Dopaminergic and cholinergic modulation of striatal tyrosine hydroxylase interneurons

Expression and function of nicotinic acetylcholine receptors in specific neuronal populations: Focus on striatal and prefrontal circuits

Nicotinic acetylcholine receptors (nAChRs) are widely expressed in the central nervous system and play an important role in the control of neural functions including neuronal activity, transmitter release and synaptic plasticity. Although the common subtypes of nAChRs are abundantly expressed throughout the brain, their expression in different brain regions and by individual neuronal types is not homogeneous or incidental. In recent years, several studies have emerged showing that particular subtypes of nAChRs are expressed by specific neuronal populations in which they have major influence on the activity of local circuits and behavior. It has been demonstrated that even nAChRs expressed by relatively rare neuronal types can induce significant changes in behavior and contribute to pathological processes. Depending on the identity and connectivity of the particular nAChRs-expressing neuronal populations, the activation of nAChRs can have distinct or even opposing effects on local neuronal signaling. In this review, we will summarize the available literature describing the expression of individual nicotinic subunits by different neuronal types in two crucial brain regions, the striatum and the prefrontal cortex. The review will also briefly discuss nicotinic expression in non-neuronal, glial cells, as they cannot be ignored as potential targets of nAChRs-modulating drugs. The final section will discuss options that could allow us to target nAChRs in a neuronal-type-specific manner, not only in the experimental field, but also eventually in clinical practice.

Investigating cell-specific effects of FMRP deficiency on spiny projection neurons in a mouse model of Fragile X syndrome

Introduction

Fragile X syndrome (FXS), resulting from a mutation in the Fmr1 gene, is the most common monogenic cause of autism and inherited intellectual disability. Fmr1 encodes the Fragile X Messenger Ribonucleoprotein (FMRP), and its absence leads to cognitive, emotional, and social deficits compatible with the nucleus accumbens (NAc) dysfunction. This structure is pivotal in social behavior control, consisting mainly of spiny projection neurons (SPNs), distinguished by dopamine D1 or D2 receptor expression, connectivity, and associated behavioral functions. This study aims to examine how FMRP absence differentially affects SPN cellular properties, which is crucial for categorizing FXS cellular endophenotypes.

Methods

We utilized a novel Fmr1-/y::Drd1a-tdTomato mouse model, which allows in-situ identification of SPN subtypes in FXS mice. Using RNA-sequencing, RNAScope and ex-vivo patch-clamp in adult male mice NAc, we comprehensively compared the intrinsic passive and active properties of SPN subtypes.

Results

Fmr1 transcripts and their gene product, FMRP, were found in both SPNs subtypes, indicating potential cell-specific functions for Fmr1. The study found that the distinguishing membrane properties and action potential kinetics typically separating D1- from D2-SPNs in wild-type mice were either reversed or abolished in Fmr1-/y::Drd1a-tdTomato mice. Interestingly, multivariate analysis highlighted the compound effects of Fmr1 ablation by disclosing how the phenotypic traits distinguishing each cell type in wild-type mice were altered in FXS.

Discussion

Our results suggest that the absence of FMRP disrupts the standard dichotomy characterizing NAc D1- and D2-SPNs, resulting in a homogenous phenotype. This shift in cellular properties could potentially underpin select aspects of the pathology observed in FXS. Therefore, understanding the nuanced effects of FMRP absence on SPN subtypes can offer valuable insights into the pathophysiology of FXS, opening avenues for potential therapeutic strategies.

Striatal Neurons Partially Expressing a Dopaminergic Phenotype: Functional Significance and Regulation

Since the discovery of striatal neurons expressing dopamine-synthesizing enzymes, researchers have attempted to identify their phenotype and functional significance. In this study, it was shown that in transgenic mice expressing green fluorescent protein (GFP) under the tyrosine hydroxylase (TH) gene promoter, (i) there are striatal neurons expressing only TH, only aromatic L-amino acid decarboxylase (AADC), or both enzymes of dopamine synthesis; (ii) striatal neurons expressing dopamine-synthesizing enzymes are not dopaminergic since they lack a dopamine transporter; (iii) monoenzymatic neurons expressing individual complementary dopamine-synthesizing enzymes produce this neurotransmitter in cooperation; (iv) striatal nerve fibers containing only TH, only AADC, or both enzymes project into the lateral ventricles, providing delivery pathways for L-3,4-dihydroxyphenylalanine and dopamine to the cerebrospinal fluid; and (v) striatal GFP neurons express receptor genes for various signaling molecules, i.e., classical neurotransmitters, neuropeptides, and steroids, indicating fine regulation of these neurons. Based on our data, it is assumed that the synthesis of dopamine by striatal neurons is a compensatory response to the death of nigral dopaminergic neurons in Parkinson’s disease, which opens broad prospects for the development of a fundamentally novel antiparkinsonian therapy.

