Expression of long-term potentiation in aged rats involves perforated synapses but dendritic spine branching results from high-frequency stimulation alone

STED microscopy reveals dendrite-specificity of spines in turtle cortex

Dendritic spines are key structures for neural communication, learning and memory. Spine size and shape probably reflect synaptic strength and learning. Imaging with superresolution STED microscopy the detailed shape of the majority of the spines of individual neurons in turtle cortex (Trachemys scripta elegans) revealed several distinguishable shape classes. Dendritic spines of a given class were not distributed randomly, but rather decorated significantly more often some dendrites than others. The individuality of dendrites was corroborated by significant inter-dendrite differences in other parameters such as spine density and length. In addition, many spines were branched or possessed spinules. These findings may have implications for the role of individual dendrites in this cortex.

Spine apparatus modulates Ca 2+ in spines through spatial localization of sources and sinks

Dendritic spines are small protrusions on dendrites in neurons and serve as sites of postsynaptic activity. Some of these spines contain smooth endoplasmic reticulum (SER), and sometimes an even further specialized SER known as the spine apparatus (SA). In this work, we developed a stochastic spatial model to investigate the role of the SER and the SA in modulating Ca 2+ dynamics. Using this model, we investigated how ryanodine receptor (RyR) localization, spine membrane geometry, and SER geometry can impact Ca 2+ transients in the spine and in the dendrite. Our simulations found that RyR opening is dependent on where it is localized in the SER and on the SER geometry. In order to maximize Ca 2+ in the dendrites (for activating clusters of spines and spine-spine communication), a laminar SA was favorable with RyRs localized in the neck region, closer to the dendrite. We also found that the presence of the SER without the laminar structure, coupled with RyR localization at the head, leads to higher Ca 2+ presence in the spine. These predictions serve as design principles for understanding how spines with an ER can regulate Ca 2+ dynamics differently from spines without ER through a combination of geometry and receptor localization.

Highlights

1RyR opening in dendritic spine ER is location dependent and spine geometry dependent. Ca 2+ buffers and SERCA can buffer against runaway potentiation of spines even when CICR is activated. RyRs located towards the ER neck allow for more Ca 2+ to reach the dendrites. RyRs located towards the spine head are favorable for increased Ca 2+ in spines.

Dendritic spinule-mediated structural synaptic plasticity: Implications for development, aging, and psychiatric disease

Dendritic spines are highly dynamic and changes in their density, size, and shape underlie structural synaptic plasticity in cognition and memory. Fine membranous protrusions of spines, termed dendritic spinules, can contact neighboring neurons or glial cells and are positively regulated by neuronal activity. Spinules are thinner than filopodia, variable in length, and often emerge from large mushroom spines. Due to their nanoscale, spinules have frequently been overlooked in diffraction-limited microscopy datasets. Until recently, our knowledge of spinules has been interpreted largely from single snapshots in time captured by electron microscopy. We summarize herein the current knowledge about the molecular mechanisms of spinule formation. Additionally, we discuss possible spinule functions in structural synaptic plasticity in the context of development, adulthood, aging, and psychiatric disorders. The literature collectively implicates spinules as a mode of structural synaptic plasticity and suggests the existence of morphologically and functionally distinct spinule subsets. A recent time-lapse, enhanced resolution imaging study demonstrated that the majority of spinules are small, short-lived, and dynamic, potentially exploring their environment or mediating retrograde signaling and membrane remodeling via trans-endocytosis. A subset of activity-enhanced, elongated, long-lived spinules is associated with complex PSDs, and preferentially contacts adjacent axonal boutons not presynaptic to the spine head. Hence, long-lived spinules can form secondary synapses with the potential to alter synaptic connectivity. Published studies further suggest that decreased spinules are associated with impaired synaptic plasticity and intellectual disability, while increased spinules are linked to hyperexcitability and neurodegenerative diseases. In summary, the literature indicates that spinules mediate structural synaptic plasticity and perturbations in spinules can contribute to synaptic dysfunction and psychiatric disease. Additional studies would be beneficial to further delineate the molecular mechanisms of spinule formation and determine the exact role of spinules in development, adulthood, aging, and psychiatric disorders.

