Heterogeneity and Diversity of Striatal GABAergic Interneurons: Update 2018

Role of the striatal dopamine, GABA and opioid systems in mediating feeding and fat intake

Food intake, which is a highly reinforcing behavior, provides nutrients required for survival in all animals. However, when fat and sugar consumption goes beyond the daily needs, it can favor obesity. The prevalence and severity of this health problem has been increasing with time. Besides covering nutrient and energy needs, food and in particular its highly palatable components, such as fats, also induce feelings of joy and pleasure. Experimental evidence supports a role of the striatal complex and of the mesolimbic dopamine system in both feeding and food-related reward processing, with the nucleus accumbens as a key target for reward or reinforcing-associated signaling during food intake behavior. In this review, we provide insights concerning the impact of feeding, including fat intake, on different types of receptors and neurotransmitters present in the striatal complex. Reciprocally, we also cover the evidence for a modulation of palatable food intake by different neurochemical systems in the striatal complex and in particular the nucleus accumbens, with a focus on dopamine, GABA and the opioid system.

GABAergic interneurons expressing the α2 nicotinic receptor subunit are functionally integrated in the striatal microcircuit

The interactions between the striatal cholinergic and GABAergic systems are crucial in shaping reward-related behavior and reinforcement learning; however, the synaptic pathways mediating them are largely unknown. Here, we use Chrna2-Cre mice to characterize striatal interneurons (INs) expressing the nicotinic α2 receptor subunit. Using triple patch-clamp recordings combined with optogenetic stimulations, we characterize the electrophysiological, morphological, and synaptic properties of striatal Chrna2-INs. Striatal Chrna2-INs have diverse electrophysiological properties, distinct from their counterparts in other brain regions, including the hippocampus and neocortex. Unlike in other regions, most striatal Chrna2-INs are fast-spiking INs expressing parvalbumin. Striatal Chrna2-INs are intricately integrated in the striatal microcircuit, forming inhibitory synaptic connections with striatal projection neurons and INs, including other Chrna2-INs. They receive excitatory inputs from primary motor cortex mediated by both AMPA and NMDA receptors. A subpopulation of Chrna2-INs responds to nicotinic input, suggesting reciprocal interactions between this GABAergic interneuron population and striatal cholinergic synapses.

A tonic nicotinic brake controls spike timing in striatal spiny projection neurons

Striatal spiny projection neurons (SPNs) transform convergent excitatory corticostriatal inputs into an inhibitory signal that shapes basal ganglia output. This process is fine-tuned by striatal GABAergic interneurons (GINs), which receive overlapping cortical inputs and mediate rapid corticostriatal feedforward inhibition of SPNs. Adding another level of control, cholinergic interneurons (CINs), which are also vigorously activated by corticostriatal excitation, can disynaptically inhibit SPNs by activating α4β2 nicotinic acetylcholine receptors (nAChRs) on various GINs. Measurements of this disynaptic inhibitory pathway, however, indicate that it is too slow to compete with direct GIN-mediated feedforward inhibition. Moreover, functional nAChRs are also present on populations of GINs that respond only weakly to phasic activation of CINs, such as parvalbumin-positive fast-spiking interneurons (PV-FSIs), making the overall role of nAChRs in shaping striatal synaptic integration unclear. Using acute striatal slices from mice we show that upon synchronous optogenetic activation of corticostriatal projections blockade of α4β2 nAChRs shortened SPN spike latencies and increased postsynaptic depolarizations. The nAChR-dependent inhibition was mediated by downstream GABA release, and data suggest that the GABA source was not limited to GINs that respond strongly to phasic CIN activation. In particular, the observed decrease in spike latency caused by nAChR blockade was associated with a diminished frequency of spontaneous inhibitory postsynaptic currents in SPNs, a parallel hyperpolarization of PV-FSIs, and was occluded by pharmacologically preventing cortical activation of PV-FSIs. Taken together, we describe a role for tonic (as opposed to phasic) activation of nAChRs in striatal function. We conclude that tonic activation of nAChRs by CINs maintains a GABAergic brake on cortically-driven striatal output by ‘priming’ feedforward inhibition, a process that may shape SPN spike timing, striatal processing, and synaptic plasticity.

Neuron types in the primate striatum: stereological analysis of projection neurons and interneurons in control and parkinsonian monkeys

AIMS

The striatum is mainly composed of projection neurons. It also contains interneurons, which modulate and control striatal output. The aim of the present study was to assess the percentages of projection neurons and interneuron populations in the striatum of control monkeys and of parkinsonian monkeys.

METHODS

Unbiased stereology was used to estimate the volume density of every neuron population in the caudate, putamen and ventral striatum of control monkeys and of monkeys treated with MPTP, which results in striatal dopamine depletion. The various neuron population phenotypes were identified by immunohistochemistry. All analyses were performed within the same subjects using similar processing and analysis parameters, thus allowing for reliable data comparisons.

RESULTS

In control monkeys, the projection neurons, which express the Dopamine-and-cAMP-Regulated-Phosphoprotein, 32-KDa (DARPP-32), were the most abundant: ~86% of the total neurons counted. The interneurons accounted for the remaining 14%. Among the interneurons, those expressing Calretinin were the most abundant (Cr+: ~57%; ~8% of the total striatal neurons counted), followed those expressing Parvalbumin (Pv+: ~18 %; 2.6%), Dinucleotide Phosphate-Diaphorase (NADPH+: ~13 %; 1.8%), Choline Acetyltransferase (ChAT+: ~11%; 1.5%) and Tyrosine Hydroxylase (TH+: ~0.5%; 0.1%). No significant changes in volume densities occurred in any population following dopamine depletion, except for the TH+ interneurons, which increased in parkinsonian non-symptomatic monkeys and even more in symptomatic monkeys.

CONCLUSIONS

These data are relevant for translational studies targeting specific neuron populations of the striatum. The fact that dopaminergic denervation does not cause neuron loss in any population has potential pathophysiological implications.

Dysregulation of the Basal Ganglia Indirect Pathway in Early Symptomatic Q175 Huntington's Disease Mice

The debilitating psychomotor symptoms of Huntington's disease (HD) are linked partly to degeneration of the basal ganglia indirect pathway. At early symptomatic stages, before major cell loss, indirect pathway neurons exhibit numerous cellular and synaptic changes in HD and its models. However, the impact of these alterations on circuit activity remains poorly understood. To address this gap, optogenetic- and reporter-guided electrophysiological interrogation was used in early symptomatic male and female Q175 HD mice. D2 dopamine receptor-expressing striatal projection neurons (D2-SPNs) were hypoactive during synchronous cortical slow-wave activity, consistent with known reductions in dendritic excitability and cortical input strength. Downstream prototypic parvalbumin-expressing external globus pallidus (PV+ GPe) neurons discharged at 2-3 times their normal rate, even during periods of D2-SPN inactivity, arguing that defective striatopallidal inhibition was not the only cause of their hyperactivity. Indeed, PV+ GPe neurons also exhibited abnormally elevated autonomous firing ex vivo Optogenetic inhibition of PV+ GPe neurons in vivo partially and fully ameliorated the abnormal hypoactivity of postsynaptic subthalamic nucleus (STN) and putative PV- GPe neurons, respectively. In contrast to STN neurons whose autonomous firing is impaired in HD mice, putative PV- GPe neuron activity was unaffected ex vivo, implying that excessive inhibition was responsible for their hypoactivity in vivo Together with previous studies, these data demonstrate that (1) indirect pathway nuclei are dysregulated in Q175 mice through changes in presynaptic activity and/or intrinsic cellular and synaptic properties; and (2) prototypic PV+ GPe neuron hyperactivity and excessive target inhibition are prominent features of early HD pathophysiology.SIGNIFICANCE STATEMENT The early symptoms of Huntington's disease (HD) are linked to degenerative changes in the action-suppressing indirect pathway of the basal ganglia. Consistent with this linkage, the intrinsic properties of cells in this pathway exhibit complex alterations in HD and its models. However, the impact of these changes on activity is poorly understood. Using electrophysiological and optogenetic approaches, we demonstrate that the indirect pathway is highly dysregulated in early symptomatic HD mice through changes in upstream activity and/or intrinsic properties. Furthermore, we reveal that hyperactivity of external globus pallidus neurons and excessive inhibition of their targets are key features of early HD pathophysiology. Together, these findings could help to inform the development and targeting of viral-based, gene therapeutic approaches for HD.

