The external globus pallidus: progress and perspectives

Striosomes Target Nigral Dopamine-Containing Neurons via Direct-D1 and Indirect-D2 Pathways Paralleling Classic Direct-Indirect Basal Ganglia Systems

The classic output pathways of the basal ganglia are known as the direct-D1 and indirect-D2, or "Go/No-Go", pathways. Balance of the activity in these canonical direct-indirect pathways is considered a core requirement for normal movement control, and their imbalance is a major etiologic factor in movement disorders including Parkinson's disease. We present evidence for a conceptually equivalent parallel system of direct-D1 and indirect-D2 pathways that arise from striatal projection neurons (SPNs) of the striosome compartment rather than from the matrix. These striosomal direct (S-D1) and indirect (S-D2) pathways, as a pair, target dopamine-containing neurons of the substantia nigra (SNpc) instead of the motor output nuclei of the basal ganglia. The novel anatomically and functionally distinct indirect-D2 striosomal pathway targets dopaminergic SNpc cells indirectly via a core region of the external pallidum (GPe). We demonstrate that these S-D1 and S-D2 pathways oppositely modulate striatal dopamine release in freely behaving mice under open-field conditions and oppositely modulate locomotor and other movements. These S-D1 and S-D2 pathways further exhibit different, time-dependent responses during performance of a probabilistic decision-making maze task and respond differently to rewarding and aversive stimuli. These contrasts depend on mediolateral and anteroposterior striatal locations of the SPNs as are the classic direct and indirect pathways. The effects of S-D1 and S-D2 stimulation on striatal dopamine release and voluntary locomotion are nearly opposite. The parallelism of the direct-indirect circuit design motifs of the striosomal S-D and S-D2 circuits and canonical matrix M-D1 and M-D2, and their contrasting behavioral effects, call for a major reformulation of the classic direct-indirect pathway model of basal ganglia function. Given that some striosomes receive limbic and association cortical inputs, the S-D1 and S-D2 circuits likely influence motivation for action and behavioral learning, complementing and possibly reorienting the motoric activities of the canonical matrix pathways. At a fundamental level, these findings suggest a unifying framework for aligning two sets of circuits that share the organizational motif of opponent D1 and D2 regulation, but that have different outputs and can even have opposite polarities in their targets and effects, albeit conditioned by striatal topography. Our findings further delineate a potentially therapeutically important set of pathways influencing dopamine, including a D2 receptor-linked S-D2 pathway likely unknowingly targeted by administration of many therapeutic drugs including those for Parkinson's disease. The novel parallel pathway model that we propose here could help to account for the normally integrated modulatory influence of the basal ganglia on motivation for actions as well as the actions themselves.

Molecular and circuit determinants in the globus pallidus mediating control of cocaine-induced behavioral plasticity

The globus pallidus externus (GPe) is a central component of the basal ganglia circuit, receiving strong input from the indirect pathway and regulating a variety of functions, including locomotor output and habit formation. We recently showed that it also acts as a gatekeeper of cocaine-induced behavioral plasticity, as inhibition of parvalbumin-positive cells in the GPe (GPe PV ) prevents the development of cocaine-induced reward and sensitization. However, the molecular and circuit mechanisms underlying this function are unknown. Here we show that GPe PV cells control cocaine reward and sensitization by inhibiting GABAergic neurons in the substantia nigra pars reticulata (SNr GABA ), and ultimately, selectively modulating the activity of ventral tegmental area dopamine (VTA DA ) cells projecting to the lateral shell of the nucleus accumbens (NAcLat). A major input to GPe PV cells is the indirect pathway of the dorsomedial striatum (DMS D 2 ), which receives DAergic innervation from collaterals of VTA DA →NAcLat cells, making this a closed-loop circuit. Cocaine likely facilitates reward and sensitization not directly through actions in the GPe, but rather in the upstream DMS, where the cocaine-induced elevation of DA triggers a depression in DMS D 2 cell activity. This cocaine-induced elevation in DA levels can be blocked by inhibition of GPe PV cells, closing the loop. Interestingly, the level of GPe PV cell activity prior to cocaine administration is correlated with the extent of reward and sensitization that animals experience in response to future administration of cocaine, indicating that GPe PV cell activity is a key predictor of future behavioral responses to cocaine. Single nucleus RNA-sequencing of GPe cells indicated that genes encoding voltage-gated potassium channels KCNQ3 and KCNQ5 that control intrinsic cellular excitability are downregulated in GPe PV cells following a single cocaine exposure, contributing to the elevation in GPe PV cell excitability. Acutely activating channels containing KCNQ3 and/or KCNQ5 using the small molecule carnosic acid, a key psychoactive component of Salvia rosmarinus (rosemary) extract, reduced GPe PV cell excitability and also impaired cocaine reward, sensitization, and volitional cocaine intake, indicating its potential as a therapeutic to counteract psychostimulant use disorder. Our findings illuminate the molecular and circuit mechanisms by which the GPe orchestrates brain-wide changes in response to cocaine that are required for reward, sensitization, and self-administration behaviors.

Neurochemistry and circuit organization of the lateral spiriform nucleus of birds: A uniquely nonmammalian direct pathway component of the basal ganglia

We used diverse methods to characterize the role of avian lateral spiriform nucleus (SpL) in basal ganglia motor function. Connectivity analysis showed that SpL receives input from globus pallidus (GP), and the intrapeduncular nucleus (INP) located ventromedial to GP, whose neurons express numerous striatal markers. SpL-projecting GP neurons were large and aspiny, while SpL-projecting INP neurons were medium sized and spiny. Connectivity analysis further showed that SpL receives inputs from subthalamic nucleus (STN) and substantia nigra pars reticulata (SNr), and that the SNr also receives inputs from GP, INP, and STN. Neurochemical analysis showed that SpL neurons express ENK, GAD, and a variety of pallidal neuron markers, and receive GABAergic terminals, some of which also contain DARPP32, consistent with GP pallidal and INP striatal inputs. Connectivity and neurochemical analysis showed that the SpL input to tectum prominently ends on GABAA receptor-enriched tectobulbar neurons. Behavioral studies showed that lesions of SpL impair visuomotor behaviors involving tracking and pecking moving targets. Our results suggest that SpL modulates brainstem-projecting tectobulbar neurons in a manner comparable to the demonstrated influence of GP internus on motor thalamus and of SNr on tectobulbar neurons in mammals. Given published data in amphibians and reptiles, it seems likely the SpL circuit represents a major direct pathway-type circuit by which the basal ganglia exerts its motor influence in nonmammalian tetrapods. The present studies also show that avian striatum is divided into three spatially segregated territories with differing connectivity, a medial striato-nigral territory, a dorsolateral striato-GP territory, and the ventrolateral INP motor territory.

Rethinking the external globus pallidus and information flow in cortico-basal ganglia-thalamic circuits

For decades, the external globus pallidus (GPe) has been viewed as a passive way-station in the indirect pathway of the cortico-basal ganglia-thalamic (CBGT) circuit, sandwiched between striatal inputs and basal ganglia outputs. According to this model, one-way descending striatal signals in the indirect pathway amplify the suppression of downstream thalamic nuclei by inhibiting GPe activity. Here, we revisit this assumption, in light of new and emerging work on the cellular complexity, connectivity and functional role of the GPe in behaviour. We show how, according to this new circuit-level logic, the GPe is ideally positioned for relaying ascending and descending control signals within the basal ganglia. Focusing on the problem of inhibitory control, we illustrate how this bidirectional flow of information allows for the integration of reactive and proactive control mechanisms during action selection. Taken together, this new evidence points to the GPe as being a central hub in the CBGT circuit, participating in bidirectional information flow and linking multifaceted control signals to regulate behaviour.

Dopamine D3 receptor modulates D2 receptor effects on cAMP and GABA release at striatopallidal terminals-Modulation by the Ca2+-Calmodulin-CaMKII system

Dopamine D2 receptor (D2R) is expressed in striatopallidal neurons and decreases forskolin-stimulated cyclic adenine monophosphate (cAMP) accumulation and gamma-aminobutyric acid (GABA) release. Dopamine D3 receptor (D3R) mRNA is expressed in a population of striatal D2R-expressing neurons. Also, D3R protein and binding have been reported in the neuropil of globus pallidus. We explore whether D2R and D3R colocalize in striatopallidal terminals and whether D3R modulates the D2R effect on forskolin-stimulated [3H]cAMP accumulation in pallidal synaptosomes and high K+ stimulated-[3H]GABA release in pallidal slices. Previous reports in heterologous systems indicate that calmodulin (CaM) and CaMKII modulate D2R and D3R functions; thus, we study whether this system regulates its functional interaction. D2R immunoprecipitates with CaM, and pretreatment with ophiobolin A or depolarization of synaptosomes with 15 mM of K+ decreases it. Both treatments increase the D2R inhibition of forskolin-stimulated [3H]cAMP accumulation when activated with quinpirole, indicating a negative modulation of CaM on D2R function. Quinpirole also activates D3R, potentiating D2R inhibition of cAMP accumulation in the ophiobolin A-treated synaptosomes. D2R and D3R immunoprecipitate in pallidal synaptosomes and decrease after the kainic acid striatal lesion, indicating the striatal origin of the presynaptic receptors. CaM-kinase II alfa (CaMKIIα) immunoprecipitates with D3R and increases after high K+ depolarization. In the presence of KN62, a CaMKIIα blocker, D3R potentiates D2R effects on cAMP accumulation in depolarized synaptosomes and GABA release in pallidal slices, indicating D3R function regulation by CaMKIIα. Our data indicate that D3R potentiates the D2R effect on cAMP accumulation and GABA release at pallidal terminals, an interaction regulated by the CaM-CaMKIIα system.