The Structural Basis of Long-Term Potentiation in Hippocampal Synapses, Revealed by Electron Microscopy Imaging of Lanthanum-Induced Synaptic Vesicle Recycling

Hippocampal neurons in dissociated cell cultures were exposed to the trivalent cation lanthanum for short periods (15–30 min) and prepared for electron microscopy (EM), to evaluate the stimulatory effects of this cation on synaptic ultrastructure. Not only were characteristic ultrastructural changes of exaggerated synaptic vesicle turnover seen within the presynapses of these cultures—including synaptic vesicle depletion and proliferation of vesicle-recycling structures—but the overall architecture of a large proportion of the synapses in the cultures was dramatically altered, due to large postsynaptic “bulges” or herniations into the presynapses. Moreover, in most cases, these postsynaptic herniations or protrusions produced by lanthanum were seen by EM to distort or break or “perforate” the so-called postsynaptic densities (PSDs) that harbor receptors and recognition molecules essential for synaptic function. These dramatic EM observations lead us to postulate that such PSD breakages or “perforations” could very possibly create essential substrates or “tags” for synaptic growth, simply by creating fragmented free edges around the PSDs, into which new receptors and recognition molecules could be recruited more easily, and thus, they could represent the physical substrate for the important synaptic growth process known as “long-term potentiation” (LTP). All of this was created simply in hippocampal dissociated cell cultures, and simply by pushing synaptic vesicle recycling way beyond its normal limits with the trivalent cation lanthanum, but we argued in this report that such fundamental changes in synaptic architecture—given that they can occur at all—could also occur at the extremes of normal neuronal activity, which are presumed to lead to learning and memory.

Ultrastructural effects of sleep and wake on the parallel fiber synapses of the cerebellum

Multiple evidence in rodents shows that the strength of excitatory synapses in the cerebral cortex and hippocampus is greater after wake than after sleep. The widespread synaptic weakening afforded by sleep is believed to keep the cost of synaptic activity under control, promote memory consolidation, and prevent synaptic saturation, thus preserving the brain's ability to learn day after day. The cerebellum is highly plastic and the Purkinje cells, the sole output neurons of the cerebellar cortex, are endowed with a staggering number of excitatory parallel fiber synapses. However, whether these synapses are affected by sleep and wake is unknown. Here, we used serial block face scanning electron microscopy to obtain the full 3D reconstruction of more than 7000 spines and their parallel fiber synapses in the mouse posterior vermis. This analysis was done in mice whose cortical and hippocampal synapses were previously measured, revealing that average synaptic size was lower after sleep compared to wake with no major changes in synapse number. Here, instead, we find that while the average size of parallel fiber synapses does not change, the number of branched synapses is reduced in half after sleep compared to after wake, corresponding to ~16% of all spines after wake and ~8% after sleep. Branched synapses are harbored by two or more spines sharing the same neck and, as also shown here, are almost always contacted by different parallel fibers. These findings suggest that during wake, coincidences of firing over parallel fibers may translate into the formation of synapses converging on the same branched spine, which may be especially effective in driving Purkinje cells to fire. By contrast, sleep may promote the off-line pruning of branched synapses that were formed due to spurious coincidences.

Stress Resilience is Associated with Hippocampal Synaptoprotection in the Female Rat Learned Helplessness Paradigm

The synaptogenic hypothesis of major depressive disorder implies that preventing the onset of depressive-like behavior also prevents the loss of hippocampal spine synapses. By applying the psychoactive drugs, diazepam and fluoxetine, we investigated whether blocking the development of helpless behavior by promoting stress resilience in the rat learned helplessness paradigm is associated with a synaptoprotective action in the hippocampus. Adult ovariectomized and intact female Sprague-Dawley rats (n = 297) were treated with either diazepam, fluoxetine, or vehicle, exposed to inescapable footshocks or sham stress, and tested in an active escape task to assess helpless behavior. Escape-evoked corticosterone secretion, as well as remodeling of hippocampal spine synapses at a timepoint representing the onset of escape testing were also analyzed. In ovariectomized females, treatment with diazepam prior to stress exposure prevented helpless behavior, blocked the loss of hippocampal spine synapses, and muted the corticosterone surge evoked by escape testing. Although fluoxetine stimulated escape performance and hippocampal synaptogenesis under non-stressed conditions, almost all responses to fluoxetine were abolished following exposure to inescapable stress. Only a much higher dose of fluoxetine was capable of partly reproducing the strong protective actions of diazepam. Importantly, these protective actions were retained in the presence of ovarian hormones. Our findings indicate that stress resilience is associated with the preservation of spine synapses in the hippocampus, raising the possibility that, besides synaptogenesis, hippocampal synaptoprotection is also implicated in antidepressant therapy.

