Dopamine receptors set the pattern of activity generated in subthalamic neurons

The Devastating Effects of Sleep Deprivation on Memory: Lessons from Rodent Models

In this narrative review article, we discuss the role of sleep deprivation (SD) in memory processing in rodent models. Numerous studies have examined the effects of SD on memory, with the majority showing that sleep disorders negatively affect memory. Currently, a consensus has not been established on which damage mechanism is the most appropriate. This critical issue in the neuroscience of sleep remains largely unknown. This review article aims to elucidate the mechanisms that underlie the damaging effects of SD on memory. It also proposes a scientific solution that might explain some findings. We have chosen to summarize literature that is both representative and comprehensive, as well as innovative in its approach. We examined the effects of SD on memory, including synaptic plasticity, neuritis, oxidative stress, and neurotransmitters. Results provide valuable insights into the mechanisms by which SD impairs memory function.

A translational perspective on pathophysiological changes of oscillatory activity in dystonia and parkinsonism

Intracerebral recordings from movement disorders patients undergoing deep brain stimulation have allowed the identification of pathophysiological patterns in oscillatory activity that correlate with symptom severity. Changes in oscillatory synchrony occur within and across brain areas, matching the classification of movement disorders as network disorders. However, the underlying mechanisms of oscillatory changes are difficult to assess in patients, as experimental interventions are technically limited and ethically problematic. This is why animal models play an important role in neurophysiological research of movement disorders. In this review, we highlight the contributions of translational research to the mechanistic understanding of pathological changes in oscillatory activity, with a focus on parkinsonism and dystonia, while addressing the limitations of current findings and proposing possible future directions.

An electrophysiological perspective on Parkinson's disease: symptomatic pathogenesis and therapeutic approaches

Parkinson's disease (PD), or paralysis agitans, is a common neurodegenerative disease characterized by dopaminergic deprivation in the basal ganglia because of neuronal loss in the substantia nigra pars compacta. Clinically, PD apparently involves both hypokinetic (e.g. akinetic rigidity) and hyperkinetic (e.g. tremor/propulsion) symptoms. The symptomatic pathogenesis, however, has remained elusive. The recent success of deep brain stimulation (DBS) therapy applied to the subthalamic nucleus (STN) or the globus pallidus pars internus indicates that there are essential electrophysiological abnormalities in PD. Consistently, dopamine-deprived STN shows excessive burst discharges. This proves to be a central pathophysiological element causally linked to the locomotor deficits in PD, as maneuvers (such as DBS of different polarities) decreasing and increasing STN burst discharges would decrease and increase the locomotor deficits, respectively. STN bursts are not so autonomous but show a "relay" feature, requiring glutamatergic synaptic inputs from the motor cortex (MC) to develop. In PD, there is an increase in overall MC activities and the corticosubthalamic input is enhanced and contributory to excessive burst discharges in STN. The increase in MC activities may be relevant to the enhanced beta power in local field potentials (LFP) as well as the deranged motor programming at the cortical level in PD. Moreover, MC could not only drive erroneous STN bursts, but also be driven by STN discharges at specific LFP frequencies (~ 4 to 6 Hz) to produce coherent tremulous muscle contractions. In essence, PD may be viewed as a disorder with deranged rhythms in the cortico-subcortical re-entrant loops, manifestly including STN, the major component of the oscillating core, and MC, the origin of the final common descending motor pathways. The configurations of the deranged rhythms may play a determinant role in the symptomatic pathogenesis of PD, and provide insight into the mechanism underlying normal motor control. Therapeutic brain stimulation for PD and relevant disorders should be adaptively exercised with in-depth pathophysiological considerations for each individual patient, and aim at a final normalization of cortical discharge patterns for the best ameliorating effect on the locomotor and even non-motor symptoms.

Symmetric and Asymmetric Synapses Driving Neurodegenerative Disorders

In 1959, E. G. Gray described two different types of synapses in the brain for the first time: symmetric and asymmetric. Later on, symmetric synapses were associated with inhibitory terminals, and asymmetric synapses to excitatory signaling. The balance between these two systems is critical to maintain a correct brain function. Likewise, the modulation of both types of synapses is also important to maintain a healthy equilibrium. Cerebral circuitry responds differently depending on the type of damage and the timeline of the injury. For example, promoting symmetric signaling following ischemic damage is beneficial only during the acute phase; afterwards, it further increases the initial damage. Synapses can be also altered by players not directly related to them; the chronic and long-term neurodegeneration mediated by tau proteins primarily targets asymmetric synapses by decreasing neuronal plasticity and functionality. Dopamine represents the main modulating system within the central nervous system. Indeed, the death of midbrain dopaminergic neurons impairs locomotion, underlying the devastating Parkinson’s disease. Herein, we will review studies on symmetric and asymmetric synapses plasticity after three different stressors: symmetric signaling under acute damage—ischemic stroke; asymmetric signaling under chronic and long-term neurodegeneration—Alzheimer’s disease; symmetric and asymmetric synapses without modulation—Parkinson’s disease.

