lead to widespread activation or inhibition of targets remote from the site of stimulation. These effects should be included when interpreting the results of functional imaging or evoked potential studies on the mechanisms of action of DBS.

Bilateral high-frequency stimulation of the subthalamic nucleus on attentional performance: transient deleterious effects and enhanced motivation in both intact and parkinsonian rats

It is now well established that subthalamic nucleus high-frequency stimulation (STN HFS) alleviates motor problems in Parkinson's disease. However, its efficacy for cognitive function remains a matter of debate. The aim of this study was to assess the effects of STN HFS in rats performing a visual attentional task. Bilateral STN HFS was applied in intact and in bilaterally dopamine (DA)-depleted rats. In all animals, STN HFS had a transient debilitating effect on all the variables measured in the task. In DA-depleted rats, STN HFS did not alleviate the deficits induced by the DA lesion such as omissions and latency to make correct responses, but induced perseverative approaches to the food magazine, an indicator of enhanced motivation. In sham-operated controls, STN HFS significantly reduced accuracy and induced perseverative behaviour, mimicking partially the effects of bilateral STN lesions in the same task. These results are in line with the hypothesis that STN HFS only partially mimics inactivation of STN produced by lesioning and confirm the motivational exacerbation induced by STN inactivation.

Reversible and irreversible knockout of the ventroposterolateral thalamic nucleus measured by intracerebral SEP recordings in the rat brain--an aid to neuronavigation in small nuclei

Centrally active drugs are often hard to administer because of the blood brain barrier, and frequently high systemic doses are required to reach sufficient brain parenchyma concentrations, since these drugs are, additionally, diluted in the total blood volume. Moreover, topical administration via the systemic route is not possible. We here propose a technique for the local, quantitative deposition of active substances at defined intracerebral targets, e.g. the thalamic nuclei. We used a long micropipette and stereotactically advanced it to the desired coordinates under electrophysiological control. The pipette acted as both an electrode for intracerebral recordings and as a transportation means for the drug. The amplitude of intracerebral evoked potentials relayed by the thalamic nucleus to the sensorimotor cortex indicated the distance between the pipette tip and the neurons of the targeted nucleus. Data were obtained from anesthetized rats, where the micropipette was advanced towards the nucleus ventralis posterolateralis (VPL) during contralateral electrical forepaw stimulation and intracerebral recording of somatosensory evoked potentials. Within the VPL we either injected lidocaine or kainic acid, both resulting in an attenuation of the intracerebral as well as the cortical evoked potentials. This proposed tool may be useful for functional investigations of deep brain structures.

Treatment of late lithium-pilocarpine-induced status epilepticus with diazepam

PURPOSE

It is well known that status epilepticus (SE) becomes increasingly difficult to control over time. Previous studies have indicated that the electroencephalographic pattern at the time of intervention is predictive of the probability of successful treatment. However, these findings are based on studies limited to the first 2h of SE onset. Little data is available on the efficacy of treating SE at later time points.

METHODS

The aim of the present study was to investigate the efficacy of diazepam (DZP) treatment given at two different phases of SE in a lithium-pilocarpine rat model: during continuous ictal discharges (CIDs, phase 3), and during late periodic epileptiform discharges (late PEDs, phase 5). Changes in cortical and hippocampal electroencephalographs (EEGs) were observed continuously during the phases of SE, as well as at 24, 36, 48, and 72h after SE onset. The effects of DZP treatment during CIDs or during late PEDs were compared to control DZP-untreated rats.

RESULTS

In all three groups, hippocampal and cortical EEGs displayed five distinct phases of SE. There were no statistical differences in the duration of phases 1 and 2 among the three groups. Although DZP administration during CIDs did not terminate CIDs in most rats, it did significantly shorten the duration of phases 3 and 4 of SE. Importantly, DZP given during phase 5 successfully ended behavioral and electrographic seizures in most rats.

