Differences in the expression of AMPA and NMDA receptors between axospinous perforated and nonperforated synapses are related to the configuration and size of postsynaptic densities

Synaptic contact number and size in stratum radiatum CA1 of APP/PS1DeltaE9 transgenic mice

Synaptic changes occur early in the course of Alzheimer's disease and are key to understanding the initial events in associated neurodegenerative processes. The quantitative analysis of synaptic morphology in transgenic mouse models of Alzheimer's disease can provide important insights into these processes. To this end, the total number and the distribution of the diameters of synaptic contacts in the stratum radiatum of the CA1 region of the hippocampus of 12-month-old APP/PS1DeltaE9 transgenic mice and wild type littermates have been evaluated by applying design-based stereological methods to material prepared for electron microscopy. Although there were no differences in the size of the synaptic contacts, the total number of synaptic contacts was significantly larger in the transgenic mice, suggesting that the transgenic effect at this age is synaptotrophic and that the presence of amyloid plaques and an elevated Abeta42/40 ratio are not necessarily detrimental to populations of synapses. The potential of this type of data in evaluating synaptic changes related to Alzheimer's disease is discussed and the methodology described in detail.

Low-frequency stimulation reverses kindling-induced neocortical motor map expansion

Repeated application of low-frequency stimulation can interrupt the development and progression of seizures. Low-frequency stimulation applied to the corpus callosum can also induce long-term depression in the neocortex of awake freely moving rats as well as reduce the size of neocortical movement representations (motor maps). We have previously shown that seizures induced through electrical stimulation of the corpus callosum, amygdala or hippocampus can expand the topographical expression of neocortical motor maps. The purpose of the present study was to determine if low-frequency stimulation administered to the corpus callosum could reverse the expansion of neocortical motor maps induced by seizures propagating from the hippocampus. Adult Long-Evans hooded rats were electrically stimulated in the right ventral hippocampus, twice daily until 30 neocortical seizures were recorded. Subsequently, low-frequency stimulation was administered to the corpus callosum once daily for 20 sessions. High-resolution intracortical microstimulation was then utilized to derive forelimb-movement representations in the left (un-implanted) sensorimotor neocortex. Our results show that hippocampal seizures result in expanded motor maps and that subsequent low-frequency application can reduce the size of the expanded motor maps. Low-frequency stimulation may be an effective treatment for reversing seizure-induced reorganization of brain function.

Caloric restriction and age affect synaptic proteins in hippocampal CA3 and spatial learning ability

Caloric restriction (CR) is a daily reduction of total caloric intake without a decrease in micronutrients or disproportionate reduction of any one dietary component. CR can increase lifespan reliably in a wide range of species and appears to counteract some aspects of the aging process throughout the body. The effects on the brain are less clear, but moderate CR seems to attenuate age-related cognitive decline. Thus, we determined the effects of age and CR on key synaptic proteins in the CA3 region of the hippocampus and whether these changes were correlated with differences in behavior on a hippocampal-dependent learning and memory task. We observed an overall, age-related decline in the NR1, N2A and N2B subunits of the N-methyl-d-aspartate (NMDA)-type and the GluR1 and GluR2 subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA)-type ionotropic glutamate receptors. Interestingly, we found that CR initially lowers the glutamate receptor subunit levels as compared to young AL animals, and then stabilizes the levels across lifespan. Synaptophysin, a presynaptic vesicle protein, showed a similar pattern. We also found that both CR and ad libitum (AL) fed animals exhibited age-related cognitive decline on the Morris water maze task. However, AL animals declined between young and middle age, and between middle age and old, whereas CR rats only declined between young and middle age. Thus, the decrease in key synaptic proteins in CA3 and cognitive decline occurring across lifespan are stabilized by CR. This age-related decrease and CR-induced stabilization are likely to affect CA3 synaptic plasticity and, as a result, hippocampal function.

