The segregation and expression of glutamate receptor subunits in cultured hippocampal neurons

High Concentrations of Tranexamic Acid Inhibit Ionotropic Glutamate Receptors

BACKGROUND

The antifibrinolytic drug tranexamic acid is structurally similar to the amino acid glycine and may cause seizures and myoclonus by acting as a competitive antagonist of glycine receptors. Glycine is an obligatory co-agonist of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors. Thus, it is plausible that tranexamic acid inhibits NMDA receptors by acting as a competitive antagonist at the glycine binding site. The aim of this study was to determine whether tranexamic acid inhibits NMDA receptors, as well as α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and kainate subtypes of ionotropic glutamate receptors.

METHODS

Tranexamic acid modulation of NMDA, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, and kainate receptors was studied using whole cell voltage-clamp recordings of current from cultured mouse hippocampal neurons.

RESULTS

Tranexamic acid rapidly and reversibly inhibited NMDA receptors (half maximal inhibitory concentration = 241 ± 45 mM, mean ± SD; 95% CI, 200 to 281; n = 5) and shifted the glycine concentration-response curve for NMDA-evoked current to the right. Tranexamic acid also inhibited α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (half maximal inhibitory concentration = 231 ± 91 mM; 95% CI, 148 to 314; n = 5 to 6) and kainate receptors (half maximal inhibitory concentration = 90 ± 24 mM; 95% CI, 68 to 112; n = 5).

CONCLUSIONS

Tranexamic acid inhibits NMDA receptors likely by reducing the binding of the co-agonist glycine and also inhibits α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and kainate receptors. Receptor blockade occurs at high millimolar concentrations of tranexamic acid, similar to the concentrations that occur after topical application to peripheral tissues. Glutamate receptors in tissues including bone, heart, and nerves play various physiologic roles, and tranexamic acid inhibition of these receptors may contribute to adverse drug effects.

Sequestration of CaMKII in dendritic spines in silico

Calcium calmodulin dependent kinase II (CaMKII) is sequestered in dendritic spines within seconds upon synaptic stimulation. The program Smoldyn was used to develop scenarios of single molecule CaMKII diffusion and binding in virtual dendritic spines. We first validated simulation of diffusion as a function of spine morphology. Additional cellular structures were then incorporated to simulate binding of CaMKII to the post-synaptic density (PSD); binding to cytoskeleton; or their self-aggregation. The distributions of GFP tagged native and mutant constructs in dissociated hippocampal neurons were measured to guide quantitative analysis. Intra-spine viscosity was estimated from fluorescence recovery after photo-bleach (FRAP) of red fluorescent protein. Intra-spine mobility of the GFP-CaMKIIα constructs was measured, with hundred-millisecond or better time resolution, from FRAP of distal spine tips in conjunction with fluorescence loss (FLIP) from proximal regions. Different FRAP \ FLIP profiles were predicted from our Scenarios and provided a means to differentiate binding to the PSDs from self-aggregation. The predictions were validated by experiments. Simulated fits of the Scenarios provided estimates of binding and rate constants. We utilized these values to assess the role of self-aggregation during the initial response of native CaMKII holoenzymes to stimulation. The computations revealed that self-aggregation could provide a concentration-dependent switch to amplify CaMKII sequestration and regulate its activity depending on its occupancy of the actin cytoskeleton.

Organization of NMDA receptors at extrasynaptic locations

NMDA receptors are found in neurons both at synapses and in extrasynaptic locations. Extrasynaptic locations are poorly characterized. Here we used preembedding immunoperoxidase and postembedding immunogold electron microscopy and fluorescence light microscopy to characterize extrasynaptic NMDA receptor locations in dissociated hippocampal neurons in vitro and in the adult and postnatal hippocampus in vivo. We found that extrasynaptic NMDA receptors on neurons in vivo and in vitro were usually concentrated at points of contact with adjacent processes, which were mainly axons, axon terminals, or glia. Many of these contacts were shown to contain adhesion factors such as cadherin and catenin. We also found associations of extrasynaptic NMDA receptors with the membrane associated guanylate kinase (MAGUKs), postsynaptic density (PSD)-95 and SAP102. Developmental differences were also observed. At postnatal day 2 in vivo, extrasynaptic NMDA receptors could often be found at sites with distinct densities whereas dense material was seen only rarely at sites of extrasynaptic NMDA receptors in the adult hippocampus in vivo. This difference probably indicates that many sites of extrasynaptic NMDA receptors in early postnatal ages represent synapse formation or possibly sites for synapse elimination. At all ages, as suggested in both in vivo and in vitro studies, extrasynaptic NMDA receptors on dendrites or the sides of spines may form complexes with other proteins, in many cases, at stable associations with adjacent cell processes. These associations may facilitate unique functions for extrasynaptic NMDA receptors.

The SALM family of adhesion-like molecules forms heteromeric and homomeric complexes

Synaptic adhesion-like molecules (SALMs) are a newly discovered family of adhesion molecules that play roles in synapse formation and neurite outgrowth. The SALM family is comprised of five homologous molecules that are expressed largely in the central nervous system. SALMs 1-3 contain PDZ-binding domains, whereas SALMs 4 and 5 do not. We are interested in characterizing the interactions of the SALMs both among the individual members and with other binding partners. In the present study, we focused on the interactions formed by the five SALM members in rat brain and heterologous cells. In brain, we found that SALMs 1-3 strongly co-immunoprecipitated with each other, whereas SALMs 4 and 5 did not, suggesting that SALMs 4 and 5 mainly form homomeric complexes. In heterologous cells transfected with SALMs, co-immunoprecipitation studies showed that all five SALMs form heteromeric and homomeric complexes. We also determined if SALMs could form trans-cellular associations between transfected heterologous cells. Both SALMs 4 and 5 formed homophilic, but not heterophilic associations, whereas no trans associations were formed by the other SALMs. The ability of SALM4 to form trans interactions is due to its extracellular N terminus because chimeras of SALM4 N terminus and SALM2 C terminus can form trans interactions, whereas chimeras of SALM2 N terminus and SALM4 C terminus cannot. Co-culture experiments using HeLa cells and rat hippocampal neurons expressing the SALMs showed that SALM4 is recruited to points of contact between the cells. In neurons, these points of contact were seen in both axons and dendrites.

Endocytosis and recycling of AMPA receptors lacking GluR2/3

Excitatory synapses in the mammalian brain contain two types of ligand-gated ion channels: AMPA receptors (AMPARs) and NMDA receptors (NMDARs). AMPARs are responsible for generating excitatory synaptic responses, whereas NMDAR activation triggers long-lasting changes in these responses by modulating the trafficking of AMPARs toward and away from synapses. AMPARs are tetramers composed of four subunits (GluR1-GluR4), which current models suggest govern distinct AMPAR trafficking behavior during synaptic plasticity. Here, we address the roles of GluR2 and GluR3 in controlling the recycling- and activity-dependent endocytosis of AMPARs by using cultured hippocampal neurons prepared from knockout (KO) mice lacking these subunits. We find that synapses and dendritic spines form normally in cells lacking GluR2/3 and that upon NMDAR activation, GluR2/3-lacking AMPARs are endocytosed in a manner indistinguishable from GluR2-containing AMPARs in wild-type (WT) neurons. AMPARs lacking GluR2/3 also recycle to the plasma membrane identically to WT AMPARs. However, because of their permeability to calcium, GluR2-lacking but not WT AMPARs exhibited robust internalization throughout the dendritic tree in response to AMPA application. Dendritic endocytosis of AMPARs also was observed in GABAergic neurons, which express a high proportion of GluR2-lacking AMPARs. These results demonstrate that GluR2 and GluR3 are not required for activity-dependent endocytosis of AMPARs and suggest that the most important property of GluR2 in the context of AMPAR trafficking may be its influence on calcium permeability.