CONCLUSIONS

Hippocampal and cortical EEGs displayed five distinct phases of SE that were similarly responsive to DZP treatment. Termination of electrographic seizures with DZP treatment was more effective in the last phase of SE (late PEDs) than in phase 3 (CIDs). These findings suggest that previous reports of DZPs decrease in efficacy over time may not be applicable to DZP treatment at 4h-post onset.

High-frequency stimulation of the subthalamic nucleus modulates the activity of pedunculopontine neurons through direct activation of excitatory fibres as well as through indirect activation of inhibitory pallidal fibres in the rat

Recent data suggest a potential role of pedunculopontine nucleus (PPN) electrical stimulation in improving gait and posture in Parkinson's disease. Because the PPN receives fibres from the subthalamic nucleus (STN), we investigated the effects of STN-high-frequency stimulation (HFS) on PPN neuronal activity in intact rats and in rats bearing either an ibotenate lesion of the entopeduncular nucleus (EP) or a lesion of the substantia nigra (SN). The main response of PPN neurons to STN single-shock stimulations in the three experimental groups was a short latency (4.5 +/- 2.1 ms) and brief (15.3 +/- 6.5 ms) excitation. This response was maintained during 1-5 s of STN-HFS (130 Hz, 60 micros, 100-1000 microA). In EP-lesioned rats the percentage (75.0%) of PPN neurons showing a modulation of activity following STN-HFS was significantly higher compared with that observed in intact (39.7%) and in SN-lesioned rats (35.4%). Furthermore, in EP-lesioned rats the most frequent response of PPN neurons following STN-HFS was a 5-20 s excitation, which was present in 76.6% of responsive neurons in comparison to 15.4% and 9.1% of neurons responsive in intact and in 6-hydroxydopamine-lesioned rats, respectively. Neurons responsive to STN-HFS in the three experimental groups showed either a sharp positively skewed distribution of interspike intervals or multisecond oscillations in autocorrelograms. The results support that STN-HFS modulates the PPN through a balance of excitatory and inhibitory influences, which may be independent from the dopaminergic nigral neurons. In the absence of inhibitory EP fibres, the direct excitatory influence exerted by the STN on the PPN appears to predominate.

Suppression of axonal conduction by sinusoidal stimulation in rat hippocampusin vitro

Deep brain stimulation (DBS), also known as high frequency stimulation (HFS), is a well-established therapy for Parkinson's disease and essential tremor, and shows promise for the therapeutic control of epilepsy. However, the direct effect of DBS on neural elements close to the stimulating electrode remains an important unanswered question. Computational studies have suggested that HFS has a dual effect on neural elements inhibiting cell bodies, while exciting axons. Prior experiments have shown that sinusoidal HFS (50 Hz) can suppress synaptic and non-synaptic cellular activity in several in vitro epilepsy models, in all layers of the hippocampus. However, the effects of HFS on axons near the electrode are still unclear. In the present study, we tested the hypothesis that HFS suppresses axonal conduction in vitro. Sinusoidal HFS was applied to the alvear axon field of transverse rat hippocampal slices. The results show that HFS suppresses the alvear compound action potential (CAP) as well as the CA1 antidromic evoked potential (AEP). Complete suppression was observed as a 100% reduction in the amplitude of the evoked field potential for the duration of the stimulus. Evoked potential width and latency were not significantly affected by sinusoidal HFS. Suppression was dependent on HFS amplitude and frequency, but independent of stimulus duration and synaptic transmission. The frequency dependence of sinusoidal HFS is similar to that observed in clinical DBS, with maximal suppression between 50 and 200 Hz. HFS produced not only suppression of axonal conduction but also a correlated rise in extracellular potassium. These data provide new insights into the effects of HFS on neuronal elements, and show that HFS can block axonal activity through non-synaptic mechanisms.

