A novel functionally distinct subtype of striatal neuropeptide Y interneuron

The nucleus accumbens in reward and aversion processing: insights and implications

The nucleus accumbens (NAc), a central component of the brain's reward circuitry, has been implicated in a wide range of behaviors and emotional states. Emerging evidence, primarily drawing from recent rodent studies, suggests that the function of the NAc in reward and aversion processing is multifaceted. Prolonged stress or drug use induces maladaptive neuronal function in the NAc circuitry, which results in pathological conditions. This review aims to provide comprehensive and up-to-date insights on the role of the NAc in motivated behavior regulation and highlights areas that demand further in-depth analysis. It synthesizes the latest findings on how distinct NAc neuronal populations and pathways contribute to the processing of opposite valences. The review examines how a range of neuromodulators, especially monoamines, influence the NAc's control over various motivational states. Furthermore, it delves into the complex underlying mechanisms of psychiatric disorders such as addiction and depression and evaluates prospective interventions to restore NAc functionality.

Interneuron diversity in the human dorsal striatum

Deciphering the striatal interneuron diversity is key to understanding the basal ganglia circuit and to untangling the complex neurological and psychiatric diseases affecting this brain structure. We performed snRNA-seq and spatial transcriptomics of postmortem human caudate nucleus and putamen samples to elucidate the diversity and abundance of interneuron populations and their inherent transcriptional structure in the human dorsal striatum. We propose a comprehensive taxonomy of striatal interneurons with eight main classes and fourteen subclasses, providing their full transcriptomic identity and spatial expression profile as well as additional quantitative FISH validation for specific populations. We have also delineated the correspondence of our taxonomy with previous standardized classifications and shown the main transcriptomic and class abundance differences between caudate nucleus and putamen. Notably, based on key functional genes such as ion channels and synaptic receptors, we found matching known mouse interneuron populations for the most abundant populations, the recently described PTHLH and TAC3 interneurons. Finally, we were able to integrate other published datasets with ours, supporting the generalizability of this harmonized taxonomy.

The transcriptomic and spatial organization of telencephalic GABAergic neuronal types

The telencephalon of the mammalian brain comprises multiple regions and circuit pathways that play adaptive and integrative roles in a variety of brain functions. There is a wide array of GABAergic neurons in the telencephalon; they play a multitude of circuit functions, and dysfunction of these neurons has been implicated in diverse brain disorders. In this study, we conducted a systematic and in-depth analysis of the transcriptomic and spatial organization of GABAergic neuronal types in all regions of the mouse telencephalon and their developmental origins. This was accomplished by utilizing 611,423 single-cell transcriptomes from the comprehensive and high-resolution transcriptomic and spatial cell type atlas for the adult whole mouse brain we have generated, supplemented with an additional single-cell RNA-sequencing dataset containing 99,438 high-quality single-cell transcriptomes collected from the pre- and postnatal developing mouse brain. We present a hierarchically organized adult telencephalic GABAergic neuronal cell type taxonomy of 7 classes, 52 subclasses, 284 supertypes, and 1,051 clusters, as well as a corresponding developmental taxonomy of 450 clusters across different ages. Detailed charting efforts reveal extraordinary complexity where relationships among cell types reflect both spatial locations and developmental origins. Transcriptomically and developmentally related cell types can often be found in distant and diverse brain regions indicating that long-distance migration and dispersion is a common characteristic of nearly all classes of telencephalic GABAergic neurons. Additionally, we find various spatial dimensions of both discrete and continuous variations among related cell types that are correlated with gene expression gradients. Lastly, we find that cortical, striatal and some pallidal GABAergic neurons undergo extensive postnatal diversification, whereas septal and most pallidal GABAergic neuronal types emerge simultaneously during the embryonic stage with limited postnatal diversification. Overall, the telencephalic GABAergic cell type taxonomy can serve as a foundational reference for molecular, structural and functional studies of cell types and circuits by the entire community.

Towards Standardizing Nomenclature in Huntington's Disease Research

The field of Huntington's disease research covers many different scientific disciplines, from molecular biology all the way through to clinical practice, and as our understanding of the disease has progressed over the decades, a great deal of different terminology has accrued. The field is also renowned for its collaborative spirit and use of standardized reagents, assays, datasets, models, and clinical measures, so the use of standardized terms is especially important. We have set out to determine, through a consensus exercise involving basic and clinical scientists working in the field, the most appropriate language to use across disciplines. Nominally, this article will serve as the style guide for the Journal of Huntington's Disease (JHD), the only journal devoted exclusively to HD, and we lay out the preferred and standardized terminology and nomenclature for use in JHD publications. However, we hope that this article will also serve as a useful resource to the HD research community at large and that these recommended naming conventions will be adopted widely.

