Anatomy and Physiology of the Subthalamic Nucleus: A Driving Force of the Basal Ganglia

Long-lasting antiseizure effects of chronic intrasubthalamic convection-enhanced delivery of valproate

Intracerebral drug delivery is an experimental approach for the treatment of drug-resistant epilepsies that allows for pharmacological intervention in targeted brain regions. Previous studies have shown that targeted pharmacological inhibition of the subthalamic nucleus (STN) via modulators of the GABAergic system produces antiseizure effects. However, with chronic treatment, antiseizure effects are lost as tolerance develops. Here, we report that chronic intrasubthalamic microinfusion of valproate (VPA), an antiseizure medication known for its wide range of mechanisms of action, can produce long-lasting antiseizure effects over three weeks in rats. In the intravenous pentylenetetrazole seizure-threshold test, seizure thresholds were determined before and during chronic VPA application (480 μg/d, 720 μg/d, 960 μg/d) to the bilateral STN. Results indicate a dose-dependent variation in VPA-induced antiseizure effects with mean increases in seizure threshold of up to 33%, and individual increases of up to 150%. The lowest VPA dose showed a complete lack of tolerance development with long-lasting antiseizure effects. Behavioral testing with all doses revealed few, acceptable adverse effects. VPA concentrations were high in STN and low in plasma and liver. In vitro electrophysiology with bath applied VPA revealed a reduction in spontaneous firing rate, increased background membrane potential, decreased input resistance and a significant reduction in peak NMDA, but not AMPA, receptor currents in STN neurons. Our results suggest an advantage of VPA over purely GABAergic modulators in preventing tolerance development with chronic intrasubthalamic drug delivery and provide first mechanistic insights in intracerebral pharmacotherapy targeting the STN.

Effects of diffusion signal modeling and segmentation approaches on subthalamic nucleus parcellation

The subthalamic nucleus (STN) is commonly used as a surgical target for deep brain stimulation in movement disorders such as Parkinson's Disease. Tractography-derived connectivity-based parcellation (CBP) has been recently proposed as a suitable tool for non-invasive in vivo identification and pre-operative targeting of specific functional territories within the human STN. However, a well-established, accurate and reproducible protocol for STN parcellation is still lacking. The present work aims at testing the effects of different tractography-based approaches for the reconstruction of STN functional territories. We reconstructed functional territories of the STN on the high-quality dataset of 100 unrelated healthy subjects and on the test-retest dataset of the Human Connectome Project (HCP) repository. Connectivity-based parcellation was performed with a hypothesis-driven approach according to cortico-subthalamic connectivity, after dividing cortical areas into three groups: associative, limbic and sensorimotor. Four parcellation pipelines were compared, combining different signal modeling techniques (single-fiber vs multi-fiber) and different parcellation approaches (winner takes all parcellation vs fiber density thresholding). We tested these procedures on STN regions of interest obtained from three different, commonly employed, subcortical atlases. We evaluated the pipelines both in terms of between-subject similarity, assessed on the cohort of 100 unrelated healthy subjects, and of within-subject similarity, using a second cohort of 44 subjects with available test-retest data. We found that each parcellation provides converging results in terms of location of the identified parcels, but with significative variations in size and shape. All pipelines obtained very high within-subject similarity, with tensor-based approaches outperforming multi-fiber pipelines. On the other hand, higher between-subject similarity was found with multi-fiber signal modeling techniques combined with fiber density thresholding. We suggest that a fine-tuning of tractography-based parcellation may lead to higher reproducibility and aid the development of an optimized surgical targeting protocol.

Functional analysis of distinct populations of subthalamic nucleus neurons on Parkinson's disease and OCD-like behaviors in mice

The subthalamic nucleus (STN) is a component of the basal ganglia and plays a key role to control movement and limbic-associative functions. STN modulation with deep brain stimulation (DBS) improves the symptoms of Parkinson's disease (PD) and obsessive-compulsive disorder (OCD) patients. However, DBS does not allow for cell-type-specific modulation of the STN. While extensive work has focused on elucidating STN functionality, the understanding of the role of specific cell types is limited. Here, we first performed an anatomical characterization of molecular markers for specific STN neurons. These studies revealed that most STN neurons express Pitx2, and that different overlapping subsets express Gabrr3, Ndnf, or Nos1. Next, we used optogenetics to define their roles in regulating locomotor and limbic functions in mice. Specifically, we showed that optogenetic photoactivation of STN neurons in Pitx2-Cre mice or of the Gabrr3-expressing subpopulation induces locomotor changes, and improves locomotion in a PD mouse model. In addition, photoactivation of Pitx2 and Gabrr3 cells induced repetitive grooming, a phenotype associated with OCD. Repeated stimulation prompted a persistent increase in grooming that could be reversed by fluoxetine treatment, a first-line drug therapy for OCD. Conversely, repeated inhibition of STN^Gabrr3 neurons suppressed grooming in Sapap3 KO mice, a model for OCD. Finally, circuit and functional mapping of STN^Gabrr3 neurons showed that these effects are mediated via projections to the globus pallidus/entopeduncular nucleus and substantia nigra reticulata. Altogether, these data identify Gabrr3 neurons as a key population in mediating the beneficial effects of STN modulation thus providing potential cellular targets for PD and OCD drug discovery.

The basal ganglia and the cerebellum: nodes in an integrated network

The basal ganglia and the cerebellum are considered to be distinct subcortical systems that perform unique functional operations. The outputs of the basal ganglia and the cerebellum influence many of the same cortical areas but do so by projecting to distinct thalamic nuclei. As a consequence, the two subcortical systems were thought to be independent and to communicate only at the level of the cerebral cortex. Here, we review recent data showing that the basal ganglia and the cerebellum are interconnected at the subcortical level. The subthalamic nucleus in the basal ganglia is the source of a dense disynaptic projection to the cerebellar cortex. Similarly, the dentate nucleus in the cerebellum is the source of a dense disynaptic projection to the striatum. These observations lead to a new functional perspective that the basal ganglia, the cerebellum and the cerebral cortex form an integrated network. This network is topographically organized so that the motor, cognitive and affective territories of each node in the network are interconnected. This perspective explains how synaptic modifications or abnormal activity at one node can have network-wide effects. A future challenge is to define how the unique learning mechanisms at each network node interact to improve performance.

Basal ganglia, movement disorders and deep brain stimulation: advances made through non-human primate research

Studies in non-human primates (NHPs) have led to major advances in our understanding of the function of the basal ganglia and of the pathophysiologic mechanisms of hypokinetic movement disorders such as Parkinson's disease and hyperkinetic disorders such as chorea and dystonia. Since the brains of NHPs are anatomically very close to those of humans, disease states and the effects of medical and surgical approaches, such as deep brain stimulation (DBS), can be more faithfully modeled in NHPs than in other species. According to the current model of the basal ganglia circuitry, which was strongly influenced by studies in NHPs, the basal ganglia are viewed as components of segregated networks that emanate from specific cortical areas, traverse the basal ganglia, and ventral thalamus, and return to the frontal cortex. Based on the presumed functional domains of the different cortical areas involved, these networks are designated as 'motor', 'oculomotor', 'associative' and 'limbic' circuits. The functions of these networks are strongly modulated by the release of dopamine in the striatum. Striatal dopamine release alters the activity of striatal projection neurons which, in turn, influences the (inhibitory) basal ganglia output. In parkinsonism, the loss of striatal dopamine results in the emergence of oscillatory burst patterns of firing of basal ganglia output neurons, increased synchrony of the discharge of neighboring basal ganglia neurons, and an overall increase in basal ganglia output. The relevance of these findings is supported by the demonstration, in NHP models of parkinsonism, of the antiparkinsonian effects of inactivation of the motor circuit at the level of the subthalamic nucleus, one of the major components of the basal ganglia. This finding also contributed strongly to the revival of the use of surgical interventions to treat patients with Parkinson's disease. While ablative procedures were first used for this purpose, they have now been largely replaced by DBS of the subthalamic nucleus or internal pallidal segment. These procedures are not only effective in the treatment of parkinsonism, but also in the treatment of hyperkinetic conditions (such as chorea or dystonia) which result from pathophysiologic changes different from those underlying Parkinson's disease. Thus, these interventions probably do not counteract specific aspects of the pathophysiology of movement disorders, but non-specifically remove the influence of the different types of disruptive basal ganglia output from the relatively intact portions of the motor circuitry downstream from the basal ganglia. Knowledge gained from studies in NHPs remains critical for our understanding of the pathophysiology of movement disorders, of the effects of DBS on brain network activity, and the development of better treatments for patients with movement disorders and other neurologic or psychiatric conditions.

