Nigral and pallidal inputs to functionally segregated thalamostriatal neurons in the centromedian/parafascicular intralaminar nuclear complex in monkey

Modulation of neuronal activity in human centromedian nucleus during an auditory attention and working memory task

Centromedian nucleus (CM) is one of several intralaminar nuclei of the thalamus and is thought to be involved in consciousness, arousal, and attention. CM has been suggested to play a key role in the control of attention, by regulating the flow of information to different brain regions such as the ascending reticular system, basal ganglia, and cortex. While the neurophysiology of attention in visual and auditory systems has been studied in animal models, combined single unit and LFP recordings in human have not, to our knowledge, been reported. Here, we recorded neuronal activity in the CM nucleus in 11 patients prior to insertion of deep brain stimulation electrodes for the treatment of epilepsy while subjects performed an auditory attention task. Patients were requested to attend and count the infrequent (p = 0.2) odd or "deviant" tones, ignore the frequent standard tones and report the total number of deviant tones at trial completion. Spikes were discriminated, and LFPs were band pass filtered (5-45 Hz). Average peri‑stimulus time histograms and spectra were constructed by aligning on tone onsets and statistically compared. The firing rate of CM neurons showed selective, multi-phasic responses to deviant tones in 81% of the tested neurons. Local field potential analysis showed selective beta and low gamma (13-45 Hz) modulations in response to deviant tones, also in a multi-phasic pattern. The current study demonstrates that CM neurons are under top-down control and participate in the selective processing during auditory attention and working memory. These results, taken together, implicate the CM in selective auditory attention and working memory and support a role of beta and low gamma oscillatory activity in cognitive processes. It also has potential implications for DBS therapy for epilepsy and non-motor symptoms of PD, such as apathy and other disorders of attention.

Structural Plasticity of GABAergic Pallidothalamic Terminals in MPTP-Treated Parkinsonian Monkeys: A 3D Electron Microscopic Analysis

The internal globus pallidus (GPi) is a major source of tonic GABAergic inhibition to the motor thalamus. In parkinsonism, the firing rate of GPi neurons is increased, and their pattern switches from a tonic to a burst mode, two pathophysiological changes associated with increased GABAergic pallidothalamic activity. In this study, we used high-resolution 3D electron microscopy to demonstrate that GPi terminals in the parvocellular ventral anterior nucleus (VApc) and the centromedian nucleus (CM), the two main GPi-recipient motor thalamic nuclei in monkeys, undergo significant morphometric changes in parkinsonian monkeys including (1) increased terminal volume in both nuclei; (2) increased surface area of synapses in both nuclei; (3) increased number of synapses/GPi terminals in the CM, but not VApc; and (4) increased total volume, but not number, of mitochondria/terminals in both nuclei. In contrast to GPi terminals, the ultrastructure of putative GABAergic nonpallidal terminals was not affected. Our results also revealed striking morphological differences in terminal volume, number/area of synapses, and volume/number of mitochondria between GPi terminals in VApc and CM of control monkeys. In conclusion, GABAergic pallidothalamic terminals are endowed with a high level of structural plasticity that may contribute to the development and maintenance of the abnormal increase in pallidal GABAergic outflow to the thalamus in the parkinsonian state. Furthermore, the evidence for ultrastructural differences between GPi terminals in VApc and CM suggests that morphologically distinct pallidothalamic terminals from single pallidal neurons may underlie specific physiological properties of pallidal inputs to VApc and CM in normal and diseased states.

Causality methods to study the functional connectivity in brain networks: the basal ganglia - thalamus causal interactions

This study uses methods recently developed to study the complex evolution of atmospheric phenomena which have some similarities with the dynamics of the human brain. In both cases, it is possible to record the activity of particular centers (geographic regions or brain nuclei) but not to make an experimental modification of their state. The study of "causality", which is necessary to understand the dynamics of these complex systems and to develop robust models that can predict their evolution, is hampered by the experimental restrictions imposed by the nature of both systems. The study was performed with data obtained in the thalamus and basal ganglia of awake humans executing different tasks. This work studies the linear, non-linear and more complex relationships of these thalamic centers with the cortex and main BG nuclei, using three complementary techniques: the partial correlation regression method, the Gaussian process regression/distance correlation and a model-free method based on nearest-neighbor that computes the conditional mutual information. These causality methods indicated that the basal ganglia present a different functional relationship with the anterior-ventral (motor), intralaminar and medio-dorsal thalamic centers, and that more than 60% of these thalamus-basal ganglia relationships present a non-linear dynamic (35 of the 57 relationships found). These functional interactions were observed for basal ganglia nuclei with direct structural connections with the thalamus (primary somatosensory and motor cortex, striatum, internal globus pallidum and substantia nigra pars reticulata), but also for basal ganglia without structural connections with the thalamus (external globus pallidum and subthalamic nucleus). The motor tasks induced rapid modifications of the thalamus-basal ganglia interactions. These findings provide new perspectives of the thalamus - BG interactions, many of which may be supported by indirect functional relationships and not by direct excitatory/inhibitory interactions.

Micropopulation mapping of the mouse parafascicular nucleus connections reveals diverse input-output motifs

Introduction

In primates, including humans, the centromedian/parafascicular (CM-Pf) complex is a key thalamic node of the basal ganglia system. Deep brain stimulation in CM-Pf has been applied for the treatment of motor disorders such as Parkinson's disease or Tourette syndrome. Rodents have become widely used models for the study of the cellular and genetic mechanisms of these and other motor disorders. However, the equivalence between the primate CM-Pf and the nucleus regarded as analogous in rodents (Parafascicular, Pf) remains unclear.

Methods

Here, we analyzed the neurochemical architecture and carried out a brain-wide mapping of the input-output motifs in the mouse Pf at micropopulation level using anterograde and retrograde labeling methods. Specifically, we mapped and quantified the sources of cortical and subcortical input to different Pf subregions, and mapped and compared the distribution and terminal structure of their axons.

Results

We found that projections to Pf arise predominantly (>75%) from the cerebral cortex, with an unusually strong (>45%) Layer 5b component, which is, in part, contralateral. The intermediate layers of the superior colliculus are the main subcortical input source to Pf. On its output side, Pf neuron axons predominantly innervate the striatum. In a sparser fashion, they innervate other basal ganglia nuclei, including the subthalamic nucleus (STN), and the cerebral cortex. Differences are evident between the lateral and medial portions of Pf, both in chemoarchitecture and in connectivity. Lateral Pf axons innervate territories of the striatum, STN and cortex involved in the sensorimotor control of different parts of the contralateral hemibody. In contrast, the mediodorsal portion of Pf innervates oculomotor-limbic territories in the above three structures.

Discussion

Our data thus indicate that the mouse Pf consists of several neurochemically and connectively distinct domains whose global organization bears a marked similarity to that described in the primate CM-Pf complex.

