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Two subtypes of striatal spiny projection neurons, iSPNs and dSPNs, whose axons form the "indirect" and "direct" pathways of the basal ganglia, respectively, both make synaptic connections in the external globus pallidus (GPe) but are usually found to have different effects on behavior. Activation of the terminal fields of iSPNs or dSPNs generated compound currents in almost all GPe neurons. To determine whether iSPNs and dSPNs have the same or different effects on pallidal neurons, we studied the unitary synaptic currents generated in GPe neurons by action potentials in single striatal neurons. We used optogenetic excitation to elicit repetitive firing in a small number of nearby SPNs, producing sparse barrages of inhibitory postsynaptic currents (IPSCs) in GPe neurons. From these barrages, we isolated sequences of IPSCs with similar time courses and amplitudes, which presumably arose from the same SPN. There was no difference between the amplitudes of unitary IPSCs generated by the indirect and direct pathways. Most unitary IPSCs were small, but a subset from each pathway were much larger. To determine the effects of these unitary synaptic currents on the action potential firing of GPe neurons, we drove SPNs to fire as before and recorded the membrane potential of GPe neurons. Large unitary potentials from iSPNs and dSPNs perturbed the spike timing of GPe neurons in a similar way. Most SPN-GPe neuron pairs are weakly connected, but a subset of pairs in both pathways are strongly connected. NEW & NOTEWORTHY This is the first study to record the synaptic currents generated by single identified direct or indirect pathway striatal neurons on single pallidal neurons. Each GPe neuron receives synaptic inputs from both pathways. Most striatal neurons generate small synaptic currents that become influential when occurring together, but a few are powerful enough to be individually influential.

Parvalbumin-expressing dorsal striatal fast-spiking interneurons, comprising ∼1% of the total dorsal striatal neuronal population, are necessary for the expression of compulsive-like ethanol consumption mice. Fast-spiking interneurons are driven to fire by glutamatergic inputs derived primarily from the cortex. However, these neurons also receive substantial GABAergic input from two sources: the globus pallidus and the reticular nucleus of the thalamus. How ethanol modulates inhibitory input onto fast-spiking neurons is unclear and, more broadly, alcohol effects on GABAergic synaptic transmission onto GABAergic interneurons are understudied. Examining this, we found that acute bath application of ethanol (50 mM) potentiated GABAergic transmission from both the globus pallidus and the reticular nucleus of the thalamus onto fast-spiking interneurons in mouse of both sexes. This ethanol-induced potentiation required postsynaptic calcium and was not accompanied by a sustained change in presynaptic GABA release probability. Examining whether this ethanol effect persisted following chronic intermittent ethanol exposure, we found attenuated acute-ethanol potentiation of GABAergic transmission from both the globus pallidus and the reticular nucleus of the thalamus onto striatal fast-spiking interneurons. These data underscore the impact of ethanol on GABAergic signaling in the dorsal striatum and support the notion that ethanol may disinhibit the dorsolateral striatum., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Recent studies have shown that temporal stability of the neuronal activity over time can be estimated by the structure of the spike-count autocorrelation of neuronal populations. This estimation, called the intrinsic timescale, has been computed for several cortical areas and can be used to propose a cortical hierarchy reflecting a scale of temporal receptive windows between areas. In this study, we performed an autocorrelation analysis on neuronal populations of three basal ganglia (BG) nuclei, including the striatum and the subthalamic nucleus (STN), the input structures of the BG, and the external globus pallidus (GPe). The analysis was performed during the baseline period of a motivational visuomotor task in which monkeys had to apply different amounts of force to receive different amounts of reward. We found that the striatum and the STN have longer intrinsic timescales than the GPe. Moreover, our results allow for the placement of these subcortical structures within the already-defined scale of cortical temporal receptive windows. Estimates of intrinsic timescales are important in adding further constraints in the development of computational models of the complex dynamics among these nuclei and throughout cortico-BG-thalamo-cortical loops., (© 2021. The Author(s).)

Symptoms of neurological diseases emerge through the dysfunction of neural circuits whose diffuse and intertwined architectures pose serious challenges for delivering therapies. Deep brain stimulation (DBS) improves Parkinson’s disease symptoms acutely but does not differentiate between neuronal circuits, and its effects decay rapidly if stimulation is discontinued. Recent findings suggest that optogenetic manipulation of distinct neuronal subpopulations in the external globus pallidus (GPe) provides long-lasting therapeutic effects in dopamine-depleted (DD) mice. We used synaptic differences to excite parvalbumin-expressing GPe neurons and inhibit lim-homeobox-6–expressing GPe neurons simultaneously using brief bursts of electrical stimulation. In DD mice, circuit-inspired DBS provided long-lasting therapeutic benefits that far exceeded those induced by conventional DBS, extending several hours after stimulation. These results establish the feasibility of transforming knowledge of circuit architecture into translatable therapeutic approaches.

Specification of the progenitors' regional identity is a pivotal step during development of the cerebral cortex and basal ganglia. The molecular mechanisms underlying progenitor regionalization, however, are poorly understood. Here we showed that the transcription factor Vax1 was highly expressed in the developing subpallium. In its absence, the RNA-Seq analysis, in situ RNA hybridization, and immunofluorescence staining results showed that the cell proliferation was increased in the subpallium, but the neuronal differentiation was blocked. Moreover, the dLGE expands ventrally, and the vLGE, MGE, and septum get smaller. Finally, overexpressed VAX1 in the LGE progenitors strongly inhibits Gsx2 expression. Taken together, our findings show that Vax1 is crucial for subpallium regionalization by repressing Gsx2., (© 2021. The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.)

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 ( DMS dSPNs), optogenetic stimulation of dSPNs in the dorsolateral striatum ( DLS dSPNs) 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 DLS dSPN-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., (Copyright © 2021 the authors.)

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., (Copyright © 2021 the authors.)

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., (Copyright © 2021 The Authors. Published by Elsevier Inc. All rights reserved.)

The basal ganglia (BG) inhibit movements through two independent circuits: the striatal neuron-indirect and the subthalamic nucleus-hyperdirect pathways. These pathways exert opposite effects onto external globus pallidus (GPe) neurons, whose functional importance as a relay has changed drastically with the discovery of two distinct cell types, namely the prototypic and the arkypallidal neurons. However, little is known about the synaptic connectivity scheme of different GPe neurons toward both motor-suppressing pathways, as well as how opposite changes in GPe neuronal activity relate to locomotion inhibition. Here, we optogenetically dissect the input organizations of prototypic and arkypallidal neurons and further define the circuit mechanism and behavioral outcome associated with activation of the indirect or hyperdirect pathways. This work reveals that arkypallidal neurons are part of a novel disynaptic feedback loop differentially recruited by the indirect or hyperdirect pathways and that broadcasts inhibitory control onto locomotion only when arkypallidal neurons increase their activity., Competing Interests: Declaration of Interests The authors declare no competing interests., (Copyright © 2020 The Authors. Published by Elsevier Inc. All rights reserved.)

