Role of the parafascicular thalamic nucleus and N-methyl-D-aspartate transmission in the D1-dependent control of in vivo acetylcholine release in rat striatum

Micropopulation mapping of the mouse parafascicular nucleus connections reveals diverse input-output motifs

Introduction

In primates, including humans, the centromedian/parafascicular (CM-Pf) complex is a key thalamic node of the basal ganglia system. Deep brain stimulation in CM-Pf has been applied for the treatment of motor disorders such as Parkinson's disease or Tourette syndrome. Rodents have become widely used models for the study of the cellular and genetic mechanisms of these and other motor disorders. However, the equivalence between the primate CM-Pf and the nucleus regarded as analogous in rodents (Parafascicular, Pf) remains unclear.

Methods

Here, we analyzed the neurochemical architecture and carried out a brain-wide mapping of the input-output motifs in the mouse Pf at micropopulation level using anterograde and retrograde labeling methods. Specifically, we mapped and quantified the sources of cortical and subcortical input to different Pf subregions, and mapped and compared the distribution and terminal structure of their axons.

Results

We found that projections to Pf arise predominantly (>75%) from the cerebral cortex, with an unusually strong (>45%) Layer 5b component, which is, in part, contralateral. The intermediate layers of the superior colliculus are the main subcortical input source to Pf. On its output side, Pf neuron axons predominantly innervate the striatum. In a sparser fashion, they innervate other basal ganglia nuclei, including the subthalamic nucleus (STN), and the cerebral cortex. Differences are evident between the lateral and medial portions of Pf, both in chemoarchitecture and in connectivity. Lateral Pf axons innervate territories of the striatum, STN and cortex involved in the sensorimotor control of different parts of the contralateral hemibody. In contrast, the mediodorsal portion of Pf innervates oculomotor-limbic territories in the above three structures.

Discussion

Our data thus indicate that the mouse Pf consists of several neurochemically and connectively distinct domains whose global organization bears a marked similarity to that described in the primate CM-Pf complex.

Cognitive effects of thalamostriatal degeneration are ameliorated by normalizing striatal cholinergic activity

The loss of neurons in parafascicular thalamus (Pf) and their inputs to dorsomedial striatum (DMS) in Lewy body disease (LBD) and Parkinson's disease dementia (PDD) have been linked to the effects of neuroinflammation. We found that, in rats, these inputs were necessary for both the function of striatal cholinergic interneurons (CINs) and the flexible encoding of the action-outcome (AO) associations necessary for goal-directed action, producing a burst-pause pattern of CIN firing but only during the remapping elicited by a shift in AO contingency. Neuroinflammation in the Pf abolished these changes in CIN activity and goal-directed control after the shift in contingency. However, both effects were rescued by either the peripheral or the intra-DMS administration of selegiline, a monoamine oxidase B inhibitor that we found also enhances adenosine triphosphatase activity in CINs. These findings suggest a potential treatment for the cognitive deficits associated with neuroinflammation affecting the function of the Pf and related structures.

Analysis of morphological and neurochemical changes in subthalamic nucleus neurons in response to a unilateral 6-OHDA lesion of the substantia nigra in adult rats

Background

Subthalamic nucleus (STN) neurons undergo changes in their pattern of activity and morphology during the clinical course of Parkinson’s disease (PD). Striatal dopamine depletion and hyperactivity of neurons in the parafascicular nucleus (Pf) of the intralaminar thalamus are predicted to contribute to the STN changes.

Objective

This study investigated possible morphological and neurochemical changes in STN neurons in a rat model of unilateral, nigral dopamine neuron loss, in relation to previously documented alterations in Pf neurons.

Methods

Male Sprague-Dawley rats received a unilateral injection of 6-hydroxydopamine (6-OHDA) into the substantia nigra pars compacta (SNpc). Rats were randomly divided into two groups (6/group) for study at 1 and 5 months by post-treatment. The extent of SNpc dopamine neuron damage was assessed in an amphetamine-induced rotation test and postmortem assessment of tyrosine hydroxylase mRNA levels using in situ hybridization histochemistry. Neural cross-sectional measurements and assessment of vesicular glutamate transporter-2 (vGlut2) mRNA levels were performed to measure the impact on neurons in the STN.

Results

A unilateral SNpc dopaminergic neuron lesion significantly decreased the cross-sectional area of STN neurons ipsilateral to the lesion, at 1 month (P < 0.05) and 5 months (P < 0.01) post-lesion, while bilateral vGlut2 mRNA levels in STN neurons were unaltered.

Conclusions

Decreased size of STN neurons in the presence of sustained vGlut2 mRNA levels following a unilateral SNpc 6-OHDA lesion, indicate altered STN physiology. This study presents further details of changes within the STN, coincident with observed alterations in Pf neurons and behaviour.

Data availability

The data associated with the findings of this study are available from the corresponding author upon request.

Selective remodeling of glutamatergic transmission to striatal cholinergic interneurons after dopamine depletion

The widely held view that the pathophysiology of Parkinson's disease arises from an under-activation of the direct pathway striatal spiny neurons (dSPNs) has gained support from a recently described weakening of the glutamatergic projection from the parafascicular nucleus (PfN) to dSPNs in experimental parkinsonism. However, the impact of the remodeling of the thalamostriatal projection cannot be fully appreciated without considering its impact on cholinergic interneurons (ChIs) that themselves preferentially activate indirect pathway spiny neurons (iSPNs). To study this thalamostriatal projection, we virally transfected with Cre-dependent channelrhodopsin-2 (ChR2) the PfN of Vglut2-Cre mice that were dopamine-depleted with 6-hydroxydopamine (6-OHDA). In parallel, we studied the corticostriatal projection to ChIs in 6-OHDA-treated transgenic mice expressing ChR2 under the Thy1 promoter. We found the 6-OHDA lesions failed to affect short-term synaptic plasticity or the size of unitary responses evoked optogenetically in either of these projections. However, we found that NMDA-to-AMPA ratios at PfN synapses-that were significantly larger than NMDA-to-AMPA ratios at cortical synapses-were reduced by 6-OHDA treatment, thereby impairing synaptic integration at PfN synapses onto ChIs. Finally, we found that application of an agonist of the D5 dopamine receptors on ChIs potentiated NMDA currents without affecting AMPA currents or short-term plasticity selectively at PfN synapses. We propose that dopamine depletion leads to an effective de-potentiation of NMDA currents at PfN synapses onto ChIs which degrades synaptic integration. This selective remodeling of NMDA currents at PfN synapses may counter the selective weakening of PfN synapses onto dSPNs in parkinsonism.

