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

The metabotropic glutamate receptor subtype mGluR 2/3 is located at extrasynaptic loci in rat spinal dorsal horn synapses

The position of neurotransmitter receptors relative to active neurotransmitter release sites may be a major factor influencing neuronal responses. The location of the metabotropic glutamate receptor subtype mGluR2/3 was investigated in synaptic structures in the rat superficial spinal dorsal horn laminae by using a pre-embedding immunogold technique. Immunostaining for mGluR2/3 occurred in laminae I through III. Gold particles were encountered both in the cytosol and along the plasma membrane. Distinctive plasmalemmal immunodeposits were detected in vesicle-containing profiles, where they were located to membrane compartments distant from active release sites rather than in the close vicinity of synaptic specialisations. No distinct immunolabelling was observed in profiles meeting characteristics of primary afferent terminals. The extrasynaptic occurrence of mGluR2/3 suggests a presynaptic heteroreceptor role for these receptor subtypes in the spinal dorsal horn.

AMPA and NMDA receptors: similarities and differences in their synaptic distribution

Recent technical developments, including antigen-retrieval and electron microscopic immunogold methods, are making it possible to determine some of the basic principles governing the subcellular distribution of ionotropic glutamate receptors. Distinct AMPA and NMDA receptor subtypes are selectively targeted to functionally different synapses of a single cell, resulting in an input-selective fine-tuning and regulation of the postsynaptic responses. The amount, density and variability of AMPA receptors at a given glutamatergic synapse is governed by both pre- and postsynaptic factors, resulting in functionally distinct glutamatergic connections that display characteristic patterns of receptor expression.

Calpain-mediated truncation of rat brain AMPA receptors increases their Triton X-100 solubility

Previous studies have indicated that calpain activation results in the truncation of the C-terminal domains of AMPA and NMDA receptor subunits. The present study determined the distribution of the truncated species of the subunits between Triton-soluble and -insoluble fractions. Western blots were performed with various antibodies to quantify the amounts of the various species of GluR1, GluR2, GluR3 and NR2B subunits. The results indicate that calpain activation decreased the amount of all the intact subunits in Triton-insoluble fractions. Calpain-generated truncated forms of GluR1 and GluR2, but not NR2B, were absent in these fractions, and were recovered in Triton-soluble fractions. These findings suggest that calpain-mediated truncation of AMPA but not NMDA receptor C-terminal domains results in modifications of the interactions between the receptors and postsynaptic densities, and that this mechanism could be involved in activity-dependent changes in the subcellular distribution of AMPA receptors.

Organization of ionotropic glutamate receptors at dendrodendritic synapses in the rat olfactory bulb

Dendrodendritic synapses between mitral (or tufted) and granule cells of the olfactory bulb play a major role in the processes of odor discrimination and olfactory learning. Release of glutamate at these synapses activates postsynaptic receptors on the dendritic spines of granule cells, as well as presynaptic NMDA receptors in the mitral cell membrane. However, immunocytochemical studies have failed to demonstrate the presence of ionotropic glutamate receptors in granule cell dendrites. By using a postembedding immunogold procedure, we describe here the precise organization of neurotransmitter receptors at dendrodendritic synapses. We show that there is a selective localization of glutamate and GABA receptors at asymmetric and symmetric synaptic junctions, respectively. In addition, we demonstrate that NMDA and AMPA receptors are clustered at postsynaptic specializations on granule cell spines and that they are extensively colocalized. Conversely, glutamate receptors do not appear to be concentrated in clusters on mitral cell dendrites, suggesting that the presynaptic effects of glutamate are mediated by a small complement of extrasynaptic receptors. By analyzing the subsynaptic distribution of the NR1 and GluR2/3 subunits, we show that they are distributed along the entire extent of the postsynaptic specialization, indicating that both NMDA and AMPA receptors are available for dendrodendritic signaling between mitral and granule cells. These results indicate that the principles recently found to underlie the organization of glutamate receptors at axospinous synapses also apply to dendrodendritic synapses.

Use of proteoliposomes to generate phage antibodies against native AMPA receptor

To isolate antibodies against ionotropic glutamate receptors (GluRs), we prepared a phage antibody library from mice immunized with proteoliposomes containing purified alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), a selective GluRD receptor. Specific binders were selected by repeated rounds of affinity panning against immobilized GluRD liposomes. Using this approach, we obtained a panel of high-affinity antibody fragments that immunoprecipitated both recombinant and native GluRD receptors, but not GluR6, a kainate receptor subunit with a 40% sequence similarity. The antibody fragments showed subunit selectivity, some being strictly specific for GluRD, whereas others also recognized the GluRB and GluRC but not GluRA subunits. Further experiments indicated that the epitopes recognized were conformational in nature and reside in the N-terminal extracellular 400-residue X domain of GluRD. Our results suggest that proteoliposomes, in combination with phage display technology, provide an effective tool for the generation of high-affinity conformation-sensitive monoclonal antibodies against predetermined membrane proteins.

Digital analysis of light microscope immunofluorescence: high-resolution co-localization of synaptic proteins in cultured neurons

A protocol is presented for determining the subcellular distribution of fluorescently labeled proteins in neurons using deconvolved images gathered with a wide-field microscope. The protocol includes optimal settings for the numerical algorithm used to deconvolve the images and an objective method for thresholding the deconvolved images to retain only high-intensity, specific labeling. The effectiveness of the protocol is demonstrated using a fluorescent antibody stain directed towards the alpha1 subunit of the GABA(A) receptor in cultured neurons. We also show, using an antibody against the presynaptic vesicular protein synaptophysin, that the technique can detect presumptive regions of synaptic contact between neurons. Double-labeling with the anti-alpha1 and anti-synaptophysin antibodies in a cultured neuron reveals regions of both synaptic and non-synaptic alpha1 labeling. Thus, numerical postprocessing of wide-field images can be used to efficiently locate receptor proteins in neurons in relation to functionally important structures. This confocal-like functionality is attained without the excessive bleaching and phototoxicity associated with the intense laser excitation light used in confocal techniques.

