Physiology of MPTP tremor

Animal models of Parkinson's disease: a guide to selecting the optimal model for your research

Parkinson's disease (PD) is a complex, multisystem disorder characterised by α-synuclein (SNCA) pathology, degeneration of nigrostriatal dopaminergic neurons, multifactorial pathogenetic mechanisms and expression of a plethora of motor and non-motor symptoms. Animal models of PD have already been instructive in helping us unravel some of these aspects. However, much remains to be discovered, requiring continued interrogation by the research community. In contrast with the situation for many neurological disorders, PD benefits from of a wide range of available animal models (pharmacological, toxin, genetic and α-synuclein) but this makes selection of the optimal one for a given study difficult. This is especially so when a study demands a model that displays a specific combination of features. While many excellent reviews of animal models already exist, this review takes a different approach with the intention of more readily informing this decision-making process. We have considered each feature of PD in turn - aetiology, pathology, pathogenesis, motor dysfunctions and non-motor symptoms (NMS) - highlighting those animal models that replicate each. By compiling easily accessible tables and a summary figure, we aim to provide the reader with a simple, go-to resource for selecting the optimal animal model of PD to suit their research needs.

Impaired Hand Dexterity Function in a Non-human Primate Model with Chronic Parkinson's Disease

Symptoms of Parkinson’s disease (PD) caused by loss of dopaminergic neurons are accompanied by movement disorders, including tremors, rigidity, bradykinesia, and akinesia. Non-human primate (NHP) models with PD play an essential role in the analysis of PD pathophysiology and behavior symptoms. As impairments of hand dexterity function can affect activities of daily living in patients with PD, research on hand dexterity function in NHP models with chronic PD is essential. Traditional rating scales previously used in the evaluation of animal spontaneous behavior were insufficient due to factors related to subjectivity and passivity. Thus, experimentally designed applications for an appropriate apparatus are necessary. In this study, we aimed to longitudinally assess hand dexterity function using hand dexterity task (HDT) in NHP-PD models. To validate this assessment, we analyzed the alteration in Parkinsonian tremor signs and the functionality of presynaptic dopaminergic neuron using positron emission tomography imaging of dopamine transporters in these models. In addition, a significant inverse correlation between HDT and DAT level was identified, but no local bias was found. The correlation with intention tremor signs was lower than the resting tremor. In conclusion, the evaluation of HDT may reflect behavioral symptoms of NHP-PD models. Furthermore, HDT was effectively used to experimentally distinguish intention tremors from other tremors.

Generation of Pluripotent Stem Cells Using Somatic Cell Nuclear Transfer and Induced Pluripotent Somatic Cells from African Green Monkeys

Patient-specific stem cells derived from somatic cell nuclear transfer (SCNT) embryos or from induced pluripotent stem cells (iPSCs) could be used to treat various diseases with minimal immune rejection. Many studies using these cells have been conducted in rats and mice; however, there exist numerous dissimilarities between the rodents and humans limiting the clinical predictive power and experimental utility of rodent experiments alone. Nonhuman primates (NHPs) share greater homology to human than rodents in all respects, including genomics, physiology, biochemistry, and the immune system. Thus, experimental data obtained from monkey studies would be more predictive for designing an effective cell replacement therapy in humans. Unfortunately, there are few iPSC lines and even fewer SCNT lines that have been derived in NHPs, hampering broader studies in regenerative medicine. One promising potential therapy would be the replacement of dopamine neurons that are lost in Parkinson's disease. After dopamine depletion by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), the African green monkey (Chlorocebus sabaeus) shows the most complete model of Parkinsonism compared with other species and brain pathology and behavioral changes are almost identical to those in humans after accidental exposure to MPTP. Therefore, we have developed a SCNT procedure to generate multiple pluripotent stem cell lines in this species for studies of possible treatment of Parkinsonism and for comparing with cells derived from iPSCs. Using 24 female monkeys as egg donors and 7 somatic cell donor monkeys, we have derived 11 SCNT embryonic stem cell lines that expressed typical stemness genes and formed all three germ layer derivatives. We also derived two iPSC lines using an episome-mediated reprogramming factor delivery system. This report describes the process for deriving these cell lines and proving their pluripotency for differentiation into various potentially therapeutic cells.

Characteristics of oscillatory pallidal neurons in patients with Parkinson's disease

BACKGROUND

Excessive neuronal activity in the globus pallidus internus (GPi) is believed to promote parkinsonian akinesia/bradykinesia, but not tremor. Parkinsonian tremor is thought to result from dysfunction in the basal ganglia and cerebello-thalamo-cortical circuits. Whether the GPi is involved in tremorgenesis has not been fully elucidated. This study was designed to quantify the characteristics of oscillatory GPi neurons in patients with Parkinson's disease.

METHODS

Nine patients undergoing surgery were studied. Microelectrode recordings in the GPi and electromyographic (EMG) activity in the limbs were recorded and the mean spontaneous firing rates (MSFRs) were calculated. Spectral analysis was used to assess neuronal oscillatory patterns. Coherence analysis was applied to explore the relationship between oscillatory neurons and EMG.

RESULTS

Of 79 GPi neurons, 50.6% oscillated at the tremor frequency; 25.3% oscillated at β frequency, and 24.1% did not oscillate. The MSFR of all neurons was 81.5 ± 7.4 spikes/s. Among neurons oscillating at tremor frequency, 40% were coherent with the tremor. In four neurons, the pattern changed from tremor frequency to β frequency or vice versa. It appeared that the tremor began before the GPi fired bursts.

CONCLUSION

Some neuronal activity in the GPi correlates with tremor and this correlation might be due to either feedback, maintenance, or initiation. Since there were examples of EMG tremor prior to GPi activity, initiation seems least likely. The data further support the prediction of the classic pathophysiology model of Parkinson's disease.

Animal Models of Tremor: Relevance to Human Tremor Disorders

Background

Tremor is the most common movement disorder; however, the pathophysiology of tremor remains elusive. While several neuropathological alterations in tremor disorders have been observed in post-mortem studies of human brains, a full understanding of the relationship between brain circuitry alterations and tremor requires testing in animal models. Additionally, tremor animal models are critical for our understanding of tremor pathophysiology, and/or to serve as a platform for therapy development.

Methods

A PubMed search was conducted in May 2018 to identify published papers for review.

Results

The methodology used in most studies on animal models of tremor lacks standardized measurement of tremor frequency and amplitude; instead, these studies are based on the visual inspection of phenotypes, which may fail to delineate tremor from other movement disorders such as ataxia. Of the animal models with extensive tremor characterization, harmaline-induced rodent tremor models provide an important framework showing that rhythmic and synchronous neuronal activities within the olivocerebellar circuit can drive action tremor. In addition, dopamine-depleted monkey and mouse models may develop rest tremor, highlighting the role of dopamine in rest tremor generation. Finally, other animal models of tremor have involvement of the cerebellar circuitry, leading to altered Purkinje cell physiology.