Cellular and Synaptic Dysfunctions in Parkinson's Disease: Stepping out of the Striatum

The basal ganglia (BG) are a collection of interconnected subcortical nuclei that participate in a great variety of functions, ranging from motor programming and execution to procedural learning, cognition, and emotions. This network is also the region primarily affected by the degeneration of midbrain dopaminergic neurons localized in the substantia nigra pars compacta (SNc). This degeneration causes cellular and synaptic dysfunctions in the BG network, which are responsible for the appearance of the motor symptoms of Parkinson’s disease. Dopamine (DA) modulation and the consequences of its loss on the striatal microcircuit have been extensively studied, and because of the discrete nature of DA innervation of other BG nuclei, its action outside the striatum has been considered negligible. However, there is a growing body of evidence supporting functional extrastriatal DA modulation of both cellular excitability and synaptic transmission. In this review, the functional relevance of DA modulation outside the striatum in both normal and pathological conditions will be discussed.

Different Dopaminergic Dysfunctions Underlying Parkinsonian Akinesia and Tremor

Although the occurrence of Parkinsonian akinesia and tremor is traditionally associated to dopaminergic degeneration, the multifaceted neural processes that cause these impairments are not fully understood. As a consequence, current dopamine medications cannot be tailored to the specific dysfunctions of patients with the result that generic drug therapies produce different effects on akinesia and tremor. This article proposes a computational model focusing on the role of dopamine impairments in the occurrence of akinesia and resting tremor. The model has three key features, to date never integrated in a single computational system: (a) an architecture constrained on the basis of the relevant known system-level anatomy of the basal ganglia-thalamo-cortical loops; (b) spiking neurons with physiologically-constrained parameters; (c) a detailed simulation of the effects of both phasic and tonic dopamine release. The model exhibits a neural dynamics compatible with that recorded in the brain of primates and humans. Moreover, it suggests that akinesia might involve both tonic and phasic dopamine dysregulations whereas resting tremor might be primarily caused by impairments involving tonic dopamine release and the responsiveness of dopamine receptors. These results could lead to develop new therapies based on a system-level view of the Parkinson's disease and targeting phasic and tonic dopamine in differential ways.

Bidirectional dopamine modulation of excitatory and inhibitory synaptic inputs to subthalamic neuron subsets containing α4β2 or α7 nAChRs

The subthalamic nucleus (STN) possesses microcircuits distinguished by subtypes of nicotinic acetylcholine receptors (nAChRs). Although dysfunction of the STN is well-known in Parkinson's disease, there is still little information about whether dopamine differentially modulates excitatory and inhibitory synaptic inputs to STN neurons expressing different nAChR subtypes. To address this issue, we performed brain slice patch-clamp recordings on STN neurons, while we pharmacologically manipulated dopaminergic inputs. In STN neuron subsets containing either α4β2 or α7 nAChRs, D₁ and D₂ receptors respectively enhanced and inhibited spontaneous inhibitory and excitatory postsynaptic currents (sIPSCs and sEPSCs) and firing rates. The elevation of dopamine levels resulted in diverse regulations of synaptic transmission in these two neuron subsets, and interestingly, the dopamine regulation of sIPSCs significantly correlated with that of sEPSCs. Surprisingly, depletion of dopamine either by reserpine treatment or by unilateral 6-OHDA lesion of nigrostriatal dopaminergic neurons did not alter synaptic inputs to STN neurons, but STN neurons in the 6-OHDA-lesioned side exhibited hyperactivity. In summary, dopamine regulated both GABAergic and glutamatergic synaptic inputs to STN neuron subsets containing either α4β2 or α7 nAChRs, forming a balancing machinery to control neuronal activity. In parkinsonian mice, postsynaptic mechanisms may exist and contribute to the hyperactivity of STN neurons.

D5 dopamine receptors control glutamatergic AMPA transmission between the motor cortex and subthalamic nucleus

Corticofugal fibers target the subthalamic nucleus (STN), a component nucleus of the basal ganglia, in addition to the striatum, their main input. The cortico-subthalamic, or hyperdirect, pathway, is thought to supplement the cortico-striatal pathways in order to interrupt/change planned actions. To explore the previously unknown properties of the neurons that project to the STN, retrograde and anterograde tools were used to specifically identify them in the motor cortex and selectively stimulate their synapses in the STN. The cortico-subthalamic neurons exhibited very little sag and fired an initial doublet followed by non-adapting action potentials. In the STN, AMPA/kainate synaptic currents had a voltage-dependent conductance, indicative of GluA2-lacking receptors and were partly inhibited by Naspm. AMPA transmission displayed short-term depression, with the exception of a limited bandpass in the 5 to 15 Hz range. AMPA synaptic currents were negatively controlled by dopamine D5 receptors. The reduction in synaptic strength was due to postsynaptic D5 receptors, mediated by a PKA-dependent pathway, but did not involve a modified rectification index. Our data indicated that dopamine, through post-synaptic D5 receptors, limited the cortical drive onto STN neurons in the normal brain.

