Thalamic input to parvalbumin-immunoreactive GABAergic interneurons: organization in normal striatum and effect of neonatal decortication

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.

Input-specific control of interneuron numbers in nascent striatal networks

The assembly of functional neuronal circuits requires appropriate numbers of distinct classes of neurons, but the mechanisms through which their relative proportions are established remain poorly defined. Investigating the mouse striatum, we found that the two most prominent subtypes of striatal interneurons, parvalbumin-expressing (PV+) GABAergic and cholinergic (ChAT+) interneurons, undergo extensive programmed cell death between the first and second postnatal weeks. Remarkably, the survival of PV+ and ChAT+ interneurons is regulated by distinct mechanisms mediated by their specific afferent connectivity. While long-range cortical inputs control PV+ interneuron survival, ChAT+ interneuron survival is regulated by local input from the medium spiny neurons. Our results identify input-specific circuit mechanisms that operate during the period of programmed cell death to establish the final number of interneurons in nascent striatal networks.

Dystonia and Cerebellum: From Bench to Bedside

Dystonia pathogenesis remains unclear; however, findings from basic and clinical research suggest the importance of the interaction between the basal ganglia and cerebellum. After the discovery of disynaptic pathways between the two, much attention has been paid to the cerebellum. Basic research using various dystonia rodent models and clinical studies in dystonia patients continues to provide new pieces of knowledge regarding the role of the cerebellum in dystonia genesis. Herein, we review basic and clinical articles related to dystonia focusing on the cerebellum, and clarify the current understanding of the role of the cerebellum in dystonia pathogenesis. Given the recent evidence providing new hypotheses regarding dystonia pathogenesis, we discuss how the current evidence answers the unsolved clinical questions.

Morphological Study of the Cortical and Thalamic Glutamatergic Synaptic Inputs of Striatal Parvalbumin Interneurons in Rats

Parvalbumin-immunoreactive (Parv+) interneurons is an important component of striatal GABAergic microcircuits, which receive excitatory inputs from the cortex and thalamus, and then target striatal projection neurons. The present study aimed to examine ultrastructural synaptic connection features of Parv+ neruons with cortical and thalamic input, and striatal projection neurons by using immuno-electron microscopy (immuno-EM) and immunofluorescence techniques. Our results showed that both Parv+ somas and dendrites received numerous asymmetric synaptic inputs, and Parv+ terminals formed symmetric synapses with Parv- somas, dendrites and spine bases. Most interestingly, spine bases targeted by Parv+ terminals simultaneously received excitatory inputs at their heads. Electrical stimulation of the motor cortex (M1) induced higher proportion of striatal Parv+ neurons express c-Jun than stimulation of the parafascicular nucleus (PFN), and indicated that cortical- and thalamic-inputs differentially modulate Parv+ neurons. Consistent with that, both Parv + soma and dendrites received more VGlut1+ than VGlut2+ terminals. However, the proportion of VGlut1+ terminal targeting onto Parv+ proximal and distal dendrites was not different, but VGlut2+ terminals tended to target Parv+ somas and proximal dendrites than distal dendrites. These functional and morphological results suggested excitatory cortical and thalamic glutamatergic inputs differently modulate Parv+ interneurons, which provided inhibition inputs onto striatal projection neurons. To maintain the balance between the cortex and thalamus onto Parv+ interneurons may be an important therapeutic target for neurological disorders.

Progression of basal ganglia pathology in heterozygous Q175 knock-in Huntington's disease mice

We used behavioral testing and morphological methods to detail the progression of basal ganglia neuron type-specific pathology and the deficits stemming from them in male heterozygous Q175 mice, compared to age-matched WT males. A rotarod deficit was not present in Q175 mice until 18 months, but increased open field turn rate (reflecting hyperkinesia) and open field anxiety were evident at 6 months. No loss of striatal neurons was seen out to 18 months, but ENK+ and DARPP32+ striatal perikarya were fewer by 6 months, due to diminished expression, with further decline by 18 months. No reduction in SP+ striatal perikarya or striatal interneurons was seen in Q175 mice at 18 months, but cholinergic interneurons showed dendrite attenuation by 6 months. Despite reduced ENK expression in indirect pathway striatal perikarya, ENK-immunostained terminals in globus pallidus externus (GPe) were more abundant at 6 months and remained so out to 18 months. Similarly, SP-immunostained terminals from striatal direct pathway neurons were more abundant in globus pallidus internus and substantia nigra at 6 months and remained so at 18 months. FoxP2+ arkypallidal GPe neurons and subthalamic nucleus neurons were lost by 18 months but not prototypical PARV+ GPe neurons or dopaminergic nigral neurons. Our results show that striatal projection neuron abnormalities and behavioral abnormalities reflecting them develop between 2 and 6 months of age in Q175 male heterozygotes, indicating early effects of the HD mutation. The striatal pathologies resemble those in human HD, but are less severe at 18 months than even in premanifest 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.

•

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

•

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

•

LTSIs are primarily excited by contralateral M1

•

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

Intrinsic electrophysiological properties predict variability in morphology and connectivity among striatal Parvalbumin-expressing Pthlh-cells

Determining the cellular content of the nervous system in terms of cell types and the rules of their connectivity represents a fundamental challenge to the neurosciences. The recent advent of high-throughput techniques, such as single-cell RNA-sequencing has allowed for greater resolution in the identification of cell types and/or states. Although most of the current neuronal classification schemes comprise discrete clusters, several recent studies have suggested that, perhaps especially, within the striatum, neuronal populations exist in continua, with regards to both their molecular and electrophysiological properties. Whether these continua are stable properties, established during development, or if they reflect acute differences in activity-dependent regulation of critical genes is currently unknown. We set out to determine whether gradient-like molecular differences in the recently described Pthlh-expressing inhibitory interneuron population, which contains the Pvalb-expressing cells, correlate with differences in morphological and connectivity properties. We show that morphology and long-range inputs correlate with a spatially organized molecular and electrophysiological gradient of Pthlh-interneurons, suggesting that the processing of different types of information (by distinct anatomical striatal regions) has different computational requirements.

Thalamostriatal degeneration contributes to dystonia and cholinergic interneuron dysfunction in a mouse model of Huntington's disease

Huntington’s disease (HD) is an autosomal dominant trinucleotide repeat disorder characterized by choreiform movements, dystonia and striatal neuronal loss. Amongst multiple cellular processes, abnormal neurotransmitter signalling and decreased trophic support from glutamatergic cortical afferents are major mechanisms underlying striatal degeneration. Recent work suggests that the thalamostriatal (TS) system, another major source of glutamatergic input, is abnormal in HD although its phenotypical significance is unknown. We hypothesized that TS dysfunction plays an important role in generating motor symptoms and contributes to degeneration of striatal neuronal subtypes. Our results using the R6/2 mouse model of HD indicate that neurons of the parafascicular nucleus (PF), the main source of TS afferents, degenerate at an early stage. PF lesions performed prior to motor dysfunction or striatal degeneration result in an accelerated dystonic phenotype and are associated with premature loss of cholinergic interneurons. The progressive loss of striatal medium spiny neurons and parvalbumin-positive interneurons observed in R6/2 mice is unaltered by PF lesions. Early striatal cholinergic ablation using a mitochondrial immunotoxin provides evidence for increased cholinergic vulnerability to cellular energy failure in R6/2 mice, and worsens the dystonic phenotype. The TS system therefore contributes to trophic support of striatal interneuron subtypes in the presence of neurodegenerative stress, and TS deafferentation may be a novel cell non-autonomous mechanism contributing to the pathogenesis of HD. Furthermore, behavioural experiments demonstrate that the TS system and striatal cholinergic interneurons are key motor-network structures involved in the pathogenesis of dystonia. This work suggests that treatments aimed at rescuing the TS system may preserve important elements of striatal structure and function and provide symptomatic relief in HD.

