Metabotropic glutamate receptors and cell-type-specific vulnerability in the striatum: implication for ischemia and Huntington's disease

Targeting calciumopathy for neuroprotection: focus on calcium channels Cav1, Orai1 and P2X7

As the uncontrolled entry of calcium ions (Ca2+) through plasmalemmal calcium channels is a cell death trigger, the conjecture is here raised that mitigating such an excess of Ca2+ entry should rescue from death the vulnerable neurons in neurodegenerative diseases (NDDs). However, this supposition has failed in some clinical trials (CTs). Thus, a recent CT tested whether isradipine, a blocker of the Cav1 subtype of voltage-operated calcium channels (VOCCs), exerted a benefit in patients with Parkinson's disease (PD); however, outcomes were negative. This is one more of the hundreds of CTs done under the principle of one-drug-one-target, that have failed in Alzheimer's disease (AD) and other NDDs during the last three decades. As there are myriad calcium channels to let Ca2+ ions gain the cell cytosol, it seems reasonable to predict that blockade of Ca2+ entry through a single channel may not be capable of preventing the Ca2+ flood of cells by the uncontrolled Ca2+ entry. Furthermore, as Ca2+ signaling is involved in the regulation of myriad functions in different cell types, it seems also reasonable to guess that a therapy should be more efficient by targeting different cells with various drugs. Here, we propose to mitigate Ca2+ entry by the simultaneous partial blockade of three quite different subtypes of plasmalemmal calcium channels that is, the Cav1 subtype of VOCCs, the Orai1 store-operated calcium channel (SOCC), and the purinergic P2X7 calcium channel. All three channels are expressed in both microglia and neurons. Thus, by targeting the three channels with a combination of three drug blockers we expect favorable changes in some of the pathogenic features of NDDs, namely (i) to mitigate Ca2+ entry into microglia; (ii) to decrease the Ca2+-dependent microglia activation; (iii) to decrease the sustained neuroinflammation; (iv) to decrease the uncontrolled Ca2+ entry into neurons; (v) to rescue vulnerable neurons from death; and (vi) to delay disease progression. In this review we discuss the arguments underlying our triad hypothesis in the sense that the combination of three repositioned medicines targeting Cav1, Orai1, and P2X7 calcium channels could boost neuroprotection and delay the progression of AD and other NDDs.

Glutamate receptor endocytosis and signaling in neurological conditions

The non-essential amino acid glutamate acts as a major excitatory neurotransmitter and plays a significant role in the central nervous system (CNS). It binds with two different types of receptors, ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs), responsible for the postsynaptic excitation of neurons. They are important for memory, neural development and communication, and learning. Endocytosis and subcellular trafficking of the receptor are essential for the regulation of receptor expression on the cell membrane and excitation of the cells. The endocytosis and trafficking of the receptor are dependent on its type, ligand, agonist, and antagonist present. This chapter discusses the types of glutamate receptors, their subtypes, and the regulation of their internalization and trafficking. The roles of glutamate receptors in neurological diseases are also briefly discussed.

ImShot: An Open-Source Software for Probabilistic Identification of Proteins In Situ and Visualization of Proteomics Data

Imaging mass spectrometry (IMS) has developed into a powerful tool allowing label-free detection of numerous biomolecules in situ. In contrast to shotgun proteomics, proteins/peptides can be detected directly from biological tissues and correlated to its morphology leading to a gain of crucial clinical information. However, direct identification of the detected molecules is currently challenging for MALDI-IMS, thereby compelling researchers to use complementary techniques and resource intensive experimental setups. Despite these strategies, sufficient information could not be extracted because of lack of an optimum data combination strategy/software. Here, we introduce a new open-source software ImShot that aims at identifying peptides obtained in MALDI-IMS. This is achieved by combining information from IMS and shotgun proteomics (LC-MS) measurements of serial sections of the same tissue. The software takes advantage of a two-group comparison to determine the search space of IMS masses after deisotoping the corresponding spectra. Ambiguity in annotations of IMS peptides is eliminated by introduction of a novel scoring system that identifies the most likely parent protein of a detected peptide in the corresponding IMS dataset. Thanks to its modular structure, the software can also handle LC-MS data separately and display interactive enrichment plots and enriched Gene Ontology terms or cellular pathways. The software has been built as a desktop application with a conveniently designed graphic user interface to provide users with a seamless experience in data analysis. ImShot can run on all the three major desktop operating systems and is freely available under Massachusetts Institute of Technology license.

Cell competition from development to neurodegeneration

Cell competition is a process by which suboptimal cells are eliminated to the benefit of cells with higher fitness. It is a surveillance mechanism that senses differences in the fitness status by several modes, such as expression of fitness fingerprints, survival factor uptake rate and resistance to mechanical stress. Fitness fingerprints-mediated cell competition recognizes isoforms of the transmembrane protein Flower, and translates the relative fitness of cells into distinct fates through the Flower code. Impairments in cell competition potentiate the development of diseases like cancer and ageing-related pathologies. In cancer, malignant cells acquire a supercompetitor behaviour, killing the neighbouring cells and overtaking the tissue, thus avoiding elimination. Neurodegenerative disorders affect millions of people and are characterized by cognitive decline and locomotor deficits. Alzheimer's disease is the most common form of dementia, and one of the largely studied diseases. However, the cellular processes taking place remain unclear. Drosophila melanogaster is an emerging neurodegeneration model due to its versatility as a tool for genetic studies. Research in a Drosophila Alzheimer's disease model detected fitness markers in the suboptimal and hyperactive neurons, thus establishing a link between cell competition and Alzheimer's disease. In this Review, we overview cell competition and the new insights related to neurodegenerative disorders, and discuss how research in the field might contribute to the development of new therapeutic targets for these diseases.

Group 1 Metabotropic Glutamate Receptors in Neurological and Psychiatric Diseases: Mechanisms and Prospective

Metabotropic glutamate receptors (mGluRs) are G-protein coupled receptors that are activated by glutamate in the central nervous system (CNS). Basically, mGluRs contribute to fine-tuning of synaptic efficacy and control the accuracy and sharpness of neurotransmission. Among eight subtypes, mGluR1 and mGluR5 belong to group 1 (Gp1) family, and are implicated in multiple CNS disorders, such as Alzheimer’s disease, autism, Parkinson’s disease, and so on. In the present review, we systematically discussed underlying mechanisms and prospective of Gp1 mGluRs in a group of neurological and psychiatric diseases, including Alzheimer’s disease, Parkinson’s disease, autism spectrum disorder, epilepsy, Huntington’s disease, intellectual disability, Down’s syndrome, Rett syndrome, attention-deficit hyperactivity disorder, addiction, anxiety, nociception, schizophrenia, and depression, in order to provide more insights into the therapeutic potential of Gp1 mGluRs.