Inverse pattern of GABAergic system impairment in the external versus internal globus pallidus in male heroin addicts

Opioid addiction is a global problem that has been exacerbated in the USA and Europe by the COVID-19 pandemic. The globus pallidus (GP) plays a prominent neurobiological role in the regulation of behaviour as an output station of the striato-pallidal system. GABAergic large projection neurons are the main neuronal type in the external (EGP) and internal (IGP) parts of the GP, where addiction-specific molecular and functional abnormalities occur. In these neurons, glutamate decarboxylase (GAD) with isoforms GAD 65 and 67 is a key enzyme in GABA synthesis, and experimental studies suggest GAD dysregulation in the GP of heroin addicts. Our study, which was performed on paraffin-embedded brains from the Magdeburg Brain Bank, aimed to investigate abnormalities in the GABAergic function of large GP neurons by densitometric evaluation of their GAD 65/67-immunostained thick dendrites. The study revealed a bilaterally decreased fibres density in the EGP paralleled by the increase in the IGP in 11 male heroin addicts versus 11 healthy controls (significant U-test P values). The analysis of confounding variables found no interference of age, brain volume, and duration of formalin fixation with the results. Our findings suggest a dysregulation of GABAergic activity in the GP of heroin addicts, which is consistent with experimental data from animal models and plays potentially a role in the disturbed function of basal ganglia circuit in opioid addiction.

Distinct neurochemical influences on fMRI response polarity in the striatum

The striatum, known as the input nucleus of the basal ganglia, is extensively studied for its diverse behavioral roles. However, the relationship between its neuronal and vascular activity, vital for interpreting functional magnetic resonance imaging (fMRI) signals, has not received comprehensive examination within the striatum. Here, we demonstrate that optogenetic stimulation of dorsal striatal neurons or their afferents from various cortical and subcortical regions induces negative striatal fMRI responses in rats, manifesting as vasoconstriction. These responses occur even with heightened striatal neuronal activity, confirmed by electrophysiology and fiber-photometry. In parallel, midbrain dopaminergic neuron optogenetic modulation, coupled with electrochemical measurements, establishes a link between striatal vasodilation and dopamine release. Intriguingly, in vivo intra-striatal pharmacological manipulations during optogenetic stimulation highlight a critical role of opioidergic signaling in generating striatal vasoconstriction. This observation is substantiated by detecting striatal vasoconstriction in brain slices after synthetic opioid application. In humans, manipulations aimed at increasing striatal neuronal activity likewise elicit negative striatal fMRI responses. Our results emphasize the necessity of considering vasoactive neurotransmission alongside neuronal activity when interpreting fMRI signal.

The cortico-striatal circuitry in autism-spectrum disorders: a balancing act

The basal ganglia are major targets of cortical inputs and, in turn, modulate cortical function via their projections to the motor and prefrontal cortices. The role of the basal ganglia in motor control and reward is well documented and there is also extensive evidence that they play a key role in social and repetitive behaviors. The basal ganglia influence the activity of the cerebral cortex via two major projections from the striatum to the output nuclei, the globus pallidus internus and the substantia nigra, pars reticulata. This modulation involves a direct projection known as the direct pathway and an indirect projection via the globus pallidus externus and the subthalamic nucleus, known as the indirect pathway. This review discusses the respective contribution of the direct and indirect pathways to social and repetitive behaviors in neurotypical conditions and in autism spectrum disorders.

Rethinking the external globus pallidus and information flow in cortico-basal ganglia-thalamic circuits

For decades the external globus pallidus (GPe) has been viewed as a passive way-station in the indirect pathway of the cortico-basal ganglia-thalamic (CBGT) circuit, sandwiched between striatal inputs and basal ganglia outputs. According to this model, one-way descending striatal signals in the indirect pathway amplify the suppression of downstream thalamic nuclei by inhibiting GPe activity. Here we revisit this assumption, in light of new and emerging work on the cellular complexity, connectivity, and functional role of the GPe in behavior. We show how, according to this new circuit-level logic, the GPe is ideally positioned for relaying ascending and descending control signals within the basal ganglia. Focusing on the problem of inhibitory control, we illustrate how this bidirectional flow of information allows for the integration of reactive and proactive control mechanisms during action selection. Taken together, this new evidence points to the GPe as being a central hub in the CBGT circuit, participating in bidirectional information flow and linking multifaceted control signals to regulate behavior.

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.

Selective encoding of reward predictions and prediction errors by globus pallidus subpopulations

Basal ganglia (BG) circuits help guide and invigorate actions using predictions of future rewards (values). Within the BG, the globus pallidus pars externa (GPe) may play an essential role in aggregating and distributing value information. We recorded from the GPe in unrestrained rats performing both Pavlovian and instrumental tasks to obtain rewards and distinguished neuronal subtypes by their firing properties across the wake/sleep cycle and optogenetic tagging. In both tasks, the parvalbumin-positive (PV+), faster-firing "prototypical" neurons showed strong, sustained modulation by value, unlike other subtypes, including the "arkypallidal" cells that project back to striatum. Furthermore, we discovered that a distinct minority (7%) of GP cells display slower, pacemaker-like firing and encode reward prediction errors (RPEs) almost identically to midbrain dopamine neurons. These cell-specific forms of GPe value representation help define the circuit mechanisms by which the BG contribute to motivation and reinforcement learning.

Gestational and postnatal age associations for striatal tissue iron deposition in early infancy

Striatal development is crucial for later motor, cognitive, and reward behavior, but age-related change in striatal physiology during the neonatal period remains understudied. An MRI-based measure of tissue iron deposition, T2*, is a non-invasive way to probe striatal physiology neonatally, linked to dopaminergic processing and cognition in children and adults. Striatal subregions have distinct functions that may come online at different time periods in early life. To identify if there are critical periods before or after birth, we measured if striatal iron accrued with gestational age at birth [range= 34.57-41.85 weeks] or postnatal age at scan [range= 5-64 days], using MRI to probe the T2* signal in N = 83 neonates in three striatal subregions. We found iron increased with postnatal age in the pallidum and putamen but not the caudate. No significant relationship between iron and gestational age was observed. Using a subset of infants scanned at preschool age (N = 26), we show distributions of iron shift between time points. In infants, the pallidum had the least iron of the three regions but had the most by preschool age. Together, this provides evidence of distinct change for striatal subregions, a possible differentiation between motor and cognitive systems, identifying a mechanism that may impact future trajectories.

Effects of Repetitive Transcranial Magnetic Stimulation on Pallidum GABAergic Neurons and Motor Function in Rat Models of Kernicterus

Kernicterus is a serious complication of hyperbilirubinemia, caused by neuronal injury due to excessive unconjugated bilirubin (UCB) in specific brain areas. This injury induced by this accumulation in the globus pallidus can induce severe motor dysfunction. Repetitive transcranial magnetic stimulation (rTMS) has shown neuroprotective effects in various neurological diseases. This study aimed to investigate the effects of rTMS on pallidal nerve damage and motor dysfunction in a rat model of kernicterus. Rats were divided into a sham group (n = 16), a model group (bilirubin with sham rTMS; n = 16) and an rTMS group (bilirubin with rTMS; n = 16). High-frequency rTMS (10 Hz) was applied starting from 24 h postmodeling for 7 days. The rotarod test, western blotting and immunohistochemical staining were performed to measure motor function and protein expression levels. The rTMS mitigated the negative effects of UCB on the general health of kernicterus-model rats and improved their growth and development. Furthermore, the rTMS alleviated UCB-induced motor dysfunction and increased the expression of GABAergic neuronal marker GAD67 in the globus pallidus. Notably, it also inhibited apoptosis-related protein caspase-3 activation. In conclusion, rTMS could alleviate motor dysfunction by inhibiting apoptosis and increasing globus pallidus GAD67 in kernicterus rat models, indicating that it may be a promising treatment for kernicterus.