Cholinergic modulation of striatal microcircuits

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

Striatal cholinergic interneurons and Parkinson's disease

Giant, aspiny cholinergic interneurons (ChIs) have long been known to be key nodes in the striatal circuitry controlling goal-directed actions and habits. In recent years, new experimental approaches, like optogenetics and monosynaptic rabies virus mapping, have expanded our understanding of how ChIs contribute to the striatal activity underlying action selection and the interplay of dopaminergic and cholinergic signaling. These approaches also have begun to reveal how ChI function is distorted in disease states affecting the basal ganglia, like Parkinson's disease (PD). This review gives a brief overview of our current understanding of the functional role played by ChIs in striatal physiology and how this changes in PD. The translational implications of these discoveries, as well as the gaps that remain to be bridged, are discussed as well.

Access to the CNS: Biomarker Strategies for Dopaminergic Treatments

Despite substantial research carried out over the last decades, it remains difficult to understand the wide range of pharmacological effects of dopaminergic agents. The dopaminergic system is involved in several neurological disorders, such as Parkinson's disease and schizophrenia. This complex system features multiple pathways implicated in emotion and cognition, psychomotor functions and endocrine control through activation of G protein-coupled dopamine receptors. This review focuses on the system-wide effects of dopaminergic agents on the multiple biochemical and endocrine pathways, in particular the biomarkers (i.e., indicators of a pharmacological process) that reflect these effects. Dopaminergic treatments developed over the last decades were found to be associated with numerous biochemical pathways in the brain, including the norepinephrine and the kynurenine pathway. Additionally, they have shown to affect peripheral systems, for example the hypothalamus-pituitary-adrenal (HPA) axis. Dopaminergic agents thus have a complex and broad pharmacological profile, rendering drug development challenging. Considering the complex system-wide pharmacological profile of dopaminergic agents, this review underlines the needs for systems pharmacology studies that include: i) proteomics and metabolomics analysis; ii) longitudinal data evaluation and mathematical modeling; iii) pharmacokinetics-based interpretation of drug effects; iv) simultaneous biomarker evaluation in the brain, the cerebrospinal fluid (CSF) and plasma; and v) specific attention to condition-dependent (e.g., disease) pharmacology. Such approach is considered essential to increase our understanding of central nervous system (CNS) drug effects and substantially improve CNS drug development.

Cortical Control of Striatal Dopamine Transmission via Striatal Cholinergic Interneurons

Corticostriatal regulation of striatal dopamine (DA) transmission has long been postulated, but ionotropic glutamate receptors have not been localized directly to DA axons. Striatal cholinergic interneurons (ChIs) are emerging as major players in striatal function, and can govern DA transmission by activating nicotinic receptors (nAChRs) on DA axons. Cortical inputs to ChIs have historically been perceived as sparse, but recent evidence indicates that they strongly activate ChIs. We explored whether activation of M1/M2 corticostriatal inputs can consequently gate DA transmission, via ChIs. We reveal that optogenetic activation of channelrhodopsin-expressing corticostriatal axons can drive striatal DA release detected with fast-scan cyclic voltammetry and requires activation of nAChRs on DA axons and AMPA receptors on ChIs that promote short-latency action potentials. By contrast, DA release driven by optogenetic activation of intralaminar thalamostriatal inputs involves additional activation of NMDA receptors on ChIs and action potential generation over longer timescales. Therefore, cortical and thalamic glutamate inputs can modulate DA transmission by regulating ChIs as gatekeepers, through ionotropic glutamate receptors. The different use of AMPA and NMDA receptors by cortical versus thalamic inputs might lead to distinct input integration strategies by ChIs and distinct modulation of the function of DA and striatum.

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

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

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

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

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.

Influence of parafascicular thalamic input on neuronal activity within the nucleus accumbens is mediated by nitric oxide - an in vivo study

AIMS

Thalamostriatal fibers are involved in cognitive tasks such as acquisition, learning, processing of sensory events, and behavioral flexibility and might play a role in Parkinson's disease. The aim of the present study was the in vivo electrochemical characterization of the projection from the lateral aspect of the parafascicular thalamus (Pfl) to the dorsolateral aspect of the nucleus accumbens (dNAc). Since nitric oxide (NO) plays a crucial role in striatal synaptic transmission, its implication in Pfl-evoked signaling within the dNAc was investigated.

MAIN METHODS

The Pfl was electrically stimulated utilizing paired pulses and extracellular potentials were recorded within the dNAc. Simultaneously, the dNAc was superfused using the push-pull superfusion technique for local application of compounds and for assessing the influence of NO on release of glutamate, aspartate and GABA.

KEY FINDINGS

Stimulation of the Pfl evoked a negative-going component at 9-14 ms followed by a positive-going component at 39-48 ms. The early response was current-dependent and diminished by superfusion of the dNAc with tetrodotoxin, kynurenic acid or N(G)-nitro-l-arginine methyl ester (L-NAME), while 3-(2-hydroxy-2-nitroso-1-propylhydrazino)-1-propanamine (PAPA/NO) increased this evoked potential. Transmitter release was inhibited by L-NAME and facilitated by PAPA/NO.

SIGNIFICANCE

This study describes for the first time in vivo extracellular electrical responses of the dNAc on stimulation of the Pfl. Synaptic transmission within the dNAc on stimulation of the Pfl seems to be facilitated by NO.

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.

The thalamostriatal pathway and cholinergic control of goal-directed action: interlacing new with existing learning in the striatum

The capacity for goal-directed action depends on encoding specific action-outcome associations, a learning process mediated by the posterior dorsomedial striatum (pDMS). In a changing environment, plasticity has to remain flexible, requiring interference between new and existing learning to be minimized, yet it is not known how new and existing learning are interlaced in this way. Here we investigated the role of the thalamostriatal pathway linking the parafascicular thalamus (Pf) with cholinergic interneurons (CINs) in the pDMS in this process. Removing the excitatory input from Pf to the CINs was found to reduce the firing rate and intrinsic activity of these neurons and produced an enduring deficit in goal-directed learning after changes in the action-outcome contingency. Disconnection of the Pf-pDMS pathway produced similar behavioral effects. These data suggest that CINs reduce interference between new and existing learning, consistent with claims that the thalamostriatal pathway exerts state control over learning-related plasticity.