Alterations of Calcium Channels in a Mouse Model of Huntington's Disease and Neuroprotection by Blockage of CaV1 Channels

Huntington’s disease (HD) is a neurodegenerative autosomal dominant disorder, characterized by symptoms of involuntary movement of the body, loss of cognitive function, psychiatric disorder, leading inevitably to death. It has been previously described that higher levels of brain expression of Cav1 channels are involved in major neurodegenerative disorders, such as Alzheimer’s disease and Parkinson’s disease. Our results demonstrate that a bacterial artificial chromosome (BAC)-mediated transgenic mouse model (BACHD mice) at the age of 3 and 12 months exhibits significantly increased Cav1.2 protein levels in the cortex, as compared with wild-type littermates. Importantly, electrophysiological analyses confirm a significant increase in L-type Ca2+ currents and total Ca2+ current density in cortical neurons from BACHD mice. By using an in vitro assay to measure neuronal cell death, we were able to observe neuronal protection against glutamate toxicity after treatment with Cav1 blockers, in wild-type and, more importantly, in BACHD neurons. According to our data, Cav1 blockers may offer an interesting strategy for the treatment of HD. Altogether, our results show that mutant huntingtin (mHtt) expression may cause a dysregulation of Cav1.2 channels and we hypothesize that this contributes to neurodegeneration during HD.

Role of Metabotropic Glutamate Receptors in Neurological Disorders

Glutamate is a fundamental excitatory neurotransmitter in the mammalian central nervous system (CNS), playing key roles in memory, neuronal development, and synaptic plasticity. Moreover, excessive glutamate release has been implicated in neuronal cell death. There are both ionotropic and metabotropic glutamate receptors (mGluRs), the latter of which can be divided into eight subtypes and three subgroups based on homology sequence and their effects on cell signaling. Indeed, mGluRs exert fine control over glutamate activity by stimulating several cell-signaling pathways via the activation of G protein-coupled (GPC) or G protein-independent cell signaling. The involvement of specific mGluRs in different forms of synaptic plasticity suggests that modulation of mGluRs may aid in the treatment of cognitive impairments related to several neurodevelopmental/psychiatric disorders and neurodegenerative diseases, which are associated with a high economic and social burden. Preclinical and clinical data have shown that, in the CNS, mGluRs are able to modulate presynaptic neurotransmission by fine-tuning neuronal firing and neurotransmitter release in a dynamic, activity-dependent manner. Current studies on drugs that target mGluRs have identified promising, innovative pharmacological tools for the treatment of neurodegenerative and neuropsychiatric conditions, including chronic pain.

Cause or compensation?-Altered neuronal Ca2+ handling in Huntington's disease

Huntington's disease (HD) is a hereditary neurodegenerative disorder of typically middle-aged onset for which there is no disease-modifying treatment. Caudate and putamen medium-sized spiny projection neurons (SPNs) most severely degenerate in HD. However, it is unclear why mutant huntingtin protein (mHTT) is preferentially toxic to these neurons or why symptoms manifest only relatively late in life. mHTT interacts with numerous neuronal proteins. Likewise, multiple SPN cellular processes have been described as altered in various HD models. Among these, altered neuronal Ca²⁺ influx and intracellular Ca²⁺ handling feature prominently and are addressed here. Specifically, we focus on extrasynaptic NMDA-type glutamate receptors, endoplasmic reticulum IP3 receptors, and mitochondria. As mHTT is expressed throughout development, compensatory processes will likely be mounted to mitigate any deleterious effects. Although some compensations can lessen mHTT's disruptive effects, others-such as upregulation of the ER-refilling store-operated Ca²⁺ channel response-contribute to pathogenesis. A causation-based approach is therefore necessary to decipher the complex sequence of events linking mHTT to neurodegeneration, and to design rational therapeutic interventions. With this in mind, we highlight evidence, or lack thereof, that the above alterations in Ca²⁺ handling occur early in the disease process, clearly interact with mHTT, and show disease-modifying potential when reversed in animals.

Striatal Vulnerability in Huntington's Disease: Neuroprotection Versus Neurotoxicity

Huntington’s disease (HD) is an autosomal dominant neurodegenerative disease caused by the expansion of a CAG trinucleotide repeat encoding an abnormally long polyglutamine tract (PolyQ) in the huntingtin (Htt) protein. In HD, striking neuropathological changes occur in the striatum, including loss of medium spiny neurons and parvalbumin-expressing interneurons accompanied by neurodegeneration of the striosome and matrix compartments, leading to progressive impairment of reasoning, walking and speaking abilities. The precise cause of striatal pathology in HD is still unknown; however, accumulating clinical and experimental evidence suggests multiple plausible pathophysiological mechanisms underlying striatal neurodegeneration in HD. Here, we review and discuss the characteristic neurodegenerative patterns observed in the striatum of HD patients and consider the role of various huntingtin-related and striatum-enriched proteins in neurotoxicity and neuroprotection.

Metabotropic glutamate receptor 5 upregulates surface NMDA receptor expression in striatal neurons via CaMKII

Metabotropic and ionotropic glutamate receptors are closely clustered in postsynaptic membranes and are believed to interact actively with each other to control excitatory synaptic transmission. Metabotropic glutamate receptor 5 (mGluR5), for example, has been well documented to potentiate ionotropic NMDA receptor activity, although underlying mechanisms are poorly understood. In this study, we investigated the role of mGluR5 in regulating trafficking and subcellular distribution of NMDA receptors in adult rat striatal neurons. We found that the mGluR1/5 agonist DHPG concentration-dependently increased NMDA receptor GluN1 and GluN2B subunit expression in the surface membrane. Meanwhile, DHPG reduced GluN1 and GluN2B levels in the intracellular compartment. The effect of DHPG was blocked by an mGluR5 selective antagonist MTEP but not by an mGluR1 selective antagonist 3-MATIDA. Pretreatment with an inhibitor or a specific inhibitory peptide for synapse-enriched Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) also blocked the DHPG-stimulated redistribution of GluN1 and GluN2B. In addition, DHPG enhanced CaMKIIα activity and elevated GluN2B phosphorylation at a CaMKII-sensitive site (serine 1303). These results demonstrate that mGluR5 regulates trafficking of NMDA receptors in striatal neurons. Activation of mGluR5 appears to induce rapid trafficking of GluN1 and GluN2B to surface membranes through a signaling pathway involving CaMKII.

Intravenous Bone Marrow Stem Cell Grafts Preferentially Migrate to Spleen and Abrogate Chronic Inflammation in Stroke

BACKGROUND AND PURPOSE

Adult stem cell therapy is an experimental stroke treatment. Here, we assessed homing and anti-inflammatory effects of bone marrow stromal cells (hBMSCs) in chronic stroke.

METHODS

At 60 days post stroke, adult Sprague-Dawley rats received intravenous hBMSCs (4×10(6) labeled or nonlabeled cells) or vehicle (saline). A sham surgery group served as additional control. In vivo imaging was conducted between 1 hour and 11 days post transplantation, followed by histological examination.

RESULTS

Labeled hBMSCs migrated to spleen which emitted significantly higher fluorescent signal across all time points, especially during the first hour, and were modestly detected in the head region at the 12 hours and 11 days, compared with nonlabeled hBMSCs and vehicle-infused stroke animals, or sham (P<0.05). At 11 days post transplantation, ex vivo imaging confirmed preferential hBMSC migration to the spleen over the brain. Hematoxylin and eosin staining revealed significant 15% and 30% reductions in striatal infarct and peri-infarct area, and a trend of rescue against neuronal loss in the hippocampus. Unbiased stereology showed significant 75% and 60% decrements in major histocompatibility complex II-activated inflammatory cells in gray and white matter, and a 43% diminution in tumor necrosis factor-α cell density in the spleen of transplanted stroke animals compared with vehicle-infused stroke animals (P<0.05). Human antigen immunostaining revealed 0.03% hBMSCs survived in spleen and only 0.0007% in brain. MSC migration to spleen, but not brain, inversely correlated with reduced infarct, peri-infarct, and inflammation.