Neuron glial communication at synapses: insights from vertebrates and invertebrates

Glial cells are instrumental for many aspects of nervous-system function. Interestingly, complex neuron-glial interactions at synapses are commonly found in both invertebrates and vertebrates. Although these interactions are known to be important for synaptic physiology, the cellular processes and molecular mechanisms involved have not been fully uncovered. Identifying the common and unique features of neuron-glial interactions between invertebrates and vertebrates may provide valuable insights into the relationship of neuron-glial cross-talk to nervous-system function. This review highlights selected studies that have revealed structural and functional insights into neuron-glial interactions at synapses in invertebrate and vertebrate model systems.

Evidence that long-term potentiation occurs within individual hippocampal synapses during learning

Stabilization of long-term potentiation (LTP) depends on multiple signaling cascades linked to actin polymerization. We used one of these, involving phosphorylation of the regulatory protein cofilin, as a marker to test whether LTP-related changes occur in hippocampal synapses during unsupervised learning. Well handled rats were allowed to explore a compartmentalized environment for 30 min after an injection of vehicle or the NMDA receptor antagonist (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP). Another group of rats consisted of vehicle-injected, home-cage controls. Vehicle-treated rats that explored the environment had 30% more spines with dense phosphorylated (p) cofilin immunoreactivity in hippocampal field CA1 than did rats in the home-cage group. The increase in pCofilin-positive spines and behavioral evidence for memory of the explored environment were both eliminated by CPP. Coimmunostaining for pCofilin and the postsynaptic density protein 95 (PSD-95) showed that synapses on pCofilin-positive spines were substantially larger than those on neighboring (pCofilin-negative) spines. These results establish that uncommon cellular events associated with LTP, including changes in synapse size, occur in individual spines during learning, and provide a technique for mapping potential engrams.

Changes in synaptic morphology accompany actin signaling during LTP

Stabilization of long-term potentiation (LTP) is commonly proposed to involve changes in synaptic morphology and reorganization of the spine cytoskeleton. Here we tested whether, as predicted from this hypothesis, induction of LTP by theta-burst stimulation activates an actin regulatory pathway and alters synapse morphology within the same dendritic spines. TBS increased severalfold the numbers of spines containing phosphorylated (p) p21-activated kinase (PAK) or its downstream target cofilin; the latter regulates actin filament assembly. The PAK/cofilin phosphoproteins were increased at 2 min but not 30 s post-TBS, peaked at 7 min, and then declined. Double immunostaining for the postsynaptic density protein PSD95 revealed that spines with high pPAK or pCofilin levels had larger synapses (+60-70%) with a more normal size frequency distribution than did neighboring spines. Based on these results and simulations of shape changes to synapse-like objects, we propose that theta stimulation markedly increases the probability that a spine will enter a state characterized by a large, ovoid synapse and that this morphology is important for expression and later stabilization of LTP.

High-resolution quantitative visualization of glutamate and GABA receptors at central synapses

Glutamate and GABA are the main transmitters in the central nervous system and their effects are mediated by ionotropic and metabotropic receptors. Immunogold electron microscopy has revealed the quantitative localization of these receptors at 20-30nm resolution. SDS-digested freeze-fracture replica labeling (SDS-FRL), a newly developed immunogold method, provides an accurate estimate of molecule numbers. Here, we summarize the recent advances in quantitative receptor localization, including use of SDS-FRL analyses to determine numbers of AMPA-type glutamate receptors in the cerebellum. The two-dimensional view and high sensitivity of SDS-FRL have revealed small, irregularly shaped AMPA receptor clusters within cerebellar synapses.

Do thin spines learn to be mushroom spines that remember?

Dendritic spines are the primary site of excitatory input on most principal neurons. Long-lasting changes in synaptic activity are accompanied by alterations in spine shape, size and number. The responsiveness of thin spines to increases and decreases in synaptic activity has led to the suggestion that they are 'learning spines', whereas the stability of mushroom spines suggests that they are 'memory spines'. Synaptic enhancement leads to an enlargement of thin spines into mushroom spines and the mobilization of subcellular resources to potentiated synapses. Thin spines also concentrate biochemical signals such as Ca(2+), providing the synaptic specificity required for learning. Determining the mechanisms that regulate spine morphology is essential for understanding the cellular changes that underlie learning and memory.

