GluR1-immunopositive interneurons in rat neocortex

Ionotropic glutamate receptors: Which ones, when, and where in the mammalian neocortex

A multitude of 18 iGluR receptor subunits, many of which are diversified by splicing and RNA editing, localize to >20 excitatory and inhibitory neocortical neuron types defined by physiology, morphology, and transcriptome in addition to various types of glial, endothelial, and blood cells. Here we have compiled the published expression of iGluR subunits in the areas and cell types of developing and adult cortex of rat, mouse, carnivore, bovine, monkey, and human as determined with antibody- and mRNA-based techniques. iGluRs are differentially expressed in the cortical areas and in the species, and all have a unique developmental pattern. Differences are quantitative rather than a mere absence/presence of expression. iGluR are too ubiquitously expressed and of limited use as markers for areas or layers. A focus has been the iGluR profile of cortical interneuron types. For instance, GluK1 and GluN3A are enriched in, but not specific for, interneurons; moreover, the interneurons expressing these subunits belong to different types. Adressing the types is still a major hurdle because type-specific markers are lacking, and the frequently used neuropeptide/CaBP signatures are subject to regulation by age and activity and vary as well between species and areas. RNA-seq reveals almost all subunits in the two morphofunctionally characterized interneuron types of adult cortical layer I, suggesting a fairly broad expression at the RNA level. It remains to be determined whether all proteins are synthesized, to which pre- or postsynaptic subdomains in a given neuron type they localize, and whether all are involved in synaptic transmission. J. Comp. Neurol. 525:976-1033, 2017. © 2016 Wiley Periodicals, Inc.

AMPA Receptors: Players in Calcium-Mediated Neuronal Injury?

AMPA receptors mediate a majority of fast, excitatory neurotransmissions in the CNS. AMPA receptors are multimeric proteins composed of one or more of four subunits, termed GluR1-4. Structure-function studies using recombinant AMPA receptor subunits have demonstrated the importance of an arginine residue at the Q/R site within the second transmembrane domain of the GluR2 subunit in conferring the selectivity of the receptor for monovalent cations. Native AMPA receptors in the adult, mammalian CNS are multimeric proteins that are largely calcium-impermeable because of the presence of the dominant GluR2 subunit. Populations of cells in both the developing and adult brain, however, express AMPA receptors that lack the GluR2 subunit and thus are cal cium-permeable. The function of these calcium-permeable AMPA receptors (CPARs) is unknown. Such cells may, however, be selectively vulnerable to excitotoxic injury. After severe global cerebral ischemia in rats, GluR2 expression is selectively downregulated in the vulnerable CA1 region of the hippocampus before the onset of ischemic cell death. Thus AMPA receptor activation may contribute to the excessive accumulation of intra cellular calcium that is thought to mediate irreversible cell injury. This article reviews the molecular basis of CPARs and discusses their possible role in mediating excitotoxic injury. NEUROSCIENTIST 4:149-153, 1998

Neuregulin-1 signals from the periphery regulate AMPA receptor sensitivity and expression in GABAergic interneurons in developing neocortex

Neuregulin-1 (NRG1) signaling is thought to contribute to both neuronal development and schizophrenia neuropathology. Here, we describe the developmental effects of excessive peripheral NRG1 signals on synaptic activity and AMPA receptor expression of GABAergic interneurons in postnatal rodent neocortex. A core peptide common to all NRG1 variants (eNRG1) was subcutaneously administered to mouse pups. Injected eNRG1 penetrated the blood-brain barrier and activated ErbB4 NRG1 receptors in the neocortex, in which ErbB4 mRNA is predominantly expressed by parvalbumin-positive GABAergic interneurons. We prepared neocortical slices from juvenile mice that were receiving eNRG1 subchronically and recorded inhibitory synaptic activity from layer V pyramidal neurons. Postnatal eNRG1 treatment significantly enhanced polysynaptic IPSCs, although monosynaptic IPSCs were not affected. Examination of excitatory inputs to parvalbumin-containing GABAergic interneurons revealed that eNRG1 treatment significantly increased AMPA-triggered inward currents and the amplitudes and frequencies of miniature EPSCs (mEPSCs). Similar effects on mEPSCs were observed in mice treated with a soluble, full-length form of NRG1 type I. Consistent with the electrophysiologic data, expression of the AMPA receptor GluA1 (i.e., GluR1, GluRA) was upregulated in the postsynaptic density/cytoskeletal fraction prepared from eNRG1-treated mouse neocortices. Cortical GABAergic neurons cultured with eNRG1 exhibited a significant increase in surface GluA1 immunoreactivity at putative synaptic sites on their dendrites. These results indicate that NRG1 circulating in the periphery influences postnatal development of synaptic AMPA receptor expression in cortical GABAergic interneurons and may play a role in conditions characterized by GABA-associated neuropathologic processes.

Developmental course of EphA4 cellular and subcellular localization in the postnatal rat hippocampus

From embryonic development to adulthood, the EphA4 receptor and several of its ephrin-A or -B ligands are expressed in the hippocampus, where they presumably play distinct roles at different developmental stages. To help clarify these diverse roles in the assembly and function of the hippocampus, we examined the cellular and subcellular localization of EphA4 in postnatal rat hippocampus by light and electron microscopic immunocytochemistry. On postnatal day (P) 1, the EphA4 immunostaining was robust in most layers of CA1, CA3, and dentate gyrus and then decreased gradually, until P21, especially in the cell body layers. At the ultrastructural level, focal spots of EphA4 immunoreactivity were detected all over the plasma membrane of pyramidal and granule cells, between P1 and P14, from the perikarya to the dendritic and axonal extremities, including growth cones and filopodia. This cell surface immunoreactivity then became restricted to the synapse-associated dendritic spines and axon terminals by P21. In astrocytes, the EphA4 immunolabeling showed a similar cell surface redistribution, from the perikarya and large processes at P1-P7, to small perisynaptic processes at P14-P21. In both cell types, spots of EphA4 immunoreactivity were also detected, with an incidence decreasing with maturation, on the endoplasmic reticulum, Golgi apparatus, and vesicles, organelles involved in protein synthesis, posttranslational modifications, and transport. The cell surface evolution of EphA4 localization in neuronal and glial cells is consistent with successive involvements in the developmental movements of cell bodies first, followed by process outgrowth and guidance, synaptogenesis, and finally synaptic maintenance and plasticity.

