Heterogeneity and diversity of striatal GABAergic interneurons

Inhibitory interneurons mediate autism-associated behaviors via 4E-BP2

Translational control plays a key role in regulation of neuronal activity and behavior. Deletion of the translational repressor 4E-BP2 in mice alters excitatory and inhibitory synaptic functions, engendering autistic-like behaviors. The contribution of 4E-BP2-dependent translational control in excitatory and inhibitory neurons and astrocytic cells to these behaviors remains unknown. To investigate this, we generated cell-type-specific conditional 4E-BP2 knockout mice and tested them for the salient features of autism, including repetitive stereotyped behaviors (self-grooming and marble burying), sociability (3-chamber social and direct social interaction tests), and communication (ultrasonic vocalizations in pups). We found that deletion of 4E-BP2 in GABAergic inhibitory neurons, defined by Gad2, resulted in impairments in social interaction and vocal communication. In contrast, deletion of 4E-BP2 in forebrain glutamatergic excitatory neurons, defined by Camk2a, or in astrocytes, defined by Gfap, failed to cause autistic-like behavioral abnormalities. Taken together, we provide evidence for an inhibitory-cell-specific role of 4E-BP2 in engendering autism-related behaviors.

Transcriptional profiling aligned with in situ expression image analysis reveals mosaically expressed molecular markers for GABA neuron sub-groups in the ventral tegmental area

γ‐Aminobutyric acid (GABA) neurons in the ventral tegmental area (VTA) provide local inhibitory control of dopamine neuron activity and send long‐range projections to several target regions including the nucleus accumbens. They play diverse roles in reward and aversion, suggesting that they be comprised of several functionally distinct sub‐groups, but our understanding of this diversity has been limited by a lack of molecular markers that might provide genetic entry points for cell type‐specific investigations. To address this, we conducted transcriptional profiling of GABA neurons and dopamine neurons using immunoprecipitation of tagged polyribosomes (RiboTag) and RNAseq. First, we directly compared these two transcriptomes in order to obtain a list of genes enriched in GABA neurons compared with dopamine neurons. Next, we created a novel bioinformatic approach, that used the PANTHER (Protein ANalysis THrough Evolutionary Relationships) gene ontology database and VTA gene expression data from the Allen Mouse Brain Atlas, from which we obtained 6 candidate genes: Cbln4, Rxfp3, Rora, Gpr101, Trh and Nrp2. As a final step, we verified the selective expression of these candidate genes in sub‐groups of GABA neurons in the VTA (and neighbouring substantia nigra pars compacta) using immunolabelling. Taken together, our study provides a valuable toolbox for the future investigation of GABA neuron sub‐groups in the VTA.

Three divisions of the mouse caudal striatum differ in the proportions of dopamine D1 and D2 receptor-expressing cells, distribution of dopaminergic axons, and composition of cholinergic and GABAergic interneurons

The greater part of the striatum is composed of striosomes and matrix compartments, but we recently demonstrated the presence of a region that has a distinct structural organization in the ventral half of the mouse caudal striatum (Miyamoto et al. in Brain Struct Funct 223:4275-4291, 2018). This region, termed the tri-laminar part based upon its differential immunoreactivities for substance P and enkephalin, consists of medial, intermediate, and lateral divisions. In this study, we quantitatively analyzed the distributions of both projection neurons and interneurons in each division using immunohistochemistry. Two types of projection neurons expressing either the dopamine D1 receptor (D1R) or D2 receptor (D2R) showed complementary distributions throughout the tri-laminar part, but the proportions significantly differed among the three divisions. The proportion of D1R-expressing neurons in the medial, intermediate, and lateral divisions was 88.6 ± 8.2% (651 cells from 3 mice), 14.7 ± 3.8% (1025 cells), and 49.3 ± 4.5% (873 cells), respectively. The intermediate division was further characterized by poor innervation of tyrosine hydroxylase immunoreactive axons. The numerical density of choline acetyltransferase immunoreactive neurons differed among the three divisions following the order from the medial to lateral divisions. In contrast, PV-positive somata were distributed throughout all three divisions at a constant density. Two types of GABAergic interneurons labeled for nitric oxide synthase and calretinin showed the highest cell density in the medial division. The present results characterize the three divisions of the mouse caudal striatum as distinct structures, which will facilitate studies of novel functional loops in the basal ganglia.

Loss of striatal tyrosine-hydroxylase interneurons impairs instrumental goal-directed behavior

The striatum mediates a broad range of cognitive and motor functions. Within the striatum, recently discovered tyrosine hydroxylase expressing interneurons (THINs) provide a source of intrastriatal synaptic connectivity that is critical for regulating striatal activity, yet the role of THIN's in behavior remains unknown. Given the important role of the striatum in reward-based behaviors, we investigated whether loss of striatal THINs would impact instrumental behavior in mice. We selectively ablated striatal THINs in TH-Cre mice using chemogenetic techniques, and then tested THIN-lesioned or control mice on three reward-based striatal-dependent instrumental tests: (a) progressive ratio test; (b) choice test following selective-satiety induced outcome devaluation; (c) outcome reinstatement test. Both striatal-THIN-lesioned and control mice acquired an instrumental response for flavored food pellets, and their behavior did not differ in the progressive ratio test, suggesting intact effort to obtain rewards. However, striatal THIN lesions markedly impaired choice performance following selective-satiety induced outcome devaluation. Unlike control mice, THIN-lesioned mice did not adjust their choice of actions following a change in outcome value. In the outcome reinstatement test THIN-lesioned and control mice showed response invigoration by outcome presentation, suggesting the incentive properties of outcomes were not disrupted by THIN lesions. Overall, we found that striatal THIN lesions selectively impaired goal-directed behavior, while preserving motoric and appetitive behaviors. These findings are the first to describe a function of striatal THINs in reward-based behavior, and further illustrate the important role for intrastriatal interneuronal connectivity in behavioral functions ascribed to the striatum more generally.

The neurobiological basis for novel experimental therapeutics in dystonia

Dystonia is a movement disorder characterized by involuntary muscle contractions, twisting movements, and abnormal postures that may affect one or multiple body regions. Dystonia is the third most common movement disorder after Parkinson's disease and essential tremor. Despite its relative frequency, small molecule therapeutics for dystonia are limited. Development of new therapeutics is further hampered by the heterogeneity of both clinical symptoms and etiologies in dystonia. Recent advances in both animal and cell-based models have helped clarify divergent etiologies in dystonia and have facilitated the identification of new therapeutic targets. Advances in medicinal chemistry have also made available novel compounds for testing in biochemical, physiological, and behavioral models of dystonia. Here, we briefly review motor circuit anatomy and the anatomical and functional abnormalities in dystonia. We then discuss recently identified therapeutic targets in dystonia based on recent preclinical animal studies and clinical trials investigating novel therapeutics.

Ensemble encoding of action speed by striatal fast-spiking interneurons

Striatal fast-spiking interneurons (FSIs) potently inhibit the output neurons of the striatum and, as such, powerfully modulate action learning. Through electrical synaptic coupling, FSIs are theorized to temporally coordinate their activity. This has important implications for their ability to temporally summate inhibition on downstream striatal projection neurons. While some in vivo single-unit electrophysiological recordings of putative FSIs support coordinated firing, others do not. Moreover, it is unclear as to what aspect of action FSIs encode. To address this, we used in vivo calcium imaging of genetically identified FSIs in freely moving mice and applied machine learning analyses to decipher the relationship between FSI activity and movement. We report that FSIs exhibit ensemble activity that encodes the speed of action sub-components, including ambulation and head movements. These results suggest FSI population dynamics fit within a Hebbian model for ensemble inhibition of striatal output guiding action.