Striatal synaptic adaptations in Parkinson's disease

The striatum is densely innervated by mesencephalic dopaminergic neurons that modulate acquisition and vigor of goal-directed actions and habits. This innervation is progressively lost in Parkinson's disease (PD), contributing to the defining movement deficits of the disease. Although boosting dopaminergic signaling with levodopa early in the course of the disease alleviates these deficits, later this strategy leads to the emergence of debilitating dyskinesia. Here, recent advances in our understanding of how striatal cells and circuits adapt to this progressive de-innervation and to levodopa therapy are discussed. First, we discuss how dopamine (DA) depletion triggers cell type-specific, homeostatic changes in spiny projection neurons (SPNs) that tend to normalize striatal activity but also lead to disruption of the synaptic architecture sculpted by experience. Second, we discuss the roles played by cholinergic and nitric oxide-releasing interneurons in these adaptations. Third, we examine recent work in freely moving mice suggesting that alterations in the spatiotemporal dynamics of striatal ensembles contributes to PD movement deficits. Lastly, we discuss recently published evidence from a progressive model of PD suggesting that contrary to the classical model, striatal pathway imbalance is necessary but not sufficient to produce frank parkinsonism.

Cocaine restricts nucleus accumbens feedforward drive through a monoamine-independent mechanism

Parvalbumin-expressing fast-spiking interneurons (PV-INs) within feedforward microcircuits in the nucleus accumbens (NAc) coordinate goal-directed motivational behavior. Feedforward inhibition of medium spiny projection neurons (MSNs) is initiated by glutamatergic input from corticolimbic brain structures. While corticolimbic synapses onto MSNs are targeted by the psychostimulant, cocaine, it remains unknown whether cocaine also exerts acute neuromodulatory actions at collateralizing synapses onto PV-INs. Using whole-cell patch-clamp electrophysiology, optogenetics, and pharmacological tools in transgenic reporter mice, we found that cocaine decreases thalamocortical glutamatergic drive onto PV-INs by engaging a monoamine-independent mechanism. This mechanism relies on postsynaptic sigma-1 (σ₁) activity, leading to the mobilization of intracellular Ca²⁺ stores that trigger retrograde endocannabinoid signaling at presynaptic type-1 cannabinoid receptors (CB₁R). Cocaine-evoked CB₁R activity occludes the expression of CB₁R-dependent long-term depression (LTD) at this synaptic locus. These findings provide evidence that acute cocaine exposure targets feedforward microcircuits in the NAc and extend existing models of cocaine action on mesolimbic reward circuits.

LRRK2 at Striatal Synapses: Cell-Type Specificity and Mechanistic Insights

Mutations in leucine-rich repeat kinase 2 (LRRK2) cause Parkinson’s disease with a similar clinical presentation and progression to idiopathic Parkinson’s disease, and common variation is linked to disease risk. Recapitulation of the genotype in rodent models causes abnormal dopamine release and increases the susceptibility of dopaminergic neurons to insults, making LRRK2 a valuable model for understanding the pathobiology of Parkinson’s disease. It is also a promising druggable target with targeted therapies currently in development. LRRK2 mRNA and protein expression in the brain is highly variable across regions and cellular identities. A growing body of work has demonstrated that pathogenic LRRK2 mutations disrupt striatal synapses before the onset of overt neurodegeneration. Several substrates and interactors of LRRK2 have been identified to potentially mediate these pre-neurodegenerative changes in a cell-type-specific manner. This review discusses the effects of pathogenic LRRK2 mutations in striatal neurons, including cell-type-specific and pathway-specific alterations. It also highlights several LRRK2 effectors that could mediate the alterations to striatal function, including Rabs and protein kinase A. The lessons learned from improving our understanding of the pathogenic effects of LRRK2 mutations in striatal neurons will be applicable to both dissecting the cell-type specificity of LRRK2 function in the transcriptionally diverse subtypes of dopaminergic neurons and also increasing our understanding of basal ganglia development and biology. Finally, it will inform the development of therapeutics for Parkinson’s disease.

Neuroadaptations and TGF-β signaling: emerging role in models of neuropsychiatric disorders

Neuropsychiatric diseases are manifested by maladaptive behavioral plasticity. Despite the greater understanding of the neuroplasticity underlying behavioral adaptations, pinpointing precise cellular mediators has remained elusive. This has stymied the development of pharmacological interventions to combat these disorders both at the level of progression and relapse. With increased knowledge on the putative role of the transforming growth factor (TGF- β) family of proteins in mediating diverse neuroadaptations, the influence of TGF-β signaling in regulating maladaptive cellular and behavioral plasticity underlying neuropsychiatric disorders is being increasingly elucidated. The current review is focused on what is currently known about the TGF-β signaling in the central nervous system in mediating cellular and behavioral plasticity related to neuropsychiatric manifestations.

Cocaine-induced projection-specific and cell type-specific adaptations in the nucleus accumbens

Cocaine craving, seeking, and relapse are mediated, in part, by cocaine-induced adaptive changes in the brain reward circuits. The nucleus accumbens (NAc) integrates and prioritizes different emotional and motivational inputs to the reward system by processing convergent glutamatergic projections from the medial prefrontal cortex, basolateral amygdala, ventral hippocampus, and other limbic and paralimbic brain regions. Medium spiny neurons (MSNs) are the principal projection neurons in the NAc, which can be divided into two major subpopulations, namely dopamine receptor D1- versus D2-expressing MSNs, with complementing roles in reward-associated behaviors. After cocaine experience, NAc MSNs exhibit complex and differential adaptations dependent on cocaine regimen, withdrawal time, cell type, location (NAc core versus shell), and related input and output projections, or any combination of these factors. Detailed characterization of these cellular adaptations has been greatly facilitated by the recent development of optogenetic/chemogenetic techniques combined with transgenic tools. In this review, we discuss such cell type- and projection-specific adaptations induced by cocaine experience. Specifically, (1) D1 and D2 NAc MSNs frequently exhibit differential adaptations in spinogenesis, glutamatergic receptor trafficking, and intrinsic membrane excitability, (2) cocaine experience differentially changes the synaptic transmission at different afferent projections onto NAc MSNs, (3) cocaine-induced NAc adaptations exhibit output specificity, e.g., being different at NAc-ventral pallidum versus NAc-ventral tegmental area synapses, and (4) the input, output, subregion, and D1/D2 cell type may together determine cocaine-induced circuit plasticity in the NAc. In light of the projection- and cell-type specificity, we also briefly discuss ensemble and circuit mechanisms contributing to cocaine craving and relapse after drug withdrawal.