Cortical and thalamic inputs exert cell type-specific feedforward inhibition on striatal GABAergic interneurons

The classical view of striatal GABAergic interneuron function has been that they operate as largely independent, parallel, feedforward inhibitory elements providing inhibitory inputs to spiny projection neurons (SPNs). Much recent evidence has shown that the extrinsic innervation of striatal interneurons is not indiscriminate but rather very specific, and that striatal interneurons are themselves interconnected in a cell type-specific manner. This suggests that the ultimate effect of extrinsic inputs on striatal neuronal activity depends critically on synaptic interactions within interneuronal circuitry. Here, we compared the cortical and thalamic input to two recently described subtypes of striatal GABAergic interneurons, tyrosine hydroxylase-expressing interneurons (THINs), and spontaneously active bursty interneurons (SABIs) using transgenic TH-Cre and Htr3a-Cre mice of both sexes. Our results show that both THINs and SABIs receive strong excitatory input from the motor cortex and the thalamic parafascicular nucleus. Cortical optogenetic stimulation also evokes disynaptic inhibitory GABAergic responses in THINs but not in SABIs. In contrast, optogenetic stimulation of the parafascicular nucleus induces disynaptic inhibitory responses in both interneuron populations. However, the short-term plasticity of these disynaptic inhibitory responses is different suggesting the involvement of different intrastriatal microcircuits. Altogether, our results point to highly specific interneuronal circuits that are selectively engaged by different excitatory inputs.

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.

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.

Hippocampal-Evoked Feedforward Inhibition in the Nucleus Accumbens

The nucleus accumbens (NAc) is critical for motivated behavior and is rewired following exposure to drugs of abuse. Medium spiny neurons (MSNs) in the NAc express either D1 or D2 receptors and project to distinct downstream targets. Differential activation of these MSNs depends on both excitation from long-range inputs and inhibition via the local circuit. Assessing how long-range excitatory inputs engage inhibitory circuitry is therefore important for understanding NAc function. Here, we use slice electrophysiology and optogenetics to study ventral hippocampal (vHPC)-evoked feedforward inhibition in the NAc of male and female mice. We find that vHPC-evoked excitation is stronger at D1+ than D1- MSNs, whereas inhibition is unbiased at the two cell types. vHPC inputs contact both parvalbumin-positive (PV+) and somatostatin-positive (SOM+) interneurons, but PV+ cells are preferentially activated. Moreover, suppressing PV+ interneurons indicates they are primarily responsible for vHPC-evoked inhibition. Finally, repeated cocaine exposure alters the excitation of D1+ and D1- MSNs, without concomitant changes to inhibition, shifting the excitation/inhibition balance. Together, our results highlight the contributions of multiple interneuron populations to feedforward inhibition in the NAc. Moreover, they demonstrate that inhibition provides a stable backdrop on which drug-evoked changes to excitation occur within this circuit.SIGNIFICANCE STATEMENT Given the importance of the nucleus accumbens (NAc) in reward learning and drug-seeking behaviors, it is critical to understand what controls the activity of cells in this region. While excitatory inputs to projection neurons in the NAc have been identified, it is unclear how the local inhibitory network becomes engaged. Here, we identify a sparse population of interneurons responsible for feedforward inhibition evoked by ventral hippocampal input and characterize their connections within the NAc. We also demonstrate that the balance of excitation and inhibition that projection neurons experience is altered by exposure to cocaine. Together, this work provides insight into the fundamental circuitry of this region as well as the effects of drugs of abuse.

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.

Quantitative Analyses of the Projection of Individual Neurons from the Midline Thalamic Nuclei to the Striosome and Matrix Compartments of the Rat Striatum

A fundamental organizing principle of the striatum is the striosome/matrix system that is defined by inputs/outputs and neurochemical markers. The thalamostriatal projection is highly heterogeneous originating in many subnuclei of the thalamus including the midline (ML) and intralaminar (IL) nuclei. We examined the dendritic morphology and axonal trajectory of 15 ML and 11 IL neurons by single-neuron labeling with viral vectors in combination with mu-opioid receptor immunostaining in rat brains. Dendritic and axonal morphology defined ML neurons as type II cells consisting of at least two subclasses according to the presence or absence of striatal axon collaterals. In the striatum, ML neurons preferentially innervated striosomes, whereas parafascicular neurons preferentially innervated the matrix. Almost all single thalamostriatal neurons favoring striosome or matrix compartments also innervated the cerebral cortical areas that supplied cortical input to the same striatal compartment. We thus revealed that thalamostriatal projections are highly organized 1) by the similarity in morphological characteristics and 2) their preference for the striatal compartments and cortical areas. These findings demonstrate that the functional properties of striatal compartments are influenced by both their cortical and thalamic afferents presumably with a different time latency and support selective dynamics for the striosome and matrix compartments.

Parvalbumin-producing striatal interneurons receive excitatory inputs onto proximal dendrites from the motor thalamus in male mice

In rodents, the dorsolateral striatum regulates voluntary movement by integrating excitatory inputs from the motor-related cerebral cortex and thalamus to produce contingent inhibitory output to other basal ganglia nuclei. Striatal parvalbumin (PV)-producing interneurons receiving this excitatory input then inhibit medium spiny neurons (MSNs) and modify their outputs. To understand basal ganglia function in motor control, it is important to reveal the precise synaptic organization of motor-related cortical and thalamic inputs to striatal PV interneurons. To examine which domains of the PV neurons receive these excitatory inputs, we used male bacterial artificial chromosome transgenic mice expressing somatodendritic membrane-targeted green fluorescent protein in PV neurons. An anterograde tracing study with the adeno-associated virus vector combined with immunodetection of pre- and postsynaptic markers visualized the distribution of the excitatory appositions on PV dendrites. Statistical analysis revealed that the density of thalamostriatal appositions along the dendrites was significantly higher on the proximal than distal dendrites. In contrast, there was no positional preference in the density of appositions from axons of the dorsofrontal cortex. Population observations of thalamostriatal and corticostriatal appositions by immunohistochemistry for pathway-specific vesicular glutamate transporters confirmed that thalamic inputs preferentially, and cortical ones less preferentially, made apposition on proximal dendrites of PV neurons. This axodendritic organization suggests that PV neurons produce fast and reliable inhibition of MSNs in response to thalamic inputs and process excitatory inputs from motor cortices locally and plastically, possibly together with other GABAergic and dopaminergic dendritic inputs, to modulate MSN inhibition.

