Keeping in check painful synapses in central amygdala

Glutamatergic projections from the parabrachial nucleus to the central amygdala are implicated in pain transmission. In this issue of Neuron, Delaney et al. identify a new form of adrenergic modulation at these synapses, demonstrating that noradrenaline-induced suppression of glutamate release is mediated by a decrease in the number of sites of synaptic transmission without changes in probability of release.     

Role of glutamate in locomotor rhythm generating neuronal circuitry

Central pattern generators (CPGs) are defined as neuronal circuits capable of producing a rhythmic and coordinated output without the influence of sensory input. The locomotor and respiratory neuronal circuits are two of the better-characterized CPGs, although much work remains to fully understand how these networks operate. Glutamatergic neurons are involved in most neuronal circuits of the nervous system and considerable efforts have been made to study glutamate receptors in nervous system signaling using a variety of approaches. Because of the complexity of glutamate-mediated signaling and the variety of receptors triggered by glutamate, it has been difficult to pinpoint the role of glutamatergic neurons in neuronal circuits. In addition, glutamate is an amino acid used by every cell, which has hampered identification of glutamatergic neurons. Glutamatergic excitatory neurotransmission is dependent on the release from glutamate-filled presynaptic vesicles loaded by three members of the solute carrier family, Slc17a6-8, which function as vesicular glutamate transporters (VGLUTs). Recent data describe that Vglut2 (Slc17a6) null mutant mice die immediately after birth due to a complete loss of the stable autonomous respiratory rhythm generated by the pre-B?tzinger complex. Surprisingly, we found that basal rhythmic locomotor activity is not affected in Vglut2 null mutant embryos. With this perspective, we discuss data regarding presence of VGLUT1, VGLUT2 and VGLUT3 positive neuronal populations in the spinal cord.     

谷氨酸性突触在痛觉和记忆中的突触和分子机制

谷氨酸是哺乳动物脑中的兴奋性递质。中枢神经系统的谷氨酸性突触广泛参与痛觉传递,突触可塑性和递质的调节。谷氨酸的NMDA受体参与前脑相关的学习及功能。在这篇综述中,我们提出前脑的NMDA受体通过增强谷氨酸性突触传递导致长期性的炎痛。具有增强NMDA受体功能的小鼠会产生更多的慢性痛。NMDA NR2B受体抑制剂在未来可能被用来控制人类的慢性痛。     

Microdomains in Forebrain Spines: an Ultrastructural Perspective

Glutamatergic axons in the mammalian forebrain terminate predominantly onto dendritic spines. Long-term changes in the efficacy of these excitatory synapses are tightly coupled to changes in spine morphology. The reorganization of the actin cytoskeleton underlying this spine “morphing” involves numerous proteins that provide the machinery needed for adaptive cytoskeletal remodeling. Here, we review recent literature addressing the chemical architecture of the spine, focusing mainly on actin-binding proteins (ABPs). Accumulating evidence suggests that ABPs are organized into functionally distinct microdomains within the spine cytoplasm. This functional compartmentalization provides a structural basis for regulation of the spinoskeleton, offering a novel window into mechanisms underlying synaptic plasticity.     

NMDA受体与Ⅰ组代谢型谷氨酸受体的相互作用

谷氨酸是哺乳动物中枢神经系统重要兴奋性神经递质,参与学习、记忆、药物依赖成瘾及神经系统退行性疾病等多种病理生理过程。谷氨酸通过激活离子型（iGluRs）和代谢型谷氨酸受体（mGluRs）发挥作用。业已有研究提示iGluRs和mGluRs之间存在相互作用,但具体机制尚待阐明。本文从蛋白分子结构、突触可塑性、相互作用可能涉及的信号分子和通路等方面综述了NMDAR与Ⅰ组mGluRs之间的相互作用,旨在为深入研究谷氨酸受体之间的相互作用提供线索。     

Synaptic communication between neurons and NG2+ cells

Chemical synaptic transmission provides the basis for much of the rapid signaling that occurs within neuronal networks. However, recent studies have provided compelling evidence that synapses are not used exclusively for communication between neurons. Physiological and anatomical studies indicate that a distinct class of glia known as NG2(+) cells also forms direct synaptic junctions with both glutamatergic and GABAergic neurons. Glutamatergic signaling can influence intracellular Ca(2+) levels in NG2(+) cells by activating Ca(2+) permeable AMPA receptors, and these inputs can be potentiated through high frequency stimulation. Although the significance of this highly differentiated form of communication remains to be established, these neuro-glia synapses might enable neurons to influence rapidly the behavior of this ubiquitous class of glial progenitors.     

