Selective modulation of desensitization at AMPA versus kainate receptors by cyclothiazide and concanavalin A

Potentiation by cyclothiazide of recombinant glutamate receptor responses in Xenopus oocytes showed absolute selectivity for AMPA versus kainate receptors. In contrast, concanavalin A strongly potentiated responses at kainate but not AMPA receptors. Rapid desensitization in HEK 293 cells transfected with AMPA receptors was blocked by cyclothiazide, but only weakly attenuated by concanavalin A. Desensitization at kainate receptors was blocked by concanavalin A but unaffected by cyclothiazide. Selective effects of these modulators following coexpression of subunits from different families suggest independent assembly of functional AMPA and kainate receptors. Northern blot analysis of mRNA for dorsal root ganglia revealed a predominant expression of GluR5, indicating that modulation of desensitization by concanavalin A but not cyclothiazide in sensory neurons accurately predicts subunit expression for native glutamate receptors.

Differential roles of NR2A- and NR2B-containing NMDA receptors in Ras-ERK signaling and AMPA receptor trafficking

NMDA receptors (NMDARs) control bidirectional synaptic plasticity by regulating postsynaptic AMPA receptors (AMPARs). Here we show that NMDAR activation can have differential effects on AMPAR trafficking, depending on the subunit composition of NMDARs. In mature cultured neurons, NR2A-NMDARs promote, whereas NR2B-NMDARs inhibit, the surface expression of GluR1, primarily by regulating its surface insertion. In mature neurons, NR2B is coupled to inhibition rather than activation of the Ras-ERK pathway, which drives surface delivery of GluR1. Moreover, the synaptic Ras GTPase activating protein (GAP) SynGAP is selectively associated with NR2B-NMDARs in brain and is required for inhibition of NMDAR-dependent ERK activation. Preferential coupling of NR2B to SynGAP could explain the subtype-specific function of NR2B-NMDARs in inhibition of Ras-ERK, removal of synaptic AMPARs, and weakening of synaptic transmission.

Activity-dependent regulation of dendritic synthesis and trafficking of AMPA receptors

Regulation of AMPA receptor (AMPAR) trafficking is important for neural plasticity. Here we examined the trafficking and synthesis of the GluR1 and GluR2 subunits using ReAsH-EDT(2) and FlAsH-EDT(2) staining. Activity blockade of rat cultured neurons increased dendritic GluR1, but not GluR2, levels. Examination of transected dendrites revealed that both AMPAR subunits were synthesized in dendrites and that activity blockade enhanced dendritic synthesis of GluR1 but not GluR2. In contrast, acute pharmacological manipulations increased dendritic synthesis of both subunits. AMPARs synthesized in dendrites were inserted into synaptic plasma membranes and, after activity blockade, the electrophysiological properties of native synaptic AMPARs changed in the manner predicted by the imaging experiments. In addition to providing a novel mechanism for synaptic modifications, these results point out the advantages of using FlAsH-EDT(2) and ReAsH-EDT(2) for studying the trafficking of newly synthesized proteins in local cellular compartments such as dendrites.

Activity regulates the synaptic localization of the NMDA receptor in hippocampal neurons

We describe here a novel effect of activity on the subcellular distribution of NMDA receptors in hippocampal neurons in culture. In spontaneously active neurons, NMDA receptors were clustered at a few synaptic and nonsynaptic sites. Chronic blockade of NMDA receptor activity induced a 380% increase in the number of NMDA receptor clusters and a shift to a more synaptic distribution. This effect was reversible. The distributions of the presynaptic marker synaptophysin, the AMPA-type glutamate receptor subunit GluR1, and the putative NMDA receptor clustering protein PSD-95 were not affected by blockade. Regulation of the synaptic localization of NMDA receptors by activity may define a novel mechanism by which input controls a neuron's ability to modify its synapses.