Rapid 3D Enhanced Resolution Microscopy Reveals Diversity in Dendritic Spinule Dynamics, Regulation, and Function

Dendritic spinules are thin protrusions, formed by neuronal spines, not adequately resolved by diffraction-limited light microscopy, which has limited our understanding of their behavior. Here we performed rapid structured illumination microscopy and enhanced resolution confocal microscopy to study spatiotemporal spinule dynamics in cortical pyramidal neurons. Spinules recurred at the same locations on mushroom spine heads. Most were short-lived, dynamic, exploratory, and originated near simple PSDs, whereas a subset was long-lived, elongated, and associated with complex PSDs. These subtypes were differentially regulated by Ca²⁺ transients. Furthermore, the postsynaptic Rac1-GEF kalirin-7 regulated spinule formation, elongation, and recurrence. Long-lived spinules often contained PSD fragments, contacted distal presynaptic terminals, and formed secondary synapses. NMDAR activation increased spinule number, length, and contact with distal presynaptic elements. Spinule subsets, dynamics, and recurrence were validated in cortical neurons of acute brain slices. Thus, we identified unique properties, regulatory mechanisms, and functions of spinule subtypes, supporting roles in neuronal connectivity.

The role of CaMKII-Tiam1 complex on learning and memory

A stimulation inducing long-term potentiation (LTP) of synaptic transmission induces a persistent expansion of dendritic spines, a phenomenon known as structural LTP (sLTP). We previously proposed that the formation of a reciprocally activating kinase-effector complex (RAKEC) between CaMKII and Tiam1, an activator of the small G-protein Rac1, locks CaMKII into an active conformation, which in turn maintains the phosphorylation status of Tiam1. This makes Rac1 persistently active, specifically in the stimulated spine. To understand the significance of the CaMKII-Tiam1 RAKEC in vivo, we generated a Tiam1 mutant knock-in mouse line in which critical residues for CaMKII binding were mutated into alanines. We confirmed the central role of this interaction on sLTP by observing that KI mice showed reduced Rac1 activity, had smaller spines and a diminished sLTP as compared to their wild-type littermates. Moreover, behavioral tests showed that the novel object recognition memory of these animals was impaired. We thus propose that the CaMKII-Tiam1 interaction regulates spine morphology in vivo and is required for memory storage.

Age-Induced Loss of Mossy Fibre Synapses on CA3 Thorns in the CA3 Stratum Lucidum

Advanced ageing is associated with hippocampal deterioration and mild cognitive decline. The hippocampal subregion CA3 stratum lucidum (CA3-SL) receives neuronal inputs from the giant mossy fibre boutons of the dentate gyrus, but relatively little is known about the integrity of this synaptic connection with ageing. Using serial electron microscopy and unbiased stereology, we examined age-related changes in mossy fibre synapses on CA3 thorny excrescences within the CA3-SL of young adults (4-month-old), middle-aged (12-month-old), and old-aged (28-month-old) Wistar rats. Our data show that while there is an increase in CA3 volume with ageing, there is a significant (40–45%) reduction in synaptic density within the CA3-SL of 12- and 28-month-old animals compared with 4-month-old animals. We also present preliminary data showing that the CA3 neuropil in advanced ageing was conspicuously full of lipofuscin and phagolysosome positive, activated microglial cellular processes, and altered perivascular pathology. These data suggest that synaptic density in the CA3-SL is significantly impaired in ageing, accompanied by underlying prominent ultrastructural glial and microvascular changes.