Evidence for a task-dependent switch in subthalamo-nigral basal ganglia signaling

Basal ganglia (BG) can either facilitate or inhibit movement through excitatory and inhibitory pathways; however whether these opposing signals are dynamically regulated during healthy behavior is not known. Here, we present compelling neurophysiological evidence from three complimentary experiments in non-human primates, indicating task-specific changes in tonic BG pathway weightings during saccade behavior with different cognitive demands. First, simultaneous local field potential recording in the subthalamic nucleus (STN; BG input) and substantia nigra pars reticulata (SNr; BG output) reveals task-dependent shifts in subthalamo-nigral signals. Second, unilateral electrical stimulation of the STN, SNr, and caudate nucleus results in strikingly different saccade directionality and latency biases across the BG. Third, a simple artificial neural network representing canonical BG signaling pathways suggests that pathway weightings can be altered by cortico-BG input activation. Overall, inhibitory pathways (striato-pallidal-subthalamo-nigral) dominate during goal-driven behavior with instructed rewards, while facilitatory pathways (striato-nigral and subthalamo-pallidal-nigral) dominate during unconstrained (free reward) conditions.

Basal ganglia can both facilitate or inhibit movement through excitatory and inhibitory pathways; however whether these opposing signals are dynamically regulated during behavior is not known. Here the authors use multinucleus LFP recordings and electrical microstimulation in monkeys performing saccade based tasks to show task specific changes in the tonic weighting of these pathways.

MDMA modulates spontaneous firing of subthalamic nucleus neurons in vitro

3,4-Methylene-dioxy-N-methylamphetamine (MDMA, 'ecstasy') has a broad spectrum of molecular targets in the brain, among them receptors and transporters of the serotonergic (5-hydroxytryptamine, 5-HT) and noradrenergic systems. Its action on the serotonergic system modulates motor systems in rodents and humans. Although parts of the basal ganglia could be identified as mediators of the motor effects of MDMA, very little is known about the role of the subthalamic nucleus (STN). Therefore, this study investigated the modulation of spontaneous action potential activity of the STN by MDMA (2.5-20 µM) in vitro. MDMA had very heterogeneous effects, ranging from a complete but reversible inhibition to a more than twofold increase in firing at 5 µM. On average, MDMA excited STN neurons moderately, but lost its excitatory effect in the presence of the 5-HT(2A) antagonist MDL 11,939. 5-HT(1A) receptors did not appear to play a major role. Effects of MDMA on transporters for serotonin (SERT) and norepinephrine (NET) were investigated by coapplication of the reuptake inhibitors citalopram and desipramine, respectively. Similar to the effects of 5-HT(2A) receptor blockade, antagonism of SERT and NET bestowed an inhibitory effect on MDMA. From these results, we conclude that both the 5-HT and the noradrenergic system mediate MDMA-induced effects on STN neurons.

Mechanism of parkinsonian neuronal oscillations in the primate basal ganglia: some considerations based on our recent work

Accumulating evidence suggests that abnormal neuronal oscillations in the basal ganglia (BG) contribute to the manifestation of parkinsonian symptoms. In this article, we would like to summarize our recent work on the mechanism underlying abnormal oscillations in the parkinsonian state and discuss its significance in pathophysiology of Parkinson’s disease. We recorded neuronal activity in the BG of parkinsonian monkeys treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Systemic administration of L-DOPA alleviated parkinsonian motor signs and decreased abnormal neuronal oscillations (8–15 Hz) in the internal (GPi) and external (GPe) segments of the globus pallidus and the subthalamic nucleus (STN). Inactivation of the STN by muscimol (GABA_A receptor agonist) injection also ameliorated parkinsonian signs and suppressed GPi oscillations. The blockade of glutamatergic inputs to the STN by local microinjection of a mixture of 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (glutamatergic NMDA receptor antagonist) and 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (glutamatergic AMPA/kainate receptor antagonist) suppressed neuronal oscillations in the STN. STN oscillations were also attenuated by the blockade of GABAergic neurotransmission from the GPe to the STN by muscimol inactivation of the GPe. These results suggest that cortical glutamatergic inputs to the STN and reciprocal GPe-STN interconnections are both important for the generation and amplification of the oscillatory activity of GPe and STN neurons in the parkinsonian state. The oscillatory activity in the STN is subsequently transmitted to the GPi and may contribute to manifestation of parkinsonian symptoms.

Involvement of dopamine loss in extrastriatal basal ganglia nuclei in the pathophysiology of Parkinson's disease

Parkinson's disease (PD) is a neurological disorder characterized by the manifestation of motor symptoms, such as akinesia, muscle rigidity and tremor at rest. These symptoms are classically attributed to the degeneration of dopamine neurons in the pars compacta of substantia nigra (SNc), which results in a marked dopamine depletion in the striatum. It is well established that dopamine neurons in the SNc innervate not only the striatum, which is the main target, but also other basal ganglia nuclei including the two segments of globus pallidus and the subthalamic nucleus (STN). The role of dopamine and its depletion in the striatum is well known, however, the role of dopamine depletion in the pallidal complex and the STN in the genesis of their abnormal neuronal activity and in parkinsonian motor deficits is still not clearly determined. Based on recent experimental data from animal models of Parkinson's disease in rodents and non-human primates and also from parkinsonian patients, this review summarizes current knowledge on the role of dopamine in the modulation of basal ganglia neuronal activity and also the role of dopamine depletion in these nuclei in the pathophysiology of Parkinson's disease.