The roles of surround inhibition for the intrinsic function of the striatum, analyzed in silico

The basal ganglia are important for action initiation, selection, and motor learning. The input level, the striatum, receives input preferentially from the cortex and thalamus and is to 95% composed of striatal projection neurons (SPNs) with sparse GABAergic collaterals targeting distal dendrites of neighboring SPNs, in a distance-dependent manner. The remaining 5% are GABAergic and cholinergic interneurons. Our aim here is to investigate the role of surround inhibition for the intrinsic function of the striatum. Large-scale striatal networks of 20 to 40 thousand neurons were simulated with detailed multicompartmental models of different cell types, corresponding to the size of a module of the dorsolateral striatum, like the forelimb area (mouse). The effect of surround inhibition on dendritic computation and network activity was investigated, while groups of SPNs were activated. The SPN-induced surround inhibition in distal dendrites shunted effectively the corticostriatal EPSPs. The size of dendritic plateau-like potentials within the specific dendritic segment was both reduced and enhanced by inhibition, due to the hyperpolarized membrane potential of SPNs and the reversal-potential of GABA. On a population level, the competition between two subpopulations of SPNs was found to depend on the distance between the two units, the size of each unit, the activity level in each subgroup and the dopaminergic modulation of the dSPNs and iSPNs. The SPNs provided the dominating source of inhibition within the striatum, while the fast-spiking interneuron mainly had an initial effect due to short-term synaptic plasticity as shown in with ablation of the synaptic interaction.

Developmental emergence of cortical neurogliaform cell diversity

GABAergic interneurons are key regulators of cortical circuit function. Among the dozens of reported transcriptionally distinct subtypes of cortical interneurons, neurogliaform cells (NGCs) are unique: they are recruited by long-range excitatory inputs, are a source of slow cortical inhibition and are able to modulate the activity of large neuronal populations. Despite their functional relevance, the developmental emergence and diversity of NGCs remains unclear. Here, by combining single-cell transcriptomics, genetic fate mapping, and electrophysiological and morphological characterization, we reveal that discrete molecular subtypes of NGCs, with distinctive anatomical and molecular profiles, populate the mouse neocortex. Furthermore, we show that NGC subtypes emerge gradually through development, as incipient discriminant molecular signatures are apparent in preoptic area (POA)-born NGC precursors. By identifying NGC developmentally conserved transcriptional programs, we report that the transcription factor Tox2 constitutes an identity hallmark across NGC subtypes. Using CRISPR-Cas9-mediated genetic loss of function, we show that Tox2 is essential for NGC development: POA-born cells lacking Tox2 fail to differentiate into NGCs. Together, these results reveal that NGCs are born from a spatially restricted pool of Tox2+ POA precursors, after which intra-type diverging molecular programs are gradually acquired post-mitotically and result in functionally and molecularly discrete NGC cortical subtypes.

Morphology and morphometry of interneuron subpopulations of the marmoset monkey (Callithrix jacchus) striatum

The mammalian striatum has long been considered a homogeneous entity. However, neuroanatomical and histochemical studies reveal that the striatum is much more heterogeneous than previously suspected. The caudate (Cd) and putamen (Pu) are composed of two chemical compartments: the matrix and the striosomes. Striatal interneurons have been classified into a variety of morphological and neurochemical subtypes. In this study, we compared the distribution of multiple neurochemical markers in the striatum of marmosets and described the morphology of different types of striatum interneurons. The immunoreactivities of choline-acetyl transferase (ChAT), neuropeptide Y (NPY), nitric oxide synthase (NOS), calretinin (CR), parvalbumin (PV) were analyzed along the entire rostrocaudal extent of the marmoset striatum. Calbindin immunohistochemistry is useful in identifying medium spiny neurons (MSNs), with efficient soma staining. Based on the size of the CB-positive cells, considered medium-sized, as expected, cholinergic cells are larger in area and diameter than the other subpopulations investigated, followed by NOS, NPY, PV and CR. In adjacent CB and PV-stained sections, the matrix and striosomes were clearly distinguished. The matrix is strongly reactive to CB and PV neuropils, while the striosomes exhibit low reactivity, especially in the dorsal Cd. Therefore, we provide a detailed description morphology and distribution of striatal interneuron populations in a model as a valuable tool for studying neurodegenerative pathogenesis, progression and treatment strategies.