A striatal interneuron circuit for continuous target pursuit

Most adaptive behaviors require precise tracking of targets in space. In pursuit behavior with a moving target, mice use distance to target to guide their own movement continuously. Here, we show that in the sensorimotor striatum, parvalbumin-positive fast-spiking interneurons (FSIs) can represent the distance between self and target during pursuit behavior, while striatal projection neurons (SPNs), which receive FSI projections, can represent self-velocity. FSIs are shown to regulate velocity-related SPN activity during pursuit, so that movement velocity is continuously modulated by distance to target. Moreover, bidirectional manipulation of FSI activity can selectively disrupt performance by increasing or decreasing the self-target distance. Our results reveal a key role of the FSI-SPN interneuron circuit in pursuit behavior and elucidate how this circuit implements distance to velocity transformation required for the critical underlying computation.

Many natural behaviours involve tracking of a target in space. Here, the authors describe a task to assess this behaviour in mice and use in vivo electrophysiology, calcium imaging, optogenetics, and chemogenetics to investigate the role of the striatum in target pursuit.

The role of the neuropeptide PEN receptor, GPR83, in the reward pathway: Relationship to sex-differences

GPR83, the receptor for the neuropeptide PEN, exhibits high expression in the nucleus accumbens of the human and rodent brain, suggesting that it plays a role in modulating the mesolimbic reward pathway. However, the cell-type specific expression of GPR83, its functional impact in the reward pathway, and in drug reward-learning has not been fully explored. Using GPR83/eGFP mice, we show high GPR83 expression on cholinergic interneurons in the nucleus accumbens and moderate expression on ventral tegmental area dopamine neurons. In GPR83 knockout mice, baseline dopamine release in the nucleus accumbens is enhanced which disrupts the ratio of tonic vs phasic release. Additionally, GPR83 knockout leads to changes in the expression of dopamine-related genes. Using the morphine conditioned place preference model, we identify sex differences in morphine reward-learning, show that GPR83 is upregulated in the nucleus accumbens following morphine conditioned place preference, and show that shRNA-mediated knockdown of GPR83 in the nucleus accumbens leads to attenuation morphine reward. Together, these findings detect GPR83 expression in the reward-pathway, and show its involvement in dopamine release and morphine reward-learning.

A Novel Type of Neuron Within the Dorsal Striatum

The striatum is predominantly composed of medium spiny projection neurons, with the remaining neurons consisting of several types of interneurons. Among the interneurons are a group of cells that express tyrosine hydroxylase (TH). Although the intrinsic electrical properties of these TH-expressing interneurons have been characterized, there is no agreement on the number of TH-expressing cell types and their electrical properties. Here, we have used transgenic mice in which YFP-tagged channelrhodopsin-2 (ChR2) was expressed in potential TH-expressing cells in a Cre-dependent manner. We found that the YFP+ neurons in the striatum were heterogeneous in their intrinsic electrical properties; unbiased clustering indicated that there are three main neuronal subtypes. One population of neurons had aspiny dendrites with high-frequency action potential (AP) firing and plateau potentials, resembling the TH interneurons (THIN) described previously. A second, very small population of labeled neurons resembled medium-sized spiny neurons (MSN). The third population of neurons had dendrites with an intermediate density of spines, showed substantial AP adaptation and generated prolonged spikes. This type of striatal neuron has not been previously identified in the adult mouse and we have named it the Frequency-Adapting Neuron with Spines (FANS). Because of their distinctive properties, FANS may play a unique role in striatal information processing.

Shared Behavioral and Neurocircuitry Disruptions in Drug Addiction, Obesity, and Binge Eating Disorder: Focus on Group I mGluRs in the Mesolimbic Dopamine Pathway

Accumulated data from clinical and preclinical studies suggest that, in drug addiction and states of overeating, such as obesity and binge eating disorder (BED), there is an imbalance in circuits that are critical for motivation, reward saliency, executive function, and self-control. Central to these pathologies and the extensive topic of this Review are the aberrations in dopamine (DA) and glutamate (Glu) within the mesolimbic pathway. Group I metabotropic glutamate receptors (mGlus) are highly expressed in the mesolimbic pathway and are poised in key positions to modulate disruptions in synaptic plasticity and neurotransmitter release observed in drug addiction, obesity, and BED. The use of allosteric modulators of group I mGlus has been studied in drug addiction, as they offer several advantages over traditional orthosteric agents. However, they have yet to be studied in obesity or BED. With the substantial overlap between the neurocircuitry involved in drug addiction and eating disorders, group I mGlus may also provide novel targets for obesity and BED.

Major Contribution of Somatostatin-Expressing Interneurons and Cannabinoid Receptors to Increased GABA Synaptic Activity in the Striatum of Huntington's Disease Mice

Huntington’s disease (HD) is a heritable neurological disorder that affects cognitive and motor performance in patients carrying the mutated huntingtin (HTT) gene. In mouse models of HD, previous reports showed a significant increase in spontaneous GABA_A receptor-mediated synaptic activity in striatal spiny projection neurons (SPNs). In this study, using optogenetics and slice electrophysiology, we examined the contribution of γ-aminobutyric acid (GABA)-ergic parvalbumin (PV)- and somatostatin (SOM)-expressing interneurons to the increase in GABA neurotransmission using the Q175 (heterozygote) mouse model of HD. Patch clamp recordings in voltage-clamp mode were performed on SPNs from brain slices of presymptomatic (2 months) and symptomatic (8 and 12 months) Q175 mice and wildtype (WT) littermates. While inhibitory postsynaptic currents (IPSCs) evoked in SPNs following optical activation of PV- and SOM-expressing interneurons differed in amplitude, no genotype-dependent differences were observed at all ages from both interneuron types; however, responses evoked by either type were found to have faster kinetics in symptomatic mice. Since SOM-expressing interneurons are constitutively active in striatal brain slices, we then examined the effects of acutely silencing these neurons in symptomatic mice with enhanced Natronomonas pharaonis halorhodopsin (eNpHR). Optically silencing SOM-expressing interneurons resulted in a greater decrease in the frequency of spontaneous IPSCs (sIPSCs) in a subset of SPNs from Q175 mice compared to WTs, suggesting that SOM-expressing interneurons are the main contributors to the overall increased GABA synaptic activity in HD SPNs. Additionally, the effects of activating GABA_B and cannabinoid (CB1) receptors were investigated to determine whether these receptors were involved in modulating interneuron-specific GABA synaptic transmission and if this modulation differed in HD mice. When selectively activating PV- and SOM-expressing interneurons in the presence of the CB1 receptor agonist WIN-55,212, the magnitudes of the evoked IPSCs in SPNs decreased for both interneuron types although this change was less prominent in symptomatic Q175 SPNs during SOM-expressing interneuron activation. Overall, these findings show that dysfunction of SOM-expressing interneurons contributes to the increased GABA synaptic activity found in HD mouse models and that dysregulation of the endocannabinoid system may contribute to this effect.

Parvalbumin Interneurons Determine Emotional Valence Through Modulating Accumbal Output Pathways

Parvalbumin (PV) expressing GABAergic interneurons provide large source of GABA to spiny projection neurons (SPNs) in the striatum. However, the roles of PV⁺ interneurons in the regulation of SPNs in the ventral striatum and emotional states are largely unknown. Here, we investigated whether stimulation of ventral striatal (accumbal) PV⁺ interneurons would drive emotional valence in mice. We found that during conditioned place preference (CPP) training, activation of accumbal PV⁺ interneurons evoked place preference while suppressing them resulted in conditioned place aversion (CPA). Activation of PV⁺ interneurons during place conditioning increased Fos expression in SPNs in the direct pathway (dSPNs) and impaired lithium chloride-induced CPA. Activation of dSPNs and SPNs in the indirect pathway (iSPNs) induced CPP and CPA, respectively; conversely, suppression of dSPNs or iSPNs induced CPA or CPP. In addition, activation or suppression of calretinin-expressing (CR) GABAergic interneurons did not induce place preference or aversion. These data suggest that PV⁺ interneurons can bidirectionally determine the emotional valence through their regulation of accumbal SPN activities and raise the possibility that manipulation of PV⁺ interneuron activity may have the potential to alter emotional valence and treat related mental disorders.