Functional comparison of corticostriatal and thalamostriatal postsynaptic responses in striatal neurons of the mouse

Synaptic inputs from cortex and thalamus were compared in electrophysiologically defined striatal cell classes: direct and indirect pathways' striatal projection neurons (dSPNs and iSPNs), fast-spiking interneurons (FS), cholinergic interneurons (ChINs), and low-threshold spiking-like (LTS-like) interneurons. Our purpose was to observe whether stimulus from cortex or thalamus had equivalent synaptic strength to evoke prolonged suprathreshold synaptic responses in these neuron classes. Subthreshold responses showed that inputs from either source functionally mix up in their dendrites at similar electrotonic distances from their somata. Passive and active properties of striatal neuron classes were consistent with the previous studies. Cre-dependent adeno-associated viruses containing Td-Tomato or eYFP fluorescent proteins were used to identify target cells. Transfections with ChR2-eYFP driven by the promoters CamKII or EF1.DIO in intralaminar thalamic nuclei using Vglut-2-Cre mice, or CAMKII in the motor cortex were used to stimulate cortical or thalamic afferents optogenetically. Both field stimuli in the cortex or photostimulation of ChR2-YFP cortical fibers evoked similar prolonged suprathreshold responses in SPNs. Photostimulation of ChR2-YFP thalamic afferents also evoked suprathreshold responses. Differences previously described between responses of dSPNs and iSPNs were observed in both cases. Prolonged suprathreshold responses could also be evoked from both sources onto all other neuron classes studied. However, to evoke thalamostriatal suprathreshold responses, afferents from more than one thalamic nucleus had to be stimulated. In conclusion, both thalamus and cortex are capable to generate suprathreshold responses converging on diverse striatal cell classes. Postsynaptic properties appear to shape these responses.

Neostriatal GABAergic Interneurons Mediate Cholinergic Inhibition of Spiny Projection Neurons

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

SIGNIFICANCE STATEMENT

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

Differential processing of thalamic information via distinct striatal interneuron circuits

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

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

Optogenetic stimulation reveals distinct modulatory properties of thalamostriatal vs corticostriatal glutamatergic inputs to fast-spiking interneurons

Parvalbumin-containing fast-spiking interneurons (FSIs) exert a powerful feed-forward GABAergic inhibition on striatal medium spiny neurons (MSNs), playing a critical role in timing striatal output. However, how glutamatergic inputs modulate their firing activity is still unexplored. Here, by means of a combined optogenetic and electrophysiological approach, we provide evidence for a differential modulation of cortico- vs thalamo-striatal synaptic inputs to FSIs in transgenic mice carrying light-gated ion channels channelrhodopsin-2 (ChR2) in glutamatergic fibers. Corticostriatal synapses show a postsynaptic facilitation, whereas thalamostriatal synapses present a postsynaptic depression. Moreover, thalamostriatal synapses exhibit more prominent AMPA-mediated currents than corticostriatal synapses, and an increased release probability. Furthermore, during current-evoked firing activity, simultaneous corticostriatal stimulation increases bursting activity. Conversely, thalamostriatal fiber activation shifts the canonical burst-pause activity to a more prolonged, regular firing pattern. However, this change in firing pattern was accompanied by a significant rise in the frequency of membrane potential oscillations. Notably, the responses to thalamic stimulation were fully abolished by blocking metabotropic glutamate 1 (mGlu1) receptor subtype, whereas both acetylcholine and dopamine receptor antagonists were ineffective. Our findings demonstrate that cortical and thalamic glutamatergic input differently modulate FSIs firing activity through specific intrinsic and synaptic properties, exerting a powerful influence on striatal outputs.

Characteristics of fast-spiking neurons in the striatum of behaving monkeys

Inhibitory interneurons are the fundamental constituents of neural circuits that organize network outputs. The striatum as part of the basal ganglia is involved in reward-directed behaviors. However, the role of the inhibitory interneurons in this process remains unclear, especially in behaving monkeys. We recorded the striatal single neuron activity while monkeys performed reward-directed hand or eye movements. Presumed parvalbumin-containing GABAergic interneurons (fast-spiking neurons, FSNs) were identified based on narrow spike shapes in three independent experiments, though they were a small population (4.2%, 42/997). We found that FSNs are characterized by high-frequency and less-bursty discharges, which are distinct from the basic firing properties of the presumed projection neurons (phasically active neurons, PANs). Besides, the encoded information regarding actions and outcomes was similar between FSNs and PANs in terms of proportion of neurons, but the discharge selectivity was higher in PANs than that of FSNs. The coding of actions and outcomes in FSNs and PANs was consistently observed under various behavioral contexts in distinct parts of the striatum (caudate nucleus, putamen, and anterior striatum). Our results suggest that FSNs may enhance the discharge selectivity of postsynaptic output neurons (PANs) in encoding crucial variables for a reward-directed behavior.

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.

The glycoprotein Ten-m3 mediates topography and patterning of thalamostriatal projections from the parafascicular nucleus in mice

The striatum is the key input nucleus of the basal ganglia, and is implicated in motor control and learning. Despite the importance of striatal circuits, the mechanisms associated with their development are not well established. Previously, Ten-m3, a member of the Ten-m/teneurin/odz family of transmembrane glycoproteins, was found to be important in the mapping of binocular visual pathways. Here, we investigated a potential role for Ten-m3 in striatal circuit formation. In situ hybridisation revealed a patchy distribution of Ten-m3 mRNA expression superimposed on a high-dorsal to low-ventral gradient in a subregion of the striatal matrix. A survey of afferent/efferent structures associated with the matrix identified the parafascicular thalamic nucleus (PF) as a potential locus of action. Ten-m3 was also found to be expressed in a high-dorsal to low-ventral gradient in the PF, corresponding topographically to its expression in the striatum. Further, a subset of thalamic terminal clusters overlapped with Ten-m3-positive domains within the striatal matrix. Studies in wild-type (WT) and Ten-m3 knockout (KO) mice revealed no differences in overall striatal or PF structure. Thalamostriatal terminals in KOs, however, while still confined to the matrix subregion, lost their clustered appearance. Topography was also altered, with terminals from the lateral PF projecting ectopically to ventral and medial striatum, rather than remaining confined dorsolaterally as in WTs. Behaviorally, Ten-m3 KOs displayed delayed motor skill acquisition. This study demonstrates that Ten-m3 plays a key role in directing the formation of thalamostriatal circuitry, the first molecular candidate reported to regulate connectivity within this pathway.

Striatal firing rate reflects head movement velocity

Although the basal ganglia have long been implicated in the initiation of actions, their contribution to movement remains a matter of dispute. Using wireless multi-electrode recording and motion tracking, we examined the relationship between single-unit activity in the sensorimotor striatum and movement kinematics. We recorded single-unit activity from medium spiny projection neurons and fast-spiking interneurons while monitoring the movements of mice using motion tracking. In Experiment 1, we trained mice to generate movements reliably by water-depriving them and giving them periodic cued sucrose rewards. We found high correlations between single-unit activity and movement velocity in particular directions. This correlation was found in both putative medium spiny projection neurons and fast-spiking interneurons. In Experiment 2, to rule out the possibility that the observed correlations were due to reward expectancy, we repeated the same procedure but added trials in which sucrose delivery was replaced by an aversive air puff stimulus. The air puff generated avoidance movements that were clearly different from movements on rewarded trials, but the same neurons that showed velocity correlation on reward trials exhibited a similar correlation on air puff trials. These experiments show for the first time that the firing rate of striatal neurons reflects movement velocity for different types of movements, whether to seek rewards or to avoid harm.

