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PubMed2002

Ultrastructural organization of lamprey reticulospinal synapses in three dimensions.

The giant reticulospinal synapse in lamprey provides a unique model to study synaptic vesicle traffic. The axon permits microinjections, and the active zones are often separated from each other, which makes it possible to track vesicle cycling at individual release sites. However, the proportion of reticulospinal synapses with individual active zones ("simple synapses") is unknown and a quantitative description of their organization is lacking. Here, we report such data obtained by serial section analysis, intermediate-voltage electron microscopy, and electron tomography. The simple synapse was the most common type (78%). It consisted of one active zone contacting one dendritic process. The remaining synapses were "complex," mostly containing one vesicle cluster and two to three active zones synapsing with distinct dendritic shafts. Occasional axosomatic synapses with multiple active zones forming synapses with the same cell were also observed. The vast majority of active zones in all synapse types contained both chemical and electrotonic synaptic specializations. Quantitative analysis of simple synapses showed that the majority had active zones with a diameter of 0.8-1.8 microm. The number of synaptic vesicles and the height of the vesicle cluster in middle sections of serially cut synapses correlated with the active zone length within, but not above, this size range. Electron tomography of simple synapses revealed small filaments between the clustered synaptic vesicles. A single vesicle could be in contact with up to 12 filaments. Another type of filament, also associated with synaptic vesicles, emerged from dense projections. Up to six filaments could be traced from one dense projection.

Animals↗
PubMed2003

Changes in mouse barrel synapses consequent to sensory deprivation from birth.

Neonatal sensory deprivation induced by whisker trimming affects significantly the functional organization of receptive fields in adult barrel cortex. In this study, the effects of deprivation on thalamocortical synapses and on asymmetrical and symmetrical synapses not of thalamic origin were examined. Thalamocortical synapses were labeled by lesion-induced degeneration in adult (postnatal day 60) mice subjected to whisker trimming from birth, other synaptic types were unlabeled. Brains were processed for electron microscopy, and numerical densities of synapses were evaluated by using stereologic approaches for whisker trimmed vs. control animals. Results demonstrated no change in nonthalamic, asymmetrical synapses; however, a decrease of 52% in the numerical density of symmetrical synapses (46.3 vs. 88.5 million per mm(3); Z = -2.121; P < 0.05) and a decrease of 43% in the numerical density of thalamocortical synapses (57.5 vs. 102.33 million per mm(3); Z = -2.121; P < 0.05) were observed after deprivation. Thus, experience-dependent plasticity of receptive fields in barrel cortex involves directly axons of both extrinsic and intracortical origin. The proportion of thalamocortical axospinous to axodendritic synapses was the same in control vs. deprived animals: in each instance, 80% of the synapses were axospinous (Z = 0.85; P = 0.2). These results suggest that neither excitatory neurons, whose thalamocortical synapses are primarily axospinous, nor inhibitory neurons, whose thalamocortical synapses are mainly axodendritic (White [1989] Cortical Circuits. Synaptic Organization of the Cerebral Cortex; Structure, Function, and Theory. 1989; Boston: Birkhauser), are affected preferentially by the deprivation-associated decrease in thalamocortical synapses.

Animals↗
PubMed1976

Synapses in the interpeduncular nucleus: electron microscopy of normal and habenula lesioned rats.

Four types of synapses have been identified in the rat interpeduncular nucleus. The S synapses were formed by the mainhorizontal plexus of habenulointerpeduncular axons. They contained spherical synaptic vesicles and formed asymmetrical en passant contacts with dendritic processes. The crest synapses had all of these features in common with the S synapses, but occurred in pairs with the two contact zones coextensive and largely parallel on opposite sides of markedly attenuated dendritic processes. Synaptic glomeruli found in this nucleus consisted of several crest synapses engulfed by sheet-like dendritic processes. Identical crest synapses also occurred on dendritic crests. In a limited number of cases, both S and crest synapses arose in common from single axons. Following destruction of the habenular nuclei unilaterally or bilaterally, as expected from the above observations, both S and crest synapses underwent dense degeneration. In some cases with bilateral lesions and in all cases with unilateral lesions, only one of the two axonal endings forming a crest synapse degenerated while the other remained unaffected. Axons containing flat vesicles forming symmetrical axodendritic synapses, axosomatic synapses, and subjunctional bodies associated with S or crest synapses were all minor features of this nucleus. Aspects of the unusual synaptology of this nucleus are discussed in terms of morphology, physiology and transmitter chemistry.

