Ablation of the subthalamic nucleus supports the survival of nigral dopaminergic neurons after nigrostriatal lesions induced by the mitochondrial toxin 3-nitropropionic acid

Identification of effective features of LFP signal for making closed-loop deep brain stimulation in parkinsonian rats

Traditional deep brain stimulation (DBS) is one of the acceptable methods to relieve the clinical symptoms of Parkinson's disease in its advanced stages. Today, the use of closed-loop DBS to increase stimulation efficiency and patient satisfaction is one of the most important issues under investigation. The present study was aimed to find local field potential (LFP) features of parkinsonian rats, which can determine the timing of stimulation with high accuracy. The LFP signals from rats were recorded in three groups of parkinsonian rat models receiving stimulation (stimulation), without getting stimulation (off-stimulation), and sham-controlled group. The frequency domain and chaotic features of signals were extracted for classifying three classes by support vector machine (SVM) and neural networks. The best combination of features was selected using the genetic algorithm (GA). Finally, the effective features were introduced to determine the on/off stimulation time, and the optimal stimulation parameters were identified. It was found that a combination of frequency domain and chaotic features with an accuracy of about 99% was able to determine the time the DBS must switch on. In about 80.67% of the 1861 different stimulation parameters, the brain was able to maintain its state for about 3 min after stimulation discontinuation.

Subthalamic nucleus deep brain stimulation induces sustained neurorestoration in the mesolimbic dopaminergic system in a Parkinson's disease model

BACKGROUND

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) is an established therapeutic principle in Parkinson's disease, but the underlying mechanisms, particularly mediating non-motor actions, remain largely enigmatic.

OBJECTIVE/HYPOTHESIS

The delayed onset of neuropsychiatric actions in conjunction with first experimental evidence that STN-DBS causes disease-modifying effects prompted our investigation on how cellular plasticity in midbrain dopaminergic systems is affected by STN-DBS.

METHODS

We applied unilateral or bilateral STN-DBS in two independent cohorts of 6-hydroxydopamine hemiparkinsonian rats four to eight weeks after dopaminergic lesioning to allow for the development of a stable dopaminergic dysfunction prior to DBS electrode implantation.

RESULTS

After 5 weeks of STN-DBS, stimulated animals had significantly more TH⁺ dopaminergic neurons and fibres in both the nigrostriatal and the mesolimbic systems compared to sham controls with large effect sizes of g_Hedges = 1.9-3.4. DBS of the entopeduncular nucleus as the homologue of the human Globus pallidus internus did not alter the dopaminergic systems. STN-DBS effects on mesolimbic dopaminergic neurons were largely confirmed in an independent animal cohort with unilateral STN stimulation for 6 weeks or for 3 weeks followed by a 3 weeks washout period. The latter subgroup even demonstrated persistent mesolimbic dopaminergic plasticity after washout. Pilot behavioural testing showed that augmentative dopaminergic effects on the mesolimbic system by STN-DBS might translate into improvement of sensorimotor neglect.

CONCLUSIONS

Our data support sustained neurorestorative effects of STN-DBS not only in the nigrostriatal but also in the mesolimbic system as a potential factor mediating long-latency neuropsychiatric effects of STN-DBS in Parkinson's disease.

The potential neuromodulatory impact of subthalamic nucleus deep brain stimulation on Parkinson's disease progression

INTRODUCTION

STN-DBS has been claimed to change progressionsymptomsin animal models of PD, but information is lacking about the possible neuromodulatory role of STN-DBS in humans. The aim of this prospective controlled study was to evaluate the long-term impact of STN-DBS on motor disabilities and cognitive impairment in PD patients in comparison to Best-Medical-Therapy (BMT) and Long-term-Post-Operative (POP) groups.

MATERIAL AND METHODS

Patients were divided into 3 groups: the BMT-group consisted of 20 patients treated only with pharmacotherapy, the DBS-group consisted of 20 PD patients who underwent bilateral STN-DBS (examined pre- and postoperatively) and the POP-group consisted of 14 long-term postoperative patients in median 30 month-time after DBS. UPDRS III scale was measured during 3 visits in 9 ± 2 months periods (V1, V2, V3) in total-OFF phase. Cognitive assessment was performed during each visit in total-ON phase.