The Arousal-Related "Central Thalamus" Stimulation Site Simultaneously Innervates Multiple High-Level Frontal and Parietal Areas

In human and nonhuman primates, deep brain stimulation applied at or near the internal medullary lamina of the thalamus [a region referred to as "central thalamus," (CT)], but not at nearby thalamic sites, elicits major changes in the level of consciousness, even in some minimally conscious brain-damaged patients. The mechanisms behind these effects remain mysterious, as the connections of CT had not been specifically mapped in primates. In marmoset monkeys (Callithrix jacchus) of both sexes, we labeled the axons originating from each of the various CT neuronal populations and analyzed their arborization patterns in the cerebral cortex and striatum. We report that, together, these CT populations innervate an array of high-level frontal, posterior parietal, and cingulate cortical areas. Some populations simultaneously target the frontal, parietal, and cingulate cortices, while others predominantly target the dorsal striatum. Our data indicate that CT stimulation can simultaneously engage a heterogeneous set of projection systems that, together, target the key nodes of the attention, executive control, and working-memory networks of the brain. Increased functional connectivity in these networks has been previously described as a signature of consciousness.SIGNIFICANCE STATEMENT In human and nonhuman primates, deep brain stimulation at a specific site near the internal medullary lamina of the thalamus ["central thalamus," (CT)] had been shown to restore arousal and awareness in anesthetized animals, as well as in some brain-damaged patients. The mechanisms behind these effects remain mysterious, as CT connections remain poorly defined in primates. In marmoset monkeys, we mapped with sensitive axon-labeling methods the pathways originated from CT. Our data indicate that stimulation applied in CT can simultaneously engage a heterogeneous set of projection systems that, together, target several key nodes of the attention, executive control, and working-memory networks of the brain. Increased functional connectivity in these networks has been previously described as a signature of consciousness.

The role of the parafascicular thalamic nucleus in action initiation and steering

The parafascicular (Pf) nucleus of the thalamus has been implicated in arousal and attention, but its contributions to behavior remain poorly characterized. Here, using in vivo and in vitro electrophysiology, optogenetics, and 3D motion capture, we studied the role of the Pf nucleus in behavior using a continuous reward-tracking task in freely moving mice. We found that many Pf neurons precisely represent vector components of velocity, with a strong preference for ipsiversive movements. Their activity usually leads velocity, suggesting that Pf output is critical for self-initiated orienting behavior. To test this hypothesis, we expressed excitatory or inhibitory opsins in VGlut2+ Pf neurons to manipulate neural activity bidirectionally. We found that selective optogenetic stimulation of these neurons consistently produced ipsiversive head turning, whereas inhibition stopped turning and produced downward movements. Taken together, our results suggest that the Pf nucleus can send continuous top-down commands that specify detailed action parameters (e.g., direction and speed of the head), thus providing guidance for orienting and steering during behavior.

Hyperkinetic Rat Model Induced by Optogenetic Parafascicular Nucleus Stimulation

OBJECTIVE

The parafascicular nucleus (PF) plays important roles in controlling the basal ganglia. It is not well known whether the PF affects the development of abnormal involuntary movements (AIMs). This study was aimed to find a role of the PF in development of AIMs using optogenetic methods in an animal model.

METHODS

Fourteen rats were underwent stereotactic operation, in which they were injected with an adeno-associated virus with channelrhodopsin (AAV2-hSyn-ChR2-mCherry) to the lateral one third of the PF. Behavior test was performed with and without optical stimulation 14 days after the injection of the virus. AIM of rat was examined using AIM score. After the behavior test, rat's brain was carefully extracted and the section was examined using a fluorescence microscope to confirm transfection of the PF.

RESULTS

Of the 14 rats, seven rats displayed evident involuntary abnormal movements. AIM scores were increased significantly after the stimulation compared to those at baseline. In rats with AIMs, mCherry expression was prominent in the PF, while the rats without AIM lacked with the mCherry expression.

CONCLUSION

AIMs could be reversibly induced by stimulating the PF through an optogenetic method.

Novel targets in deep brain stimulation for movement disorders

The neurosurgical treatment of movement disorders, primarily via deep brain stimulation (DBS), is a rapidly expanding and evolving field. Although conventional targets including the subthalamic nucleus (STN) and internal segment of the globus pallidus (GPi) for Parkinson's disease and ventral intermediate nucleus of the thalams (VIM) for tremor provide substantial benefit in terms of both motor symptoms and quality of life, other targets for DBS have been explored in an effort to maximize clinical benefit and also avoid undesired adverse effects associated with stimulation. These novel targets primarily include the rostral zona incerta (rZI), caudal zona incerta (cZI)/posterior subthalamic area (PSA), prelemniscal radiation (Raprl), pedunculopontine nucleus (PPN), substantia nigra pars reticulata (SNr), centromedian/parafascicular (CM/PF) nucleus of the thalamus, nucleus basalis of Meynert (NBM), dentato-rubro-thalamic tract (DRTT), dentate nucleus of the cerebellum, external segment of the globus pallidus (GPe), and ventral oralis (VO) complex of the thalamus. However, reports of outcomes utilizing these targets are scattered and disparate. In order to provide a comprehensive resource for researchers and clinicians alike, we have summarized the existing literature surrounding these novel targets, including rationale for their use, neurosurgical techniques where relevant, outcomes and adverse effects of stimulation, and future directions for research.

Disentangling the influences of multiple thalamic nuclei on prefrontal cortex and cognitive control

The prefrontal cortex (PFC) has a complex relationship with the thalamus, involving many nuclei which occupy predominantly medial zones along its anterior-to-posterior extent. Thalamocortical neurons in most of these nuclei are modulated by the affective and cognitive signals which funnel through the basal ganglia. We review how PFC-connected thalamic nuclei likely contribute to all aspects of cognitive control: from the processing of information on internal states and goals, facilitating its interactions with mnemonic information and learned values of stimuli and actions, to their influence on high-level cognitive processes, attentional allocation and goal-directed behavior. This includes contributions to transformations such as rule-to-choice (parvocellular mediodorsal nucleus), value-to-choice (magnocellular mediodorsal nucleus), mnemonic-to-choice (anteromedial nucleus) and sensory-to-choice (medial pulvinar). Common mechanisms appear to be thalamic modulation of cortical gain and cortico-cortical functional connectivity. The anatomy also implies a unique role for medial PFC in modulating processing in thalamocortical circuits involving other orbital and lateral PFC regions. We further discuss how cortico-basal ganglia circuits may provide a mechanism through which PFC controls cortico-cortical functional connectivity.

Circuit Mechanisms of L-DOPA-Induced Dyskinesia (LID)

L-DOPA is the criterion standard of treatment for Parkinson disease. Although it alleviates some of the Parkinsonian symptoms, long-term treatment induces L-DOPA–induced dyskinesia (LID). Several theoretical models including the firing rate model, the firing pattern model, and the ensemble model are proposed to explain the mechanisms of LID. The “firing rate model” proposes that decreasing the mean firing rates of the output nuclei of basal ganglia (BG) including the globus pallidus internal segment and substantia nigra reticulata, along the BG pathways, induces dyskinesia. The “firing pattern model” claimed that abnormal firing pattern of a single unit activity and local field potentials may disturb the information processing in the BG, resulting in dyskinesia. The “ensemble model” described that dyskinesia symptoms might represent a distributed impairment involving many brain regions, but the number of activated neurons in the striatum correlated most strongly with dyskinesia severity. Extensive evidence for circuit mechanisms in driving LID symptoms has also been presented. LID is a multisystem disease that affects wide areas of the brain. Brain regions including the striatum, the pallidal–subthalamic network, the motor cortex, the thalamus, and the cerebellum are all involved in the pathophysiology of LID. In addition, although both amantadine and deep brain stimulation help reduce LID, these approaches have complications that limit their wide use, and a novel antidyskinetic drug is strongly needed; these require us to understand the circuit mechanism of LID more deeply.