Animals↗
PubMed1981

Quantitative study of synapse formation in the duck olfactory bulb.

Synaptic emergence and development in the duck olfactory bulb was quantitatively studied by electron microscopy from the 14th day of incubation (E 14) to the adult stage. Overall synaptic density in this bulb grew considerably during the last weeks of embryonic life and the first postnatal week. The pattern of synaptic density development was similar in the four main architectonic layers of the bulb. However, lower density values were observed in the mitral and inner granule cell layers. In the glomerular layer (GL), axodendritic synapse density was always higher than dendrodentritic synapse density. In the external plexiform layer, most synapses were dendrodendritic and were established between the gemmules of the granule cells (GC) and the dendrites of the mitral cells (MC) or tufted cells (TC). Synapses established by MC and TC on GC gemmules, or by GC on MC and TC dendrites had densities very similar to each other at all the stages studied. Reciprocal synapses already appeared at E 14; their density grew until a week after birth (P7) and thereafter remained stable. In the internal granular layer, the density of asymmetrical synapses was always higher than that of symmetrical synapses. Excitatory synapses formed earlier on MC and TC than inhibitory synapses. The ratio of inhibitory-to-excitatory synapses rose rapidly after birth, reaching 2.5 in the adult duck. The density of excitatory synapses received by granule cells was as high in the external plexiform layer as in the inner granule layer, at all stages of GC development. However, the ratio of received-to-formed synapses fell in these cells from 8.42 at E 14 to 2 after birth. These results are discussed as a function of the evolution of the different synaptic balances during olfactory bulb development. Synaptic development in the duck olfactory bulb at birth is relatively close to the adult state. It appears sufficiently advanced to enable the olfactory system to function in a way compatible with the relatively independent behavior displayed by the duckling.

Aging↗
PubMed1988

Optic synapse number but not density is constrained during regeneration onto surgically halved tectum in goldfish: HRP-EM evidence that optic fibers compete for fixed numbers of postsynaptic sites on the tectum.

The number of optic synapses in the half tectum of goldfish was counted by using an improved HRP-labeling protocol and a columnar sampling method that spanned the entire optic innervation layer, S-SO-SFGS. It was previously found by using this procedure in intact tectum that the normal number of optic synapses was regenerated by 30 days and maintained thereafter even in the absence of impulse activity. This suggested that the number of synapses in this system was intrinsically fixed. In order to examine whether this limit was imposed by optic fibers or by target cells, optic synapses were counted in surgically halved tecta which received compressed optic projections consisting of regenerating optic fibers from the entire retina. We reasoned that if synapse number is a function of the number of afferents, then there should be twice the normal number of optic synapses per column; on the other hand, if their number is fixed by target, then their number per column should be normal. We found that the number of optic (labeled) synapses was normal in sample columns from fish at 70 days and 160 days after optic nerve crush. Thus, retinal ganglion cells, on average, formed half as many synapses on the half tectum compared to intact tectum, indicating the number of optic synapses was limited by the tectum. The number of nonoptic (unlabeled) synapses was also found to be normal. By contrast, the S-SO-SFGS was found to be 88-103% thicker compared to normal fish, apparently because of a 20-fold increase in the number of optic fibers. As a result, the density of synapses was about half normal in half tecta, and so, in contrast to synapse number, synaptic density is not constrained during regeneration. We infer from these data that optic fibers compete for limited numbers of postsynaptic sites during regeneration and suggest that this competition promotes neural map refinement and the various plasticities described for this projection.

Animals↗
PubMed1987

Determination of the numerical density of perforated synapses in rat neocortex.

