Commentary: the pedunculopontine nucleus: clinical experience, basic questions and future directions

This issue is dedicated to a potential new target for the treatment of movement disorders, the pedunculopontine tegmental nucleus (PPTg), or, more simply, the pedunculopontine nucleus, that some authors abbreviate as PPN. We provide an overview of the field as an introduction to the general reader, beginning with the clinical experience to date of Mazzone and co-workers in Rome, some basic questions that need to be addressed, and potential future directions required in order to ensure that the potential benefits of this work are realized.

[Stimulation therapies for Parkinson's disease: over the past two decades]

Levodopa has been the mainstay of treatment for Parkinson's disease since the 1960s, but the dyskinesias it induces are a major drawback. High-frequency deep brain stimulation offers a safe, reversible alternative. Targets include the thalamus, pallidum, subthalamic nucleus and, more recently, the pedunculopntine nucleus, which requires low-frequency excitation. The subthalamic nucleus is the preferred target in Parkinson's disease. Other treatments such as gene therapy are in the pipeline.

Intracranial neurostimulation for pain control: a review

Intracranial neurostimulation for pain relief is most frequently delivered by stimulating the motor cortex, the sensory thalamus, or the periaqueductal and periventricular gray matter. The stimulation of these sites through MCS (motor cortex stimulation) and DBS (deep brain stimulation) has proven effective for treating a number of neuropathic and nociceptive pain states that are not responsive or amenable to other therapies or types of neurostimulation. Prospective randomized clinical trials to confirm the efficacy of these intracranial therapies have not been published. Intracranial neurostimulation is somewhat different than other forms of neurostimulation in that its current primary application is for the treatment of medically intractable movement disorders. However, the increasing use of intracranial neurostimulation for the treatment of chronic pain, especially for pain not responsive to other neuromodulation techniques, reflects the efficacy and relative safety of these intracranial procedures. First employed in 1954, intracranial neurostimulation represents one of the earliest uses of neurostimulation to treat chronic pain that is refractory to medical therapy. Currently, 2 kinds of intracranial neurostimulation are commonly used to control pain: motor cortex stimulation and deep brain stimulation. MCS has shown particular promise in the treatment of trigeminal neuropathic pain and central pain syndromes such as thalamic pain syndrome. DBS may be employed for a number of nociceptive and neuropathic pain states, including cluster headaches, chronic low back pain, failed back surgery syndrome, peripheral neuropathic pain, facial deafferentation pain, and pain that is secondary to brachial plexus avulsion. The unique lack of stimulation-induced perceptual experience with MCS makes MCS uniquely suited for blinded studies of its effectiveness. This article will review the scientific rationale, indications, surgical techniques, and outcomes of intracranial neuromodulation procedures for the treatment of chronic pain.

[Advances in application of deep brain stimulation in treatment of neuropsychological diseases]

Deep brain stimulation has drawn more and more concerns as a method to treat neuropsychological diseases. Compared with surgery and other methods using electrical stimulation, deep brain stimulation has advantages of clear targets, high selectivity, reversibility, titratability and non-ablation. A large body of clinical trials has shown that deep brain stimulation targeting various brain structures is able to alleviate the symptoms of Parkinson's disease, epilepsy, chronic pain and depression that are intractable with medicines and other methods, with few complications or side effects. Deep brain stimulation is now emerging as a promising approach for the treatment of resistant neuropsychological diseases.

The hippocampus and nucleus accumbens as potential therapeutic targets for neurosurgical intervention in schizophrenia

Schizophrenia is a chronic and disabling psychiatric illness that is often refractory to treatment. Psychotic symptoms (e.g. hallucinations and delusions) in schizophrenia are reliably correlated with excess dopamine levels in the striatum, and have more recently been related to excess metabolic activity in the hippocampus. Multiple lines of evidence suggest that aberrantly high hippocampal activity may, via hippocampal connections with the limbic basal ganglia, drive excessive dopamine release into the striatum. In the present paper, we hypothesize that inhibition or stabilization of neural activity with high-frequency electrical stimulation of the hippocampus or nucleus accumbens, through different mechanisms, would treat the positive symptoms of schizophrenia. Thus, we suggest a direction for further experimentation aimed at developing neurosurgical therapeutic approaches for this devastating disease.