Improved post-embedding immunocytochemistry of myelinated nervous tissue for electron microscopy

The particularly high lipid content of normal mature adult myelin sheaths, together with the light fixation protocols usually necessary to retain antigenicity, combine to make white matter nervous tissue an especially problematical subject for post-embedding immuno-electron microscopy using modern acrylic resins. Fixation and infiltration modifications to standard processing schedules for Lowicryl were found to greatly improve the embedding and therefore the resulting morphology. This in turn improved the signal to noise ratio by reducing the high non-specific backgrounds usually found in poorly infiltrated areas. Using Lowicryl HM20, we have been able to obtain satisfactory immunostaining for myelin basic protein with good retention of structural integrity in the myelin of both normal and lesioned adult cortico spinal tract.

Serotoninergic neurons and serotonin receptors: gains from cytochemical approaches

Serotonergic systems, their phylogeny and ontogeny have been thoroughly described up to the ultrastructural level, thanks to the multiplicity of methodological approaches. They have often been referred to as a 'Rosetta stone', as several features first described for serotonin neurons or paraneurons have been then extended to other neurotransmitter systems: coexistence with neuropeptides or even a canonical neurotransmitter (GABA), volume transmission, regrowth after lesioning, and characterization of multiple receptor subtypes. This review deals with the contributions of neuroanatomical approaches for studying serotoninergic systems, and focuses on recent advances concerning the topological relationships between serotonergic innervation, receptors and target cells. This aspect is particularly important with regard to the possibility for serotonin to act through classical synaptic transmission and/or non-junctional transmission. Serotonin then can selectively regulate different neuronal systems through the activation of distinct receptor subtypes, which in turn can be linked to different transduction pathways. Neurocytochemical approaches constitute unique tools to analyse both anatomical and functional characteristics of complex neuronal systems.

Role of AMPA receptor cycling in synaptic transmission and plasticity

Compounds known to disrupt exocytosis or endocytosis were introduced into CA1 pyramidal cells while monitoring excitatory postsynaptic currents (EPSCs). Disrupting exocytosis or the interaction of GluR2 with NSF caused a gradual reduction in the AMPAR EPSC, while inhibition of endocytosis caused a gradual increase in the AMPAR EPSC. These manipulations had no effect on the NMDAR EPSC but prevented the subsequent induction of LTD. These results suggest that AMPARs, but not NMDARs, cycle into and out of the synaptic membrane at a rapid rate and that certain forms of synaptic plasticity may utilize this dynamic process.

Clustering of the group III metabotropic glutamate receptor 4a at parallel fiber synaptic terminals in the rat cerebellar molecular layer

We report in this study with a pre-embedding immunogold method, the clustering of the group III metabotropic glutamate receptor 4a (mGluR4a) along the presynaptic membrane of parallel fiber synaptic terminals in the cerebellar molecular layer. The mGluR4a clusters were homogeneously distributed and interspaced by about 60 nm. These results suggest a particular arrangement of mGluR4a which might help to a rapid and effective activation of this receptor by glutamate.

Distribution of AMPA receptor subunits GluR1-4 in the dorsal vagal complex of the rat: a light and electron microscope immunocytochemical study

The dorsal vagal complex, localized in the dorsomedial medulla, includes the nucleus tractus solitarii (NTS), the dorsal motor nucleus of the vagus nerve (DMN) and the area postrema (AP). The distribution of AMPA-preferring glutamate receptors (AMPA receptors) within this region was investigated using immunohistochemistry and antibodies recognizing either one (GluR1 or GluR4) or two (GluR2 and GluR3) AMPA receptors subunits. The distribution of GluR1 immunoreactivity showed high contrast of staining between strongly and lightly labeled areas. Labeling was intense in the AP and weak in the NTS, except for its medial and dorsalmost parts which exhibited moderate staining. Almost no GluR1 immunoreactivity was found in the DMN. GluR2/3 immunolabeling was present in the entire dorsal vagal complex. This labeling was strong in the AP, the DMN and the medial half of the NTS and moderate in the lateral half of the NTS, except for the interstitial subdivision which exhibited intense staining. Labeling induced by the GluR4 antibody was very weak throughout the dorsal vagal complex. Ultrastructural examination showed that GluR1 and GluR2/3 immunoreactivity was localized in neuronal cell bodies and dendrites. No labeled axon terminal or glial cell body was found. Immunoperoxidase staining in labeled cell bodies and dendrites was associated with intracellular organelles (microtubules, mitochondria, cisternae of the endoplasmic reticulum,.) and/or parts of the plasma membrane. Plasma membrane labeling was often associated with asymmetrical synaptic differentiations. No labeled symmetrical synapse was found using either GluR1 or GluR2/3 antibody. The present results show that AMPA receptors have a widespread distribution in neuronal perikarya and dendrites of the rat dorsal vagal complex. They suggest differences in subunit composition between AMPA receptors localized in the NTS, the DMN and the AP. Ultrastructural data are consistent with the fact that AMPA receptors associated with the plasma membrane are mostly synaptic receptors. However, they also suggest the existence of a large intracellular pool of receptor subunits in neuronal soma and dendrites.

Precisely localized LTD in the neocortex revealed by infrared-guided laser stimulation

In a direct approach to elucidate the origin of long-term depression (LTD), glutamate was applied onto dendrites of neurons in rat neocortical slices. An infrared-guided laser stimulation was used to release glutamate from caged glutamate in the focal spot of an ultraviolet laser. A burst of light flashes caused an LTD-like depression of glutamate receptor responses, which was highly confined to the region of "tetanic" stimulation (<10 micrometers). A similar depression of glutamate receptor responses was observed during LTD of synaptic transmission. A spatially highly specific postsynaptic mechanism can account for the LTD induced by glutamate release.

Synaptic control of glycine and GABA(A) receptors and gephyrin expression in cultured motoneurons

We have evaluated the influence of the secretory phenotype of presynaptic boutons on the accumulation of postsynaptic glycine receptors (GlyRs), type A GABA receptors (GABA(A)Rs), and gephyrin clusters. The cellular distribution of these components was analyzed on motoneurons cultured either alone or with glycinergic and/or GABAergic neurons. In motoneurons cultured alone, we observed gephyrin clusters at nonsynaptic sites and in front of cholinergic boutons, whereas glycine and GABA(A) receptors formed nonsynaptic clusters. These receptors are functionally and pharmacologically similar to those found in cultures of all spinal neurons. Motoneurons receiving GABAergic innervation from dorsal root ganglia neurons displayed postsynaptic clusters of gephyrin and GABA(A)Rbeta but not of GlyRalpha/beta subunits. In motoneurons receiving glycinergic and GABAergic innervation from spinal interneurons, gephyrin, GlyRalpha/beta, and GABA(A)Rbeta formed mosaics at synaptic loci. These results indicate that (1) the transmitter phenotype of the presynaptic element determines the postsynaptic accumulation of specific receptors but not of gephyrin and (2) the postsynaptic accumulation of gephyrin alone cannot account for the formation of GlyR-rich microdomains.