The numerical density and frequency of perforated synapses in the molecular layer of rat parietal cortex have been determined using 4 procedures in an attempt to overcome problems associated with the size and complex three-dimensional shape of perforated synapses. The following procedures were compared: A, single-section analysis; B, adjacent-section analysis; C, semi-serial-section analysis; and D, complete serial-section analysis. All procedures made use of an unbiased counting rule. Estimates of the numerical density of perforated synapses ranged from 0.06 to 0.27 X 10(9) mm-3, and that of all synapses (non-perforated and perforated) from 1.88 to 2.50 X 10(9) mm-3. The frequency of perforated synapses varied from 4.5 to 18.0%. Procedures B (adjacent-section analysis) and D (complete serial-section analysis), neither of which utilize assumptions regarding the shape of synapses, produced comparable results (numerical density of perforated synapses 0.19-0.27 X 10(9) mm-3, and of all synapses 2.24-2.45 X 10(9) mm-3; frequency of perforated synapses 8.6-10.9%). The frequency of perforated synapses appeared to be underestimated by procedure A (single section analysis; 4.5%) and overestimated by C (semi-serial section analysis; 18%). It is concluded that adjacent-section analysis is the most efficient and effective procedure for determining the numerical density and frequency of complex particles, such as perforated synapses. There is, however, no significant difference in the performance of this procedure compared with that of single-section analysis, for determining the numerical density of synapses in general. Nevertheless, inherent problems of bias within the single-section procedure make the adjacent section method the procedure of choice.

Animals↗
PubMed1996

Ultrastructure of synapses in the first-evolved nervous systems.

The phylum Cnidaria represents the first group of animals to evolve a recognizable nervous system. A comparison of the ultrastructural features of synaptic loci in animals representing all four classes of the cnidaria has provided an overview of the first-evolved synapses that can be compared morphologically to synapses in higher forms. Synapses in these watery jellylike animals with unmyelinated axons are sparse and difficult to fix well. However, we now have sufficient evidence to define an early synapse as one with paired electron dense plasma membranes separated by a 13-25 nm gap containing intracleft filaments and with vesicles on one or both sides of the synaptic cleft. The vesicles are of three types: dense-cored, clear, and opaque. Neuromuscular synapses resemble neuronal synapses and lack the postsynaptic specializations of higher animals. However, some coelenterates, such as the jellyfish Chrysaora, have a postsynaptic cisterna in the muscle. Neuromuscular and neuronematocyte synapses can have either clear or dense-cored vesicles. Opaque vesicles at two-way interneuronal synapses and at neuromuscular synapses in the oral sphincter muscle of sea anemones can be labelled with antisera to the neuropeptides Antho-RFamide (Antho-Arg-Phe-NH2) and Antho-RWamides (Antho-Arg-Trp-NH2) I and II, respectively. That suggests that neuropeptides evolved as neurotransmitters early in the animal kingdom. The basic differences between first evolved synapses and synapses of higher animals are the lack of postjunctional folds at neuromuscular synapses and the presence of fewer and somewhat larger synaptic vesicles, generally containing granular cores, in the more primitive animals.

Animals↗
PubMed2004

Schwann cells and astrocytes induce synapse formation by spinal motor neurons in culture.

Glia constitute 90% of cells in the human nervous system, but relatively little is known about their functions. We have been focusing on the potential synaptic roles of glia in the CNS. We recently found that astrocytes increase the number of mature, functional synapses on retinal ganglion cells (RGCs) by sevenfold and are required for synaptic maintenance in vitro. These observations raised the question of whether glia similarly enhance synapse formation by other neuron types. Here we have investigated whether highly purified motor neurons isolated from developing rat spinal cords are able to form synapses in the absence of glia or whether glia similarly enhance synapse number. We show that spinal motor neurons (SMNs) form few synapses unless Schwann cells or astrocytes are present. Schwann cells increase the number of functional synapses by ninefold as measured by immunostaining, and increase spontaneous synaptic activity by several hundredfold. Surprisingly, the synapses formed between spinal motor neurons were primarily glutamatergic, as they could be blocked by CNQX. This synapse-promoting activity is not mediated by direct glial-neuronal cell contact but rather is mediated by secreted molecule(s) from the Schwann cells, as we previously found for astrocytes. Interestingly, the synapse-promoting activity from astrocytes and Schwann cells was functionally similar: Schwann cells also promoted synapse formation between retinal ganglion cells, and astrocytes promoted synapse formation between spinal motor neurons. These studies show that both astrocytes and Schwann cells strongly promote synapse formation between spinal motor neurons and demonstrate that glial regulation of synaptogenesis extends to other neuron types.