The morphological characteristics of corticostriatal and thalamostriatal neurons and their intrastriatal terminals in rats

PURPOSE

The glutamatergic projection from the cerebral cortex and the thalamus extensively innervates the neostriatal neurons. However, some conflicts in the published literatures about cortical and thalamic intrastriatal synaptic terminals still need to be resolved. The present study intends to further elucidate the morphological characteristics of these two types of the terminals and their neurons.

METHODS

The corticostriatal and thalamostriatal terminals were immunolabeled for vesicular glutamate transporter type 1 (VGluT1) and 2 (VGluT2), respectively, and their neurons were retrograde labeled by biotinylated dextran amine 3,000 molecular weight (BDA3k) injection into the dorsolateral striatum of rats. The characteristics of the corticostriatal and thalamostriatal terminals were observed at the LM and EM levels, and the data were statistically analyzed with SPSS10.0 software.

RESULTS

We observed that 63.53% of VGluT1+ terminals synapsed on dendritic spines, which was different from VGluT2+ terminals with the equal percentage of synapses on spines and dendrites (14.88 and 17.86%, respectively). Notably, VGluT1+ axospinous synaptic terminals were remarkably larger than VGluT2+ axospinous synaptic terminals. Terminal size-frequency distribution analysis showed that VGluT1+ terminals were within the size ranges of 0.4-0.5 and 0.8-0.9 μm, and VGluT2+ terminals were in the ranges of 0.4-0.5 and 0.6-0.7 μm. Perforated-postsynaptic densities (-PSDs) were more frequently found in VGluT1+ axospinous synaptic terminals than in VGluT2+ axospinous terminals. Furthermore, BDA3k-labeled corticostrital neurons were larger in perikaryal diameter than the thalamostriatal neurons, and they were also categorized as the two main populations based on their size-frequency distribution.

CONCLUSIONS

The morphological characteristics of corticostriatal and thalamostriatal terminals and neurons have implications for understanding the roles of synaptic plasticity in adaptive motor control by the basal ganglia, and they have facilitations for understanding the complexities of basal ganglia function.

Long-term treadmill exposure protects against age-related neurodegenerative change in the rat hippocampus

The potential of exercise or environmental enrichment to prevent or reverse age-related cognitive decline in rats has been widely investigated. The data suggest that the efficacy of these interventions as neuroprotectants may depend upon the duration and nature of the protocols and age of onset. Investigations of the mechanisms underlying these neuroprotective strategies indicate a potential role for the neurotrophin family of proteins, including nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF). In this study, we have assessed the effects of 8 months of forced exercise, begun in middle-age, on the expression of long-term potentiation (LTP) and on spatial learning in the Morris water maze in aged Wistar rats. We also assessed these measures in a cage control group and in a group of rats exposed to the stationary treadmill for the same duration as the exercised rats. Our data confirm an age-related decline in expression of LTP and in spatial learning concomitant with decreased expression of NGF and BDNF mRNA in dentate gyrus (DG). The age-related impairments in both plasticity and growth factor expression were prevented in the long-term exercised group and, surprisingly, the treadmill control group. Given the extensive handling that the treadmill control group received and their regular exposure to an environment outside the home cage, this group can be considered to have experienced environmentally enriched conditions when compared with the cage control group. Significant correlations were observed between both learning and LTP and the expression of NGF and BDNF mRNA in the dentate gyrus. We conclude that decreased expression of NGF and BDNF in the dentate gyrus of aged rats is associated with impaired LTP and spatial learning. We suggest that the reversal of these age-related impairments by enrichment and exercise may be linked with prevention of the age-related decline in expression of these growth factors and, furthermore, that enrichment is as efficacious as exercise in preventing this age-related decline.