Novel Nuclear Protein Complexes of Dystrophin 71 Isoforms in Rat Cultured Hippocampal GABAergic and Glutamatergic Neurons

The precise functional role of the dystrophin 71 in neurons is still elusive. Previously, we reported that dystrophin 71d and dystrophin 71f are present in nuclei from cultured neurons. In the present work, we performed a detailed analysis of the intranuclear distribution of dystrophin 71 isoforms (Dp71d and Dp71f), during the temporal course of 7-day postnatal rats hippocampal neurons culture for 1h, 2, 4, 10, 15 and 21 days in vitro (DIV). By immunofluorescence assays, we detected the highest level of nuclear expression of both dystrophin Dp71 isoforms at 10 DIV, during the temporal course of primary culture. Dp71d and Dp71f were detected mainly in bipolar GABAergic (≥60%) and multipolar Glutamatergic (≤40%) neurons, respectively. We also characterized the existence of two nuclear dystrophin-associated protein complexes (DAPC): dystrophin 71d or dystrophin 71f bound to β-dystroglycan, α1-, β-, α2-dystrobrevins, α-syntrophin, and syntrophin-associated protein nNOS (Dp71d-DAPC or Dp71f-DAPC, respectively), in the hippocampal neurons. Furthermore, both complexes were localized in interchromatin granule cluster structures (nuclear speckles) of neuronal nucleoskeleton preparations. The present study evinces that each Dp71’s complexes differ slightly in dystrobrevins composition. The results demonstrated that Dp71d-DAPC was mainly localized in bipolar GABAergic and Dp71f-DAPC in multipolar Glutamatergic hippocampal neurons. Taken together, our results show that dystrophin 71d, dystrophin 71f and DAP integrate protein complexes, and both complexes were associated to nuclear speckles structures.     

A model of cooperative effect of AMPA and NMDA receptors in glutamatergic synapses

Glutamatergic synapses play a pivotal role in brain excitation. The synaptic response is mediated by the activity of two receptor types (AMPA and NMDA). In the present paper we propose a model of glutamatergic synaptic activity where the fast current generated by the AMPA conductance produces a local depolarization which activates the voltage- and [Mg²⁺]-dependent NMDA conductance. This cooperative effect is dependent on the biophysical properties of the synaptic spine which can be considered a high input resistance specialized compartment. Herein we present results of simulations where different values of the spine resistance and of the Mg²⁺ concentrations determine different levels of cooperativeness between AMPA and NMDA receptors in shaping the post-synaptic response.     

Contribution of glutamatergic signaling to nitrosative stress-induced protein misfolding in normal brain aging and neurodegenerative diseases

Glutamatergic hyperactivity, associated with Ca2+ influx and consequent production of nitric oxide (NO), is potentially involved in both normal brain aging and age-related neurodegenerative disorders. Many neurodegenerative diseases are characterized by conformational changes in proteins that result in their misfolding and aggregation. Normal protein degradation by the ubiquitin-proteasome system can prevent accumulation of aberrantly folded proteins. Our recent studies have linked nitrosative stress to protein misfolding and neuronal cell death. In particular, molecular chaperones - such as protein disulfide isomerase, glucose regulated protein 78, and heat shock proteins - can provide neuroprotection from misfolded proteins by facilitating proper folding and thus preventing aggregation. Here, we present evidence for the hypothesis that NO contributes to normal brain aging and degenerative conditions by S-nitrosylating specific chaperones that would otherwise prevent accumulation of misfolded proteins.     

Repeated Restraint Stress Alters Hippocampal Glutamate Uptake and Release in the Rat

Glutamatergic mechanisms are thought to be involved in stress-induced changes of brain function, especially in the hippocampus. We hypothesized that alterations caused by the hormonal changes associated with chronic and acute stress may affect glutamate uptake and release from hippocampal synaptosomes in Wistar rats. It was found that [3H]glutamate uptake and release by hippocampal nerve endings, when measured 24 h after 1 h of acute restraint, presented no significant difference. The exposure to repeated restraint stress for 40 days increased neuronal presynaptic [3H]glutamate uptake as well as basal and K+-stimulated glutamate release when measured 24 h after the last stress session. Chronic treatment also caused a significant decrease in [3H]glutamate binding to hippocampal membranes. We suggest that changes in the glutamatergic system are likely to take part in the mechanisms involved in nervous system plasticity following repeated stress exposure.     

Glutamate-induced metabolic changes influence the cytoplasmic redox state of hippocampal neurons

Brain cell metabolism is intimately associated with intracellular oxidation–reduction (redox) balance. Glutamatergic transmission is accompanied with changes in substrate preference in neurons. Therefore, we studied cytoplasmatic redox changes in hippocampal neurons in culture exposed to glutamate. Neurons were transfected with HyPer, a genetically encoded redox biosensor for hydrogen peroxide which allows real-time imaging of the redox state. The rate of fluorescence decay, corresponding to the reduction of the biosensor was found to be augmented by low doses of glutamate (10 μM) as well as by pharmacological stimulation of NMDA glutamate receptors. Acute chelation of extracellular Ca²⁺ abolished the glutamate-induced effect observed on HyPer fluorescence. Additional experiments indicated that mitochondrial function and hence energetic substrate availability commands the redox state of neurons and is required for the glutamate effect observed on the biosensor signal. Furthermore, our results implicated astrocytic metabolism in the changes of neuronal redox state observed with glutamate.     