Striatal Low-Threshold Spiking Interneurons Regulate Goal-Directed Learning

The dorsomedial striatum (DMS) is critically involved in motor control and reward processing, but the specific neural circuit mediators are poorly understood. Recent evidence highlights the extensive connectivity of low-threshold spiking interneurons (LTSIs) within local striatal circuitry; however, the in vivo function of LTSIs remains largely unexplored. We employed fiber photometry to assess LTSI calcium activity in a range of DMS-mediated behaviors, uncovering specific reward-related activity that is down-modulated during goal-directed learning. Using two mechanistically distinct manipulations, we demonstrated that this down-modulation of LTSI activity is critical for acquisition of novel contingencies, but not for their modification. In contrast, continued LTSI activation slowed instrumental learning. Similar manipulations of fast-spiking interneurons did not reproduce these effects, implying a specific function of LTSIs. Finally, we revealed a role for the γ-aminobutyric acid (GABA)ergic functions of LTSIs in learning. Together, our data provide new insights into this striatal interneuron subclass as important gatekeepers of goal-directed learning.

A Motivational and Neuropeptidergic Hub: Anatomical and Functional Diversity within the Nucleus Accumbens Shell

The mesocorticolimbic pathway is canonically known as the "reward pathway." Embedded within the center of this circuit is the striatum, a massive and complex network hub that synthesizes motivation, affect, learning, cognition, stress, and sensorimotor information. Although striatal subregions collectively share many anatomical and functional similarities, it has become increasingly clear that it is an extraordinarily heterogeneous region. In particular, the nucleus accumbens (NAc) medial shell has repeatedly demonstrated that the rules dictated by more dorsal aspects of the striatum do not apply or are even reversed in functional logic. These discrepancies are perhaps most easily captured when isolating the functions of various neuromodulatory peptide systems within the striatum. Endogenous peptides are thought to play a critical role in modulating striatal signals to either amplify or dampen evoked behaviors. Here we describe the anatomical-functional backdrop upon which several neuropeptides act within the NAc to modulate behavior, with a specific emphasis on nucleus accumbens medial shell and stress responsivity. Additionally, we propose that, as the field continues to dissect fast neurotransmitter systems within the NAc, we must also provide considerable contextual weight to the roles local peptides play in modulating these circuits to more comprehensively understand how this important subregion gates motivated behaviors.

Opposing Influence of Sensory and Motor Cortical Input on Striatal Circuitry and Choice Behavior

The striatum is the main input nucleus of the basal ganglia and is a key site of sensorimotor integration. While the striatum receives extensive excitatory afferents from the cerebral cortex, the influence of different cortical areas on striatal circuitry and behavior is unknown. Here, we find that corticostriatal inputs from whisker-related primary somatosensory (S1) and motor (M1) cortex differentially innervate projection neurons and interneurons in the dorsal striatum and exert opposing effects on sensory-guided behavior. Optogenetic stimulation of S1-corticostriatal afferents in ex vivo recordings produced larger postsynaptic potentials in striatal parvalbumin (PV)-expressing interneurons than D1- or D2-expressing spiny projection neurons (SPNs), an effect not observed for M1-corticostriatal afferents. Critically, in vivo optogenetic stimulation of S1-corticostriatal afferents produced task-specific behavioral inhibition, which was bidirectionally modulated by striatal PV interneurons. Optogenetic stimulation of M1 afferents produced the opposite behavioral effect. Thus, our results suggest opposing roles for sensory and motor cortex in behavioral choice via distinct influences on striatal circuitry.

Dopaminergic modulation of striatal function and Parkinson's disease

The striatum is richly innervated by mesencephalic dopaminergic neurons that modulate a diverse array of cellular and synaptic functions that control goal-directed actions and habits. The loss of this innervation has long been thought to be the principal cause of the cardinal motor symptoms of Parkinson's disease (PD). Moreover, chronic, pharmacological overstimulation of striatal dopamine (DA) receptors is generally viewed as the trigger for levodopa-induced dyskinesia (LID) in late-stage PD patients. Here, we discuss recent advances in our understanding of the relationship between the striatum and DA, particularly as it relates to PD and LID. First, it has become clear that chronic perturbations of DA levels in PD and LID bring about cell type-specific, homeostatic changes in spiny projection neurons (SPNs) that tend to normalize striatal activity. Second, perturbations in DA signaling also bring about non-homeostatic aberrations in synaptic plasticity that contribute to disease symptoms. Third, it has become evident that striatal interneurons are major determinants of network activity and behavior in PD and LID. Finally, recent work examining the activity of SPNs in freely moving animals has revealed that the pathophysiology induced by altered DA signaling is not limited to imbalance in the average spiking in direct and indirect pathways, but involves more nuanced disruptions of neuronal ensemble activity.

Optogenetic Stimulation Enhanced Neuronal Plasticities in Motor Recovery after Ischemic Stroke

Motor capability recovery after ischemic stroke involves dynamic remodeling processes of neural connectomes in the nervous system. Various neuromodulatory strategies combining direct stimulating interventions with behavioral trainings for motor recovery after ischemic stroke have been developed. However, the effectiveness of these interventions varies widely due to unspecific activation or inhibition of undefined neuronal subtypes. Optogenetics is a functional and structural connection-based approach that can selectively activate or inhibit specific subtype neurons with a higher precision, and it has been widely applied to build up neuronal plasticities of the nervous system, which shows a great potential in restoring motor functions in stroke animal models. Here, we reviewed neurobiological mechanisms of enhanced brain plasticities underlying motor recovery through the optogenetic stimulation after ischemic stroke. Several brain sites and neural circuits that have been previously proven effective for motor function rehabilitation were identified, which would be helpful for a more schematic understanding of effective neuronal connectomes in the motor function recovery after ischemic stroke.

Viral labeling of neurons synaptically connected to nucleus accumbens somatostatin interneurons

The nucleus accumbens, a key brain reward region, receives synaptic inputs from a range of forebrain and brainstem regions. Many of these projections have been established using electrophysiology or fluorescent tract tracing. However, more recently developed viral tracing techniques have allowed for fluorescent labeling of synaptic afferents in a cell type-specific manner. Since the NAc is comprised of multiple cell types, these methods have enabled the delineation of the cell type-specific connectivity of principal medium spiny neurons in the region. The synaptic connectivity of somatostatin interneurons, which account for <5% of the neurons in the region, has been inferred from electrophysiological and immunohistochemical data, but has not yet been visualized using modern viral tracing techniques. Here, we use the pseudorabies virus (PRV)-Introvert-GFP virus, an alphaherpes virus previously shown to label synaptic afferents in a cell type-specific manner, to label first order afferents to NAc somatostatin interneurons. While we find GFP(+) labeling in several well established projections to the NAc, we also observe that several known projections to NAc did not contain GFP(+) cells, suggesting they do not innervate somatostatin interneurons in the region. A subset of the GFP(+) afferents are c-FOS(+) following acute administration of cocaine, showing that NAc somatostatin interneurons are innervated by some cells that respond to rewarding stimuli. These results provide a foundation for future studies aimed toward elucidating the cell type-specific connectivity of the NAc, and identify specific circuits that warrant future functional characterization.