Synaptic clustering of AMPA receptors by the extracellular immediate-early gene product Narp

Narp (neuronal activity-regulated pentraxin) is a secreted immediate-early gene (IEG) regulated by synaptic activity in brain. In this study, we demonstrate that Narp possesses several properties that make it likely to play a key role in excitatory synaptogenesis. Narp is shown to be selectively enriched at excitatory synapses on neurons from both the hippocampus and spinal cord. Overexpression of recombinant Narp increases the number of excitatory but not inhibitory synapses in cultured spinal neurons. In transfected HEK 293T cells, Narp interacts with itself, forming large surface clusters that coaggregate AMPA receptor subunits. Moreover, Narp-expressing HEK 293T cells can induce the aggregation of neuronal AMPA receptors. These studies support a model in which Narp functions as an extracellular aggregating factor for AMPA receptors.

Dysregulated metabotropic glutamate receptor-dependent translation of AMPA receptor and postsynaptic density-95 mRNAs at synapses in a mouse model of fragile X syndrome

Fragile X syndrome, a common form of inherited mental retardation, is caused by the loss of fragile X mental retardation protein (FMRP), an mRNA binding protein that is hypothesized to regulate local mRNA translation in dendrites downstream of gp1 metabotropic glutamate receptors (mGluRs). However, specific FMRP-associated mRNAs that localize to dendrites in vivo and show altered mGluR-dependent translation at synapses of Fmr1 knock-out mice are unknown so far. Using fluorescence in situ hybridization, we discovered that GluR1/2 and postsynaptic density-95 (PSD-95) mRNAs are localized to dendrites of cortical and hippocampal neurons in vivo. Quantitative analyses of their dendritic mRNA levels in cultured neurons and synaptoneurosomes did not detect differences between wild-type and Fmr1 knock-out (KO) mice. In contrast, PSD-95, GluR1/2, and calcium/calmodulin-dependent kinase IIalpha (CaMKIIalpha) mRNA levels in actively translating polyribosomes were dysregulated in synaptoneurosomes from Fmr1 knock-out mice in response to mGluR activation. [35S]methionine incorporation into newly synthesized proteins similarly revealed impaired stimulus-induced protein synthesis of CaMKIIalpha and PSD-95 in synaptoneurosomes from Fmr1 KO mice. Quantitative analysis of mRNA levels in FMRP-specific immunoprecipitations from synaptoneurosomes demonstrated the association of FMRP with CaMKIIalpha, PSD-95, and GluR1/2 mRNAs. These findings suggest a novel mechanism whereby FMRP regulates the local synthesis AMPA receptor (AMPAR) subunits, PSD-95, and CaMKIIalpha downstream of mGluR-activation. Dysregulation of local translation of AMPAR and associated factors at synapses may impair control of the molecular composition of the postsynaptic density and consequently alter synaptic transmission, causing impairments of neuronal plasticity observed in Fmr1 knock-out mice and fragile X syndrome.

Surface expression of AMPA receptors in hippocampal neurons is regulated by an NSF-dependent mechanism

Here, we show that disruption of N-ethylmaleimide-sensitive fusion protein- (NSF-) GluR2 interaction by infusion into cultured hippocampal neurons of a blocking peptide (pep2m) caused a rapid decrease in the frequency but no change in the amplitude of AMPA receptor-mediated miniature excitatory postsynaptic currents (mEPSCs). N-methyl-D-aspartate (NMDA) receptor-mediated mEPSCs were not changed. Viral expression of pep2m reduced the surface expression of alpha-amino-3-hydroxy-5-methyl-isoxazolepropionate (AMPA) receptors, whereas NMDA receptor surface expression in the same living cells was unchanged. In permeabilized neurons, the total amount of GluR2 immunoreactivity was unchanged, and a punctate distribution of GluR2 was observed throughout the dendritic tree. These data suggest that the NSF-GluR2 interaction is required for the surface expression of GluR2-containing AMPA receptors and that disruption of the interaction leads to the functional elimination of AMPA receptors at synapses.