Discussion

Both the cerebellum and the basal ganglia are likely to play a role in tremor generation. While the cerebellar circuitry can generate rhythmic movements, the nigrostriatal system is likely to modulate the tremor circuit. Tremor disorders are heterogeneous in nature. Therefore, each animal model may represent a subset of tremor disorders, which collectively can advance our understanding of tremor.

Charting the onset of Parkinson-like motor and non-motor symptoms in nonhuman primate model of Parkinson's disease

Parkinson’s disease is a progressive neurodegenerative disease increasingly affecting our aging population. Remarkable advances have been made in developing novel therapies to control symptoms, halt or cure the disease, ranging from physiotherapy and small molecules to cell and gene therapy. This progress was enabled by the existence of reliable animal models. The nonhuman primate model of Parkinson’s disease emulates the cardinal symptoms of the disease, including tremor, rigidity, bradykinesia, postural instability, freezing and cognitive impairment. However, this model is established through the specific loss of midbrain dopaminergic neurons, while our current knowledge reflects the reality of Parkinson’s disease as a multisystem disease. Parkinson’s disease involves both motor and non-motor symptoms, such as sleep disturbance, olfaction, gastrointestinal dysfunctions, depression and cognitive deficits. Some of the non-motor symptoms emerge earlier at the prodromal phase and worsen with disease progression, yet in basic and translational studies, they are rarely considered as endpoints. In this study, we set to characterize an ensemble of less described motor and non-motor dysfunctions in the marmoset MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) model. We provide evidence that this animal model expresses postural head tremor and a progressive worsening of fine motor skills, movement coordination and cognitive abilities over a 6-month period. We report for the first time a non-invasive approach showing detailed analysis of daytime and nighttime sleep and circadian rhythm disturbance remarkably similar to Parkinson’s disease patients. This study describes the incidence of tremors, motor and non-motor dysfunctions in a preclinical model and highlights the need for their consideration in translating effective new therapeutic approaches for Parkinson’s disease.

Not all boronic acids with a five-membered cycle induce tremor, neuronal damage and decreased dopamine

Several striatal toxins can be used to induce motor disruption. One example is MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), whose toxicity is accepted as a murine model of parkinsonism. Recently, 3-Thienylboronic acid (3TB) was found to produce motor disruption and biased neuronal damage to basal ganglia in mice. The aim of this study was to examine the toxic effects of four boronic acids with a close structural relationship to 3TB (all having a five-membered cycle), as well as boric acid and 3TB. These boron-containing compounds were compared to MPTP regarding brain access, morphological disruption of the CNS, and behavioral manifestations of such disruption. Data was collected through acute toxicity evaluations, motor behavior tests, necropsies, determination of neuronal survival by immunohistochemistry, Raman spectroscopic analysis of brain tissue, and HPLC measurement of dopamine in substantia nigra and striatum tissue. Each compound showed a distinct profile for motor disruption. For example, motor activity was not disrupted by boric acid, but was decreased by two boronic acids (caused by a sedative effect). 3TB, 2-Thienyl and 2-furanyl boronic acid gave rise to shaking behavior. The various manifestations generated by these compounds can be linked, in part, to different levels of dopamine (measured by HPLC) and degrees of neuronal damage in the basal ganglia and cerebellum. Clearly, motor disruption is not induced by all boronic acids with a five-membered cycle as substituent. Possible explanations are given for the diverse chemico-morphological changes and degrees of disruption of the motor system, considering the role of boron and the structure-toxicity relationship.

Nonhuman primate model in clinical modeling of diseases for stem cell therapy

Nonhuman primates (NHPs) are alike humans in size, behavior, physiology, biochemistry, and immunology. Given close similarities to humans, the NHP model offers exceptional opportunities to understand the biological mechanisms and translational applications with direct relevance to human conditions. Here, we evaluate the opportunities and limitations of NHPs as animal models for translational regenerative medicine. NHP models of human disease propose exceptional opportunities to advance stem cell-based therapy by addressing pertinent translational concerns related to this research. Nonetheless, the value of these primates must be carefully assessed, taking into account the expense of specialized equipment and requirement of highly specialized staff. Well-designed initial fundamental studies in small animal models are essential before translating research into NHP models and eventually into human trials. In addition, we suggest that applying a directed and collaborative approach, as seen in the evolution of stroke NHP models, will greatly benefit the translation of stem cell therapy in other NHP disease models.

Modeling Parkinson's disease in the common marmoset (Callithrix jacchus): overview of models, methods, and animal care

The common marmoset (Callithrix jacchus) is a small-bodied, popular New World monkey and is used widely in reproductive biology, neuroscience, and drug development, due to its comparative ease of handling, high reproductive efficiency, and its unique behavioral characters. In this review, we discuss the marmoset models in Parkinson's disease (PD), which is a neurological movement disorder primarily resulting from a degeneration of dopaminergic neurons with clinical features of tremor, rigidity, postural instability, and akinesia. The most common PD models involve the administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) or 6-hydroxydopamine to study the pathogenesis and to evaluate novel therapies. Following the systemic or local administration of these neurotoxins, the marmosets with very severe Parkinson's symptoms are recommended to be placed in an intensive care unit with artificial feeding to increase survival rate. All procedures with MPTP should be conducted in a special room with enclosed cages under negative-pressure by trained researchers with personal protection. Behavioral tests are conducted to provide an external measure of the brain pathology. Along with several biomarkers, including α-synuclein and DJ-1, non-invasive neuroimaging techniques such as positron emission tomography and magnetic resonance imaging are used to evaluate the functional changes associated with PD. With the recent growing interest in potential and novel therapies such as stem cell and gene therapy for PD in Korea, the marmoset can be considered as a suitable non-human primate model in PD research to bridge the gap between rodent studies and clinical applications.

Mechanisms of deep brain stimulation

Deep brain stimulation (DBS) is widely used for the treatment of movement disorders including Parkinson's disease, essential tremor, and dystonia and, to a lesser extent, certain treatment-resistant neuropsychiatric disorders including obsessive-compulsive disorder. Rather than a single unifying mechanism, DBS likely acts via several, nonexclusive mechanisms including local and network-wide electrical and neurochemical effects of stimulation, modulation of oscillatory activity, synaptic plasticity, and, potentially, neuroprotection and neurogenesis. These different mechanisms vary in importance depending on the condition being treated and the target being stimulated. Here we review each of these in turn and illustrate how an understanding of these mechanisms is inspiring next-generation approaches to DBS.