Animals↗
PubMedDate not supplied

Silent synapses in neural plasticity: current evidence.

Silent synapses, defined as structural specializations for neurotransmission that do not produce a physiological response in the receiving cell, may occur frequently in neural circuits. Their recruitment to physiological effectiveness may be an important component of circuit modification. In several nervous systems, evidence from electrophysiological and optophysiological measurements has established a strong case for the existence of silent synapses and for their emergence as active synapses with appropriate stimulation. During normal development and aging, synapses of individual neurons change in number, and many of these may be functionally silent at certain stages of their developmental trajectory. Changes in their status may contribute to shaping the properties of neural pathways during development, often in response to neural activity. In general, it is often difficult to distinguish physiological emergence of pre-established silent synapses from developmental maturation or de novo formation of new synapses. Several possible mechanisms for silent synapses and their recruitment are reviewed. These include incompletely assembled synapses that lack structural components, insufficient availability of key presynaptic proteins, and nonfunctional postsynaptic receptors, or presence of receptors that do not mediate a postsynaptic response except under specific conditions (conditionally silent synapses). The available silent synapses can often be rapidly activated, and conversely, active synapses appear to be rapidly silenced in many instances. These properties enable silent synapses to participate in short-term facilitation and depression. In addition, they may contribute to long-term facilitation and potentiation, especially during development.

Animals↗
PubMed2006

Differential dependence of axo-dendritic and axo-somatic GABAergic synapses on GABAA receptors containing the alpha1 subunit in Purkinje cells.

Synapse formation and maintenance require extensive transsynaptic interactions involving multiple signal transduction pathways. In the cerebellum, Purkinje cells (PCs) receive GABAergic, axo-dendritic synapses from stellate cells and axo-somatic synapses from basket cells, both with GABAA receptors containing the alpha1 subunit. Here, we investigated the effects of a targeted deletion of the alpha1 subunit gene on GABAergic synaptogenesis in PCs, using electrophysiology and immunoelectron microscopy. Whole-cell patch-clamp recordings in acute slices revealed that PCs from alpha1(0/0) mice lack spontaneous and evoked IPSCs, demonstrating that assembly of functional GABAA receptors requires the alpha1 subunit. Ultrastructurally, stellate cell synapses on PC dendrites were reduced by 75%, whereas basket cell synapses on the soma were not affected, despite the lack of GABAA-mediated synaptic transmission. Most strikingly, GABAergic terminals were retained in the molecular layer of adult alpha1(0/0) mice and formed heterologous synapses with PC spines characterized by a well differentiated asymmetric postsynaptic density. These synapses lacked presynaptic glutamatergic markers and postsynaptic AMPA-type glutamate receptors but contained delta2-glutamate receptors. During postnatal development, initial steps of GABAergic synapse formation were qualitatively normal, and heterologous synapses appeared in parallel with maturation of dendritic spines. These results suggest that synapse formation in the cerebellum is governed by neurotransmitter-independent mechanisms. However, in the absence of GABAA-mediated transmission, GABAergic terminals in the molecular layer apparently become responsive to synaptogenic signals from PC spines and form stable heterologous synapses. In contrast, maintenance of axo-somatic GABAergic synapses does not depend on functional GABAA receptors, suggesting differential regulation in distinct subcellular compartments.

Animals↗
PubMed1999

Three-dimensional relationships between hippocampal synapses and astrocytes.