Adopting new technologies in stroke rehabilitation: the influence of the US health care system

Stroke rehabilitation is entering a new era of technological innovation, including the development of robotic aids for therapy, peripheral electrical stimulation devices, and brain stimulation systems. These technologies have the potential to significantly improve the efficiency and efficacy of stroke rehabilitation. The United States health care system creates both opportunities for new technologies to be created and adopted, as well as important barriers. Inadequate support of clinical trials of the efficacy of new non-invasive devices is a particular concern for practitioners seeking to determine if new devices are clinically useful. Government support of clinical trials of efficacy, coupled with reform of FDA approval processes for novel therapies, is needed to create an evidence-based approach to improving stroke rehabilitation.

The history and future of deep brain stimulation

The advancement of electrical stimulation of the central nervous system has been a story of fits and bursts with numerous setbacks. In many ways, this history has paralleled the history of medicine and physics. We have moved from anecdotal observation to double-blinded, prospective randomized trials. We have moved from faradic stimulation to systems that lie completely under the skin and can deliver complex electrical currents to discrete areas of the brain while controlled through a device that is not much bigger than a PDA. This review will discuss how deep brain stimulation has developed into its current form, where we see the field going and the potential pitfalls along the way.

Deep brain stimulation for treatment-refractory obsessive-compulsive disorder: the search for a valid target

Obsessive-compulsive disorder (OCD) is a common psychiatric disease that is marked by recurring, anxiety-provoking thoughts (obsessions) accompanied by repetitive and time-consuming behaviors (compulsions). Among the controversies in the OCD literature is the issue of the origin of the disease and whether brain changes observed with modern imaging techniques are the causes or results of OCD behaviors and thoughts. These issues remain unresolved; however, significant strides have been made in understanding the illness from both phenomenological and pathophysiological perspectives. The current staple of OCD management remains pharmacological in nature and often occurs in conjunction with cognitive behavioral therapy. Refractory cases, however, are occasionally referred for neurosurgical consultation, and several procedures have been examined. Success in the treatment of Parkinson's disease, the reversibility of the therapy, and a relatively safe side-effect profile have allowed deep brain stimulation (DBS) to be examined as an alternative treatment for some psychiatric conditions. Here we assess the possibility of applying DBS to the treatment of OCD. Morphological, functional metabolic, and volumetric data point to several brain regions that are important to the etiology and maintenance of OCD. Converging evidence from the genetics and neurocircuitry literature suggests that several subcortical structures play prominent roles in the disease. The functional modification of these structures could potentially provide symptom relief. Here, we review the ablative and DBS procedures for refractory OCD, and provide a research-driven hypothesis that highlights the ventromedial head of the caudate nucleus, and structures up- and downstream from it, as potential DBS targets for treatment-resistant disease. We hope that a research-driven approach, premised on converging evidence and previous experience, will lead to a safe and effective DBS procedure that will benefit patients who remain disabled despite presently available therapies.

Translational principles of deep brain stimulation

Deep brain stimulation (DBS) has shown remarkable therapeutic benefits for patients with otherwise treatment-resistant movement and affective disorders. This technique is not only clinically useful, but it can also provide new insights into fundamental brain functions through direct manipulation of both local and distributed brain networks in many different species. In particular, DBS can be used in conjunction with non-invasive neuroimaging methods such as magnetoencephalography to map the fundamental mechanisms of normal and abnormal oscillatory synchronization that underlie human brain function. The precise mechanisms of action for DBS remain uncertain, but here we give an up-to-date overview of the principles of DBS, its neural mechanisms and its potential future applications.

The effects of subthalamic nucleus deep brain stimulation on parkinsonian tremor

Deep brain stimulation (DBS) of the ventral intermediate (Vim) nucleus of the thalamus has been the target of choice for patients with disabling essential tremor or medication refractory parkinsonian tremor. Recently there is evidence that the subthalamic nucleus (STN) should be the targets for patients with tremor associated with Parkinson's disease (PD). To assess the effects of STN DBS on parkinsonian tremor, eight consecutive patients with PD and disabling tremor were videotaped using a standardized tremor protocol. Evaluations were performed at least 12 h after last dose of medication with the DBS turned off followed by optimal DBS on state. A rater blinded to DBS status evaluated randomized video segments with the tremor components of the Unified Parkinson Disease Rating Scale (UPDRS) and Tremor Rating Scale (TRS). Compared with DBS off state there were significant improvements in mean UPDRS tremor score 79.4% (p=0.008), total TRS score 69.9% (p=0.008) and upper extremity 92.5% (p=0.008) TRS subscore. Functional improvement was noted with pouring liquids. Our findings provide support that STN DBS is an effective treatment of tremor associated with PD.