Adenosine and dopamine interact antagonistically in living mammals. These interactions are mediated via adenosine A 2A and dopamine D 2 receptors (R). Stimulation of A 2A R inhibits and blockade of A 2A R enhances D 2 R-mediated locomotor activation and goal-directed behavior in rodents. In striatal membrane preparations, adenosine decreases both the affinity and the signal transduction of D 2 R via its interaction with A 2A R. Reciprocal A 2A R/D 2 R interactions occur mainly in striatopallidal GABAergic medium spiny neurons (MSNs) of the indirect pathway that are involved in motor control, and in striatal astrocytes. In the nucleus accumbens, they also take place in MSNs involved in reward-related behavior. A 2A R and D 2 R co-aggregate, co-internalize, and co-desensitize. They are at very close distance in biomembranes and form heteromers. Antagonistic interactions between adenosine and dopamine are (at least partially) caused by allosteric receptor-receptor interactions within A 2A R/D 2 R heteromeric complexes. Such interactions may be exploited in novel strategies for the treatment of Parkinson's disease, schizophrenia, substance abuse, and perhaps also attention deficit-hyperactivity disorder. Little is known about shifting A 2A R/D 2 R heteromer/homodimer equilibria in the brain. Positron emission tomography with suitable ligands may provide in vivo information about receptor crosstalk in the living organism. Some experimental approaches, and strategies for the design of novel imaging agents (e.g., heterobivalent ligands) are proposed in this review.

Progenitors in the dorsal lateral ganglionic eminence (dLGE) are known to give rise to olfactory bulb (OB) interneurons and intercalated cells (ITCs) of the amygdala. The dLGE enriched transcription factor Sp8 is required for the normal generation of ITCs as well as OB interneurons, particularly the calretinin (CR)-expressing subtype. In this study, we used a genetic gain-of-function approach in mice to examine the roles Sp8 plays in controlling the development of dLGE-derived neuronal subtypes. Misexpression of Sp8 throughout the ventral telencephalic subventricular zone (SVZ) from early embryonic stages, led to an increased generation of ITCs which was dependent on Tshz1 gene dosage. Additionally, Sp8 misexpression impaired rostral migration of OB interneurons with clusters of CR interneurons seen in the SVZ along with decreased differentiation of calbindin OB interneurons. Sp8 misexpression throughout the ventral telencephalon also reduced ventral LGE neuronal subtypes including striatal projection neurons. Delaying Sp8 misexpression until E14-15 rescued the striatal and amygdala phenotypes but only partially rescued OB interneuron reductions, consistent with an early window of striatal and amygdala neurogenesis and ongoing OB interneuron generation at this late stage. Our results demonstrate critical roles for the timing and neuronal cell-type specificity of Sp8 expression in mouse LGE neurogenesis., (© The Author(s) 2020. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permission@oup.com.)

In the current study, we use tyrosine hydroxylase (TH) immunohistochemistry to detail the nuclear parcellation and cellular morphology of neurons belonging to the catecholaminergic system in the brain of the Nile crocodile. In general, our results are similar to that found in another crocodilian (the spectacled caiman) and indeed other vertebrates, but certain differences of both evolutionary and functional significance were noted. TH immunopositive (TH+) neurons forming distinct nuclei were observed in the olfactory bulb (A16), hypothalamus (A11, A13-15), midbrain (A8-A10), pons (A5-A7) and medulla oblongata (area postrema, C1, C2, A1, A2), encompassing the more commonly observed nuclear complexes of this system across vertebrates. In addition, TH + neurons forming distinct nuclei not commonly identified in vertebrates were observed in the anterior olfactory nucleus, the pretectal nuclear complex, adjacent to the posterior commissure, and within nucleus laminaris, nucleus magnocellularis lateralis and the lateral vestibular nucleus. Palely stained TH + neurons were observed in some of the serotonergic nuclei, including the medial and lateral divisions of the superior raphe nucleus and the inferior raphe and inferior reticular nucleus, but not in other serotonergic nuclei. In birds, a high density of TH + fibres and pericellular baskets in the dorsal ventricular ridge marks the location of the nidopallium caudolaterale (NCL), a putative avian analogue of mammalian prefrontal cortex. In the dorsal ventricular ridge (DVR) of the crocodile a small region in the caudolateral anterior DVR (ADVRcl) revealed a slightly higher density of TH + fibres and some pericellular baskets (formed by only few TH + fibres). These results are discussed in an evolutionary and functional framework., (Copyright © 2020 Elsevier B.V. All rights reserved.)

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., (Copyright © 2020 the authors.)

Neural stem cells directly or indirectly generate all neurons and macroglial cells and guide migrating neurons by using a palisade-like scaffold made of their radial fibers. Here, we describe an unexpected role for the radial fiber scaffold in directing corticospinal and other axons at the junction between the striatum and globus pallidus. The maintenance of this scaffold, and consequently axon pathfinding, is dependent on the expression of an atypical RHO-GTPase, RND3/RHOE, together with its binding partner ARHGAP35/P190A, a RHO GTPase-activating protein, in the radial glia-like neural stem cells within the ventricular zone of the medial ganglionic eminence. This role is independent of RND3 and ARHGAP35 expression in corticospinal neurons, where they regulate dendritic spine formation, axon elongation, and pontine midline crossing in a FEZF2-dependent manner. The prevalence of neural stem cell scaffolds and their expression of RND3 and ARHGAP35 suggests that these observations might be broadly relevant for axon guidance and neural circuit formation., Competing Interests: Declaration of Interests The authors declare no competing interests., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Short-term depression is a low-pass filter of synaptic information, reducing synaptic information transfer at high presynaptic firing frequencies. Consequently, during elevated presynaptic firing, little information passes to the postsynaptic neuron. However, many neurons fire at relatively high frequencies all the time. Does depression silence their synapses? We tested this apparent contradiction in the indirect pathway of the basal ganglia. Using numerical modeling and whole-cell recordings from single entopeduncular nucleus (EP) neurons in rat brain slices, we investigated how different firing rates of globus pallidus (GP) neurons affect information transmission to the EP. Whole-cell recordings showed significant variability in steady-state depression, which decreased as stimulation frequency increased. Modeling predicted that this variability would translate into different postsynaptic noise levels during constitutive presynaptic activity. Our simulations further predicted that individual GP-EP synapses mediate gain control. However, when we consider the integration of multiple inputs, the broad range of GP firing rates would enable different modes of information transmission. Finally, we predict that changes in dopamine levels can shift the action of GP neurons from rate coding to gain modulation. Our results thus demonstrate how short-term depression shapes information transmission in the basal ganglia in particular and via GABAergic synapses in general.

