Pathological basal ganglia activity in movement disorders

Dystonia and dopamine: From phenomenology to pathophysiology

A line of evidence suggests that the pathophysiology of dystonia involves the striatum, whose activity is modulated among other neurotransmitters, by the dopaminergic system. However, the link between dystonia and dopamine appears complex and remains unclear. Here, we propose a physiological approach to investigate the clinical and experimental data supporting a role of the dopaminergic system in the pathophysiology of dystonic syndromes. Because dystonia is a disorder of motor routines, we first focus on the role of dopamine and striatum in procedural learning. Second, we consider the phenomenology of dystonia from every angle in order to search for features giving food for thought regarding the pathophysiology of the disorder. Then, for each dystonic phenotype, we review, when available, the experimental and imaging data supporting a connection with the dopaminergic system. Finally, we propose a putative model in which the different phenotypes could be explained by changes in the balance between the direct and indirect striato-pallidal pathways, a process critically controlled by the level of dopamine within the striatum. Search strategy and selection criteria References for this article were identified through searches in PubMed with the search terms « dystonia », « dopamine", « striatum », « basal ganglia », « imaging data », « animal model », « procedural learning », « pathophysiology », and « plasticity » from 1998 until 2018. Articles were also identified through searches of the authors' own files. Only selected papers published in English were reviewed. The final reference list was generated on the basis of originality and relevance to the broad scope of this review.

Changing views of the pathophysiology of Parkinsonism

Studies of the pathophysiology of parkinsonism (specifically akinesia and bradykinesia) have a long history and primarily model the consequences of dopamine loss in the basal ganglia on the function of the basal ganglia/thalamocortical circuit(s). Changes of firing rates of individual nodes within these circuits were originally considered central to parkinsonism. However, this view has now given way to the belief that changes in firing patterns within the basal ganglia and related nuclei are more important, including the emergence of burst discharges, greater synchrony of firing between neighboring neurons, oscillatory activity patterns, and the excessive coupling of oscillatory activities at different frequencies. Primarily focusing on studies obtained in nonhuman primates and human patients with Parkinson's disease, this review summarizes the current state of this field and highlights several emerging areas of research, including studies of the impact of the heterogeneity of external pallidal neurons on parkinsonism, the importance of extrastriatal dopamine loss, parkinsonism-associated synaptic and morphologic plasticity, and the potential role(s) of the cerebellum and brainstem in the motor dysfunction of Parkinson's disease. © 2019 International Parkinson and Movement Disorder Society.

State transitions in the substantia nigra reticulata predict the onset of motor deficits in models of progressive dopamine depletion in mice

Parkinson's disease (PD) is a progressive neurodegenerative disorder whose cardinal motor symptoms are attributed to dysfunction of basal ganglia circuits under conditions of low dopamine. Despite well-established physiological criteria to define basal ganglia dysfunction, correlations between individual parameters and motor symptoms are often weak, challenging their predictive validity and causal contributions to behavior. One limitation is that basal ganglia pathophysiology is studied only at end-stages of depletion, leaving an impoverished understanding of when deficits emerge and how they evolve over the course of depletion. In this study, we use toxin- and neurodegeneration-induced mouse models of dopamine depletion to establish the physiological trajectory by which the substantia nigra reticulata (SNr) transitions from the healthy to the diseased state. We find that physiological progression in the SNr proceeds in discrete state transitions that are highly stereotyped across models and correlate well with the prodromal and symptomatic stages of behavior.

Correlative study using structural MRI and super-resolution microscopy to detect structural alterations induced by long-term optogenetic stimulation of striatal medium spiny neurons

Striatal medium spiny neurons (MSNs) control motor function. Hyper- or hypo-activity of MSNs coincides with basal ganglia-related movement disorders. Based on the assumption that lasting alterations in neuronal activity lead to structural changes in the brain, understanding these structural alterations may be used to infer MSN functional abnormalities. To infer MSN function from structural data, understanding how long-lasting alterations in MSN activity affect brain morphology is essential. To address this, we utilized a simplified model of functional induction by stimulating MSNs expressing channelrhodopsin 2 (ChR2). Subsequent structural alterations which induced long-term activity changes in these MSNs were investigated in the striatal pathway and its associated regions by diffusion tensor imaging (DTI) and histological assessment with super-resolution microscopy. DTI detected changes in the striatum, substantia nigra, and motor cortex. Histological assessment found a reduction in the diameter of myelinated cortical axons as well as MSN dendrites and axons. The structural changes showed a high correlation between DTI parameters and histological data. These results demonstrated that long-term neural activation in the MSNs alters the diameter of MSN and cortical neurons fibers. This study provides a tool for understanding the causal relationship between functional and structural alterations.

Mapping of excitatory and inhibitory postsynaptic potentials of neuronal populations in hippocampal slices using the GEVI, ArcLight

To understand the circuitry of the brain, it is essential to clarify the functional connectivity among distinct neuronal populations. For this purpose, neuronal activity imaging using genetically-encoded calcium sensors such as GCaMP has been a powerful approach due to its cell-type specificity. However, calcium (Ca²⁺) is an indirect measure of neuronal activity. A more direct approach would be to use genetically encoded voltage indicators (GEVIs) to observe subthreshold, synaptic activities. The GEVI, ArcLight, which exhibits large fluorescence transients in response to voltage, was expressed in excitatory neurons of the mouse CA1 hippocampus. Fluorescent signals in response to the electrical stimulation of the Schaffer collateral axons were observed in brain slice preparations. ArcLight was able to map both excitatory and inhibitory inputs projected to excitatory neurons. In contrast, the Ca²⁺ signal detected by GCaMP6f, was only associated with excitatory inputs. ArcLight and similar voltage sensing probes are also becoming powerful paradigms for functional connectivity mapping of brain circuitry.

Properties of oscillatory neuronal activity in the basal ganglia and thalamus in patients with Parkinson's disease

Background

The cardinal features of Parkinson’s disease (PD) are bradykinesia, rigidity and rest tremor. Abnormal activity in the basal ganglia is predicted to underlie the mechanism of motor symptoms. This study aims to characterize properties of oscillatory activity in the basal ganglia and motor thalamus in patients with PD.

Methods

Twenty-nine patients with PD who underwent bilateral or unilateral electrode implantation for subthalamic nucleus (STN) DBS (n = 11), unilateral pallidotomy (n = 9) and unilateral thalamotomy (n = 9) were studied. Microelectrode recordings in the STN, globus pallidus internus (GPi) and ventral oral posterior/ventral intermediate of thalamus (Vop/Vim) were performed. Electromyography of the contralateral limbs was recorded. Single unit characteristics including interspike intervals were analyzed. Spectral and coherence analyses were assessed. Mean spontaneous firing rate (MSFR) of neurons was calculated. Analysis of variance and X² test were performed.

Results

Of 76 STN neurons, 39.5% were 4–6 Hz band oscillatory neurons and 28.9% were β frequency band (βFB) oscillatory neurons. The MSFR was 44.2 ± 7.6 Hz. Of 62 GPi neurons, 37.1% were 4–6 Hz band oscillatory neurons and 27.4% were βFB neurons. The MSFR was 80.9 ± 9.6 Hz. Of 44 Vop neurons, 65.9% were 4–6 Hz band oscillatory neurons and 9% were βFB neurons. The MSFR was 24.4 ± 4.2 Hz. Of 30 Vim oscillatory neurons, 70% were 4–6 Hz band oscillatory neurons and 13.3% were βFB neurons. The MSFR was 30.3 ± 3.6 Hz. Further analysis indicated that proportion of βFB oscillatory neurons in STN and GPi was higher than that of similar neurons in the Vop and Vim (P < 0.05). Conversely, the proportion of 4–6 Hz band oscillatory neurons and tremor related neurons in the Vim and Vop was higher than that of STN and GPi (P < 0.05). The highest MSFR was for GPi oscillatory neurons whereas the lowest MSFR was for Vop oscillatory neurons (P < 0.005).