Receptor trafficking and the plasticity of excitatory synapses

Newly discovered features of the trafficking of AMPA receptors to and from the postsynaptic membrane of excitatory synapses are now bringing the mechanisms of synaptic plasticity into focus. Recent advances, including the existence of slots, anchors, transport factors and pathways for activity-dependent control, have elucidated the role of the individual AMPA receptor subunits and their binding partners. The latest views describe how subunit type dictates the assembly of heteromeric receptors, and how these heteromers interact with the receptor trafficking machinery and synaptic anchorage factors. Moreover, phosphorylation may play an important role in receptor transport and synaptic turnover.

Differential effects of enrichment on learning and memory function in NR2B transgenic mice

It has been known that environmental enrichment leads to better learning and memory in mice. However, the molecular mechanisms are not known. In this study, we used the 10th-12th of the NR2B transgenic (Tg) lines, in which the NMDA receptor function is enhanced via the NR2B subunit transgene in neurons of the forebrain, to test the hypothesis of the involvement of NMDA receptor function in enrichment-induced better learning and memory. Consistent with our previous results, both larger long-term potentiation (LTP) in the hippocampus and superior learning and memory were observed in naive NR2B Tg mice even after the 10th-12th generation of breeding. After enrichment, wild-type mice exhibited overall improvement in their performances in contextual and cued conditioning, fear extinctions, and novel object recognition tasks. Interestingly, the same enrichment procedures could not further increase the performance of NR2B Tg mice in contextual conditioning, cued conditioning, or fear extinction, thereby indicating that enhanced NMDA receptor function can occlude these enrichment effects. However, we found that in the novel object recognition task enriched NR2B Tg mice exhibited much longer recognition memory (up to 1 week), compared to that (up to 3 days) in naive NR2B Tg mice. Furthermore, our biochemical experiments showed that enrichment significantly increased protein levels of GluR1, NR2B, and NR2A subunits of glutamate receptors in both wild-type and NR2B Tg mice. Therefore, our results suggest an interactive nature of molecular pathways involved in both environmental and genetic NMDA receptor manipulations for enhancing learning and memory.

Rapid synthesis and synaptic insertion of GluR2 for mGluR-LTD in the ventral tegmental area

The activation of metabotropic glutamate receptors (mGluRs) leads to long-term depression (mGluR-LTD) at many synapses of the brain. The induction of mGluR-LTD is well characterized, whereas the mechanisms underlying its expression remain largely elusive. mGluR-LTD in the ventral tegmental area (VTA) efficiently reverses cocaine-induced strengthening of excitatory inputs onto dopamine neurons. We show that mGluR-LTD is expressed by an exchange of GluR2-lacking AMPA receptors for GluR2-containing receptors with a lower single-channel conductance. The synaptic insertion of GluR2 depends on de novo protein synthesis via rapid messenger RNA translation of GluR2. Regulated synthesis of GluR2 in the VTA is therefore required to reverse cocaine-induced synaptic plasticity.

Visual deprivation reactivates rapid ocular dominance plasticity in adult visual cortex

Brief monocular deprivation (< or =3 d) induces a rapid shift in the ocular dominance of binocular neurons in the juvenile rodent visual cortex but is ineffective in adults. Here, we report that persistent, rapid, juvenile-like ocular dominance plasticity can be reactivated in adult rodent visual cortex when monocular deprivation is preceded by visual deprivation. Ocular dominance shifts in visually deprived adults are caused by a rapid depression of the response to stimulation of the deprived eye, previously only reported in juveniles, and a simultaneous potentiation of the response to stimulation of the nondeprived eye. The enhanced ocular dominance plasticity induced by visual deprivation persists for days, even if binocular vision precedes monocular deprivation. Visual deprivation also induces a significant decrease in the level of GABAA receptors relative to AMPA receptors and a return to the juvenile form of NMDA receptors in the visual cortex, two molecular changes that we propose enable the persistent reactivation of rapid ocular dominance plasticity.