Optogenetic inhibition of indirect pathway neurons in the dorsomedial striatum reduces excessive grooming in Sapap3-knockout mice

Excessive grooming of Sapap3-KO mice has been used as a model of obsessive-compulsive disorder (OCD). Previous studies suggest that dysregulation of cortico-striatal circuits is critically important in the generation of compulsive behaviors, and it has been proposed that the alteration in the activity patterns of striatal circuitry underlies the excessive grooming observed in Sapap3-KO mice. To test this hypothesis, we used in-vivo calcium imaging of individual cells to record striatal activity in these animals and optogenetic inhibition to manipulate this activity. We identified striatal neurons that are modulated during grooming behavior and found that their proportion is significantly larger in Sapap3-KO mice compared to wild-type littermates. Inhibition of striatal cells in Sapap3-KO mice increased the number of grooming episodes observed. Remarkably, the specific inhibition of indirect pathway neurons decreased the occurrence of grooming events. Our results indicate that there is striatal neural activity related to excessive grooming engagement in Sapap3-KO mice. We also demonstrate, for the first time, that specific inhibition of striatal indirect pathway neurons reduces this compulsive phenotype, suggesting that treatments that alleviate compulsive symptoms in OCD patients may exert their effects through this specific striatal population.

Symmetric and Asymmetric Synapses Driving Neurodegenerative Disorders

In 1959, E. G. Gray described two different types of synapses in the brain for the first time: symmetric and asymmetric. Later on, symmetric synapses were associated with inhibitory terminals, and asymmetric synapses to excitatory signaling. The balance between these two systems is critical to maintain a correct brain function. Likewise, the modulation of both types of synapses is also important to maintain a healthy equilibrium. Cerebral circuitry responds differently depending on the type of damage and the timeline of the injury. For example, promoting symmetric signaling following ischemic damage is beneficial only during the acute phase; afterwards, it further increases the initial damage. Synapses can be also altered by players not directly related to them; the chronic and long-term neurodegeneration mediated by tau proteins primarily targets asymmetric synapses by decreasing neuronal plasticity and functionality. Dopamine represents the main modulating system within the central nervous system. Indeed, the death of midbrain dopaminergic neurons impairs locomotion, underlying the devastating Parkinson’s disease. Herein, we will review studies on symmetric and asymmetric synapses plasticity after three different stressors: symmetric signaling under acute damage—ischemic stroke; asymmetric signaling under chronic and long-term neurodegeneration—Alzheimer’s disease; symmetric and asymmetric synapses without modulation—Parkinson’s disease.

From Progenitors to Progeny: Shaping Striatal Circuit Development and Function

Understanding how neurons of the striatum are formed and integrate into complex synaptic circuits is essential to provide insight into striatal function in health and disease. In this review, we summarize our current understanding of the development of striatal neurons and associated circuits with a focus on their embryonic origin. Specifically, we address the role of distinct types of embryonic progenitors, found in the proliferative zones of the ganglionic eminences in the ventral telencephalon, in the generation of diverse striatal interneurons and projection neurons. Indeed, recent evidence would suggest that embryonic progenitor origin dictates key characteristics of postnatal cells, including their neurochemical content, their location within striatum, and their long-range synaptic inputs. We also integrate recent observations regarding embryonic progenitors in cortical and other regions and discuss how this might inform future research on the ganglionic eminences. Last, we examine how embryonic progenitor dysfunction can alter striatal formation, as exemplified in Huntington's disease and autism spectrum disorder, and how increased understanding of embryonic progenitors can have significant implications for future research directions and the development of improved therapeutic options.SIGNIFICANCE STATEMENT This review highlights recently defined novel roles for embryonic progenitor cells in shaping the functional properties of both projection neurons and interneurons of the striatum. It outlines the developmental mechanisms that guide neuronal development from progenitors in the embryonic ganglionic eminences to progeny in the striatum. Where questions remain open, we integrate observations from cortex and other regions to present possible avenues for future research. Last, we provide a progenitor-centric perspective onto both Huntington's disease and autism spectrum disorder. We suggest that future investigations and manipulations of embryonic progenitor cells in both research and clinical settings will likely require careful consideration of their great intrinsic diversity and neurogenic potential.

Selective Manipulation of G-Protein γ7 Subunit in Mice Provides New Insights into Striatal Control of Motor Behavior

Stimulatory coupling of dopamine D₁ (D₁R) and adenosine A_2A receptors (A_2AR) to adenylyl cyclase within the striatum is mediated through a specific Gα_olfβ₂γ₇ heterotrimer to ultimately modulate motor behaviors. To dissect the individual roles of the Gα_olfβ₂γ₇ heterotrimer in different populations of medium spiny neurons (MSNs), we produced and characterized conditional mouse models, in which the Gng7 gene was deleted in either the D₁R- or A_2AR/D₂R-expressing MSNs. We show that conditional loss of γ₇ disrupts the cell type-specific assembly of the Gα_olfβ₂γ₇ heterotrimer, thereby identifying its circumscribed roles acting downstream of either the D₁Rs or A_2ARs in coordinating motor behaviors, including in vivo responses to psychostimulants. We reveal that Gα_olfβ₂γ₇/cAMP signal in D₁R-MSNs does not impact spontaneous and amphetamine-induced locomotor behaviors in male and female mice, while its loss in A_2AR/D₂R-MSNs results in a hyperlocomotor phenotype and enhanced locomotor response to amphetamine. Additionally, Gα_olfβ₂γ₇/cAMP signal in either D₁R- or A_2AR/D₂R-expressing MSNs is not required for the activation of PKA signaling by amphetamine. Finally, we show that Gα_olfβ₂γ₇ signaling acting downstream of D₁Rs is selectively implicated in the acute locomotor-enhancing effects of morphine. Collectively, these results support the general notion that receptors use specific Gαβγ proteins to direct the fidelity of downstream signaling pathways and to elicit a diverse repertoire of cellular functions. Specifically, these findings highlight the critical role for the γ₇ protein in determining the cellular level, and hence, the function of the Gα_olfβ₂γ₇ heterotrimer in several disease states associated with dysfunctional striatal signaling.SIGNIFICANCE STATEMENT Dysfunction or imbalance of cAMP signaling in the striatum has been linked to several neurologic and neuropsychiatric disorders, including Parkinson's disease, dystonia, schizophrenia, and drug addiction. By genetically targeting the γ₇ subunit in distinct striatal neuronal subpopulations in mice, we demonstrate that the formation and function of the Gα_olfβ₂γ₇ heterotrimer, which represents the rate-limiting step for cAMP production in the striatum, is selectively disrupted. Furthermore, we reveal cell type-specific roles for Gα_olfβ₂γ₇-mediated cAMP production in the control of spontaneous locomotion as well as behavioral and molecular responses to psychostimulants. Our findings identify the γ₇ protein as a novel therapeutic target for disease states associated with dysfunctional striatal cAMP signaling.