Glutamate receptor dynamics in dendritic microdomains

Among diverse factors regulating excitatory synaptic transmission, the abundance of postsynaptic glutamate receptors figures prominently in molecular memory and learning-related synaptic plasticity. To allow for both long-term maintenance of synaptic transmission and acute changes in synaptic strength, the relative rates of glutamate receptor insertion and removal must be tightly regulated. Interactions with scaffolding proteins control the targeting and signaling properties of glutamate receptors within the postsynaptic membrane. In addition, extrasynaptic receptor populations control the equilibrium of receptor exchange at synapses and activate distinct signaling pathways involved in plasticity. Here, we review recent findings that have shaped our current understanding of receptor mobility between synaptic and extrasynaptic compartments at glutamatergic synapses, focusing on AMPA and NMDA receptors. We also examine the cooperative relationship between intracellular trafficking and surface diffusion of glutamate receptors that underlies the expression of learning-related synaptic plasticity.

Ionotropic glutamate receptor GluR1 in the visual cortex of hamster: distribution and co-localization with calcium-binding proteins and GABA

The subunit composition of the AMPA receptor is critical to its function. AMPA receptors that display very low calcium permeability include the GluR2 subunit, while AMPA receptors that contain other subunits, such as GluR1, display high calcium permeability. We have studied the distribution and morphology of neurons containing GluR1 in the hamster visual cortex with antibody immunocytochemistry. We compared this labeling to that for calbindin D28K, parvalbumin, and GABA. Anti-GluR1-immunoreactive (IR) neurons were located in all layers. The highest density of GluR1-IR neurons was found in layers II/III. The labeled neurons were non-pyramidal neurons, but were varied in morphology. The majority of the labeled neurons were round or oval cells. However, stellate, vertical fusiform, pyriform, and horizontal neurons were also labeled with the anti-GluR1 antibody. Two-color immunofluorescence revealed that many of the GluR1-IR neurons in the hamster visual cortex were double-labeled with either calbindin D28K (31.50%), or parvalbumin (22.91%), or GABA (63.89%). These results indicate that neurons in the hamster visual cortex express GluR1 differently according to different layers and selective cell types, and that many of the GluR1-IR neurons are limited to neurons that express calbindin D28K, parvalbumin, or GABA. The present study elucidates the neurochemical structure of GluR1, a useful clue in understanding the differential vulnerability of GluR1-containing neurons with regard to calcium-dependent excitotoxic mechanisms.

Transforming growth factor alpha attenuates the functional expression of AMPA receptors in cortical GABAergic neurons

In the developing neocortex, brain-derived neurotrophic factor (BDNF) exerts a trophic activity to increase the expression and channel activity of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptor subunits. Here, we demonstrate that the epidermal growth factor (EGF) receptor (ErbB1) ligands exert the opposite biological activity in cultured neocortical neurons. Subchronic stimulation of ErbB1 with transforming growth factor alpha (TGFalpha), EGF, or heparin-binding EGF (HB-EGF) down-regulated protein expression of the GluR1 AMPA receptor subunit in cultured neocortical neurons. In agreement, TGFalpha treatment decreased the Bmax of [3H] AMPA binding and GluR1 mRNA levels. Immunocytochemistry revealed that the decrease in GluR1 was most pronounced in multipolar GABAergic neurons. To examine the physiological consequences, we recorded AMPA-evoked currents as well as miniature excitatory postsynaptic currents in morphologically identified putative GABAergic neurons in culture. Subchronic TGFalpha treatment decreased AMPA-triggered currents as well as the amplitude and frequency of miniature excitatory postsynaptic currents. An ErbB1 tyrosine kinase inhibitor, PD153035, inhibited the TGFalpha effect. Moreover, TGFalpha counteracted the neurotrophic activity of BDNF on AMPA receptor expression. Co-application of TGFalpha with BDNF blocked the BDNF-triggered up-regulation of AMPA receptor expression and currents. These observations reveal a negative regulatory activity of the ErbB1 ligand, TGFalpha, which reduces the input sensitivity of cortical GABAergic neurons to attenuate their inhibitory function.

Localization of glutamate receptors in developing cortical neurons in culture and relationship to susceptibility to excitotoxicity

Overactivation of glutamate receptors leading to excitotoxicity has been implicated in the neurodegenerative alterations of a range of central nervous system (CNS) disorders. We have investigated the cell-type-specific changes in glutamate receptor localization in developing cortical neurons in culture, as well as the relationship between glutamate receptor subunit distribution with synapse formation and susceptibility to excitotoxicity. Glutamate receptor subunit clustering was present prior to the formation of synapses. However, different receptor types showed distinctive temporal patterns of subunit clustering, localization to spines, and apposition to presynaptic terminals. N-methyl-D-aspartate (NMDA) receptor subunit immunolabelling was present in puncta along dendrites prior to the formation of synapses, with relatively little localization to spines. Vulnerability to NMDA receptor-mediated excitotoxicity occurred before receptor subunits became localized in apposition to presynaptic terminals. Clustering of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors occurred concurrently with development of vulnerability to excitotoxicity and was related to localization of AMPA receptors at synapses and in spines. Different AMPA receptor subunits demonstrated cell-type-specific localization as well as distribution to spines, dendrites, and extrasynaptic subunit clusters. A subclass of neurons demonstrated substantial perineuronal synaptic innervation, and these neurons expressed relatively high levels of GluR1 and/or GluR4 at receptor puncta, indicating the presence of calcium-permeable AMPA receptors and suggesting alternative synaptic signalling mechanisms and vulnerability to excitotoxicity. These data demonstrate the relationship between glutamate receptor subunit expression and localization with synaptogenesis and development of neuronal susceptibility to excitotoxicity. These data also suggest that excitotoxicity can be mediated through extrasynaptic receptor subunit complexes along dendrites.

Differential expression of NMDA and AMPA receptor subunits in DARPP-32-containing neurons of the cerebral cortex, hippocampus and neostriatum of rats

Dopamine and cyclic adenosine 3',5'-monophosphate-regulated phosphoprotein, 32 kDa (DARPP-32) is a key element of dopamine/D1/DARPP-32/protein phosphatase-1 (PP-1) signaling cascades of mammalian brain. We are interested in the expression patterns of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors in DARPP-32-containing neurons, which may constitute morphological basis for interaction between dopamine and ionotropic glutamate receptors in dopaminoceptive cells. Double immunofluorescence was performed to visualize neurons showing coexpression of DARPP-32 with NMDA or AMPA receptor subunits (i.e., NR1, NR2a/b, glutamate receptor subunit 1 [GluR1], GluR2/3, and GluR4) in the forebrains of rats. Distribution of DARPP-32-positive neurons completely or partially overlapped with that of NMDA receptor- or AMPA receptor-immunoreactive ones in the frontal and parietal cortex, hippocampus and neostriatum, and neurons double-labeled with DARPP-32/NR1, DARPP-32/NR2a/b, DARPP-32/GluR1, DARPP-32/GluR2/3, or DARPP-32/GluR4 immunoreactivity were numerously observed. Semiquantification analysis indicated that most of DARPP-32-containing neurons (86-98%) expressed NR1, NR2a/b and GluR2/3, while less of them (14-90%) expressed GluR1 and GluR4. Although high rates (90-98%) of DARPP-32-positive cells expressed NMDA receptors in all regions above, variant percentages of them expressing AMPA receptor subunits were observed among the cortex (54-90%), hippocampus (59-97%) and neostriatum (14-97%). The study presents differential expression patterns of NMDA and AMPA receptors in DARPP-32-postive neurons in these forebrain regions. Taken together with previous reports, the present data suggest that interaction between dopamine and glutamate receptors may occur in the dopaminoceptive neurons with distinct receptor compositions and may be involved in modulating neuronal properties and excitotoxicity in mammalian forebrain.