Alteration of hormone and neurotransmitter production in cultured cells by high and low frequency electrical stimulation

HFS has become a widely used method in functional neurosurgery. However, its mechanism is not well understood, and its cellular and molecular effects have not yet been investigated. The aim of the study was to understand which cellular events, unrelated to the network organization of cells or neurons, participate in the mechanism of action of HFS. In vitro cellular effects of high (HFS) and low (LFS) frequency electrical stimulation on prolactin secretion in GH3 cell lines (prolactinoma), as well as the catecholaminergic secretion on PC12 cells (pheochromocytoma) were investigated. Cells were cultured in dishes with integrated electrodes to deliver stimulation at the same parameters as those used in clinical conditions to treat advanced forms of Parkinson's disease. Prolactin production was measured in GH3 using a Radio-Immuno-Assay. Dopamine, epinephrine and norepinephrine were measured in PC12 using Enzymo-immuno-assays. HFS for 24 hours reduced prolactin secretion by 40.3%, dopamine by 32.7%, epinephrine by 18.1% (non significant) and norepinephrine by 27.0%. LFS did not induce significant changes. These results suggest that HFS has an inhibitory impact on the cellular machinery responsible for hormone and neurotransmitter production. In this model of isolated cultured cells, network interactions and particularly presynaptic actions are discarded. HFS has inhibitory effects on cellular mechanisms responsible for the production and release of molecules participating in intercellular communication. This HFS-induced inhibition might participate in the lesion-like effect of therapeutic HFS in the basal ganglia during various movement disorders.

Studies of the neural mechanisms of deep brain stimulation in rodent models of Parkinson's disease

Several rodent models of deep brain stimulation (DBS) have been developed in recent years. Electrophysiological and neurochemical studies have been performed to examine the mechanisms underlying the effects of DBS. In vitro studies have provided deep insights into the role of ion channels in response to brain stimulation. In vivo studies reveal neural responses in the context of intact neural circuits. Most importantly, recording of neural responses to behaviorally effective DBS in freely moving animals provides a direct means for examining how DBS modulates the basal ganglia thalamocortical circuits and thereby improves motor function. DBS can modulate firing rate, normalize irregular burst firing patterns and reduce low-frequency oscillations associated with the Parkinsonian state. Our current efforts are focused on elucidating the mechanisms by which DBS effects on neural circuitry improve motor performance. New behavioral models and improved recording techniques will aide researchers conducting future DBS studies in a variety of behavioral modalities and enable new treatment strategies to be explored, such as closed-loop stimulations based on real-time computation of ensemble neural activity.

Repetitive Activation of Glutamatergic Inputs Evokes a Long-Lasting Excitation in Rat Globus Pallidus Neurons In Vitro

External globus pallidus (GPe) neurons express abundant metabotropic glutamate receptor 1 (mGluR1) in their somata and dendrites and receive glutamatergic inputs mainly from the subthalamic nucleus. We investigated whether synaptically released glutamate could activate mGluR1s using whole cell and cell-attached recordings in rat brain slice preparations. Repetitive internal capsule stimulation evoked EPSPs followed by a slow depolarizing response (sDEPO) lasting 10–20 s. Bath application of both GABAA and GABAB receptor antagonists increased the amplitude of sDEPOs. A mixture of AMPA/kainate and N-methyl-d-aspartate receptor antagonists did not alter sDEPOs. The induction of sDEPOs was only partially mediated by mGluR1 because mGluR1 antagonists reduced but failed to completely block the responses. Voltage-clamp recordings revealed that slow inward currents sensitive to mGluR1 antagonist were larger at −60 than at −100 mV, whereas the currents insensitive to mGluR1 antagonist were larger at −100 than at −60 mV. In cell-attached recordings, repetitive internal capsule stimulation evoked long-lasting excitations in GPe neurons, which were also partially suppressed by mGluR1 antagonists. Application of a glutamate uptake inhibitor or an mGluR1 agonist significantly increased the spontaneous firing rate but decreased the excitations to repetitive stimulation. These results suggest that synaptically released glutamate can activate mGluR1, contributing to the induction of long-lasting excitation in GPe neurons and that background mGluR1 activation suppresses the slow mGluR1 responses. Thus mGluR1 may play important roles in the control of GPe neuronal activity.

High frequency deep brain stimulation: what are the therapeutic mechanisms?