Integrated anatomical and physiological mapping of striatal afferent projections

The dorsomedial striatum, a key site of reward-sensitive motor output, receives extensive afferent input from cortex, thalamus and midbrain. These projections are integrated by striatal microcircuits containing both spiny projection neurons and local circuit interneurons. To explore target cell specificity of these projections, we compared inputs onto D1-dopamine receptor-positive spiny neurons, parvalbumin-positive fast-spiking interneurons and somatostatin-positive low-threshold-spiking interneurons, using cell type-specific rabies virus tracing and optogenetic-mediated projection neuron recruitment in mice. While the relative proportion of retrogradely labelled projection neurons was similar between target cell types, the convergence of inputs was systematically higher for projections onto fast-spiking interneurons. Rabies virus is frequently used to assess cell-specific anatomical connectivity but it is unclear how this correlates to synaptic connectivity and efficacy. To test this, we compared tracing data with target cell-specific measures of synaptic efficacy for anterior cingulate cortex and parafascicular thalamic projections using novel quantitative optogenetic measures. We found that target-specific patterns of convergence were extensively modified according to region of projection neuron origin and postsynaptic cell type. Furthermore, we observed significant divergence between cell type-specific anatomical connectivity and measures of excitatory synaptic strength, particularly for low-threshold-spiking interneurons. Taken together, this suggests a basic uniform connectivity map for striatal afferent inputs upon which presynaptic-postsynaptic interactions impose substantial diversity of physiological connectivity.

Unique contributions of parvalbumin and cholinergic interneurons in organizing striatal networks during movement

Striatal parvalbumin (PV) and cholinergic interneurons (CHIs) are poised to play major roles in behavior by coordinating the networks of medium spiny cells that relay motor output. However, the small numbers and scattered distribution of these cells have hindered direct assessment of their contribution to activity in networks of medium spiny neurons (MSNs) during behavior. Here, we build on recent improvements in single-cell calcium imaging combined with optogenetics to test the capacity of PVs and CHIs to affect MSN activity and behavior in mice engaged in voluntary locomotion. We find that PVs and CHIs have unique effects on MSN activity and dissociable roles in supporting movement. PV cells facilitate movement by refining the activation of MSN networks responsible for movement execution. CHIs, in contrast, synchronize activity within MSN networks to signal the end of a movement bout. These results provide new insights into the striatal network activity that supports movement.

The Functional Role of Striatal Cholinergic Interneurons in Reinforcement Learning From Computational Perspective

In this study, we explore the functional role of striatal cholinergic interneurons, hereinafter referred to as tonically active neurons (TANs), via computational modeling; specifically, we investigate the mechanistic relationship between TAN activity and dopamine variations and how changes in this relationship affect reinforcement learning in the striatum. TANs pause their tonic firing activity after excitatory stimuli from thalamic and cortical neurons in response to a sensory event or reward information. During the pause striatal dopamine concentration excursions are observed. However, functional interactions between the TAN pause and striatal dopamine release are poorly understood. Here we propose a TAN activity-dopamine relationship model and demonstrate that the TAN pause is likely a time window to gate phasic dopamine release and dopamine variations reciprocally modulate the TAN pause duration. Furthermore, this model is integrated into our previously published model of reward-based motor adaptation to demonstrate how phasic dopamine release is gated by the TAN pause to deliver reward information for reinforcement learning in a timely manner. We also show how TAN-dopamine interactions are affected by striatal dopamine deficiency to produce poor performance of motor adaptation.

A Hippocampus-Accumbens Tripartite Neuronal Motif Guides Appetitive Memory in Space

Retrieving and acting on memories of food-predicting environments are fundamental processes for animal survival. Hippocampal pyramidal cells (PYRs) of the mammalian brain provide mnemonic representations of space. Yet the substrates by which these hippocampal representations support memory-guided behavior remain unknown. Here, we uncover a direct connection from dorsal CA1 (dCA1) hippocampus to nucleus accumbens (NAc) that enables the behavioral manifestation of place-reward memories. By monitoring neuronal ensembles in mouse dCA1→NAc pathway, combined with cell-type selective optogenetic manipulations of input-defined postsynaptic neurons, we show that dCA1 PYRs drive NAc medium spiny neurons and orchestrate their spiking activity using feedforward inhibition mediated by dCA1-connected parvalbumin-expressing fast-spiking interneurons. This tripartite cross-circuit motif supports spatial appetitive memory and associated NAc assemblies, being independent of dorsal subiculum and dispensable for both spatial novelty detection and reward seeking. Our findings demonstrate that the dCA1→NAc pathway instantiates a limbic-motor interface for neuronal representations of space to promote effective appetitive behavior.

Changes in Dendritic Spine Density and Inhibitory Perisomatic Connectivity onto Medium Spiny Neurons in L-Dopa-Induced Dyskinesia

Using bacterial artificial chromosome-double transgenic mice expressing tdTomato in D1 receptor-medium spiny neurons (MSNs) and enhanced green fluorescent protein in D2 receptor-MSNs, we have studied changes in spine density and perisomatic GABAergic boutons density in MSNs of both the D1R and D2R pathways, in an experimental model of parkinsonism (mouse injected with 6-hydroxydopamine in the medial forebrain bundle), both in the parkinsonian and dyskinetic condition induced by L-DOPA treatment. To assess changes in perisomatic GABAergic connectivity onto MSNs, we measured the number of contacts originated from parvalbumin (PV)-containing striatal "fast-spiking" interneurons (FSIs), the major component of a feed-forward inhibition mechanism that regulates spike timing in MSNs, in both cell types as well as the number of vesicular GABA transporter (VGAT) contacts. Furthermore, we determined changes in PV-immunoreactive cell density by PV immunolabeling combined with Wisteria floribunda agglutinin (WFA) labeling to detect FSI in a PV-independent manner. We also explored the differential expression of striatal activity-regulated cytoskeleton-associated protein (Arc) and c-Fos in both types of MSNs as a measure of neuronal activation. Our results confirm previous findings of major structural changes in dendritic spine density after nigrostriatal denervation, which are further modified in the dyskinetic condition. Moreover, the finding of differential modifications in perisomatic GABAergic connectivity and neuronal activation in MSNs suggests an attempt by the system to regain homeostasis after denervation and an imbalance between excitation and inhibition leading to the development of dyskinesia after exposure to L-DOPA.

Neuropeptide Y and representation of salience in human nucleus accumbens

Neuropeptide Y (NPY) produces anxiolytic effects in rodent models, and naturally occurring low NPY expression in humans has been associated with negative emotional phenotypes. Studies in rodent models have also demonstrated that NPY elicits reward behaviors through its action in the nucleus accumbens (NAc), but the impact of NPY on the human NAc is largely unexplored. We recruited 222 healthy young adults of either sex and genetically selected 53 of these subjects at the extremes of NPY expression (Low-NPY and High-NPY) to participate in functional magnetic resonance imaging. Responses of the NAc and surrounding ventral striatum were quantified during a monetary incentive delay task in which stimuli varied by salience (high versus low) and valence (win versus loss). We found that bilateral NAc responses to high-salience versus low-salience stimuli were greater for Low-NPY subjects relative to High-NPY subjects, regardless of stimulus valence. To our knowledge, these results provide the first evidence in humans linking NPY with salience sensitivity of the NAc, raising the possibility that individual differences in NPY expression moderate the risk for disorders of mesoaccumbal function such as addictions and mood disorders. Additionally, we found that head motion was greater among High-NPY subjects, consistent with previous reports linking NPY with hyperactivity. Future studies in animal models are warranted to elucidate the neural mechanisms through which NPY influences NAc function and related behaviors.