The thalamostriatal system in normal and diseased states

Because of our limited knowledge of the functional role of the thalamostriatal system, this massive network is often ignored in models of the pathophysiology of brain disorders of basal ganglia origin, such as Parkinson's disease (PD). However, over the past decade, significant advances have led to a deeper understanding of the anatomical, electrophysiological, behavioral and pathological aspects of the thalamostriatal system. The cloning of the vesicular glutamate transporters 1 and 2 (vGluT1 and vGluT2) has provided powerful tools to differentiate thalamostriatal from corticostriatal glutamatergic terminals, allowing us to carry out comparative studies of the synaptology and plasticity of these two systems in normal and pathological conditions. Findings from these studies have led to the recognition of two thalamostriatal systems, based on their differential origin from the caudal intralaminar nuclear group, the center median/parafascicular (CM/Pf) complex, or other thalamic nuclei. The recent use of optogenetic methods supports this model of the organization of the thalamostriatal systems, showing differences in functionality and glutamate receptor localization at thalamostriatal synapses from Pf and other thalamic nuclei. At the functional level, evidence largely gathered from thalamic recordings in awake monkeys strongly suggests that the thalamostriatal system from the CM/Pf is involved in regulating alertness and switching behaviors. Importantly, there is evidence that the caudal intralaminar nuclei and their axonal projections to the striatum partly degenerate in PD and that CM/Pf deep brain stimulation (DBS) may be therapeutically useful in several movement disorders.

Confocal laser scanning microscopy and ultrastructural study of VGLUT2 thalamic input to striatal projection neurons in rats

We examined thalamic input to striatum in rats using immunolabeling for the vesicular glutamate transporter (VGLUT2). Double immunofluorescence viewed with confocal laser scanning microscopy (CLSM) revealed that VGLUT2+ terminals are distinct from VGLUT1+ terminals. CLSM of Phaseolus vulgaris-leucoagglutinin (PHAL)-labeled cortical or thalamic terminals revealed that VGLUT2 is rare in corticostriatal terminals but nearly always present in thalamostriatal terminals. Electron microscopy revealed that VGLUT2+ terminals made up 39.4% of excitatory terminals in striatum (with VGLUT1+ corticostriatal terminals constituting the rest), and 66.8% of VGLUT2+ terminals synapsed on spines and the remainder on dendrites. VGLUT2+ axospinous terminals had a mean diameter of 0.624 μm, while VGLUT2+ axodendritic terminals a mean diameter of 0.698 μm. In tissue in which we simultaneously immunolabeled thalamostriatal terminals for VGLUT2 and striatal neurons for D1 (with about half of spines immunolabeled for D1), 54.6% of VGLUT2+ terminals targeted D1+ spines (i.e., direct pathway striatal neurons), and 37.3% of D1+ spines received VGLUT2+ synaptic contacts. By contrast, 45.4% of VGLUT2+ terminals targeted D1-negative spines (i.e., indirect pathway striatal neurons), and only 25.8% of D1-negative spines received VGLUT2+ synaptic contacts. Similarly, among VGLUT2+ axodendritic synaptic terminals, 59.1% contacted D1+ dendrites, and 40.9% contacted D1-negative dendrites. VGLUT2+ terminals on D1+ spines and dendrites tended to be slightly smaller than those on D1-negative spines and dendrites. Thus, thalamostriatal terminals contact both direct and indirect pathway striatal neurons, with a slight preference for direct. These results are consistent with physiological studies indicating slightly different effects of thalamic input on the two types of striatal projection neurons.

Striatal parvalbuminergic neurons are lost in Huntington's disease: implications for dystonia

Although dystonia represents a major source of motor disability in Huntington's disease (HD), its pathophysiology remains unknown. Because recent animal studies indicate that loss of parvalbuminergic (PARV+) striatal interneurons can cause dystonia, we investigated if loss of PARV+ striatal interneurons occurs during human HD progression, and thus might contribute to dystonia in HD. We used immunolabeling to detect PARV+ interneurons in fixed sections, and corrected for disease-related striatal atrophy by expressing PARV+ interneuron counts in ratio to interneurons co-containing somatostatin and neuropeptide Y (whose numbers are unaffected in HD). At all symptomatic HD grades, PARV+ interneurons were reduced to less than 26% of normal abundance in rostral caudate. In putamen rostral to the level of globus pallidus, loss of PARV+ interneurons was more gradual, not dropping off to less than 20% of control until grade 2. Loss of PARV+ interneurons was even more gradual in motor putamen at globus pallidus levels, with no loss at grade 1, and steady grade-wise decline thereafter. A large decrease in striatal PARV+ interneurons, thus, occurs in HD with advancing disease grade, with regional variation in the loss per grade. Given the findings of animal studies and the grade-wise loss of PARV+ striatal interneurons in motor striatum in parallel with the grade-wise appearance and worsening of dystonia, our results raise the possibility that loss of PARV+ striatal interneurons is a contributor to dystonia in HD.

Convergence of cortical and thalamic input to direct and indirect pathway medium spiny neurons in the striatum

The major afferent innervation of the basal ganglia is derived from the cortex and the thalamus. These excitatory inputs mainly target the striatum where they innervate the principal type of striatal neuron, the medium-sized spiny neurons (MSNs), and are critical in the expression of basal ganglia function. The aim of this work was to test directly whether corticostriatal and thalamostriatal terminals make convergent synaptic contact with individual direct and indirect pathway MSNs. Individual MSNs were recorded in vivo and labelled by the juxtacellular method in the striatum of BAC transgenic mice in which green fluorescent protein reports the expression of dopamine D1 or D2 receptors. After recovery of the neurons, the tissue was immunolabelled for vesicular glutamate transporters type 1 and 2, as markers of cortical and thalamic terminals, respectively. Three of each class of MSNs were reconstructed in 3D and second-order dendrites selected for electron microscopic analysis. Our findings show that direct and indirect pathway MSNs, located in the matrix compartment of the striatum, receive convergent input from cortex and thalamus preferentially on their spines. There were no differences in the pattern of innervation of direct and indirect pathway MSNs, but the cortical input is more prominent in both and synaptic density is greater for direct pathway neurons. The 3D reconstructions revealed no morphological differences between direct and indirect MSNs. Overall, our findings demonstrate that direct and indirect pathway MSNs located in the matrix receive convergent cortical and thalamic input and suggest that both cortical and thalamic inputs are involved in the activation of MSNs.