Spatiomolecular Characterization of Dopamine D2 Receptors Cells in the Mouse External Globus Pallidus

The external globus pallidus (GPe) is part of the basal ganglia circuit and plays a key role in controlling the actions. Although, many evidence indicate that dopamine through its activation of D2 receptors (D2Rs) modulates the GPe neuronal activity, the precise spatiomolecular characterization of cell populations expressing D2Rs in the mouse GPe is still lacking. By combining single molecule in situ hybridization, cell type-specific imaging analyses, and electrophysiology slice recordings, we found that GPe D2R cells are neurons preferentially localized in the caudal portion of GPe. These neu- rons comprising pallido-striatal, pallido-nigral, and pallido-cortical neurons segregate into two distinct populations displaying molecular and electrophysiological features of GPe GABAergic PV/NKX2.1 and cholinergic neurons respectively. By clarifying the spatial molecular identity of GPe D2R neurons in the mouse, this work provides the basis for future studies aiming at disentangling the action of do- pamine within the GPe.

Cell and circuit complexity of the external globus pallidus

The external globus pallidus (GPe) of the basal ganglia has been underappreciated owing to poor understanding of its cells and circuits. It was assumed that the GPe consisted of a homogeneous neuron population primarily serving as a 'relay station' for information flowing through the indirect basal ganglia pathway. However, the advent of advanced tools in rodent models has sparked a resurgence in interest in the GPe. Here, we review recent data that have unveiled the cell and circuit complexity of the GPe. These discoveries have revealed that the GPe does not conform to traditional views of the basal ganglia. In particular, recent evidence confirms that the afferent and efferent connections of the GPe span both the direct and the indirect pathways. Furthermore, the GPe displays broad interconnectivity beyond the basal ganglia, consistent with its emerging multifaceted roles in both motor and non-motor functions. In summary, recent data prompt new proposals for computational rules of the basal ganglia.

Gestational and postnatal age associations for striatal tissue iron deposition in early infancy

Striatal development is crucial for later motor, cognitive, and reward behavior, but age-related change in striatal physiology during the neonatal period remains understudied. An MRI-based measure of tissue iron deposition, T2*, is a non-invasive way to probe striatal physiology neonatally, linked to dopaminergic processing and cognition in children and adults. Striatal subregions have distinct functions that may come online at different time periods in early life. To identify if there are critical periods before or after birth, we measured if striatal iron accrued with gestational age at birth [range=34.57-41.85 weeks] or postnatal age at scan [range=5-64 days], using MRI to probe the T2* signal in N=83 neonates in three striatal subregions. We found iron increased with postnatal age in the pallidum and putamen but not the caudate. No significant relationship between iron and gestational age was observed. Using a subset of infants scanned at preschool age (N=26), we show distributions of iron shift between timepoints. In infants, the pallidum had the least iron of the three regions but had the most by preschool age. Together, this provides evidence of distinct change for striatal subregions, a possible differentiation between motor and cognitive systems, identifying a mechanism that may impact future trajectories.

Highlights

Neonatal striatal tissue iron can be measured using the T2* signal from rsfMRInT2* changed with postnatal age in the pallidum and putamen but not in the caudatenT2* did not change with gestational age in any of the three regionsPatterns of iron deposition (nT2*) among regions shift from infancy to preschool.

Optogenetic Globus Pallidus Stimulation Improves Motor Deficits in 6-Hydroxydopamine-Lesioned Mouse Model of Parkinson's Disease

Excessive inhibition of the external globus pallidus (GPe) by striatal GABAergic neurons is considered a central mechanism contributing to motor symptoms of Parkinson's disease (PD). While electrophysiological findings support this view, behavioral studies assessing the beneficial effects of global GPe activations are scarce and the reported results are controversial. We used an optogenetic approach and the standard unilateral 6-hydroxydopamine nigrostriatal dopamine (DA) lesion model of PD to explore the effects of GPe photostimulation on motor deficits in mice. Global optogenetic GPe inhibition was used in normal mice to verify whether it reproduced the typical motor impairment induced by DA lesions. GPe activation improved ipsilateral circling, contralateral forelimb akinesia, locomotor hypoactivity, and bradykinesia in 6-OHDA-lesioned mice at ineffective photostimulation parameters (532 nm, 5 Hz, 3 mW) in normal mice. GPe photoinhibition (450 nm, 12 mW) had no effect on locomotor activity and forelimb use in normal mice. Bilateral photoinhibition (450 nm, 6 mW/side) reduced directed exploration and improved working memory performances indicating that recruitment of GPe in physiological conditions may depend on the behavioral task involved. Collectively, these findings shed new light on the functional role of GPe and suggest that it is a promising target for neuromodulatory restoration of motor deficits in PD.

Spontaneous Activity of the Local GABAergic Synaptic Network Causes Irregular Neuronal Firing in the External Globus Pallidus

Autonomously firing GABAergic neurons in the external globus pallidus (GPe) form a local synaptic network. In slices, most GPe neurons receive a continuous inhibitory synaptic barrage from 1 or 2 presynaptic GPe neurons. We measured the barrage's effect on the firing rate and regularity of GPe neurons in male and female mice using perforated patch recordings. Silencing the firing of parvalbumin-positive (PV+) GPe neurons by activating genetically expressed Archaerhodopsin current increased the firing rate and regularity of PV- neurons. In contrast, silencing Npas1+ GPe neurons with Archaerhodopsin had insignificant effects on Npas1- neuron firing. Blocking spontaneous GABAergic synaptic input with gabazine reproduced the effects of silencing PV+ neuron firing on the firing rate and regularity of Npas1+ neurons and had similar effects on PV+ neuron firing. To simulate the barrage, we constructed conductance waveforms for dynamic clamp based on experimentally measured inhibitory postsynaptic conductance trains from 1 or 2 unitary local connections. The resulting inhibition replicated the effect on firing seen in the intact active network in the slice. We then increased the number of unitary inputs to match estimates of local network connectivity in vivo As few as 5 unitary inputs produced large increases in firing irregularity. The firing rate was also reduced initially, but PV+ neurons exhibited a slow spike-frequency adaptation that partially restored the rate despite sustained inhibition. We conclude that the irregular firing pattern of GPe neurons in vivo is largely due to the ongoing local inhibitory synaptic barrage produced by the spontaneous firing of other GPe neurons.SIGNIFICANCE STATEMENT Functional roles of local axon collaterals in the external globus pallidus (GPe) have remained elusive because of difficulty in isolating local inhibition from other GABAergic inputs in vivo, and in preserving the autonomous firing of GPe neurons and detecting their spontaneous local inputs in slices. We used perforated patch recordings to detect spontaneous local inputs during rhythmic firing. We found that the autonomous firing of single presynaptic GPe neurons produces inhibitory synaptic barrages that significantly alter the firing regularity of other GPe neurons. Our findings suggest that, although GPe neurons receive input from only a few other GPe neurons, each local connection has a large impact on their firing.

Amino acid transporter Asc-1 (SLC7A10) expression is altered in basal ganglia in experimental Parkinsonism and L-dopa-induced dyskinesia model mice

In Parkinson's disease (PD), a decrease in dopamine levels in the striatum causes abnormal circuit activity in the basal ganglia, resulting in increased output via the substantia nigra pars reticulata (SNr). A characteristic feature of glutamatergic synaptic transmission in the basal ganglia circuitry under conditions of dopamine depletion is enhanced synaptic activity of NMDA receptors. However, the cause of this NMDA receptor hyperactivity is not fully understood. We focused on Asc-1 (SLC7A10), an alanine-serine-cysteine transporter, as one of the factors that regulate NMDA receptor activity by modulating D-serine and glycine concentration in synaptic clefts. We generated PD model mice by injection of 6-hydroxydopamine into the unilateral medial forebrain bundle and analyzed the expression level of Asc-1 mRNA in the nuclei of basal ganglia (the external segment of the globus pallidus (GPe), subthalamic nucleus (STN), and SNr) compared to control mice. Each nucleus was dissected using laser microdissection, and RNA was extracted and quantified by quantitative PCR. Asc-1 mRNA expression was significantly higher in the GPe and lower in the SNr under the PD state than that in control naïve mice. The STN showed no change in Asc-1 mRNA expression. We further modeled L-dopa-induced dyskinesia by administering L-dopa continuously for 14 days to the PD model mice and found that Asc-1 mRNA expression in the GPe and SNr became close to that of control mice, regardless of the presence of abnormal involuntary movements. The present study revealed that Asc-1 mRNA expression is differentially regulated in the basal ganglionic nuclei in response to striatal dopamine concentration (depleted or replenished) and suggests that Asc-1 can be a therapeutic target for the amelioration of motor symptoms of PD.