Physiological Markers of Motor Inhibition during Human Behavior

Transcranial magnetic stimulation (TMS) studies in humans have shown that many behaviors engage processes that suppress excitability within the corticospinal tract. Inhibition of the motor output pathway has been extensively studied in the context of action stopping, where a planned movement needs to be abruptly aborted. Recent TMS work has also revealed markers of motor inhibition during the preparation of movement. Here, we review the evidence for motor inhibition during action stopping and action preparation, focusing on studies that have used TMS to monitor changes in the excitability of the corticospinal pathway. We discuss how these physiological results have motivated theoretical models of how the brain selects actions, regulates movement initiation and execution, and switches from one state to another.

Subthalamic, not striatal, activity correlates with basal ganglia downstream activity in normal and parkinsonian monkeys

The striatum and the subthalamic nucleus (STN) constitute the input stage of the basal ganglia (BG) network and together innervate BG downstream structures using GABA and glutamate, respectively. Comparison of the neuronal activity in BG input and downstream structures reveals that subthalamic, not striatal, activity fluctuations correlate with modulations in the increase/decrease discharge balance of BG downstream neurons during temporal discounting classical condition task. After induction of parkinsonism with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), abnormal low beta (8-15 Hz) spiking and local field potential (LFP) oscillations resonate across the BG network. Nevertheless, LFP beta oscillations entrain spiking activity of STN, striatal cholinergic interneurons and BG downstream structures, but do not entrain spiking activity of striatal projection neurons. Our results highlight the pivotal role of STN divergent projections in BG physiology and pathophysiology and may explain why STN is such an effective site for invasive treatment of advanced Parkinson's disease and other BG-related disorders.

DOI:http://dx.doi.org/10.7554/eLife.16443.001

The symptoms of Parkinson’s disease include tremor and slow movement, as well as loss of balance, depression and problems with sleep and memory. The death of neurons in a region of the brain called the substantia nigra pars compacta is one of the major hallmarks of Parkinson’s disease. These neurons produce a chemical called dopamine, and their death reduces dopamine levels in another area of the brain called the striatum. This structure is one of five brain regions known collectively as the basal ganglia, which form a circuit that helps to control movement.

The most effective treatment currently available for advanced Parkinson’s disease entails lowering electrodes deep into the brain in order to shut down the activity of part of the basal ganglia. However, the target is not the striatum; instead it is a structure called the subthalamic nucleus. The striatum and the subthalamic nucleus are the two input regions of the basal ganglia: each sends signals to the other three structures downstream. So why does targeting the subthalamic nucleus, but not the striatum, reduce the symptoms of Parkinson’s disease?

To shed some light on this issue, Deffains et al. recorded the activity of neurons in the basal ganglia before and after injecting two monkeys with a drug called MPTP. Related to heroin, MPTP produces symptoms in animals that resemble those of Parkinson’s disease. Before the injections, spontaneous fluctuations in the activity of the subthalamic nucleus produced matching changes in the activity of the three downstream basal ganglia structures. Fluctuations in the activity of the striatum, by contrast, had no such effect. Moreover, injecting the monkeys with MPTP caused the basal ganglia to fire in an abnormal highly synchronized rhythm, similar to that seen in Parkinson’s disease. Crucially, the subthalamic nucleus contributed to this abnormal rhythm, whereas the striatum did not.

The results presented by Deffains et al. provide a concrete explanation for why inactivating the subthalamic nucleus, but not the striatum, reduces the symptoms of Parkinson’s disease. Further research is now needed to explore how the striatum controls the activity of downstream regions of the basal ganglia, both in healthy people and in those with Parkinson's disease.

DOI:http://dx.doi.org/10.7554/eLife.16443.002

Chemical anatomy of pallidal afferents in primates

Neurons of the globus pallidus receive massive inputs from the striatum and the subthalamic nucleus, but their activity, as well as those of their striatal and subthalamic inputs, are modulated by brainstem afferents. These include serotonin (5-HT) projections from the dorsal raphe nucleus, cholinergic (ACh) inputs from the pedunculopontine tegmental nucleus, and dopamine (DA) afferents from the substantia nigra pars compacta. This review summarizes our recent findings on the distribution, quantitative and ultrastructural aspects of pallidal 5-HT, ACh and DA innervations. These results have led to the elaboration of a new model of the pallidal neuron based on a precise knowledge of the hierarchy and chemical features of the various synaptic inputs. The dense 5-HT, ACh and DA innervations disclosed in the associative and limbic pallidal territories suggest that these brainstem inputs contribute principally to the planification of motor behaviors and the regulation of attention and mood. Although 5-HT, ACh and DA inputs were found to modulate pallidal neurons and their afferents mainly through asynaptic (volume) transmission, genuine synaptic contacts occur between these chemospecific axon varicosities and pallidal dendrites, revealing that these brainstem projections have a direct access to pallidal neurons, in addition to their indirect input through the striatum and subthalamic nucleus. Altogether, these findings reveal that the brainstem 5-HT, ACh and DA pallidal afferents act in concert with the more robust GABAergic inhibitory striatopallidal and glutamatergic excitatory subthalamopallidal inputs. We hypothesize that a fragile equilibrium between forebrain and brainstem pallidal afferents plays a key role in the functional organization of the primate basal ganglia, in both health and disease.

Oscillators and Oscillations in the Basal Ganglia

What is the meaning of an action potential? There must be different answers for neurons that fire spontaneously, even in the absence of synaptic input, and those driven to fire from a resting membrane potential. In spontaneously firing neurons, the occurrence of the next action potential is guaranteed; only variations in its timing can carry the message. In the basal ganglia, the globus pallidus, the substantia nigra, and the subthalamic nucleus consist of neurons firing spontaneously. They each receive thousands of synaptic inputs, but these are not required to maintain their background firing. Instead, synaptic interactions among basal ganglia nuclei comprise a system of coupled oscillators that produces a complex resting pattern of activity. Normally, this pattern is highly irregular and uncorrelated, so that the firing of each cell is statistically independent of the others. This maximizes the potential information that may be transmitted by the basal ganglia to its target structures. In Parkinson's disease, the resting pattern of activity is dominated by a slow oscillation shared by nearly all of the neurons. Treatment with deep brain stimulation may gain its therapeutic value by disrupting this shared pathological oscillation, and restoring independent action by each neuron in the network.

Mood and behavioural effects of subthalamic stimulation in Parkinson's disease

Deep-brain stimulation (DBS) of the subthalamic nucleus (STN) is an established treatment for motor complications in Parkinson's disease. 20 years of experience with this procedure have contributed to improved understanding of the role of the STN in motor, cognitive, and emotional control. In Parkinson's disease, the pathological STN neuronal activity leads to motor, cognitive, and emotional inhibition. Deafferentation of the STN by DBS can reverse such behavioural inhibition. The release of this brake allows both motor and non-motor improvement, but can also be associated with excessive motor, cognitive, and emotional behavioural disinhibition. Conversely, the notable reduction in anti-parkinsonian drug dose allowed by motor improvement can unveil mesolimbic hypodopaminergic behaviours such as apathy, anxiety, or depression. Fine-tuning of stimulation parameters with dopaminergic drugs is necessary to prevent or improve pathological behaviours.

The basal ganglia communicate with the cerebellum

The basal ganglia and cerebellum are major subcortical structures that influence not only movement, but putatively also cognition and affect. Both structures receive input from and send output to the cerebral cortex. Thus, the basal ganglia and cerebellum form multisynaptic loops with the cerebral cortex. Basal ganglia and cerebellar loops have been assumed to be anatomically separate and to perform distinct functional operations. We investigated whether there is any direct route for basal ganglia output to influence cerebellar function that is independent of the cerebral cortex. We injected rabies virus (RV) into selected regions of the cerebellar cortex in cebus monkeys and used retrograde transneuronal transport of the virus to determine the origin of multisynaptic inputs to the injection sites. We found that the subthalamic nucleus of the basal ganglia has a substantial disynaptic projection to the cerebellar cortex. This pathway provides a means for both normal and abnormal signals from the basal ganglia to influence cerebellar function. We previously showed that the dentate nucleus of the cerebellum has a disynaptic projection to an input stage of basal ganglia processing, the striatum. Taken together these results provide the anatomical substrate for substantial two-way communication between the basal ganglia and cerebellum. Thus, the two subcortical structures may be linked together to form an integrated functional network.

