Striatal interneurons expressing calretinin, parvalbumin or NADPH-diaphorase: a comparative study in the rat, monkey and human

Sex differences in the distribution and density of regulatory interneurons in the striatum

Introduction

Dysfunction of the cortico-basal circuitry - including its primary input nucleus, the striatum - contributes to neuropsychiatric disorders, such as autism and Tourette Syndrome (TS). These conditions show marked sex differences, occurring more often in males than in females. Regulatory interneurons, such as cholinergic interneurons (CINs) and parvalbumin-expressing GABAergic fast spiking interneurons (FSIs), are implicated in human neuropsychiatric disorders such as TS, and ablation of these interneurons produces relevant behavioral pathology in male mice, but not in females. Here we investigate sex differences in the density and distribution of striatal interneurons.

Methods

We use stereological quantification of CINs, FSIs, and somatostatin-expressing (SOM) GABAergic interneurons in the dorsal striatum (caudate-putamen) and the ventral striatum (nucleus accumbens) in male and female mice.

Results

Males have a higher density of CINs than females, especially in the dorsal striatum; females have equal distribution between dorsal and ventral striatum. FSIs showed similar distributions, with a greater dorsal-ventral density gradient in males than in females. SOM interneurons were denser in the ventral than in the dorsal striatum, with no sex differences.

Discussion

These sex differences in the density and distribution of FSIs and CINs may contribute to sex differences in basal ganglia function, particularly in the context of psychopathology.

Recapitulation of Perturbed Striatal Gene Expression Dynamics of Donors' Brains With Ventral Forebrain Organoids Derived From the Same Individuals With Schizophrenia

OBJECTIVE

Schizophrenia is a brain disorder that originates during neurodevelopment and has complex genetic and environmental etiologies. Despite decades of clinical evidence of altered striatal function in affected patients, studies examining its cellular and molecular mechanisms in humans are limited. To explore neurodevelopmental alterations in the striatum associated with schizophrenia, the authors established a method for the differentiation of induced pluripotent stem cells (iPSCs) into ventral forebrain organoids (VFOs).

METHODS

VFOs were generated from postmortem dural fibroblast-derived iPSCs of four individuals with schizophrenia and four neurotypical control individuals for whom postmortem caudate genotypes and transcriptomic data were profiled in the BrainSeq neurogenomics consortium. Individuals were selected such that the two groups had nonoverlapping schizophrenia polygenic risk scores (PRSs).

RESULTS

Single-cell RNA sequencing analyses of VFOs revealed differences in developmental trajectory between schizophrenia and control individuals in which inhibitory neuronal cells from the patients exhibited accelerated maturation. Furthermore, upregulated genes in inhibitory neurons in schizophrenia VFOs showed a significant overlap with upregulated genes in postmortem caudate tissue of individuals with schizophrenia compared with control individuals, including the donors of the iPSC cohort.

CONCLUSIONS

The findings suggest that striatal neurons derived from high-PRS individuals with schizophrenia carry abnormalities that originated during early brain development and that the VFO model can recapitulate disease-relevant cell type-specific neurodevelopmental phenotypes in a dish.

The dual role of striatal interneurons: circuit modulation and trophic support for the basal ganglia

ABSTRACT

Striatal interneurons play a key role in modulating striatal-dependent behaviors, including motor activity and reward and emotional processing. Interneurons not only provide modulation to the basal ganglia circuitry under homeostasis but are also involved in changes to plasticity and adaptation during disease conditions such as Parkinson's or Huntington's disease. This review aims to summarize recent findings regarding the role of striatal cholinergic and GABAergic interneurons in providing circuit modulation to the basal ganglia in both homeostatic and disease conditions. In addition to direct circuit modulation, striatal interneurons have also been shown to provide trophic support to maintain neuron populations in adulthood. We discuss this interesting and novel role of striatal interneurons, with a focus on the maintenance of adult dopaminergic neurons from interneuron-derived sonic-hedgehog.

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.

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.

Reprogramming Human Adult Fibroblasts into GABAergic Interneurons

Direct reprogramming is an appealing strategy to generate neurons from a somatic cell by forced expression of transcription factors. The generated neurons can be used for both cell replacement strategies and disease modelling. Using this technique, previous studies have shown that γ-aminobutyric acid (GABA) expressing interneurons can be generated from different cell sources, such as glia cells or fetal fibroblasts. Nevertheless, the generation of neurons from adult human fibroblasts, an easily accessible cell source to obtain patient-derived neurons, has proved to be challenging due to the intrinsic blockade of neuronal commitment. In this paper, we used an optimized protocol for adult skin fibroblast reprogramming based on RE1 Silencing Transcription Factor (REST) inhibition together with a combination of GABAergic fate determinants to convert human adult skin fibroblasts into GABAergic neurons. Our results show a successful conversion in 25 days with upregulation of neuronal gene and protein expression levels. Moreover, we identified specific gene combinations that converted fibroblasts into neurons of a GABAergic interneuronal fate. Despite the well-known difficulty in converting adult fibroblasts into functional neurons in vitro, we could detect functional maturation in the induced neurons. GABAergic interneurons have relevance for cognitive impairments and brain disorders, such as Alzheimer’s and Parkinson’s diseases, epilepsy, schizophrenia and autism spectrum disorders.