The Striatum's Role in Executing Rational and Irrational Economic Behaviors

The striatum is a critical component of the brain that controls motor, reward, and executive function. This ancient and phylogenetically conserved structure forms a central hub where rapid instinctive, reflexive movements and behaviors in response to sensory stimulation or the retrieval of emotional memory intersect with slower planned motor movements and rational behaviors. This review emphasizes two distinct pathways that begin in the thalamus and converge in the striatum to differentially affect movements, behaviors, and decision making. The convergence of excitatory glutamatergic activity from the thalamus and cortex, along with dopamine release in response to novel stimulation, provide the basis for motor learning, reward seeking, and habit formation. We outline how the rules derived through research on neural pathways may enhance the predictability of reflexive actions and rational responses studied in behavioral economics.

Cartography of hevin-expressing cells in the adult brain reveals prominent expression in astrocytes and parvalbumin neurons

Hevin, also known as SPARC-like 1, is a member of the secreted protein acidic and rich in cysteine family of matricellular proteins, which has been implicated in neuronal migration and synaptogenesis during development. Unlike previously characterized matricellular proteins, hevin remains strongly expressed in the adult brain in both astrocytes and neurons, but its precise pattern of expression is unknown. The present study provides the first systematic description of hevin mRNA distribution in the adult mouse brain. Using isotopic in situ hybridization, we showed that hevin is strongly expressed in the cortex, hippocampus, basal ganglia complex, diverse thalamic nuclei and brainstem motor nuclei. To identify the cellular phenotype of hevin-expressing cells, we used double fluorescent in situ hybridization in mouse and human adult brains. In the mouse, hevin mRNA was found in the majority of astrocytes but also in specific neuronal populations. Hevin was expressed in almost all parvalbumin-positive projection neurons and local interneurons. In addition, hevin mRNA was found in: (1) subsets of other inhibitory GABAergic neuronal subtypes, including calbindin, cholecystokinin, neuropeptide Y, and somatostatin-positive neurons; (2) subsets of glutamatergic neurons, identified by the expression of the vesicular glutamate transporters VGLUT1 and VGLUT2; and (3) the majority of cholinergic neurons from motor nuclei. Hevin mRNA was absent from all monoaminergic neurons and cholinergic neurons of the ascending pathway. A similar cellular profile of expression was observed in human, with expression of hevin in parvalbumin interneurons and astrocytes in the cortex and caudate nucleus as well as in cortical glutamatergic neurons. Furthermore, hevin transcript was enriched in ribosomes of astrocytes and parvalbumin neurons providing a direct evidence of hevin mRNAs translation in these cell types. This study reveals the unique and complex expression profile of the matricellular protein hevin in the adult brain. This distribution is compatible with a role of hevin in astrocytic-mediated adult synaptic plasticity and in the regulation of network activity mediated by parvalbumin-expressing neurons.

Striatal GABAergic interneuron dysfunction in the Q175 mouse model of Huntington's disease

The pathological hallmark of Huntington's disease (HD) is the massive loss of striatal and cortical neurons. Until recently, it was believed that striatal interneurons were spared from degeneration. This view has changed after the demonstration that parvalbumin (PV)-expressing interneurons also are vulnerable in humans. Here we compared morphological and functional changes of striatal fast-spiking interneurons (FSIs) and low-threshold spiking (LTS) interneurons in the Q175 mouse model of HD at presymptomatic (2 months) and symptomatic (12 months) stages of the disease. Electrophysiological intrinsic and synaptic properties of FSIs were significantly altered in symptomatic mice compared to wild-type (WT) littermates. Overall, FSIs became more excitable with disease progression. Sholl analysis also revealed a significant loss of dendritic complexity and excitatory synaptic inputs. The basic membrane and synaptic properties of LTS interneurons were similar in Q175 and WT mice regardless of disease stage. The resilience of LTS interneurons could be related to their sparsity of excitatory synaptic inputs compared with FSIs. However, in symptomatic mice, a subpopulation of LTS interneurons displayed an increase in action potential firing within oscillating bursts. Thus, we conclude that while both FSI and LTS interneurons demonstrate increases in excitability, the HD mutation differentially affects their membrane and synaptic properties as well as their ability to respond to compensatory challenges presented during the late stage of the disease. Alterations in GABAergic interneuron intrinsic activity and responsiveness to incoming signals may significantly affect SPN output thus contributing to abnormal motor movements in patients afflicted with HD.

A Cell Atlas for the Mouse Brain

Despite vast numbers of studies of stained cells in the mouse brain, no current brain atlas provides region-by-region neuron counts. In fact, neuron numbers are only available for about 4% of brain of regions and estimates often vary by as much as 3-fold. Here we provide a first 3D cell atlas for the whole mouse brain, showing cell positions constructed algorithmically from whole brain Nissl and gene expression stains, and compared against values from the literature. The atlas provides the densities and positions of all excitatory and inhibitory neurons, astrocytes, oligodendrocytes, and microglia in each of the 737 brain regions defined in the AMBA. The atlas is dynamic, allowing comparison with previously reported numbers, addition of cell types, and improvement of estimates as new data is integrated. The atlas also provides insights into cellular organization only possible at this whole brain scale, and is publicly available.

Cell-type-specific and projection-specific brain-wide reconstruction of single neurons

We developed a dual-adeno-associated-virus expression system that enables strong and sparse labeling of individual neurons with cell-type and projection specificity. We demonstrated its utility for whole-brain reconstruction of midbrain dopamine neurons and striatum-projecting cortical neurons. We further extended the labeling method for rapid reconstruction in cleared thick brain sections and simultaneous dual-color labeling. This labeling system may facilitate the process of generating mesoscale single-neuron projectomes of mammalian brains.

Heterogeneity and Diversity of Striatal GABAergic Interneurons: Update 2018

Our original review, “Heterogeneity and Diversity of Striatal GABAergic Interneurons,” to which this is an invited update, was published in December, 2010 in Frontiers is Neuroanatomy. In that article, we reviewed several decades’ worth of anatomical and electrophysiological data on striatal parvalbumin (PV)-, neuropeptide Y (NPY)- and calretinin(CR)-expressing GABAergic interneurons from many laboratories including our own. In addition, we reported on a recently discovered novel tyrosine hydroxylase (TH) expressing GABAergic interneuron class first revealed in transgenic TH EGFP reporter mouse line. In this review, we report on further advances in the understanding of the functional properties of previously reported striatal GABAergic interneurons and their synaptic connections. With the application of new transgenic fluorescent reporter and Cre-driver/reporter lines, plus optogenetic, chemogenetic and viral transduction methods, several additional subtypes of novel striatal GABAergic interneurons have been discovered, as well as the synaptic networks in which they are embedded. These findings make it clear that previous hypotheses in which striatal GABAergic interneurons modulate and/or control the firing of spiny neurons principally by simple feedforward and/or feedback inhibition are at best incomplete. A more accurate picture is one in which there are highly selective and specific afferent inputs, synaptic connections between different interneuron subtypes and spiny neurons and among different GABAergic interneurons that result in the formation of functional networks and ensembles of spiny neurons.

Inhibition enhances spatially-specific calcium encoding of synaptic input patterns in a biologically constrained model

Synaptic plasticity, which underlies learning and memory, depends on calcium elevation in neurons, but the precise relationship between calcium and spatiotemporal patterns of synaptic inputs is unclear. Here, we develop a biologically realistic computational model of striatal spiny projection neurons with sophisticated calcium dynamics, based on data from rodents of both sexes, to investigate how spatiotemporally clustered and distributed excitatory and inhibitory inputs affect spine calcium. We demonstrate that coordinated excitatory synaptic inputs evoke enhanced calcium elevation specific to stimulated spines, with lower but physiologically relevant calcium elevation in nearby non-stimulated spines. Results further show a novel and important function of inhibition-to enhance the difference in calcium between stimulated and non-stimulated spines. These findings suggest that spine calcium dynamics encode synaptic input patterns and may serve as a signal for both stimulus-specific potentiation and heterosynaptic depression, maintaining balanced activity in a dendritic branch while inducing pattern-specific plasticity.