Striatal topographical organization: Bridging the gap between molecules, connectivity and behavior

The striatum represents the major hub of the basal ganglia, receiving projections from the entire cerebral cortex and it is assumed to play a key role in a wide array of complex behavioral tasks. Despite being extensively investigated during the last decades, the topographical organization of the striatum is not well understood yet. Ongoing efforts in neuroscience are focused on analyzing striatal anatomy at different spatial scales, to understand how structure relates to function and how derangements of this organization are involved in various neuropsychiatric diseases. While being subdivided at the macroscale level into dorsal and ventral divisions, at a mesoscale level the striatum represents an anatomical continuum sharing the same cellular makeup. At the same time, it is now increasingly ascertained that different striatal compartments show subtle histochemical differences, and their neurons exhibit peculiar patterns of gene expression, supporting functional diversity across the whole basal ganglia circuitry. Such diversity is further supported by afferent connections which are heterogenous both anatomically, as they originate from distributed cortical areas and subcortical structures, and biochemically, as they involve a variety of neurotransmitters. Specifically, the cortico-striatal projection system is topographically organized delineating a functional organization which is maintained throughout the basal ganglia, subserving motor, cognitive and affective behavioral functions. While such functional heterogeneity has been firstly conceptualized as a tripartite organization, with sharply defined limbic, associative and sensorimotor territories within the striatum, it has been proposed that such territories are more likely to fade into one another, delineating a gradient-like organization along medio-lateral and ventro-dorsal axes. However, the molecular and cellular underpinnings of such organization are less understood, and their relations to behavior remains an open question, especially in humans. In this review we aimed at summarizing the available knowledge on striatal organization, especially focusing on how it links structure to function and its alterations in neuropsychiatric diseases. We examined studies conducted on different species, covering a wide array of different methodologies: from tract-tracing and immunohistochemistry to neuroimaging and transcriptomic experiments, aimed at bridging the gap between macroscopic and molecular levels.

Striatal low-threshold spiking interneurons locally gate dopamine

The dorsomedial striatum (DMS) is a central hub supporting goal-directed learning and motor performance. Recent evidence has revealed unexpected roles for local inhibitory GABAergic networks in modulating striatal output and behavior.¹ The sparse low-threshold spiking interneuron subtype (LTSI), which exhibits robust reward-circumscribed population activity, is a bidirectional regulator of initial goal-directed learning.² Striatal dopamine signaling is a central reward-related neuromodulatory system mediating goal-directed action and performance, serving as a teaching signal,³ facilitating synaptic plasticity,⁴ and invigorating motor behaviors.⁵ Given the dynamic modulation of LTSIs during goal-directed behavior, we hypothesized that they could provide a novel GABAergic mechanism of local striatal dopaminergic regulation to shape early learning. We provide anatomical evidence for close proximation of LTSI terminals and dopaminergic processes in striatum, suggesting that LTSIs directly control dopaminergic axon activity. Using in vitro fast scan cyclic voltammetry, we demonstrate that LTSIs directly attenuate optogenetically evoked dopamine via GABA_B receptor signaling. In vivo, GRAB_DA dopamine sensor imaging shows that LTSIs strongly modulate striatal dopamine dynamics during operant learning, while pharmacological stabilization of dopamine via intra-striatal aripiprazole microinjection suppresses the effects of LTSI inhibition on learning. Together, these results uncover an unexpected function for LTSIs in gating striatal dopamine to facilitate goal-directed learning.

Endocannabinoid Modulation of Nucleus Accumbens Microcircuitry and Terminal Dopamine Release

The nucleus accumbens (NAc) is located in the ventromedial portion of the striatum and is vital to valence-based predictions and motivated action. The neural architecture of the NAc allows for complex interactions between various cell types that filter incoming and outgoing information. Dopamine (DA) input serves a crucial role in modulating NAc function, but the mechanisms that control terminal DA release and its effect on NAc neurons continues to be elucidated. The endocannabinoid (eCB) system has emerged as an important filter of neural circuitry within the NAc that locally shapes terminal DA release through various cell type- and site-specific actions. Here, we will discuss how eCB signaling modulates terminal DA release by shaping the activity patterns of NAc neurons and their afferent inputs. We then discuss recent technological advancements that are capable of dissecting how distinct cell types, their afferent projections, and local neuromodulators influence valence-based actions.

Astrocyte-derived neurons provide excitatory input to the adult striatal circuitry

Astrocytes have emerged as a potential source for new neurons in the adult mammalian brain. In mice, adult striatal neurogenesis can be stimulated by local damage, which recruits striatal astrocytes into a neurogenic program by suppression of active Notch signaling (J. P. Magnusson et al., Science 346, 237-241 [2014]). Here, we induced adult striatal neurogenesis in the intact mouse brain by the inhibition of Notch signaling in astrocytes. We show that most striatal astrocyte-derived neurons are confined to the anterior medial striatum, do not express established striatal neuronal markers, and exhibit dendritic spines, which are atypical for striatal interneurons. In contrast to striatal neurons generated during development, which are GABAergic or cholinergic, most adult astrocyte-derived striatal neurons possess distinct electrophysiological properties, constituting the only glutamatergic striatal population. Astrocyte-derived neurons integrate into the adult striatal microcircuitry, both receiving and providing synaptic input. The glutamatergic nature of these neurons has the potential to provide excitatory input to the striatal circuitry and may represent an efficient strategy to compensate for reduced neuronal activity caused by aging or lesion-induced neuronal loss.

Making decisions in the dark basement of the brain: A look back at the GPR model of action selection and the basal ganglia

How does your brain decide what you will do next? Over the past few decades compelling evidence has emerged that the basal ganglia, a collection of nuclei in the fore- and mid-brain of all vertebrates, are vital to action selection. Gurney, Prescott, and Redgrave published an influential computational account of this idea in Biological Cybernetics in 2001. Here we take a look back at this pair of papers, outlining the "GPR" model contained therein, the context of that model's development, and the influence it has had over the past twenty years. Tracing its lineage into models and theories still emerging now, we are encouraged that the GPR model is that rare thing, a computational model of a brain circuit whose advances were directly built on by others.

Dorsal and ventral striatal neuronal subpopulations differentially disrupt male mouse copulatory behavior

The specific role of the striatum, especially its dorsolateral (DLS) and dorsomedial (DMS) parts, in male copulatory behavior is still debated. In order to clarify their contribution to male sexual behavior, we specifically ablated the major striatal neuronal subpopulations, direct and indirect medium spiny neurons (dMSNs and iMSNs) in DMS or DLS, and dMSNs, iMSNs and cholinergic interneurons in nucleus accumbens (NAc), The main results of this study can be summarized as follows: In DMS, dMSN ablation causes a reduction in the percent of mice that mount a receptive female, and a complex alteration in the parameters of the copulatory performance, that is largely opposite to the alterations induced by iMSN ablation. In DLS, dMSN ablation causes a widespread alteration in the copulatory behavior parameters, that tends to disappear at repetition of the test; iMSN ablation induces minor copulatory behavior alterations that are complementary to those observed after dMSN ablation. In NAc, dMSN ablation causes a marked reduction in the percent of mice that mount a receptive female and a disruption of copulatory behavior, while iMSN ablation induces minor copulatory behavior alterations that are opposite to those observed with dMSN ablation, and cholinergic neuron ablation induces a selective decrease in mount latency. Overall, present data point to a complex region and cell-specific contribution to copulatory behavior of the different neuronal subpopulations of both dorsal and ventral striatum, with a prominent role of the dMSNs of the different subregions.

Cellular context shapes cyclic nucleotide signaling in neurons through multiple levels of integration

Intracellular signaling with cyclic nucleotides are ubiquitous signaling pathways, yet the dynamics of these signals profoundly differ in different cell types. Biosensor imaging experiments, by providing direct measurements in intact cellular environment, reveal which receptors are activated by neuromodulators and how the coincidence of different neuromodulators is integrated at various levels in the signaling cascade. Phosphodiesterases appear as one important determinant of cross-talk between different signaling pathways. Finally, analysis of signal dynamics reveal that striatal medium-sized spiny neuron obey a different logic than other brain regions such as cortex, probably in relation with the function of this brain region which efficiently detects transient dopamine.