Firing Differences Between Adult Intralaminar Thalamo-striatal Neurons

Differences in the intrinsic properties of intralaminar thalamo-striatal neurons such as expressing low-threshold-spikes (LTS) or after hyperpolarizing potentials (AHPs) of different duration have been attributed to different maturation stages. However, two morphological types: "diffuse" and "bushy" have been described. Therefore, we explored whether electrophysiological differences persist in adult mice using whole cell recordings. Some recorded neurons were identified by intracellular labeling with biocytin and double labeling with retrograde or anterograde tracings using Cre-mice. We classified these neurons by their AHPs during spontaneous firing. Neurons with long duration AHPs, with fast and slow components, were mostly found in the parafascicular (Pf) nucleus. Neurons with brief AHPs were mainly found in the central lateral (CL) nucleus. However, neurons with both AHPs were found in both nuclei in different proportions. Firing frequency adaptation differed between these neuron classes: those with prolonged AHPs exhibited firing frequency adaptation with fast and slow time constants whereas those with brief AHPs were slow adapters. Neurons with more prolonged AHPs had significant higher input resistances than neurons with brief AHPs. Both cell classes could fire in two modes: trains of single action potentials at depolarized potentials or high frequency bursts on top of LTS at more hyperpolarized potentials. LTS were probably generated by Cav3 calcium channels since they were blocked by the selective antagonist TTA-P2. About 11% of neurons with brief AHPs and 55% of neurons with prolonged AHPs do not show LTS and bursts, even when potassium currents are blocked.

The centromedian nucleus: Anatomy, physiology, and clinical implications

Of all the truncothalamic nuclei, the centromedian-parafascicular nuclei complex (CM-Pf) is the largest and is considered the prototypic thalamic projection system. Located among the caudal intralaminar thalamic nuclei, the CM-Pf been described by Jones as "the forgotten components of the great loop of connections joining the cerebral cortex via the basal ganglia". The CM, located lateral relative to the Pf, is a major source of direct input to the striatum and also has connections to other, distinct region of the basal ganglia as well as the brainstem and cortex. Functionally, the CM participates in sensorimotor coordination, cognition (e.g. attention, arousal), and pain processing. The role of CM as 'gate control' function by propagating only salient stimuli during attention-demanding tasks has been proposed. Given its rich connectivity and diverse physiologic role, recent studies have explored the CM as potential target for neuromodulation therapy for Tourette syndrome, Parkinson's disease, generalized epilepsy, intractable neuropathic pain, and in restoring consciousness. This comprehensive review summarizes the structural and functional anatomy of the CM and its physiologic role with a focus on clinical implications.

The thalamus in drug addiction: from rodents to humans

Impairments in response inhibition and salience attribution (iRISA) have been proposed to underlie the clinical symptoms of drug addiction as mediated by cortico-striatal-thalamo-cortical networks. The bulk of evidence supporting the iRISA model comes from neuroimaging research that has focused on cortical and striatal influences with less emphasis on the role of the thalamus. Here, we highlight the importance of the thalamus in drug addiction, focusing on animal literature findings on thalamic nuclei in the context of drug-seeking, structural and functional changes of the thalamus as measured by imaging studies in human drug addiction, particularly during drug cue and non-drug reward processing, and response inhibition tasks. Findings from the animal literature suggest that the paraventricular nucleus of the thalamus, the lateral habenula and the mediodorsal nucleus may be involved in the reinstatement, extinction and expression of drug-seeking behaviours. In support of the iRISA model, the human addiction imaging literature demonstrates enhanced thalamus activation when reacting to drug cues and reduced thalamus activation during response inhibition. This pattern of response was further associated with the severity of, and relapse in, drug addiction. Future animal studies could widen their field of focus by investigating the specific role(s) of different thalamic nuclei in different phases of the addiction cycle. Similarly, future human imaging studies should aim to specifically delineate the structure and function of different thalamic nuclei, for example, through the application of advanced imaging protocols at higher magnetic fields (7 Tesla).This article is part of a discussion meeting issue 'Of mice and mental health: facilitating dialogue between basic and clinical neuroscientists'.

Anatomy, Physiology, and Clinical Syndromes of the Basal Ganglia: A Brief Review

Movement disorders typically arise from dysfunction of the basal ganglia (BG), cerebellum, or both. The BG-a group of deep, subcortical structures-form complex circuits that shape motor control and motor learning, as well as limbic and associative functions. In this article, we summarize the anatomy and physiology of the BG and cerebellum, and briefly highlight the clinical syndromes that may arise in the context of their injury or dysfunction.

Thalamic interactions of cerebellum and basal ganglia

Cerebellum and basal ganglia are reciprocally interconnected with the neocortex via oligosynaptic loops. The signal pathways of these loops predominantly converge in motor areas of the frontal cortex and are mainly segregated on subcortical level. Recent evidence, however, indicates subcortical interaction of these systems. We have reviewed literature that addresses the question whether, and to what extent, projections of main output nuclei of basal ganglia (reticular part of the substantia nigra, internal segment of the globus pallidus) and cerebellum (deep cerebellar nuclei) interact with each other in the thalamus. To this end, we compiled data from electrophysiological and anatomical studies in rats, cats, dogs, and non-human primates. Evidence suggests the existence of convergence of thalamic projections originating in basal ganglia and cerebellum, albeit sparse and restricted to certain regions. Four regions come into question to contain converging inputs: (1) lateral parts of medial dorsal nucleus (MD); (2) parts of anterior intralaminar nuclei and centromedian and parafascicular nuclei (CM/Pf); (3) ventromedial nucleus (VM); and (4) border regions of cerebellar and ganglia terminal territories in ventral anterior and ventral lateral nuclei (VA-VL). The amount of convergences was found to exhibit marked interspecies differences. To explain the rather sparse convergences of projection territories and to estimate their physiological relevance, we present two conceivable principles of anatomical organization: (1) a "core-and-shell" organization, in which a central core is exclusive to one projection system, while peripheral shell regions intermingle and occasionally converge with other projection systems and (2) convergences that are characteristic to distinct functional networks. The physiological relevance of these convergences is not yet clear. An oculomotor network proposed in this work is an interesting candidate to examine potential ganglia and cerebellar subcortical interactions.

Seventy Years with the Globus Pallidus: Pallidal Surgery for Movement Disorders Between 1947 and 2017

The year 2017 marks the 70th anniversary of the birth of human stereotactic neurosurgery. The first procedure was a pallidotomy for Huntington's disease. However, it was for Parkinson's disease that pallidotomy was soon adopted worldwide. Pallidotomy was abandoned in the late 1950s in favor of thalamotomy because of the latter's more striking effect on tremor. The advent of levodopa put a halt to all surgery for PD. In the mid-1980s, Laitinen reintroduced the posteroventral pallidotomy of Leksell, and this procedure spread worldwide thanks to its efficacy on most parkinsonian symptoms including levodopa-induced dyskinesias and thanks to basic scientific work confirming the role of the globus pallidus internus in the pathophysiology of PD. With the advent of deep brain stimulation of the subthalamic nucleus, pallidotomy was again abandoned, and even DBS of the GPi has been overshadowed by STN DBS. The GPi reemerged in the late 1990s as a major stereotactic target for DBS in dystonia and, recently, in Tourette syndrome. Lately, lesioning of the GPI is being proposed to treat refractory status dystonicus or to treat DBS withdrawal syndrome in PD patients. Hence, the pallidum as a stereotactic target for either lesioning or DBS has been the phoenix of functional stereotactic neurosurgery, constantly abandoned and then rising again from its ashes. This review is a tribute to the pallidum on its 70th anniversary as a surgical target for movement disorders, analyzing its ebbs and flows and highlighting its merits, its versatility, and its resilience. © 2017 International Parkinson and Movement Disorder Society.