Adenosine 2A receptor (A 2A R)-containing indirect medium spiny neurons (iMSNs) in the dorsomedial striatum (DMS) contribute to reward-seeking behaviors. However, those roles for ethanol-seeking behaviors remain unknown. To investigate ethanol-seeking behaviors, we used an ethanol-containing reward (10% ethanol and 10% sucrose solution; 10E10S). Upon conditioning with 10E10S, mice that initially only preferred 10% sucrose, not 10E10S, showed a stronger preference for 10E10S. Then, we investigated whether the manipulation of the DMS-external globus pallidus (GPe) iMSNs circuit alters the ethanol-containing reward (10E10S) seeking behaviors using the combination of pharmacologic and optogenetic approaches. DMS A 2A R activation dampened operant conditioning-induced ethanol-containing reward, whereas A 2A R antagonist abolished the effects of the A 2A R agonist and restored ethanol-containing reward-seeking. Moreover, pre-ethanol exposure potentiated the A 2A R-dependent reward-seeking. Interestingly, mice exhibiting ethanol-containing reward-seeking showed the reduction of the DMS iMSNs activity, suggesting that disinhibiting iMSNs decreases reward-seeking behaviors. In addition, we found that A 2A R activation reversed iMSNs neural activity in the DMS. Similarly, optogenetic stimulation of the DMS-GPe iMSNs reduced ethanol-containing reward-seeking, whereas optogenetic inhibition of the DMS-GPe iMSNs reversed this change. Together, our study demonstrates that DMS A 2A R and iMSNs regulate ethanol-containing reward-seeking behaviors. SIGNIFICANCE STATEMENT Our findings highlight the mechanisms of how operant conditioning develops the preference of ethanol-containing conditioned reward. Mice exhibiting ethanol-containing reward-seeking showed a reduction of the indirect medium spiny neuronal activity in the dorsomedial striatum. Pharmacological activation of adenosine A 2A receptor (A 2A R) or optogenetic activation of indirect medium spiny neurons dampened operant conditioned ethanol-containing reward-seeking, whereas inhibiting this neuronal activity restored ethanol-containing reward-seeking. Furthermore, repeated intermittent ethanol exposure potentiated A 2A R-dependent reward-seeking. Therefore, our finding suggests that A 2A R-containing indirect medium spiny neuronal activation reduces ethanol-containing reward-seeking, which may provide a potential therapeutic target for alcohol use disorder., (Copyright © 2019 the authors.)

Variations in the human FTO gene have been linked to obesity and altered connectivity of the dopaminergic neurocircuitry. Here, we report that fat mass and obesity-associated protein (FTO) in dopamine D2 receptor-expressing medium spiny neurons (D2 MSNs) of mice regulate the excitability of these cells and control their striatopallidal globus pallidus external (GPe) projections. Lack of FTO in D2 MSNs translates into increased locomotor activity to novelty, associated with altered timing behavior, without impairing the ability to control actions or affecting reward-driven and conditioned behavior. Pharmacological manipulations of dopamine D1 receptor (D1R)- or D2R-dependent pathways in these animals reveal altered responses to D1- and D2-MSN-mediated control of motor output. These findings reveal a critical role for FTO to control D2 MSN excitability, their projections to the GPe, and behavioral responses to novelty., (Copyright © 2019 The Authors. Published by Elsevier Inc. All rights reserved.)

Dopamine neurotransmission is suspected to play important physiological roles in multiple sparsely innervated brain nuclei, but there has not been a means to measure synaptic dopamine release in such regions. The globus pallidus externa (GPe) is a major locus in the basal ganglia that displays a sparse innervation of en passant dopamine axonal fibers. Due to the low levels of innervation that preclude electrochemical analysis, it is unknown if these axons engage in neurotransmission. To address this, we introduce an optical approach using a pH-sensitive fluorescent false neurotransmitter, FFN102, that exhibits increased fluorescence upon exocytosis from the acidic synaptic vesicle to the neutral extracellular milieu. In marked contrast to the striatum, FFN102 transients in the mouse GPe were spatially heterogeneous and smaller than in striatum with the exception of sparse hot spots. GPe transients were also significantly enhanced by high frequency stimulation. Our results support hot spots of dopamine release from substantia nigra axons., Competing Interests: JM, TC, ME, UK, DS, CK, DS No competing interests declared, (© 2018, Meszaros et al.)

Stress evokes directed movement to escape or hide from potential danger. Corticotropin-releasing factor (CRF) neurons are highly activated by stress; however, it remains unclear how this activity participates in stress-evoked movement. The external globus pallidus (GPe) expresses high levels of the primary receptor for CRF, CRFR1, suggesting the GPe may serve as an entry point for stress-relevant information to reach basal ganglia circuits, which ultimately gate motor output. Indeed, projections from CRF neurons are present within the GPe, making direct contact with CRFR1-positive neurons. CRFR1 expression is heterogenous in the GPe; prototypic GPe neurons selectively express CRFR1, while arkypallidal neurons do not. Moreover, CRFR1-positive GPe neurons are excited by CRF via activation of CRFR1, while nearby CRFR1-negative neurons do not respond to CRF. Using monosynaptic rabies viral tracing techniques, we show that CRF neurons in the stress-activated paraventricular nucleus of the hypothalamus (PVN), central nucleus of the amygdala (CeA), and bed nucleus of the stria terminalis (BST) make synaptic connections with CRFR1-positive neurons in the GPe an unprecedented circuit connecting the limbic system with the basal ganglia. CRF neurons also make synapses on Npas1 neurons, although the majority of Npas1 neurons are arkypallidal and do not express CRFR1. Interestingly, prototypic and arkypallidal neurons receive different patterns of innervation from CRF-rich nuclei. Hypothalamic CRF neurons preferentially target prototypic neurons, while amygdalar CRF neurons preferentially target arkypallidal neurons, suggesting that these two inputs to the GPe may have different impacts on GPe output. Together, these data describe a novel neural circuit by which stress-relevant information carried by the limbic system signals in the GPe via CRF to influence motor output.

The striatum and globus pallidus are principal nuclei of the basal ganglia. Nissl- and acetylcholinesterase-stained sections of the tree shrew brain showed the neuroanatomical features of the caudate nucleus (Cd), internal capsule (ic), putamen (Pu), accumbens, internal globus pallidus, and external globus pallidus. The ic separated the dorsal striatum into the Cd and Pu in the tree shrew, but not in rats and mice. In addition, computer-based 3D images allowed a better understanding of the position and orientation of these structures. These data provided a large-scale atlas of the striatum and globus pallidus in the coronal, sagittal, and horizontal planes, the first detailed distribution of parvalbumin-immunoreactive cells in the tree shrew, and the differences in morphological characteristics and density of parvalbumin-immunoreactive neurons between tree shrew and rat. Our findings support the tree shrew as a potential model for human striatal disorders.

