Age-dependent pre- and postsynaptic distribution of AMPA receptors at synapses in CA3 stratum radiatum of hippocampal slice cultures compared with intact brain

Presynaptic glutamate receptors in nociception

Chronic pain is a frequent, distressing and poorly understood health problem. Plasticity of synaptic transmission in the nociceptive pathways after inflammation or injury is assumed to be an important cellular basis for chronic, pathological pain. Glutamate serves as the main excitatory neurotransmitter at key synapses in the somatosensory nociceptive pathways, in which it acts on both ionotropic and metabotropic glutamate receptors. Although conventionally postsynaptic, compelling anatomical and physiological evidence demonstrates the presence of presynaptic glutamate receptors in the nociceptive pathways. Presynaptic glutamate receptors play crucial roles in nociceptive synaptic transmission and plasticity. They modulate presynaptic neurotransmitter release and synaptic plasticity, which in turn regulates pain sensitization. In this review, we summarize the latest understanding of the expression of presynaptic glutamate receptors in the nociceptive pathways, and how they contribute to nociceptive information processing and pain hypersensitivity associated with inflammation / injury. We uncover the cellular and molecular mechanisms of presynaptic glutamate receptors in shaping synaptic transmission and plasticity to mediate pain chronicity, which may provide therapeutic approaches for treatment of chronic pain.

Presynaptic AMPA Receptors in Health and Disease

AMPA receptors (AMPARs) are ionotropic glutamate receptors that play a major role in excitatory neurotransmission. AMPARs are located at both presynaptic and postsynaptic plasma membranes. A huge number of studies investigated the role of postsynaptic AMPARs in the normal and abnormal functioning of the mammalian central nervous system (CNS). These studies highlighted that changes in the functional properties or abundance of postsynaptic AMPARs are major mechanisms underlying synaptic plasticity phenomena, providing molecular explanations for the processes of learning and memory. Conversely, the role of AMPARs at presynaptic terminals is as yet poorly clarified. Accruing evidence demonstrates that presynaptic AMPARs can modulate the release of various neurotransmitters. Recent studies also suggest that presynaptic AMPARs may possess double ionotropic-metabotropic features and that they are involved in the local regulation of actin dynamics in both dendritic and axonal compartments. In addition, evidence suggests a key role of presynaptic AMPARs in axonal pathology, in regulation of pain transmission and in the physiology of the auditory system. Thus, it appears that presynaptic AMPARs play an important modulatory role in nerve terminal activity, making them attractive as novel pharmacological targets for a variety of pathological conditions.

The Dual Role of Glutamatergic Neurotransmission in Alzheimer's Disease: From Pathophysiology to Pharmacotherapy

Alzheimer's disease (AD) is an age-related dementia and neurodegenerative disorder, characterized by Aβ and tau protein deposition impairing learning, memory and suppressing synaptic plasticity of neurons. Increasing evidence suggests that there is a link between the glucose and glutamate alterations with age that down-regulates glucose utilization reducing glutamate levels in AD patients. Deviations in brain energy metabolism reinforce the development of AD by hampering glutamate levels in the brain. Glutamate is a nonessential amino acid and the major excitatory neurotransmitter synthesized from glucose. Alterations in cerebral glucose and glutamate levels precede the deposition of Aβ plaques. In the brain, over 40% of neuronal synapses are glutamatergic and disturbances in glutamatergic function have been implicated in pathophysiology of AD. Nevertheless, targeting the glutamatergic system seems to be a promising strategy to develop novel, improved therapeutics for AD. Here, we review data supporting the involvement of the glutamatergic system in AD pathophysiology as well as the efficacy of glutamatergic agents in this neurodegenerative disorder. We also discuss exciting new prospects for the development of improved therapeutics for this devastating disorder.

Effect of TGF-beta1 on long-term synaptic plasticity and distribution of AMPA receptors in the CA1 field of the hippocampus

Using the methods of electrophysiology and immunohistochemistry, the effect of the transforming factor beta-1(TGF-β1), an anti-inflammatory cytokine, on the long-term post-tetanic potentiation (LTP) in CA1 field hippocampal slices and the distribution of the GluR1 subunit of the AMPA receptor has been studied. It was shown that TGF-β1 at a concentration of 10 ng/ml did not significantly affect the initial stage of LTP and substantially changed the distribution of synaptic AMPA receptors in response to tetanic stimulation. Twenty five minutes after the tetanization, the main pool of AMPA receptors (90%) was due to the postsynaptic density (PSD). By contrast, LTP in the presence of TGF-β1 was accompanied by less pronounced changes in the distribution of AMPA receptors. Their localization in both pre- and postsynaptic regions remained nearly the same as that in the control. It may be suggested that the normal distribution of AMPA receptors in spinous synapses promotes the stabilization of potentiated synapses, thereby retaining LTP for longer terms.