Convergent evidence for abnormal striatal synaptic plasticity in dystonia

Dystonia is a functionally disabling movement disorder characterized by abnormal movements and postures. Although substantial recent progress has been made in identifying genetic factors, the pathophysiology of the disease remains a mystery. A provocative suggestion gaining broader acceptance is that some aspect of neural plasticity may be abnormal. There is also evidence that, at least in some forms of dystonia, sensorimotor "use" may be a contributing factor. Most empirical evidence of abnormal plasticity in dystonia comes from measures of sensorimotor cortical organization and physiology. However, the basal ganglia also play a critical role in sensorimotor function. Furthermore, the basal ganglia are prominently implicated in traditional models of dystonia, are the primary targets of stereotactic neurosurgical interventions, and provide a neural substrate for sensorimotor learning influenced by neuromodulators. Our working hypothesis is that abnormal plasticity in the basal ganglia is a critical link between the etiology and pathophysiology of dystonia. In this review we set up the background for this hypothesis by integrating a large body of disparate indirect evidence that dystonia may involve abnormalities in synaptic plasticity in the striatum. After reviewing evidence implicating the striatum in dystonia, we focus on the influence of two neuromodulatory systems: dopamine and acetylcholine. For both of these neuromodulators, we first describe the evidence for abnormalities in dystonia and then the means by which it may influence striatal synaptic plasticity. Collectively, the evidence suggests that many different forms of dystonia may involve abnormal plasticity in the striatum. An improved understanding of these altered plastic processes would help inform our understanding of the pathophysiology of dystonia, and, given the role of the striatum in sensorimotor learning, provide a principled basis for designing therapies aimed at the dynamic processes linking etiology to pathophysiology of the disease.

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.

Cholinergic innervation and thalamic input in rat nucleus accumbens

Cholinergic interneurons are the only known source of acetylcholine in the rat nucleus accumbens (nAcb); yet there is little anatomical data about their mode of innervation and the origin of their excitatory drive. We characterized the cholinergic and thalamic innervations of nAcb with choline acetyltransferase (ChAT) immunocytochemistry and anterograde transport of Phaseolus vulgaris-leucoagglutinin (PHA-L) from the midline/intralaminar/paraventricular thalamic nuclei. The use of a monoclonal ChAT antiserum against whole rat ChAT protein allowed for an optimal visualization of the small dendritic branches and fine varicose axons of cholinergic interneurons. PHA-L-labeled thalamic afferents were heterogeneously distributed throughout the core and shell regions of nAcb, overlapping regionally with cholinergic somata and dendrites. At the ultrastructural level, several hundred single-section profiles of PHA-L and ChAT-labeled axon terminals were analyzed for morphology, synaptic frequency, and the nature of their synaptic targets. The cholinergic profiles were small and apposed to various neuronal elements, but rarely exhibited a synaptic membrane specialization (5% in single ultrathin sections). Stereological extrapolation indicated that less than 15% of these cholinergic varicosities were synaptic. The PHA-L-labeled profiles were comparatively large and often synaptic (37% in single ultrathin sections), making asymmetrical contacts primarily with dendritic spines (>90%). Stereological extrapolation indicated that all PHA-L-labeled terminals were synaptic. In double-labeled material, some PHA-L-labeled terminals were directly apposed to ChAT-labeled somata or dendrites, but synapses were never seen between the two types of elements. These observations demonstrate that the cholinergic innervation of rat nAcb is largely asynaptic. They confirm that the afferents from midline/intralaminar/paraventricular thalamic nuclei to rat nAcb synapse mostly on dendritic spines, presumably of medium spiny neurons, and suggest that the excitatory drive of nAcb cholinergic interneurons from thalamus is indirect, either via substance P release from recurrent collaterals of medium spiny neurons and/or by extrasynaptic diffusion of glutamate.

Effect of interleukin-1beta on NMDA-induced 45Ca2+ uptake by synaptosomes of rat brain cortex

The effect of interleukin-1beta on presynaptic NMDA receptors was evaluated by studying NMDA-induced 45Ca2+ uptake by synaptosomes from rat brain cortex. Interleukin-1beta inhibited 45Ca2+ uptake by synaptosomes. Our results indicate that interleukin-1beta modulates presynaptic NMDA receptors and is probably involved in the regulation of synaptic transmission in the central nervous system.

A proposed hypothalamic-thalamic-striatal axis for the integration of energy balance, arousal, and food reward

We elaborate herein a novel theory of basal ganglia function that accounts for why palatable, energy-dense foods retain high incentive value even when immediate physiological energy requirements have been met. Basal ganglia function has been studied from the perspective of topographical segregation of processing within parallel circuits, with primary focus on motor control and cognition. Recent findings suggest, however, that the striatum can act as an integrated unit to modulate motivational state. We describe evidence that the striatal enkephalin system, which regulates the hedonic impact of preferred foods, undergoes coordinated gene expression changes that track current motivational state with regard to food intake. Striatal enkephalin gene expression is also downregulated by an intrastriatal infusion of a cholinergic muscarinic antagonist, a manipulation that greatly suppresses food intake. To account for these findings, we propose that signaling through a hypothalamic-midline thalamic-striatal axis impinges on the cholinergic interneurons of the striatum, which via their large, overlapping axonal fields act as a network to modulate enkephalin-containing striatal output neurons. A key relay in this circuit is the paraventricular thalamic nucleus, which receives convergent input from orexin-coded hypothalamic energy-sensing and behavioral state-regulating neurons, as well as from circadian oscillators, and projects to cholinergic interneurons throughout the striatal complex. We hypothesize that this system evolved to coordinate feeding and arousal, and to prolong the feeding central motivational state beyond the fulfillment of acute energy needs, thereby promoting "overeating" and the consequent development of an energy reserve for potential future food shortages.

Restricted Expression of LGR8 in Intralaminar Thalamic Nuclei of Rat Brain Suggests a Role in Sensorimotor Systems

LGR8 was recently identified as a cognate receptor for insulin-like peptide-3 (INSL3), and INSL3-LGR8 signaling is best known for its role in testis descent during development. LGR8 mRNA has been detected in various human tissues including brain, but the regional and cellular distribution of LGR8 expression in the mammalian central nervous system is unknown. Therefore, in this study we investigated the presence and localization of LGR8 mRNA in rat brain using reverse transcription-polymerase chain reaction and in situ hybridization histochemistry. Results revealed a distinct distribution of LGR8 in forebrain, with transcripts principally restricted to the posterior thalamus and highest densities detected in the parafascicular nucleus of both adult and developing rats. Unexpectedly, INSL3 mRNA was not detected in brain by similar methods, but preliminary electrophysiologic studies of parafascicular neurons revealed that INSL3 altered their activity. These findings suggest that LGR8 signaling may be involved in sensorimotor control in the rat and perhaps other species, particularly via actions on parafascicular neurons that project to basal ganglia and are depleted in Parkinson's disease.