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.

Light microscopic study of GluR1 and calbindin expression in interneurons of neocortical microgyral malformations

Rat neocortex that has been injured on the first or second postnatal day (P0-1) develops an epileptogenic, aberrantly layered malformation called a microgyrus. To investigate the effects of this developmental plasticity on inhibitory interneurons, we studied a sub-population of GABAergic cells that co-express the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor GluR1 subunit and the calcium-binding protein, calbindin (CB). Both malformed and control cortex of adult (P40-60) animals contained numerous interneurons double-stained for CB and GluR1. Immunoreactivity (IR) for CB was up-regulated in perikarya of interneurons within supragranular layers of control cortex between P12 and P40. However, in malformed adult (P40) cortex, CB-IR levels were significantly lower than in adult controls, and fell midway between levels in immature and adult control animals. Between P12 and P40, GluR1-IR was down-regulated in perikarya of interneurons in control cortex. Somatic GluR1-IR levels in malformed adult (P40) cortex were not different from adult controls. These neurons formed a dense plexus of highly GluR1-positive spiny dendrites within layer II. The dendritic plexus in the malformation was more intensely GluR1-immunoreactive than that in layer II of control cortex. This was due to apparent changes in thickness and length of dendrites, rather than to significant changes in the number of interneuronal perikarya in the microgyral cortex. Results indicate that the population of GluR1/CB-containing interneurons is spared in malformed microgyral cortex, but that these cells sustain lasting decreases in their somatic expression of calbindin and alterations of dendritic structure. Potential functional implications of these findings are discussed.

AMPA receptor trafficking and long-term potentiation

Activity-dependent changes in synaptic function are believed to underlie the formation of memories. A prominent example is long-term potentiation (LTP), whose mechanisms have been the subject of considerable scrutiny over the past few decades. I review studies from our laboratory that support a critical role for AMPA receptor trafficking in LTP and experience-dependent plasticity.

AMPA glutamate receptor subunits in the guinea pig hypothalamus: distribution and colocalization with progesterone receptor

Excitatory amino acids (EAAs), particularly glutamate, have been implicated in the control of luteinizing hormone (LH) secretion through facilitation of gonadotropin-releasing hormone release. The effects of EAAs are mediated by means of ionotropic glutamate receptors, which are divided into N-methyl-D-aspartate (NMDA) and non-NMDA (kainate and AMPA) subtypes. Moreover, ovarian steroids are responsible for inducing the preovulatory surge of LH and are involved in the actions of EAAs on LH release. Progesterone is directly involved in the potentiating effect of ovarian steroids on the stimulating effect of AMPA neurotransmission on gonadotropin secretion. To broaden our understanding of the role of hypothalamic AMPA receptors in the steroid-induced LH surge, we determined the cellular localization of AMPA receptors in the hypothalamus of guinea pigs by using antibodies that recognize the GluR1, GluR2, GluR2/3, or GluR4 subunits, and then we examined the neuroanatomic relationships between these receptors and the progesterone receptor (PR). Different patterns of immunostaining within the preoptic area and hypothalamus were evident with the antibodies to the four subunits with marked contrasts between moderate staining for GluR1, intensely stained structures for GluR2 and GluR2/3, and little specific staining for GluR4. Immunoreactive (IR) neurons were visualized in many regions, including the two regions known to contain a dense population of estradiol-induced PR-IR cells: the preoptic periventricular and ventrolateral hypothalamic nuclei. Approximately 60% of GluR1-IR and 39% of GluR2-IR cells in the preoptic region possessed PR, whereas 46% of GluR1-IR and 54% of GluR2-IR cells in the ventrolateral nucleus expressed PR. These neuroanatomic results suggest that the coordinated actions of progesterone and glutamatergic inputs on mammalian reproductive functions are integrated at the cellular level.

AMPA receptor trafficking and synaptic plasticity

Activity-dependent changes in synaptic function are believed to underlie the formation of memories. Two prominent examples are long-term potentiation (LTP) and long-term depression (LTD), whose mechanisms have been the subject of considerable scrutiny over the past few decades. Here we review the growing literature that supports a critical role for AMPA receptor trafficking in LTP and LTD, focusing on the roles proposed for specific AMPA receptor subunits and their interacting proteins. While much work remains to understand the molecular basis for synaptic plasticity, recent results on AMPA receptor trafficking provide a clear conceptual framework for future studies.

Interspecific differences in the expression of the AMPA-type glutamate receptors and parvalbumin in the nucleus of Edinger-Westphal of chicks and pigeons

The distribution of AMPA-type glutamate receptor (GluR) subunits was studied in the Edinger-Westphal nucleus (EW) of chicks and pigeons. GluR1, GluR2, GluR3 and GluR4 subunits appeared to be present in EW neurons of both species, but interspecific differences were observed in the abundance of the different types of subunits found in EW neurons. Of particular note, GluR2 immunoreactivity was present in the vast majority (ca. 80%) of neurons of pigeon EW but was found in only a small fraction (ca. 15%) of chick EW neurons. Scarcity of the GluR2 subunit in chick EW was confirmed by in situ hybridization. Because of the tendency for parvalbumin to be localized to neurons that are selectively deficient in GluR2, we also studied the localization of parvalbumin, as well as other calcium-binding proteins, in EW of chick and pigeon. Parvalbumin was found in more than 50% of chick EW neurons but was not detected in pigeon EW neurons. Our results suggest that there are major glutamatergic inputs to EW neurons in both pigeons and chicks. Furthermore, there are likely to be more AMPA-type calcium-permeable glutamate receptors in EW neurons of chick than in pigeon, since it is known that the subtype containing the edited GluR2 subunit is not calcium permeable.