The Reactive Plasticity of Hippocampal Ionotropic Glutamate Receptors in Animal Epilepsies

Ionotropic glutamate receptors (iGluRs) mediate the synaptic and metabolic actions of glutamate. These iGluRs are classified within the α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type, kainate-type, and N-methyl-d-aspartate (NMDA)-type functional receptor families. The iGluR assemblies are regulated by transcription, alternative splicing, and cytoplasmic post-translational modifications. The iGluR subunit proteins are transported from the endoplasmic reticulum, inserted into the synaptic membranes, and anchored at their action site by different scaffolding and interacting proteins. The functional properties of iGluRs depend on their subunit composition, the amino acid sequence of the protein domains, and the scaffolding proteins in the synaptic membranes. The iGluRs are removed from the membranes by enzymatic action and endocytosis. Hippocampal iGluRs are rearranged through the upregulation and downregulation of the subunits following deafferentation and epileptic seizures. The rearrangement of iGluRs and the alteration of their subunit composition transform neurons into “pathological” cells, determining the further plasticity or pathology of the hippocampal formation. In the present review, we summarize the expression of AMPA, kainate, and NMDA receptor subunits following deafferentation, repeated mild seizures, and status epilepticus. We compare our results to literature descriptions, and draw conclusions as to the reactive plasticity of iGluRs in the hippocampus.

Physiological signature of a novel potentiator of AMPA receptor signalling

We have synthesized a novel small molecule based on the pyrrolidinone–containing core structure of clausenamide, which is a candidate anti–dementia drug. The synthetic route yielded multi–gram quantities of an isomeric racemate mixture in a short number of steps. When tested in hippocampal slices from young adult rats the compound enhanced AMPA receptor–mediated signalling at mossy fibre synapses, and potentiated inward currents evoked by local application of l–glutamate onto CA3 pyramidal neurons. It facilitated the induction of mossy fibre LTP, but the magnitude of potentiation was smaller than that observed in untreated slices. The racemic mixture was separated and it was shown that only the (−) enantiomer was active. Toxicity analysis indicated that cell lines tolerated the compound at concentrations well above those enhancing synaptic transmission. Our results unveil a small molecule whose physiological signature resembles that of a potent nootropic drug.

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A small molecule was developed using a novel approach towards the synthesis of clausenamide, which is thought to have anti–dementia properties.

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When tested in hippocampal slices the compound enhanced AMPA receptor signalling as well as glutamate–evoked currents in CA3 pyramidal neurons.

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It facilitated the induction of mossy fibre LTP.

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Cytotoxicity experiments revealed no deleterious effect to cells at sub–millimolar concentrations.

Organotypic slice cultures for studies of postnatal neurogenesis

Here we describe a technique for studying hippocampal postnatal neurogenesis in the rodent brain using the organotypic slice culture technique. This method maintains the characteristic topographical morphology of the hippocampus while allowing direct application of pharmacological agents to the developing hippocampal dentate gyrus. Additionally, slice cultures can be maintained for up to 4 weeks and thus, allow one to study the maturation process of newborn granule neurons. Slice cultures allow for efficient pharmacological manipulation of hippocampal slices while excluding complex variables such as uncertainties related to the deep anatomic location of the hippocampus as well as the blood brain barrier. For these reasons, we sought to optimize organotypic slice cultures specifically for postnatal neurogenesis research.

The synaptic proteome

Synapses are focal hot spots for signal transduction and plasticity in the brain. A synapse comprises an axon terminus, the presynapse, the synaptic cleft containing extracellular matrix proteins as well as adhesion molecules, and the postsynaptic density as target structure for chemical signaling. The proteomes of the presynaptic and postsynaptic active zones control neurotransmitter release and perception. These tasks demand short- and long-term structural and functional dynamics of the synapse mediated by its proteinaceous inventory. This review addresses subcellular fractionation protocols and the related proteomic approaches to the various synaptic subcompartments with an emphasis on the presynaptic active zone (PAZ). Furthermore, it discusses major constituents of the PAZ including the amyloid precursor protein family members. Numerous proteins regulating the rearrangement of the cytoskeleton are indicative of the functional and structural dynamics of the pre- and postsynapse. The identification of protein candidates of the synapse provides the basis for further analyzing the interaction of synaptic proteins with their targets, and the effect of their deletion opens novel insights into the functional role of these proteins in neuronal communication. The knowledge of the molecular interactome is also a prerequisite for understanding numerous neurodegenerative diseases.