Tolerance to the procholinergic action of the D1 receptor full agonist dihydrexidine

RATIONALE

It has been well established that dopamine D1 receptor agonists increase acetylcholine (ACh) release in the brain, an effect that has been suggested to contribute to their procognitive properties.

OBJECTIVES

We sought to examine whether the ACh-releasing action of the D1 receptor full agonist dihydrexidine HCl (DHX) is altered after repeated administration.

RESULTS

DHX dose-dependently (3, 9, 18, and 36 mg/kg, i.p.) increased ACh efflux in the hippocampus of freely moving rats through D1 receptor stimulation, as the D1 receptor antagonist SCH 23390 (0.3 mg/kg, i.p.) abolished the effect of DHX (18 mg/kg, i.p.). Daily injections of DHX (18 mg/kg, i.p.) over 14 days did not significantly affect basal hippocampal ACh concentrations, but they significantly curtailed the stimulatory action of a challenge injection of DHX (18 mg/kg, i.p.) on ACh efflux.

CONCLUSIONS

Tolerance to the procholinergic action of DHX develops with repeated administration.

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.

In vivo temporal sequence of rat striatal glutamate, aspartate and dopamine efflux during apomorphine, nomifensine, NMDA and PDC in situ administration

In vivo microdialysis was used to investigate the interactions between dopamine (DA), glutamate (Glu) and aspartate (Asp) in anaesthetised-rat striatum. The combination of brain microdialysis and capillary electrophoresis with laser-induced fluorescence detection (CE-LIFD) allows the simultaneous monitoring of the efflux of these neurotransmitters up to every 10 s. DA and Glu reuptake inhibitors, nomifensine and L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) and, dopaminergic and glutamatergic receptor agonists, apomorphine and NMDA respectively, were administered by reverse dialysis. Reverse dialysis of 20 micro M nomifensine induced a rapid and marked increase (+3200% at 5 min) in extracellular DA, while a decrease in Glu and Asp (-11 and -25%, respectively) was observed simultaneously. Reverse dialysis of 10 micro M apomorphine led to progressive changes: -63% decrease in DA and +25% Glu increase at 36 min. Reverse dialysis of 1 mM NMDA induced a simultaneous increase in DA, Glu and Asp which peaked at +2 min (+840%, +40% and +150%, respectively). Surprisingly, a second increase in Glu was observed 5 min after the end of NMDA perfusion. Reverse dialysis of PDC (1 mM and 10 mM) induced a rapid increase in Glu and Asp levels, while DA increased with a 26-s delay. These findings indicate that, in the striatum, endogenous DA and Glu may act in opposition to regulate each other's efflux. These results have been obtained due to unique features offered by microdialysis coupled with CE-LIFD.

Re-examination of the thalamostriatal projections in the rat with retrograde tracers

Topographical arrangements of thalamostriatal projection neurons was examined in the rat by the retrograde tract-tracing method. After injecting Fluoro-Gold (FG) and/or cholera toxin beta-subunit (CTB) in different regions of the caudate-putamen (CPu), distribution of retrogradely labeled neurons was observed in the thalamus. The main findings were as follows: (1) Retrogradely labeled neurons were seen in the midline-intralaminar thalamic nuclei in all rats examined in the present study. Neurons in the ventral lateral and posterior thalamic nuclear groups were also labeled in the rats which were injected with the tracer into the dorsal part of Cpu, but not in the rats which were injected with the tracer into the nucleus accumbens (Acb) and its adjavent regions in the ventromedial part of the Cpu. (2) Topographical organization was observed in the projections from the midline-intralaminar thalamic nuclei to the CPu. After the tracer injection into the dorsal part of the CPu or the ventral part of the CPu (including the Acb), labeled neurons in the midline-intralaminar thalamic nuclei were distributed predominantly in the lateral part of the intralaminar nuclei or the midline nuclei, respectively. On the other hand, after the tracer injection into the medial or the lateral part of the CPu, labeled neurons in the midline-intralaminar nuclei were distributed mainly in the dorsal or the ventral part of these nuclei, respectively. (3) Topographical organization was also observed in the thalamostriatal projections from the ventral and Pos. After the tracer injection into the rostral part of the CPu, labeled neurons were distributed mainly in the rostral part of the ventral nuclear group. On the other hand, after the tracer injection into the caudal part of the CPu, labeled neurons were distributed mainly in the caudal part of the ventral nuclear group, as well as in the posterior nuclear group.

An outline of brain function

An outline of how the brain may compute is proposed. In the cerebral cortex memories are stored through long-term potentiation at synapses from layer 1 cortical inputs (representing contexts) on layer 2/3 pyramidal cells linked with the thalamus in a cortico-thalamic (CT) unit. The signals which are memorized are the layer 3 inputs from the thalamus or other cortical areas. Signals are memorized (and later recalled) at the gamma frequency. A conscious thought comprises the outputs of layer 5 cells in CT units in different cortical regions firing in synchrony through the contribution of oscillatory thalamic and cortical inputs. This cortical output influences sub-cortical areas to cause or participate in a movement. Cerebral cortical outputs may be stored in the cerebellum and generated later in a particular context by the basal ganglia and cerebellum. Thus the brain may either generate 'conscious' outputs using the cerebral cortex or 'automatic' outputs using the basal ganglia and cerebellum. When contexts are recognized by the basal ganglia it permits outputs stored in the cerebellum to commence and in this way the basal ganglia can control complex sequences of outputs or movements. Working memory involves the prefrontal cortex using similarly the basal ganglia and cerebellum. The hippocampus has a role in the storage and recall of cortical outputs by providing unique layer 1 contexts to all the CT loops in different cortical areas in a conscious thought. With further recall of the thought new layer 1 contexts may become associated with the CT loops enabling recall without the hippocampal input.