Alpha7 nicotinic acetylcholine receptors occur at postsynaptic densities of AMPA receptor-positive and -negative excitatory synapses in rat sensory cortex

NMDA receptor (NMDAR) activation requires concurrent membrane depolarization, and glutamatergic synapses lacking AMPA receptors (AMPARs) are often considered "silent" in the absence of another source of membrane depolarization. During the second postnatal week, NMDA currents can be enhanced in rat auditory cortex through activation of the alpha7 nicotinic acetylcholine receptor (alpha7nAChR). Electrophysiological results support a mainly presynaptic role for alpha7nAChR at these synapses. However, immunocytochemical evidence that alpha7nAChR is prevalent at postsynaptic sites of glutamatergic synapses in hippocampus and neocortex, along with emerging electrophysiological evidence for postsynaptic nicotinic currents in neocortex and hippocampus, has prompted speculation that alpha7nAChR allows for activation of NMDAR postsynaptically at synapses lacking AMPAR. Here we used dual immunolabeling and electron microscopy to examine the distribution of alpha7nAChR relative to AMPAR (GluR1, GluR2, and GluR3 subunits combined) at excitatory synapses in somatosensory cortex of adult and 1-week-old rats. alpha7nAChR occurred discretely over most of the thick postsynaptic densities in all cortical layers of both age groups. AMPAR immunoreactivity was also detectable at most synapses; its distribution was independent of that of alpha7nAChR. In both age groups, approximately one-quarter of asymmetrical synapses were alpha7nAChR positive and AMPAR negative. The variability of postsynaptic alpha7nAChR labeling density was greater at postnatal day (PD) 7 than in adulthood, and PD 7 neuropil contained a subset of small AMPA receptor-negative synapses with a high density of alpha7nAChR immunoreactivity. These observations support the idea that acetylcholine receptors can aid in activating glutamatergic synapses and work together with AMPA receptors to mediate postsynaptic excitation throughout life.

AMPA GluR2 subunit is differentially distributed on GABAergic neurons and pyramidal cells in the macaque monkey visual cortex

The cellular and synaptic distribution of the AMPA receptor subunit GluR2 was analyzed in the monkey primary visual cortex (area V1), by immunocytochemistry and postembedding immunogold methods. GluR2 immunoreactivity was widely distributed in all of the layers of area V1. A quantitative double labeling analysis in layers II and III revealed that the vast majority of GABAergic interneurons in this area also contained GluR2. Postembedding immunogold analysis revealed that GluR2 immunoreactivity was present at asymmetric synapses on both GABAergic interneurons and pyramidal cells. A quantitative study indicated that the number of GluR2 immunogold particles at asymmetric synapses on pyramidal cells was significantly higher than that on GABAergic interneurons. These results from the primate neocortex are in agreement with and extend our previous studies on the rat hippocampus and amygdala. In view of the dominant role of the GluR2 subunit in regulating calcium flux through AMPA receptors, the differential synaptic distribution of GluR2 on different neuronal types might provide a mechanism for cell-specific response properties to glutamate as well as clues to selective neuronal vulnerability and cell death mediated by calcium-dependent excitotoxic mechanisms.

Changes of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate receptors in layer V of epileptogenic, chronically isolated rat neocortex

In vivo chronic partial isolation of neocortical islands results in epileptogenesis that involves pyramidal neurons of layer V. To test whether an alteration in glutamate receptors might contribute to the epileptiform activity, we analysed the time-course of light microscopic changes in expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors using subunit-specific antibodies. The isolation caused a rapid down-regulation of immunoreactivity for GluR1 and GluR2/3 subunits in deep layer V pyramidal neurons within the neocortical island which was evident 24h post-lesion, and within three days was reduced to about 40-60% of the control level. Many pyramidal cells in deep layer V completely lacked GluR2. Between one and four weeks of survival, down-regulation of GluR2/3 and GluR2 involved the majority of pyramidal layer V neurons, except for cells in the upper part of layer V, and those within narrow areas of all sub-laminae of layer V ("micro-islands"). Initial down-regulation was also observed one to three days post-lesion for subunits 1 and 2 of the N-methyl-D-aspartate receptor, but in contrast to GluR2/3 immunoreactivity, NMDAR2A/B immunoreactivity was enhanced three weeks post-lesion. The present data provide evidence for plastic changes in glutamate receptors in neurons of partially isolated neocortical island. A sub-population of layer V neurons remains relatively unaffected, and would presumably be capable of generating fast glutamatergic synaptic potentials necessary for the development of synchronous epileptiform activity.

SAP97 concentrates at the postsynaptic density in cerebral cortex

SAP97, a PDZ-containing protein, is reported to concentrate in axon terminals, where its function remains unknown. Using highly specific new antibodies, we show that SAP97 in rat cerebral cortex is associated with heteromeric AMPA receptors via a selective biochemical interaction between SAP97 and the GluR1 subunit. Using light and electron microscopic immunocytochemistry, we demonstrate cellular and synaptic colocalization of SAP97 and GluR1, and show that SAP97 concentrates at synapses that contain GluR1 but not necessarily GluR2 or GluR3. Using quantitative postembedding immunogold electron microscopy, we find that SAP97 is at highest concentration within the postsynaptic density of asymmetric synapses. These data suggest that SAP97 may help to anchor GluR1-containing AMPA receptors at the synapse. As a multifunctional scaffolding protein, SAP97 may organize components of AMPA-related intracellular signalling pathways, including those associated with calcium-permeable homomeric GluR1 channels.

Zinc-rich synaptic boutons in human temporal cortex biopsies

The distribution of zinc-rich synaptic boutons in biopsies of the temporal cortex from epileptic patients who had undergone surgery is described. Unfixed cryostat sections were exposed to H(2)S vapour to precipitate endogenous zinc, which was subsequently shown by silver enhancement. In the temporal cortex, the stain for zinc was arranged in bands: stain was heavy in layers II and VI, moderate-to-heavy in layers I, III and V, and low in layer IV. The white matter was virtually devoid of staining. At the electron microscope level, labelling was found in synaptic boutons that made asymmetric synaptic contacts. Immunohistochemical staining for glutamate receptor subunits GluR2/3 was observed in cell bodies in layers II, III, V and VI, coincident with the layers that showed heavy staining for zinc. Immunostaining for glutamate receptor subunit GluR1 was prominent in non-pyramidal neurons in deep cortical layers. These results support findings in other mammals and indicate that the human neocortex may contain an extensive system of zinc-rich cortico-cortical connections. This system may be altered in pathological conditions.