Nonhuman primate models in translational regenerative medicine

Humans and nonhuman primates (NHPs) are similar in size, behavior, physiology, biochemistry, structure and function of organs, and complexity of the immune system. Research on NHPs generates complementary data that bridge translational research from small animal models to humans. NHP models of human disease offer unique opportunities to develop stem cell-based therapeutic interventions that directly address relevant and challenging translational aspects of cell transplantation therapy. These include the use of autologous induced pluripotent stem cell-derived cellular products, issues related to the immune response in autologous and allogeneic setting, pros and cons of delivery techniques in a clinical setting, as well as the safety and efficacy of candidate cell lines. The NHP model allows the assessment of complex physiological, biochemical, behavioral, and imaging end points, with direct relevance to human conditions. At the same time, the value of using primates in scientific research must be carefully evaluated and timed due to expense and the necessity for specialized equipment and highly trained personnel. Often it is more efficient and useful to perform initial proof-of-concept studies for new therapeutics in rodents and/or other species before the pivotal studies in NHPs that may eventually lead to first-in-human trials. In this report, we present how the Southwest National Primate Research Center, one of seven NIH-funded National Primate Research Centers, may help the global community in translating promising technologies to the clinical arena.

Pathophysiology of L-dopa-induced motor and non-motor complications in Parkinson's disease

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa (L-dopa) therapy for Parkinson's disease (PD). L-dopa-induced dyskinesia (LID) are ultimately experienced by the vast majority of patients. In addition, psychiatric conditions often manifested as compulsive behaviours, are emerging as a serious problem in the management of L-dopa therapy. The present review attempts to provide an overview of our current understanding of dyskinesia and other L-dopa-induced dysfunctions, a field that dramatically evolved in the past twenty years. In view of the extensive literature on LID, there appeared a critical need to re-frame the concepts, to highlight the most suitable models, to review the central nervous system (CNS) circuitry that may be involved, and to propose a pathophysiological framework was timely and necessary. An updated review to clarify our understanding of LID and other L-dopa-related side effects was therefore timely and necessary. This review should help in the development of novel therapeutic strategies aimed at preventing the generation of dyskinetic symptoms.

Modulations in oscillatory frequency and coupling in globus pallidus with increasing parkinsonian severity

While beta oscillations often occur within the parkinsonian basal ganglia, how these oscillations emerge from a naive state and change with disease severity is not clear. To address this question, a progressive, nonhuman primate model of Parkinson's disease was developed using staged injections of MPTP. Within each parkinsonian state (naive, mild, moderate, and severe), spontaneous local field potentials were recorded throughout the sensorimotor globus pallidus. In the naive state, beta oscillations (11-32 Hz) occurred in half of the recordings, indicating spontaneous beta oscillations in globus pallidus are not pathognomonic. Mild and moderate states were characterized by a narrower distribution of beta frequencies that shifted toward the 8-15 Hz range. Additionally, coupling between the phase of beta and the amplitude of high-frequency oscillations (256-362 Hz) emerged in the mild state and increased with severity. These findings provide a novel mechanistic framework to understand how progressive loss of dopamine translates into abnormal information processing in the pallidum through alterations in oscillatory activity. The results suggest that rather than the emergence of oscillatory activity in one frequency spectrum or the other, parkinsonian motor signs may relate more to the development of altered coupling across multiple frequency spectrums.

Acute morphine treatments alleviate tremor in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated monkeys

Parkinson’s disease (PD) is a chronic and progressive neurodegenerative disorder associated with decreased striatal dopamine levels. Morphine has been found to elevate dopamine levels, which indicates a potential therapeutic effect in PD treatment that has not been investigated previously. To evaluate this hypothesis, an investigation of the acute effects of morphine on PD symptoms was carried out in male rhesus PD monkeys that had been induced with MPTP. All MPTP induced monkeys displayed progressive and irreversible PD motor symptoms. The behavioral response of these animals to morphine and L-Dopa were quantified with the Kurlan scale. It was found that L-Dopa alleviated bradykinesia, but did not significantly improve tremor. In contrast, acute morphine alleviated tremor significantly. These results suggested that, compared to L-Dopa, morphine has different therapeutic effects in PD therapy and may act through different biological mechanisms to alleviate PD symptoms.

Neuropathological diagnostic considerations in hyperkinetic movement disorders

Neuropathology of hyperkinetic movement disorders can be very challenging. This paper starts with basic functional anatomy of the basal ganglia in order to appreciate that focal lesions like for instance tumor or infarction can cause hyperkinetic movement disorders like (hemi)ballism. The neuropathology of different causes of chorea (amongst others Huntington’s disease, neuroacanthosis, and HLD-2) and dystonia (DYT1, PD, and Dopa-Responsive Dystonia) are described. Besides the functional anatomy of the basal ganglia a wider anatomical network view is provided. This forms the basis for the overview of the neuropathology of different forms of tremor.

PET analysis of dopaminergic neurodegeneration in relation to immobility in the MPTP-treated common marmoset, a model for Parkinson's disease

Background

Positron Emission Tomography (PET) measurement was applied to the brain of the common marmoset, a small primate species, treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The marmoset shows prominent Parkinson’s disease (PD) signs due to dopaminergic neural degeneration. Recently, the transgenic marmoset (TG) carrying human PD genes is developing. For phenotypic evaluations of TG, non-invasive PET measurement is considered to be substantially significant. As a reference control for TG, the brain of the MPTP-marmoset as an established and valid model was scanned by PET. Behavioral analysis was also performed by recording locomotion of the MPTP-marmoset, as an objective measure of PD signs.

Methodology/Principal Findings

Marmosets received several MPTP regimens (single MPTP regimen: 2 mg/kg, s.c., per day for 3 consecutive days) were used for PET measurement and behavioral observation. To measure immobility as a central PD sign, locomotion of marmosets in their individual living cages were recorded daily by infrared sensors. Daily locomotion counts decreased drastically after MPTP regimens and remained diminished for several months or more. PET scan of the brain, using [¹¹C]PE2I as a ligand of the dopamine (DA) transporter, was performed once several months after the last MPTP regimen. The mean binding potential (BP_ND) in the striatum (putamen and caudate) of the MPTP-marmoset group was significantly lower than that of the MPTP-free control group (n = 5 for each group). In the MPTP-marmosets, the decrease of BP_ND in the striatum closely correlated with the decrease in locomotion counts (r = 0.98 in putamen and 0.91 in caudate).

Conclusion/Significance

The present characterization of neural degeneration using non-invasive PET imaging and of behavioral manifestation in the MPTP marmoset mimics typical PD characteristics and can be useful in evaluating the phenotype of TG marmosets being developed.