Parallel basal ganglia circuits for decision making

The basal ganglia control body movements, mainly, based on their values. Critical for this mechanism is dopamine neurons, which sends unpredicted value signals, mainly, to the striatum. This mechanism enables animals to change their behaviors flexibly, eventually choosing a valuable behavior. However, this may not be the best behavior, because the flexible choice is focused on recent, and, therefore, limited, experiences (i.e., short-term memories). Our old and recent studies suggest that the basal ganglia contain separate circuits that process value signals in a completely different manner. They are insensitive to recent changes in value, yet gradually accumulate the value of each behavior (i.e., movement or object choice). These stable circuits eventually encode values of many behaviors and then retain the value signals for a long time (i.e., long-term memories). They are innervated by a separate group of dopamine neurons that retain value signals, even when no reward is predicted. Importantly, the stable circuits can control motor behaviors (e.g., hand or eye) quickly and precisely, which allows animals to automatically acquire valuable outcomes based on historical life experiences. These behaviors would be called 'skills', which are crucial for survival. The stable circuits are localized in the posterior part of the basal ganglia, separately from the flexible circuits located in the anterior part. To summarize, the flexible and stable circuits in the basal ganglia, working together but independently, enable animals (and humans) to reach valuable goals in various contexts.

Distinct temporal spike and local field potential activities in the thalamic parafascicular nucleus of parkinsonian rats during rest and limb movement

Several studies have suggested that the thalamic centromedian-parafascicular (CM/PF or the PF in rodents) is implicated in the pathophysiology of Parkinson's disease (PD). However, inconsistent changes in the neuronal firing rate and pattern have been reported in parkinsonian animals. To investigate the impact of a dopaminergic cell lesion on PF extracellular discharge in behaving rats, the PF neural activities in the spike and local field potential (LFP) were recorded in unilaterally 6-hydroxydopamine- (6-OHDA) lesioned and neurologically intact control rats during rest and limb movement. During rest, the two PF neuronal subtypes was less spontaneously active, with no difference in the spike firing rates between the control and lesioned rats; only the lesioned rats reshaped their spike firing pattern. Furthermore, the simultaneously recorded LFP in the lesioned rats exhibited a significant increase in power at 12-35 and 35-70Hz and a decrease in power at 0.7-12Hz. During the execution of a voluntary movement, two subtypes of PF neurons were identified by a rapid increase in the discharge activity in both the control and lesioned rats. However, dopamine lesioning was associated with a decrease in neuronal spiking fire rate and reshaping in the firing pattern in the PF. The simultaneously recorded LFP activity exhibited a significant increase in power at 12-35Hz and a decrease in power at 0.7-12Hz compared with the control rats. These findings indicate that 6-OHDA induces modifications in PF spike and LFP activities in rats during rest and movement and suggest that PF dysfunction may be an important contributor to the pathophysiology of parkinsonian motor impairment.

Participation of the thalamic CM-Pf complex in movement performance in patients with dystonia

INTRODUCTION

The centrum medianum- parafascicular complex of the human thalamus has a critical influence on cortical activity and significantly influences somatosensory function, arousal, and attention. In addition to its cortical connections, this region of the intralaminar thalamic nuclei is also connected to motor areas of the basal ganglia and the brain stem.

OBJECTIVE

The goal of this study was to identify movement-related neurons in the centrum medianum-parafascicular complex and analyze the changes in their activity during voluntary movements in patients with cervical dystonia.

METHODS

Single-unit activity was recorded during the micro-electrode-guided surgical ablation procedures in patients with cervical dystonia. The neural responses and synchronous electromyographic signals of the neck and finger flexor muscles were simultaneously recorded.

RESULTS

We found the following 3 types of movement-sensitive neurons in the centrum medianum-parafascicular complex: neurons that responded selectively to voluntary hand movement (hand-only neurons), neurons that selectively responded to neck movements (neck-only neurons), neurons responding to both hand and neck movements (combined neurons). We discovered the following 3 patterns of movement-related changes in neural activity: an increase in the firing rate, a reduction in the bursting activity, and short-term oscillations and synchronization with neighboring neurons. The most pronounced and prolonged responses were observed during movements involving neck muscles as well as during involuntary dystonic movements.

CONCLUSION

The centrum medianum-parafascicular complex of the thalamus is a component of the subcortical network that participates in motor behavior and may be involved in the pathophysiology of cervical dystonia. © 2016 International Parkinson and Movement Disorder Society.

Automatic target validation based on neuroscientific literature mining for tractography

Target identification for tractography studies requires solid anatomical knowledge validated by an extensive literature review across species for each seed structure to be studied. Manual literature review to identify targets for a given seed region is tedious and potentially subjective. Therefore, complementary approaches would be useful. We propose to use text-mining models to automatically suggest potential targets from the neuroscientific literature, full-text articles and abstracts, so that they can be used for anatomical connection studies and more specifically for tractography. We applied text-mining models to three structures: two well-studied structures, since validated deep brain stimulation targets, the internal globus pallidus and the subthalamic nucleus and, the nucleus accumbens, an exploratory target for treating psychiatric disorders. We performed a systematic review of the literature to document the projections of the three selected structures and compared it with the targets proposed by text-mining models, both in rat and primate (including human). We ran probabilistic tractography on the nucleus accumbens and compared the output with the results of the text-mining models and literature review. Overall, text-mining the literature could find three times as many targets as two man-weeks of curation could. The overall efficiency of the text-mining against literature review in our study was 98% recall (at 36% precision), meaning that over all the targets for the three selected seeds, only one target has been missed by text-mining. We demonstrate that connectivity for a structure of interest can be extracted from a very large amount of publications and abstracts. We believe this tool will be useful in helping the neuroscience community to facilitate connectivity studies of particular brain regions. The text mining tools used for the study are part of the HBP Neuroinformatics Platform, publicly available at http://connectivity-brainer.rhcloud.com/.

Turning a Negative into a Positive: Ascending GABAergic Control of Cortical Activation and Arousal

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the brain. Recent technological advances have illuminated the role of GABAergic neurons in control of cortical arousal and sleep. Sleep-promoting GABAergic neurons in the preoptic hypothalamus are well-known. Less well-appreciated are GABAergic projection neurons in the brainstem, midbrain, hypothalamus, and basal forebrain, which paradoxically promote arousal and fast electroencephalographic (EEG) rhythms. Thus, GABA is not purely a sleep-promoting neurotransmitter. GABAergic projection neurons in the brainstem nucleus incertus and ventral tegmental nucleus of Gudden promote theta (4-8 Hz) rhythms. Ventral tegmental area GABAergic neurons, neighboring midbrain dopamine neurons, project to the frontal cortex and nucleus accumbens. They discharge faster during cortical arousal and regulate reward. Thalamic reticular nucleus GABAergic neurons initiate sleep spindles in non-REM sleep. In addition, however, during wakefulness, they tonically regulate the activity of thalamocortical neurons. Other GABAergic inputs to the thalamus arising in the globus pallidus pars interna, substantia nigra pars reticulata, zona incerta, and basal forebrain regulate motor activity, arousal, attention, and sensory transmission. Several subpopulations of cortically projecting GABAergic neurons in the basal forebrain project to the thalamus and neocortex and preferentially promote cortical gamma-band (30-80 Hz) activity and wakefulness. Unlike sleep-active GABAergic neurons, these ascending GABAergic neurons are fast-firing neurons which disinhibit and synchronize the activity of their forebrain targets, promoting the fast EEG rhythms typical of conscious states. They are prominent targets of GABAergic hypnotic agents. Understanding the properties of ascending GABAergic neurons may lead to novel treatments for diseases involving disorders of cortical activation and wakefulness.