Deep brain stimulation in craniofacial pain: seven years' experience

Cluster headache (CH) is a primary headache with excruciatingly painful attacks that are strictly unilateral. About 10% of cases experience no significant remission, and about 15% of these do not respond to medication, so surgery is considered. Neuroimaging studies show that the posterior inferior hypothalamus is activated during CH attacks and is plausibly the CH generator. We report on 16 chronic CH patients, with headaches refractory to all medication, who received long-term hypothalamic stimulation following electrode implant to the posterior inferior hypothalamus. After a mean follow-up of 23 months, a persistent pain-free to almost pain-free state was achieved in 13/16 patients (15/18 implants; 83.3%) a mean of 42 days (range 1-86 days) after monopolar stimulation initiation. Ten patients (11 implants) are completely pain-free. A common side effect was transient diplopia, which limited stimulation amplitude. In one patient, a small non-symptomatic haemorrhage into the 3rd ventricle occurred following implant, but regressed 24 h later. Persistent side effects are absent except in one patient with bilateral stimulation, in whom stimulation was stopped to resolve vertigo and worsened bradycardia, but was resumed later without further problems. Hypothalamic stimulation is an effective, safe and well tolerated treatment for chronic drug-refractory CH. It appears as a valid alternative to destructive surgical modalities, and has the additional advantage of being reversible.

Transcranial and deep brain stimulation approaches as treatment for depression

Given that a considerable portion of depressed patients does not respond to or remit during pharmacotherapy, there is increasing interest in non-pharmacological strategies to treat depressive disorders. Several brain stimulation approaches are currently being investigated as novel therapeutic interventions beside electroconvulsive therapy (ECT), a prototypic method in this field with proven effectiveness. These neurostimulation methods include repetitive transcranial magnetic stimulation (rTMS), magnetic seizure therapy (MST), vagus nerve stimulation (VNS), deep brain stimulation (DBS) and transcranial direct current stimulation (tDCS). It is via different neuroanatomically defined "windows" that the various approaches access the neuronal networks showing an altered function in depression. Also, the methods vary regarding their degree of invasiveness. One or the other method may finally achieve antidepressant effectiveness with minimized side effects and constitute a new effective treatment for major depression.

Recent advances in the treatment of chronic pain with non-invasive brain stimulation techniques

BACKGROUND

Brain stimulation is a technique that can guide brain plasticity and thus be suitable to treat chronic pain-a disorder that is associated with substantial reorganisation of CNS activity. In fact, the idea of using invasive and non-invasive brain stimulation for pain relief is not new. Studies from the 1950s investigated the use of this therapeutic method for the treatment of chronic pain. However, recent advancements in the techniques of non-invasive brain stimulation have enhanced their modulatory effects and thus become a new, attractive alternative for chronic pain treatment.

RECENT DEVELOPMENTS

Recent studies with non-invasive brain stimulation--eg, repetitive transcranial magnetic stimulation (rTMS) and transcranial direct current stimulation (tDCS)--using new parameters of stimulation have shown encouraging results. These studies explored alternative sites of stimulation, such as the secondary somatosensory cortex (rather than primary motor cortex) for the treatment of chronic visceral pain and new parameters of stimulation, such as repeated sessions of tDCS with 2 mA for the treatment of chronic central pain. WHERE NEXT?: The investigation of non-invasive brain stimulation for therapeutic effects is in its at initial stages; but the preliminary data make us optimistic. Several questions still need to be addressed before any firm conclusion about this therapy is made. Other parameters of stimulation need to be further explored such as theta-burst stimulation and the combination of tDCS and rTMS. The duration of the therapeutic effects is another important issue to be considered, especially because the current devices for brain stimulation do not allow patients to receive this therapy in their homes; therefore, maintenance therapy regimens, as well as the development of portable stimulators, need to be investigated. Further trials must determine the optimum parameters of stimulation. After that, confirmatory, larger studies are mandatory.