Human T cells express a functional ionotropic glutamate receptor GluR3, and glutamate by itself triggers integrin-mediated adhesion to laminin and fibronectin and chemotactic migration

T cells may encounter glutamate, the major excitatory neurotransmitter in the nervous system, when patrolling the brain and in glutamate-rich peripheral organs. Moreover, glutamate levels increase in the CNS in many pathological conditions in which T cells exert either beneficial or detrimental effects. We discovered that normal human T cells, human T leukemia cells, and mouse anti-myelin basic protein T cells express high levels of glutamate ion channel receptor (ionotropic) of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) subtype 3 (GluR3). The evidence for GluR3 on T cells includes GluR3-specific RT-PCR, Western blot, immunocytochemical staining and flow cytometry. Sequencing showed that the T cell-expressed GluR3 is identical with the brain GluR3. Glutamate (10 nM), in the absence of any additional molecule, triggered T cell function: integrin-mediated T cell adhesion to laminin and fibronectin, a function normally performed by activated T cells only. The effect of glutamate was mimicked by AMPA receptor-agonists and blocked specifically by the selective receptor-antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 6-nitro-7-sulfamoylbenzo[f]quinoxalin-2,3-dione (NBQX), and by relevant anti-integrin mAbs. Glutamate also increased the CXCR4-mediated T cell chemotactic migration toward the key chemokine CXCL12/stromal cell-derived factor-1. GluR3 expression on normal, cancer and autoimmune-associated T cells and the ability of glutamate to directly activate T cell function could be of substantial scientific and clinical importance to normal neuroimmune dialogues and to CNS diseases and injury, and especially to: 1) T cell transmigration to the CNS and patrolling in the brain, 2) T cell-mediated multiple sclerosis, and 3) autoimmune epilepsy, as neurotoxic anti-GluR3 Abs are found and suspected to cause/potentiate seizures and neuropathology in several types of human epilepsies. Thus far, GluR3 was found only on neurons and glia cells; our results reveal a novel peripheral source of this antigenic receptor.

Brain-derived neurotrophic factor regulates the expression and synaptic delivery of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor subunits in hippocampal neurons

Brain-derived neurotrophic factor (BDNF) plays an important role in synaptic plasticity in the hippocampus, but the mechanisms involved are not fully understood. The neurotrophin couples synaptic activation to changes in gene expression underlying long term potentiation and short term plasticity. Here we show that BDNF acutely up-regulates GluR1, GluR2, and GluR3 alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor subunits in 7-day in vitro cultured hippocampal neurons. The increase in GluR1 and GluR2 protein levels in developing cultures was impaired by K252a, a tropomyosin-related [corrected] kinase (Trk) inhibitor, and by translation (emetine and anisomycin) and transcription (alpha-amanitine and actinomycin D) inhibitors [corrected] The increase in GluR1 and GluR2 protein levels in developing cultures was impaired by K252a, a Trk inhibitor, and by translation (emetine and anisomycin) and transcription (alpha-amanitine and actinomycin D) inhibitors. Accordingly, BDNF increased the mRNA levels for GluR1 and GluR2 subunits. Biotinylation studies showed that stimulation with BDNF for 30 min selectively increased the amount of GluR1 associated with the plasma membrane, and this effect was abrogated by emetine. Under the same conditions, BDNF induced GluR1 phosphorylation on Ser-831 through activation of protein kinase C and Ca(2+)-calmodulin-dependent protein kinase II. Chelation of endogenous extracellular BDNF with TrkB-IgG selectively decreased GluR1 protein levels in 14-day in vitro cultures of hippocampal neurons. Moreover, BDNF promoted synaptic delivery of homomeric GluR1 AMPA receptors in cultured organotypic slices, by a mechanism independent of NMDA receptor activation. Taken together, the results indicate that BDNF up-regulates the protein levels of AMPA receptor subunits in hippocampal neurons and induces the delivery of AMPA receptors to the synapse.