Conclusion

The alterations in neuronal activity in basal ganglia play a critical role in generation of parkinsonism. β oscillatory activity is more prominent in basal ganglia than in thalamus suggesting that the activity likely results from dopaminergic depletion. While both basal ganglia and thalamus have tremor activity, the thalamus appears to play a more important role in tremor production, and basal ganglia β oscillatory activity might be the trigger.

NMDA receptor blockade ameliorates abnormalities of spike firing of subthalamic nucleus neurons in a parkinsonian nonhuman primate

N-methyl-D-aspartate receptors (NMDARs) are ion channels comprising tetrameric assemblies of GluN1 and GluN2 receptor subunits that mediate excitatory neurotransmission in the central nervous system. Of the four different GluN2 subunits, the GluN2D subunit-containing NMDARs have been suggested as a target for antiparkinsonian therapy because of their expression pattern in some of the basal ganglia nuclei that show abnormal firing patterns in the parkinsonian state, specifically the subthalamic nucleus (STN). In this study, we demonstrate that blockade of NMDARs altered spike firing in the STN in a male nonhuman primate that had been rendered parkinsonian by treatment with the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. In accompanying experiments in male rodents, we found that GluN2D-NMDAR expression in the STN was reduced in acutely or chronically dopamine-depleted animals. Taken together, our data suggest that blockade of NMDARs in the STN may be a viable antiparkinsonian strategy, but that the ultimate success of this approach may be complicated by parkinsonism-associated changes in NMDAR expression in the STN.

The neural circuitry of restricted repetitive behavior: Magnetic resonance imaging in neurodevelopmental disorders and animal models

Restricted, repetitive behaviors (RRBs) are patterns of behavior that exhibit little variation in form and have no obvious function. RRBs although transdiagonstic are a particularly prominent feature of certain neurodevelopmental disorders, yet relatively little is known about the neural circuitry of RRBs. Past work in this area has focused on isolated brain regions and neurotransmitter systems, but implementing a neural circuit approach has the potential to greatly improve understanding of RRBs. Magnetic resonance imaging (MRI) is well-suited to studying the structural and functional connectivity of the nervous system, and is a highly translational research tool. In this review, we synthesize MRI research from both neurodevelopmental disorders and relevant animal models that informs the neural circuitry of RRB. Together, these studies implicate distributed neural circuits between the cortex, basal ganglia, and cerebellum. Despite progress in neuroimaging of RRB, there are many opportunities for conceptual and methodological improvement. We conclude by suggesting future directions for MRI research in RRB, and how such studies can benefit from complementary approaches in neuroscience.

Segmentation of the Globus Pallidus Internus Using Probabilistic Diffusion Tractography for Deep Brain Stimulation Targeting in Parkinson Disease

BACKGROUND AND PURPOSE

Although globus pallidus internus deep brain stimulation is a widely accepted treatment for Parkinson disease, there is persistent variability in outcomes that is not yet fully understood. In this pilot study, we aimed to investigate the potential role of globus pallidus internus segmentation using probabilistic tractography as a supplement to traditional targeting methods.

MATERIALS AND METHODS

Eleven patients undergoing globus pallidus internus deep brain stimulation were included in this retrospective analysis. Using multidirection diffusion-weighted MR imaging, we performed probabilistic tractography at all individual globus pallidus internus voxels. Each globus pallidus internus voxel was then assigned to the 1 ROI with the greatest number of propagated paths. On the basis of deep brain stimulation programming settings, the volume of tissue activated was generated for each patient using a finite element method solution. For each patient, the volume of tissue activated within each of the 10 segmented globus pallidus internus regions was calculated and examined for association with a change in the Unified Parkinson Disease Rating Scale, Part III score before and after treatment.

RESULTS

Increasing volume of tissue activated was most strongly correlated with a change in the Unified Parkinson Disease Rating Scale, Part III score for the primary motor region (Spearman r = 0.74, P = .010), followed by the supplementary motor area/premotor cortex (Spearman r = 0.47, P = .15).

CONCLUSIONS

In this pilot study, we assessed a novel method of segmentation of the globus pallidus internus based on probabilistic tractography as a supplement to traditional targeting methods. Our results suggest that our method may be an independent predictor of deep brain stimulation outcome, and evaluation of a larger cohort or prospective study is warranted to validate these findings.

Dopamine Induces Oscillatory Activities in Human Midbrain Neurons with Parkin Mutations

Locomotor symptoms in Parkinson's disease (PD) are accompanied by widespread oscillatory neuronal activities in basal ganglia. Here, we show that activation of dopamine D1-class receptors elicits a large rhythmic bursting of spontaneous excitatory postsynaptic currents (sEPSCs) in midbrain neurons differentiated from induced pluripotent stem cells (iPSCs) of PD patients with parkin mutations, but not normal subjects. Overexpression of wild-type parkin, but not its PD-causing mutant, abolishes the oscillatory activities in patient neurons. Dopamine induces a delayed enhancement in the amplitude of spontaneous, but not miniature, EPSCs, thus increasing quantal content. The results suggest that presynaptic regulation of glutamatergic transmission by dopamine D1-class receptors is significantly potentiated by parkin mutations. The aberrant dopaminergic regulation of presynaptic glutamatergic transmission in patient-specific iPSC-derived midbrain neurons provides a mechanistic clue to PD pathophysiology, and it demonstrates the usefulness of this model system in understanding how mutations of parkin cause movement symptoms in Parkinson's disease.

The cerebellum and dystonia

Dystonia is a heterogeneous disorder characterized by involuntary muscle contractions, twisting movements, and abnormal postures in various body regions. It is widely accepted that the basal ganglia are involved in the pathogenesis of dystonia. A growing body of evidence, however, is challenging the traditional view and suggest that the cerebellum may also play a role in dystonia. Studies on animals indicate that experimental manipulations of the cerebellum lead to dystonic-like movements. Several clinical observations, including those from secondary dystonia cases as well as neurophysiologic and neuroimaging studies in human patients, provide further evidence in humans of a possible relationship between cerebellar abnormalities and dystonia. Claryfing the role of the cerebellum in dystonia is an important step towards providing alternative treatments based on noninvasive brain stimulation techniques.

Effect of optogenetic modulation on entopeduncular input affects thalamic discharge and behavior in an AAV2-α-synuclein-induced hemiparkinson rat model

OBJECTIVE

Neuromodulation of the globus pallidus internus(GPi) alleviates Parkinson's disease symptoms. The primate GPi is homologous to the rat entopeduncular nucleus (EP). The aim of the present study was to determine if optogenetic modulation of the EP could alter parkinsonian behavior or thalamic discharge in a hemiparkinson rat model.

METHODS

We injected an adeno-associated virus type-2 expressing α-synuclein (AAV2-α-syn) into the substantia nigra pars compacta (SNc) of the right hemisphere and confirmed parkinsonian behavior using an amphetamine-induced rotation test. Then we injected activated or inhibited neurons, using the channelrhodopsin2 (ChR2)/halorhodopsin (NpHR) system in the EP of the hemiparkinson rat model and examined downstream effects in vivo. We assessed alterations in parkinsonian behaviors using the stepping and cylinder tests before, during, and after optogenetic stimulation.

