Dinesh V. Raju

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.

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 Parkinsons disease. Although the available clinical evidence is encouraging, these procedures remain empirical at this stage because of the limited understanding of the thalamostriatal systems.

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. J. Comp. Neurol. 499:231–243, 2006.

Differential synaptic plasticity of the corticostriatal and thalamostriatal systems in an MPTP‐treated monkey model of parkinsonism

Two cardinal features of Parkinsons disease (PD) pathophysiology are a loss of glutamatergic synapses paradoxically accompanied by an increased glutamatergic transmission to the striatum. The exact substrate of this increased glutamatergic drive remains unclear. The striatum receives glutamatergic inputs from the thalamus and the cerebral cortex. Using vesicular glutamate transporters (vGluTs) 1 and 2 as markers of the corticostriatal and thalamostriatal afferents, respectively, we examined changes in the synaptology and relative prevalence of striatal glutamatergic inputs in methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP)‐treated monkeys using electron microscopic immunoperoxidase and confocal immunofluorescence methods. Our findings demonstrate that the prevalence of vGluT1‐containing terminals is significantly increased in the striatum of MPTP‐treated monkeys (51.9 ± 3.5% to 66.5 ± 3.4% total glutamatergic boutons), without any significant change in the pattern of synaptic connectivity; more than 95% of vGluT1‐immunolabeled terminals formed axo‐spinous synapses in both conditions. In contrast, the prevalence of vGluT2‐immunoreactive terminals did not change after MPTP treatment (21.7 ± 1.3% vs. 21.6 ± 1.2% total glutamatergic boutons). However, a substantial increase in the ratio of axo‐spinous to axo‐dendritic synapses formed by vGluT2‐immunoreactive terminals was found in the pre‐caudate and post‐putamen striatal regions of MPTP‐treated monkeys, suggesting a certain degree of synaptic reorganization of the thalamostriatal system in parkinsonism. About 20% of putative glutamatergic terminals did not show immunoreactivity in striatal tissue immunostained for both vGluT1 and vGluT2, suggesting the expression of another vGluT in these boutons. These findings provide striking evidence that suggests a differential degree of plasticity of the corticostriatal and thalamostriatal system in PD.

Metabotropic glutamate receptor 2 modulates excitatory synaptic transmission in the rat globus pallidus.

While group II metabotropic glutamate receptors (mGluRs) are known to be expressed in the rat globus pallidus (GP), their functions remain poorly understood. We used standard patch clamping technique in GP slices to determine the effect of group II mGluR activation on excitatory transmission in this region. Activation of group II mGluRs with the group-selective agonist DCG-IV or APDC reduced the amplitude of the evoked excitatory postsynaptic currents (EPSCs) and significantly increased the paired pulse ratio suggesting a presynaptic site of action. This was further supported by double-labeling electron microscopy data showing that group II mGluRs (mGluR2 and 3) immunoreactivity is localized in glutamatergic pre-terminal axons and terminals in the GP. Furthermore, we found that LY 487379, an mGluR2-specific allosteric modulator, significantly potentiated the inhibitory effect of DCG-IV on the excitatory transmission in the GP. Co-incubation with 30 microM LY 487379 increased the potency of DCG-IV about 10-fold in the GP. We were thus able to pharmacologically isolate the mGluR2-mediated function in the rat GP using an mGluR2-specific allosteric modulator. Therefore, our findings do not only shed light on the functions of group II mGluRs in the GP, they also illustrate the therapeutic potential of mGluR-targeting allosteric modulators in neurological disorders such as Parkinsons disease.

Striatal spine plasticity in Parkinson’s disease: pathological or not?

Parkinsons disease (PD) is characterized by a dramatic loss of dopamine that underlies complex structural and functional changes in striatal projection neurons. A key alteration that has been reported in various rodent models and PD patients is a significant reduction in striatal dendritic spine density. Our recent findings indicate that striatal spine loss is also a prominent feature of parkinsonism in MPTP-treated monkeys. In these animals, striatal spine plasticity is tightly linked with the degree of striatal dopamine denervation. It affects predominantly the sensorimotor striatal territory (i.e. the post-commissural putamen) and targets both direct and indirect striatofugal neurons. However, electron microscopic 3D reconstruction studies demonstrate that the remaining spines in the dopamine-denervated striatum of parkinsonian monkeys undergo major morphological and ultrastructural changes characteristic of increased synaptic efficacy. Although both corticostriatal and thalamostriatal glutamatergic afferents display such plastic changes, the ultrastructural features of pre- and post-synaptic elements at these synapses are consistent with a higher strength of corticostriatal synapses over thalamic inputs in both normal and pathological conditions. Thus, striatal projection neurons and their glutamatergic afferents are endowed with a high degree of structural and functional plasticity. In parkinsonism, the striatal dopamine denervation induces major spine loss on medium spiny neurons and generates a significant remodeling of corticostriatal and thalamostriatal glutamatergic synapses, consistent with increased synaptic transmission. Future studies are needed to further characterize the mechanisms underlying striatal spine plasticity, and determine if it represents a pathological feature or compensatory process of PD.

