Miniature synaptic events maintain dendritic spines via AMPA receptor activation

We investigated the influence of synaptically released glutamate on postsynaptic structure by comparing the effects of deafferentation, receptor antagonists and blockers of glutamate release in hippocampal slice cultures. CA1 pyramidal cell spine density and length decreased after transection of Schaffer collaterals and after application of AMPA receptor antagonists or botulinum toxin to unlesioned cultures. Loss of spines induced by lesion or by botulinum toxin was prevented by simultaneous AMPA application. Tetrodotoxin did not affect spine density. Synaptically released glutamate thus exerts a trophic effect on spines by acting at AMPA receptors. We conclude that AMPA receptor activation by spontaneous vesicular glutamate release is sufficient to maintain dendritic spines.     

Quantitative studies on regional differences in Purkinje cell dendritic spines and parallel fiber synaptic density

Volume densities, surface densities, length densities and numerical densities of several structures in the neocerebellar lobule VIa and the archicerebellar lobule X of six-month old male Han: WIST-rats were estimated by point- and intersection-counting. The volume densities of dendritic spines (ca. 6.5%), parallel fiber varicosities (ca. 25%) and processes of Bergmann glial cells (ca. 21%) were similar in the upper third of the molecular layer of lobule VIa and X respectively. The surface density of the spine membrane was 31 mm2/mm3 in lobule X and 32 mm2/mm3 in lobule VIa (p = 0.4375; paired Pitman permutation test). The length density of dendritic spines varied from 793 meters/mm3 in lobule VIa to 675 meters/mm3 in lobule X (p = 0.0938). The mean caliper diameter of parallel fiber-Purkinje cell synapses was estimated by Mayhew's (1979) method and calculated by Cruz-Orive's (1983) computer program. Both tests yielded nearly identical numerical densities of parallel fiber synapses in lobule VIa (6.558 X 10(8)/mm3) and in lobule X (4.892 X 10(8)/mm3; p = 0.0313). The area of synaptic apposition relative to the postsynaptic dendritic spine surface was higher in lobule VIa (13.3%) than in lobule X (10.4%; p = 0.0313). The data provide electron microscopic evidence of regional differences in spine morphology, which together with different spiny branchlet diameter and numerical density of parallel fiber synapses may be of importance in Purkinje cell physiology.     

Three-dimensional analysis of dendritic spines

Summary The glial envelope of dendritic spines in the visual and cerebellar cortices was evaluated by analysis of serial sections. Three-dimensional reconstructions of the protoplasmic astrocyte processes were made and the quantitative proportions of the glial cover on dendritic spines on spiny branchlets of Purkinje cells are, with the exception of afferent axon terminals, completely covered by the glial sheath (74.44%), dendritic spines of pyramidal cells are only partially covered (28.89%), so that spine stalks and even synaptic clefts frequently lack glial isolation. A new, relatively frequent configuration of subsurface cistern-astrocyte process — dendritic spine is described. A possible functional significance of the differences in the glial ensheathment of dendritic spines in visual and cerebellar cortices is discussed.     

Three-Dimensional analysis of dendritic spines

Summary A total of 212 dendritic spines (108 from the visual and 104 from cerebellar cortices of the mouse) were analyzed in serial sections. Dendritic spines (DS) and synaptic active zones (SAZ) were classified according to their shape, and the following quantitative data were measured: DS stalk and bulb diameters, DS length and volume, number of cisterns of the spine apparatus, DS and SAZ surface areas and their mutual proportions. Quantitative relationships between the spine apparatus and the size of DS and SAZ, between the volume and surface area of DS and between the size of DS and the size of SAZ were studied. Thin, mushroom-shaped and stubby DS with simple (circular or oval), complex (perforated, annulate or horseshoe-shaped) and multifocal SAZ were found on terminal branches of pyramidal cell apical dendrites and club-shaped DS with simple (circular or oval) SAZ on spiny branchlets of Purkinje cells.Statistically significant differences were found between all values measured on various DS types in the visual cortex. Linear dependencies of the DS surface area on DS volume and of the SAZ surface area on the DS surface area were established. Only a limited area of DS plasma membrane (7–10%) was occupied by SAZ. This finding indicates a possible functional importance of the SAZ/DS (and possibly also of the total SAZ/total postsynaptic membrane) surface ratio.     