Whole-Brain Afferent Inputs to the Caudate Nucleus, Putamen, and Accumbens Nucleus in the Tree Shrew Striatum

Day-active tree shrews have a well-developed internal capsule (ic) that clearly separates the caudate nucleus (Cd) and putamen (Pu). The striatum consists of the Cd, ic, Pu, and accumbens nucleus (Acb). Here, we characterized the cytoarchitecture of the striatum and the whole-brain inputs to the Cd, Pu, and Acb in tree shrews by using immunohistochemistry and the retrograde tracer Fluoro-Gold (FG). Our data show the distribution patterns of parvalbumin (PV), nitric oxide synthase (NOS), calretinin (CR), and tyrosine hydroxylase (TH) immunoreactivity in the striatum of tree shrews, which were different from those observed in rats. The Cd and Pu mainly received inputs from the thalamus, motor cortex, somatosensory cortex, subthalamic nucleus, substantia nigra, and other cortical and subcortical regions, whereas the Acb primarily received inputs from the anterior olfactory nucleus, claustrum, infralimbic cortex, thalamus, raphe nucleus, parabrachial nucleus, ventral tegmental area, and so on. The Cd, Pu, and Acb received inputs from different neuronal populations in the ipsilateral (60, 67, and 63 brain regions, respectively) and contralateral (23, 20, and 36 brain regions, respectively) brain hemispheres. Overall, we demonstrate that there are species differences between tree shrews and rats in the density of PV, NOS, CR, and TH immunoreactivity in the striatum. Additionally, we mapped for the first time the distribution of whole-brain input neurons projecting to the striatum of tree shrews with FG injected into the Cd, Pu, and Acb. The similarities and differences in their brain-wide input patterns may provide new insights into the diverse functions of the striatal subregions.

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

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

Antipsychotic-like profile of CIQ isomers in animal models of schizophrenia

Earlier, we have shown the efficacy of racemic (±) CIQ, a positive allosteric modulator of GluN2C/2D receptor against MK-801 induced impairment of prepulse inhibition as well as working memory. The present study investigated the antipsychotic-like profile of different CIQ (±, +, -) isomers against schizophrenia-like symptoms in series of behavioural animal models like apomorphine climbing, social isolation behaviour and NMDA receptor antagonist MK-801 induced cognitive deficits. Further, we also tested CIQ (±, +, -) isomers in neurodevelopmental model against MK-801induced deficits using open field test, Y-maze test and novel object recognition test. CIQ (±, +, -) isomers decreased climbing behaviour, increased social interaction and improved the MK-801 induced deficits in working memory in Y-maze. Further, CIQ (±, +) but not CIQ (-) improved the recognition memory in novel object recognition test as well as reduced hyperlocomotion and stereotyped behaviour. We conclude that CIQ (±, +) but not CIQ (-) exhibit the significant antipsychotic-like profile.

A Quantitative Comparison of Inhibitory Interneuron Size and Distribution between Mouse and Macaque V1, Using Calcium-Binding Proteins

The mouse is a useful and popular model for studying of visual cortical function. To facilitate the translation of results from mice to primates, it is important to establish the extent of cortical organization equivalence between species and to identify possible differences. We focused on the different types of interneurons as defined by calcium-binding protein (CBP) expression in the layers of primary visual cortex (V1) in mouse and rhesus macaque. CBPs parvalbumin (PV), calbindin (CB), and calretinin (CR) provide a standard, largely nonoverlapping, labeling scheme in macaque, with preserved corresponding morphologies in mouse, despite a slightly higher overlap. Other protein markers, which are relevant in mouse, are not preserved in macaque. We fluorescently tagged CBPs in V1 of both species, using antibodies raised against preserved aminoacid sequences. Our data demonstrate important similarities between the expression patterns of interneuron classes in the different layers between rodents and primates. However, in macaque, expression of PV and CB is more abundant, CR expression is lower, and the laminar distribution of interneuron populations is more differentiated. Our results reveal an integrated view of interneuron types that provides a basis for translating results from rodents to primates, and suggest a reconciliation of previous results.

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

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

Neurons Expressing Parvalbumin and Calretinin in the Human Amygdaloid Complex: A Quantitative and Qualitative Analysis in Every Nucleus and Nuclear Subdivision

The primate amygdaloid complex (AC) contains projection neurons as well as subsets of interneurons (IN), many of which express calcium-binding proteins, that through their local circuits control the activity of the projection neurons. The inhibitory parvalbumin (PV) and calretinin (CR)-positive (+) AC IN have a crucial role in the appearance of synchronized oscillations in local ensembles of projection neurons that mediate the consolidation and recall of fear memories. The GABAergic transmission of these subsets of IN is modulated by dopamine. To expand the knowledge regarding the cellular composition and distribution of IN in the human AC, we focused on two non-overlapping populations: the PV+ and CR+. We have analyzed the distribution of these IN throughout the AC from subjects without any neurological or psychiatric disorders and estimated their absolute number and density using stereological methods. We have also provided percentages of the IN with respect to the total AC neurons. The CR + IN were distributed throughout the AC, whereas the PV+ were only present in the basolateral nuclear group. The quantity of CR + IN was four times higher than that of PV+ and the percentages varied from less than 1% for PV + IN to 6-20% for CR+. The differences in quantity and distribution of CR+ and PV + IN could be related to their differential inhibitory properties and to the intrinsic and extrinsic connections of every amygdaloid region.