High frequency deep brain stimulation (HFS) used to treat the symptoms of Parkinson's disease (PD) was first assumed to act by reducing an excessive tonic GABAergic inhibitory output from the internal globus pallidus (GPi). Stimulation in GPi might produce this directly by mechanisms such as depolarization block or activation of presynaptic inhibitory fibers, and the same mechanisms evoked by HFS in the subthalamic nucleus (STN) could reduce the excitatory action of STN on GPi neurons. Although somatic recordings from neurons near the stimulation site may appear to support this potential mechanism, the action downstream from the site of stimulation often is not consistent with this interpretation. A more parsimonious explanation for the similar effects of HFS in STN or GPi and a lesion of either of these structures is that both HFS and pallidotomy interrupt an abnormal pattern of firing in cortico-basal ganglia-thalamocortical loops that is responsible for the symptoms of PD.

Subthalamic nucleus lesions increase impulsive action and decrease impulsive choice - mediation by enhanced incentive motivation?

The subthalamic nucleus (STN) is traditionally thought of as part of a system involved in motor control but recent evidence suggests that it may also play a role in other psychological processes. Here we examined the effects of STN lesions on two measures of impulsivity and found that STN lesions increased 'impulsive action' (produced behavioral disinhibition), as measured by performance on a differential reinforcement of low rates of responding task, but decreased 'impulsive choice' (impulsive decision making), as measured by a delay discounting task. In addition, amphetamine and food restriction increased 'impulsive action' and decreased 'impulsive choice' to a greater extent in STN-lesioned animals than in sham controls. We speculate that these apparently discrepant effects may be because STN lesions enhance the incentive salience assigned to rewards. These findings suggest that the STN may serve as a novel target for the treatment of psychological disorders characterized by deficits in behavioral control, such as drug addiction and attention deficit hyperactivity disorder.

Atomoxetine produces changes in cortico-basal thalamic loop circuits: assessed by phMRI BOLD contrast

Atomoxetine is a selective noradrenaline reuptake inhibitor used in the treatment of attention deficit hyperactivity disorder (ADHD) which has not yet been assessed using pharmacological neuroimaging for its effects on rat brain activity. The pharmacological magnetic resonance imaging (phMRI) blood oxygenation level dependent (BOLD) response was determined in rat brain regions following administration of atomoxetine. Rats were individually placed into a 2.35T Bruker magnet for 60min to achieve basal recording of changes in signal intensity. Either saline (n=9) or atomoxetine hydrochloride (2mg/kg; i.p.; n=10) was then administered and recording continued for a further 90min. Data were analysed for BOLD random effects using statistical parametric maps and time course analysis. The main changes observed were widespread negative BOLD responses in the caudate putamen and changes in brain regions associated with the cortico-basal thalamic loop circuits. BOLD changes in the basal ganglia help explain its efficacy in reducing hyperactivity observed in ADHD patients. Although positive BOLD changes in the prefrontal cortex were limited to the ventral orbital cortex this is an area associated with behavioral control and may be of relevance to the use of the drug in ADHD.

Surgical Treatment of Movement Disorders

A substantial body of evidence has accumulated regarding the efficacy and safety of neurosurgery for Parkinson's disease, essential tremor, and dystonia. Surgery for movement disorders (thalamotomy, pallidotomy, and subthalamic nucleotomy or subthalamotomy) was largely ablative (lesion-based). Given the safety and anatomy-preservation advantage, long-term electrical stimulation of these same targets (thalamus, globus pallidus, and subthalamic nucleus) is discussed as the treatment of choice. High-frequency deep brain stimulation procedures replicate the effects of ablative interventions, but do not require making a destructive brain lesion. This article outlines patient eligibility for surgery, targeting techniques, intraoperative findings, and potential complications and discusses the outcomes expected for each of the major interventions for which clinical trial data are available.