Immunogold localization of AMPA and NMDA receptors in somatic sensory cortex of albino rat

We performed an electron microscopic study of S-1 cortex by using postembedding immunogold histochemistry to examine the subcellular distribution of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors (assessed with an antibody recognizing the glutamate receptor 2 and 3 [GluR2 and GluR3] subunits) and to compare this distribution with that of N-methyl-D-aspartate (NMDA) receptors (assessed with an antibody for the NR1 subunit). Both receptors were concentrated at active zones of asymmetric synapses, often directly apposed to presynaptic dense bodies. GluR2/3 showed a bias for long active zones, whereas short active zones expressed GluR2/3 at substantially lower levels; in contrast, labeling for NR1 was independent of synaptic size. Particle counts suggested that synaptic labeling was Poisson distributed and implied that the majority of synapses express both receptors. Quantitative analysis indicates that approximately one-half of synapses express high levels of GluR2/3 and that the remainder express GluR2/3 at a much lower level. Approximately three-fourths of synapses express NR1 at a uniform level; the remainder, which may lack NR1 completely, include synapses with especially large active zones. The present results suggest that the smallest active zones may play a special role in synaptic plasticity.

Mu-opioid and NMDA-type glutamate receptors are often colocalized in spiny neurons within patches of the caudate-putamen nucleus

The patch compartments of the caudate-putamen nucleus (CPN) are enriched in mu-opioid receptors (MORs) and have been recently implicated in reward-related behaviors. This function has been established more clearly in the nucleus accumbens, where physiological and anatomical studies show reward-associated interactions involving MORs and N-methyl-D-aspartate-type glutamate receptors (NMDARs). We examined the immunolabeling for MOR and NMDAR subunit NR1 in patches of the rat CPN to determine the potential relevance of dual activation of the respective receptors. Electron microscopy showed the presence of MOR and/or NR1 immunoreactivity (IR) in many perikarya, dendrites, and spines and in morphologically heterogeneous axon terminals. In each 1,000-microm(2) area, the dually labeled dendrites and spines constituted 65% (37/57) and 37% (9/25) of the total NR1-labeled and 34% (37/109) and 13% (9/71) of the total MOR-labeled dendritic profiles. Dually labeled spines received asymmetric excitatory-type synapses from terminals, which were generally unlabeled, but also occasionally contained MOR and/or NR1. The asymmetric synapses comprised the majority (81%) of the total 263 synaptic contacts between MOR- and NR1-labeled neuronal profiles. In dendrites and spines, MOR-IR was localized mainly along nonsynaptic plasma membranes, whereas NR1-IR was more often associated with asymmetric postsynaptic densities and cytoplasmic organelles. In contrast to dendrites, 6% (1.3/22) of NR1-IR and 4% (1.3/33) of MOR-IR axon terminals were dually labeled in each 1,000-microm(2) area. Most singly or dually labeled terminals formed asymmetric synapses with MOR- or NR1-labeled spines. Our results suggest that opioids acting through MOR and excitatory neurotransmitters through NMDAR dually regulate the output of single spiny neurons and some of their excitatory afferents in the CPN.

Association of AMPA receptors with a subset of glutamate receptor-interacting protein in vivo

The NMDA and AMPA classes of ionotropic glutamate receptors are concentrated at postsynaptic sites in excitatory synapses. NMDA receptors interact via their NR2 subunits with PSD-95/SAP90 family proteins, whereas AMPA receptors bind via their GluR2/3 subunits to glutamate receptor-interacting protein (GRIP), AMPA receptor-binding protein (ABP), and protein interacting with C kinase 1 (PICK1). We report here a novel cDNA (termed ABP-L/GRIP2) that is virtually identical to ABP except for additional GRIP-like sequences at the N-terminal and C-terminal ends. Like GRIP (which we now term GRIP1), ABP-L/GRIP2 contains a seventh PDZ domain at its C terminus. Using antibodies that recognize both these proteins, we examined the subcellular localization of GRIP1 and ABP-L/GRIP2 (collectively termed GRIP) and their biochemical association with AMPA receptors. Immunogold electron microscopy revealed the presence of GRIP at excitatory synapses and also at nonsynaptic membranes and within intracellular compartments. The association of native GRIP and AMPA receptors was confirmed biochemically by coimmunoprecipitation from rat brain extracts. A majority of detergent-extractable GluR2/3 was complexed with GRIP in the brain. However, only approximately half of GRIP was associated with AMPA receptors. Unexpectedly, immunocytochemistry of cultured hippocampal neurons and rat brain at the light microscopic level showed enrichment of GRIP in GABAergic neurons and in GABAergic nerve terminals. Thus GRIP is associated with inhibitory as well as excitatory synapses. Collectively, these findings support a role for GRIP in the synaptic anchoring of AMPA receptors but also suggest that GRIP has additional functions unrelated to the binding of AMPA receptors.

Cochlear nerve projections to the small cell shell of the cochlear nucleus: the neuroanatomy of extremely thin sensory axons

Labeling cochlear nerve fibers in the inner ear of chinchillas with biotinylated dextran polyamine was used to trace the thin fibers (Type II), which likely innervate outer hair cells. These axons, 0. 1-0.5 microm in diameter, were distinguished from the thicker Type I, fibers innervating inner hair cells, and traced to small-cell clusters in the cochlear nucleus. This study provided two major new insights into the outer hair cell connections in the cochlear nucleus and the potential significance of very thin axons and synaptic nests, which are widespread in the CNS. 1) EM serial reconstructions of labeled and unlabeled material revealed that Type II axons rarely formed synapses with conventional features (vesicles gathered at junctions). Rather, their endings contained arrays of endoplasmic reticulum and small spherical vesicles without junctions. 2) Type II axons projected predominantly to synaptic nests, where they contacted other endings and dendrites of local interneurons (small stellate and mitt cells, but not granule cells). Synaptic nests lacked intrinsic glia and, presumably, their high-affinity amino acid transporters. As functional units, nests and their Type II inputs from outer hair cells may contribute to an analog processing mode, which is slower, more diffuse, longer-lasting, and potentially more plastic than the digital processors addressed by inner hair cells.