Lesions causing post-stroke spasticity localize to a common brain network

Objective

The efficacy of clinical interventions for post-stroke spasticity (PSS) has been consistently unsatisfactory, probably because lesions causing PSS may occur at different locations in the brain, leaving the neuroanatomical substrates of spasticity unclear. Here, we investigated whether heterogeneous lesions causing PSS were localized to a common brain network and then identified the key nodes in this network.

Methods

We used 32 cases of PSS and the Human Connectome dataset (n = 1,000), using a lesion network mapping method to identify the brain regions that were associated with each lesion in patients with PSS. Functional connectivity maps of all lesions were overlaid to identify common connectivity. Furthermore, a split-half replication method was used to evaluate reproducibility. Then, the lesion network mapping results were compared with those of patients with post-stroke non-spastic motor dysfunction (n = 29) to assess the specificity. Next, both sensitive and specific regions associated with PSS were identified using conjunction analyses, and the correlation between these regions and PSS was further explored by correlation analysis.

Results

The lesions in all patients with PSS were located in different cortical and subcortical locations. However, at least 93% of these lesions (29/32) had functional connectivity with the bilateral putamen and globus pallidus. These connections were highly repeatable and specific, as compared to those in non-spastic patients. In addition, the functional connectivity between lesions and bilateral putamen and globus pallidus in patients with PSS was positively correlated with the degree of spasticity.

Conclusion

We identified that lesions causing PSS were localized to a common functional connectivity network defined by connectivity to the bilateral putamen and globus pallidus. This network may best cover the locations of lesions causing PSS. The putamen and globus pallidus may be potential key regions in PSS. Our findings complement previous neuroimaging studies on PSS, contributing to identifying patients with stroke at high risk for spasticity at an early stage, and may point to PSS-specific brain stimulation targets.

Activation of pallidal H2 receptors induces catalepsy in Wistar rats: A regulatory role of CRF1 receptors

An increased concentration of histamine was found in the globus pallidus of parkinsonian patients. The role of this abnormality in the development of parkinsonism is unclear. We examined cataleptogenic activity of histamine injected into the globus pallidus (GP); also, the role of H2 receptors in histamine effect was evaluated. Given a possible role of the GP in integration and processing of stress signals, we tested the involvement of CRF1 receptors in the regulation of histamine effect. The experiments were conducted with male Wistar rats, catalepsy was assessed using bar test. The entopeduncular nucleus (EPN) was used as a neuroanatomical control. Intrapallidal injections of histamine (1.0 and 10.0 µg) produced clear cataleptic response whereas intra-EPN injections were ineffective. Histamine-induced catalepsy was dose-dependently attenuated by H2 receptor antagonist ranitidine and CRF1 receptor antagonist NBI 35965. The results suggest the involvement of pallidal H2 and CRF1 receptors in the development of catalepsy in rats. These findings may provide novel insight into the mechanism of parkinsonian-like disorders. In light of the presented data, H2 and CRF1 receptors might be potential targets for therapy of parkinsonism.

Bilirubin Encephalopathy

PURPOSE OF REVIEW

Hyperbilirubinemia is commonly seen in neonates. Though hyperbilirubinemia is typically asymptomatic, severe elevation of bilirubin levels can lead to acute bilirubin encephalopathy and progress to kernicterus spectrum disorder, a chronic condition characterized by hearing loss, extrapyramidal dysfunction, ophthalmoplegia, and enamel hypoplasia. Epidemiological data show that the implementation of universal pre-discharge bilirubin screening programs has reduced the rates of hyperbilirubinemia-associated complications. However, acute bilirubin encephalopathy and kernicterus spectrum disorder are still particularly common in low- and middle-income countries.

RECENT FINDINGS

The understanding of the genetic and biochemical processes that increase the susceptibility of defined anatomical areas of the central nervous system to the deleterious effects of bilirubin may facilitate the development of effective treatments for acute bilirubin encephalopathy and kernicterus spectrum disorder. Scoring systems are available for the diagnosis and severity grading of these conditions. The treatment of hyperbilirubinemia in newborns relies on the use of phototherapy and exchange transfusion. However, novel therapeutic options including deep brain stimulation, brain-computer interface, and stem cell transplantation may alleviate the heavy disease burden associated with kernicterus spectrum disorder. Despite improved screening and treatment options, the prevalence of acute bilirubin encephalopathy and kernicterus spectrum disorder remains elevated in low- and middle-income countries. The continued presence and associated long-term disability of these conditions warrant further research to improve their prevention and management.

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.

A Multi-Scale Computational Model of Levodopa-Induced Toxicity in Parkinson's Disease

Parkinson's disease (PD) is caused by the progressive loss of dopaminergic cells in substantia nigra pars compacta (SNc). The root cause of this cell loss in PD is still not decisively elucidated. A recent line of thinking has traced the cause of PD neurodegeneration to metabolic deficiency. Levodopa (L-DOPA), a precursor of dopamine, used as a symptom-relieving treatment for PD, leads to positive and negative outcomes. Several researchers inferred that L-DOPA might be harmful to SNc cells due to oxidative stress. The role of L-DOPA in the course of the PD pathogenesis is still debatable. We hypothesize that energy deficiency can lead to L-DOPA-induced toxicity in two ways: by promoting dopamine-induced oxidative stress and by exacerbating excitotoxicity in SNc. We present a systems-level computational model of SNc-striatum, which will help us understand the mechanism behind neurodegeneration postulated above and provide insights into developing disease-modifying therapeutics. It was observed that SNc terminals are more vulnerable to energy deficiency than SNc somas. During L-DOPA therapy, it was observed that higher L-DOPA dosage results in increased loss of terminals in SNc. It was also observed that co-administration of L-DOPA and glutathione (antioxidant) evades L-DOPA-induced toxicity in SNc neurons. Our proposed model of the SNc-striatum system is the first of its kind, where SNc neurons were modeled at a biophysical level, and striatal neurons were modeled at a spiking level. We show that our proposed model was able to capture L-DOPA-induced toxicity in SNc, caused by energy deficiency.

Topographic connectivity and cellular profiling reveal detailed input pathways and functionally distinct cell types in the subthalamic nucleus

The subthalamic nucleus (STN) controls psychomotor activity and is an efficient therapeutic deep brain stimulation target in individuals with Parkinson's disease. Despite evidence indicating position-dependent therapeutic effects and distinct functions within the STN, the input circuit and cellular profile in the STN remain largely unclear. Using neuroanatomical techniques, we construct a comprehensive connectivity map of the indirect and hyperdirect pathways in the mouse STN. Our circuit- and cellular-level connectivities reveal a topographically graded organization with three types of indirect and hyperdirect pathways (external globus pallidus only, STN only, and collateral). We confirm consistent pathways into the human STN by 7 T MRI-based tractography. We identify two functional types of topographically distinct glutamatergic STN neurons (parvalbumin [PV+/-]) with synaptic connectivity from indirect and hyperdirect pathways. Glutamatergic PV+ STN neurons contribute to burst firing. These data suggest a complex interplay of information integration within the basal ganglia underlying coordinated movement control and therapeutic effects.

The neural bases of vertebrate motor behaviour through the lens of evolution

The primary driver of the evolution of the vertebrate nervous system has been the necessity to move, along with the requirement of controlling the plethora of motor behavioural repertoires seen among the vast and diverse vertebrate species. Understanding the neural basis of motor control through the perspective of evolution, mandates thorough examinations of the nervous systems of species in critical phylogenetic positions. We present here, a broad review of studies on the neural motor infrastructure of the lamprey, a basal and ancient vertebrate, which enjoys a unique phylogenetic position as being an extant representative of the earliest group of vertebrates. From the central pattern generators in the spinal cord to the microcircuits of the pallial cortex, work on the lamprey brain over the years, has provided detailed insights into the basic organization (a bauplan) of the ancestral vertebrate brain, and narrates a compelling account of common ancestry of fundamental aspects of the neural bases for motion control, maintained through half a billion years of vertebrate evolution. This article is part of the theme issue 'Systems neuroscience through the lens of evolutionary theory'.