Localization and function of dopamine receptors in the subthalamic nucleus of normal and parkinsonian monkeys

The subthalamic nucleus (STN) receives a dopaminergic innervation from the substantia nigra pars compacta, but the role of this projection remains poorly understood, particularly in primates. To address this issue, we used immuno-electron microscopy to localize D1, D2, and D5 dopamine receptors in the STN of rhesus macaques and studied the electrophysiological effects of activating D1-like or D2-like receptors in normal and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated parkinsonian monkeys. Labeling of D1 and D2 receptors was primarily found presynaptically, on preterminal axons and putative glutamatergic and GABAergic terminals, while D5 receptors were more significantly expressed postsynaptically, on dendritic shafts of STN neurons. The electrical spiking activity of STN neurons, recorded with standard extracellular recording methods, was studied before, during, and after intra-STN administration of the dopamine D1-like receptor agonist SKF82958, the D2-like receptor agonist quinpirole, or artificial cerebrospinal fluid (control injections). In normal animals, administration of SKF82958 significantly reduced the spontaneous firing but increased the rate of intraburst firing and the proportion of pause-burst sequences of firing. Quinpirole only increased the proportion of such pause-burst sequences in STN neurons of normal monkeys. In MPTP-treated monkeys, the D1-like receptor agonist also reduced the firing rate and increased the proportion of pause-burst sequences, while the D2-like receptor agonist did not change any of the chosen descriptors of the firing pattern of STN neurons. Our data suggest that dopamine receptor activation can directly modulate the electrical activity of STN neurons by pre- and postsynaptic mechanisms in both normal and parkinsonian states, predominantly via activation of D1 receptors.

Constitutive activities and inverse agonism in dopamine receptors

The concept of activation in the absence of agonists has been demonstrated for many GPCRs and is now solidified as one of the principal aspects of GPCR signaling. In this chapter, we review how dopamine receptors demonstrate this ability. Although difficult to prove in vivo due to the presence of endogenous dopamine and lack of subtype-selective inverse agonists and "pure" antagonists (neutral ligands), in vitro assays such as measuring intracellular cAMP, [(35)S]GTPγS binding, and [(3)H]thymidine incorporation have uncovered the constitutive activation of D1- and D2-class receptors. Nevertheless, because of limited and inconsistent findings, the existence of constitutive activity for D2-class receptors is currently not well established. Mutagenesis studies have shown that basal signaling, notably by D1-class receptors, is governed by the collective contributions of transmembrane domains and extracellular/intracellular loops, such as the third extracellular loop, the third intracellular loop, and C-terminal tail. Furthermore, constitutive activities of D1-class receptors are subjected to regulation by kinases. Among the dopamine receptor family, the D5 receptor subtype exhibits a higher basal signaling and bears resemblance to constitutively active mutant forms of GPCRs. The presence of its constitutive activity in vivo and its pathophysiological relevance, with a brief mention of other subtypes, are also discussed.

Inhibiting subthalamic D5 receptor constitutive activity alleviates abnormal electrical activity and reverses motor impairment in a rat model of Parkinson's disease

Burst firing has been reported as a pathological activity of subthalamic nucleus (STN) neurons in Parkinson's disease. However, the origin of bursts and their causal link with motor deficits remain unknown. Here we tested the hypothesis that dopamine D5 receptors (D5Rs), characterized by a high constitutive activity, may contribute to the emergence of burst firing in STN. We tested whether inhibiting D5R constitutive activity depresses burst firing and alleviates motor impairments in the 6-OHDA rat model of Parkinson's disease. Intrasubthalamic microinjections of either an inverse agonist of D5Rs, flupenthixol, or a D2R antagonist, raclopride, were applied. Behavioral experiments, in vivo and in vitro electrophysiological recordings, and ex vivo functional neuroanatomy studies were performed. Using [(5)S]GTPγ binding autoradiography, we show that application of flupenthixol inhibits D5R constitutive activity within the STN. Furthermore, flupenthixol reduced evoked burst in brain slices and converted pathological burst firing into physiological tonic, single-spike firing in 6-OHDA rats in vivo. This later action was mimicked by calciseptine, a Cav1 channel blocker. Moreover, the same treatment dramatically attenuated motor impairment in this model and normalized metabolic hyperactivity in both STN and substantia nigra pars reticulata, the main output structure of basal ganglia in rats. In contrast, raclopride as well as saline did not reverse burst firing and motor deficits, confirming the selective action of flupenthixol on D5Rs. These results are the first to demonstrate that subthalamic D5Rs are involved in the pathophysiology of Parkinson's disease and that administering an inverse agonist of these receptors may lessen motor symptoms.