Localization and function of pre‐ and postsynaptic kainate receptors in the rat globus pallidus

Kainate receptors (KARs) are widely expressed the basal ganglia. In this study, we used electron microscopic immunocytochemistry and whole‐cell recording techniques to examine the localization and function of KARs in the rat globus pallidus (GP). Dendrites were the most common immunoreactive elements, while terminals forming symmetric or asymmetric synapses and unmyelinated axons comprised most of the presynaptic labeling. To determine whether synaptically released glutamate activates KARs, we recorded excitatory postsynaptic currents (EPSCs) in the GP following single‐pulse stimulation of the internal capsule. 4‐(8‐Methyl‐9H‐1,3‐dioxolo[4,5 h]{2,3}benzodiazepine‐5‐yl)‐benzenamine hydrochloride (GYKI 52466, 100 µm), an α‐amino‐3‐hydroxyl‐5‐methyl‐4‐isoxazole propionic acid (AMPA) receptor antagonist, reduced but did not completely block evoked EPSCs. The remaining EPSC component was mediated through activation of KARs because it was abolished by 6‐cyano‐7‐nitroquinoxaline‐2, 3‐dione (CNQX), an AMPA/KAR antagonist. The rise time (10–90%) and decay time constant (τ) for those EPSCs were longer than those of AMPA‐mediated EPSCs recorded before GYKI 52466 application. KAR activation inhibited EPSCs. This inhibition was associated with a significant increase in paired‐pulse facilitation ratio, suggesting a presynaptic action of KAR. KAR inhibition of EPSCs was blocked by the G‐protein inhibitor, N‐ethylmaleimide (NEM), or the protein kinase C (PKC) inhibitor calphostin C. Our results demonstrate that KAR activation has dual effects on glutamatergic transmission in the rat GP: (1) it mediates small‐amplitude EPSCs; and (2) it reduces glutamatergic synaptic transmission through a presynaptic G‐protein coupled, PKC‐dependent, metabotropic mechanism. These findings provide evidence for the multifarious functions of KARs in regulating synaptic transmission, and open up the possibility for the development of pharmacotherapies to reduce the hyperactive subthalamofugal projection in Parkinsons disease.

Anterograde Axonal Tract Tracing

The mammalian brain contains a myriad of interconnected regions. An examination of the complex circuitry of these areas requires sensitive neuroanatomical tract tracing techniques. The anterograde tracers, Phaseolus vulgaris leucoagglutinin (PHA‐L) and biotinylated dextran amines (BDA) are powerful tools that can be used to label fiber tracts that project from one particular brain region. When injected iontophoretically, PHA‐L and BDA are readily taken up by neurons and transported anterogradely along their axonal tracts. Combined with immunocytochemistry for neurotransmitters, neuropeptides, and receptors, tract tracing methods may be used to elucidate the phenotype of synapses that form the microcircuitry of specific neural systems.

GABAB receptors in the centromedian/parafascicular thalamic nuclear complex: An ultrastructural analysis of GABABR1 and GABABR2 in the monkey thalamus

Strong γ‐aminobutyric acid type B (GABAB) receptor binding has been shown throughout the thalamus, but the distribution of the two GABAB receptor subunits, GABAB receptor subunit 1 (GABABR1) and GABAB receptor subunit 2 (GABABR2), remains poorly characterized. In primates, the caudal intralaminar nuclei, centromedian and parafascicular (CM/PF), are an integral part of basal ganglia circuits and a main source of inputs to the striatum. In this study, we analyzed the subcellular and subsynaptic distribution of GABAB receptor subunits by using light and electron microscopic immunocytochemical techniques. Quantitative immunoperoxidase and immunogold analysis showed that both subunits display a similar pattern of distribution in CM/PF, being expressed largely at extrasynaptic and perisynaptic sites in neuronal cell bodies, dendrites, and axon‐like processes and less abundantly in axon terminals. Postsynaptic GABABR1 labeling was found mostly on the plasma membrane (70–80%), whereas GABABR2 was more evenly distributed between the plasma membrane and intracellular compartments of CM/PF neurons. A few axon terminals forming symmetric and asymmetric synapses were also labeled for GABABR1 and GABABR2, but the bulk of presynaptic labeling was expressed in small axon‐like processes. About 20% of presynaptic vesicle‐containing dendrites of local circuit neurons displayed GABABR1/R2 immunoreactivity. Vesicular glutamate transporters (vGluT1)‐containing terminals forming asymmetric synapses expressed GABABR1 and/or displayed postsynaptic GABABR1 at the edges of their asymmetric specialization. Overall, these findings provide evidence for multiple sites where GABAB receptors could modulate GABAergic and glutamatergic transmission in the primate CM/PF complex. J. Comp. Neurol. 496:269–287, 2006.

Anatomical and functional Relationships Between Intralaminar Thalamic Nuclei and Basal Ganglia in Monkeys

The basal ganglia are major telencephalic subcortical structures involved in the control of motor, cognitive and psychoaffective behaviours. In primate, these nuclei include (1) the caudate nucleus, putamen and nucleus accumbens which commonly form the striatum, (2) the globus pallidus which comprises an external (GPe) and internal segments (GPi), as well as the ventral pallidum (VP), (3) the subthalamic nucleus (STN) and (4) the substantia nigra that includes the pars compacta (SNc) which contains dopaminergic neurons, and the pars reticulata (SNr) which contains GABAergic neurons. The striatum is the largest component and the main entrance of information to the basal ganglia. It receives major excitatory inputs from the entire cerebral cortex and the thalamus.