A high GluR1 : GluR2 expression ratio is correlated with expression of Ca2+-binding proteins in rat forebrain neurons

alpha-Amino-3-hydroxy-5-methyl-4-isoxazle propionic acid (AMPA) receptors are ubiquitously expressed; however, their subtypes and abundance vary from region to region. We classified the neurons in various forebrain regions (hippocampus, striatum, amygdala, piriform cortex and somatosensory cortex) into six types: [R1+/R2+], [R1-/R2+], [R1+/R2-], [R1-/R2-], [R1++/R2+] and [R1++/R2-], and analysed the expression of Ca2+-binding proteins, such as parvalbumin and calbindin-D28k, using a triple-staining method. The neurons showing a high GluR1 : GluR2 expression ratio, [R1+/R2-], [R1++/R2+] and [R1++/R2-] neurons, comprised 13-30% of the total neuronal population. In addition, the expression of Ca2+-binding proteins was mainly observed in these three types of neurons. The results suggest that Ca2+-binding protein-positive neurons express Ca2+-permeable AMPA receptors, because the Ca2+-permeability of AMPA receptors is enhanced by the relative scarcity of the GluR2 subunit. To directly test the possibility that Ca2+-binding protein-positive neurons express Ca2+-permeable AMPA receptors, we performed Ca2+-imaging experiments in cultured cortical neurons. Ca2+ influx through AMPA receptors was measured selectively by addition of AMPA together with cyclothiazide in the presence of blockers of other Ca2+ influx routes. More than half of the calbindin-D28k-positive neurons showed a large increase in the intracellular Ca2+ concentration ([Ca2+]i), whilst most of the calbindin-D28k-undetectable neurons exhibited only a slight rise in [Ca2+]i after AMPA addition. These results suggest that the expression of calbindin-D28k is related to the expression of Ca2+-permeable AMPA receptors.

Presynaptic NMDA receptor subunit immunoreactivity in GABAergic terminals in rat brain

N-methyl-D-aspartate (NMDA) receptors are commonly found post-synaptically; they mediate fast excitatory neurotransmission in the central nervous system. In this study, we provide immunocytochemical data supporting the existence of presynaptic NMDA receptors in GABAergic terminals using polyclonal antisera raised against the C-terminus of the NMDAR1 subunit. At the light microscope level, rich plexuses of NMDAR1-positive varicose fibers were found in various nuclei in the basal forebrain (bed nucleus of stria terminalis, septum, parastrial nucleus, vascular organ of the lamina terminalis), thalamus (paraventricular nucleus, midline nuclei), and hypothalamus (parvocellular paraventricular nucleus, arcuate nucleus, preoptic nucleus, suprachiasmatic nucleus). In the brainstem, labeled fibers were much less abundant and were confined to the ventral tegmental area, periaqueductal gray, parabrachial nucleus, and locus coeruleus. At the electron microscope level, NMDAR1-immunoreactive terminals examined in the bed nucleus of stria terminalis, parvocellular paraventricular hypothalamic nucleus, and arcuate nucleus formed symmetric synapses, contained darkly stained large dense-core vesicles, and displayed gamma-aminobutyric acid (GABA) immunoreactivity. Terminals with similar ultrastructural features were found in the paraventricular thalamic nucleus. These findings demonstrate the existence of NMDAR1 subunit immunoreactivity in subsets of GABAergic terminals, which raises questions about the potential roles and mechanisms of activation of presynaptic NMDA heteroreceptors in the rat central nervous system. The pattern of distribution and ultrastructural features of these boutons suggest that they may arise from local GABAergic projections interconnecting a group of brain structures mediating stress responses and/or other endocrine, autonomic, and limbic functions.

Immunolocalization of mGluR1alpha in specific populations of local circuit neurons in the cerebral cortex

By coupling glutamate to the IP(3) signaling pathway, group I metabotropic receptors can increase intracellular Ca(2+) concentration, and might thus contribute to excitotoxicity. To identify neurons that might be vulnerable to such injury, we performed immunofluorescence histochemistry for metabotropic glutamate receptor 1alpha (mGluR1alpha) in the cerebral cortex of adult rat. mGluR1alpha was in somata and dendrites of a subset of non-pyramidal neurons scattered throughout the cerebral cortex. To further characterize mGluR1alpha-positive neurons, we investigated its colocalization with several neurochemical markers. Nearly all mGluR1alpha-positive cells were interneurons immunopositive for gamma-aminobutyric acid. The majority (70-80%) of mGluR1alpha-immunopositive neurons were double-labeled for somatostatin. Approximately half of calretinin-positive neurons and 30% of calbindin-positive neurons expressed mGluR1alpha. In contrast, parvalbumin-expressing neurons were rarely positive for mGluR1alpha. Neurons staining strongly for mGluR1alpha were also positive for GluR1. These results indicated that mGluR1alpha is expressed by specific classes of GABAergic neurons in the neocortex, and suggests a mechanism by which these neurons may be especially vulnerable to excitotoxic injury.

The intra-spine electric force can drive vesicles for fusion: a theoretical model for long-term potentiation

We have estimated the intensity of intra-spine electric fields triggered by stimulation of excitatory spine synapses. We show that this electric force can cause fast electrophoretic movement of negatively charged vesicles which bring new postsynaptic receptors and membrane for insertion during the induction of long-term potentiation (LTP). Due to the direction of an intra-spine electric field, movement of vesicles is electrophoretically directed along the longitudinal spine axis towards the spine head. Thus, the number of fused vesicles may be proportional not only to the increased calcium concentration within the spine head during the induction of LTP but also to the magnitude of electric force which drives vesicles towards the postsynaptic membrane.

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.

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.

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.

Subpopulations of GABAergic and non-GABAergic rat dorsal horn neurons express Ca2+-permeable AMPA receptors

Subpopulations of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors that are either permeable or impermeable to Ca2+ are expressed on dorsal horn neurons in culture. While both mediate synaptic transmission, the Ca2+ -permeable AMPA receptors provide a Ca2+ signal that may result in a transient change in synaptic strength [Gu, J.G., Albuquerque, C., Lee, C.J. & MacDermott, A.B. (1996) Nature, 381, 793]. To appreciate the relevance of these receptors to dorsal horn physiology, we have investigated whether they show selective expression in identified subpopulations of dorsal horn neurons. Expression of Ca2+-permeable AMPA receptors was assayed using the kainate-induced cobalt loading technique first developed by Pruss et al. [Pruss, R.M., Akeson, R.L., Racke, M.M. & Wilburn, J.L. (1991) Neuron, 7, 509]. Subpopulations of dorsal horn neurons were identified using immunocytochemistry for gamma-aminobutyric acid (GABA), glycine, substance P receptor (NK1 receptor) and the Ca2+-binding proteins, calretinin and calbindin D28K. We demonstrate that, in dorsal horn neurons in culture, kainate-induced cobalt uptake is selectively mediated by Ca2+-permeable AMPA receptors, and that a majority of GABA and NK1 receptor-expressing neurons express Ca2+-permeable AMPA receptors. GABAergic dorsal horn neurons are important in local inhibition as well as in the regulation of transmitter release from primary afferent terminals. NK1 receptor-expressing dorsal horn neurons include many of the projection neurons in the nociceptive spino-thalamic pathway. Thus, we have identified two populations of dorsal horn neurons representing important components of dorsal horn function that express Ca2+-permeable AMPA receptors. Furthermore, we show that several subpopulations of putative excitatory interneurons defined by calretinin and calbindin expression do not express Ca2+-permeable AMPA receptors.