Dopaminergic stimulation of local protein synthesis enhances surface expression of GluR1 and synaptic transmission in hippocampal neurons

The use-dependent modification of synapses is strongly influenced by dopamine, a transmitter that participates in both the physiology and pathophysiology of animal behavior. In the hippocampus, dopaminergic signaling is thought to play a key role in protein synthesis-dependent forms of synaptic plasticity. The molecular mechanisms by which dopamine influences synaptic function, however, are not well understood. Using a GFP-based reporter, as well as a small-molecule reporter of endogenous protein synthesis, we show that dopamine D1/D5 receptor activation stimulates local protein synthesis in the dendrites of hippocampal neurons. We also identify the GluR1 subunit of AMPA receptors as one protein upregulated by dopamine receptor activation, with increased incorporation of surface GluR1 at synaptic sites. The insertion of new GluRs is accompanied by an increase in the frequency of miniature synaptic events. Together, these data suggest a local protein synthesis-dependent activation of previously silent synapses as a result of dopamine receptor stimulation.

Maintenance of late-phase LTP is accompanied by PKA-dependent increase in AMPA receptor synthesis

Long-term potentiation (LTP) is a form of synaptic plasticity that has been extensively studied as a putative mechanism underlying learning and memory. A late phase of LTP occurring 3-5 hours after stimulation and depending on transcription, protein synthesis and cyclic-AMP-dependent protein kinase (protein kinase A, or PKA) has been described, but it is not known whether transcription of presynaptic and/or postsynaptic genes is required to support late-phase LTP. Here we show that late-phase LTP can be obtained in rat hippocampal CA1 mini-slices in which the cell bodies of presynaptic Schaffer collateral/commissural fibres are removed. Thus, transcription of presynaptic genes is not necessary to support maintenance of late-phase LTP. The AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate) receptor is the predominant mediator of the ionotropic response to synaptically released glutamate in the hippocampus and it has been implicated in LTP maintenance. We find that synthesis of AMPA receptor subunits is increased three hours after LTP induction: this effect on the synthesis of the AMPA receptor is blocked by inhibitors of PKA and of transcription. Our results support the idea of a postsynaptic mechanism maintaining late-phase LTP, in which AMPA receptor synthesis is increased as a result of PKA-dependent gene transcription.

CPEB3 and CPEB4 in neurons: analysis of RNA-binding specificity and translational control of AMPA receptor GluR2 mRNA

CPEB is a sequence-specific RNA-binding protein that promotes polyadenylation-induced translation in oocytes and neurons. Vertebrates contain three additional genes that encode CPEB-like proteins, all of which are expressed in the brain. Here, we use SELEX, RNA structure probing, and RNA footprinting to show that CPEB and the CPEB-like proteins interact with different RNA sequences and thus constitute different classes of RNA-binding proteins. In transfected neurons, CPEB3 represses the translation of a reporter RNA in tethered function assays; in response to NMDA receptor activation, translation is stimulated. In contrast to CPEB, CPEB3-mediated translation is unlikely to involve cytoplasmic polyadenylation, as it requires neither the cis-acting AAUAAA nor the trans-acting cleavage and polyadenylation specificity factor, both of which are necessary for CPEB-induced polyadenylation. One target of CPEB3-mediated translation is GluR2 mRNA; not only does CPEB3 bind this RNA in vitro and in vivo, but an RNAi knockdown of CPEB3 in neurons results in elevated levels of GluR2 protein. These results indicate that CPEB3 is a sequence-specific translational regulatory protein.