On the origin of tremor in Parkinson's disease

The exact origin of tremor in Parkinson’s disease remains unknown. We explain why the existing data converge on the basal ganglia-thalamo-cortical loop as a tremor generator and consider a conductance-based model of subthalamo-pallidal circuits embedded into a simplified representation of the basal ganglia-thalamo-cortical circuit to investigate the dynamics of this loop. We show how variation of the strength of dopamine-modulated connections in the basal ganglia-thalamo-cortical loop (representing the decreasing dopamine level in Parkinson’s disease) leads to the occurrence of tremor-like burst firing. These tremor-like oscillations are suppressed when the connections are modulated back to represent a higher dopamine level (as it would be the case in dopaminergic therapy), as well as when the basal ganglia-thalamo-cortical loop is broken (as would be the case for ablative anti-parkinsonian surgeries). Thus, the proposed model provides an explanation for the basal ganglia-thalamo-cortical loop mechanism of tremor generation. The strengthening of the loop leads to tremor oscillations, while the weakening or disconnection of the loop suppresses them. The loop origin of parkinsonian tremor also suggests that new tremor-suppression therapies may have anatomical targets in different cortical and subcortical areas as long as they are within the basal ganglia-thalamo-cortical loop.

Cerebral causes and consequences of parkinsonian resting tremor: a tale of two circuits?

Tremor in Parkinson's disease has several mysterious features. Clinically, tremor is seen in only three out of four patients with Parkinson's disease, and tremor-dominant patients generally follow a more benign disease course than non-tremor patients. Pathophysiologically, tremor is linked to altered activity in not one, but two distinct circuits: the basal ganglia, which are primarily affected by dopamine depletion in Parkinson's disease, and the cerebello-thalamo-cortical circuit, which is also involved in many other tremors. The purpose of this review is to integrate these clinical and pathophysiological features of tremor in Parkinson's disease. We first describe clinical and pathological differences between tremor-dominant and non-tremor Parkinson's disease subtypes, and then summarize recent studies on the pathophysiology of tremor. We also discuss a newly proposed ‘dimmer-switch model’ that explains tremor as resulting from the combined actions of two circuits: the basal ganglia that trigger tremor episodes and the cerebello-thalamo-cortical circuit that produces the tremor. Finally, we address several important open questions: why resting tremor stops during voluntary movements, why it has a variable response to dopaminergic treatment, why it indicates a benign Parkinson's disease subtype and why its expression decreases with disease progression.

A tale on animal models of Parkinson's disease

Parkinson's disease is a neurodegenerative disorder whose cardinal manifestations are due primarily to a profound deficit in brain dopamine. Since the 1980s, several therapeutic strategies have been discovered to treat the symptoms of this neurological disorder, but as of yet, none halts or retards the neurodegenerative process. In an attempt to shed light on the neurobiology of Parkinson's disease, a number of experimental models have been developed, especially during the last 25 years. They come essentially in 3 flavors: pharmacological (eg, reserpine), toxic (eg, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), and genetic (eg, transgenic synuclein mice). These models can also be recast as etiologic, pathogenic, and symptomatic/pathophysiologic, as each may contribute to our understanding of the cause, the mechanisms, and the treatment of Parkinson's disease. In this review, we will discuss the question of Parkinson's disease models, starting from the period when this journal was born to today. During this journey of 25 years, we will discuss both the significant contributions of the Parkinson's disease models and hurdles that remain to be overcome to one day cure this neurological disease.

Regulation of Substantia Nigra Pars Reticulata GABAergic Neuron Activity by H₂O₂ via Flufenamic Acid-Sensitive Channels and KATP Channels

Substantia nigra pars reticulata (SNr) GABAergic neurons are key output neurons of the basal ganglia. Given the role of these neurons in motor control, it is important to understand factors that regulate their firing rate and pattern. One potential regulator is hydrogen peroxide (H₂O₂), a reactive oxygen species that is increasingly recognized as a neuromodulator. We used whole-cell current clamp recordings of SNr GABAergic neurons in guinea-pig midbrain slices to determine how H₂O₂ affects the activity of these neurons and to explore the classes of ion channels underlying those effects. Elevation of H₂O₂ levels caused an increase in the spontaneous firing rate of SNr GABAergic neurons, whether by application of exogenous H₂O₂ or amplification of endogenous H₂O₂ through inhibition of glutathione peroxidase with mercaptosuccinate. This effect was reversed by flufenamic acid (FFA), implicating transient receptor potential (TRP) channels. Conversely, depletion of endogenous H₂O₂ by catalase, a peroxidase enzyme, decreased spontaneous firing rate and firing precision of SNr neurons, demonstrating tonic control of firing rate by H₂O₂. Elevation of H₂O₂ in the presence of FFA revealed an inhibition of tonic firing that was prevented by blockade of ATP-sensitive K(+) (K(ATP)) channels with glibenclamide. In contrast to guinea-pig SNr neurons, the dominant effect of H₂O₂ elevation in mouse SNr GABAergic neurons was hyperpolarization, indicating a species difference in H₂O₂-dependent regulation. Thus, H₂O₂ is an endogenous modulator of SNr GABAergic neurons, acting primarily through presumed TRP channels in guinea-pig SNr, with additional modulation via K(ATP) channels to regulate SNr output.

Restoration of locomotive function in Parkinson's disease by spinal cord stimulation: mechanistic approach

Specific motor symptoms of Parkinson's disease (PD) can be treated effectively with direct electrical stimulation of deep nuclei in the brain. However, this is an invasive procedure, and the fraction of eligible patients is rather low according to currently used criteria. Spinal cord stimulation (SCS), a minimally invasive method, has more recently been proposed as a therapeutic approach to alleviate PD akinesia, in light of its proven ability to rescue locomotion in rodent models of PD. The mechanisms accounting for this effect are unknown but, from accumulated experience with the use of SCS in the management of chronic pain, it is known that the pathways most probably activated by SCS are the superficial fibers of the dorsal columns. We suggest that the prokinetic effect of SCS results from direct activation of ascending pathways reaching thalamic nuclei and the cerebral cortex. The afferent stimulation may, in addition, activate brainstem nuclei, contributing to the initiation of locomotion. On the basis of the striking change in the corticostriatal oscillatory mode of neuronal activity induced by SCS, we propose that, through activation of lemniscal and brainstem pathways, the locomotive increase is achieved by disruption of antikinetic low-frequency (<30 Hz) oscillatory synchronization in the corticobasal ganglia circuits.