Different modes of expression of AMPA and NMDA receptors in hippocampal synapses

Postembedding immunogold labeling was used to determine the relationship between AMPA and NMDA receptor density and size of Schaffer collateral-commissural (SCC) synapses of the adult rat. All SCC synapses expressed NMDA receptors. AMPA and NMDA receptors were colocalized in at least 75% of SCC synapses; the ratio of AMPA to NMDA receptors was a linear function of postsynaptic density (PSD) diameter, with AMPA receptor number dropping to zero at a PSD diameter of approximately 180 nm. These findings indicate that 'silent' SCC synapses are smaller than the majority of SCC synapses at which AMPA and NMDA receptors are colocalized. Thus synapse size may determine important properties of SCC synapses.

Pathophysiological implications of the structural organization of the excitatory synapse

The glutamatergic synapse is the key structure in the development of activity-dependent synaptic plasticity in the central nervous system. The analysis of the complex biochemical mechanisms at the basis of the long-term changes in synaptic efficacy have received a tremendous impulse by the observation that the post-synaptic constituents of the synapse can be separated and purified through a simple procedure involving detergent treatment of synaptosomes and differential centrifugation. In this fraction, called post-synaptic density (PSD), the functional interactions of its constituents are preserved. The various subunits of ionotropic glutamate receptors are held in register with the presynaptic active zone through their interaction with linker proteins. N-methyl-D-aspartate (NMDA) subunits NR2A and NR2B, bind to the PSD protein called PSD-95, which in turn binds neuroligins, providing a handle for interacting with neurexin, located in the plasma membrane at the presynaptic active zone. Additional clustering of NMDA receptors is provided through the binding of NRI subunits to the cytoskeletal protein alpha-actinin-2. AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) and kainate receptors are other important constituents of PSDs and bind to different anchoring proteins. Phosphorylation processes have long been known to modulate NMDA receptor functional activity: the finding that several protein kinases, particularly Ca2+/Calmodulin-dependent protein kinase II and protein tyrosine kinases of the src family, are major constituents of PSDs has allowed to demonstrate that these enzymes are localized in a strategic position of the glutamatergic synapse, so that their activation provides a means for NMDA receptor function regulation upon its activation. The relevance of these mechanisms has been demonstrated in experimental models of pathologies involving deficits in synaptic plasticity, such as in streptozotocin-induced diabetes and in an animal model of prenatal induced ablation of hippocampal neurons. Both animal models display disturbances in long-term potentiation and cognitive deficits, thus providing in vivo models to study pathology related changes in both the structure and the function of the excitatory synapse.

Importance of AMPA receptors for hippocampal synaptic plasticity but not for spatial learning

Gene-targeted mice lacking the L-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor subunit GluR-A exhibited normal development, life expectancy, and fine structure of neuronal dendrites and synapses. In hippocampal CA1 pyramidal neurons, GluR-A-/- mice showed a reduction in functional AMPA receptors, with the remaining receptors preferentially targeted to synapses. Thus, the CA1 soma-patch currents were strongly reduced, but glutamatergic synaptic currents were unaltered; and evoked dendritic and spinous Ca2+ transients, Ca2+-dependent gene activation, and hippocampal field potentials were as in the wild type. In adult GluR-A-/- mice, associative long-term potentiation (LTP) was absent in CA3 to CA1 synapses, but spatial learning in the water maze was not impaired. The results suggest that CA1 hippocampal LTP is controlled by the number or subunit composition of AMPA receptors and show a dichotomy between LTP in CA1 and acquisition of spatial memory.

Differential synaptic distribution of the AMPA-GluR2 subunit on GABAergic and non-GABAergic neurons in the basolateral amygdala

The cellular and ultrastructural distribution patterns of the AMPA glutamate receptor subunit, GluR2, were determined in the rat basolateral amygdala. GluR2 immunoreactivity was widely and uniformly distributed in the basolateral nucleus, with both pyramidal and non-pyramidal neurons labelled. In fact, double label immunohistochemical analyses demonstrated that over 90% of the GABAergic interneurons were labelled for GluR2. Electron microscopic analyses further confirmed the presence of GluR2 in the soma and dendrites of GABAergic interneurons as well as in the soma, spines and dendritic shafts of pyramidal cells. As in our parallel study in the rat hippocampus, immunogold analyses revealed that GluR2 immunoreactivity was frequently preferentially located at asymmetric synapses on both pyramidal cell spines and shafts, as well as the dendritic processes and soma of GABAergic interneurons. However, the number of immunogold particles per labelled synapse on GABAergic neurons was significantly lower than at similar labelled asymmetric synapses on spines of presumed pyramidal cells. Given that the presence of GluR2 within the AMPA receptor complex decreases calcium flux, these data indicate that GABAergic local circuit neurons might possess AMPA receptors with higher calcium permeability on average than pyramidal cells, as has been suggested for hippocampus. Such cell class-specific differences in the subunit representation and resultant channel properties of AMPA receptors have implications for response properties as well as selective vulnerability of neurons within the basolateral nucleus of the amygdala.