Synaptic determinants of cholinergic interneurons hyperactivity during parkinsonism

Parkinson’s disease is a neurodegenerative ailment generated by the loss of dopamine in the basal ganglia, mainly in the striatum. The disease courses with increased striatal levels of acetylcholine, disrupting the balance among these modulatory transmitters. These modifications disturb the excitatory and inhibitory balance in the striatal circuitry, as reflected in the activity of projection striatal neurons. In addition, changes in the firing pattern of striatal tonically active interneurons during the disease, including cholinergic interneurons (CINs), are being searched. Dopamine-depleted striatal circuits exhibit pathological hyperactivity as compared to controls. One aim of this study was to show how striatal CINs contribute to this hyperactivity. A second aim was to show the contribution of extrinsic synaptic inputs to striatal CINs hyperactivity. Electrophysiological and calcium imaging recordings in Cre-mice allowed us to evaluate the activity of dozens of identified CINs with single-cell resolution in ex vivo brain slices. CINs show hyperactivity with bursts and silences in the dopamine-depleted striatum. We confirmed that the intrinsic differences between the activity of control and dopamine-depleted CINs are one source of their hyperactivity. We also show that a great part of this hyperactivity and firing pattern change is a product of extrinsic synaptic inputs, targeting CINs. Both glutamatergic and GABAergic inputs are essential to sustain hyperactivity. In addition, cholinergic transmission through nicotinic receptors also participates, suggesting that the joint activity of CINs drives the phenomenon; since striatal CINs express nicotinic receptors, not expressed in striatal projection neurons. Therefore, CINs hyperactivity is the result of changes in intrinsic properties and excitatory and inhibitory inputs, in addition to the modification of local circuitry due to cholinergic nicotinic transmission. We conclude that CINs are the main drivers of the pathological hyperactivity present in the striatum that is depleted of dopamine, and this is, in part, a result of extrinsic synaptic inputs. These results show that CINs may be a main therapeutic target to treat Parkinson’s disease by intervening in their synaptic inputs.

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.

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.

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.

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.

Neural substrates of habit

Active reward pursuit is supported by the balance between the cognitive and habitual control of behavior. The cognitive, goal-directed strategy relies on the prospective evaluation of anticipated consequences, which allows behavior to readily adapt when circumstances change. Repetition of successful actions promotes less cognitively taxing habits, in which behavior is automatically executed without prospective consideration. Disruption in either of these behavioral regulatory systems contributes to the symptoms that underlie many psychiatric disorders. Here, I review recently identified neural substrates, at multiple neural levels, that contribute to habits and outline gaps in knowledge that must be addressed to fully understand the neural mechanisms of behavioral control.

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.

Cholinergic modulation of striatal microcircuits

The purpose of this review is to bridge the gap between earlier literature on striatal cholinergic interneurons and mechanisms of microcircuit interaction demonstrated with the use of newly available tools. It is well known that the main source of the high level of acetylcholine in the striatum, compared to other brain regions, is the cholinergic interneurons. These interneurons provide an extensive local innervation that suggests they may be a key modulator of striatal microcircuits. Supporting this idea requires the consideration of functional properties of these interneurons, their influence on medium spiny neurons, other interneurons, and interactions with other synaptic regulators. Here, we underline the effects of intrastriatal and extrastriatal afferents onto cholinergic interneurons and discuss the activation of pre‐ and postsynaptic muscarinic and nicotinic receptors that participate in the modulation of intrastriatal neuronal interactions. We further address recent findings about corelease of other transmitters in cholinergic interneurons and actions of these interneurons in striosome and matrix compartments. In addition, we summarize recent evidence on acetylcholine‐mediated striatal synaptic plasticity and propose roles for cholinergic interneurons in normal striatal physiology. A short examination of their role in neurological disorders such as Parkinson's, Huntington's, and Tourette's pathologies and dystonia is also included.

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.

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.