Dynamics in the ultrastructure of asymmetric axospinous synapses in the frontal cortex of hibernating European ground squirrels (Spermophilus citellus)

Recent theories on the function of arousals from torpor in hibernating mammals focus on the repair of the central nervous system from damage accumulating during prolonged hypothermia. In this framework, we investigated the synaptic ultrastructure in Layer 2 of the frontal cortex from hibernating European ground squirrels (Spermophilus citellus) sacrificed at four different phases in the torpor-arousal cycle. Using electron microscopy, we quantified synapse number and morphometric data on asymmetric axospinous synapses. Length, width, and surface area of postsynaptic densities (PSDs), and the synaptic apposition length of the analyzed synapse were measured. Five groups of animals were compared during entrance into torpor (Torpor Early, TE, n = 6), late torpor (Torpor Late, TL, n = 5), beginning of euthermic arousal episodes (Arousal Early, AE, n = 5), late in the euthermic arousal episode (Arousal Late, AL, n = 5), and during continuous euthermy in spring (EU, n = 6). The results showed that during torpor and at the beginning of arousals the PSD length and synaptic apposition length are significantly increased compared to synapses during late arousal and in spring conditions. In contrast, the width and surface area of the PSDs are decreased in torpor. At the beginning of an arousal the width of the PSD increases and gains maximum value in late arousals (AL), returning to spring (EU) values. No differences were found in total number of synapses during the torpor-arousal cycle. The results indicate reversible changes in ultrastructure of (asymmetric axospinous) synapses in the frontal cortex, which may be critical for the maintenance of cortical neuronal networks and for protection against potential deleterious effects of prolonged hypothermic phases of hibernation.

The morphology of excitatory central synapses: from structure to function

Synapses are the key elements for signal transduction and plasticity in the brain. For a better understanding of the functional signal cascades underlying synaptic transmission, a quantitative morphological analysis of the pre- and postsynaptic structures that represent morphological correlates for synaptic transmission is important. In particular, realistic values of the number, distribution, and geometry of synaptic contacts and the organization of the pool of synaptic vesicles provide important constraints for realistic models and numerical simulations of those parameters of synaptic transmission that, at present, are still not accessible to experiment. Although all synapses are composed of almost the same structural elements, the composition of these elements within a given synapse and the microcircuit in which they are embedded are the deciding factors determining its function. One possible way to analyze these structures is by computer-assisted three-dimensional reconstructions of synapses and their subsequent quantitative analysis based on ultrathin serial sections. The present review summarizes and discusses the morphology of five central excitatory synapses that are quantitatively well described: (1) a giant synapse, the so-called Calyx of Held, in the medial nucleus of the trapezoid body in the auditory brain stem, (2) the mossy fiber terminal establishing synapses with multiple cerebellar granule cell dendrites, (3) the mossy fiber bouton in the hippocampus predominantly terminating on proximal dendrites of CA3 pyramidal neurons, (4) the climbing fiber-Purkinje cell synapse in the cerebellum, and (5) cortical input synapses on the basal dendrites of layer 5 pyramidal cells. The detailed morphological description of these synaptic structures may help to define the morphological correlates of the functional parameters of synaptic transmission, such as the readily releasable pool of synaptic vesicles, of release, and of the variability of quantal size and might therefore explain the existing differences in the function between individual synapses embedded in different microcircuits.

Distance-dependent differences in synapse number and AMPA receptor expression in hippocampal CA1 pyramidal neurons

The ability of synapses throughout the dendritic tree to influence neuronal output is crucial for information processing in the brain. Synaptic potentials attenuate dramatically, however, as they propagate along dendrites toward the soma. To examine whether excitatory axospinous synapses on CA1 pyramidal neurons compensate for their distance from the soma to counteract such dendritic filtering, we evaluated axospinous synapse number and receptor expression in three progressively distal regions: proximal and distal stratum radiatum (SR), and stratum lacunosum-moleculare (SLM). We found that the proportion of perforated synapses increases as a function of distance from the soma and that their AMPAR, but not NMDAR, expression is highest in distal SR and lowest in SLM. Computational models of pyramidal neurons derived from these results suggest that they arise from the compartment-specific use of conductance scaling in SR and dendritic spikes in SLM to minimize the influence of distance on synaptic efficacy.