Anatomic localization of dyskinesia in children with "profound" perinatal hypoxic-ischemic injury

BACKGROUND AND PURPOSE

CP is a common feature of perinatal HIBD in the context of "acute profound" injury, and in this article, we have studied the possible anatomic substrates of dyskinesia. We have reviewed the extent of brain injury in children with dyskinetic and spastic CP due to acute profound hypoxia to identify sites of injury that explain why only some children develop movement disorders. It is known that the STN has a role in the development of movement disorders; therefore, we have specifically studied it.

MATERIALS AND METHODS

We retrospectively reviewed MR imaging of 40 consecutive children referred to our center with CP confirmed to be due to acute profound hypoxic-ischemic injury. All children received the same high-resolution MR imaging protocol with the same 1.5T scanner. Two pediatric neuroradiologists reviewed the imaging. Logistic regression was applied to identify multivariable predictors that differentiate dyskinetic and spastic CP.

RESULTS

Twenty children had dyskinetic CP and 20 had spastic CP. Children with dyskinetic CP had more frequent injury to the STN, as manifest by increased T2 signal intensity. Children with spastic CP had more severe damage to white matter in the vicinity of the paracentral lobule. Injuries to the putamen, caudate, and globus pallidus were not significant predictors of dyskinesia.

CONCLUSIONS

We have shown an association between hypoxic-ischemic injury to the STN at birth and the emergence of dyskinesia later in life.

Striatal information signaling and integration in globus pallidus: timing matters

Advances in research on globus pallidus (GP) suggest that this 'long thought to be' relay in the 'indirect pathway' plays a unique and critical role in basal ganglia function. The traditional idea of parallel processing within the basal ganglia is also challenged by recent findings. It is now clear that axons of GP neurons form large, perisomatic baskets around target neurons in all major basal ganglia nuclei, thereby exerting a profound influence on the output of the entire basal ganglia. GP neurons are autonomously active both in vivo and in vitro. It is believed that temporal information carried along the corticostriatopallidal pathway is critical for proper motor execution. The importance of appropriately controlled discharge of GP neurons is highlighted by psychomotor disorders such as Parkinson's disease, in which alterations in the pattern and synchrony of discharge in GP neurons are thought to contribute to motor symptoms. Several lines of evidence suggest that the aberrant activity of GP neurons following dopamine depletion is caused by alteration in the synaptic input from both striatum and subthalamic nucleus. In normal subjects, the capability of striatal input in translating cortical input into precisely timed responses in GP neurons is mediated by (1) the expression of postsynaptic GABA(A) receptor composed of subunits with fast kinetic properties; (2) an effective GABA reuptake system in terminating the action of synaptically released GABA, and (3) the existence of dendritic HCN channels that actively abbreviate the time course of the inhibitory postsynaptic potentials and reset rhythmic discharge. Despite the rapid pace in uncovering the elements that shape the activity along the striatopallidosubthalamic pathway, the origin of rhythmic, synchronized bursting of GP neurons seen in parkinsonism has not been fully established experimentally. Further elucidation of the factors that control the information transfer in the striatopallidal synapses is thus critical to our understanding of basal ganglia function and establishing treatment for Parkinson's disease and other basal ganglia disorders.

Cellular Localization of AMPA Type Glutamate Receptor Subunits in the Basal Ganglia of Pigeons (Columba livia)

Corticostriatal and thalamostriatal projections utilize glutamate as a neurotransmitter in mammals and birds. The influence on striatum is mediated, in part, by ionotropic AMPA-type glutamate receptors, which are heteromers composed of GluR1-4 subunits. Although the cellular localization of AMPA-type subunits has been well characterized in mammalian basal ganglia, their localization in avian basal ganglia has not. We thus carried out light microscopic single- and double-label and electron microscopic single-label immunohistochemical studies of GluR1-4 distribution and cellular localization in pigeon basal ganglia. Single-label studies showed that the striatal neuropil is rich in GluR1, GluR2, and GluR2/3 immunolabeling, suggesting the localization of GluR1, GluR2 and/or GluR3 to the dendrites and spines of striatal projection neurons. Double-label studies and perikaryal size distribution determined from single-label material indicated that about 25% of enkephalinergic and 25% of substance P-containing striatal projection neuron perikarya contained GluR1, whereas GluR2 was present in about 75% of enkephalinergic neurons and all substance-P -containing neurons. The perikaryal size distribution for GluR2 compared to GluR2/3 suggested that enkephalinergic neurons might more commonly contain GluR3 than do substance P neurons. Parvalbuminergic and calretininergic striatal interneurons were rich in GluR1 and GluR4, a few cholinergic striatal interneurons possessed GluR2, but somatostatinergic striatal interneurons were devoid of all subunits. The projection neurons of globus pallidus all possessed GluR1, GluR2, GluR2/3 and GluR4 immunolabeling. Ultrastructural analysis of striatum revealed that GluR1 was preferentially localized to dendritic spines, whereas GluR2/3 was found in spines, dendrites, and perikarya. GluR2/3-rich spines were generally larger than GluR1 spines and more frequently possessed perforated post-synaptic densities. These results show that the diverse basal ganglia neuron types each display different combinations of AMPA subunit localization that shape their responses to excitatory input. For striatal projection neurons and parvalbuminergic interneurons, the combinations resemble those for the corresponding cell types in mammals, and thus their AMPA responses to glutamate are likely to be similar.

GABAergic interneurons in human subthalamic nucleus

The subthalamic nucleus (STN) is considered a homogeneous structure composed essentially of projection neurons that exert a profound glutamate-mediated excitatory influence upon the main output structures of the basal ganglia. It is currently the most efficient target for deep brain stimulations designed to alleviate symptoms of Parkinson's disease. STN neurons were analyzed by applying stereological methods and single/double-immunostaining procedures to postmortem material obtained from normal individuals. Besides a multitude of closely packed projection neurons ( approximately 24.7 mum in diameter), the human STN (mean volume, 174.5 +/- 20.4 mm3; total neuronal density, 239.5 +/- 31.9 x 10(3)) contained smaller neurons (approximately 12.2 microm), which displayed glutamic acid decarboxylase (GAD)(65/67) immunoreactivity and shared the morphological features of interneurons described in Golgi studies of primate STN. These putative gamma-aminobutyric acid (GABA)ergic interneurons accounted for 7.5% of the total neuronal population of the STN. Although present throughout the nucleus, they were significantly more numerous in its posterior-ventral-medial sector, which belongs to the limbic/associative functional territory. Many projection neurons located dorsolaterally in the STN showed parvalbumin immunoreactivity and others lying ventromedially displayed calretinin immunostaining, but none of the GAD-positive interneurons expressed these calcium-binding proteins. Although less abundant than projection neurons, GABAergic interneurons might play a important role in the intrinsic organization of the STN. The morphological and chemical heterogeneity of the human STN reported here might have important implications in the functional organization of the basal ganglia.

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.

Information processing, dimensionality reduction and reinforcement learning in the basal ganglia

Modeling of the basal ganglia has played a major role in our understanding of this elusive group of nuclei. Models of the basal ganglia have undergone evolutionary and revolutionary changes over the last 20 years, as new research in the fields of anatomy, physiology and biochemistry of these nuclei has yielded new information. Early models dealt with a single pathway through the nuclei and focused on the nature of the processing performed within it, convergence of information versus parallel processing of information. Later, the Albin-DeLong "box-and-arrow" model characterized the inter-nuclei interaction as multiple pathways while maintaining a simplistic scalar representation of the nuclei themselves. This model made a breakthrough by providing key insights into the behavior of these nuclei in hypo- and hyper-kinetic movement disorders. The next generation of models elaborated the intra-nuclei interactions and focused on the role of the basal ganglia in action selection and sequence generation which form the most current consensus regarding basal ganglia function in both normal and pathological conditions. However, new findings challenge these models and point to a different neural network approach to information processing in the basal ganglia. Here, we take an in-depth look at the reinforcement driven dimensionality reduction (RDDR) model which postulates that the basal ganglia compress cortical information according to a reinforcement signal using optimal extraction methods. The model provides new insights and experimental predictions on the computational capacity of the basal ganglia and their role in health and disease.