Both the subthalamic nucleus (STN) and the globus pallidus pars interna (GPi) are major targets for neuromodulation therapy for movement disorders. An example of such a therapy is deep brain stimulation (DBS). The striatum is the primary target for pharmacological treatment of these disorders. To further our understanding of both the functional relationships among motor nuclei and the mechanisms of therapies for movement disorders, it is important to clarify how changing the neuronal activity of one target, either by medication or by artificial electrical stimulation, affects the other connected nuclei. To investigate this point, we recorded single-unit activity from tonically active neurons (TANs), which are putative cholinergic interneurons in the striatum, of healthy monkeys (Macaca fuscata) during electrical stimulation of the STN or GPi. Both STN stimulation and GPi stimulation reduced the TAN spike rate. Local infusion of a D2 receptor antagonist in the striatum blocked the reduction in spike rate induced by STN stimulation but not that induced by GPi stimulation. Further, STN stimulation induced phasic dopamine release in the striatum as revealed by in vivo fast-scan cyclic voltammetry. These results suggest the presence of multiple, strong functional relationships among the STN, GPi, and striatum that have different pathways and imply distinct therapeutic mechanisms for STN- and GPi-DBS., (© 2017 Federation of European Neuroscience Societies and John Wiley & Sons Ltd.)

Previous studies have suggested that the neurokinin-1 receptor (NK-1R) expressing neurons in the globus pallidus (GP) receive substance P (SP), presumably released by axon collaterals of striatal direct neurons. However, the effect of SP on the GP remains unclear. In this study, we identified that the SP-responsive cells comprise a highly specific cell type in the GP with regard to immunofluorescence, electrophysiology, and projection properties. Morphologically, NK-1R-immunoreactive neurons occasionally co-expressed parvalbumin (PV) and/or Lim-homeobox 6 (Lhx6), but not Forkhead box protein P2 (FoxP2), which is mainly expressed by arkypallidal neurons. Retrograde tracing experiments also showed that some of GP neurons projecting to the subthalamic nucleus (namely prototypic neurons) expressed NK-1R as well as Lhx6 and/or PV, but not FoxP2. In vitro electrophysiological study revealed that, among 48 GP neurons, the SP agonist induced inward current in 21 neurons. The response was prevented by bath application of the NK-1R antagonist. Based on the firing properties, 92 recorded GP neurons were classified into three distinct types, i.e., CL1, 2, and 3. Interestingly, all the SP-responsive neurons were found to be in CL2 and CL3 types, but not in CL1. Moreover, active and passive membrane properties of the neurons in those clusters and immunofluorescent identification suggested that CL1 and CL2/3 could be considered as arkypallidal and prototypic neurons, respectively. Therefore, SP-responsive neurons were one of the populations of prototypic neurons based on both anatomical and electrophysiological results. Altogether, the striatal direct pathway neurons could affect the indirect pathway in the way of prototypic neurons, via the action of SP to NK-1R.

The ability to alter neuronal gene expression, either to affect levels of endogenous molecules or to express exogenous ones, is a powerful tool for linking brain and behavior. Scientists continue to finesse genetic manipulation in mice. Yet mice do not exhibit every behavior of interest. For example, Mus musculus do not readily imitate sounds, a trait known as vocal learning and a feature of speech. In contrast, thousands of bird species exhibit this ability. The circuits and underlying molecular mechanisms appear similar between disparate avian orders and are shared with humans. An advantage of studying vocal learning birds is that the neurons dedicated to this trait are nested within the surrounding brain regions, providing anatomical targets for relating brain and behavior. In songbirds, these nuclei are known as the song control system. Molecular function can be interrogated in non-traditional model organisms by exploiting the ability of viruses to insert genetic material into neurons to drive expression of experimenter-defined genes. To date, the use of viruses in the song control system is limited. Here, we review prior successes and test additional viruses for their capacity to transduce basal ganglia song control neurons. These findings provide a roadmap for troubleshooting the use of viruses in animal champions of fascinating behaviors-nowhere better featured than at the 12th International Congress!

Basal ganglia (BG) circuits orchestrate complex motor behaviors predominantly via inhibitory synaptic outputs. Although these inhibitory BG outputs are known to reduce the excitability of postsynaptic target neurons, precisely how this change impairs motor performance remains poorly understood. Here, we show that optogenetic photostimulation of inhibitory BG inputs from the globus pallidus induces a surge of action potentials in the ventrolateral thalamic (VL) neurons and muscle contractions during the post-inhibitory period. Reduction of the neuronal population with this post-inhibitory rebound firing by knockout of T-type Ca 2+ channels or photoinhibition abolishes multiple motor responses induced by the inhibitory BG input. In a low dopamine state, the number of VL neurons showing post-inhibitory firing increases, while reducing the number of active VL neurons via photoinhibition of BG input, effectively prevents Parkinson disease (PD)-like motor symptoms. Thus, BG inhibitory input generates excitatory motor signals in the thalamus and, in excess, promotes PD-like motor abnormalities. VIDEO ABSTRACT., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Orexin, released from the hypothalamus, has been implicated in various basic non-somatic functions including feeding, the sleep-wakefulness cycle, emotion, and cognition. However, the role of orexin in somatic motor control is still little known. Here, using whole-cell patch clamp recording and immunostaining, we investigated the effect and the underlying receptor mechanism of orexin-A on neurons in the globus pallidus internus (GPi), a critical structure in the basal ganglia and an effective target for deep brain stimulation therapy. Our results showed that orexin-A induced direct postsynaptic excitation of GPi neurons in a concentration-dependent manner. The orexin-A-induced excitation was mediated via co-activation of both OX1 and OX2 receptors. Furthermore, the immunostaining results showed that OX1 and OX2 receptors were co-localized in the same GPi neurons. These results suggest that the central orexinergic system actively modulates the motor functions of the basal ganglia via direct innervation on GPi neurons and presumably participates in somatic-non-somatic integration.

Here, we unravel the mechanism of action of the Ikaros family zinc finger protein Helios (He) during the development of striatal medium spiny neurons (MSNs). He regulates the second wave of striatal neurogenesis involved in the generation of striatopallidal neurons, which express dopamine 2 receptor and enkephalin. To exert this effect, He is expressed in neural progenitor cells (NPCs) keeping them in the G 1 /G 0 phase of the cell cycle. Thus, a lack of He results in an increase of S-phase entry and S-phase length of NPCs, which in turn impairs striatal neurogenesis and produces an accumulation of the number of cycling NPCs in the germinal zone (GZ), which end up dying at postnatal stages. Therefore, He -/- mice show a reduction in the number of dorso-medial striatal MSNs in the adult that produces deficits in motor skills acquisition. In addition, overexpression of He in NPCs induces misexpression of DARPP-32 when transplanted in mouse striatum. These findings demonstrate that He is involved in the correct development of a subset of striatopallidal MSNs and reveal new cellular mechanisms for neuronal development., (© 2017. Published by The Company of Biologists Ltd.)