Age-dependent impairment of spine morphology and synaptic plasticity in hippocampal CA1 neurons of a presenilin 1 transgenic mouse model of Alzheimer's disease

Presenilin 1 (PS1) mutations are responsible for a majority of early onset familial Alzheimer's disease (FAD) cases, in part by increasing the production of Abeta peptides. However, emerging evidence suggests other possible effects of PS1 on synaptic dysfunction where PS1 might contribute to the pathology independent of Abeta. We chose to study the L286V mutation, an aggressive FAD mutation which has never been analyzed at the electrophysiological and morphological levels. In addition, we analyzed for the first time the long term effects of wild-type human PS1 overexpression. We investigated the consequences of the overexpression of either wild-type human PS1 (hPS1) or the L286V mutated PS1 variant (mutPS1) on synaptic functions by analyzing synaptic plasticity and associated spine density changes from 3 to 15 months of age. We found that mutPS1 induces a transient increase observed only in 4- to 5-month-old mutPS1 animals in NMDA receptor (NMDA-R)-mediated responses and LTP compared with hPS1 mice and nontransgenic littermates. The increase in synaptic functions is concomitant with an increase in spine density. With increasing age, however, we found that the overexpression of human wild-type PS1 progressively decreased NMDA-R-mediated synaptic transmission and LTP, without neurodegeneration. These results identify for the first time a transient increase in synaptic function associated with L286V mutated PS1 variant in an age-dependent manner. In addition, they support the view that the PS1 overexpression promotes synaptic dysfunction in an Abeta-independent manner and underline the crucial role of PS1 during both normal and pathological aging.

Balancing structure and function at hippocampal dendritic spines

Dendritic spines are the primary recipients of excitatory input in the central nervous system. They provide biochemical compartments that locally control the signaling mechanisms at individual synapses. Hippocampal spines show structural plasticity as the basis for the physiological changes in synaptic efficacy that underlie learning and memory. Spine structure is regulated by molecular mechanisms that are fine-tuned and adjusted according to developmental age, level and direction of synaptic activity, specific brain region, and exact behavioral or experimental conditions. Reciprocal changes between the structure and function of spines impact both local and global integration of signals within dendrites. Advances in imaging and computing technologies may provide the resources needed to reconstruct entire neural circuits. Key to this endeavor is having sufficient resolution to determine the extrinsic factors (such as perisynaptic astroglia) and the intrinsic factors (such as core subcellular organelles) that are required to build and maintain synapses.

WITHDRAWN: Dendritic Spine and Synapse Morphological Alterations Induced by a Neural Cell Adhesion Molecule (NCAM) Mimetic

The neural cell adhesion molecule (NCAM) is a glycoprotein expressed on the surface of neurons and glial cells. It plays a key role in morphogenesis of the nervous system, regeneration of damaged neural tissue and synaptic plasticity. The extracellular domain of NCAM engages in homophilic interactions (NCAM binding to NCAM) and in heterophilic interactions between NCAM and other proteins such as the fibroblast growth factor (FGF) receptor. It promotes synaptogenesis and activity-dependent remodelling of synapses but less is know of its influence on synaptic and dendritic morphology. Recently, quantitative electron microscopy and 3-dimensional reconstruction (3-D) of ultrathin serial sections has been used to examine the morphology of synapses and dendritic spines in the hippocampus of rats treated with a neural cell adhesion molecule-derived fibroblast growth factor receptor agonist, FGL-peptide (an NCAM mimetic). These data show clearly that the FGL peptide has marked influences on both spine and synaptic form.

Pathway specificity of dendritic spine morphology in identified synapses onto rat hippocampal CA1 neurons in organotypic slices

The output of the hippocampus is largely determined by interaction of the three excitatory pathways that impinge on CA1 pyramidal neurons. These synapses, formed by axons of: (1) CA3 pyramidal neurons; (2) neurons of the entorhinal cortex (EC); and (3) neighboring CA1 neurons, are all potentially plastic. Here, we take advantage of the accessibility of the organotypic slice preparation to identify the type of spines with which each of these pathways forms synapses, at different developmental stages. Recent reports have shown that morphology of dendritic spines is activity-dependent with large mushroom spines being thought to represent stronger synaptic connections than thin or stubby spines. Although in a wide range of preparations, mushroom spines represent only 15% of spines across the whole dendritic tree, we find that this proportion is highly pathway specific. Thus in organotypic slices, the axons of CA3 neurons form synapses with mushroom spines on CA1 neurons in approximately 50% of cases, whereas this spine type is rare (<10%) in either of the other two pathways. This high proportion of mushroom spines only occurs after spontaneous excitatory activity in the CA1 cells increases over the second week in vitro. Previous studies suggest that pathway specificity also occurs in vivo. In tissue fixed in vivo, it is the synapses of distal apical dendrites thought to be formed by axons originating in the EC that are richer in mushroom spines. Hence, contrary to previous suggestions, the proportion of mushroom spines is clearly not an intrinsic property of the pathway but rather a characteristic dependent on the environment. We suggest that this is most likely a result of the previous activity of the synapses. The fact that, despite the large differences in pathway specificity between preparations, the overall proportion of different spine types remains unchanged, suggests a strong influence of homeostasis across the network.