Recent studies show that glutamate transporter-mediated currents occur in astrocytes when glutamate is released from hippocampal synapses. These transporters remove excess glutamate from the extracellular space, thereby facilitating synaptic input specificity and preventing neurotoxicity. Little is known about the position of astrocytic processes at hippocampal synapses. Serial electron microscopy and three-dimensional analyses were used to investigate structural relationships between astrocytes and synapses in stratum radiatum of hippocampal area CA1 in the mature rat in vivo and in slices. Only 57 +/- 11% of the synapses had astrocytic processes apposed to them. Of these, the astrocytic processes surrounded less than half (0.43 +/- 22) of the synaptic interface. Other studies suggest that astrocytes extend processes toward higher concentrations of glutamate; thus the presence of astrocytic processes at particular hippocampal synapses might signal which ones are releasing glutamate. The distance between nearest neighboring synapses was usually (approximately 95%) <1 microgram. Astrocytic processes occurred along the extracellular path between 33% of the neighboring synapses, neuronal processes occurred along the path between another 66% of the neighboring synapses, and only 1% of the synapses were close enough such that neither astrocytic nor neuronal processes occurred between them. These morphological arrangements suggest that the glutamate released at approximately two-thirds of hippocampal synapses might diffuse to other synapses, unless neuronal glutamate transporters are more effective than previously reported. The findings also suggest that physiological recordings made from hippocampal astrocytes do not uniformly sample the glutamate released from all hippocampal synapses.

Animals↗
PubMed1992

Hormonal regulation of motor unit size and synaptic strength during synapse elimination in the rat levator ani muscle.

Previous anatomical studies suggest that androgen regulates synapse elimination in the androgen-sensitive levator ani(LA) muscle of the rat. Androgen treatment beginning on postnatal day 7 (P7) prevents some of the normal loss of multiaxonal innervation in this muscle. The present study used physiological techniques to measure the number and size of LA motor units during the synapse elimination period in muscles from normals, and castrates treated with either testosterone propionate or oil. The number of increments in LA twitch tension as nerve stimulation intensity increased, a measure of the number of motor units, was the same at the end (P28) of synapse elimination as near the beginning (P7) of this process. This result indicates that motoneuronal cell death does not contribute to synapse elimination in the LA. Moreover, androgen during this period did not influence the number of LA motor units. In contrast, between P7 and P28, there was a dramatic decline in the size of LA motor units, as indicated by a decrease in the percentage of twitch or tetanus tension of individual motor units relative to the maximal twitch or tetanus tension of the whole muscle. In addition, androgen treatment of castrated males during this period prevented some of the normal decline in the size of LA motor units. Estimates of the number of inputs per LA muscle fiber derived from the number of LA motor units and their average size indicate that androgen maintains polyneuronal innervation in the LA muscle. This finding supports previous anatomical studies suggesting that androgen can prevent synapse elimination in this muscle. The strength of LA synapses was also examined by measuring the tetanus: twitch ratio of individual motor units and by measuring the safety margin of LA synapses. Both measurements indicated that the average strength of LA synapses increases during synapse elimination. Moreover, androgen appeared to spare synapses from elimination without increasing their strength, since androgen-treated muscles generally had larger motor units but the same mean tetanus:twitch ratio and safety margins as untreated LA muscles except at P28, when synapses in androgen-treated LA muscles had appreciably lower safety margins than normal. These results suggest that androgen regulates synapse elimination through a mechanism(s) independent of synaptic strength.

Androgens↗
PubMed2003

Ultrastructural features of sprouted mossy fiber synapses in kindled and kainic acid-treated rats.

The mossy fiber pathway in the dentate gyrus undergoes sprouting and synaptic reorganization in response to seizures. The types of new synapses, their location and number, and the identity of their postsynaptic targets determine the functional properties of the reorganized circuitry. The goal of this study was to characterize the types and proportions of sprouted mossy fiber synapses in kindled and kainic acid-treated rats. In normal rats, synapses labeled by Timm histochemistry or dynorphin immunohistochemistry were rarely observed in the supragranular region of the inner molecular layer when examined by electron microscopy. In epileptic rats, sprouted mossy fiber synaptic terminals were frequently observed. The ultrastructural analysis of the types of sprouted synapses revealed that 1) in the supragranular region, labeled synaptic profiles were more frequently axospinous than axodendritic, and many axospinous synapses were perforated; 2) sprouted mossy fiber synaptic terminals formed exclusively asymmetric, putatively excitatory synapses with dendritic spines and shafts in the supragranular region and with the soma of granule cells in the granule cell layer; 3) in contrast to the large sprouted mossy fiber synapses in resected human epileptic hippocampus, the synapses formed by sprouted mossy fibers in rats were smaller; and 4) in several cases, the postsynaptic targets of sprouted synapses were identified as granule cells, but, in one case, a sprouted synaptic terminal formed a synapse with an inhibitory interneuron. The results demonstrate that axospinous asymmetric synapses are the most common type of synapse formed by sprouted mossy fiber terminals, supporting the viewpoint that most sprouted mossy fibers contribute to recurrent excitation in epilepsy.