Reduced Child-Oriented Face Mirroring Brain Responses in Mothers With Opioid Use Disorder: An Exploratory Study

While the prevalence of opioid use disorder (OUD) among pregnant women has multiplied in the United States in the last decade, buprenorphine treatment (BT) for peripartum women with OUD has been administered to reduce risks of repeated cycles of craving and withdrawal. However, the maternal behavior and bonding in mothers with OUD may be altered as the underlying maternal behavior neurocircuit (MBN) is opioid sensitive. In the regulation of rodent maternal behaviors such as licking and grooming, a series of opioid-sensitive brain regions are functionally connected, including the ventral pallidum (VP). In humans, these brain regions, interact with the supplementary motor area (SMA) to regulate maternal behaviors and are functionally dysregulated by opioids. It is unclear how these brain regions respond to the emotions of their child for mothers receiving BT. In this functional magnetic resonance imaging (fMRI) pilot study in 22 mothers within the first postpartum year, including six mothers receiving BT and 16 non-OUD mothers as a comparison group (CG), we devised a child face mirroring task in fMRI settings to assess maternal responses to pictures of facial expressions of own child and an unknown child in an empathic mirroring condition (Join) and a non-mirroring observation condition (Observe). In each condition, faces of neutral, ambiguous, distressed, and joyful expressions of each child were repeatedly displayed in a random order. The response of SMA during empathic mirroring (Join) vs. non-mirroring (Observe) of own child was reduced among BT/OUD vs. CG. Within MBN, the left VP, critical for parental sensitivity, had a similar deficit. This study outlines potential mechanisms for investigating the risks of deficits in the neural responses to actual maternal sensitivity and parenting behavior in mothers with OUD, and potential targets for interventions that reduce stress and augment maternal behavior and child outcome.

MGE-Like Neural Progenitor Cell Survival and Expression of Parvalbumin and Proenkephalin in a Jaundiced Rat Model of Kernicterus

Kernicterus is a permanent condition caused by brain damage from bilirubin toxicity. Dystonia is one of the most debilitating symptoms of kernicterus and results from damage to the globus pallidus (GP). One potential therapeutic strategy to treat dystonia in kernicterus is to replace lost GP neurons and restore basal ganglia circuits through stem cell transplantation. Toward this end, we differentiated human embryonic stem cells (hESCs) into medial ganglion eminence (MGE; the embryological origin of most of the GP neurons)-like neural precursor cells (NPCs). We determined neurochemical phenotype in cell culture and after transplanting into the GP of jaundiced Gunn rats. We also determined grafted cell survival as well as migration, distribution, and morphology after transplantation. As in the GP, most cultured MGE-like NPCs expressed γ-aminobutyric acid (GABA), with some co-expressing markers for parvalbumin (PV) and others expressing markers for pro-enkephalin (PENK). MGE-like NPCs survived in brains at least 7 weeks after transplantation, with most aggregating near the injection site. Grafted cells expressed GABA and PV or PENK as in the normal GP. Although survival was low and the maturity of grafted cells varied, many cells produced neurite outgrowth. While promising, our results suggest the need to further optimize the differentiation protocol for MGE-like NPC for potential use in treating dystonia in kernicterus.

Synaptic pathology in Huntington's disease: Beyond the corticostriatal pathway

Huntington's disease (HD) is a heritable, fatal neurodegenerative disorder caused by a mutation in the Huntingtin gene. It is characterized by chorea, as well as cognitive and psychiatric symptoms. Histopathologically, there is a massive loss of striatal projection neurons and less but significant loss in other areas throughout the cortico-basal ganglia-thalamocortical (CBGTC) loop. The mutant huntingtin protein has been implicated in numerous functions, including an important role in synaptic transmission. Most studies on anatomical and physiological alterations in HD have focused on striatum and cerebral cortex. However, based on recent CBGTC projectome evidence, the need to study other pathways has become increasingly clear. In this review, we examine the current status of our knowledge of morphological and electrophysiological alterations of those pathways in animal models of HD. Based on recent studies, there is accumulating evidence that synaptic disconnection, particularly along excitatory pathways, is pervasive and almost universal in HD, thus supporting a critical role of the huntingtin protein in synaptic transmission.

Astrocyte-neuron interaction in the dorsal striatum-pallidal circuits and alcohol-seeking behaviors

In the striatum, two main types of GABAergic medium spiny neurons (MSNs), denoted striatonigral (or direct-pathway MSNs, dMSNs) and striatopallidal neurons (indirect-pathway MSNs, iMSNs), form circuits with distinct pallidal nuclei, which sends "GO" or "NO-GO" signals through the thalamus. These striatopallidal circuits evaluate and execute reward-seeking and taking behaviors. Especially, the dorsal striatum can be further divided into the dorsomedial striatum (DMS, equivalent to caudate in primates and humans) and dorsolateral striatum (DLS, equivalent to putamen), which orchestrates goal-directed and habitual reward-seeking and taking behaviors, respectively. Using optogenetics, chemogenetics and in vivo calcium imaging technologies combined with electrophysiology and digitalized behavior phenotyping, recent studies have revealed cell-, circuit- and context-specific functions of these microcircuits in addictive behaviors. Also, region-specific astrocytes regulate the homeostatic activities of the dMSNs and iMSNs as well as the downstream circuits, which determine the net balance of cortico-striato-pallidal activities to the thalamic neurons. This review will summarize the recent progress of striatopallidal circuits focusing on astrocyte-neuron interaction and, reward- and alcohol-seeking behaviors. Our review will also discuss the translational and clinical implications of these microcircuit studies. This article is part of the special Issue on "Neurocircuitry Modulating Drug and Alcohol Abuse".

On the Role of Arkypallidal and Prototypical Neurons for Phase Transitions in the External Pallidum

The external pallidum (globus pallidus pars externa [GPe]) plays a central role for basal ganglia functions and dynamics and, consequently, has been included in most computational studies of the basal ganglia. These studies considered the GPe as a homogeneous neural population. However, experimental studies have shown that the GPe contains at least two distinct cell types (prototypical and arkypallidal cells). In this work, we provide in silico insight into how pallidal heterogeneity modulates dynamic regimes inside the GPe and how they affect the GPe response to oscillatory input. We derive a mean-field model of the GPe system from a microscopic spiking neural network of recurrently coupled prototypical and arkypallidal neurons. Using bifurcation analysis, we examine the influence of dopamine-dependent changes of intrapallidal connectivity on the GPe dynamics. We find that increased self-inhibition of prototypical cells can induce oscillations, whereas increased inhibition of prototypical cells by arkypallidal cells leads to the emergence of a bistable regime. Furthermore, we show that oscillatory input to the GPe, arriving from striatum, leads to characteristic patterns of cross-frequency coupling observed at the GPe. Based on these findings, we propose two different hypotheses of how dopamine depletion at the GPe may lead to phase-amplitude coupling between the parkinsonian beta rhythm and a GPe-intrinsic γ rhythm. Finally, we show that these findings generalize to realistic spiking neural networks of sparsely coupled Type I excitable GPe neurons.SIGNIFICANCE STATEMENT Our work provides (1) insight into the theoretical implications of a dichotomous globus pallidus pars externa (GPe) organization, and (2) an exact mean-field model that allows for future investigations of the relationship between GPe spiking activity and local field potential fluctuations. We identify the major phase transitions that the GPe can undergo when subject to static or periodic input and link these phase transitions to the emergence of synchronized oscillations and cross-frequency coupling in the basal ganglia. Because of the close links between our model and experimental findings on the structure and dynamics of prototypical and arkypallidal cells, our results can be used to guide both experimental and computational studies on the role of the GPe for basal ganglia dynamics in health and disease.

In vivo Bidirectional Modulation of Cannabinoid on the Activity of Globus Pallidus in Rats

Endocannabinoids are bioactive substances which participate in central motor control. The globus pallidus (GP) is a major nucleus in the basal ganglia circuit, which plays an important function in movement regulation. Both cannabinoid receptor type 1 (CB₁R) and cannabinoid receptor type 2 (CB₂R) are expressed in the GP suggesting GP as a main action area of endocannabinoids. To investigate the direct electrophysiological and behavioral effects of cannabinoids in GP, in vivo single unit extracellular recordings and behavioral tests were performed in rats. Administration of WIN 55,212-2 exerted three neuronal response patterns from all sampled neurons of GP, including (1) increase of the firing rate; (2) decrease of the firing rate; (3) increase and then decrease of the firing rate. Selectively blocking CB₁R by AM 251 decreased the firing rate and increased the firing rate. Selectively blocking CB₂R by AM 630 did not change the firing rate significantly, which suggested that endocannabinoids modulated the spontaneous firing activity of pallidal neurons mainly via CB₁R. Furthermore, co-application of AM 251, but not AM 630, blocked WIN 55,212-2-induced modulation of firing activity of pallidal neurons. Finally, both haloperidol-induced postural behavioral test and elevated body swing test (EBST) showed that unilateral microinjection of WIN 55,212-2 mainly induced contralateral-biased swing and deflection behaviors. Meanwhile, AM 251 produced opposite effect. The present in vivo study revealed that cannabinoids produced complicated electrophysiological and behavioral effects in the GP, which further demonstrated that the GP is a major functional region of endocannabinoid.