Nigral GABAergic inhibition upon cholinergic neurons in the rat pedunculopontine tegmental nucleus

We investigated, in a midbrain parasagittal slice preparation of Wistar rats (postnatal day 9-17), the synaptic inhibition of neurons in the pedunculopontine tegmental nucleus (PPN), which was mediated by gamma (gamma)-amino-butyric acid (GABA). Whole-cell patch-clamp recording was used, in combination with a single-cell reverse transcription-polymerase chain reaction amplification technique, to record synaptic potentials and to identify the phenotype of the recorded PPN neuron. In the presence of the ionotropic glutamate receptor antagonists, 6-cyano-2, 3-dihydroxy-7-nitro-quinoxaline-2, 3, dione, and dl-2-amino-5-phosphonovaleric acid, single electrical stimuli were applied to the substantia nigra pars reticulata (SNr), one of the basal ganglia output nuclei. Stimulation of the SNr evoked inhibitory postsynaptic potentials (IPSPs) in 73 of the 104 neurons in the PPN. The IPSPs were abolished with a GABAA receptor antagonist, bicuculline. Inhibitory postsynaptic currents of the neurons were reversed in polarity at approximately -93.5 mV, which was close to the value of the equilibrium potential for chloride ions of -88.4 mV. Single-cell reverse transcription-polymerase chain reactions revealed that approximately 30% (9/32) of the PPN neurons that received inhibition from the SNr expressed detectable levels of choline acetyltransferase mRNA. These findings show that output from the SNr regulates the activity of cholinergic PPN neurons through GABAA receptors.

"The little engine that could": voltage-dependent Na(+) channels and the subthalamic nucleus

The most effective treatment for late-stage Parkinson's disease is to electrically stimulate the subthalamic nucleus (STN) at high frequencies. Why this strategy works is unclear. The work by Do and Bean shows that the Na channels in STN neurons have distinctive features--like resurgence--that regulate their spiking behavior, providing new insights into the mechanism of DBS.

The Effects of Activation or Inhibition of the Subthalamic Nucleus on the Metabolic and Electrophysiological Activities Within the Pallidal Complex and Substantia Nigra in the Rat

The changes in local cerebral glucose utilization (LCGU) and neuronal unit activities in the subthalamic nucleus and its major target structures (the pars reticulata of the substantia nigra, the globus pallidus and the entopeduncular nucleus) following microinjection of a GABAergic antagonist (bicuculline methiodide, 0.08 nmol) or agonist (muscimol, 0.2 nmol) into the subthalamic nucleus were determined. The metabolic effect was assessed by measuring LCGU by quantitative [14C]-2-deoxyglucose autoradiography in ketamine-anaesthetized rats. Bicuculline methiodide induced increased LCGU in the ipsilateral globus pallidus, the entopeduncular nucleus and the substantia nigra pars reticulata. In contrast, muscimol decreased LCGU in these structures. The neuronal activities in the subthalamic nucleus and related structures increased following injection of bicuculline and decreased after injection of muscimol. The changes in LCGU within the structures directly related to the subthalamic nucleus were correlated with the changes in the unit activity either in the subthalamic nucleus and/or its projection structures. However, the amplitude of the relative changes in neuronal unit activity were greater than the changes in LCGU. Nevertheless, the results emphasize the functional role of the subthalamic nucleus as an activating structure within the basal ganglia.

Complementary Distribution of Calbindin D-28k and Parvalbumin in the Basal Forebrain and Midbrain of the Squirrel Monkey

The distribution of cell bodies expressing either calbindin D-28k or parvalbumin immunoreactivity in the basal forebrain and midbrain of squirrel monkeys (Saimiri sciureus) was studied on contiguous sections incubated with monoclonal antibodies raised against calbindin or parvalbumin. In the nucleus accumbens, medium-sized calbindin-positive neurons formed two cell bridges joining the ventral part of the striatum to the olfactory tubercle, whereas medium-sized parvalbumin-positive cells in the same area were much less numerous and more uniformly distributed. The medial and dorsal septal nuclei contained a small number of elongated calbindin-positive neurons and only a few parvalbumin-immunoreactive cells. In the nucleus of the diagonal band of Broca, calbindin and parvalbumin were found to label two distinct but closely intermingled neuronal populations. In the striatum, medium-sized calbindin-immunoreactive cells occurred in very large numbers and appeared to be confined to the extrastriosomal matrix. Medium-sized, parvalbumin-immunoreactive neurons were also present in the striatum but they were less numerous than the calbindin-positive cells. The calbindin-positive neurons in the dorsal portion of the striatum were less intensely stained than those in the ventral portion, whereas this pattern did not occur for neurons expressing parvalbumin immunoreactivity. At the pallidal level, neurons in both segments were devoid of calbindin but displayed a very strong parvalbumin immunoreactivity. Most of the large neurons of the nucleus basalis of Meynert were strongly calbindin-immunoreactive and many of them invaded dorsally the medullary laminae of the pallidal complex. The neurons of the subthalamic nucleus were markedly enriched with parvalbumin but displayed only light calbindin staining. In the substantia nigra/ventral tegmental area complex, calbindin-immunoreactive cells abounded in the ventral tegmental area and in the dorsal tier of the pars compacta of the substantia nigra, but were absent in the ventral tier of the pars compacta and in the entire pars reticulata of the substantia nigra. In contrast, numerous parvalbumin-immunoreactive neurons occurred in the pars reticulata and pars lateralis, but none were found in the pars compacta and ventral tegmental area. These findings reveal that the patterns of calbindin and parvalbumin distribution in primate basal forebrain and midbrain are strikingly complementary, suggesting a synergistic role for these calcium-binding proteins in basal forebrain and midbrain function.

Neostriatal and globus pallidus stimulation induced inhibitory postsynaptic potentials in entopeduncular neurons in rat brain slice preparations

Recent anatomical studies revealed that the entopeduncular nucleus of the rat receives GABAergic inputs from both the neostriatum and the globus pallidus. The present study was undertaken to reveal the physiological features of these inputs using the intracellular recording method in rat brain slice preparations. Most of the entopeduncular nucleus neurons generated repetitive firing without spike accommodation with intracellular current stimulation and thus were classified as Type-I. A small number of neurons were classified as Type-II since they generated spikes with pronounced accommodation. Most of the Type-I, but none of Type-II, entopeduncular nucleus neurons exhibited monosynaptic GABAergic inhibitory postsynaptic potentials (IPSPs) after stimulation of the neostriatum and the globus pallidus. Neostriatal stimulation induced long latency IPSPs while pallidal stimulation induced long latency IPSPs compounded with short latency IPSPs. The IPSPs were mediated by GABA(A) receptors. The unitary IPSPs to striatal stimulation were small while those to pallidal stimulation were large in amplitude and able to reset ongoing rhythmic firing. The short latency IPSPs induced by pallidal stimulation reversed at a somatic membrane potential that was a few millivolts more depolarized than the long latency IPSPs, suggesting that the striatal inputs were evoked in more distal portions of the neurons than the pallidal inputs. Repetitive activation of these inputs resulted in a poor amplitude summation but a prolongation of the duration of the IPSPs. The results of the present study indicate that the pallidal projection to the entopeduncular nucleus is physiologically significant and that the neostriatum and the globus pallidus play important roles in controlling the activity of the entopeduncular nucleus, although in different ways.

Jules Bernard Luys and the subthalamic nucleus

Jules Bernard Luys, a highly energetic French investigator who made important contributions to anatomopathology, clinical neurology, and psychiatry, discovered the subthalamic nucleus in 1865. Unfortunately, Luys' description of the nucleus that still bears his name, as well as his highly original insights on the role of this important motor structure, are rarely taken into account in the current thinking about the basal ganglia organization. Thus, we thought it of interest to review Luys' contribution to our knowledge of the subthalamic nucleus and also briefly examine the other remarkable achievements of this singular figure of 19th century neurology.