Genetic evidence of aberrant striatal synaptic maturation and secretory pathway alteration in a dystonia mouse model

Animal models of DYT-TOR1A dystonia consistently demonstrate abnormalities of striatal cholinergic function, but the molecular pathways underlying this pathophysiology are unclear. To probe these molecular pathways in a genetic model of DYT-TOR1A, we performed laser microdissection in juvenile mice to isolate striatal cholinergic interneurons and non-cholinergic striatal tissue largely comprising spiny projection neurons during maturation. Both cholinergic and GABAergic enriched samples demonstrated a defined set of gene expression changes consistent with a role of torsinA in the secretory pathway. GABAergic enriched striatum samples also showed alteration to genes regulating synaptic transmission and an upregulation of activity dependent immediate early genes. Reconstruction of Golgi-Cox stained striatal spiny projection neurons from adult mice demonstrated significantly increased spiny density, suggesting that torsinA null striatal neurons have increased excitability during striatal maturation and long lasting increases in afferent input. These findings are consistent with a developmental role for torsinA in the secretory pathway and link torsinA loss of function with functional and structural changes of striatal cholinergic and GABAergic neurons. These transcriptomic datasets are freely available as a resource for future studies of torsinA loss of function-mediated striatal dysfunction.

Characterization and mu opioid receptor sensitivity of neuropeptide Y interneurons in the mouse nucleus accumbens

Inhibitory interneurons represent less than 5% of neurons within the nucleus accumbens, but are critical for proper microcircuit function within this brain region. In the dorsal striatum, neuropeptide Y is expressed by two interneuron subtypes (low-threshold spiking interneurons and neurogliaform interneurons) that exhibit mu opioid receptor sensitivity in other brain regions. However, few studies have assessed the molecular and physiological properties of neuropeptide Y interneurons within the nucleus accumbens. We used a transgenic reporter mouse to identify and characterize neuropeptide Y interneurons in acute nucleus accumbens brain slices. Nearly all cells exhibited electrophysiological properties of low-threshold spiking interneurons, with almost no neurogliaform interneurons observed among neuropeptide Y interneurons. We corroborated this pattern using fluorescent in situ hybridization, and also identified a high level of mu opioid receptor expression by low-threshold spiking interneurons, which led us to examine the functional consequences of mu opioid receptor activation in these cells using electrophysiology. Mu opioid receptor activation caused a reduction in the rate of spontaneous action potentials in low-threshold spiking interneurons, as well as a decrease in optogenetically-evoked GABA release onto medium spiny neurons. The latter effect was more robust in female versus male mice, and when the postsynaptic medium spiny neuron expressed the Drd1 dopamine receptor. This work is the first to examine the physiological properties of neuropeptide Y interneurons in the nucleus accumbens, and show they may be an important target for mu opioid receptor modulation by endogenous and exogenous opioids.

Connectivity of the corticostriatal and thalamostriatal systems in normal and parkinsonian states: An update

The striatum receives abundant glutamatergic afferents from the cortex and thalamus. These inputs play a major role in the functions of the striatal neurons in normal conditions, and are significantly altered in pathological states, such as Parkinson's disease. This review summarizes the current knowledge of the connectivity of the corticostriatal and thalamostriatal pathways, with emphasis on the most recent advances in the field. We also discuss novel findings regarding structural changes in cortico- and thalamostriatal connections that occur in these connections as a consequence of striatal loss of dopamine in parkinsonism.

Oscillation/Coincidence-Detection Models of Reward-Related Timing in Corticostriatal Circuits

The major tenets of beat-frequency/coincidence-detection models of reward-related timing are reviewed in light of recent behavioral and neurobiological findings. This includes the emphasis on a core timing network embedded in the motor system that is comprised of a corticothalamic-basal ganglia circuit. Therein, a central hub provides timing pulses (i.e., predictive signals) to the entire brain, including a set of distributed satellite regions in the cerebellum, cortex, amygdala, and hippocampus that are selectively engaged in timing in a manner that is more dependent upon the specific sensory, behavioral, and contextual requirements of the task. Oscillation/coincidence-detection models also emphasize the importance of a tuned ‘perception’ learning and memory system whereby target durations are detected by striatal networks of medium spiny neurons (MSNs) through the coincidental activation of different neural populations, typically utilizing patterns of oscillatory input from the cortex and thalamus or derivations thereof (e.g., population coding) as a time base. The measure of success of beat-frequency/coincidence-detection accounts, such as the Striatal Beat-Frequency model of reward-related timing (SBF), is their ability to accommodate new experimental findings while maintaining their original framework, thereby making testable experimental predictions concerning diagnosis and treatment of issues related to a variety of dopamine-dependent basal ganglia disorders, including Huntington’s and Parkinson’s disease.

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.

Decoding molecular and cellular heterogeneity of mouse nucleus accumbens

The nucleus accumbens (NAc) plays an important role in regulating multiple behaviors, and its dysfunction has been linked to many neural disorders. However, the molecular, cellular and anatomic heterogeneity underlying its functional diversity remains incompletely understood. In this study, we generated a cell census of the mouse NAc using single-cell RNA sequencing and multiplexed error-robust fluorescence in situ hybridization, revealing a high level of cell heterogeneity in this brain region. Here we show that the transcriptional and spatial diversity of neuron subtypes underlie the NAc's anatomic and functional heterogeneity. These findings explain how the seemingly simple neuronal composition of the NAc achieves its highly heterogenous structure and diverse functions. Collectively, our study generates a spatially resolved cell taxonomy for understanding the structure and function of the NAc, which demonstrates the importance of combining molecular and spatial information in revealing the fundamental features of the nervous system.