Evidence for Decreased Density of Calretinin-Immunopositive Neurons in the Caudate Nucleus in Patients With Schizophrenia

Schizophrenia (SCH) and autism spectrum disorder (ASD) share several common aetiological and symptomatic features suggesting they may be included in a common spectrum. For example, recent results suggest that excitatory/inhibitory imbalance is relevant in the etiology of SCH and ASD. Numerous studies have investigated this imbalance in regions like the ventromedial and dorsolateral prefrontal cortex (DLPFC). However, relatively little is known about neuroanatomical changes that could reduce inhibition in subcortical structures, such as the caudate nucleus (CN), in neuropsychiatric disorders. We recently showed a significant decrease in calretinin-immunopositive (CR-ip) interneuronal density in the CN of patients with ASD without significant change in the density of neuropeptide Y-immunopositive (NPY-ip) neurons. These subtypes together constitute more than 50% of caudate interneurons and are likely necessary for maintaining excitatory/inhibitory balance. Consequently, and since SCH and ASD share characteristic features, here we tested the hypothesis, that the density of CR-ip neurons in the CN is decreased in patients with SCH. We used immunohistochemistry and qPCR for CR and NPY in six patients with schizophrenia and six control subjects. As expected, small, medium and large CR-ip interneurons were detected in the CN. We found a 38% decrease in the density of all CR-ip interneurons (P < 0.01) that was driven by the loss of the small CR-ip interneurons (P < 0.01) in patients with SCH. The densities of the large CR-ip and of the NPY-ip interneurons were not significantly altered. The lower density detected could have been due to inflammation-induced degeneration. However, the state of microglial activation assessed by quantification of ionized calcium-binding adapter molecule 1 (Iba1)- and transmembrane protein 119 (TMEM119)-immunopositive cells showed no significant difference between patients with SCH and controls. Our results warrant further studies focussing on the role of CR-ip neurons and on the striatum being a possible hub for information selection and regulation of associative cortical fields whose function have been altered in SCH.

Anatomy and Connectivity of the Subthalamic Nucleus in Humans and Non-human Primates

The Subthalamic Nucleus (STh) is an oval-shaped diencephalic structure located ventrally to the thalamus, playing a fundamental role in the circuitry of the basal ganglia. In addition to being involved in the pathophysiology of several neurodegenerative disorders, such as Huntington’s and Parkinson’s disease, the STh is one of the target nuclei for deep brain stimulation. However, most of the anatomical evidence available derives from non-human primate studies. In this review, we will present the topographical and morphological organization of the nucleus and its connections to structurally and functionally related regions of the basal ganglia circuitry. We will also highlight the importance of additional research in humans focused on validating STh connectivity, cytoarchitectural organization, and its functional subdivision.

Decreased density of cholinergic interneurons in striatal territories in Williams syndrome

Williams syndrome (WS) is a rare neurodevelopmental disorder caused by the hemideletion of approximately 25-28 genes at 7q11.23. Its unusual social and cognitive phenotype is most strikingly characterized by the disinhibition of social behavior, in addition to reduced global IQ, with a relative sparing of language ability. Hypersociality and increased social approach behavior in WS may represent a unique inability to inhibit responses to specific social stimuli, which is likely associated with abnormalities of frontostriatal circuitry. The striatum is characterized by a diversity of interneuron subtypes, including inhibitory parvalbumin-positive interneurons (PV+) and excitatory cholinergic interneurons (Ch+). Animal model research has identified an important role for these specialized cells in regulating social approach behavior. Previous research in humans identified a depletion of interneuron subtypes associated with neuropsychiatric disorders. Here, we examined the density of PV+ and Ch+ interneurons in the striatum of 13 WS and neurotypical (NT) subjects. We found a significant reduction in the density of Ch+ interneurons in the medial caudate nucleus and nucleus accumbens, important regions receiving cortical afferents from the orbitofrontal and ventromedial prefrontal cortex, and circuitry involved in language and reward systems. No significant difference in the distribution of PV+ interneurons was found. The pattern of decreased Ch+ interneuron densities in WS differs from patterns of interneuron depletion found in other disorders.

Structural organization, GABAergic and tyrosine hydroxylase expression in the striatum and globus pallidus of the South American plains vizcacha, Lagostomus maximus (Rodentia, Caviomorpha)

The striatum is an essential component of the basal ganglia that regulatessensory processing, motor, cognition, and behavior. Depending on the species, the striatum shows a unique structure called caudate-putamen as in mice, or its separation into two regions called caudate and lenticular nuclei, the latter formed by putamen and globus pallidus areas, as in primates. These structures have two compartments, striosome and matrix. We investigated the structural organization, GABAergic and tyrosine hydroxylase (TH) expression in the striatum and globus pallidus of the South American plains vizcacha, Lagostomus maximus. Its striatum showed regionalization arising from the presence of an internal capsule, and a similar organization to a striosome-matrix compartmentalization. GABAergic neurons in the matrix of caudate exhibited parvalbumin, calretinin, calbindin, GAD65, and NADPH-d-immunoreactivity. These were also expressed in cells of the putamen with the exception of calretinin showing neurofibers localization. Globus pallidus showed parvalbumin- and GAD65-immunoreactive cells, and calretinin- and calbindin-immunoreactive neuropil, plus GABA-A-immunoreactive neurofibers. NADPH-d-, GAD65- and GABA-A-immunoreactive neurons were larger than parvalbumin-, calretinin-, and calbindin-immunoreactive cells, whereas calbindin-immunoreactive cells were the most abundant. In addition, TH-immunoreactive neuropil was observed in the matrix of the striatum. A significant larger TH-immunoreactive area and neuron number was found in females compared to males. The presence of an internal capsule suggests an adaptive advantage concerning motor and cognitive abilities favoring reaction time in response to predators. In an anatomy-evolutive perspective, the striatum of vizcacha seems to be closer to that of humans than to that of laboratory traditional models such as mouse.