Camk2a-Cre and Tshz3 Expression in Mouse Striatal Cholinergic Interneurons: Implications for Autism Spectrum Disorder

Camk2a-Cre mice have been widely used to study the postnatal function of several genes in forebrain projection neurons, including cortical projection neurons (CPNs) and striatal medium-sized spiny neurons (MSNs). We linked heterozygous deletion of TSHZ3/Tshz3 gene to autism spectrum disorder (ASD) and used Camk2a-Cre mice to investigate the postnatal function of Tshz3, which is expressed by CPNs but not MSNs. Recently, single-cell transcriptomics of the adult mouse striatum revealed the expression of Camk2a in interneurons and showed Tshz3 expression in striatal cholinergic interneurons (SCINs), which are attracting increasing interest in the field of ASD. These data and the phenotypic similarity between the mice with Tshz3 haploinsufficiency and Camk2a-Cre-dependent conditional deletion of Tshz3 (Camk2a-cKO) prompted us to better characterize the expression of Tshz3 and the activity of Camk2a-Cre transgene in the striatum. Here, we show that the great majority of Tshz3-expressing cells are SCINs and that all SCINs express Tshz3. Using lineage tracing, we demonstrate that the Camk2a-Cre transgene is expressed in the SCIN lineage where it can efficiently elicit the deletion of the Tshz3-floxed allele. Moreover, transcriptomic and bioinformatic analysis in Camk2a-cKO mice showed dysregulated striatal expression of a number of genes, including genes whose human orthologues are associated with ASD and synaptic signaling. These findings identifying the expression of the Camk2a-Cre transgene in SCINs lineage lead to a reappraisal of the interpretation of experiments using Camk2a-Cre-dependent gene manipulations. They are also useful to decipher the cellular and molecular substrates of the ASD-related behavioral abnormalities observed in Tshz3 mouse models.

Gap Junctions Between Striatal D1 Neurons and Cholinergic Interneurons

The striatum participates in numerous important behaviors. Its principal projection neurons use GABA and peptides as neurotransmitters and interact extensively with interneurons, including cholinergic interneurons (ChIs) that are tonically active. Dissecting the interactions between projection neurons and ChIs is important for uncovering the role and mechanisms of the striatal microcircuits. Here, by combining several optogenetic tools with cell type-specific electrophysiological recordings, we uncovered direct electrical coupling between D1-type projection neurons and ChIs, in addition to the chemical transmission between these two major cell types. Optogenetic stimulation or inhibition led to bilateral current exchanges between D1 neurons and ChIs, which can be abolished by gap junction blockers. We further confirmed the presence of gap junctions through paired electrophysiological recordings and dye microinjections. Finally, we found that activating D1 neurons promotes basal activity of ChIs via gap junctions. Collectively, these results reveal the coexistence of the chemical synapse and gap junctions between D1 neurons and ChIs, which contributes to maintaining the tonically active firing patterns of ChIs.

BACHD Mice Recapitulate the Striatal Parvalbuminergic Interneuron Loss Found in Huntington's Disease

Huntington’s disease (HD) is a dominantly inherited, adult-onset neurodegenerative disease characterized by motor, psychiatric, and cognitive abnormalities. Neurodegeneration is prominently observed in the striatum where GABAergic medium spiny neurons (MSN) are the most affected neuronal population. Interestingly, recent reports of pathological changes in HD patient striatal tissue have identified a significant reduction in the number of parvalbumin-expressing interneurons which becomes more robust in tissues of higher disease grade. Analysis of other interneuron populations, including somatostatin, calretinin, and cholinergic, did not reveal significant neurodegeneration. Electrophysiological experiments in BACHD mice have identified significant changes in the properties of parvalbumin and somatostatin expressing interneurons in the striatum. Furthermore, their interactions with MSNs are altered as the mHTT expressing mouse models age with increased input onto MSNs from striatal somatostatin and parvalbumin-expressing neurons. In order to determine whether BACHD mice recapitulate the alterations in striatal interneuron number as observed in HD patients, we analyzed the number of striatal parvalbumin, somatostatin, calretinin, and choline acetyltransferase positive cells in symptomatic 12–14 month-old mice by immunofluorescent labeling. We observed a significant decrease in the number of parvalbumin-expressing interneurons as well as a decrease in the area and perimeter of these cells. No significant changes were observed for somatostatin, calretinin, or cholinergic interneuron numbers while a significant decrease was observed for the area of cholinergic interneurons. Thus, the BACHD mice recapitulate the degenerative phenotype observed in the parvalbumin interneurons in HD patient striata without affecting the number of other interneuron populations in the striatum.

Reduced Expression of GABA A Receptor Alpha2 Subunit Is Associated With Disinhibition of DYT-THAP1 Dystonia Patient-Derived Striatal Medium Spiny Neurons

DYT-THAP1 dystonia (formerly DYT6) is an adolescent-onset dystonia characterized by involuntary muscle contractions usually involving the upper body. It is caused by mutations in the gene THAP1 encoding for the transcription factor Thanatos-associated protein (THAP) domain containing apoptosis-associated protein 1 and inherited in an autosomal-dominant manner with reduced penetrance. Alterations in the development of striatal neuronal projections and synaptic function are known from transgenic mice models. To investigate pathogenetic mechanisms, human induced pluripotent stem cell (iPSC)-derived medium spiny neurons (MSNs) from two patients and one family member with reduced penetrance carrying a mutation in the gene THAP1 (c.474delA and c.38G > A) were functionally characterized in comparison to healthy controls. Calcium imaging and quantitative PCR analysis revealed significantly lower Ca²⁺ amplitudes upon GABA applications and a marked downregulation of the gene encoding the GABA_A receptor alpha2 subunit in THAP1 MSNs indicating a decreased GABAergic transmission. Whole-cell patch-clamp recordings showed a significantly lower frequency of miniature postsynaptic currents (mPSCs), whereas the frequency of spontaneous action potentials (APs) was elevated in THAP1 MSNs suggesting that decreased synaptic activity might have resulted in enhanced generation of APs. Our molecular and functional data indicate that a reduced expression of GABA_A receptor alpha2 subunit could eventually lead to limited GABAergic synaptic transmission, neuronal disinhibition, and hyperexcitability of THAP1 MSNs. These data give pathophysiological insight and may contribute to the development of novel treatment strategies for DYT-THAP1 dystonia.

The density of calretinin striatal interneurons is decreased in 6-OHDA-lesioned mice

Interneurons play a significant role in the functional organization of the striatum and some of them display marked plastic changes in dopamine-depleted conditions. Here, we applied immunohistochemistry on brain sections from 6-hydroxydopamine (6-OHDA) mouse model of Parkinson's disease and sham animals to characterize the regional distribution and the morphological and neurochemical changes of striatal interneurons expressing the calcium-binding protein calretinin (CR). Two morphological subtypes of calretinin-immunostained (CR +) interneurons referred, respectively, as small- and medium-sized CR + interneurons were detected in 6-OHDA- and sham-lesioned animals. The small cells (9-12 µm) prevail in the anterior and dorsal striatal regions; they stain intensely for CR and display a single slightly varicose and moderately arborized process. The medium-sized CR + interneurons (15-20 µm) are more numerous than the small CR + cells and rather uniformly distributed within the striatum; they stain weakly for CR and display 2-3 long, slightly varicose and poorly branched dendrites. The density of medium CR + interneurons is significantly decreased in the dopamine-depleted striatum (158 ± 15 neurons/mm³), when compared to sham animals (370 ± 41 neurons/mm³), whereas that of the small-sized CR + interneurons is unchanged (174 ± 46 neurons/mm³ in 6-OHDA-lesioned striatum and 164 ± 22 neurons/mm³ in sham-lesioned striatum). The nucleus accumbens is populated only by medium-sized CR + interneurons, which are distributed equally among the core and shell compartments and whose density is unaltered after dopamine denervation. Our results provide the first evidence that the medium-sized striatal interneurons expressing low level of CR are specifically targeted by dopamine denervation, while the small and intensely immunoreactive CR + cells remain unaffected. These findings suggest that high expression of the calcium-binding protein CR might protect striatal interneurons against an increase in intracellular calcium level that is believed to arise from altered glutamate corticostriatal transmission in Parkinson's disease.