RESULTS

Importantly, optogenetic inhibition of the EP improved parkinsonian motor behaviors. When we monitored thalamic neuronal activity following optogenetic neuromodulation in vivo, and we observed alterations in thalamic discharge The thalamic neuronal activity is increased for optogenetic inhibition stimulation, whereas decreased for optogenetic activation stimulation.

CONCLUSIONS

Taken together, our data demonstrate that optical neuromodulation of the EP can successfully control contralateral forelimb movement and thalamic discharge in an AAV2-α-synuclein-induced hemiparkinson rat model.

Antiarrhythmics cure brain arrhythmia: The imperativeness of subthalamic ERG K+ channels in parkinsonian discharges

ERG K⁺ channels have long been known to play a crucial role in shaping cardiac action potentials and, thus, appropriate heart rhythms. The functional role of ERG channels in the central nervous system, however, remains elusive. We demonstrated that ERG channels exist in subthalamic neurons and have similar gating characteristics to those in the heart. ERG channels contribute crucially not only to the setting of membrane potential and, consequently, the firing modes, but also to the configuration of burst discharges and, consequently, the firing frequency and automaticity of the subthalamic neurons. Moreover, modulation of subthalamic discharges via ERG channels effectively modulates locomotor behaviors. ERG channel inhibitors ameliorate parkinsonian symptoms, whereas enhancers render normal animals hypokinetic. Thus, ERG K⁺ channels could be vital to the regulation of both cardiac and neuronal rhythms and may constitute an important pathophysiological basis and pharmacotherapeutic target for the growing list of neurological disorders related to "brain arrhythmias."

Altered somatosensory cortex neuronal activity in a rat model of Parkinson's disease and levodopa-induced dyskinesias

Several findings support the concept that sensorimotor integration is disturbed in Parkinson's disease (PD) and in levodopa-induced dyskinesias. In this study, we explored the neuronal firing activity of excitatory pyramidal cells and inhibitory interneurons in the forelimb region of the primary somatosensory cortex (S1FL-Ctx), along with its interaction with oscillatory activity of the primary motor cortex (MCtx) in 6-hydroxydopamine lesioned hemiparkinsonian (HP) and levodopa-primed dyskinetic (HP-LID) rats as compared to controls under urethane (1.4g/kg, i.p.) anesthesia. Further, gene expression patterns of distinct markers for inhibitory GABAergic neurons were analyzed in both cortical regions. While firing frequency and burst activity of S1FL-Ctx inhibitory interneurons were reduced in HP and HP-LID rats, measures of irregularity were enhanced in pyramidal cells. Further, enhanced coherence of distinct frequency bands of the theta/alpha, high-beta, and gamma frequency, together with enhanced synchronization of putative pyramidal cells and interneurons with MCtx oscillatory activity were observed. While GABA level was similar, gene expression levels of interneuron and GABAergic markers in S1FL-Ctx and MCtx of HP-LID rats differed to some extent. Our study shows that in a rat model of PD with dyskinesias, neuronal activity in putative interneurons was reduced, which was accompanied by high beta and gamma coherence between S1FL-Ctx and MCtx, together with changes in gene expression, indicating maladaptive neuroplasticity after long term levodopa treatment.

TRPM2 Promotes Neurotoxin MPP+/MPTP-Induced Cell Death

In neurons, Ca²⁺ is essential for a variety of physiological processes that regulate gene transcription to neuronal growth and their survival. 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridinium ions (MPP⁺) are potent neurotoxins that selectively destroys the dopaminergic (DA) neurons and mimics Parkinson's disease (PD) like symptoms, but the mechanism as how MPP⁺/MPTP effects DA neuron survival is not well-understood. In the present study, we found that MPP⁺ treatment increased the level of reactive oxygen species (ROS) that activates and upregulates the expression and function of melastatin-like transient receptor potential (TRPM) subfamily member, melastatin-like transient receptor potential channel 2 (TRPM2). Correspondingly, TRPM2 expression was also increased in substantia nigra of MPTP-induced PD mouse model and PD patients. ROS-mediated activation of TRPM2 resulted in an increased intracellular Ca²⁺, which in turn promoted cell death in SH-SY5Y cells. Intracellular Ca²⁺ overload caused by MPP⁺-induced ROS also affected calpain activity, followed by increased caspase 3 activities and activation of downstream apoptotic pathway. On the other hand, quenching of H₂O₂ by antioxidants, resveratrol (RSV), or N-acetylcysteine (NAC) effectively blocked TRPM2-mediated Ca²⁺ influx, decreased intracellular Ca²⁺ overload, and increased cell survival. Importantly, pharmacological inhibition of TRPM2 or knockdown of TRPM2 using siRNA, but not control siRNA, showed an increased protection by preventing MPP⁺-induced Ca²⁺ increase and inhibited apoptosis. Taken together, we show here a novel role for TRPM2 expression and function in MPP⁺-induced dopaminergic neuronal cell death.

Dendritic Release of Neurotransmitters

Release of neuroactive substances by exocytosis from dendrites is surprisingly widespread and is not confined to a particular class of transmitters: it occurs in multiple brain regions, and includes a range of neuropeptides, classical neurotransmitters, and signaling molecules, such as nitric oxide, carbon monoxide, ATP, and arachidonic acid. This review is focused on hypothalamic neuroendocrine cells that release vasopressin and oxytocin and midbrain neurons that release dopamine. For these two model systems, the stimuli, mechanisms, and physiological functions of dendritic release have been explored in greater detail than is yet available for other neurons and neuroactive substances. © 2017 American Physiological Society. Compr Physiol 7:235-252, 2017.

Neuroprotective effect of Portulaca oleracea extracts against 6-hydroxydopamine-induced lesion of dopaminergic neurons

The Portulaca oleracea L. (Portulacaceae) is a cosmopolitan species with a wide range of biological activities, including antioxidant and neuroprotective actions. We investigated the effects of P. oleracea extracts in a 6-hydroxydopamine rat model of Parkinson's disease, a debilitating disorder without effective treatments. Chemical profiles of aqueous and ethanolic extracts of whole plant were analyzed by thin layer chromatography and the antioxidant activity was assessed by 2,2-diphenyl-1-picrilhidrazila method. Male Wistar rats received intrastriatal 6-hydroxydopamine and were treated with vehicle or extracts (oral, 200 and 400 mg/kg) daily for two weeks. The behavioral open field test was conducted at days 1 and 15. Immunohistochemical analysis was performed 4 weeks after surgery to quantify tyrosine-hydroxylase cell counts in the substantia nigra pars compacta. Extracts presented antioxidant activity in concentrations above 300 µg/kg. The chromatographic analysis revealed the presence of Levodopa, alkaloids, flavonoids, saponins, tannins, terpenoids and polysaccharides. Both extracts improved motor recovery 15 days after lesion and protected from tyrosine-hydroxylase cell loss after 4 weeks, but these effects were more evident for the aqueous extract. Because the dopamine precursor is present, in addition to antioxidant compounds and neuroprotective effects, P. oleracea can be considered as potential strategy for treating Parkinson's disease.