Heterogeneous properties of central lateral and parafascicular thalamic synapses in the striatum

To understand the principles of operation of the striatum it is critical to elucidate the properties of the main excitatory inputs from cortex and thalamus, as well as their ability to activate the main neurons of the striatum, the medium spiny neurons (MSNs). As the thalamostriatal projection is heterogeneous, we set out to isolate and study the thalamic afferent inputs to MSNs using small localized injections of adeno-associated virus carrying fusion genes for channelrhodopsin-2 and YFP, in either the rostral or caudal regions of the intralaminar thalamic nuclei (i.e. the central lateral or parafascicular nucleus). This enabled optical activation of specific thalamic afferents combined with whole-cell, patch-clamp recordings of MSNs and electrical stimulation of cortical afferents, in adult mice. We found that thalamostriatal synapses differ significantly in their peak amplitude responses, short-term dynamics and expression of ionotropic glutamate receptor subtypes. Our results suggest that central lateral synapses are most efficient in driving MSNs to depolarization, particularly those of the direct pathway, as they exhibit large amplitude responses, short-term facilitation and predominantly express postsynaptic AMPA receptors. In contrast, parafascicular synapses exhibit small amplitude responses, short-term depression and predominantly express postsynaptic NMDA receptors, suggesting a modulatory role, e.g. facilitating Ca(2+)-dependent processes. Indeed, pairing parafascicular, but not central lateral, presynaptic stimulation with action potentials in MSNs, leads to NMDA receptor- and Ca(2+)-dependent long-term depression at these synapses. We conclude that the main excitatory thalamostriatal afferents differ in many of their characteristics and suggest that they each contribute differentially to striatal information processing.

Functional neuroanatomy of the basal ganglia

The "basal ganglia" refers to a group of subcortical nuclei responsible primarily for motor control, as well as other roles such as motor learning, executive functions and behaviors, and emotions. Proposed more than two decades ago, the classical basal ganglia model shows how information flows through the basal ganglia back to the cortex through two pathways with opposing effects for the proper execution of movement. Although much of the model has remained, the model has been modified and amplified with the emergence of new data. Furthermore, parallel circuits subserve the other functions of the basal ganglia engaging associative and limbic territories. Disruption of the basal ganglia network forms the basis for several movement disorders. This article provides a comprehensive account of basal ganglia functional anatomy and chemistry and the major pathophysiological changes underlying disorders of movement. We try to answer three key questions related to the basal ganglia, as follows: What are the basal ganglia? What are they made of? How do they work? Some insight on the canonical basal ganglia model is provided, together with a selection of paradoxes and some views over the horizon in the field.

Plastic changes in striatal fast-spiking interneurons following hemicerebellectomy and environmental enrichment

Recent findings suggest marked interconnections between the cerebellum and striatum, thus challenging the classical view of their segregated operation in motor control. Therefore, this study was aimed at further investigating this issue by analyzing the effects of hemicerebellectomy (HCb) on density and dendritic length of striatal fast-spiking interneurons (FSi). First, we analyzed the plastic rearrangements of striatal FSi morphology in hemicerebellectomized animals reared in standard conditions. Then, since environmental enrichment (EE) induces structural changes in experimental models of brain disease, we evaluated FSi morphology in lesioned animals exposed to an enriched environment after HCb. Although HCb did not affect FSi density, it progressively shrank dendritic branching of striatal FSi of both sides. These plastic changes, already evident 15 days after the cerebellar ablation, became very marked 30 days after the lesion. Such a relevant effect was completely abolished by postoperative enrichment. EE not only counteracted shrinkage of FSi dendritic arborization but also provoked a progressive increase in dendritic length which surpassed that of the controls as the enrichment period lengthened. These data confirm that the cerebellum and striatum are more interconnected than previously retained. Furthermore, cerebellar damage likely evokes a striatal response through cortical mediation. The EE probably modifies HCb-induced plastic changes in the striatum by increasing the efficiency of the cortical circuitry. This is the first study describing the morphological rearrangement of striatal FSi following a cerebellar lesion; it provides the basis for further studies aimed at investigating the mechanisms underlying cerebello-striatal "talking."

The primate thalamostriatal systems: Anatomical organization, functional roles and possible involvement in Parkinson's disease

The striatum receives glutamatergic inputs from two main thalamostriatal systems that originate either from the centre median/parafascicular complex (CM/PF-striatal system) or the rostral intralaminar, midline, associative and relay thalamic nuclei (non-CM/PF-striatal system). These dual thalamostriatal systems display striking differences in their anatomical and, most likely, functional organization. The CM/PF-striatal system is topographically organized, and integrated within functionally segregated basal ganglia-thalamostriatal circuits that process sensorimotor, associative and limbic information. CM/PF neurons are highly responsive to attention-related sensory stimuli, suggesting that the CM/PF-striatal system, through its strong connections with cholinergic interneurons, may play a role in basal ganglia-mediated learning, behavioral switching and reinforcement. In light of evidence for prominent CM/PF neuronal loss in Parkinson's disease, we propose that the significant CM-striatal system degeneration, combined with the severe nigrostriatal dopamine loss in sensorimotor striatal regions, may alter normal automatic actions, and shift the processing of basal ganglia-thalamocortical motor programs towards goal-directed behaviors.

Corticostriatal projection neurons - dichotomous types and dichotomous functions

The mammalian striatum receives its main excitatory input from the two types of cortical pyramidal neurons of layer 5 of the cerebral cortex - those with only intratelencephalic connections (IT-type) and those sending their main axon to the brainstem via the pyramidal tract (PT-type). These two neurons types are present in layer 5 of all cortical regions, and thus they appear to project together to all parts of striatum. These two neuron types, however, differ genetically, morphologically, and functionally, with IT-type neurons conveying sensory and motor planning information to striatum and PT-type neurons conveying an efference copy of motor commands (for motor cortex at least). Anatomical and physiological data for rats, and more recent data for primates, indicate that these two cortical neuron types also differ in their targeting of the two main types of striatal projection neurons, with the IT-type input preferentially innervating direct pathway neurons and the PT-type input preferentially innervating indirect pathway striatal neurons. These findings have implications for understanding how the direct and indirect pathways carry out their respective roles in movement facilitation and movement suppression, and they have implications for understanding the role of corticostriatal synaptic plasticity in adaptive motor control by the basal ganglia.

The search for a role of the caudal intralaminar nuclei in the pathophysiology of Parkinson's disease

The situation of the caudal intralaminar thalamic nuclei within basal ganglia circuits has gained increased attention over the past few years. Although initially considered as a "non-specific" thalamic nuclei, tract-tracing studies carried out over the past two decades have demonstrated that the centromedian-parafascicular thalamic complex (CM-Pf) is connected to virtually all basal ganglia components and related nuclei. Although the anatomical basis sustaining the thalamic modulation of basal ganglia circuits has long been characterized, the functional significance of these transverse circuits still remain to be properly accommodated within the basal ganglia model, both under normal conditions as well as in situations of dopaminergic depletion. However, the recent demonstration of primary (e.g., non-dopamine related) neurodegenerative phenomena restricted to the CM-Pf in Parkinson's disease (PD) has renewed interest in the role played by the caudal intralaminar nuclei in the pathophysiology of PD. Concomitantly, evidence has become available of increased metabolic activity in the caudal intralaminar nuclei in rodent models of PD. Finally, CM-Pf neurosurgery in patients suffering from PD has produced contrasting outcomes, indicating that a consensus is still to be reached regarding the potential usefulness of targeting the caudal intralaminar nuclei to treat movement disorders of basal ganglia origin.