CONCLUSIONS

hBMSC transplantation is therapeutic in chronic stroke possibly by abrogating the inflammation-plagued secondary cell death.

Striatal cholinergic interneuron regulation and circuit effects

The striatum plays a central role in motor control and motor learning. Appropriate responses to environmental stimuli, including pursuit of reward or avoidance of aversive experience all require functional striatal circuits. These pathways integrate synaptic inputs from limbic and cortical regions including sensory, motor and motivational information to ultimately connect intention to action. Although many neurotransmitters participate in striatal circuitry, one critically important player is acetylcholine (ACh). Relative to other brain areas, the striatum contains exceptionally high levels of ACh, the enzymes that catalyze its synthesis and breakdown, as well as both nicotinic and muscarinic receptor types that mediate its postsynaptic effects. The principal source of striatal ACh is the cholinergic interneuron (ChI), which comprises only about 1-2% of all striatal cells yet sends dense arbors of projections throughout the striatum. This review summarizes recent advances in our understanding of the factors affecting the excitability of these neurons through acute effects and long term changes in their synaptic inputs. In addition, we discuss the physiological effects of ACh in the striatum, and how changes in ACh levels may contribute to disease states during striatal dysfunction.

Metabotropic glutamate receptor 5 positive allosteric modulators are neuroprotective in a mouse model of Huntington's disease

Background and Purpose

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by a polyglutamine expansion in the huntingtin protein. We have previously demonstrated that the cell signalling of the metabotropic glutamate receptor 5 (mGluR5) is altered in a mouse model of HD. Although mGluR5-dependent protective pathways are more activated in HD neurons, intracellular Ca²⁺ release is also more pronounced, which could contribute to excitotoxicity. In the present study, we aim to investigate whether mGluR5 positive allosteric modulators (PAMs) could activate protective pathways without triggering high levels of Ca²⁺ release and be neuroprotective in HD.

Experimental Approach

We performed a neuronal cell death assay to determine which drugs are neuroprotective, Western blot and Ca²⁺ release experiments to investigate the molecular mechanisms involved in this neuroprotection, and object recognition task to determine whether the tested drugs could ameliorate HD memory deficit.

Key Results

We find that mGluR5 PAMs can protect striatal neurons from the excitotoxic neuronal cell death promoted by elevated concentrations of glutamate and NMDA. mGluR5 PAMs are capable of activating Akt without triggering increased intracellular Ca²⁺ concentration ([Ca²⁺]_i); and Akt blockage leads to loss of PAM-mediated neuroprotection. Importantly, PAMs' potential as drugs that may be used to treat neurodegenerative diseases is highlighted by the neuroprotection exerted by mGluR5 PAMs on striatal neurons from a mouse model of HD, BACHD. Moreover, mGluR5 PAMs can activate neuroprotective pathways more robustly in BACHD mice and ameliorate HD memory deficit.

Conclusions and Implications

mGluR5 PAMs are potential drugs that may be used to treat neurodegenerative diseases, especially HD.

The mGluR5 positive allosteric modulator, CDPPB, ameliorates pathology and phenotypic signs of a mouse model of Huntington's disease

Huntington's disease (HD) is an autosomal-dominant neurodegenerative disorder caused by a polyglutamine expansion in the amino-terminal region of the huntingtin protein (htt), leading to motor dysfunction, cognitive decline, psychiatric alterations, and death. The metabotropic glutamate receptor 5 (mGluR5) has been implicated in HD and we have recently demonstrated that mGluR5 positive allosteric modulators (PAMs) are neuroprotective in vitro. In the present study we demonstrate that the mGluR5 PAM, CDPPB, is a potent neuroprotective drug, in vitro and in vivo, capable of delaying HD-related symptoms. The HD mouse model, BACHD, exhibits many HD features, including neuronal cell loss, htt aggregates, motor incoordination and memory impairment. However, chronic treatment of BACHD mice with CDPPB 1.5 mg/kg s.c. for 18 weeks increased the activation of cell signaling pathways important for neuronal survival, including increased AKT and ERK1/2 phosphorylation and augmented the BDNF mRNA expression. CDPPB chronic treatment was also able to prevent the neuronal cell loss that takes place in the striatum of BACHD mice and decrease htt aggregate formation. Moreover, CDPPB chronic treatment was efficient to partially ameliorate motor incoordination and to rescue the memory deficit exhibited by BACHD mice. Importantly, no toxic effects or stereotypical behavior were observed upon CDPPB chronic treatment. Thus, CDPPB is a potential drug to treat HD, preventing neuronal cell loss and htt aggregate formation and delaying HD symptoms.

Metabotropic glutamate receptor 5 as a potential therapeutic target in Huntington's disease

INTRODUCTION

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by a polyglutamine expansion in the amino-terminal region of the huntingtin (htt) protein, which underlies the loss of striatal and cortical neurons. Glutamate has been implicated in a number of neurodegenerative diseases, and several studies suggest that the metabotropic glutamate receptor 5 (mGluR5) may represent a target for the treatment of HD.

AREAS COVERED

The main goal of this review is to discuss the current data in the literature regarding the role of mGluR5 in HD and evaluate the potential of mGluR5 as a therapeutic target for the treatment of HD. mGluR5 is highly expressed in the brain regions affected in HD and is involved in movement control. Moreover, mGluR5 interacts with htt and mutated htt profoundly affects mGluR5 signaling. However, mGluR5 stimulation can activate both neuroprotective and neurotoxic signaling pathways, depending on the context of activation.

EXPERT OPINION

Although the data published so far strongly indicate that mGluR5 plays a major role in HD-associated neurodegeneration, htt aggregation and motor symptoms, it is not clear whether mGluR5 stimulation can diminish or intensify neuronal cell loss and HD progression. Thus, future experiments will be necessary to further investigate the outcome of drugs acting on mGluR5 for the treatment of neurodegenerative diseases.

A real-time method for measuring cAMP production modulated by Gαi/o-coupled metabotropic glutamate receptors

Group II and group III metabotropic glutamate (mGlu) receptors are G protein-coupled receptors (GPCRs) that inhibit adenylyl cyclase via activation of Gαi/o. The purpose of this study was to design a universal method that overcomes previous challenges in consistently measuring group II and group III mGlu-receptor (mGluR) activation in stably transfected systems. In Chinese hamster ovary (CHO) cells stably transfected with the GloSensor cAMP biosensor, we optimized conditions for simple and highly reproducible (<5% S.E.M.) measurements of cAMP in real time. The GloSensor cAMP biosensor is a recombinant firefly luciferase conjugated to a cAMP-binding domain, where cAMP binding promotes a conformational shift within the GloSensor protein, inducing luciferase activity; cAMP levels are positively correlated with light output resulting from the luciferase-mediated breakdown of d-luciferin. Each group II and group III mGluR was then stably transfected into the CHO-GloSensor cell line, and experimental conditions were optimized for each receptor. During assay optimization, we observed ion sensitivity of several receptors and inverse agonist activity of the antagonist, LY341495 [2-[(1S,2S)-2-carboxycyclopropyl]-3-(9H-xanthen-9-yl)-d-alanine]. Although these phenomena have been previously reported, they remain poorly understood, emphasizing the GloSensor assay as an important tool with which to study group II and group III mGlu receptors. Our results highlight many advantages of using the GloSensor method for measuring activation of group II and group III mGlu receptors, and they further suggest that corresponding methods designed to measure activation of any Gαi/o- or Gαs-coupled GPCR will be similarly advantageous.