DBS in tourette syndrome: rationale, current status and future prospects

Tourette syndrome is a neuropsychiatric disorder with onset in early childhood and characterized by tics, often associated with behavioural abnormalities. Symptoms often disappear before or during adulthood. Treatment consists of psychotherapy or pharmacotherapy. A small percentage of patients is treatment refractory. After the introduction of deep brain stimulation (DBS) of the thalamus as a new therapeutical approach in 1999, several other brain nuclei have been targeted in a small number of patients, like the globus pallidus internus, anteromedial and ventroposterolateral part, and the nucleus accumbens. In the published reports, a tic reduction rate of at least 66% is described. The effects of DBS on associated behavioural disorders are more variable. The number of treated patients is small and it is unclear whether the effects of DBS are dependent on the target nucleus. The pathophysiology of Tourette syndrome is not well understood. On the basis of our current knowledge of cortico-basal ganglia-thalamocortical circuits, an explanation for the beneficial effects of DBS on tics is proposed. It is concluded that a meticulous evaluation of the electrode position, and a blinded assessment of the clinical effects on tics and behavioural disorders, is absolutely mandatory in order to identify the best target of DBS for Tourette syndrome.

Deep brain stimulation improves orthostatic regulation of patients with Parkinson disease

OBJECTIVE

To evaluate whether subthalamic nucleus (STN) stimulation has an effect on the orthostatic regulation of patients with Parkinson disease (PD), we studied cardiovascular regulation during on and off phases of STN stimulation.

METHODS

We examined 14 patients with PD (mean age 58.1 +/- 5.8 years, 4 women, 10 men) with bilateral STN stimulators. Patients underwent 3 minutes of head-up tilt (HUT) testing during STN stimulation and after 90 minutes interruption of stimulation. We monitored arterial blood pressure (BP), RR intervals (RRI), respiration, and skin blood flow (SBF). Baroreflex sensitivity (BRS) was assessed as the square root of the ratio of low-frequency power of RRI to the low-frequency power of systolic BP for coherences above 0.5.

RESULTS

During the on phase of the STN stimulation, HUT induced no BP decrease, a significant tachycardia, and a significant decrease of SBF. During the off phase of stimulation, HUT resulted in significant decreases in BPsys and RRI and only a slight SBF decrease. HUT induced no change of BRS during stimulation, but lowered BRS when the stimulator was off (p < 0.05).

CONCLUSIONS

STN stimulation of patients with PD increases peripheral vasoconstriction and BRS and stabilizes BP, thereby improving postural hypotension in patients with PD. The results indicate that STN stimulation not only alleviates motor deficits but also influences autonomic regulation in patients with PD.

International diffusion of new health technologies: a ten-country analysis of six health technologies

OBJECTIVES

The objective of this study was to examine and explain the differential international diffusion of six health innovations.

METHODS

A retrospective diffusion study was undertaken of sildenafil, cyclooxygenase-II (COX II) inhibitors, beta interferon, verteporfin, deep brain stimulators, and drug-eluting coronary stents in ten countries-Australia, Canada, Denmark, France, The Netherlands, Norway, Spain, Sweden, Switzerland, and the United Kingdom. We plotted diffusion curves of daily defined doses per quarter, vials or implants per million population, and examined the association between diffusion and five key variables.

RESULTS

Canada, Switzerland, and Sweden are generally high users of new technologies; Spain, Denmark, and particularly the United Kingdom are low users. Almost all countries experienced rapid adoption of sildenafil with diffusion to a similar level; there was variable adoption and diffusion of COX II inhibitors, verteporfin, and interferon beta; drug-eluting stents penetrated the market in a similar way in all but one country; and two countries had very different adoption patterns for deep brain stimulators. Above average health spending and the presence of health technology assessment (HTA) or other guidance reports are consistently associated with increased diffusion. Early warning activity and a national coverage decision being taken are more likely to be associated with a reduced diffusion.