Effect of microiontophoretic application of dopamine on subthalamic nucleus neuronal activity in normal rats and in rats with unilateral lesion of the nigrostriatal pathway

The subthalamic nucleus (STN) receives dopamine inputs from the substantia nigra but their implication in the pathophysiology of parkinsonism is still debated. Extracellular microrecordings were used to study the effect of microiontophoretic injection of dopamine and the D1 receptor agonist SKF 38393 on the activity of STN neurons in normal and 6-hydroxydopamine-lesioned rats under urethane anaesthesia. Dopamine and SKF induced an increase in the firing rate of the majority of STN neurons in both normal and 6-OHDA rats. In rats with 6-OHDA lesions, the percentage of firing rate increase did not differ from that of controls. When GABA, glutamate and dopamine were all applied to the same individual STN neurons, GABA induced an inhibitory effect and glutamate and dopamine caused an excitatory effect in both groups. This excitatory response was suppressed by the application of GABA. Systemic administration of apomorphine provoked a decrease in the firing rate of STN neurons in rats with 6-OHDA lesions. These results show that dopamine exerts an excitatory influence on STN neurons, suggesting that the inhibitory effect induced by the systemic injection of apomorphine is due to the GABAergic inputs from the globus pallidus as predicted by the current model of basal ganglia organization. In addition, we show that dopamine, GABA and glutamate can act on the same STN neuron and that GABA can reverse the excitatory effect of dopamine and glutamate, suggesting the predominant influence of GABAergic inputs to the subthalamic nucleus.

Entopeduncular lesions facilitate and thalamic lesions depress spontaneous and drug-evoked motor behavior in the hemiparkinsonian rat

Pallidotomy is a neurosurgical procedure designed to ameliorate the akinesia and bradykinesia associated with Parkinson's disease. In the present study, the effects of pallidal-like lesions on motor behavior in the hemiparkinsonian rat were compared to the effects of lesions in the ventrolateral thalamus, a target of entopeduncular projections feeding motor-related information to motor cortex. Six aspects of spontaneous and evoked behavior induced by amphetamine and apomorphine in the hemiparkinsonian rat with either bilateral electrolytic entopeduncular lesions or bilateral electrolytic ventrolateral thalamic lesions were measured for 60 min. Saline or amphetamine, 5 mg/kg, or apomorphine, 0.3 mg/kg, were administered IP 5 min before the tests. The results show that on all measures except time spent resting the hemiparkinsonian rats with the entopeduncular lesions were more active than the hemiparkinsonian rats with the thalamic lesions. The asymmetrical rotation responses to dopamine receptor stimulation evoked by amphetamine and apomorphine were influenced by the general effect on gross motor behavior, as shown by the response being very large in the entopeduncular group and very small in the thalamic group. These results are consistent with current thinking about the functional organization of the basal ganglia according to which damage of the entopeduncular nucleus reduces its inhibitory control of the thalamic motor regions, thereby promoting thalamic facilitation of motor cortex, and damage to the thalamic motor regions has the opposite effect. These effects of the lesions translate, respectively, into hyperactivity and hypoactivity without blocking the asymmetrical rotation response of the hemiparkinsonian rat.

The subthalamic nucleus, hemiballismus and Parkinson's disease: reappraisal of a neurosurgical dogma

The subthalamic nucleus (STN) currently is considered to play a key role in the pathophysiological origin of the parkinsonian state and is therefore the main target for surgical treatment of Parkinson's disease. The authors review the incidence of hemichorea/ballism (HCB) as a complication of thalamotomy, pallidotomy or campotomy procedures before the introduction of levodopa therapy, including the few reported cases accompanied by a neuropathological study. The literature shows that only a small number of parkinsonian patients with HCB had a lesion of the STN. Preliminary data in Parkinson's disease patients submitted to a subthalamotomy with current functional stereotaxy also indicate that HCB is a very rare complication. To explain this observation, we suggest that the parkinsonian state is characterized by an increased threshold for the induction of dyskinesia following STN lesioning. This arises as a consequence of reduced activity in the 'direct' GABA projection to the globus pallidus medialis (GPm) which accompanies dopamine depletion. Lesioning of the STN reduces excitation of the GPm, and theoretically this should induce dyskinesias. However, an STN lesion also, simultaneously, further reduces the hypoactivity in the globus pallidus lateralis (GPl) that is a feature of Parkinson's disease, and hence may compensate for GPm hypoactivity, thus self-stabilizing basal ganglia output activity and reducing the risk of HCB. We conclude that lesioning of the STN in Parkinson's disease is a feasible approach in some circumstances.

Dopaminergic innervation of human basal ganglia

This paper summarises the results of some of our recent tyrosine hydroxylase (TH) immunohistochemical studies of the dopaminergic innervation of the human basal ganglia. It also reports new findings on the presence of TH-immunoreactive (ir) neurons in the striatum. Our data show the existence of nigrostriatal TH-ir axons that provide collaterals arborizing in the globus pallidus and subthalamic nucleus. These thin and varicose collaterals emerge from thick and smooth axons that course along the main output pathways of the basal ganglia, including the ansa lenticularis, the lenticular fasciculus and Wilson's pencils. We postulate that this extrastriatal innervation, which allows nigral dopaminergic neurons to directly affect the pallidum and subthalamic nucleus, plays a critical role in the functional organisation of human basal ganglia. The TH-ir fibres that reach the striatum arborize according to a highly heterogeneous pattern. At rostral striatal levels, numerous small TH-poor zones embedded in a TH-rich matrix correspond to calbindin-poor striosomes and calbindin-rich extrastriosomal matrix, respectively. At caudal striatal levels, in contrast, striosomes display a TH immunostaining that is more intense than that of the matrix. A significant number of small, oval, aspiny TH-ir neurons scattered throughout the rostrocaudal extent of the caudate nucleus and putamen, together with a few larger, multipolar, spiny TH-ir neurons lying principally within the ventral portion of the putamen, were disclosed in human. This potential source of intrinsic striatal dopamine might play an important role in the functional organisation of the human striatum, particularly in case of Parkinson's disease.

Unrelated course of subthalamic nucleus and globus pallidus neuronal activities across vigilance states in the rat

The pallido-subthalamic pathway powerfully controls the output of the basal ganglia circuitry and has been implicated in movement disorders observed in Parkinson's disease (PD). To investigate the normal functioning of this pathway across the sleep-wake cycle, single-unit activities of subthalamic nucleus (STN) and globus pallidus (GP) neurons were examined, together with cortical electroencephalogram and nuchal muscular activity, in non-anaesthetized head-restrained rats. STN neurons shifted from a random discharge in wakefulness (W) to a bursting pattern in slow wave sleep (SWS), without any change in their mean firing rate. This burst discharge occurred in the 1-2 Hz range, but was not correlated with cortical slow wave activity. In contrast, GP neurons, with a mean firing rate higher in W than in SWS, exhibited a relatively regular discharge whatever the state of vigilance. During paradoxical sleep, both STN and GP neurons increased markedly their mean firing rate relative to W and SWS. Our results are not in agreement with the classical 'direct/indirect' model of the basal ganglia organization, as an inverse relationship between STN and GP activities is not observed under normal physiological conditions. Actually, because the STN discharge pattern appears dependent on coincident cortical activity, this nucleus can hardly be viewed as a relay along the indirect pathway, but might rather be considered as an input stage conveying corticothalamic information to the basal ganglia.