Huntington's disease and Group I metabotropic glutamate receptors

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterized by involuntary body movement, cognitive impairment and psychiatric disturbance. A polyglutamine expansion in the amino-terminal region of the huntingtin (htt) protein is the genetic cause of HD. Htt protein interacts with a wide variety of proteins, and htt mutation causes cell signaling alterations in various neurotransmitter systems, including dopaminergic, glutamatergic, and cannabinoid systems, as well as trophic factor systems. This review will overview recent findings concerning htt-promoted alterations in cell signaling that involve different neurotransmitters and trophic factor systems, especially involving mGluR1/5, as glutamate plays a crucial role in neuronal cell death. The neuronal cell death that takes place in the striatum and cortex of HD patients is the most important factor underlying HD progression. Metabotropic glutamate receptors (mGluR1 and mGluR5) have a very controversial role in neuronal cell death and it is not clear whether mGluR1/5 activation either protects or exacerbates neuronal death. Thus, understanding how mutant htt protein affects glutamatergic receptor signaling will be essential to further establish a role for glutamate receptors in HD and develop therapeutic strategies to treat HD.

Increased GAD expression in the striatum after transient cerebral ischemia

Striatum is one of the brain regions that are highly sensitive to transient cerebral ischemia. Most of the striatal neurons die shortly after ischemia but interneurons including large aspiny (LA) neurons survive the same insult. Previous studies have shown that inhibitory synaptic transmission is enhanced in LA neurons after ischemia. The present study is aimed at revealing the mechanisms underlying this phenomenon. Immunohistochemical studies and Western blotting were performed to examine the expression of glutamic decarboxylase (GAD), the key enzyme in the synthesis of GABA, in the striatum. GAD65 expression and the number of GAD67-positive cells were increased after ischemia. GAD67-positive cells in the striatum co-expressed GAD65 after ischemia. The increase of GAD67-positive cells did not result from neurogenesis. Double-labeling of GAD67 and SOM indicates that some of the GAD67-positive cells are from the phenotypic shift of pre-existing somatostatin (SOM)-containing GABAergic interneurons after ischemia. Facilitation of inhibitory synaptic transmission by muscimol, a specific GABA(A) receptor agonist, increased the number of survived cells in the striatum after ischemia. Altogether, these data suggest that GAD expression is increased in the striatum after ischemia, which might contribute to the facilitated inhibitory synaptic transmission and the consequent survival of LA neurons.

Impaired plasticity at specific subset of striatal synapses in the Ts65Dn mouse model of Down syndrome

BACKGROUND

Trisomy 21 or Down syndrome (DS) is the most frequent genetic cause of mental retardation. There is limited insight into the biological basis for the cognitive and motor deficits in DS. Because the striatum plays a key role in the regulation and learning of voluntary movements and in cognitive processes, our study was aimed at investigating striatal synaptic transmission and plasticity in a well-accepted genetic model of DS.

METHODS

Electrophysiological recordings were performed in a corticostriatal slice preparation from trisomic (Ts65Dn) and wild-type mice. Synaptic properties and plasticity, long-term potentiation (LTP) and long-term depression (LTD), were investigated.

RESULTS

The basal electrophysiological properties of striatal principal spiny neurons and cholinergic interneurons were spared in the Ts65Dn mouse model of DS. Striatal principal spiny neurons from Ts65Dn mice maintained their ability to undergo LTP and LTD. Conversely, LTP was lost in striatal cholinergic interneurons of Ts65Dn mice. The loss of LTP in striatal cholinergic interneurons of Ts65Dn mice was accompanied by a severe impairment of endogenous cholinergic signaling within the striatum.

CONCLUSIONS

The intrastriatal cholinergic system that was thought to be spared in DS is functionally altered in the Ts65Dn genetic model of DS. Altered cholinergic transmission might play a critical role in the pathophysiology of motor and cognitive deficits in DS, leading to an abnormal processing of neuronal inputs within the basal ganglia. Targeting striatal cholinergic transmission might represent a new therapeutic strategy in DS.

Metabotropic glutamate receptor-mediated cell signaling pathways are altered in a mouse model of Huntington's disease

Huntington's disease (HD) is an autosomal-dominant neurodegenerative disorder caused by a polyglutamine expansion in the huntingtin protein (Htt). Group I metabotropic glutamate receptors (mGluRs) are coupled to G(alphaq) and play an important role in neuronal survival. We have previously demonstrated that mGluRs interact with Htt. Here we used striatal neuronal primary cultures and acute striatal slices to demonstrate that mGluR-mediated signaling pathways are altered in a presymptomatic mouse model of HD (Hdh(Q111/Q111)), as compared to those of control mice (Hdh(Q20/Q20)). mGluR1/5-mediated inositol phosphate (InsP) formation is desensitized in striatal slices from Hdh(Q111/Q111) mice and this desensitization is PKC-mediated. Despite of decreased InsP formation, (S)-3,5-dihydroxylphenylglycine (DHPG)-mediated Ca(2+) release is higher in Hdh(Q111/Q111) than in Hdh(Q20/Q20) neurons. Furthermore, mGluR1/5-stimulated AKT and extracellular signal-regulated kinase (ERK) activation is altered in Hdh(Q111/Q111) mice. Basal AKT activation is higher in Hdh(Q111/Q111) neurons and this increase is mGluR5 dependent. Moreover, mGluR5 activation leads to higher levels of ERK activation in Hdh(Q111/Q111) than in Hdh(Q20/Q20) striatum. PKC inhibition not only brings Hdh(Q111/Q111) DHPG-stimulated InsP formation to Hdh(Q20/Q20) levels, but also causes an increase in neuronal cell death in Hdh(Q111/Q111) neurons. However, PKC inhibition does not modify neuronal cell death in Hdh(Q20/Q20) neurons, suggesting that PKC-mediated desensitization of mGluR1/5 in Hdh(Q111/Q111) mice might be protective in HD. Together, these data indicate that group I mGluR-mediated signaling pathways are altered in HD and that these cell signaling adaptations could be important for striatal neurons survival.

Agonist-induced internalization of mGluR1alpha is mediated by caveolin

Agonist-induced internalization of metabotropic glutamate receptors (mGluRs) plays an important role in neuronal signaling. Although internalization of mGluRs has been reported to be mediated by clathrin-dependent pathway, studies describing clathrin-independent pathways are emerging. Here, we report that agonist-induced internalization of mGluR1alpha is mediated by caveolin. We show that two caveolin-binding motifs of mGluR1alpha interact with caveolin1/2. Using cell surface-immunoprecipitation and total internal reflection fluorescence imaging, we found that agonist-induced internalization of mGluR1alpha is regulated by caveolin-binding motifs of the receptor in heterologous cells. Moreover, in the cerebellum, group I mGluR agonist dihydroxyphenylglycol increased the interaction of phosphorylated caveolin with mGluR1alpha. This interaction was blocked by methyl-beta-cyclodextrin, known to disrupt caveolin/caveolae-dependent signaling by cholesterol depletion. Methyl-beta-cyclodextrin also blocked the agonist-induced internalization of mGluR1alpha. Thus, these findings represent the evidence for agonist-induced internalization of mGluR1alpha via caveolin and suggest that caveolin might play a role in synaptic metaplasticity by regulating internalization of mGluR1alpha in the cerebellum.