Synaptic organization of projections from the amygdala to visual cortical areas TE and V1 in the macaque monkey

The primate amygdaloid complex projects to a number of visual cortices, including area V1, primary visual cortex, and area TE, a higher-order unimodal visual area involved in object recognition. We investigated the synaptic organization of these projections by injecting anterograde tracers into the amygdaloid complex of Macaca fascicularis monkeys and examining labeled boutons in areas TE and V1 using the electron microscope. The 256 boutons examined in area TE formed 263 synapses. Two hundred twenty-three (84%) of these were asymmetric synapses onto dendritic spines and 40 (15%) were asymmetric synapses onto dendritic shafts. Nine boutons (3.5%) formed double asymmetric synapses, generally on dendritic spines, and 2 (1%) of the boutons did not form a synapse. The 200 boutons examined in area V1 formed 211 synapses. One hundred eighty-nine (90%) were asymmetric synapses onto dendritic spines and 22 (10%) were asymmetric synapses onto dendritic shafts. Eleven boutons (5.5%) formed double synapses, usually with dendritic spines. We conclude from these observations that the amygdaloid complex provides an excitatory input to areas TE and V1 that primarily influences spiny, probably pyramidal, neurons in these cortices.

Activity-dependent movements of postsynaptic scaffolds at inhibitory synapses

Dendritic spines show an activity-dependent cytoskeleton-based remodeling coupled with variations in receptor number and the functional properties of excitatory synapses. In this study, we analyzed the dynamics of gephyrin containing inhibitory postsynaptic scaffolds imaging a Venus::gephyrin (VeGe) chimera in dissociated spinal cord neurons. We provide evidence that the postsynaptic scaffolds at mature synapses display a submicrometric rapid lateral motion and are continuously moving on the dendritic shaft. This dynamic behavior is calcium dependent and is controlled by the cytoskeleton. Minute rearrangement within the gephyrin scaffold as well as the scaffold lateral displacements are F-actin dependent. The lateral movements are counteracted by microtubules. Moreover, the action of the potassium channel blocker 4-aminopyridine and receptor antagonists indicate that the dynamics of postsynaptic gephyrin scaffolds are controlled by synaptic activity.

Light microscopic identification and immunocytochemical characterization of glutamatergic synapses in brain sections

Presynaptic proteins are readily identified by light microscopic immunocytochemistry, but immunodetection of postsynaptic proteins in brain sections proves difficult. We performed immunofluorescent double labeling for the NR1 subunit of the N-methyl-D-aspartate receptor (NMDAR) and the vesicular glutamate transporter 1 (VGLUT1). In material fixed with 4% paraformaldehyde, NMDAR staining in somatosensory cortex was restricted to the section surface, whereas presynaptic staining extended deeper into the tissue. Staining for postsynaptic proteins was enhanced in weakly fixed material and in tissue treated with pepsin, as previously reported, but tissue quality was impaired. Staining was also markedly enhanced, and without impairment of tissue quality, by treatment during perfusion with a mixture of inhibitors of proteases and the ubiquitin/proteosome system. We performed quantitative analysis of confocal images to study how immunostaining varies with depth into the tissue. Virtually all puncta immunopositive for VGLUT1 colocalized with synaptophysin puncta; these presynaptic puncta were most numerous 1-2 microm beneath the section surface. In contrast, puncta immunopositive for the NR1 subunit were most numerous at the surface, as were puncta immunopositive for the NR2 subunit, SynGAP, and CaMKII. Punctate staining for all postsynaptic proteins, but not presynaptic markers, was substantially enhanced in material pretreated with antiproteolytic agents. The large majority of NR1-positive puncta at the surface associated with VGLUT1 in this material are likely to represent synaptic contacts. Approximately eighty-five percent of VGLUT1-positive puncta in layers II-III of SI are associated with NR1-positive puncta, and approximately 80% are associated with NR2, SynGAP, and CaMKII. This approach may permit systematic analysis of the chemistry of glutamatergic synapses with light microscopic immunocytochemistry.