Potential mechanisms for imperfect synchronization in parkinsonian basal ganglia

Neural activity in the brain of parkinsonian patients is characterized by the intermittently synchronized oscillatory dynamics. This imperfect synchronization, observed in the beta frequency band, is believed to be related to the hypokinetic motor symptoms of the disorder. Our study explores potential mechanisms behind this intermittent synchrony. We study the response of a bursting pallidal neuron to different patterns of synaptic input from subthalamic nucleus (STN) neuron. We show how external globus pallidus (GPe) neuron is sensitive to the phase of the input from the STN cell and can exhibit intermittent phase-locking with the input in the beta band. The temporal properties of this intermittent phase-locking show similarities to the intermittent synchronization observed in experiments. We also study the synchronization of GPe cells to synaptic input from the STN cell with dependence on the dopamine-modulated parameters. Earlier studies showed how the strengthening of dopamine-modulated coupling may lead to transitions from non-synchronized to partially synchronized dynamics, typical in Parkinson's disease. However, dopamine also affects the cellular properties of neurons. We show how the changes in firing patterns of STN neuron due to the lack of dopamine may lead to transition from a lower to a higher coherent state, roughly matching the synchrony levels observed in basal ganglia in normal and parkinsonian states. The intermittent nature of the neural beta band synchrony in Parkinson's disease is achieved in the model due to the interplay of the timing of STN input to pallidum and pallidal neuronal dynamics, resulting in sensitivity of pallidal output to the phase of the arriving STN input. Thus the mechanism considered here (the change in firing pattern of subthalamic neurons through the dopamine-induced change of membrane properties) may be one of the potential mechanisms responsible for the generation of the intermittent synchronization observed in Parkinson's disease.

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.

Intermittent neural synchronization in Parkinson's disease

Motor symptoms of Parkinson's disease are related to the excessive synchronized oscillatory activity in the beta frequency band (around 20Hz) in the basal ganglia and other parts of the brain. This review explores the dynamics and potential mechanisms of these oscillations employing ideas and methods from nonlinear dynamics. We present extensive experimental documentation of the relevance of synchronized oscillations to motor behavior in Parkinson's disease, and we discuss the intermittent character of this synchronization. The reader is introduced to novel time-series analysis techniques aimed at the detection of the fine temporal structure of intermittent phase locking observed in the brains of parkinsonian patients. Modeling studies of brain networks are reviewed, which may describe the observed intermittent synchrony, and we discuss what these studies reveal about brain dynamics in Parkinson's disease. The parkinsonian brain appears to exist on the boundary between phase-locked and nonsynchronous dynamics. Such a situation may be beneficial in the healthy state, as it may allow for easy formation and dissociation of transient patterns of synchronous activity which are required for normal motor behavior. Dopaminergic degeneration in Parkinson's disease may shift the brain networks closer to this boundary, which would still permit some motor behavior while accounting for the associated motor deficits. Understanding the mechanisms of the intermittent synchrony in Parkinson's disease is also important for biomedical engineering since efficient control strategies for suppression of pathological synchrony through deep brain stimulation require knowledge of the dynamics of the processes subjected to control.

Regulation of polysynaptic subthalamonigral transmission by D2, D3 and D4 dopamine receptors in rat brain slices

Dopamine depletion in experimental models of Parkinson's disease promotes burst firing of neurons in the subthalamic nucleus (STN) and substantia nigra zona reticulata (SNR). A synaptically generated form of burst firing has been shown to arise from complex excitatory postsynaptic currents (EPSCs) that are evoked in SNR neurons by STN stimulation. The present experiments were designed to characterize actions of dopamine on complex EPSCs in slices of rat brain. Using patch pipettes to record whole-cell currents under voltage clamp, dopamine (30 μm) caused a reversible 64% reduction in complex EPSC charge. This effect was partially mimicked by D(2), D(3) and D(4) receptor agonists, and the action of dopamine could be nearly completely blocked by the combined effects of the D(2/3) antagonist sulpiride and the D(4) antagonist L-745,870. Local application of dopamine to the STN caused a larger inhibition of the complex EPSC (55% reduction) than did dopamine application to the SNR (15% reduction). Simple, monophasic EPSCs, which were evoked in SNR neurons by stimulating the SNR close to the recording pipette, were inhibited to a smaller extent compared to complex EPSCs. Bursts of action potentials evoked in SNR neurons by STN stimulation were inhibited by dopamine to a greater extent than was spontaneous firing. These results show that dopamine D(2)-like receptors inhibit complex EPSCs and burst discharges in the SNR by acting within the STN to suppress transmission in the subthalamonigral pathway. Dopamine receptor-mediated inhibition of polysynaptic connections in the STN might be beneficial in the treatment of Parkinson's disease.