Cytosolic Ca2+ changes during in vitro ischemia in rat hippocampal slices: major roles for glutamate and Na+-dependent Ca2+ release from mitochondria

This work determined Ca2+ transport processes that contribute to the rise in cytosolic Ca2+ during in vitro ischemia (deprivation of oxygen and glucose) in the hippocampus. The CA1 striatum radiatum of rat hippocampal slices was monitored by confocal microscopy of calcium green-1. There was a 50-60% increase in fluorescence during 10 min of ischemia after a 3 min lag period. During the first 5 min of ischemia the major contribution was from Ca2+ entering via NMDA receptors; most of the fluorescence increase was blocked by MK-801. Approximately one-half of the sustained increase in fluorescence during 10 min of ischemia was caused by activation of Ca2+ release from mitochondria via the mitochondrial 2Na+-Ca2+ exchanger. Inhibition of Na+ influx across the plasmalemma using lidocaine, low extracellular Na+, or the AMPA/kainate receptor blocker CNQX reduced the fluorescence increase by 50%. The 2Na+-Ca2+ exchange blocker CGP37157 also blocked the increase, and this effect was not additive with the effects of blocking Na+ influx. When added together, CNQX and lidocaine inhibited the fluorescence increase more than CGP37157 did. Thus, during ischemia, Ca2+ entry via NMDA receptors accounts for the earliest rise in cytosolic Ca2+. Approximately 50% of the sustained rise is attributable to Na+ entry and subsequent Ca2+ release from the mitochondria via the 2Na+-Ca2+ exchanger. Sodium entry is also hypothesized to compromise clearance of cytosolic Ca2+ by routes other than mitochondrial uptake, probably by enhancing ATP depletion, accounting for the large inhibition of the Ca2+ increase by the combination of CNQX and lidocaine.

Differential expression of ionotropic glutamate receptor subunits in the outer retina

Ionotropic glutamate receptors (iGluRs) are extremely diverse in their subunit compositions. To understand the functional consequences of this diversity, it is necessary to know the subunits that are expressed by known cell types. By using immunocytochemistry with light and electron microscopy, we localized several subunits (GluR2/3, GluR4, and GluR6/7) in cat retinal neurons, postsynaptic to photoreceptors. Type A horizontal cells express all three subunits strongly, whereas type B horizontal cells express GluR2/3 strongly, GluR6/7 weakly, and do not express GluR4. When they are present, the subunits are expressed strongly throughout the cytoplasm of the somata and primary dendrites; however, in the terminals, they are concentrated at the postsynaptic region, just opposite the presumed site of photoreceptor glutamate release. Surprisingly, all bipolar cell classes (OFF cone bipolar cells, ON cone bipolar cells, and rod bipolar cells) express at least one iGluR subunit at their dendritic tips. Cone bipolar cells forming basal contacts with the cones (presumably OFF cells) express all three subunits in association with the electron-dense postsynaptic membrane. Invaginating dendrites of cone bipolar cells (presumably ON cells) express GluR2/3 and GluR4. Rod bipolar cells (ON cells) express GluR2/3 in their invaginating dendrites. The function of iGluRs in horizontal cells and OFF bipolar cells clearly is to mediate their light responses. GluR6/7 subunit in the receptor of these cells may be responsible for the dopamine-mediated enhancement of glutamate responses that have been observed previously in these cells. The function of iGluRs in ON bipolar cells remains an enigma.

Dopamine selectively inhibits the direct cortical pathway to the CA1 hippocampal region

The perforant path input (pp) is a major direct source of specific sensory information for the CA1 hippocampal region. The termination area of this pathway, the stratum lacunosum-moleculare, has the highest concentration of dopamine receptors in the hippocampus. We have examined the properties of the pp input and its modulation by dopamine. The input is glutamatergic and has a larger NMDA component than the Schaffer collateral (sc) input. Dopamine strongly inhibits the response to pp stimulation (IC50 approximately 3 microM) but not the response to sc stimulation. Dopamine reduces both the NMDA and AMPA components of transmission at the pp and increases paired-pulse facilitation. In the sc, the NMDA component but not the AMPA component is decreased, and paired-pulse facilitation is not affected. The effect of dopamine on the pp does not depend on GABAA inhibition but is reduced by the antagonists of both D1 and D2 families of dopamine receptors. The effect is not completely blocked by the combination of D1 and D2 antagonists, but is completely blocked by the atypical neuroleptic clozapine. Our results provide the first evidence for strong dopaminergic control of transmission in the perforant path. By inhibiting this pathway, dopamine hyperfunction and/or NMDA hypofunction abnormalities implicated in schizophrenia may isolate CA1 from its main source of sensory information.

The connection from cortical area V1 to V5: a light and electron microscopic study

Area V5 (middle temporal) in the superior temporal sulcus of macaque receives a direct projection from the primary visual cortex (V1). By injecting anterograde tracers (biotinylated dextran and Phaseolus vulgaris lectin) into V1, we have examined the synaptic boutons that they form in V5 in the electron microscope. Nearly 80% of the target cells in V5 were spiny (excitatory). The boutons formed asymmetric (Gray's type 1) synapses with spines (54%), dendrites (33%), and somata (13%). All somatic targets and some (26%) of the target dendritic shafts showed features characteristic of smooth (inhibitory) cells. Each bouton formed, on average, 1.7 synapses. The larger boutons formed multiple synapses with the same neuron and completely enveloped the entire spine head. On most dendritic shafts and all somata the postsynaptic density en face was disk-shaped but in about half the cases the reconstructed postsynaptic densities of synapses on spines appeared as complete or partial annuli. Even in the zones of densest innervation only 3% of the asymmetric synapses were formed by the labeled boutons. Although the V1 projection forms only a small minority of synapses in V5, its affect could be considerably amplified by local circuits in V5, in a way analogous to the amplification of the small thalamic input to area V1.

Variability of neurotransmitter concentration and nonsaturation of postsynaptic AMPA receptors at synapses in hippocampal cultures and slices

To understand the elementary unit of synaptic communication between CNS neurons, one must know what causes the variability of quantal postsynaptic currents and whether unitary packets of transmitter saturate postsynaptic receptors. We studied single excitatory synapses between hippocampal neurons in culture. Focal glutamate application at individual postsynaptic sites evoked currents (I(glu)) with little variability compared with quantal excitatory postsynaptic currents (EPSCs). The maximal I(glu) was >2-fold larger than the median EPSC. Thus, variations in [glu]cleft are the main source of variability in EPSC size, and glutamate receptors are generally far from saturation during quantal transmission. This conclusion was verified by molecular antagonism experiments in hippocampal cultures and slices. The general lack of glutamate receptor saturation leaves room for increases in [glu]cleft as a mechanism for synaptic plasticity.