Restoration of locomotive function in Parkinson's disease by spinal cord stimulation: mechanistic approach

Specific motor symptoms of Parkinson's disease (PD) can be treated effectively with direct electrical stimulation of deep nuclei in the brain. However, this is an invasive procedure, and the fraction of eligible patients is rather low according to currently used criteria. Spinal cord stimulation (SCS), a minimally invasive method, has more recently been proposed as a therapeutic approach to alleviate PD akinesia, in light of its proven ability to rescue locomotion in rodent models of PD. The mechanisms accounting for this effect are unknown but, from accumulated experience with the use of SCS in the management of chronic pain, it is known that the pathways most probably activated by SCS are the superficial fibers of the dorsal columns. We suggest that the prokinetic effect of SCS results from direct activation of ascending pathways reaching thalamic nuclei and the cerebral cortex. The afferent stimulation may, in addition, activate brainstem nuclei, contributing to the initiation of locomotion. On the basis of the striking change in the corticostriatal oscillatory mode of neuronal activity induced by SCS, we propose that, through activation of lemniscal and brainstem pathways, the locomotive increase is achieved by disruption of antikinetic low-frequency (<30 Hz) oscillatory synchronization in the corticobasal ganglia circuits.

Dissociation of motor symptoms during deep brain stimulation of the subthalamic nucleus in the region of the internal capsule

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) can be an effective treatment for the motor symptoms of Parkinson's disease. The therapeutic benefits are voltage-dependent and, in many cases, limited by the appearance of side effects, including muscle contractions. We have observed a number of clinical cases where improvements in rigidity were accompanied by a worsening of bradykinesia. Considering the anatomic position of STN and current approaches to implantation of the DBS lead, we hypothesized that this dissociation of motor symptoms arises from activation of pyramidal tract fibers in the adjacent internal capsule. The objective of this study was to assess the physiological basis for this dissociation and to test our hypothesis that the underlying etiology of this paradox is activation of fibers of the internal capsule. The effect of STN DBS at 80% of motor threshold for each of the four contacts was evaluated for its effect on rigidity, bradykinesia, and akinesia in a single primate with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced parkinsonism. Consistent with our observations in humans, this near-threshold stimulation was found to improve rigidity while bradykinesia and akinesia worsened. Worsening bradykinesia in the face of improvement of other motor signs in Parkinson's disease (PD) patients is suggestive of activation of pyramidal tract (PT) fibers during stimulation. This phenomenon may occur without overt muscle contraction and improved rigidity.

Pathophysiology of parkinsonism

The motor signs of Parkinson's disease are thought to result in large part from a reduction of the level of dopamine in the basal ganglia. Over the last few years, many of the functional and anatomical consequences of dopamine loss in these structures have been identified, both in the basal ganglia and in related areas in thalamus and cortex. This knowledge has contributed significantly to our understanding of the link between the degeneration of dopamine neurons in the midbrain and the development of parkinsonism. This review discusses the evidence that implicates electrophysiologic changes (including altered discharge rates, increased incidence of burst firing, interneuronal synchrony, oscillatory activity, and altered sensorimotor processing) in basal ganglia, thalamus, and cortex, in parkinsonism. From these studies, parkinsonism emerges as a complex network disorder, in which abnormal activity in groups of neurons in the basal ganglia strongly affects the excitability, oscillatory activity, synchrony and sensory responses of areas of the cerebral cortex that are involved in the planning and execution of movement, as well as in executive, limbic or sensory functions. Detailed knowledge of these changes will help us to develop more effective and specific symptomatic treatments for patients with Parkinson's disease.

[123I]FP-CIT striatal binding in early Parkinson's disease patients with tremor vs. akinetic-rigid onset

We performed [123I]FP-CIT/SPECT in 20 drug-naive Parkinson's disease (PD) patients, 10 with unilateral akinesia/rigidity at onset (arPD) and 10 with additional tremor-at-rest (tPD), to evaluate whether resting tremor at onset is associated with differences in striatal dopamine transporter binding. Patients of the two cohorts were matched for age, disease duration (<3 years) and severity of non-tremor motor symptoms; 31 healthy participants served as controls. Mean striatal dopamine transporter binding reduction in PD patients vs. controls was 42% for arPD and 50% for tPD; mean ipsilateral striatum and caudate nucleus uptake values were lower by 12 and 24%, respectively, in tPD than arPD. We conclude that widespread degeneration of the nigrostriatal dopaminergic pathway might be necessary for the development of parkinsonian tremor-at-rest.

Oscillations in the basal ganglia under normal conditions and in movement disorders

A substantial body of work within the last decade has demonstrated that there is a variety of oscillatory phenomena that occur in the basal ganglia and in associated regions of the thalamus and cortex. Most of the earlier studies focused on recordings in rodents and primates. More recently, significant advances have been made in this field of research through the analysis of basal ganglia field potentials recorded from implanted deep brain stimulation electrodes in the basal ganglia of human patients with Parkinson's disease and other disorders. It now appears that oscillatory activity may play a significant role in the pathogenesis of these diseases. The most significant finding is that in Parkinson's disease synchronized oscillatory activity in the 10- to 35-Hz band (often termed "beta-band") is prevalent in the basal ganglia-thalamocortical circuits, and that such activity can be reduced by dopaminergic treatments. The entrainment of large portions of these circuits may disrupt information processing in them and may lead to parkinsonian akinesia (and perhaps tremor). Although less firmly established than the role of oscillations in movement disorders, oscillatory activities at higher frequencies may also be a component of normal basal ganglia physiology.

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

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

Is the rhythm of physiological tremor involved in cortico-cortical interactions?

The function of low-frequency oscillations as correlates of physiological tremor in supplementary motor area (SMA) and M1 remains unclear. In epicortical recordings from M1 and SMA and surface electromyographic (EMG) recordings in an epileptic patient we found reproducibly significant coherence between all three recording sites in the 6- to 15-Hz band. The partial coherence between SMA and muscle, however, was not significant. There was a constant phase shift between SMA and M1 indicating synchronized activity. We conclude that the cortical correlates of physiological tremor may be involved in linking different cortical motor centers and might therefore play a role in cortical motor planning.