AMPA, KA and NMDA receptors are expressed in the rat DRG neurones

The localization of AMPA receptor subunits (GluR1-4), a KA receptor subunit (GluR5) and NMDA receptor subunits (NR1 and NRgbs; the glutamate binding subunits of an NMDA receptor complex) was investigated using immunohistochemistry and in situ hybridization histochemistry in the rat dorsal root ganglion. Small neurones expressed GluR1-, GluR2/3-like immunoreactivities and GluR5, NR1, NRgbs mRNAs, while large neurones expressed GluR2/3-like immunoreactivity and NR1 and NRgbs mRNAs. These data suggest that the glutamatergic system plays an important role in the primary sensory afferent systems and that the composition of glutamate receptors differs according to the cell size.

Dimensions and ion selectivity of recombinant AMPA and kainate receptor channels and their dependence on Q/R site residues

1. Recombinant alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor (AMPAR) subunits (GluR-A or GluR-B) and kainate receptor (KAR) subunit (GluR-6) in their unedited (Q)- and edited (R)-forms were expressed in HEK 293 cells. To estimate the dimensions of the narrow portion of these channels, biionic reversal potentials for organic cations of different mean diameters were determined with Cs+ as the internal reference ion. 2. Homomeric channels assembled from Q-form subunits were cation selective. The relation between the relative permeability and the mean size of different organic cations suggests that the diameter of the narrow portion of Q-form channels is approximately 0.78 nm for AMPAR and 0.75 nm for KAR channels. 3. Homomeric channels assembled from R-form subunits were permeant for anions and cations. When probed with CsC1 gradients the relative chloride permeability (PC1/PCs) was estimated as 0.14 for GluR-B(R) and 0.74 for GluR-6(R)-subunit channels. The permeability versus mean size relation for large cations measured with the weakly permeant F- as anion, indicates that for the R-form KAR channels the apparent pore diameter is close to 0.76 nm. 4. Heteromeric AMPAR and KAR channels co-assembled from Q- and R-form subunits were cation selective. The diameter of the narrow portion of these channels is estimated to be in the range between 0.70 and 0.74 nm. 5. The results indicated that the diameters of the narrow portion of AMPAR and KAR channels of different subunit composition and of widely different ion selectivity are comparable. Therefore, the differences in the anion versus cation selectivity, in Ca2+ permeability and in channel conductance are likely to be determined by the difference in charge density of the channel.

NSF ATPase and alpha-/beta-SNAPs disassemble the AMPA receptor-PICK1 complex

AMPA receptor (AMPAR) trafficking is crucial for synaptic plasticity that may be important for learning and memory. NSF and PICK1 bind the AMPAR GluR2 subunit and are involved in trafficking of AMPARs. Here, we show that GluR2, PICK1, NSF, and alpha-/beta-SNAPs form a complex in the presence of ATPgammaS. Similar to SNARE complex disassembly, NSF ATPase activity disrupts PICK1-GluR2 interactions in this complex. Alpha- and beta-SNAP have differential effects on this reaction. SNAP overexpression in hippocampal neurons leads to corresponding changes in AMPAR trafficking by acting on GluR2-PICK1 complexes. This demonstrates that the previously reported synaptic stabilization of AMPARs by NSF involves disruption of GluR2-PICK1 interactions. Furthermore, we are reporting a non-SNARE substrate for NSF disassembly activity.

Molecular mechanisms involved in the growth stimulation of breast cancer cells by leptin

Obesity is a risk factor for breast cancer in postmenopausal women. Leptin, an adipocyte-derived cytokine, elicits proliferative effects in some cell types and potentially stimulates the growth of mammary epithelium. Here we show that leptin induced time- and dose-dependent signal transducer and activator of transcription 3 (STAT3) phosphorylation and extracellular signal-regulated kinase (ERK) 1/2 kinase activation in breast carcinoma cells. Blocking STAT3 phosphorylation with a specific inhibitor, AG490, abolished leptin-induced proliferation of MCF-7 cells, whereas blocking ERK1/2 activation by a specific ERK1/2 kinase inhibitor, U0126, did not result in any significant changes in leptin-induced cell proliferation. Our experiments also showed that one member of the p160 family of steroid receptor coactivators, steroid receptor coactivator (SRC)-1, but not glucocorticoid receptor interacting protein 1 (GRIP1) or amplified in breast cancer 1 (AIB1), also functioned in gene transactivation in response to leptin treatment. Glutathione S-transferase pull-down experiments showed that SRC-1 physically interacted with the activation domain of STAT3 and that chromatin immunoprecipitation experiments detected the occupancy of SRC-1, but not GRIP1 or AIB1, on the promoter of STAT3 target genes. Our experiments collectively showed that SRC-1 is involved in STAT3 signaling pathway that is implicated in leptin-stimulated cell growth.