Functional pharmacology in human brain

Most neurological and psychiatric disorders involve selective or preferential impairments of neurotransmitter systems. Therefore, studies of functional transmitter pathophysiology in human brain are of unique importance in view of the development of effective, mechanism-based, therapeutic modalities. It is well known that central nervous system functional proteins, including receptors, transporters, ion channels, and enzymes, can exhibit high heterogeneity in terms of structure, function, and pharmacological profile. If the existence of types and subtypes of functional proteins amplifies the possibility of developing selective drugs, such heterogeneity certainly increases the likelihood of interspecies differences. It is therefore essential, before choosing animal models to be used in preclinical pharmacology experimentation, to establish whether functionally corresponding proteins in men and animals also display identical pharmacological profiles. Because of evidence that scaffolding proteins, trafficking between plasma membrane and intracellular pools, phosphorylation and allosteric modulators can affect the function of receptors and transporters, experiments with human clones expressed in host cells where the environment of native receptors is rarely reproduced should be interpreted with caution. Thus, the use of neurosurgically removed fresh human brain tissue samples in which receptors, transporters, ion channels, and enzymes essentially retain their natural environment represents a unique experimental approach to enlarge our understanding of human brain processes and to help in the choice of appropriate animal models. Using this experimental approach, many human brain functional proteins, in particular transmitter receptors, have been characterized in terms of localization, function, and pharmacological properties.

Cellular mechanisms preventing sustained activation of cortex during subcortical high-frequency stimulation

Axonal excitation has been proposed as a key mechanism in therapeutic brain stimulation. In this study we examined how high-frequency stimulation (HFS) of subcortical white matter tracts projecting to motor cortex affects downstream postsynaptic responses in cortical neurons. Whole cell recordings were performed in the primary motor cortex (M1) and ventral thalamus of rat brain slices. In M1, neurons showed only an initial depolarization in response to HFS, after which the membrane potential returned to prestimulation levels. The prolonged suppression of excitation during stimulation was neither associated with GABAergic inhibition nor complete action potential failure in stimulated axons. Instead we found that HFS caused a depression of excitatory synaptic currents in postsynaptic neurons that was specific to the stimulated subcortical input. These data are consistent with the hypothesis that axonal HFS produces a functional deafferentation of postsynaptic targets likely from depletion of neurotransmitter.

Subthalamic nucleus lesions enhance the psychomotor-activating, incentive motivational, and neurobiological effects of cocaine

The subthalamic nucleus (STN) is traditionally thought to be involved in motor control, and dysfunction of the STN is thought to contribute to movement disorders. Here, we show that the STN also plays an important role in motivational processes and the response to drugs of abuse. Specifically, bilateral STN lesions produced a dose-dependent increase in the psychomotor-activating effects of cocaine, the rate at which animals acquired cocaine self-administration, and the motivation for cocaine assessed using a progressive ratio schedule. Furthermore, bilateral STN lesions enhanced the ability of cocaine to induce gene expression in the nucleus accumbens and caudate-putamen, two structures known to be involved in mediating the psychomotor-activating and incentive motivational effects of drugs of abuse. These findings suggest that engagement of the STN serves to dampen the psychomotor-activating and incentive motivational effects of drugs of abuse. Thus, the STN may serve as a novel target for therapeutic interventions aimed at treating drug dependence.

Subthalamic oscillatory activities at beta or higher frequency do not change after high-frequency DBS in Parkinson's disease