Somatostatin Neurons in the Mouse Pontine Nucleus Activate GABAA Receptor Mediated Synaptic Currents in Locus Coeruleus Neurons

The pontine nuclei comprising the locus coeruleus (LC) and Barrington’s nucleus (BRN) amongst others form the neural circuitry(s) that coordinates arousal and voiding behaviors. However, little is known about the synaptic connectivity of neurons within or across these nuclei. These include corticotropin-releasing factor (CRF⁺) expressing neurons in the BRN that control bladder contraction and somatostatin expressing (SST⁺) neurons whose role in this region has not been discerned. To determine the synaptic connectivity of these neurons, we employed optogenetic stimulation with recordings from BRN and LC neurons in brain stem slices of channelrhodopsin-2 expressing SST or CRF neurons. Optogenetic stimulation of CRF⁺ BRN neurons of Crf^Cre;chr2-yfp mice had little effect on either CRF⁺ BRN neurons, CRF^– BRN neurons, or LC neurons. In contrast, in Sst^Cre;chr2-yfp mice light-activated inhibitory postsynaptic currents (IPSCs) were reliably observed in a majority of LC but not BRN neurons. The GABA_A receptor antagonist, bicuculline, completely abolished the light-induced IPSCs. To ascertain if these neurons were part of the neural circuitry that controls the bladder, the trans-synaptic tracer, pseudorabies virus (PRV) was injected into the bladder wall of Crf^Cre;tdTomato or Sst^Cre;tdTomato mice. At 68–72 h post-viral infection, PRV labeled neurons were present only in the BRN, being preponderant in CRF⁺ neurons with few SST⁺ BRN neurons labeled from the bladder. At 76 and 96 h post-virus injection, increased labeling was observed in both BRN and LC neurons. Our results suggest SST⁺ neurons rather than CRF⁺ neurons in BRN can regulate the activity of LC neurons.

Reprogramming reactive glia into interneurons reduces chronic seizure activity in a mouse model of mesial temporal lobe epilepsy

Reprogramming brain-resident glial cells into clinically relevant induced neurons (iNs) is an emerging strategy toward replacing lost neurons and restoring lost brain functions. A fundamental question is now whether iNs can promote functional recovery in pathological contexts. We addressed this question in the context of therapy-resistant mesial temporal lobe epilepsy (MTLE), which is associated with hippocampal seizures and degeneration of hippocampal GABAergic interneurons. Using a MTLE mouse model, we show that retrovirus-driven expression of Ascl1 and Dlx2 in reactive hippocampal glia in situ, or in cortical astroglia grafted in the epileptic hippocampus, causes efficient reprogramming into iNs exhibiting hallmarks of interneurons. These induced interneurons functionally integrate into epileptic networks and establish GABAergic synapses onto dentate granule cells. MTLE mice with GABAergic iNs show a significant reduction in both the number and cumulative duration of spontaneous recurrent hippocampal seizures. Thus glia-to-neuron reprogramming is a potential disease-modifying strategy to reduce seizures in therapy-resistant epilepsy.

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Retroviruses target reactive hippocampal glia proliferating in a mouse model of mesial temporal lobe epilepsy

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Ascl1 and Dlx2 reprogram reactive glia into GABAergic interneurons in the epileptic hippocampus

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Induced interneurons establish GABAergic synapses onto dentate granule cells

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Induced interneurons reduce chronic epileptic activity in the hippocampus

Oxycodone in the Opioid Epidemic: High 'Liking', 'Wanting', and Abuse Liability

It is estimated that nearly a third of people who abuse drugs started with prescription opioid medicines. Approximately, 11.5 million Americans used prescription drugs recreationally in 2016, and in 2018, 46,802 Americans died as the result of an opioid overdose, including prescription opioids, heroin, and illicitly manufactured fentanyl (National Institutes on Drug Abuse (2020) Opioid Overdose Crisis. https://www.drugabuse.gov/drugs-abuse/opioids/opioid-overdose-crisis . Accessed 06 June 2020). Yet physicians will continue to prescribe oral opioids for moderate-to-severe pain in the absence of alternative therapeutics, underscoring the importance in understanding how drug choice can influence detrimental outcomes. One of the opioid prescription medications that led to this crisis is oxycodone, where misuse of this drug has been rampant. Being one of the most highly prescribed opioid medications for treating moderate-to-severe pain as reflected in the skyrocketed increase in retail sales of 866% between 1997 and 2007, oxycodone was initially suggested to be less addictive than morphine. The false-claimed non-addictive formulation of oxycodone, OxyContin, further contributed to the opioid crisis. Abuse was often carried out by crushing the pills for immediate burst release, typically by nasal insufflation, or by liquefying the pills for intravenous injection. Here, we review oxycodone pharmacology and abuse liability as well as present the hypothesis that oxycodone may exhibit a unique pharmacology that contributes to its high likability and abuse susceptibility. We will discuss various mechanisms that likely contribute to the high abuse rate of oxycodone including clinical drug likability, pharmacokinetics, pharmacodynamics, differences in its actions within mesolimbic reward circuity compared to other opioids, and the possibility of differential molecular and cellular receptor interactions that contribute to its selective effects. We will also discuss marketing strategies and drug difference that likely contributes to the oxycodone opioid use disorders and addiction.