Optogenetic Activation of Striatopallidal Neurons Reveals Altered HCN Gating in DYT1 Dystonia

Firing activity of external globus pallidus (GPe) is crucial for motor control and is severely perturbed in dystonia, a movement disorder characterized by involuntary, repetitive muscle contractions. Here, we show that GPe projection neurons exhibit a reduction of firing frequency and an irregular pattern in a DYT1 dystonia model. Optogenetic activation of the striatopallidal pathway fails to reset pacemaking activity of GPe neurons in mutant mice. Abnormal firing is paralleled by alterations in motor learning. We find that loss of dopamine D2 receptor-dependent inhibition causes increased GABA input at striatopallidal synapses, with subsequent downregulation of hyperpolarization-activated, cyclic nucleotide-gated cation (HCN) channels. Accordingly, enhancing in vivo HCN channel activity or blocking GABA release restores both the ability of striatopallidal inputs to pause ongoing GPe activity and motor coordination deficits. Our findings demonstrate an impaired striatopallidal connectivity, supporting the central role of GPe in motor control and, more importantly, identifying potential pharmacological targets for dystonia.

Assessing brain iron and volume of subcortical nuclei in idiopathic rapid eye movement sleep behavior disorder

STUDY OBJECTIVES

The relationship of iron with cognitive and motor impairment in idiopathic rapid eye movement sleep behavior disorder (iRBD) remains unknown.

METHODS

Twenty-nine (29) patients and 28 healthy controls (HCs) underwent susceptibility weighted imaging and susceptibility mapping. These images were used to evaluate the nigrosome-1 (N1) sign in the substantia nigra (SN), global and regional high-iron (RII) content and volume of subcortical nuclei.

RESULTS

The number of iRBD patients with N1 loss (12) was significantly higher than HCs (2) (P=0.005). Compared with HCs, the iRBD patients had reduced volume of the right caudate nucleus (RCN) (P<0.05, FDR correction) but no significant changes in global and RII iron of the subcortical nuclei (all P>0.05, FDR correction). Multiple regression analysis revealed that: for cognitive function, the RII iron of the RCN was significantly correlated with visuospatial function and the global iron of the right dentate nucleus (RDN) was correlated with memory function; for motor function, the RII iron of the left DN (LDN) and global iron of the left CN correlated with the Alternate-Tap test (left, average), the global iron of the LDN correlated with the Alternate-Tap test (right), and the global iron of the left GP correlated with the 3-meter Timed Up and Go test (all P<0.05, FDR correction).

CONCLUSIONS

Our exploratory analysis found that iRBD patients had a higher incidence of N1 loss and reduced RCN volume after FDR correction. Cognitive and motor impairment were associated with iron deposition in several cerebral nuclei after FDR correction.

Striatal Direct Pathway Targets Npas1+ Pallidal Neurons

The classic basal ganglia circuit model asserts a complete segregation of the two striatal output pathways. Empirical data argue that, in addition to indirect-pathway striatal projection neurons (iSPNs), direct-pathway striatal projection neurons (dSPNs) innervate the external globus pallidus (GPe). However, the functions of the latter were not known. In this study, we interrogated the organization principles of striatopallidal projections and their roles in full-body movement in mice (both males and females). In contrast to the canonical motor-promoting response of dSPNs in the dorsomedial striatum (DMSdSPNs), optogenetic stimulation of dSPNs in the dorsolateral striatum (DLSdSPNs) suppressed locomotion. Circuit analyses revealed that dSPNs selectively target Npas1+ neurons in the GPe. In a chronic 6-hydroxydopamine lesion model of Parkinson's disease, the dSPN-Npas1+ projection was dramatically strengthened. As DLSdSPN-Npas1+ projection suppresses movement, the enhancement of this projection represents a circuit mechanism for the hypokinetic symptoms of Parkinson's disease that has not been previously considered. In sum, our results suggest that dSPN input to the GPe is a critical circuit component that is involved in the regulation of movement in both healthy and parkinsonian states.SIGNIFICANCE STATEMENT In the classic basal ganglia model, the striatum is described as a divergent structure: it controls motor and adaptive functions through two segregated, opposing output streams. However, the experimental results that show the projection from direct-pathway neurons to the external pallidum have been largely ignored. Here, we showed that this striatopallidal subpathway targets a select subset of neurons in the external pallidum and is motor-suppressing. We found that this subpathway undergoes changes in a Parkinson's disease model. In particular, our results suggest that the increase in strength of this subpathway contributes to the slowness or reduced movements observed in Parkinson's disease.

Dissociable Roles of Pallidal Neuron Subtypes in Regulating Motor Patterns

We have previously established that PV+ neurons and Npas1+ neurons are distinct neuron classes in the external globus pallidus (GPe): they have different topographical, electrophysiological, circuit, and functional properties. Aside from Foxp2+ neurons, which are a unique subclass within the Npas1+ class, we lack driver lines that effectively capture other GPe neuron subclasses. In this study, we examined the utility of Kcng4-Cre, Npr3-Cre, and Npy2r-Cre mouse lines (both males and females) for the delineation of GPe neuron subtypes. By using these novel driver lines, we have provided the most exhaustive investigation of electrophysiological studies of GPe neuron subtypes to date. Corroborating our prior studies, GPe neurons can be divided into two statistically distinct clusters that map onto PV+ and Npas1+ classes. By combining optogenetics and machine learning-based tracking, we showed that optogenetic perturbation of GPe neuron subtypes generated unique behavioral structures. Our findings further highlighted the dissociable roles of GPe neurons in regulating movement and anxiety-like behavior. We concluded that Npr3+ neurons and Kcng4+ neurons are distinct subclasses of Npas1+ neurons and PV+ neurons, respectively. Finally, by examining local collateral connectivity, we inferred the circuit mechanisms involved in the motor patterns observed with optogenetic perturbations. In summary, by identifying mouse lines that allow for manipulations of GPe neuron subtypes, we created new opportunities for interrogations of cellular and circuit substrates that can be important for motor function and dysfunction.SIGNIFICANCE STATEMENT Within the basal ganglia, the external globus pallidus (GPe) has long been recognized for its involvement in motor control. However, we lacked an understanding of precisely how movement is controlled at the GPe level as a result of its cellular complexity. In this study, by using transgenic and cell-specific approaches, we showed that genetically-defined GPe neuron subtypes have distinct roles in regulating motor patterns. In addition, the in vivo contributions of these neuron subtypes are in part shaped by the local, inhibitory connections within the GPe. In sum, we have established the foundation for future investigations of motor function and disease pathophysiology.

Progression of basal ganglia pathology in heterozygous Q175 knock-in Huntington's disease mice

We used behavioral testing and morphological methods to detail the progression of basal ganglia neuron type-specific pathology and the deficits stemming from them in male heterozygous Q175 mice, compared to age-matched WT males. A rotarod deficit was not present in Q175 mice until 18 months, but increased open field turn rate (reflecting hyperkinesia) and open field anxiety were evident at 6 months. No loss of striatal neurons was seen out to 18 months, but ENK+ and DARPP32+ striatal perikarya were fewer by 6 months, due to diminished expression, with further decline by 18 months. No reduction in SP+ striatal perikarya or striatal interneurons was seen in Q175 mice at 18 months, but cholinergic interneurons showed dendrite attenuation by 6 months. Despite reduced ENK expression in indirect pathway striatal perikarya, ENK-immunostained terminals in globus pallidus externus (GPe) were more abundant at 6 months and remained so out to 18 months. Similarly, SP-immunostained terminals from striatal direct pathway neurons were more abundant in globus pallidus internus and substantia nigra at 6 months and remained so at 18 months. FoxP2+ arkypallidal GPe neurons and subthalamic nucleus neurons were lost by 18 months but not prototypical PARV+ GPe neurons or dopaminergic nigral neurons. Our results show that striatal projection neuron abnormalities and behavioral abnormalities reflecting them develop between 2 and 6 months of age in Q175 male heterozygotes, indicating early effects of the HD mutation. The striatal pathologies resemble those in human HD, but are less severe at 18 months than even in premanifest HD.

Striatal cholinergic transmission. Focus on nicotinic receptors' influence in striatal circuits

The critical role of acetylcholine (ACh) in the basal ganglia is evident from the effect of cholinergic agents in patients suffering from several related neurological disorders, such as Parkinson's disease, Tourette syndrome, or dystonia. The striatum possesses the highest density of ACh markers in the basal ganglia underlying the importance of ACh in this structure. Striatal cholinergic interneurons (CINs) are responsible for the bulk of striatal ACh, although extrinsic cholinergic afferents from brainstem structures may also play a role. CINs are tonically active, and synchronized pause in their activity occurs following the presentation of salient stimuli during behavioral conditioning. However, the synaptic mechanisms involved are not fully understood in this physiological response. ACh modulates striatal circuits by acting on muscarinic and nicotinic receptors existing in several combinations both presynaptically and postsynaptically. While the effects of ACh in the striatum through muscarinic receptors have received particular attention, nicotinic receptors function has been less studied. Here, after briefly reviewing relevant results regarding muscarinic receptors expression and function, I will focus on striatal nicotinic receptor expressed presynaptically on glutamatergic and dopaminergic afferents and postsynaptically on diverse striatal interneurons populations. I will also review recent evidence suggesting the involvement of different GABAergic sources in two distinct nicotinic-receptor-mediated striatal circuits: the disynaptic inhibition of striatal projection neurons and the recurrent inhibition among CINs. A better understanding of striatal nicotinic receptors expression and function may help to develop targeted pharmacological interventions to treat brain disorders such as Parkinson's disease, Tourette syndrome, dystonia, or nicotine addiction.