Parafascicular thalamic nucleus deafferentation reduces c-fos expression induced by dopamine D-1 receptor stimulation in rat striatum

We investigated the role played by the parafascicular thalamostriatal pathway, one of the major excitatory inputs to the striatum, in the D-1 receptor induction of c-fos messenger RNA expression in the rat striatum. The full D-1 receptor agonist, SKF-82958 (0.05, 0.1, 0.5 and 1 mg/kg, s.c., 30 min), dose-dependently induced c-fos messenger RNA in naive rat striatum as determined by northern blot analysis. One day following electrolytic lesion of the parafascicular thalamostriatal nucleus, striatal c-fos signal by itself was not altered but the stimulated expression of c-fos was strongly decreased. Sections of sham-operated and acute-lesioned dorsal striata of vehicle- or SKF-82958-treated rats were processed for in situ hybridization histochemistry at the single-cell level with an RNA probe for c-fos. The basal expression of striatal c-fos was poorly detectable in sham and lesioned groups. Sections of dorsal striata from sham-operated rats treated with SKF-82958 showed two types of labeled neurons for c-fos: the lightly and the very densely labeled neurons which are few in number. Thalamic lesion strongly reduced SKF-82958 stimulated expression of c-fos RNA in both types of labeled cells. The frequency distribution of c-fos labeling per neuron in dorsal striata of lesioned rats treated with SKF-82958 was shifted to the left, and its median was lower than in the sham-operated striata treated with the D-1 receptor agonist. The results provide evidence that the excitatory projections from the parafascicular nucleus of the thalamus, thought to operate primarily through the N-methyl-D-aspartate receptors, exert a facilitatory control over D-1 receptor-induced c-fos gene expression specifically in the dorsal striatum.

Unilateral lesion of the nigrostriatal pathway induces a transient decrease of firing rate with no change in the firing pattern of neurons of the parafascicular nucleus in the rat

Electrophysiological recordings of thalamic parafascicular nucleus neurons were done in normal rats and in three groups of rats at different time intervals after injection of 6-hydroxydopamine into the pars compacta of substantia nigra. In normal rats, parafascicular neurons exhibited low firing rates (3.88+/-0.80 spikes/s). Concerning the pattern, 59% of the units discharged irregularly and 41% exhibited bursty pattern. In rats with 6-hydroxydopamine lesions, the firing rate decreased significantly during the first week post-lesion (1.15+/-0.36 spikes/s, P<0.01). During the second week, the firing rate was slightly, but not significantly, lower (2.59+/-0.41 spikes/s, P>0.05) than that of normal rats to return to the basal level three weeks post-lesion (3. 66+/-0.41 spikes/s, P>0.05). In these three groups of 6-hydroxydopamine-lesioned rats, the firing pattern showed no change when compared to control animals. These results show that the lesion of nigral dopaminergic neurons induced a transient decrease of the firing rate of parafascicular neurons with no change in the firing pattern demonstrating the absence of a stable influence of the dopaminergic system on the spontaneous activity of parafascicular neurons.

Glutamatergic influences on the basal ganglia

Glutamate is the predominant excitatory neurotransmitter of the basal ganglia, where it acts on ionotropic and metabotropic receptors. In the best studied of the basal ganglia disorders, Parkinson's disease, there is compelling evidence that the activities of glutamatergic pathways are altered. Of particular importance, the glutamatergic subthalamic nucleus becomes overactive. Pharmacologic blockade of subthalamic neurotransmission has antiparkinsonian symptomatic effects and may also help to protect the remaining dopamine neurons of the substantia nigra from excitotoxic neurodegeneration. Development of drugs to manipulate the glutamatergic system with appropriate pharmacologic and anatomic selectivity is likely to dramatically improve our ability to treat disorders of the basal ganglia.

NMDA receptors in the basal ganglia

The basal ganglia consist of several interconnected nuclei located in the telecephalon, diencephalon and mesencephalon that are involved in a variety of motor and non-motor behavioural functions. Glutamate receptors play a major role in neurotransmission within the basal ganglia and are present in all nuclei of the basal ganglia. This review focuses on the contribution of the NMDA class of glutamatergic receptors to various movement disorders whose primary pathology lies within the basal ganglia and discusses how pharmacological manipulation of such receptors may be therapeutically useful.

Dopamine D(1) receptor-mediated control of striatal acetylcholine release by endogenous dopamine

The role of dopamine D(1) and D(2) receptors in the control of acetylcholine release in the dorsal striatum by endogenous dopamine was investigated by monitoring with microdialysis the effect of the separate or combined administration of the dopamine D(1) receptor antagonist, SCH 39166 ¿(-)-trans-6,7,7a,8,9, 13b-exahydro-3-chloro-2-hydroxy-N-methyl-5H-benzo-[d]-nap hto-[2, 1b]-azepine hydrochloride¿ (50 microg/kg subcutaneous (s.c.)), of the dopamine D(2)/D(3) receptor agonist, quinpirole (trans-(-)-4aR, 4a,5,6,7,8,8a,9-octahydro-5-propyl-1H-pyrazolo-(3,4-g)-quinoline hydrochloride) (5 and 10 microg/kg s.c.), and of the D(3) receptor selective agonist, PD 128,907 [S(+)-(4aR,10bR)-3,4,4a, 10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano-[4,3-b]-1,4-oxazin -9-ol hydrochloride] (50 microg/kg s.c.), on in vivo dopamine and acetylcholine release. Microdialysis was performed with a Ringer containing low concentrations (0.01 microM) of the acetylcholinesterase inhibitor, neostigmine. Quinpirole (10 microg/kg s.c.) decreased striatal dopamine and acetylcholine release. Administration of PD 128,907 (50 microg/kg) decreased dopamine but failed to affect acetylcholine release. SCH 39166 (50 microg/kg s.c.) stimulated dopamine release and reduced acetylcholine release. Pretreatment with quinpirole reduced (5 microg/kg s.c.) or completely prevented (10 microg/kg s.c.) the stimulation of dopamine release elicited by SCH 39166 (50 microg/kg s.c.); on the other hand, pretreatment with quinpirole (5 and 10 microg/kg) potentiated the reduction of striatal acetylcholine release induced by SCH 39166 (50 microg/kg s.c.). Similarly, pretreatment with PD 128,907 (50 microg/kg) which prevented the increase of dopamine release induced by SCH 39166 (50 microg/kg), potentiated the reduction of striatal acetylcholine transmission elicited by SCH 39166. Thus, pretreatment with low doses of quinpirole or PD 128,907 influences in opposite manner the effect of SCH 39166 on striatal dopamine and acetylcholine release, counteracting the increase of dopamine release and potentiating the decrease in acetylcholine release. These results provide further evidence for the existence of a tonic stimulatory input of endogenous dopamine on striatal acetylcholine transmission mediated by dopamine D(1) receptors.