Introducing an expanded CAG tract into the huntingtin gene causes a wide spectrum of ultrastructural defects in cultured human cells

Modeling of neurodegenerative diseases in vitro holds great promise for biomedical research. Human cell lines harboring a mutations in disease-causing genes are thought to recapitulate early stages of the development an inherited disease. Modern genome-editing tools allow researchers to create isogenic cell clones with an identical genetic background providing an adequate "healthy" control for biomedical and pharmacological experiments. Here, we generated isogenic mutant cell clones with 150 CAG repeats in the first exon of the huntingtin (HTT) gene using the CRISPR/Cas9 system and performed ultrastructural and morphometric analyses of the internal organization of the mutant cells. Electron microscopy showed that deletion of three CAG triplets or an HTT gene knockout had no significant influence on the cell structure. The insertion of 150 CAG repeats led to substantial changes in quantitative and morphological parameters of mitochondria and increased the association of mitochondria with the smooth and rough endoplasmic reticulum while causing accumulation of small autolysosomes in the cytoplasm. Our data indicate for the first time that expansion of the CAG repeat tract in HTT introduced via the CRISPR/Cas9 technology into a human cell line initiates numerous ultrastructural defects that are typical for Huntington's disease.

The role of the dorsal striatum in choice impulsivity

It has long been recognized that the dorsal striatum is an essential brain region for control of action selection based on action-outcome contingency learning, particularly when the available actions are bound to rewarding outcomes. In principle, intertemporal choice in the delay-discounting task-a validated measure of choice impulsivity-involves reward-associated actions that require the recruitment of the dorsal striatum. Here, we conjecture about ways the dorsal striatum is involved in choice impulsivity. Based on a selective body of studies, we begin with a brief history of research on choice impulsivity and the dorsal striatum, and then provide a comprehensive summary of contemporary studies utilizing human neuroimaging and animal models to search for links between choice impulsivity and the dorsal striatum. In particular, we discuss in-depth the converging evidence for the associations of choice impulsivity with the reward valuation coded by the caudate, a ventral-to-dorsal gradient in the dorsal striatum, the origins of striatal afferents, and developmental maturation of frontostriatal connectivity during adolescence.

A thermodynamic description for physiological transmembrane transport

A general formulation for both passive and active transmembrane transport is derived from basic thermodynamical principles. The derivation takes into account the energy required for the motion of molecules across membranes and includes the possibility of modeling asymmetric flow. Transmembrane currents can then be described by the general model in the case of electrogenic flow. As it is desirable in new models, it is possible to derive other well-known expressions for transmembrane currents as particular cases of the general formulation. For instance, the conductance-based formulation for current turns out to be a linear approximation of the general formula for current. Also, under suitable assumptions, other formulas for current based on electrodiffusion, like the constant field approximation by Goldman, can be recovered from the general formulation. The applicability of the general formulations is illustrated first with fits to existing data, and after, with models of transmembrane potential dynamics for pacemaking cardiocytes and neurons. The general formulations presented here provide a common ground for the biophysical study of physiological phenomena that depend on transmembrane transport.

nNOS-Expressing Neurons in the Ventral Tegmental Area and Substantia Nigra Pars Compacta

GABA neurons in the VTA and SNc play key roles in reward and aversion through their local inhibitory control of dopamine neuron activity and through long-range projections to several target regions including the nucleus accumbens. It is not clear whether some of these GABA neurons are dedicated local interneurons or if they all collateralize and send projections externally as well as making local synaptic connections. Testing between these possibilities has been challenging in the absence of interneuron-specific molecular markers. We hypothesized that one potential candidate might be neuronal nitric oxide synthase (nNOS), a common interneuronal marker in other brain regions. To test this, we used a combination of immunolabelling (including antibodies for nNOS that we validated in tissue from nNOS-deficient mice) and cell type-specific virus-based anterograde tracing in mice. We found that nNOS-expressing neurons, in the parabrachial pigmented (PBP) part of the VTA and the SNc were GABAergic and did not make detectable projections, suggesting they may be interneurons. In contrast, nNOS-expressing neurons in the rostral linear nucleus (RLi) were mostly glutamatergic and projected to a number of regions, including the lateral hypothalamus (LH), the ventral pallidum (VP), and the median raphe (MnR) nucleus. Taken together, these findings indicate that nNOS is expressed by neurochemically- and anatomically-distinct neuronal sub-groups in a sub-region-specific manner in the VTA and SNc.

Neuronal mechanisms mediating pathological reward-related behaviors: A focus on silent synapses in the nucleus accumbens

The compulsive drive to seek drugs despite negative consequences relies heavily on drug-induced alterations that occur within the reward neurocircuit. These alterations include changes in neuromodulator and neurotransmitter systems that ultimately lock behaviors into an inflexible and permanent state. To provide clinicians with improved treatment options, researchers are trying to identify, as potential targets of therapeutic intervention, the neural mechanisms mediating an "addictive-like state". Here, we discuss how drug-induced generation of silent synapses in the nucleus accumbens may be a potential therapeutic target capable of reversing drug-related behaviors.

Region-specific and state-dependent action of striatal GABAergic interneurons

Striatum processes a wide range of functions including goal-directed behavior and habit formation, respectively encoded by the dorsomedial striatum (DMS) and dorsolateral striatum (DLS). GABAergic feedforward inhibition is known to control the integration of cortical information by striatal projection neurons (SPNs). Here we questioned whether this control is specific between distinct striatal functional territories. Using opto-activation and opto-inhibition of identified GABAergic interneurons, we found that different circuits are engaged in DLS and DMS, both ex vivo and in vivo: while parvalbumin interneurons efficiently control SPNs in DLS, somatostatin interneurons control SPNs in DMS. Moreover, both parvalbumin and somatostatin interneurons use a dual hyperpolarizing/depolarizing effect to control cortical input integration depending on SPN activity state: GABAergic interneurons potently inhibit spiking SPNs while in resting SPNs, they favor cortical activity summation via a depolarizing effect. Our findings establish that striatal GABAergic interneurons exert efficient territory-specific and state-dependent control of SPN activity and functional output.

Transcriptional and physiological adaptations in nucleus accumbens somatostatin interneurons that regulate behavioral responses to cocaine

The role of somatostatin interneurons in nucleus accumbens (NAc), a key brain reward region, remains poorly understood due to the fact that these cells account for < 1% of NAc neurons. Here, we use optogenetics, electrophysiology, and RNA-sequencing to characterize the transcriptome and functioning of NAc somatostatin interneurons after repeated exposure to cocaine. We find that the activity of somatostatin interneurons regulates behavioral responses to cocaine, with repeated cocaine reducing the excitability of these neurons. Repeated cocaine also induces transcriptome-wide changes in gene expression within NAc somatostatin interneurons. We identify the JUND transcription factor as a key regulator of cocaine action and confirmed, by use of viral-mediated gene transfer, that JUND activity in somatostatin interneurons influences behavioral responses to cocaine. Our results identify alterations in NAc induced by cocaine in a sparse population of somatostatin interneurons, and illustrate the value of studying brain diseases using cell type-specific whole transcriptome RNA-sequencing.