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.

NDNF is selectively expressed by neocortical, but not habenular neurogliaform cells

The lateral habenula (LHb) is a brain structure which is known to be pathologically hyperactive in depression, whereby it shuts down the brains' reward systems. Interestingly, inhibition of the LHb has been shown to have an antidepressant effect, hence making the LHb a fascinating subject of study for developing novel antidepressant therapies. Despite this however, the exact mechanisms by which inhibitory signalling is processed within the LHb remain incompletely understood. Some studies have proposed the existence of locally targeting inhibitory interneuron populations within the LHb. One such population is believed to be akin to neocortical neurogliaform cells, yet specific molecular markers for studying these neurons are sparse and hence their function remains elusive. Recently, neuron-derived neurotrophic factor (NDNF) has been proposed as one such marker for neocortical neurogliaform cells. Using a combination of histological, physiological and optogenetic tools, we hence sought to first validate if NDNF was selectively expressed by such inhibitory neurons within the neocortex, and then if it was confined to a similar population within the LHb. While we report this to be true for the neocortex, we find no such evidence within the LHb; rather that NDNF is expressed without restriction to a particular neuronal subpopulation. These results hence indicate that molecular markers can represent broadly diverse populations of neurons on a region-to-region basis and that therefore each population as defined by molecular marker expression should be validated in each brain structure.

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.

Enhanced GABAergic Inhibition of Cholinergic Interneurons in the zQ175+/- Mouse Model of Huntington's Disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder that initially manifests itself in the striatum. How intrastriatal circuitry is altered by the disease is poorly understood. To help fill this gap, the circuitry linking spiny projection neurons (SPNs) to cholinergic interneurons (ChIs) was examined using electrophysiological and optogenetic approaches in ex vivo brain slices from wildtype mice and zQ175^+/− models of HD. These studies revealed a severalfold enhancement of GABAergic inhibition of ChIs mediated by collaterals of indirect pathway SPNs (iSPNs), but not direct pathway SPNs (dSPNs). This cell-specific alteration in synaptic transmission appeared in parallel with the emergence of motor symptoms in the zQ175^+/− model. The adaptation had a presynaptic locus, as it was accompanied by a reduction in paired-pulse ratio but not in the postsynaptic response to GABA. The alterations in striatal GABAergic signaling disrupted spontaneous ChI activity, potentially contributing to the network dysfunction underlying the hyperkinetic phase of HD.

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

Broadband Entrainment of Striatal Low-Threshold Spike Interneurons

Striatal interneurons and spiny projection (SP) neurons are differentially tuned to spectral components of their input. Previous studies showed that spike responses of somatostatin/NPY-expressing low threshold spike (LTS) interneurons have broad frequency tuning, setting these cells apart from other striatal GABAergic interneurons and SP neurons. We investigated the mechanism of LTS interneuron spiking resonance and its relationship to non-spiking membrane impedance resonance, finding that abolition of impedance resonance did not alter spiking resonance. Because LTS interneurons are pacemakers whose rhythmic firing is perturbed by synaptic input, we tested the hypothesis that their spiking resonance arises from their phase resetting properties. Phase resetting curves (PRCs) were measured in LTS interneurons and SP neurons and used to make phase-oscillator models of both cell types. The models reproduced the broad tuning of LTS interneurons, and the differences from SP neurons. The spectral components of the PRC predicted each cell's sensitivity to corresponding input frequencies. LTS interneuron PRCs contain larger high-frequency components than SP neuron PRCs, providing enhanced responses to input frequencies above the cells' average firing rates. Thus, LTS cells can be entrained by input oscillations to which SP neurons are less responsive. These findings suggest that feedforward inhibition by LTS interneurons may regulate SP neurons' entrainment by oscillatory afferents.