The basal ganglia (BG) integrate inputs from diverse sensorimotor, limbic, and associative regions to guide action-selection and goal-directed behaviors. The entopeduncular nucleus (EP) is a major BG output nucleus and has been suggested to channel signals from distinct BG nuclei to target regions involved in diverse functions. Here we use single-cell transcriptional and molecular analyses to demonstrate that the EP contains at least three classes of projection neurons-glutamate/GABA co-releasing somatostatin neurons, glutamatergic parvalbumin neurons, and GABAergic parvalbumin neurons. These classes comprise functionally and anatomically distinct output pathways that differentially affect EP target regions, such as the lateral habenula (LHb) and thalamus. Furthermore, LHb- and thalamic-projecting EP neurons are differentially innervated by subclasses of striatal and pallidal neurons. Therefore, we identify previously unknown subdivisions within the EP and reveal the existence of cascading, molecularly distinct projections through striatum and globus pallidus to EP targets within epithalamus and thalamus., (Copyright © 2017 Elsevier Inc. All rights reserved.)

The basal ganglia are a forebrain network of interconnected nuclei that are involved in action selection, reward circuits and coordinating movement. PDE10A inhibition has been proposed as a novel way to modulate basal ganglia circuitry and to ameliorate symptoms in Huntington's disease, Parkinson's disease and Schizophrenia. However, despite encouraging results from pre-clinical models, PDE10A inhibitors failed to show efficacy as an antipsychotic in several clinical trials. PDE10A is expressed in the medium spiny neurons of the striatum and works to limit cyclic nucleotide signaling in response to modulatory neurotransmitters like dopamine. In this chapter, we will review the current literature on PDE10A and discuss how inhibition of PDE10A will result in alterations of the basal ganglia circuitry at the biochemical, physiological and behavioral level.

Cyclic nucleotide phosphodiesterase (PDE) enzymes catalyze the hydrolysis and inactivation of cyclic nucleotides (cAMP/cGMP) in the brain. Several classes of PDE enzymes with distinct tissue distributions, cyclic nucleotide selectivity, and regulatory factors are highly expressed in brain regions subserving cognitive and motor processes known to be disrupted in neurodegenerative diseases such as Parkinson's disease (PD). Furthermore, small-molecule inhibitors of several different PDE family members alter cyclic nucleotide levels and favorably enhance motor performance and cognition in animal disease models. This chapter will explore the roles and therapeutic potential of non-selective and selective PDE inhibitors on neural processing in fronto-striatal circuits in normal animals and models of DOPA-induced dyskinesias (LIDs) associated with PD. The impact of selective PDE inhibitors and augmentation of cAMP and cGMP signaling on the membrane excitability of striatal medium-sized spiny projection neurons (MSNs) will be discussed. The effects of cyclic nucleotide signaling and PDE inhibitors on synaptic plasticity of striatonigral and striatopallidal MSNs will be also be reviewed. New data on the efficacy of PDE10A inhibitors for reversing behavioral and electrophysiological correlates of L-DOPA-induced dyskinesias in a rat model of PD will also be presented. Together, these data will highlight the potential of novel PDE inhibitors for treatment of movement disorders such as PD which are associated with abnormal corticostriatal transmission.

Deep brain stimulation of the internal globus pallidus (GPi) is a major treatment for advanced Parkinson's disease. The effects of this intervention on electrical activity patterns in targets of GPi output, specifically in the thalamus, are poorly understood. The experiments described here examined these effects using electrophysiological recordings in two Rhesus monkeys rendered moderately parkinsonian through treatment with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), after sampling control data in the same animals. Analysis of spontaneous spiking activity of neurons in the basal ganglia-receiving areas of the ventral thalamus showed that MPTP-induced parkinsonism is associated with a reduction of firing rates of segments of the data that contained neither bursts nor decelerations, and with increased burst firing. Spectral analyses revealed an increase of power in the 3- to 13-Hz band and a reduction in the γ-range in the spiking activity of these neurons. Electrical stimulation of the ventrolateral motor territory of GPi with macroelectrodes, mimicking deep brain stimulation in parkinsonian patients (bipolar electrodes, 0.5 mm intercontact distance, biphasic stimuli, 120 Hz, 100 μs/phase, 200 μA), had antiparkinsonian effects. The stimulation markedly reduced oscillations in thalamic firing in the 13- to 30-Hz range and uncoupled the spiking activity of recorded neurons from simultaneously recorded local field potential (LFP) activity. These results confirm that oscillatory and nonoscillatory characteristics of spontaneous activity in the basal ganglia receiving ventral thalamus are altered in MPTP-induced parkinsonism. Electrical stimulation of GPi did not entrain thalamic activity but changed oscillatory activity in the ventral thalamus and altered the relationship between spikes and simultaneously recorded LFPs., (Copyright © 2016 the American Physiological Society.)

The external globus pallidus (GP) is known as a relay nucleus of the indirect pathway of the basal ganglia. Recent studies in dopamine-depleted and healthy rats indicate that the GP comprises two main types of pallidofugal neurons: the so-called "prototypic" and "arkypallidal" neurons. However, the reconstruction of complete arkypallidal neurons in healthy rats has not been reported. Here we visualized the entire axonal arborization of four single arkypallidal neurons and six single prototypic neurons in rat brain using labeling with a viral vector expressing membrane-targeted green fluorescent protein and examined the distribution of axon boutons in the target nuclei. Results revealed that not only the arkypallidal neurons but nearly all of the prototypic neurons projected to the striatum with numerous axon varicosities. Thus, the striatum is a major target nucleus for pallidal neurons. Arkypallidal and prototypic GP neurons located in the calbindin-positive and calbindin-negative regions mainly projected to the corresponding positive and negative regions in the striatum. Because the GP and striatum calbindin staining patterns reflect the topographic organization of the striatopallidal projection, the striatal neurons in the sensorimotor and associative regions constitute the reciprocal connection with the GP neurons in the corresponding regions.

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

The basal ganglia, a group of subcortical nuclei, play a crucial role in decision-making by selecting actions and evaluating their outcomes. While much is known about the function of the basal ganglia circuitry in selection, how these nuclei contribute to outcome evaluation is less clear. Here we show that neurons in the habenula-projecting globus pallidus (GPh) in mice are essential for evaluating action outcomes and are regulated by a specific set of inputs from the basal ganglia. We find in a classical conditioning task that individual mouse GPh neurons bidirectionally encode whether an outcome is better or worse than expected. Mimicking these evaluation signals with optogenetic inhibition or excitation is sufficient to reinforce or discourage actions in a decision-making task. Moreover, cell-type-specific synaptic manipulations reveal that the inhibitory and excitatory inputs to the GPh are necessary for mice to appropriately evaluate positive and negative feedback, respectively. Finally, using rabies-virus-assisted monosynaptic tracing, we show that the GPh is embedded in a basal ganglia circuit wherein it receives inhibitory input from both striosomal and matrix compartments of the striatum, and excitatory input from the 'limbic' regions of the subthalamic nucleus. Our results provide evidence that information about the selection and evaluation of actions is channelled through distinct sets of basal ganglia circuits, with the GPh representing a key locus in which information of opposing valence is integrated to determine whether action outcomes are better or worse than expected., Competing Interests: The authors declare no competing financial interests. Readers are welcome to comment on the online version of the paper