Synapse formation by hippocampal neurons from agrin-deficient mice

Agrin, a proteoglycan secreted by motoneurons, is a critical organizer of synaptic differentiation at skeletal neuromuscular junctions. Agrin is widely expressed in the nervous system so other functions seem likely, but none have been demonstrated. To test roles for agrin in interneuronal synapse formation, we studied hippocampi from mutant mice that completely lack the z+ splice form of agrin essential for neuromuscular differentiation and also exhibit severely ( approximately 90%) reduced levels of all agrin isoforms (M. Gautam et al., 1996, Cell 85, 525-535). The brains of neonatal homozygous agrin mutants were often smaller than those of heterozygous and wild-type littermates, but were morphologically and histologically indistinguishable. In particular, antibodies to pre- and postsynaptic components of glutamatergic synapses were similarly coaggregated at synaptic sites in both mutants and controls. Because mutants die at birth due to neuromuscular defects, we cultured neurons to assess later stages of synaptic maturation. In primary cultures, the agrin-deficient neurons formed MAP2-positive dendrites and tau-1-positive axons. Synaptic vesicle proteins, AMPA- and NMDA-type glutamate receptors, GABAA receptors, and the putative synapse-organizing proteins PSD-95, GKAP, and gephyrin formed numerous clusters at synaptic sites. Quantitatively, the number of SV2-labeled contacts per neuron at day 5 and the number of PSD-95 clusters per dendrite length at day 18 in culture showed no significant differences between genotypes. Furthermore, exogenous z+ agrin was unable to induce ectopic accumulation of components of central glutamatergic or GABAergic synapses as it does for neuromuscular cholinergic synapses. These results indicate that the z+ forms of agrin are dispensable for glutamatergic and GABAergic synaptic differentiation in the central nervous system.

Subcellular redistribution of m2 muscarinic acetylcholine receptors in striatal interneurons in vivo after acute cholinergic stimulation

The purpose of our work was to investigate how the cholinergic environment influences the targeting and the intracellular trafficking of the muscarinic receptor m2 (m2R) in vivo. To address this question, we have used immunohistochemical approaches at light and electron microscopic levels to detect the m2R in control rats and rats treated with muscarinic receptor agonists. In control animals, m2Rs were located mostly at postsynaptic sites at the plasma membrane of perikarya and dendrites of cholinergic and NPY-somatostatin interneurons as autoreceptors and heteroreceptors, respectively. Presynaptic receptors were also detected in boutons. The m2Rs were usually detected at extrasynaptic sites, but they could be found rarely in association with symmetrical synapses, suggesting that the cholinergic transmission mediated by m2R occurs via synaptic and nonsynaptic mechanisms. The stimulation of muscarinic receptors with oxotremorine provoked a dramatic alteration of m2R compartmentalization, including endocytosis with a decrease of the density of m2R at the membrane (-63%) and an increase of those associated with endosomes (+86%) in perikarya. The very strong increase of m2R associated with multivesicular bodies (+732%) suggests that oxotremorine activated degradation. The slight increase in the Golgi apparatus (+26%) suggests that the m2R stimulation had an effect on the maturation of m2R. The substance P receptor located at the membrane of the same neurons was unaffected by oxotremorine. Our data demonstrate that cholinergic stimulation dramatically influences the subcellular distribution of m2R in striatal interneurons in vivo. These events may have key roles in controlling abundance and availability of muscarinic receptors via regulation of receptor endocytosis, degradation, and/or neosynthesis. Further, the control of muscarinic receptor trafficking may influence the activity of striatal interneurons, including neurotransmitter release and/or electric activity.

Subcellular and subsynaptic distribution of the NR1 subunit of the NMDA receptor in the neostriatum and globus pallidus of the rat: co-localization at synapses with the GluR2/3 subunit of the AMPA receptor

Glutamatergic neurotransmission in the neostriatum and the globus pallidus is mediated through NMDA-type as well as other glutamate receptors and is critical in the expression of basal ganglia function. In order to characterize the cellular, subcellular and subsynaptic localization of NMDA receptors in the neostriatum and globus pallidus, multiple immunocytochemical techniques were applied using antibodies that recognize the NR1 subunit of the NMDA receptor. In order to determine the spatial relationship between NMDA receptors and AMPA receptors, double labelling was performed with the NR1 antibodies and an antibody that recognizes the GluR2 and 3 subunits of the AMPA receptor. In the neostriatum all neurons with characteristics of spiny projection neurons, some interneurons and many dendrites and spines were immunoreactive for NR1. In the globus pallidus most perikarya and many dendritic processes were immunopositive. Immunogold methods revealed that most NR1 labelling is associated with asymmetrical synapses and, like the labelling for GluR2/3, is evenly spread across the synapse. Double immunolabelling revealed that in neostriatum, over 80% of NR1-positive axospinous synapses are also positive for GluR2/3. In the globus pallidus most NR1-positive synapses are positive for GluR2/3. In both regions many synapses labelled only for GluR2/3 were also detected. These results, together with previous data, suggest that NMDA and AMPA receptor subunits are expressed by the same neurons in the neostriatum and globus pallidus and that NMDA and AMPA receptors are, at least in part, colocalized at individual asymmetrical synapses. The synaptic responses to glutamate in these regions are thus likely be mediated by both AMPA and NMDA receptors at the level of individual synapses.

Surface expression and metabolic half-life of AMPA receptors in cultured rat cerebellar granule cells

The surface expression and metabolic turnover time of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors are important factors in determining the involvement of these proteins in synaptic function. We used the membrane-impermeant chemical crosslinking reagent BS3 and subsequent immunoblot analysis to show that a large proportion of each of the AMPA receptor subunits GluR1-4 is surface expressed in cultured rat cerebellar granule cells. The apparent molecular mass of the individual subunits was 105 kDa and the surface expressed crosslinked complex was 530 kDa for each of the antibodies tested. These results are consistent with functional ionotropic glutamate receptors being pentameric subunit assemblies. We investigated the time course of AMPA receptor surface expression using the membrane-impermeant biotinylating agent NHS-SS-biotin. The half-life of surface expressed AMPA receptors was found to be 30 h. To estimate the mean degradation rate of AMPA receptors and total trichloracetic acid (TCA)-precipitable protein we used [35S]methionine/cysteine pulse-chase labelling. The half-life of AMPA receptors immunoprecipitated with anti-GluR1 antibody was approximately 48 h and the half-life of total TCA-precipitable protein from the same samples was 37 h.