CONCLUSIONS

The significant differences in diffusion between different countries are not consistent with a neat evidence-based world. The tools available to policy makers to control diffusion (early warning systems, HTA, and a fourth hurdle) play some part in influencing diffusion but need close scrutiny of how successfully they operate.

Deep brain stimulation: overview and update

OBJECTIVE

To summarize the techniques for physiological localization that contribute to increased accuracy in the surgical treatment of movement disorders.

METHODS

Initial targeting through imaging referenced to stereotactic atlas are confirmed through physiological localization. Spontaneous recording, elicited recording, evoked potentials and stimulation provided physiological localization for target confirmation in the placement of lesions or DBS.

RESULTS

Imaging and stereotactic techniques produce inaccuracy that may be address by physiological localization. Microelectrode recording from basal ganglia and thalamic sites provides signature neuronal patterns to confirm and guide the trajectory. Spontaneous and elicited neuronal response are recorded from microelectrodes.

CONCLUSIONS

Accuracy of movement disorders surgery is enhance through use of physiological localization. A multimodality approach provides techniques that allow localization in a variety of patient and environmental conditions.

A review of diagnostic and functional imaging in headache

The neuroimaging of headache patients has revolutionised our understanding of the pathophysiology of primary headaches and provided unique insights into these syndromes. Modern imaging studies point, together with the clinical picture, towards a central triggering cause. The early functional imaging work using positron emission tomography shed light on the genesis of some syndromes, and has recently been refined, implying that the observed activation in migraine (brainstem) and in several trigeminal-autonomic headaches (hypothalamic grey) is involved in the pain process in either a permissive or triggering manner rather than simply as a response to first-division nociception per se. Using the advanced method of voxel-based morphometry, it has been suggested that there is a correlation between the brain area activated specifically in acute cluster headache--the posterior hypothalamic grey matter--and an increase in grey matter in the same region. No structural changes have been found for migraine and medication overuse headache, whereas patients with chronic tension-type headache demonstrated a significant grey matter decrease in regions known to be involved in pain processing. Modern neuroimaging thus clearly suggests that most primary headache syndromes are predominantly driven from the brain, activating the trigeminovascular reflex and needing therapeutics that act on both sides: centrally and peripherally.

Deep brain stimulation for pain relief: a meta-analysis

Deep brain stimulation (DBS) has been used to treat intractable pain for over 50 years. Variations in targets and surgical technique complicate the interpretation of many studies. To better understand its efficacy, we performed a meta-analysis of DBS for pain relief. MEDLINE (1966 to February 2003) and EMBASE (1980 to January 2003) databases were searched using key words deep brain stimulation, sensory thalamus, periventricular gray and pain. Inclusion criteria were based on patient characteristics and protocol clarity. Six studies (between 1977-1997) fitting the criteria were identified. Stimulation sites included the periventricular/periaqueductal grey matter (PVG/PAG), internal capsule (IC), and sensory thalamus (ST). The long-term pain alleviation rate was highest with DBS of the PVG/PAG (79%), or the PVG/PAG plus sensory thalamus/internal capsule (87%). Stimulation of the sensory thalamus alone was less effective (58% long-term success) (p < 0.05). DBS was more effective for nociceptive than deafferentation pain (63% vs 47% long-term success; p < 0.01). Long-term success was attained in over 80% of patients with intractable low back pain (failed back surgery) following successful trial stimulation. Trial stimulation was successful in approximately 50% of those with post-stroke pain, and 58% of patients permanently implanted achieved ongoing pain relief. Higher rates of success were seen with phantom limb pain and neuropathies. We conclude that DBS is frequently effective when used in well-selected patients. Neuroimaging and neuromodulation technology advances complicate the application of these results to modern practice. Ongoing investigations should shed further light on this complex clinical conundrum.