The primate centromedian-parafascicular complex: anatomical organization with a note on neuromodulation

In addition to the cerebral cortex, the striatum receives excitatory input from the thalamus. The centromedian (centre median, CM) and parafascicular (Pf) nuclei are an important source of thalamostriatal projections. Anterograde tract-tracing indicates the CM-Pf complex provides dense afferents to the matrix compartment of the striatum. Whereas CM projects to the entire sensorimotor territory of the striatum, the Pf provides complementary input to the entire associative sector. The Pf also provides lighter input to the nucleus accumbens. Both CM and Pf provide light to moderately dense inputs to other components of the basal ganglia in a largely complementary manner, covering motor or associative-limbic territories of the subthalamic nucleus, globus pallidus and ventral midbrain. In turn, the CM and Pf receive mainly segregated input from parallel motor and associative-limbic circuits of the basal ganglia. The CM and Pf may therefore be considered important participants in parallel processing of motor and associative-limbic information in the basal ganglia. Connections of the CM and Pf with other thalamic nuclei suggest they also participate in integrative functions within the thalamus. In addition, inputs from the brainstem reticular core, reciprocal connections with the cerebral cortex and reticular thalamic nucleus suggest a role in state-dependant information processing. Consideration of the differential connections of the CM and Pf, and better understanding of their role in pathophysiology, may eventually lead to development of an important new target for relief of a variety of neurological and psychiatric disorders.

The thalamostriatal systems: anatomical and functional organization in normal and parkinsonian states

Although we have gained significant knowledge in the anatomy and microcircuitry of the thalamostriatal system over the last decades, the exact function(s) of these complex networks remain(s) poorly understood. It is now clear that the thalamostriatal system is not a unique entity, but consists of multiple neural systems that originate from a wide variety of thalamic nuclei and terminate in functionally segregated striatal territories. The primary source of thalamostriatal projections is the caudal intralaminar nuclear group which, in primates, comprises the centromedian and parafascicular nuclei (CM/Pf). These two nuclei provide massive, functionally organized glutamatergic inputs to the whole striatal complex. There are several anatomical and physiological features that distinguish this system from other thalamostriatal projections. Although all glutamatergic thalamostriatal neurons express vGluT2 and release glutamate as neurotransmitter, CM/Pf neurons target preferentially the dendritic shafts of striatal projection neurons, whereas all other thalamic inputs are almost exclusively confined to the head of dendritic spines. This anatomic arrangement suggests that transmission of input from sources other than CM/Pf to the striatal neurons is likely regulated by dopaminergic afferents in the same manner as cortical inputs, while the CM/Pf axo-dendritic synapses do not display any particular relationships with dopaminergic terminals. A better understanding of the role of these systems in the functional circuitry of the basal ganglia relies on future research of the physiology and pathophysiology of these networks in normal and pathological basal ganglia conditions. Although much remains to be known about the role of these systems, recent electrophysiological studies from awake monkeys have provided convincing evidence that the CM/Pf-striatal system is the entrance for attention-related stimuli to the basal ganglia circuits. However, the processing and transmission of this information likely involves intrinsic GABAergic and cholinergic striatal networks, thereby setting the stage for complex physiological responses of striatal output neurons to CM/Pf activation. Finally, another exciting development that will surely generate significant interest towards the thalamostriatal systems in years to come is the possibility that CM/Pf may be a potential surgical target for movement disorders, most particularly Tourette syndrome and Parkinson's disease. Although the available clinical evidence is encouraging, these procedures remain empirical at this stage because of the limited understanding of the thalamostriatal systems.

A dopamine-acetylcholine cascade: simulating learned and lesion-induced behavior of striatal cholinergic interneurons

The giant cholinergic interneurons of the striatum are tonically active neurons (TANs) that respond with pauses to appetitive and aversive cues and to novel events. Whereas tonic activity emerges from intrinsic properties of these neurons, glutamatergic inputs from intralaminar thalamic nuclei and dopaminergic inputs from midbrain are required for genesis of pause responses. No prior computational models encompass both intrinsic and synaptically gated dynamics. We present a mathematical model that robustly accounts for behavior-related electrophysiological properties of TANs in terms of their intrinsic physiological properties and known afferents. In the model, balanced intrinsic hyperpolarizing and depolarizing currents engender tonic firing and glutamatergic inputs from thalamus (and cortex) both directly excite and indirectly inhibit TANs. If this inhibition, probably mediated by GABAergic nitric oxide synthase interneurons, exceeds a threshold, a persistent K+ conductance current amplifies its effect to generate a prolonged pause. Dopamine (DA) signals modulate both the intrinsic mechanisms and the external inputs of TANs. Simulations revealed that many learning-dependent behaviors of TANs, including acquired pauses to task-relevant cues, are explicable without recourse to learning-dependent changes in synapses onto TANs, due to a tight coupling between DA bursts and TAN pauses. These interactions imply that reward-predicting cues often cause striatal projection neurons to receive a cascade of signals: an adaptively scaled DA burst, a brief acetylcholine (ACh) burst, and an ACh pause. A sensitivity analysis revealed a unique TAN response surface, which shows that DA inputs robustly cooperate with thalamic inputs to control cue-dependent pauses of ACh release, which strongly affects performance- and learning-related dynamics in the striatum.

Lesion of the centromedian thalamic nucleus in MPTP-treated monkeys

The caudal intralaminar nuclei are a major source of glutamatergic afferents to the basal ganglia. Experiments in the 6-hydroxydopamine rat model have shown that the parafascicular nucleus is overactive and its lesion alleviates basal ganglia neurochemical abnormalities associated with dopamine depletion. Accordingly, removal of this excitatory innervation of the basal ganglia could have a beneficial value in the parkinsonian state. To test this hypothesis, unilateral kainate-induced chemical ablation of the centromedian thalamic nucleus (CM) has been performed in MPTP-treated monkeys. Successful lesions restricted to the CM boundaries (n = 2) without spreading over other neighboring thalamic nuclei showed an initial, short-lasting, and mild change in the parkinsonian motor scale but no effect against levodopa-induced dyskinesias. The lack of significant and persistent motor improvement leads us to conclude that unilateral selective lesion of the CM alone cannot be considered as a suitable surgical approach for the treatment of PD or levodopa-induced dyskinesias. The role of the caudal intralaminar nuclei in the pathophysiology of movement disorders of basal ganglia origin remains to be clarified.

High-resolution neuroanatomical tract-tracing for the analysis of striatal microcircuits

Although currently available retrograde tracers are useful tools for identifying striatal projection neurons, transported tracers often remained restricted within the neuronal somata and the thickest, main dendrites. Indeed, thin dendrites located far away from the cell soma as well as post-synaptic elements such as dendritic spines cannot be labeled unless performing intracellular injections. In this regard, the subsequent use of anterograde tracers for the labeling of striatal afferents often failed to unequivocally elucidate whether a given afferent makes true contacts with striatal projections neurons. Here we show that such a technical constraint can now be circumvented by retrograde tracing using rabies virus (RV). Immunofluorescence detection with a monoclonal antibody directed against the viral phosphoprotein resulted in a consistent Golgi-like labeling of striatal projection neurons, allowing clear visualization of small-size elements such as thin dendrites as well as dendritic spines. The combination of this retrograde tracing together with dual anterograde tracing of cortical and thalamic afferents has proven to be a useful tool for ascertaining striatal microcircuits. Indeed, by taking advantage of the trans-synaptic spread of RV, different subpopulations of local-circuit neurons modulating striatal efferent neurons can also be identified. At the striatal level, structures displaying labeling were visualized under the confocal laser-scanning microscope at high resolution. Once acquired, confocal stacks of images were firstly deconvoluted and then processed through 3D-volume rendering in order to unequivocally identify true contacts between pre-synaptic elements (axon terminals from cortical or thalamic sources) and post-synaptic elements (projection neurons and/or interneurons labeled with RV).