The functional connectivity of intralaminar thalamic nuclei in the human basal ganglia

Projections of the centromedian-parafasicularis neurons of the intralaminar thalamus are major inputs of the striatum. Their functional role in the activity of human basal ganglia (BG) is not well known. The aim of this work was to study the functional connectivity of intralaminar thalamic nuclei with other BG by using the correlations of the BOLD signal recorded during "resting" and a motor task. Intralaminar nuclei showed a marked functional connectivity with all the tested BG, which was observed during "resting" and did not change with the motor task. As regards the intralaminar nuclei, BG connectivity was much lower for the medial dorsal nucleus (a thalamic nucleus bordering the intralaminar nuclei) and for the default mode network (although intralaminar nuclei showed a negative correlation with the default mode network). After the "regression" of intralaminar nuclei activity (partial correlation), the functional connectivity of the caudate and putamen nuclei with other BG decreased (but not with the primary sensorimotor cortex). Present data provide evidence that intralaminar nuclei are not only critical for striatal activity but also for the global performance of human BG, an action involving subcortical BG loops more than cortico-subcortical loops. The high correlation found between BG suggest that, similarly to that reported in other brain centers, the very-slow frequency fluctuations are relevant for the functional activity of these centers.

Role of functional imaging in the development and refinement of invasive neuromodulation for psychiatric disorders

Deep brain stimulation (DBS) is emerging as a powerful tool for the alleviation of targeted symptoms in treatment-resistant neuropsychiatric disorders. Despite the expanding use of neuropsychiatric DBS, the mechanisms responsible for its effects are only starting to be elucidated. Several modalities such as quantitative electroencephalography as well a intraoperative recordings have been utilized to attempt to understand the underpinnings of this new treatment modality, but functional imaging appears to offer several unique advantages. Functional imaging techniques like positron emission tomography, single photon emission computed tomography and functional magnetic resonance imaging have been used to examine the effects of focal DBS on activity in a distributed neural network. These investigations are critical for advancing the field of invasive neuromodulation in a safe and effective manner, particularly in terms of defining the neuroanatomical targets and refining the stimulation protocols. The purpose of this review is to summarize the current functional neuroimaging findings from neuropsychiatric DBS implantation for three disorders: treatment-resistant depression, obsessive-compulsive disorder, and Tourette syndrome. All of the major targets will be discussed (Nucleus accumbens, anterior limb of internal capsule, subcallosal cingulate, Subthalamic nucleus, Centromedial nucleus of the thalamus-Parafasicular complex, frontal pole, and dorsolateral prefrontal cortex). We will also address some apparent inconsistencies within this literature, and suggest potential future directions for this promising area.

Cerebellum in levodopa-induced dyskinesias: the unusual suspect in the motor network

The exact mechanisms that generate levodopa-induced dyskinesias (LID) during chronic levodopa therapy for Parkinson’s disease (PD) are not yet fully established. The most widely accepted theories incriminate the non-physiological synthesis, release and reuptake of dopamine generated by exogenously administered levodopa in the striatum, and the aberrant plasticity in the cortico-striatal loops. However, normal motor performance requires the correct recruitment of motor maps. This depends on a high level of synergy within the primary motor cortex (M1) as well as between M1 and other cortical and subcortical areas, for which dopamine is necessary. The plastic mechanisms within M1, which are crucial for the maintenance of this synergy, are disrupted both during “OFF” and dyskinetic states in PD. When tested without levodopa, dyskinetic patients show loss of treatment benefits on long-term potentiation and long-term depression-like plasticity of the intracortical circuits. When tested with the regular pulsatile levodopa doses, they show further impairment of the M1 plasticity, such as inability to depotentiate an already facilitated synapse and paradoxical facilitation in response to afferent input aimed at synaptic inhibition. Dyskinetic patients have also severe impairment of the associative, sensorimotor plasticity of M1 attributed to deficient cerebellar modulation of sensory afferents to M1. Here, we review the anatomical and functional studies, including the recently described bidirectional connections between the cerebellum and the basal ganglia that support a key role of the cerebellum in the generation of LID. This model stipulates that aberrant neuronal synchrony in PD with LID may propagate from the subthalamic nucleus to the cerebellum and “lock” the cerebellar cortex in a hyperactive state. This could affect critical cerebellar functions such as the dynamic and discrete modulation of M1 plasticity and the matching of motor commands with sensory information from the environment during motor performance. We propose that in dyskinesias, M1 neurons have lost the ability to depotentiate an activated synapse when exposed to acute pulsatile, non-physiological, dopaminergic surges and become abnormally receptive to unfiltered, aberrant, and non-salient afferent inputs from the environment. The motor program selection in response to such non-salient and behaviorally irrelevant afferent inputs would be abnormal and involuntary. The motor responses are worsened by the lack of normal subcortico–cortical inputs from cerebellum and basal ganglia, because of the aberrant plasticity at their own synapses. Artificial cerebellar stimulation might help re-establish the cerebellar and basal ganglia control over the non-salient inputs to the motor areas during synaptic dopaminergic surges.

Effect of deep brain stimulation in rats selectively bred for reduced prepulse inhibition

BACKGROUND

Sensorimotor gating, measured as prepulse inhibition (PPI) of the acoustic startle reaction (ASR), is disturbed in certain neuropsychiatric disorders, such as schizophrenia, obsessive compulsive disorder, and Tourette's syndrome (TS). Deep brain stimulation (DBS) of the centromedian-parafascicular complex (CM-Pf), globus pallidus internus (in rats the entopeduncular nucleus - EPN), and the ventral striatum (in rats the nucleus accumbens - NAC) has been used for treatment in TS.

OBJECTIVE

We tested whether DBS of these regions would alleviate breeding-induced low PPI in rats.

METHODS

Rats with breeding-induced low and high PPI were bilaterally implanted with electrodes in the CM-Pf, the EPN, or the NAC. After two weeks, they were stimulated or sham stimulated for epochs of 6 days (in the EPN with a current of 20% below the individual threshold for stimulation-induced side effects, in the NAC or CM-Pf with 100 μA and 150 μA). On the 6th day the rats were tested for PPI of ASR.

RESULTS

Stimulation in the CM-Pf with 150 μA significantly alleviated PPI, while NAC stimulation was less effective. In PPI low rats electrode implantation in the EPN already improved PPI, while subsequent stimulation had no additional effect. Startle reaction of PPI low rats was not affected by stimulation of either region.

CONCLUSION

The CM-Pf and the EPN are important for the modulation of sensorimotor gating in rats with breeding-induced low PPI. These rats may therefore be useful to further investigate the pathophysiological mechanisms of deficient sensorimotor gating and also mechanisms of action of DBS in these circumstances.

The intralaminar thalamus-an expressway linking visual stimuli to circuits determining agency and action selection

Anatomical investigations have revealed connections between the intralaminar thalamic nuclei and areas such as the superior colliculus (SC) that receive short latency input from visual and auditory primary sensory areas. The intralaminar nuclei in turn project to the major input nucleus of the basal ganglia, the striatum, providing this nucleus with a source of subcortical excitatory input. Together with a converging input from the cerebral cortex, and a neuromodulatory dopaminergic input from the midbrain, the components previously found necessary for reinforcement learning in the basal ganglia are present. With this intralaminar sensory input, the basal ganglia are thought to play a primary role in determining what aspect of an organism's own behavior has caused salient environmental changes. Additionally, subcortical loops through thalamic and basal ganglia nuclei are proposed to play a critical role in action selection. In this mini review we will consider the anatomical and physiological evidence underlying the existence of these circuits. We will propose how the circuits interact to modulate basal ganglia output and solve common behavioral learning problems of agency determination and action selection.

Centromedian-parafascicular deep brain stimulation induces differential functional inhibition of the motor, associative, and limbic circuits in large animals

BACKGROUND

Deep brain stimulation (DBS) of the centromedian-parafascicular (CM-Pf) thalamic nuclei has been considered an option for treating Tourette syndrome. Using a large animal DBS model, this study was designed to explore the network effects of CM-Pf DBS.