Pathological basal ganglia activity in movement disorders

Our understanding of the pathophysiology of movement disorders and associated changes in basal ganglia activities has significantly changed during the last few decades. This process began with the development of detailed anatomical models of the basal ganglia, followed by studies of basal ganglia activity patterns in animal models of common movement disorders and electrophysiological recordings in movement disorder patients undergoing functional neurosurgical procedures. These investigations first resulted in an appreciation of global activity changes in the basal ganglia in parkinsonism and other disorders, and later in the detailed description of pathological basal ganglia activity patterns, specifically burst patterns and oscillatory synchronous discharge of basal ganglia neurons. In this review, we critically summarize our current knowledge of the pathological discharge patterns of basal ganglia neurons in Parkinson's disease, dystonia, and dyskinesias.

Extrastriatal dopaminergic circuits of the Basal Ganglia

The basal ganglia are comprised of the striatum, the external and internal segment of the globus pallidus (GPe and GPi, respectively), the subthalamic nucleus (STN), and the substantia nigra pars compacta and reticulata (SNc and SNr, respectively). Dopamine has long been identified as an important modulator of basal ganglia function in the striatum, and disturbances of striatal dopaminergic transmission have been implicated in diseases such as Parkinson's disease (PD), addiction and attention deficit hyperactivity disorder. However, recent evidence suggests that dopamine may also modulate basal ganglia function at sites outside of the striatum, and that changes in dopaminergic transmission at these sites may contribute to the symptoms of PD and other neuropsychiatric disorders. This review summarizes the current knowledge of the anatomy, functional effects and behavioral consequences of the dopaminergic innervation to the GPe, GPi, STN, and SNr. Further insights into the dopaminergic modulation of basal ganglia function at extrastriatal sites may provide us with opportunities to develop new and more specific strategies for treating disorders of basal ganglia dysfunction.

A population level computational model of the basal ganglia that generates parkinsonian Local Field Potential activity

Recordings from the basal ganglia's subthalamic nucleus are acquired via microelectrodes immediately prior to the application of Deep Brain Stimulation (DBS) treatment for Parkinson's Disease (PD) to assist in the selection of the final point for the implantation of the DBS electrode. The acquired recordings reveal a persistent characteristic beta band peak in the power spectral density function of the Local Field Potential (LFP) signals. This peak is considered to lie at the core of the causality-effect relationships of the parkinsonian pathophysiology. Based on LFPs acquired from human subjects during DBS for PD, we constructed a computational model of the basal ganglia on the population level that generates LFPs to identify the critical pathophysiological alterations that lead to the expression of the beta band peak. To this end, we used experimental data reporting that the strengths of the synaptic connections are modified under dopamine depletion. The hypothesis that the altered dopaminergic modulation may affect both the amplitude and the time course of the postsynaptic potentials is validated by the model. The results suggest a pivotal role of both of these parameters to the pathophysiology of PD.

Probing the constitutive activity among dopamine D1 and D5 receptors and their mutants

Dopamine D1 and D5 receptors are prototypical cell-surface seven-transmembrane (TM) G protein-coupled receptors (GPCRs) mediating elevation of intracellular cAMP levels. The high level of constitutive activity of D5 receptor mediating intracellular cAMP production is one of the functional hallmarks distinguishing the closely related D1-like dopaminergic subtypes (D1 and D5). D1-like subtypes share over 80% identity within their TM regions. Thus, D1 and D5 receptors can serve as unparalleled and useful molecular tools to gain structural and mechanistic insights into subtype-specific determinants regulating GPCR constitutive activation and inverse agonism. A method has been developed that relies on the use of transfected human embryonic kidney 293 cells with wild-type (WT), epitope-tagged, chimeric, truncated, and mutant forms of mammalian D1 and D5 receptors using a modified DNA and calcium phosphate precipitation procedure. Receptor expression levels are quantified by a radioligand binding using [(3)H]-SCH23390, a D1-like selective drug. Regulation of ligand-independent and dependent activity of WT and mutated D1 and D5 receptors is determined by whole cell cAMP assays using metabolic [(3)H]-adenine labeling and sequential purification radiolabeled nucleotides over Dowex and alumina resin columns. Results on the regulation of D1 and D5 constitutive activity are presented here. Our studies indicate that dopamine-mediated D5 receptor stimulation in a dose-dependent manner is not always detectable, suggesting that D5 receptors can exist in a "locked" constitutively activated state. This "locked" constitutively active state of D5 receptor is not linked to aberrant high receptor expression levels or cell behavior, as D1 receptor function remains essentially unchanged in these cells. In fact, we show that phorbol ester treatment of cells harboring "locked" constitutively active D5 receptors abrogates constitutive activation of D5R to allow its stimulation by dopamine in a dose-dependent manner.