Orexin is a member of neuropeptides which was first identified in the hypothalamus. The globus pallidus is a key structure in the basal ganglia, which is involved in both normal motor function and movement disorders. Morphological studies have shown the expression of both OX1 and OX2 receptors in the globus pallidus. Employing single unit extracellular recordings and behavioural tests, the direct in vivo electrophysiological and behavioural effects of orexin-A in the globus pallidus were studied. Micro-pressure administration of orexin-A significantly increased the spontaneous firing rate of pallidal neurons. Correlation analysis revealed a negative correlation between orexin-A induced excitation and the basal firing rate. Furthermore, application of the specific OX1 receptor antagonist, SB-334867, decreased the firing rate of pallidal neurons, suggesting that endogenous orexinergic systems modulate the firing activity of pallidal neurons. Orexin-A increased the excitability of pallidal neurons through both OX1 and OX2 receptors. In 6-hydroxydopamine parkinsonian rats, orexin-A-induced increase in firing rate of pallidal neurons was stronger than that in normal rats. Immunostaining revealed positive OX1 receptor expression in the globus pallidus of both normal and parkinsonian rats. Finally, postural test showed that unilateral microinjection of orexin-A led to contralateral deflection in the presence of systemic haloperidol administration. Further elevated body swing test revealed that pallidal orexin-A and SB-334867 induced contralateral-biased swing and ipsilateral-biased swing respectively. Based on the electrophysiological and behavioural findings of orexin-A in the globus pallidus, the present findings may provide a rationale for the pathogenesis and treatment of Parkinson's disease., (© 2016 Federation of European Neuroscience Societies and John Wiley & Sons Ltd.)

The external globus pallidus (GPe) is a key nucleus within basal ganglia circuits that are thought to be involved in action selection. A class of computational models assumes that, during action selection, the basal ganglia compute for all actions available in a given context the probabilities that they should be selected. These models suggest that a network of GPe and subthalamic nucleus (STN) neurons computes the normalization term in Bayes' equation. In order to perform such computation, the GPe needs to send feedback to the STN equal to a particular function of the activity of STN neurons. However, the complex form of this function makes it unlikely that individual GPe neurons, or even a single GPe cell type, could compute it. Here, we demonstrate how this function could be computed within a network containing two types of GABAergic GPe projection neuron, so-called 'prototypic' and 'arkypallidal' neurons, that have different response properties in vivo and distinct connections. We compare our model predictions with the experimentally-reported connectivity and input-output functions (f-I curves) of the two populations of GPe neurons. We show that, together, these dichotomous cell types fulfil the requirements necessary to compute the function needed for optimal action selection. We conclude that, by virtue of their distinct response properties and connectivities, a network of arkypallidal and prototypic GPe neurons comprises a neural substrate capable of supporting the computation of the posterior probabilities of actions.

Unlabelled: Altered dopamine D2 receptor (D2R) binding in the striatum has been associated with abnormal motivation in neuropsychiatric disorders, including schizophrenia. Here, we tested whether motivational deficits observed in mice with upregulated D2Rs (D2R-OEdev mice) are reversed by decreasing function of the striatopallidal "no-go" pathway. To this end, we expressed the Gαi-coupled designer receptor hM4D in adult striatopallidal neurons and activated the receptor with clozapine-N-oxide (CNO). Using a head-mounted miniature microscope we confirmed with calcium imaging in awake mice that hM4D activation by CNO inhibits striatopallidal function measured as disinhibited downstream activity in the globus pallidus. Mice were then tested in three operant tasks that address motivated behavior, the progressive ratio task, the progressive hold-down task, and outcome devaluation. Decreasing striatopallidal function in the dorsomedial striatum or nucleus accumbens core enhanced motivation in D2R-OEdev mice and control littermates. This effect was due to increased response initiation but came at the cost of goal-directed efficiency. Moreover, response vigor and the sensitivity to changes in reward value were not altered. Chronic activation of hM4D by administering CNO for 2 weeks in drinking water did not affect motivation due to a tolerance effect. However, the acute effect of CNO on motivation was reinstated after discontinuing chronic treatment for 48 h. Used as a therapeutic approach, striatopallidal inhibition should consider the risk of impairing goal-directed efficiency and behavioral desensitization., Significance Statement: Motivation involves a directional component that allows subjects to efficiently select the behavior that will lead to an optimal outcome and an activational component that initiates and maintains the vigor and persistence of actions. Striatal output pathways modulate motivated behavior, but it remains unknown how these pathways regulate specific components of motivation. Here, we found that the indirect pathway controls response initiation without affecting response vigor or the sensitivity to changes in the reward outcome. A specific enhancement in the activational component of motivation, however, can come at the cost of goal-directed efficiency when a sustained response is required to obtain the goal. These data should inform treatment strategies for brain disorders with impaired motivation such as schizophrenia and Parkinson's disease., (Copyright © 2016 the authors 0270-6474/16/365989-14$15.00/0.)

Unlabelled: 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., (Copyright © 2016 the authors 0270-6474/16/365556-16$15.00/0.)

Unlabelled: Compelling evidence demonstrates that the external globus pallidus (GPe) plays a key role in processing sensorimotor information. An anatomical projection from the GPe to the dorsal striatum has been described for decades. However, the cellular target and functional impact of this projection remain unknown. Using cell-specific transgenic mice, modern monosynaptic tracing techniques, and optogenetics-based mapping, we discovered that GPe neurons provide inhibitory inputs to direct and indirect pathway striatal projection neurons (SPNs). Our results indicate that the GPe input to SPNs arises primarily from Npas1-expressing neurons and is strengthened in a chronic Parkinson's disease (PD) model. Alterations of the GPe-SPN input in a PD model argue for the critical position of this connection in regulating basal ganglia motor output and PD symptomatology. Finally, chemogenetic activation of Npas1-expressing GPe neurons suppresses motor output, arguing that strengthening of the GPe-SPN connection is maladaptive and may underlie the hypokinetic symptoms in PD., Significance Statement: An anatomical projection from the pallidum to the striatum has been described for decades, but little is known about its connectivity pattern. The authors dissect the presynaptic and postsynaptic neurons involved in this projection, and show its cell-specific remodeling and strengthening in parkinsonian mice. Chemogenetic activation of Npas1(+) pallidal neurons that give rise to the principal pallidostriatal projection increases the time that the mice spend motionless. This argues that maladaptive strengthening of this connection underlies the paucity of volitional movements, which is a hallmark of Parkinson's disease., (Copyright © 2016 the authors 0270-6474/16/365472-17$15.00/0.)