The use of connexin-based therapeutic approaches to target inflammatory diseases

Alterations in Connexin43 (Cx43) expression levels have been shown to play a role in inflammatory processes including skin wounding and neuroinflammation. Cx43 protein levels increase following a skin wound and can inhibit wound healing. Increased Cx43 has been observed following stroke, epilepsy, ischemia, optic nerve damage, and spinal cord injury with gap junctional communication and hemichannel opening leading to increased secondary damage via the inflammatory response. Connexin43 modulation has been identified as a potential target for protection and repair in neuroinflammation and skin wound repair. This review describes the use of a Cx43 specific antisense oligonucleotide (Cx43 AsODN) and peptide mimetics of the connexin extracellular loop domain to modulate Cx43 expression and/or function in inflammatory disorders of the skin and central nervous system. An overview of the role of connexin43 in inflammatory conditions, how antisense and peptide have allowed us to elucidate the role of Cx43 in these diseases, create models of diseases to test interventions and their potential for use clinically or in current clinical trials is presented. Antisense oligonucleotides are applied topically and have been used to improve wound healing following skin injury. They have also been used to develop ex vivo models of neuroinflammatory diseases that will allow testing of intervention strategies. The connexin mimetic peptides have shown potential in a number of neuroinflammatory disorders in ex vivo models as well as in vivo when delivered directly to the injury site or when delivered systemically.

The proteome of the presynaptic active zone from mouse brain

Neurotransmitter release as well as the structural and functional dynamics of the presynaptic active zone is controlled by proteinaceous components. Here we describe for the first time an experimental approach for the isolation of the presynaptic active zone from individual mouse brains, a prerequisite for understanding the functional inventory of the presynaptic protein network and for the later analysis of changes occurring in mutant mice. Using a monoclonal antibody against the ubiquitous synaptic vesicle protein SV2 we immunopurified synaptic vesicles docked to the presynaptic plasma membrane. Enrichment studies by means of Western blot analysis and mass spectrometry identified 485 proteins belonging to an impressive variety of functional categories. Our data suggest that presynaptic active zones represent focal hot spots that are not only involved in the regulation of neurotransmitter release but also in multiple structural and functional alterations the adult nerve terminal undergoes during neural activity in adult CNS. They furthermore open new avenues for characterizing alterations in the active zone proteome of mutant mice and their corresponding controls, including the various mouse models of neurological diseases.

Organotypic hippocampal slice culture from the adult mouse brain: a versatile tool for translational neuropsychopharmacology

One of the most significant barriers towards translational neuropsychiatry would be an unavailability of living brain tissues. Although organotypic brain tissue culture could be a useful alternative enabling observation of temporal changes induced by various drugs in living brain tissues, a proper method to establish a stable organotypic brain slice culture system using adult (rather than neonatal) hippocampus has been still elusive. In this study, we evaluated our simple method using the serum-free culture medium for successful adult organotypic hippocampal slice culture. Several tens of hippocampal slices from a single adult mouse (3-5 months old) were cultured in serum-free versus serum-containing conventional culture medium for 30 days and underwent various experiments to validate the effects of the existence of serum in the culture medium. Neither the excessive regression of neuronal viability nor metabolic deficiency was observed in the serum-free medium culture in contrast to the serum-containing medium culture. Despite such viability, newly generated immature neurons were scarcely detected in the serum-free culture, suggesting that the original neurons in the brain slice persist rather than being replaced by neurogenesis. Key structural features of in vivo neural tissue constituting astrocytes, neural processes, and pre- and post-synapses were also well preserved in the serum-free culture. In conclusion, using the serum-free culture medium, the adult hippocampal slice culture system will serve as a promising ex vivo tool for various fields of neuroscience, especially for studies on aging-related neuropsychiatric disorders or for high throughput screening of potential agents working against such disorders.