Cholinergic neurons of the adult rat striatum are immunoreactive for glutamatergic N-methyl-d-aspartate 2D but not N-methyl-d-aspartate 2C receptor subunits

Cholinergic neurons of the striatum play a crucial role in controlling output from this region. Their firing is under the control of a relatively limited glutamatergic input, deriving principally from the thalamus. Glutamate transmission is effected via three major subtypes of receptors, including those with affinity for N-methyl-d-aspartate (NMDA) and the properties of individual receptors reflect their precise subunit composition. We examined the distribution of NMDA2C and NMDA2D subunits in the rat striatum using immunocytochemistry and show that a population of large neurons is strongly immunoreactive for NMDA2D subunits. From their morphology and ultrastructure, these neurons were presumed to be cholinergic and this was confirmed with double immunofluorescence. We also show that NMDA2C is present in a small number of septal and olfactory cortical neurons but absent from the striatum. Receptors that include NMDA2D subunits are relatively insensitive to magnesium ion block making neurons more likely to fire at more negative membrane potentials. Their localization to cholinergic neurons may enable very precise regulation of firing of these neurons by relatively small glutamatergic inputs.

Differential synaptology of vGluT2-containing thalamostriatal afferents between the patch and matrix compartments in rats

The striatum is divided into two compartments named the patch (or striosome) and the matrix. Although these two compartments can be differentiated by their neurochemical content or afferent and efferent projections, the synaptology of inputs to these striatal regions remains poorly characterized. By using the vesicular glutamate transporters vGluT1 and vGluT2, as markers of corticostriatal and thalamostriatal projections, respectively, we demonstrate a differential pattern of synaptic connections of these two pathways between the patch and the matrix compartments. We also demonstrate that the majority of vGluT2-immunolabeled axon terminals form axospinous synapses, suggesting that thalamic afferents, like corticostriatal inputs, terminate preferentially onto spines in the striatum. Within both compartments, more than 90% of vGluT1-containing terminals formed axospinous synapses, whereas 87% of vGluT2-positive terminals within the patch innervated dendritic spines, but only 55% did so in the matrix. To characterize further the source of thalamic inputs that could account for the increase in axodendritic synapses in the matrix, we undertook an electron microscopic analysis of the synaptology of thalamostriatal afferents to the matrix compartments from specific intralaminar, midline, relay, and associative thalamic nuclei in rats. Approximately 95% of PHA-L-labeled terminals from the central lateral, midline, mediodorsal, lateral dorsal, anteroventral, and ventral anterior/ventral lateral nuclei formed axospinous synapses, a pattern reminiscent of corticostriatal afferents but strikingly different from thalamostriatal projections arising from the parafascicular nucleus (PF), which terminated onto dendritic shafts. These findings provide the first evidence for a differential pattern of synaptic organization of thalamostriatal glutamatergic inputs to the patch and matrix compartments. Furthermore, they demonstrate that the PF is the sole source of significant axodendritic thalamic inputs to striatal projection neurons. These observations pave the way for understanding differential regulatory mechanisms of striatal outflow from the patch and matrix compartments by thalamostriatal afferents.

Synaptic microcircuitry of tyrosine hydroxylase-containing neurons and terminals in the striatum of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated monkeys

A population of tyrosine hydroxylase (TH)-containing neurons that is up-regulated after lesion of the nigrostriatal dopaminergic pathway has been described in the primate striatum. The goal of this study was to examine the morphology, synaptology, and chemical phenotype of these neurons and TH-immunoreactive (-ir) terminals in the striatum of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated rhesus monkeys. TH-ir perikarya were small (10-12 microm), displayed nuclear invaginations, and received very few synaptic inputs. On the other hand, TH-containing dendrites were typically large in diameter (>1.0 microm) and received scarce synaptic innervation from putative excitatory and inhibitory terminals forming asymmetric and symmetric synapses, respectively. More than 70% of TH-positive intrastriatal cell bodies were found in the caudate nucleus and the precommissural putamen, considered as the associative functional territories of the primate striatum. Under 10% of these cells displayed calretinin immunoreactivity. TH-ir terminals rarely formed clear synaptic contacts, except for a few that established asymmetric axodendritic synapses. Almost two-thirds of TH-containing boutons displayed gamma-aminobutyric acid (GABA) immunoreactivity in the striatum of parkinsonian monkeys, whereas under 5% did so in the normal striatum. These findings provide strong support for the existence of a population of putative catecholaminergic interneurons in the associative territory of the striatum in parkinsonian monkeys. Their sparse synaptic innervation raises interesting issues regarding synaptic and nonsynaptic mechanisms involved in the regulation and integration of these neurons in the striatal microcircuitry. Finally, the coexpression of GABA in TH-positive terminals in the striatum of dopamine-depleted monkeys suggests dramatic neurochemical changes in the catecholaminergic modulation of striatal activity in Parkinson's disease.

Postnatal maturational properties of rat parafascicular thalamic neurons recorded in vitro

Thalamic relay neurons have homogeneous, adult-like firing properties and similar morphology by 12 days postnatally (PN 12). Parafascicular (Pf) neurons have a different morphology compared with typical thalamic relay neurons, but the development of their electrophysiological properties is not well studied. Intracellular recordings in PN 12-50 Pf neurons revealed several heterogeneous firing patterns different from those in thalamic relay neurons. Two types of cells were identified: Type I cells displayed a fast afterhyperpolarization (AHP) followed by a large-amplitude, slow AHP; whereas Type II cells had only a fast AHP. These cell types had overlapping membrane properties but differences in excitability. Some properties of Pf neurons were adult-like by PN 12, but, unlike thalamic relay neurons, there were significant maturational changes thereafter, including decreased action potential (AP) duration, increased fast AHP amplitude and increased excitability. Pf neurons did not exhibit rhythmic bursting and generally lacked low-threshold spike (LTS) responses that characterize thalamic relay neurons. Pf neurons exhibited nonlinear I-V relationships, and only a third of the cells expressed the time and voltage-dependent hyperpolarization activated (Ih) current, which declined with age. These results indicate that the morphological differences between Pf neurons and typical thalamic relay neurons are paralleled by electrophysiological differences, and that Pf membrane properties change during postnatal development.

Basal ganglia neural responses during behaviorally effective deep brain stimulation of the subthalamic nucleus in rats performing a treadmill locomotion test

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) is an effective treatment for Parkinson's disease (PD). In spite of proven therapeutic success, the mechanism underlying the benefits of DBS has not been resolved. A multiple-channel single-unit recording technique was used in the present study to investigate basal ganglia (BG) neural responses during behaviorally effective DBS of the STN in a rat model of PD. Rats underwent unilateral dopamine (DA) depletion by injection of 6-hydroxyDA (6-OHDA) into one side of the medial forebrain bundle and subsequently developed a partial akinesia, which was assessed during the treadmill locomotion task. High frequency stimulation (HFS) of the STN restored normal treadmill locomotion behavior. Simultaneous recording of single unit activity in the striatum (STR), globus pallidus (GP), substantia nigra pars reticulata (SNr), and STN revealed a variety of neural responses during behaviorally effective HFS of the STN. Predominant inhibitory responses appeared in the STN stimulation site. Nearly equal numbers of excitatory and inhibitory responses were found in the GP and SNr, whereas more rebound excitatory responses were found in the STR. Mean firing rate did not change significantly in the STR, GP, and SNr, but significantly decreased in both sides of STN during DBS. A decrease in firing rate in the contralateral side of STN provides neural substrate for the clinical observation that unilateral DBS produces bilateral benefits in patients with PD. In addition to the firing rate changes, a decrease in burst firing was observed in the GP and STN. The present study indicates that DBS induces complex modulations of the BG circuit and further suggests that BG network reorganization, rather than a simple excitation or inhibition, may underlie the therapeutic effects of DBS in patients with PD.