Striatal Interneurons in Transgenic Nonhuman Primate Model of Huntington's Disease

Huntington's disease is an autosomal dominant neurodegenerative disorder associated with progressive motor and cognitive impairments, and the expansion of a cysteine-adenine-guanine trinucleotide (polyglutamine) repeats in exon one of the human huntingtin gene. The pathology of the disease is characterized by a profound degeneration of striatal GABAergic projection neurons with relative sparing of interneurons accompanied with astrogliosis. Here, we describe the striatal pathology in two genotypically different transgenic HD monkeys that exhibit degrees of disease progression that resembled those seen in juvenile- (rHD1) and adult-onset (rHD7) HD. The caudate nucleus and putamen underwent severe neuronal loss in both animals, while the striatal volume was reduced only in rHD1, the most severely affected monkey. The number of GABAergic (calretinin- and parvalbumin-positive) and cholinergic interneurons was also reduced in most striatal regions of these two monkeys, but to variable degrees. Overall, the density of interneurons was increased in rHD1, but not in rHD7, suggesting a relative sparing of interneurons over projection neurons in the striatum of the most affected HD monkey. The neuropil of both the caudate nucleus and putamen was invaded with reactive astrocytes in rHD1, while astrogliosis was much less severe in rHD7 and absent from control. Combined with behavioral data collected from these monkeys, our findings further demonstrate that transgenic HD monkeys share similar disease patterns with HD patients, making them a highly reliable preclinical HD animal model.

Human Pluripotent Stem Cell-Derived Striatal Interneurons: Differentiation and Maturation In Vitro and in the Rat Brain

Striatal interneurons are born in the medial and caudal ganglionic eminences (MGE and CGE) and play an important role in human striatal function and dysfunction in Huntington's disease and dystonia. MGE/CGE-like neural progenitors have been generated from human pluripotent stem cells (hPSCs) for studying cortical interneuron development and cell therapy for epilepsy and other neurodevelopmental disorders. Here, we report the capacity of hPSC-derived MGE/CGE-like progenitors to differentiate into functional striatal interneurons. In vitro, these hPSC neuronal derivatives expressed cortical and striatal interneuron markers at the mRNA and protein level and displayed maturing electrophysiological properties. Following transplantation into neonatal rat striatum, progenitors differentiated into striatal interneuron subtypes and were consistently found in the nearby septum and hippocampus. These findings highlight the potential for hPSC-derived striatal interneurons as an invaluable tool in modeling striatal development and function in vitro or as a source of cells for regenerative medicine.

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hPSCs differentiate into cortical and striatal interneuron-like cells in vitro

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They present mature electrophysiological and morphological properties in vitro

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They express striatal interneuron subtype markers upon transplantation in rat brain

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hPSC-interneuron-like cells adopt region-specific morphologies in vivo

Brain volumetric alterations accompanied with loss of striatal medium-sized spiny neurons and cortical parvalbumin expressing interneurons in Brd1+/- mice

Schizophrenia is a common and severe mental disorder arising from complex gene-environment interactions affecting brain development and functioning. While a consensus on the neuroanatomical correlates of schizophrenia is emerging, much of its fundamental pathobiology remains unknown. In this study, we explore brain morphometry in mice with genetic susceptibility and phenotypic relevance to schizophrenia (Brd1^+/− mice) using postmortem 3D MR imaging coupled with histology, immunostaining and regional mRNA marker analysis. In agreement with recent large-scale schizophrenia neuroimaging studies, Brd1^+/− mice displayed subcortical abnormalities, including volumetric reductions of amygdala and striatum. Interestingly, we demonstrate that structural alteration in striatum correlates with a general loss of striatal neurons, differentially impacting subpopulations of medium-sized spiny neurons and thus potentially striatal output. Akin to parvalbumin interneuron dysfunction in patients, a decline in parvalbumin expression was noted in the developing cortex of Brd1^+/− mice, mainly driven by neuronal loss within or near cortical layer V, which is rich in corticostriatal projection neurons. Collectively, our study highlights the translational value of the Brd1^+/− mouse as a pre-clinical tool for schizophrenia research and provides novel insight into its developmental, structural, and cellular pathology.

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.

Baseline striatal and nigral interneuronal protein levels in two distinct mice strains differ in accordance with their MPTP susceptibility

Epidemiological studies reveal an ethnicity-based bias in prevalence of Parkinson's disease (PD), deriving from the differences that exist between Caucasians and African or Asian populations. Experimental mice models provide a scope to analyse the cellular mechanisms of differential susceptibility to PD. C57BL/6J mice, for instance, are more susceptible to MPTP-induced Parkinsonism whereas CD-1 mice are resistant. In PD-pathogenesis, interneuronal contribution is also likely, although they comprise only 5-10% of the striatal cells. The interneurons harbour calcium binding proteins, like calretinin (Cal-R) and parvalbumin (PV), which are crucial in Ca²⁺ homeostasis for preventing calcium-induced excitotoxicity. GAD-67-immunoreactive interneurons are the other prominent set of GABAergic interneurons. In PD, dopamine loss up-regulates GAD-67 expression in striatal projection neurons and other basal ganglia circuit. We studied the possible contribution of interneurons in determining variable susceptibility by assessing the expression of calretinin, PV and GAD-67 in both striatum and substantia nigra pars compacta (SNpc) in two distinct mice strains, i.e. C57BL/6J and CD-1 under normal conditions, using unbiased stereology for quantification of immunoreactive cells and immunoblotting. The vulnerable C57BL/6J had lesser basal parvalbumin expression in both nigra and striatum whereas the calretinin levels were low only in the striatum. GAD-67 expression showed no perceptible differences in the striatum or SNpc of either of the strains. Differential expression of calcium buffering/binding proteins under normal physiological condition proffers a role for interneurons in the differential susceptibility to PD. Thus, even the baseline susceptibility indices i.e. without using the neurotoxin; can provide vital mechanistic insights into PD pathogenesis.