Adult rat hippocampal slices as in vitro models for neurodegeneration: Studies on cell viability and apoptotic processes

Adult hippocampal slice cultures were used in the modeling of apoptotic aspects of neurodegeneration. Slice viability was determined by the use of trypan blue (TB) staining, and apoptosis was assessed by caspase-3 immunohistochemistry. A large number of pyramidal cells showed signs of degeneration 30 min after sectioning (58.4% of the total number of pyramidal cells), as they exhibited TB uptake, and about 71.6% of these neurons became stained by the third day in culture, when patches in the stratum oriens also demonstrated distinct TB staining. By the sixth day of culturing, almost all cells in the pyramidal cell layer became TB positive (88.4%). The caspase-3 immunoreactivity displayed a different pattern, as the most intense immunoreactivity, detected mainly in the pyramidal cells, peaked 6 h after culturing, and then decreased steadily. The present data show that in adult hippocampal slices a large number of pyramidal cells initiate apoptotic processes as a result of irreparable damage sustained during slice preparation and culture maintenance, and support the notion that apoptosis is an integral part of the neurodegenerative processes not only in vivo but also in vitro. Elucidation of mechanisms for the apoptotic processes in adult hippocampal slice cultures could lead to the development of new therapeutic strategies; moreover, the utilization of adult hippocampal slice cultures could be a viable alternative technique to in vivo experiments in studying the mechanisms responsible for neurodegeneration.

Ultrastructural relationship between the AMPA-GluR2 receptor subunit and the mu-opioid receptor in the mouse central nucleus of the amygdala

Activation of GluR2-expressing non-calcium-permeable AMPA-type glutamate receptors in the central nucleus of the amygdala (CeA) may play an important role in integrating emotion and memory with goal-directed behaviors involved in opioid addiction. The location of non-calcium-permeable AMPA receptors within distinct neuronal compartments (i.e., soma, dendrite, or axon) is an important functional feature of these proteins; however, their ultrastructural location and subcellular relationship with mu-opioid receptors (μOR) in the CeA are unknown. Immunocytochemical electron microscopy was used to characterize the ultrastructural distribution of GluR2 and its association with μOR in the mouse CeA. A single-labeling analysis of GluR2 distribution employing immunoperoxidase or immunogold markers revealed that this protein was frequently affiliated with intracellular vesicular organelles, as well as the plasma membrane of CeA neuronal profiles. Among all GluR2-labeled neuronal structures, over 85% were dendrites or somata. Unlabeled axon terminals frequently formed asymmetric excitatory-type synaptic junctions with GluR2-labeled dendritic profiles. Dual-labeling immunocytochemical analysis showed that GluR2 and μOR were co-localized in neuronal compartments. Among all dual-labeled structures, approximately 80% were dendritic. Synaptic inputs to these dual-labeled dendrites were frequently from unlabeled axon terminals forming asymmetric excitatory-type synapses. The presence of GluR2 in dendritic profiles receiving asymmetric synapses suggests that activation of the non-calcium-permeable AMPA receptor plays a role in the postsynaptic modulation of excitatory signaling involving CeA neuronal circuits that coordinate sensory, affective, and behavioral processes involved in drug addiction. Given the critical role of non-calcium-permeable AMPA receptor function in neural and behavioral adaptability, their dendritic association with μOR in CeA dendrites provides a neuronal substrate for opioid-mediated plasticity.

Polarized sorting of AMPA receptors to the somatodendritic domain is regulated by adaptor protein AP-4

Neurons are highly polarized cells comprising somatodendritic and axonal domains. For proper neuronal function, such as neurotransmission and synaptic plasticity, membrane proteins must be transported to precise positions. alpha-Amino-3-hydroxy-5-methyl-4-isoxazole propionic acid-type glutamate receptors (AMPA receptors) are membrane proteins localized in the somatodendritic domain. AMPA receptors mediate excitatory synaptic transmission; thus, regulation of the intracellular trafficking of AMPA receptors has a critical role in synaptic plasticity. An understanding of the molecular mechanisms that regulate AMPA receptor trafficking is essential for gaining further insight into neuronal function. Despite its importance, however, how neurons selectively transport AMPA receptors to the somatodendritic domain is largely unknown. In this Update Article, we discuss recent progress in studies of the mechanisms underlying the somatodendritic targeting of AMPA receptors in neurons.

Accumulation of AMPA receptors in autophagosomes in neuronal axons lacking adaptor protein AP-4

AP-4 is a member of the adaptor protein complexes, which control vesicular trafficking of membrane proteins. Although AP-4 has been suggested to contribute to basolateral sorting in epithelial cells, its function in neurons is unknown. Here, we show that disruption of the gene encoding the beta subunit of AP-4 resulted in increased accumulation of axonal autophagosomes, which contained AMPA receptors and transmembrane AMPA receptor regulatory proteins (TARPs), in axons of hippocampal neurons and cerebellar Purkinje cells both in vitro and in vivo. AP-4 indirectly associated with the AMPA receptor via TARPs, and the specific disruption of the interaction between AP-4 and TARPs caused the mislocalization of endogenous AMPA receptors in axons of wild-type neurons. These results indicate that AP-4 may regulate proper somatodendritic-specific distribution of its cargo proteins, including AMPA receptor-TARP complexes and the autophagic pathway in neurons.