METHODS

The combination of DBS and functional magnetic resonance imaging is a powerful means of tracing brain circuitry and testing the modulatory effects of electrical stimulation on a neuronal network in vivo. With a within-subjects design, we tested the proportional effects of CM and Pf DBS by manipulating current spread and varying stimulation contacts in healthy pigs (n = 5).

RESULTS

Our results suggests that CM-Pf DBS has an inhibitory modulating effect in areas that have been suggested as contributing to impaired sensory-motor and emotional processing. The results also help to define the differential neural circuitry effects of the CM and Pf with evidence of prominent sensorimotor/associative effects for CM DBS and prominent limbic/associative effects for Pf DBS.

CONCLUSIONS

Our results support the notion that stimulation of deep brain structures, such as the CM-Pf, modulates multiple networks with cortical effects. The networks affected by CM-Pf stimulation in this study reinforce the conceptualization of Tourette syndrome as a condition with psychiatric and motor symptoms and of CM-Pf DBS as a potentially effective tool for treating both types of symptoms.

Deep brain stimulation for Tourette syndrome

Gilles de la Tourette syndrome is a movement disorder characterized by repetitive stereotyped motor and phonic movements with varying degrees of psychiatric comorbidity. Deep brain stimulation (DBS) has emerged as a novel therapeutic intervention for patients with refractory Tourette syndrome. Since 1999, more than 100 patients have undergone DBS at various targets within the corticostriatothalamocortical network thought to be implicated in the underlying pathophysiology of Tourette syndrome. Future multicenter clinical trials and the use of a centralized online database to compare the results are necessary to determine the efficacy of DBS for Tourette syndrome.

Frontal-thalamic circuits associated with language

Thalamic nuclei associated with language including the ventral lateral, ventral anterior, intralaminar and mediodorsal form a hub that uniquely receives the output of the basal ganglia and cerebellum, and is connected with frontal (premotor and prefrontal) cortices through two parallel circuits: a thalamic pathway targets the middle frontal cortical layers focally, and the other innervates widely cortical layer 1, poised to recruit other cortices and thalamic nuclei for complex cognitive operations. Return frontal pathways to the thalamus originate from cortical layers 6 and 5. Information through this integrated thalamo-cortical system is gated by the inhibitory thalamic reticular nucleus and modulated by dopamine, representing a specialization in primates. The intricate dialogue of distinct thalamic nuclei with the basal ganglia, cerebellum, and specific dorsolateral prefrontal and premotor cortices associated with language, suggests synergistic roles in the complex but seemingly effortless sequential transformation of cognitive operations for speech production in humans.

Biology of Parkinson's disease: pathogenesis and pathophysiology of a multisystem neurodegenerative disorder

Parkinson's disease (PD) is the second most common movement disorder. The characteristic motor impairments - bradykinesia, rigidity, and resting tremor - result from degenerative loss of midbrain dopamine (DA) neurons in the substantia nigra, and are responsive to symptomatic treatment with dopaminergic medications and functional neurosurgery. PD is also the second most common neurodegenerative disorder. Viewed from this perspective, PD is a disorder of multiple functional systems, not simply the motor system, and of multiple neurotransmitter systems, not merely that of DA. The characteristic pathology - intraneuronal Lewy body inclusions and reduced numbers of surviving neurons - is similar in each of the targeted neuron groups, suggesting a common neurodegenerative process. Pathological and experimental studies indicate that oxidative stress, proteolytic stress, and inflammation figure prominently in the pathogenesis of PD. Yet, whether any of these mechanisms plays a causal role in human PD is unknown, because to date we have no proven neuroprotective therapies that slow or reverse disease progression in patients with PD. We are beginning to understand the pathophysiology of motor dysfunction in PD, but its etiopathogenesis as a neurodegenerative disorder remains poorly understood.

Deep brain stimulation for Tourette syndrome: target selection

BACKGROUND/AIMS

Tourette syndrome (TS) is a complex neurological disorder manifested chiefly by motor and phonic tics and a variety of behavioral comorbidities, including attention disorder, obsessive-compulsive disorder and impulse control problems. Surgical treatment is increasingly considered when tics become troublesome or even disabling or self-injurious despite optimal medical therapy. In this review, we describe the surgical techniques, stimulation parameters, outcomes of deep brain stimulation (DBS) in TS, and critically review target choices.

METHODS

A search of the PubMed database was performed to identify all articles discussing DBS and TS. 'Tourette' and 'Stimulation' were used as MeSH headings.

RESULTS

Since the first report of thalamic DBS for TS in 1999, follow-up on less than 100 patients has been reported in the literature. Reported targets for DBS include the thalamic centromedian nucleus and substantia periventricularis, posteroventral globus pallidus internus, ventromedial globus pallidus internus, globus pallidus externus, anterior limb of the internal capsule and nucleus accumbens.

CONCLUSIONS

Determination of the optimal surgical target will require a multicenter, randomized trial, and an expanded understanding of the neurobiology of TS.

Acetylcholine-based entropy in response selection: a model of how striatal interneurons modulate exploration, exploitation, and response variability in decision-making

The basal ganglia play a fundamental role in decision-making. Their contribution is typically modeled within a reinforcement learning framework, with the basal ganglia learning to select the options associated with highest value and their dopamine inputs conveying performance feedback. This basic framework, however, does not account for the role of cholinergic interneurons in the striatum, and does not easily explain certain dynamic aspects of decision-making and skill acquisition like the generation of exploratory actions. This paper describes basal ganglia acetylcholine-based entropy (BABE), a model of the acetylcholine system in the striatum that provides a unified explanation for these phenomena. According to this model, cholinergic interneurons in the striatum control the level of variability in behavior by modulating the number of possible responses that are considered by the basal ganglia, as well as the level of competition between them. This mechanism provides a natural way to account for the role of basal ganglia in generating behavioral variability during the acquisition of certain cognitive skills, as well as for modulating exploration and exploitation in decision-making. Compared to a typical reinforcement learning model, BABE showed a greater modulation of response variability in the face of changes in the reward contingences, allowing for faster learning (and re-learning) of option values. Finally, the paper discusses the possible applications of the model to other domains.

The effects of nigrostriatal dopamine depletion on the thalamic parafascicular nucleus

Neuronal loss in Parkinson's disease (PD) is seen in a number of brain regions in addition to the substantia nigra (SN). Among these is the thalamic parafascicular nucleus (PF), which sends glutamatergic projections to the striatum and receives GABAergic inputs from the SN. Recent data suggest that lesions of nigrostriatal dopamine axons cause a loss of PF neurons, which has been interpreted to suggest that the PF cell loss seen in PD is secondary to dopamine denervation. However, the extent of a PF dopamine innervation in the rat is unclear, and it is possible that PF cell loss in parkinsonism is independent of nigrostriatal dopamine degeneration. We characterized the dopamine innervation of the PF in the rat and determined if 6-hydroxydopamine SN lesions cause PF neuron degeneration. Dual-label immunohistochemistry revealed that almost all tyrosine hydroxylase-immunoreactive (TH-ir) axons in the PF also expressed dopamine-beta-hydroxylase and were therefore noradrenergic or adrenergic. Moreover, an antibody directed against dopamine revealed only very rare PF dopaminergic axons. Retrograde-tract tracing-immunohistochemistry did not uncover an innervation of the PF from midbrain dopamine neurons. Nigrostriatal dopamine neuron lesions did not elicit degeneration of PF cells, as reflected by a lack of FluoroJade C staining. Similarly, neither unilateral 6-OHDA lesions of nigrostriatal axons nor the dorsal noradrenergic bundle decreased the number of PF neurons or the number of PF neurons retrogradely-labeled from the striatum. These data suggest that the loss of thalamostriatal PF neurons in Parkinson's Disease is a primary event rather than secondary to nigrostriatal dopamine degeneration.