Genetically identified neurons in avian auditory pallium mirror core principles of their mammalian counterparts

In vertebrates, advanced cognitive abilities are typically associated with the telencephalic pallium. In mammals, the pallium is a layered mixture of excitatory and inhibitory neuronal populations with distinct molecular, physiological, and network phenotypes. This cortical architecture is proposed to support efficient, high-level information processing. Comparative perspectives across vertebrates provide a lens to understand the common features of pallium that are important for advanced cognition. Studies in songbirds have established strikingly parallel features of neuronal types between mammalian and avian pallium. However, lack of genetic access to defined pallial cell types in non-mammalian vertebrates has hindered progress in resolving connections between molecular and physiological phenotypes. A definitive mapping of the physiology of pallial cells onto their molecular identities in birds is critical for understanding how synaptic and computational properties depend on underlying molecular phenotypes. Using viral tools to target excitatory versus inhibitory neurons in the zebra finch auditory association pallium (calmodulin-dependent kinase alpha [CaMKIIα] and glutamate decarboxylase 1 [GAD1] promoters, respectively), we systematically tested predictions derived from mammalian pallium. We identified two genetically distinct neuronal populations that exhibit profound physiological and computational similarities with mammalian excitatory and inhibitory pallial cells, definitively aligning putative cell types in avian caudal nidopallium with these molecular identities. Specifically, genetically identified CaMKIIα and GAD1 cell types in avian auditory association pallium exhibit distinct intrinsic physiological parameters, distinct auditory coding principles, and inhibitory-dependent pallial synchrony, gamma oscillations, and local suppression. The retention, or convergence, of these molecular and physiological features in both birds and mammals clarifies the characteristics of pallial circuits for advanced cognitive abilities.

Interpreting the role of the striatum during multiple phases of motor learning

The synaptic pathways in the striatum are central to basal ganglia functions including motor control, learning and organization, action selection, acquisition of motor skills, cognitive function, and emotion. Here, we review the role of the striatum and its connections in motor learning and performance. The development of new techniques to record neuronal activity and animal models of motor disorders using neurotoxin, pharmacological, and genetic manipulations are revealing pathways that underlie motor performance and motor learning, as well as how they are altered by pathophysiological mechanisms. We discuss approaches that can be used to analyze complex motor skills, particularly in rodents, and identify specific questions central to understanding how striatal circuits mediate motor learning.

Calcium-Permeable AMPA Receptors Promote Endocannabinoid Signaling at Parvalbumin Interneuron Synapses in the Nucleus Accumbens Core

Synaptic plasticity is a key mechanism of learning and memory. Synaptic plasticity mechanisms within the nucleus accumbens (NAc) mediate differential behavioral adaptations. Feedforward inhibition in the NAc occurs when glutamatergic afferents onto medium spiny neurons (MSNs) collateralize onto fast-spiking parvalbumin (PV)-expressing interneurons (PV-INs), which exert GABAergic control over MSN action potential generation. Here, we find that feedforward glutamatergic synapses onto PV-INs in the NAc core selectively express Ca²⁺-permeable AMPA receptors (CP-AMPARs). Ca²⁺ influx by CP-AMPARs on PV-INs triggers long-term depression (LTD) mediated by endocannabinoid (eCB) signaling at presynaptic cannabinoid type-1 (CB₁) receptors (CB₁Rs). Moreover, CP-AMPARs authorize tonic eCB signaling to negatively regulate glutamate release probability. Blockade of CP-AMPARs in the NAc core in vivo is sufficient to disinhibit locomotor output. These findings elucidate mechanisms by which PV-IN-embedded microcircuits in the NAc undergo activity-dependent shifts in synaptic strength.

Manz et al. show that CP-AMPARs are expressed at glutamatergic synapses onto PV-INs but not D1- or D2-expressing MSNs in the NAc core. Ca²⁺ influx through CP-AMPARs triggers endocannabinoid-dependent tone and synaptic plasticity. Intra-NAc blockade of CP-AMPARs in vivo increases basal locomotion.

Chronic nicotine impairs sparse motor learning via striatal fast-spiking parvalbumin interneurons

Nicotine can diversely affect neural activity and motor learning in animals. However, the impact of chronic nicotine on striatal activity in vivo and motor learning at long-term sparse timescale remains unknown. Here, we demonstrate that chronic nicotine persistently suppresses the activity of striatal fast-spiking parvalbumin interneurons, which mediate nicotine-induced deficit in sparse motor learning. Six weeks of longitudinal in vivo single-unit recording revealed that mice show reduced activity of fast-spiking interneurons in the dorsal striatum during chronic nicotine exposure and withdrawal. The reduced firing of fast-spiking interneurons was accompanied by spike broadening, diminished striatal delta oscillation power, and reduced sample entropy in local field potential. In addition, chronic nicotine withdrawal impaired motor learning with a weekly sparse training regimen but did not affect general locomotion and anxiety-like behavior. Lastly, the excitatory DREADD hM3Dq-mediated activation of striatal fast-spiking parvalbumin interneurons reversed the chronic nicotine withdrawal-induced deficit in sparse motor learning. Taken together, we identified that chronic nicotine withdrawal impairs sparse motor learning via disruption of activity in striatal fast-spiking parvalbumin interneurons. These findings suggest that sparse motor learning paradigm can reveal the subtle effect of nicotine withdrawal on motor function and that striatal fast-spiking parvalbumin interneurons are a neural substrate of nicotine's effect on motor learning.

Recurrent Implication of Striatal Cholinergic Interneurons in a Range of Neurodevelopmental, Neurodegenerative, and Neuropsychiatric Disorders

Cholinergic interneurons are "gatekeepers" for striatal circuitry and play pivotal roles in attention, goal-directed actions, habit formation, and behavioral flexibility. Accordingly, perturbations to striatal cholinergic interneurons have been associated with many neurodevelopmental, neurodegenerative, and neuropsychiatric disorders. The role of acetylcholine in many of these disorders is well known, but the use of drugs targeting cholinergic systems fell out of favor due to adverse side effects and the introduction of other broadly acting compounds. However, in response to recent findings, re-examining the mechanisms of cholinergic interneuron dysfunction may reveal key insights into underlying pathogeneses. Here, we provide an update on striatal cholinergic interneuron function, connectivity, and their putative involvement in several disorders. In doing so, we aim to spotlight recurring physiological themes, circuits, and mechanisms that can be investigated in future studies using new tools and approaches.

Er81 Transcription Factor Fine-Tunes Striatal Cholinergic Interneuron Activity and Drives Habit Formation

The molecular mechanisms tuning cholinergic interneuron (CIN) activity, although crucial for striatal function and behavior, remain largely unexplored. Previous studies report that the Etv1/Er81 transcription factor is vital for regulating neuronal maturation and activity. While Er81 is known to be expressed in the striatum during development, its specific role in defining CIN properties and the resulting consequences on striatal function is unknown. We report here that Er81 is expressed in CINs and its specific ablation leads to prominent changes in their molecular, morphologic, and electrophysiological features. In particular, the lack of Er81 amplifies intrinsic delayed-rectifier and hyperpolarization-activated currents, which subsequently alters the tonic and phasic activity of CINs. We further reveal that Er81 expression is required for normal CIN pause and time-locked responses to sensorimotor inputs in awake mice. Overall, this study uncovers a new cell type-specific control of CIN function in the striatum which drives habit formation in adult male mice.SIGNIFICANCE STATEMENT Although previous studies have shown that cholinergic interneurons drive striatal activity and habit formation, the underlying molecular mechanisms controlling their function are unknown. Here we reveal that key cholinergic interneuron physiological properties are controlled by Er81, a transcription factor regulating neuronal activity and development in a cell-specific manner. Moreover, our findings uncover a link between the Er81-dependent molecular control of cholinergic interneuron function and habit formation in mice. These insights will contribute to the future enhancement of our understanding of disorders that involve behavioral inflexibility, such as autism and addiction.