Dynamics of postsynaptic glutamate receptor targeting

Targeting of glutamate receptors to synapses is an important event in both developing and mature neurons. Glutamate receptors are delivered to nascent synapses during synaptogenesis and to existing synapses during activity-dependent synaptic strengthening. Increasing evidence suggests that glutamate receptors are inserted into the plasma membrane before they accumulate at the synapse. Lateral diffusion of receptors occurs at both synaptic and non-synaptic membranes, and glutamate receptors can exchange rapidly between synaptic and extrasynaptic sites. In addition, recent studies show that postsynaptic scaffold molecules can be highly mobile. The dynamic nature of the synapse suggests that many mechanisms might be involved in regulating synapse formation and synaptic plasticity.

Analysis of the synaptic vesicle proteome using three gel-based protein separation techniques

Synaptic vesicles are key organelles in neurotransmission. Their functions are governed by a unique set of integral and peripherally associated proteins. To obtain a complete protein inventory, we immunoisolated synaptic vesicles from rat brain to high purity and performed a gel-based analysis of the synaptic vesicle proteome. Since the high hydrophobicity of integral membrane proteins hampers their resolution by gel electrophoretic techniques, we applied in parallel three different gel electrophoretic methods for protein separation prior to MS. Synaptic vesicle proteins were subjected to either 1-D SDS-PAGE along with nano-LC ESI-MS/MS or to the 2-D gel electrophoretic techniques benzyldimethyl-n-hexadecylammonium chloride (BAC)/SDS-PAGE, and double SDS (dSDS)-PAGE in combination with MALDI-TOF-MS. We demonstrate that the combination of all three methods provides a comprehensive survey of the proteinaceous inventory of the synaptic vesicle membrane compartment. The identified synaptic vesicle proteins include transporters, soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), synapsins, rab and rab-interacting proteins, additional guanine nucleotide triphosphate (GTP) binding proteins, cytoskeletal proteins, and proteins modulating synaptic vesicle exo- and endocytosis. In addition, we identified novel proteins of unknown function. Our results demonstrate that the parallel application of three different gel-based approaches in combination with mass spectrometry permits a comprehensive analysis of the synaptic vesicle proteome that is considerably more complex than previously anticipated.

Application of two-photon microscopy to the study of cellular pharmacology of central neurons

Two-photon microscopy is an especially powerful tool for combining anatomical and physiological experiments in the central nervous system: the possibility of simultaneously studying physiological phenomena in well-defined anatomical compartments allows fluorescence imaging of neurons in deeper layers of the brain. In this review we summarize the most commonly used brain preparation techniques together with the methods of loading neurons with fluorescent indicators. We will focus primarily on issues of drug delivery specifically related to two-photon experiments highlighting the different ways of drug administration. Methods of chemical stimulation via caged neurotransmitters are also discussed. Finally a few specific areas of two-photon applications in drug research on neuronal tissue are highlighted.

Suppression of neuronal network excitability and seizure-like events by 2-methyl-4-oxo-3H-quinazoline-3-acetyl piperidine in juvenile rat hippocampus: involvement of a metabotropic glutamate receptor

We present data on the antiepileptic potency of 2-methyl-4-oxo-3H-quinazoline-3-acetyl piperidine (Q5) in juvenile (P9-13) rat hippocampal slices and in particular Q5's action mechanism and target. Q5 (200-500 microM), but not alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/Kainate receptor antagonists blocked low-[Mg2+]-induced seizure-like events (SLE) in the CA3 region. Q5 (100 microM) decreased Glu-induced [35S]guanosine 5'-O-(3-thiotriphosphate) binding enhancement in brain homogenates, without interaction with ionotropic Glu receptor sites and Glu transport. In voltage-clamped CA3 pyramidal cells, Q5 (500 microM) depressed activities of spontaneous excitatory and inhibitory postsynaptic currents without affecting miniature inhibitory currents. Metabotropic Glu receptor (mGluR) subtype antagonists affected network excitability dissimilarly. Intracellular Ca2+ ion transients induced by the mGluR agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) were suppressed by Q5. Agreeing predictions obtained by modelling Q5 binding to different experimental conformations of mGlu1, Q5 was bound partially to an mGluR binding site in the presence of 1mM ACPD. Findings suggest the apparent involvement of a novel phenotype of action or a new mGluR subtype in the specific suppression of epileptiform activity by Q5 through the depression of network excitability.