The connections of the dopaminergic system with the striatum in rats and primates: an analysis with respect to the functional and compartmental organization of the striatum

This Commentary compares the connections of the dopaminergic system with the striatum in rats and primates with respect to two levels of striatal organization: a tripartite functional (motor, associative and limbic) subdivision and a compartmental (patch/striosome-matrix) subdivision. The topography of other basal ganglia projections to the dopaminergic system with respect to their tripartite functional subdivision is also reviewed. This examination indicates that, in rats and primates, the following observations can be made. (1) The limbic striatum reciprocates its dopaminergic input and in addition innervates most of the dopaminergic neurons projecting to the associative and motor striatum, whereas the motor and associative striatum reciprocate only part of their dopaminergic input. Therefore, the connections of the three striatal subregions with the dopaminergic system are asymmetrical, but the direction of asymmetry differs between the limbic versus the motor and associative striatum. (2) The limbic striatum provides the main striatal input to dopamine cell bodies and proximal dendrites, with some contribution from a subset of neurons in the associative and motor striatum (patch neurons in rats; an unspecified group of neurons in primates), while striatal input to the ventrally extending dopamine dendrites arises mainly from a subset of neurons in the associative and motor striatum (matrix neurons in rats; an unspecified group of neurons in primates). (3) Projections from functionally corresponding subdivisions of the striatum, pallidum and subthalamic nucleus to the dopaminergic system overlap, but the specific targets (dopamine cells, dopamine dendrites, GABA cells) of these projections differ. Major differences include the following. (1) In rats, neurons projecting to the motor and associative striatum reside in distinct regions, while in primates they are arranged in interdigitating clusters. (2) In rats, the terminal fields of projections arising from the motor and associative striatum are largely segregated, while in primates they are not. (3) In rats, patch- and matrix-projecting dopamine cells are organized in spatially, morphologically, histochemically and hodologically distinct ventral and dorsal tiers, while in primates there is no (bi)division of the dopaminergic system that results in two areas which have all the characteristics of the two tiers in rats. Based on the anatomical data and known dopamine cell physiology, we forward an hypothesis regarding the influence of the basal ganglia on dopamine cell activity which captures at least part of the complex interplay taking place within the substantia nigra between projections arising from the different basal ganglia nuclei. Finally, we incorporate the striatal connections with the dopaminergic system into an open-interconnected scheme of basal ganglia-thalamocortical circuitry.

Subthalamic responses to amphetamine and apomorphine in the behaving rat with a unilateral 6-OHDA lesion in the substantia nigra

The activity of neurons in the subthalamic nucleus (STN) of the behaving rat, before and after a unilateral 6-OHDA lesion of the substantia nigra, was recorded with the extracellular technique to determine whether it was altered following systemic amphetamine, 5 mg/kg, apomorphine, 3 mg/kg, and apomorphine, 0.3 mg/kg, and whether in cases of altered activity, it was related to the drug-induced motor response expressed concurrently. Activity in the STN of intact rats increased dramatically after amphetamine, 5 mg/kg. This excitatory response had the same latency, similar magnitude, and the same duration as the motor response expressed in terms of locomotion and oral stereotypy. Motor and unit responses were also induced by amphetamine after the lesion with 6-hydroxydopamine (6-OHDA), but now the excitatory response was attenuated while the motor response was not. The effects of the 6-OHDA lesion were the same in all animals with loss of the nigra dopamine neurons, regardless of whether they were rotators or non-rotators. Activity in the STN of intact rats also increased after apomorphine, 3 mg/kg, and again, this increase was correlated with the increase in motor behavior, but both responses were of shorter duration than the responses to amphetamine. The increases in unit activity and motor behavior induced by apomorphine in the 6-OHDA-lesioned rats had the same magnitude but lasted longer than in the intact rats. Treatment with apomorphine, 0.3 mg/kg, of the intact rats produced small and very brief increases in the activity of the STN and in motor behavior. The same treatment given the 6-OHDA-lesioned rats produced responses of larger magnitude but no change in duration. These findings demonstrate a role for STN neurons in the mediation of the motor behaviors induced by stimulation of the dopamine receptor. The results also show that a unilateral lesion of the substantia nigra with 6-OHDA did not block these responses but altered them in a manner consistent with a dopaminergic deafferentation of the basal ganglia. The increased activity in the STN during the expression of dopamine-dependent motor behavior conflicts with the current model of basal ganglia function that assumes prejudicial effects of excessive STN activity on the expression of motor behavior. An explanation for this conflict suggests that it is more apparent than real.

The axonal arborization of single nigrostriatal neurons in rats

Neurons of the substantia nigra pars compacta (SNc) were iontophoretically injected with biotin dextran and their anterogradely labeled axons individually reconstructed from serial sagittal sections. Most nigrostriatal axons travelled directly to the striatum, where they branched abundantly. Other axons arborized profusely in various extrastriatal structures, including the globus pallidus, the entopeduncular and subthalamic nuclei, and branched only sparsely in the striatum. This heterogeneous organization of the nigrostriatal projection allows single SNc neurons to influence differently striatal neurons and to act directly upon extrastriatal components of the basal ganglia via a highly patterned set of collaterals.

Extrastriatal dopaminergic innervation of human basal ganglia

A tyrosine-hydroxylase immunohistochemical analysis of the brains of normal human individuals has revealed nigrostriatal axons providing collaterals that arborize in the pallidum and subthalamic nucleus. These thin and varicose collaterals emerge from thick and smooth axons that course backward along the main output pathways of the basal ganglia, including the ansa lenticularis, the lenticular fasciculus and Wilson's pencils. Many of these fibers run within pallidal medullary laminae before reaching the putamen, whereas others climb along the reticular thalamic nucleus to reach the caudate nucleus. This extrastriatal innervation, which allows nigral dopaminergic neurons to directly affect the pallidum and subthalamic nucleus, may play a crucial role in the functional organization of human basal ganglia, in both health and disease.

Unilateral dopamine depletion paradoxically enhances amphetamine-induced Fos expression in basal ganglia output structures

The ability of amphetamine to induce expression of the immediate early gene protein, Fos, was examined by immunocytochemistry in animals with unilateral 6-hydroxydopamine lesions of the nigrostriatal bundle. Amphetamine induced Fos expression in the globus pallidus (GP) on the intact side of the brain, but this response was greatly attenuated on the dopamine-depleted side. In contrast, amphetamine induced little Fos expression in the entopeduncular nucleus (EPN) and the substantia nigra pars reticulata (SNpr) on the intact side of the brain, but resulted in pronounced expression in these structures on the lesioned side. These findings demonstrate that unilateral dopamine depletion results in a pathophysiological state in which some responses to amphetamine are attenuated while others are paradoxically potentiated. One explanation of these effects is that amphetamine may indirectly activate excitatory inputs to the SNpr and the EPN on both sides of the brain. On the intact side, these effects would be opposed by the simultaneous activation of inhibitory pathways arising in the striatum and the GP, with the result that little Fos expression would be seen. On the dopamine-depleted side, however, engagement of these inhibitory pathways would be attenuated and the unopposed effects of the excitatory inputs mobilized by amphetamine would result in exaggerated Fos synthesis.

Localization of calcium-binding proteins and GABA transporter (GAT-1) messenger RNA in the human subthalamic nucleus

The distribution of messenger RNA encoding the human GAT-1 (a high-affinity GABA transporter) was investigated in the subthalamic nucleus of 10 neurologically normal human post mortem cases. Further, the distribution of messenger RNA and protein encoding the three neuronally expressed calcium-binding proteins (calbindin D28k, parvalbumin and calretinin) was similarly investigated using in situ hybridization and immunohistochemical techniques. Cellular sites of calbindin D28k, parvalbumin, calretinin and GAT-1 messenger RNA expression were localized using human-specific oligonucleotide probes radiolabelled with [35S]dATP. Sites of protein localization were visualized using specific anti-calbindin D28k, anti-parvalbumin and anti-calretinin antisera. Examination of emulsion-coated tissue sections processed for in situ hybridization revealed an intense signal for GAT-1 messenger RNA within the human subthalamic nucleus, indeed the majority of Methylene Blue-counterstained cells were enriched in this transcript. Further, a marked heterogeneity was noted with regard to the expression of the messenger RNA's encoding the three calcium-binding proteins; this elliptical nucleus was highly enriched in parvalbumin messenger RNA-positive neurons and calretinin mRNA-positive cells but not calbindin messenger RNA-positive cells. Indeed, only an occasional calbindin messenger RNA-positive cell was detected within the mediolateral extent of the nucleus. In marked contrast, numerous parvalbumin messenger RNA-positive cells and calretinin messenger RNA-positive cells were detected and they were topographically distributed; parvalbumin messenger RNA-positive cells were highly enriched in the dorsal subthalamic nucleus extending mediolaterally; calretinin messenger RNA-positive cells were more enriched ventrally although some degree of overlap was apparent. Computer-assisted analysis of the average cross-sectional somatic area of parvalbumin, calretinin and GAT-1 messenger RNA-positive neurons revealed them all to be in the range of 300 microm2. The unique patterns of calcium-binding protein gene expression were similarly reflected at the protein level; an abundance of parvalbumin- and calretinin-immunopositive neurons were observed whereas only occasional intensely-labelled calbindin-immunopositive fibres were seen, no calbindin-immunopositive cells were detected. Single and double labelling studies show that parvalbumin-immunopositive neurons were mainly localized in the dorsal region of the nucleus, and calretinin-immunopositive neurons were mainly localized in the ventral region although there was overlap with double-labelled neurons located in the middle and dorsal regions. The significance of these findings, in particular the expression of GAT-1, a high-affinity GABA uptake protein, for basal ganglia signalling is discussed.