Long-term potentiation is associated with changes in synaptic ultrastructure in the rat neocortex

Long-term potentiation (LTP) in the sensorimotor cortex of freely moving rats has been associated with changes in dendritic morphology and dendritic spine density. The current research examined changes in synaptic number and ultrastructure associated with LTP in this cortical region. LTP was induced over a 1 h period and the animals were sacrificed 2 h after the initial stimulation of the LTP group. Synapses within the terminal area of the apical dendrites from layer III pyramidal neurons were quantified by determining the total number of synapses per neuron, the number of excitatory and inhibitory contacts, number of synapses with different curvature subtypes, number of perforated synapses, and synaptic length. Several changes in synaptic morphology of excitatory synapses were revealed but no overall increase in the number of synapses per neuron was evident. Specifically, the induction of LTP was associated with an increased number of excitatory perforated and concave shaped synapses. Increased numbers of perforated concave synapses were also found to be significantly correlated with the degree of potentiation in the LTP animals. These and previous results suggest similar synaptic changes in both the cortex and hippocampus during the early phases of LTP maintenance and distinct synaptic changes during later phases of LTP maintenance.

Hippocampal synaptic loss in early Alzheimer's disease and mild cognitive impairment

One of the major neuropathological findings in the brains of individuals with Alzheimer's disease (AD) is a loss of synaptic contacts in both the neocortex and hippocampus. Here we report, for the first time, an estimate of the total number of synapses in the outer molecular layer (OML) of the human dentate gyrus, in individuals with early Alzheimer's disease (eAD), mild cognitive impairment (MCI), or no cognitive impairment (NCI). An unbiased stereologic sampling scheme coupled with transmission electron microscopy to directly visualize synaptic contacts, was used to estimate the total number of synapses in short postmortem autopsy tissue. Individuals with eAD had significantly fewer synapses than the other two diagnostic groups. Seventy-five percent of the individuals with MCI had synaptic values that were lower than the NCI group mean. The number of synapses showed a significant correlation with the subject's Mini-Mental State score and with cognitive tests involving delayed recall. Synaptic loss showed no relationship to Braak stage or to apoE genotype. The volume of the OML was significantly reduced in eAD compared to the other two diagnositic groups that were not different from each other. These data suggest that a loss of afferents from the entorhinal cortex underlie the synapse loss seen in eAD. This study supports the concept that synapse loss is an early event in the disease process and suggests that MCI may be a transition stage between eAD and NCI with synaptic loss a structural correlate involved in cognitive decline.

The roles of dendritic spine shapes in Purkinje cells

Shapes of dendritic spines are changed by various physiological or pathological states. The high degree of spine shape heterogeneity suggests that they would be the morphological basis for synaptic plasticity. An increasing number of proteins and signal transduction pathways have recently been shown to be associated with structural modifications of spines. Here, we review the possible functional roles of spine shapes in cerebellar Purkinje neurons. Several studies have suggested that spine shapes in Purkinje cells are regulated by both intrinsic and environmental factors, and different spine shapes could have significantly different consequences for brain function. Clearly constricted necks observed in thin, mushroom-shaped, and branched spines serve for compartmentalization of calcium and other second messenger molecules, influencing different signaling mechanisms and synaptic plasticity. Mushroom-shaped spines frequently have perforated postsynaptic density and the area of the spine head is much larger than simple spines, implying that membrane dynamics and receptor turnover are occurring. Branched spines might form additional synapses with afferent inputs resulting in the modification of neuronal circuits. Taken together, all these studies suggest that each spine shape is likely to have a distinct role in Purkinje cell function.