Alterations in AMPA receptor phosphorylation in the rat striatum following acute and repeated cocaine administration

Protein phosphorylation is an important mechanism for the posttranslational modulation of ionotropic glutamate receptors and is subject to regulation by changing synaptic inputs. In this study, we investigated the regulation of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor GluR1 subunit phosphorylation by cocaine exposure in the rat dorsal striatum in vivo. We found that acute cocaine challenge followed by 6 days of repeated systemic injections of cocaine (20 mg/kg once daily) enhanced the sensitivity of the GluR1 subunit in its phosphorylation at serine 831 (Ser831) in the dorsal striatum. This enhancement of the sensitivity of Ser831 phosphorylation was reduced, at the receptor and ion channel level, by blocking (1) group I metabotropic glutamate receptors (mGluRs), (2) N-methyl-D-aspartate receptors, and (3) L-type voltage-operated Ca(2+) channels. Similar reduction of the enhancement was also induced, at the protein kinase level, by inhibiting (1) protein kinase C, (2) calcium/calmodulin-dependent protein kinases, and (3) c-Jun N-terminal kinases. In addition, inhibition of protein phosphatase 1/2A or calcineurin increased GluR1-Ser831 phosphorylation in the dorsal striatum of normal rats, whereas inhibition of these phosphatases did not further enhance the Ser831 phosphorylation in rats pretreated with 7 daily injections of cocaine. These data suggest that the phosphorylation of AMPA receptor GluR1 subunits at Ser831 is subject to upregulation by acute and repeated cocaine administration. Complex signaling integrations among glutamate receptors, Ca(2+) channels, protein kinases, and protein phosphatases participate in this upregulation.

Metabotropic glutamate 1 receptor: current concepts and perspectives

Almost 25 years after the first report that glutamate can activate receptors coupled to heterotrimeric G-proteins, tremendous progress has been made in the field of metabotropic glutamate receptors. Now, eight members of this family of glutamate receptors, encoded by eight different genes that share distinctive structural features have been identified. The first cloned receptor, the metabotropic glutamate (mGlu) receptor mGlu1 has probably been the most extensively studied mGlu receptor, and in many respects it represents a prototypical subtype for this family of receptors. Its biochemical, anatomical, physiological, and pharmacological characteristics have been intensely investigated. Together with subtype 5, mGlu1 receptors constitute a subgroup of receptors that couple to phospholipase C and mobilize Ca(2+) from intracellular stores. Several alternatively spliced variants of mGlu1 receptors, which differ primarily in the length of their C-terminal domain and anatomical localization, have been reported. Use of a number of genetic approaches and the recent development of selective antagonists have provided a means for clarifying the role played by this receptor in a number of neuronal systems. In this article we discuss recent advancements in the pharmacology and concepts about the intracellular transduction and pathophysiological role of mGlu1 receptors and review earlier data in view of these novel findings. The impact that this new and better understanding of the specific role of these receptors may have on novel treatment strategies for a variety of neurological and psychiatric disorders is considered.

Enhancement of inhibitory synaptic transmission in large aspiny neurons after transient cerebral ischemia

Large aspiny neurons and most of the GABAergic interneurons survive transient cerebral ischemia while medium spiny neurons degenerate in 24 h. Expression of a long-term enhancement of excitatory transmission in medium spiny neurons but not in large aspiny neurons has been indicated to contribute to this selective vulnerability. Because neuronal excitability is determined by the counterbalance of excitation and inhibition, the present study examined inhibitory synaptic transmission in large aspiny neurons after ischemia in rats. Transient cerebral ischemia was induced for 22 min using the four-vessel occlusion method and whole-cell voltage-clamp recording was performed on striatal slices. The amplitudes of evoked inhibitory postsynaptic currents in large aspiny neurons were significantly increased at 3 and 24 h after ischemia, which was mediated by the increase of presynaptic release. Postsynaptic responses were depressed at 24 h after ischemia. Inhibitory postsynaptic currents could be evoked in large aspiny neurons at 24 h after ischemia, suggesting that they receive GABAergic inputs from the survived GABAergic interneurons. Muscimol, a GABA(A) receptor agonist, presynaptically facilitated inhibitory synaptic transmission at 24 h after ischemia. Such facilitation was dependent on the extracellular calcium and voltage-gated sodium channels. The present study demonstrates an enhancement of inhibitory synaptic transmission in large aspiny neurons after ischemia, which might reduce excitotoxicity and contribute, at least in part, to the survival of large aspiny neurons. Our data also suggest that large aspiny neurons might receive inhibitory inputs from GABAergic interneurons.

Distinct roles of group I mGlu receptors in striatal function

In the recent past, evidence accumulated in favour of a central role of group I metabotropic glutamate (mGlu) receptors, mGlu1 and mGlu5, in the modulation of cell excitability both of striatal medium spiny projection neurons (MSNs) and interneuronal population. Electrophysiological and pharmacological studies have clearly shown that activation of mGlu1 and mGlu5 receptors exerts distinct actions, depending on the neuronal subtype involved. MGlu5 receptor activation mediates the potentiation of NMDA responses in MSNs, and underlies the retrograde inhibitory signaling by endocannabinoids at corticostriatal synapses. Conversely, both group I mGlu receptors are involved in long-term synaptic plasticity of MSNs. Likewise, either mGlu1 or mGlu5 receptors are engaged in shaping the excitability of large cholinergic interneurons, playing different roles in the membrane responses. Differently, although GABAergic parvalbumin-positive, fast-spiking interneurons have been shown to express both group I receptors, only mGlu1 receptor seems to mediate membrane and synaptic responses. This review provides a brief survey of the cellular and synaptic actions of group I mGlu receptors, and discusses the potential relevance of these findings in neostriatal function and motor control.

TrpC3/C7 and Slo2.1 are molecular targets for metabotropic glutamate receptor signaling in rat striatal cholinergic interneurons

Large aspiny cholinergic interneurons provide the sole source of striatal acetylcholine, a neurotransmitter critical for basal ganglia function; these tonically active interneurons receive excitatory inputs from corticostriatal glutamatergic afferents that act, in part, via metabotropic glutamate receptors (mGluRs). We combined electrophysiological recordings in brain slices with molecular neuroanatomy to identify distinct ion channel targets for mGluR1/5 receptors in striatal cholinergic interneurons: transient receptor potential channel 3/7 (TrpC3/C7) and Slo2.1. In recordings obtained with methanesulfonate-based internal solutions, we found an mGluR-activated current with voltage-dependent and pharmacological properties reminiscent of TrpC3 and TrpC7; expression of these TrpC subunits in cholinergic interneurons was verified by combined immunohistochemistry and in situ hybridization, and modulation of both TrpC channels was reconstituted in HEK293 (human embryonic kidney 293) cells cotransfected with mGluR1 or mGluR5. With a chloride-based internal solution, mGluR agonists did not activate interneuron TrpC-like currents. Instead, a time-dependent, outwardly rectifying K(+) current developed after whole-cell access, and this Cl(-)-activated K(+) current was strongly inhibited by volatile anesthetics and mGluR activation. This modulation was recapitulated in cells transfected with Slo2.1, a Na(+)- and Cl(-)-activated K(+) channel, and Slo2.1 expression was confirmed histochemically in striatal cholinergic interneurons. By using gramicidin perforated-patch recordings, we established that the predominant agonist-activated current was TrpC-like when ambient intracellular chloride was preserved, although a small K(+) current contribution was observed in some cells. Together, our data indicate that mGluR1/5-mediated glutamatergic excitation of cholinergic interneurons is primarily a result of activation of TrpC3/TrpC7-like cationic channels; under conditions when intracellular NaCl is elevated, a Slo2.1 background K(+) channel may also contribute.