Unlabelled: The basal ganglia (BG) control action selection, motor programs, habits, and goal-directed learning. The striatum, the principal input structure of BG, is predominantly composed of medium-sized spiny neurons (MSNs). Arising from these spatially intermixed MSNs, two inhibitory outputs form two main efferent pathways, the direct and indirect pathways. Striatonigral MSNs give rise to the activating, direct pathway MSNs and striatopallidal MSNs to the inhibitory, indirect pathway (iMSNs). BG output nuclei integrate information from both pathways to fine-tune motor procedures and to acquire complex habits and skills. Therefore, balanced activity between both pathways is crucial for harmonious functions of the BG. Despite the increase in knowledge concerning the role of glutamate NMDA receptors (NMDA-Rs) in the striatum, understanding of the specific functions of NMDA-R iMSNs is still lacking. For this purpose, we generated a conditional knock-out mouse to address the functions of the NMDA-R in the indirect pathway. At the cellular level, deletion of GluN1 in iMSNs leads to a reduction in the number and strength of the excitatory corticostriatopallidal synapses. The subsequent scaling down in input integration leads to dysfunctional changes in BG output, which is seen as reduced habituation, delay in goal-directed learning, lack of associative behavior, and impairment in action selection or skill learning. The NMDA-R deletion in iMSNs causes a decrease in the synaptic strength of striatopallidal neurons, which in turn might lead to a imbalanced integration between direct and indirect MSN pathways, making mice less sensitive to environmental change. Therefore, their ability to learn and adapt to the environment-based experience was significantly affected., Significance Statement: The striatum controls habits, locomotion, and goal-directed behaviors by coordinated activation of two antagonistic pathways. Insofar as NMDA receptors (NMDA-Rs) play a key role in synaptic plasticity essential for sustaining these behaviors, we generated a mouse model lacking NMDA-Rs specifically in striatopallidal neurons. To our knowledge, this is the first time that a specific deletion of inhibitory, indirect pathway medium-sized spiny neuron (iMSN) NMDA-Rs has been used to address the role of these receptors in the inhibitory pathway. Importantly, we found that this specific deletion led to a significant reduction in the number and strength of the cortico-iMSN synapses, which resulted in the significant impairments of behaviors orchestrated by the basal ganglia. Our findings indicate that the NMDA-Rs of the indirect pathway are essential for habituation, action selection, and goal-directed learning., (Copyright © 2016 the authors 0270-6474/16/364977-17$15.00/0.)

The globus pallidus external segment (GPe) constitutes part of the indirect pathway of the basal ganglia. Because of inhibitory projections from the striatum, most GPe neurons are expected to reduce activity during movements. However, many GPe neurons in fact display increased activity. We previously found that both excitatory and inhibitory responses were modulated during antisaccades, when eyes were directed away from a visual stimulus. To elucidate the roles of these neurons during antisaccades, we examined neuronal activities as monkeys performed antisaccades, prosaccades, and NoGo tasks under 2 conditions. In the Deliberate condition, the task-rule was instructed by color of the fixation point, while in the Immediate condition, it was given by color of the target. Under both conditions, the increase-type neurons exhibited greater activity during antisaccades compared with the other tasks and neuronal activity negatively correlated with saccade latency. The decrease-type neurons also showed greater modulation during antisaccades but their activity was comparable between NoGo and antisaccade trials in the Immediate condition. These results suggest that the increase-type neurons might play a role in facilitating antisaccades, whereas the decrease-type neurons could mediate signals for reflexive saccade suppression. We propose that these GPe neurons are differently involved in basal ganglia pathways., (© The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.com.)

The globus pallidus externus (GP) is a nucleus of the basal ganglia (BG), containing GABAergic projection neurons that arborize widely throughout the BG, thalamus and cortex. Ongoing work seeks to map axonal projection patterns from GP cell types, as defined by their electrophysiological and molecular properties. Here we use transgenic mice and recombinant viruses to characterize parvalbumin expressing (PV+) GP neurons within the BG circuit. We confirm that PV+ neurons 1) make up ~40% of the GP neurons 2) exhibit fast-firing spontaneous activity and 3) provide the major axonal arborization to the STN and substantia nigra reticulata/compacta (SNr/c). PV+ neurons also innervate the striatum. Retrograde labeling identifies ~17% of pallidostriatal neurons as PV+, at least a subset of which also innervate the STN and SNr. Optogenetic experiments in acute brain slices demonstrate that the PV+ pallidostriatal axons make potent inhibitory synapses on low threshold spiking (LTS) and fast-spiking interneurons (FS) in the striatum, but rarely on spiny projection neurons (SPNs). Thus PV+ GP neurons are synaptically positioned to directly coordinate activity between BG input nuclei, the striatum and STN, and thalamic-output from the SNr.

The suppression of inappropriate actions is critical for flexible behavior. Cortical-basal ganglia networks provide key gating mechanisms for action suppression, yet the specific roles of neuronal subpopulations are poorly understood. Here, we examine Arkypallidal (‘‘Arky’’) and Prototypical (‘‘Proto’’) globus pallidus neurons during a Stop task, which requires abrupt cancellation of an imminent action. We first establish that Arky neurons can be identified by their firing properties across the natural sleep/wake cycle. We then show that Stop responses are earlier and stronger in the Arky compared to the Proto subpopulation. In contrast to other basal ganglia neurons, pallidal Stop responses are selective to Stop, rather than Go, cues. Furthermore, the timing of these Stop responses matches the suppression of developing striatal Go-related activity. Our results support a two-step model of action suppression: actions-inpreparation are first paused via a subthalamic-nigral pathway, then cancelled via Arky GABAergic projections to striatum.

The striatum is a large brain nucleus with an important role in the control of movement and emotions. Medium spiny neurons (MSNs) are striatal output neurons forming prominent descending axon tracts that target different brain nuclei. However, how MSN axon tracts in the forebrain develop remains poorly understood. Here, we implicate the Wnt binding receptor Frizzled3 in several uncharacterized aspects of MSN pathway formation [i.e., anterior-posterior guidance of MSN axons in the striatum and their subsequent growth into the globus pallidus (GP), an important (intermediate) target]. In Frizzled3 knock-out mice, MSN axons fail to extend along the anterior-posterior axis of the striatum, and many do not reach the GP. Wnt5a acts as an attractant for MSN axons in vitro, is expressed in a posterior high, anterior low gradient in the striatum, and Wnt5a knock-out mice phenocopy striatal anterior-posterior defects observed in Frizzled3 mutants. This suggests that Wnt5a controls anterior-posterior guidance of MSN axons through Frizzled3. Axons that reach the GP in Frizzled3 knock-out mice fail to enter this structure. Surprisingly, entry of MSN axons into the GP non-cell-autonomously requires Frizzled3, and our data suggest that GP entry may be contingent on the correct positioning of "corridor" guidepost cells for thalamocortical axons by Frizzled3. Together, these data dissect MSN pathway development and reveal (non)cell-autonomous roles for Frizzled3 in MSN axon guidance. Further, they are the first to identify a gene that provides anterior-posterior axon guidance in a large brain nucleus and link Frizzled3 to corridor cell development., Significance Statement: Striatal axon pathways mediate complex physiological functions and are an important therapeutic target, underscoring the need to define how these connections are established. Remarkably, the molecular programs regulating striatal pathway development remain poorly characterized. Here, we determine the embryonic ontogeny of the two main striatal pathways (striatonigral and striatopallidal) and identify novel (non)cell-autonomous roles for the axon guidance receptor Frizzled3 in uncharacterized aspects of striatal pathway formation (i.e., anterior-posterior axon guidance in the striatum and axon entry into the globus pallidus). Further, our results link Frizzled3 to corridor guidepost cell development and suggest that an abnormal distribution of these cells has unexpected, widespread effects on the development of different axon tracts (i.e., striatal and thalamocortical axons)., (Copyright © 2015 the authors 0270-6474/15/3514206-15$15.00/0.)