Animals↗
PubMed1984

Intramembrane organization of synapses in the lobster stretch receptor organ.

The intramembrane organization of axodendritic and neuromuscular synapses in the lobster stretch receptor organ was investigated by freeze-fracturing. Based on ultrastructural criteria which are known to be correlated with physiological properties, we identified three types of synapse: the inhibitory axodendritic, the inhibitory neuromuscular, and the excitatory neuromuscular synapse. Although these synapses have some features in common, each has a characteristic arrangement of intramembrane particles in both the presynaptic and postsynaptic membranes. All three have, in their presynaptic membranes, aggregates of P-face particles and associated depressions representing sites of synaptic vesicle exocytosis, features which together define active zones. However, in the inhibitory axodendritic synapse the P-face contains short ridges in this region. These ridges may occur singly or in pairs oriented in V-shaped configurations. The ridges are decorated with particles along their entire length. In the inhibitory neuromuscular synapse, no ridges are present. Clusters of particles are present, but they are scattered randomly over a large expanse of presynaptic membrane. In the excitatory neuromuscular synapse, isolated clusters of particles are associated with the P-face and are occasionally located on circular elevations of the membrane. The postsynaptic membrane also shows structural diversity in the three types of synapse. In the inhibitory axodendritic synapse, there is no apparent specialization. However, in the inhibitory neuromuscular synapse, P-face particles are arranged in double rows which are separated by particle-free strips of membrane. In the excitatory neuromuscular synapse, particles are confined to a narrow band that borders the synaptic cleft. This band is demarcated by a single intermittent strand of particles arranged in the direction of the long axis of the muscle fibre. Therefore, intramembrane specializations of both the presynaptic and postsynaptic membranes are sufficiently distinctive that three different types of synapse can be recognized.

Animals↗
PubMed1997

Compound synapses within the GABAergic innervation of the auditory inner hair cells in the adolescent mouse.

Ultrastructural investigation of the gamma-aminobutyric acid (GABA) component of the inner spiral bundle in adolescent mice revealed a pathway of glutamic acid decarboxylase (GAD)-positive and -negative fibers and vesiculated endings that contact inner hair cells and their afferents through a complex of axosomatic and axodendritic synapses. Ultrastructural details were investigated by using conventional electron microscopy. Several synaptic arrangements were observed: Main axosomatic synapses form between vesiculated endings and individual or adjoining inner hair cells (interreceptor synapses). Spinous synapses form on long, spinelike processes that protrude from inner hair cells to reach distant efferent endings. The efferent endings associate with inner hair cells and their synaptic afferents through compound synapses-serial, "converging," and triadic-otherwise characteristic of sensory relay nuclei. Serial synapses form by the sequential presynaptic alignment of the efferent-->receptor-->afferent components. Converging synapses result from the simultaneous apposition of a receptor ribbon synapse and a presynaptic efferent terminal on a recipient afferent dendrite. Triadic synapses comprise a vesiculated efferent ending in contact with an inner hair cell and with its synaptic afferent. Additionally, efferent endings may form simple axodendritic and axoaxonal synapses with GAD-negative vesiculated endings. The combination of different synaptic arrangements leads to short chains of compound synapses. It is assumed that these synaptic patterns seen in the adolescent mouse represent adult synaptology. The patterns of synaptic connectivity suggest an integrative role for the GABA/GAD lateral efferent system, and imply its involvement in the pre- and postsynaptic modulation of auditory signals.

Animals↗
PubMed1999

Ultrastructure of neuro-spirocyte synapses in the sea anemone Aiptasia pallida (Cnidaria, Anthozoa, Zoantharia).