High-frequency stimulation of the subthalamic nucleus suppresses experimental resting tremor in the monkey

The effect of high-frequency stimulation of the subthalamic nucleus on parkinsonian-like resting tremor was investigated in two monkeys (Macaca fascicularis). Unilateral tremor of the arm and leg was induced by electrical coagulation of the brainstem area including the substantia nigra and the red nucleus. The tremor was only seen at rest condition with a very stable frequency of 4.46+/-0.59 Hz (mean+/-S.D.). Apomorphine (0.10-0.4 mg/kg, s.c.) completely blocked the tremor, suggesting that it was a dopaminergic-dependent symptom just like the parkinsonian tremor. When the stimulating frequency varied from 20 to 1000 Hz, both mono- and bipolar stimulation (square pulses, 0-5 mA, 0.06 ms) of the subthalamic nucleus suppressed resting tremor in a frequency-dependent manner but monopolar stimulation was more effective. These effects remained stable for more than two years. The present results suggest that the subthalamic nucleus is involved in the control and mechanism of resting tremor and that the high-frequency stimulation of the subthalamic nucleus can be used as an alternative therapy in parkinsonian patients with akinesia, rigidity and resting tremor.

Structural and functional evolution of the basal ganglia in vertebrates

While a basal ganglia with striatal and pallidal subdivisions is 1 clearly present in many extant anamniote species, this basal ganglia is cell sparse and receives only a relatively modest tegmental dopaminergic input and little if any cortical input. The major basal ganglia influence on motor functions in anamniotes appears to be exerted via output circuits to the tectum. In contrast, in modern mammals, birds, and reptiles (i.e., modern amniotes), the striatal and pallidal parts of the basal ganglia are very neuron-rich, both consist of the same basic populations of neurons in all amniotes, and the striatum receives abundant tegmental dopaminergic and cortical input. The functional circuitry of the basal ganglia also seems very similar in all amniotes, since the major basal ganglia influences on motor functions appear to be exerted via output circuits to both cerebral cortex and tectum in sauropsids (i.e., birds and reptiles) and mammals. The basal ganglia, output circuits to the cortex, however, appear to be considerably more developed in mammals than in birds and reptiles. The basal ganglia, thus, appears to have undergone a major elaboration during the evolutionary transition from amphibians to reptiles. This elaboration may have enabled amniotes to learn and/or execute a more sophisticated repertoire of behaviors and movements, and this ability may have been an important element of the successful adaptation of amniotes to a fully terrestrial habitat. The mammalian lineage appears, however, to have diverged somewhat from the sauropsid lineage with respect to the emergence of the cerebral cortex as the major target of the basal ganglia circuitry devoted to executing the basal ganglia-mediated control of movement.

Globus pallidus lesions depress the excitatory responses to apomorphine but not amphetamine in the subthalamic nucleus of the behaving rat with a 6-OHDA nigra lesion

The role of the dopaminergic innervation of the basal ganglia on the activity in the subthalamic nucleus (STN) evoked by amphetamine and apomorphine in the behaving rat was examined. The aim was to determine the relationship between that neural activity and the movements evoked by the drugs. Bilateral electrolytic lesions of the globus pallidus (GP), superimposed on the earlier unilateral lesion in substantia nigra (SN) with 6-hydroxydopamine (6-OHDA) affected differently the excitatory responses in the STN evoked by amphetamine and apomorphine and the motor responses to the drugs recorded concurrently. Before the GP lesions, the administration of amphetamine, 5 mg/kg, to the unilaterally deafferented rat induced increased activity in the STN and simultaneously increased movement in the animal. After the GP lesions, the excitatory response to amphetamine in the STN was not different from that seen before the GP lesions. The motor response was also unchanged. In contrast, the GP lesions altered the excitatory response to apomorphine, 3 mg/kg. Before these lesions, the administration of apomorphine to the 6-OHDA lesioned animal evoked a robust and long-lasting excitatory response in the STN and, concurrently, a long-lasting motor response. After the GP lesions, both responses to apomorphine were attenuated. These differential effects of the GP lesions on the unit and motor responses to the two drugs are viewed as representing the effects of the damage in the GP on the dopaminergic innervation contributing to the regulation of activity in the STN. In the 6-OHDA animal, the dopamine afferents innervating the basal ganglia had already been dramatically reduced by 6-OHDA. The GP lesions did not significantly add to the number of these afferents previously eliminated; therefore, the excitatory and motor responses to amphetamine were not changed by the GP lesions. But the GP damage served to eliminate the dopamine receptor in the GP and thus reduced the density of the dopamine receptor in the basal ganglia available for binding to apomorphine. Therefore, the excitatory and motor responses to apomorphine were attenuated after the GP lesions compared to the responses before these lesions.

Morphological organization of the globus pallidus-subthalamic nucleus system studied in organotypic cultures

The morphological organization of the globus pallidus (GP), the subthalamic nucleus (STN), and the pallidosubthalamic projection was studied in organotypic cultures. Coronal slices from the GP, the STN, the striatum (CPu), and the cortex (Cx) were taken from the rat after postnatal days 0-2 and grown for 2 or 5-6 weeks. For analysis, immunocytochemistry against glutamate (GLU), parvalbumin (PV), and calretinin (CR) was combined with confocal microscopy. After 2 weeks in vitro, the STN showed a densely packed, homogeneous GLU-immunoreactive (ir) cell population. Pallidal GLU-ir neurons were heterogeneous, consisting of large-sized weakly GLU-ir neurons and small-sized intensively GLU-ir neurons. After 5-6 weeks in vitro, pallidal axons had radiated from numerous large-sized PV-ir cells and selectively innervated the STN, where they heavily ramified. Cultured STN neurons were not stained for PV; however, multipolar intensely PV-ir neurons were located at the border of the STN with their dendrites oriented towards the STN. Double labeling for PV and CR in both mature cultures and in the adult rat revealed that the culture CR-ir neurons from the GP, the Cpu, and from areas adjacent to the STN were different from cultured PV-ir neurons and their morphologies and distribution corresponded to that in vivo. These results demonstrate that 1) cultured CP and STN neurons display similar morphologies found in in vivo, 2) PV-ir pallidal neurons heavily and selectively innervate the STN; 3) there is a specific class of STN border neurons; and 4) in contrast to the in vivo situation, most cultured STN neurons are PV-negative.

Prefrontal cortex-basal ganglia circuits in the rat: involvement of ventral pallidum and subthalamic nucleus

The core of the nucleus accumbens (NAcc core) is the principal input structure to the basal ganglia circuitry for the prelimbic and medial orbital areas (PL/MO) of the prefrontal cortex. As is now well recognized in the rat, the main basal ganglia output of this prefrontal channel is the dorsomedial part of the substantia nigra pars reticulata (SNR) and not the ventral pallidum as previously suggested. There is evidence suggesting that the ventral pallidum is rather involved with the subthalamic nucleus (STN) in an indirect NAcc-SNR pathway. Indeed, we have recently shown that the NAcc core sends an inhibitory input to the lateral ventral pallidum (VPl), which projects to the medial STN. In the present study, we injected biocytin into the medial STN, at a site where neurons presented an inhibitory response to VPl stimulation. This produced anterogradely labelled fibres in the medial SNR and in the VPl. Furthermore, the stimulation of the VPl induced an inhibition in a majority of the STN cells identified, by the antidromic activation method, as projecting to SNR (76.6%) and/or back to the VPl (72.7%). In conclusion, these data further demonstrate the existence of an indirect striato-nigral pathway in the PL/MO channel and indicate that VPl is involved in an inhibitory feedback circuit, which modulates the discharge of medial STN. These results indicate that the medial STN is implicated in the limbic/cognitive functions of the basal ganglia.