The mental retardation protein PAK3 contributes to synapse formation and plasticity in hippocampus

Mutations of the gene coding for PAK3 (p21-activated kinase 3) are associated with X-linked, nonsyndromic forms of mental retardation (MRX) in which the only distinctive clinical feature is the cognitive deficit. The mechanisms through which PAK3 mutation produces the mental handicap remain unclear, although an involvement in the mechanisms that regulate the formation or plasticity of synaptic networks has been proposed. Here we show, using a transient transfection approach, that antisense and small interfering RNA-mediated suppression of PAK3 or expression of a dominant-negative PAK3 carrying the human MRX30 mutation in rat hippocampal organotypic slice cultures results in the formation of abnormally elongated dendritic spines and filopodia-like protrusions and a decrease in mature spine synapses. Ultrastructural analysis of the changes induced by expression of PAK3 carrying the MRX30 mutation reveals that many elongated spines fail to express postsynaptic densities or contact presynaptic terminals. These defects are associated with a reduced spontaneous activity, altered expression of AMPA-type glutamate receptors, and defective long-term potentiation. Together, these data identify PAK3 as a key regulator of synapse formation and plasticity in the hippocampus and support interpretations that these defects might contribute to the cognitive deficits underlying this form of mental retardation.

Reduction in size of perforated postsynaptic densities in hippocampal axospinous synapses and age-related spatial learning impairments

A central problem in the neurobiology of normal aging is why learning is preserved in some aged individuals yet impaired in others. To investigate this issue, we examined whether age-related deficits in spatial learning are associated with a reduction in postsynaptic density (PSD) area in hippocampal excitatory synapses (i.e., with a structural modification that is likely to have a deleterious effect on synaptic function). A hippocampus-dependent version of the Morris water maze task was used to separate Long-Evans male rats into young adult, aged learning-unimpaired, and equally aged learning-impaired groups. Axospinous synapses from the CA1 stratum radiatum were analyzed using systematic random sampling and serial section analyses. We report that aged learning-impaired rats exhibit a marked ( approximately 30%) and significant reduction in PSD area, whereas aged learning-unimpaired rats do not. The observed structural alteration involves a substantial proportion of perforated synapses but is not observed in nonperforated synapses. These findings support the notion that many hippocampal perforated synapses become less efficient in aged learning-impaired rats, which may contribute to cognitive decline during normal aging.

Unilateral ischemic sensorimotor cortical damage induces contralesional synaptogenesis and enhances skilled reaching with the ipsilateral forelimb in adult male rats

Unilateral damage to the forelimb representation area of the sensorimotor cortex (SMC) results in a compensatory reliance on the unimpaired (ipsilateral to the lesion) forelimb as well as reorganization of neuronal structure and connectivity in the contralateral motor cortex. Recently, male rats with unilateral electrolytic SMC lesions were found to have enhanced skilled reaching performance with the ipsilesional forelimb compared with sham-operated controls. The present study was performed to determine whether these behavioral findings are replicable using an ischemic lesion and whether there is a link between the enhanced learning and synaptogenesis in motor cortical layer V opposite the trained limb and lesion, as assessed using stereological methods for light and electron microscopy. Rats were given a sham operation or an endothelin-1 (ET-1) induced ischemic SMC lesion. They were then trained for 20 days on a skilled reaching task with the unimpaired limb or received control procedures. As with previous findings using electrolytic lesions, rats with unilateral ischemic SMC lesions performed significantly better using the unimpaired forelimb than did sham-operates. Lesions, but not training, significantly increased the total number of motor cortical layer V synapses per neuron as well as the number of perforated and multisynaptic bouton (MSB) synapses per neuron compared with shams. Thus, in addition to a net increase in synapses, the improved reaching ability was coupled with an increase in synapse subtypes that have previously been linked to enhanced synaptic efficacy. The failure to induce synaptogenesis in layer V with reach training alone is in contrast to previous findings and may be related to training intensity.