Labeling of dendritic spines with the carbocyanine dye DiI for confocal microscopic imaging in lightly fixed cortical slices

Visualization of dendritic spines is an important tool for researches on structural synaptic plasticity. Fluorescent labeling of the dendrites and spines followed by confocal microscopy permits imaging a large population of dendritic spines with a higher resolution. We sought to establish an optimal protocol to label neurons in cortical slices with the carbocyanine dye DiI for confocal microscopic imaging of dendritic spines. DiI finely labeled dendrites and spines in slices prefixed (by cardiac perfusion) with 1.5% paraformaldehyde to the similar extent that could be achieved in live preparation. In contrast, fixation with 4% paraformaldehyde severely compromised dye diffusion. Confocal microscopy showed that structural integrity of dendrites and spines was preserved much better in lightly (1.5%) fixed slices than those prepared without fixation. Quantitative measurement revealed that spine density was lower in live slices than that counted in lightly fixed slices, suggesting that fixation is necessary for an adequate evaluation of spine density. The quality of confocal microscopic images obtained from lightly fixed slices allowed us to observe distinctive morphologies such as branched spines and dendritic filopodium, which may be indicative of structural changes at synapses. This method will thus be useful for studying structural synaptic plasticity.

Chemically induced long-term potentiation increases the number of perforated and complex postsynaptic densities but does not alter dendritic spine volume in CA1 of adult mouse hippocampal slices

Examination of the morphological correlates of long-term potentiation (LTP) in the hippocampus requires the analysis of both the presynaptic and postsynaptic elements. However, ultrastructural measurements of synapses and dendritic spines following LTP induced via tetanic stimulation presents the difficulty that not all synapses examined are necessarily activated. To overcome this limitation, and to ensure that a very large proportion of the synapses and spines examined have been potentiated, we induced LTP in acute hippocampal slices of adult mice by addition of tetraethylammonium (TEA) to a modified CSF containing an elevated concentration of Ca(2+) and no Mg(+). Quantitative electron microscope morphometric analyses and three-dimensional (3-D) reconstructions of both dendritic spines and postsynaptic densities (PSDs) in CA1 stratum radiatum were made on serial ultrathin sections. One hour after chemical LTP induction the proportion of macular (unperforated) synapses decreased (50%) whilst the number of synapses with simple perforated and complex PSDs (nonmacular) increased significantly (17%), without significant changes in volume and surface area of the PSD. In addition, the surface area of mushroom spines increased significantly (13%) whilst there were no volume differences in either mushroom or thin spines, or in surface area of thin spines. CA1 stratum radiatum contained multiple-synapse en passant axons as well as multiple-synapse spines, which were unaffected by chemical LTP. Our results suggest that chemical LTP induces active dendritic spine remodelling and correlates with a change in the weight and strength of synaptic transmission as shown by the increase in the proportion of nonmacular synapses.

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.