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.

Laminar and cellular distribution of AMPA, kainate, and NMDA receptor subunits in monkey sensory-motor cortex

In situ hybridization histochemistry and immunocytochemistry were used to examine lamina- and cell-specific expression of glutamate receptor (GluR) mRNAs and polypeptide subunits in motor and somatosensory cortex of macaque monkeys. Radioactive complementary RNA (cRNA) probes were prepared from cDNAs specific for alpha-amino-3-hydroxy-5-methylisoxozolepropionate (AMPA)/kainate (GluR1-GluR4), kainate (GluR5-GluR7), and N-methyl-D-aspartate (NMDA; NR1, NR2A-NR2D) receptor subunits. AMPA/kainate and NR1, NR2A, and NR2B receptor transcripts show higher expression than other transcripts. All transcripts show lamina-specific patterns of distribution. GluR2 and GluR4 mRNAs show higher expression than do GluR1 and GluR3 mRNAs. GluR6 transcript expression is higher than that of GluR5 and GluR7. NR1 mRNA expression is much higher than that of NR2 mRNAs. NR2C subunit expression is very low except for a very distinct band of high expression in layer IV of area 3b. Immunocytochemistry, using subunit-specific antisera and double labeling for calbindin, parvalbumin, or alpha type II Ca2+/calmodulin-dependent protein kinase (CaMKII-alpha), allowed identification of cell types expressing different subunit genes. GluR1 and GluR5/6/7 immunoreactivity is found in both pyramidal cells and gamma-amino butyric acid (GABA) cells; GluR2/3 immunoreactivity is preferentially found in pyramidal cells, whereas GluR4 immunoreactivity is largely restricted to GABA cells; NMDA receptor subunit immunoreactivity is far greater in excitatory cells than in GABA cells. The density of expression of AMPA/kainate, kainate, and NMDA receptor subunit mRNAs differed within and across the architectonic fields of sensory-motor cortex. This finding and the lamina- and cell-specific patterns of expression suggest assembly of functional receptors from different arrangements of available subunits in specific neuronal populations.

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.

Immunocytochemical localization of the AMPA receptor subunits in the mesencephalic trigeminal nucleus of the rat

The mesencephalic trigeminal nucleus is composed of large (35-50 microns) pseudo-unipolar neurons. Closely associated with them are small (< 20 microns) multipolar neurons. An unique peculiarity of the pseudo-unipolar perikarya is that they receive synaptic input from various sources, which sets them apart from the dorsal root and cranial nerves sensory ganglia neurons. Whereas glutamate is the best neurotransmitter candidate in pseudo-unipolar neurons, glutamatergic input into them has not yet been reported. AMPA glutamate receptors are implicated in fast excitatory glutamatergic synaptic transmission. They have been localized ultrastructurally at postsynaptic sites. This study demonstrates that the pseudo-unipolar neurons of the mesencephalic trigeminal nucleus express AMPA glutamate receptor subunits, which indicates that these neurons receive glutamatergic input. Serial sections from the rostral pons and midbrain of Sprague-Dawley rats were immunostained with antibodies against C-terminus of AMPA receptor subunits: GluR1, GluR2/3, and GluR4. The immunoreaction was visualized with avidin-biotin-peroxidase/DAB for light and electron microscopy. With GluR1 antibody only the smallest multipolar neurons were recognized as immunopositive within the mesencephalic trigeminal nucleus. GluR2/3 stained the pseudo-unipolar neurons intensely within the entire rostro-caudal extent of the nucleus. In addition the former antibody stained small multipolar neurons within the mesencephalic trigeminal nucleus, though with somewhat larger dimensions than those immunoreactive for GluR1. Whereas the overall staining with GluR4 antibody was scant, those pseudo-unipolar neurons that were stained, were strongly stained. Furthermore, a considerable number of microglial cells within and surrounding the mesencephalic trigeminal nucleus displayed very intense immunoreactivity for GluR4. These results are discussed in the light of the glutamate receptor subunit composition.

Development and characterization of monoclonal antibodies specific to the serotonin 5-HT2A receptor

Serotonin (5-hydroxytryptamine, 5-HT) mediates many functions of the central and peripheral nervous systems by its interaction with specific neuronal and glial receptors. Fourteen serotonin receptors belonging to seven families have been identified through physiological, pharmacological, and molecular cloning studies. Monoclonal antibodies (MAbs) specific for each of these receptor subtypes are needed to characterize their expression, distribution, and function in embryonic, adult, and pathological tissues. In this article we report the development and characterization of MAbs specific to the serotonin 5-HT2A receptor. To generate MAbs against 5-HT2AR, mice were immunized with the N-terminal domain of the receptor. The antigens were produced as glutathionine S-transferase (GST) fusion proteins in insect cells using a Baculovirus expression system. The hybridomas were initially screened by ELISA against the GST-5-HT2AR recombinant proteins and subsequently against GST control proteins to eliminate clones with unwanted reactivity. They were further tested by Western blotting against recombinant GST-5-HT2AR, rat and human brain lysate, and lysate from cell lines transfected with 5-HT2AR cDNA. One of the MAbs G186-1117, which recognizes a portion of the 5-HT2AR N-terminus, was selected for further characterization. G186-1117 reacted with a band of molecular size 55 kD corresponding to the predicted size of 5-HT2AR in lysates from rat brain and a 5-HT2AR-transfected cell line. Its specificity was further confirmed by adsorption of immunoreactivity with recombinant 5-HT2AR but not with recombinant 5-HT2BR and 5-HT2CR. Rat brain sections and Schwann cell cultures were immunohistochemically labeled with this MAb. G186-1117 showed differential staining in various regions of the rat brain, varying from regions with no staining to regions of intense reactivity. In particular, staining of cell bodies and dendrites of the pyramidal neurons in the cortex was observed, which is in agreement with observations of electrophysiological studies.

AMPA receptor-mediated EPSCs in rat neocortical layer II/III interneurons have rapid kinetics

The properties of spontaneous and miniature (m) AMPA receptor-mediated excitatory postsynaptic currents (EPSCs) were studied in rat neocortical layer II/III fast spiking interneurons. Under optimal recording conditions, averaged mEPSCs had a 10-90% rise time of about 0.3 ms. The decay of averaged mEPSCs was double exponential with time constants of about 1 and 4 ms. Kinetics were observed to slow as series resistance increased. The amplitudes of mEPSCs were much smaller at +50 mV than at -50 mV indicating that the currents were inwardly rectifying. These results suggest that synaptic AMPA receptors on neocortical inhibitory interneurons have a deactivation time constant less than 1 ms which largely determines the decay of the synaptic currents. The receptors appear to lack GluR-2 subunits and may be Ca2+ permeable.