Variations in the tangential distribution of postsynaptic glutamate receptors in Purkinje cell parallel and climbing fiber synapses during development

Major factors affecting the responses of a neuron to release of glutamate include the kinds and distribution of glutamate receptors in the neuron and their distribution along the surface of the postsynaptic membrane (tangential distribution). The latter distribution pattern is established during the development of the synapse and could be modified during maturation of synapse structure/function and through adult synapse plasticity. Parallel and climbing fiber synapses of cerebellar Purkinje cells are good models for studying this pattern because they contain two major kinds of ionotropic glutamate receptors, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and delta, that are involved in adult plasticity and show differences in distribution, and because these two synapse types show complex changes in architecture and glutamate receptor distributions during development. In the present study, both AMPA and delta receptors showed variations in tangential distributions during many stages of development from postnatal day 2 to adult; i.e. qualitative assessment showed that receptors are concentrated either near the center or in outer portions of the synapse, while they are rare or absent from the perisynaptic region. Quantitative analysis showed statistically significant nonuniformities at some ages; the most common nonuniformity in these cases appears to be a drop-off in receptor density in the outer 20% of the synapse. Statistical analyses also indicated that distribution patterns did not change significantly with age. In contrast to the ionotropic receptors, the metabotropic glutamate receptor, mGluR1alpha, was found mainly in the perisynaptic region both during development and in adults. Differences in the distribution of glutamate receptors may be necessary to assure an effective response to glutamate release and may be modified through synaptic plasticity. Our findings show that the basic patterns of distribution of ionotropic and metabotropic glutamate receptors in synapses are established early in development, indicating that the postsynaptic density/membrane region is highly organized even in the immature synapse.

Calcium-evoked dendritic exocytosis in cultured hippocampal neurons. Part II: mediation by calcium/calmodulin-dependent protein kinase II

Calcium-evoked dendritic exocytosis (CEDE), demonstrated in cultured hippocampal neurons, is a novel mechanism that could play a role in synaptic plasticity. A number of forms of neuronal plasticity are thought to be mediated by calcium/calmodulin-dependent protein kinase II (CaMKII). Here, we investigate the role of CaMKII in CEDE. We find that the developmental time course of CEDE parallels the expression of alphaCaMKII, a dominant subunit of CaMKII. An inhibitor of this enzyme, KN-62, blocks CEDE. Furthermore, 7 d in vitro neurons (which normally do not express alphaCaMKII nor show CEDE) can undergo CEDE when infected with a recombinant virus producing alphaCaMKII. Expression of a constitutively active CaMKII produces dendritic exocytosis in the absence of calcium stimulus, and this exocytosis is blocked by nocodazole, an inhibitor of microtubule polymerization that also blocks CEDE. These results indicate that CEDE is mediated by the activation of CaMKII, consistent with the view that CEDE plays a role in synaptic plasticity.

Calcium-evoked dendritic exocytosis in cultured hippocampal neurons. Part I: trans-Golgi network-derived organelles undergo regulated exocytosis

Exocytosis is a widely observed cellular mechanism for delivering transmembrane proteins to the cell surface and releasing signaling molecules into the extracellular space. Calcium-evoked exocytosis, traditionally thought to be restricted to presynaptic specializations in neurons, has been described recently in many cells. Here, calcium-evoked dendritic exocytosis (CEDE) is visualized in living cultured hippocampal neurons. Organelles that undergo CEDE are in somata, dendrites, and perisynaptic regions, identified by using immunocytochemistry and correlative light and electron microscopy. CEDE is regulated developmentally: neurons <9 d in vitro do not show CEDE. In addition, CEDE is blocked by tetanus toxin, an inhibitor of regulated exocytosis, and nocodazole, an inhibitor of microtubule polymerization. Organelles that undergo CEDE often are found on the base of spines, putative sites of synaptic plasticity. CEDE therefore could be involved in structural and functional modification of spines and could play a role in synaptic plasticity, where it might involve changes in receptor/channel density, release of active compounds having effect on pre- and postsynaptic function, and/or growth of synaptic structures.

N-cadherin redistribution during synaptogenesis in hippocampal neurons

Cadherins are homophilic adhesion molecules that, together with their intracellular binding partners the catenins, mediate adhesion and signaling at a variety of intercellular junctions. This study shows that neural (N)-cadherin and beta-catenin, an intracellular binding partner for the classic cadherins, are present in axons and dendrites before synapse formation and then cluster at developing synapses between hippocampal neurons. N-cadherin is expressed initially at all synaptic sites but rapidly becomes restricted to a subpopulation of excitatory synaptic sites. Sites of GABAergic, inhibitory synapses in mature cultures therefore lack N-cadherin but are associated with clusters of beta-catenin, implying that they contain a different classic cadherin. These findings indicate that N-cadherin adhesion may stabilize early synapses that can then be remodeled to express a different cadherin and that cadherins systematically differentiate between functionally (excitatory and inhibitory) and spatially distinct synaptic sites on single neurons. These results suggest that differential cadherin expression may orchestrate the point-to-point specificity displayed by developing synapses.

Dentate granule cells form novel basal dendrites in a rat model of temporal lobe epilepsy

Mossy fibre sprouting and re-organization in the inner molecular layer of the dentate gyrus is a characteristic of many models of temporal lobe epilepsy including that induced by perforant-path stimulation. However, neuroplastic changes on the dendrites of granule cells have been less-well studied. Basal dendrites are a transient morphological feature of rodent granule cells during development. The goal of the present study was to examine whether granule cell basal dendrites are generated in rats with epilepsy induced by perforant-path stimulation. Adult Wistar rats were stimulated for 24 h at 2 Hz and with intermittent (1/min) trains (10 s duration) of single stimuli at 20 Hz (20 V, 0.1 ms) delivered 1/min via an electrode placed in the angular bundle. The brains of these experimental rats and age- and litter-matched control animals were processed for the rapid Golgi method. All rats with perforant-path stimulation displayed basal dendrites on many Golgi-impregnated granule cells. These basal dendrites mainly originated from their somata at the hilar side and then extended into the hilus. Quantitative analysis of more than 800 granule cells in the experimental and matched control brains showed that 6-15% (mean=8.7%) of the impregnated granule cells have spiny basal dendrites on the stimulated side, as well as the contralateral side (mean=3.1%, range=2.9-3.9%) of experimental rats, whereas no basal dendrites were observed in the dentate gyrus from control animals. The formation of basal dendrites appears to be an adaptive morphological change for granule cells in addition to the previously described mossy fibre sprouting, as well as dendritic and somatic spine formation observed in the dentate gyrus of animal and human epileptic brains. The presence of these dendrites in the subgranular region of the hilus suggests that they may be postsynaptic targets of the mossy fibre collaterals.