Calcium currents in striatal fast-spiking interneurons: dopaminergic modulation of CaV1 channels

BACKGROUND

Striatal fast-spiking interneurons (FSI) are a subset of GABAergic cells that express calcium-binding protein parvalbumin (PV). They provide feed-forward inhibition to striatal projection neurons (SPNs), receive cortical, thalamic and dopaminergic inputs and are coupled together by electrical and chemical synapses, being important components of the striatal circuitry. It is known that dopamine (DA) depolarizes FSI via D₁-class DA receptors, but no studies about the ionic mechanism of this action have been reported. Here we ask about the ion channels that are the effectors of DA actions. This work studies their Ca²⁺ currents.

RESULTS

Whole-cell recordings in acutely dissociated and identified FSI from PV-Cre transgenic mice were used to show that FSI express an array of voltage gated Ca²⁺ channel classes: Ca_V1, Ca_V2.1, Ca_V2.2, Ca_V2.3 and Ca_V3. However, Ca_V1 Ca²⁺ channel carries most of the whole-cell Ca²⁺ current in FSI. Activation of D₁-like class of DA receptors by the D₁-receptor selective agonist SKF-81297 (SKF) enhances whole-cell Ca²⁺ currents through Ca_V1 channels modulation. A previous block of Ca_V1 channels with nicardipine occludes the action of the DA-agonist, suggesting that no other Ca²⁺ channel is modulated by D₁-receptor activation. Bath application of SKF in brain slices increases the firing rate and activity of FSI as measured with both whole-cell and Ca²⁺ imaging recordings. These actions are reduced by nicardipine.

CONCLUSIONS

The present work discloses one final effector of DA modulation in FSI. We conclude that the facilitatory action of DA in FSI is in part due to Ca_V1 Ca²⁺ channels positive modulation.

Striatal Cholinergic Interneurons in a Knock-in Mouse Model of L-DOPA-Responsive Dystonia

Striatal cholinergic dysfunction is a common phenotype associated with various forms of dystonia in which anti-cholinergic drugs have some therapeutic benefits. However, the underlying substrate of striatal cholinergic defects in dystonia remain poorly understood. In this study, we used a recently developed knock-in mouse model of dopamine-responsive dystonia (DRD) with strong symptomatic responses to anti-cholinergic drugs, to assess changes in the prevalence and morphology of striatal cholinergic interneurons (ChIs) in a model of generalized dystonia. Unbiased stereological neuronal counts and Sholl analysis were used to address these issues. To determine the potential effect of aging on the number of ChIs, both young (3 months old) and aged (15 months old) mice were used. For purpose of comparisons with ChIs, the number of GABAergic parvalbumin (PV)-immunoreactive striatal interneurons was also quantified in young mice. Overall, no significant change in the prevalence of ChIs and PV-immunoreactive cells was found throughout various functional regions of the striatum in young DRD mice. Similar results were found for ChIs in aged animals. Subtle changes in the extent and complexity of the dendritic tree of ChIs were found in middle and caudal regions of the striatum in DRD mice. Additional immunohistochemical data also suggested lack of significant change in the expression of striatal cholinergic M1 and M4 muscarinic receptors immunoreactivity in DRD mice. Thus, together with our previous data from a knock-in mouse model of DYT-1 dystonia (Song et al., 2013), our data further suggest that the dysregulation of striatal cholinergic transmission in dystonia is not associated with major neuroplastic changes in the morphology or prevalence of striatal ChIs.

Highlights-

There is no significant change in the number of striatal ChIs in young and aged mice model of DRD

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There is no significant change in the prevalence of striatal GABAergic PV-containing interneurons in the striatum of young mice models of DRD

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Subtle morphological changes in the dendritic arborization of striatal ChIs are found in the middle and caudal tiers of the striatum in young mice models of DRD

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The levels of both M1 and M4 muscarinic receptors immunoreactivity are not significantly changed in the striatum of DRD mice

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Major changes in the prevalence and morphology of striatal ChIs are unlikely to underlie striatal cholinergic dysfunction in DRD

Coding of self-motion-induced and self-independent visual motion in the rat dorsomedial striatum

Evolutionary development of vision has provided us with the capacity to detect moving objects. Concordant shifts of visual features suggest movements of the observer, whereas discordant changes are more likely to be indicating independently moving objects, such as predators or prey. Such distinction helps us to focus attention, adapt our behavior, and adjust our motor patterns to meet behavioral challenges. However, the neural basis of distinguishing self-induced and self-independent visual motions is not clarified in unrestrained animals yet. In this study, we investigated the presence and origin of motion-related visual information in the striatum of rats, a hub of action selection and procedural memory. We found that while almost half of the neurons in the dorsomedial striatum are sensitive to visual motion congruent with locomotion (and that many of them also code for spatial location), only a small subset of them are composed of fast-firing interneurons that could also perceive self-independent visual stimuli. These latter cells receive their visual input at least partially from the secondary visual cortex (V2). This differential visual sensitivity may be an important support in adjusting behavior to salient environmental events. It emphasizes the importance of investigating visual motion perception in unrestrained animals.

Identification and Characterization of a Novel Spontaneously Active Bursty GABAergic Interneuron in the Mouse Striatum

The recent availability of different transgenic mouse lines coupled with other modern molecular techniques has led to the discovery of an unexpectedly large cellular diversity and synaptic specificity in striatal interneuronal circuitry. Prior research has described three spontaneously active interneuron types in mouse striatal slices: the cholinergic interneuron, the neuropeptide Y-low threshold spike interneuron, and the tyrosine hydroxylase interneurons (THINs). Using transgenic Htr3a-Cre mice, we now characterize a fourth population of spontaneously active striatal GABAergic interneurons termed spontaneously active bursty interneurons (SABIs) because of their unique burst-firing pattern in cell-attached recordings. Although they bear some qualitative similarity in intrinsic electrophysiological properties to THINs in whole-cell recordings, detailed analysis revealed significant differences in many intrinsic properties and in their morphology. Furthermore, all previously identified striatal GABAergic interneurons have been shown to innervate striatal spiny projection neurons (SPNs), contributing to the suggestion that the principal function of striatal GABAergic interneurons is to provide feedforward inhibition to SPNs. Here, very surprisingly, paired recordings show that SABIs do not innervate SPNs significantly. Further, optogenetic inhibition of striatal Htr3a-Cre interneurons triggers barrages of IPSCs in SPNs. We hypothesize that these IPSCs result from disinhibition of a population of GABAergic interneurons with activity that is constitutively suppressed by the SABIs. We suggest that the SABIs represent the first example of a striatal interneuron-selective interneuron and, further, that their existence, along with previously defined interneuronal networks, may participate in the formation of SPN ensembles observed by others.SIGNIFICANCE STATEMENT Before ∼2010, the main function of the three known subtypes of striatal GABAergic interneurons was assumed to mediate feedforward inhibition of the spiny neurons (SPNs). During the past decade, we and others have described several novel populations of striatal GABAergic interneurons and their synaptic connections and have shown that striatal interneurons and SPNs interact through extensive and highly cell-type-specific connections that form specialized networks. Here, we describe a novel population of striatal GABAergic interneuron and provide several lines of evidence suggesting that it represents the first interneuron-selective interneuron in striatum. Striatal interneurons and their synaptic connections are suggested to play an important role in the formation of ensembles of striatal SPNs interconnected by inhibitory axon collaterals.