The development of excitatory synapses in cultured spinal neurons

Immunohistochemical studies of synapses in the CNS have demonstrated that glutamate receptors (GluRs) are concentrated at postsynaptic sites in vivo and in vitro (Baude et al., 1995). The mechanisms leading to receptor clustering at excitatory synapses are far less understood than those governing acetylcholine receptor accumulation at the neuromuscular junction () or glycine receptor aggregation at central inhibitory synapses (). Using cultured rat spinal cord neurons, we demonstrate that clustering of the AMPA receptor subunit GluR1 is among the earliest events in excitatory synapse formation in vitro, coincident with the onset of miniature EPSCs and in many cases preceding presynaptic vesicle accumulation. Postsynaptic receptor clustering is induced in a highly specific and reiterative pattern, independent of receptor activation, by contact with a subset of axons capable of inducing receptor clusters. The subunit composition of AMPA receptor clusters varied significantly between neurons but was invariant within a given neuron. The presence of either GluR2 or GluR3 was common to all receptor clusters. Neither high-affinity glutamate transporters nor NMDA receptors appeared to be concentrated with AMPA receptor subunits at these excitatory synapses.

Redistribution and stabilization of cell surface glutamate receptors during synapse formation

Although the regulation of neurotransmitter receptors during synaptogenesis has been studied extensively at the neuromuscular junction, little is known about the control of excitatory neurotransmitter receptors during synapse formation in central neurons. Using antibodies against extracellular N-terminal (N-GluR1) and intracellular C-terminal (C-GluR1) domains of the AMPA receptor subunit GluR1, combined with surface biotinylation and metabolic labeling studies, we have characterized the redistribution and metabolic stabilization of the AMPA receptor subunit GluR1 during synapse formation in culture. Before synapse formation, GluR1 is distributed widely, both on the surface and within the dendritic cytoplasm of these neurons. The diffuse cell surface pool of receptor appears to be mobile within the membrane and can be induced to cluster by the addition of N-GluR1 to live neurons. As cultures mature and synapses form, there is a redistribution of surface GluR1 into clusters at excitatory synapses where it appears to be immobilized. The change in the distribution of GluR1 is accompanied by an increase in both the half-life of the receptor and the percentage of the total pool of GluR1 that is present on the cell surface. Blockade of postsynaptic AMPA and NMDA receptors had no effect on the redistribution of GluR1. These results begin to characterize the events regulating the distribution of AMPA receptors and demonstrate similarities between synapse formation at the neuromuscular junction and at excitatory synapses in cultured neurons.

Characterization of guanylate kinase-associated protein, a postsynaptic density protein at excitatory synapses that interacts directly with postsynaptic density-95/synapse-associated protein 90

The structure of central synapses is poorly understood at the molecular level. A recent advance came with the identification of the postsynaptic density-95 (PSD-95)/synapse-associated protein 90 family of proteins as important mediators of the synaptic clustering of certain classes of ion channels. By yeast two-hybrid screening, a novel protein termed guanylate kinase-associated protein (GKAP) has been isolated that binds to the GK-like domain of PSD-95 (). Here we present a detailed characterization of GKAP expression in the rat brain and report the cloning of a novel GKAP splice variant. By Northern blot, GKAP mRNAs (4, 6.5, and 8 kB) are expressed predominantly in the rat brain. By in situ hybridization, GKAP is expressed widely in neurons of cortex and hippocampus and in the Purkinje and granule cells of the cerebellum. On brain immunoblots, two prominent bands of 95 and 130 kDa are detected that correspond to products of short and long N-terminal splice variants of GKAP. Two independent GKAP antibodies label somatodendritic puncta in neocortical and hippocampal neurons in a pattern consistent with synaptic elements. Immunogold electron microscopy reveals GKAP to be predominantly postsynaptic and present at asymmetric synapses and in dendritic spines. The distribution of GKAP immunogold particles is uniform in the lateral plane of the PSD but peaks in the perpendicular axis approximately 20 nm from the postsynaptic membrane. In cultured hippocampal neurons GKAP immunoreactive puncta colocalize with the AMPA receptor subunit Glu receptor 1 but not with the GABAA receptor subunits beta2 and beta3. Thus GKAP is a widely expressed neuronal protein localized specifically in the PSD of glutamatergic synapses, consistent with its direct interaction with PSD-95 family proteins.

NMDA and AMPA receptors in the lateral nucleus of the amygdala are postsynaptic to auditory thalamic afferents

Projections from the medial geniculate body (MGB) to the lateral nucleus of the amygdala (LA) have been implicated in the conditioning of emotional reactions to acoustic stimuli. Anatomical and physiological studies indicate that this pathway uses the excitatory amino acid L-glutamate as a transmitter. Recent physiological studies have demonstrated that synaptic transmission in the thalamo-amygdala pathway requires the activation of both N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors, two of the major classes of ionotrophic glutamate receptors. In order to characterize the nature of thalamoamygdala interactions, we examined the synaptic associations between thalamic afferents and amygdala neurons that contain at least one glutamate receptor subtype. Thalamic afferents to the amygdala were identified by lesion-induced anterograde degeneration and anterograde transport of biotinylated dextran-amine, while postsynaptic glutamate receptors were labeled immunocytochemically using antisera directed the R1 subunit of the NMDA receptor and the GluR1 and GluR2/3 subunits of the AMPA receptors. Both methods demonstrated that the majority (77%) of thalamic afferents contact dendritic spines, and most (60%) of these spines express at least one glutamate receptor subtype. To a lesser extent, identified afferents also contacted small and large dendritic shafts, and many of these were immunoreactive. Thalamic afferents terminated on approximately the same proportion (60%) of immunoreactive targets for each glutamate receptor studied. These data provide morphological evidence that thalamic afferents directly synapse onto amygdala neurons that express glutamate receptors and suggest ways in which thalamic afferents activate and influence amygdala circuitry.