Loss of GLUR2 alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid receptor subunit differentially affects remaining synaptic glutamate receptors in cerebellum and cochlear nuclei

The alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) type of ionotropic glutamate receptor is the major mediator of fast neurotransmission in the brain and spinal cord. Most AMPA receptors are impermeable to calcium because they contain the GluR2 subunit. However, some AMPA receptors lack GluR2 and pass calcium which can mediate synaptic plasticity and, in excess, neurotoxicity. Previously, we showed a decrease in the density of synaptic AMPA receptors in the hippocampus of mice lacking GluR2. In this study, using these GluR2-lacking mice, we examined other areas of the brain that differ in the amount of GluR2 normally present. Like hippocampal spines, cerebellar Purkinje spines normally express AMPA receptors with high GluR2 and showed a decrease in synaptic AMPA receptors in mutant mice. In contrast, neurons that normally express AMPA receptors with little or no GluR2, such as in the anteroventral cochlear nucleus, showed no decrease in AMPA receptors and even showed an increase in one AMPA receptor subunit. These two different patterns may relate to preadaptations to prevent calcium neurotoxicity; such mechanisms might be absent in Purkinje and hippocampal spines so that these neurons must decrease their total expression of synaptic AMPA receptors (calcium permeable in mutant mice) to prevent calcium neurotoxicity. In addition, we found that another glutamate receptor, GluRdelta2, which is abundant only in parallel fibre synapses on Purkinje cells and in the dorsal cochlear nucleus, is up-regulated at these synapses in mutant mice; this probably reflects some change in GluRdelta2 targeting to these synapses.

Aging, spatial learning, and total synapse number in the rat CA1 stratum radiatum

The aim of this study was to determine whether spatial learning deficits in aged rats are associated with a loss of hippocampal synapses. The Morris water maze task was used to assess the spatial learning capacity of young and aged rats and to attribute aged animals to learning-impaired and learning-unimpaired groups. The number of axospinous synapses in the entire volume of the CA1 stratum radiatum was estimated with unbiased stereological techniques. The results show that the total number of all axospinous synapses and of their perforated and nonperforated subtypes remains constant in the CA1 stratum radiatum of aged learning-impaired rats as compared to aged learning-unimpaired rats and to young adults. Thus, neither age-related deficits in spatial learning nor advanced chronological age are associated with a loss of axospinous synapses from the rat CA1 stratum radiatum.

Synapses with a segmented, completely partitioned postsynaptic density express more AMPA receptors than other axospinous synaptic junctions

Axospinous perforated synapses of one morphological subtype exhibit multiple transmission zones, each one being formed by an axon terminal protrusion apposing a postsynaptic density (PSD) segment and separated from others by complete spine partitions. Such segmented, completely partitioned (SCP) synapses have been implicated in synaptic plasticity and postulated to be exceptionally efficacious. The present study explored the validity of this supposition. Postembedding immunogold electron microscopy was used for quantifying the postsynaptic AMPA receptor (AMPAR) expression, which is widely regarded as a major determinant of synaptic efficacy. Various subtypes of axospinous synapses were examined in the rat CA1 stratum radiatum. The results showed that the number of immunogold particles for AMPARs in SCP synapses markedly and significantly exceeded that in other perforated subtypes (by 101% on the average) and in nonperforated immunopositive synapses (by 1086%). Moreover, the particle number per single PSD segment, each of which also contained NMDA receptors, was significantly higher than that per nonperforated PSD (by 485%). SCP synapses also exhibited a higher particle density per unit PSD area, as well as a larger overall PSD area as compared with other synaptic subtypes. Analysis of covariance revealed that the high AMPAR expression in SCP synapses was related to the segmented PSD configuration, not only to the PSD size. Moreover, the subpopulations of SCP and other perforated synapses with either overlapping or equal PSD sizes differed in AMPAR content and concentration, with both measures being significantly higher in SCP synapses. Thus, the elevated AMPAR expression in SCP synapses is associated with the presence of separate PSD segments, not only with their large PSD area. These findings are consistent with the idea that SCP synapses have a relatively greater efficacy and may support maximal levels of synaptic enhancement characteristic of certain forms of synaptic plasticity such as the early LTP phase.