A novel pathway for presynaptic mitogen-activated kinase activation via AMPA receptors

AMPA-type glutamate receptors play a key role in mediating postsynaptic responses of excitatory neurotransmitters. It is now well accepted that AMPA receptors are also present at the presynapse, where they are thought to modulate neurotransmitter release. However, the mechanisms through which they control synaptic vesicle traffic have remained elusive. We used cultured hippocampal neurons and growth cone particles prepared from fetal rat brain to investigate the functional role of presynaptic AMPA receptors. We show here that stimulation of presynaptic AMPA receptors induces activation of mitogen-activated protein kinase (MAPK) through a nonreceptor tyrosine kinase-dependent and Na+/Ca2+-independent mechanism. This pathway is activated predominantly in axonal growth cones compared with the somatodendritic compartment. After stimulation of presynaptic AMPA receptors, synapsin I is phosphorylated at MAPK-specific sites. These events are paralleled by a prominent increase in evoked synaptic vesicle recycling that is blocked by the specific MAPK inhibitor 2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one. Similarly, in synaptosomes isolated from adult brain, AMPA stimulation induces MAPK activation and phosphorylation of synapsin I at MAPK-dependent sites and enhances significantly synaptic vesicle recycling. These results reveal a novel pathway for activation of presynaptic MAPK and suggest a role of this pathway in the regulation of short-term presynaptic plasticity.

The molecular pharmacology and cell biology of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPARs) are of fundamental importance in the brain. They are responsible for the majority of fast excitatory synaptic transmission, and their overactivation is potently excitotoxic. Recent findings have implicated AMPARs in synapse formation and stabilization, and regulation of functional AMPARs is the principal mechanism underlying synaptic plasticity. Changes in AMPAR activity have been described in the pathology of numerous diseases, such as Alzheimer's disease, stroke, and epilepsy. Unsurprisingly, the developmental and activity-dependent changes in the functional synaptic expression of these receptors are under tight cellular regulation. The molecular and cellular mechanisms that control the postsynaptic insertion, arrangement, and lifetime of surface-expressed AMPARs are the subject of intense and widespread investigation. For example, there has been an explosion of information about proteins that interact with AMPAR subunits, and these interactors are beginning to provide real insight into the molecular and cellular mechanisms underlying the cell biology of AMPARs. As a result, there has been considerable progress in this field, and the aim of this review is to provide an account of the current state of knowledge.

G protein-dependent presynaptic inhibition mediated by AMPA receptors at the calyx of Held

The alpha-amino-3-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) is an ionotropic receptor mediating excitatory synaptic transmission, but it can also interact with intracellular messengers. Here we report that, at the calyx of Held in the rat auditory brainstem, activation of AMPARs induced inward currents in the nerve terminal and inhibited presynaptic Ca2+ currents (I(pCa)), thereby attenuating glutamatergic synaptic transmission. The AMPAR-mediated I(pCa) inhibition was disinhibited by a strong depolarizing pulse and occluded by the nonhydrolyzable GTP analog GTPgammaS loaded into the terminal. We conclude that functional AMPARs are expressed at the calyx of Held nerve terminal and that their activation inhibits voltage-gated Ca2+ channels by an interaction with heterotrimeric GTP-binding proteins (G proteins). Thus, at a central glutamatergic synapse, presynaptic AMPARs have a metabotropic nature and regulate transmitter release by means of G proteins.

NMDA-receptor mediated efflux of N-acetylaspartate: physiological and/or pathological importance?

N-Acetylaspartate (NAA) is a largely neuron specific dianionic amino acid present in high concentration in vertebrate brain. Many fundamental questions concerning N-acetylaspartate in brain remain unanswered. One such issue is the predominantly neuronal synthesis and largely glial catabolism which implies the existence of a regulated efflux from neurons. Here we show that transient (5 min) NMDA-receptor activation (60 microM) induces a long lasting Ca2+ -dependent efflux of N-acetylaspartate from organotypic slices of rat hippocampus. The NMDA-receptor stimulated efflux was unaffected by hyper-osmotic conditions (120 mM sucrose) and no efflux of N-acetylaspartate was evoked by high K+ -depolarization (50 mM) or kainate (300 microM). These results indicate that the efflux induced by NMDA is not related directly to either cell swelling or depolarization but is coupled to Ca2+ -influx via the NMDA-receptor. The efflux of N-acetylaspartate persisted at least 20 min after the omission of NMDA, similar to the efflux of the organic anions glutathione and phosphoethanolamine. The efflux of taurine and hypotaurine was also stimulated by NMDA but returned more quickly to basal levels. The NMDA-receptor stimulated efflux of N-acetylaspartate, glutathione, phosphoethanolamine, taurine and hypotaurine correlated with delayed nerve cell death measured 24 h after the transient NMDA-receptor stimulation. However, exogenous administration of high concentrations of N-acetylaspartate to the culture medium was non-toxic. The results suggest that Ca2+ -influx via the NMDA-receptor regulates the efflux of N-acetylaspartate from neurons which may have both physiological and pathological importance.