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.

Atlas of the Striatum and Globus Pallidus in the Tree Shrew: Comparison with Rat and Mouse

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

Structural and molecular heterogeneity of calretinin-expressing interneurons in the rodent and primate striatum

Calretinin‐expressing (CR+) interneurons are the most common type of striatal interneuron in primates. However, because CR+ interneurons are relatively scarce in rodent striatum, little is known about their molecular and other properties, and they are typically excluded from models of striatal circuitry. Moreover, CR+ interneurons are often treated in models as a single homogenous population, despite previous descriptions of their heterogeneous structures and spatial distributions in rodents and primates. Here, we demonstrate that, in rodents, the combinatorial expression of secretagogin (Scgn), specificity protein 8 (SP8) and/or LIM homeobox protein 7 (Lhx7) separates striatal CR+ interneurons into three structurally and topographically distinct cell populations. The CR+/Scgn+/SP8+/Lhx7− interneurons are small‐sized (typically 7–11 µm in somatic diameter), possess tortuous, partially spiny dendrites, and are rostrally biased in their positioning within striatum. The CR+/Scgn−/SP8−/Lhx7− interneurons are medium‐sized (typically 12–15 µm), have bipolar dendrites, and are homogenously distributed throughout striatum. The CR+/Scgn−/SP8−/Lhx7+ interneurons are relatively large‐sized (typically 12–20 µm), and have thick, infrequently branching dendrites. Furthermore, we provide the first in vivo electrophysiological recordings of identified CR+ interneurons, all of which were the CR+/Scgn−/SP8−/Lhx7− cell type. In the primate striatum, Scgn co‐expression also identified a topographically distinct CR+ interneuron population with a rostral bias similar to that seen in both rats and mice. Taken together, these results suggest that striatal CR+ interneurons comprise at least three molecularly, structurally, and topographically distinct cell populations in rodents. These properties are partially conserved in primates, in which the relative abundance of CR+ interneurons suggests that they play a critical role in striatal microcircuits.

Calretinin interneuron density in the caudate nucleus is lower in autism spectrum disorder

Autism spectrum disorder is a debilitating condition with possible neurodevelopmental origins but unknown neuroanatomical correlates. Whereas investigators have paid much attention to the cerebral cortex, few studies have detailed the basal ganglia in autism. The caudate nucleus may be involved in the repetitive movements and limbic changes of autism. We used immunohistochemistry for calretinin and neuropeptide Y in 24 age- and gender-matched patients with autism spectrum disorder and control subjects ranging in age from 13 to 69 years. Patients with autism had a 35% lower density of calretinin+ interneurons in the caudate that was driven by loss of small calretinin+ neurons. This was not caused by altered size of the caudate, as its cross-sectional surface areas were similar between diagnostic groups. Controls exhibited an age-dependent increase in the density of medium and large calretinin+ neurons, whereas subjects with autism did not. Diagnostic groups did not differ regarding ionized calcium-binding adapter molecule 1+ immunoreactivity for microglia, suggesting chronic inflammation did not cause the decreased calretinin+ density. There was no statistically significant difference in the density of neuropeptide Y+ neurons between subjects with autism and controls. The decreased calretinin+ density may disrupt the excitation/inhibition balance in the caudate leading to dysfunctional corticostriatal circuits. The description of such changes in autism spectrum disorder may clarify pathomechanisms and thereby help identify targets for drug intervention and novel therapeutic strategies.

Progress in developing transgenic monkey model for Huntington's disease

Huntington's disease (HD) is a complex neurodegenerative disorder that has no cure. Although treatments can often be given to relieve symptoms, the neuropathology associated with HD cannot be stopped or reversed. HD is characterized by degeneration of the striatum and associated pathways that leads to impairment in motor and cognitive functions as well as psychiatric disturbances. Although cell and rodent models for HD exist, longitudinal study in a transgenic HD nonhuman primate (i.e., rhesus macaque; HD monkeys) shows high similarity in its progression with human patients. Progressive brain atrophy and changes in white matter integrity examined by magnetic resonance imaging are coherent with the decline in cognitive behaviors related to corticostriatal functions and neuropathology. HD monkeys also express higher anxiety and irritability/aggression similar to human HD patients that other model systems have not yet replicated. While a comparative model approach is critical for advancing our understanding of HD pathogenesis, HD monkeys could provide a unique platform for preclinical studies and long-term assessment of translatable outcome measures. This review summarizes the progress in the development of the transgenic HD monkey model and the opportunities for advancing HD preclinical research.