Real-time imaging of discrete exocytic events mediating surface delivery of AMPA receptors

We directly resolved discrete exocytic fusion events mediating insertion of AMPA-type glutamate receptors (AMPARs) to the somatodendritic surface of rat hippocampal pyramidal neurons, in slice and dissociated cultures, using protein tagging with a pH-sensitive GFP (green fluorescent protein) variant and rapid (10 frames/s) fluorescence microscopy. AMPAR-containing exocytic events occurred under basal culture conditions in both the cell body and dendrites; potentiating chemical stimuli produced an NMDA receptor-dependent increase in the frequency of individual exocytic events. The number of AMPARs inserted per exocytic event, estimated using single-molecule analysis, was quite uniform but individual events differed significantly in kinetic properties affecting the subsequent surface distribution of receptors. "Transient" events, from which AMPARs dispersed laterally immediately after surface insertion, generated a pronounced but short-lived (dissipating within approximately 1 s) increase in surface AMPAR fluorescence extending locally (2-5 microm) from the site of exocytosis. "Persistent" events, from which inserted AMPARs dispersed slowly (typically over 5-10 s), affected local surface receptor concentration to a much smaller degree. Both modes of exocytic insertion occurred throughout the dendritic shaft, but remarkably, neither mode of insertion was observed directly into synaptic spines. AMPARs entered spines preferentially from transient events occurring in the adjoining dendritic shaft, driven apparently by mass action and short-range lateral diffusion, and locally delivered AMPARs remained mostly in the mobile fraction. These results suggest a highly dynamic mechanism for both constitutive and activity-dependent surface delivery of AMPARs, mediated by kinetically distinct exocytic modes that differ in propensity to drive lateral entry of receptors to nearby synapses.

Ionotropic glutamate receptor activation increases intracellular calcium in prolactin-releasing cells of the adenohypophysis

Endocrine cells of the anterior pituitary are controlled by the central nervous system through hormonal interactions and are not believed to receive direct synaptic connections from the brain. Studies suggest that some pituitary cells may be modulated by the neurotransmitter glutamate. We investigated prolactin (PRL)-releasing cells of the anterior pituitary of a euryhaline fish, the tilapia (Oreochromis mossambicus), for the presence of possible glutamate receptors (GluRs). Fura-2 imaging addressed the ability of glutamate to increase intracellular calcium. We observed a dose-dependent increase in intracellular calcium with transient perfusion (1-2 min) of glutamate (10 nM to 1 mM) in two-thirds of imaged cells. This increase was attenuated by the ionotropic GluR antagonist kynurenic acid (0.5-1.0 mM). The increase was also blocked or attenuated by antagonists of L-type voltage-gated calcium channels. The GluR agonist alpha-amino-3-hydroxy-5-methylisoxazole propionic acid (AMPA; 100 microM) produced intracellular calcium increases that were reversibly blocked by the selective AMPA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). In contrast, the selective agonist N-methyl-D-aspartate (NMDA; 100 microM to 1 mM in magnesium-free solution with 10 microM glycine) had no effect on intracellular calcium. Radioimmunoassays demonstrated that glutamate stimulated PRL release. CNQX but not the NMDA receptor antagonist 2-amino-5-phosphonovaleric acid blocked this release. Antibodies for mammalian AMPA- and NMDA-type GluR produced a similar punctate immunoreactivity in the periphery of PRL cells. However, the NMDA antibody recognized a protein of a different molecular mass in PRL cells compared with brain cells. These results clearly indicate the presence of GluRs on tilapia PRL cells that can stimulate PRL release.

TNFalpha-induced AMPA-receptor trafficking in CNS neurons; relevance to excitotoxicity?

Injury and disease in the CNS increases the amount of tumor necrosis factor alpha (TNFalpha) that neurons are exposed to. This cytokine is central to the inflammatory response that occurs after injury and during prolonged CNS disease, and contributes to the process of neuronal cell death. Previous studies have addressed how long-term apoptotic-signaling pathways that are initiated by TNFalpha might influence these processes, but the effects of inflammation on neurons and synaptic function in the timescale of minutes after exposure are largely unexplored. Our published studies examining the effect of TNFalpha on trafficking of AMPA-type glutamate receptors (AMPARs) in hippocampal neurons demonstrate that glial-derived TNFalpha causes a rapid (<15 minute) increase in the number of neuronal, surface-localized, synaptic AMPARs leading to an increase in synaptic strength. This indicates that TNFalpha-signal transduction acts to facilitate increased surface localization of AMPARs from internal postsynaptic stores. Importantly, an excess of surface localized AMPARs might predispose the neuron to glutamate-mediated excitotoxicity and excessive intracellular calcium concentrations, leading to cell death. This suggests a new mechanism for excitotoxic TNFalpha-induced neuronal death that is initiated minutes after neurons are exposed to the products of the inflammatory response. Here we review the importance of AMPAR trafficking in normal neuronal function and how abnormalities that are mediated by glial-derived cytokines such as TNFalpha can be central in causing neuronal disorders. We have further investigated the effects of TNFalpha on different neuronal cell types and present new data from cortical and hippocampal neurons in culture. Finally, we have expanded our investigation of the temporal profile of the action of this cytokine relevant to neuronal damage. We conclude that TNFalpha-mediated effects on AMPAR trafficking are common in diverse neuronal cell types and very rapid in their onset. The abnormal AMPAR trafficking elicited by TNFalpha might present a novel target to aid the development of new neuroprotective drugs.

Brain-specific potential guanine nucleotide exchange factor for Arf, synArfGEF (Po), is localized to postsynaptic density

We cloned from a rat brain cDNA library a novel cDNA and named it a potential synaptic guanine nucleotide exchange factor (GEF) for Arf (synArfGEF (Po)) (GenBank Accession no. AB057643) based on its domain structure and localization. The cloned gene was 7410 bases long with a 3585-bp coding sequence encoding a protein of 1194 amino acids. The deduced protein contained a coiled-coil structure in the N-terminal portion followed by Sec7 and Plekstrin homology (PH) domains. Thus, the protein was a member of the Sec7 family of proteins, GEFs. Conservation of the ADP-ribosylation factor (Arf)-binding sequence suggested that the protein was a GEF for Arf. The gene was expressed specifically in the brain, where it exhibited region-specific expression. The protein was highly enriched in the postsynaptic density (PSD) fraction prepared from the rat forebrain. Uniquely, the protein interacted with PSD-95, SAP97 and Homer/Vesl 1/PSD-Zip45 via its C-terminal PDZ-binding motif and co-localized with these proteins in cultured cortical neurons. These results supported its localization in the PSD. The postsynaptic localization was also supported by immunohistochemical examination of the rat brain. The mRNA for the synArfGEF was also localized to dendrites, as well as somas, of neuronal cells. Thus, both the mRNA and the protein were localized in the postsynaptic compartments. These results suggest a postsynaptic role of synArfGEF in the brain.

Internalization at glutamatergic synapses during development

Glutamate receptors are internalized from the cell membrane via clathrin-coated pits. However, little is known about where this occurs - whether at or near the synapse or at some distance from it. In this study we used immunogold localization in the rat brain (mainly hippocampus) to show that clathrin-coated pits are found both at the edge of the synaptic active zone and at further postsynaptic distances, including on the sides of the spine; we also localize these pits specifically to glutamatergic synapses. In addition, we show that clathrin-coated pits can internalize both N-methyl-d-aspartate (in vivo) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (in vitro data only) receptors at extrasynaptic sites not associated directly with synapses. Also, caveolin might be prevalent at excitatory synapses, although it is not known whether it is involved in receptor internalization, receptor stabilization, or some other function.

AMPA receptor subunit expression in chick vestibular nucleus neurons

The principal cells of the chick tangential nucleus are vestibular nucleus neurons whose responses on vestibular nerve stimulation are abolished by glutamate receptor antagonists. Using confocal microscopy, we quantified immunolabeling for AMPA receptor subunits GluR1, GluR2, GluR2/3, and GluR4 in principal cells that were identified by the neuronal marker, microtubule-associated protein 2 (MAP2). This work was focused primarily on 9 days after hatching (H9) when the principal cells have acquired some important mature electrophysiologic properties. At H9, the principal cell bodies stained strongly with GluR2/3 and GluR4, whereas GluR1 and GluR2 produced weak signals. Moreover, GluR2/3 and GluR4 receptor subunit clusters in principal cell bodies and dendrites were localized at sites contacted by biocytin-labeled vestibular nerve terminals and synaptotagmin-labeled terminals. Developmental expression of AMPA receptor immunolabeling was studied in the principal cell bodies at embryonic day 16 (E16) and hatching (H1). At E16, labeling for GluR4 was already strong, and continued to increase at H1 and H9. In contrast, GluR2/3 labeling was weak at E16, but increased significantly at H1, and more so by H9. GluR1 and GluR2 were present at low levels at E16 and H1. From E16 to H9, overall AMPA receptor subunit expression increased steadily, with H9 showing the strongest labeling. Ultrastructural observations at E16 and H3 confirmed the presence of immunogold labeling for AMPA receptor subunits at the vestibular nerve and non-vestibular nerve synapses on the principal cell bodies. In summary, these results indicate that GluR3 and GluR4 are the major AMPA receptor subunits involved in excitatory synaptic transmission in principal cells during the perinatal period.