The microcircuits of striatum in silico

The basal ganglia play an important role in decision making and selection of action primarily based on input from cortex, thalamus, and the dopamine system. Their main input structure, striatum, is central to this process. It consists of two types of projection neurons, together representing 95% of the neurons, and 5% of interneurons, among which are the cholinergic, fast-spiking, and low threshold-spiking subtypes. The membrane properties, soma-dendritic shape, and intrastriatal and extrastriatal synaptic interactions of these neurons are quite well described in the mouse, and therefore they can be simulated in sufficient detail to capture their intrinsic properties, as well as the connectivity. We focus on simulation at the striatal cellular/microcircuit level, in which the molecular/subcellular and systems levels meet. We present a nearly full-scale model of the mouse striatum using available data on synaptic connectivity, cellular morphology, and electrophysiological properties to create a microcircuit mimicking the real network. A striatal volume is populated with reconstructed neuronal morphologies with appropriate cell densities, and then we connect neurons together based on appositions between neurites as possible synapses and constrain them further with available connectivity data. Moreover, we simulate a subset of the striatum involving 10,000 neurons, with input from cortex, thalamus, and the dopamine system, as a proof of principle. Simulation at this biological scale should serve as an invaluable tool to understand the mode of operation of this complex structure. This platform will be updated with new data and expanded to simulate the entire striatum.

Increased GABAergic transmission in neuropeptide Y-expressing neurons in the dopamine-depleted murine striatum

As the main input nucleus of the basal ganglia, the striatum plays a central role in planning, control, and execution of movement and motor skill learning. More than 90% of striatal neurons, so-called medium spiny neurons (MSN), are GABAergic projection neurons, innervating primarily the substantia nigra pars reticulata or the globus pallidus internus. The remaining neurons are GABAergic and cholinergic interneurons, synchronizing and controlling striatal output by reciprocal connections with MSN. Besides prominent local cholinergic influence, striatal function is globally regulated by dopamine (DA) from the nigrostriatal pathway. Little is known about whether DA depletion, as occurs in Parkinson's disease, affects the activity of striatal interneurons. Here we focused on neuropeptide Y (NPY)-expressing interneurons, which are among the major subgroups of GABAergic interneurons in the striatum. We investigated the effects of striatal DA depletion on GABAergic transmission in NPY interneurons by electrophysiologically recording GABAergic spontaneous (s) and miniature (m) inhibitory postsynaptic currents (IPSCs) in identified NPY interneurons in slices from 6-hydroxydopamine (6-OHDA)- and vehicle-injected transgenic NPY-humanized Renilla green fluorescent protein (hrGFP) mice with the whole cell patch-clamp technique. We report a significant increase in sIPSC and mIPSC frequency as well as the occurrence of giant synaptic and burst sIPSCs in the 6-OHDA group, suggesting changes in GABAergic circuit activity and synaptic transmission. IPSC kinetics remained unchanged, pointing to mainly presynaptic changes in GABAergic transmission. These results show that chronic DA depletion following 6-OHDA injection causes activity-dependent and -independent increase of synaptic GABAergic inhibition onto striatal NPY interneurons, confirming their involvement in the functional impairments of the DA-depleted striatum.NEW & NOTEWORTHY Neuropeptide Y (NPY) interneurons regulate the function of striatal projection neurons and are upregulated upon dopamine depletion in the striatum. Here we investigated how dopamine depletion affects NPY circuits and show electrophysiologically that it leads to the occurrence of giant synaptic and burst GABAergic spontaneous inhibitory postsynaptic currents (IPSCs) and to an activity-independent increase in GABAergic miniature IPSC frequency in NPY neurons. We suggest that degeneration of dopaminergic terminals in the striatum causes functional changes in striatal GABAergic function.

Diversity of Interneurons in the Dorsal Striatum Revealed by Single-Cell RNA Sequencing and PatchSeq

Striatal locally projecting neurons, or interneurons, act on nearby circuits and shape functional output to the rest of the basal ganglia. We performed single-cell RNA sequencing of striatal cells enriching for interneurons. We find seven discrete interneuron types, six of which are GABAergic. In addition to providing specific markers for the populations previously described, including those expressing Sst/Npy, Th, Npy without Sst, and Chat, we identify two small populations of cells expressing Cck with or without Vip. Surprisingly, the Pvalb-expressing cells do not constitute a discrete cluster but rather are part of a larger group of cells expressing Pthlh with a spatial gradient of Pvalb expression. Using PatchSeq, we show that Pthlh cells exhibit a continuum of electrophysiological properties correlated with expression of Pvalb. Furthermore, we find significant molecular differences that correlate with differences in electrophysiological properties between Pvalb-expressing cells of the striatum and those of the cortex.