Lamotrigine in Parkinson's disease--a double blind study

Antiglutamatergic acting substances are considered to be useful tools for the treatment of hypokinesia in animal models for Parkinson's disease (PD). Moreover, most known antiglutamatergic compounds act postsynaptically and are either toxic or weak with regard to their clinical potency. The antiepileptic drug "Lamotrigine (LTG)" inhibits presynaptic glutamate release and may therefore provide a novel approach for PD therapy. Encouraging results from a pilot project led us to establish a placebo controlled trial including 20 patients with PD. The substance was generally well tolerated. There was a significant difference in the investigator's overall assessment of efficacy (6/10 vs. 2/10 improvement; p < 0.05) and a tendency for LTG to exhibit a beneficial effect in some registration parameters, but no significant differences in motor response were found between the two groups. We failed to confirm that LTG mediates a strong antiparkinsonian effect in this small study, but to clearly demonstrate slight or moderate beneficial effects larger groups are required.

Dystonic posturing and circling behaviors induced by dopaminergic agents in rats with unilateral globus pallidus lesions

The present study examined the behavioral effects of dopamine receptor agonists, antagonists, or N-methyl-D-aspartate (NMDA) glutamate receptor antagonist in rats with a unilateral excitotoxic lesion of the globus pallidus (GP). After the unilateral GP lesions were made by injections of the ibotenic acid, drugs were systemically given and the elicited behaviors were quantitatively assessed. Systemic administration of haloperidol, but not SCH23390, dose-dependently induced contraversive dystonic posturing in unilateral GP-lesioned rats. On the other hand, systemic administration of quinpirole, but not SKF38393, induced ipsiversive circling. MK-801, only when given at a high dose, unilateral GP-lesioned rat, the D2 receptor agonist and antagonist caused ipsiversive and contraversive posturing or circling, respectively. Since the rotational behavior is induced on the basis of asymmetry of the basal ganglia output activity, there must be a marked difference between the GP ablation and the administration of D2 receptor blockade on the basal ganglia output activity, supporting a speculation that overactivity of the basal ganglia under dopamine depletion is not solely a result of the disinhibition from the inhibitory GP efferents. The present unilateral GP-lesion model appears to be a useful one for the pharmacobehavioral investigation of D2-mediated mechanisms.

Corticosubthalamic input zones from forelimb representations of the dorsal and ventral divisions of the premotor cortex in the macaque monkey: comparison with the input zones from the primary motor cortex and the supplementary motor area

Employing double anterograde axonal tracing in combination with intracortical microstimulation, we examined the distribution patterns of corticosubthalamic inputs from forelimb representations of the dorsal (PMd) and ventral (PMv) divisions of the premotor cortex in the macaque monkey. The inputs from the PMd and PMv were distributed mainly in the medial aspect of the subthalamic nucleus (STN), in which their distribution areas overlapped each other. By the same experimental approach, we further compared corticosubthalamic input zones from the PMd/PMv with those from the primary motor cortex (MI) and the supplementary motor area (SMA). The input zones from the PMd/PMv and SMA largely overlapped in the medial aspect of the STN, whereas the input zones from the PMd/PMv and MI were virtually segregated mediolaterally in the STN.

Electrophysiological and Fos immunohistochemical evidence for the excitatory nature of the parafascicular projection to the globus pallidus

Extracellular recordings and immunohistological detection of c-Fos-like immunoreactive proteins were used to determine the synaptic effect of the parafascicular projection to the globus pallidus. Electrical stimulation of the parafascicular neurons induced a single-spike excitatory response with a stable latency of 2.3 ms, suggesting a monosynaptically driven effect. Pharmacological stimulation of the parafascicular nucleus with carbachol increased tonically the pallidal discharge rate by 142%. The discharge rate of the pallidal neurons was described by 37% in parafascicular-lesioned rats. These results demonstrate the excitatory nature and the tonic action of the parafasciculopallidal projection. Carbachol activation of parafascicular neurons also induced the synthesis of c-Fos-like immunoreactive proteins in the pallidal neurons. Control experiments in subthalamic-lesioned rats showed that the parafascicular excitation of the pallidal neurons remained, but both electrophysiological and expression of c-Fos-like immunoreactive proteins were attenuated. This suggests that the direct parafascicular excitation of the pallidal neurons is indirectly reinforced by the previously described parafascicular excitatory input to the subthalamic nucleus. Conversely, the effect of this last input to the subthalamic nucleus is dramatically enhanced in rats with pallidal lesion. Our results demonstrate the complex role of the parafascicular nucleus in activating both the globus pallidus and the subthalamic nucleus, two closely related structures. These results illustrate the integrative capacities of the globus pallidus, whose activity is modulated by multiple afferents.

Excitation of rat subthalamic nucleus neurones in vitro by activation of a group I metabotropic glutamate receptor

The subthalamic nucleus (SThN) provides a glutamate mediated excitatory drive to several other component nuclei of the basal ganglia, thereby significantly influencing locomotion and control of voluntary movement. We have characterised functionally the metabotropic glutamate (mGlu) receptors in the SThN using extracellular single unit recording from rat midbrain slices. SThN neurones fired action potentials spontaneously at a rate of 10 Hz which was increased by the group I/II mGlu receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (1S,3 R-ACPD; 1-30 microM) and the group I selective agonist (S, R)-dihydroxyphenylglycine (DHPG; 1-30 microM). However, both the group II selective agonist (1S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV; 1 microM) and the group III selective agonist (S)-2-amino-4-phosphonobutanoic acid (L-AP4; 10 microM) were without effect, indicating that the excitation was mediated by a group I mGlu receptor. The excitation caused by DHPG (3 microM) was reversed by co-application of the mGlu receptor antagonist (+)-alpha-methyl-4-carboxyphenylglycine (MCPG; 500 microM). Thus a group I mGlu receptor mediates excitation of SThN neurones, and suggests a use for group I mGlu receptor ligands for treatment of both hypo- and hyperkinetic disorders of basal ganglia origin, such as Parkinson's disease and Huntington's disease.

Two types of cholinergic neurons in the rat tegmental pedunculopontine nucleus: electrophysiological and morphological characterization

Two types of tegmental pedunculopontine nucleus neurons have been reported previously based on their electrophysiological characteristics: type I neurons were characterized by low-threshold Ca spikes and type II neurons displayed a transient outward current. This report describes the membrane properties, synaptic inputs, morphologies and axonal projections of two subgroups of type II neurons examined in an in vitro slice preparation. Type II neurons were divided into two groups based on their spike durations: short-duration neurons with an action potential duration of 0.7-1.5 ms and long-duration neurons with an action potential duration of 1.6- 2.9 ms. Choline acetyltransferase immunohistochemistry combined with biocytin labeling indicated that 56% of short-duration neurons and 61% of long-duration neurons were immunopositive for choline acetyltransferase. Short-duration neurons had a high input resistance and the capacity to discharge with high frequency. By contrast, long-duration neurons had a low input resistance and low firing frequency and upon current injection displayed an accommodation (spike-frequency adaptation) before reaching a steady firing frequency. Microstimulation of the substantia nigra pars compacta evoked antidromic responses in both short-duration neurons (n=5/14, 36%) and long-duration neurons (n=20/39. 51%). Stimulations of the subthalamic nucleus and the substantia nigra pars reticulata induced in these neurons excitatory and inhibitory postsynaptic potentials, respectively. Short-duration neurons were dispersed equally throughout the extent of the tegmental pedunculopontine nucleus area, while long-duration neurons were located more in the rostral tegmental pedunculopontine nucleus. Short-duration neurons were small with two to four thin primary dendrites. Long-duration neurons were medium to large with three to six thick primary dendrites. Cell size was positively correlated with spike duration and axonal conduction velocity, but negatively with input resistance and spontaneous firing frequency. Both groups of neurons had ascending (toward thalamus, pretectal areas and tectum) and descending (toward pontomedullary reticular formation) axons in addition to nigropetal axons. Ascending axons were observed in 75% (6/8) of short-duration neurons and in 45% (15/33) of long-duration neurons, while nigropetal axons were observed in 50% (4/8) of short-duration neurons and in 76% (25/33) of long-duration neurons. These results suggest that the tegmental pedunculopontine nucleus cholinergic projection system is composed of heterogeneous populations of neurons in terms of electrophysiological and morphological characteristics as well as their distribution patterns in the nucleus.