Nucleus Accumbens Microcircuit Underlying D2-MSN-Driven Increase in Motivation

The nucleus accumbens (NAc) plays a central role in reinforcement and motivation. Around 95% of the NAc neurons are medium spiny neurons (MSNs), divided into those expressing dopamine receptor D1 (D1R) or dopamine receptor D2 (D2R). Optogenetic activation of D2-MSNs increased motivation, whereas inhibition of these neurons produced the opposite effect. Yet, it is still unclear how activation of D2-MSNs affects other local neurons/interneurons or input terminals and how this contributes for motivation enhancement. To answer this question, in this work we combined optogenetic modulation of D2-MSNs with in loco pharmacological delivery of specific neurotransmitter antagonists in rats. First, we showed that optogenetic activation of D2-MSNs increases motivation in a progressive ratio (PR) task. We demonstrated that this behavioral effect relies on cholinergic-dependent modulation of dopaminergic signalling of ventral tegmental area (VTA) terminals, which requires D1R and D2R signalling in the NAc. D2-MSN optogenetic activation decreased ventral pallidum (VP) activity, reducing the inhibitory tone to VTA, leading to increased dopaminergic activity. Importantly, optogenetic activation of D2-MSN terminals in the VP was sufficient to recapitulate the motivation enhancement. In summary, our data suggests that optogenetic stimulation of NAc D2-MSNs indirectly modulates VTA dopaminergic activity, contributing for increased motivation. Moreover, both types of dopamine receptors signalling in the NAc are required in order to produce the positive behavioral effects.

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.

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.

Dysregulation of Corticostriatal Connectivity in Huntington's Disease: A Role for Dopamine Modulation

Aberrant communication between striatum, the main information processing unit of the basal ganglia, and cerebral cortex plays a critical role in the emergence of Huntington’s disease (HD), a fatal monogenetic condition that typically strikes in the prime of life. Although both striatum and cortex undergo substantial cell loss over the course of HD, corticostriatal circuits become dysfunctional long before neurons die. Understanding the dysfunction is key to developing effective strategies for treating a progressively worsening triad of motor, cognitive, and psychiatric symptoms. Cortical output neurons drive striatal activity through the release of glutamate, an excitatory amino acid. Striatal outputs, in turn, release γ-amino butyric acid (GABA) and exert inhibitory control over downstream basal ganglia targets. Ample evidence from transgenic rodent models points to dysregulation of corticostriatal glutamate transmission along with corresponding changes in striatal GABA release as underlying factors in the HD behavioral phenotype. Another contributor is dysregulation of dopamine (DA), a modulator of both glutamate and GABA transmission. In fact, pharmacological manipulation of DA is the only currently available treatment for HD symptoms. Here, we review data from animal models and human patients to evaluate the role of DA in HD, including DA interactions with glutamate and GABA within the context of dysfunctional corticostriatal circuitry.

Striatal Local Circuitry: A New Framework for Lateral Inhibition

This Perspective will examine the organization of intrastriatal circuitry, review recent findings in this area, and discuss how the pattern of connectivity between striatal neurons might give rise to the behaviorally observed synergism between the direct/indirect pathway neurons. The emphasis of this Perspective is on the underappreciated role of lateral inhibition between striatal projection cells in controlling neuronal firing and shaping the output of this circuit. We review some classic studies in combination with more recent anatomical and functional findings to lay out a framework for an updated model of the intrastriatal lateral inhibition, where we explore its contribution to the formation of functional units of processing and the integration and filtering of inputs to generate motor patterns and learned behaviors.

Neostriatal GABAergic Interneurons Mediate Cholinergic Inhibition of Spiny Projection Neurons

Synchronous optogenetic activation of striatal cholinergic interneurons ex vivo produces a disynaptic inhibition of spiny projection neurons composed of biophysically distinct GABAAfast and GABAAslow components. This has been shown to be due, at least in part, to activation of nicotinic receptors on GABAergic NPY-neurogliaform interneurons that monosynaptically inhibit striatal spiny projection neurons. Recently, it has been proposed that a significant proportion of this inhibition is actually mediated by activation of presynaptic nicotinic receptors on nigrostriatal terminals that evoke GABA release from the terminals of the dopaminergic nigrostriatal pathway. To disambiguate these the two mechanisms, we crossed mice in which channelrhodopsin is endogenously expressed in cholinergic neurons with Htr3a-Cre mice, in which Cre is selectively targeted to several populations of striatal GABAergic interneurons, including the striatal NPY-neurogliaform interneuron. Htr3a-Cre mice were then virally transduced to express halorhodopsin to allow activation of channelrhodopsin and halorhodopsin, individually or simultaneously. Thus we were able to optogenetically disconnect the interneuron-spiny projection neuron (SPN) cell circuit on a trial-by-trial basis. As expected, optogenetic activation of cholinergic interneurons produced inhibitory currents in SPNs. During simultaneous inhibition of GABAergic interneurons with halorhodopsin, we observed a large, sometimes near complete reduction in both fast and slow components of the cholinergic-evoked inhibition, and a delay in IPSC latency. This demonstrates that the majority of cholinergic-evoked striatal GABAergic inhibition is derived from GABAergic interneurons. These results also reinforce the notion that a semiautonomous circuit of striatal GABAergic interneurons is responsible for transmitting behaviorally relevant cholinergic signals to spiny projection neurons.