RESULTS

The comparable UPDRS III OFF gain was observed in both BMT-group and POP-group evaluations (p < 0.05). UPDRS III OFF results in DBS-group revealed significant UPDRS III OFF increase in ΔV2-V1 assessment (p < 0.05) with no significant UPDRS III OFF alteration in ΔV3-V2 DBS-group evaluation (p > 0.05). Cognitive assessment revealed significant alterations between DBS-group and BMT-group in working memory, executive functions and learning abilities (p < 0.05).

CONCLUSIONS

The impact of STN-DBS on UPDRS III OFF score and cognitive alterations suggest its neuromodulatory role, mainly during the first 9-18 months after surgery.

Deep brain stimulation: potential for neuroprotection

Over the last two decades there has been an exponential rise in the number of patients receiving deep brain stimulation (DBS) to manage debilitating neurological symptoms in conditions such as Parkinson's disease, essential tremor, and dystonia. Novel applications of DBS continue to emerge including treatment of various psychiatric conditions (e.g. obsessive-compulsive disorder, major depression) and cognitive disorders such as Alzheimer's disease. Despite widening therapeutic applications, our understanding of the mechanisms underlying DBS remains limited. In addition to modulation of local and network-wide neuronal activity, growing evidence suggests that DBS may also have important neuroprotective effects in the brain by limiting synaptic dysfunction and neuronal loss in neurodegenerative disorders. In this review, we consider evidence from preclinical and clinical studies of DBS in Parkinson's disease, Alzheimer's disease, and epilepsy that suggest chronic stimulation has the potential to mitigate neuronal loss and disease progression.

Neuroprotective Surgical Strategies in Parkinson's Disease: Role of Preclinical Data

Although there have been many pharmacological agents considered to be neuroprotective therapy in Parkinson's disease (PD) patients, neurosurgical approaches aimed to neuroprotect or restore the degenerative nigrostriatal system have rarely been the focus of in depth reviews. Here, we explore the neuroprotective strategies involving invasive surgical approaches (NSI) using neurotoxic models 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 6-hydroxydopamine (6-OHDA), which have led to clinical trials. We focus on several NSI approaches, namely deep brain stimulation of the subthalamic nucleus, glial neurotrophic derived factor (GDNF) administration and cell grafting methods. Although most of these interventions have produced positive results in preclinical animal models, either from behavioral or histological studies, they have generally failed to pass randomized clinical trials to validate each approach. We argue that NSI are promising approaches for neurorestoration in PD, but preclinical studies should be planned carefully in order not only to detect benefits but also to detect potential adverse effects. Further, clinical trials should be designed to be able to detect and disentangle neuroprotection from symptomatic effects. In summary, our review study evaluates the pertinence of preclinical models to study NSI for PD and how this affects their efficacy when translated into clinical trials.

Subthalamic nucleus deep brain stimulation is neuroprotective in the A53T α-synuclein Parkinson's disease rat model

Objective

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) is a highly effective symptomatic therapy for motor deficits in Parkinson's disease (PD). An additional, disease‐modifying effect has been suspected from studies in toxin‐based PD animal models, but these models do not reflect the molecular pathology and progressive nature of PD that would be required to evaluate a disease‐modifying action. Defining a disease‐modifying effect could radically change the way in which DBS is used in PD.

Methods

We applied STN‐DBS in an adeno‐associated virus (AAV) 1/2‐driven human mutated A53T α‐synuclein (aSyn)‐overexpressing PD rat model (AAV1/2‐A53T‐aSyn). Rats were injected unilaterally, in the substantia nigra (SN), with AAV1/2‐A53T‐aSyn or control vector. Three weeks later, after behavioral and nigrostriatal dopaminergic deficits had developed, rats underwent STN‐DBS electrode implantation ipsilateral to the vector‐injected SN. Stimulation lasted for 3 weeks. Control groups remained OFF stimulation. Animals were sacrificed at 6 weeks.

Results

Motor performance in the single pellet reaching task was impaired in the AAV1/2‐A53T‐aSyn–injected stim‐OFF group, 6 weeks after AAV1/2‐A53T‐aSyn injection, compared to preoperative levels (–82%; p < 0.01). Deficits were reversed in AAV1/2‐A53T‐aSyn, stim‐ON rats after 3 weeks of active stimulation, compared to the AAV1/2‐A53T‐aSyn stim‐OFF rats (an increase of ∼400%; p < 0.05), demonstrating a beneficial effect of DBS. This motor improvement was maintained when the stimulation was turned off and was accompanied by a higher number of tyrosine hydroxylase⁺ SN neurons (increase of ∼29%), compared to AAV1/2‐A53T‐aSyn stim‐OFF rats (p < 0.05).