Cholinergic modulation of striatal nitric oxide-producing interneurons

Striatal GABAergic interneurons that express nitric oxide synthase-so-called low-threshold spike interneurons (LTSIs)-play several key roles in the striatum. But what drives the activity of these interneurons is less well defined. To fill this gap, a combination of monosynaptic rabies virus mapping (msRVm), electrophysiological and optogenetic approaches were used in transgenic mice in which LTSIs expressed either Cre recombinase or a fluorescent reporter. The rabies virus studies revealed a striking similarity in the afferent connectomes of LTSIs and neighboring cholinergic interneurons, particularly regarding connections arising from the parafascicular nucleus of the thalamus and cingulate cortex. While optogenetic stimulation of cingulate inputs excited both cholinergic interneurons and LTSIs, thalamic stimulation excited cholinergic interneurons, but inhibited LTSIs. This inhibition was dependent on cholinergic interneurons and had two components: a previously described GABAergic element and one that was mediated by M4 muscarinic acetylcholine receptors. In addition to this phasic signal, cholinergic interneurons tonically excited LTSIs through a distinct, M1 muscarinic acetylcholine receptor pathway. This coordinated cholinergic modulation of LTSIs predisposed them to rhythmically burst in response to phasic thalamic activity, potentially reconfiguring striatal circuitry in response to salient environmental stimuli.

GABAergic modulation of serotonergic neurons in the dorsal raphe nucleus

The dorsal raphe nucleus (DRN), located in the brainstem, is involved in several functions such as sleep, temperature regulation, stress responses, and anxiety behaviors. This nucleus contains the largest population of serotonin expressing neurons in the brain. Serotonergic DRN neurons receive tonic γ-aminobutyric acid (GABA)inhibitory inputs from several brain areas, as well as from interneurons within the same nucleus. Serotonergic and GABAergic neurons in the DRN can be distinguished by their size, location, pharmacological responses, and electrophysiological properties. GABAergic neurons regulate the excitability of DRN serotonergic neurons and the serotonin release in different brain areas. Also, it has been shown that GABAergic neurons can synchronize the activity of serotonergic neurons across functions such as sleep or alertness. Moreover, dysregulation of GABA signaling in the DRN has been linked to psychiatric disorders such as anxiety and depression. This review focuses on GABAergic transmission in the DRN. The interaction between GABAergic and serotonergic neurons is discussed considering some physiological implications. Also, the main electrophysiological and morphological characteristics of serotonergic and GABAergic neurons are described.

Pedunculopontine Glutamatergic Neurons Provide a Novel Source of Feedforward Inhibition in the Striatum by Selectively Targeting Interneurons

The main excitatory inputs to the striatum arising from the cortex and the thalamus innervate both striatal spiny projection neurons and interneurons. These glutamatergic inputs to striatal GABAergic interneurons have been suggested to regulate the spike timing of striatal projection neurons via feedforward inhibition. Understanding how different excitatory inputs are integrated within the striatal circuitry and how they regulate striatal output is crucial for understanding basal ganglia function and related behaviors. Here, using VGLUT2 mice from both sexes, we report the existence of a glutamatergic projection from the mesencephalic locomotor region to the striatum that avoids the spiny neurons and selectively innervates interneurons. Specifically, optogenetic activation of glutamatergic axons from the pedunculopontine nucleus induced monosynaptic excitation in most recorded striatal cholinergic interneurons and GABAergic fast-spiking interneurons. Optogenetic stimulation in awake head-fixed mice consistently induced an increase in the firing rate of putative cholinergic interneurons and fast-spiking interneurons. In contrast, this stimulation did not induce excitatory responses in spiny neurons but rather disynaptic inhibitory responses ex vivo and a decrease in their firing rate in vivo, suggesting a feedforward mechanism mediating the inhibition of spiny projection neurons through the selective activation of striatal interneurons. Furthermore, unilateral stimulation of pedunculopontine nucleus glutamatergic axons in the striatum induced ipsilateral head rotations consistent with the inhibition of striatal output neurons. Our results demonstrate the existence of a unique interneuron-specific midbrain glutamatergic input to the striatum that exclusively recruits feedforward inhibition mechanisms.SIGNIFICANCE STATEMENT Glutamatergic inputs to the striatum have been shown to target both striatal projection neurons and interneurons and have been proposed to regulate spike timing of the projection neurons in part through feedforward inhibition. Here, we reveal the existence of a midbrain source of glutamatergic innervation to the striatum, originating in the pedunculopontine nucleus. Remarkably, this novel input selectively targets striatal interneurons, avoiding the projection neurons. Furthermore, we show that this selective innervation of interneurons can regulate the firing of the spiny projection neurons and inhibit the striatal output via feedforward inhibition. Together, our results describe a unique source of excitatory innervation to the striatum which selectively recruits feedforward inhibition of spiny neurons without any accompanying excitation.