Vomeronasal neurons promote synaptic formation on dendritic spines but not dendritic shafts in primary culture of accessory olfactory bulb neurons

To investigate the morphological changes of accessory olfactory bulb (AOB) neurons arising from pheromonal signals, a coculture system of AOB neurons and vomeronasal (VN) neurons had been established. Our previous study indicates that under coculture condition, the density of dendritic spines of an AOB neuron is less and the individual spine-head volume is larger than those under monoculture condition. In this study, to determine whether these differences in the dendrites of AOB neurons reflect the differences in synapse formation and synaptic properties, we observed these cultured cells by electron microscopy. Various synapses were observed under each culture condition. Synapses were classified on the basis of their postsynaptic structure and the size of postsynaptic density (PSD) was measured. Under the coculture condition with VN neurons, synapses on dendritic spines, which formed between AOB neurons, were observed frequently. In contrast, many synapses were formed on dendritic shafts under monoculture condition. The PSD of asymmetrical synapses on the spines under coculture condition was larger than that under monoculture condition. Moreover, some dendrodendritic reciprocal synapses were found only in coculture. We confirmed synapse formation between VN axons and AOB dendrites by immunohistochemical electron microscopy; thus, the characteristics of synapses between AOB neurons are considered to be modified by the synaptic contacts with VN axons.     

Tyrosine hydroxylase-immunoreactive boutons in synaptic contact with identified striatonigral neurons, with particular reference to dendritic spines

Tyrosine hydroxylase-immunoreactive fibres in the rat neostriatum were studied in the electron microscope in order to determine the nature of the contacts they make with other neural elements. The larger varicose parts of such fibres contained relatively few vesicles and rarely displayed synaptic membrane specializations; however, thinner parts of axons (0.1-0.4 micron) contained many vesicles and had symmetrical membrane specializations, indicative of en passant type synapses. By far the most common postsynaptic targets of tyrosine hydroxylase-immunoreactive boutons were dendritic spines and shafts, although neuronal cell bodies and axon initial segments also received such input. Six striatonigral neurons in the ventral striatum were identified by retrograde labelling with horseradish peroxidase and their dendritic processes were revealed by Golgi impregnation using the section-Golgi procedure. The same sections were also developed to reveal tyrosine hydroxylase immunoreactivity and so we were able to study immunoreactive boutons in contact with the Golgi-impregnated striatonigral neurons. Each of the 280 immunoreactive boutons examined in the electron microscope displayed symmetrical synaptic membrane specializations: 59% of the boutons were in synaptic contact with the dendritic spines, 35% with the dendritic shafts and 6% with the cell bodies of striatonigral neurons. The dendritic spines of striatonigral neurons that received input from immunoreactive boutons invariably also received input, usually more distally, from unstained boutons that formed asymmetrical synaptic specializations. A study of 87 spines along the dendrites of an identified striatonigral neuron showed that the most common type of synaptic input was from an individual unstained bouton making asymmetrical synaptic contact (53%), while 39% of the spines received one asymmetrical synapse and one symmetrical immunoreactive synapse. It is proposed that the spatial distribution of presumed dopaminergic terminals in synaptic contact with different parts of striatonigral neurons has important functional implications. Those synapses on the cell body and proximal dendritic shafts might mediate a relatively non-selective inhibition. In contrast, the major dopaminergic input that occurs on the necks of dendritic spines is likely to be highly selective since it could prevent the excitatory input to the same spines from reaching the dendritic shaft. One of the main functions of dopamine released from nigrostriatal fibres might thus be to alter the pattern of firing of striatal output neurons by regulating their input.     