Patch-clamp and multi-electrode array electrophysiological analysis in acute mouse brain slices

Patch-clamp and multi-electrode array electrophysiology techniques are used to measure dynamic functional properties of neurons. Whole-cell and cell-attached patch-clamp recordings in brain slices can be performed in voltage-clamp and current-clamp configuration to reveal cell-type-specific synaptic and cellular parameters governing neurotransmission. Multi-electrode array electrophysiology can provide spike activity recordings from multiple neurons, enabling larger sample sizes, and long-term recordings. We provide our guide to preparing acute rodent brain slices with example experiments and analyses intended for novice and expert electrophysiologists.

For complete details on the use and execution of this protocol, please refer to Manz et al. (2020b).

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Viable and efficient preparation of mouse brain tissue

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Comprehensive material source for acute brain slice electrophysiology

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Detailed approach to whole-cell patch-clamp recording of neurons

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Utility and application of multi-electrode array electrophysiology in mouse brain

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

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

Endocannabinoid system in the neurodevelopment of GABAergic interneurons: implications for neurological and psychiatric disorders

In mature mammalian brains, the endocannabinoid system (ECS) plays an important role in the regulation of synaptic plasticity and the functioning of neural networks. Besides, the ECS also contributes to the neurodevelopment of the central nervous system. Due to the increase in the medical and recreational use of cannabis, it is inevitable and essential to elaborate the roles of the ECS on neurodevelopment. GABAergic interneurons represent a group of inhibitory neurons that are vital in controlling neural network activity. However, the role of the ECS in the neurodevelopment of GABAergic interneurons remains to be fully elucidated. In this review, we provide a brief introduction of the ECS and interneuron diversity. We focus on the process of interneuron development and the role of ECS in the modulation of interneuron development, from the expansion of the neural stem/progenitor cells to the migration, specification and maturation of interneurons. We further discuss the potential implications of the ECS and interneurons in the pathogenesis of neurological and psychiatric disorders, including epilepsy, schizophrenia, major depressive disorder and autism spectrum disorder.

Axonal Modulation of Striatal Dopamine Release by Local γ-Aminobutyric Acid (GABA) Signalling

Striatal dopamine (DA) release is critical for motivated actions and reinforcement learning, and is locally influenced at the level of DA axons by other striatal neurotransmitters. Here, we review a wealth of historical and more recently refined evidence indicating that DA output is inhibited by striatal γ-aminobutyric acid (GABA) acting via GABAA and GABAB receptors. We review evidence supporting the localisation of GABAA and GABAB receptors to DA axons, as well as the identity of the striatal sources of GABA that likely contribute to GABAergic modulation of DA release. We discuss emerging data outlining the mechanisms through which GABAA and GABAB receptors inhibit the amplitude as well as modulate the short-term plasticity of DA release. Furthermore, we highlight recent data showing that DA release is governed by plasma membrane GABA uptake transporters on striatal astrocytes, which determine ambient striatal GABA tone and, by extension, the tonic inhibition of DA release. Finally, we discuss how the regulation of striatal GABA-DA interactions represents an axis for dysfunction in psychomotor disorders associated with dysregulated DA signalling, including Parkinson's disease, and could be a novel therapeutic target for drugs to modify striatal DA output.

How predictive learning influences choice: Evidence for a GPCR-based memory process necessary for Pavlovian-instrumental transfer

Predictive learning endows stimuli with the capacity to signal both the sensory-specific and general motivational properties of their associated rewards or outcomes. These two signals can be distinguished behaviorally by their influence on the selection and performance of instrumental actions, respectively. This review focuses on how sensory-specific predictive learning guides choice between actions that earn otherwise equally desirable outcomes. We describe evidence that outcome-specific predictive learning is encoded in the basolateral amygdala and drives the accumulation of delta-opioid receptors on the surface of cholinergic interneurons located in the nucleus accumbens shell. This accumulation constitutes a novel form of cellular memory, not for outcome-specific predictive learning per se but for the selection of, and choice between, future instrumental actions. We describe recent evidence regarding the cascade of events necessary for the formation and expression of this cellular memory and point to open questions for future research into this process. Beyond these mechanistic considerations, the discovery of this new form of memory is consistent with recent evidence suggesting that intracellular rather than synaptic changes can mediate learning-related plasticity to modify brain circuitry to prepare for future significant events.

Functional Connectome Analysis of the Striatum with Optogenetics

Neural circuit function is determined not only by anatomical connections but also by the strength and nature of the connections, that is functional or physiological connectivity. To elucidate functional connectivity, selective stimulation of presynaptic terminals of an identified neuronal population is crucial. However, in the central nervous system, intermingled input fibers make selective electrical stimulation impossible. With optogenetics, this becomes possible, and enables the comprehensive study of functional synaptic connections between an identified population of neurons and defined postsynaptic targets to determine the functional connectome. By stimulating convergent synaptic inputs impinging on individual postsynaptic neurons, low frequency and small amplitude synaptic connections can be detected. Further, the optogenetic approach enables the measurement of cotransmission and its relative strength. Recently, optogenetic methods have been more widely used to study synaptic connectivity and revealed novel synaptic connections and revised connectivity of known projections. In this chapter, I focus on functional synaptic connectivity in the striatum, the main input structure of the basal ganglia, involved in the motivated behavior, cognition, and motor control, and its disruption in a range of neuropsychiatric disorders.

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

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

The Functional Organization of Cortical and Thalamic Inputs onto Five Types of Striatal Neurons Is Determined by Source and Target Cell Identities

To understand striatal function, it is essential to know the functional organization of the numerous inputs targeting the diverse population of striatal neurons. Using optogenetics, we activated terminals from ipsi- or contralateral primary somatosensory cortex (S1) or primary motor cortex (M1), or thalamus while obtaining simultaneous whole-cell recordings from pairs or triplets of striatal medium spiny neurons (MSNs) and adjacent interneurons. Ipsilateral corticostriatal projections provided stronger excitation to fast-spiking interneurons (FSIs) than to MSNs and only sparse and weak excitation to low threshold-spiking interneurons (LTSIs) and cholinergic interneurons (ChINs). Projections from contralateral M1 evoked the strongest responses in LTSIs but none in ChINs, whereas thalamus provided the strongest excitation to ChINs but none to LTSIs. In addition, inputs varied in their glutamate receptor composition and their short-term plasticity. Our data revealed a highly selective organization of excitatory striatal afferents, which is determined by both pre- and postsynaptic neuronal identity.