A computational study of the interdependencies between neuronal impulse pattern, noise effects and synchronization

Alterations of individual neurons dynamics and associated changes of the activity pattern, especially the transition from tonic firing (single-spikes) to bursts discharges (impulse groups), play an important role for neuronal information processing and synchronization in many physiological processes (sensory encoding, information binding, hormone release, sleep-wake cycles) as well as in disease (Parkinson, epilepsy). We have used Hodgkin-Huxley-type model neurons with subthreshold oscillations to examine the impact of noise on neuronal encoding and thereby have seen significant differences depending on noise implementation as well as on the neuron's dynamic state. The importance of the individual neurons' dynamics is further elucidated by simulation studies with electrotonically coupled model neurons which revealed mutual interdependencies between the alterations of the network's coupling strength and neurons' activity patterns with regard to synchronization. Remarkably, a pacemaker-like activity pattern which revealed to be much more noise sensitive than the bursting patterns also requires much higher coupling strengths for synchronization. This seemingly simple pattern is obviously governed by more complex dynamics than expected from a conventional pacemaker which may explain why neurons more easily synchronize in the bursting than in the tonic firing mode.

Neuronal networks and synaptic plasticity in Parkinson's disease: beyond motor deficits

The excitatory corticostriatal pathway, which plays a critical role in the building up and storage of adaptive motor behaviours, can undergo long-lasting, activity-dependent changes in the efficacy of synaptic transmission, named long-term potentiation (LTP) and long- term depression (LTD). Both forms of plasticity are thought to underlie motor learning and depend upon the concomitant activation of glutamatergic corticostriatal and dopaminergic nigrostriatal pathways. Accordingly, corticostriatal LTP and LTD are altered in Parkinson's Disease (PD) models. The dopamine (DA)/acetylcholine(Ach) synaptic unbalance could be responsible of some of the cognitive deficits described in PD patients. The impairment of DA/ACh-dependent cellular learning could lead to the storage of unessential memory traces, as it has been postulated for the induction of L-DOPA-induced dyskinesias. Other non-motor symptoms involve not only the central dopaminergic system, but also in the noradrenergic, serotoninergic and cholinergic transmitter systems.

Variety of synchronous regimes in neuronal ensembles

We consider a Hodgkin-Huxley-type model of oscillatory activity in neurons of the snail Helix pomatia. This model has a distinctive feature: It demonstrates multistability in oscillatory and silent modes that is typical for the thalamocortical neurons. A single neuron cell can demonstrate a variety of oscillatory activity: Regular and chaotic spiking and bursting behavior. We study collective phenomena in small and large arrays of nonidentical cells coupled by models of electrical and chemical synapses. Two single elements coupled by electrical coupling show different types of synchronous behavior, in particular in-phase and antiphase synchronous regimes. In an ensemble of three inhibitory synaptically coupled elements, the phenomenon of sequential synchronous dynamics is observed. We study the synchronization phenomena in the chain of nonidentical neurons at different oscillatory behavior coupled with electrical and chemical synapses. Various regimes of phase synchronization are observed: (i) Synchronous regular and chaotic spiking; (ii) synchronous regular and chaotic bursting; and (iii) synchronous regular and chaotic bursting with different numbers of spikes inside the bursts. We detect and study the effect of collective synchronous burst generation due to the cluster formation and the oscillatory death.

D2-like dopamine receptor-mediated modulation of activity-dependent plasticity at GABAergic synapses in the subthalamic nucleus

Reciprocally connected glutamatergic subthalamic nucleus (STN) and GABAergic external globus pallidus (GP) neurons normally exhibit weakly correlated, irregular activity but following the depletion of dopamine in Parkinson's disease they express more highly correlated, rhythmic bursting activity. Patch clamp recording was used to test the hypothesis that dopaminergic modulation reduces the capability of GABAergic inputs to pattern 'pathological' activity in STN neurons. Electrically evoked GABA(A) receptor-mediated IPSCs exhibited activity-dependent plasticity in STN neurons, i.e. IPSCs evoked at frequencies between 1 and 50 Hz exhibited depression that increased with the frequency of activity. Dopamine, the D(2)-like dopamine receptor agonist quinpirole and external media containing a low [Ca(2+)] reduced both the magnitude of IPSCs evoked at 1-50 Hz and synaptic depression at 10-50 Hz. Dopamine/quinpirole also reduced the frequency but not the amplitude of miniature IPSCs recorded in the presence of tetrodotoxin. D(1)-like and D(4) agonists were ineffective and D(2/3) but not D4 receptor antagonists reversed the effects of dopamine or quinpirole. Together these data suggest that presynaptic D(2/3) dopamine receptors modulate the short-term dynamics of GABAergic transmission in the STN by lowering the initial probability of transmitter release. Simulated GABA(A) receptor-mediated synaptic conductances representative of control or modulated transmission were then generated in STN neurons using the dynamic clamp technique. Dopamine-modulated transmission was less effective at resetting autonomous activity or generating rebound burst firing than control transmission. The data therefore support the conclusion that dopamine acting at presynaptic D(2)-like receptors reduces the propensity for GABAergic transmission to generate correlated, bursting activity in STN neurons.

Neural synchronization at tonic-to-bursting transitions

We studied the synchronous behavior of two electrically-coupled model neurons as a function of the coupling strength when the individual neurons are tuned to different activity patterns that ranged from tonic firing via chaotic activity to burst discharges. We observe asynchronous and various synchronous states such as out-of-phase, in-phase and almost in-phase chaotic synchronization. The highest variety of synchronous states occurs at the transition from tonic firing to chaos where the highest coupling strength is also needed for in-phase synchronization which is, essentially, facilitated towards the bursting range. This demonstrates that tuning of the neuron's internal dynamics can have significant impact on the synchronous states especially at the physiologically relevant tonic-to-bursting transitions.