Rapid turnover of actin in dendritic spines and its regulation by activity

Dendritic spines are motile structures that contain high concentrations of filamentous actin. Using hippocampal neurons expressing fluorescent actin and the method of fluorescence recovery after photobleaching, we found that 85 +/- 2% of actin in the spine was dynamic, with a turnover time of 44.2 +/- 4.0 s. The rapid turnover is not compatible with current models invoking a large population of stable filaments and static coupling of filaments to postsynaptic components. Low-frequency stimulation known to induce long-term depression in these neurons stabilized nearly half the dynamic actin in the spine. This effect depended on the activation of N-methyl-D-aspartate (NMDA) receptors and the influx of calcium. In neurons from mice lacking gelsolin, a calcium-dependent actin-binding protein, activity-dependent stabilization of actin was impaired. Our studies provide new information on the kinetics of actin turnover in spines, its regulation by neural activity and the mechanisms involved in this regulation.     

Coincidence detection in single dendritic spines mediated by calcium release

Cerebellar long-term depression (LTD) is a calcium-dependent process in which coincident activity of parallel fiber (PF) and climbing fiber (CF) synapses causes a long-lasting decrease in PF synaptic strength onto Purkinje cells. Here we show that pairing CF activation with bursts of PF activity triggers large (>10 microM) calcium signals in Purkinje cell dendrites. When PFs are densely activated, signals span whole dendritic branchlets and are mediated by voltage-dependent calcium entry. When PFs are sparsely activated, however, signals are restricted to single spines and blocked by metabotropic glutamate receptor antagonists. Single-spine signals and sparse-stimulation LTD are also blocked by thapsigargin, indicating that calcium must be released from stores. Single-spine signals and sparse-stimulation LTD are greatest when PF activation precedes the CF activation within 50-200 ms. This timing rule matches the properties of several forms of motor learning, providing a link between behavior and functional properties of cerebellar synaptic plasticity.     

Plasticity of calcium channels in dendritic spines

Voltage-sensitive Ca2+ channels (VSCCs) constitute a major source of calcium ions in dendritic spines, but their function is unknown. Here we show that R-type VSCCs in spines of rat CA1 pyramidal neurons are depressed for at least 30 min after brief trains of back-propagating action potentials. Populations of channels in single spines are depressed stochastically and synchronously, independent of channels in the parent dendrite and other spines, implying that depression is the result of signaling restricted to individual spines. Induction of VSCC depression blocks theta-burst-induced long-term potentiation (LTP), indicating that postsynaptic action potentials can modulate synaptic plasticity by tuning VSCCs. Induction of depression requires [Ca2+] elevations and activation of L-type VSCCs, which activate Ca2+/calmodulin-dependent kinase II (CaMKII) and a cyclic adenosine monophosphate (cAMP)-dependent pathway. Given that L-type VSCCs do not contribute measurably to Ca2+ influx in spines, they must activate downstream effectors either directly through voltage-dependent conformational changes or via [Ca2+] microdomains.     

Spatial organization of cofilin in dendritic spines

Synaptic plasticity is associated with morphological changes in dendritic spines. The actin-based cytoskeleton plays a key role in regulating spine structure, and actin reorganization in spines is critical for the maintenance of long term potentiation. To test the hypothesis that a stable pool of F-actin rests in the spine "core," while a dynamic pool lies peripherally in its "shell," we performed immunoelectron microscopy in the stratum radiatum of rat hippocampus to elucidate the subcellular distribution of cofilin, an actin-depolymerizing protein that mediates reorganization of the actin cytoskeleton. We provide direct evidence that cofilin in spines avoids the core, and instead concentrates in the shell and within the postsynaptic density. These data suggest that cofilin may link synaptic plasticity to the actin remodeling that underlies changes in spine morphology.     