Huntingtin-interacting protein-1-related protein of rat (rHIP1R) is localized in the postsynaptic regions

We cloned a rHIP1R (GenBank Accession No., AB005052) encoding a Sla2/huntingtin-interacting protein (HIP1) family protein from a rat brain cDNA library. Localization of rHIP1R was investigated in the rat brain using an antibody specific to the HIP1R antibody. The rHIP1R protein was enriched in the synaptic plasma membrane fraction along with huntingtin, a synaptic protein and a causal protein for Huntington's disease. The electron microscopic examination revealed that HIP1R was localized at postsynaptic spines. Localization of HIP1R in the small vesicular structures in the spine, possible sites of vesicular transport of synaptic proteins, together with the structure-based analysis, suggested a role of HIP1R for vesicle trafficking through interaction with F-actin and working together with huntingtin and HIP1 at the synaptic sites.

A labile component of AMPA receptor-mediated synaptic transmission is dependent on microtubule motors, actin, and N-ethylmaleimide-sensitive factor

Glutamate receptor channels are synthesized in the cell body, are inserted into intracellular vesicles, and move to dendrites where they become incorporated into synapses. Dendrites contain abundant microtubules that have been implicated in the vesicle-mediated transport of ion channels. We have examined how the inhibition of microtubule motors affects synaptic transmission. Monoclonal antibodies that inactivate the function of dynein or kinesin were introduced into hippocampal CA1 pyramidal cells through a patch pipette. Both antibodies substantially reduced the AMPA receptor-mediated responses within 1 hr but had no effect on the NMDA receptor-mediated response. Heat-inactivated antibody or control antibodies had a much smaller effect. A component of transmission appeared to be resistant even to the combination of these inhibitors, and we therefore explored whether other agents also produce only a partial inhibition of transmission. A similar resistant component was found by using an actin inhibitor (phalloidin) or an inhibitor of NSF (N-ethylmaleimide-sensitive fusion protein)/GluR2 interaction. We then examined whether these effects were independent or occluded each other. We found that a combination of phalloidin and NSF/GluR2 inhibitor reduced the response to approximately 30% of baseline level, an effect only slightly larger than that produced by each agent alone. The addition of microtubule motor inhibitors to this combination produced no further inhibition. We conclude that there are two components of AMPA receptor-mediated transmission; one is a labile pool sensitive to NSF/GluR2 inhibitors, actin inhibitors, and microtubule motor inhibitors. A second, nonlabile pool resembles NMDA receptor channels in being nearly insensitive to any of these agents on the hour time scale of our experiments.

A common exocytotic mechanism mediates axonal and dendritic outgrowth

Outgrowth of the dendrites and the axon is the basis of the establishment of the neuronal shape, and it requires addition of new membrane to both growing processes. It is not yet clear whether one or two exocytotic pathways are responsible for the respective outgrowth of axons and dendrites. We have previously shown that tetanus neurotoxin-insensitive vesicle-associated membrane protein (TI-VAMP) defines a novel network of tubulovesicular structures present both at the leading edge of elongating dendrites and axons of immature hippocampal neurons developing in primary culture and that TI-VAMP is an essential protein for neurite outgrowth in PC12 cells. Here we show that the expression of the N-terminal domain of TI-VAMP inhibits the outgrowth of both dendrites and axons in neurons in primary culture. This effect is more prominent at the earliest stages of the development of neurons in vitro. Expression of the N-terminal domain deleted form of TI-VAMP has the opposite effect. This constitutively active form of TI-VAMP localizes as the endogenous protein, particularly concentrating at the leading edge of growing axons. Our results suggest that a common exocytotic mechanism that relies on TI-VAMP mediates both axonal and dendritic outgrowth in developing neurons.

Glutamate receptor subunit expression in primary neuronal and secondary glial cultures

We report on the expression of ionotropic glutamate receptor subunits in primary neuronal cultures from rat cortex, hippocampus and cerebellum and of metabotropic glutamate (mGlu) receptor subtypes in these neuronal cultures as well as in cortical astroglial cultures. We found that the NMDA receptor (NR) subunits NR1, NR2A and NR2B were expressed in all three cultures. Each of the three cultures showed also expression of the four AMPA receptor subunits. Although RT-PCR detected mRNA of all kainate (KA) subunits in the three cultures, western blot showed only expression of Glu6 and KA2 receptor subunits. The expression analysis of mGlu receptors indicated the presence of all mGlu receptor subtype mRNAs in the three neuronal cultures, except for mGlu2 receptor mRNA, which was not detected in the cortical and cerebellar culture. mGlu1a/alpha, -2/3 and -5 receptor proteins were present in all three cultures, whereas mGlu4a and mGlu8a receptor proteins were not detected. Astroglial cultures were grown in either serum-containing or chemically defined medium. Only mGlu5 receptor protein was found in astroglial cultures grown in serum-containing medium. When astrocytes were cultured in chemically defined medium, mGlu3, -5 and -8 receptor mRNAs were detected, but at the protein level, still only mGlu5 receptor was found.

Interferon-gamma-induced changes in synaptic activity and AMPA receptor clustering in hippocampal cultures

Extended release of interferon-gamma (IFN-gamma) in the nervous system during immunological and infectious conditions may trigger demyelinating disorders and cause disturbances in brain function. The aim of this study was to examine the effects of IFN-gamma on neuronal function in rat hippocampal cell cultures by using whole cell patch clamp analysis together with quantitative immunocytochemistry. Acute application of IFN-gamma to differentiated neurons in culture caused no immediate neurophysiological responses, but recordings after 48 h of incubation displayed an increase in frequency of AMPA receptor (AMPAR)-mediated spontaneous excitatory postsynaptic currents (EPSCs). Quantitative immunocytochemistry for the AMPAR subunit GluR1 showed no alteration in receptor clustering at this time point. However, prolonged treatment with IFN-gamma for 2 weeks resulted in a significant reduction in AMPAR clustering on dendrites but no marked differences in EPSC frequency between treated neurons and controls could be observed. On the other hand, treatment of hippocampal neurons for 4 weeks, instituted at an immature stage (1 day in culture), caused a significant reduction in spontaneous EPSC frequency. These neurons developed with no overt alterations in dendritic arborization or in the appearance of dendritic spines as visualized by alpha-actinin immunocytochemistry. Nonetheless, there was a marked reduction in AMPAR clustering on dendrites. These observations show that a key immunomodulatory molecule, IFN-gamma, can cause long-term modifications of synaptic activity and perturb glutamate receptor clustering.

GluR3 autoantibodies destroy neural cells in a complement-dependent manner modulated by complement regulatory proteins

GluR3 autoantibodies have been implicated in the development of Rasmussen's encephalitis, a rare neurodegenerative disease of humans characterized by epilepsy and degeneration of a single cerebral hemisphere. GluR3 autoantibodies are found in some Rasmussen's encephalitis patients, and GluR3 antibodies raised in rabbits destroy cultured cortical cells in a complement-dependent manner. In this study, the cellular targets of anti-GluR3 antisera-mediated cytotoxicity were examined in mixed primary neuronal-glial cultures of rat cortex. Unexpectedly, astrocytes were the principal target of the cytotoxic effects as assessed by immunohistochemistry and lactate dehydrogenase activity; neurons were destroyed to a lesser extent. Astrocyte vulnerability was rescued by transfection with complement regulatory proteins, and neuronal resistance was defeated by impairing complement regulatory protein function. Astrocyte death may occur in Rasmussen's encephalitis, and destruction of this cell type may play a critical role in the progression of this disorder. The present findings suggest complement regulatory protein expression may in part determine the nature and severity of Rasmussen's encephalitis and other complement-dependent nervous system diseases and thus underscore the need for a systematic investigation of the expression of all known complement regulatory proteins in healthy and diseased nervous system tissues.