Influence of Rearing Environment on Longitudinal Brain Development, Object Recognition Memory, and Exploratory Behaviors in the Domestic Pig (Sus scrofa)

Introduction

Over the last 40 years, the domestic pig has emerged as a prominent preclinical model as this species shares similarities with humans with regard to immunity, gastrointestinal physiology, and neurodevelopment. Artificial rearing of pigs provides a number of advantages over conventional rearing (i.e., true maternal care), including careful control of nutrient intake and environment conditions. Yet there remains a gap in knowledge when comparing brain development between sow-reared and artificially reared domestic pigs. Thus, our research sought to model brain development and assess recognition memory in a longitudinal manner by directly comparing rearing environments.

Methods

Forty-four intact (i.e., not castrated) male pigs were artificially reared or sow-reared from postnatal day 2 until postnatal week 4. After postnatal week 4, all pigs were housed in a group setting within the same environment until postnatal week 24. Magnetic resonance imaging was conducted on pigs at 8 longitudinal time-points to model developmental trajectories of brain macrostructural and microstructural outcomes. Additionally, pigs behavior were tested using the novel object recognition task at postnatal weeks 4 and 8.

Results

Throughout the 24-week study, no differences between rearing groups were noted in weekly body weights, average growth and feed intake patterns, or feed efficiency. Whole brain, gray matter, white matter, and cerebrospinal fluid growth patterns also did not differ between pigs assigned to different early-life rearing environments. Moreover, minimal differences in regional absolute volumes and fractional anisotropy developmental trajectories were identified, though artificially reared pigs exhibited higher initial rates of myelination in multiple brain regions compared with sow-reared pigs. Furthermore, behavioral assessment at both PNW 4 and 8 suggested little influence of rearing environment on recognition memory, however, an age-dependent increase in object recognition memory was observed in the sow-reared group.

Conclusion

Our findings suggest that early-life rearing environment influences the rate of development in some brain regions but has little influence on overall brain growth and object recognition memory and exploratory behaviors in the domestic pig. Artificial rearing may promote maturation in certain brain areas but does not appear to elicit long-term effects in outcomes including brain structure or object recognition memory.

Connectivity and Functionality of the Globus Pallidus Externa Under Normal Conditions and Parkinson's Disease

The globus pallidus externa (GPe) functions as a central hub in the basal ganglia for processing motor and non-motor information through the creation of complex connections with the other basal ganglia nuclei and brain regions. Recently, with the adoption of sophisticated genetic tools, substantial advances have been made in understanding the distinct molecular, anatomical, electrophysiological, and functional properties of GPe neurons and non-neuronal cells. Impairments in dopamine transmission in the basal ganglia contribute to Parkinson's disease (PD), the most common movement disorder that severely affects the patients' life quality. Altered GPe neuron activity and synaptic connections have also been found in both PD patients and pre-clinical models. In this review, we will summarize the main findings on the composition, connectivity and functionality of different GPe cell populations and the potential GPe-related mechanisms of PD symptoms to better understand the cell type and circuit-specific roles of GPe in both normal and PD conditions.

Cellular transcriptomics reveals evolutionary identities of songbird vocal circuits

Birds display advanced behaviors, including vocal learning and problem-solving, yet lack a layered neocortex, a structure associated with complex behavior in mammals. To determine whether these behavioral similarities result from shared or distinct neural circuits, we used single-cell RNA sequencing to characterize the neuronal repertoire of the songbird song motor pathway. Glutamatergic vocal neurons had considerable transcriptional similarity to neocortical projection neurons; however, they displayed regulatory gene expression patterns more closely related to neurons in the ventral pallium. Moreover, while γ-aminobutyric acid-releasing neurons in this pathway appeared homologous to those in mammals and other amniotes, the most abundant avian class is largely absent in the neocortex. These data suggest that songbird vocal circuits and the mammalian neocortex have distinct developmental origins yet contain transcriptionally similar neurons.

A change of mind: Globus pallidus activity and effective connectivity during changes in choice selections

The basal ganglia are a group of interconnected subcortical nuclei that plays a key role in multiple motor and cognitive processes, in a close interplay with several cortical regions. Two conflicting theories postulate that the basal ganglia pathways can either foster or suppress the cortico-striatal output or, alternatively, they can stabilize or destabilize the cortico-striatal circuit dynamics. These different approaches significantly impact the understanding of observable behaviours and cognitive processes in healthy, as well as clinical populations. We investigated the predictions of these models in healthy participants (N = 28), using dynamic causal modeling of fMRI BOLD activity to estimate time- and context-dependent changes in the indirect pathway effective connectivity, in association with repetitions or changes of choice selections. We used two multi-option tasks that required the participants to adapt to uncontrollable environmental changes, by performing sequential choice selections, with and without value-based feedbacks. We found that, irrespective of the task, the trials that were characterized by changes in choice selections (switch trials) were associated with a neural response that mostly overlapped with a network commonly described for the encoding of uncertainty. More interestingly, dynamic causal modeling and family-wise model comparison identified with high likelihood a directed causal relation from the external to the internal part of the globus pallidus (i.e., the short indirect pathway in the basal ganglia), in association with the switch trials. This finding supports the hypothesis that the short indirect pathway in the basal ganglia drives instability in the network dynamics, resulting in changes in choice selection.

Facilitation of GluN2C-containing NMDA receptors in the external globus pallidus increases firing of fast spiking neurons and improves motor function in a hemiparkinsonian mouse model

Globus pallidus externa (GPe) is a nucleus in the basal ganglia circuitry involved in the control of movement. Recent studies have demonstrated a critical role of GPe cell types in Parkinsonism. Specifically increasing the function of parvalbumin (PV) neurons in the GPe has been found to facilitate motor function in a mouse model of Parkinson’s disease (PD). The knowledge of contribution of NMDA receptors to GPe function is limited. Here, we demonstrate that fast spiking neurons in the GPe express NMDA receptor currents sensitive to GluN2C/GluN2D-selective inhibitors and glycine site agonist with higher efficacy at GluN2C-containing receptors. Furthermore, using a novel reporter model, we demonstrate the expression of GluN2C subunits in PV neurons in the GPe which project to subthalamic nuclei. GluN2D subunit was also found to localize to PV neurons in GPe. Ablation of GluN2C subunit does not affect spontaneous firing of fast spiking neurons. In contrast, facilitating the function of GluN2C-containing receptors using glycine-site NMDA receptor agonists, D-cycloserine (DCS) or AICP, increased the spontaneous firing frequency of PV neurons in a GluN2C-dependent manner. Finally, we demonstrate that local infusion of DCS or AICP into the GPe improved motor function in a mouse model of PD. Together, these results demonstrate that GluN2C-containing receptors and potentially GluN2D-containing receptors in the GPe may serve as a therapeutic target for alleviating motor dysfunction in PD and related disorders.

Apelin-13 regulates electrical activity in the globus pallidus and induces postural changes in rats

The globus pallidus is the relay nucleus of the basal ganglia, and changes in its electrical activity can cause motor impairment. Apelin-13 is widely distributed in the central and peripheral nervous systems. It has been demonstrated that apelin-13 plays important roles in the regulation of blood pressure and other non-motor functions. However, its role in motor function has rarely been reported. In the present study, apelin-13 (10 μM/100 μM) was injected into the globus pallidus of rats. The results showed that apelin-13 increased the spontaneous discharges in the majority of pallidal neurons. However, an apelin-13-induced inhibitory effect on the firing rate was also observed in a few pallidal neurons. In postural tests, after the systemic administration of haloperidol, unilateral pallidal injection of apelin-13 caused a contralateral deflection. Together, these findings suggest that apelin-13 regulates the electrical activity of pallidal neurons and thus participates in central motor control in rats. The study was approved by the Animal Ethics Committee of Qingdao University (approval No. 20200615Wistar0451003020) on June 15, 2020.