Density and morphology of dendritic spines in mouse neocortex

Dendritic spines of pyramidal cells are the main postsynaptic targets of cortical excitatory synapses and as such, they are fundamental both in neuronal plasticity and for the integration of excitatory inputs to pyramidal neurons. There is significant variation in the number and density of dendritic spines among pyramidal cells located in different cortical areas and species, especially in primates. This variation is believed to contribute to functional differences reported among cortical areas. In this study, we analyzed the density of dendritic spines in the motor, somatosensory and visuo-temporal regions of the mouse cerebral cortex. Over 17,000 individual spines on the basal dendrites of layer III pyramidal neurons were drawn and their morphologies compared among these cortical regions. In contrast to previous observations in primates, there was no significant difference in the density of spines along the dendrites of neurons in the mouse. However, systematic differences in spine dimensions (spine head size and spine neck length) were detected, whereby the largest spines were found in the motor region, followed by those in the somatosensory region and those in visuo-temporal region.     

Three-dimensional reconstruction of synapses and dendritic spines in the rat and ground squirrel hippocampus: New structural-functional paradigms for synaptic function

Published data are reviewed along with our own data on synaptic plasticity and rearrangements of synaptic organelles in the central nervous system. Contemporary laser scanning and confocal microscopy techniques are discussed, along with the use of serial ultrathin sections for in vivo and in vitro studies of dendritic spines, including those addressing relationships between morphological changes and the efficiency of synaptic transmission, especially in conditions of the long-term potentiation model. Different categories of dendritic spines and postsynaptic densities are analyzed, as are the roles of filopodia in originating spines. The role of serial ultrathin sections for unbiased quantitative stereological analysis and three-dimensional reconstruction is assessed. The authors data on the formation of more than two synapses on single mushroom spines on neurons in hippocampal field CA1 are discussed. Analysis of these data provides evidence for new paradigms in both the organization and functioning of synapses.Translated from Zhurnal Vysshei Nervnoi Deyatelnosti, Vol. 54, No. 1, pp, 120–129, January–February, 2004.     

Focal motility determines the geometry of dendritic spines

The geometry of dendritic spines has a major impact on signal transmission at excitatory synapses. To study it in detail we raised transgenic mice expressing an intrinsic green fluorescent protein-based plasma membrane marker that directly visualizes the cell surface of living neurons throughout the brain. Confocal imaging of developing hippocampal slices showed that as dendrites mature they switch from producing labile filopodia and polymorphic spine precursors to dendritic spines with morphologies similar to those reported from studies of adult brain. In images of live dendrites these mature spines are fundamentally stable structures, but retain morphological plasticity in the form of actin-rich lamellipodia at the tips of spine heads. In live mature dendrites up to 50% of spines had cup-shaped heads with prominent terminal lamellipodia whose motility produced constant alterations in the detailed geometry of the synaptic contact zone. The partial enveloping of presynaptic terminals by these cup-shaped spines coupled with rapid actin-driven changes in their shape may operate to fine-tune receptor distribution and neurotransmitter cross-talk at excitatory synapses.     