Interpretation

Our data support the putative neuroprotective and disease‐modifying effect of STN‐DBS in a mechanistically relevant model of PD. Ann Neurol 2017;81:825–836

Progression and survival in Parkinson's disease with subthalamic nucleus stimulation

BACKGROUND

Treatment for Parkinson's disease (PD) is symptomatic. Surgical treatment with continuous high-frequency stimulation of the subthalamic nucleus (STN-DBS) is established as a safe symptomatic treatment with long-term beneficial effects. It has been postulated that STN-DBS could halt the progression of PD through a disease modifying or neuroprotective effect.

OBJECTIVE

To investigate the postulated disease modifying or neuroprotective effect of STN-DBS by comparing the rate of deterioration of parkinsonism and mortality over time in two selected and matched groups of patients with PD with and without surgery.

METHODS

Group A was derived from all patients who received STN-DSB surgery at Oslo University Hospital, from January 2001 to December 2007. Group B was derived from a prevalence study of PD in the Stavanger area of Western Norway in 1993. The two groups were individually matched and the disease progression measured by Unified Parkinson's Disease Rating Scale-motor scores, and the mortality was compared.

RESULTS

The mean annual change based on baseline and last observation scores in individually matched groups was 0.97 (SD = 3.57) for the surgery group and 1.04 (SD = 3.33) for the controls and thus not significantly different, F(1, 104) = .21, P = 0.89. The long-term mortality was also similar in the two groups during long-term follow-up, hazard ratio = 1.76, CL 0.91-3.40, P = 0.091.

CONCLUSION

This study gives no support to a postulated disease modifying or neuroprotective effect of STN-DBS in patients with PD.

Subthalamotomy in the treatment of Parkinson's disease: clinical aspects and mechanisms of action

Parkinson's disease (PD) is a neurodegenerative condition that can be pharmacologically treated with levodopa. However, important motor and nonmotor symptoms appear with its long-term use. The subthalamic nucleus (STN) is known to be involved in the pathophysiology of PD and to contribute to levodopa-induced complications. Surgery is considered in patients who have advanced PD that is refractory to pharmacotherapy and who display disabling dyskinesia. Deep brain stimulation of the STN is currently the main surgical procedure for PD, but lesioning is still performed. This review covers the clinical aspects and complications of subthalamotomy as one of the lesion-based options for PD patients with levodopa-induced dyskinesias. Moreover, the authors discuss the possible effects of subthalamic lesioning.

The impact of subthalamic deep brain stimulation on nigral neuroprotection-myth or reality?

OBJECTIVE

  In the present review article we summarize available clinical and preclinical evidence, if modulation of the subthalamic nucleus (STN) could be a target for neuroprotection in Parkinson's disease (PD).

BACKGROUND

  Chronic deep brain stimulation (DBS) of the STN has emerged as a powerful therapeutic alternative for the treatment of PD, ensuring stable symptom control for up to five years despite the progressive nature PD.

MATERIALS AND METHODS

  Comparative review of literature in PuBMed available up to December 2008.

RESULTS

  The assessment of neuroprotection has been proven difficult in the clinical situation, as medical or surgical therapeutic options that improve PD symptoms could be erroneously considered to be neuroprotective because of the difficulty of differentiating between symptomatic effects and potential neuromodulative disease-related effects of various treatment options applied in PD. The methodological limitations of clinical trials underline the importance of putative neuroprotective compounds to be tested in clinically driven preclinical studies. Thus, animal models, mimicking progressive nigrostriatal cell death, are indispensable to further advance the important issue of neuroprotection or neuromodulation following DBS.

CONCLUSION

  Clear clinical evidence for STN-DBS-related neuroprotection in PD is missing. However, numerous preclinical studies show (and are discussed) that silencing of the STN via lesion or DBS may exert neuromodulative effects on nigral dopamine neurons.

Deep brain stimulation in early Parkinson's disease: enrollment experience from a pilot trial

BACKGROUND

Deep brain stimulation (DBS) of the subthalamic nucleus is an accepted therapy for advanced Parkinson's disease (PD). In animal models, pharmacologic ablation and stimulation of the subthalamic nucleus have resulted in clinical improvement and, in some cases, improved survival of dopaminergic neurons. DBS has not been studied in the early stages of PD, but early application should be explored to evaluate safety, efficacy, and the potential to alter disease progression.