A new outlook on cholinergic interneurons in Parkinson's disease and L-DOPA-induced dyskinesia

Traditionally, dopamine (DA) and acetylcholine (ACh) striatal systems were considered antagonistic and imbalances or aberrant signaling between these neurotransmitter systems could be detrimental to basal ganglia activity and pursuant motor function, such as in Parkinson's disease (PD) and L-DOPA-induced dyskinesia (LID). Herein, we discuss the involvement of cholinergic interneurons (ChIs) in striatally-mediated movement in a healthy, parkinsonian, and dyskinetic state. ChIs integrate numerous neurotransmitter signals using intrinsic glutamate, serotonin, and DA receptors and convey the appropriate transmission onto nearby muscarinic and nicotinic ACh receptors to produce movement. In PD, severe DA depletion causes abnormal rises in ChI activity which promote striatal signaling to attenuate normal movement. When treating PD with L-DOPA, hyperkinetic side effects, or LID, develop due to increased striatal DA; however, the role of ChIs and ACh transmission, until recently has been unclear. Fortunately, new technology and pharmacological agents have facilitated understanding of ChI function and ACh signaling in the context of LID, thus offering new opportunities to modify existing and discover future therapeutic strategies in movement disorders.

Striatal cholinergic interneurons and Parkinson's disease

Giant, aspiny cholinergic interneurons (ChIs) have long been known to be key nodes in the striatal circuitry controlling goal-directed actions and habits. In recent years, new experimental approaches, like optogenetics and monosynaptic rabies virus mapping, have expanded our understanding of how ChIs contribute to the striatal activity underlying action selection and the interplay of dopaminergic and cholinergic signaling. These approaches also have begun to reveal how ChI function is distorted in disease states affecting the basal ganglia, like Parkinson's disease (PD). This review gives a brief overview of our current understanding of the functional role played by ChIs in striatal physiology and how this changes in PD. The translational implications of these discoveries, as well as the gaps that remain to be bridged, are discussed as well.

Differential inputs to striatal cholinergic and parvalbumin interneurons imply functional distinctions

Striatal cholinergic (ChAT) and parvalbumin (PV) interneurons exert powerful influences on striatal function in health and disease, yet little is known about the organization of their inputs. Here using rabies tracing, electrophysiology and genetic tools, we compare the whole-brain inputs to these two types of striatal interneurons and dissect their functional connectivity in mice. ChAT interneurons receive a substantial cortical input from associative regions of cortex, such as the orbitofrontal cortex. Amongst subcortical inputs, a previously unknown inhibitory thalamic reticular nucleus input to striatal PV interneurons is identified. Additionally, the external segment of the globus pallidus targets striatal ChAT interneurons, which is sufficient to inhibit tonic ChAT interneuron firing. Finally, we describe a novel excitatory pathway from the pedunculopontine nucleus that innervates ChAT interneurons. These results establish the brain-wide direct inputs of two major types of striatal interneurons and allude to distinct roles in regulating striatal activity and controlling behavior.

On the neuronal circuitry mediating L-DOPA-induced dyskinesia

With the advent of rodent models of l-DOPA-induced dyskinesia (LID), a growing literature has linked molecular changes in the striatum to the development and expression of abnormal involuntary movements. Changes in information processing at the striatal level are assumed to impact on the activity of downstream basal ganglia nuclei, which in turn influence brain-wide networks, but very little is actually known about systems-level mechanisms of dyskinesia. As an aid to approach this topic, we here review the anatomical and physiological organisation of cortico-basal ganglia-thalamocortical circuits, and the changes affecting these circuits in animal models of parkinsonism and LID. We then review recent findings indicating that an abnormal cerebellar compensation plays a causal role in LID, and that structures outside of the classical motor circuits are implicated too. In summarizing the available data, we also propose hypotheses and identify important knowledge gaps worthy of further investigation. In addition to informing novel therapeutic approaches, the study of LID can provide new clues about the interplay between different brain circuits in the control of movement.

Disrupted striatal neuron inputs and outputs in Huntington's disease

Huntington's disease (HD) is a hereditary progressive neurodegenerative disorder caused by a CAG repeat expansion in the gene coding for the protein huntingtin, resulting in a pathogenic expansion of the polyglutamine tract in the N-terminus of this protein. The HD pathology resulting from the mutation is most prominent in the striatal part of the basal ganglia, and progressive differential dysfunction and loss of striatal projection neurons and interneurons account for the progression of motor deficits seen in this disease. The present review summarizes current understanding regarding the progression in striatal neuron dysfunction and loss, based on studies both in human HD victims and in genetic mouse models of HD. We review evidence on early loss of inputs to striatum from cortex and thalamus, which may be the basis of the mild premanifest bradykinesia in HD, as well as on the subsequent loss of indirect pathway striatal projection neurons and their outputs to the external pallidal segment, which appears to be the basis of the chorea seen in early symptomatic HD. Later loss of direct pathway striatal projection neurons and their output to the internal pallidal segment account for the severe akinesia seen late in HD. Loss of parvalbuminergic striatal interneurons may contribute to the late dystonia and rigidity.

Parvalbumin Interneurons of the Mouse Nucleus Accumbens are Required For Amphetamine-Induced Locomotor Sensitization and Conditioned Place Preference

To determine the requirement for parvalbumin (PV) expressing GABAergic interneurons of the nucleus accumbens (NAc) in the behavioral adaptations induced by amphetamine (AMPH), we blocked synaptic vesicle release from these neurons using Cre-inducible viral expression of the tetanus toxin light chain in male and female PV-Cre mice. Silencing PV+ interneurons of the NAc selectively inhibited the expression of locomotor sensitization following repeated injections of AMPH and blocked AMPH-induced conditioned place preference (CPP). AMPH induced significantly more expression of the activity-dependent gene Fos in both D1 and D2 dopamine receptor-expressing medium spiny neurons (MSNs) of the NAc of PV+ interneuron silenced mice, suggesting a function for PV+ interneuron-mediated MSN inhibition in the expression of AMPH-induced locomotor sensitization and CPP. These data show a requirement for PV+ interneurons of the NAc in behavioral responses to AMPH, and they raise the possibility that modulation of PV+ interneuron function may alter the development or expression of psychostimulant-induced behavioral adaptations.

Recent Insights into Corticostriatal Circuit Mechanisms underlying Habits: Invited review for Current Opinions in Behavioral Sciences

Habits have been studied for decades, but it was not until recent years that experiments began to elucidate the underlying cellular and circuit mechanisms. The latest experiments have been enabled by advances in cell-type specific monitoring and manipulation of activity in large neuronal populations. Here we will review recent efforts to understand the neural substrates underlying habit formation, focusing on rodent studies on corticostriatal circuits.

Excitatory extrinsic afferents to striatal interneurons and interactions with striatal microcircuitry

The striatum constitutes the main input structure of the basal ganglia and receives two major excitatory glutamatergic inputs, from the cortex and the thalamus. Excitatory cortico- and thalamostriatal connections innervate the principal neurons of the striatum, the spiny projection neurons (SPNs), which constitute the main cellular input as well as the only output of the striatum. In addition, corticostriatal and thalamostriatal inputs also innervate striatal interneurons. Some of these inputs have been very well studied, for example the thalamic innervation of cholinergic interneurons and the cortical innervation of striatal fast-spiking interneurons, but inputs to most other GABAergic interneurons remain largely unstudied, due in part to the relatively recent identification and characterization of many of these interneurons. In this review, we will discuss and reconcile some older as well as more recent data on the extrinsic excitatory inputs to striatal interneurons. We propose that the traditional feed-forward inhibitory model of the cortical input to the fast-spiking interneuron then inhibiting the SPN, often assumed to be the prototype of the main functional organization of striatal interneurons, is incomplete. We provide evidence that the extrinsic innervation of striatal interneurons is not uniform but shows great cell-type specificity. In addition, we will review data showing that striatal interneurons are themselves interconnected in a highly cell-type-specific manner. These data suggest that the impact of the extrinsic inputs on striatal activity critically depends on synaptic interactions within interneuronal circuitry.