A within-subjects microdialysis/behavioural study of the role of striatal acetylcholine in D1-dependent turning

In rats lesioned with 6-hydroxydopamine (6-OHDA) the effect of the noncompetitive N-methyl D-aspartate (NMDA) receptor antagonist, MK-801, the dopamine (DA) D2 receptor agonist quinpirole and the A2A adenosine antagonist SCH 58261 was studied on acetylcholine (ACh) release in the lesioned striatum and contralateral turning behaviour stimulated by the administration of the DA D1 receptor agonist CY 208-243. Administration of CY 208-243 (75, 100 and 200 microg/kg) to 6-OHDA-lesioned rats dose-dependently stimulated ACh release and induced contralateral turning. MK-801 (50 and 100 microg/kg) reduced basal ACh release (max 22%) and did not elicit any turning. MK-801 (50 and 100 microg/kg) potentiated the contralateral turning, but failed to modify the stimulation of ACh release elicited by 100 and 200 microg/kg of CY 208-243. MK-801 (100 microg/kg) prevented the increase in striatal ACh release evoked by the lower dose of CY 208-243 (75 microg/kg) but contralateral turning was not observed. The D2 receptor agonist quinpirole (30 and 60 microg/kg) elicited low-intensity contralateral turning and decreased basal ACh release. Quinpirole potentiated the D1-mediated contralateral turning behaviour elicited by CY 208-243 (100 microg/kg), but failed to affect the increase in ACh release elicited by the D1 agonist. The adenosine A2A receptor antagonist SCH 58261 (1 microg/kg i.v.) failed per se to elicit contralateral turning behaviour. SCH 58261 potentiated the contraversive turning induced by CY 208-243 but failed to affect the increase of ACh release. The results of the present study indicate that blockade of NMDA receptors by MK-801. stimulation of DA D2 receptors by quinpirole and blockade of adenosine A2A receptors by SCH 58261 potentiate the D1-mediated contralateral turning behaviour in DA denervated rats without affecting the action of the D1 agonist on ACh release. These observations do not support the hypothesis that the potentiation of D1-dependent contralateral turning by MK-801, quinpirole or SCH 58261 is mediated by changes in D1-stimulated release of ACh in the striatum.

Distribution of the nociceptin and nocistatin precursor transcript in the mouse central nervous system

The distribution of prepronociceptin messenger RNA, the recently identified endogenous ligand of the ORL1 receptor (opioid receptor-like-1), has been studied in the adult mouse central nervous system using in situ hybridization. Prepronociceptin is a new peptide precursor that generates, upon maturation, at least three bioactive peptides: nociceptin, noc2 and the recently described nocistatin. Considering both the density of labeled neurons per region and their intensity of labeling, the distribution of prepronociceptin messenger RNA-containing neurons can be summarized as follows: the highest level of prepronociceptin messenger RNA expression was detected in the septohippocampal nucleus, bed nucleus of the stria terminalis, central amygdaloid nucleus, and in selective thalamic nuclei such as the parafascicular, reticular, ventral lateral geniculate and zona incerta. High to moderate levels of prepronociceptin messenger RNA expression were detected in the lateral, ventral and medial septum, and were evident in brainstem structures implicated in descending antinociceptive pathways (e.g., the gigantocellular nucleus, raphe magnus nucleus, periaqueductal gray matter), and also observed in association with auditory relay nuclei such as the inferior colliculi, lateral lemniscus nucleus, medioventral preolivary nucleus and lateral superior nucleus. A moderate level of prepronociceptin messenger RNA expression was observed in the medial preoptic nucleus, ventromedial preoptic nucleus, periventricular nucleus, pedonculopontine tegmental nucleus, solitary tract nucleus and spinal trigeminal nucleus. A weak level of prepronociceptin messenger RNA expression was present in some areas, such as the cerebral cortex, endopiriform cortex, hippocampal formation, medial amygdaloid nucleus, anterior hypothalamic area, medial mammillary hypothalamic nuclei, retrorubral field and substantia nigra pars compacta. No labeled cells could be found in the caudate-putamen, nucleus accumbens and ventral tegmental area. The present data confirm that nociceptin is expressed in a broad array of regions of the central nervous system. In good correlation with the presently known physiological actions of nociceptin, they include, amongst others, brain areas conveying/integrating pain and auditory sensory afferences.

Different sensitivity of in vivo acetylcholine transmission to D1 receptor stimulation in shell and core of nucleus accumbens

We investigated whether D1 dopaminergic receptors modulate in vivo acetylcholine output in the shell and core areas of rat nucleus accumbens using the microdialysis technique. Subcutaneous injection (1, 2 and 3 mg/kg) of the D1 agonist SKF 82958 enhanced acetylcholine output in both areas of the nucleus accumbens while the selective D1 antagonist SCH 39166 (0.15 and 0.30 mg/kg, s.c.) lowered it. Both SKF 82958 and SCH 39166 were more effective in the shell than in the core region. The increase in acetylcholine release induced by SKF 82958 in the shell was tetrodotoxin-sensitive. The dopamine release inducer d-amphetamine (1 and 2mg/kg, s.c.) and the dopamine uptake inhibitor cocaine (10 and 20 mg/kg, i.p.) dose-dependently raised acetylcholine release in the shell and core areas. The dopaminergic stimulants, like the direct-acting D1 compounds, were more effective in the shell than in the core compartment of the nucleus accumbens. The acetylcholine increases in the shell induced by d-amphetamine (2 mg/kg), cocaine (20 mg/kg) and SKF 82958 (3 mg/kg) were antagonized by the D1 antagonists SCH 39166 (5 microM) and SCH 23390 (10 microM), applied locally by reverse dialysis. The results suggest that dopamine acting at the D1 receptors exerts a tonic stimulatory control over the cholinergic function of the shell and core compartments of the nucleus accumbens with the shell being more strongly influenced.

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

The neocortex and thalamus send dense glutaminergic projections to the neostriatum. The neocortex makes synaptic contact with spines of striatal projection neurons, and also targets a distinct class of GABAergic interneurons immunoreactive for the calcium-binding protein parvalbumin. We determined whether the parafascicular thalamic nucleus also targets striatal parvalbumin-immunoreactive interneurons. The anterograde tracer biotinylated dextranamine was injected into the parafascicular nucleus of adult rats. Double-labeled histochemistry/immunohistochemistry revealed overlapping thalamic fibers and parvalbumin-immunoreactive neurons in the neostriatum. Areas of overlap within the sensorimotor striatum were analysed by electron microscopy. Of 311 synaptic boutons originating from the parafascicular nucleus, 75.9% synapsed with unlabeled dendrites, 22.5% with unlabeled spines, and 1.3% had parvalbumin-immunoreactive dendrites as a postsynaptic target. Only 4% of all asymmetric synapses on parvalbumin-immunoreactive dendrites were derived from the parafascicular nucleus. A separate group of animals underwent bilateral neocortical deafferentation on the third postnatal day, prior to injection of anterograde tracer into the parafascicular nucleus of adult animals. These experiments were performed with the dual purpose of (i) reducing the possibility that thalamic inputs to parvalbumin-immunoreactive neurons are the result of transsynaptic uptake of tracer by a thalamo-cortico-striatal route, and (ii) determining whether competitive interactions between developing corticostriatal and thalamostriatal fibers may account for the relatively sparse thalamic input onto parvalbumin-immunoreactive interneurons. In decorticates, 219 striatal synaptic contacts derived from the parafascicular nucleus, out of which 77.2% were on unlabeled dendrites, 20.9% were upon unlabeled spines, and 0.9% targeted parvalbumin-immunoreactive dendrites. We conclude that the thalamic parafascicular nucleus indeed sends synaptic input to parvalbumin-immunoreactive striatal neurons. Parafascicular nucleus inputs to striatal parvalbumin-immunoreactive interneurons are sparse in comparison to other asymmetric inputs, most of which are likely to be of cortical origin. The synaptic profile of thalamostriatal inputs to parvalbumin-immunoreactive neurons and unlabeled elements is unchanged following neonatal decortication. This suggests that competitive interaction between developing thalamostriatal and corticostriatal projections is not a major mechanism determining synaptic input to striatal subpopulations.