Using transmission electron microscopy of serially sectioned tentacles from the sea anemone Aiptasia pallida, we located and characterized two types of neuro-spirocyte synapses. Clear vesicles were observed at 10 synapses and dense-cored vesicles at five synapses. The diameters of vesicles at each neuro-spirocyte synapse were averaged; clear vesicles ranged from 49-89 nm in diameter, whereas the dense-cored vesicles ranged from 97-120 nm in diameter. One sequential pair of synapses included a neuro-spirocyte synapse with clear vesicles (81 nm) and a neuro-neuronal synapse with dense-cored vesicles (168 nm). A second synapse on the same cell had dense-cored vesicles (103 nm). An Antho-RFamide-labeled ganglion cell and three different neurites were observed adjacent to spirocytes, but no neuro-spirocyte synapses were present. Many of the spirocytes also were immunoreactive to Antho-RFamide. The presence of sequential neuro-neuro-spirocyte synapses suggests that synaptic modulation may be involved in the neural control of spirocyst discharge. The occurrence of either dense-cored or clear vesicles at neuro-spirocyte synapses suggests that at least two types of neurotransmitter substances control the discharge of spirocysts in sea anemones.

Animals↗
PubMed2006

Structural basis for developmentally regulated changes in cadherin function at synapses.

Members of the cadherin family of calcium-dependent cell adhesion molecules can bind homophilically across central nervous system (CNS) synapses, but experimental evidence indicates the nature of their contribution to synapse structure and function changes over time. We asked whether changes in function correspond to differences in intrasynaptic distribution. Using quantitative immuno-electronmicroscopy, we determined where cadherins are localized within synapses at key developmental stages in cultured hippocampal neurons and in hippocampus in vivo. At 5-6 days in culture, when most synapses are newly formed, cadherins are regularly and evenly distributed at synaptic clefts throughout the active zone. In contrast, at 14 days, when the majority of synapses are comparatively mature, cadherin labeling concentrates in discrete clusters. Such clusters can occur at any place within or immediately surrounding synaptic clefts. To assess whether this change in distribution is unique to neurons grown in culture, we compared the distribution of cadherins in the CA1 region of hippocampus at postnatal days 2, 3 (P2-3) and in adult. Consistent with our observations in cultured neurons, synapses in P2-3 hippocampus most often exhibit cadherins distributed regularly throughout the cleft, while adult synapses show predominantly discrete concentrations at single sites. The early developmental pattern of cadherin distribution can also be detected at occasional synapses in adult tissue. Such synapses also have morphological features consistent with immature synapses, suggesting that intrasynaptic cadherin distribution is a feature that may distinguish synapse age.

Age Factors↗
PubMed1985

A laminar analysis of the number of round-asymmetrical and flat-symmetrical synapses on spines, dendritic trunks, and cell bodies in area 17 of the cat.

The number of synapses per unit volume of tissue (NV) has been estimated in individual laminae of the binocular and monocular regions of area 17 in six adult cats by using a method of size-frequency distribution. Separate estimates were obtained for RA synapses (containing round vesicles associated with asymmetric membrane differentiations) and for FS synapses (containing flat vesicles associated with symmetric membrane differentiations). For the total cortical thickness, the NV of all synapses is not statistically different between binocular (286 million per mm1(3] and monocular (281 million) regions, nor is it different between the two regions for any of the laminae. Eighty-four percent of synapses are of the RA type. Of those, 79% are found on dendritic spines, 21% on dendritic trunks, 0.1% on somata. FS synapses represent 16% of the total, with 31% of them on spines, 62% on dendritic trunks, and 7% on somata. The ratio of RA to FS synapses is kept relatively constant throughout the layers. A two-way analysis of variance shows no difference in the NV of either RA or FS synapses in the two regions nor in the NV or RA synapses between cats. It does, however, clearly demonstrate (p less than 0.001) interindividual differences for FS synapses. These variations between individual cats may be due to differences in age, breed, or environmental factors. In contrast to the relative uniformity of the NV of synapses between regions, the number of each type under 1 mm2 of cortical surface is 33% higher in the binocular region. This is due mainly to the greater thickness of the binocular region.

Animals↗