Thalamocortical interactions

Glutamatergic pathways dominate information processing in the brain, but these are not homogeneous. They include two distinct types: Class 1, which carries the main information for processing, and Class 2, which serves a modulatory role. Identifying the Class 1 inputs in a circuit can lead to a better understanding of its function. Also, identifying Class 1 inputs to a thalamic nucleus tells us its main function (e.g. the lateral geniculate nucleus, or LGN, is the relay of retinal Class 1 input), and such identification leads to a division of thalamic relays into first and higher order: the former receives Class 1 inputs from subcortical sources; the latter, from layer 5 of cortex, which it then relays to another cortical area. When a cortical area directly connects with another, it often has a parallel, transthalamic connection through these higher order relays. This leads to a novel appreciation of cortical functioning and raises many new questions.     

NMDA受体参与小鼠的前额扣带回的神经突触传递

谷氨酸性突触是哺乳动物神经系统的主要兴奋性突触。在正常条件下,大多数的突触反应是由谷氨酸的AMPA受体传递的。NMDA受体在静息电位下为镁离子抑制。在被激活时,NMDA受体主要参与突触的可塑性变化。但是,许多NMDA受体拮抗剂在全身或局部注射时能产生行为效应,提示NMDA受体可能参与静息状态的生理功能。此文中,我们在离体的前额扣带回脑片上进行电生理记录,发现NMDA受体参与前额扣带回的突触传递。在重复刺激或近于生理性温度时,NMDA受体传递的反应更为明显。本文直接显示了NMDA受体参与前额扣带回的突触传递,并提示NMDA受体在前额扣带回中起着调节神经元兴奋的重要作用。     

Effects of Depressive-Like Behavior of Rats on Brain Glutamate Uptake

Learned helplessness paradigm is a widely accepted animal model of depressive-like behavior based on stress. Glutamatergic system is closely involved with the stress-neurotoxicity in the brain and recently it is pointed to have a relevant role in the pathophysiology of depression disorder. Glutamate uptake is the main mechanism to terminate the glutamatergic physiological activity and to neuroprotection against excitotoxicity. We investigated the profile of glutamate uptake in female rats submitted to the learned helplessness paradigm and to different classes of stress related to the paradigm, in slices of brain cortex, striatum and hippocampus. Glutamate uptake in slices of hippocampus differ between learned helplessness (LH) and non-learned helplessness (NLH) animals immediately persisting up to 21 days after the paradigm. In addition, there were a decrease of glutamate uptake in the three brain structures analyzed at 21 days after the paradigm for LH animals. These results may contribute to better understand the role of the glutamatergic system on the depressive-like behavior.     

Novel approaches to targeting glutamate receptors for the treatment of chronic pain: review article

Summary.  Glutamatergic mechanisms are implicated in acute and chronic pain, and there is a great diversity of glutamate receptors that can be used as targets for novel analgesics. Some approaches, e.g. NMDA receptor antagonism, have been validated clinically, however, the central side-effects have remained the main problem with most compounds. Recently, some novel approaches have been explored as new compounds targeting some modulatory sites at the NMDA receptor (glycine_B and NR2B-subtype selective antagonists), as well as kainate and metabotropic glutamate receptors, have been discovered. Many of these compounds have demonstrated efficacy in animal models of chronic pain, and some of them appear to have a reduced side-effect liability compared to clinically tested NMDA antagonists. These recent advances are reviewed in the present work. Received July 6, 2001 Accepted August 6, 2001 Published online June 26, 2002     

Neurotoxins from venoms of the Hymenoptera--twenty-five years of research in Amsterdam

1. In co-operation with colleagues in Europe, Japan and the U.S.A., 25 years of research in Amsterdam have provided new views on the way some hymenopteran insects incapacitate their prey by a diversity of neurotoxins, resulting in block of synaptic transmission in CNS or neuromuscular junctions, or affecting voltage dependent phenomena in nerve and muscle fibers. 2. Nicotinic synaptic transmission in the insect CNS is irreversibly blocked at the presynaptic side by kinins, or reversibly and postsynaptically blocked by philanthotoxins. 3. Glutamatergic neuromuscular transmission is reversibly blocked by philanthotoxins at the pre- and/or postsynaptic side. 4. A presynaptic block of neuromuscular transmission was found with the Microbracon toxins. 5. An irreversible deactivation, without paralysis, of cockroaches is caused by a sting of Ampulex compressa into the suboesophageal ganglion. 6. Poneratoxin, a 25 amino acid residue polypeptide, isolated from an ant venom, is the first described hymenopteran neurotoxin affecting excitability of nerve and muscle fibres by changing the kinetics of the voltage-dependent sodium channel.     