Deep brain stimulation of the substantia nigra pars reticulata exerts long lasting suppression of amygdala-kindled seizures

Deep brain stimulation (DBS) has been used to treat a variety of neurological disorders including epilepsy. However, we have limited knowledge about effective target areas, optimal stimulation parameters, and long-term effect of DBS on epileptic seizures. Here we examined the effects of DBS of the substantia nigra pars reticulata (SNr) on amygdala-kindled seizures. Microwire electrodes were implanted into the SNr and amygdala of adult male rats. When stage 5-kindled seizures were achieved by daily amygdala kindling, high frequency stimulation was delivered to the SNr bilaterally 1 s after cessation of kindling. Our DBS protocol completely blocked kindled seizures in 10 out of 23 (43.5%) rats studied. Furthermore, when the same amygdala kindling procedure was performed 24 h later without DBS, the kindling failed to elicit any seizure signs in 6 of these 10 rats. Some of the post-DBS period of seizure suppression lasted for up to 4 days. In other 3 rats, only mild stage 1 to 2 seizures appeared following amygdala kindling. Only 1 of the 10 rats for which DBS had blocked kindled seizures exhibited full-scale 5 stage-kindled seizures 24 h after DBS. These results suggest that highly plastic neural networks are involved in amygdala-kindled seizures and that DBS, if well timed with the onset of amygdala kindling, may exert long lasting effects on the networks that may prevent the recurrence of kindled seizures.

Brain stimulation techniques in the treatment of obsessive-compulsive disorder: current and future directions

Recent studies on the epidemiology of obsessive-compulsive disorder (OCD) estimate 50 million patients suffer from OCD worldwide, thus making it a global problem. The treatment of OCD has changed substantially over the last 2 decades following the introduction of selective serotonin reuptake inhibitors, which provide symptom improvement in approximately 60% of patients. However, some patients remain resistant to the standard pharmacologic and behavioral treatments. Although some treatment-resistant patients respond to pharmacologic augmentations, others do not, and there is evidence that some of the most severe cases benefit from treatment with neurosurgical interventions. Besides pharmacologic, behavioral, and neurosurgical approaches, different brain stimulation methods-transcranial magnetic stimulation, deep brain stimulation, and electroconvulsive therapy-have been investigated in treatment-resistant patients with OCD. However, available data about the use of these techniques in OCD treatment are quite limited in terms of sample size and study design, given the difficulty in conducting standard blinded trials for these procedures. In addition, none of the mentioned treatments have received Food and Drug Administration approval for the treatment of OCD. Nevertheless, promising findings regarding efficacy, tolerability, and non-invasiveness and/or reversibility of these techniques have increased interest in investigating their use in treatment-resistant OCD.

Pallidal vs subthalamic nucleus deep brain stimulation in Parkinson disease

BACKGROUND

Deep brain stimulation (DBS) of the globus pallidus interna (GPi) and subthalamic nucleus (STN) has been reported to relieve motor symptoms and levodopa-induced dyskinesia in patients with advanced Parkinson disease (PD). Although it has been suggested that stimulation of the STN may be superior to stimulation of the GPi, comparative trials are limited.

OBJECTIVE

To extend our randomized, blinded pilot comparison of the safety and efficacy of STN and GPi stimulation in patients with advanced PD.

DESIGN

This study represents the combined results from our previously published, randomized, blinded, parallel-group pilot study and additional patients enrolled in our single-center extension study.

SETTING

Oregon Health and Science University in Portland.Patients Twenty-three patients with idiopathic PD, levodopa-induced dyskinesia, and response fluctuations were randomized to implantation of bilateral GPi or STN stimulators. Patients and evaluating clinicians were blinded to stimulation site. All patients were tested preoperatively while taking and not taking medications and after 3, 6, and 12 months of DBS.

MAIN OUTCOME MEASURES

Postoperatively, response of symptoms to DBS, medication, and combined medication and DBS was evaluated. Twenty patients (10 in the GPi group and 10 in the STN group) completed 12-month follow-up.

RESULTS

Off-medication Unified Parkinson's Disease Rating Scale motor scores were improved after 12 months of both GPi and STN stimulation (39% vs 48%). Bradykinesia tended to improve more with STN than GPi stimulation. No improvement in on-medication function was observed in either group. Levodopa dose was reduced by 38% in STN stimulation patients compared with 3% in GPi stimulation patients (P = .08). Dyskinesia was reduced by stimulation at both GPi and STN (89% vs 62%). Cognitive and behavioral complications were observed only in combination with STN stimulation.

CONCLUSION

Stimulation of either the GPi or STN improves many features of advanced PD. It is premature to exclude GPi as an appropriate target for DBS in patients with advanced disease.

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.