Neural mechanisms of transient neocortical beta rhythms: Converging evidence from humans, computational modeling, monkeys, and mice

Human neocortical 15-29-Hz beta oscillations are strong predictors of perceptual and motor performance. However, the mechanistic origin of beta in vivo is unknown, hindering understanding of its functional role. Combining human magnetoencephalography (MEG), computational modeling, and laminar recordings in animals, we present a new theory that accounts for the origin of spontaneous neocortical beta. In our MEG data, spontaneous beta activity from somatosensory and frontal cortex emerged as noncontinuous beta events typically lasting <150 ms with a stereotypical waveform. Computational modeling uniquely designed to infer the electrical currents underlying these signals showed that beta events could emerge from the integration of nearly synchronous bursts of excitatory synaptic drive targeting proximal and distal dendrites of pyramidal neurons, where the defining feature of a beta event was a strong distal drive that lasted one beta period (∼50 ms). This beta mechanism rigorously accounted for the beta event profiles; several other mechanisms did not. The spatial location of synaptic drive in the model to supragranular and infragranular layers was critical to the emergence of beta events and led to the prediction that beta events should be associated with a specific laminar current profile. Laminar recordings in somatosensory neocortex from anesthetized mice and awake monkeys supported these predictions, suggesting this beta mechanism is conserved across species and recording modalities. These findings make several predictions about optimal states for perceptual and motor performance and guide causal interventions to modulate beta for optimal function.

Orexin-A increases the activity of globus pallidus neurons in both normal and parkinsonian rats

Orexin is a member of neuropeptides which was first identified in the hypothalamus. The globus pallidus is a key structure in the basal ganglia, which is involved in both normal motor function and movement disorders. Morphological studies have shown the expression of both OX1 and OX2 receptors in the globus pallidus. Employing single unit extracellular recordings and behavioural tests, the direct in vivo electrophysiological and behavioural effects of orexin-A in the globus pallidus were studied. Micro-pressure administration of orexin-A significantly increased the spontaneous firing rate of pallidal neurons. Correlation analysis revealed a negative correlation between orexin-A induced excitation and the basal firing rate. Furthermore, application of the specific OX1 receptor antagonist, SB-334867, decreased the firing rate of pallidal neurons, suggesting that endogenous orexinergic systems modulate the firing activity of pallidal neurons. Orexin-A increased the excitability of pallidal neurons through both OX1 and OX2 receptors. In 6-hydroxydopamine parkinsonian rats, orexin-A-induced increase in firing rate of pallidal neurons was stronger than that in normal rats. Immunostaining revealed positive OX1 receptor expression in the globus pallidus of both normal and parkinsonian rats. Finally, postural test showed that unilateral microinjection of orexin-A led to contralateral deflection in the presence of systemic haloperidol administration. Further elevated body swing test revealed that pallidal orexin-A and SB-334867 induced contralateral-biased swing and ipsilateral-biased swing respectively. Based on the electrophysiological and behavioural findings of orexin-A in the globus pallidus, the present findings may provide a rationale for the pathogenesis and treatment of Parkinson's disease.

Optogenetic Inhibition of the Subthalamic Nucleus Reduces Levodopa-Induced Dyskinesias in a Rat Model of Parkinson's Disease

BACKGROUND

The inhibition of neuronal activity by electrical deep brain stimulation is one of the mechanisms explaining the amelioration of levodopa-induced dyskinesia. However, electrical deep brain stimulation cannot specifically activate or inactivate selected types of neurons.

OBJECTIVES

We applied optogenetics as an alternative treatment to deep brain stimulation for levodopa-induced dyskinesia, and also to confirm that the mechanism of levodopa-induced dyskinesia amelioration by subthalamic nucleus deep brain stimulation is mediated through neuronal inhibition.

METHODS

6-hydroxydopamine-induced hemiparkinsonian rats received injections of hSynapsin1-NpHR-YFP adeno-associated virus (AAV) or hSynapsin1-YFP AAV. Two weeks after viral injections, all rats were treated with daily injections of levodopa. Then, the optic fiber was implanted into the ipsilateral subthalamic nucleus. We performed various behavioral tests to evaluate the changes in levodopa-induced dyskinesias after optogenetic expression and illumination in the subthalamic nucleus.

RESULTS

The behavioral tests revealed that optical inhibition of the subthalamic nucleus significantly ameliorated levodopa-induced dyskinesia by reducing the duration of the dyskinesias as well as the severity of axial dyskinesia.

CONCLUSIONS

These findings will provide a useful foundation for the future development of optogenetic modulation systems that could be considered as an approach to dyskinesia therapy.

The Striatum and Subthalamic Nucleus as Independent and Collaborative Structures in Motor Control

The striatum and the subthalamic nucleus (STN) are two separate input structures into the basal ganglia (BG). Accordingly, research to date has primarily focused on the distinct roles of these structures in motor control and cognition, often through investigation of Parkinson’s disease (PD). Both structures are divided into sensorimotor, associative, and limbic subdivisions based on cortical connectivity. The more recent discovery of the STN as an input structure into the BG drives comparison of these two structures and their respective roles in cognition and motor control. This review compares the role of the striatum and STN in motor response inhibition and execution, competing motor programs, feedback based learning, and response planning. Through comparison, it is found that the striatum and STN have highly independent roles in motor control but also collaborate in order to execute desired actions. There is also the possibility that inhibition or activation of one of these structures indirectly contributes to the function of other connected anatomical structures. Both structures contribute to selective motor response inhibition, which forms the basis of many tasks, but the STN additionally contributes to global inhibition through the hyperdirect pathway. Research is warranted on the functional connectivity of the network for inhibition involving the rIFG, preSMA, striatum, and STN.

Somatodendritic dopamine release: recent mechanistic insights

Dopamine (DA) is a key transmitter in motor, reward and cogitative pathways, with DA dysfunction implicated in disorders including Parkinson's disease and addiction. Located in midbrain, DA neurons of the substantia nigra pars compacta project via the medial forebrain bundle to the dorsal striatum (caudate putamen), and DA neurons in the adjacent ventral tegmental area project to the ventral striatum (nucleus accumbens) and prefrontal cortex. In addition to classical vesicular release from axons, midbrain DA neurons exhibit DA release from their cell bodies and dendrites. Somatodendritic DA release leads to activation of D2 DA autoreceptors on DA neurons that inhibit their firing via G-protein-coupled inwardly rectifying K(+) channels. This helps determine patterns of DA signalling at distant axonal release sites. Somatodendritically released DA also acts via volume transmission to extrasynaptic receptors that modulate local transmitter release and neuronal activity in the midbrain. Thus, somatodendritic release is a pivotal intrinsic feature of DA neurons that must be well defined in order to fully understand the physiology and pathophysiology of DA pathways. Here, we review recent mechanistic aspects of somatodendritic DA release, with particular emphasis on the Ca(2+) dependence of release and the potential role of exocytotic proteins.

Neuroimaging of Parkinson's disease: Expanding views

Advances in molecular and structural and functional neuroimaging are rapidly expanding the complexity of neurobiological understanding of Parkinson's disease (PD). This review article begins with an introduction to PD neurobiology as a foundation for interpreting neuroimaging findings that may further lead to more integrated and comprehensive understanding of PD. Diverse areas of PD neuroimaging are then reviewed and summarized, including positron emission tomography, single photon emission computed tomography, magnetic resonance spectroscopy and imaging, transcranial sonography, magnetoencephalography, and multimodal imaging, with focus on human studies published over the last five years. These included studies on differential diagnosis, co-morbidity, genetic and prodromal PD, and treatments from L-DOPA to brain stimulation approaches, transplantation and gene therapies. Overall, neuroimaging has shown that PD is a neurodegenerative disorder involving many neurotransmitters, brain regions, structural and functional connections, and neurocognitive systems. A broad neurobiological understanding of PD will be essential for translational efforts to develop better treatments and preventive strategies. Many questions remain and we conclude with some suggestions for future directions of neuroimaging of PD.