This study aimed to assess whether changes in the patterns of local field potential (LFP) oscillations of the subthalamic nucleus (STN) underlie to the clinical improvement within 60 s after turning off subthalamic DBS. We studied by spectral analysis the STN LFPs recorded in 13 nuclei from 7 patients with Parkinson's disease before and immediately after unilateral high-frequency (130 Hz) stimulation of the same nucleus, when the clinical benefit of DBS was unchanged. The results were compared with LFP data previously reported [A. Priori, G. Foffani, A. Pesenti, F. Tamma, A.M. Bianchi, M. Pellegrini et al., Rhythm-specific pharmacological modulation of subthalamic activity in Parkinson's disease. Exp. Neurol. 189 (2004) 369-379]--namely 13 STN from 9 parkinsonian patients recorded before and after levodopa administration--which were used as a control. Before DBS, in the 'off' clinical state after overnight withdrawal of dopaminergic therapy, the STN spectrum did not significantly differ from the control nuclei, showing prominent activity at beta frequencies (13-20 and 20-35 Hz). After DBS (10-15 min) of the STN, the recorded nuclei significantly differed from the control, failing to show significant changes either in the beta bands or at higher frequencies (60-90 and 250-350 Hz). The patterns of subthalamic LFP oscillations after DBS therefore differ from those after dopaminergic medication. These results suggest (1) that subthalamic LFP modulations are not the epiphenomenon of peripheral motor improvement and (2) that the transitory clinical efficacy maintained after discontinuation of subthalamic DBS is not associated with local modulation of LFP activity at beta or higher frequencies within the STN.

Dopamine neurons derived from embryonic stem cells efficiently induce behavioral recovery in a Parkinsonian rat model

To test the in vivo effect of a high yield of dopaminergic (DA) neurons (90% of total neurons) which had been generated from a genetically modified mouse embryonic stem cell line, N2, the cells were transplanted into a rat model of Parkinson's disease (PD). The PD animals grafted with N2-derived cells showed significant behavior improvements compared with sham controls from 2 weeks posttransplantation, whereas animals with naïve D3-derived cells ( approximately 28% DA neurons of total neurons) showed only a modest recovery. Furthermore, hyperactivity observed in the subthalamic nucleus, pedunculopontine nucleus, and substantia nigra pars reticulata of PD rat models was dramatically reduced by the grafting of N2-derived cells. The number of DA neurons in the striatum which originated from N2 grafting was much higher compared to that from D3 grafting, and the neurons efficiently released DA in the brain, showing a good correlation with behavioral recovery.

Dopamine efflux in the rat striatum evoked by electrical stimulation of the subthalamic nucleus: potential mechanism of action in Parkinson's disease

The precise mechanism whereby continuous high-frequency electrical stimulation of the subthalamic nucleus ameliorates motor symptoms of Parkinson's disease is unknown. We examined the effects of high-frequency stimulation of regions dorsal to and within the subthalamic nucleus on dopamine efflux in the striatum of urethane-anaesthetized rats using constant potential amperometry. Complementary extracellular electrophysiological studies determined the activity of subthalamic nucleus neurons in response to similar electrical stimulation of the subthalamic nucleus. High-frequency stimulation of the subthalamic nucleus increased action potential firing in the subthalamic nucleus only during the initial stimulation period and was followed by a cessation of firing over the remainder of stimulation. Electrical stimulation of the subthalamic nucleus with 15 pulses elicited stimulus-time-locked increases in striatal dopamine efflux with maximal peak effects occurring at 50 Hz frequency and 300 microA intensity. Extended subthalamic nucleus stimulation (1000 pulses at 50 Hz; 300 microA) elicited a similar peak increase in striatal dopamine efflux that was followed by a relatively lower steady-state elevation in extracellular dopamine over the course of stimulation. In contrast, extended stimulation immediately adjacent and dorsal to the subthalamic nucleus resulted in an 11-fold greater increase in dopamine efflux that remained elevated over the course of the stimulation. Immunohistochemical staining for tyrosine hydroxylase revealed catecholaminergic fibers running immediately dorsal to and through the subthalamic nucleus. Taken together, these results suggest that enhanced dopamine release within the basal ganglia may be an important mechanism whereby high-frequency stimulation of the subthalamic nucleus improves motor symptoms of Parkinson's disease.