The parafascicular thalamic nucleus but not the prefrontal cortex facilitates the nitric oxide/cyclic GMP pathway in rat striatum

We investigated whether the parafascicular thalamic nucleus and the prefrontal cortex, the two major excitatory inputs to the striatum, modulate the nitric oxide/cyclic GMP pathway in rat striatum. Electrical stimulation (10 pulses of 0.5 ms, 10 V applied at 10 Hz, 140 microA) delivered bilaterally to the parafascicular thalamic nucleus for a total of 4, 10 and 20 min, time-dependently facilitated cyclic GMP output in the dorsal striatum of freely moving rats, assessed by trans-striatal microdialysis. Electrical stimulation to the prefrontal cortex for a total duration of 20 min did not affect striatal cyclic GMP levels. The facilitatory effect observed after electrical stimulation of the parafascicular thalamic nucleus was blocked by co-perfusion with tetrodotoxin, suggesting that the effect is mediated by neuronal process(es). The non-competitive N-methyl-D-aspartate receptor antagonist, dizocilpine maleate (30 microM infused into the dorsal striatum), and the competitive one, 3-[(R)-carboxypiperazin-4-yl]-propyl-phosphonic acid (50 microM infused), but not local perfusion of the alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid antagonist, 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione (15 microM perfused locally), abolished the cyclic GMP response in the striatum. The nitric oxide synthase inhibitor, 7-nitroindazole, applied locally (1 mM), blocked the electrically evoked increase in striatal extracellular cyclic GMP. This increase was also prevented by local application (100 and 300 microM) of 1H-(1,2,4)-oxadiazolo-(4,3a)-quinoxalin-1-one, a selective inhibitor of soluble guanylyl cyclase. The results provide direct functional evidence of selective thalamic facilitation of the nitric oxide/cyclic GMP pathway in the dorsal striatum, through activation of N-methyl-D-aspartate receptors.

Subcellular and subsynaptic distribution of the NR1 subunit of the NMDA receptor in the neostriatum and globus pallidus of the rat: co-localization at synapses with the GluR2/3 subunit of the AMPA receptor

Glutamatergic neurotransmission in the neostriatum and the globus pallidus is mediated through NMDA-type as well as other glutamate receptors and is critical in the expression of basal ganglia function. In order to characterize the cellular, subcellular and subsynaptic localization of NMDA receptors in the neostriatum and globus pallidus, multiple immunocytochemical techniques were applied using antibodies that recognize the NR1 subunit of the NMDA receptor. In order to determine the spatial relationship between NMDA receptors and AMPA receptors, double labelling was performed with the NR1 antibodies and an antibody that recognizes the GluR2 and 3 subunits of the AMPA receptor. In the neostriatum all neurons with characteristics of spiny projection neurons, some interneurons and many dendrites and spines were immunoreactive for NR1. In the globus pallidus most perikarya and many dendritic processes were immunopositive. Immunogold methods revealed that most NR1 labelling is associated with asymmetrical synapses and, like the labelling for GluR2/3, is evenly spread across the synapse. Double immunolabelling revealed that in neostriatum, over 80% of NR1-positive axospinous synapses are also positive for GluR2/3. In the globus pallidus most NR1-positive synapses are positive for GluR2/3. In both regions many synapses labelled only for GluR2/3 were also detected. These results, together with previous data, suggest that NMDA and AMPA receptor subunits are expressed by the same neurons in the neostriatum and globus pallidus and that NMDA and AMPA receptors are, at least in part, colocalized at individual asymmetrical synapses. The synaptic responses to glutamate in these regions are thus likely be mediated by both AMPA and NMDA receptors at the level of individual synapses.

Dopamine D1-like receptor activation excites rat striatal large aspiny neurons in vitro

The aim of this study was to elucidate electrophysiologically the actions of dopamine and SKF38393, a D1-like dopamine receptor agonist, on the membrane excitability of striatal large aspiny neurons (cholinergic interneurons). Whole-cell and perforated patch-clamp recordings were made of striatal cholinergic neurons in rat brain slice preparations. Bath application of dopamine (1-100 microM) evoked a depolarization/inward current with an increase, a decrease, or no change in membrane conductance in a dose-dependent manner. This effect was antagonized by SCH23390, a D1-like dopamine receptor antagonist. The current-voltage relationships of the dopamine-induced current determined in 23 cells suggested two conductances. In 10 cells the current reversed at -94 mV, approximately equal to the K+ equilibrium potential (EK); in three cells the I-V curves remained parallel, whereas in 10 cells the current reversed at -42 mV, which suggested an involvement of a cation permeable channel. Change in external K+ concentration shifted the reversal potential as expected for Ek in low Na+ solution. The current observed in 2 mM Ba2+-containing solution reversed at -28 mV. These actions of dopamine were mimicked by application of SKF38393 (1-50 microM) or forskolin (10 microM), an adenylyl cyclase activator, and were blocked by SCH23390 (10 microM) or SQ22536 (300 microM), an inhibitor of adenylyl cyclase. These data indicate, first, that dopamine depolarizes the striatal large aspiny neurons by a D1-mediated suppression of resting K+ conductance and an opening of a nonselective cation channel and, second, that both mechanisms are mediated by an adenylyl cyclase-dependent pathway.