Pathophysiology of Parkinson's disease: the MPTP primate model of the human disorder

The striatum is viewed as the principal input structure of the basal ganglia, while the internal pallidal segment (GPi) and the substantia nigra pars reticulata (SNr) are output structures. Input and output structures are linked via a monosynaptic "direct" pathway and a polysynaptic "indirect" pathway involving the external pallidal segment (GPe) and the subthalamic nucleus (STN). According to current schemes, striatal dopamine (DA) enhances transmission along the direct pathway (via D1 receptors), and reduces transmission over the indirect pathway (via D2 receptors). DA also acts on receptors in GPe, GPi, SNr, and STN. Electrophysiologic and other studies in primates rendered parkinsonian by treatment with the dopaminergic neurotoxin MPTP have demonstrated a reduction of neuronal activity of GPe and an increase of neuronal discharge in STN, GPi. and SNr. These findings are compatible with the view that striatal DA loss results in increased activity over the indirect pathway. Prominent bursting, oscillatory discharge patterns, and increased synchronization of neighboring neurons are found throughout the basal ganglia. These may result from changes in the activity of local circuits (e.g., the GPe-STN "pacemaker") or from more global abnormalities of the basal ganglia-thalamocortical network. These findings have been replicated in human patients undergoing microelectrode-guided stereotactic procedures targeted at GPi or STN. PET studies in patients with Parkinson's disease have lent further support to the proposed circuit abnormalities. The current models of basal ganglia function have recently been criticized. For instance, the strict separation of direct and indirect pathways and the segregation of D1 and D2 receptors have been questioned, and the almost complete absence of motor side effects of pallidal or thalamic lesions in human patients and animals is inconsistent. These results suggest that changes in discharge patterns and synchronization between basal ganglia neurons, abnormal network interactions, and compensatory mechanisms are at least as important in the pathophysiology of parkinsonism as changes in discharge rates in individual basal ganglia nuclei. Lesions of GPi or STN are effective in treating parkinsonism, because they reduce or abolish abnormal basal ganglia output, enabling remaining circuits to function more normally.

Tremor-related activity of neurons in the 'motor' thalamus: changes in firing rate and pattern in the MPTP vervet model of parkinsonism

The pathophysiology of parkinsonian tremor remains a matter of debate with two opposing hypotheses proposing a peripheral and a central origin, respectively. A central origin of tremor could arise either from a rhythmic activity of the internal segment of the globus pallidus (GPi) or from a structure such as the thalamus, outside the basal ganglia. In this study, single-unit recordings were performed in three 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated monkeys within the GPi and within three territories of the motor thalamus (delimited by their afferent inputs from the GPi, the substantia nigra and the cerebellum, respectively). For each recorded neuron, we compared the variations in firing rate and pattern in tremor and no tremor periods. Tremor either occurred spontaneously or was induced by external stimulation. When the animals entered into a tremor period we observed: (i) an increase in the mean firing rate in about half of the recorded neurons of the motor thalamus; and (ii), a change from an irregular to a rhythmic discharge within the range of tremor frequency (5-7 Hz) in about 10% of the recorded neurons of the motor thalamus (pallidal and cerebellar territories) and the GPi. Most of the thalamic neurons that exhibited a rhythmic discharge during tremor were found to be sensitive to external stimulation. Because the changes in firing rate occurred predominantly in the motor thalamus and not in the GPi, and because a fast rhythmic discharge of 10-15 Hz was frequently observed in the GPi and not in the motor thalamus, we conclude that thalamic activity is not a simple reproduction of basal ganglia output. Moreover, we suggest that thalamic processing of basal ganglia outputs could participate in the genesis of tremor, and that this thalamic processing could be influenced by sensory inputs and/or changes in attentional level elicited by external stimulation.

Evidence for a glutamatergic projection from the zona incerta to the basal ganglia of rats

This study explores the organisation and neurochemical nature of the projections from the zona incerta (ZI) to the basal ganglia. Sprague-Dawley rats were anaesthetised with ketamine (100 mg/kg) and Rompun (10 mg/kg), and injections of cholera toxin subunit B were made into each of the following nuclei: the ZI, the substantia nigra (SN), the pedunculopontine tegmental nucleus (PpT), and the entopeduncular nucleus (Ep). Brains were aldehyde fixed, sectioned, and processed using standard methods. Tracer-labelled sections were then doubly labelled with antibodies to glutamate (Glu), nitric oxide synthase (NOS), parvalbumin (Pv), or glutamic acid decarboxylase (GAD; the latter two are markers for GABAergic cells); these neurochemicals characterise most types of ZI cells. After ZI injections, labelling was nonuniform across the different basal ganglia nuclei. The bulk of labelling, both anterograde and retrograde, was seen in the SN and PpT and, to a lesser extent, within the other nuclei of the basal ganglia (e.g., caudate-putamen, globus pallidus, subthalamus, Ep). In the SN, labelling was found in both major parts of the nucleus, the pars compacta and pars reticulata. Within the PpT, however, the bulk of labelling was limited to only one of the two sectors of the nucleus, namely, the pars dissipata (PpTd). The pars compacta of the PpT (PpTc) remained largely free of labelled profiles. After CTb injections into three basal ganglia nuclei (SN, PpT, Ep), most labelled cells in the ZI were glutamate+ and very few were NOS+ or gamma-aminobutyric acidergic. Overall, the results indicate that the ZI is in a position to influence preferentially the activity of the SN and PpTd of the basal ganglia via an excitatory, glutamatergic input.

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

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

Cellular replacement therapy for Parkinson's disease--where we are today?

The concept of replacing lost dopamine neurons in Parkinson's disease using mesencephalic brain cells from fetal cadavers has been supported by over 20 years of research in animals and over a decade of clinical studies. The ambitious goal of these studies was no less than a molecular and cellular "cure" for Parkinson's disease, other neurodegenerative diseases, and spinal cord injury. Much research has been done in rodents, and a few studies have been done in nonhuman primate models. Early uncontrolled clinical reports were enthusiastic, but the outcome of the first randomized, double blind, controlled study challenged the idea that dopamine replacement cells can cure Parkinson's disease, although there were some significant positive findings. Were the earlier animal studies and clinical reports wrong? Should we give up on the goal? Some aspects of the trial design and implantation methods may have led to lack of effects and to some side effects such as dyskinesias. But a detailed review of clinical neural transplants published to date still suggests that neural transplantation variably reverses some aspects of Parkinson's disease, although differing methods make exact comparisons difficult. While the randomized clinical studies have been in progress, new methods have shown promise for increasing transplant survival and distribution, reconstructing the circuits to provide dopamine to the appropriate targets and with normal regulation. Selected promising new strategies are reviewed that block apoptosis induced by tissue dissection, promote vascularization of grafts, reduce oxidant stress, provide key growth factors, and counteract adverse effects of increased age. New sources of replacement cells and stem cells may provide additional advantages for the future. Full recovery from parkinsonism appears not only to be possible, but a reliable cell replacement treatment may finally be near.