Dendritic branch typing and spine expression patterns in cortical nonpyramidal cells

To understand the dendritic differentiation in various types of cortical nonpyramidal cells, we analyzed quantitatively their dendritic branching and spine expression. The dendritic internode and interspine interval obeyed exponential distributions with type-specific decay constants. The initial branching pattern, internode interval and spine density at the light microscopic level divided nonpyramidal cells into three dendritic types, correlated with axonal, neurochemical and firing types. The initial branching pattern determined the overall vertical spread of dendrites. Basket cell subtypes with different firing and chemical expression patterns were distinct in the vertical and horizontal spatial spread, providing diverse input territories. Internode densities of dendritic spines, as well as those of axonal synaptic boutons, did not correlate with the tortuosities and intervals, suggesting a tendency to distribute synapses homogeneously over the arbor. Dendritic spines identified at the electron microscopic level were different in length and shape among subtypes. Although the density was lower than that of pyramidal cells, spines themselves were also composed of several morphological types such as mushroom and multihead ones, which were expressed differentially among subtypes. Correlation of dendritic branching characteristics with differences in spine structure suggests distinct ways to receive specific inputs among the subtypes.

Glutamate induces the rapid formation of spine head protrusions in hippocampal slice cultures

Synaptic plasticity at neuronal connections has been well characterized functionally by using electrophysiological approaches, but the structural basis for this phenomenon remains controversial. We have studied the dynamic interactions between presynaptic and postsynaptic structures labeled with FM 4-64 and a membrane-targeted GFP, respectively, in hippocampal slices. Under conditions of reduced neuronal activity (1 muM tetrodotoxin), we observed extension of glutamate receptor-dependent processes from dendritic spines of CA1 pyramidal cells to presynaptic boutons. The formation of these spine head protrusions is blocked by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor antagonists and by agents that reduce the release of glutamate from presynaptic terminals. Moreover, spine head protrusions form in response to exogenously applied glutamate, with clear directionality toward the glutamate electrode. Our results suggest that spontaneously released glutamate is sufficient to activate nearby spines, which can then lead to the growth of new postsynaptic processes connecting to a presynaptic site. Spines thus can compare their recent history with that of neighboring synapses and modify local connectivity accordingly.

Reconstruct: a free editor for serial section microscopy

Many microscopy studies require reconstruction from serial sections, a method of analysis that is sometimes difficult and time-consuming. When each section is cut, mounted and imaged separately, section images must be montaged and realigned to accurately analyse and visualize the three-dimensional (3D) structure. Reconstruct is a free editor designed to facilitate montaging, alignment, analysis and visualization of serial sections. The methods used by Reconstruct for organizing, transforming and displaying data enable the analysis of series with large numbers of sections and images over a large range of magnifications by making efficient use of computer memory. Alignments can correct for some types of non-linear deformations, including cracks and folds, as often encountered in serial electron microscopy. A large number of different structures can be easily traced and placed together in a single 3D scene that can be animated or saved. As a flexible editor, Reconstruct can reduce the time and resources expended for serial section studies and allows a larger tissue volume to be analysed more quickly.

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.

Remodelling of synaptic morphology but unchanged synaptic density during late phase long-term potentiation(ltp): A serial section electron micrograph study in the dentate gyrus in the anaesthetised rat

In anaesthetised rats, long-term potentiation (LTP) was induced unilaterally in the dentate gyrus by tetanic stimulation of the perforant path. Animals were killed 6 h after LTP induction and dendritic spines and synapses in tetanised and untetanised (contralateral) hippocampal tissue from the middle molecular layer (MML) were examined in the electron microscope using stereological analysis. Three-dimensional reconstructions were also used for the first time in LTP studies in vivo, with up to 130 ultrathin serial sections analysed per MML dendritic segment. A volume sampling procedure revealed no significant changes in hippocampal volume after LTP and an unbiased counting method demonstrated no significant changes in synapse density in potentiated compared with control tissue. In the potentiated hemisphere, there were changes in the proportion of different spine types and their synaptic contacts. We found an increase in the percentage of synapses on thin dendritic spines, a decrease in synapses on both stubby spines and dendritic shafts, but no change in the proportion of synapses on mushroom spines. Analysis of three-dimensional reconstructions of thin and mushroom spines following LTP induction revealed a significant increase in their volume and area. We also found an increase in volume and area of unperforated (macular) and perforated (segmented) postsynaptic densities. Our data demonstrate that whilst there is no change in synapse density 6 h after the induction of LTP in vivo, there is a considerable restructuring of pre-existing synapses, with shaft and stubby spines transforming to thin dendritic spines, and mushroom spines changing only in shape and volume.