Thalamic contributions to Basal Ganglia-related behavioral switching and reinforcement

Although the existence of prominent connections between the intralaminar thalamic nuclei and the basal ganglia has long been established, the limited knowledge of the functional relevance of this network has considerably hampered progress in our understanding of the neural mechanisms by which the thalamostriatal system integrates and regulates the basal ganglia circuitry. In this brief commentary, we will address this gap of knowledge through a discussion of the key points of a symposium entitled "Thalamic Contributions to Basal Ganglia-Related Behavioral Switching and Reinforcement" that will be presented at the 2011 Society for Neuroscience meeting. Recent anatomical and physiological data that support the role of the thalamostriatal system in action selection, attentional shifting, and reinforcement will be discussed. We will also address the possibility that degeneration of the thalamostriatal system could underlie some of the deficits in redirection of attention in response to salient stimuli seen in Parkinson's disease.

Cognitive consilience: primate non-primary neuroanatomical circuits underlying cognition

Interactions between the cerebral cortex, thalamus, and basal ganglia form the basis of cognitive information processing in the mammalian brain. Understanding the principles of neuroanatomical organization in these structures is critical to understanding the functions they perform and ultimately how the human brain works. We have manually distilled and synthesized hundreds of primate neuroanatomy facts into a single interactive visualization. The resulting picture represents the fundamental neuroanatomical blueprint upon which cognitive functions must be implemented. Within this framework we hypothesize and detail 7 functional circuits corresponding to psychological perspectives on the brain: consolidated long-term declarative memory, short-term declarative memory, working memory/information processing, behavioral memory selection, behavioral memory output, cognitive control, and cortical information flow regulation. Each circuit is described in terms of distinguishable neuronal groups including the cerebral isocortex (9 pyramidal neuronal groups), parahippocampal gyrus and hippocampus, thalamus (4 neuronal groups), basal ganglia (7 neuronal groups), metencephalon, basal forebrain, and other subcortical nuclei. We focus on neuroanatomy related to primate non-primary cortical systems to elucidate the basis underlying the distinct homotypical cognitive architecture. To display the breadth of this review, we introduce a novel method of integrating and presenting data in multiple independent visualizations: an interactive website (http://www.frontiersin.org/files/cognitiveconsilience/index.html) and standalone iPhone and iPad applications. With these tools we present a unique, annotated view of neuroanatomical consilience (integration of knowledge).

The primate thalamostriatal systems: Anatomical organization, functional roles and possible involvement in Parkinson's disease

The striatum receives glutamatergic inputs from two main thalamostriatal systems that originate either from the centre median/parafascicular complex (CM/PF-striatal system) or the rostral intralaminar, midline, associative and relay thalamic nuclei (non-CM/PF-striatal system). These dual thalamostriatal systems display striking differences in their anatomical and, most likely, functional organization. The CM/PF-striatal system is topographically organized, and integrated within functionally segregated basal ganglia-thalamostriatal circuits that process sensorimotor, associative and limbic information. CM/PF neurons are highly responsive to attention-related sensory stimuli, suggesting that the CM/PF-striatal system, through its strong connections with cholinergic interneurons, may play a role in basal ganglia-mediated learning, behavioral switching and reinforcement. In light of evidence for prominent CM/PF neuronal loss in Parkinson's disease, we propose that the significant CM-striatal system degeneration, combined with the severe nigrostriatal dopamine loss in sensorimotor striatal regions, may alter normal automatic actions, and shift the processing of basal ganglia-thalamocortical motor programs towards goal-directed behaviors.

Probing basal ganglia functions by saccade eye movements

The basal ganglia (BG) are a group of subcortical structures involved in diverse functions, such as motor, cognition and emotion. However, the BG do not control these functions directly, but rather modulate functional processes occurring in structures outside the BG. The BG form multiple functional loops, each of which controls different functions with similar architectures. Accordingly, to understand the modulatory role of the BG, it is strategic to uncover the mechanisms of signal processing within specific functional loops that control simple neural circuits outside the BG, and then extend the knowledge to other BG loops. The saccade control system is one of the best-understood neural circuits in the brain. Furthermore, sophisticated saccade paradigms have been used extensively in clinical research in patients with BG disorders as well as in basic research in behaving monkeys. In this review, we describe recent advances of BG research from the viewpoint of saccade control. Specifically, we account for experimental results from neuroimaging and clinical studies in humans based on the updated knowledge of BG functions derived from neurophysiological experiments in behaving monkeys by taking advantage of homologies in saccade behavior. It has become clear that the traditional BG network model for saccade control is too limited to account for recent evidence emerging from the roles of subcortical nuclei not incorporated in the model. Here, we extend the traditional model and propose a new hypothetical framework to facilitate clinical and basic BG research and dialogue in the future.

Reduced striatal acetylcholine efflux in the R6/2 mouse model of Huntington's disease: an examination of the role of altered inhibitory and excitatory mechanisms

Huntington's disease (HD) is a genetic neurodegenerative disorder that is characterized by the progressive onset of cognitive, psychiatric, and motor symptoms. In parallel, the neuropathology of HD is characterized by progressive loss of projection neurons in cortex and striatum; striatal cholinergic interneurons are relatively spared. Nonetheless, there is evidence that striatal acetylcholine (ACh) function is altered in HD. The present study is the first to examine striatal ACh function in awake, behaving animals, using the R6/2 mouse model of HD, which is transgenic for exon 1 of the mutant huntingtin gene. Physiological levels of extracellular striatal ACh were monitored in R6/2 mice and wild type controls using in vivo microdialysis. Results indicate that spontaneous ACh release is reduced in R6/2 mice relative to controls. Intrastriatal application of the GABA(A) antagonist bicuculline methiodide (10.0 μM) significantly elevated ACh levels in both R6/2 mice and wild type controls, while overall ACh levels were reduced in the R6/2 mice compared to the wild type group. In contrast, systemic administration of the D(1) dopamine receptor partial agonist, SKF-38393 (10.0mg/kg, IP), elevated ACh levels in control animals, but not R6/2 mice. Taken together, the present results suggest that GABA-mediated inhibition of striatal ACh release is intact in R6/2 mice, further demonstrating that cholinergic interneurons are capable of increased ACh release, whereas D(1) receptor-dependent activation of excitatory inputs to striatal cholinergic interneurons is dysfunctional in R6/2 mice. Reduced levels of extracellular striatal ACh in HD may reflect abnormalities in the excitatory innervation of cholinergic interneurons, which may have implications ACh-dependent processes that are altered in HD, including corticostriatal plasticity.

Pallidothalamic-projecting neurons in Macaca fascicularis co-express GABAergic and glutamatergic markers as seen in control, MPTP-treated and dyskinetic monkeys

GABAergic neurons within the internal division of the globus pallidus (GPi) are the main source of basal ganglia output reaching the thalamic ventral nuclei in monkeys. Following dopaminergic denervation, pallidothalamic-projecting neurons are known to be hyperactive, whereas a reduction in GPi activity is typically observed in lesioned animals showing levodopa-induced dyskinesia. Besides the mRNAs coding for GABAergic markers (GAD65 and GAD67), we show that all GPi neurons innervating thalamic targets also express transcripts for the isoforms 1 and 2 of the vesicular glutamate transporter (vGlut1 and vGlut2 mRNA). Indeed, dual immunofluorescent detection of GAD67 and vGlut1/2 confirmed the data gathered from in situ hybridization experiments, therefore demonstrating that the detected mRNAs are translated into the related proteins. Furthermore, the dopaminergic lesion resulted in an up-regulation of expression levels for both GAD65 and GAD67 mRNA within identified pallidothalamic-projecting neurons. This was coupled with a down-regulation of GAD65/67 mRNA expression levels in GPi neurons innervating thalamic targets in monkeys showing levodopa-induced dyskinesia. By contrast, the patterns of gene expression for both vGlut1 and vGlut2 mRNAs remained unchanged across GPi projection neurons in control, MPTP-treated and dyskinetic monkeys. In summary, both GABAergic and glutamatergic markers were co-expressed by GPi efferent neurons in primates. Although the status of the dopaminergic system directly modulates the expression levels of GAD65/67 mRNA, the observed expression of vGlut1/2 mRNA is not regulated by either dopaminergic removal or by continuous stimulation with dopaminergic agonists.