Effective desynchronization with bipolar double-pulse stimulation

This paper is devoted to the desynchronizing effects of bipolar stimuli on a synchronized cluster of globally coupled phase oscillators. The bipolar pulses considered here are symmetrical and consist of a positive and a negative monopolar pulse. A bipolar single pulse with the right intensity and duration desynchronizes a synchronized cluster provided the stimulus is administered at a vulnerable initial phase of the cluster's order parameter. A considerably more effective desynchronization is achieved with a bipolar double pulse consisting of two qualitatively different bipolar pulses. The first bipolar pulse is stronger and resets the cluster, so that the second bipolar pulse, which follows after a constant delay, hits the cluster in a vulnerable state and desynchronizes it. A bipolar double pulse desynchronizes the cluster independently of the cluster's dynamical state at the beginning of the stimulation. The dynamics of the order parameter during a bipolar single pulse or a bipolar double pulse is different from the dynamics during a monopolar single pulse or a monopolar double pulse. Nevertheless, concerning their desynchronizing effects the monopolar and the bipolar stimuli are comparable, respectively. This is significant for applications where bipolar stimulation is required. For example, in medicine and physiology charge-balanced stimulation is typically necessary in order to avoid tissue damage. Based on the results presented here, demand-controlled bipolar double-pulse stimulation is suggested as a milder and more efficient therapy compared to the standard permanent high-frequency deep brain stimulation in neurological patients.

Nigrostriatal lesion and dopamine agonists affect firing patterns of rodent entopeduncular nucleus neurons

Altered activity of the entopeduncular nucleus, the rodent homologue of the globus pallidus internal segment in primates, is thought to mediate behavioral consequences of midbrain dopamine depletion in rodents. Few studies, however, have examined dopaminergic modulation of spiking activity in this nucleus. This study characterizes changes in entopeduncular neuronal activity after nigrostriatal dopaminergic lesion and the effects of systemic treatment with selective D(1) (SKF 38393) and D(2) (quinpirole) agonists in lesioned rats. Extracellular single-unit recordings were performed in awake immobilized rats, either in neurologically intact animals (n = 42) or in animals that had received unilateral 6-hydroxydopamine infusion into the medial forebrain bundle several weeks previously (n = 35). Nigrostriatal lesion altered baseline activity of entopeduncular neurons in several ways. Interspike interval distributions had significantly decreased modes and significantly increased coefficient of variation, skewness and kurtosis; yet interspike interval mean (the inverse of firing rate) was not affected. Also, spectral analysis of autocorrelograms indicated that lesion significantly reduced the incidence of regular-spiking neurons and increased the incidence of neurons with 4-18 Hz oscillations. Dopamine agonist treatment reversed some lesion-induced effects: quinpirole reversed changes in interspike interval distribution mode and coefficient of variation, while combined quinpirole and SKF 38393 blocked the appearance of 4-18 Hz oscillations. However, no agonist treatment normalized all aspects of entopeduncular activity. Additionally, inhibition of firing rates by D(1) or combined D(1)/D(2) receptor activation indicated that dopamine agonists affected the overall level of entopeduncular activity in a manner similar to that found in the substantia nigra pars reticulata and globus pallidus internal segment after dopamine neuron lesion. These data demonstrate that lesion of the nigrostriatal tract leads to modifications of several aspects of firing pattern in the rodent entopeduncular nucleus and so expand on similar findings in the rodent substantia nigra pars reticulata and in the globus pallidus internal segment in humans and nonhuman primates. The results support the view that dysfunction in the basal ganglia after midbrain dopamine neuron loss relates more consistently to abnormal activity patterns than to net changes in firing rate in the basal ganglia output nuclei, while overall decreases in firing rate in these structures may play a more important role in adverse motor reactions to dopamine agonist treatments.

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

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

Pathophysiology of Parkinson's disease: from clinical neurology to basic neuroscience and back

Parkinson's disease (PD) is characterized by motor and nonmotor (cognitive and limbic) deficits. The motor signs of PD include hypokinetic signs such as akinesia/bradykinesia, rigidity and loss of normal postural reflexes, and hyperkinetic signs such as tremor. Dopamine depletion in the striatum is the hallmark of PD and of its animal models, still the pathophysiology of the parkinsonian symptoms and especially of parkinsonian tremor are under debate. The most extreme hypotheses argue about peripheral versus central nervous system origin, intrinsic cellular oscillator versus network oscillators, and basal ganglia-based pathophysiology versus cerebellar-thalamic based pathophysiology. Recent studies support the view that parkinsonian symptoms are most likely due to abnormal synchronous oscillating neuronal activity within the basal ganglia. Peripheral factors do only play a minor role for the generation, maintenance, and modulation of PD tremor and other signs. The most likely candidates producing these neuronal oscillations are the weakly coupled neural networks of the basal ganglia-thalamo-cortical loops. However, the present evidence supports the view that the basal ganglia loops are influenced by other neuronal structures and systems and that the tuning of these loops by cerebello-thalamic mechanisms and by other modulator neurotransmitter systems entrain the abnormal synchronized oscillations. Neurosurgical procedures, such as lesions or high-frequency stimulation of different parts of the loop, might resume the normal unsynchronized activity of the basal ganglia circuitry, and, therefore, ameliorate the clinical symptoms of Parkinson's disease.

Activity patterns in a model for the subthalamopallidal network of the basal ganglia

Based on recent experimental data, we have developed a conductance-based computational network model of the subthalamic nucleus and the external segment of the globus pallidus in the indirect pathway of the basal ganglia. Computer simulations and analysis of this model illuminate the roles of the coupling architecture of the network, and associated synaptic conductances, in modulating the activity patterns displayed by this network. Depending on the relationships of these coupling parameters, the network can support three general classes of sustained firing patterns: clustering, propagating waves, and repetitive spiking that may show little regularity or correlation. Each activity pattern can occur continuously or in discrete episodes. We characterize the mechanisms underlying these rhythms, as well as the influence of parameters on details such as spiking frequency and wave speed. These results suggest that the subthalamopallidal circuit is capable both of correlated rhythmic activity and of irregular autonomous patterns of activity that block rhythmicity. Increased striatal input to, and weakened intrapallidal inhibition within, the indirect pathway can switch the behavior of the circuit from irregular to rhythmic. This may be sufficient to explain the emergence of correlated oscillatory activity in the subthalamopallidal circuit after destruction of dopaminergic neurons in Parkinson's disease and in animal models of parkinsonism.

Synchronized neuronal discharge in the basal ganglia of parkinsonian patients is limited to oscillatory activity

It has been proposed that an increase in synchronization between neurons in the basal ganglia contributes to the clinical features of Parkinson's disease (PD). To examine this hypothesis, we looked for correlations in the discharge activity of pairs of neurons in the globus pallidus internus (GPi), globus pallidus externus (GPe), and the substantia nigra pars reticulata (SNr). Recordings were performed in PD patients undergoing functional stereotactic mapping for pallidotomy (eight patients) or subthalamic nucleus deep brain stimulation (four patients). A double-microelectrode setup was used to simultaneously record from neurons separated by distances as small as 250 microm. In the five pallidotomy patients without limb tremor during the procedure, none of the 73 GPi pairs and 15 GPe pairs displayed synchronous activity. In the three pallidotomy patients with limb tremor, 6 of 21 GPi pairs and 5 of 29 GPe pairs displayed oscillatory synchronization in the frequency range of the ongoing limb tremor (3-6 Hz) or at higher frequencies (15-30 Hz). Synchronized activity was not observed in the SNr (10 pairs). The findings indicate that oscillatory synchronization between pairs of GPi or GPe neurons is found in patients with limb tremor. These results also suggest that overt neuronal synchronization, which may be attributable to an increase in direct synaptic connections or common collateral afferent inputs, is not present in the basal ganglia of patients with PD.