A physiologically plausible model of action selection and oscillatory activity in the basal ganglia

The basal ganglia (BG) have long been implicated in both motor function and dysfunction. It has been proposed that the BG form a centralized action selection circuit, resolving conflict between multiple neural systems competing for access to the final common motor pathway. We present a new spiking neuron model of the BG circuitry to test this proposal, incorporating all major features and many physiologically plausible details. We include the following: effects of dopamine in the subthalamic nucleus (STN) and globus pallidus (GP), transmission delays between neurons, and specific distributions of synaptic inputs over dendrites. All main parameters were derived from experimental studies. We find that the BG circuitry supports motor program selection and switching, which deteriorates under dopamine-depleted and dopamine-excessive conditions in a manner consistent with some pathologies associated with those dopamine states. We also validated the model against data describing oscillatory properties of BG. We find that the same model displayed detailed features of both gamma-band (30-80 Hz) and slow (approximately 1 Hz) oscillatory phenomena reported by Brown et al. (2002) and Magill et al. (2001), respectively. Only the parameters required to mimic experimental conditions (e.g., anesthetic) or manipulations (e.g., lesions) were changed. From the results, we derive the following novel predictions about the STN-GP feedback loop: (1) the loop is functionally decoupled by tonic dopamine under normal conditions and recoupled by dopamine depletion; (2) the loop does not show pacemaking activity under normal conditions in vivo (but does after combined dopamine depletion and cortical lesion); (3) the loop has a resonant frequency in the gamma-band.

GABAergic control of the subthalamic nucleus

The glutamatergic subthalamic nucleus (STN) is a key component of the basal ganglia, a group of subcortical brain nuclei important for voluntary movement and the site of dysfunction in Parkinson's disease. The rate and pattern of STN activity is precisely regulated by the reciprocally connected GABAergic external globus pallidus (GP(e)) and glutamatergic afferents from the cortex. Subthalamic neurons possess intrinsic membrane properties that underlie the autonomous generation of action potentials and complex forms of synaptic integration. Thus, GABA acting at GABA(A) and/or GABA(B) receptors can inhibit/reset autonomous activity by deactivating postsynaptic voltage-dependent Na(+) (Na(v)) channels and generate sufficient hyperpolarization for rebound burst firing, through the de-inactivation of postsynaptic voltage-dependent Ca(2+) (Ca(v)) and Na(v) channels. Feedback inhibition from the GP(e) can therefore paradoxically and transiently increase the efficacy of subsequent excitatory synaptic inputs, and thus enhance the response of the STN to rhythmic input from the cortex. Evidence is also provided that dopamine acting at post- and presynaptic receptors in the STN may, through actions on the integrative properties of STN neurons and activity-dependent synaptic plasticity, be critical for the patterning of STN neuronal activity in vivo. Taken together, these discoveries may be relevant for the emergence of correlated, rhythmic, burst firing in the dopamine-depleted STN of patients with PD.

Cellular principles underlying normal and pathological activity in the subthalamic nucleus

The motor symptoms of Parkinson's disease are associated with abnormal, correlated, low frequency, rhythmic burst activity in the subthalamic nucleus and connected nuclei. Research into the mechanisms controlling the pattern of subthalamic activity has intensified because therapies that manipulate the pattern of subthalamic activity, such as deep brain stimulation and levodopa administration, improve motor function in Parkinson's disease. Recent findings suggest that dopamine denervation of the striatum and extrastriatal basal ganglia profoundly alters the transmission and integration of glutamatergic cortical and GABAergic pallidal inputs to subthalamic neurons, leading to pathological activity that resonates throughout the basal ganglia and wider motor system.

Effect of kinetic redundancy on hand digit control in children with DCD

We investigated finger strength and the ability to control digit force/torque production in children with developmental coordination disorder (DCD) using manipulative tasks with different kinetic redundancies (KNR). Age-related changes in finger strength and finger force/torque control in typically developing (TD) children were also examined to provide a developmental landscape that allows a comparison with children with DCD. Forty-eight TD children (7-, 9-, and 11- year-olds) and sixteen 9-year-old children with DCD participated in the study. Three isometric tasks with different KNR were tested: constant index finger pressing force production (KNR=0), constant thumb-index finger pinching force production (KNR=1), and constant thumb-index finger torque production (KNR=5). Each subject performed two conditions for each isometric task: maximum voluntary force/torque production and constant force/torque control (40% of maximum force/torque). The results showed that the maximum force/torque production increased and the variability of constant force/torque control decreased with age in all tasks in TD children. Children with DCD showed larger variability than TD children in the constant thumb-index finger pinching torque production. These results suggest that children with DCD, as compared to TD children, are capable of producing the same level of maximum finger force, but have poor control in manipulation tasks with a large number of kinetic redundancies.