Propagation of dendritic spikes mediated by excitable spines: a continuum theory

1. Neuroscientists are currently hypothesizing on how voltage-dependent channels, in dendrites with spines, may be spatially distributed or how their numbers may divide between spine heads and the dendritic base. A new cable theory is formulated to investigate electrical interactions between many excitable and/or passive dendritic spines. The theory involves a continuum formulation in which the spine density, the membrane potential in spine heads, and the spine stem current vary continuously in space and time. The spines, however, interact only indirectly by voltage spread along the dendritic shaft. Active membrane in the spine heads is modeled with Hodgkin-Huxley (HH) kinetics. Synaptic currents are generated by transient conductance increases. For most simulations the membrane of spine stems and dendritic shaft is assumed passive. 2. Action-potential generation and propagation occur as localized excitatory synaptic input into spine heads causes a few excitable spines to fire, which then initiates a chain reaction of spine firings along a branch. This sustained wavelike response is possible for a certain range of spine densities and electrical parameters. Propagation is precluded for spine stem resistance (Rss) either too large or too small. Moreover, even if Rss lies in a suitable range for the local generation of an action potential (resulting from local synaptic excitatory input), this range may not be suitable to initiate a chain reaction of spine firings along the dendrite; success or failure of impulse propagation depends on an even narrower range of Rss values. 3. The success or failure of local excitation to spread as a chain reaction depends on the spatial distribution of spines. Impulse propagation is unlikely if the excitable spines are spaced too far apart. However, propagation may be recovered by redistributing the same number of equally spaced spines into clusters. 4. The spread of excitation in a distal dendritic arbor is also influenced by the branching geometry. Input to one branch can initiate a chain reaction that accelerates into the sister branch but rapidly attenuates as it enters the parent branch. In branched dendrites with many excitable and passive spines, regions of decreased conductance load (e.g., near sealed ends) can facilitate attenuating waves and enhance waves that are successfully propagating. Regions of increased conductance load (e.g., near common branch points) promote attenuation and tend to block propagation. Non-uniform loading and/or nonuniform spine densities can lead to complex propagation characteristics. 5. Some analytic results of classical cable theory are generalized for the case of a passive spiny dendritic cable.(ABSTRACT TRUNCATED AT 400 WORDS)     

Loss of dendritic spines in aging cerebral cortex

Summary Previous work has shown that the dendritic spines of pyramidal neurons of the cerebral cortex are sensitive to a wide variety of environmental and surgical manipulations. The present study shows that the normal aging process also affects these spines. The spines were studied with the light microscope in Golgi preparations from rats ranging in age from 3 to 29.5 months. Visible spines were counted on either 25 or 50 segments of the basal dendrites, apical dendrites, oblique branches, and terminal tufts of layer V pyramidal cells in area 17. A progressive loss of spines occurred at each of these loci. The smallest observed spine loss (24%) occurred on the dendrites of the terminal tuft, and the largest (40%) on the oblique branches. Age-related spine loss appears to affect all animals, and for animals of any one age the overall loss is similar. However, the cell-to-cell variability within an individual animal is pronounced, some cells with high spine densities being present at every age examined. As a general rule, there is a positive relationship between visible spine density along the apical dendrite as it traverses layer IV and the thickness of the dendrite. With advancing age, the relatively thick dendrites decrease in number so that the thinner dendrites make up an increasingly larger proportion of the total apical dendrite population. Questions that remain for the future include the genesis of the spine loss, its relation to other aging changes, and its functional significance for the neuron.Supported by United States Public Health Service Program Project Grant HDO-5796-03 and Research Grant NB-07016     

Lateral organization of endocytic machinery in dendritic spines

Postsynaptic membrane trafficking plays an important role in synaptic plasticity, but the organization of trafficking machinery within dendritic spines is poorly understood. We use immunocytochemical analysis of rat hippocampal neurons to show that proteins mediating endocytosis are systematically arrayed within dendritic spines, tangential to the synapse. Thus, previously unrecognized lateral domains of the spine organize endocytic protein machinery at sites removed from the postsynaptic density.     

Passive normalization of synaptic integration influenced by dendritic architecture