Presynaptic localization of an AMPA-type glutamate receptor in corticostriatal and thalamostriatal axon terminals

The neostriatum is known to receive glutamatergic projections from the cerebral cortex and thalamic nuclei. Vesicular glutamate transporters 1 and 2 (VGluT1 and VGluT2) are located on axon terminals of corticostriatal and thalamostriatal afferents, respectively, whereas VGluT3 is found in axon terminals of cholinergic interneurons in the neostriatum. In the present study, the postsynaptic localization of ionotropic glutamate receptors was examined in rat neostriatum by the postembedding immunogold method for double labelling of VGluT and glutamate receptors. Immunoreactive gold particles for AMPA receptor subunits GluR1 and GluR2/3 were frequently found not only on postsynaptic but also on presynaptic profiles immunopositive for VGluT1 and VGluT2 in the neostriatum, and GluR4-immunoreactive particles were observed on postsynaptic and presynaptic profiles positive for VGluT1. Quantitative analysis revealed that 27-45% of GluR1-, GluR2-, GluR2/3- and GluR4-immunopositive particles found in VGluT1- or VGluT2-positive synaptic structures in the neostriatum were associated with the presynaptic profiles of VGluT-positive axons. In contrast, VGluT-positive presynaptic profiles in the neostriatum showed almost no immunoreactivity for NMDA receptor subunits NR1 or NR2A/B. Furthermore, almost no GluR2/3-immunopositive particles were observed in presynaptic profiles of VGluT3-positive (cholinergic) terminals that made asymmetric synapses in the neostriatum, or in those of VGluT1- or VGluT2-positive terminals in the neocortex. The present results indicate that AMPA receptor subunits but not NMDA receptor subunits are located on axon terminals of corticostriatal and thalamostriatal afferents, and suggest that glutamate released from these axon terminals controls the activity of the terminals through the presynaptic AMPA autoreceptors.

Functional validation of adult hippocampal organotypic cultures as an in vitro model of brain injury

To determine whether hippocampal pyramidal neurons retain authentic functional properties in mature organotypic culture, hippocampal slice cultures were established from young adult rats (P20-21). Cultures maintained 7 days in vitro retained tight organization of neuronal layers, as opposed to the widening restructure of pyramidal neurons often observed in perinatal slices. CA3 and CA1 pyramidal neurons fired action potentials in response to current injection and exhibited spontaneous and evoked synaptic currents, indicating intact neuronal function and normal hippocampal neural circuitry. We also tested neuronal sensitivity of slice cultures to ischemic injury. Acute ischemic paradigm resulted in selective death of pyramidal neurons in the CA1 region, which was prevented by treatment with an NMDA-antagonist, MK-801. Robust efflux of excitatory and inhibitory amino acid neurotransmitters was detected during ischemia, consistent with changes shown in acute slices. In summary, hippocampal organotypic cultures prepared from young adult rats maintained neuronal architecture and synaptic activity in vitro and can be used in parallel with an acute slice system to model mature brain tissue to examine ischemic pathophysiology and neuroprotective treatment.

Presynaptic N-methyl-D-aspartate receptor expression is increased by estrogen in an aromatase-rich area of the songbird hippocampus

The vertebrate hippocampus (HP) is sensitive to estrogens, in part via effects on N-methyl-D-aspartate (NMDA)-type glutamate receptors (NR). Although the precise mechanism of this interaction is unclear, it constitutes a key interface in the plasticity of the adult vertebrate HP. The songbird HP expresses high levels of aromatase (estrogen synthase), suggesting that locally generated steroid may affect excitatory pathways. By using light, confocal, and electron microscopy with antibodies that specifically recognize aromatase and NR, we have 1) mapped their distribution in the zebra finch brain, 2) documented their coexpression in HP neurons, 3) studied the ultrastructure of NR-expressing cells in the HP, and 4) tested the influence of estrogen on the cellular and subcellular characteristics of NR-positive HP neurons. Aromatase and NR are coexpressed in HP neurons. NRs are detectable in presynaptic boutons of the songbird HP in addition to postsynaptic loci. Treatment with estrogen increased the somal size and innervation of NR-positive neurons and the frequency of presynaptic NR. Autoreception of excitatory neurotransmission via presynaptic NR may promote the strengthening of activity-dependent, excitatory synapses, thereby enhancing learning. NR-mediated autoreception may underlie estrogenic enhancement of HP structural and functional plasticity.