Dystonia and Paroxysmal Dyskinesias: Under-Recognized Movement Disorders in Domestic Animals? A Comparison with Human Dystonia/Paroxysmal Dyskinesias

Dystonia is defined as a neurological syndrome characterized by involuntary sustained or intermittent muscle contractions causing twisting, often repetitive movements, and postures. Paroxysmal dyskinesias are episodic movement disorders encompassing dystonia, chorea, athetosis, and ballism in conscious individuals. Several decades of research have enhanced the understanding of the etiology of human dystonia and dyskinesias that are associated with dystonia, but the pathophysiology remains largely unknown. The spontaneous occurrence of hereditary dystonia and paroxysmal dyskinesia is well documented in rodents used as animal models in basic dystonia research. Several hyperkinetic movement disorders, described in dogs, horses and cattle, show similarities to these human movement disorders. Although dystonia is regarded as the third most common movement disorder in humans, it is often misdiagnosed because of the heterogeneity of etiology and clinical presentation. Since these conditions are poorly known in veterinary practice, their prevalence may be underestimated in veterinary medicine. In order to attract attention to these movement disorders, i.e., dystonia and paroxysmal dyskinesias associated with dystonia, and to enhance interest in translational research, this review gives a brief overview of the current literature regarding dystonia/paroxysmal dyskinesia in humans and summarizes similar hereditary movement disorders reported in domestic animals.

Deep Brain Stimulation for Movement Disorders

Disruption in the interaction between the central nervous system, nerves, and muscles cause movement disorders. These disorders can negatively affect quality of life. Deep brain stimulation (DBS) has been identified as a therapy for Parkinson disease and essential tremor that has significant advantages compared with medicinal therapies. Surgical intervention for these disorders before DBS included ablative therapies such as thalamotomy and pallidotomy. These procedures were not reversible and did not allow for treatment adjustments. The advent of DBS progressed therapies for significant movement disorders into the realm of being reversible and adjustable based on patient symptoms.

Combination of acamprosate and baclofen as a promising therapeutic approach for Parkinson's disease

Parkinson’s disease (PD) is a progressive neurodegenerative disorder characterised by the loss of dopaminergic nigrostriatal neurons but which involves the loss of additional neurotransmitter pathways. Mono- or polytherapeutic interventions in PD patients have declining efficacy long-term and no influence on disease progression. The systematic analysis of available genetic and functional data as well as the substantial overlap between Alzheimer’s disease (AD) and PD features led us to repurpose and explore the effectiveness of a combination therapy (ABC) with two drugs – acamprosate and baclofen – that was already effective in AD animal models, for the treatment of PD. We showed in vitro that ABC strongly and synergistically protected neuronal cells from oxidative stress in the oxygen and glucose deprivation model, as well as dopaminergic neurons from cell death in the 6-hydroxydopamine (6-OHDA) rat model. Furthermore, we showed that ABC normalised altered motor symptoms in vivo in 6-OHDA-treated rats, acting by protecting dopaminergic cell bodies and their striatal terminals. Interestingly, ABC also restored a normal behaviour pattern in lesioned rats suggesting a symptomatic effect, and did not negatively interact with L-dopa. Our results demonstrate the potential value of combining repurposed drugs as a promising new strategy to treat this debilitating disease.

Inhibiting cholesterol degradation induces neuronal sclerosis and epileptic activity in mouse hippocampus

Elevations in neuronal cholesterol have been associated with several degenerative diseases. An enhanced excitability and synchronous firing in surviving neurons are among the sequels of neuronal death in these diseases and also in some epileptic syndromes. Here, we attempted to increase neuronal cholesterol levels, using a short hairpin RNA to suppress expression of the enzyme cytochrome P450 family 46, subfamily A, polypeptide 1 gene (CYP46A1). This protein hydroxylates cholesterol and so facilitates transmembrane extrusion. A short hairpin RNA CYP46A1construction coupled to the adeno-associated virus type 5 was injected focally and unilaterally into mouse hippocampus. It was selectively expressed first in neurons of the cornu ammonis (hippocampus) (CA)3a region. Cytoplasmic and membrane cholesterol increased, and the neuronal soma volume increased and then decreased before pyramidal cells died. As CA3a pyramidal cells died, interictal electroencephalographic (EEG) events occurred during exploration and non-rapid eye movement sleep. With time, neuronal death spread to involve pyramidal cells and interneurons of the CA1 region. CA1 neuronal death was correlated with a delayed local expression of phosphorylated tau. Astrocytes were activated throughout the hippocampus and microglial activation was specific to regions of neuronal death. CA1 neuronal death was correlated with distinct aberrant EEG activity. During exploratory behaviour and rapid eye movement sleep, EEG oscillations at 7-10 Hz (theta) could accelerate to 14-21 Hz (beta) waves. They were accompanied by low-amplitude, high-frequency oscillations of peak power at ~300 Hz and a range of 250-350 Hz. Although episodes of EEG acceleration were not correlated with changes in exploratory behaviour, they were followed in some animals by structured seizure-like discharges. These data strengthen links between increased cholesterol, neuronal sclerosis and epileptic behaviour.

Pallidal activity in myoclonus dystonia correlates with motor signs

BACKGROUND

Myoclonus-dystonia related to epsilon-sarcoglycan gene mutations is characterized by myoclonic jerks and mild to moderate dystonia. The role of basal ganglia dysfunction in the pathogenesis is unknown.

METHODS

Pallidal neuronal activity was recorded in six myoclonus-dystonia and six primary generalized dystonia patients operated on for internal globus pallidus deep brain stimulation.

RESULTS

In myoclonus-dystonia patients compared with primary-dystonia patients, internal pallidum neurons showed higher burst frequency, lower mean burst, and pause durations. External pallidum neurons showed higher mean pause frequency. Oscillatory activity was present in 33% and 35% of internal pallidum neurons in myoclonus-dystonia and primary-dystonia patients, respectively, predominantly in the theta frequency band (3-8 Hz). In myoclonus-dystonia patients with more severe myoclonus, internal pallidum neurons exhibited a higher bursting activity with high intraburst frequency and lower oscillatory activity frequency.

CONCLUSIONS

Myoclonus-dystonia appears to be related to specific changes in internal pallidum activity, leading to disruption in striato-pallido-thalamo-cortical circuits. © 2015 International Parkinson and Movement Disorder Society.

Reduced cortical innervation of the subthalamic nucleus in MPTP-treated parkinsonian monkeys

The striatum and the subthalamic nucleus are the main entry points for cortical information to the basal ganglia. Parkinson's disease affects not only the function, but also the morphological integrity of some of these inputs and their synaptic targets in the basal ganglia. Significant morphological changes in the cortico-striatal system have already been recognized in patients with Parkinson's disease and in animal models of the disease. To find out whether the primate cortico-subthalamic system is also subject to functionally relevant morphological alterations in parkinsonism, we used a combination of light and electron microscopy anatomical approaches and in vivo electrophysiological methods in monkeys rendered parkinsonian following chronic exposure to low doses of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). At the light microscopic level, the density of vesicular glutamate transporter 1-positive (i.e. cortico-subthalamic) profiles in the dorsolateral part of the subthalamic nucleus (i.e. its sensorimotor territory) was 26.1% lower in MPTP-treated parkinsonian monkeys than in controls. These results were confirmed by electron microscopy studies showing that the number of vesicular glutamate transporter 1-positive terminals and of axon terminals forming asymmetric synapses in the dorsolateral subthalamic nucleus was reduced by 55.1% and 27.9%, respectively, compared with controls. These anatomical findings were in line with in vivo electrophysiology data showing a 60% reduction in the proportion of pallidal neurons that responded to electrical stimulation of the cortico-subthalamic system in parkinsonian monkeys. These findings provide strong evidence for a partial loss of the hyperdirect cortico-subthalamic projection in MPTP-treated parkinsonian monkeys.

Alterations in neuronal activity in basal ganglia-thalamocortical circuits in the parkinsonian state

In patients with Parkinson’s disease and in animal models of this disorder, neurons in the basal ganglia and related regions in thalamus and cortex show changes that can be recorded by using electrophysiologic single-cell recording techniques, including altered firing rates and patterns, pathologic oscillatory activity and increased inter-neuronal synchronization. In addition, changes in synaptic potentials or in the joint spiking activities of populations of neurons can be monitored as alterations in local field potentials (LFPs), electroencephalograms (EEGs) or electrocorticograms (ECoGs). Most of the mentioned electrophysiologic changes are probably related to the degeneration of diencephalic dopaminergic neurons, leading to dopamine loss in the striatum and other basal ganglia nuclei, although degeneration of non-dopaminergic cell groups may also have a role. The altered electrical activity of the basal ganglia and associated nuclei may contribute to some of the motor signs of the disease. We here review the current knowledge of the electrophysiologic changes at the single cell level, the level of local populations of neural elements, and the level of the entire basal ganglia-thalamocortical network in parkinsonism, and discuss the possible use of this information to optimize treatment approaches to Parkinson’s disease, such as deep brain stimulation (DBS) therapy.

Ultra-high field magnetic resonance imaging of the basal ganglia and related structures

Deep brain stimulation is a treatment for Parkinson's disease and other related disorders, involving the surgical placement of electrodes in the deeply situated basal ganglia or thalamic structures. Good clinical outcome requires accurate targeting. However, due to limited visibility of the target structures on routine clinical MR images, direct targeting of structures can be challenging. Non-clinical MR scanners with ultra-high magnetic field (7T or higher) have the potential to improve the quality of these images. This technology report provides an overview of the current possibilities of visualizing deep brain stimulation targets and their related structures with the aid of ultra-high field MRI. Reviewed studies showed improved resolution, contrast- and signal-to-noise ratios at ultra-high field. Sequences sensitive to magnetic susceptibility such as T2^* and susceptibility weighted imaging and their maps in general showed the best visualization of target structures, including a separation between the subthalamic nucleus and the substantia nigra, the lamina pallidi medialis and lamina pallidi incompleta within the globus pallidus and substructures of the thalamus, including the ventral intermediate nucleus (Vim). This shows that the visibility, identification, and even subdivision of the small deep brain stimulation targets benefit from increased field strength. Although ultra-high field MR imaging is associated with increased risk of geometrical distortions, it has been shown that these distortions can be avoided or corrected to the extent where the effects are limited. The availability of ultra-high field MR scanners for humans seems to provide opportunities for a more accurate targeting for deep brain stimulation in patients with Parkinson's disease and related disorders.

AMP kinase regulates K-ATP currents evoked by NMDA receptor stimulation in rat subthalamic nucleus neurons

Our lab recently showed that N-methyl-D-aspartate (NMDA) evokes ATP-sensitive K(+) (K-ATP) currents in subthalamic nucleus (STN) neurons in slices of the rat brain. Both K-ATP channels and 5'-adenosine monophosphate-activated protein kinase (AMPK) are considered cellular energy sensors because their activities are influenced by the phosphorylation state of adenosine nucleotides. Moreover, AMPK has been shown to regulate K-ATP function in a variety of tissues including pancreas, cardiac myocytes, and hypothalamus. We used whole-cell patch clamp recordings to study the effect of AMPK activation on K-ATP channel function in STN neurons in slices of the rat brain. We found that bath or intracellular application of the AMPK activators A769662 and PT1 augmented tolbutamide-sensitive K-ATP currents evoked by NMDA receptor stimulation. The effect of AMPK activators was blocked by the AMPK inhibitor dorsomorphin (compound C), and by STO609, an inhibitor of the upstream AMPK activator CaMKKβ. AMPK augmentation of NMDA-induced K-ATP current was also blocked by intracellular BAPTA and by inhibitors of nitric oxide synthase and guanylyl cyclase. However, A769662 did not augment currents evoked by the K-ATP channel opener diazoxide. In the presence of NMDA, A769662 inhibited depolarizing plateau potentials and burst firing, both of which could be antagonized by tolbutamide or dorsomorphin. These studies show that AMPK augments NMDA-induced K-ATP currents by a Ca(2+)-dependent process that involves nitric oxide and cGMP. By augmenting K-ATP currents, AMPK activation would be expected to dampen the excitatory effect of glutamate-mediated transmission in the STN.

Distinct effects of mGlu4 receptor positive allosteric modulators at corticostriatal vs. striatopallidal synapses may differentially contribute to their antiparkinsonian action

Metabotropic glutamate 4 (mGlu4) receptor is a promising target for the treatment of motor deficits in Parkinson's disease (PD). This is due in part to its localization at key basal ganglia (BG) synapses that become hyperactive in this pathology, particularly striatopallidal synapses. In this context, mGlu4 receptor activation using either orthosteric agonists or positive allosteric modulators (PAMs) improves motor symptoms in rodent PD models in certain conditions. However, literature data show that mGlu4 receptor PAMs have no effect at striatopallidal GABAergic synapses (unless combined with an orthosteric agonist) and on the firing activity of pallidal neurons, and fail to provide significant motor improvement in relevant PD models. This questions the mechanistic hypothesis that mGlu4 receptor PAMs should act at striatopallidal synapses to alleviate PD motor symptoms. To shed light on this issue, we performed brain slice electrophysiology experiments. We show that Lu AF21934, an mGlu4 PAM small-molecule probe-compound, was ineffective at striatopallidal synapses at all concentrations tested, while it significantly inhibited corticostriatal synaptic transmission. Similarly, Lu AF21934 did not affect electrophysiology readouts at striatopallidal synapses in the presence of haloperidol or in 6-hydroxydopamine-lesioned rats. Interestingly, co-application of Lu AF21934 with a glutamate transporter inhibitor revealed a significant inhibitory action at striatopallidal synapses. Possibly, this effect could rely on increased level/permanence of glutamate in the synaptic cleft. Such differential efficacy of mGlu4 receptor PAMs at corticostriatal vs. striatopallidal synapses raises several issues regarding the synaptic target(s) of these drugs in the BG, and challenges the mechanisms by which they alleviate motor deficits in experimental PD models.

A movable microelectrode array for chronic basal ganglia single-unit electrocorticogram co-recording in freely behaving rats

The basal ganglia-cortical circuits are important for information process to brain function. However, chronic recording of single-unit activities in the basal ganglia nucleus has not yet been well established. We present a movable bundled microwire array for chronic subthalamic nucleus (STN) single-unit electrocorticogram co-recording. The electrode assembly contains a screw-advanced microdrive and a microwire array. The array consists of a steel guide tube, five recording wires and one referenced wire which form the shape of a guiding hand, and one screw electrode for cortico-recording. The electrode can acquire stable cortex oscillation-driven STN firing units in rats under different behaving conditions for 8 weeks. We achieved satisfying signal-to-noise ratio, portions of cells retaining viability, and spike waveform similarities across the recording sections. Using this method, we investigated neural correlations of the basal ganglia-cortical circuits in different behaving conditions. This method will become a powerful tool for multi-region recording to study normal statements or movement disorders.

Abnormal Bursting as a Pathophysiological Mechanism in Parkinson's Disease

Despite remarkable advances in Parkinson's disease (PD) research, the pathophysiological mechanisms causing motor dysfunction remain unclear, possibly delaying the advent of new and improved therapies. Several such mechanisms have been proposed including changes in neuronal firing rates, the emergence of pathological oscillatory activity, increased neural synchronization, and abnormal bursting. This review focuses specifically on the role of abnormal bursting of basal ganglia neurons in PD, where a burst is a physiologically-relevant, transient increase in neuronal firing over some reference period or activity. After reviewing current methods for how bursts are detected and what the functional role of bursts may be under normal conditions, existing studies are reviewed that suggest that bursting is abnormally increased in PD and that this increases with worsening disease. Finally, the influence of therapeutic approaches for PD such as dopamine-replacement therapy with levodopa or dopamine agonists, lesions, or deep brain stimulation on bursting is discussed. Although there is insufficient evidence to conclude that increased bursting causes motor dysfunction in PD, current evidence suggests that targeted investigations into the role of bursting in PD may be warranted.

Optogenetic activation of nigral inhibitory inputs to motor thalamus in the mouse reveals classic inhibition with little potential for rebound activation

The inhibitory output from the internal pallidum and substantia nigra to the thalamus forms an important link in the transmission of basal ganglia processing to cortex. Two hypotheses consider either inhibition of thalamic activity or thalamic excitation via post-inhibitory rebound burst firing as the functional mode of this link. We used optogenetics to characterize the synaptic properties of nigral input to motor thalamus in adult mouse brain slices, and to determine in what conditions the nigral inhibition of motor thalamus is transmitted via inhibition or rebound firing. Our results are more consistent with graded inhibition of spiking for conditions expected in normal awake animals, because inhibitory potentials from nigral input were generally not sufficient to elicit rebound spikes when the thalamic neurons were actively firing. However, with bursty or fast trains of nigral input low-threshold rebound spike bursts could be triggered for low levels of excitation. This may form the basis of pathological burst generation and transmission in parkinsonian conditions.

Remote brain network changes after unilateral cortical impact injury and their modulation by acetylcholinesterase inhibition

We explored whether cerebral cortical impact injury (CCI) effects extend beyond direct lesion sites to affect remote brain networks, and whether acetylcholinesterase (AChE) inhibition elicits discrete changes in functional activation of motor circuits following CCI. Adult male rats underwent unilateral motor-sensory CCI or sham injury. Physostigmine (AChE inhibitor) or saline were administered subcutaneously continuously via implanted minipumps (1.6 micromoles/kg/day) for 3 weeks, followed by cerebral perfusion mapping during treadmill walking using [(14)C]-iodoantipyrine. Quantitative autoradiographs were analyzed by statistical parametric mapping and functional connectivity (FC) analysis. CCI resulted in functional deficits in the ipsilesional basal ganglia, with increased activation contralesionally. Recruitment was also observed, especially contralesionally, of the red nucleus, superior colliculus, pedunculopontine tegmental nucleus, thalamus (ventrolateral n., central medial n.), cerebellum, and sensory cortex. FC decreased significantly within ipsi- and contralesional motor circuits and between hemispheres, but increased between midline cerebellum and select regions of the basal ganglia within each hemisphere. Physostigmine significantly increased functional brain activation in the cerebellar thalamocortical pathway (midline cerebellum→ventrolateral thalamus→motor cortex), subthalamic nucleus/zona incerta, and red nucleus and bilateral sensory cortex. In conclusion, CCI resulted in increased functional recruitment of contralesional motor cortex and bilateral subcortical motor regions, as well as recruitment of the cerebellar-thalamocortical circuit and contralesional sensory cortex. This phenomenon, augmented by physostigmine, may partially compensate motor deficits. FC decreased inter-hemispherically and in negative, but not positive, intra-hemispherical FC, and it was not affected by physostigmine. Circuit-based approaches into functional brain reorganization may inform future behavioral or molecular strategies to augment targeted neurorehabilitation.

Glutamate spillover drives endocannabinoid production and inhibits GABAergic transmission in the Substantia Nigra pars compacta

Endocannabinoids (eCBs) modulate synaptic transmission in the brain, but little is known of their regulatory role in nigral dopaminergic neurons, and whether transmission to these neurons is tonically inhibited by eCBs as seen in some other brain regions. Using whole-cell recording in midbrain slices, we observed potentiation of evoked IPSCs (eIPSCs) in these neurons after blocking CB1 receptors with rimonabant or LY-320,135, indicating the presence of an eCB tone reducing inhibitory synaptic transmission. Increased postsynaptic calcium buffering and block of mGluR1 or postsynaptic G-protein coupled receptors prevented this potentiation. Increasing spillover of endogenous glutamate by inhibiting uptake attenuated eIPSC amplitude, while enhancing the potentiation by rimonabant. Group I mGluR activation transiently inhibited eIPSCs, which could be prevented by GDP-β-S, increased calcium buffering or rimonabant. We explored the possibility that the dopamine-derived eCB N-arachidonoyl dopamine (NADA) is involved. The eCB tone was abolished by preventing dopamine synthesis, and enhanced by l-DOPA. It was not detected in adjacent non-dopaminergic neurons. Preventing 2-AG synthesis did not affect the tone, while inhibition of NADA production abolished it. Quantification of ventral midbrain NADA suggested a basal level that increased following prolonged depolarization or mGluR activation. Since block of the tone was not always accompanied by attenuation of depolarization-induced suppression of inhibition (DSI) and vice versa, our results indicate DSI and the eCB tone are mediated by distinct eCBs. This study provides evidence that dopamine modulates the activity of SNc neurons not only by conventional dopamine receptors, but also by CB1 receptors, potentially via NADA.

Basal ganglia output to the thalamus: still a paradox

The basal ganglia (BG)-recipient thalamus controls motor output but it remains unclear how its activity is regulated. Several studies report that thalamic activation occurs via disinhibition during pauses in the firing of inhibitory pallidal inputs from the BG. Other studies indicate that thalamic spiking is triggered by pallidal inputs via post-inhibitory 'rebound' calcium spikes. Finally excitatory cortical inputs can drive thalamic activity, which becomes entrained, or time-locked, to pallidal spikes. We present a unifying framework where these seemingly distinct results arise from a continuum of thalamic firing 'modes' controlled by excitatory inputs. We provide a mechanistic explanation for paradoxical pallidothalamic coactivations observed during behavior that raises new questions about what information is integrated in the thalamus to control behavior.

Differential contributions of basal ganglia and thalamus to song initiation, tempo, and structure

Basal ganglia-thalamocortical circuits are multistage loops critical to motor behavior, but the contributions of individual components to overall circuit function remain unclear. We addressed these issues in a songbird basal ganglia-thalamocortical circuit (the anterior forebrain pathway, AFP) specialized for singing and critical for vocal plasticity. The major known afferent to the AFP is the premotor cortical nucleus, HVC. Surprisingly, previous studies found that lesions of HVC alter song but do not eliminate the ability of the AFP to drive song production. We therefore used this AFP-driven song to investigate the role of basal ganglia and thalamus in vocal structure, tempo, and initiation. We found that lesions of the striatopallidal component (Area X) slowed song and simplified its acoustic structure. Elimination of the thalamic component (DLM) further simplified the acoustic structure of song and regularized its rhythm but also dramatically reduced song production. The acoustic structure changes imply that sequential stages of the AFP each add complexity to song, but the effects of DLM lesions on song initiation suggest that thalamus is a locus of additional inputs important to initiation. Together, our results highlight the cumulative contribution of stages of a basal ganglia-thalamocortical circuit to motor output along with distinct involvement of thalamus in song initiation or "gating."

In vivo electrophysiology of nigral and thalamic neurons in alpha-synuclein-overexpressing mice highlights differences from toxin-based models of parkinsonism

Numerous studies have suggested that alpha-synuclein plays a prominent role in both familial and idiopathic Parkinson's disease (PD). Mice in which human alpha-synuclein is overexpressed (ASO) display progressive motor deficits and many nonmotor features of PD. However, it is unclear what in vivo pathophysiological mechanisms drive these motor deficits. It is also unknown whether previously proposed pathophysiological features (i.e., increased beta oscillations, bursting, and synchronization) described in toxin-based, nigrostriatal dopamine-depletion models are also present in ASO mice. To address these issues, we first confirmed that 5- to 6-mo-old ASO mice have robust motor dysfunction, despite the absence of significant nigrostriatal dopamine degeneration. In the same animals, we then recorded simultaneous single units and local field potentials (LFPs) in the substantia nigra pars reticulata (SNpr), the main basal ganglia output nucleus, and one of its main thalamic targets, the ventromedial nucleus, as well as LFPs in the primary motor cortex in anesthetized ASO mice and their age-matched, wild-type littermates. Neural activity was examined during slow wave activity and desynchronized cortical states, as previously described in 6-hydroxydopamine-lesioned rats. In contrast to toxin-based models, we found a small decrease, rather than an increase, in beta oscillations in the desynchronized state. Similarly, synchronized burst firing of nigral neurons observed in toxin-based models was not observed in ASO mice. Instead, we found more subtle changes in pauses of SNpr firing compared with wild-type control mice. Our results suggest that the pathophysiology underlying motor dysfunction in ASO mice is distinctly different from striatal dopamine-depletion models of parkinsonism.

Supersensitive presynaptic dopamine D2 receptor inhibition of the striatopallidal projection in nigrostriatal dopamine-deficient mice

The dopamine (DA) D2 receptor (D2R)-expressing medium spiny neurons (D2-MSNs) in the striatum project to and inhibit the GABAergic neurons in the globus pallidus (GP), forming an important link in the indirect pathway of the basal ganglia movement control circuit. These striatopallidal axon terminals express presynaptic D2Rs that inhibit GABA release and thus regulate basal ganglion function. Here we show that in transcription factor Pitx3 gene mutant mice with a severe DA loss in the dorsal striatum mimicking the DA denervation in Parkinson's disease (PD), the striatopallidal GABAergic synaptic transmission displayed a heightened sensitivity to presynaptic D2R-mediated inhibition with the dose-response curve shifted to the left, although the maximal inhibition was not changed. Functionally, low concentrations of DA were able to more efficaciously reduce the striatopallidal inhibition-induced pauses of GP neuron activity in DA-deficient Pitx3 mutant mice than in wild-type mice. These results demonstrate that presynaptic D2R inhibition of the striatopallidal synapse becomes supersensitized after DA loss. These supersensitive D2Rs may compensate for the lost DA in PD and also induce a strong disinhibition of GP neuron activity that may contribute to the motor-stimulating effects of dopaminergic treatments in PD.

Subtle microstructural changes of the striatum in a DYT1 knock-in mouse model of dystonia

The dystonias are comprised of a group of disorders that share common neurological abnormalities of involuntary twisting or repetitive movements and postures. The most common inherited primary dystonia is DYT1 dystonia, which is due to loss of a GAG codon in the TOR1A gene that encodes torsinA. Autopsy studies of brains from patients with DYT1 dystonia have revealed few abnormalities, although recent neuroimaging studies have implied the existence of microstructural defects that might not be detectable with traditional histopathological methods. The current studies took advantage of a knock-in mouse model for DYT1 dystonia to search for subtle anatomical abnormalities in the striatum, a region often implicated in studies of dystonia. Multiple abnormalities were identified using a combination of quantitative stereological measures of immunohistochemical stains for specific neuronal populations, morphometric studies of Golgi-stained neurons, and immuno-electron microscopy of synaptic connectivity. In keeping with other studies, there was no obvious loss of striatal neurons in the DYT1 mutant mice. However, interneurons immunoreactive for choline acetyltransferase or parvalbumin were larger in the mutants than in control mice. In contrast, interneurons immunoreactive for neuronal nitric oxide synthase were smaller in the mutants than in controls. Golgi histochemical studies of medium spiny projection neurons in the mutant mice revealed slightly fewer and thinner dendrites, and a corresponding loss of dendritic spines. Electron microscopic studies showed a reduction in the ratio of axo-spinous to axo-dendritic synaptic inputs from glutamatergic and dopaminergic sources in mutant mice compared with controls. These results suggest specific anatomical substrates for altered signaling in the striatum and potential correlates of the abnormalities implied by human imaging studies of DYT1 dystonia.

Presynaptic serotonergic gating of the subthalamonigral glutamatergic projection

The GABAergic projection neurons in the substantia nigra pars reticulata (SNr) are key basal ganglia output neurons. The activity of these neurons is critically influenced by the glutamatergic projection from the subthalamic nucleus (STN). The SNr also receives an intense serotonin (5-HT) innervation, raising the possibility that 5-HT may regulate the STN→SNr glutamatergic transmission and the consequent STN-triggered spike firing in SNr neurons. Here we show that 5-HT reduced STN stimulation-evoked long-lasting polysynaptic complex EPSCs in SNr GABA neurons. This inhibitory 5-HT effect was mimicked by the 5-HT1B receptor agonist CP93129 and blocked by the 5-HT1B antagonist NAS-181. 5-HT1A receptor ligands were ineffective. Additionally, 5-HT and CP93129 reduced the frequency but not the amplitude of miniature EPSCs, suggesting a reduced vesicular release. 5-HT and CP93129 also decreased the amplitude but increased the paired pulse ratio of the monosynaptic EPSCs in SNr GABA neurons, indicating a presynaptic 5-HT1B receptor-mediated inhibition of glutamate release. Furthermore, 5-HT and CP93129 inhibited STN-triggered burst firing in SNr GABA neurons, and CP93129's inhibitory effect was strongest when puffed to STN→SNr axon terminals in SNr, indicating a primary role of the 5-HT1B receptors in these axon terminals. Finally, the 5-HT1B receptor antagonist NAS-181 increased the STN-triggered complex EPSCs and burst firing in SNr GABA neurons, demonstrating the effects of endogenous 5-HT. These results suggest that nigral 5-HT, via presynaptic 5-HT1B receptor activation, gates the excitatory STN→SNr projection, reduces burst firing in SNr GABA neurons, and thus may play a critical role in movement control.

The balance of striatal feedback transmission is disrupted in a model of parkinsonism

Inhibitory connections among striatal projection neurons (SPNs) called "feedback inhibition," have been proposed to endow the striatal microcircuit with computational capabilities, such as motor sequence selection, filtering, and the emergence of alternating network states. These properties are disrupted in models of Parkinsonism. However, the impact of feedback inhibition in the striatal network has remained under debate. Here, we test this inhibition at the microcircuit level. We used optical and electrophysiological recordings in mice and rats to demonstrate the action of striatal feedback transmission in normal and pathological conditions. Dynamic calcium imaging with single-cell resolution revealed the synchronous activation of a pool of identified SPNs by antidromic stimulation. Using bacterial artificial chromosome-transgenic mice, we demonstrate that the activated neuron pool equally possessed cells from the direct and indirect basal ganglia pathways. This pool inhibits itself because of its own GABA release when stimuli are frequent enough, demonstrating functional and significant inhibition. Blockade of GABAA receptors doubled the number of responsive neurons to the same stimulus, revealing a second postsynaptic neuron pool whose firing was being arrested by the first pool. Stronger connections arise from indirect SPNs. Dopamine deprivation impaired striatal feedback transmission disrupting the ability of a neuronal pool to arrest the firing of another neuronal pool. We demonstrate that feedback inhibition among SPNs is strong enough to control the firing of cell ensembles in the striatal microcircuit. However, to be effective, feedback inhibition should arise from synchronized pools of SPNs whose targets are other SPNs pools.