Brain-derived neurotrophic factor differentially regulates excitatory and inhibitory synaptic transmission in hippocampal cultures

Brain-derived neurotrophic factor (BDNF) has been postulated to be a key signaling molecule in regulating synaptic strength and overall circuit activity. In this context, we have found that BDNF dramatically increases the frequency of spontaneously initiated action potentials in hippocampal neurons in dissociated culture. Using analysis of unitary synaptic transmission and immunocytochemical methods, we determined that chronic treatment with BDNF potentiates both excitatory and inhibitory transmission, but that it does so via different mechanisms. BDNF strengthens excitation primarily by augmenting the amplitude of AMPA receptor-mediated miniature EPSCs (mEPSCs) but enhances inhibition by increasing the frequency of mIPSC and increasing the size of GABAergic synaptic terminals. In contrast to observations in other systems, BDNF-mediated increases in AMPA-receptor mediated mEPSC amplitudes did not require activity, because blocking action potentials with tetrodotoxin for the entire duration of BDNF treatment had no effect on the magnitude of this enhancement. These forms of synaptic regulations appear to be a selective action of BDNF because intrinsic excitability, synapse number, and neuronal survival are not affected in these cultures. Thus, although BDNF induces a net increase in overall circuit activity, this results from potentiation of both excitatory and inhibitory synaptic drive through distinct and selective physiological mechanisms.

One GABA and two acetylcholine receptors function at the C. elegans neuromuscular junction

We describe an electrophysiological preparation of the neuromuscular junction of the nematode C. elegans, which adds to its considerable genetic and genomic resources. Mutant analysis, pharmacology and patch-clamp recording showed that the body wall muscles of wild-type animals expressed a GABA receptor and two acetylcholine receptors. The muscle GABA response was abolished in animals lacking the GABA receptor gene unc-49. One acetylcholine receptor was activated by the nematocide levamisole. This response was eliminated in mutants lacking either the unc-38 or unc-29 genes, which encode alpha and non-alpha acetylcholine receptor subunits, respectively. The second, previously undescribed, acetylcholine receptor was activated by nicotine, desensitized rapidly and was selectively blocked by dihydro-beta-erythroidine, thus explaining the residual motility of unc-38 and unc-29 mutants. By recording spontaneous endogenous currents and selectively eliminating each of these receptors, we demonstrated that all three receptor types function at neuromuscular synapses.

Calnexin and the immunoglobulin binding protein (BiP) coimmunoprecipitate with AMPA receptors

To identify proteins that interact with alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors, we carried out coimmunoprecipitation analyses on detergent-solubilized rat forebrain membranes. Membranes were solubilized with Triton X-100, and immunoprecipitation was done using subunit-specific antibodies to GluR1, GluR2/3, and GluR4 attached to protein Aagarose. Proteins bound to the antibodies were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by silver staining and western blotting. With solubilization in low ionic strength buffer, several coimmunoprecipitating proteins, with Mr = 17,000-100,000, were identified in silver-stained gels. Western blots were then probed with antibodies to a series of candidate proteins that were chosen based on the molecular masses of the copurifying proteins. Two of these were identified as the molecular chaperones calnexin (90 kDa) and the immunoglobulin binding protein (BiP; 78 kDa). Immunoprecipitation with antibodies to calnexin and BiP demonstrated that glycosylated AMPA receptor subunits were associated. The relationship between AMPA receptors and calnexin and BiP was further studied with immunocytochemistry of the hippocampus. Both calnexin and BiP labeling was present not only in the cell body but also in dendrites of hippocampal pyramidal neurons, where double-label immunofluorescence also showed the presence of AMPA receptor subunits.

Autoantibodies to the glutamate receptor kill neurons via activation of the receptor ion channel

Antibodies to the glutamate/AMPA receptor subunit 3 (GluR3), are found in a human epilepsy, Rasmussen's encephalitis [RE], and were hypothesized as the major cause for the neuronal loss, chronic inflammatory changes and epileptic seizures characteristic of the disease. To establish the pathogenic potential and mechanism of action of such antibodies, we raised murine antibodies against specific peptides of the GluR3 protein and studied their ability to bind, activate, and kill neurons. Mice were immunized with two GluR3 specific peptides: GluR3A (amino acids 245-274) and GluR3B (amino acids 372-395), and with a scrambled GluR3B peptide for control. High levels of antibodies to each of these peptides were obtained, with no cross reactivity between them. Antibodies to the GluR3B peptide were found to bind to cultured neurons, evoke GluR ion channel activity, and kill neurons. In contrast, antibodies against GluR3A peptide bound to neurons but failed to activate the receptor or kill neurons. Anti-scrambled-GluR3B antibodies had no effect. Both the activation of the GluRs and the neuronal death induced by anti-GluR3B antibodies were blocked by CNQX, a specific glutamate/AMPA receptor antagonist; killing was independent of complement. This indicates a mechanism of excitotoxicity-neuronal death due to over-activation of the receptor, a phenomenon known to be caused by excess of glutamate. Purified anti-GluR3B IgGs retained the neuronal killing capacity, and killing was completely and specifically blocked by preincubation with the GluR3B peptide. Excitotoxic neuronal death induced by anti-GluR3B antibodies took place primarily via apoptosis. Taken together, these results show that antibodies to a specific peptide of the GluR can kill neurons by an excitotoxic mechanism, thus mimicking the effects of excess of glutamate. This is the first example that antibodies can lead to neuronal death in a non-classical complement-independent manner, via activation of a membranal neurotransmitter receptor.

Differential distribution of intracellular glutamate receptors in dendrites

Glutamate receptors are synthesized in the cell body and transported in intracellular compartments to the target synapse. The objective of the present study was to analyze the intracellular pool of glutamate receptors and determine whether the intracellular pool was related to the synaptic distribution of the receptors. As a model system, we chose the fusiform cell of the dorsal cochlear nucleus for which we have previously demonstrated that receptors are selectively targeted to synapses on apical and basal dendrites. A combination of retrograde tracing and postembedding immunogold labeling was used to quantify intracellular receptors in segments of apical and basal dendrites. Immunolabeling for GluR4 and mGluR1alpha is present at synapses on basal dendrites but not on apical dendrites, whereas immunolabeling for GluR2/3 is present at both populations of synapses. In the analysis of intracellular pools, we find that GluR2/3 is equally distributed in apical and basal dendrites, whereas GluR4 and mGluR1alpha are more concentrated in basal dendrites than in apical dendrites. These findings indicate that the distribution of intracellular receptors is related to that of synaptic receptors and suggest that a mechanism exists in neurons to target proteins to dendritic domains soon after synthesis. We found no evidence for the existence of a pool of intracellular receptors, which could represent a receptor reserve, near the postsynaptic density. Receptors were often found in clusters associated with tubulovesicular membranes of the endoplasmic reticulum, identified with immunoglobulin binding protein (BIP) or calnexin, suggesting that this organelle is involved in receptor transport in dendrites.