Endocannabinoids limit metabotropic glutamate 5 receptor-mediated synaptic inhibition of striatal principal neurons

Synaptic transmission in the striatum is regulated by metabotropic glutamate (mGlu) receptors through pre- and postsynaptic mechanisms. We investigated the involvement of mGlu 1 and 5 receptors in the control of both excitatory and inhibitory transmission in the striatum. The mGlu 1 and 5 receptor agonist 3,5-DHPG failed to affect glutamate transmission, while it caused a biphasic effect on GABA transmission, characterized by early increase and late decrease in the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) recorded from striatal principal neurons. Both mGlu 1 and 5 receptors were involved in the early response to 3,5-DHPG, through membrane depolarization of striatal GABAergic interneurons and action potential generation. The 3,5-DHPG-mediated late depression of inhibitory inputs to striatal principal neurons was conversely secondary to mGlu 5 receptor activation and subsequent endocannabinoid release. In conclusion, we have identified an mGlu-dependent mechanism of GABA transmission regulation of potential relevance for physiological neuronal activity.

Striatal and extrastriatal atrophy in Huntington's disease and its relationship with length of the CAG repeat

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder that affects the striatum most severely. However, except for juvenile forms, relative preservation of the cerebellum has been reported. The objective of the present study was to perform MRI measurements of caudate, putamen, cerebral, and cerebellar volumes and correlate these findings with the length of the CAG repeat and clinical parameters. We evaluated 50 consecutive patients with HD using MRI volumetric measurements and compared them to normal controls. Age at onset of the disease ranged from 4 to 73 years (mean: 43.1 years). The length of the CAG repeat ranged from 40 to 69 (mean: 47.2 CAG). HD patients presented marked atrophy of the caudate and putamen, as well as reduced cerebellar and cerebral volumes. There was a significant correlation between age at onset of HD and length of the CAG repeat, as well as clinical disability and age at onset. The degree of basal ganglia atrophy correlated with the length of the CAG repeat. There was no correlation between cerebellar or cerebral volume and length of the CAG repeat. However, there was a tendency to a positive correlation between duration of disease and cerebellar atrophy. While there was a negative correlation of length of the CAG repeat with age at disease onset and with striatal degeneration, its influence on extrastriatal atrophy, including the cerebellum, was not clear. Extrastriatal atrophy occurs later in HD and may be related to disease duration.

Is the functional interaction between adenosine A(2A) receptors and metabotropic glutamate 5 receptors a general mechanism in the brain? Differences and similarities between the striatum and the hippocampus

The aim of the present paper was to examine, in a comparative way, the occurrence and the mechanisms of the interactions between adenosine A_2A receptors (A_2ARs) and metabotropic glutamate 5 receptors (mGlu5Rs) in the hippocampus and the striatum. In rat hippocampal and corticostriatal slices, combined ineffective doses of the mGlu5R agonist 2-chloro-5-hydroxyphenylglycine (CHPG) and the A_2AR agonist CGS 21680 synergistically reduced the slope of excitatory postsynaptic field potentials (fEPSPs) recorded in CA1 and the amplitude of field potentials (FPs) recorded in the dorsomedial striatum. The cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) pathway appeared to be involved in the effects of CGS 21680 in corticostriatal but not in hippocampal slices. In both areas, a postsynaptic locus of interaction appeared more likely. N-methyl-D-aspartate (NMDA) reduced the fEPSP slope and FP amplitude in hippocampal and corticostriatal slices, respectively. Such an effect was significantly potentiated by CHPG in both areas. Interestingly, the A_2AR antagonist ZM 241385 significantly reduced the NMDA-potentiating effect of CHPG. In primary cultures of rat hippocampal and striatal neurons (ED 17, DIV 14), CHPG significantly potentiated NMDA-induced lactate dehydrogenase (LDH) release. Again, such an effect was prevented by ZM 241385. Our results show that A_2A and mGlu5 receptors functionally interact both in the hippocampus and in the striatum, even though different mechanisms seem to be involved in the two areas. The ability of A_2ARs to control mGlu5R-dependent effects may thus be a general feature of A_2ARs in different brain regions (irrespective of their density) and may represent an additional target for the development of therapeutic strategies against neurological disorders.

Differential expression of group I metabotropic glutamate receptors in human motoneurons at low and high risk of degeneration in amyotrophic lateral sclerosis

Glutamate excitotoxicity has been suggested to play a role in amyotrophic lateral sclerosis, since overstimulation of post-synaptic glutamate receptors by accumulated extracellular glutamate leads to motoneuron cell death. It is however unclear as to why some groups of motoneurons degenerate in this disease while other groups remain relatively intact even during terminal stages of the disease. Our previous studies in the rat showed differential expression of group I metabotropic glutamate receptors in motoneurons at low and high risk of degeneration in amyotrophic lateral sclerosis. Here we have extended this study to normal human brains. In situ hybridization showed that transcripts of both metabotropic glutamate receptor (mGluR) 1 and mGluR5 were expressed in motoneurons in both the resistant motor nucleus III and the vulnerable motor nucleus XII. Immunolabeling of mGluR1alpha and mGluR5 was found in both motoneurons and glia-like cells in all the motor nuclei and the ventral horn of the cervical spinal cord studied. Quantitative analysis of optical density measurements showed higher levels of mGluR1alpha protein expression but lower levels of mGluR5 protein expression in the vulnerable motoneuron pool (VII, XII and spinal cord) than in the resistant motoneuron pool (III, IV and VI). This differential expression of group I metabotropic glutamate receptor proteins within vulnerable motoneuron pools may predispose these neurons to degeneration as seen in amyotrophic lateral sclerosis.