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Whole-cell recordings are obtained from neighboring striatal neurons of different types

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FSIs receive the strongest inputs from S1, M1, and thalamic PF

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LTSIs are primarily excited by contralateral M1

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ChINs are primarily excited by PF and receive no input from contralateral M1 and S1

Polysynaptic inhibition between striatal cholinergic interneurons shapes their network activity patterns in a dopamine-dependent manner

Striatal activity is dynamically modulated by acetylcholine and dopamine, both of which are essential for basal ganglia function. Synchronized pauses in the activity of striatal cholinergic interneurons (ChINs) are correlated with elevated activity of midbrain dopaminergic neurons, whereas synchronous firing of ChINs induces local release of dopamine. The mechanisms underlying ChIN synchronization and its interplay with dopamine release are not fully understood. Here we show that polysynaptic inhibition between ChINs is a robust network motif and instrumental in shaping the network activity of ChINs. Action potentials in ChINs evoke large inhibitory responses in multiple neighboring ChINs, strong enough to suppress their tonic activity. Using a combination of optogenetics and chemogenetics we show the involvement of striatal tyrosine hydroxylase-expressing interneurons in mediating this inhibition. Inhibition between ChINs is attenuated by dopaminergic midbrain afferents acting presynaptically on D2 receptors. Our results present a novel form of interaction between striatal dopamine and acetylcholine dynamics.

The Spatial Extent of Optogenetic Silencing in Transgenic Mice Expressing Channelrhodopsin in Inhibitory Interneurons

Optogenetic stimulation of inhibitory interneurons has become a commonly used strategy for silencing neuronal activity. This is typically achieved using transgenic mice expressing excitatory opsins in inhibitory interneurons throughout the brain, raising the question of how spatially extensive the resulting inhibition is. Here, we characterize neuronal silencing in VGAT-ChR2 mice, which express channelrhodopsin-2 in inhibitory interneurons, as a function of light intensity and distance from the light source in several cortical and subcortical regions. We show that light stimulation, even at relatively low intensities, causes inhibition not only in brain regions targeted for silencing but also in their subjacent areas. In contrast, virus-mediated expression of an inhibitory opsin enables robust silencing that is restricted to the region of opsin expression. Our results reveal important constraints on using inhibitory interneuron activation to silence neuronal activity and emphasize the necessity of carefully controlling light stimulation parameters when using this silencing strategy.

Striatal circuit development and its alterations in Huntington's disease

Huntington's disease (HD) is an inherited neurodegenerative disorder that usually starts during midlife with progressive alterations of motor and cognitive functions. The disease is caused by a CAG repeat expansion within the huntingtin gene leading to severe striatal neurodegeneration. Recent studies conducted on pre-HD children highlight early striatal developmental alterations starting as soon as 6 years old, the earliest age assessed. These findings, in line with data from mouse models of HD, raise the questions of when during development do the first disease-related striatal alterations emerge and whether they contribute to the later appearance of the neurodegenerative features of the disease. In this review we will describe the different stages of striatal network development and then discuss recent evidence for its alterations in rodent models of the disease. We argue that a better understanding of the striatum's development should help in assessing aberrant neurodevelopmental processes linked to the HD mutation.

Fast spiking interneuron activity in primate striatum tracks learning of attention cues

Cognitive flexibility depends on a fast neural learning mechanism for enhancing momentary relevant over irrelevant information. A possible neural mechanism realizing this enhancement uses fast spiking interneurons (FSIs) in the striatum to train striatal projection neurons to gate relevant and suppress distracting cortical inputs. We found support for such a mechanism in nonhuman primates during the flexible adjustment of visual attention in a reversal learning task. FSI activity was modulated by visual attention cues during feature-based learning. One FSI subpopulation showed stronger activation during learning, while another FSI subpopulation showed response suppression after learning, which could indicate a disinhibitory effect on the local circuit. Additionally, FSIs that showed response suppression to learned attention cues were activated by salient distractor events, suggesting they contribute to suppressing bottom-up distraction. These findings suggest that striatal fast spiking interneurons play an important role when cues are learned that redirect attention away from previously relevant to newly relevant visual information. This cue-specific activity was independent of motor-related activity and thus tracked specifically the learning of reward predictive visual features.

The Nucleus Accumbens: A Common Target in the Comorbidity of Depression and Addiction

The comorbidity of depression and addiction has become a serious public health issue, and the relationship between these two disorders and their potential mechanisms has attracted extensive attention. Numerous studies have suggested that depression and addiction share common mechanisms and anatomical pathways. The nucleus accumbens (NAc) has long been considered a key brain region for regulating many behaviors, especially those related to depression and addiction. In this review article, we focus on the association between addiction and depression, highlighting the potential mediating role of the NAc in this comorbidity via the regulation of changes in the neural circuits and molecular signaling. To clarify the mechanisms underlying this association, we summarize evidence from overlapping reward neurocircuitry, the resemblance of cellular and molecular mechanisms, and common treatments. Understanding the interplay between these disorders should help guide clinical comorbidity prevention and the search for a new target for comorbidity treatment.

Disentangling neuronal inhibition and inhibitory pathways in the lateral habenula

The lateral habenula (LHb) is hyperactive in depression, and thus potentiating inhibition of this structure makes an interesting target for future antidepressant therapies. However, the circuit mechanisms mediating inhibitory signalling within the LHb are not well-known. We addressed this issue by studying LHb neurons expressing either parvalbumin (PV) or somatostatin (SOM), two markers of particular sub-classes of neocortical inhibitory neurons. Here, we find that both PV and SOM are expressed by physiologically distinct sub-classes. Furthermore, we describe multiple sources of inhibitory input to the LHb arising from both local PV-positive neurons, from PV-positive neurons in the medial dorsal thalamic nucleus, and from SOM-positive neurons in the ventral pallidum. These findings hence provide new insight into inhibitory control within the LHb, and highlight that this structure is more neuronally diverse than previously thought.

Human Pluripotent Stem Cell-Derived Neurons Are Functionally Mature In Vitro and Integrate into the Mouse Striatum Following Transplantation

Human pluripotent stem cells (hPSCs) are a powerful tool for modelling human development. In recent years, hPSCs have become central in cell-based therapies for neurodegenerative diseases given their potential to replace affected neurons. However, directing hPSCs into specific neuronal types is complex and requires an accurate protocol that mimics endogenous neuronal development. Here we describe step-by-step a fast feeder-free neuronal differentiation protocol to direct hPSCs to mature forebrain neurons in 37 days in vitro (DIV). The protocol is based upon a combination of specific morphogens, trophic and growth factors, ions, neurotransmitters and extracellular matrix elements. A human-induced PSC line (Ctr-Q33) and a human embryonic stem cell line (GEN-Q18) were used to reinforce the potential of the protocol. Neuronal activity was analysed by single-cell calcium imaging. At 8 DIV, we obtained a homogeneous population of hPSC-derived neuroectodermal progenitors which self-arranged in bi-dimensional neural tube-like structures. At 16 DIV, we generated hPSC-derived neural progenitor cells (NPCs) with mostly a subpallial identity along with a subpopulation of pallial NPCs. Terminal in vitro neuronal differentiation was confirmed by the expression of microtubule associated protein 2b (Map 2b) by almost 100% of hPSC-derived neurons and the expression of specific-striatal neuronal markers including GABA, CTIP2 and DARPP-32. HPSC-derived neurons showed mature and functional phenotypes as they expressed synaptic markers, voltage-gated ion channels and neurotransmitter receptors. Neurons displayed diverse spontaneous activity patterns that were classified into three major groups, namely “high”, “intermediate” and “low” firing neurons. Finally, transplantation experiments showed that the NPCs survived and differentiated within mouse striatum for at least 3 months. NPCs integrated host environmental cues and differentiated into striatal medium-sized spiny neurons (MSNs), which successfully integrated into the endogenous circuitry without teratoma formation. Altogether, these findings demonstrate the potential of this robust human neuronal differentiation protocol, which will bring new opportunities for the study of human neurodevelopment and neurodegeneration, and will open new avenues in cell-based therapies, pharmacological studies and alternative in vitro toxicology.