Basal ganglia neural responses during behaviorally effective deep brain stimulation of the subthalamic nucleus in rats performing a treadmill locomotion test

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) is an effective treatment for Parkinson's disease (PD). In spite of proven therapeutic success, the mechanism underlying the benefits of DBS has not been resolved. A multiple-channel single-unit recording technique was used in the present study to investigate basal ganglia (BG) neural responses during behaviorally effective DBS of the STN in a rat model of PD. Rats underwent unilateral dopamine (DA) depletion by injection of 6-hydroxyDA (6-OHDA) into one side of the medial forebrain bundle and subsequently developed a partial akinesia, which was assessed during the treadmill locomotion task. High frequency stimulation (HFS) of the STN restored normal treadmill locomotion behavior. Simultaneous recording of single unit activity in the striatum (STR), globus pallidus (GP), substantia nigra pars reticulata (SNr), and STN revealed a variety of neural responses during behaviorally effective HFS of the STN. Predominant inhibitory responses appeared in the STN stimulation site. Nearly equal numbers of excitatory and inhibitory responses were found in the GP and SNr, whereas more rebound excitatory responses were found in the STR. Mean firing rate did not change significantly in the STR, GP, and SNr, but significantly decreased in both sides of STN during DBS. A decrease in firing rate in the contralateral side of STN provides neural substrate for the clinical observation that unilateral DBS produces bilateral benefits in patients with PD. In addition to the firing rate changes, a decrease in burst firing was observed in the GP and STN. The present study indicates that DBS induces complex modulations of the BG circuit and further suggests that BG network reorganization, rather than a simple excitation or inhibition, may underlie the therapeutic effects of DBS in patients with PD.

Matching geometry and stimulation parameters of electrodes for deep brain stimulation experiments—Numerical considerations

Deep brain stimulation, the electric stimulation of basal ganglia nuclei, is a treatment for movement disorders such as Parkinson's disease. The underlying mechanisms are studied in animals, e.g. rodents. Designs and materials of commercially available microelectrodes, as well as experimentally applied driving signals vary tremendously. We used finite integration modeling to compare the electric field and current density distributions induced by various electrodes. Current density or field strength "hot spots", which are located particularly at sites of high curvature and material interfaces coincided with corrosion and erosion at poles and insulation, respectively, as shown by scanning electron microscopy of stainless steel electrodes. Cell constants, i.e. geometry factors relating the electrode impedance to the specific medium conductivity, were calculated to determine the electrode voltage for a given stimulation current. Nevertheless, for electrodes of the same cell constant but of different geometry, current and field distributions may be very dissimilar. We found geometry-dependent limiting values of the stimulation current, above which electric tissue damage may occur. These values limit the reach of the stimulation signal for a given electrode geometry. Also, electrode geometries determine the shape of the stimulated tissue volume. This study provides tools for choosing the most appropriate geometry for targeting different-sized brain areas.

Surgical therapy for Parkinson's disease

High frequency stimulation (HFS) has become the main alternative to medical treatment, due to its reversibility, adaptability, and low morbidity. Initiated in the thalamus (Vim) for the control of tremor, HFS has been applied to the Pallidum (GPi), and then to the subthalamic nucleus (STN), suggested by experiments in MPTP monkeys. STN-HFS is highly efficient on tremor, rigidity and bradykinesia and is now widely applied. Criteria for success are correct patient selection and precise electrode placement. The best outcome predictor is the response to Levodopa. The mechanisms of action might associate inhibition of cell firing, jamming of neuronal message and exhaustion of synaptic neurotransmitter release. The inhibition of glutamate STN release could be neuroprotective on nigral cells. Animal experiments support this hypothesis, not contradicted by the long-term follow up of patients. Neuroprotection might have considerable impact on the management of PD patient and warrants clinical trials.