GABA(B) receptors at glutamatergic synapses in the rat striatum

Although multiple effects of GABA(B) receptor activation on synaptic transmission in the striatum have been described, the precise locations of the receptors mediating these effects have not been determined. To address this issue, we carried out pre-embedding immunogold electron microscopy in the rat using antibodies against the GABA(B) receptor subunits, GABA(B1) and GABA(B2). In addition, to investigate the relationship between GABA(B) receptors and glutamatergic striatal afferents, we used antibodies against the vesicular glutamate transporters, vesicular glutamate transporter 1 and vesicular glutamate transporter 2, as markers for glutamatergic terminals. Immunolabeling for GABA(B1) and GABA(B2) was widely and similarly distributed in the striatum, with immunogold particles localized at both presynaptic and postsynaptic sites. The most commonly labeled structures were dendritic shafts and spines, as well as terminals forming asymmetric and symmetric synapses. In postsynaptic structures, the majority of labeling associated with the plasma membrane was localized at extrasynaptic sites, although immunogold particles were also found at the postsynaptic specialization of some symmetric, putative GABAergic synapses. Labeling in axon terminals was located within, or at the edge of, the presynaptic active zone, as well as at extrasynaptic sites. Double labeling for GABA(B) receptor subunits and vesicular glutamate transporters revealed that labeling for both GABA(B1) and GABA(B2) was localized on glutamatergic axon terminals that expressed either vesicular glutamate transporter 1 or vesicular glutamate transporter 2. The patterns of innervation of striatal neurons by the vesicular glutamate transporter 1- and vesicular glutamate transporter 2-positive terminals suggest that they are selective markers of corticostriatal and thalamostriatal afferents, respectively. These results thus provide evidence that presynaptic GABA(B) heteroreceptors are in a position to modulate the two major excitatory inputs to striatal spiny projection neurons arising in the cortex and thalamus. In addition, presynaptic GABA(B) autoreceptors are present on the terminals of spiny projection neurons and/or striatal GABAergic interneurons. Furthermore, the data indicate that GABA may also affect the excitability of striatal neurons via postsynaptic GABA(B) receptors.

Thalamic innervation of the direct and indirect basal ganglia pathways in the rat: Ipsi- and contralateral projections

The present study describes the thalamic innervation coming from the rat parafascicular nucleus (PF) onto striatal and subthalamic efferent neurons projecting either to the globus pallidus (GP) or to the substantia nigra pars reticulata (SNr) by using a protocol for multiple neuroanatomical tracing. Both striatofugal neurons targeting the ipsilateral SNr (direct pathway) as well as striatal efferent neurons projecting to the ipsilateral GP (indirect pathway) were located within the terminal fields of the thalamostriatal afferents. In the subthalamic nucleus (STN), both neurons projecting to ipsilateral GP as well as neurons projecting to ipsilateral SNr also appear to receive thalamic afferents. Although the projections linking the caudal intralaminar nuclei with the ipsilateral striatum and STN are far more prominent, we also noticed that thalamic axons could gain access to the contralateral STN. Furthermore, a small number of STN neurons were seen to project to both the contralateral GP and PF nuclei. These ipsi- and contralateral projections enable the caudal intralaminar nuclei to modulate the activity of both the direct and the indirect pathway.

Thalamic regulation of striatal acetylcholine efflux is both direct and indirect and qualitatively altered in the dopamine-depleted striatum

Striatal cholinergic interneurons play a pivotal role in the integrative sensorimotor functions of the basal ganglia. The major excitatory input to these interneurons arises from glutamatergic neurons of the parafascicular nucleus of the thalamus (Pf). Thalamic regulation of cholinergic interneurons, however, may also include an indirect inhibitory component mediated by the axon collaterals of GABAergic medium spiny neurons that are also innervated by Pf. The present study examined thalamic regulation of striatal cholinergic interneurons by employing dual probe in vivo microdialysis in freely moving animals to determine the effect of pharmacological manipulation of Pf on acetylcholine (ACh) efflux in intact and dopamine-lesioned striata. In intact animals, reverse dialysis application of the GABA(A) antagonist bicuculline (50 microM) into Pf, likely disinhibiting Pf neurons, significantly decreased striatal ACh efflux. When striatal GABA(A) receptors were blocked by simultaneous reverse dialysis application of bicuculline (10 microM), however, the same manipulation significantly increased ACh efflux. Qualitatively similar results were obtained in experiments employing a higher concentration of bicuculline (200 microM). Application of the GABA agonist muscimol (500 microM) into Pf, likely inhibiting Pf neurons, decreased ACh efflux only when the experiment was conducted under blockade of striatal GABA(A) receptors. These data are consistent with the existence of an indirect, inhibitory, GABA(A) receptor-mediated component of ACh regulation that is most clearly manifested when Pf is disinhibited and with the existence of a direct excitatory component of ACh regulation, evident when Pf is inhibited. Manipulation of Pf using very high concentrations of drug (500 microM bicuculline, 2 mM muscimol), however, yielded data consistent only with direct excitatory thalamic regulation. In contrast to results obtained in intact animals, in animals with prior (3 weeks) unilateral lesion of the dopaminergic nigrostriatal pathway, bicuculline application (50 muM) in Pf significantly increased striatal ACh efflux, irrespective of simultaneous blockade of striatal GABA(A) receptors. The results of experiments in which muscimol (500 microM) was applied in Pf were similar to those obtained in intact animals, however. Baseline ACh efflux was not significantly elevated in dopamine-lesioned animals. These results indicate a qualitative alteration in the effectiveness of an inhibitory component of the thalamic regulation of ACh efflux in the dopamine depleted striatum, evident during increased thalamostriatal input. Such altered regulation of striatal ACh output is likely to have profound consequences for integrative function in the parkinsonian basal ganglia.

The thalamostriatal system: a highly specific network of the basal ganglia circuitry

Although the existence of thalamostriatal projections has long been known, the role(s) of this system in the basal ganglia circuitry remains poorly characterized. The intralaminar and ventral motor nuclei are the main sources of thalamic inputs to the striatum. This review emphasizes the high degree of anatomical and functional specificity of basal ganglia-thalamostriatal projections and discusses various aspects of the synaptic connectivity and neurochemical features that differentiate this glutamate system from the corticostriatal network. It also discusses the importance of thalamostriatal projections from the caudal intralaminar nuclei in the process of attentional orientation. A major task of future studies is to characterize the role(s) of corticostriatal and thalamostriatal pathways in regulating basal ganglia activity in normal and pathological conditions.