Regulation of metabotropic glutamate receptor signaling, desensitization and endocytosis

Metabotropic glutamate receptors (mGluRs) comprise a unique family of G protein-coupled receptors (GPCR) that can be classified into 3 groups based on G protein coupling specificity and sequence similarity. Group I mGluRs (mGluR1 and mGluR5) are coupled to the heterotrimeric G protein Galpha(q/11) and trigger the release of calcium from intracellular stores. In the present review, we discuss the molecular mechanisms involved in the desensitization and endocytosis of group I mGluRs. Group I mGluRs desensitize in response to both second-messenger-dependent protein kinases and G protein-coupled receptor kinases (GRK). However, GRK2-mediated mGluR1 desensitization appears to be both phosphorylation- and beta-arrestin-independent. In addition to GRK-mediated uncoupling of mGluRs from heterotrimeric G proteins, the huntingtin-interacting protein, optineurin, also contributes to mGluR1 and mGluR5 desensitization. The G protein-uncoupling activity of optineurin appears to be facilitated by the presence of polyglutamine-expanded mutant huntingtin but not wild-type huntingtin. Group I mGluRs also undergo both agonist-dependent and -independent endocytosis in both heterologous cell expression systems and primary neuronal cultures. The present review overviews the current understanding of the contribution of second messenger-dependent protein kinases, beta-arrestins and a novel Ral/phospholipase D2 (PLD2)-mediated endocytic pathway to the regulation of Group I mGluR endocytosis. Overall, the regulation of Group I mGluR desensitization and endocytosis appears to be mediated by the same molecular intermediates as have been described for more typical GPCR such as the beta(2)-adrenergic receptor. However, there appears to be subtle, but important, differences in the mechanisms by which these intermediates are employed to regulate Group I mGluR desensitization and endocytosis.

Physiological role for casein kinase 1 in glutamatergic synaptic transmission

Casein kinase 1 (CK1) is a highly conserved serine/threonine kinase, present in virtually all cell types, in which it phosphorylates a wide variety of substrates. So far, no role has been found for this ubiquitous protein kinase in the physiology of nerve cells. In the present study, we show that CK1 regulates fast synaptic transmission mediated by glutamate, the major excitatory neurotransmitter in the brain. Through the use of CK1 inhibitors, we present evidence that activation of CK1 decreases NMDA receptor activity in the striatum via a mechanism that involves activation by this kinase of protein phosphatase 1 and/or 2A and resultant increased dephosphorylation of NMDA receptors. Indeed, inhibition of CK1 increases NMDA-mediated EPSCs in medium spiny striatal neurons. This effect is associated with an increased phosphorylation of the NR1 and NR2B subunits of the NMDA receptor and is occluded by the phosphatase inhibitor okadaic acid. The mGluR1, but not mGluR5, subclass of metabotropic glutamate receptors uses CK1 to inhibit NMDA-mediated synaptic currents. These results provide the first evidence for a role of CK1 in the regulation of synaptic transmission in the brain.

Group-II metabotropic glutamate receptors negatively modulate NMDA transmission at striatal cholinergic terminals: Role of P/Q-type high voltage activated Ca++ channels and endogenous dopamine

Striatal cholinergic nerve terminals express functional group-II metabotropic (mGlu) and NMDA glutamate receptors. To investigate whether these receptors interact to regulate ACh release, LY354740 (a group-II mGlu receptor agonist) and NMDA were co-applied in striatal synaptosomes and slices. LY354740 prevented the NMDA-evoked [3H]-choline release from synaptosomes and ACh release from slices. In synaptosomes, this modulation was prevented by omega-agatoxin IVA, suggesting that it was mediated by P/Q-type high voltage activated Ca++ channels. In slices, LY341495 (a group-II mGlu receptor antagonist) enhanced the NMDA-induced ACh release, suggesting that group-II mGlu receptor activation by endogenous glutamate inhibits NMDA transmission. Co-immunoprecipitation studies excluded direct group-II mGlu-NMDA receptor interactions. Finally, group-II mGlu negative modulation of NMDA transmission was abolished in dopamine-depleted synaptosomes and slices, suggesting that it relied on endogenous dopamine. We conclude that group-II mGlu receptors attenuate NMDA inputs at striatal cholinergic terminals via Ca++ channel modulation and dopamine-sensitive pathways.

Driving cellular plasticity and survival through the signal transduction pathways of metabotropic glutamate receptors

Metabotropic glutamate receptors (mGluRs) share a common molecular morphology with other G protein-linked receptors, but there expression throughout the mammalian nervous system places these receptors as essential mediators not only for the initial development of an organism, but also for the vital determination of a cell's fate during many disorders in the nervous system that include amyotrophic lateral sclerosis, Parkinson's disease, Alzheimer's disease, Huntington's disease, Multiple Sclerosis, epilepsy, trauma, and stroke. Given the ubiquitous distribution of these receptors, the mGluR system impacts upon neuronal, vascular, and glial cell function and is activated by a wide variety of stimuli that includes neurotransmitters, peptides, hormones, growth factors, ions, lipids, and light. Employing signal transduction pathways that can modulate both excitatory and inhibitory responses, the mGluR system drives a spectrum of cellular pathways that involve protein kinases, endonucleases, cellular acidity, energy metabolism, mitochondrial membrane potential, caspases, and specific mitogen-activated protein kinases. Ultimately these pathways can converge to regulate genomic DNA degradation, membrane phosphatidylserine (PS) residue exposure, and inflammatory microglial activation. As we continue to push the envelope for our understanding of this complex and critical family of metabotropic receptors, we should be able to reap enormous benefits for both clinical disease as well as our understanding of basic biology in the nervous system.

mGluRI targets microglial activation and selectively prevents neuronal cell engulfment through Akt and caspase dependent pathways

Metabotropic glutamate receptors (mGluRs) are expressed throughout the mammalian central nervous system and integrate a host of signal transduction pathways that determine cellular function, plasticity and injury. Yet, one of the more unique regulatory functions of this family of GTP-binding proteins involves cytoprotection in the nervous system. Here, we demonstrate that activation of group I metabotropic glutamate receptors (mGluRIs) in primary hippocampal neurons not only provides intrinsic cellular protection for the maintenance of genomic DNA integrity, but also prevents inflammatory microglial activation and specific neuronal cell engulfment during free radical oxidative stress. Loss of cellular membrane asymmetry and exposure of membrane phosphatidylserine (PS) residues were necessary and sufficient to result in microglial activation and proliferation, since administration of an antibody to the PS receptor could block microglial activity. Through the continuous assessment of individual neurons in real time, activation of mGluRIs was documented to block neuronal PS exposure and prevented subsequent neuronal cell engulfment by microglia seeking "PS tagged" neurons. Furthermore, regulation of both cellular integrity and microglial activity by mGluRI activation was dependent upon the activation and phosphorylation of protein kinase B (Akt1), prevention of mitochondrial membrane depolarization with associated permeability transition pore complex formation, and the down regulation of caspase 9-like activity. Our work defines a significant role of mGluRIs for the modulation of cellular survival and inflammation in the nervous system during oxidative stress.