Cell type and pathway dependence of synaptic AMPA receptor number and variability in the hippocampus

It has been suggested that some glutamatergic synapses lack functional AMPA receptors. We used quantitative immunogold localization to determine the number and variability of synaptic AMPA receptors in the rat hippocampus. Three classes of synapses show distinct patterns of AMPA receptor content. Mossy fiber synapses on CA3 pyramidal spines and synapses on GABAergic interneurons are all immunopositive, have less variability, and contain 4 times as many AMPA receptors as synapses made by Schaffer collaterals on CA1 pyramidal spines and by commissural/ associational (C/A) terminals on CA3 pyramidal spines. Up to 17% of synapses in the latter two connections are immunonegative. After calibrating the immunosignal (1 gold = 2.3 functional receptors) at mossy synapses of a 17-day-old rat, we estimate that the AMPA receptor content of C/A synapses on CA3 pyramidal spines ranges from <3 to 140. A similar range is found in adult Schaffer collateral and C/A synapses.

Endogenous NMDA-receptor activation regulates glutamate release in cultured spinal neurons

N-methyl--aspartate (NMDA) receptor activation plays a fundamental role in the genesis of electrical activity of immature neurons and may participate in activity-dependent aspects of CNS development. A recent study has suggested that NMDA-receptor-mediated glutamatergic neurotransmission might occur in the developing spinal cord via activation of nonsynaptic receptors, but the details of NMDA-receptor activation in the developing CNS are not yet well understood. We describe here a model of cultured spinal neurons that display ongoing alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor activity characterized by spontaneous excitatory postsynaptic currents (EPSCs), with NMDA-receptor activity detectable only as single channel events. -2-amino-5-phosphonovaleric acid (100 microM) and tetrodotoxin (TTX) 100 nM each reduced the occurrence of spontaneous AMPA EPSCs; quantal analysis showed a decrease in the number of released quanta but no changes in quantal size, indicating that NMDA-receptor activation and Na+ channel activity affect the generation of spontaneous AMPA EPSCs, at least in part, via mechanisms that impinge on the presynaptic terminal. Once the Mg2+-block was released, activity of NMDA receptors dramatically increased the release of quantal and multiquantal amounts of glutamate, indicating that the NMDA receptors are physiologically coupled to glutamate release. In Mg2+-free solution, TTX application elicited an increase in the number of quantal AMPA EPSCs and a reduction in the number of multiquantal EPSCs, consistent with an effect of NMDA-receptor activation on presynaptic terminals. Our results suggest that endogenous activity at a small number of NMDA receptors can regulate the release of neurotransmitters at developing AMPA synapses.

Molecular mechanisms of glutamate receptor clustering at excitatory synapses

The targeting of AMPA- and NMDA-type glutamate receptors to synapses in the central nervous system is essential for efficient excitatory synaptic transmission. Recent studies have indicated that protein-protein interactions of these receptors with synaptic proteins that contain PDZ domains are crucial for receptor targeting. NMDA receptors have been found to bind to the PSD-95 family of proteins, whereas AMPA receptors interact with the PDZ-domain-containing protein GRIP (glutamate receptor interacting protein). PSD-95 and GRIP contain multiple PDZ domains as well as other protein-protein interaction motifs that help to form large macromolecular complexes that may be important for the formation and plasticity of synapses.

Salient features of synaptic organisation in the cerebral cortex

The neuronal and synaptic organisation of the cerebral cortex appears exceedingly complex, and the definition of a basic cortical circuit in terms of defined classes of cells and connections is necessary to facilitate progress of its analysis. During the last two decades quantitative studies of the synaptic connectivity of identified cortical neurones and their molecular dissection revealed a number of general rules that apply to all areas of cortex. In this review, first the precise location of postsynaptic GABA and glutamate receptors is examined at cortical synapses, in order to define the site of synaptic interactions. It is argued that, due to the exclusion of G protein-coupled receptors from the postsynaptic density, the presence of extrasynaptic receptors and the molecular compartmentalisation of the postsynaptic membrane, the synapse should include membrane areas beyond the membrane specialisation. Subsequently, the following organisational principles are examined: 1. The cerebral cortex consists of: (i) a large population of principal neurones reciprocally connected to the thalamus and to each other via axon collaterals releasing excitatory amino acids, and, (ii) a smaller population of mainly local circuit GABAergic neurones. 2. Differential reciprocal connections are also formed amongst GABAergic neurones. 3. All extrinsic and intracortical glutamatergic pathways terminate on both the principal and the GABAergic neurones, differentially weighted according to the pathway. 4. Synapses of multiple sets of glutamatergic and GABAergic afferents subdivide the surface of cortical neurones and are often co-aligned on the dendritic domain. 5. A unique feature of the cortex is the GABAergic axo-axonic cell, influencing principal cells through GABAA receptors at synapses located exclusively on the axon initial segment. The analysis of these salient features of connectivity has revealed a remarkably selective array of connections, yet a highly adaptable design of the basic circuit emerges when comparisons are made between cortical areas or layers. The basic circuit is most obvious in the hippocampus where a relatively homogeneous set of spatially aligned principal cells allows an easy visualization of the organisational rules. Those principles which have been examined in the isocortex proved to be identical or very similar. In the isocortex, the basic circuit, scaled to specific requirements, is repeated in each layer. As multiple sets of output neurones evolved, requiring subtly different needs for their inputs, the basic circuit may be superimposed several times in the same layer. Tangential intralaminar connections in both the hippocampus and isocortex also connect output neurones with similar properties, as best seen in the patchy connections in the isocortex. The additional radial superposition of several laminae of distinct sets of output neurones, each representing and supported by its basic circuit, requires a co-ordination of their activity that is mediated by highly selective interlaminar connections, involving both the GABAergic and the excitatory amino acid releasing neurones. The remarkable specificity in the geometry of cells and the selectivity in placement of neurotransmitter receptors and synapses on their surface, strongly suggest a predominant role for time in the coding of information, but this does not exclude an important role also for the rate of action potential discharge in cortical representation of information.