SIGNIFICANCE STATEMENT

The circuitry between neurons of the striatum has been recently described to be far more complex than originally imagined. One example of this phenomenon is that striatal cholinergic interneurons have been shown to provide intrinsic nicotinic excitation of local GABAergic interneurons, which then inhibit the projection neurons of the striatum. As deficits of cholinergic interneurons are reported in patients with Tourette syndrome, the normal functions of these interneurons are of great interest. Whether this novel route of nicotinic input constitutes a major output of cholinergic interneurons remains unknown. The study addressed this question using excitatory and inhibitory optogenetic technology, so that cholinergic interneurons could be selectively activated and GABAergic interneurons selectively inhibited to determine the causal relationship in this circuit.

Differential processing of thalamic information via distinct striatal interneuron circuits

Recent discoveries of striatal GABAergic interneurons require a new conceptualization of the organization of intrastriatal circuitry and their cortical and thalamic inputs. We investigated thalamic inputs to the two populations of striatal neuropeptide Y (NPY) interneurons, plateau low threshold spike (PLTS) and NPY-neurogliaform (NGF) cells. Optogenetic activation of parafascicular inputs evokes suprathreshold monosynaptic glutamatergic excitation in NGF interneurons and a disynaptic, nicotinic excitation through cholinergic interneurons. In contrast, the predominant response of PLTS interneurons is a disynaptic inhibition dependent on thalamic activation of striatal tyrosine hydroxylase interneurons (THINs). In contrast, THINs do not innervate NGF or fast spiking interneurons, showing significant specificity in THINs outputs. Chemospecific ablation of THINs impairs prepulse inhibition of the acoustic startle response suggesting an important behavioural role of this disynaptic pathway. Our findings demonstrate that the impact of the parafascicular nucleus on striatal activity and some related behaviour critically depend on synaptic interactions within interneuronal circuits.

The responses of striatal GABAergic interneurons to thalamic inputs are not well characterised. Here, the authors demonstrate that complex intrastriatal circuitry is responsible for thalamic-evoked monosynaptic and disynaptic excitation in NPY-NGF interneurons but a disynaptic inhibition in the NPY-PLTS.

Reappraising striatal D1- and D2-neurons in reward and aversion

The striatum has been involved in complex behaviors such as motor control, learning, decision-making, reward and aversion. The striatum is mainly composed of medium spiny neurons (MSNs), typically divided into those expressing dopamine receptor D1, forming the so-called direct pathway, and those expressing D2 receptor (indirect pathway). For decades it has been proposed that these two populations exhibit opposing control over motor output, and recently, the same dichotomy has been proposed for valenced behaviors. Whereas D1-MSNs mediate reinforcement and reward, D2-MSNs have been associated with punishment and aversion. In this review we will discuss pharmacological, genetic and optogenetic studies that indicate that there is still controversy to what concerns the role of striatal D1- and D2-MSNs in this type of behaviors, highlighting the need to reconsider the early view that they mediate solely opposing aspects of valenced behaviour.

Are striatal tyrosine hydroxylase interneurons dopaminergic?

Striatal GABAergic interneurons that express the gene for tyrosine hydroxylase (TH) have been identified previously by several methods. Although generally assumed to be dopaminergic, possibly serving as a compensatory source of dopamine (DA) in Parkinson's disease, this assumption has never been tested directly. In TH-Cre mice whose nigrostriatal pathway had been eliminated unilaterally with 6-hydroxydopamine, we injected a Cre-dependent virus coding for channelrhodopsin-2 and enhanced yellow fluorescent protein unilaterally into the unlesioned midbrain or bilaterally into the striatum. Fast-scan cyclic voltammetry in striatal slices revealed that both optical and electrical stimulation readily elicited DA release in control striata but not from contralateral striata when nigrostriatal neurons were transduced. In contrast, neither optical nor electrical stimulation could elicit striatal DA release in either the control or lesioned striata when the virus was injected directly into the striatum transducing only striatal TH interneurons. This demonstrates that striatal TH interneurons do not release DA. Fluorescence immunocytochemistry in enhanced green fluorescent protein (EGFP)-TH mice revealed colocalization of DA, l-amino acid decarboxylase, the DA transporter, and vesicular monoamine transporter-2 with EGFP in midbrain dopaminergic neurons but not in any of the striatal EGFP-TH interneurons. Optogenetic activation of striatal EGFP-TH interneurons produced strong GABAergic inhibition in all spiny neurons tested. These results indicate that striatal TH interneurons are not dopaminergic but rather are a type of GABAergic interneuron that expresses TH but none of the other enzymes or transporters necessary to operate as dopaminergic neurons and exert widespread GABAergic inhibition onto direct and indirect spiny neurons.