The globus pallidus plays a significant role in motor control under both health and pathological states. Recent studies have revealed that hyperpolarization-activated cyclic nucleotide-gated (HCN) channels occupy a critical position in globus pallidus pacemaking activity. Morphological studies have shown the expression of HCN channels in the globus pallidus. To investigate the in vivo effects of HCN channels in the globus pallidus, extracellular recordings and behavioral tests were performed in the present study. In normal rats, micro-pressure ejection of 0.05mM ZD7288, the selective HCN channel blocker, decreased the frequency of spontaneous firing in 21 out of the 40 pallidal neurons. The average decrease was 50.4±5.4%. Interestingly, in another 18 out of the 40 pallidal neurons, ZD7288 increased the firing rate by 137.1±27.6%. Similar bidirectional modulation on the firing rate was observed by a higher concentration of ZD7288 (0.5mM) as well as another HCN channel blocker, CsCl. Furthermore, activation of HCN channels by 8-Br-cAMP increased the firing rate by 63.0±9.3% in 15 out of the 25 pallidal neurons and decreased the firing rate by 46.9±9.4% in another 8 out of the 25 pallidal neurons. Further experiments revealed that modulation of glutamatergic but not GABAergic transmission may be involved in ZD7288-induced increase in firing rate. Consistent with electrophysiological results, further studies revealed that modulation of HCN channels also had bidirectional effects on behavior. Taken together, the present studies suggest that HCN channels may modulate the activity of pallidal neurons by different pathways in vivo., (Copyright © 2015 Elsevier Inc. All rights reserved.)

In instrumental conditioning, newly acquired actions are generally goal-directed and are mediated by the relationship between the action and its consequences or outcome. With continued training, however, the performance of such actions can become automatic, reflexive or habitual and under the control of antecedent stimuli rather than their consequences. Recent evidence suggests that habit learning is mediated by plasticity in the dorsolateral striatum (DLS). To date, however, no direct evidence of learning-related plasticity associated with overtraining has been reported in this region, nor is it known whether, or which, specific cell types are involved in this learning process. The striatum is primarily composed of two classes of spiny projection neurons, the striatonigral and striatopallidal spiny projection neurons, which express dopamine D1 and D2 receptors, and control direct and indirect pathways, respectively. Here we found evidence of a post-synaptic depression in DLS striatopallidal projecting neurons in the indirect pathway during habit learning in mice. Moreover, this training-induced depression occluded post-synaptic depression induced by co-activation of D2 receptors and transient receptor potential vanilloid 1 (TRPV1) channels, implying that this pathway is involved in habit learning. This hypothesis was further tested by disrupting this signal pathway by knocking out TRPV1 channels, resulting in compromised habit learning. Our findings suggest that post-synaptic plasticity at D2 neurons in the DLS mediates habit learning and, by implicating an interaction between the D2 receptor and TRPV1 channel activity, provide a potential drug target for influencing habitual action control., (© 2015 Federation of European Neuroscience Societies and John Wiley & Sons Ltd.)

We report the presence of a small population of cholinergic neurons closely intermingled with external pallidal neurons in cynomolgus monkeys (Macaca fascicularis). The majority of these cholinergic pallidal neurons are devoid of Nerve Growth Factor receptor (NGFr), which sets them apart from the population of corticopetal NGFr-rich neurons of the nucleus basalis of Meynert and its ectopic elements that impinge dorsally upon the pallidum via the medullary laminae.

The basal ganglia are phylogenetically conserved subcortical nuclei necessary for coordinated motor action and reward learning. Current models postulate that the basal ganglia modulate cerebral cortex indirectly via an inhibitory output to thalamus, bidirectionally controlled by direct- and indirect-pathway striatal projection neurons (dSPNs and iSPNs, respectively). The basal ganglia thalamic output sculpts cortical activity by interacting with signals from sensory and motor systems. Here we describe a direct projection from the globus pallidus externus (GP), a central nucleus of the basal ganglia, to frontal regions of the cerebral cortex (FC). Two cell types make up the GP-FC projection, distinguished by their electrophysiological properties, cortical projections and expression of choline acetyltransferase (ChAT), a synthetic enzyme for the neurotransmitter acetylcholine (ACh). Despite these differences, ChAT(+) cells, which have been historically identified as an extension of the nucleus basalis, as well as ChAT(-) cells, release the inhibitory neurotransmitter GABA (γ-aminobutyric acid) and are inhibited by iSPNs and dSPNs of dorsal striatum. Thus, GP-FC cells comprise a direct GABAergic/cholinergic projection under the control of striatum that activates frontal cortex in vivo. Furthermore, iSPN inhibition of GP-FC cells is sensitive to dopamine 2 receptor signalling, revealing a pathway by which drugs that target dopamine receptors for the treatment of neuropsychiatric disorders can act in the basal ganglia to modulate frontal cortices.

Studies in dopamine-depleted rats indicate that the external globus pallidus (GPe) contains two main types of GABAergic projection cell; so-called "prototypic" and "arkypallidal" neurons. Here, we used correlative anatomical and electrophysiological approaches in rats to determine whether and how this dichotomous organization applies to the dopamine-intact GPe. Prototypic neurons coexpressed the transcription factors Nkx2-1 and Lhx6, comprised approximately two-thirds of all GPe neurons, and were the major GPe cell type innervating the subthalamic nucleus (STN). In contrast, arkypallidal neurons expressed the transcription factor FoxP2, constituted just over one-fourth of GPe neurons, and innervated the striatum but not STN. In anesthetized dopamine-intact rats, molecularly identified prototypic neurons fired at relatively high rates and with high regularity, regardless of brain state (slow-wave activity or spontaneous activation). On average, arkypallidal neurons fired at lower rates and regularities than prototypic neurons, and the two cell types could be further distinguished by the temporal coupling of their firing to ongoing cortical oscillations. Complementing the activity differences observed in vivo, the autonomous firing of identified arkypallidal neurons in vitro was slower and more variable than that of prototypic neurons, which tallied with arkypallidal neurons displaying lower amplitudes of a "persistent" sodium current important for such pacemaking. Arkypallidal neurons also exhibited weaker driven and rebound firing compared with prototypic neurons. In conclusion, our data support the concept that a dichotomous functional organization, as actioned by arkypallidal and prototypic neurons with specialized molecular, structural, and physiological properties, is fundamental to the operations of the dopamine-intact GPe., (Copyright © 2015 Abdi et al.)