Solubilization and immunological identification of presynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors in the rat hippocampus

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors have been identified mostly as postsynaptic receptors mediating fast glutamatergic synaptic transmission. However, neurochemical studies based on the modulation of neurotransmitter release have suggested the existence of presynaptic AMPA receptors. We have used a recently described technique that allows a high-purity fractionation of the pre- and postsynaptic proteins of synaptic junctions to evaluate the distribution of the different AMPA receptor subunits in rat hippocampal synapses. Surprisingly, we found very high levels of GluR1- and GluR2/3-like immunoreactivity in the presynaptic fraction, but also in the postsynaptic and extrasynaptic fractions. GluR4-like immunoreactivity was much less abundant but was still detected, predominantly in the postsynaptic fraction. This methodology appears to be far more sensitive than the classical immunogold electron microscopy to determine the localization of synaptic receptors.

Primary afferent terminals in spinal cord express presynaptic AMPA receptors

Larger dorsal root ganglion neurons are stained by an antibody for the C terminus of glutamate receptor subunit 2 (GluR2) and GluR3 (GluR2/3) rather than by an antibody for GluR4. In dorsal roots, anti-GluR2/3 stains predominantly myelinated fibers; anti-GluR4 or anti-GluR2/4 stains predominantly unmyelinated fibers. In the dorsal horn, puncta immunopositive for synaptophysin and GluR2/3 are predominantly in laminas III and IV, whereas puncta immunopositive for synaptophysin and GluR4 or GluR2/4 are predominantly in laminas I and II. Puncta immunopositive for GluR2/3 costain with the B subunit of cholera toxin, whereas puncta immunopositive for GluR2/4 costain with isolectin B4 after injections of these tracers in the sciatic nerve. No puncta costain with calcitonin gene-related peptide and AMPA receptor subunits. Electron microscopy indicates that AMPA receptor-immunopositive terminals are more numerous than suggested by confocal microscopy. Of all synapses in which immunostaining is presynaptic, postsynaptic, or both, the percentage of presynaptic immunostain is approximately 70% with anti-GluR4 or anti-GluR2/4 (in laminas I-III), 25-30% with anti-GluR2/3 (in laminas III and IV), and 5% with anti-GluR2 (in laminas I-III). Because of fixation constraints, the types of immunostained terminals could be identified only on the basis of morphological characteristics. Many terminals immunostained for GluR2/3, GluR4, or GluR2/4 have morphological features of endings of primary afferents. Terminals with morphological characteristics of presumed GABAergic terminals are also immunostained with anti-GluR2/4 and anti-GluR4 in laminas I and II and with anti-GluR2/3 in laminas III and IV. The conspicuous and selective expression of presynaptic AMPA receptor subunits may contribute to the characteristic physiological profile of different classes of primary afferents and suggests an important mechanism for the modulation of transmitter release by terminals of both myelinated and unmyelinated primary afferents.

Major vault protein is a substrate of endogenous protein kinases in CHO and PC12 cells

Major vault protein (MVP) is the predominant member of a large cytosolic ribonucleoprotein particle, termed vault. We have previously shown that MVP derived from electric ray electric organ becomes phosphorylated by protein kinase C in vitro and by tyrosine kinase in vivo. Here we show that MVP from two mammalian cell lines (CHO and PC12 cell) becomes highly phosphorylated by endogenous protein kinases in cell-free systems. The susceptibility to protein kinases differs substantially from those observed in MVP derived from electric organ. Phosphorylation of MVP depends on the presence of Mg2+ and can be inhibited by the chelating agent EDTA. Inhibitors of casein kinase II attenuate the phosphorylation of MVP. In contrast to CHO cells, addition of recombinant casein kinase II enhances the phosphorylation of MVP in PC12 cells. Endogenous kinase activity is of particulate nature and copurifies with vault particles. Immuno-affinity purified vaults containing recombinant tagged MVP expressed in CHO cells reveal no autophosphorylation, suggesting that protein kinase activity is not an intrinsic property of vaults. Our results suggest that cell-specific phosphorylation of MVP may play a critical role in vault function.