METHODS

We are conducting a prospective, randomized, single-blind clinical trial of optimal drug therapy (ODT) compared to medication plus DBS (ODT + DBS) in subjects with Hoehn & Yahr Stage II idiopathic PD who are without motor fluctuations or dementia. We report here subject screening, enrollment, baseline characteristics, and adverse events.

RESULTS

30 subjects (average age 60 ± 6.9 years, average duration of medicine 2.1 ± 1.3 years, average UPDRS-III scores 14.9 on medication and 27.0 off medication) are enrolled in the ongoing study. Twelve of 15 subjects randomized to DBS experienced perioperative adverse events, the majority of which were related to the procedure or device and resolved without sequelae. Frequently reported adverse events included wound healing problems, headache, edema, and confusion.

CONCLUSION

This report demonstrates that subjects with early stage PD can be successfully recruited, consented and retained in a long-term clinical trial of DBS. Our ongoing pilot investigation will provide important preliminary safety and tolerability data concerning the application of DBS in early stage PD.

Deep brain stimulation of the subthalamic nucleus for the treatment of Parkinson's disease

High-frequency deep brain stimulation (DBS) of the subthalamic nucleus (STN-HFS) is the preferred surgical treatment for advanced Parkinson's disease. In the 15 years since its introduction into clinical practice, many studies have reported on its benefits, drawbacks, and insufficiencies. Despite limited evidence-based data, STN-HFS has been shown to be surgically safe, and improvements in dopaminergic drug-sensitive symptoms and reductions in subsequent drug dose and dyskinesias are well documented. However, the procedure is associated with adverse effects, mainly neurocognitive, and with side-effects created by spread of stimulation to surrounding structures, depending on the precise location of electrodes. Quality of life improves substantially, inducing sudden global changes in patients' lives, often requiring societal readaptation. STN-HFS is a powerful method that is currently unchallenged in the management of Parkinson's disease, but its long-term effects must be thoroughly assessed. Further improvements, through basic research and methodological innovations, should make it applicable to earlier stages of the disease and increase its availability to patients in developing countries.

Frontiers in the surgical treatment of Parkinson's disease

Despite the continued refinement of medical and surgical therapies, the treatment of Parkinson's disease (PD) remains challenging. Current treatment strategies are largely focused on managing the motor symptoms of the disease, either by dopamine-based medications or, in advanced stages, by the application of deep brain stimulation to more stably alter the function of the basal ganglia. Important advances have been made in the last decade, but unfortunately a number of the motor symptoms of late-stage PD remain poorly treated, and while currently available therapies address the symptoms of the disease, they fail to alter the course of the disease itself. This has spurred basic and clinical exploration on a number of fronts. Several centers have examined novel stimulation targets to treat refractory symptoms of gait difficulty and axial imbalance. Basic and clinical researchers are examining whether the use of deep brain stimulation might slow the progress of the disease and thus be a useful neuroprotective therapy if initiated earlier in the progression of the disease. An expanded understanding of the genetic and cellular events that underlie PD has led some researchers to explore the use of neurotrophic factors or genetic restoration to preserve threatened neuronal populations. Finally, there has been much research on the use of fetal mesencephalic or stem cell populations to restore dopaminergic function. In this report, we will examine each of these potential new surgical therapies and the promise they may hold for the future treatment of PD.

THE DOPAMINERGIC NIGROSTRIATAL SYSTEMAND PARKINSON'S DISEASE

For several decades, the clinical study of Parkinson's disease has driven an increasingly sophisticated understanding of the dopaminergic system and its complex role in modulating motor behavior. This article reviews salient areas of research in this field, commencing with the molecular biology of the development of the mesencephalic dopaminergic system. We then discuss events thought to be crucial in the cellular and molecular pathology of Parkinson's disease, proposed mechanisms of cell death, and relevant toxin models. These advancements are used as a template to review emerging therapeutic techniques, including neuroprotection strategies, surgical treatment of trophic factors, gene therapy, and neural transplantation.

Protection of nigral cell death by bilateral subthalamic nucleus stimulation

In Parkinson disease (PD), the subthalamic nucleus (STN) becomes hyperactive (disinhibited), which is reported to cause excitotoxic damage to midbrain dopaminergic neurons. Here, we examined whether silencing of the hyperactive STN by chronic bilateral deep brain stimulation (DBS) increased the survival of midbrain dopaminergic neurons in a rat model of PD. High-precision design-based stereologic examination of the total number of neurons and tyrosine tydroxylase (TH) immunoreactive neurons in the substantia nigra pars compacta revealed that STN DBS resulted in a significant survival of these neurons. These data provide the first evidence in vivo that bilateral STN DBS is useful for protecting midbrain dopaminergic neurons from cell death in PD.

Neural stimulation for Parkinson's disease: current therapies and future directions

Neural stimulation has rapidly become an integral tool in the treatment of Parkinson's disease and other movement disorders. Today it serves as an important adjunct to medical therapy that continues to gain applicability to patients in whom the disease has progressed significantly. Studies have demonstrated efficacy in several deep-brain targets, with prolonged benefit exceeding 5-year follow-up times. Continuing study is teaching us more about the mechanism of deep-brain stimulation effect. New targets, which may treat the disease more successfully, are being examined. In this review, the history of deep-brain stimulation, the rationale for the known targets of stimulation; the clinical evidence demonstrating their benefit and, finally, future perspectives on new treatments that are being investigated and may have an impact on the field are discussed.

Toxin-induced models of Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disease that appears essentially as a sporadic condition. It results mainly from the death of dopaminergic neurons in the substantia nigra. PD etiology remains mysterious, whereas its pathogenesis begins to be understood as a multifactorial cascade of deleterious factors. Most insights into PD pathogenesis come from investigations performed in experimental models of PD, especially those produced by neurotoxins. Although a host of natural and synthetic molecules do exert deleterious effects on dopaminergic neurons, only a handful are used in living laboratory animals to recapitulate some of the hallmarks of PD. In this review, we discuss what we believe are the four most popular parkinsonian neurotoxins, namely 6-hydroxydopamine (6-OHDA), 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), rotenone, and paraquat. The main goal is to provide an updated summary of the main characteristics of each of these four neurotoxins. However, we also try to provide the reader with an idea about the various strengths and the weaknesses of these neurotoxic models.

Subthalamic nucleus lesioning inhibits expression and phosphorylation of c-Jun in nigral neurons in the rat's 6-OHDA model of Parkinson's disease

Parkinson's Disease (PD) is characterized by a loss of nigral dopamine (DA) neurons, followed by a striatal DA deficit. Inhibition of the subthalamic nucleus (STN) reverses L-DOPA sensitive motor symptoms and improves efficacy of pharmacotherapy in PD-patients. The underlying mechanism of these effects, however, remains largely unknown. Previously, we could show in the rat's 6-hydroxyDA (6-OHDA) model of PD that ablative STN-lesioning exerts functionally neuroprotective effects on the DAergic nigrostriatal pathway against 6-OHDA toxicity, in terms of elevating the number of tyrosine hydroxylase (TH)-expressing neurons rather than enhancing the total number of cells surviving 2 and 6 weeks post lesioning, as assessed via fluorogold staining. These data were correlated with increased functional recovery of 6-OHDA-lesioned rats with preceding STN-lesioning. Here, we extend the previous study design to observation periods of up to 12 weeks to assess long-term effects. Furthermore, to elucidate cellular mechanisms underlying potential neuroprotective effects, we explore the regulation of cellular markers involved in neurodegenerative cascades via immunocytochemistry. We show that preceding STN-lesioning significantly inhibits 6-OHDA induced expression/phosphorylation of the transcription factor c-Jun in surviving nigral neurons in comparison with controls. However, we also demonstrate that functionally neuroprotective effects of preceding STN-lesioning subside after 12 weeks, as assessed with TH immunostaining. We therefore conclude that c-Jun induction/phosphorylation is involved in 6-OHDA toxicity and that STN-lesioning transiently preserves of dopaminergic phenotype of nigral neurons partially via delaying the induction and attenuating the expression and phosphorylation of c-Jun.

Turning the heterogeneous into homogeneous: studies on selectively isolated GABAergic interneuron subsets

The amazing morphological and electrophysiological diversity of cortical GABAergic interneurons subserves the broad diversity of processes these cells modulate in neuronal networks. Until recently, interneuron development and functions have been extensively studied in heterogeneous in vitro and in vivo systems containing both excitatory and inhibitory components. However, mechanisms of interneuron specification during development, key signaling mechanisms controlling the establishment of particular inhibitory neuron subsets, and the spatial and temporal regulation of their integration in neuronal microcircuits remain poorly understood. Selective isolation of particular interneuron subsets may significantly extend our knowledge on the scenario of neurochemical and electrophysiological specification of developing interneurons, identification of signaling cues directing their axon growth, and principles of their anterograde and retrograde synaptic communication with other cell types. Here, we show that selective isolation of perisomatic inhibitory cells containing either parvalbumin or cholecystokinin reveals major differences in the temporal dynamics of their functional differentiation, and their dependence on target-derived signals like brain-derived neurotrophic factor and endocannabinoids. In addition, we discuss therapeutic prospects of modulating increased excitatory output in the hippocampus and subthalamic nucleus by re-adjusting the inhibitory control of principal cells.

Challenges in Parkinson's disease: restoration of the nigrostriatal dopamine system is not enough

Levodopa remains the most effective treatment for Parkinson's disease (PD). However, the drug is complicated by a wide range of adverse effects, most notably motor fluctuations and dyskinesias. Long-acting dopamine agonists are associated with a reduced incidence of these complications and modern surgical approaches and pharmacological methods of providing more continuous dopaminergic stimulation have a substantial ameliorative effect on these problems. Despite these advances, disease progression remains unaffected. For this reason there has been much enthusiasm for cellular therapies designed to replace degenerating nigrostriatal dopaminergic neurons. However, recent fetal transplant trials have failed to show expected benefit and have been complicated by medication dyskinesias". Even if successful, such treatment may be predestined to provide no better outcome than available treatments given current medical and surgical experience that emphasises the increasingly critical role of "non-dopaminergic" symptoms to quality of life in late-stage PD. Knowledge of the widespread, multisystem nature of the neurodegeneration that accounts for these problems suggests that restoration of the nigrostriatal dopamine system should not be the ultimate goal of future research.

Ablation of the subthalamic nucleus protects dopaminergic phenotype but not cell survival in a rat model of Parkinson's disease

Inhibition or ablation of the hyperactive subthalamic nucleus (STN) in Parkinson's disease (PD) does not only reverse motor deficits, silencing the glutamatergic output of the subthalamic nucleus, but has also been implicated to have neuroprotective effects on nigral neurons in animal models of Parkinson's disease. Ablation of the subthalamic nucleus has been shown to increase the number of tyrosinhydroxylase-immunopositive cells and partially restores behavioral deficits in animal models of Parkinson's disease. However, it is unclear whether subthalamic nucleus ablation indeed prevents cell death or whether the effect is due to the rescue of the dopaminergic (DA) phenotype of impaired cells by upregulating tyrosine hydroxylase (TH). We therefore investigated the potential neuroprotective effects of a preceding subthalamic nucleus lesion on 6-hydroxydopamine (6-OHDA)-induced nigral cell death and compared the retrograde tracer fluorogold (FG) as a marker of cell survival with tyrosinhydroxylase immunoreactivity as a marker of the dopaminergic phenotype. In the present study, we show that ablation of the subthalamic nucleus does not affect the number of fluorogold-labeled cells but increases the number of tyrosinhydroxylase-positive neurons in subthalamic nucleus-lesioned hemiparkinsonian animals and leads to partial behavioral recovery of the rats. We conclude that subthalamic nucleus ablation exerts neuroprotective properties on the dopaminergic nigrostriatal pathway against 6-hydroxydopamine toxicity in terms of rescuing the neurotransmitter phenotype in the remaining neurons rather than enhancing the total number of nigral cells.

Deep brain stimulation for Parkinson’s disease

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

Kainic acid lesion-induced nigral neuronal death

Parkinson's disease (PD) is characterized by progressive death of dopamine (DA) neurons in the substantia nigra pars compacta. We report a rat model that exhibits progressive death of nigral neurons following unilateral injection of kainic acid in the striatum. In situ end-labeling revealed significant numbers of dying nigral neurons ipsilateral to the lesion during the first 3 weeks following injection. An indication of the gradual nature of death was that similar small numbers of cells were detected at each time point. These early morphological markers of neuronal death led to a significant reduction (20%) at 5 months of tyrosine hydroxylase-positive neurons and total number of neurons in the ipsilateral substantia nigra compared with the contralateral control. To examine the role of nigrostriatal DA metabolism in the observed nigral neuronal death, we manipulated DA metabolism during the initial 2 weeks following kainic acid lesion. Neurons in the ventral tier of the substantia nigra pars compacta were protected from death by treatment with 2,4-diamino-6-hydroxy-pyrimidine (DAHP), an inhibitor of GTP cyclohydrolase, the initial enzyme in the synthesis of the tyrosine hydroxylase co-substrate, tetrahydrobiopterin (BH(4)). Neurons in both the dorsal and ventral tier of substantia nigra pars compacta were protected from death by treatment with DAHP and L-DOPA. These experiments suggest that intrastriatal kainic acid lesion is an in vivo model of trophic support withdrawal. This experimental procedure is useful for studying mechanisms underlying protracted death of nigral DA neurons and may provide valuable mechanistic information relevant to understanding the etiology of PD.

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

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

Subthalamic GAD gene therapy in a Parkinson's disease rat model

The motor abnormalities of Parkinson's disease (PD) are caused by alterations in basal ganglia network activity, including disinhibition of the subthalamic nucleus (STN), and excessive activity of the major output nuclei. Using adeno-associated viral vector-mediated somatic cell gene transfer, we expressed glutamic acid decarboxylase (GAD), the enzyme that catalyzes synthesis of the neurotransmitter GABA, in excitatory glutamatergic neurons of the STN in rats. The transduced neurons, when driven by electrical stimulation, produced mixed inhibitory responses associated with GABA release. This phenotypic shift resulted in strong neuroprotection of nigral dopamine neurons and rescue of the parkinsonian behavioral phenotype. This strategy suggests that there is plasticity between excitatory and inhibitory neurotransmission in the mammalian brain that could be exploited for therapeutic benefit.

Acute administration of haloperidol induces apoptosis of neurones in the striatum and substantia nigra in the rat

Chronic administration of typical neuroleptics is associated with tardive dyskinesia in some patients. This dyskinetic syndrome has been associated with loss of GABAergic markers in the basal ganglia but the cause of these GABAergic depletions remains uncertain. Haloperidol, a commonly prescribed typical neuroleptic, is known to be toxic in vitro, possibly as a consequence of its conversion to pyridinium-based metabolites and potentially by raising glutamate-mediated transmission. We report here that the in vivo, acute administration of a large dose of haloperidol resulted in a microglial response indicative of neuronal damage. This was accompanied by an increase in the number of apoptotic cells in the striatum (especially in the dorsomedial caudate putamen) and in the substantia nigra pars reticulata. These apoptotic cells were characterised by the stereotaxic injection of a retrograde neuroanatomical tracer into the projection targets of the striatum and substantia nigra pars reticulata prior to the systemic injection of haloperidol. This procedure confirmed that the dying cells were neurones and demonstrated that within the striatum the majority were striatopallidal neurones though relatively high levels of apoptotic striatoentopeduncular neurones were also seen.The possibility that chronic administration of haloperidol could induce cumulative neuronal loss in the substantia nigra pars reticulata and thereby induce the pathological changes which lead to tardive dyskinesia is discussed.

Prevention of neurotoxin damage of 6-OHDA to dopaminergic nigral neuron by subthalamic nucleus lesions

OBJECT

To investigate the possibility that subthalamic nucleus (STN) ablation could prevent the toxicity of the selective dopaminergic neurotoxin 6-hydroxydopamine (6-OHDA).

METHODS

Sixty rats were divided into 6 groups (n = 10). The control group received a unilateral microinjection of 6-OHDA into the right ventral tegmental area (VTA) and the right median forebrain bundle (MFB). Group 1 received an administration of kainic acid (KA) into the right STN and, 1-week later, an injection of 6-OHDA in the right VTA and MFB. Groups 2-5 received an injection of 6-OHDA in the right VTA and MFB, 1 h, 2 h, 3 days, and 7 days before KA in the right STN respectively. Four weeks later, the changes of tyrosine hydroxylase (TH)-positive (dopaminergic) neurons in the SNc were investigated with immunocytochemical and morphometrical methods.

RESULTS

The number of TH-positive cells in the SNc on the injected side of treated groups (groups 1-5) and control group were 71.46 +/- 6.84, 57.07 +/- 5.54, 51.09 +/- 4.85, 12.68 +/- 2.67, 4.15 +/- 1.60 and 3.40 +/- 1.54/slice, which decreased to 96.7, 72.9, 69.8, 17.2, 5.6 and 4.4% of the non-injected side, respectively. The number of TH-positive neurons in groups 1-4 significantly increased in comparison with the controls (p < 0.05, 0.01). In group 5, there were no remarkable differences in contrast to the number of TH-positive neurons of the controls (p > 0.05). The difference in the number of TH-positive neurons between groups 1-5 was statistically significant (p < 0.01).