N-methyl-D-aspartate-evoked release of [3H]acetylcholine in striatal compartments of the rat: regulatory roles of dopamine and GABA

The N-methyl-D-aspartate-evoked release of [3H]acetylcholine previously formed from [3H]choline was estimated in striosome- (identified by [3H]naloxone binding) or matrix-enriched areas of the rat striatum using an in vitro microsuperfusion procedure. Experiments were performed in either the absence or the presence of dopaminergic and/or GABAergic receptor antagonists. Although the cell bodies of the cholinergic interneurons were mainly found in the matrix, in the absence of magnesium, N-methyl-D-aspartate (50 microM) stimulated the release of [3H]acetylcholine in both striatal compartments. These responses were blocked by either magnesium, dizocilpine maleate, 7-chlorokynurenate or tetrodotoxin. N-Methyl-D-aspartate responses were concentration-dependent, but the 1 mM N-methyl-D-aspartate response was higher in striosomes than in the matrix. The co-application of D-serine (10 microM) enhanced the 10 microM N-methyl-D-aspartate response in both compartments, but reduced those induced by 1 mM N-methyl-D-aspartate, this reduction being higher in striosomes. The blockade of dopaminergic transmission with the D2 and D1 dopaminergic receptor antagonists, (-)-sulpiride (1 microM) and SCH23390 (1 microM), was without effect on the 50 microM N-methyl-D-aspartate-evoked release of [3H]acetylcholine, but markedly enhanced the 1 mM N-methyl-D-aspartate+D-serine-evoked response in striosomes and to a lesser extent in the matrix. Disinhibitory responses of similar amplitude were observed not only in striosomes but also in the matrix when (-)-sulpiride was used alone, while SCH23390 alone enhanced the 1 mM N-methyl-D-aspartate+D-serine response only in striosomes and to a lower extent than (-)-sulpiride. These results indicate that D2 receptors are mainly involved in the inhibitory effect of dopamine on the 1 mM N-methyl-D-aspartate+D-serine-evoked release of [3H]acetylcholine. They also show that the stimulation of D1 receptors can either reduce (striosomes) or enhance (matrix) this response, since in the latter case the effect induced by the combined application of the D1 and D2 receptor antagonists was smaller than that observed with the D2 receptor antagonist alone. Indicating that released GABA facilitates N-methyl-D-aspartate responses, the blockade of GABAA receptors with bicuculline (5 microM) reduced the 50 microM N-methyl-D-aspartate-evoked release of [3H]acetylcholine in both striatal compartments and the 1 mM N-methyl-D-aspartate+D-serine response in the matrix. These effects result from an inhibition by GABA of the evoked release of dopamine, since the reducing effects of bicuculline on N-methyl-D-aspartate responses were not observed under the complete blockade of dopaminergic transmission by the D1 and D2 receptor antagonists. Further demonstrating a facilitatory role of GABA in the control of N-methyl-D-aspartate-evoked release of [3H]acetylcholine, in the presence of bicuculline, (-)-sulpiride and SCH23390 alone or in combination enhanced, in both compartments, the responses induced not only by 1 mM N-methyl-D-aspartate+D-serine, but also by 50 microM N-methyl-D-aspartate.

The parafascicular thalamic nucleus modulates messenger RNA encoding glutamate decarboxylase 67 in rat striatum

We investigated whether the parafascicular thalamostriatal pathway, one of the major excitatory inputs to the striatum, regulates the expression in rat striatum of messenger RNA encoding two isoforms of glutamate decarboxylase (mol. wt 67,000: glutamate decarboxylase 67 and mol. wt 65,000: glutamate decarboxylase 65). Acute (one day) and chronic (14 days) electrolytic lesions of the parafascicular nucleus resulted in 58% and 23% decreases in glutamate decarboxylase 67 messenger RNA expression, respectively, as determined by northern blot analysis. Glutamate decarboxylase 65 messenger RNA was not modified by either lesion. Sections of sham- and acute-lesioned striata were processed for in situ hybridization histochemistry at the single cell level with an RNA probe for glutamate decarboxylase 67. Labelling of glutamate decarboxylase 67 messenger RNA was decreased in both types of cells known to be present in the striatum, i.e. the lightly and the very densely-labelled neurons. The frequency distribution of glutamate decarboxylase 67 labelling per neuron in the lesioned striata, in fact, was shifted to the left and its median was lower than in the sham-lesioned striata. In view of the excitatory nature of the thalamostriatal pathway, we examined the subtype of glutamate receptors modulating the glutamate decarboxylase 67 gene expression. The N-methyl-D-aspartate-type receptor antagonist, dizocilpine, at 0.1-0.5 mg/kg i.p., produced a marked and persistent reduction in striatal glutamate decarboxylase 67 messenger RNA. The non-N-methyl-D-aspartate receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (12 nmol/side, i.c.v.) had no such effect. The results provide evidence that excitatory thalamostriatal afferents selectively modulate the gene expression of glutamate decarboxylase 67, probably through the N-methyl-D-aspartate subtype of glutamate receptors.

The cerebral cortex and parafascicular thalamic nucleus facilitate in vivo acetylcholine release in the rat striatum through distinct glutamate receptor subtypes

Electrical stimulation (ten pulses of 0.5 ms, 10 V applied over 10 s at 10 Hz, 140 microA) delivered bilaterally to the prefrontal cortex or the parafascicular thalamic nucleus of freely moving rats facilitated acetylcholine release in dorsal striata, assessed by trans-striatal microdialysis. The facilitatory effects were blocked by coperfusion with 5 microM tetrodotoxin, suggesting that the release was of neuronal origin. The response of the striatal cholinergic neurons to prefrontal cortical stimulation was short-lived and required a longer period of stimulation (20 min) that the response to thalamic stimulation (4 min) to reach maximal effect. The alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate glutamatergic receptor antagonist 6,7-dinitroquinoxaline-2,3-dione [DNQX; 12 nmol per side, intracerebroventricularly (i.c.v.)] and the AMPA antagonist 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione (NBQX; 12 nmol per side, i.c.v. or 12.8 microM infused into the striatum), but not the NMDA-type receptor antagonist MK-801 (0.2 mg/kg, i.p.), abolished the facilitatory effect on striatal acetylcholine release evoked by stimulation of the prefrontal cortex. By contrast, DNQX or NBQX did not prevent the increase in striatal acetylcholine release evoked by parafascicular nucleus stimulation, but MK-801, in accordance with previous results, did so. MK-801 by itself lowered striatal acetylcholine output while DNQX and NBQX did not. The results provide in vivo evidence that the cerebral cortex facilitates cholinergic activity in the dorsal striatum apparently through the non-tonic activation of AMPA-type glutamatergic receptors while the parafascicular nucleus does this through tonic activation of NMDA receptors. Both glutamate receptor types are probably located in the striatum. The overall results suggest that the two pathways operate independently to regulate striatal cholinergic activity through distinct mechanisms.