The functional role of the subthalamic nucleus in cognitive and limbic circuits

Once it was believed that the subthalamic nucleus (STN) was no more than a relay station serving as a "gate" for ascending basal ganglia-thalamocortical circuits. Nowadays, the STN is considered to be one of the main regulators of motor function related to the basal ganglia. The role of the STN in the regulation of associative and limbic functions related to the basal ganglia has generally received little attention. In the present review, the functional role of the STN in the control of cortico-basal ganglia-thalamocortical associative and limbic circuits is discussed. In the past 20 years the concepts about the functional role of the STN have changed dramatically: from being an inhibitory nucleus to a potent excitatory nucleus, and from being involved in hyperkinesias to hypokinesias. However, it has been demonstrated only recently, mainly by reports on the behavioral (side-) effects of STN deep brain stimulation (DBS), which is a popular surgical technique in the treatment of patients suffering from advanced Parkinson Disease (PD), that the STN is clinically involved in associative and limbic functions. These findings were confirmed by results from animal studies. Experimental studies applying STN DBS or STN lesions to investigate the neuronal mechanisms involved in these procedures found profound effects on cognitive and motivational parameters. The anatomical, electrophysiological and behavioral data presented in this review point towards a potent regulatory function of the STN in the processing of associative and limbic information towards cortical and subcortical regions. In conclusion, it can be stated that the STN has anatomically a central position within the basal ganglia thalamocortical associative and limbic circuits and is functionally a potent regulator of these pathways.

Suppression of secondary generalization of limbic seizures by stimulation of subthalamic nucleus in rats

OBJECT

Deep brain stimulation (DBS) of subcortical nuclei such as the subthalamic nucleus (STN) or the substantia nigra pars reticulata (SNR) may provide an alternative therapy for intractable epilepsy. The authors attempted to evaluate the antiepileptic effects of DBS to these structures in an experimental seizure model.

METHODS

Three groups of rats were prepared. In the first two groups, the rats underwent unilateral implantation of stimulation electrodes in the STN (six rats) or the SNR (six rats). A control group received no electrodes (six rats). Kainic acid (KA) was systemically administered to induce limbic seizures, which started with focal seizures and became generalized secondarily. High-frequency electrical stimulation of the STN or SNR was begun immediately after KA administration, and changes on electroencephalography (EEG) and the magnitude of clinical seizures were evaluated. Results showed that STN stimulation significantly reduced the duration of generalized seizures on EEG, although the total duration of seizures (generalized plus focal seizures) was unchanged. The duration of focal seizures on EEG was prolonged by STN DBS, a result possibly due to the suppression of secondary generalization. In addition, STN DBS reduced the severity of clinical seizures. On the other hand, stimulation of the SNR demonstrated no effect.

CONCLUSIONS

Unilateral STN DBS showed significant suppression of the secondary generalization of limbic seizures. Note, however, that SNR DBS was less effective, which implies that in addition to the nigral control of the epilepsy system, another antiepileptic mechanism such as antidromic stimulation of the corticosubthalamic pathway should be considered.

Pallidal origin of GABA release within the substantia nigra pars reticulata during high-frequency stimulation of the subthalamic nucleus

High-frequency stimulation of the subthalamic nucleus (HFS-STN) is an effective treatment for alleviating the motor symptoms of parkinsonian patients. However, the neurochemical basis of its effects remains unknown. We showed previously that 1 h of HFS-STN in normal rats increases extracellular glutamate (Glu) level in the output nuclei of the STN, the globus pallidus (GP), and the substantia nigra pars reticulata (SNr), consistent with an increase in the activity of STN neurons. HFS-STN also increases GABA levels in the SNr, but the origin of this increase is unclear. We investigated the effectiveness of HFS-STN for improving Parkinson's disease symptoms, using intracerebral microdialysis to determine the extracellular Glu and GABA levels of the GP and SNr in response to HFS-STN in anesthetized hemiparkinsonian rats [6-hydroxydopamine lesion of the substantia nigra pars compacta (SNc)]. Basal levels of Glu and GABA in the GP and SNr were significantly higher in hemiparkinsonian than in intact rats. HFS-STN did not affect extracellular Glu level in the SNr of hemiparkinsonian rats but doubled the level of GABA. Ibotenic acid lesion of the GP abolished the increase in GABA levels in the SNr induced by HFS-STN in SNc-lesioned rats. These results provide neurochemical confirmation of the hyperactivity of the STN after dopaminergic denervation and suggest that the therapeutic effects of HFS-STN may result partly from the stimulation of pallidonigral fibers, thereby revealing a potential role for pallidal GABA in the inhibition of basal ganglial output structures during HFS-STN.