The calretinin interneurons of the striatum: comparisons between rodents and primates under normal and pathological conditions

This paper reviews the major organizational features of calretinin interneurons in the dorsal striatum of rodents and primates, with some insights on the state of these neurons in Parkinson's disease and Huntington's chorea. The rat striatum harbors medium-sized calretinin-immunoreactive (CR⁺) interneurons, whereas the mouse striatum is pervaded by medium-sized CR⁺ interneurons together with numerous small and highly immunoreactive CR⁺ cells. The CR interneuronal network is even more elaborated in monkey and human striatum where, in addition to the small- and medium-sized CR⁺ interneurons, a set of large CR⁺ interneurons occurs. The majority of these giant CR⁺ interneurons, which are unique to the primate striatum, also display immunoreactivity for choline acetyltransferase (ChAT), a faithful marker of cholinergic neurons. The expression of CR and/or ChAT by the large striatal interneurons appears to be seriously compromised in Parkinson's disease and Huntington's chorea. The species differences noted above have to be considered to better understand the role of CR interneurons in striatal organization in both normal and pathological conditions.

Striatal synaptic dysfunction and altered calcium regulation in Huntington disease

Synaptic dysfunction and altered calcium homeostasis in the brain is common to many neurodegenerative disorders. Among these, Huntington disease (HD), which is inherited in an autosomal dominant fashion, can serve as a model for investigating these mechanisms. HD generally manifests in middle age as a disorder of movement, mood and cognition. An expanded polymorphic CAG repeat in the HTT gene results in progressive neurodegeneration that impacts striatal spiny projection neurons (SPNs) earliest and most severely. Striatal SPNs receive massive glutamatergic input from cortex and thalamus, and these excitatory synapses are a focus for early changes that can trigger aberrant downstream signaling to disrupt synaptic plasticity and lead to later degeneration. Mitochondrial dysfunction and altered intracellular calcium-induced calcium release and sequestration mechanisms add to the impairments in circuit function that may underlie prodromal cognitive and subtle motor deficits. These mechanisms and implications for developing disease-modifying therapy will be reviewed here.

The number of striatal cholinergic interneurons expressing calretinin is increased in parkinsonian monkeys

The most abundant interneurons in the primate striatum are those expressing the calcium-binding protein calretinin (CR). The present immunohistochemical study provides detailed assessments of their morphological traits, number, and topographical distribution in normal monkeys (Macaca fascicularis) and in monkeys rendered parkinsonian (PD) by MPTP intoxication. In primates, the CR+ striatal interneurons comprise small (8-12μm), medium (12-20μm) and large-sized (20-45μm) neurons, each with distinctive morphologies. The small CR+ neurons were 2-3 times more abundant than the medium-sized CR+ neurons, which were 20-40 times more numerous than the large CR+ neurons. In normal and PD monkeys, the density of small and medium-sized CR+ neurons was twice as high in the caudate nucleus than in the putamen, whereas the inverse occurred for the large CR+ neurons. Double immunostaining experiments revealed that only the large-sized CR+ neurons expressed choline acetyltransferase (ChAT). The number of large CR+ neurons was found to increase markedly (4-12 times) along the entire anteroposterior extent of both the caudate nucleus and putamen of PD monkeys compared to controls. Comparison of the number of large CR-/ChAT+ and CR+/ChAT+ neurons together with experiments involving the use of bromo-deoxyuridine (BrdU) as a marker of newly generated cells showed that it is the expression of CR by the large ChAT+ striatal interneurons, and not their absolute number, that is increased in the dopamine-depleted striatum. These findings reveal the modulatory role of dopamine in the phenotypic expression of the large cholinergic striatal neurons, which are known to play a crucial role in PD pathophysiology.

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.

Cytoarchitecture of nitrergic neurons in the human striatum and subthalamic nucleus

BACKGROUND

Nitric oxide (NO) is a gaseous molecule that modulates several physiological processes, including signal transmission in the central nervous system. There is evidence supporting NO as a major neurotransmitter involved in motor and emotion/behavior control. We investigated the distribution and morphology of nitrergic neurons in the two main input structures of the basal ganglia of human brain: the striatum and subthalamic nucleus.

METHODS

We studied samples of striatum (caudate and putamen) and subthalamic nucleus of 20 human brains from subjects without neurological/psychiatric diseases. The tissues were stained by histochemistry for nicotinamide adenine dinucleotide phosphate diaphorase activity and by immunohistochemistry for neuronal NO synthase (nNOS). Subsequently, we analyzed the nitrergic neuronal profile and its morphometric parameters.

RESULTS

Our data corroborate that approximately 2% of neurons in striatum express nNOS and these exhibited morphology characteristic of interneurons. Posterior regions of the striatum have a higher nitrergic neuronal profile than anterior regions of this nucleus suggesting an anteroposterior gradient of nitrergic neurons. Posterior limbic-associated areas of the striatum have a higher nitrergic neuronal profile compared to other functional subdivisions. Also, approximately 90% of neurons in the subthalamic nucleus express nNOS.

CONCLUSIONS

A remarkable presence of nitrergic neurons in the human striatum and subthalamic nucleus suggests that NO may play a critical role in the physiology of these nuclei.

New neurons in the adult striatum: from rodents to humans

Most neurons are generated during development and are not replaced during adulthood, even if they are lost to injury or disease. However, it is firmly established that new neurons are generated in the dentate gyrus of the hippocampus of almost all adult mammals, including humans. Nevertheless, many questions remain regarding adult neurogenesis in other brain regions and particularly in humans, where standard birth-dating methods are not generally feasible. Exciting recent evidence indicates that calretinin-expressing interneurons are added to the adult human striatum at a substantial rate. The role of new neurons is unknown, but studies in rodents will be able to further elucidate their identity and origin and then we may begin to understand their regulation and function.