Differential distribution of ionotropic glutamate receptor subunits in the rat olfactory bulb

The subcellular localization of ionotropic glutamate receptor (GluR) subunits was examined with light and electron microscopy in the rat olfactory bulb by using antibodies to alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subunits: GluR1, GluR2/3, and GluR4; and kainate (KA) receptor subunits: GluR5/6/7. Immunoreactivity to GluR1 was heavy in the glomerular layer, moderate in the external plexiform layer, and localized to periglomerular somata and dendrites, short axon somata and dendrites, mitral cell somata, and mitral/tufted dendrites. GluR2/3 immunoreactivity was heavy in the external plexiform and glomerular layers and localized to periglomerular somata and dendrites, mitral cell somata, mitral/tufted dendrites, granule cell somata, and olfactory nerve-associated glia. GluR4 immunoreactivity showed heavy staining in the external plexiform and olfactory nerve layers with localization to mitral cells, mitral/tufted dendritic processes, and olfactory nerve glial processes. GluR5/6/7 immunoreactivity was heavy in the external plexiform layer, moderate in the olfactory nerve and glomerular layers, and localized to granule cells, mitral cells, and mitral/tufted dendritic processes. Ultrastructural immunolabeling for all antibodies examined showed immunoreactivity in the postsynaptic membrane and densities, adjacent dendritic cytoplasm, and somatic cytoplasm. These data demonstrate a highly specific laminar, cellular, and subcellular distribution of ionotropic GluR subunits within the primary afferent and local synaptic circuits of the olfactory bulb. The results are consistent with the notion that the different roles subserved by glutamate in the olfactory bulb are actuated, in part, by a differential distribution of GluR subunits.

Increased densities of AMPA GluR1 subunit proteins and presynaptic mossy fiber sprouting in the fascia dentata of human hippocampal epilepsy

In human hippocampal epilepsy, there is a consistent pathology of cell loss and reactive synaptic reorganization of 'excitatory' mossy fibers (MF) into the inner molecular layer (IML) of the fascia dentata (FD). In this study, neo-Timm's histochemistry of MFs and immunocytochemistry of GluR1 were used to determine, in patients with or without hippocampal sclerosis (HS), if there was a correlation between aberrant supragranular (IML) mossy fiber sprouting and increased densities of AMPA GluR1 subunit proteins in the IML of the FD. Computerized quantified densitometric grey values of Timm and GluR1 densities were corrected for the densities of granule cell losses using cell counts. In the IML of the HS group, despite the losses of granule cells, mossy fiber sprouting was significantly greater (P<0.000001) and GluR1 protein densities were significantly higher (P<0.0005) than those of the non-HS group. Unlike supragranular mossy fiber sprouting, which was limited to the IML, the increased GluR1 stainings were distributed throughout the whole molecular layer. For all cases, MF synaptic reorganization in the supragranular ML was correlated with GluR1 subunit protein densities in the IML (R=0.784, P<0.0093). These data demonstrate that in the human epileptic fascia dentata, there are significantly increased AMPA GluR1 subunit proteins associated with aberrant MF synaptic reorganizations. This suggests that the hyperexcitability of sclerotic hippocampus occurs, at least in part, from the associated changes of both presynaptic mossy fiber glutamatergic neoinnervation and increased GluR1 subunit proteins in the dendritic domains of the FD.

Potentiation of GABAergic synaptic transmission by AMPA receptors in mouse cerebellar stellate cells: changes during development

1. The effects of low concentrations of domoate, an agonist at both alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate and kainate receptors (AMPARs and KARs, respectively), were investigated in stellate cells in slices of mouse cerebellum at two developmental stages (postnatal day (PN) 11-13 and PN21-25). 2. Low concentrations of domoate enhanced the frequency of miniature IPSCs (mIPSCs) recorded in the presence of tetrodotoxin (TTX) at PN11-13 but not at PN21-25. 3. The effects of low concentrations of domoate on synaptic activity were probably mediated by the activation of AMPARs and not KARs, since they were blocked by GYKI 53655 (LY300168), a selective AMPAR antagonist. 4. Domoate increased mIPSC frequency in part by activation of presynaptic voltage-dependent Ca2+ channels since potentiation was reduced by 60 % in the presence of Cd2+. AMPARs in stellate cells were found to be permeable to Ca2+. The residual potentiation in the presence of Cd2+ could thus be due to a direct entry of Ca2+ through AMPAR channels. 5. In the presence of TTX, potentiation of synaptic activity by focal application of domoate was not restricted to the region of the cell body, but was observed within distances of 120 micro(m). These experiments also revealed a strong spatial correlation between the location of the presynaptic effects of domoate and the activation of postsynaptic AMPARs. 6. Our data show a developmentally regulated presynaptic potentiation of synaptic transmission between cerebellar interneurones mediated by AMPARs. We discuss the possibility that the developmental switch could be due to a shift in the localization of AMPARs from the axonal to the somato-dendritic compartment.

Activity differentially regulates the surface expression of synaptic AMPA and NMDA glutamate receptors

Distinct subtypes of glutamate receptors often are colocalized at individual excitatory synapses in the mammalian brain yet appear to subserve distinct functions. To address whether neuronal activity may differentially regulate the surface expression at synapses of two specific subtypes of ionotropic glutamate receptors we epitope-tagged an AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) receptor subunit (GluR1) and an NMDA (N-methyl-D-aspartate) receptor subunit (NR1) on their extracellular termini and expressed these proteins in cultured hippocampal neurons using recombinant adenoviruses. Both receptor subtypes were appropriately targeted to the synaptic plasma membrane as defined by colocalization with the synaptic vesicle protein synaptophysin. Increasing activity in the network of cultured cells by prolonged blockade of inhibitory synapses with the gamma-aminobutyric acid type A receptor antagonist picrotoxin caused an activity-dependent and NMDA receptor-dependent decrease in surface expression of GluR1, but not NR1, at synapses. Consistent with this observation identical treatment of noninfected cultures decreased the contribution of endogenous AMPA receptors to synaptic currents relative to endogenous NMDA receptors. These results indicate that neuronal activity can differentially regulate the surface expression of AMPA and NMDA receptors at individual synapses.

Postsynaptic membrane fusion and long-term potentiation

The possibility that membrane fusion events in the postsynaptic cell may be required for the change in synaptic strength resulting from long-term potentiation (LTP) was examined. Introducing substances into the postsynaptic cell that block membrane fusion at a number of different steps reduced LTP. Introducing SNAP, a protein that promotes membrane fusion, into cells enhanced synaptic transmission, and this enhancement was significantly less when generated in synapses that expressed LTP. Thus, postsynaptic fusion events, which could be involved either in retrograde signaling or in regulating postsynaptic receptor function or both, contribute to LTP.

Glutamate receptor GluR3 antibodies and death of cortical cells

Rasmussen's encephalitis (RE), a childhood disease characterized by epileptic seizures associated with progressive destruction of a single cerebral hemisphere, is an autoimmune disease in which one of the autoantigens is a glutamate receptor, GluR3. The improvement of some affected children following plasma exchange that removed circulating GluR3 antibodies (anti-GluR3) suggested that anti-GluR3 gained access to the central nervous system where it exerted deleterious effects. Here, we demonstrate that a subset of rabbits immunized with a GluR3 fusion protein develops a neurological disorder mimicking RE. Anti-GluR3 IgG isolated from serum of both ill and healthy GluR3-immunized animals promoted death of cultured cortical cells by a complement-dependent mechanism. IgG immunoreactivity decorated neurons and their processes in neocortex and hippocampus in ill but not in healthy rabbits. Moreover, both IgG and complement membrane attack complex (MAC) immunoreactivity was evident on neurons and their processes in the cortex of a subset of patients with RE. We suggest that access of IgG to epitopes in the central nervous system triggers complement-mediated neuronal damage and contributes to the pathogenesis of both this animal model and RE.

Glutamate receptor subunit 2-selective antibody shows a differential distribution of calcium-impermeable AMPA receptors among populations of neurons

Alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors are the major excitatory neurotransmitter receptors of the central nervous system. AMPA receptor complexes that contain the AMPA-type glutamate receptor subunit 2 (GluR2) are responsible for the low calcium permeability typical of most AMPA receptors, and the absence of GluR2 may be a key factor in neurotoxicity. A polyclonal antibody was produced to a 16 amino acid peptide near the C-terminus of GluR2 and was affinity-purified in a three-step procedure. The antibody did not recognize other AMPA subunits in transfected cells with the use of either Western blots or immunocytochemistry. This highly specific GluR2 antibody was used to provide a specific morphological study of GluR2 protein distribution in neurons and synapses of the rat. GluR2 is prevalent in most principal neurons throughout the telencephalon. Neurons with few or no GluR2 subunits include two major types: 1) some populations of interneurons of the telencephalon and of some other areas and 2) many populations of principal neurons in the brainstem and spinal cord. Immunofluorescence showed that GluR2 immunolabeling was widespread, including in dendrites and puncta, in the hippocampus and neocortex. Where they were present, GluR2 subunits colocalized with other AMPA receptor subunits in individual neurons. Electron microscopy of the hippocampus showed GluR2-bearing, calcium-impermeable AMPA receptors postsynaptic to dendrite synapses of forebrain principal neurons. In addition, electron microscopy of the neocortex showed significant staining in postsynaptic profiles. Electron microscopy of the cerebellum revealed the presence of GluR2 subunits in the postsynaptic profiles of many parallel fiber/Purkinje cell spine synapses, whereas electron microscopy of the spinal cord showed substantial staining in the postsynaptic profiles of dorsal horn synapses, but not in ventral horn synapses. Both ultrastructural and immunofluorescence data showed that calcium-impermeable AMPA receptors are widespread in dendrite arborizations.