We examined how biophysical properties and neuronal morphology affect the propagation of individual postsynaptic potentials (PSPs) from synaptic inputs to the soma. This analysis is based on evidence that individual synaptic activations do not reduce local driving force significantly in most central neurons, so each synapse acts approximately as a current source. Therefore the spread of PSPs throughout a dendritic tree can be described in terms of transfer impedance (Z(c)), which reflects how a current applied at one location affects membrane potential at other locations. We addressed this topic through four lines of study and uncovered new implications of neuronal morphology for synaptic integration. First, Z(c) was considered in terms of two-port theory and contrasted with dendrosomatic voltage transfer. Second, equivalent cylinder models were used to compare the spatial profiles of Z(c) and dendrosomatic voltage transfer. These simulations showed that Z(c) is less affected by dendritic location than voltage transfer is. Third, compartmental models based on morphological reconstructions of five different neuron types were used to calculate Z(c), input impedance (Z(N)), and voltage transfer throughout the dendritic tree. For all neurons, there was no significant variation of Z(c) with location within higher-order dendrites. Furthermore, Z(c) was relatively independent of synaptic location throughout the entire cell in three of the five neuron types (CA3 interneurons, CA3 pyramidal neurons, and dentate granule cells). This was quite unlike Z(N), which increased with distance from the soma and was responsible for a parallel decrease of voltage transfer. Fourth, simulations of fast excitatory PSPs (EPSPs) were consistent with the analysis of Z(c); peak EPSP amplitude varied <20% in the same three neuron types, a phenomenon that we call "passive synaptic normalization" to underscore the fact that it does not require active currents. We conclude that the presence of a long primary dendrite, as in CA1 or neocortical pyramidal cells, favors substantial location-dependent variability of somatic PSP amplitude. In neurons that lack long primary dendrites, however, PSP amplitude at the soma will be much less dependent on synaptic location.     

The function of dendritic spines: a theoretical study

A modeling procedure is proposed which introduces the cable equivalent of dendritic spines into the Rall model of spiny interneurons in the spinal cord. At this point combined morphological and physiological works have given some insight into the possible role of a single spine and the function of a single spine has been studied by theoretical computations [Jack, Noble and Tsien (1975) Electric Current Flow in Excitable Cells, pp. 218-223. Oxford University Press, Oxford; Koch and Poggio (1983) Trends Neurosci. 6, 80-83; Perkel (1983) J. Physiol., Paris 78, 695-699]. The goal of the present paper is two-fold: (a) to stress the gross function of the spine system in the excitability of dendrites; and (b) to emphasize the role of spines in the dynamic input/output function of neurons. The simulation procedure is based on the well-known compartmental method. (1) The kinetics of active somatic and dendritic compartments are taken from a currently available spinal interneuron model to match the physiological data of large dorsal horn neurons carrying spines. (2) Beside the prolongation of the somatic excitatory postsynaptic potential, the model suggests that the spiny neuron increases the differences in the latency and height of excitatory postsynaptic potential as a function of the electrotonic position of input. The characteristics of the excitatory postsynaptic potential can be modified by the changes in spine geometry and the ratio of cytoplasmic resistances of spine stalk to that of main dendritic shaft. (3) Dendritic electroresponsiveness, which was already postulated for dorsal horn neurons, is analysed by the model including calcium and slow potassium systems. It is concluded that the participation of the spine stalk in active processes can highly modify the input dependence of response pattern. Depolarization-dependent Ca2+ accumulation in spines may reflect the interaction of spine stalks. (4) Passive antidromic spread of action potential can be suppressed in spiny cells. Analysis of active antidromic spread shows the probable importance of spines located near the soma. Centripetal vs centrifugal conduction of dendritic action potential may depend on the spine distribution along the tree and change in electrical parameters of spines.     

一种快速简单显示神经元树突棘的Golgi-Cox法(英文)

现存很多高尔基染色法有成功率低、产生沉淀或操作程序复杂等缺点。本文报道了我们发展的显示大脑皮层神经元树突棘的另一种Gogli-Cox染色方法。其方法是将动物灌注固定后取脑并将全脑浸于Golgi-Cox液中2周,然后浸于30%蔗糖中;反应采用100微米厚的震动切片,经过蒸馏水洗涤、氨化、酸性坚膜定影液反应、蒸馏水再洗涤,然后切片经上升梯度酒精脱水、透明,裱片后观察。结果显示,神经元的形状和树突棘标记十分清楚;相比之下,我们采用的另一种应用K2S的Golgi-Cox法则标记质量很差。本研究结果提示我们使用酸性坚膜定影液的变通Golgi-Cox染色法是一种简单而有效的标记神经元树突棘的方法。