Hyperactive striatal neurons in symptomatic Huntington R6/2 mice: variations with behavioral state and repeated ascorbate treatment

Membrane and morphological abnormalities occur in the striatum of R6/2 transgenics, a widely used mouse model of Huntington's disease. To assess changes in behavior-related neuronal activity, we implanted micro-wire bundles in the striatum of symptomatic R6/2 mice and wild-type controls. Unit activity was recorded in an open-field arena once weekly for the next several weeks. For each recording session, firing rate was monitored before, during, and after a period of light anesthesia to assess the influence of behavioral arousal. Because low ascorbate in striatal extracellular fluid may contribute to Huntington's disease symptoms, all animals received an injection of either 300 mg/kg sodium ascorbate or vehicle for three consecutive days prior to each recording session. In R6/2 mice, regardless of treatment, striatal unit activity was significantly faster than in wild-type controls. The difference in mean (+/-S.E.M.) firing was most apparent during wakefulness (6.4+/-0.8 vs. 3.5+/-0.3 spikes/s) but also persisted during anesthesia (2.0+/-0.3 vs. 0.7+/-0.1 spikes/s). Assessment of treatment duration indicated that R6/2 mean waking discharge rate was significantly slower after three weeks than after one week of ascorbate treatment (3.1+/-0.6 vs. 10.2+/-2.7 spikes/s). Vehicle-treated R6/2s showed no such decline in striatal activity ruling out an age- or injection-related effect. Slow-scan voltammetry in separate animals confirmed that ascorbate-injections returned the level of striatal extracellular ascorbate in R6/2 mice to that of wild-type controls. Our results indicate that although striatal neurons modulate firing in relation to behavioral state, impulse activity is consistently elevated in transgenic relative to wild-type mice. Restoring extracellular ascorbate to the wild-type level reverses this effect suggesting a role for ascorbate in normalizing neuronal function in Huntington's disease striatum.

Functional anatomy of the basal ganglia in X-linked recessive dystonia-parkinsonism

Dystonia is a neurological syndrome characterized by sustained muscle contractions that produce repetitive twisting movements or abnormal postures. X-linked recessive dystonia parkinsonism (XDP; DYT3; Lubag) is an adult-onset disorder that manifests severe and progressive dystonia with a high frequency of generalization. In search for the anatomical basis for dystonia, we performed postmortem analyses of the functional anatomy of the basal ganglia based on the striatal compartments (ie, the striosomes and the matrix compartment) in XDP. Here, we provide anatomopathological evidence that, in the XDP neostriatum, the matrix compartment is relatively spared in a unique fashion, whereas the striosomes are severely depleted. We also document that there is a differential loss of striatal neuron subclasses in XDP. In view of the three-pathway basal ganglia model, we postulate that the disproportionate involvement of neostriatal compartments and their efferent projections may underlie the manifestation of dystonia in patients with XDP. This study is the first to our knowledge to show specific basal ganglia pathology that could explain the genesis of dystonia in human heredodegenerative movement disorders, suggesting that dystonia may result from an imbalance in the activity between the striosomal and matrix-based pathways.

Inhibition of metabotropic glutamate receptor signaling by the huntingtin-binding protein optineurin

Huntington disease is caused by a polyglutamine expansion in the huntingtin protein (Htt) and is associated with excitotoxic death of striatal neurons. Group I metabotropic glutamate receptors (mGluRs) that are coupled to inositol 1,4,5-triphosphate formation and the release of intracellular Ca(2+) stores play an important role in regulating neuronal function. We show here that mGluRs interact with the Htt-binding protein optineurin that is also linked to normal pressure open angled glaucoma and, when expressed in HEK 293 cells, optineurin functions to antagonize agonist-stimulated mGluR1a signaling. We find that Htt is co-precipitated with mGluR1a and that mutant Htt functions to facilitate optineurin-mediated attenuation of mGluR1a signaling. In striatal cell lines derived from Htt(Q111/Q111) mutant knock-in mice mGluR5-stimulated inositol phosphate formation is also severely impaired when compared with striatal cells derived from Htt(Q7/Q7) knock-in mice. In addition, we show that a missense single nucleotide polymorphism optineurin H486R variant previously identified to be associated with glaucoma is selectively impaired in mutant Htt binding. Although optineurin H486R retains the capacity to bind to mGluR1a, optineurin H486R-dependent attenuation of mGluR1a signaling is not enhanced by the expression of mutant Htt. Because G protein-coupled receptor kinase 2 (GRK2) protein expression is relatively low in striatal tissue, we propose that optineurin may substitute for GRK2 in the striatum to mediate mGluR desensitization. Taken together, these studies identify a novel mechanism for mGluR desensitization and an additional biochemical link between altered glutamate receptor signaling and Huntington disease.

Focal cerebral ischemia preferentially affects neurons distant from their neighboring microvessels

Developing cerebral infarction obscures the relationship of neurons to their local supply microvessels. We tested the notion that in the basal ganglia (i) an ordered relationship between neurons and their nearest neighboring microvessel exists, and (ii) focal ischemia predictably affects neuron integrity based on microvessel-neuron proximity. Distances between individual microvessels and their nearest neurons ([m-n distance]s) were measured in normal primates and ischemic subjects undergoing middle cerebral artery occlusion for 2 hours. An ordered microvessel-neuron relationship exists in the normal nonischemic basal ganglia within the early hours of focal ischemia. During ischemia normal (n) and sensitive (n*) neurons are interspersed. On average, neurons more distant from their nearest microvessel are most sensitive ([m-n distance]=16.2+/-11.2 microm versus [m-n* distance]=22.2+/-13.0 microm, 2P<0.00000001). Neurons not expressing glutamic acid decarboxylase were more likely to be sensitive than those with a normal microvessel-neuron relationship. In contrast, the [m-n distance] distribution of injured tyrosine hydroxylase-containing neurons was similar to those without tyrosine hydroxylase. Hence, the [m-n distance] relationship in the normal and ischemic basal ganglia is highly ordered, and distant neurons are consistently perturbed by ischemia, although this is not uniformly dependent on neurotransmitter type.

Developmental changes of transient potassium currents in large aspiny neurons in the neostriatum

Developmental regulation of the potassium conductance is important for the maturation of neuronal excitability and the formation of functional circuitry in the central nervous system (CNS). The rapidly inactivating A-type current is a major component of the voltage-dependent outward potassium currents in the large aspiny (LA) neurons in the neostriatum. The large aspiny neurons play important roles in the function of neostriatum in physiological and pathological conditions. Whole-cell patch-clamp recording was performed on acutely dissociated neurons and brain slices to investigate the postnatal development of A-type current in the large aspiny neurons. The current density of A-type current in large aspiny neurons was the highest at postnatal 1-3 days and gradually decreased during the development with the lowest levels in adult animals. In comparison to postnatal 1-3 days, the steady-state inactivation curve shifted in depolarizing direction in mature neurons. No significant changes in the voltage dependence of steady-state activation were observed during development. Consistent with the decrease in the current density of A-type current during development, the latency to the first spike was dramatically shortened in mature large aspiny neurons. These results suggest that the decrease of rapidly inactivating A-type potassium current during development might contribute, at least in part, to the maturation of the membrane excitability of large aspiny neurons in the neostriatum.

Glutamate receptors and Parkinson's disease: opportunities for intervention

Parkinson's disease is a debilitating neurodegenerative movement disorder that is the result of a degeneration of dopaminergic neurons in the substantia nigra pars compacta. The resulting loss of striatal dopaminergic tone is believed to underlie a series of changes in the circuitry of the basal ganglia that ultimately lead to severe motor disturbances due to excessive basal ganglia outflow. Glutamate plays a central role in the disruption of normal basal ganglia function, and it has been hypothesised that agents acting to restore normal glutamatergic function may provide therapeutic interventions that bypass the severe motor side effects associated with current dopamine replacement strategies. Analysis of the effects of glutamate receptor ligands in the basal ganglia circuit suggests that both ionotropic and metabotropic glutamate receptors could have antiparkinsonian actions. In particular, NMDA receptor antagonists that selectively target the NR2B subunit and antagonists of the metabotropic glutamate receptor mGluR5 appear to hold promise and deserve future attention.