Global ischemia induces downregulation of Glur2 mRNA and increases AMPA receptor-mediated Ca2+ influx in hippocampal CA1 neurons of gerbil

Transient, severe forebrain or global ischemia leads to delayed cell death of pyramidal neurons in the hippocampal CA1. The precise molecular mechanisms underlying neuronal cell death after global ischemia are as yet unknown. Glutamate receptor-mediated Ca2+ influx is thought to play a critical role in this cell death. In situ hybridization revealed that the expression of mRNA encoding GluR2 (the subunit that limits Ca2+ permeability of AMPA-type glutamate receptors) was markedly and specifically reduced in gerbil CA1 pyramidal neurons after global ischemia but before the onset of neurodegeneration. To determine whether the change in GluR2 expression is functionally significant, we examined the AMPA receptor-mediated rise in cytoplasmic free Ca2+ level ([Ca2+]i) in individual CA1 pyramidal neurons by optical imaging with the Ca2+ indicator dye fura-2 and by intracellular recording. Seventy-two hours after ischemia, CA1 neurons that retained the ability to fire action potentials exhibited a greatly enhanced AMPA-elicited rise in [Ca2+]i. Basal [Ca2+]i in these neurons was unchanged. These findings provide evidence for Ca2+ entry directly through AMPA receptors in pyramidal neurons destined to die. Downregulation of GluR2 gene expression and an increase in Ca2+ influx through AMPA receptors in response to endogenous glutamate are likely to contribute to the delayed neuronal death after global ischemia.

A monoclonal antibody shows discrete cellular and subcellular localizations of mGluR1 alpha metabotropic glutamate receptors

The metabotropic glutamate receptor, mGluR1 alpha, is postsynaptic in excitatory synapses in many populations of neurons and mediates long-term responses. The present study defines the distribution of this receptor using a new, highly specific monoclonal antibody to mGluR1 alpha. Overall distribution of immunostaining was similar to that described previously with polyclonal antibodies, including prominent staining in the olfactory bulb, interneurons of the CA1 hippocampus stratum oriens/alveus, globus pallidus, thalamus, Purkinje cells and in cells of the outer dorsal cochlear nucleus and with little or low staining in principal cells of the cerebral cortex and hippocampus. Interestingly, the well-known association of mGluR1 alpha receptors with neocortical interneurons was even more prevalent than previously noted with polyclonal antibodies. Ultrastructural studies in the hippocampus and cerebellum showed dense immunoperoxidase staining in postsynaptic membranes and densities and in perisynaptic and extrasynaptic membranes, as well as substantial cytoplasmic staining associated with organelles, especially the endoplasmic reticulum.

Glutamate receptors are selectively targeted to postsynaptic sites in neurons

The objective of the present study was to determine if a neuron that expresses multiple glutamate receptors targets the same receptors to all glutamatergic postsynaptic populations, or if the receptors are differentially targeted to specific postsynaptic populations. As a model for this study, we chose the fusiform cell of the dorsal cochlear nucleus. This neuron expresses multiple glutamate receptors and receives two distinct glutamatergic inputs: parallel fibers synapse on apical dendrites, and auditory nerve fibers synapse on basal dendrites. Pre- and postembedding immunocytochemistry were combined with retrograde tracing to identify the receptors expressed on postsynaptic membranes of parallel fiber and auditory nerve synapses. Most receptors were found at both populations of synapses, but the AMPA receptor subunit, GluR4, and the metabotropic receptor, mGluR1 alpha, were found only at the auditory nerve synapse. These results demonstrate that glutamate receptors are targeted to specific postsynaptic populations of glutamatergic synapses.

Differential modulation of AMPA receptor mediated currents by evans blue in postnatal rat hippocampal neurones

1. The modulation of non-N-methyl-D-aspartate (NMDA) receptor-mediated whole cell currents and of glutamatergic synaptic transmission by purified Evans Blue (EB) was investigated in rat cultured postnatal hippocampal neurones by use of patch clamp recordings and a fast drug application system. 2. Three different groups of neurones could be distinguished with respect to the type of modulation obtained with 10 microM EB: EB was either a predominant inhibitor of desensitization (13% of the neurones), a predominant inhibitor of current amplitudes (42%) or a mixed inhibitor of both properties (45%). Both effects were not use-dependent and reached maximal levels after 30 s of pre-equilibration with the diazo dye. 3. Dose-response curves obtained from glutamate activated whole cell currents yielded an IC50 value for EB of 13.3 microM (Hill coefficient: 1.3) for the inhibition of desensitization, and an IC50 value of 10.7 microM (Hill coefficient: 1.2) for the inhibition of current amplitudes. 4. Chicago acid SS (100 microM) which is one of the synthesis precursors of EB had no effect on current amplitudes of glutamate activated whole cell currents but was a weak inhibitor of desensitization in all hippocampal neurones investigated, irrespective of the type of modulation obtained with EB in the same neurone. 5. Oxidatively modified EB (the so-called VIMP (10 microM)) had no effect on the kinetics but was a partial inhibitor of glutamate-activated whole cell currents in all hippocampal neurones investigated. 6. EB (10 microM) inhibited the amplitudes of non-NMDA receptor mediated autaptic currents to the same extent (to 39 +/- 19% of control) as observed for glutamate activated whole cell currents (to 41 +/- 17% and 56 +/- 20%). However, the decay of the autaptic responses remained uninfluenced upon EB application, indicating that either receptor desensitization does not dominate the time course of the synaptic response or that the non-NMDA receptors sensitive to modulation of desensitization by EB are not present in the postsynaptic membrane. 7. In conclusion, EB differentially modulates alpha-amino-3-hydroxy-5-methyl -4-isoxazole propionic acid (AMPA) receptor gating in different subsets of neurones. Upon identification of the cellular determinants for the differential modulation (e.g. AMPA receptor subunit composition) EB could become a useful tool to investigate receptor subtypes during electrophysiological recordings.

Quantitation of AMPA receptor surface expression in cultured hippocampal neurons

Protein and messenger RNA levels of the AMPA-type glutamate receptor subunits 1-3 are high in many brain regions, but it is not known how much of the glutamate receptor protein is expressed on the surface of neurons in the form of functional receptors. To provide insight into this matter, western blot immunoreactivities for glutamate receptors 1 and 2/3, as well as binding of the specific ligand [3H]AMPA, were quantified following three independent treatments modifying surface receptors in intact primary hippocampal cultures: (i) proteolysis of surface receptors by chymotrypsin, (ii) cross-linking of surface receptors with the membrane-impermeant reagent bis(sulfosuccinimidyl)suberate, and (iii) biotinylation of surface receptors with the membrane-impermeant reagent sulfosuccinimidyl-2(biotinamido)ethyl-1,3-dithiopropionate. All three of these methods demonstrated that 60-70% of total glutamate receptor subunit 1 protein and 40-50% of total glutamate receptor 2/3 protein are expressed on the surface of hippocampal neurons. Parallel studies revealed that 52% of total [3H]AMPA binding sites could be precipitated with avidin beads following biotinylation of intact cultures, providing an estimate of [3H]AMPA binding site surface expression in accord with the estimates of the surface expression of glutamate receptor subunits 1-3. Experiments examining the surface expression of 32P-labeled glutamate receptor subunit 1 demonstrated that approximately 65% of the phosphorylated form of the subunit is located in the plasma membrane, an estimate similar to the that derived via western blot for the entire glutamate receptor subunit 1 population in the same samples. Moreover, no significant change in the surface expression profile of the glutamate receptor subunits 1-3 was observed following stimulatory treatments known to increase glutamate receptor phosphorylation. These data indicate that slightly more than half of the AMPA receptors in cultured hippocampal neurons are located in the plasma membrane, and that AMPA receptor surface expression is not rapidly altered by glutamate receptor phosphorylation.