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

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

Tremor in Parkinson's disease

Rest tremor is a common feature of Parkinson's disease, but its underlying pathophysiology remains unknown. This review hypothesizes that tremor is related to selective loss of components of the substantia nigra. The relative scarcity of tremor in related Parkinsonian conditions may indicate a dissociation associated with different pathological involvement of the substantia nigra and its connections. Connections of the subthalamic nucleus with the pallidum, modified by cortical and nigral inputs, allow for the transfer of tremorogenic activity to the thalamus. Thalamo-cortical interactions, tempered by cerebellar input, generate the final common pathway for tremor production.

The pathophysiology of tremor

Tremor is defined as rhythmic oscillatory activity of body parts. Four physiological basic mechanisms for such oscillatory activity have been described: mechanical oscillations; oscillations based on reflexes; oscillations due to central neuronal pacemakers; and oscillations because of disturbed feedforward or feedback loops. New methodological approaches with animal models, positron emission tomography, and mathematical analysis of electromyographic and electroencephalographic signals have provided new insights into the mechanisms underlying specific forms of tremor. Physiological tremor is due to mechanical and central components. Psychogenic tremor is considered to depend on a clonus mechanism and is thus believed to be mediated by reflex mechanisms. Symptomatic palatal tremor is most likely due to rhythmic activity of the inferior olive, and there is much evidence that essential tremor is also generated within the olivocerebellar circuits. Orthostatic tremor is likely to originate in hitherto unidentified brainstem nuclei. Rest tremor of Parkinson's disease is probably generated in the basal ganglia loop, and dystonic tremor may also originate within the basal ganglia. Cerebellar tremor is at least in part caused by a disturbance of the cerebellar feedforward control of voluntary movements, and Holmes' tremor is due to the combination of the mechanisms producing parkinsonian and cerebellar tremor. Neuropathic tremor is believed to be caused by abnormally functioning reflex pathways and a wide variety of causes underlies toxic and drug-induced tremors. The understanding of the pathophysiology of tremor has made significant progress but many hypotheses are not yet based on sufficient data. Modern neurology needs to develop and test such hypotheses, because this is the only way to develop rational medical and surgical therapies.

High-frequency synchronization of neuronal activity in the subthalamic nucleus of parkinsonian patients with limb tremor

It has been hypothesized that in Parkinson's disease (PD) there is increased synchronization of neuronal firing in the basal ganglia. This study examines the discharge activity of 121 pairs of subthalamic nucleus (STN) neurons in nine PD patients undergoing functional stereotactic mapping. Four patients had a previous pallidotomy. A double microelectrode setup was used to simultaneously record from two neurons separated by distances as small as 250 micrometer. In the six patients who had limb tremor during the recording session (n = 76 pairs), the discharge pattern of 12 pairs of tremor cells (TCs) was found to be coherent at the frequency of the limb tremor. Both in-phase and out-of-phase relationships were observed between TCs. Interestingly, in these six patients, 63/129 single neurons displayed 15-30 Hz oscillations, whereas 36/76 pairs were coherent in this frequency range. Although the oscillatory frequencies were variable between patients, they were highly clustered within a patient. The phase difference between these pairs was found to be close to 0. High-frequency synchronization was observed during periods of limb tremor as well as during intermittent periods with no apparent limb tremor. In contrast, in the three patients without limb tremor during the recording session, only 1/84 neurons had high-frequency oscillatory activity, and no TCs or synchronous high-frequency oscillatory activity was observed (n = 45 pairs). These findings demonstrate that in PD patients with limb tremor, many STN neurons display high-frequency oscillations with a high degree of in-phase synchrony. The results suggest that high-frequency synchronized oscillatory activity may be associated with the pathology that gives rise to tremor in PD patients.

Firing patterns and correlations of spontaneous discharge of pallidal neurons in the normal and the tremulous 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine vervet model of parkinsonism

To investigate the role of the basal ganglia in parkinsonian tremor, we recorded hand tremor and simultaneous activity of several neurons in the external and internal segments of the globus pallidus (GPe and GPi) in two vervet monkeys, before and after systemic treatment with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and development of parkinsonism with tremor of 5 and 11 Hz. In healthy monkeys, only 11% (20/174) of the GPe cells and 3% (1/29) of the GPi cells displayed significant 3-19 Hz oscillations. After MPTP treatment, 39% (107/271) of the GPe cells and 43% (26/61) of the GPi cells developed significant oscillations. Oscillation frequencies of single cells after MPTP treatment were bimodally distributed around 7 and 13 Hz. For 10% of the oscillatory cells that were recorded during tremor periods, there was a significant tendency for the tremor and neuronal oscillations to appear simultaneously. Cross-correlation analysis revealed a very low level of correlated activity between pallidal neurons in the normal state; 95.6% (477/499) of the pairs were not correlated, and oscillatory cross-correlograms were found in only 1% (5/499) of the pairs. After MPTP treatment, the correlations increased dramatically, and 40% (432/1080) of the cross-correlograms had significant oscillations, centered around 13-14 Hz. Phase shifts of the cross-correlograms of GPe pairs, but not of GPi, were clustered around 0 degrees. The results illustrate that MPTP treatment changes the pattern of activity and synchronization in the GPe and GPi. These changes are related to the symptoms of Parkinson's disease and especially to the parkinsonian tremor.

Concurrent Parkinson tremors

1. Concurrent resting and postural tremors of patients with idiopathic Parkinson's disease were monitored using transducers responding to angular velocity of rotation. Spectra and correlation functions were calculated for each pair of records. 2. When concurrent tremor spectra share indistinguishable fundamental frequencies, have statistically significant peaks in their coherence spectra at those fundamental frequencies, and show significant peaks in their cross-correlation functions near zero delay, they are classified as linearly dependent. When such tremor records are superimposed, their phase-locked behaviour is evident. 3. Pairs of correlated concurrent tremors, of varying duration, have been observed in both hands, both feet and in either hand and the contralateral or ipsilateral foot. Correlated tremors may be concurrent with other tremors that are independent. We hypothesize that correlated Parkinson tremors arise from one or more common (and possibly unilateral) central sources.