Calcium-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors: a molecular determinant of selective vulnerability in amyotrophic lateral sclerosis

The cause of the selective degeneration of motor neurons in amyotrophic lateral sclerosis (ALS) remains unexplained. One potential pathogenetic mechanism is chronic toxicity due to disturbances of the glutamatergic neurotransmitter system, mediated via alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-sensitive glutamate receptors. Functional AMPA receptors consist of various combinations of four subunits (designated GluR1-4). The GluR2 subunit is functionally dominant and renders AMPA receptors impermeable to calcium. Most native AMPA receptors in the mammalian central nervous system (CNS) contain the GluR2 subunit and are calcium impermeable. We have investigated the composition of AMPA receptors expressed on normal human spinal motor neurons by in situ hybridization to determine their likely subunit stoichiometry. Highly significant levels of mRNA were detected for the GluR1, GluR3, and GluR4 subunits. However, GluR2 subunit mRNA was not detectable in this cell group. The absence of detectable GluR2 mRNA in normal human spinal motor neurons predicts that they express calcium-permeable AMPA receptors unlike most neuronal groups in the human CNS. Expression of atypical calcium-permeable AMPA receptors by human motor neurons provides a possible mechanism whereby disturbances of glutamate neurotransmission in ALS may selectively injure this cell group.

Activity of 2,3-benzodiazepines at native rat and recombinant human glutamate receptors in vitro: stereospecificity and selectivity profiles

The activity and selectivity of the glutamate receptor antagonists belonging to the 2,3-benzodiazepine class of compounds have been examined at recombinant human non-NMDA glutamate receptors expressed in HEK293 cells and on native rat NMDA and non-NMDA receptors in vitro. The racemic 2,3-benzodiazepines GYKI52466, LY293606 (GYKI53405) and LY300168 (GYKI53655) inhibited AMPA (10 microM)-mediated responses in recombinant human GluR1 receptors expressed in HEK293 cells with approximate IC50 values of 18 microM, 24 microM and 6 microM, respectively and AMPA (10 microM) responses in recombinant human GluR4 expressing HEK293 cells with approximate IC50 values of 22 microM, 28 microM and 5 microM, respectively. GYKI 52466, LY293606 and LY300168 were non-competitive antagonists of AMPA receptor-mediated responses in acutely isolated rat cerebellar Purkinje neurons with approximate IC50 values of 10 microM, 8 microM and 1.5 microM, respectively. The activity of racemic compounds LY293606 and LY300168 was established to reside in the (-) isomer of each compound. At a concentration of 100 microM, GYKI52466, LY293606 and LY300168 produced < 30% inhibition of kainate-activated currents evoked in HEK293 cells expressing either human homomeric GluR5 or GluR6 receptors or heteromeric GluR6+KA2 kainate receptors. The activity of the 2,3-benzodiazepines at 100 microM was weak at kainate receptors, but was stereoselective. Similar levels of inhibition were observed for kainate-induced currents in dorsal root ganglion neurons. Intact tissue preparations were also used to examine the stereoselective actions of the 2,3-benzodiazepines. In the cortical wedge preparation, the active isomer of LY300168, LY303070, produced a non-competitive antagonism of AMPA-evoked depolarizations with smaller changes in depolarizations induced by kainate and no effect on NMDA-dependent depolarizations. LY303070 was also effective in preventing 30 microM AMPA-induced depolarizations in isolated spinal cord dorsal roots with an approximate IC50 value of 1 microM. Synaptic transmission in the hemisected spinal cord preparation was stereoselectively antagonized by the active isomers of LY300168 and LY293606. In summary, these results indicate that 2,3-benzodiazepines are potent, selective and stereospecific antagonists of the AMPA subtype of the non-NMDA glutamate receptor.