Frontal Cortical and Left Temporal Glutamatergic Dysfunction in Schizophrenia

Glutamatergic mechanisms have been investigated in postmortem brain samples from schizophrenics and controls. D-[3H]Aspartate binding to glutamate uptake sites was used as a marker for glutamatergic neurones, and [3H]kainate binding for a subclass of postsynaptic glutamate receptors. There were highly significant increases in the binding of both ligands to membranes from orbital frontal cortex on both the left and right sides of schizophrenic brains. The changes are unlikely to be due to antemortem neuroleptic drug treatment, because no similar changes were recorded in other areas. A predicted left-sided reduction in D-[3H]aspartate binding was refuted at 5% probability, but not at 10%. Previously reported high concentrations of dopamine in left amygdala were strongly associated with low concentrations of D-[3H]aspartate binding in left polar temporal cortex in the schizophrenics. The findings are compatible with an overabundant glutamatergic innervation of orbital frontal cortex in schizophrenia. The results also suggest that schizophrenia may involve left-sided abnormalities in the relationship between temporal glutamatergic and dopaminergic projections to amygdala.     

Synaptic Efficacy as a Function of Ionotropic Receptor Distribution: A Computational Study

Glutamatergic synapses are the most prevalent functional elements of information processing in the brain. Changes in pre-synaptic activity and in the function of various post-synaptic elements contribute to generate a large variety of synaptic responses. Previous studies have explored postsynaptic factors responsible for regulating synaptic strength variations, but have given far less importance to synaptic geometry, and more specifically to the subcellular distribution of ionotropic receptors. We analyzed the functional effects resulting from changing the subsynaptic localization of ionotropic receptors by using a hippocampal synaptic computational framework. The present study was performed using the EONS (Elementary Objects of the Nervous System) synaptic modeling platform, which was specifically developed to explore the roles of subsynaptic elements as well as their interactions, and that of synaptic geometry. More specifically, we determined the effects of changing the localization of ionotropic receptors relative to the presynaptic glutamate release site, on synaptic efficacy and its variations following single pulse and paired-pulse stimulation protocols. The results indicate that changes in synaptic geometry do have consequences on synaptic efficacy and its dynamics.     

Spatio‐temporal regulation of the formation of the somatosensory system

The somatosensory system in the brain has been widely used for investigating the mechanisms underlying neural circuit formation and developmental neural plasticity. In the primary somatosensory cortex (S1) of rodents, there are discrete cytoarchitectonic units called barrels. Reverse genetic analyses using knockout mice have revealed molecules that control spatial pattern formation of barrels in S1. Glutamatergic receptors such as the NMDA receptor and mGluR5, and molecules related to serotonin such as serotonin transporter and monoamine oxidase A are essential for the formation of barrels. In addition to the mechanisms of spatial pattern formation, those regulating the timing of developmental processes were uncovered recently. Barrels are formed soon after the birth of newborn mouse pups from their mothers, and it was shown that the timing of barrel formation was determined by the timing of the birth of mouse pups. The mechanisms downstream of birth were also examined. It would be intriguing to examine if the mechanisms found using the somatosensory system are applicable to other brain regions.     

TNFα controls glutamatergic gliotransmission in the hippocampal dentate gyrus

Glutamatergic gliotransmission provides a stimulatory input to excitatory synapses in the hippocampal dentate gyrus. Here, we show that tumor necrosis factor-alpha (TNFα) critically controls this process. With constitutive TNFα present, activation of astrocyte P2Y1 receptors induces localized [Ca(2+)](i) elevations followed by glutamate release and presynaptic NMDA receptor-dependent synaptic potentiation. In preparations lacking TNFα, astrocytes respond with identical [Ca(2+)](i) elevations but fail to induce neuromodulation. We find that TNFα specifically controls the glutamate release step of gliotransmission. In cultured astrocytes lacking TNFα glutamate exocytosis is dramatically slowed down due to altered vesicle docking. Addition of low picomolar TNFα promptly reconstitutes both normal exocytosis in culture and gliotransmission in situ. Alternatively, gliotransmission can be re-established without adding TNFα, by limiting glutamate uptake, which compensates slower release. These findings demonstrate that gliotransmission and its synaptic effects are controlled not only by astrocyte [Ca(2+)](i) elevations but also by permissive/homeostatic factors like TNFα. VIDEO ABSTRACT: