Dopamine depletion weakens direct pathway modulation of SNr neurons

Neurons in the substantia nigra reticulata (SNr) transmit information about basal ganglia output to dozens of brain regions in thalamocortical and brainstem motor networks. Activity of SNr neurons is regulated by convergent input from upstream basal ganglia nuclei, including GABAergic inputs from the striatum and the external globus pallidus (GPe). GABAergic inputs from the striatum convey information from the direct pathway, while GABAergic inputs from the GPe convey information from the indirect pathway. Chronic loss of dopamine, as occurs in Parkinson's disease, disrupts the balance of direct and indirect pathway neurons at the level of the striatum, but the question of how dopamine loss affects information propagation along these pathways outside of the striatum is less well understood. Using a combination of in vivo and slice electrophysiology, we find that dopamine depletion selectively weakens the direct pathway's influence over neural activity in the SNr due to changes in the decay kinetics of GABA-mediated synaptic currents. GABAergic signaling from GPe neurons in the indirect pathway was not affected, resulting in an inversion of the normal balance of inhibitory control over basal ganglia output through the SNr. These results highlight the contribution of cellular mechanisms outside of the striatum that impact the responses of basal ganglia output neurons to the direct and indirect pathways in disease.

The transcription regulator Lmo3 is required for the development of medial ganglionic eminence derived neurons in the external globus pallidus

The external globus pallidus (GPe) is an essential component of the basal ganglia, a group of subcortical nuclei that are involved in control of action. Changes in the firing of GPe neurons are associated with both passive and active body movements. Aberrant activity of GPe neurons has been linked to motor symptoms of a variety of movement disorders, such as Parkinson's Disease, Huntington's disease and dystonia. Recent studies have helped delineate functionally distinct subtypes of GABAergic GPe projection neurons. However, not much is known about specific molecular mechanisms underlying the development of GPe neuronal subtypes. We show that the transcriptional regulator Lmo3 is required for the development of medial ganglionic eminence derived Nkx2.1+ and PV+ GPe neurons, but not lateral ganglionic eminence derived FoxP2+ neurons. As a consequence of the reduction in PV+ neurons, Lmo3-null mice have a reduced GPe input to the subthalamic nucleus.

Shared GABA transmission pathology in dopamine agonist- and antagonist-induced dyskinesia

Dyskinesia is involuntary movement caused by long-term medication with dopamine-related agents: the dopamine agonist 3,4-dihydroxy-L-phenylalanine (L-DOPA) to treat Parkinson's disease (L-DOPA-induced dyskinesia [LID]) or dopamine antagonists to treat schizophrenia (tardive dyskinesia [TD]). However, it remains unknown why distinct types of medications for distinct neuropsychiatric disorders induce similar involuntary movements. Here, we search for a shared structural footprint using magnetic resonance imaging-based macroscopic screening and super-resolution microscopy-based microscopic identification. We identify the enlarged axon terminals of striatal medium spiny neurons in LID and TD model mice. Striatal overexpression of the vesicular gamma-aminobutyric acid transporter (VGAT) is necessary and sufficient for modeling these structural changes; VGAT levels gate the functional and behavioral alterations in dyskinesia models. Our findings indicate that lowered type 2 dopamine receptor signaling with repetitive dopamine fluctuations is a common cause of VGAT overexpression and late-onset dyskinesia formation and that reducing dopamine fluctuation rescues dyskinesia pathology via VGAT downregulation.

Opposing functions of glutamatergic inputs between the globus pallidus external segment and substantia nigra pars reticulata

The indirect pathway of the basal ganglia, including the subthalamic nucleus (STN) and globus pallidus external segment (GPe), is believed to play a crucial role in suppressing involuntary movements. However, recent evidence suggests the STN and GPe also facilitate voluntary movements. This study hypothesized that excitatory inputs from the STN to the GPe contribute to this facilitation, and that excitatory projections to the substantia nigra pars reticulata (SNr) are involved in the inhibition. To disrupt the STN-GPe or STN-SNr projections in monkeys during choice and fixation tasks, glutamate receptor inhibitors were injected into the GPe or SNr, which induced delayed saccade latencies toward good choices in the choice task (GPe) and caused frequent reflexive saccades to objects in the fixation task (SNr). Our findings suggest excitatory inputs to the GPe and SNr work in opposing manners, providing new insights that redefine our understanding of the functions of basal ganglia pathways.

The indirect pathway of the basal ganglia promotes transient punishment but not motor suppression

Optogenetic stimulation of Adora2a receptor-expressing spiny projection neurons (A2A-SPNs) in the striatum drives locomotor suppression and transient punishment, results attributed to activation of the indirect pathway. The sole long-range projection target of A2A-SPNs is the external globus pallidus (GPe). Unexpectedly, we found that inhibition of the GPe drove transient punishment but not suppression of movement. Within the striatum, A2A-SPNs inhibit other SPNs through a short-range inhibitory collateral network, and we found that optogenetic stimuli that drove motor suppression shared a common mechanism of recruiting this inhibitory collateral network. Our results suggest that the indirect pathway plays a more prominent role in transient punishment than in motor control and challenges the assumption that activity of A2A-SPNs is synonymous with indirect pathway activity.

Pallidostriatal Projections Promote β Oscillations in a Dopamine-Depleted Biophysical Network Model

In the basal ganglia, focused rhythmicity is an important feature of network activity at certain stages of motor processing. In disease, however, the basal ganglia develop amplified rhythmicity. Here, we demonstrate how the cellular architecture and network dynamics of an inhibitory loop in the basal ganglia yield exaggerated synchrony and locking to β oscillations, specifically in the dopamine-depleted state. A key component of this loop is the pallidostriatal pathway, a well-characterized anatomical projection whose function has long remained obscure. We present a synaptic characterization of this pathway in mice and incorporate these data into a computational model that we use to investigate its influence over striatal activity under simulated healthy and dopamine-depleted conditions. Our model predicts that the pallidostriatal pathway influences striatal output preferentially during periods of synchronized activity within GPe. We show that, under dopamine-depleted conditions, this effect becomes a key component of a positive feedback loop between the GPe and striatum that promotes synchronization and rhythmicity. Our results generate novel predictions about the role of the pallidostriatal pathway in shaping basal ganglia activity in health and disease.

SIGNIFICANCE STATEMENT

This work demonstrates that functional connections from the globus pallidus externa (GPe) to striatum are substantially stronger onto fast-spiking interneurons (FSIs) than onto medium spiny neurons. Our circuit model suggests that when GPe spikes are synchronous, this pallidostriatal pathway causes synchronous FSI activity pauses, which allow a transient window of disinhibition for medium spiny neurons. In simulated dopamine-depletion, this GPe-FSI activity is necessary for the emergence of strong synchronization and the amplification and propagation of β oscillations, which are a hallmark of parkinsonian circuit dysfunction. These results suggest that GPe may play a central role in propagating abnormal circuit activity to striatum, which in turn projects to downstream basal ganglia structures. These findings warrant further exploration of GPe as a target for interventions for Parkinson's disease.

Asymmetric pallidal neuronal activity in patients with cervical dystonia

The origin of asymmetric clinical manifestation of symptoms in patients suffering from cervical dystonia (CD) is hitherto poorly understood. Dysregulated neuronal activity in the basal ganglia has been suggested to have a role in the pathophysiology of CD. Here, we re-assessed the question to what extent relative changes occur in the direct vs. indirect basal ganglia pathway in CD, whether these circuit changes are lateralized, and how these alterations relate to CD symptoms. To this end, we recorded ongoing single cell and local field potential (LFP) activity from the external (GPe) and internal pallidal segment (GPi) of 13 CD patients undergoing microelectrode-guided stereotactic surgery for deep brain stimulation in the GPi. We compared pallidal recordings from CD patients operated under local anaesthesia (LA) with those obtained in CD patients operated under general anaesthesia (GA). In awake patients, mean GPe discharge rate (52 Hz) was lower than that of GPi (72 Hz). Mean GPi discharge ipsilateral to the side of head turning was higher than contralateral and correlated with torticollis symptom severity. Lateralized differences were absent at the level of the GPe and in recordings from patients operated under GA. Furthermore, in the GPi of CD patients there was a subpopulation of theta-oscillatory cells with unique bursting characteristics. Power and coherence of GPe– and GPi–LFPs were dominated by a theta peak and also exhibited band-specific interhemispheric differences. Strong cross-frequency coupling of low-gamma amplitude to theta phase was a feature of pallidal LFPs recorded under LA, but not GA. These results indicate that CD is associated with an asymmetric pallidal outflow. Based on the finding of symmetric neuronal discharges in the GPe, we propose that an imbalanced interhemispheric direct pathway gain may be involved in CD pathophysiology.

Active decorrelation in the basal ganglia

The cytoarchitecturally-homogeneous appearance of the globus pallidus, subthalamic nucleus and substantia nigra has long been said to imply a high degree of afferent convergence and sharing of inputs by nearby neurons. Moreover, axon collaterals of neurons in the external segment of the globus pallidus and the substantia nigra pars reticulata arborize locally and make inhibitory synapses on other cells of the same type. These features suggest that the connectivity of the basal ganglia may impose spike-time correlations among the cells, and it has been puzzling that experimental studies have failed to demonstrate such correlations. One possible solution arises from studies of firing patterns in basal ganglia cells, which reveal that they are nearly all pacemaker cells. Their high rate of firing does not depend on synaptic excitation, but they fire irregularly because a dense barrage of synaptic inputs normally perturbs the timing of their autonomous activity. Theoretical and computational studies show that the responses of repetitively-firing neurons to shared input or mutual synaptic coupling often defy classical intuitions about temporal synaptic integration. The patterns of spike-timing among such neurons depend on the ionic mechanism of pacemaking, the level of background uncorrelated cellular and synaptic noise, and the firing rates of the neurons, as well as the properties of their synaptic connections. Application of these concepts to the basal ganglia circuitry suggests that the connectivity and physiology of these nuclei may be configured to prevent the establishment of permanent spike-timing relationships between neurons. The development of highly synchronous oscillatory patterns of activity in Parkinson's disease may result from the loss of pacemaking by some basal ganglia neurons, and accompanying breakdown of the mechanisms responsible for active decorrelation.

Chronic treatment with MPEP, an mGlu5 receptor antagonist, normalizes basal ganglia glutamate neurotransmission in L-DOPA-treated parkinsonian monkeys

Metabotropic glutamate 5 (mGlu5) receptor antagonists reduce L-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesias (LID) in Parkinson's disease (PD). The aim of this study was to investigate the long-term effect of the prototypal mGlu5 receptor antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) on glutamate receptors known to be involved in the development of LID in the de novo chronic treatment of monkeys lesioned with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). MPTP monkeys were treated for one month with L-DOPA and developed dyskinesias while those treated with L-DOPA and MPEP (10 mg/kg) developed significantly less. Normal control and saline-treated MPTP monkeys were also included. All MPTP monkeys were extensively and similarly denervated. The basal ganglia [(3)H]ABP688 specific binding (mGlu5 receptors) was elevated in L-DOPA-treated MPTP monkeys compared to controls but not in those treated with L-DOPA and MPEP; dyskinesia scores of these monkeys correlated positively with their [(3)H]ABP688 specific binding. Striatal density (B(max)) of [(3)H]ABP688 specific binding increased in L-DOPA-treated MPTP monkeys compared to other groups and affinity (Kd) remained unchanged. Striatal mGlu5 receptor mRNA remained unchanged following treatments. Elevated basal ganglia specific binding of [(3)H]Ro 25-6981 (NMDA NR1/NR2B receptors), [(3)H]Ro 48-8587 (AMPA receptors) but not [(3)H]CGP-39653 (NMDA NR1/NR2A receptors) was observed only in L-DOPA-treated MPTP monkeys; dyskinesias scores correlated with binding. By contrast, basal ganglia [(3)H]LY341495 specific binding (mGlu2/3 receptors) decreased in L-DOPA-treated MPTP monkeys compared to controls, saline and L-DOPA + MPEP treated MPTP monkeys; dyskinesias scores correlated negatively with this binding. Hence, chronic MPEP treatment reduces the development of LID and is associated with a normalization of glutamate neurotransmission.

GABAA-receptor activation in the subthalamic nucleus compensates behavioral asymmetries in the hemiparkinsonian rat

The subthalamic nucleus (STN) has a pivotal role in the pathophysiology of Parkinson's disease (PD). Modulation of STN activity (by lesions, pharmacological or electrical stimulation) has been shown to improve motor parameters in PD patients and in animal models of PD. In an attempt to characterize the neurochemical bases for such antiparkinsonian action, we address specific neurotransmitter systems via local pharmacological manipulation of the STN in hemiparkinsonian rats. Here, we have focused on the GABAergic and glutamatergic receptors in the STN. In animals with unilateral 6-hydroxydopamine lesions of the nigro-striatal tract, we administered either the selective GABAA-agonist muscimol (0.5 μg and 1.0 μg), the non-competitive N-methyl-d-aspartate (NMDA)-antagonist MK-801 (dizocilpine; 2.5 μg), or vehicle (0.25 μl) into the STN. The effects of GABAergic and glutamatergic modulation of the STN on motor parameters were assessed by gauging rotational behavior and locomotion. Application of muscimol ipsilateral to the side of dopamine-depletion influenced turning behavior in a dose-dependent fashion, with the low dose re-adjusting turning behavior to a non-biased distribution, and the high dose evoking contraversive turning. The administration of MK-801 did not have such effects. These findings give evidence for the involvement of GABAergic activation in the STN in the compensation of motor asymmetries in the hemiparkinsonian rat, whereas N-methyl-d-aspartate (NMDA)-antagonism was ineffective in this model of PD.