(-)Epicatechin stimulates ERK-dependent cyclic AMP response element activity and up-regulates GluR2 in cortical neurons

Emerging evidence suggests that the cellular actions of flavonoids relate not simply to their antioxidant potential but also to the modulation of protein kinase signalling pathways. We investigated in primary cortical neurons, the ability of the flavan-3-ol, (-)epicatechin, and its human metabolites at physiologically relevant concentrations, to stimulate phosphorylation of the transcription factor cAMP-response element binding protein (CREB), a regulator of neuronal viability and synaptic plasticity. (-)Epicatechin at 100-300 nmol/L stimulated a rapid, extracellular signal-regulated kinase (ERK)- and PI3K-dependent, increase in CREB phosphorylation. At micromolar concentrations, stimulation was no longer apparent and at the highest concentration tested (30 mumol/L) (-)epicatechin was inhibitory. (-)Epicatechin also stimulated ERK and Akt phosphorylation with similar bell-shaped concentration-response characteristics. The human metabolite 3'-O-methyl-(-)epicatechin was as effective as (-)epicatechin at stimulating ERK phosphorylation, but (-)epicatechin glucuronide was inactive. (-)Epicatechin and 3'-O-methyl-(-)epicatechin treatments (100 nmol/L) increased CRE-luciferase activity in cortical neurons in a partially ERK-dependent manner, suggesting the potential to increase CREB-mediated gene expression. mRNA levels of the glutamate receptor subunit GluR2 increased by 60%, measured 18 h after a 15 min exposure to (-)epicatechin and this translated into an increase in GluR2 protein. Thus, (-)epicatechin has the potential to increase CREB-regulated gene expression and increase GluR2 levels and thus modulate neurotransmission, plasticity and synaptogenesis.

Molecular determinants of AMPA receptor subunit assembly

AMPA-type (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate) glutamate receptors (AMPARs) mediate post-synaptic depolarization and fast excitatory transmission in the central nervous system. AMPARs are tetrameric ion channels that assemble in the endoplasmic reticulum (ER) in a poorly understood process. The subunit composition determines channel conductance properties and gating kinetics, and also regulates vesicular traffic to and from synaptic sites, and is thus critical for synaptic function and plasticity. The distribution of functionally different AMPARs varies within and between neuronal circuits, and even within individual neurons. In addition, synapses employ channels with specific subunit stoichiometries, depending on the type of input and the frequency of stimulation. Taken together, it appears that assembly is not simply a stochastic process. Recently, progress has been made in understanding the molecular mechanisms underlying subunit assembly and receptor biogenesis in the ER. These processes ultimately determine the size and shape of the postsynaptic response, and are the subject of this review.

AMPA and NMDA glutamate receptor trafficking: multiple roads for reaching and leaving the synapse

Glutamate receptor trafficking in and out of synapses is one of the core mechanisms for rapid changes in the number of functional receptors during synaptic plasticity. Recent data have shown that the fast gain and loss of receptors from synaptic sites are accounted for by endocytic/exocytic processes and by their lateral diffusion in the plane of the membrane. These events are interdependent and regulated by neuronal activity and interactions with scaffolding proteins. We review here the main cellular steps for AMPA and NMDA receptor synthesis, traffic within intracellular organelles, membrane exocytosis/endocytosis and surface trafficking. We focus on new findings that shed light on the regulation of receptor cycling events and surface trafficking and the way that this might reshape our thinking about the specific regulation of receptor accumulation at synapses.