Cell Type-Specific Oxidative Stress Genomic Signatures in the Globus Pallidus of Dopamine-Depleted Mice

Neuron subtype dysfunction is a key contributor to neurologic disease circuits, but identifying associated gene regulatory pathways is complicated by the molecular complexity of the brain. For example, parvalbumin-expressing (PV+) neurons in the external globus pallidus (GPe) are critically involved in the motor deficits of dopamine-depleted mouse models of Parkinson's disease, where cell type-specific optogenetic stimulation of PV+ neurons over other neuron populations rescues locomotion. Despite the distinct roles these cell types play in the neural circuit, the molecular correlates remain unknown because of the difficulty of isolating rare neuron subtypes. To address this issue, we developed a new viral affinity purification strategy, Cre-Specific Nuclear Anchored Independent Labeling, to isolate Cre recombinase-expressing (Cre+) nuclei from the adult mouse brain. Applying this technology, we performed targeted assessments of the cell type-specific transcriptomic and epigenetic effects of dopamine depletion on PV+ and PV- cells within three brain regions of male and female mice: GPe, striatum, and cortex. We found GPe PV+ neuron-specific gene expression changes that suggested increased hypoxia-inducible factor 2α signaling. Consistent with transcriptomic data, regions of open chromatin affected by dopamine depletion within GPe PV+ neurons were enriched for hypoxia-inducible factor family binding motifs. The gene expression and epigenomic experiments performed on PV+ neurons isolated by Cre-Specific Nuclear Anchored Independent Labeling identified a transcriptional regulatory network mediated by the neuroprotective factor Hif2a as underlying neural circuit differences in response to dopamine depletion.SIGNIFICANCE STATEMENT Cre-Specific Nuclear Anchored Independent Labeling is an enhanced, virus-based approach to isolate nuclei of a specific cell type for transcriptome and epigenome interrogation that decreases dependency on transgenic animals. Applying this technology to GPe parvalbumin-expressing neurons in a mouse model of Parkinson's disease, we discovered evidence for an upregulation of the oxygen homeostasis maintaining pathway involving Hypoxia-inducible factor 2α. These results provide new insight into how neuron subtypes outside the substantia nigra pars compacta may be compensating at a molecular level for differences in the motor production neural circuit during the progression of Parkinson's disease. Furthermore, they emphasize the utility of cell type-specific technologies, such as Cre-Specific Nuclear Anchored Independent Labeling, for isolated assessment of specific neuron subtypes in complex systems.

Comparative analyses of transgene expression patterns after intra-striatal injections of rAAV2-retro in rats and rhesus monkeys: A light and electron microscopic study

Retrogradely-transducing viral vectors are versatile tools for anatomical and functional interrogations of neural circuits. These vectors can be applied in nonhuman primates (NHPs), powerful model species for neuroscientific studies with limited genetic tractability, but limited data are available regarding the tropism and transgene expression patterns of such viruses after injections in NHP brains. Consequently, NHP researchers must often rely on related data available from other species for experimental planning. To evaluate the suitability of rAAV2-retro in the NHP basal ganglia, we studied the transgene expression patterns at the light and electron microscope level after injections of rAAV2-retro vector encoding the opsin Jaws conjugated to a green fluorescent protein (GFP) in the putamen of rhesus macaques. For inter-species comparison, we injected the same vector in the rat dorsal striatum. In both species, GFP expression was observed in numerous cortical and subcortical regions with known striatal projections. However, important inter-species differences in pathway transduction were seen, including labeling of the intralaminar thalamostriatal projection in rats, but not monkeys. Electron microscopic ultrastructural observations within the basal ganglia revealed GFP labeling in both postsynaptic dendrites and presynaptic axonal terminals; the latter likely derived from anterograde transgene transport in neurons that project to the striatum, and from collaterals of these neurons. Our results suggest that certain neural pathways may be refractory to transduction by retrograde vectors in a species-specific manner, highlighting the need for caution when determining the suitability of a retrograde vector for NHP studies based solely on rodent data.

Synaptic Dysfunction in Huntington's Disease: Lessons from Genetic Animal Models

The understanding of the functional and structural changes occurring in the cerebral cortex and basal ganglia in Huntington's disease (HD) has benefited considerably from the generation of genetic animal models. Most studies of synaptic alterations in HD models have focused on the striatum, but a more complete picture of synaptic dysfunction in the cortico-basal ganglia-cortical loop is emerging. Here, we provide a review and analysis of current developments in the study of synaptic alterations in these areas using HD rodent models. Recent evidence indicates that cortical maldevelopment plays a role in synaptic dysfunction along the corticostriatal pathway that may have its roots in the way mutant huntingtin interacts with synaptic proteins. Furthermore, a progressive disconnection in the corticostriatal pathway leads to abnormal function engaging extrasynaptic N-methyl-D-aspartate glutamate receptors that contribute to eventual cell degeneration. In addition, biphasic increases followed by decreases in glutamate and dopamine release in the striatum could explain contrasting symptomatology in early and late stages of the disease. Changes in striatal output regions also are beginning to be examined. Finally, we highlight some therapeutic avenues aimed at rescuing synaptic dysfunction.

Dopamine D4 receptor modulates inhibitory transmission in pallido-pallidal terminals and regulates motor behavior

Two major groups of terminals release GABA within the Globus pallidus; one group is constituted by projections from striatal neurons, while endings of the intranuclear collaterals form the other one. Each neurons' population expresses different subtypes of dopamine D2-like receptors: D₂ R subtype is expressed by encephalin-positive MSNs, while pallidal neurons express the D₄ R subtype. The D₂ R modulates the firing rate of striatal neurons and GABA release at their projection areas, while the D₄ R regulates Globus pallidus neurons excitability and GABA release at their projection areas. However, it is unknown if these receptors control GABA release at pallido-pallidal collaterals and regulate motor behavior. Here, we present neurochemical evidence of protein content and binding of D₄ R in pallidal synaptosomes, control of [³ H] GABA release in pallidal slices of rat, electrophysiological evidence of the presence of D₄ R on pallidal recurrent collaterals in mouse slices, and turning behavior induced by D₄ R antagonist microinjected in amphetamine challenged rats. As in projection areas of pallidal neurons, GABAergic transmission in pallido-pallidal recurrent synapses is under modulation of D₄ R, while the D₂ R subtype, as known, modulates striato-pallidal projections. Also, as in projection areas, D₄ R contributes to control the motor activity differently than D₂ R. This study could help to understand the organization of intra-pallidal circuitry.

Parvalbumin+ and Npas1+ Pallidal Neurons Have Distinct Circuit Topology and Function

The external globus pallidus (GPe) is a critical node within the basal ganglia circuit. Phasic changes in the activity of GPe neurons during movement and their alterations in Parkinson's disease (PD) argue that the GPe is important in motor control. Parvalbumin-positive (PV+) neurons and Npas1+ neurons are the two principal neuron classes in the GPe. The distinct electrophysiological properties and axonal projection patterns argue that these two neuron classes serve different roles in regulating motor output. However, the causal relationship between GPe neuron classes and movement remains to be established. Here, by using optogenetic approaches in mice (both males and females), we showed that PV+ neurons and Npas1+ neurons promoted and suppressed locomotion, respectively. Moreover, PV+ neurons and Npas1+ neurons are under different synaptic influences from the subthalamic nucleus (STN). Additionally, we found a selective weakening of STN inputs to PV+ neurons in the chronic 6-hydroxydopamine lesion model of PD. This finding reinforces the idea that the reciprocally connected GPe-STN network plays a key role in disease symptomatology and thus provides the basis for future circuit-based therapies.SIGNIFICANCE STATEMENT The external pallidum is a key, yet an understudied component of the basal ganglia. Neural activity in the pallidum goes awry in neurologic diseases, such as Parkinson's disease. While this strongly argues that the pallidum plays a critical role in motor control, it has been difficult to establish the causal relationship between pallidal activity and motor function/dysfunction. This was in part because of the cellular complexity of the pallidum. Here, we showed that the two principal neuron types in the pallidum have opposing roles in motor control. In addition, we described the differences in their synaptic influence. Importantly, our research provides new insights into the cellular and circuit mechanisms that explain the hypokinetic features of Parkinson's disease.

Weakly correlated activity of pallidal neurons in behaving monkeys

The basal ganglia play a crucial role in the control of voluntary movements. Neurons in both the external and internal segments of the globus pallidus, the connecting and output nuclei of the basal ganglia, respectively, change their firing rates in relation to movements. Firing rate changes of movement-related neurons seem to convey signals for motor control. On the other hand, coincident spikes among neurons, that is, correlated activity, may also contribute to motor control. To address this issue, we first identified multiple pallidal neurons receiving inputs from the forelimb regions of the primary motor cortex and supplementary motor area, recorded neuronal activity of these neurons simultaneously, and analyzed their spike correlations while monkeys performed a hand-reaching task. Most (79%) pallidal neurons exhibited task-related firing rate changes, whereas only a small fraction (20%) showed significant but small and short correlated activity during the task performance. These results suggest that motor control signals are conveyed primarily by firing rate changes in the external and internal segments of the globus pallidus and that the contribution of correlated activity may play only a minor role in the healthy state.