Nicotine enhances but does not normalize visual sustained attention and the associated brain network in schizophrenia

Sustained attention abnormality in schizophrenia is usually refractory to available treatment. Nicotine can transiently improve sustained attention in schizophrenia patients, although its neural mechanisms are unknown. Understanding the neural basis of this effect may lead to new treatment strategies for this cognitive deficit. Twenty schizophrenia patients and 24 healthy comparison smokers participated in a double-blind, placebo-controlled, crossover, randomized functional magnetic resonance imaging study comparing nicotine vs placebo patch on sustained attention, using the rapid visual information-processing task. Schizophrenia patients had impaired visual sustained attention accuracy and processing speed (all P's <.001) and showed significantly reduced activation in the frontal-parietal-cingulate-thalamic attention network compared with healthy comparison subjects. Nicotine administration enhanced accuracy and processing speed compared with placebo (all P's ≤.006), with no drug × diagnosis interactions. However, schizophrenia patients' task performance remained impaired during the nicotine condition, even when compared with healthy comparison subjects in the placebo condition (all P's ≤.01). Nicotine exerted no significant reversal of the impaired attention network associated with schizophrenia. Activations in brain regions associated with nicotine-induced behavioral improvement were not significantly different between patients and comparison subjects. Thus, nicotine transiently enhanced sustained attention similarly in schizophrenia patients and in healthy comparison smokers. The neural mechanisms for this nicotinic effect in schizophrenia appear similar to those for healthy comparison subjects. However, nicotine, at least in a single sustained dose, does not normalize impaired sustained attention and its associated brain network in schizophrenia. These findings provide guidance for developing new treatment strategies for the sustained attention deficit in schizophrenia.

Basal Ganglia circuits underlying the pathophysiology of levodopa-induced dyskinesia

Involuntary movements or dyskinesia, represent a debilitating complication of levodopa therapy for Parkinson's disease. Dyskinesia is, ultimately, experienced by the vast majority of the patients. Despite the importance of this problem, little was known about the cause of dyskinesia, a situation that has dramatically evolved in the last few years with a focus upon the molecular and signaling changes induced by chronic levodopa treatment. Departing from this, we here review the progress made in functional anatomy and neuroimaging that have had a tremendous impact on our understanding of the anatomo-functional organization of the basal ganglia in Parkinsonism and dyskinetic states, notably the demonstration that dyskinesia are linked to a pathological processing of limbic and cognitive information.

Deep brain stimulation of the center median-parafascicular complex of the thalamus has efficient anti-parkinsonian action associated with widespread cellular responses in the basal ganglia network in a rat model of Parkinson's disease

The thalamic centromedian-parafascicular (CM/Pf) complex, mainly represented by Pf in rodents, is proposed as an interesting target for the neurosurgical treatment of movement disorders, including Parkinson's disease. In this study, we examined the functional impact of subchronic high-frequency stimulation (HFS) of Pf in the 6-hydroxydopamine-lesioned hemiparkinsonian rat model. Pf-HFS had significant anti-akinetic action, evidenced by alleviation of limb use asymmetry (cylinder test). Whereas this anti-akinetic action was moderate, Pf-HFS totally reversed lateralized neglect (corridor task), suggesting potent action on sensorimotor integration. At the cellular level, Pf-HFS partially reversed the dopamine denervation-induced increase in striatal preproenkephalin A mRNA levels, a marker of the neurons of the indirect pathway, without interfering with the markers of the direct pathway (preprotachykinin and preprodynorphin). Pf-HFS totally reversed the lesion-induced changes in the gene expression of cytochrome oxidase subunit I in the subthalamic nucleus, the globus pallidus, and the substantia nigra pars reticulata, and partially in the entopeduncular nucleus. Unlike HFS of the subthalamic nucleus, Pf-HFS did not induce per se dyskinesias and directly, although partially, alleviated L-3,4-dihydroxyphenylalanine (L-DOPA)-induced forelimb dyskinesia. Conversely, L-DOPA treatment negatively interfered with the anti-parkinsonian effect of Pf-HFS. Altogether, these data show that Pf-DBS, by recruiting a large basal ganglia circuitry, provides moderate to strong anti-parkinsonian benefits that might, however, be affected by L-DOPA. The widespread behavioral and cellular outcomes of Pf-HFS evidenced here demonstrate that CM/Pf is an important node for modulating the pathophysiological functioning of basal ganglia and related disorders.

Basal ganglia control of substantia nigra dopaminergic neurons

Although substantia nigra dopaminergic neurons are spontaneously active both in vivo and in vitro, this activity does not depend on afferent input as these neurons express an endogenous calcium-dependent oscillatory mechanism sufficient to drive action potential generation. However, afferents to these neurons, a large proportion of them GABAergic and arising from other nuclei in the basal ganglia, play a crucial role in modulating the activity of dopaminergic neurons. In the absence of afferent activity or when in brain slices, dopaminergic neurons fire in a very regular, pacemaker-like mode. Phasic activity in GABAergic, glutamatergic, and cholinergic inputs modulates the pacemaker activity into two other modes. The most common is a random firing pattern in which interspike intervals assume a Poisson-like distribution, and a less common pattern, often in response to a conditioned stimulus or a reward in which the neurons fire bursts of 2-8 spikes time-locked to the stimulus. Typically in vivo, all three firing patterns are observed, intermixed, in single nigrostriatal neurons varying over time. Although the precise mechanism(s) underlying the burst are currently the focus of intensive study, it is obvious that bursting must be triggered by afferent inputs. Most of the afferents to substantia nigra pars compacta dopaminergic neurons comprise monosynaptic inputs from GABAergic projection neurons in the ipsilateral neostriatum, the globus pallidus, and the substantia nigra pars reticulata. A smaller fraction of the basal ganglia inputs, something less than 30%, are glutamatergic and arise principally from the ipsilateral subthalamic nucleus and pedunculopontine nucleus. The pedunculopontine nucleus also sends a cholinergic input to nigral dopaminergic neurons. The GABAergic pars reticulata projection neurons also receive inputs from all of these sources, in some cases relaying them disynaptically to the dopaminergic neurons, thereby playing a particularly significant role in setting and/or modulating the firing pattern of the nigrostriatal neurons.

A key role of the basal ganglia in pain and analgesia--insights gained through human functional imaging

The basal ganglia (BG) are composed of several nuclei involved in neural processing related to the execution of motor, cognitive and emotional activities. Preclinical and clinical data have implicated a role for these structures in pain processing. Recently neuroimaging has added important information on BG activation in conditions of acute pain, chronic pain and as a result of drug effects. Our current understanding of alterations in cortical and sub-cortical regions in pain suggests that the BG are uniquely involved in thalamo-cortico-BG loops to integrate many aspects of pain. These include the integration of motor, emotional, autonomic and cognitive responses to pain.