CONCLUSION

The results indicate that STN ablation can provide antiglutamate-based neuroprotection of the dopaminergic nigrostriatal pathway against 6-OHDA toxicity.

Deep brain stimulation for Parkinson's disease

Deep brain stimulation (DBS) is making a major impact in patients with advanced Parkinson's disease who continue to be disabled despite the best available medical therapy. Stimulation of the internal segment of the globus pallidus (GPi) or the subthalamic nucleus (STN) can improve tremor, rigidity, bradykinesia and gait disturbances in Parkinson's disease and improve the day-to-day activities of patients with these disabling symptoms. While the mechanism of action of DBS remains poorly understood, the success of technique in the treatment of movement disorders is bringing into question traditional concepts of the organization and of the basal ganglia and spearheading a re-examination of the nature and function of brain areas involved in the control of movement. Future developments in this rapidly advancing area will include the elucidation of the mechanism of action of DBS and technical advances in surgical techniques, in electrode design and in choosing better stimulation parameters. These advances will improve the scope and effectiveness of DBS and expand its clinical indications.

Functional changes of the basal ganglia circuitry in Parkinson's disease

The basal ganglia circuitry processes the signals that flow from the cortex, allowing the correct execution of voluntary movements. In Parkinson's disease, the degeneration of dopaminergic neurons of the substantia nigra pars compacta triggers a cascade of functional changes affecting the whole basal ganglia network. The most relevant alterations affect the output nuclei of the circuit, the medial globus pallidus and substantia nigra pars reticulata, which become hyperactive. Such hyperactivity is sustained by the enhanced glutamatergic inputs that the output nuclei receive from the subthalamic nucleus. The mechanisms leading to the subthalamic disinhibition are still poorly understood. According to the current model of basal ganglia organization, the phenomenon is due to a decrease in the inhibitory control exerted over the subthalamic nucleus by the lateral globus pallidus. Recent data, however, suggest that additional if not alternative mechanisms may underlie subthalamic hyperactivity. In particular, given the reciprocal innervation of the substantia nigra pars compacta and the subthalamic nucleus, the dopaminergic deficit might influence the subthalamic activity, directly. In addition, the increased excitatory drive to the dopaminergic nigral neurons originating from the hyperactive subthalamic nucleus might sustain the progression of the degenerative process. The identification of the role of the subthalamic nucleus and, more in general, of the glutamatergic mechanisms in the pathophysiology of Parkinson's disease might lead to a new approach in the pharmacological treatment of the disease. Current therapeutic strategies rely on the use of L-DOPA and/or dopamine agonists to correct the dopaminergic deficit. Drugs capable of antagonizing the effects of glutamate might represent, in the next future, a valuable tool for the development of new symptomatic and neuroprotective strategies for therapy of Parkinson's disease.

The putative neuroprotective role of dopamine agonists in Parkinson's disease

Parkinson's disease is probably caused by a combination of genetic and environmental factors, which trigger a cascade of events that lead to the cell death of the dopamine-containing neurons of the substantia nigra pars compacta. These processes include oxidative stress, mitochondrial dysfunction, excitotoxicity with excess of nitric oxide formation, glial and inflammatory abnormalities and apoptosis. Dopamine agonists are chemical compounds that act directly on the dopamine receptors without any previous enzymatic biotransformation. Besides their symptomatic antiparkinsonian effect, these drugs may have neuroprotective properties in Parkinson's disease through different possible mechanisms: (a) stimulation of dopamine auoreceptors, reducing thereby dopamine turnover; (b) direct antioxidant effects; (c) reduction of excitotoxicity induced by excessive subthalamic nucleus firing; (d) inhibition of mitochondrial permeability; (e) induction of trophic factors. Dopamine agonists have already shown neuroprotective effects on dopaminergic cells against a variety of neurotoxins in several in vitro and in vivo studies. Clinical studies to detect changes in the progression of the underlying neurodegenerative process in patients with Parkinson's disease treated with dopamine agonists, by assessing the dopamine terminal function in the striatum by means of PET and SPECT techniques are under way.