SOX2 reprograms resident astrocytes into neural progenitors in the adult brain

Glial cells can be in vivo reprogrammed into functional neurons in the adult CNS; however, the process by which this reprogramming occurs is unclear. Here, we show that a distinct cellular sequence is involved in SOX2-driven in situ conversion of adult astrocytes to neurons. This includes ASCL1⁺ neural progenitors and DCX⁺ adult neuroblasts (iANBs) as intermediates. Importantly, ASCL1 is required, but not sufficient, for the robust generation of iANBs in the adult striatum. These progenitor-derived iANBs predominantly give rise to calretinin⁺ interneurons when supplied with neurotrophic factors or the small-molecule valproic acid. Patch-clamp recordings from the induced neurons reveal subtype heterogeneity, though all are functionally mature, fire repetitive action potentials, and receive synaptic inputs. Together, these results show that SOX2-mediated in vivo reprogramming of astrocytes to neurons passes through proliferative intermediate progenitors, which may be exploited for regenerative medicine.

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SOX2 induces ASCL1-positive neural progenitors in the adult mouse brain

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Ascl1 in resident astrocytes is required for SOX2-mediated in vivo reprogramming

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Induced ASCL1-positive neural progenitors generate mature calretinin neurons

Cholinergic interneurons in the dorsal and ventral striatum: anatomical and functional considerations in normal and diseased conditions

Striatal cholinergic interneurons (ChIs) are central for the processing and reinforcement of reward-related behaviors that are negatively affected in states of altered dopamine transmission, such as in Parkinson's disease or drug addiction. Nevertheless, the development of therapeutic interventions directed at ChIs has been hampered by our limited knowledge of the diverse anatomical and functional characteristics of these neurons in the dorsal and ventral striatum, combined with the lack of pharmacological tools to modulate specific cholinergic receptor subtypes. This review highlights some of the key morphological, synaptic, and functional differences between ChIs of different striatal regions and across species. It also provides an overview of our current knowledge of the cellular localization and function of cholinergic receptor subtypes. The future use of high-resolution anatomical and functional tools to study the synaptic microcircuitry of brain networks, along with the development of specific cholinergic receptor drugs, should help further elucidate the role of striatal ChIs and permit efficient targeting of cholinergic systems in various brain disorders, including Parkinson's disease and addiction.

Striatal cholinergic dysfunction as a unifying theme in the pathophysiology of dystonia

Dystonia is a movement disorder of both genetic and non-genetic causes, which typically results in twisted posturing due to abnormal muscle contraction. Evidence from dystonia patients and animal models of dystonia indicate a crucial role for the striatal cholinergic system in the pathophysiology of dystonia. In this review, we focus on striatal circuitry and the centrality of the acetylcholine system in the function of the basal ganglia in the control of voluntary movement and ultimately clinical manifestation of movement disorders. We consider the impact of cholinergic interneurons (ChIs) on dopamine-acetylcholine interactions and examine new evidence for impairment of ChIs in dysfunction of the motor systems producing dystonic movements, particularly in animal models. We have observed paradoxical excitation of ChIs in the presence of dopamine D2 receptor agonists and impairment of striatal synaptic plasticity in a mouse model of DYT1 dystonia, which are improved by administration of recently developed M1 receptor antagonists. These findings have been confirmed across multiple animal models of DYT1 dystonia and may represent a common endophenotype by which to investigate dystonia induced by other types of genetic and non-genetic causes and to investigate the potential effectiveness of pharmacotherapeutics and other strategies to improve dystonia.

Distribution and morphological characteristics of striatal interneurons expressing calretinin in mice: a comparison with human and nonhuman primates

Striatal interneurons display a morphological and chemical heterogeneity that has been particularly well characterized in rats, monkeys and humans. By comparison much less is known of striatal interneurons in mice, although these animals are now widely used as transgenic models of various neurodegenerative diseases. The present immunohistochemical study aimed at characterizing striatal interneurons expressing calretinin (CR) in mice compared to those in squirrel monkeys and humans. The mouse striatum contains both small (9-12 μm) and medium-sized (15-20 μm) CR+ cells. The small cells are intensely stained with a single, slightly varicose and moderately arborized process. They occur throughout the striatum (77±9 cells/mm(3)), but prevail in the area of the subventricular zone and subcallosal streak, with statistically significant anteroposterior and dorsoventral decreasing gradients. The medium-sized cells are less intensely immunoreactive and possess 2-3 long, slightly varicose and poorly branched dendrites. They are rather uniformly scattered throughout the striatum and three times more numerous (224±31 cells/mm(3)) than the smaller CR+ cells. Double immunostaining experiments with choline acetyltransferase (ChAT) as a cholinergic marker in normal and Drd1a-tdTomato/Drd2-EGFP double transgenic mice reveal that none of the small or medium-sized CR+ cells express ChAT or D1 and D2 dopamine receptors. In contrast, the striatum in human and nonhuman primates harbors small and medium-sized CR+/ChAT- cells, as well as large CR+/ChAT+ interneurons that are absent in mice. Such a difference between rodents and primates must be taken into consideration if one hopes to better understand the striatal function in normal and pathological conditions.