AMPA receptors in cultured vestibular ganglion neurons: detection and activation

The presence and the activity of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) glutamate receptors were investigated in mouse cultured vestibular ganglion neurons using immunocytochemistry and measurement of intracellular calcium concentration ([Ca2+]i) by spectrofluorimetry. Cultures of dissociated vestibular ganglia from 18 gestation day mouse embryos were grown in vitro for 3-4 days. Immunocytochemical labelling of AMPA receptor subunits GluR2/R3 and GluR4 was detected in neuron cell bodies and proximal neurites and more lightly in glial cells. There was no clear selective subcellular localization of the different subunits. For the GluR1 subunit a signal was observed only in some neurons and neurites and was weak. Vestibular ganglion neurons responded to fast application of 1 mM glutamate and 10 mM aspartate through unknown receptors by a transient increase in [Ca2+]i. The mean amplitude of this rapid increase was about nine times the resting level and recovery was complete within 30-45 s after the application. If separated by an interval of at least 10 min, consecutive applications produced similar calcium responses. AMPA (1 mM) application induced the same type of responses. Five minutes prior to the AMPA exposure, the application of a specific AMPA antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX, 1.5 mM), in the external medium inhibited the response to AMPA. Chelation of external calcium by EGTA (1.5 mM) abolished the responses to drug applications, indicating that an influx of external calcium is involved in the [Ca2+]i increase. These observations suggest that heteromeric AMPA receptors are expressed in vestibular ganglion neurons in culture and play a functional role in their glutamate-induced depolarization. Experiments are in progress using specific AMPA and NMDA antagonists to characterize the participation of the two types of ionotropic glutamate receptors in the glutamate/aspartate-induced intracellular calcium response.

Localization of the glutamate receptor subunit GluR1 on the surface of living and within cultured hippocampal neurons

The distribution of the glutamate receptor subunit GluR1 was investigated in cultured hippocampal neurons by confocal microscopy, using polyclonal antibodies directed against either the N- or C-terminal region. On living neurons, GluR1 immunofluorescence was detected with the N-terminal antibody only. GluR1 was localized in a highly punctate manner on the surface of neuronal soma and throughout the dendritic tree. Many GluR1 puncta co-localized with the synaptic marker synaptophysin, although extrasynaptic GluR1 puncta were also observed. A comparison of GluR1 subunit distribution of living neurons labelled with N-terminal antibody with that obtained after the cells had been fixed, permeabilized and subsequently reacted with C-terminal or additional N-terminal antibody showed a number of differences. In permeabilized cells additional, diffuse labelling was observed which was very pronounced in the soma and extended into the proximal dendrites. Furthermore, some spines showed little or no labelling of their membrane surface, but labelled strongly after the cells had been fixed and permeabilized. Such spines may be the postsynaptic components of silent or suboptimal synapses.

Clustering of gephyrin at GABAergic but not glutamatergic synapses in cultured rat hippocampal neurons

The molecular mechanisms underlying the establishment of a postsynaptic receptor mosaic on CNS neurons are poorly understood. One protein thought to be involved is gephyrin, a peripheral membrane protein that binds to the inhibitory glycine receptor and functions in clustering this receptor at synapses in cultured rat spinal cord neurons. We investigated the possible association of gephyrin with synapses in cultured rat hippocampal neurons, where glutamate and GABA but not glycine are the principal transmitters. Gephyrin immunoreactivity was detected in axons as well as dendrites, changing from a predominantly axonal to a more dendritic distribution with time in culture. Gephyrin staining was not distributed uniformly, but always took the form of clusters. Small clusters of gephyrin (0.2 microns 2), present throughout development, were distributed widely and not restricted to synaptic sites. Larger clusters of gephyrin (0.4-10.0 microns 2, sometimes composed of groups of small clusters), which developed in older cells, were localized to a subset of contacts between axons and dendrites. These large clusters were not present at glutamatergic synapses (marked by immunostaining for GluR1), but were closely associated with GABAergic synapses (marked by immunostaining for GABA and glutamic acid decarboxylase). These results, together with previous findings, suggest that gephyrin may function to anchor GABA and glycine receptors, but not glutamate receptors, at postsynaptic sites on central neurons. They also raise the possibility that gephyrin has additional functions, independent of its role at synapses.

Characterization of multiple phosphorylation sites on the AMPA receptor GluR1 subunit

We have characterized the phosphorylation of the glutamate receptor subunit GluR1, using biochemical and electrophysiological techniques. GluR1 is phosphorylated on multiple sites that are all located on the C-terminus of the protein. Cyclic AMP-dependent protein kinase specifically phosphorylates SER-845 of GluR1 in transfected HEK cells and in neurons in culture. Phosphorylation of this residue results in a 40% potentiation of the peak current through GluR1 homomeric channels. In addition, protein kinase C specifically phosphorylates Ser-831 of GluR1 in HEK-293 cells and in cultured neurons. These results are consistent with the recently proposed transmembrane topology models of glutamate receptors, in which the C-terminus is intracellular. In addition, the modulation of GluR1 by PKA phosphorylation of Ser-845 suggests that phosphorylation of this residue may underlie the PKA-induced potentiation of AMPA receptors in neurons.

Differential expression of glutamate receptor subtypes in rat pancreatic islets

Immunocytochemistry was carried out on sections of rat pancreas to localize the expression of glutamate receptor subunits and the major pancreatic peptide hormones. Glutamate receptor expression was concentrated in pancreatic islets, and each islet cell type expressed different neuronal glutamate receptors of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and kainate classes. AMPA receptor subunits were expressed in alpha, beta, and pancreatic polypeptide cells, whereas kainate receptors were found predominantly in alpha and delta cells. Patch clamp electrophysiology was used to measure the functional properties of islet cell glutamate receptors. L-glutamate and other glutamate receptor agonists evoked currents in islet cells that were blocked by the selective AMPA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione and potentiated by cyclothiazide in a manner indistinguishable from that of neuronal AMPA receptors. Activation of islet cell AMPA receptors produced steady-state cation currents that depolarized the cells an average of 20.7 +/- 5.4 mV (n = 6). Currents mediated by functional kainate receptors were also observed in a line of transformed pancreatic alpha cells. Thus, L-glutamate probably regulates islet physiology via actions at both AMPA and kainate receptor classes. The pattern of receptor expression suggests that glutamate receptor activation may have multiple, complex consequences for islet physiology.

GluR1-immunopositive interneurons in rat neocortex

Recent in vitro studies suggest that inhibitory interneurons in cortex may express the GluR1 glutamate receptor subunit in the absence of GluR2, leading to calcium-permeable alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) channels. We performed a study of rat somatic sensory cortex to confirm and extend these observations, using quantitative immunocytochemistry for multiple antigens. A morphologically distinct subpopulation of nonpyramidal neurons in neocortex was intensely immunoreactive for GluR1. Electron microscopic analysis of these cells revealed somatic staining for GluR1, mainly in the rough endoplasmic reticulum. Dendritic staining was concentrated at the synaptic active zone and in the adjacent subsynaptic cytoplasm. Double immunostaining revealed that the large majority of intensely GluR1-positive cells contained gamma-aminobutyric acid or its synthetic enzyme, glutamic acid decarboxylase, but little or no GluR2. Thus, AMPA receptors on a subpopulation of inhibitory interneurons in cortex are likely to be calcium permeable. This calcium permeability is likely to influence functional properties of these neurons; it may underlie the high levels of calcium-binding proteins they contain; and may render them liable to excitotoxic injury

Expression and subcellular distribution of glutamate receptor subunits 2/3 in the developing cerebellar cortex

The expression and subcellular location of glutamate receptor subunits 2&3 was investigated in the developing postnatal cerebellum. Immunoblotting revealed that glutamate receptor subunits 2/3 is expressed in an identical pattern of immunoreactive bands of approximately 108 kDa from postnatal day zero to adult animals. Light microscopy showed that within the cerebellar cortex, GluR 2/3 immunoreactivity was essentially confined to Purkinje neurons. Strong immunostaining could be observed at postnatal days 1-3 within Purkinje cell bodies and primary dendrites. With ongoing development, the cell body and an increasingly elaborate dendritic tree was outlined by immunoreaction product. In adult animals, staining of Purkinje cell dendrites was patchy, and staining intensity of the cell body, in particular, was greatly reduced. Ultrastructural analysis revealed that during early postnatal development, immunoreaction product was localized to the cell membrane, but was not confined to postsynaptic densities. From the second postnatal week, glutamate receptor subunits 2/3 immunoreactivity was largely restricted to postsynaptic densities. These observations reveal a developmentally regulated refinement of the subcellular distribution of defining subunits of the AMPA-type glutamate receptor. The presence of membrane bond receptors prior to the formation of synapses also provides a rationale for the known transmitter-mediated modulation of Purkinje cell dendritogenesis.