Ontogeny of non-NMDA glutamate receptors in rat barrel field cortex: II. Alpha-AMPA and kainate receptors

The ontogeny of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and kainate (KA) glutamate receptors in rat barrel field cortex was characterized by using receptor autoradiography and immunocytochemistry. A somatotopic pattern of AMPA receptors with fewer [3H]AMPA sites in barrel centers than in surrounding cortex did not emerge until postnatal day 10 (P10). After reaching a peak density at P14, the density of [3H]AMPA receptors declined in both barrel centers and surrounding cortex. Compared with AMPA receptors, the density of [3H]KA sites at all ages was low, a somatotopic expression of [3H]KA sites was missing, and the developmental curve for [3H]KA sites was more shallow than that for [3H]AMPA binding sites. A differential ontogeny of AMPA and KA receptors in barrel field cortex was also demonstrated in immunocytochemical studies with antibodies to the AMPA receptor subunits GluR1 and GluR2,3 and the KA receptor subunits GluR6,7. GluR1 and GluR2,3 staining was more dense in barrel septa than in barrel centers; this pattern persisted into adulthood. GluR1 and GluR2,3 receptors were localized to cell bodies and dendrites as well as the neuropil, but different populations of cortical neurons expressed these receptors. At P10, KA receptor subunits GluR6,7 exhibited a contrasting pattern to that of AMPA receptor subunits, with slightly more neuropil staining in barrel centers than in surrounding cortex. After that point, the somatotopic pattern of GluR6,7 subunit expression was lost. The contrasting developmental patterns of expression of the AMPA and KA receptors in the barrel field suggest that they may play different roles in the whisker-to-barrel pathway.

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.

Neuronal degeneration by suicide transport following injection of volkensin into rat cerebral cortex

We have examined the time course of neurodegeneration in subcortical nuclei and other cortical areas known to project to the rat parietal cortex, following unilateral injection of the suicide transport agent, volkensin, into the cortex of one side. Degenerating neurons, visualized by Gallyas silver staining were most prominent 21 days after injection. At this time darkly staining neurons were present in nuclei and areas known to project to the injected cortical area but not in other sites. Affected subcortical nuclei included the ipsilateral ventral thalamus and intralaminar nuclei, the basal nucleus of Meynert and claustrum of the same side, and the dorsal median raphé nucleus of both sides. Within the cortex degenerating pyramidal neurons were visible in the contralateral parietal cortex and in the frontal cortex of the same side. The distribution of degenerating cells is in agreement with the conclusion that only neurons projecting to the injection site were affected. The time course of the appearance of the degeneration and its distribution are in keeping with axonal transport rather than spread by diffusion of the toxin. Neuronal counts in Nissl-stained sections of the contralateral SMI confirmed significant neuronal loss 28 days after injection. In situ hybridization studies using an oligonucleotide probe directed against GAD mRNA and counts of GAD mRNA-positive neurons in the contralateral cortex confirmed that this population of cortical interneurons, which do not project to the injection site, were unaffected.

GABAergic neurons with AMPA GluR1 and GluR2/3 immunoreactivity in the rat striate cortex

The co-localization of GABA with AMPA receptor subunits GluR1 or GluR2/3 was analysed in the striate cortex of adult rats by post-embedding immunocytochemistry in semithin sections. Adjacent 1 micron semithin sections of four brains were alternately incubated with specific antibodies against GABA and the GluR1 and GluR2/3 subunits. The post-embedding immunocytochemistry showed that 38% of GABAergic neurons contained the GluR1 subunit and 10% contained the GluR2/3 subunits. Previous work has shown GluR1 immunoreactivity in non-pyramidal neurons and GluR2/3 immunoreactivity in pyramidal neurons. However, this study is the first to demonstrate that there are GABAergic neurons co-localized with GluR2/3 AMPA subunits. Additionally, this study provides quantitative estimations of the laminar distribution of GABAergic and non-GABAergic cells containing the AMPA GluR1 and GluR2/3 subunits.

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.

Rapid kinetics and inward rectification of miniature EPSCs in layer I neurons of rat neocortex

With the use of the whole cell patch-clamp technique combined with visualization of neurons in brain slices, we studied the properties of miniature excitatory postsynaptic currents (mEPSCs) in rat neocortical layer I neurons. At holding potentials (-50 to -70 mV) near the resting membrane potential (RMP), mEPSCs had amplitudes of 5-100 pA and were mediated mostly by alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate (AMPA) receptors. Amplitude histograms were skewed toward large events. An N-methyl-D-aspartate (NMDA) component was revealed by depolarization to -30 mV or by the use of a Mg2+-free bathing solution. At RMP, averaged AMPA mEPSCs had a 10-90% rise time of approximately 0.3 ms (uncorrected for instrument filtering). The decay of averaged mEPSCs was best fit by double-exponential functions in most cases. The fast, dominating component had a decay time constant of approximately 1.2 ms and comprised approximately 80% of the total amplitude. A small slow component had a decay time constant of approximately 4 ms. Positive correlations were found between rise and decay times of both individual and averaged mEPSCs, indicative of dendritic filtering. Some large-amplitude mEPSCs and spontaneous EPSCs (recorded in the absence of tetrodotoxin) had slower kinetics, suggesting a role of asynchronous transmitter release in shaping EPSCs. The amplitudes of mEPSCs were much smaller at +60 mV than at -60 mV, indicating that synaptic AMPA-receptor-mediated currents were inwardly rectifying. These results suggest that neocortical layer I neurons receive both NMDA- and AMPA-receptor-mediated synaptic inputs. The rapid decay of EPSCs appears to be largely determined by AMPA receptor deactivation. The observed rectification of synaptic responses suggests that synaptic AMPA receptors in layer I neurons may lack GluR-2 subunits and may be Ca2+ permeable.

Heterogeneity of AMPA receptors in the dorsal column nuclei of the rat

We have combined immunocytochemistry with retrograde tracing to demonstrate that projecting neurons in the gracile and cuneate nuclei express predominantly the GluR3 subunit of the AMPA receptor while interneurons in these nuclei express predominantly the GluR1 subunit. Interneurons expressing the GluR2 subunit are also present. It is speculated that the two classes of interneurons may release different inhibitory transmitters.

AMPA receptor subunits underlying terminals of fine-caliber primary afferent fibers

Postembedding immunogold electron microscopy was used to determine the relation of primary afferent terminals in superficial laminae of the spinal dorsal horn with AMPA receptor subunits. Immunogold particles coding for GluR1 and GluR2/3 were concentrated at synaptic sites, between 30 nm outside and 40 nm inside the postsynaptic membrane. Immunopositive synapses displayed round vesicles and asymmetric specializations, characteristic of terminals releasing excitatory neurotransmitters; symmetric synapses, characteristic of terminals releasing inhibitory amino acids, were immunonegative. In superficial laminae, large terminals of two main types at the center of a synaptic glomerulus originate from primary afferents: C1 terminals are mainly endings of unmyelinated afferent fibers; C2 terminals are mainly endings of thinly myelinated afferent fibers. Terminals of both types were presynaptic to AMPA subunits, but in different proportions: C1 terminals were related more to GluR1 than to GluR2/3, whereas the reverse was true for C2 terminals. These results suggest that functional properties of peripheral afferents to the spinal cord may be specified by the density and combination of receptor subunits in the postsynaptic membrane, and raise the possibility that calcium-permeable AMPA channels may play a special role in the mediation of sensory input by unmyelinated fibers.