MeCP2-mediated alterations of striatal features accompany psychomotor deficits in a mouse model of Rett syndrome

Rett Syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the methyl-CpG-binding protein 2 (MECP2) gene. Affected individuals develop motor deficits including stereotypic hand movements, impaired motor learning and difficulties with movement. To understand the neural mechanisms of motor deficits in RTT, we characterized the molecular and cellular phenotypes in the striatum, the major input nucleus of the basal ganglia that controls psychomotor function, in mice carrying a null allele of Mecp2. These mice showed significant hypoactivity associated with impaired motor coordination and motor skill learning. We found that dopamine content was significantly reduced in the striatum of Mecp2 null mice. Reduced dopamine was accompanied by down-regulation of tyrosine hydroxylase and up-regulation of dopamine D2 receptors, particularly in the rostral striatum. We also observed that loss of MeCP2 induced compartment-specific alterations in the striatum, including reduced expression of μ-opioid receptors in the striosomes and increased number of calbindin-positive neurons in the striatal matrix. The total number of parvalbumin-positive interneurons and their dendritic arborization were also significantly increased in the striatum of Mecp2 null mice. Together, our findings support that MeCP2 regulates a unique set of genes critical for modulating motor output of the striatum, and that aberrant structure and function of the striatum due to MeCP2 deficiency may underlie the motor deficits in RTT.

Anatomical and electrophysiological changes in striatal TH interneurons after loss of the nigrostriatal dopaminergic pathway

Using transgenic mice that express enhanced green fluorescent protein (EGFP) under the control of the tyrosine hydroxylase (TH) promoter, we have previously shown that there are approximately 3,000 striatal EGFP-TH interneurons per hemisphere in mice. Here, we report that striatal TH-EGFP interneurons exhibit a small, transient but significant increase in number after unilateral destruction of the nigrostriatal dopaminergic pathway. The increase in cell number is accompanied by electrophysiological and morphological changes. The intrinsic electrophysiological properties of EGFP-TH interneurons ipsilateral to 6-OHDA lesion were similar to those originally reported in intact mice except for a significant reduction in the duration of a characteristic depolarization induced plateau potential. There was a significant change in the distribution of the four previously described electrophysiologically distinct subtypes of striatal TH interneurons. There was a concomitant increase in the frequency of both spontaneous excitatory and inhibitory post-synaptic currents, while their amplitudes did not change. Nigrostriatal lesions did not affect somatic size or dendritic length or branching, but resulted in an increase in the density of proximal dendritic spines and spine-like appendages in EGFP-TH interneurons. The changes indicate that electrophysiology properties and morphology of striatal EGFP-TH interneurons depend on endogenous levels of dopamine arising from the nigrostriatal pathway. Furthermore, these changes may serve to help compensate for the changes in activity of spiny projection neurons that occur following loss of the nigrostriatal innervation in experimental or in early idiopathic Parkinson's disease by increasing feedforward GABAergic inhibition exerted by these interneurons.

GABAergic inputs from direct and indirect striatal projection neurons onto cholinergic interneurons in the primate putamen

Striatal cholinergic interneurons (ChIs) are involved in reward-dependent learning and the regulation of attention. The activity of these neurons is modulated by intrinsic and extrinsic γ-aminobutyric acid (GABA)ergic and glutamatergic afferents, but the source and relative prevalence of these diverse regulatory inputs remain to be characterized. To address this issue, we performed a quantitative ultrastructural analysis of the GABAergic and glutamatergic innervation of ChIs in the postcommissural putamen of rhesus monkeys. Postembedding immunogold localization of GABA combined with peroxidase immunostaining for choline acetyltransferase showed that 60% of all synaptic inputs to ChIs originate from GABAergic terminals, whereas 21% are from putatively glutamatergic terminals that establish asymmetric synapses, and 19% from other (non-GABAergic) sources of symmetric synapses. Double pre-embedding immunoelectron microscopy using substance P and Met-/Leu-enkephalin antibodies to label GABAergic terminals from collaterals of "direct" and "indirect" striatal projection neurons, respectively, revealed that 47% of the indirect pathway terminals and 36% of the direct pathway terminals target ChIs. Together, substance P- and enkephalin-positive terminals represent 24% of all synapses onto ChIs in the monkey putamen. These findings show that ChIs receive prominent GABAergic inputs from multiple origins, including a significant contingent from axon collaterals of direct and indirect pathway projection neurons.

Optogenetic identification of striatal projection neuron subtypes during in vivo recordings

Optogenetics has revolutionized neuroscience over the past several years by allowing researchers to modulate the activity of specific cell types, both in vitro and in vivo. One promising application of optogenetics is to use channelrhodopsin-2 (ChR2) mediated spiking to identify distinct cell types in electrophysiological recordings from awake behaving animals. In this paper, we apply this approach to in vivo recordings of the two major projection cell types in the striatum: the direct- and indirect-pathway medium spiny neurons. We expressed ChR2 in the neurons of the direct or indirect pathways using a cre-dependent viral strategy and performed electrical recordings together with optical stimulation using an implanted microwire array that included an integrated optical fiber. Despite the apparent simplicity of identifying ChR2-expressing neurons as those that respond to light, we encountered multiple potential confounds when applying this approach. Here, we describe and address these confounds and provide a Matlab tool so that others can implement our analysis methods. This article is part of a Special Issue entitled Optogenetics (7th BRES).

Striatal microcircuitry and movement disorders

The basal ganglia network serves to integrate information about context, actions, and outcomes to shape the behavior of an animal based on its past experience. Clinically, the basal ganglia receive the most attention for their role in movement disorders. Recent advances in technology have opened new avenues of research into the structure and function of basal ganglia circuits. One emerging theme is the importance of GABAergic interneurons in coordinating and regulating network function. Here, we discuss evidence that changes in striatal GABAergic microcircuits contribute to basal ganglia dysfunction in several movement disorders. Because interneurons are genetically and neurochemically unique from striatal projection neurons, they may provide promising therapeutic targets for the treatment of a variety of striatal-based disorders.