A Novel Type of Neuron Within the Dorsal Striatum

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

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

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

Parvalbumin Interneurons Determine Emotional Valence Through Modulating Accumbal Output Pathways

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

Cell type-specific distribution of GABAA receptor subtypes in the mouse dorsal striatum

The striatum is the main input nucleus of the basal ganglia, mediating motor and cognitive functions. Striatal projection neurons are GABAergic medium spiny neurons (MSN), expressing either the dopamine receptor type 1 (D₁ -R MSN) and forming the direct, movement-promoting pathway, or dopamine receptor type 2 (D₂ -R MSN), forming the indirect movement-suppressing pathway. Locally, activity and synchronization of MSN are modulated by several subtypes of GABAergic and cholinergic interneurons. Overall, GABAergic circuits in the striatum remain poorly characterized, and little is known about the intrastriatal connectivity of interneurons and the distribution of GABA_A receptor (GABA_A R) subtypes, distinguished by their subunit composition, in striatal synapses. Here, by using immunofluorescence in mouse tissue, we investigated the distribution of GABA_A Rs containing the α₁ , α₂ , or α₃ subunit in perisomatic synapses of striatal MSN and interneurons, as well as the innervation pattern of D₁ R- and D₂ R-MSN soma and axonal initial segment (AIS) by GABAergic and cholinergic interneurons. Our results show that perisomatic GABAergic synapses of D₁ R- and D₂ R-MSN contain the GABA_A R α₁ and/or α₂ subunits, but not the α₃ subunit; D₂ R-MSN have significantly more α₁ -GABA_A Rs on their soma than D₁ R-MSN. Further, interneurons have few perisomatic synapses containing α₂ -GABA_A Rs, whereas α₃ -GABA_A Rs (along with the α₁ -GABA_A Rs) are abundant in perisomatic synapses of CCK⁺ , NPY⁺ /SOM⁺ , and vAChT⁺ interneurons. Each MSN and interneuron population analyzed received a distinct pattern of GABAergic and cholinergic innervation, complementing this postsynaptic heterogeneity. In conclusion, intra-striatal GABAergic circuits are distinguished by cell-type specific innervation patterns, differential expression and postsynaptic targeting of GABA_A R subtypes.

Neuropeptide Y and representation of salience in human nucleus accumbens

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

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

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

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.

MMP-1 overexpression selectively alters inhibition in D1 spiny projection neurons in the mouse nucleus accumbens core

Protease activated receptor-1 (PAR-1) and its ligand, matrix metalloproteinase-1 (MMP-1), are altered in several neurodegenerative diseases. PAR-1/MMP-1 signaling impacts neuronal activity in various brain regions, but their role in regulating synaptic physiology in the ventral striatum, which is implicated in motor function, is unknown. The ventral striatum contains two populations of GABAergic spiny projection neurons, D1 and D2 SPNs, which differ with respect to both synaptic inputs and projection targets. To evaluate the role of MMP-1/PAR-1 signaling in the regulation of ventral striatal synaptic function, we performed whole-cell recordings (WCR) from D1 and D2 SPNs in control mice, mice that overexpress MMP-1 (MMP-1OE), and MMP-1OE mice lacking PAR-1 (MMP-1OE/PAR-1KO). WCRs from MMP1-OE mice revealed an increase in spontaneous inhibitory post-synaptic current (sIPSC), miniature IPSC, and miniature excitatory PSC frequency in D1 SPNs but not D2 SPNs. This alteration may be partially PAR-1 dependent, as it was not present in MMP-1OE/PAR-1KO mice. Morphological reconstruction of D1 SPNs revealed increased dendritic complexity in the MMP-1OE, but not MMP-1OE/PAR-1KO mice. Moreover, MMP-1OE mice exhibited blunted locomotor responses to amphetamine, a phenotype also observed in MMP-1OE/PAR-1KO mice. Our data suggest PAR-1 dependent and independent MMP-1 signaling may lead to alterations in striatal neuronal function.

Thinking Outside the Box (and Arrow): Current Themes in Striatal Dysfunction in Movement Disorders

The basal ganglia are an intricately connected assembly of subcortical nuclei, forming the core of an adaptive network connecting cortical and thalamic circuits. For nearly three decades, researchers and medical practitioners have conceptualized how the basal ganglia circuit works, and how its pathology underlies motor disorders such as Parkinson's and Huntington's diseases, using what is often referred to as the "box-and-arrow model": a circuit diagram showing the broad strokes of basal ganglia connectivity and the pathological increases and decreases in the weights of specific connections that occur in disease. While this model still has great utility and has led to groundbreaking strategies to treat motor disorders, our evolving knowledge of basal ganglia function has made it clear that this classic model has several shortcomings that severely limit its predictive and descriptive abilities. In this review, we will focus on the striatum, the main input nucleus of the basal ganglia. We describe recent advances in our understanding of the rich microcircuitry and plastic capabilities of the striatum, factors not captured by the original box-and-arrow model, and provide examples of how such advances inform our current understanding of the circuit pathologies underlying motor disorders.

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

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

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

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

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

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

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

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

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.

Disrupted striatal neuron inputs and outputs in Huntington's disease

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

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.