High-resolution immunogold localization of AMPA type glutamate receptor subunits at synaptic and non-synaptic sites in rat hippocampus

The cellular and subcellular localization of the GluRA, GluRB/C and GluRD subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) type glutamate receptor was determined in the rat hippocampus using polyclonal antipeptide antibodies in immunoperoxidase and immunogold procedures. For the localization of the GluRD subunit a new polyclonal antiserum was developed using the C-terminal sequence of the protein (residues 869-881), conjugated to carrier protein and absorbed to colloidal gold for immunization. The purified antibodies immunoprecipitated about 25% of 3[H]AMPA binding activity from the hippocampus, cerebellum or whole brain, but very little from neocortex. These antibodies did not precipitate a significant amount of 3[H]kainate binding activity. The antibodies also recognize the GluRD subunit, but not the other AMPA receptor subunits, when expressed in transfected COS-7 cells and only when permeabilized with detergent, indicating an intracellular epitope. All subunits were enriched in the neuropil of the dendritic layers of the hippocampus and in the molecular layer of the dentate gyrus. The cellular distribution of the GluRD subunit was studied more extensively. The strata radiatum, oriens and the dentate molecular layer were more strongly immunoreactive than the stratum lacunosum moleculare, the stratum lucidum and the hilus. However, in the stratum lucidum of the CA3 area and in the hilus the weakly reacting dendrites were surrounded by immunopositive rosettes, shown in subsequent electron microscopic studies to correspond to complex dendritic spines. In the stratum radiatum, the weakly reacting apical dendrites contrasted with the surrounding intensely stained neuropil. The cell bodies of pyramidal and granule cells were moderately reactive. Some non-principal cells and their dendrites in the pyramidal cell layer and in the alveus also reacted very strongly for the GluRD subunit. At the subcellular level, silver intensified immunogold particles for the GluRA, GluRB/C and GluRD subunits were present at type 1 synaptic membrane specializations on dendritic spines of pyramidal cells throughout all layers of the CA1 and CA3 areas. The most densely labelled synapses tended to be on the largest spines and many smaller spines remained unlabelled. Immunoparticle density at type 1 synapses on dendritic shafts of some non-principal cells was consistently higher than at labelled synapses of dendritic spines of pyramidal cells. Synapses established between dendritic spines and mossy fibre terminals, were immunoreactive for all studied subunits in stratum lucidum of the CA3 area. The postembedding immunogold method revealed that the AMPA type receptors are concentrated within the main body of the anatomically defined type 1 (asymmetrical) synaptic junction. Often only a part of the membrane specialization showed clustered immunoparticles. There was a sharp decrease in immunoreactive receptor density at the edge of the synaptic specialization. Immunolabelling was consistently demonstrated at extrasynaptic sites on dendrites, dendritic spines and somata. The results demonstrate that the GluRA, B/C and D subunits of the AMPA type glutamate receptor are present in many of the glutamatergic synapses formed by the entorhinal, CA3 pyramidal and mossy fibre terminals. Some interneurons have a higher density of AMPA type receptors in their asymmetrical afferent synapses than pyramidal cells. This may contribute to a lower activation threshold of interneurons as compared to principal cells by the same afferents in the hippocampal formation.

Synaptic expression of the high-affinity kainate receptor subunit KA2 in hippocampal cultures

Non-N-methyl-D-aspartate glutamate receptors are responsible for fast excitatory neurotransmission in the mammalian CNS. These receptors are rapidly activated and desensitized in the presence of glutamate, and are often further subdivided into alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid and kainate receptors based on their selective agonists. Non-NMDA glutamate receptors are composed of multiple subunits which recently have been cloned, and studies on the recombinant glutamate receptors have helped clarify the distinctions between AMPA and kainate-preferring glutamate receptors. Although the subunits which make up both AMPA and kainate receptors have a widespread distribution, most currents recorded in vivo are characteristic of recombinant AMPA receptors. To help clarify the functional role of high-affinity kainate receptors, we have characterized the expression of a high-affinity kainate receptor subunit, KA2, in cultured hippocampal neurons. Using immunocytochemistry, we found that KA2 was expressed in hippocampal neurons at all times during the development of the cells in culture, and the subunit was enriched in dendritic spines after about 14 days. The subcellular distribution of KA2 paralleled that of the AMPA receptor subunit GluR1, with the AMPA and kainate subunits being colocalized at all times in culture. The enriched KA2 immunoreactivity co-localized with the synaptic vesicle protein synaptophysin at the resolution of light microscopy, indicating synaptic localization of KA2. Although the kainate subunit KA2 co-localized with the AMPA subunit GluR1, co-immunoprecipitation experiments demonstrated a direct interaction between the AMPA receptor subunits GluR1 and GluR2/3, but not between GluR1 and the kainate subunits GluR6/7 or KA2. We therefore, conclude that both AMPA and kainate receptor subunits are enriched in the same dendritic spines, yet do not combine to form receptor complexes.

Regulated subcellular distribution of the NR1 subunit of the NMDA receptor

NMDA (N-methyl-D-aspartate) receptors are selectively localized at the postsynaptic membrane of excitatory synapses in the mammalian brain. The molecular mechanisms underlying this localization were investigated by expressing the NR1 subunit of the NMDA receptor in fibroblasts. NR1 splice variants containing the first COOH-terminal exon cassette (NR1A and NR1D) were located in discrete, receptor-rich domains associated with the plasma membrane. NR1 splice variants lacking this exon cassette (NR1C and NR1E) were distributed throughout the cell, with large amounts of NR1 protein present in the cell interior. Insertion of this exon cassette into the COOH-terminus of the GluR1 AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate) receptor was sufficient to cause GluR1 to be localized to discrete, receptor-rich domains. Furthermore, protein kinase C phosphorylation of specific serines within this exon disrupted the receptor-rich domains. These results demonstrate that amino acid sequences contained within the NR1 molecule serve to localize this receptor subunit to discrete membrane domains in a manner that is regulated by alternative splicing and protein phosphorylation.

Differential distribution of AMPA-selective glutamate receptor subunits in the parabrachial nucleus of the rat

The parabrachial complex is made up of at least 11 cytoarchitectonically distinct subnuclei which differ in their anatomical connections and neurotransmitter content, as well as the functions they subserve. To determine whether parabrachial subnuclei also express different types of glutamate receptors, we undertook a light microscopic examination of the regional distribution of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor subunits within the parabrachial complex using antibodies directed against synthetic peptides corresponding to the C-terminal parts of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor subunits. Antibodies that recognize GluR1 subunits stained cells mainly in the central lateral parabrachial subnucleus, whereas GluR4 antibodies selectively stained cells in the internal lateral subnucleus. In contrast, antibodies directed against the GLuR2/3 subunits stained neurons in every parabrachial subnucleus, although the most dense labelling was seen in the external lateral cell group. These differences in expression of alpha-amino-3-hydroxy-5-methyl-4- isoxazolepropionate receptor subtypes may eventually allow selective activation or inhibition of specific subsets of neurons in the parabrachial complex.